pdf file - Istituto Nazionale di Oceanografia e di Geofisica
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pdf file - Istituto Nazionale di Oceanografia e di Geofisica
Vol. 51 - SUPPLEMENT Bollettino di Geofisica teorica ed applicata An International Journal of Earth Sciences Guest Editors: A. Tassone, E. Lodolo, M. Menichetti, A. Rapalini International Geological Congress on the Southern Hemisphere Scientific Contributions of the GeoSur2010 22-23 November, 2010 Mar del Plata, Argentina Istituto Nazionale di Oceanografia e di Geofisica Sperimentale ISSN 0006-6729 Responsibility for all statements made in B.G.T.A. lies with the authors Cover design: Nino Bon, OGS Printing: ArgenGraphics – Buenos Aires, Argentina Authorized by the Tribunale di Trieste, n. 242, September 17, 1960 INTERNATIONAL SYMPOSIUM International Geological Congress of the Southern Hemisphere 22-23 November 2010 Mar del Plata, Argentina SCIENTIFIC CONTRIBUTIONS GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA ORGANIZERS ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE - OGS, TRIESTE, ITALY INSTITUTO DE GEOFÍSICA “DANIEL A. VALENCIO”, UNIVERSIDAD DE BUENOS AIRES, ARGENTINA DIPARTIMENTO DI SCIENZE GEOLOGICHE, UNIVERSITÀ DI URBINO, ITALY 4 GEOSUR2004 22-23 NOVEMBER 2010 – MAR DEL PLATA Conference Organizers ALEJANDRO TASSONE - CONICET. INGEODAV. Universidad de Buenos Aires, Argentina EMANUELE LODOLO - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy MARCO MENICHETTI - Universitá di Urbino, Italy AUGUSTO RAPALINI - CONICET. INGEODAV. Universidad de Buenos Aires, Argentina JOSÉ CARCIONE - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy Local Organizing Committee JUAN FRANCISCO VILAS - CONICET. INGEODAV. Universidad de Buenos Aires, Argentina HORACIO LIPPAI - CONICET. INGEODAV. Universidad de Buenos Aires, Argentina FEDERICO ISLA - CONICET. Universidad Nacional de Mar del Plata, Argentina GABRIELE PAPARO - Italian Embassy in Buenos Aires, Argentina JOSÉ LUÍS HORMAECHEA - Estación Astronómica Rio Grande (CONICET-UNLP), Argentina FEDERICO ESTEBAN - CONICET, INGEODAV. Universidad de Buenos Aires, Argentina JAVIER PERONI - CONICET, INGEODAV. Universidad de Buenos Aires, Argentina MARIA ELENA CERREDO - CONICET. Universidad de Buenos Aires, Argentina MARÍA PAULA IGLESIA LLANOS - CONICET. INGEODAV. Universidad de Buenos Aires, Argentina Scientific Committee ASTINI RICARDO - CONICET. Universidad Nacional de Cordoba, Argentina BEN-AVRAHAM ZVI - University of Tel Aviv, Israel CANALS MIQUEL - Universitat de Barcelona, Spain CAWOOD PETER - Western Australian University CONCHEIRO ANDREA - CONICET, Dpto. Geologia, Universidad de Buenos Aires, Argentina CORDANI UMBERTO - Universidade do Sao Paulo, Brazil DALZIEL IAN - Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, USA DE BATIST MARC - Renard Centre of Marine Geology, Gent, Belgium GAMBOA LUIS - PetroBras, Brazil GHIDELLA MARTA - Instituto Antártico Argentino, Buenos Aires GOTZE JURGEN-HANS - Universität Otto Hahn, Germany HARTNADY CHRIS - Umvoto Africa (Pty) Ltd., South Africa HERNANDEZ-MOLINA JAVIER - Dpto. de Geociencias Marinas, Universidad de Vigo, Spain HERVÉ FRANCISCO - Universidad de Santiago de Chile JOKAT WILFRIED - Alfred Wegener Institute, Bremerhaven, Germany LARTER ROBERT - British Antarctic Survey, UK LEITCHENKOV GERMAN - Institute for Geology and Mineral Resources of the World Ocean, St. Petersburg, Russia LÓPEZ DE LUCHI MÓNICA - Instituto de Geología Isotópica. CONICET-UBA, Argentina MARENSSI SERGIO - Intituto Antártico Argentino, Buenos Aires PATERLINI MARCELO - Servicio de Hidrografía Naval, Argentina PERILLO GERARDO - Instituto Argentino de Oceanografía, Argentina RAMOS VICTOR - Universidad de Buenos Aires, Argentina RAPELA CARLOS - CONICET. Centro de Investigaciones Geológicas, La Plata, Argentina RENZULLI ALBERTO - Università di Urbino, Italy RUSSI MARINO - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy RUZZANTE JOSÉ - Comisión Nacional de Energía Atómica. ICES. Argentina SOMOZA LUÍS - Instituto Geológico y Minero de España, Madrid, Spain SPALETTI LUÍS - CONICET. Centro de Investigaciones Geológicas, La Plata, Argentina Conference Secretariat CAROLINA GAMBA – Fundación Ciencias Exactas y Naturales MARGHERITA PERSI – Ufficio Comunicazione Istituzionale - OGS, Trieste, Italy 5 CONTENTS Session 1 - Rodinia in South America RODINIA ANCESTRIES OF NEOPROTEROZOIC–EARLY PALEOZOIC SEDIMENTARY ROCKS OF SOUTHERN SOUTH AMERICA, THE ROSS SEA REGIONS OF ANTARCTICA, ZEALANDIA AND SOUTHEAST AUSTRALIA: POSSIBLE ORIGINS IN THE SOUTH CHINA BLOCK. . . . . . . . . . .1-01 C.J. Adams WESTERN PRECORDILLERA OPHIOLITE BELT: CORRELATIONS BETWEEN CORDÓN DEL PEÑASCO AND CORTADERAS LOCALITIES (MENDOZA PROVINCE, ARGENTINA) . . . . . . . .1-02 L. Boedo Florencia, I. Vujovich, Graciela I. PAMPIA: A FRAGMENT OF THE AUTHOCHTONOUS MESOPROTEROZOIC OROGEN OF WESTERN RÍO DE LA PLATA CRATON. ITS DETACHMENT DURING RODINIA`S BREAK-UP, AND RE-ACCRETION DURING GONDWANA´S AMALGAMATION . . . . . . . . . . . . . . . . . . . . . . . . . . .1-03 C.J. Chernicoff, E.O. Zappettini, J.O.S. Santos NEW INSIGHTS ON THE PALEOPROTEROZOIC BASEMENT OF TANDILIA BELT, RÍO DE LA PLATA CRATON, ARGENTINA: FIRST HF ISOTOPE STUDIES ON ZIRCON CRYSTALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-04 C. Cingolani, J.O.S. Santos, W. Griffin THE RODINIAN RELICS OF AUSTRALIA-MAWSON, AZANIA, NEOPROTEROZOIC INDIA AND A GREATER KALAHARI – DIVINING THEIR EXTENT AND INTERPRETING THEIR EVOLUTION. . . . . . . . . . . . . . . . . . . . .1-05 A.S. Collins, K. Selway, C. Clark, P.D. Kinny, D. Plavsa, U. Amarasinghe RODINIA: IS A RAPPROCHEMENT OF CURRENT ‘SOUTHERN OCEAN’ AND ‘NORTH ATLANTIC’ MODELS ACHIEVABLE? . . . . . . . . . . . . . . .1-06 Ian.W.D. Dalziel VOLCANISM IN THE WESTERN OUACHITA-CUYANIA BASIN AND SEPARATION OF LAURENTIA AND GONDWANA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-07 P.W. Dickerson, R.E. Hanson, J.M. Roberts, C.M. Fanning GEOCHEMISTRY, GEOCHRONOLOGY AND PALEOMAGNETISM OF PALEOPROTEROZOIC GRANITES OF THE ULKAN MASSIF, SE SIBERIAN CRATON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-08 A.N. Didenko, V.A. Guryanov, A.Yu. Peskov, A.N. Perestoronin, D.V. Avdeev THE AGE AND SIGNIFICANCE OF THE PUNCOVISCANA FORMATION WITH RESPECT TO NEOPROTEOZOIC TO CAMBRIAN TECTONIC EVOLUTION OF THE PROTO-ANDEAN MARGIN OF GONDWANA. . . . . . . . . . . . .1-09 M. Escayola, C. van Staal THE PUTUMAYO OROGEN OF NORTHWEST SOUTH AMERICA: IMPLICATIONS FOR RODINIAN CONNECTIONS BETWEEN AMAZONIA, BALTICA AND THE MIDDLE- AMERICAN OAXAQUIAN TERRANES. . . . . . . . . . . . . . . . . . . . .1-10 M. Ibanez-Mejia, J. Ruiz, V. Valencia, A. Cardona, G. Gehrels, A. Mora, P. DeCelles COUPLED DELAMINATION AND INDENTOR-ESCAPE TECTONICS IN THE SOUTHERN PART OF THE C. 650-500 MA EAST AFRICAN/ANTARCTIC OROGEN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-11 J. Jacobs, R.T. Thomas, K. Ueda, I. Kleinhanns, B. Emmel, R. Kumar, B Bingen, A. Engvik SM-ND ISOTOPIC CONSTRAINTS ON THE NEOPROTEROZOIC – EARLY PALEOZOIC EVOLUTION OF THE EASTERN SIERRAS PAMPEANAS . . . . . . . . . . . . .1-12 M.G. López de Luchi, A. Steenken, C.I. Martínez Dopic, M. Drobe, K. Wemmer, S. Siegesmund 7 CONTENTS PALEOMAGNETIC POLE FOR THE NEOPROTEROZOIC DIKES OF THE NICO PÉREZ TERRANE (URUGUAY) AND THE APPARENT POLAR WANDER PATH (APWP) FOR THE RÍO DE LA PLATA CRATON . . . . . . . . . . . . . . . . . .1-13 A.L. Lossada, A.E. Rapalini, L. Sanchez Bettucci ORDOVICIAN MAGMATISM IN THE NORTHEASTERN NORTH PATAGONIAN MASSIF: FURTHER EVIDENCE FOR THE CONTINUITY OF THE FAMATINIAN OROGEN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-14 C.I. Martínez Dopico, M.G. López de Luchi, K. Wemmer, A.E. Rapalini, E. Linares IAPETAN EVOLUTION OF APPALACHIAN PERI-LAURENTIAN AND PERI-GONDWANAN ARC COMPLEXES: A NEWFOUNDLAND COMPARISON . . . . . . . .1-15 Brian. H. O’Brien U- PB ZIRCON GEOCHRONOLOGY OF THE SIERRA VALLE FÉRTIL, FAMATINIAN ARC, ARGENTINA: PETROLOGICAL AND GEOLOGICAL IMPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-16 J.E. Otamendi, M.N. Ducea, G. Bergantz CERRO LA TUNA MAFIC TO ULTRAMAFIC COMPLEX: AN OCEAN FLOOR REMNANT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-17 E. Peel, M.A.S. Basei, L. Sánchez Bettucci GRENVILLE-AGE SOURCES IN CUESTA DE RAHUE, NORTHERN PATAGONIA: CONSTRAINS FROM U/PB SHRIMP AGES FROM DETRITAL ZIRCONS . . . . . . . . . . . . . . . . .1-18 V.A. Ramos, E García Morabito, F. Hervé, C.M. Fanning WAS THE RIO DE LA PLATA CRATON NEVER PART OF RODINIA? SOME PALEOMAGNETIC HINTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-19 A.E. Rapalini THE AFRICAN PROVENANCE OF SOUTHERN SOUTH AMERICA TERRANES A RECORD FROM RODINIA BREAK-UP TO GONDWANA ASSEMBLY . . . . . . . . . . . . . . . . . .1-20 C.W. Rapela, C.M. Fanning, C. Casquet,t R.J. Pankhurst, L.A. Spalletti, D. Poiré, E.G. Baldo THE PIEDRA ALTA TERRANE: A PALEOPROTEROZOIC JUVENILE MAGMATIC ARC, RIO DE LA PLATA CRATON, URUGUAY . . . . . . . . . . . . . . . . . . .1-21 L. Sánchez Bettucci, E. Peel, M.A.S. Basei FURTHER EVIDENCE FOR MULTIPLE REVERSALS IN THE NEOPROTEROZOIC ARARAS CAP CARBONATE (BRAZIL) . . . . . . . . . . . . . . . . . . . . .1-22 P. Sansjofre, R.I.F. Trindade, M. Ader , A.C.R. Nogueira, J.L. Soares PARAGUAY BELT FOLDING AND OROCLINAL BENDING DURING THE FINAL ASSEMBLY OF WESTERN GONDWANA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-23 R.I. Trindade, E. Tohver, A. C. Nogueira, C. Riccomini EVIDENCE FOR MIDDLE-LATE ORDOVICIAN SUBDUCTION AND A LOWER PLATE SETTING OF THE CUYANIA TERRANE DURING ITS ACCRETION TO THE PROTO-ANDEAN MARGIN OF GONDWANA . . . . . . . . . . . . . . . . . . . . .1-24 C. van Staa, G. Vujovich, K. Currie, M. Naipauer. LOW-PRESSURE ANATEXIS IN FAMATINIAN FORELAND OF ARGENTINA, SOUTH-WESTERN MARGIN OF GONDWANA: SOURCE HEAT PROBLEM. . . . . . . . . . . . . . .1-25 S.O. Verdecchia, E.G. Baldo THE MESO-NEOPROTEROZOIC SUBDUCTION-ACCRETION EVENTS AND MAGMATIC EVOLUTION ALONG THE WESTERN MARGIN OF THE SIBERIAN CRATON: TO THE PROBLEM OF RODINIA BREAK-UP . . . . . . . . . . . . . . .1-26 V.A. Vernikovsky, A.E. Vernikovskaya THE SUTURE ZONE BETWEEN CUYANIA AND CHILENIA TERRANES: A SUBDUCTION CHANNEL AND A-TYPE OROGEN? . . . . . . . . . . . . .1-27 G.I. Vujovich, F.L. Boedo, A.P Willner 8 CONTENTS Session 2 - Volcanism and Petrology EXOTIC EXHALATIONS FROM ACTIVE S-ANDES VOLCANOES: DOMUYO, TROMEN AND COPAHUE VOLCANOES, ARGENTINA . . . . . . . . . . . . . . . . . . . . . .2-01 A. Bermudez, D. Delpino, J. C. Varekamp, T. Kading CERRO NEGRO DEL GHÍO: MAGMATISM IN-BETWEEN SOUTHERN PATAGONIAN BATHOLITH AND LAGO BUENOS AIRES PLATEAU LAVAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-02 J. Castro, A. Sánchez, F. Hervé, M. de Saint Blaquat, M. Polvé MAGMATIC ACTIVITY AND STRIKE SLIP TECTONICS IN THE SOUTHERNMOST ANDES: KRANCK PLUTON, CHARACTERIZATION AND PRELIMINARY AMS SURVEY . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-03 M.E. Cerredo, M.B. Remesal, A.A. Tassone, J.I. Peroni, M. Menichetti, H. Lippai CHARACTERIZATION OF AN ULTRABASIC LAMPROPHYRE (EVOLVED DAMTJERNITE) IN THE TANDILIA BASEMENT, SOUTHERNMOST RÍO DE LA PLATA CRATON, ARGENTINA. . . . . . . . . . . . . . . . . . . . . . . . . .2-04 J.A. Dristas, J.C. Martínez, H.J. Massonne, K. Wemmer CONSTRAINTS ON OIB-TYPE PREMA AND EM1 MANTLE SOURCES FROM TRACE ELEMENT AND PB, SR, AND ND ISOTOPIC RATIOS OF PRIMITIVE EOCENE TO RECENT BACKARC PATAGONIAN BASALTS . . . . . . . . . . . . . . . .2-05 K.S. Mahlburg, J. Helen, G. Matthew SERRA GERAL VOLCANISM IN THE PROVINCE OF MISIONES (ARGENTINA): GEOCHEMICAL ASPECTS AND INTERPRETATION OF ITS GENESIS IN THE CONTEXT OF THE LARGE IGNEOUS PROVINCE PARANÁ-ETENDEKA-ANGOLA. ITS RELATION WITH THE ALKALINE VOLCANISM OF CÓRDOBA PROVINCE . . . . . . . . . . . . . . .2-06 S. Leonor Lagorio, H. Vizán LITHOLOGY AND AGE OF THE CUSHAMEN FORMATION. DEVONIAN MAGMATISM IN THE WESTERN NORTH PATAGONIAN MASSIF. ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-07 M.G. López de Luchi, M.E. Cerredo, C. Martínez Dopico METAMORPHIC EVOLUTION OF THE CINCO CERROS AREA, SIERRA DE TANDIL, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-08 H.J. Massonne, J. Dristas, J.C. Martinez PALEOMAGNETIC STUDIES OF CENOZOIC BASALTS FROM NORTHERN NEUQUÉN AND SOUTHERN MENDOZA PROVINCES: STRATIGRAPHIC IMPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-09 G.H. Re, J.F. Vilas, AMPHIBOLE MEGACRYSTS OF THE CERRO JEU-JEPÉN PLUTON: NEW CONSTRAINTS ON MAGMA SOURCE AND EVOLUTION (FUEGIAN ANDES, ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-10 F. Ridolfi, A. Renzulli, M.E. Cerredo, R. Oberti, M. Boiocchi, F. Bellatreccia, G. Della Ventura M. Menichetti, A. Tassone “CIRCULAR FEATURES” ON OLD SOLIDIFIED LAVA FLOW FIELDS ASSOCIATED WITH SOME YOUNG SCORIA CONES FROM LLANCANELO AND PAYÚN MATRU VOLCANIC FIELDS, MENDOZA PROVINCE, ARGENTINA. . . . . . . . . . . .2-11 C. Risso, K. Nemeth, F. Nullo, M. Inbar THE NEOGENE BARRIL NIYEU VOLCANIC COMPLEX SOMÚN CURÁ MAGMATIC PROVINCE. NORTHERN EXTRA ANDEAN PATAGONIA. ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-12 F.M. Salani, M. Remesal, M.E. Cerredo THE MAGNETIC SUSCEPTIBILITY OF IGNIMBRITES FROM THE ALTIPLANO- PUNA VOLCANIC COMPLEX, CENTRAL ANDES: A USEFUL TOOL TO DISTINGUISH LITHOMAGNETIC DOMAINS ACROSS THE ARC . . . . . .2-13 S.E. Singer 9 CONTENTS FIRST KIMBERLITE PIPE IN CENTRAL YAKUTIA (RUSSIA): MINERAL COMPOSITION AND THICKNESS OF LITHOSPHERIC MANTLE AND AGE. . . . . .2-14 A.P. Smelov, A.I. Zaitsev, I.V. Ashchepkov DEPOSITION AND REWORKING OF PRIMARY PYROCLASTIC DETRITUS IN PUESTO LA PALOMA MEMBER, CRETACEOUS CERRO BARCINO FORMATION, SOMUNCURÁ-CAÑADÓN ASFALTO BASIN, PATAGONIA, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-15 A.M. Umazano GEODYNAMIC THAT GENERATED THE CRETACEOUS VOLCANISM OF CÓRDOBA AND THE LARGE IGNEOUS PROVINCE OF PARANÁ, INCLUDING THE ORIGIN OF THE TRISTAN “PLUME” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-16 H. Vizán, S. Leonor Lagorio MAFIC MICROGRANULAR ENCLAVE SWARMS IN GRANITIC PLUTONS OF GASTRE, CENTRAL PATAGONIA . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-17 C.B. Zaffarana, R. Somoza Session 3 - Geophysical prospecting and Geodesy ANALYSIS OF A PRECISE REGIONAL GEOID MODEL IN BUENOS AIRES PROVINCE COMPUTED BY LEAST SQUARES COLLOCATION . . . . . . . . . .3-01 D. Bagú, D. Del Cogliano, M. Scheinert, R. Dietrich, J. Schwabe, L. Mendoza GEOPHYSICAL SURVEY IN THE NORTHERN REGION OF CUYO BASIN . . . . . . . . . . . . . . . .3-02 M. García, E. Luna, O. Alvarez, S. Spagnotto, S. Nacif, P. Martínez, M. Gimenez MONITORING AND FORECASTING THE STATE OF THE SOUTH ATLANTIC MAGNETIC ANOMALY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-03 J.C. Gianibelli IMPORTANCE AND FUTURE OF THE MAGNETIC OBSERVATORIES NETWORK IN THE SOUTH AMERICA . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-04 J.C. Gianibelli, L. Sánchez Bettucci, R.E. García, G.D. Rodriguez, N. Quaglino, R. Novo, G. Tancredi IGMAS+ A NEW 3D MODELLING TOOL FOR GRAVMAG FIELDS AND GRADIOMETRY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-05 H.J. Goetze, S. Schmidt ADVANCES IN THE DETERMINATION OF A HEIGHT REFERENCE SURFACE FOR TIERRA DEL FUEGO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-06 M. E. Gomez, R. Perdomo, D. Del Cogliano, J. L. Hormaechea FIRST MAGNETOMETRIC SURVEY IN THE ZAPICÁN AND NICO PÉREZ AREA (URUGUAY) .3-07 R. Novo, N. Seluchi, I. Suarez, L. Sánchez Bettucci, J. Gianibelli MAPS OF ABSOLUTE GRAVITY, GRAVITY ANOMALY, AND TOTAL MAGNETIC FIELD ANOMALY OF VENEZUELA FROM SATELLITE DATA. . . . . . . . . . . .3-08 N.O. Guevara, A.G. Reyes, T. Tabare GEOPHYSICAL INVESTIGATION OF THE NAVARINO ISLAND PLUTONS (BEAGLE CHANNEL, CHILE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-09 J.I. Peroni, A. Tassone, H. Lippai, F. Hervé, M. Menichetti, E. Lodolo GEOPHYSICAL CHARACTERIZATION OF FILLED ZONES ALONG THE COAST OF BUENOS AIRES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-10 C. Prezzi, R. López, C. Vásquez, S. Marcomini, S. Fazzito BAJADA DEL DIABLO IMPACT CRATER-STREWN FIELD (ARGENTINA): GROUND MAGNETIC AND ELECTROMAGNETIC SURVEYS . . . . . . . . . . . . . . . . . . . . . . . . . .3-11 C.B. Prezzi, M.J. Orgeira, R. Acevedo, F. Ponce, O. Martínez, C. Vásquez, H. Corbella, M. González , J. Rabassa 10 CONTENTS EARTH TIDE OBSERVATIONS IN TIERRA DEL FUEGO (ARGENTINA) . . . . . . . . . . . . . . . . . . .3-12 A. Richter, R. Perdomo, J. L. Hormaechea, L. Mendoza, D. Del Cogliano, M. Fritsche, M. Scheinert, R. Dietrich MORPHO-BATHYMETRIC SURVEY OF LAGO ROCA (TIERRA DEL FUEGO) . . . . . . . . . . . . . . 3-13 E. Lodolo, A. Tassone, L. Baradello, H. Lippai, M. Grossi Session 4 - Tectonic processes and Seismology NEW STRUCTURAL MAPS AND CROSS-SECTIONS OF THE PATAGONIAN FOLD-THRUST BELT NEAR SENO OTWAY, SENO MARTÍNEZ AND PENINSULA BRUNSWICK, SOUTHERN CHILE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-01 P. Betka, K. Klepeis, S. Mosher TECTONIC EVOLUTION OF THE BERMEJO BASIN FROM BROKEN PLATE FLUXURAL MODEL (PRELIMINARY STUDY) . . . . . . . . . . . . . . . . . . . . . . . . .4-02 G. Carugati, I.L. Novara, M.E. Gimenez, A. Introcaso TECTONIC IMPLICATIONS OF A PALEOMAGNETIC STUDY OF MESOZOIC MAGMATIC ARC ROCKS IN CIERVA POINT, NORTHWEST ANTARCTIC PENINSULA . . . . . .4-03 N.J. Cosentino, A.A. Tassone, H.F. Lippai, J.F.A. Vilas PALAEOTECTONIC SETTING OF PRECUYANO GROUP. UPPER TRIASSIC- LOWER JURASSIC VOLCANIC DEPOSITS OF THE NEUQUEN BASIN (37º- 39º 30´LS). ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-04 D. Delpino, A. Bermudez PRELIMINARY RESULTS OF A PALEOMAGNETIC STUDY ON THE ORDOVICIAN CALMAYO GRANITOID, SIERRAS DE CÓRDOBA, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . .4-05 S. Geuna, F. D’Eramo, L. Pinotti, A. Di Marco, D. Mutti, L. Escosteguy NORTH-SOUTH VARIATIONS IN PROVENANCE IN THE LATE PALEOZOIC ACCRETIONARY COMPLEX OF CENTRAL CHILE (34º – 40º LAT. S) AS INDICATED BY SHRIMP DETRITAL ZIRCON U-TH-PB AGES . . . . . . . . . . . . . . . . . . . . . . .4-06 F. Hervé, M.Calderon, C.M. Fanning, E. Godoy THE ROLE OF TRUE POLAR WANDER IN THE JURASSIC . . . . . . . . . . . . . . . . . . . . . . . . . . .4-07 M.P. Iglesia Llanos, C.B. Prezzi THE WEDDELL SEA REVISITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-08 L.A. Lawver, M.E. Ghidella CRUSTAL STRUCTURE AND TECTONICS OF THE EAST ANTARCTICA PASSIVE MARGIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-09 G.L. Leychenkov, G. L. Guseva STRUCTURE AND TECTONIC DEVELOPMENT OF THE SOUTHERN MARGIN OF THE SCOTIA SEA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-10 E. Lodolo, D. Civile, A. Tassone 3D DENSITY MODEL OF THE CENTRAL AMERICAN SUBDUCTION ZONE FROM SATELLITE GRAVITY DATA INTERPRETATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-11 O.H. Lücke, H.J. Götze ITALY-ARGENTINA COOPERATION IN THE FIELD OF SEISMOLOGY: THE ASAIN. HISTORICAL REVIEW AND RECENT PROGRESS . . . . . . . . . . . . . . . . . . . . . . . . .4-12 M. Russi, C. Cravos, M. P. Plasencia Linares OROGENESIS REFLECTED IN THE TRANSITION FROM EXTENSIONAL RIFT BASIN TO COMPRESSIONAL FORELAND BASIN IN THE SOUTHERNMOST ANDES (54.5°S): NEW PROVENANCE DATA FROM BAHÍA BROOKES AND SENO OTWAY J. McAtamney, K. Klepeis, C.r Mehrtens, S. Thomson .4-13 CRUSTAL DEFORMATIONS ASSOCIATED TO THE M 8.8 MAULE EARTHQUAKE IN CENTRAL CHILE, 27 FEBRUARY 2010, DETECTED BY PERMANENT GPS STATIONS IN ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-14 L. Mendoza, J. Vasquez, D. Del Cogliano 11 CONTENTS STRUCTURAL GEOLOGY OF THE EASTERN TIERRA DEL FUEGO ISLAND . . . . . . . . . . . . . .4-15 M. Menichetti, A. Tassone, H. Lippai THE PRE-FARELLONES DEFORMATION (PEHUENCHE FASE) CORDILLERA PRINCIPAL AND FRONTAL (31°45’LS), SAN JUAN PROVINCE, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-16 D. José Pérez, J.M. Sanchez Magariños CORRELATIONS OF TECTONO-MAGMATIC EVENTS IN THE SOUTH VERKHOYANSK OROGENIC BELT (EASTERN SIBERIA, NORTHEAST ASIA) . . . . . . . . . . . . . .4-17 A.V. Prokopiev THE LATE OLIGOCENE-MIOCENE ÑIRIHUAU FORMATION INTERPRETED AS A FORELAND BASIN IN THE NORTHERN PATAGONIAN ANDES . . . . . . . . . . . . . . . . . . . .4-18 M.E. Ramos, D. Orts, F. Calatayud, A. Folguera, V.A. Ramos A CASE OF PALEOHORIZONTAL RESTORATION OF PLUTONIC BODIES USING PALEOMAGNETIC DATA: THE SIERRA DE VALLE FÉRTIL MAGMATIC COMPLEX, WESTERN ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-19 A. Rapalini, L. Pinotti, F. D’Eramo, J. Otamendi, N. Vegas SEISMICITY AND EARTHQUAKE HAZARD IN TIERRA DEL FUEGO PROVINCE, . . . . . . . . . . . . . . ARGENTINA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-20 N.C. Sabbione, C. Buffoni, N. Barbosa, G. Badi, G. Connon, J.L. Hormaechea SYNOROGENIC SEQUENCES ASSOCIATED WITH THE ANDEAN FRONT AT 37º S AS A CLUE FOR AGE EXHUMATION AND STRUCTURATION OF THE FORELAND AREA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-21 L. Sagripanti, M. Naipauer, A. Folguera, V.A. Ramos FURTHER EVIDENCE OF LOWER PERMIAN REMAGNETIZATION IN THE NORTH PATAGONIAN MASSIF, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-22 R. Tomezzoli, A. E. Rapalini, M.G. Lopez de Luchi TECTONIC CONTROL ON THE EVOLUTION OF MAASTRICHTIAN-PALEOGENE SYNOROGENIC SEQUENCES OF THE FUEGIAN THRUST FOLD BELT, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-23 P.J. Torres Carbonell, E.B. Olivero, L.V. Dimieri Session 5 - Stratigraphy and Sedimentology PRESERVATION OF TOTAL ORGANIC CARBON AND EVALUATION OF CORG/NTOT ATOMIC RATIO IN A SEDIMENT OUTCROP LOCATED S-E OF THE LAGO FAGNANO (TIERRA DEL FUEGO, ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . .5-01 M. Caffau, C. Comici, M. Zecchin, M. Presti, E. Lodolo, A. Tassone, H. Lippai, M. Menichetti STRATIGRAPHIC AND STRUCTURAL REVIEW OF CAÑADÓN ASFALTO BASIN, CHUBUT PROVINCE, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-02 E. Figari, V.A. Ramos THE CONTROVERSY ABOUT MIOCENE MARINE SEDIMENTATION ALONG THE FORELAND OF ANDES, SOUTH AMERICA: THE CASE OF SANTA MARIA GROUP, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-03 I.J.C. Gavriloff, M.N. Arce GEOLOGY OF THE LAGO FAGNANO AREA (FUEGIAN ANDES, TIERRA DEL FUEGO ISLAND) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-04 M. Menichetti, A. Tassone, H. Lippai, E. Lodolo THE AGE OF DINOSAURS IN SOUTH AMERICA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-05 F. Novas SEDIMENTATION ENVIRONMENTS AND FACIES DEVELOPED DURING THE MIOCENE TO EARLY PLIOCENE IN THE EASTERN BASIN OF FALCON, WESTERN VENEZUELA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-06 B.F. Romero, P. Bastos, K. Strakos, M. Baquero 12 CONTENTS AN EXAMPLE OF COMPLEX FLUVIO-AEOLIAN SEDIMENTATION: THE UPPER MEMBER OF THE MIOCENE-PLIOCENE RÍO NEGRO FORMATION, NORTHERN PATAGONIA, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-07 A.M. Umazano, G. Visconti, M. Pérez THE BRUNHES/MATUYAMA BOUNDARY AND ROCK MAGNETIC PARAMETERS IN PLEISTOCENE LOESS DEPOSITS OF CAMET, MAR DEL PLATA (ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-08 J.C. Bidegain, M. Gomez Samus THE LATE CENOZOIC SEDIMENTARY SEQUENCES IN THE CHAPADMALAL AREA (BUENOS AIRES). POLARITY CHANGES AND MAGNETOCLIMATOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-09 J.C. Bidegain, Yamile Rico Session 6 - Surface processes and Paleoclimate RECONSTRUCTION OF LATE-GLACIAL TO HOLOCENE CLIMATE AND EARTHQUAKE HISTORIES ACROSS SOUTHERN CHILE BASED ON THE SEDIMENTARY RECORD OF 21 LAKES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-01 M. De Batist, J. Moernaut, K. Heirman, M. Van Daele, S. Bertrand RAPID CRUSTAL UPLIFT IN PATAGONIA AS A CONSEQUENCE OF INCREASED ICE LOSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-02 R. Dietrich, E.R. Ivins, G. Casassa, H. Lange, J. Wendt, M. Fritsche IMPLEMENTATION OF AQUIFER PROTECTION ZONING . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-03 S. Dustay, J. Nel, Y. Xu, H Massone TERRESTRIAL AND LACUSTRINE EVIDENCE OF HOLOCENE GLACIER ACTIVITY IN TIERRA DEL FUEGO (SOUTHERNMOST SOUTH AMERICA) . . . . . . . .6-04 M. Maurer, B. Menounos, J.J. Clague, G. Osborn, J. Rabassa, J.F Ponce, G. Bujalesky, M. Fernández, A. Coronato CK MAGNETISM STUDY ON SEDIMENTS FROM STREAMS OF THE PARANÁ DELTA (ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-05 M. Mena, J.L. Dupuy CHARACTERIZATION OF COMPLEXITY OF FRACTURED ROCK AQUIFERS . . . . . . . . . . . . .6-06 J. Nel, Y. Xu, O. Batelaan ENVIRONMENTAL MAGNETISM STUDY OF A HOLOCENE EOLIAN SEDIMENTS AND PALEOSOLS SEQUENCE IN THE NORTH OF TIERRA DEL FUEGO (ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-07 M.J. Orgeira, A. Coronato, C.A. Vásquez, A. Ponce, A. Moretto, R. Egli, M.R. Onorato MANAGEMENT AND CONTROL OF THE WATER RESOURCES OF SAN LUIS PROVINCE (ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-08 O.A. Pedersen SEDIMENTARY IMPRINT OF THE 2007 AYSÉN EARTHQUAKE AND TSUNAMI IN AYSÉN FJORD (CHILEAN PATAGONIA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-09 M. Van Daele, M. De Batist, W.. Versteeg, K. De Rycker, V. Cnudde, R. Gieles, P. Duyck, M. Pino, R. Urrutia RECONSTRUCTION OF THE EVOLUTIVE STAGES OF LLANCANELO LAKE AND SURROUNDINGS (SOUTHERN MENDOZA PROVINCE, WESTERN ARGENTINA) . . . . . .6-10 E.I. Rovere, R.A. Violante, A. Osella, M. de la Vega, E. López ROCK-MAGNETISM CHARACTERIZATION OF A LATE QUATERNARY SOIL HORIZON (SAN SEBASTIÁN BAY, ISLA GRANDE OF TIERRA DEL FUEGO) A.M. Walther, M.I.B. Raposo, J.F. Vilas . . . . . . . . .6-11 13 CONTENTS Session 7 - Marine geology and geophysics CHARACTERIZATION OF THE MAGNETIC RESPONSE OF THE NORTHERN ARGENTINE CONTINENTAL MARGIN (SOUTH ATLANTIC OCEAN) . . . . . . .7-01 D.A. Abraham, M. Ghidella, M. Paterlini, B. Schreckenberger LINKING SEAFLOOR MORPHOLOGY, HYDROSEDIMENTARY PROCESSES AND LIVING RESOURCES IN SUBMARINE CANYONS OF THE NW MEDITERRANEAN SEA: A UNIQUE STUDY CASE . . . . . . . . . . . . . . . . . . . . . . . . . . .7-02 M. Canals MORPHOSTRUCTURE OF THE WESTERN SECTOR OF THE NORTH SCOTIA RIDGE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-03 F.D. Esteban, A. Tassone, E. Lodolo, M. Menichetti GIANT MOUNDED DRIFTS IN THE ARGENTINE CONTINENTAL MARGIN . . . . . . . . . . . . . . . .7-04 F.J. Hernández-Molina, M. Paterlini, L. Somoza, R. Violante, M.A. Arecco, M. de Isasi, M. Rebesco, G. Uenzelmann-Neben, P. Marshall PLIOCENE SUBMERGED CRATERS AT THE UPPER CONTINENTAL SLOPE OF MAR DEL PLATA (ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-05 F. Isla, A. Madirolas THE 2005-2006 EXPERIMENT IN ANTARCTICA WITH MABEL SEAFLOOR MULTIDISCIPLINARY OBSERVATORY . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-06 G. Marinaro, G. Falcone, F. Frugoni, P. Favali COASTAL EROSION IN MAR DE COBO (BUENOS AIRES PROVINCE) . . . . . . . . . . . . . . . . . . . . . .7-07 L. San Martín, S.C. Marcomini, R.A. López CONDITIONING FACTORS AND RESULTING MORPHOSEDIMENTARY FEATURES IN THE UPPER-MIDDLE CONTINENTAL SLOPE OFFSHORE EASTERN BUENOS AIRES PROVINCE, ARGENTINA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-08 R.A. Violante, C.M. Paterlini, F.J. Hernández Molina, G. Bozzano, I.P. Costa, S. Marcolini CENOZOIC GROWTH PATTERNS AND PALEOCEANOGRAPHY OF THE OCEAN BASINS NEAR THE SCOTIA-ANTARCTIC PLATE BOUNDARY . . . . . . . . . . . .7-09 A. Maldonado, F. Bohoyo, J. Galindo-Zaldívar, F.J. Hernández-Molina, F.J. Lobo, Y. Martos-Martin, A.A. Schreider Session 8 - Oil and Mineral resources OCCURRENCE OF SHALLOW GAS IN THE EASTERNMOST LAGO FAGNANO (TIERRA DEL FUEGO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-01 A. Darbo, L. Baradello, E. Lodolo, M. Grossi, A. Tassone, H. Lippai GEOLOGY OF THE SAN PEDRO MINING DISTRICT, SAN RAFAEL MASSIF (ARGENTINA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-02 A.l. Gómez, N. Rubinstein INDUCED POLARIZATION–RESISTIVITY EXPLORATION IN THE POLYMETALLIC PURÍSIMA-RUMICRUZ DISTRICT, JUJUY PROVINCE (ARGENTINA) . . . . . . . . . . . . . . . . . . . .8-03 L. López, H. Echeveste, M. Tessone ROCK-MAGNETISM PROPERTIES FROM DRILL CUTTING AND THEIR RELATION WITH HYDROCARBON PRESENCE AND PETROPHYSICAL PARAMETERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-04 M. Mena, A.M. Walther USE OF ASTER IMAGERY TO IDENTIFY MINERALIZATION IN THE ANDEAN CORDILLERA FRONTAL (31º45´S), SAN JUAN PROVINCE (ARGENTINA) . . . . . . . . . . . . . . . .8-05 D.J. Pérez, P. D´Odorico SEISMIC EVIDENCE OF A GAS HYDRATE SYSTEM IN THE WESTERN ROSS SEA (ANTARCTICA) BY TOMOGRAPHY, AVO ANALYSIS AND PRESTACK DEPTH MIGRATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-06 S. Picotti, R. Geletti, D. Gei, A. Mocnik, J.M. Carcione 14 Session 1 RODINIA IN SOUTH AMERICA GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA RODINIA ANCESTRIES OF NEOPROTEROZOIC–EARLY PALEOZOIC SEDIMENTARY ROCKS OF SOUTHERN SOUTH AMERICA, THE ROSS SEA REGIONS OF ANTARCTICA, ZEALANDIA AND SOUTHEAST AUSTRALIA: POSSIBLE ORIGINS IN THE SOUTH CHINA BLOCK 1-01 Adams, C.J.* GNS Science, Private Bag 1930, Dunedin, New Zealand * Presenting author’s e-mail: [email protected] Late Neoproterozoic, Cambrian and Early Ordovician sedimentary rocks form extensive basins in Argentina, West Antarctica (Marie Byrd Land), East Antarctica (North Victoria Land), the greater New Zealand region (‘Zealandia’) and southeast Australia (Victoria and New South Wales). Their abundant Precambrian detrital mineral assemblages provide a memory bank of Rodinia ancestries. It is remarkable that detrital zircon age patterns for all these areas are similar, with important dominant ages groups falling in the late Mesoproterozoic (1000-1200 Ma) and mid-(750-850 Ma) and late (550-650 Ma) Neoproterozoic. Primary sources of these zircons are often enigmatic however, since igneous complexes of this age are sometimes absent (New Zealand and North Victoria Land), or very localised local (southeast Australia), or less extensive than might be expected to supply the very large sedimentary basins on a >1000 km-scale (Argentina). Of course, their sources may have been completely eroded during this phase, or may still remain, but now hidden, beneath younger rocks. Conversely, it is also possible that many of these sources may have themselves been buried during deposition of Neoproterozoic sediments at the late Rodina continental margin. Evidence from the Puncoviscana Formation (late Neoproterozoic-Cambrian) of northwest Argentina suggests that important sources of late Mesoproterozoic and late Neoproterozoic-Cambrian zircons could have been, respectively, in the Sunsás and Brasiliano Orogens of the Precambrian Shield to the east, Both Paleozoic and Mesozoic sedimentary rocks throughout Zealandia show the same dominant late Mesoproterozoic and late Neoproterozoic zircon sources that must, at least in part, be endemic to that region. In Antarctica, a similar pattern is recorded in early Paleozoic rocks in Marie Byrd Land and North Victoria Land. These common features suggest that late Rodinia continental margins in South America, Australia, Zealandia and Antarctica had similar cratonic hinterlands. Whilst these in part would have included the Brasilian Shield of South America, it is also possible that the South China Block was an important cratonic neighbour to the west. Recent Rodinia global syntheses place a South China Block east of Precambrian Australia, in a position now occupied by Zealandia (Li et al. Precambrian Research 160: 2008). The South China Block has distinctive Meso- and Neoproterozoic igneous complexes with age patterns similar to Precambrian detrital ages from Zealandia, Antarctica and southern South America. During late Neoproterozoic - early Paleozoic break-up of Rodinia, it is possible that a small fragment of this block remained to form a Zealandia basement. Upon later erosion this has left ‘ghost’ zircon age signatures in Phanerozoic sedimentary rocks of the region. At Rodinia break-up, the South China Block could then have supplied major sedimentary basins on its western (Australia), southern (Zealandia, Antarctica) and eastern (South America) margins. WESTERN PRECORDILLERA OPHIOLITE BELT: CORRELATIONS BETWEEN CORDÓN DEL PEÑASCO AND CORTADERAS LOCALITIES (MENDOZA PROVINCE, ARGENTINA) 1-02 Boedo, F.L.1*, Vujovich, G.I.1 (1) Laboratorio de Tectónica Andina, FCEN, Universidad de Buenos Aires/CONICET * Presenting author’s e-mail: [email protected] Cordón del Peñasco area is located in western Precordillera, northern Mendoza province, Argentina (Fig. 1). Low grade metamorphic, slope and deep marine metasiltstones and metasandstones of 17 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - Geologic map of southern Precordillera showing Cordón del Peñasco and Cortaderas localities (From Davis et al., 1999). Fig 1 - Geologic map of Precordillera fold-and-thrust belt, Argentina, showing the main exposures of ophiolite assemblages (From Ramos et al., 2000) Eopaleozoic age, associated with mafic and ultramafic rocks are the most widely exposed rock types. Mafic and ultramafic rocks correspond to an ophiolitic belt composed of discontinuous exposures along western Precordillera (Ramos et al., 1984, 2000; Haller y Ramos, 1984; Cortés y Kay, 1994; Davis et al., 1999; Fauqué y Villar, 2003). This belt can be recognized, from north to south at: río Bonete area (La Rioja province), Rodeo and Calingasta (San Juan province), Cordón del Peñasco, Sierra de las Cortaderas and Cordón de Bonilla (Mendoza province) (Fig. 1). Particularly, Cordón del Peñasco and Cortaderas localities show great similarities (Fig. 2 and 3). Cordón del Peñasco mafic and ultramafic rocks comprise serpentinized peridotites (probably dunites and harzburgites) and mafic granulites retrograded to greenschist facies, and homogeneous gabbro dikes or sills, amigdaloid metabasalts and metahialoclastic rocks with greenschist facies metamorphism. Cortaderas mafic and ultramafic bodies comprise serpentinized ultramafic rocks (wehrlites, harzburgites, lherzolite websterites and dunites), layered gabbros, gabbros, microgabbros and diabases. In both areas, these bodies are heavily deformed and in tectonic contact with slope and deep marine metasedimentary rocks. In addition, the petrography of serpentinites and mafic granulites are similar. In Cortaderas, Davis et al. (1999) have recognized that a low-grade regional metamorphism has partially replaced the igneous and high temperature assemblages in mafic and ultramafic rocks. In Cordón del Peñasco, we have recognized the same process and retrogradation evidences such as reaction rims on garnets, recristalization and formation of a fine-grained 18 GEOSUR2010 Fig 3 - Geological map of Cordón del Peñasco area. 22-23 NOVEMBER 2010 – MAR DEL PLATA clinopiroxene-plagioclase-quartz mosaic and lizarditechrysotile-talc folded veins on an antigorite mesh texture. In both localities, a similar low-grade metamorphism has affected metavolcanic and metasedimentary rocks, where primary textures are better preserved. Additionally, Davis et al. (1999) calculated metamorphic P and T conditions on garnets of mafic granulites obtaining temperatures of approximately 850-1000ºC and a minimum pressure of 9 kbar. As mafic granulites have very similar mineralogy and textures in both localities, we assume that this PT condition, that fall into high pressure granulite facies, could be similar in our study area. Geochemical analyses on mafic and ultramafic rocks of different localities plot in the E-MORB (Enriched Mid Ocean Ridge Basalt)/within plate basalt field (Haller and Ramos, 1984; Kay et al., 1984; Cortés and Kay, 1994; Fauqué and Villar, 2003). Besides, these rocks show positive values of ÂNd (+6 to +9.3) that confirm their oceanic character (Kay et al., 2005). Western Precordillera Ophiolite Belt has been interpreted as a suture zone between Chilenia and Cuyania terranes (Ramos et al., 1984). Davis et al. (1999, 2000) suggested the occurrence of two ophiolite assemblages along the suture based on different ages for the mafic and ultramafic rocks. We propose that the mechanism of exhumation in a eastward dipping subduction channel could explain the association of low and high-grade metamorphic rocks, rocks with evidence of retrogradation and the east and west structural vergence reported by many authors (Ramos et al., 1984; Davis et al., 1999; Von Gosen, 1997) along the ophiolite belt. REFERENCES: • Cortés, J. M., Kay, S. M. 1994. Una dorsal oceánica como origen de las lavas almohadilladas del Grupo Ciénaga del Medio (Silúrico-Devónico) de la Precordillera de Mendoza, Argentina. 7º Congreso Geológico Chileno, Actas 2: 1005- 1009. • Davis, J., Roeske, S., McClelland, W., Snee, L. 1999. Closing an ocean between the Precordillera terrane and Chilenia: Early Devonian ophiolite emplacement and deformation in the southwest Precordillera. En Laurentia-Gondwana Connection before Pangea. Geological Society of America Special Publication 336, 115-138. • Davis, J., Roeske, S., McClelland, W., Kay, S. 2000. Mafic and ultramafic crustal fragments of the southwestern Precordillera terrane and their bearing on tectonic models of the early Paleozoic in western Argentina. Geology, 28(2): 171-174. • Fauqué, L. E., Villar, L. M. 2003. Reinterpretación estratigráfica y petrología de la Formación Chuscho, Precordillera de La Rioja. Revista de la Asociación Geológica Argentina 58(2): 218-232. • Haller, M. J., Ramos, V. A. 1984. Las ofiolitas famitinianas (Eopaleozoico) de las provincias de San Juan y Mendoza. 9º Congreso Geológico Argentino, Actas 3: 66- 83. • Kay, S. M., Ramos, V. A., Kay, R. 1984. Elementos mayoritarios y trazas de las vulcanitas ordovícicas de la Precordillera occidental; basaltos de rift oceánico temprano(?) próximo al margen continental. 9º Congreso Geológico Argentino, Actas 2: 48-65. • Kay, S. M., Boucakis, K. A., Porch, K., Davis, J. S., Roeske, S. M., Ramos, V. A. 2005. E-MORB like mafic magmatic rocks on the western border of the Cuyania terrane, Argenina. In Pankhurst, R. J. and Veiga, G. D. (Eds.) Gondwana 12 “Geological and biological heritage of Gondwana”, p. 216, Mendoza. • Ramos, V., Jordan, T., Allmendinger, R., Kay, S., Cortés, J., Palma, M. 1984. Chilenia: un terreno alóctono en la evolución paleozoica de los Andes Centrales. 9º Congreso Geológico Argentino, Actas 2: 84-106. • Ramos, V., Escayola, M., Mutti, D., Vujovich, G., 2000. Proterozoic- Early Paleozoic ophiolites of the Andean basement of southern South America. Ophiolitic and Oceanic Crust: new insights from field studies and the Ocean Drilling Program. Geological Society of America Special Paper, 349, 331-349. • Von Gosen, W. 1997. Early Paleozoic and Andrean structural evolution in the Río Jáchal section of the Argentine Precordillera. Journal of South American Earth Sciences 10(5/6): 361-388. 19 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA PAMPIA: A FRAGMENT OF THE AUTHOCHTONOUS MESOPROTEROZOIC OROGEN OF WESTERN RÍO DE LA PLATA CRATON. ITS DETACHMENT DURING RODINIA`S BREAK-UP, AND RE-ACCRETION DURING GONDWANA´S AMALGAMATION 1-03 Chernicoff, C.J.1*, Zappettini, E.O.2, Santos, J.O.S.3 (1) Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina (2) Servicio Geológico-Minero Argentino, Argentina (3) University of Western Australia, Perth, Australia * Presenting author’s e-mail: [email protected] The Río de la Plata craton (RPC) is the southernmost craton of the South American continent, and is considered to encompass autochthonous Achaean to Palaeoproterozoic basement. We regard the RPC to have also originally encompassed an `outer´ Mesoproterozoic belt accreted to the old nucleous during the amalgamation of Rodinia, and that this ´outer` belt forms part of the RPC. Increasing evidence points to the existence of an authochtonous Mesoproterozoic orogen (southern continuation of the Sunsás orogen of SW Amazonas) at the western border of the RPC, e.g. i) Mesoproterozoic Nd model ages of the Cambrian igneous basement of Sierra de la Ventana (i.e. immediately southwest of the RPC basement exposures of Tandilia, ii) identification of abundant Mesoproterozoic detrital zircon in the Neoproterozoic supracrustal sequences of the core RPC, iii) increased relative abundance of Mesoproterozoic detrital zircon in the Neoproterozoic RPC metasediments towards the west of the craton, iv) Paleo-Mesoproterozoic Hf model ages obtained by the present authors both on magmatic zircons from Lower Paleozoic magmatic units and on detrital zircon from Neoproterozoic-Lower Cambrian cover sequences in La Pampa province. The depositional age of the RPC supracrustal sequences would preclude Cuyania as a provenance, and indicate that the authochtonous Mesoproterozoic orogen acted partly as a source for these supracrustal sequences (and also partly as topographic barrier, depending on the depocenters´ locations). It is in this tectonic context that a fragment of the authochtonous Mesoproterozoic belt of the RPC — and some RPC Paleoproterozoic crustal material as well—, would have later been rifted away from the RPC nucleous —sometime prior to 600 Ma—, defining the Pampia Terrane (PT). This extensional event would have given rise to the Puncoviscana basin, whose late-stage equivalent at the latitude of La Pampa province is regarded to be represented by the Santa Helena Schists. The detachment of the PT from the western border of the RPC would, in turn, have been roughly coeval with the inferred westward subduction of Adamastor Ocean lithosphere beneath the eastern border of the RPC. These two coeval events would coincide with the late stage of the Rodinia break-up which, in turn, partly overlaps the timing of Western Gondwana amalgamation. The PT would have been re-accreted to the RPC during the Early Cambrian Pampean orogeny, occurred during the final stage of Gondwana amalgamation at ca 530-520 Ma. We envisage the PT / RPC collision to have been preceded by west dipping subduction of oceanic crust (i.e. PT = active margin). To the north of the Sierras Pampeanas, the eastern border of Arequipa-Antofalla/Pampia would have been the active margin, and the Rio Apa cratonic fragment would have been the passive margin, this scheme being consistent with the occurrence of Lower Cambrian arc magmatism in northwestern Argentina. 20 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA NEW INSIGHTS ON THE PALEOPROTEROZOIC BASEMENT OF TANDILIA BELT, RÍO DE LA PLATA CRATON, ARGENTINA: FIRST HF ISOTOPE STUDIES ON ZIRCON CRYSTALS 1-04 Cingolani, C.1*, Santos, J.O.S.2, Griffin W.3 (1) División Científica de Geología Museo de La Plata and Centro de Investigaciones Geológicas (CONICET-UNLP), La Plata, Argentina. (2) Centre for Global Targeting, University of Western Australia, Australia. [email protected] (3) GEMOC (Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), MacQuarie University, Sydney, Australia * Presenting author’s e-mail: [email protected]; [email protected] Introduction Following the fragmentation of the Rodinia supercontinent, Archean to Mesoproterozoic cratonic blocks were amalgamated to form the Gondwana continent during Neoproterozoic-Cambrian times. One of these continental blocks place at the core of western Gondwana, is the Río de la Plata craton (Almeida et al., 1973). The southernmost outcrops of this cratonic region are located in the Tandilia belt (also know as Sierras Septentrionales de Buenos Aires) in eastern Argentina (Fig. 1). It is exposed as a 350 km long and maximum 60 km wide northwest trending orographic belt, located in the central part of the Buenos Aires province. The Tandilia belt outcrops are in between the Salado (Mesozoic) and the Claromecó (Neoproterozoic-Paleozoic) basins. Some reviews on different aspects of Tandilia basement rocks were published by Dalla Salda et al. (1988), Cingolani and Dalla Salda (2000), Hartmann et al. (2002a), Pankhurst et al. (2003), Rapela et al. (2007) and Bossi and Cingolani (2010 and references therein). The main purpose of this contribution is to give new Hf isotopic insight on zircon crystals from the Tandilia Paleo- proterozoic igneous-metamorphic rocks in order to analyze their magmatic evolution and tectonic interpretation. Fig. 1 - Geological sketch map of Tandilia belt from Iñiguez et al. (1989) and Dalla Salda et al. (1988) with the studied sample locations. A relative location of the Tandilia belt in the Río de la Plata cratonic area is shown in the inset. Geological setting The geological evolution of Tandilia comprises mainly a juvenile igneous-metamorphic Paleoproterozoic basement rocks which are covered by thin Neoproterozoic to Early Paleozoic sedimentary units. The Paleoproterozoic basement called ‘Buenos Aires Complex’ (Fig. 1) consists mainly of granitic-tonalitic gneisses; migmatites; amphibolites, some ultramafic rocks and granitoid plutons (Dalla Salda et al., 1988). Subordinate rock types include schists, marbles, and dykes of felsic and mafic composition. Tandilia was recognized as an important shear belt district with mylonitic 21 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA rocks derived mainly from granitoids. The available geochemical data show that the igneous basement rocks have a calc-alkaline signature. Crust-derived Sm–Nd model ages (Cingolani et al., 2002; Hartmann et al., 2002a; Pankhurst et al., 2003) are in between 2.69-2.4 Ga implying that although the principal rock-forming events were Paleoproterozoic, the Neoarchean derivation could be possible. After U-Pb zircon crystals SHRIMP dating (Hartmann et al., 2002a; Cingolani et al., 2005) the tectonic scenario seems related to juvenile accretion (2.25-2.12 Ga) along an active continental margin, followed by continental collision (2.1-2.08 Ga). A lack of recrystallization or new zircon growth in the Neoproterozoic, suggests that the Tandilia Paleoproterozoic basement was preserved from younger orogenies such as those of the Brasiliano cycle. This geological evolution can be correlated with the Piedra Alta terrane (Uruguay), where Rb-Sr, Sm-Nd and U-Pb data show a similar signature (Cingolani et al., 1997; Hartmann et al., 2002b and references therein) during Paleoproterozoic times. After a long weathering process there is a record of Neoproterozoic sedimentary units and the final marine transgression at the Early Paleozoic. Sampling and analytical techniques Samples were taken from tonalitic-monzonitic granitoid and gneissic rocks from the Tandil region (Ta3; Ta4 and Ta5), and Opx-gneisses from the Balcarce region (Ta9, El Triunfo Hill) during the field work for U-Pb SHRIMP research (Hartmann et al., 2002a). The same zircon crystals and spots dated by U-Pb were analyzed for Hf isotope studies (Fig. 2). Samples Ta3 and Ta4 are foliated tonalitic gneiss and monzogranite (37º22’33”S-59º12’33”W) respectively sampled on the road cut about 10 km away from the Tandil town, both from the same outcrop. The Ta5 sample, is a granitoid from Montecristo quarry (37º22’15”S-59º10’42”W) homogeneous medium-grained grey rock. Sample Ta9 from El Triunfo Hill near Balcarce city (37º49’26”S-58º12’15”W) is a mafic to intermediate orthopyroxene-bearing gneiss. Fig. 2 - Backscattered electron images of zircons analyzed by Hartmann et al. (2002a) and Cingolani et al. (2005) by U-Pb SHRIMP. Same zircon crystals and spots were analyzed for Hf isotopes by ICP-MS-LA. Hf-isotope analyses were carried out using a New Wave/ Merchantek UP213 laser-ablation microprobe, attached to a Nu Plasma multi-collector ICP-MS at MacQuarie University, Sydney. Mud Tank (MT) zircon was used as reference material which has an average 176Lu/ 177Hf ratio of 0.282522±42 (2SE) (Griffin et al., 2000). Initial 176Hf/177Hf ratios are calculated using measured 176Lu/177Hf ratios, with a typical 2 standard error uncertainty on a single analysis of 176Lu/177Hf ±1–2%. Such error reflects both analytical uncertainties and intragrain variation of Lu/Hf typically observed in zircon. Chondritic values (Blichert-Toft and Albarède, 1997) of 1.93?10?11 have been used for the calculation of ÂHf values. Whilst a model of (176Hf/177Hf)i=0.279718 at 4.56 Ga and 176Lu/177Hf=0.0384 has been used to calculate model ages (T DM) based on a depleted mantle source, producing a present-day value of 176Hf/177Hf (0.28325). TDM ages, which are calculated using measured 176Hf/177Hf of the zircon, give only the minimum age for the source material from which the original magmas were derived. We have therefore also calculated a “crustal” model age (TDM C) for each zircon which assumes that the parental magma was produced from an average continental crust (176Lu/177Hf=0.015) that was originally derived from depleted mantle. Hf isotopes data and discussion: As it is shown in Fig. 3 the coherent results on zircon crystals from all studied samples suggest that 22 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA the depleted mantle model age (crustal) is Neoarchean 2.65 Ga, older (±350 Ma) than the crystallization age. Positive ÂHf obtained data show also derivation from juvenile material. An alternative interpretation could be a mixing with juvenile (2.27 Ga?) and crustal (more than 2.65 Ga) magmatic components. TDM ages, which are calculated using measured 176Hf/177Hf of the zircon, give only the minimum age for the source material from which the original magmas were derived. The average of 28 Hf model-ages (2646 Ma) is almost coincident with the age of the only one inherited zircon of sample Ta3 (A.14-1, 2657 ± 8 Ma). This is strong evidence supporting the derivation from a Neoarchean crust (Fig. 3). Fig. 3 - Plot of 176Hf/177Hf ratios versus ages on dated zircon crystals of samples Ta3, Ta4, Ta 5 (Tandil region) and Ta9 (Balcarce region). The slope of the dashed line uses the ratio of 0.015 for the 176Lu/177Hf ratio. The present Hf isotope study confirm the Sm-Nd data published by Hartmann et al. (2002a) and Pankhurst et al. (2003) showing that the constituting material of the source region from the mantle was Neoarchean (c.2.6 Ga). These results are in agreement with precise U–Pb dating of the craton in western Uruguay and southernmost Brazil, which also indicate a relatively short-lived Paleoproterozoic orogeny. Acknowledgements This study was enabled by grants from CONICET (PIP 5027) which are gratefully acknowledged. We thank Prof. Léo A. Hartmann (UFRGS, Brazil) for stimulating discussions and Norberto Uriz and Mario Campaña (UNLP) for technical support. REFERENCES • Almeida, F.F.M., Amaral, G., Cordani, U.G., Kawashita, K. 1973. The Precambrian evolution of the South American cratonic margin, south of the Amazon River. In: Nairn, A.E., Stehli, F.G. (Eds.), The Ocean Basins and Margins1, Plenum Publishing, New York, pp. 411–446. • Blichert-Toft, J., Albarède, F. 1997. The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantle-crust system. Earth and Planetary Science Letters, 148 (1-2): 243-258. • Bossi, J., Cingolani, C.A. 2010. Extension and general evolution of the Río de la Plata Craton. In: Gaucher C., Sial A.N, Halverson G.P, Frimmel H.E. (eds.). Neoproterozoic- Cambrian tectonics, global change and evolution: a focus on southwestern Gondwana. Developments in Precambrian Geology 16:73-85 • Cingolani, C.A., Varela, R., Dalla Salda, L., Bossi, J., Campal, N., Ferrando, L., Piñeyro, D.,Schipilov, A. 1997. Rb-Sr geochronology from the Río de la Plata craton of Uruguay. South-American Symposium on Isotope Geology, Brazil, Extended Abstracts 73-75. • Cingolani, C.A., Dalla Salda, L. 2000. Buenos Aires cratonic region. In Cordani, U., Milani, E., Thomaz Filho, A., y Campos D. (eds.) Tectonic evolution of South America. 31° International Geological Congress, 139-146, Río de Janeiro, Brazil. • Cingolani, C.A., Hartmann, L.A., Santos, J.O.S., McNaughton, N.J. 2002. U–Pb SHRIMP dating of zircons from the Buenos Aires complex of the Tandilia belt, Río de La Plata cratón, Argentina, Actas CD-ROM, XV Congreso Geológico Argentino (El Calafate, 23 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Santa Cruz), Asociación Geológica Argentina, Buenos Aires. • Cingolani, C.A., Santos, J.O.S., McNaughton, N.J., Hartmann, L.A. 2005. Geocronología U-Pb SHRIMP sobre circones del Granitoide Montecristo, Tandil, Provincia de Buenos Aires, Argentina. 16º Congreso Geológico Argentino, La Plata, 1: pp. 299-302. • Dalla Salda, L., Bossi, J., Cingolani, C. 1988. The Rio de la Plata cratonic region of southwestern Gondwana. Episodes,11(4):263269. • Griffin,W.L., Pearson, N.J., Belousova, E.A., Jackson, S.R., van Achterbergh, E., O’Reilly, S.Y., Shee, S.R. 2000. The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites. Geochimica et Cosmochimica Acta 64, 133–147. • Hartmann, L.A., Santos, J.O.S., Cingolani, C.A., McNaughton, N.J. 2002a. Two Paleoproterozoic Orogenies in the Evolution of the Tandilia Belt, Buenos Aires, as evidenced by zircon U-Pb SHRIMP geochronology. International Geology Review, 44: 528-543. • Hartmann, L.A., Santos, J.O.S., Bossi, J., Campal, N., Schipilov, A., McNaughton, N.J. 2002b. Zircon and titanite U–Pb SHRIMP geochronology of Neoproterozoic felsic magmatism on the eastern border of the Río de la Plata craton, Uruguay. Journal of South American Earth Sciences 15, 229–236. • Iñiguez, A.M., Del Valle, A., Poiré, D., Spalletti, L., Zalba, P. 1989. Cuenca Precámbrica-Paleozoica inferior de Tandilia, Provincia de Buenos Aires. In: Chebli, G. y L.A. Spalletti (Eds.). Cuencas sedimentarias argentinas. Instituto Superior de Correlación Geológica, Universidad Nacional de Tucumán, Serie Correlación Geológica, 6:245-263. • Pankhurst, R.B., Ramos, A., Linares, E. 2003. Antiquity of the Rio de la Plata craton in Tandilia, southern Buenos Aires province, Argentina. Journal of South American Earth Sciences, 16 (2003) 5–13 10. • Rapela, C.W., Pankhurst, R.J., Casquet, C., Fanning, C.M., Baldo, E.G., González-Casado, J.M., Galindo, C., Dahlquist, J. 2007. The Río de la Plata Craton and the assembly of SW Gondwana. Earth Science Reviews, 83, 49-82. THE RODINIAN RELICS OF AUSTRALIA-MAWSON, AZANIA, NEOPROTEROZOIC INDIA AND A GREATER KALAHARI – DIVINING THEIR EXTENT AND INTERPRETING THEIR EVOLUTION 1-05 Collins, A.S.1,2*, Selway, K.1, Clark, C.3, Kinny, P.D.3, Plavsa, D.1, Amarasinghe, U.1 (1) Tectonics Resources and Exploration (TRaX), School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia (2) Instituto de Geociências and IAG-USP Departamento de Geofisica, Universidade de São Paulo, Butantã, São Paulo, Brazil) (3) The Institute for Geoscience Research (TIGeR), Curtin University of Technology, WA, Perth, Australia * Presenting author’s e-mail: [email protected] Rodinia broke up into numerous Australia-sized continents in the Tonian-Cryogenian that amalgamated to form an Ediacaran-Cambrian Gondwana. A number of fragments of these Neoproterozoic continents are now found in East Antarctica and parts of India, Sri Lanka, Madagascar, Australia and Africa that were contiguous in central Gondwana. Tracing the extent of these continents and gaining an understanding of the tectonic events that they were part of is complicated by the dual problem that, a) many of these Neoproterozoic continental margins have been deformed, and, b) the boundaries in East Antarctica are buried under extensive ice cover. To partially overcome this latter problem, we undertook a 180 kilometre long magnetotelluric (MT) survey parallel to the Prydz Bay coast with the joint aims of developing MT methodologies for Antarctic experiments and investigating the geological history of the Prydz Bay area. Analysis and inversion of the data has imaged a crustal-scale boundary beneath the Sørsdal Glacier separating two distinct regions that correlate with the Vestfold Hills and the Rauer Group. The successful imaging of this feature shows that MT is potentially a useful tool for finding the location of other proposed suture zones in Antarctica. What geophysical imaging cannot tell us, though, is what the continents were that now crop out as the Vestfold Hills and the Rauer Islands. For this we undertook a detrital zircon study of Proterozoic samples from the Vestfold Hills. The results show a similarity with potential sources in the Singbhum craton of India. Numerous ~2.4 Ga zircons also open the possibility that the Gawler craton of South Australia was linked to the Vestfold/Singbhum craton in the Palaeoproterozoic. Detrital zircon studies in Sri Lanka, Southern India and Madagascar are also helping divine the correlation between various rock units and Neoproterozoic continents. In Madagascar and southern 24 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA India, highly metamorphosed sedimentary rocks are shown to have many Palaeoproterozoic sources. There is a paucity of Palaeoproterozoic source rocks in the Dharwar craton of India, but considerable potential in eastern Africa, suggesting that this sedimentary basin was depositionally joined to Africa in the Proterozoic. Juvenile Neoproterozoic volcanics and sedimentary rocks to the west of this belt (south western Madagascar and east Africa) suggest that this continental (Azania) rifted off Africa in the early Neoproterozoic to later recombine in the Cryogenian/Ediacaran. Sri Lanka joins India to East Antarctica in Gondwana. Here the Wanni Complex has long resisted attempts to be correlated with adjacent parts of India. New detrital zircon data suggests that quartzites from the Wanni Complex a) are Neoproterozoic, b) have similar sources to the metasedimentary rocks found in the Madurai Block of India. RODINIA: IS A RAPPROCHEMENT OF CURRENT ‘SOUTHERN OCEAN’ AND ‘NORTH ATLANTIC’ MODELS ACHIEVABLE? 1-06 Dalziel, I.W.D. Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin 10100 Burnet Rd, Austin, TX 78758 Models of latest Mesoproterozoic-Neoproterozoic global paleogeography have recently been developing in two widely separated regions over the past few years. The question is whether they are compatible. In the ‘Southern Ocean’ model, geochemical data have emerged to support the 1990’s suggestion that the Laurentian and Mawson cratons were juxtaposed prior to the Neoproterozoic opening of the Pacific Ocean basin (Goodge et al., Science, 2008), essentially the Southwest United States-East Antarctica (so-called ‘SWEAT’) reconstruction of Moores, Geology, 1991 and of Dalziel, Geology, 1991). The Mawson craton could have rifted from the Pacific margin of Laurentia in the mid-Neoproterozoic and collided with the Weddell Sea margin of the Antarctic craton, the Coats Land block, in the latest Neoproterozoic along the Pan-African suture through the Shackleton Range. In so doing, it need not have travelled at a rate higher than plates have been measured to move during the Mesozoic and Cenozoic (Dalziel, Sp. Pub. Geol. Soc. London, No. 335, 2010). Published geologic, radiometric and paleomagnetic data together with unpublished Pb isotope data indicate that Coats Land was part of the Laurentian craton. Hence, according to this model, Laurentia and the newly amalgamated Gondwanaland formed the geologically emphemeral Pannotia supercontinent prior to the Early Cambrian opening of the Iapetus Ocean basin between the proto-Appalachian and protoAndean margins. The present northern hemisphere craton of Laurentia would have been juxtaposed with cratons now bordering the Southern Ocean. In the North Atlantic region recent geochronologic and geochemical studies have led to a reconstruction that places Baltica against the Labrador-Scotland-Greenland promontory of Laurentia in the Neoproterozoic along what is termed the ‘Valhalla orogen’ (Cawood et al., Geology, 2010). This model also places Baltica against northern Amazonia. The configuration is thought to have existed until opening of Iapetus between Laurentia on one side and Baltica plus Amazonia on the other. In this presentation I will address the question of whether these two models are compatible. Study of this problem using PLATES paleogeographic reconstruction software indicates that it is difficult to reconcile the ‘northern’ and ‘southern’ models as published. Perhaps, however a rapprochement can be reached if uncertainties over the position of the Iapetus Ocean rifted margin within Gondwanaland, a margin extensively altered by Phanerozoic tectonism and magmatism, are taken into account. 25 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA VOLCANISM IN THE WESTERN OUACHITA-CUYANIA BASIN AND SEPARATION OF LAURENTIA AND GONDWANA 1-07 Dickerson, P.W.1*, Hanson, R.E.2, Roberts, J.M.2, Fanning C.M.3 (1) American Geological Institute and Jackson School of Geosciences, University of Texas, Austin, TX 78712 USA (2) Department of Geology, Texas Christian University, Ft. Worth, TX 76129 USA (3) Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia * Presenting author’s e-mail: [email protected] Neoproterozoic through early Paleozoic times witnessed the sundering of Rodinia and the amalgamation of several allochthonous and parautochthonous blocks with Western Gondwana; Cuyania was one such terrane, which originated as an element of the central-southern Laurentian margin. The western Ouachita basin (Marathon basin, west Texas) and Cuyania (greater Precordillera, western Argentina) have fundaments of Laurentian Mesoproterozoic (Grenvillian) basement and evolved together, as evidenced by isotopic, litho-, bio- and chronostratigraphic data, as well as by recent paleomagnetic determinations. Ages, isotopic and geochemical data correspond well for Cuyania (Western Sierras Pampeanas, anorthosite massif of Sierra de Umango) and for west-central Texas crystalline basement rocks (Llano Uplift, Pecos layered mafic complex). Proterozoic through Eocambrian outcrops around the northern basin rim supplied detrital zircons to Middle Cambrian sandstones of Cuyania. Fully correlative Ordovician carbonate successions developed on platforms of both the northern and southern basin and hosted homologous sponge-algae-stromatoporoid bioherms. Within the off-shelf calcarenite debris flows and bentonitic shales of the Marathon Fm. (Floian) is an interval of megaolistoliths of shelf carbonate rocks (limestone cobble to boulder conglomerates) that were likely shed from fault-bounded blocks during extension/transtension in the basin. The olistostrome (Monument Spring Mbr.) extends more than 20 km along strike and foundered into sediments belonging to a single graptolite zone (Tetragraptus approximatus; Toomey, 1978). Along with conglomerate the unit includes abundant megaclasts of lime wackestone, many of which show unusually pervasive silicification. Immediately beneath those megaclasts, a 0.7-m basalt boulder has been found within a limestone cobble conglomerate; the basalt has also undergone pervasive silicification. The extensive silicification of limestone megaclasts is suggestive of low-T hydrothermal processes at shallow levels beneath the seafloor, which could conceivably be related to volcanism in the source area for both the boulders and blocks. At the base of the superjacent Ft. Peña Fm. (Dariwillian ) additional cobbles and boulders up to ~0.5 m across of volcanic rock (basalt to trachyte) and volcaniclastic lithic wackestone have recently been discovered within an 8.5-m-thick limestone conglomerate layer. Six boulders, including that from the underlying Marathon Fm., have so far been analyzed for major and trace elements; with these ancient altered volcanic rocks, emphasis has been on trace elements that are resistant to secondary alteration. (Analyses of additional boulders are in progress.) Preliminary geochemical data for all six basalt to trachyte boulders indicate a within-plate setting for the magmatism. Evidence of explosive volcanism is found in metabentonites of the western Ouachita-Cuyania basin. Metabentonite intervals of the Marathon (Floian) and Ft. Peña (Dariwillian) Formations are within identical faunal zones to those for ash beds within the San Juan and Gualcamayo Formations, respectively, of Cuyania. Precordilleran metabentonites have been dated at 469.5 ± 3.2 to 470.1 ± 3.3 Ma (U-Pb, SHRIMP, zircons; Fanning et al., 2004). Notably, all these pyroclastic deposits belong to a distinctly older (by ~14 Ma) suite than the well-known Deicke-Millbrig-Kinnekulle metabentonites of the central Appalachians and Baltica. New preliminary geochronologic data (U-Pb, SHRIMP) for zoned igneous zircons from a basalt boulder within the basal Ft. Peña Fm. and from superjacent metabentonite and porcellanite revealed a strong Neoproterozoic (669 – 740 Ma; Cryogenian) population. A few Grenvillian (1.0 – 1.2 Ga) grains were present in both, but neither contained Ordovician zircons. However, in southernmost exposures of the Ft. Peña, initial results indicate the presence of a 470 ± 6 Ma zircon component, as well as Grenvillian grains (U-Pb, SHRIMP), in a metabentonite containing Dariwillian graptolites. If 26 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA supported by further dating, the Ft. Peña metabentonite would be coeval with San Juan Fm. metabentonites of the Precordillera. New paleomagnetically derived plate reconstructions for Neoproterozoic through medial Ordovician time place Cuyania and Western Gondwana at low southern latitudes (~26º S) and adjacent to southern Laurentia (Rapalini, 2005; 2008, written comm.). The Western Gondwanan margin trended west – that is, ~90º clockwise from its present orientation – and faced southern Laurentia. Western Ouachita-Cuyania deformation, sedimentation, and volcanism are consonant with dextral transtension in response to north-northeastward translation of Laurentia and counterclockwise rotation of Gondwana – a regime that evolved from the dextral transpressional situation which had prevailed in Early Cambrian time, as other Laurentia-derived blocks (Arequipa-Antofalla, Western Sierras Pampeanas, Amazonia) were accreted to Gondwana. With continued oblique dextral separation of the two supercontinental masses, the attenuated Laurentian slab broke apart and Cuyania was severed from Laurentia. New insight into Laurentia-Gondwana tectonic interactions is emerging from field and geochronological investigations in the western Ouachita-Cuyania basin. Preliminary geochronological data from volcanic boulders and ash in Lower to Middle Ordovician deposits of the Marathon basin reveal the presence of Cryogenian zircons. These zircons are clearly xenocrystic in the Ordovician volcanic ash, and their significance is uncertain. However, Cryogenian zircons in the volcanic boulders may provide evidence for a previously unrecognized magmatic episode at that time in the Marathon segment of the Laurentian margin. Additional geochronological data are needed to better define the timing of within-plate magmatism recorded by the volcanic boulders. REFERENCES • Fanning, C. M., Pankhurst, R. J., Rapela, C. W., Baldo, E. G., Casquet, C., and Galindo, C., 2004, K-bentonites in the Argentine Precordillera contemporaneous with rhyolite volcanism in the Famatina Arc: London, Journal of the Geological Society, v. 161, p. 747-756. • Rapalini, A., 2005, The accretional history of southern South America from the late Proterozoic to the late Paleozoic: a paleomagnetic perspective (abstract): Córdoba, Academia Nacional de Ciencias, Gondwana 12 Conference, Abstracts, p. 305. • Toomey, D. F., 1978, Observations of the Monument Spring Member of the Lower Ordovician Marathon Formation, Marathon region, southwest Texas: Midland, Permian Basin SEPM, Publication 78-17, p. 215-221. GEOCHEMISTRY, GEOCHRONOLOGY AND PALEOMAGNETISM OF PALEOPROTEROZOIC GRANITES OF THE ULKAN MASSIF, SE SIBERIAN CRATON 1-08 Didenko, A.N.1,2*, Guryanov, V.A.1, Peskov, A.Yu.1, Perestoronin, A.N.1, Avdeev D.V.1 (1) Yu.A. Kosygin Institute of Tectonics and Geophysics, FEB RAS. 65, Kim Yu Chen St., Khabarovsk, 680000, Russia (2) Geological Institute, RAS; 7, Pyzhevsky lane, Moscow, 119017, Russia * Presenting author’s email: [email protected] The interpretation of the evolution of the Siberian craton’s SE margin in the Paleoproterozoic involves the Ulkan massif, one of the key structures filled with sedimentary-volcanogenic assemblages, which are the stratotype for the Upper Karelian (Ulkanian, according to the regional scale) of the AldanStanovoy province. Three volcano-plutonic complexes are distinguished within the massif: 1) Ulkachan, mainly gabbro-trachybasaltic, 2) Elgetei, chiefly basalt-trachyrhyolitic, and 3) Ulkan, most widely represented by granitoids, which are subdivided into three phases. The figurative points of their composition occupy the fields of alkali granites and granites on the TASdiagram. The Ulkan granites are characterized by significant dominance of Fe over Mg and a high K 27 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA content; they are oversaturated with silica, iron, alkalis, fluorine, and sometimes alumina accompanied with moderate and low Mg and Ca contents. According to classification diagrams, a) granitoids belong to ferruginous assemblages, FeOt/(FeOt+MgO) ? 0.9, b) fall into the fields of alkaline, alkaline-calc and calc-alkaline assemblages, and c) occupy all the three possible areas of the Frost binary plots (ASI vs. A/NK) – agpaitic, peraluminous, and metaluminous. All the three phase granites of the Ulkan complex are distinguished by high Rb, Th, U, Nb, Ta, and REE contents, actual absence of the negative Nb-Ta anomaly and depletion in Ba, Sr, P, Ti, which is peculiar to anorogenic rare-metal alkali granites. REE contents of the Ulkan granitoids differ substantially; their sums in the 1st, 2nd and 3rd phase granites equal 466±68, 94±38 Ë 997±455 mg/g, respectively. The 3rd phase granites are noted for extreme enrichment with LREE: the average (Lan/Ybn) = 25.6; it drops to 7.6 for the 2nd phase granites; and to 5.3, for the 1st phase. The average values of the Lan/Smn ratio for the 1st, 2nd, and 3rd phase granites are 3.6±0.3, 7.9±1.5 and 8.6±2.8, respectively. All granitoids are characterized by a deep negative Eu anomaly (Eu/Eu* = 0.04-0.11). The figurative points of the study rocks occupy areas of distribution of three granite types, the lot, on the Hf–Rb–Ta diagram – intraplate, postcollisional, and those of volcanic arcs. The figurative points of the study granites occupy two different fields on the Yb–Ta–Hf diagram – Ä1 and Ä2; the former corresponds to anorogenic intraplate granitoids, and the latter, to postcollisional (postectonic) granitoids. For four zircon fractions from the 1st phase granites, the age 1730?2 Ma was derived. For zircons of two dimensional fractions, and also of two more fractions after their abrasive treatment from the 3rd phase granites, the age 1725 ?4 Ma was obtained. Positive ÂNd(T) values, +3.5 and +0.7, respectively, were determined for the 1st and 3rd phase granites. The first paleomagnetic evidence for pilot granite collections from the Ulkan complex has been acquired. The direction of the high-temperature component of granite magnetization in the modern coordinate system is Dec=60.8°, Inc=48.4° (K=5.6, a95=10.3) (we believe that the Ulkan massif did not experience any rotations about horizontal axis after granite intrusion). This corresponds to the paleomagnetic pole with the coordinates Plat=-47.4°, Plong=64.4° (dp=8.8°, dm=13.5°) which, considering the Aldan-Stanovoy province turn correction with respect to the Angara-Anabar province in the Middle Paleozoic, is close to the paleomagnetic pole by ~1,730Ma, based on postectonic granitoids of the Angara-Kansk protrusion of the Siberian craton. The investigations were financially supported by the Russian Basic Research Foundation (Project No. 09-05-00223a). THE AGE AND SIGNIFICANCE OF THE PUNCOVISCANA FORMATION WITH RESPECT TO NEOPROTEOZOIC TO CAMBRIAN TECTONIC EVOLUTION OF THE PROTO-ANDEAN MARGIN OF GONDWANA 1-09 Escayola, M.1*, Van Staal C.2 (1) CONICET (National Research Council of Argentina). Laboratorio de Téctonica Andina, Universidad de Buenos Aires, Pabellón II, Nunez. Buenos Aires (2) Geological Survey of Canada. 625 Robson Street, Vancouver, V6B 5J3 BC, Canada *Presenting author’s email: [email protected] We present a new tectonic model for the Pampean-Tilcarian accretion of the Arequipa-Antofalla-Western Pampia (AA-WP) ribbon continent to the Proto-Andean margin of Gondwana represented by the Amazonia and Rio de La Plata cratons, based on our studies of the Puncoviscana Formation and adjacent units in northern and central Argentina. A compilation of existing detrital zircon ages of the Puncoviscana Formation and correlative units along strike in the Pampean orogenic belt to the south combined with our new U-Pb SHRIMP zircon ages and conventional single grain TIMS of Puncoviscana Formation, which are based on recently discovered felsic tuffs and mafic volcaniclastic rocks (~537±1 Ma) in the unit’s type 28 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA locality suggests that the Puncoviscana Formation mainly represents an Early Cambrian arc-trench gap to foreland basin succession formed during east-directed closure of a late Neoproterozoic oceanic back-arc basin. The back-arc basin, which probably remained relatively narrow, initially had opened behind an eastfacing ~650-570 Ma island arc (Eastern Pampia arc), built upon the rifted, leading edge of the AA-WP. The c. 537±5 Ma felsic tuffs are interpreted to represent the products of a new, short-lived Early Cambrian magmatic arc built upon the composite Proto-Andean margin, following Late Neoproterozoic, softaccretion of the Eastern Pampia arc and a subduction polarity reversal. Puncoviscana Formation conglomerates and mafic volcanics previously interpreted as early rift-related deposits are better interpreted as late-orogenic basin fills and/or were deposited after basin closure. Our new U-Pb SHRIMP and conventional single grain zircon age of the post-collision Canani tonalite (524±1 Ma), which intruded into Tilcarian deformed Puncoviscana Formation rocks in the north westernmost part of Argentina in the Puncoviscana type locality, combined with the existing 529-520 Ma zircon ages for post-collision peraluminous granites and tonalites in the Eastern Pampean Ranges to the south indicates that the synorogenic Puncoviscana basin formed between 540 and 524Ma, progressively cannibalizing its orogenic hinterland over time. In addition, the Tilcarian and Pampean orogenies represent the same event. We suggest that AA-WP rifted-off from Laurentia between 700 and 650 Ma, shortly after Amazonia’s departure during Rodinia’s break-up. We emphasize that it is the departure of AA-WP, not Amazonia that opened Iapetus in the Late Neoproterozoic. We also suggest that the Ganderia terrane in the northern Appalachians, originally formed an extension of the AA-WP, but returned later to Laurentia during Iapetus’ closure. THE PUTUMAYO OROGEN OF NORTHWEST SOUTH AMERICA: IMPLICATIONS FOR RODINIAN CONNECTIONS BETWEEN AMAZONIA, BALTICA AND THE MIDDLE- AMERICAN OAXAQUIAN TERRANES 1-10 Ibanez-Mejia, M.1*, Ruiz, J.1, Valencia, V., Cardona, A.2, Gehrels, G.1, Mora, A.3, DeCelles, P1 (1) Department of Geosciences, The University of Arizona, Tucson, Arizona, USA (2) Smithsonian Tropical Research Institute, Balboa-Ancon, Panama (3) Instituto Colombiano del Petroleo (ICP) - ECOPETROL, Piedecuesta, Colombia * Presenting author’s email: [email protected] The recognition of mobile belts and tectonometamorphic provinces in Precambrian cratons is key for the study and accurate geological reconstruction of ancient orogenic belts, where sea-floor magnetic stripes are no longer available and in cases where paleomagnetic data is limited and solutions for the existent poles become non-unique. Consequently, reconstructions of Proterozoic supercontinents and connections between different cratons heavily rely on geological correlations drawn upon finely resolved geochronology and different geochemical and isotopic tracers. The Amazon Craton is the largest of the Precambrian blocks that constitute the South American continent, and its role in the amalgamation of the supercontinent Rodinia has for long been inferred. However, there are still sizable areas that remain completely unknown as a result of dense vegetation cover, heavy tropical weathering, and the widespread development of Phanerozoic sedimentary basins. In this communication, we present new zircon U-Pb geochronological results from Proterozoic highgrade metamorphic inliers found in the Colombian Andes, and basement drilling-cores from deep exploratory wells in the adjacent north Andean foreland basins. Our results constitute the first geochronological evidence for the presence of a Grenville-age belt (s.l.) in autochthonous NW Amazonia, and also document the complex Proterozoic accretionary and collisional evolution experienced by this segment of the craton. Two distinct metamorphic events were recognized, one represented by migmatite formation under amphibolite-facies conditions at ca. 1.05- 1.02 Ga, and the other characterized by granulite-facies conditions attained at ca. 0.99 Ga. Detrital zircon U-Pb results 29 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA from metasedimentary units and protolith ages from metaigneous suites suggest a pre-collisional disconnect between the evolution of (autochthonous) Paleoproterozoic Amazonia’s leading-edge, and that of the interpreted parautochthonous terranes represented by the Cordilleran inliers, these last ones to be predominantly built on Mesoproterozoic crust. Close comparisons with the known geologic evolution of the Sunsas-Aguapei belt in SW Amazonia reveals clear contrasts in terms of the collisional and pre-collisional records of these two orogenic segments, placing fundamental differences between them. Based on this, we propose the existence of a distinct Grenville-age belt in NW South America, herein called the Putumayo Orogen, characterized by a two-stage tectonometamorphic evolutionary model comprised by 1) early terrane accretions of a periAmazonian fringing-arc system onto the continental margin and 2) final collisional interactions with the Sveconorwegian province of Baltica and the Oaxaquian terranes at the heart of Rodinia. COUPLED DELAMINATION AND INDENTOR-ESCAPE TECTONICS IN THE SOUTHERN PART OF THE C. 650-500 MA EAST AFRICAN/ANTARCTIC OROGEN 1-11 Jacobs, J.1*, Thomas, R.T.2, Ueda, K.1, Kleinhanns, I.1, Emmel, B.1, Kumar, R.1, Bingen, B3, Engvik, A.3 (1) Department of Earth Science, University of Bergen, Allegaten 41, N-5007 Bergen, Norway, (2) British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, United Kingdom (3) Geological Survey of Norway, Trondheim, Norway * Presenting author’s e-mail: [email protected] The East African/Antarctic Orogen (EAAO) is one of the largest orogenic belts on the planet, resulting from the collision of various parts of East and West- Protogondwana between 620 and 550 Ma. The central and southern parts of the orogen are typified by high-grade rocks, representing the overprinted margins of the various colliding continental blocks. The southern third of this Himalayantype orogen can be interpreted in terms of a lateral tectonic escape model, similar to the situation presently developing in SE-Asia. One of the escape-related shear zones of the EAAO is exposed as the approximately 20 km wide Heimefront transpression zone in western Dronning Maud Land (Antarctica). During Gondwana break-up, the southern part of the EAAO broke up into a number of microplates (Falkland, Ellsworth-Haag and Filchner blocks). These microplates probably represent shear zone-bound blocks, which were segmented by tectonic translation during lateral tectonic extrusion. The southern part of the EAAO is also typified by large volumes of late-tectonic A2-type granitoids that intruded at c. 530-490 Ma, and can constitute up to 50% of the exposed basement. They are likely the consequence of delamination of the orogenic root and the subsequent influx of hot asthenospheric mantle during tectonic escape. The intrusion of these voluminous melts into the lower crust was accompanied by orogenic collapse. The A2-type magmatism appears to terminate along the Lurio Belt in northern Mozambique. Therefore, the Lurio Belt could represent an accommodation zone, separating an area to the south in which the orogen underwent delamination of the orogenic root, and an area to the north, where the orogenic keel is still present. Erosional unroofing of the northern EAAO is documented by the remnants of originally extensive areas covered by Cambro-Ordovician molasse-type clastic sedimentary rocks throughout North Africa and Arabia, testifying to the size of this mega-orogen. Whilst the EAAO molasse in the north covers almost the entire North African platform, probably resulting from a long lasting high standing mountain range (no delaminated root), the molasse deposits of the southern EAAO are comparatively smaller, possibly resulting from the rapid and mechanical thinning of the orogen in the south (delaminated root). 30 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA SM-ND ISOTOPIC CONSTRAINTS ON THE NEOPROTEROZOIC – EARLY PALEOZOIC EVOLUTION OF THE EASTERN SIERRAS PAMPEANAS 1-12 López de Luchi, M.G.1*, Steenken, A.2, Martínez Dopico, C.I.1, Drobe, M., Wemmer, K., Siegesmund, S. (1) Instituto de Geocronología y Geología Isotópica (INGEIS), Pabellón INGEIS, Ciudad Universitaria, C1428EHA, Buenos Aires, Argentina (2) Universität Greifswald, Geologisches und Geographisches Institut, Friedrich-Ludwig-Jahn Strasse 17a, 17489 Greifswald, Germany (3) Geoscience Centre of the University of Göttingen (GZG), Goldschmidtstr. 1-3, 37077 Göttingen, Germany. * Presenting author’s e-mail: [email protected] The aim of this contribution is to present the result of an extensive integration of 276 already published Sm-Nd data of the Sierras de Córdoba, San Luis and Chepes in order to provide an isotopical background for the crustal average age of the Eastern Sierras Pampeanas (ESP). ESP constitute a polyphase deformed morphotectonic unit, which was structured by three main events: the Ediacaran to early Cambrian (580–510 Ma) Pampean, the late Cambrian–Ordovician (500–440 Ma) Famatinian and the Devonian-Carboniferous (400–350 Ma) Achalian orogenic cycles (among others Aceñolaza et al. 2000, López de Luchi et al. 2007, Otamedi et al. 2005, Ramos 1988, Rapalini 2005, Rapela et al. 2007, Sims et al. 1999, Siegesmund et al. 2009, Steenken et al. 2006). In the Pampean orogen a collisional stage predates a high grade granulite metamorphism resulting from the collapse of the subducting slab and delamination of the mantle lithosphere. In the Famatinian orogen high grade metamorphism is related to the active subduction with close temporal and spatial relationships between mid-crustal felsic plutonism, mafic magmatism and zones of high-grade metamorphism. Achalian cycle magmatism which would be related with a compressional event farther west from the ESP is mainly represented by extensive granitoid batholiths. This magmatism involves a transient heat anomaly that controlled melts derived from an enriched mantle source which probably mixed with melts derived from a segment of crust different from the Neoproterozoic to Ordovician continental crust of the Eastern Sierras Pampeanas. Sm-Nd results for the metamorphic basement suggest that the TDM (Nd) interval of 1.7-1.8 Ga (Fig. 1), which is associated with the less radiogenic ÂNd(540) of -6 to -8 can be considered as the average crustal composition for the Eastern Sierras Pampeanas. Increasing metamorphic grade in rocks with similar detrital sources and metamorphic ages like in the Sierras de Córdoba is associated with a younger TDM and a more positive ÂNd(540). In metaclastic rocks of different metamorphic age predominance of Grenville detrital components is associated with a relatively more positive ÂNd(540). Granitoids emplaced pre-peak metamorphism in the Pampean orogen form two clusters, one with TDM (Nd) between 1.75-2.0 and another between 1.5-1.6 Ga. Pampean post- 540 Ma granitoids exhibit more homogenous TDM (Nd) ranging 1.75-2.0 Ga. TDM (Nd) for the Ordovician Famatinian granitoids define a main interval of 1.6-1.8 Ga, except for the Ordovician TTG suites of the Sierras de Córdoba, which show a younger TDM (Nd) ranging 1.0-1.3 Ga and a variable radiogenic epsilon. Achalian magmatism exhibits more radiogenic epsilon compared to the Pampean and Famatinian Nd parameters ranging between 0.5 to -4 and TDM (Nd) younger than 1.3 Ga. Two types of Pampean related mafic rocks are recognized: one with a depleted mantle signature and LREE depleted sources that could indicate the stage of ocean crust formation and another younger group with an enriched mantle signature, which is associated with the peak of metamorphism. In the Ordovician mafic-ultramafic rocks processes of mixing/assimilation of depleted mantle signature melts and continental crust are more likely. Therefore, the geodynamic scenario for the Ordovician mafic rocks would imply a thicker continental crust. On the contrary the emplacement of the Pampean mafic rocks would imply a thinner continental crust or alternatively a fast extensional process (Pampean extensional collapse) that could prevent the modification of the mafic melts in their ascent to the emplacement level. Although some discussion could be addressed concerning the degree of crust-mantle interaction in pre-Devonian times, crustal recycling is dominant, whereas processes during the Achalian cycle led to different geochemical and isotopic signatures that reflect a major input of juvenile sources to the magmatism. 31 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1- Simplified geological map of the Sierras de Córdoba, the Sierra Norte, the Sierra de Chepes and the Sierra de San Luis (Steenken et al 2006, Siegesmund et al. 2009) with the approximate location of TDM (Nd) data for metaclastic and felsic igneous rocks. Detrital zircon spectra are included as another evidence for the source. 32 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA REFERENCES • Aceñolaza, F.G., Miller, H., Toselli, A.J., 2000. The Pampean and Famatinian cycles –superimposed orogenic events in West Gondwana. In: Miller, H., Herve, F. (Eds.), Geoscientific Cooperation with Latin America. Zeitschrift für Angewandte Geologie Sonderheft, vol. 1, pp. 337–344. • Brodtkorb, M.K. de., Ostera, H., Pezzutti, N., Tassinari, C. 2005. Sm/Nd and K-Ar data from W-bearing amphibolites of Eastern Pampean Ranges, San Luis and Córdoba, Argentina. 5° South American Symposium on Isotope Geology, 478-482,Punta del Este. • Drobe M, López de Luchi MG, Steenken A, Frei R, Naumann R, Wemmer, K., Siegesmund S., (2009) Provenance of the Late Proterozoic to Early Cambrian metaclastic sediments of the Sierra de San Luis (Eastern Sierras Pampeanas) and Cordillera oriental, Argentina. Journal of South American Earth Sciences, 28:239–262 • Escayola, M.P., Pimentel, M.M., Armstrong, R., 2007. Neoproterozoic back-arc basin: sensitive high-resolution ion microprobe U–Pb and Sm–Nd isotopic evidence from the Eastern Pampean Ranges, Argentina. Geological Society of America 35 (6): 495–498. • López de Luchi, M., Siegesmund, S., Wemmer, K., Steenken, A., Naumann, R., 2007.Geochemical constraints on the petrogenesis of the Palaeozoic granitoids of the Sierra de San Luis, Sierras Pampeanas, Argentina. Journal of South American Earth Sciences 24 (2–4), 138–166. • Otamendi, J.E., Ribaldi, A.M., Demichelis, A.H., Rabbia, O.M., 2005. Metamorphic evolution of the Río Santa Rosa granulites, north Sierra de Comechingones, Argentina. Journal of South American Earth Sciences 18:163–181. • Ramos, V.A., 1988. Late Proterozoic–early Paleozoic of South America: a collisional story. Episodes 11, 168–174. • Rapalini, A.E., 2005. The accretionary history of southern South America from the latest Proterozoic to the late Paleozoic: some paleomagnetic constraints. In: Vaughan, A.P.M., Leat, P.T., Pankhurst, R.J. (eds). Terrane processes at the margins of Gondwana, London, Geological Society of London Special Publication 246, 305-328. • Rapela, C.W., Pankhurst, R.J., Casquet, C., Fanning, C.M., Baldo, E.G., Gonzáles- Casado, J.M., Galindo, C., Dahlquist, J., 2007. The Río de la Plata craton and the assembly of Gondwana. Earth-Science Reviews 83, 49–82. • Schwartz, J.J., Gromet, L.P., 2004. Provenance of a late Proterozoic–early Cambrianbasin, Sierras de Córdoba, Argentina. Precambrian Research 129, 1–21. • Siegesmund, S., Steenken, A., Martino, R. Wemmer, K. López de Luchi, M.G., Frei R., Presniakov, S., Guereschi, A., 2009. Time constraints on the tectonic evolution of the Eastern Sierras Pampeanas (Central Argentina). International Journal of Earth Sciences, DOI: 10.1007/s00531-009-0471-z • Sims, J.P., Ireland, T.R., Camacho, A., Lyons, P., Pieters, P.E., Skirrow, R.G., Stuart-Smith, P.G., Miró, R., 1998. U–Pb, Th–Pb and Ar–Ar geochronology form the southern Sierras Pampeanas: implication for the Palaeozoic tectonic evolution of the western Gondwana margin. In: Pankhurst, R.J., Rapela, C.W. (Eds.), The Proto-Andean Margin of Gondwana. Geological Society of London Special Publication, vol. 142, pp. 259–281. • Steenken, A., Siegesmund, S., López de Luchi, M.G., Frei, R., Wemmer, K., 2006. Neoproterozoic to early Palaeozoic events in the Sierra de San Luis: implications for the Famatinian geodynamics in the Eastern Sierras Pampeanas (Argentina). Journal of the Geological Society 163, 965–982. PALEOMAGNETIC POLE FOR THE NEOPROTEROZOIC DIKES OF THE NICO PÉREZ TERRANE (URUGUAY) AND THE APPARENT POLAR WANDER PATH (APWP) FOR THE RÍO DE LA PLATA CRATON 1-13 Lossada, A.L.1, Rapalini, A.E.1, Sanchez Bettucci, L.2 (1) INGEODAV, Depto. Cs. Geológicas, FCEyN, Univ. Buenos Aires, Pabellón 2, Ciudad Universitaria, C1428EHA, Buenos Aires., Argentina (2) Departamento de Geología, Área de Geofísica-Geotectónica, Facultad de Ciencias, Universidad de la República. Iguá 4225, Malvín Norte, CP 11400, Montevideo, Uruguay Neoproterozoic basic dikes intrude the Paleoproterozoic basement rocks of the Nico Pérez terrane, in central Uruguay. They are sub-vertical, trend broadly E-W, are up to 10 meters thick and can be followed in the field for tens to hundreds of meters. The general texture is porphyritic with phenocrysts of plagioclase, inmerse in an intersertal groundmass consisting of plagioclase, clinopyroxene and intersticial glass. Opaque minerals represent a minor constituent. Samples are fresh, with very low grade of alteration and no evidence of metamorphism. The age of the dikes is poorly established, having been reported one K/Ar data that indicates an age of 581±13 Ma. One hundred and five (105) oriented samples, distributed into seven (7) sites, were collected in the surroundings of Zapicán (Lavalleja department, Uruguay, 33º31´S, 55º56´W). Anisotropy of magnetic susceptibility (AMS) results suggests subhorizontal flow direction, while isothermal remanent 33 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA magnetization (IRM) acquisiton curves indicate a ferrimagnetic phase as the carrier of magnetization. Samples were submitted to AF and thermal desmagnetization, with typical desmagnetization steps of 0, 3, 6, 9, 12, 15, 20, 25, 30, 35, 40, 50, 60, 80, 100 and 120 mT, and 0, 100, 150, 200, 250, 300, 350, 400, 450, 500 and 550ºC. All samples presented high stability of magnetization with unblocking temperatures close to 550ºC, which indicates the presence of magnetite or low-Ti titanomagnetite as the principal mineral carrier of the magnetization. Consistency of the isolated magnetic components was found in five sites, three of which showed normal and two reverse polarities. The mean of five virtual geomagnetic poles (VGPs) is located at 9.9ºS, 262.7ºE (A95=14.1º). The position of this preliminary paleomagnetic pole for the Nico Pérez terrane is consistent with the apparent polar wander path (APWP) for the Río de la Plata (RP) craton for the Ediacaran-Cambrian interval, and its location on the curve suggests an age for the intrusion at sometime between 580 and 530 Ma, consistent with the scarce geochronologic data. This PP is also consistent with coeval poles from other Gondwanan blocks of ca. 550Ma, suggesting that RP and several other plates were already assembled in a Gondwana arrangement by that time. Over one hundred and thirty oriented samples from thirteen (13) sites were recently collected in the same region for a more detailed paleomagnetic study. These and an Ar-Ar dating of these basalts, which is under way, will help in confirming or modifying these preliminary paleomagnetic results ORDOVICIAN MAGMATISM IN THE NORTHEASTERN NORTH PATAGONIAN MASSIF: FURTHER EVIDENCE FOR THE CONTINUITY OF THE FAMATINIAN OROGEN 1-14 Martínez Dopico, C.I.1 3*, López de Luchi, M.G.1, Wemmer, K.2, Rapalini, A.E.3, Linares, E.1 1 Instituto de Geocronología y Geología Isotópica (INGEIS), Pabellón INGEIS, Ciudad Universitaria, C1428EHA, Buenos Aires, Argentina 2 Geoscience Centre of the University of Göttingen (GZG), Goldschmidtstr. 1-3, 37077 Göttingen, Germany 3 Instituto de Geofísica Daniel A.Valencio (INGEODAV), Depto. Cs. Geológicas, FCEyN, Universidad de Buenos Aires (*) Presenting author’s e-mail: [email protected] Different scenarios have been proposed for the tectonic and paleogeographic evolution of Patagonia during Paleozoic times. Allochthonous hypotheses point out an accretionary event along the southwestern margin of Gondwana during the Late Paleozoic (Ramos, 1984), whereas (para-) autochthonous ideas propose that Patagonia belonged to Gondwana since the Early Paleozoic (Rapalini et al., 2010 and references therein). Pankhurst et al. (2006), on the basis of strong geochronological data, suggested that whereas since Ordovician times the North Patagonian Massif (NPM) was already part of Gondwana, southern Patagonia (Deseado Massif, DM) constituted an allochthonous terrane that might have collided with the NPM during Mid Carboniferous times. Recent studies (e.g. Rapalini et al., 2010) provide geophysical, isotopic and petrologic evidence on the autochthony of the NPM considering the continuity of the Pampean basement signature on the NPM, assuming a short Siluro-Devonian rifting stage –Sierra Grande small ocean- followed by accretion onto Gondwana in the Late Carboniferous-Early Permian. Since the first geological studies in the northeastern NPM (Valcheta area, Caminos, 1983) the existence of an Ordovician magmatism was assumed, but few sound isotopic evidence has come to light only recently (López de Luchi et al., 2008; Tovher et al., 2008, Gozálvez, 2009 among others). Further south, in the Sierra Grande area, Varela et al. (1997, 2009), among others, have provided geochronological constraints on geographically overlapped Ordovician and Permian magmatism. In this short paper we present new field mapping, petrographic correlation and a comprehensive 34 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Geologic sketch of the pre-Cenozoic geologic units near Valcheta town, northeastern North Patagonian Massif. Available Ordovician ages for intrusive bodies are pointed out in their respective location. summary of the available geochronological data which provide robust evidence of the extension of Ordovician magmatism further west in the northeastern areas of the NPM (Fig. 1). This evidence is analyzed in the framework of the tectonic evolution of the NPM and its relationship to Gondwana. Local Geology The studied magmatic products mainly correspond to undeformed muscovite bearing leucogranites formerly known as Punta Sierra granitoids (Caminos, 2001) and referred to as the Valcheta pluton by Gozálvez (2009). These rocks crop out as small and discontinuous elongated bodies between Valcheta and Nahuel Niyeu towns (Fig. 1) in the Rio Negro province of Argentina. The alignment of these outcrops might suggest a SW-NE belt which is no longer recognizable further south. Field relationships indicate that the Valcheta pluton intrudes the Early Cambrian low grade metamorphic rocks of the Nahuel Niyeu Formation. The host rock is a greenschist facies chloritemuscovite schist. The limited field observations indicate that in the central part granites are medium grained whereas most of the outcropping facies towards the external part of the belt are finer grained. Very fine grained dykes related to the main intrusions were observed in all cases. Granitoids vary 35 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Table 1- Available data for the Ordovician magmatism in the Sierra Grande area. Sources 1) López de Luchi et al., 2008; 2) Tohver et al., 2008; 3) Gozálvez 2009; 4) INGEIS K-Ar repository [K2O 6.39 wt%; 40Ar rad 51.49E-10 mol/gr.; 40Ar 93.2%]. See map for location of the samples. from medium to fine grained gray or pinkish equigranular and allotriomorphic leucogranites composed of weakly zoned oligoclase-microcline-quartz-muscovite, scarce chloritized biotite, with zircon, apatite and opaques as accessory minerals. Alteration to fine grained white mica and clay minerals affected principally the plagioclase. In one example, K-feldspar is an orthoclase in transition to microcline. As muscovite is a primary magmatic phase pressure might have been higher than 3.5 kbar at the moment of emplacement. Only in one case (sample SA110-07, table 1) plagioclase appeared as subhedral lath. No pervasive planar fabric was observed but some evidence of high temperature ductile deformation is indicated by myrmekite and scarce chessboard in quartz. Deformation in plagioclase is shown by tappered and curved twins and kink-bands and bending of the muscovite crystals. Analogue observations were reported by Gozálvez (2009), who also described amphibolite lenses and xenolithes with gneissose textures within the granites of the central part of the belt. K/Ar-dating method on muscovite and biotite might be used to characterize the post-crystallization cooling history of granitoid rocks. K/Ar biotite ages indicate the cooling history below 300° ± 50°C whereas the closure temperature for ‘normal’ fine to coarse-grained rocks white mica is 350° ± 50°C (Purdy and Jäger, 1976). Same considerations can be applied to the Ar-Ar system. Cooling Ages Implications and Constraints A first observation of the K/Ar and Ar-Ar on muscovite dataset (Table 1) indicates either a slow cooling rate or perhaps different intrusions as suggested by the heterogeneity of cooling ages ranging from the 470 to 449 Ma. The closer constraint to the age of the metamorphic peak of the Nahuel Niyeu Fm. in the area of Valcheta is provided by the younger detrital zircon age of 515Ma (Pankhurst et al., 2006). Amphibolite-grade metamorphism in the region is dated at 472±5Ma (Pankhurst et al., 2006) in quartz-feldspathic gneiss from Mina Gonzalito. These gneisses were considered as deeper crustal equivalents of both the El Jaguelito and Nahuel Niyeu formations based on zircon provenance data (Pankhurst et al., 2006). Ordovician granitoids, intruding the very low grade metamorphic rocks of the El Jaguelito Fm, in the easternmost sector of the NPM near Sierra Grande, yielded U-Pb crystallization ages between 472476 Ma (Pankhurst et al., 2006, González et al., 2008, Varela et al., 1998) and Rb-Sr ages between 483-428 Ma (Varela et al., 1997; Varela et al., 2009). The older Rb-Sr age corresponds to the Punta Sierra biotite granite whereas the younger ages were calculated for amphibole bearing biotite granitoids which exhibit mafic microgranular enclaves. The U-Pb age of 472 Ma corresponds to the El Molino pluton which is a ductile deformed garnet bearing two-mica granite. With the exception of the latter, for which no indication about the structural state of the K-feldspar is available, granitoids are characterized by orthoclase instead of microcline. Although most of the above mentioned granitoids exhibit clear–cut contacts with the low grade El Jaguelito Fm, the inferred age of the metamorphic peak suggest some overlapping. The older Ar-Ar ages in the Valcheta area (Table 1) almost overlap with the crystallization ages of the granodioritic plutons of the Sierra Grande area. Granites are either slightly younger (472 Ma for the two mica El Molino pluton) or older (483 Ma for the biotite granite of Punta Sierra) than the granodiorites. Cooling ages as determined by the K-Ar and Ar-Ar ages in the Valcheta area are broadly consistent with those U-Pb and Rb-Sr from the Sierra Grande area in the east. However, 36 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA compositional differences between the plutons in these two areas deserve further studies. Considering the youngest available biotite ages of ca 430 Ma in the Sierra Grande area (Weber 1983) and 413 Ma west of Valcheta area (Table 1), a long-lasting Ordovician thermal event associated with shallow emplacement due to crustal thinning could be inferred. In summary, our new field mapping and the geochronological analysis reinforce a wider extension of the Ordovician magmatic event in the North Patagonian Massif. REFERENCES • Caminos, R., 1983. Descripción geológica de las Hojas 39g, Cerro Tapiluke y 39h, Chipauquil, prov. de Río Negro. Servicio Geológico Nacional (inédito). Buenos Aires. • Caminos, R. 2001. Hoja Geológica N 4166-I Valcheta, provincia de Río Negro, Boletín 310, Servicio Geológico Minero Argentino, 78 pag. • González, P.D., Sato, A.M., Varela, R., Llambías, E.J., Naipauer, M., Basei, M:A:S:, Campos, H., Greco, G.A., 2008. El Molino pluton: a granite with regional metamorphism within El Jagüelito Formation, North Patagonian Massif. VI South American Symposium on Isotope Geology. Electronic files • Gozálvez, M.R., 2009. Petrografía y Edad 40Ar-39Ar de granitos peraluminosos al oeste de Valcheta. Macizo Nordpatagónico (Río Negro). Revista de la Asociación Geológica Argentina 64(2): 285-294. • López de Luchi, M.G., Wemmer, K., Rapalini, A.E., 2008. The cooling history of the North Patagonian Massif. First results for the granitoids of the Valcheta area, Río Negro, Argentina. VI South American Symposium on Isotope Geology. Electronic Files, S.C. de Bariloche • Rapalini, A.E., López de Luchi, M.G., Martínez Dopico, C., Lince Klinger, F., Giménez, M., Martínez, P., 2010. Did Patagonia collide against Gondwana in the Late Paleozoic? Some insights from a multidisciplinary study of magmatic units of the North Patagonian Massif. Geologica Acta (in press). • Varela, R., Cingolani, C., Sato, A., Dalla Salda, L., Brito Neves, B.B., Basei, M.A.S., Siga Jr., O., Teixeira, W., 1997. Proterozoic and Paleozoic evolution of the Atlantic area of North-Patagonian Massif, Argentine. South American Symposium on Isotope Geology Abstracts: 326-329. San Pablo • Varela, R., Basei, M.A.S., Sato, A.M., Siga Jr., O., Cingolani, C.A., Sato, K., 1998. Edades isotópicas Rb/Sr y U/Pb en rocas de Mina Gonzalito y Arroyo Salado, Macizo Norpatagónico Atlántico, Río Negro, Argentina. 10° Congreso Latinoamericano de Geología, Actas I: 71-76, Buenos Aires. • Varela, R., Sato, K., González, P.D., Sato, A.M., Basei, M.A.S., 2009. Geología y Geocronología Rb-Sr de los granitoides de Sierra Grande, Provincia de Río Negro. Revista de la Asociación Geológica Argentina 64(2): 275-284. • Pankhurst, R.J., Rapela, C.W. Fanning, C.M., Márquez, M. 2006. Gondwanide continental collision and the origin of Patagonia Earth-Sciences Reviews, 76, 235-257. • Purdy, J.W., Jäger, E., 1976. K-Ar ages on rock forming minerals from the Central Alps. Memorie degli Istituti di Geologia e Mineralogia dell’ Universita di Padova v. 30,. 1-31. • Ramos, V.A., 1984. Patagonia: ¿Un continente paleozoico a la deriva? 9th Congreso Geológico Argentino, San Carlos de Bariloche, Actas 2: 311-325. • Tohver, E, Cawood, P.A., Rossello, E., López de Luchi, M.G., Rapalini, A., Jourdan; F., 2008. New SHRIMP U-Pb and 40Ar/39Ar constraints on the crustal stabilization of southern South America, from the margin of the Rio de Plata (Sierra de Ventana) craton to northern Patagonia. AGU, Fall Meeting, EOS. • Weber, E.I., 1983. Descripción Geológica de la Hoja 40J, cerro El Fuerte, prov. de Río Negro. Dirección Nacional de Geología y Minería. Boletín 196:1-69. Buenos Aires. IAPETAN EVOLUTION OF APPALACHIAN PERI-LAURENTIAN AND PERI-GONDWANAN ARC COMPLEXES: A NEWFOUNDLAND COMPARISON 1-15 Brian H. O’Brien(*) Geological Survey of Newfoundland and Labrador, P.O. Box 8700, St. John’s, NL, Canada A1B 466, * Presenting author’s e-mail: [email protected] Numerous studies of the well-exposed regional cross section of the Appalachian orogen on the Island of Newfoundland have provided a basis for the proposal that suprasubduction zone ophiolites developed in marginal basins on opposing sides of the Iapetus Ocean during gaps in island arc volcanism. Older extinct oceanic arcs were commonly uplifted and disconformities mainly developed beneath cover sequences related to younger ensialic arcs. However, certain well-timed tectonic events differentiate many of the early Paleozoic arc complexes of the peri-Laurentian and peri-Gondwanan oceanic realms. 37 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA First, the oldest Cambrian arc sequence found in the peri-Gondwanan part of Iapetus is ensialic and contemporaneous with the first generation of Cambrian ophiolites in the peri-Laurentian part of Iapetus. Similarly, the main late Cambrian-early Ordovician period of ophiolite generation in periLaurentia is coeval with a renewed phase of peri-Gondwanan island arc volcanism during which the underlying Ediacaran basement was rejuvenated. Second, the peri-Gondwanan ophiolites were involved in initial arc collision outboard of the continental margin at an earlier stage than they were in the peri-Laurentian Iapetan realm. Furthermore, ophiolite obduction and final emplacement also occurred earlier in the Ordovician on the peri-Gondwanan continental margin than it had done in periLaurentia. Third, though arc complexes generally grew oceanward away from the opposing continental margins, the youngest Middle Ordovician volcanism dated in the peri-Laurentian ensialic arc is about 8 My older than the youngest dated Late Ordovician rhyodacite in the peri-Gondwanan ensialic arc. Thus, at the arc-arc suture, tectonically encroaching peri-Gondwanan rocks were thrust beneath dormant peri-Laurentian rocks and active mid Caradocian arc magmatism became focused along old fault lines in the upper and lower plates. Peri-Gondwana The peri-Gondwanan ribboned microcontinent that has been argued to comprise the basement to Appalachian Ganderia has an early Paleozoic evolution governed by its paleogeographic position near the southern margin of the Iapetus Ocean but also an earlier geological history related to the Avalonian development of juvenile crust on the Neoproterozoic margin of the Gondwanan supercontinent. In south-central Newfoundland, the earliest pre-Appalachian phase of orogenesis began with basement orthogneiss formation in the Cryogenean (ca. 686 Ma), continued with deposition of a volcanosedimentary cover sequence in the Ediacaran (ca. 585-565 Ma) and culminated with emplacement of an arc-related suite of Avalonian stitching plutons (ca. 575-560 Ma). Ediacaran remobilization of Cryogenean crystalline basement was coeval with intrusion of the youngest of these plutonic rocks; however, it is the earliest Cambrian tectonism (ca. 540-535 Ma) of the linked basement-cover complexes that is characteristic of the pre-Iapetan rocks included in Newfoundland’s Ganderia. Passive continental margin deposition of the quartz-rich turbidite prism that is one of the hallmarks of Ganderia is postulated to have begun after ca. 535 Ma in the Cambrian and may have persisted in places until the Early Ordovician (Arenig). In contrast, the oldest subduction-related volcanic strata preserved within the Gondwanan realm of Iapetus are mineralized Middle and Late Cambrian ensialic arc sequences (ca. 515-496 Ma). The younger parts of this active arc and the easterly adjacent passive margin were locally drowned and buried by black shale (Tremadoc to Arenig). However, the oldest Cambrian arc rocks, which have zircons probably inherited from subjacent Ediacaran intrusions, are interpreted to have accumulated above an uplift of Cryogenean basement on the open ocean-facing margin of the peri-Gondwanan microcontinent. The oldest suprasubduction zone ophiolites on the peri-Gondwanan Iapetan margin (ca. 500-495 Ma) formed near the ribboned microcontinent during a ca. 5 My hiatus in volcanic arc construction. Related bimodal arc plutons had crosscut the supracrustal rocks of the Ediacaran cover sequence in the pre-Appalachian basement complex during this same interval. Fragments of the peri-Gondwanan ophiolites were mylonitized along with parts of a younger group of latest Cambrian-early Ordovician ensialic volcanic arc sequences (ca. 491-485 Ma) in a narrow arc collision zone of probable latest Tremadoc age. Ophiolitic mélange tracts commonly developed where ultramafic rocks were subsequently obducted onto the passive continental margin in the early Arenig. The northern portion of the peri-Gondwanan Iapetan realm was composed of basalt-dominated primitive arcs and oceanic seamounts during the latest Cambrian to early Ordovician period. By comparison, time equivalent ensialic arcs that had lain farther south near the pre-Appalachian basement inliers have a significant component of subvolcanic quartz-feldspar porphyry and mineralized felsic pyroclastic rocks associated with island arc tholeiite. Many of the ensimatic northern arc sequences became dormant and were rifted in the Arenig (ca. 475 Ma). They were either uplifted in horst blocks or buried beneath widening grabens infilled by arc-related pyroclastic turbidites, tholeiite-derived epiclastic turbidites and distal ash tuffs sourced from continental margin 38 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA melts. By comparison, in the region to the south, a more protracted phase of compressional and extensional Middle Ordovician tectonism resulted in the infrastructure of the peri-Gondwanan Iapetan realm becoming upwelled. There, the quartz-rich turbidite prism and the crystalline basement of the southern arc volcanic sequences were syntectonically intruded by mesozonal plutonic rocks in the Arenig (ca. 474-468 Ma) and they continued to be regionally metamorphosed within Abukuma-type migmatite domes until the Llanvirn (ca. 465-460 Ma). The youngest phase of volcanism recorded in the peri-Gondwanan realm of Iapetus occurred during the late Middle Ordovician and the earliest Late Ordovician. Deposits formed as recently as the early Llanvirn possess a Celtic paleobiogeographic fauna, indicating deposition in the southern low latitudes or near islands in the middle part of the Iapetus Ocean. In the northern part of Iapetan Ganderia, dominantly alkaline mafic volcanic rocks, bioclastic limestone and ribbon chert accumulated within an oceanic back arc that was invaded by an arc-related suite of gabbro laccoliths beginning in the late Llanvirn. In places, the back-arc basin infill passed laterally northward and stratigraphically upward into a relatively thin, rhyodacite-bearing Caradocian ensialic arc sequence that evolved immediately south of the relict Iapetus Ocean. In contrast, in the area lying closer to the Arenig-obducted peri-Gondwanan ophiolites, the back-arc sequence also passed laterally into a black shale-hosted mélange marked by psammitic schist olistoliths preserving pre-incorporation amphibolite facies shear zones. Here, a temporally equivalent, unconformity-bounded volcanosedimentary sequence was deposited in a continental back-arc basin situated above the older ocean-continent suture present in southern Ganderia. Peri-Laurentia The Middle Cambrian to Middle Ordovician stratified and intrusive rocks that comprise the periLaurentian Iapetan realm extend from the external Taconic foreland-propagating thrust belt in western Newfoundland to a subduction-controlled accretionary complex-type of foldbelt in central Newfoundland. The southern boundary of the peri-Laurentian Iapetan rocks is the 350 km long Red Indian Line, an arcuate feature broadly equivalent in age to structures developed near the Appalachian front, but having the opposite tectonic polarity. Peri-Laurentian volcanic rocks occupy an upper plate position with respect to the structurally underthrust peri-Gondwanan volcanic rocks along this intraIapetus Ocean suture. Fossil-bearing peri-Laurentian strata near the Red Indian Line represent the Toquima- Table Head faunal realm which links them biogeographically to the platformal and epeirogenic strata that formed along the periphery of the North American craton, at least from early Arenig to early Llanvirn time. Such strata may have never interacted with Newfoundland’s Grenville Province basement inliers and the overlying late Neoproterozoic to early Paleozoic rift-related sedimentary prism that developed on the passive margin of ancestral Laurentia. However, some unfossiliferous older Ordovician and Cambrian magmatic arc rocks and intraoceanic ophiolites belonging to central Newfoundland’s periLaurentian realm are also found in western Newfoundland. There, they were tectonically accreted, at the structural top of the foreland thrust belt, to the Early Ordovician platformal carbonate bank, the Middle Ordovician hinterland-derived flysch and the far-travelled allochtons that carry the coeval Laurentian continental slope deposits. This occurred during a Late Ordovician phase of deformation and mélange formation, some 15-20 My after the metamorphic soles of the arc ophiolites had crystallized. At least three discrete generations of suprasubduction zone ophiolites have been recognized in the peri-Laurentian Iapetan realm: those formed in the Middle to Late Cambrian (ca. 508-500 Ma), in the late Cambrian to Early Ordovician (ca. 490-485 Ma) and in the mid Early Ordovician (ca. 481-479 Ma). In places, the mantle section of the oldest ophiolite suites and the overlying primitive arc crust were sheared near postulated spreading ridges and were then incorporated as enclaves in the intermediate-age ophiolite suites. Both generations were intruded between ca. 488-484 Ma by pretectonic suites of granodiorite-granite-tonalite plutons and by mafic dyke swarms of boninitic and tholeiitic composition. Importantly, however, the mid Cambrian and earliest Ordovician suprasubduction zone ophiolites were syntectonically intruded by continental arc plutons around 466465 Ma and by posttectonic plutons between ca. 457-455 Ma at relatively deep crustal levels. In other 39 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA locations, the crustal sections of the ophiolites and the coeval arc volcanic rocks were never tectonically buried or metamorphosed above the lower greenschist facies. To explain the presence of continental crust lying beneath the Middle to Late Cambrian intraoceanic arc ophiolites by the time that a latest Cambrian (ca. 494 Ma) suite of lamprophyric dykes were emplaced, it has been recently proposed that the eastern part of Newfoundland’s peri-Laurentian Iapetan realm was underlain by an unexposed Precambrian microcontinental block that may have also acted as a rigid buttress during accretion of the island arc and ophiolite sequences. It would also account for the continental signature of the post-ophiolite Ordovician arc metaplutonic rocks and the ubiquitous enclaves and screens of continental margin metasedimentary gneiss. Within the peri-Laurentian part of the Iapetus Ocean, fossil-bearing Early and Middle Ordovician volcanosedimentary strata evolved in several island arc and back arc complexes situated above or tectonically adjacent to the dormant Middle Cambrian primitive oceanic arc. The longest ranging group (ca. 480-467 Ma) comprises an oceanic arc to oceanic back arc basin succession that was stratigraphically continuous and deposited directly above the ca. 490-485 Ma suite of ophiolites. In contrast, those that are coeval with and younger than the ca. 481-479 Ma ophiolite suite constitute the volcanic fill of ensialic arc-back arc complexes (ca. 480-462 Ma). These peri-Laurentian basins are generally distinguished from similar peri-Gondwanan depocentres by the lack of a voluminous sedimentary infill. One such ensialic arc complex is interpreted to have formed as a cover sequence (ca. 480-464 Ma) above a basement represented by a dormant Middle Cambrian arc, although an approximate 5 My hiatus is also recorded in the cover between the late Early Ordovician and the early Middle Ordovician. In the regional hanging wall sequence of the Red Indian Line, to the east of the hypothetical microcontinent, the oldest and the youngest peri-Laurentian ophiolites succeed each other without any evidence of an intervening ca. 490-485 Ma ophiolite suite. There, another grouping of felsic volcanic-dominant ensialic arc-back arc complexes ranging in age from at least ca. 473-462 Ma may have evolved by reactivating the ca. 479 Ma tonalite-lined magma conduits in the mid Early Ordovician ophiolites. Their relationship with the partly coeval unconformity-bounded ensialic arc sequences is uncertain; however, it has been postulated that they are geodynamically unrelated and were later accreted to the peri-Laurentian margin in the region that had once lain immediately north of the relict Iapetus Ocean. U- PB ZIRCON GEOCHRONOLOGY OF THE SIERRA VALLE FÉRTIL, FAMATINIAN ARC, ARGENTINA: PETROLOGICAL AND GEOLOGICAL IMPLICATIONS 1-16 Otamendi, J.E.1*, Ducea, M.N.2, Bergantz, G.3 (1) Departamento de Geología, Universidad Nacional de Río Cuarto, X5804 Río Cuarto, Argentina (2) Department of Geosciences, University of Arizona, Tucson, 85721 AZ, USA (3) Department of Earth and Space Sciences, University of Washington, Seattle, WA98195, USA * Presenting author’s e-mail: [email protected] We analyse the significance of sixteen U-Pb zircon crystallization ages on igneous plutonic and metasedimenatry rocks from Sierra Valle Fértil that we have reported elsewhere (Ducea et al., 2010). U-Pb geochronology of zircons was conducted by laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) at the Arizona LaserChron Center. These sixteen plutonic and metasedimentary rocks were collected in an east-west transect across the plutonic section from the central Sierra Valle Fértil. Sampling encompasses a plutonic suite from diorites to leucogranites, includes a granulite-facies metasedimentary rock, and covers all the lithological diversity observed in the crystalline section from Valle Fértil. This igneous plutonic suite formed at low- to middle-crustal paleo-depths within the Early Ordovician subduction-related magmatic belt 40 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA from central- and northwestern Argentina, which is known as Famatinian arc. The most relevant outcomes to communicate from our study are: Eleven out of fifteen samples of igneous plutonic rocks yielded ages between 469 and 477 Ma. Furthermore, several crystallization ages cluster around 472 ± 2 Ma, reflecting that a large volume of middle plutonic crust was built within a few million (ca. 5) years. Based on geological mapping and petrologic constraints we estimated that 15 km of the paleoarc crust is mainly constituted by intermediate to silicic plutonism (Otamendi et al., 2009). Thus, this 15 km thick middle- to upper arc crust was created in 20 My (485 to 465 Ma) at an average magmatic growth rate of 0.75 km3/My per km2 of active arc surface. The magmatic production rate expressed as km3/My.km would depend on the width of the Famatinian paleoarc active between 485 and 465 My. For example, a typical hundred-km-wide arc section would have been formed at a magmatic rate of 75 km3/My.km (km3 over strike length times My). Timing of intermediate to silicic plutonism from Valle Fértil suggests generation at high flux rates that are similar to major high flux events reported in mature continental arcs (Ducea and Barton, 2007). 3- A statistically robust population of U-Pb spot data measured in plutonic rocks proves that Early Ordovician magmatism arc in the Valle Fértil section terminated at around 465 Ma. This is consistent with the idea that the collisional accretion of Cuyania terrane on western Gondwana margin began ca. 465 - 460 Ma (Thomas and Astini, 1996). 4- The inherited core zircon ages for all of the rocks from the Sierra Valle Fértil define mainly late Mesoproterozoic - early Neoproterozoic (ca. 1150-850 My) and Neoproterozoic - early Cambrian (ca. 720-530 My) populations, reflecting zircon inheritance from Grenvillian-type, Brasiliano-Pan African and Pampeano orogenic cycles. 5- The existence of inherited zircon cores in the tonalitic and granodioritic rocks require of widespread partial to nearly complete melting of pelitic and semi-pelitic host rocks and subsequent assimilation into the evolving magmas. From our previous work the process took place at between 22 and 10 km depths (P < 6.5 kbar). 6- Consistently with the last point, U-Pb zircon ages from the metasedimentary migmatite define clusters of inherited core ages with peaks at 531, 585 and 913 My; whereas, just two of thirty one spot analyses yielded Early Ordovician ages. This study ultimate demonstrates that the full Sierra de Valle Fértil, as currently exposed by Andean faults, was utterly built up by Early Ordovician arc-related magmatism. REFERENCES • Ducea, M. N., and Barton, M. D., 2007. Igniting flare-up events in Cordilleran arcs, Geology 35, 1047-1050. • Ducea, M.N., Otamendi, J.E., Bergantz, G., Stair, K., Valencia, V., and Gehrels, G., 2010. Timing constraints on building an intermediate plutonic arc crustal section: U-Pb zircon geochronology of the Sierra Valle Fértil, Famatinian Arc, Argentina. Tectonics, in press. • Otamendi, J.E., Vujovich, G.I., de la Rosa, J.D., Tibaldi, A.M., Castro, A., Martino, R.D., and Pinotti, L.P., 2009. Geology and petrology of a deep crustal zone from the Famatinian paleo-arc, Sierras Valle Fértil-La Huerta, San Juan, Argentina, Journal of South America Earth Sciences 27, 258-279. • Thomas, W.A., and Astini, R.A., 1996. The Argentine Precordillera: A traveler from the Ouachita embayment of North American Laurentia. Science 273, 752-757. 41 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA CERRO LA TUNA MAFIC TO ULTRAMAFIC COMPLEX: AN OCEAN FLOOR REMNANT 1-17 Peel, E.1, Basei, M.A.S.2, Sánchez Bettucci, L.1* (1) Instituto de Ciencias Geológicas, Facultad de Ciencias-UdelaR, Montevideo, Uruguay (2) Instituto de Geociencias, Universidade de São Paulo * Presenting author’s e-mail: [email protected] The Cerro La Tuna Complex is located in the northeastern portion of Uruguay. Based on petrographic and chemical data (mineral assemblages and mineral compositions) this Complex is defined as a lithotectonic association composed of mafic and ultramafic rocks. The geology of this region is separated into three major domains: northern, central and southern. The northern domain is represented by granitoids with variable deformation, formed by coarse-grained (titanite) biotite granites with pink feldspar megacrystals, possibly related to the Aigua batholith. In tectonic contact occurs the central mafic-ultramafic domain. This domain (Cerro La Tuna Complex s.s) is constituted by a major anticline structure with peridotites (harzburgite) forming the core, surrounded by a volcano-sedimentary sequence represented by BIFs, metabasalts, quartzites, calc-silicate rocks and cherts layers. Some levels contain chromite pods, dunite dismembered bodies and mafic–ultramafic dikes. The entire unit is cut by ultrabasic dikes (tremolite schist) and it is disrupted by W-NNW shear zones and strike-slip faults. To the south, this mafic-ultramafic sequence is intercalated with banded migmatites belonging to the southern “basement” domain. The banded migmatites show mesosome layers composed of biotite. The leucosome is composed of sub-milimetric banded granite-gneiss with folded quartz-feldspar showing biotitic melanosome borders suggesting in situ melting. E. Peel is thankful for the financial support given by CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior). GRENVILLE-AGE SOURCES IN CUESTA DE RAHUE, NORTHERN PATAGONIA: CONSTRAINS FROM U/PB SHRIMP AGES FROM DETRITAL ZIRCONS 1-18 Ramos, V.A.1*, García Morabito, E.1, Hervé, F.2, Fanning, C.M.3 (1) Laboratorio de Tectónica Andina, FCEyN, Universidad de Buenos Aires – CONICET, Argentina (2) Departamento de Geología, Universidad de Chile (3) Research School of Earth Sciences, Australian National University, Canberra, Australia * Presenting author’s e-mail: [email protected] The northern part of Patagonia has scarce basement exposures. They are located in the south-eastern sector of the province of Neuquén along the foothills of the Patagonian Cordillera at 39º16’S latitude and 70º50’W longitude (see Fig. 1). These exposures were studied by Turner (1965) who mapped them as part of the Colohuincul Formation. This unit at that time was assigned to the Precambrian to early Paleozoic, based on regional correlations. Digregorio (1972) considered this basement as Precambrian, although in a later contribution followed the proposal of Turner (Digregorio and Uliana, 1979). The outcrops of Cuesta de Rahue, as well as other exposures of the Cordón de la Piedra Santa located a few kilometers to the east, were included by Franzese (1995) in the Piedra Santa Complex. This author presents the first K/Ar ages of these metamorphic rocks which yielded values between 372 and 311 Ma with large errors, but that identified a late Paleozoic age for the metamorphism. The first hard evidence that the rocks of the Colohuincul Formation could be of younger age was presented by Basei et al. (1999), who found 345 ± 4.3 Ma old zircons in an amphibolite further south in the Cañadón de la Mosca, near Bariloche. This finding implies that these rocks were 42 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Geological map of the Cuesta de Rahue exposures of the Colohuincul Formation, in the southern Neuquén province, northern Patagonia (after García Morabito, 2010). metamorphosed in late Paleozoic times. An age on U-Pb in titanite ca. 360 Ma together with the previous K-Ar ages, was interpreted as the cooling age of the metamorphic peak. The objective of this study was to know more about the age of these rocks in the Cuesta de Rahue section in order to constrain the age of the basement of the Neuquén Basin in its south-western margin. Cuesta de Rahue This magnificent exposure is located in the eastern margin of the Aluminé valley, a few kilometers down waters of the town of Aluminé. It is approximately 7 km east of the junction of roads 46 and 23, along a pronounced cuesta where phyllites and fine grained quartz-schists of greenish to gray color are exposed. There are also mica-schists with fenoblasts of biotite up to 5 mm size. Heavily deformed schists are exposed in a west facing wall, where the samples were taken in the upper part of the cuesta (Fig. 1). Although there is no any visible contact with the late Paleozoic granitoids, nearby there is good evidence of emplacement of these granites in the metamorphic rocks of Colohuincul Formation (Turner, 1965; Franzese, 1995). These rocks further west were studied by Dalla Salda et al. (1992) who obtained K-Ar ages of 354 ± 4 Ma and 324 ± 6 Ma for tonalitic gneisses, and 376 ± 9 Ma for a biotitic granodiorite in the Lago Lacar area, near San Martín de Los Andes. Recent U/Pb zircon dating on these granitoids constrained the age of the plutonic rocks between 420-380 Ma (Basei et al., 2005). More precise geochronological studies were done by Pankhurst et al. (2006) obtaining ages ranging 43 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 a b Fig. 2 - a) Wetherill plot and b) relative probability ages of zircons from Colohuincul Formation sampled in the upper section of Cuesta de Rahue dated by U/Pb SHRIMP. from 395 ± 4 Ma (U/Pb SHRIMP). These dates indicate a Late Devonian to Early Carboniferous age for the plutonic rocks. U/Pb SHRIMP geochronology The zircons from the Cuesta de Rahue sample appear to be detrital in since they have round to subround shapes consistent with abrasion during surface transport. Some grains are subhedral and under CL imaging, the zircons have a dominantly zoned igneous structure. Thirty grains have been analysed. The U-Th-Pb analyses were made using SHRIMP II at the Research School of Earth Sciences. The Australian National University, Canberra, Australia, following procedures given in Williams (1998), and references therein. Each analysis consisted of 6 scans through the mass range, with the Temora reference zircon grains analyzed for every three unknown analyses. The data have been reduced using the SQUID Excel Macro of Ludwig (2001). The Pb/U ratios have been normalized relative to a value of 0.0668 for the Temora reference zircon, equivalent to an age of 417 Ma (see Black et al., 2003). Uncertainty in the U-Pb calibration was 0.72% for the SHRIMP II session. The U and Th concentrations are relative to the SL13 zircon which has 238 ppm U. A wide range of ages is recorded (Fig. 2a), with a main peak between ~950 Ma and ~1200 Ma, with a secondary peak at about ~1490 Ma (Fig. 2b). Two grains yield Ordovician ages. However, one of these is enriched in common Pb and so not considered to be a significant analysis. The youngest zircon gave an age of 364 Ma, in accordance with the maximum latest Devonian age for the Colohuincul Formation. Discussion The detrital zircons show a few Paleozoic ages within the rank expected for this unit, in accordance with previous constrains obtained for the metamorphic and plutonic rocks. The dominant peak indicates a Grenville source between 950 and 1200 Ma, an age range typical from central Argentina (see discussion in Rapela et al., 2007). The potential sources for these ages are traditionally assumed to be derived from either the Sunsas belt in southern Brazil or from the Namaqualand orogen in South Africa (Bahlburg et al., 2008). However, there are also some local sources as the Cuyania basement which has a similar range of ages as shown by Naipauer et al. (2010) and Ramos (2010). Besides, there is an increasing evidence of sources derived from some of the Patagonian massifs or the adjacent Malvinas plateau. The importance of this dominant peak, plus the late Paleozoic paleogeography favor a more local source within Patagonia as the most probable provenance of the analyzed detrital zircons. Thus, more isotopic data are needed in order to differentiate these potential Grenville-age sources, as the main known source in central-western Argentina has a mantle signature opposed to the more 44 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA recycled sources of southern Patagonia (see Augustsson and Bahlburg, 2008). REFERENCES • Augustsson, C.; Bahlburg, H., 2008. Provenance of late Palaeozoic metasediments of the Patagonian proto-Pacific margin (southernmost Chile and Argentina). International Journal of Earth Sciences. Geologische Rundschau 97:71–88. • Bahlburg, H.; Vervoort, J.D.; Du Frane, S.A.; Bock, B.; Augustsson, C.; Reimann, C., 2009. Timing of crust formation and recycling in accretionary orogens: Insights learned from the western margin of South America. Earth-Science Reviews 97(1-4): 215-241. • Basei, M.A.S.; Brito Neves, B.B.; Varela, R.; Teixeira,W.; Siga Jr.,O., Sato, A.M.; Cingolani, C., 1999. Isotopic dating on the crystalline basement rocks of the Bariloche region, Río Negro, Argentina. IIº South American Symposium on Isotope Geology, Anales SEGEMAR 34: 15-18, Villa Carlos Paz. • Basei, M.A.; Varela, R.; Passarelli, C.; Siga Jr., O.; Cingolani, C.; Sato, A.; Gonzalez, P.D., 2005. The crystalline basement in the north of Patagonia: isotopic ages and regional characteristics. In: Pankhurst, R., Veiga, G. (Eds.) Gondwana 12: Geological and Biological Heritage of Gondwana, Abstracts, Academia Nacional de Ciencias, p. 62, Córdoba. • Black, L.P.; Kamo, S.L.; Allen, C.M.; Aleinikoff, J.N.; Davis, D.W.; Korsch, R.J.; Foudoulis, C., 2003. TEMORA 1: a new zircon standard for Phanerozoic U–Pb Geochronology. Chemical Geology 200: 155-170. • Dalla Salda, L.; Cingolani, C.A.; Varela, R., 1992. El basamento cristalino de la región nordpatagónica de los lagos Gutiérrez, Mascardi y Guillelmo, provincia de Río Negro. Revista de la Asociación Geológica Argentina 46 (3-4): 263-276. • Digregorio, J., 1972. Neuquén. In: A.F. Leanza (ed.). Geología Regional Argentina. Academia Nacional de Ciencias, pp. 439-505, Córdoba. • Digregorio, J.H.; Uliana, M.A., 1980. Cuenca Neuquina. In: J.C.M. Turner (ed.) Segundo Simposio de Geología Regional Argentina, Academia Nacional de Ciencias 2: 985-1032, Córdoba. • Franzese, J.R., 1995. El Complejo Piedra Santa (Neuquén, Argentina): parte de un cinturón metamórfico neoplaeozoico del Gondwana suroccidental. Revista Geológica de Chile 22(2): 193-202. • Ezequiel García Morabito, 2010. Evolución tectónica de la Cordillera de Catán Lil, Neuquén. PhD Thesis, Universidad de Buenos Aires (unpublished), Buenos Aires. • Naipauer, M.; Vujovich. G.I.; Cingolani, C.A.; McClelland, W.C., 2010. Detrital zircon analysis from the Neoproterozoic-Cambrian sedimentary cover (Cuyania terrane), Sierra de Pie de Palo, Argentina: Evidences of a rift and passive margin system? Journal South American Earth Sciences 29 (2): 306-326. • Pankhurst, R.J.; Rapela, C.W.; Fanning, C.M.; Márquez, M., 2006. Gondwanide continental collision and the origin of Patagonia. Earth Science Reviews 76: 235-257. • Ramos, V.A., 2010. The Grenville-age basement of the Andes. Journal of South American Earth Sciences 29(1): 77-91. • Rapela, C.W.; Pankhurst, R.J.; Casquet, C.; Fanning, C.M.; Baldo, E.G.; González-Casado, J.M.; Galindo, C.; Dahlquist, J., 2007. The Río de la Plata craton and the assembly of SW Gondwana. Earth-Science Reviews 83: 49–82. • Turner, J.C.M., 1965. Estratigrafia de Aluminé y adyacencias (provincia del Neuquén). Asociación Geológica Argentina, Revista 20(2): 153184. • Williams, I.S., 1998. U-Th-Pb geochronology by ion microprobe. In: McKibben, M.A. et al., (Eds.), Applications of microanalytical techniques to understanding mineralizing processes. Review Economic Geology 7: 1–35. WAS THE RIO DE LA PLATA CRATON NEVER PART OF RODINIA? SOME PALEOMAGNETIC HINTS 1-19 Rapalini, A.E.* INGEODAV, Depto. Cs. Geológicas, FCEyN, Univ. Buenos Aires, Pabellón 2, Ciudad Universitaria, C1428EHA, Buenos Aires., Argentina * Presenting author’s e-mail: [email protected] The formation and dispersal of the Rodinia supercontinent were long and complex processes in the global paleogeographic evolution during the Meso- and Neo- Proterozoic. In most paleogeographic reconstructions of Rodinia the Rio de la Plata craton (RP) is situated attached to eastern Laurentia, in a similar relative position to Amazonia as in present-day South America. In such classical reconstructions present-day western RP and eastern North America should have been conjugate margins during break-up and dispersal of the final remnants of Rodinia. Evidence of such geological process is widespread along eastern Laurentia and has been accurately dated as occurring between 600 and 550 Ma. Lack of exposed Mesoproterozoic rocks in RP and the possible existence of a large Neoproterozoic ocean to the west (present-day coordinates) of RP, called the Brasiliano-Pampean Ocean, have been interpreted by some authors as evidence of RP never forming part of Rodinia. Direct 45 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA paleomagnetic test of the paleogeographic position of RP during Rodinian times (ca. 1.10-0.75 Ga) is not expected due to the lack of exposed rocks of such ages. However, a more restricted test can be attempted with the available paleomagnetic information from late Neoproterozoic rocks (ca. 600-540 Ma) from both Laurentia and RP. A long standing controversy on the proper paleogeographic position of Laurentia between ca. 590 and 560 Ma has not been solved yet. While at least three key paleomagnetic poles of such age suggest high to polar latitudes for this continent in that interval, some authors have preferred to ignore such poles and interpolate a very similar paleogeographic position than that indicated at ca. 600 and at 550 Ma, in order to avoid non-actualistic drift rates for the continent. This means Laurentia in ecuatorial to low southern hemisphere latitudes throughout the whole period. On the other hand, systematic paleomagnetic studies in RP in the last decade have provided a list of 12 paleomagnetic poles or virtual geomagnetic poles from ca. 600 Ma up to ca. 500 Ma. These poles are distributed systematically along a path that starts to merge with other major Gondwana blocks (e.g. Congo-Sao Francisco) at around 570 Ma. This path permits to determine that RP remained at intermediate to low latitudes during the whole Ediacaran. Comparison of coeval poles of 580-570 Ma from RP and Laurentia are highly discordant in the classical configuration of RP rifting apart from Eastern Laurentia. In the high-latitude option for Laurentia, over 40° of latitude separates both continental margins. In the low latitude option, however, latitudes are similar, but to match the polarity of the rifting margins a nearly 180° rotation of RP, incompatible with paleomagnetic and geologic data, is needed. This suggests that RP was not the crustal block that rifted apart from eastern Laurentia in the Ediacaran and indirectly supports models that portray it as a “nonRodinian” craton. THE AFRICAN PROVENANCE OF SOUTHERN SOUTH AMERICA TERRANES: A RECORD FROM RODINIA BREAK-UP TO GONDWANA ASSEMBLY 1-20 Rapela, C.W.1*, Fanning, C.M.2 Casquet, C.3, Pankhurst, R.J.4, Spalletti, L.A.1, Poiré, D.1, Baldo, E.G.5 (1) Centro de Investigaciones Geológicas (CONICET-UNLP), 1900 La Plata, Argentina (2) Research School of Earth Sciences, The Australian National University, Canberra, Australia (3) Departamento de Petrología y Geoquímica, Universidad Complutense, 28040 Madrid, Spain (4) British Geological Survey, Keyworth, Nottingham NG12 5GG, United Kingdom (5) CICTERRA (CONICET-UNC), 5000 Córdoba, Argentina * Presenting author’s e-mail: [email protected] A remarkable characteristic of southern South America, is that the 2.26-2.02 Ga Palaeoproterozoic sequences of the Río de la Plata craton that define the oldest southern core of the continent, have not been affected by the widespread Neoproterozoic deformation and magmatism associated with the assemblage of Gondwana. In Uruguay, the Sarandí del Yi megashear separates the Paleoproterozoic basement unaffected by Neoproterozoic events (Piedra Alta and Pando terranes), from the complex Archean to Mesoproterozoic Nico Pérez terrane, which was reworked during the Mesoproterozoic (Bossi and Cingolani 2009, Oyhantçabal et al., 2009 and references therein), as well as the collage of terranes accreted during the Brasiliano-Panafrican orogeny (e.g., Punta del Este terrane and Dom Feliciano belt). Further south in the Tandilia belt in Argentina, SHRIMP analyses of the 2.23-2.06 Ga Paleoproterozoic basement do not show a Neoproterozoic overprint (Hartmann et al., 2002b); the 2.19-2.09 Ga samples recovered from deep drill cores in the western side of the craton also show no such evidence (Rapela et al., 2007). Another important piece of evidence comes from the zircon provenance patterns of Neoproterozoic sedimentary and metasedimentary sequences located along the western and southern sides of the Río de la Plata craton (i.e. the Pampean Belt and the North Patagonian Massif). These sequences are dominated by a bimodal pattern, with peaks at 1250-960 Ma and 680-570 Ma and a minor peak c. 46 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA 1900 Ma, with scarce, if any, Palaeoproterozoic zircons in the Río de la Plata craton range of ages (e.g. Basei et al., 2005, 2008; Pankhurst et al., 2006; Rapela et al., 2007). To explain these bimodal patterns, “African” (Namaqua-Natal) and “Brazilian” sources have been postulated (e.g. Schwartz and Gromet, 2004; Basei et al., 2005; Rapela et al, 2007). However, it is difficult to make a coherent scenario incorporating the several geodynamic models recently proposed for the pre-Atlantic (Adamastor ocean), and those for the western side of the Río de la Plata craton. Another important issue that remains poorly explained is the relative position of the Kalahari, Congo and Río de la Plata cratons during the early Neoproterozoic. This paper presents new U-Pb SHRIMP results on drill core samples from close to the present Atlantic coast, at the tip of the Tandilia belt on the eastern margin of the Río de la Plata craton (Punta Mogotes), as well as from the Sierra Ancasti and Sierra Brava in the Pampean belt, on the western side of the craton. Zircon provenance patterns on these critically located samples, together with previous results, allow an interpretation of the Neoproterozoic rifting-drifting of the Río de la Plata craton, and infer a close connection with similar processes in southwestern Africa. This process covers the transition from Rodinia dispersal to Gondwana assembly. The analyses of 5 samples from the 504 m deep Punta Mogotes borehole (Marchese and Di Paola, 1975) show the expected sharp contrast in zircon age pattern between the Neoproterozoic low-grade metapelites of the Punta Mogotes Formation and that of the overlying quartzites of the Balcarce Formation. The patterns of the two samples of the Punta Mogotes Formation are complex but remarkably similar, suggesting a similar source for at least the upper section of this sequence. Conspicuous younger peaks at 760-790 Ma defined by concordant zircons are the most important characteristic of these patterns, with significant populations in the Mesoproterozoic (peaks at 1250 and 1270 Ma respectively), and Upper Paleoproterozoic (peaks at 1735 and 1835 Ma), and minor but concordant populations at 1420-1560 Ma, 2070-2200 Ma, together with Early Palaeoproterozoic and Archaean zircons. This pattern and those found in the metasedimentary rocks of the Pampean Belt, characterized by prominent bi-modal peaks at 560-625 Ma and 1025-1110 Ma, with minor peaks at 730-760 Ma and c. 1900 Ma, have been used to constraint a plate reconstruction for various time periods involving the Río de la Plata, Congo and Kalahari cratons. The conspicuous peaks at 760-780 Ma of the Punta Mogotes Formation are unique among the Neoproterozoic successions, and these dominantly concordant detrital zircons define a minimum age for the siliciclastic succession. There are no Brasiliano-Panafrican ages (560-680 Ma) in the Punta Mogotes Formation, although this is a widespread event in southwestern South America, suggesting that the sequence is older than 680 Ma as well as younger than or coeval with the 760-780 Ma detrital peaks. Major detrital peaks at 635-660 Ma are otherwise observed in all samples of the overlying Balcarce Formatiom. Orthogneisses with U-Pb SHRIMP ages of 762 ± 8 and 776 ± 12 Ma have been described from the Punta del Este Terrane in eastern Uruguay, and inferred to be a portion of the Coastal Terrane of the Kaoko Belt (Hartmann et al., 2002a; Oyhantçabal et al., 2009). In southwestern Africa, this period is characterized by the inception of a large alkaline igneous province associated with rifting that was eventually superseded by drifting and finally by inversion of the basins (Jacobs et al., 2008 and references therein). A comparison with the Neoproterozoic detrital patterns of southwestern Africa and southeastern South America suggests that the most suitable source for the Punta Mogotes Formation was the basement of the Kaoko belt, on the southwestern edge of the Congo craton. The Piedras de Afilar Formation, a thick siliciclastic and carbonate sequence located on the edge of the Río de la Plata craton in Uruguay, shows a similar detrital pattern, which however lacks the 760-790 Ma peak (Gaucher et al., 2008), indicating that both sequences were derived from similar sources in the Congo craton. These similarities in detrital patterns strongly suggest that the Río de la Plata craton was a conjugate rift margin of the Congo craton at the time of the 760-830 Neoproterozoic rifting. The NW-SE branch of the aulacogenic triple point located at the western end of the Damara orogen (Goscombe et al., 2005 and references therein), is here considered as initiating separation of the Río de la Plata and the Congo cratons, resulting in development of the northern Adamastor ocean at the time of Rodinia break-up. It is also considered that discrete continental terranes might have rifted away from the Congo craton. The Archaean to Mesoproterozoic Nico Pérez terrane in Uruguay (Bossi and Cingolani, 2009) may have been produced during this episode. 47 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA A second NE-SW branch of the triple point runs along the western edge of the Kalahari craton and faced an open ocean to the west. Discrete continental “African” terranes may have also rifted away during the opening of the southern Adamastor ocean. The continental terranes affected by rifting at the latitude of the Gariep Belt were mostly composed of the Mesoproterozoic complexes of the NatalNamaqua orogen (c. 1000-1100 Ma), and secondly by the 1700-2000 Ma Eburnean age basement, such as the Richtersveld terrane (Frimmel et al., 2001). Maximum expansion of the northern Adamastor and Khomas oceans took place at c. 700 Ma, while ocean opening continued in the southern Adamastor ocean. East-directed subduction started in the northern Adamastor ocean (Gray et al., 2006) at c. 680 Ma, with the possible formation of intra-oceanic arcs. Closing of the northern branch took place at c. 640 Ma, involving transpression, e.g. the Kaoko Belt (Goscombe et al., 2005; Gray et al., 2006) and Punta del Este terrane (Oyhantçabal et al., 2009). West-directed subduction started in the southern Adamastor ocean with development of 620-580 “Brasiliano” magmatic arcs preserved in the “African” terranes , now located to the west and southwest of the Río de la Plata craton (present coordinates). Southward displacement of the Río de la Plata craton with the attached Nico Pérez terrane led to the highly oblique collision against the southwestern Congo craton, developing sinistral transpression in the Kaoko and Dom Feliciano belts in the 640-600 Ma time interval. Protracted oblique subduction led to closure of the Adamastor ocean at ca. 545 Ma involving collision between the Río de la Plata and the Kalahari cratons (Frimmel and Frank, 1998). On the west and southwest, the Río de la Plata craton was involved at ca. 530-520 Ma into right-lateral collision with a large continental terrane, developing the transpressional Pampean Belt. REFERENCES • Basei, M.A.S., Frimmel, H.E., Nutman, A.P., Preciozzi, F., Jacob, J., 2005. A connection between the Neoproterozoic Dom Feliciano (Brazil/Uruguay) and Gariep (Namibia/South Africa) orogenic belts –evidence from a reconnaissance provenance study. Precambrian Research 139, 195-221. • Bossi, J., Cingolani, C., 2009. Extension and general evolution of the Río de la Plata craton. In: Gaucher, G., Sial, A.N., Halverson, G.P., Frimmel, H.E. (eds.) Neoproterozoic-Cambrian Tectonics, Global Change and Evolution: A Focus on Southwesten Gondwana. Developments in Precambrian Geology, 16, Elsevier, pp.73-85. • Basei, M.A.S., Frimmel, H.E., Nutman, A.P., Preciozzi F., 2008. West Gondwana amalgamation based on detrital zircon ages from Neoproterozoic Ribeira and Dom Feliciano belts of South America and comparison with coeval sequences from SW Africa. In: Pankhurst, R.J., Trouw, R.A., Brito Neves, B.B. and de Wit, M.J. (eds.) West Gondwana: Pre-Cenozoic Correlations Across the South Atlantic Region. Geological Society, London, Special Publications, 294, 239-256. • Frimmel, H.E., Frank, W., 1998. Neoproterozoic tectono-thermal evolution of the Gariep Belt and its basement, Namibia/South Africa. Precambrian Research 90, 1-28. • Frimmel, H.E., Zartman, R.E., Späth, A., 2001. The Richstersveld Igneous Complex, South Africa: U-Pb zircon and geochemical evidence for the beginning of the Neoproterozoic continental break-up. The Journal of Geology 109, 493-508. • Gaucher, C., Finney, S.C., Poiré, D.G., Valencia, V.A., Grove, M., Blanco, G. Paamoukaghlian, K., Gómez Peral, L., 2008. Detrial zircon ages of Neoproterozoic sedimentary successions in Uruguay and Argentina: insightsinto the geological evolution of the Río de la Plata Craton. Precambrian Research 167, 150-170. • Goscombe, B., Gray, D., Armstrong, R., Foster, D.A., Vogl, J., 2005. Event geochronology of the Pan-African Kaoko Belt, Namibia. Precambrian Research 140, 1-41. • Gray, D.R., Foster, D.A., Goscombe, B., Passchier, C.W., Trouw, R.A.J., 2006. 40Ar/39Ar thermochronology of the Pan-African Damara orogen, Namibia, with implications for tectonothermal and geodynamic evolution. Precambrian Research 150, 49-72. • Hartmann, L.A., Santos, J.O.S., Bossi, J., Campal, N., Schipilov, A., McNaughton, N.J., 2002a. Zircon and titanite U–Pb SHRIMP geochronology of Neoproterozoic felsic magmatism on the eastern border of the Río de la Plata craton, Uruguay. Journal of South American Earth Sciences 15, 229-236. • Hartmann, L.A., Santos, J.O.S., Cingolani, C.A., McNaughton, N.J., 2002b. Two Palaeoproterozoic orogenies in the evolution of the Tandilia Belt, Buenos Aires, as evidenced by zircon U–Pb SHRIMP geochronology. International Geology Review 44, 528-543. • Jacobs, J., Pisarevsky, S., Thomas, R.J., Becker, T., 2008. The Kalahari Craton during the assembly and dispersal of Rodinia. Precambrian Research 160, 142-158. • Marchese, H.G., Di Paola, E.C., 1975. Reinterpretación estratigráfica de la Perforación Punta Mogotes Nº 1, Provincia de Buenos Aires, República Argentina. Revista de la Asociación Geológica Argentina 30, 44-52. • Oyhantçabal, P., Siegesmund, S., Wemmer, K., Presnyacov, S., Layer, P., 2009. Geochronological constraintson the evolution of the southern Dom Feliciano Belt (Uruguay). Journal of the Geological Society, London, 166, 1075-1084. • Pankhurst, R.J., Rapela, C.W., Fanning, C.M., Márquez, M., 2006. Gondwanide continental collision and the origin of Patagonia. Earth Science Reviews 76, 235-257. • Rapela, C.W. , Pankhurst, R.J., Casquet, C., Fanning, C.M., Baldo, E.G., González-Casado, J.M., Galindo, C., Dahlquist, J., 2007. The Río de la Plata craton and the assembly of SW Gondwana. Earth Science Reviews 83, 49-82. • Schwartz, J.J., Gromet, L.P., 2004. Provenance of Late Proterozoic-early Cambrian basin, Sierras de Córdoba, Argentina. Precambrian Research 129, 1-21. 48 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA THE PIEDRA ALTA TERRANE: A PALEOPROTEROZOIC JUVENILE MAGMATIC ARC, RIO DE LA PLATA CRATON, URUGUAY 1-21 Sánchez Bettucci, L.1*, Peel, E.1, Basei, M.A.S.2 (1) Instituto de Ciencias Geológicas, Facultad de Ciencias-UdelaR, Montevideo, Uruguay (2) Instituto de Geociencias, Universidade de São Paulo * Presenting author’s e-mail: [email protected] The Río de la Plata craton best exposures occur in the southwest of Uruguay with a north extension into the Rivera region and Taquarembó block in southern Brazil and in the neighbourhood of the Tandil hills in Argentina to the south. The Río de la Plata craton is divided in Uruguay into two tectonostratigraphic terranes: Piedra Alta (PATT) and Nico Pérez (NPTT). The PATT is constituted by juvenile arc-related granitoids and by volcano-sedimentary belts with E-W structural trend. These sequences were metamorphosed under low to medium grade conditions. The PATT granitoids represent an excellent example of the roots of a Palaeoproterozoic magmatic arc, which was cool and stable since 1.7 Ga without recording the Neoproterozoic orogeny as it is shown by its Paleoproterozoic K-Ar ages. It is considered as the best preserved Palaeoproterozoic block of the Río de La Plata craton. The late to post-orogenic magmatism (Albornoz Complex) with calcalkaline, peraluminous and alkaline granites and gabbros presents ages ca. 2100 Ma. Available data allows to define two generations of granites, the first event at ca. 2053-2086 Ma and the second one related to mafic intrusions at ca. 2016-2033 Ma, both intruding the volcano-sedimentary belts. Anorogenic granites –like the A-type Soca Granite- are emplaced into graphite mica-schist, quartzites, gneisses, amphibolites and deformed granitoids. This subalkaline, metaluminous or slightly peraluminous Atype granite was emplaced after the Paleoproterozoic orogenic climax presenting xenoliths of metamorphic rocks (graphite-schist). This body has a U-Pb isotopic age of 2078 ± 8 Ma. The available geologic and isotopic data do not support the hypothesis that suggest another terrane named Tandilia for the outcrops of the PATT located at the south of the Santa Lucía rift. FURTHER EVIDENCE FOR MULTIPLE REVERSALS IN THE NEOPROTEROZOIC ARARAS CAP CARBONATE (BRAZIL) 1-22 Sansjofre, P. 1, Trindade, R.I.F.1, Ader, M.2, Nogueira, A.C.R.3, Soares, J.L.3 (1) Departamento de Geofisica, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, 05508-900 São Paulo, Brazil (2) Laboratoire de Géochimie des Isotopes Stables, IPGP, Université Paris-Diderot, UMR 7154, 4 place Jussieu, 75252 Paris Cedex 05, France (3) Faculdade de Geologia, Instituto de Geociências, Universidade Federal do Pará,CP 1611, 66.075-900, Belém, Brazil The extent and duration of Neoproterozoic glaciations is still a matter of contention. Paleomagnetic inclination data on glacial deposits and post-glacial cap carbonates are one of the pillars of the Snowball Earth hypothesis that postulates an ice-covered Earth at the beginning of the Ediacaran period. Paleomagnetism has also contributed in constraining the time-scale of glaciations itself and the deglaciation process. Multiple reversals have been reported for the glacial deposits (Elatina Formation, in Australia) and the cap carbonates (Nuccaleena Formation, Australia, Mirbat Formation, Oman, and Araras Formation, Brazil) suggesting that the deposition during and after glacial events spent hundreds of thousands of years at least. These estimates are useful constraints on paleoclimatic and isotopic models for these extreme climatic scenarios. Here we have revisited one of these 49 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA successions, the Araras Group in western Brazil (Mato Grosso State) in order to study two correlative sections of cap carbonates located along a platform transect from shallow water (Tangara section) to moderately deep water platform (Carmelo section). Sampling in Tangara and Carmelo quarries was stratigraphically controlled and tightly spaced in order to gather the directional variations of the magnetic field through time. Paleomagnetic results obtained in these sections were combined with those of Trindade et al. (Terra Nova, vol. 15, p. 441-446, 2003) and Font et al. (Journal of Geophysical Research, doi.10.1029/2005JB004106, 2006). Magnetic reversals observed in Terconi section were reproduced in the new sections, and a clear magnetostratigraphic correlation could be drawn between the two proximal sections (Terconi and Tangara). Samples in the top of Carmelo section show the same remagnetized magnetic component observed previously at the top of Terconi. These new results confirm the multiple reversals reported previously for the Araras cap dolostones, and indicate that transgression and sedimentation after Marinoan ice-ages was long-standing at odds with the snowball Earth model. PARAGUAY BELT FOLDING AND OROCLINAL BENDING DURING THE FINAL ASSEMBLY OF WESTERN GONDWANA 1-23 Trindade, R.I.1, Tohver, E.4, Nogueira, A.C.3, Riccomini, C.2 (1) Geofisica, Universidade de Sao Paulo, Sao Paulo, Brazil. (2) Geologia Sedimentar e Ambiental, Universidade de Sao Paulo, Sao Paulo, Brazil. (3) Geociencias, Universidade Federal do Para, Belem, Brazil. (4) Western Australia University, Perth, WA, Australia. A paleomagnetic investigation was undertaken along the Paraguai belt (Western Brazil), which marks the Neoproterozoic limit of the SE Amazon craton. This belt displays ca. 90 degrees of curvature along its ca. 1200 km extent, where the well-preserved sedimentary cover found over the craton is tightly folded but not metamorphosed. We have sampled the Araras Group, that consists of dolostones and limestones of Ediacaran age and the Alto Paraguay Group, which comprises siliciclastic deposits, starting with fluvial sandstones overlaid by turbidites and lake sediments. Sedimentological evidence (presented in a companion talk) is compatible with the inversion of the Araras passive margin after the final collision between the Amazonian Craton and the Central Gondwana. Paleomagnetic data from the Neoproterozoic Araras Formation shows that it retains a magnetization that is secondary in nature, as indicated by a negative fold test reported by Trindade et al. (Terra Nova, 2003). However, the declination of this secondary magnetization varies along strike, suggesting that the curvature of the belt was generated subsequent to an initial phase of folding and thrusting (Tohver et al., 2010), with vertical axis rotation being responsible for the E-W trend branch of the Paraguay belt, which served as a transform zone during the late Cambrian collision between the West Gondwanan elements Amazonia-Rio Apa-West Africa and the Central Gondwanan cratons: Congo-São Francisco, Rio de Plata, Kalahari. The same rotation is also observed on the siliciclastic deposits that immediately overlie the Araras carbonates but is not recorded by the upper pelites at the top of the Alto Paraguay. These data enable to continuously track the deformation along the belt during the final episodes of suturing of West Gondwana. 50 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA EVIDENCE FOR MIDDLE-LATE ORDOVICIAN SUBDUCTION AND A LOWER PLATE SETTING OF THE CUYANIA TERRANE DURING ITS ACCRETION TO THE PROTO-ANDEAN MARGIN OF GONDWANA 1-24 Van Staal, C.1*, Vujovich, G.2, Currie, K.3, Naipauer, M.2 (1) Geological Survey of Canada. 625 Robson Street, Vancouver, V6B 5J3 BC, Canada (2) CONICET (National Research Council of Argentina). Laboratorio de Téctonica Andina, Universidad de Buenos Aires, Pabellón II, Nunez. Buenos Aires 1428 (3) Geological Survey of Canada. 601 Booth Street, Ottawa, K1A 0E8 ON, Canada * Presenting author’s e-mail: e-mail: [email protected] Lithostratigraphic and structural analysis of the Caucete Group and the immediately structurally overlying Pie de Palo Complex in the Sierra de Pie de Palo, Argentina, indicate a basement-cover relationship, which we suggest was established during late Early Cambrian (~515 Ma) final rifting of Cuyania from Laurentia, The main deformation (Dm) and associated metamorphism indicate conditions typical of the blueschist to high-pressure amphibolite facies conditions, which leaves little doubt that a subduction zone existed between Cuyania and the proto-Andean margin of Gondwana. This provides strong support for the Laurentia-derived microntinent model of earlier workers. The main structures involve two phases (F1 and F2) of fold nappe formation and associated thrusting; the latter forming by shearing-out of the lower limbs of the inclined to recumbent folds. The style of deformation indicates that strain localization decreased during peak-T metamorphism and imposes a penetrative non-coaxial flow on the rocks involved in the A-subduction of the Cuyania’s leading edge beneath the proto-Andean margin. We relate this to widening of the subduction channel after entrance of Cuyania into the trench (start of A-subduction and collision) associated with thermal weakening becoming more prevalent than fabric-related softening. LOW-PRESSURE ANATEXIS IN FAMATINIAN FORELAND OF ARGENTINA, SOUTH-WESTERN MARGIN OF GONDWANA: SOURCE HEAT PROBLEM 1-25 Verdecchia, S.O.*, Baldo, E.G. CICTERRA – Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). Av. Veléz Sarsfield 1611. Facultad de Ciencias Exactas, Físicas y Naturales. Universidad Nacional de Córdoba. Argentina * Presenting author’s e-mail: [email protected] Within the foreland region of the Famatinian belt in central-western Argentina low- to mediumpressure metamorphic complexes crop out, one of them being the Ordovician metasedimentary rocks from La Cébila. This complex was affected by a main tectono-thermal event (M1) characterized by low pressure (2-2.5 kbar) and high thermal gradient (~80º C/km) conditions. In this complex, a welldefined nearly isobaric metamorphic zonation was developed. It progrades towards the intrusive contact with syn- to post-tectonic peraluminous granitic bodies (S-type; 760-780º C crystallization temperature). The metamorphic grade increases from white mica-chlorite (~300-400º C; Kübler index values from 0.23 to 0.17 ¢º2ı) to cordierite-K-feldspar (714-740º C; Ti-in-biotite thermometry) zones, through the andalusite and sillimanite stability fields, reaching anatexis conditions with the development of a wide migmatitic belt (1-3 km) adjacent to the granites. In the M1 event (~460 Ma), a well-defined secondary foliation was developed with syn-kynematic blastesis of andalusite and Kfeldspar porphyroblasts, associated with a strongly compressive episode (D1). Granitic magma by itself, because of its relatively low crystallization temperature, would not be a suitable heat source to explain the widespread melting in the country rocks, thus ruling out contact metamorphism as the single cause of metamorphism. 51 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES The paleontological record (brachiopods in quartzites metamorphosed under sillimanite zone conditions) suggests a foreland marine basin in the Early to Middle Ordovician (480-460 Ma). High heat flow during the extensional regime would explain the low-pressure metamorphism in shallow crustal levels (<9 km), but it is difficult to produce anatexis without associated mafic intrusion. The integration of sedimentary, metamorphic and igneous information suggests that heat source of the main metamorphic event could be related to a combination of mechanisms: high heat flow crustal regimen related to the extensional period and immediately a plutonic intrusion (S-type granite) related to the a compressional regime. THE MESO-NEOPROTEROZOIC SUBDUCTION-ACCRETION EVENTS AND MAGMATIC EVOLUTION ALONG THE WESTERN MARGIN OF THE SIBERIAN CRATON: TO THE PROBLEM OF RODINIA BREAK-UP 1-26 Vernikovsky, V.A.*, Vernikovskaya A.E., Institute of Petroleum Geology and Geophysics, Koptyuga ave., 3, Novosibirsk, Russia, 630090. * Presenting author’s e-mail: [email protected] The Meso-Neoproterozoic subduction-accretion events and magmatic evolution along the western margin of the Siberian Craton has attracted attention in the context of debatable problems concerning the formation and break-up of Rodinia supercontinent. Available on geological, geochronological and paleomagnetic data established that in the Neoproterozoic a transformation of the Siberian Craton western margin from a passive to an active one was taking place (Vernikovsky et al., 2003 Tectonophysics, 375, 147–168; 2009 Russian Geology and Geophysics, 50, 4, 372-387; Metelkin et al., 2007 Russian Geology and Geophysics, 48, 1, 3245.; Pisarevsky et al., 2008 Precambrian Research, 160, 66-76). The Neoproterozoic subductionaccretion processes along the Siberian Craton western margin were not synchronous; they take in a wide interval of time – from 960 Ma to 630 Ma. An island arc system started to form at the north-western margin of the Siberian Craton approximately 960 Ma (Vernikovsky et al., 2009, op. cit.). Its fragments were included in the Central Taimyr accretionary belt. Paleomagnetic poles for the 960 Ma acid volcanic rocks in the Three Sisters Lake island arc from this belt have been determined. These poles are very close to the poles of same age for Siberia (Pavlov et al., 2002 Geotectonics, 36, 278–292.). These results significantly expand the previously acquired evidences for the forming of island arcs with their subsequent accretion and obduction onto the Taimyr margin of Siberia in the interval of 750-660 Ma (Vernikovsky and Vernikovskaya, 2001 Precambrian Research, 110, 4, 127-141). The succession of the forming of Siberia’s western margin, represented by the Yenisey Ridge accretional orogen, was determined. We assume that the genesis of this structure is a result of three events: a) syn-collisional events (probably outside the Siberian Craton), which resulted in the forming of the Teya granites with the age 880-860 Ma in the Central Angara terrane; b) the collision between the Central Angara terrane and the Siberian craton and the formation of the syn- and post-collisional Ayakhta and Glushikha granites with the age 760-720 Ma; c) the formation of island arcs and ophiolites along the margin of the Siberian Craton, their accretion and obduction onto the continent in the interval of 700-630 Ma (Vernikovsky and Vernikovskaya, 2006 Russian Geology and Geophysics, 47, 1, 32-50). The last event is of special interest because at the same time in the Tatarka-Ishimba suture zone of the Yenisey Ridge, which is sub parallel to the continental margin, the forming of intrusive and volcanic rocks of various composition and heightened alkalinity was taking place, including alkaline syenites as well as carbonatites and A-type granites. They formed synchronously with the rocks of the island arc complex and their accretion and obduction onto the continental margin of Siberia in the interval of 700-630 Ma (Vernikovsky et al., 2008 Doklady Earth Sciences, 419, 2, 226-230). It is 52 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES quite probable that their formation in the back-arc suprasubduction zone was taking place at the same time that the oceanic plate was subducting below the continent from the western margin of the Siberian Craton and reached the asthenospheric layer. The obtained data and the developed models uncover the geodynamic evolution of the formation of accretional orogens in the western margin of the Siberian Craton in the Neoproterozoic, and allow us to discuss the questions of the mutual location of the Siberian and North-American Cratons within the Rodinia supercontinent and Rodinia break-up. THE SUTURE ZONE BETWEEN CUYANIA AND CHILENIA TERRANES: A SUBDUCTION CHANNEL AND A-TYPE OROGEN? 1-27 Vujovich, G.I.1*, Boedo, F.L.1, Willner, A.P.2 (1) Laboratorio de Tectónica Andina, FCEN, Universidad de Buenos Aires / CONICET, Pabellón II, C. Universitaria, Buenos Aires, Argentina. (2) Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, D-70174 Stuttgart, Germany * Presenting author’s e-mail: [email protected] A discontinuous belt of Paleozoic mafic-ultramafic rocks outcrops along the Western Precordillera Argentina and Cordillera Frontal and constitute part of the Central Andean basement between La Rioja and Mendoza provinces, Argentina (Fig. 1). This belt has been interpreted from several authors as the limit between the Cuyania and Chilenia terranes accreted to the proto-Gondwana margin during Paleozoic times. Western Precordillera area: the most important outcrops of the mafic-ultramafic rocks are represented by a massive sequence of basaltic pillow lavas and columnar-jointed flows, sills of mafic and ultramafic composition (Jachal – Rodeo and Calingasta – 114 km2) intruding a metasedimentary sequence mainly composed by metagraywackes and shales. At Río Jachal section the mafic and ultramafic belt and metasediments have been steeply folded with a westward vergence (Ramos et al., 1984; von Gosen, 1997). A thick sequence of basaltic pillow lavas are interbedded with shales and others slope-deposits outcrops along the San Juan River (Km 114) and in Calingasta area. Upper Ordovician graptolites indicate a Late Ordovician age for the sequence. Serpentinized peridotites, ultramafic cumulates, coarse grained gabbros to microgabbros, diabase, and layered gabbros (garnet granulites) outcrops at Cordón del Peñasco and Cortaderas – Bonilla areas and constitute the dominant rock types. They are in tectonic contact with mafic submarine flows (hyaloclastites), tuffs and pillow lavas interlayered with metasandstones, metasilstones and scarce chert (Davis et al., 1999; Boedo, 2010). Based on field observations, petrological, geochemical and isotopic data the basaltic pillow lavas, lava flows and mafic-ultramafic sills are interpreted as evolved tholeiites (E-MORB) related to an oceanic ridge environment (see Ramos et al., 2000 and references therein; Kay et al., 2005). On geochemical grounds the garnet granulites (layered gabbros) display a weak arc magmatic signal (Davis et al. 1999). Upper Proterozoic to Ordovician ages (U-Pb zircon) for diabases and mafic sills, and minimum Silurian ages for the layered gabbros were reported by Davis et al. (2000). Low-grade regional metamorphism has partially replaced the igneous and high temperature assemblages in the mafic and ultramafic rocks and layered gabbros. Metamorphic P-T conditions for the metabasites at Rodeo and Calingasta areas are constrained to a low-temperature/low pressure setting at ca. 250-350°C and 2-3 kbar (Robinson et al., 2005). Petrologic studies of the ultramafic cumulates and layered gabbros indicate a recristallization at granulite facies (T= 850 – 1000°C) and P= 9 – 11 kbars (Davis et al., 1999). Later retrograde metamorphism occurred under greenschist facies conditions. At Cortaderas area in the low grade units Davis et al. (1999) yielded Ar-Ar plateau ages of 384±0.5 53 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES Fig. 1 - Mafic-ultramafic belt of Western Precordillera (based on Vujovich and Ramos, 1999). Ma and 377±0.5 Ma on undefined white mica concentrates. This is interpreted as the timing of low grade peak metamorphism. Cordillera Frontal: A medium grade unit of micaschist with intercalated amphibolite, marble and serpentinite outcrops at Río de Las Tunas area (Bjerg et al., 1990; López and Gregori 2004; López et al., 2009) and it is known as the Guarguaráz Complex. Metasedimentary sequences are interpreted as metagraywackes derived from a mature cratonic continental basement. The mafic-ultramafic rocks are represented by lenses of partly garnet-bearing amphibolite with an N- or E-MORB geochemical signature, serpentinite and talc-bearing wall-rocks. Marbles and calc-silicate rocks deposited in a platform environment are part of the sedimentary sequence. Metapelitic rocks achieved the peak of the medium grade metamorphism at 13.5 kbar, 500°C followed by a decompression to mid-crustal conditions at 8 kbar, 565°C (Massonne and Calderón, 2008). The maximum depositional age of 563 Ma for the metagraywackes of Guarguaráz complex has been estimated by Willner et al. (2008), which is consistent with the Vendian-Cambrian age estimated by López et al. (2009) based on microfossils, and a whole-rock Sm-Nd isochron age of 655±76 Ma for the intercalated metabasite interpreted as the probable crystallization age of the protolith (López et al., 2009). The age of metamorphism is weakly defined by an Rb/Sr whole rock isochron of the Guarguaráz micaschist at 375±34 Ma (Basei et al., 1998) and a K/Ar whole rock age of 370±18 Ma by Caminos et al. (1979). Willner et al. (2010) yielded a 390.0±2.2 Ma age (Lu-Hf isochrons on mineral separates from metapelites and metabasites) for the peak of high pressure metamorphism, and Ar-Ar plateau age of white mica at 353±1 Ma for the late decompression event. Most of these mafic-ultramafic rocks formed in different regions are related to a marine setting and represent the upper part of an ophiolite sequence. There is not conclusive evidence to define the 54 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - Ordovician to Late Devonian evolution of Cuyania and Chilenia terranes. tectonic setting of the layered gabbros. They could be part of a magmatic arc or represent the lower section of the ophiolite assemblage. Based on the PT conditions, the tectonic contacts between low grade metamorphic units and high grade rocks retrograded to greenschist facies, and the dominant western vergence, we proposed the existence of an east-dipping subduction channel. This channel was responsible of the up-ward rock flux and exhumation during the A-type collisional orogeny between Chilenia and Cuyania terranes developed during the Late Devonian times (Fig. 2). REFERENCES • Basei, M., Ramos, V.A., Vujovich, G.I., Poma, S., 1998. El basamento metamorfico de la Cordillera Frontal de Mendoza: nuevos datos geocronologicos e isotopicos. Actas X Congreso Latinoamericano de Geología y VI Congreso Nacional de Geología Económica II: 412-417. • Bjerg, E.A., Gregori, D.A., Losada Calderón, A,, Labadía, C.H., 1990. Las metamorfitas del faldeo oriental de la Cuchilla de Guarguaráz, Cordillera Frontal, Provincia de Mendoza: Revista de la Asociación Geológica Argentina, 45: 234-245. • Boedo, F.L., 2010. Geología del área del Cordón del Peñasco, Precordillera occidental, provincia de Mendoza. Trabajo Final Licenciatura, FCEN, Univ. Buenos Aires, 143 pp. • Caminos, R., Cordani, U., Linares, E., 1979. Geología y geocronología de las rocas metamórficas y eruptivas de la Precordillera y Cordillera Frontal de Mendoza. Actas 2. Congreso Geológico Chileno Santiago 1: 43-61. • Davis, J., Roeske, S., McClelland, W., Snee, L., 1999. Closing the ocean between the Precordillera terrane and Chilenia: Early Devonian ophiolite emplacement and deformation in the southwest Precordillera. In: Ramos, V.A., Keppie, J. (Eds.) Laurentia and Gondwana Connections before Pangea. Geological Society of America, Special Papers 336: 115-138. • Davis, J.S., Roeske, S.M., McClelland, W.C., Kay, S.M., 2000. Mafic and ultramafic crustal fragments of the southwestern Precordillera terrane and their bearing on tectonic models of the early Paleozoic in western Argentina. Geology 28: 171-174. • Gerbi, C., Roeske, S.M., Davis, J.S., 2002. Geology and structural history of the southwest Precordillera margin, northern Mendoza Province, Argentina. Journal of South American Earth Sciences 14: 821-835. • Kay, S.M., Boucakis, K.A., Porch, K., Davis, J.S., Roeske, S.M., Ramos, V.A., 2005. E-MORB-like mafic magmatic rocks on the western border of the Cuyania terrane Argentina. Gondwana 12, Abstracts, p. 216. Academia Nacional de Ciencias, Córdoba. • López, V.L., Gregori D.A., 2004. Provenance and evolution of the Guarguaráz Complex, Cordillera Frontal, Argentina. Gondwana Research 7: 1197-1208. • López, V.L., Escayola, M., Azarevich, M.B., Pimentel, M.M., Tassinari, C., 2009. The Guarguaráz Complex and the Neoproterozoic- 55 GEOSUR2010 • • • • • • • • • 22-23 NOVEMBER 2010 – MAR DEL PLATA Cambrian evolution of southwestern Gondwana: Geochemical signatures and geochronological constraints. Journal of South American Earth Sciences, 28: 333-344. Massonne, H.J., Calderón, M., 2008. P-T evolution of metapelites from the Guarguaráz Complex, Argentina: evidence for Devonian crustal thickening close to the western Gondwana margin. Revista Geológica de Chile, 35: 215-231. Ramos, V.A,, Jordan, T., Allmendinger, R., Kay, S., Cortés, J., Palma, M., 1984. Chilenia: un terreno alóctono en la evolución paleozoica de los Andes Centrales. Actas IX Congreso Geológico Argentino 2: 84-106. Ramos, V.A., Escayola, M., Mutti,D.I., Vujovich, G.I., 2000. Proterozoic-early Paleozoic ophiolites of the Andean basement of southern South America. Geologival Society of America. Special Paper 349: 331-349. Robinson, D., Bevins, R.E., Rubinstein, N., 2005. Subgreenschist facies metamorphism of metabasites from the Precordillera terrane of western Argentina: constraints on la later stages of accretion onto Gondwana. European Journal of Mineralogy, 17: 441452. Von Gosen, W., 1997. Early Paleozoic and Andean structural evolution in the Río Jáchal section of the Argentine Precordillera. Journal of South American Earth Sciences, 10: 361-388. Vujovich, G.I., Ramos, V.A., 1999. Mapa geotectónico de la República Argentina (1: 2.500.000), Subsecretaría de Minería de la Nación, Buenos Aires, Servicio Geológico Minero Argentino. Willner, A.P., Gerdes, A., Massonne, H.J., 2008. History of crustal growth and recycling at the Pacific convergent margin of South America at latitudes 29°-36°S revealed by a U-Pb and Lu-Hf isotope study of detrital zircon from late Paleozoic accretionary systems. Chemical Geology, 253: 114-129. Willner, A.P., Massonne, H.J., Gerdes, A., Hervé, F., Sudo, M., Thomson, S., 2009. The contrasting evolution of collisional and coastal accretionary systems between the latitudes 30°S and 35°S: evidence for the existence of a Chilenia microplate. Abstracts XII Congreso Geológico Chileno Santiago S9-099: 223. Willner, A.P., Gerdes, A., Massonne, H., Schmidt, A, Sudo, M., Thomson, S., Vujovich, G.I. 2010. Pressure-temperature-time evolution of a collisional belt (Guarguaráz Complex, W-Argentina): Evidence for the accretion of the Chilenia microplate. AGU Joint Assembly “Meeting of the Americas”, Foz do Iguazu, Session V2. 56 Session 2 VOLCANISM AND PETROLOGY GEOSUR2010 EXOTIC EXHALATIONS FROM ACTIVE S-ANDES VOLCANOES: DOMUYO, TROMEN AND COPAHUE VOLCANOES, ARGENTINA 22-23 NOVEMBER 2010 – MAR DEL PLATA 2-01 Bermudez, A.1*, Delpino, D.2, Varekamp, J. C.3, Kading, T.3 (1) CONICET, National University of Comahue, Neuquén, Argentina (2) REPSOL-YPF, Dirección General de Exploración, Talero 360 – (8300) Neuquén, Argentina (3) Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT 06459, USA * Presenting author’s e-mail: [email protected] Introduction Active volcanoes carry some of the most extreme fluids to the surface of the earth, either very acid and/or very concentrated brines. Highly acid fluids have been described from crater lakes and hot springs on active volcanoes, which are condensed volcanic gases, rich in three strong volcanic acids H2SO4, HCl, and HF (Type 1 fluids). Sulphur is the dominant volcanogenic element in these acid fluids with up to 7% sulphate. These fluids acquire their cation loading from water-rock interaction at elevated temperatures at depth. Another class of acid fluids results from the boiling of deep volcanic aquifers, where boiled-off H2S is oxidized to sulphuric acid in the surface environment. These fluids react with surrounding surface rocks, usually at temperatures not far from above the local boiling point. The first type of acid fluids are rich in both SO4 and Cl-, may have extremely low pH values (0 or below) and very high concentrations of the rock forming elements (RFE). The second type of acid fluids have more modest pH levels (1 and up), are poor in Cl and F, and have lower RFE concentrations (type 2 fluids). Hypersaline neutral brines are rare at the surface of the earth, and may form through immiscibility from early formed magmatic vapours creating Na-K-Cl brines together with aqueous, sulphur and CO2 rich volcanic vapours that commonly escape through fumarolic fields in craters. Monitoring and analyzing these various types of fluids provides insights into volcanic degassing (volcano monitoring) and water-rock interaction processes in the underlying geothermal reservoirs. Several hot springs and crater lakes of active volcanoes in Argentina have been investigated in this study: Copahue (37.45oS, 71.17oW); Tromen (37º S -70º W, 3979 m) and Domuyo (36o40’S, 70o40’W, 5400m) (Fig. 1). The Tromen Volcanic Field (Fig. 1) is formed from andesitic volcanic products, domes and basaltic cinder cones, and developed in several cycles of igneous activity, some with multiple eruptions. Two small, cool volcanic lakes, one red and one green, occur in the Tromen volcano summit region (Bermúdez et al, 2006) and contain acid fluids, with pH values ~1. The Domuyo volcanic complex (Fig. 1) consist of an Upper Miocene-Pliocene subvolcanic granodioritic dome surrounded by numerous Quaternary acid extrusive domes associated with emissions of pyroclastic flows, ash fallout and lava flows. Domuyo has an active hydrothermal field on its western flanks, with hot springs at about 90 oC. The reservoir temperature of a vapour-dominated system was estimated at >200 oC whereas the liquid-dominated fluids had temperatures of 160-200 oC. The hot spring area is strongly altered into silica with Smectite and Kaolinite and rare zeolites. Copahue volcano (Fig. 1) occurs at the rim of a 2 Ma mega caldera (19x15km) and during the Pleistocene a composite large cone was formed. Both Pliocene and Pleistocene edifices are dissected when a large ice sheet totally filled the caldera depression. Post - glacial Copahue active volcano summit area is formed by eight cinder cones aligned along a N60ºE fissure of 2.5 km length. Craters are partially covered by glaciers and inside the active cone, a small glacier lobe covers its western wall and supplies melt water to a circular (250 m diameter) crater lake. Copahue Volcano had minor phreato-magmatic eruptions in 1992-1995 and a magmatic eruption in 2000 (Delpino D. and Bermúdez, 2002 and Bermúdez et al., 2002). Chemical composition of the hydrothermal fluids The chemical compositions of water samples from Tromen, Domuyo and Copahue are listed in Table 1. Copahue fluids are characterized by extremely low pH values, and very high anion and cation concentrations. The Domuyo fluids on the other hand are pH-neutral with very high Cl and Na contents. The Tromen lake has a low pH, low Cl contents and modest cation concentrations. The active volcanohydrothermal system at depth in Copahue (pH~0, T~300 oC) scrubs all volcanic volatiles, and SO2 59 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA disproportionates into bi-sulphate and liquid elemental Sulphur (Varekamp et al., 2001). These acid fluids are injected into a crater lake after modest water rock interaction, and emerge as acid hot springs after more extensive water-rock interaction. The hot springs feed a glacial meltwater river (Upper Rio Agrio) that drains into the large glacial Lake Caviahue. We measured the water fluxes and river water compositions of the Upper Rio Agrio just before it enters Lake Caviahue from 1997 to 2009, which includes the 2000 eruptive period. This has provided a flux record of volcanic elements (F, Cl, S, B, As) and major RFE (Al, Fe, Mg, Ca, K, Na). The annual river flux measurements are complemented with analytical data from vertical water profiles through Lake Caviahue, which Fig. 1 - Localization of active S-Andes volcanoes: Domuyo, through its ~3.5 years water residence Tromen and Copahue. time has a ‘chemical memory’ of past element fluxes. The latter can be used to reconstruct mean element fluxes over the years based on changes in the element contents of the lake. The mean, time-weighted measured volcanogenic fluxes (tonnes/month) were 1510 S, 1180 Cl and 104 F, with peak values in 1999-2000 and very low yields in 2001-2002 after the eruption. The F-ClS concentrations in Lake Caviahue waters have dropped from their high values in 2000 as a result of the more modest element fluxes in the last few years (Varekamp, 2008). The RFE fluxes also peaked during the 2000 eruption, and went through a minimum in 2001-2002. Precipitation of Jarosite / Alunite in the hydrothermal reservoir since 2000 has strongly reduced the flux of K, Al and SO4 into the lake. The S flux rate can be translated into a volumetric magma degassing rate of ~ 3.5 108 m3/decade (using a high estimate of 200 ppm S released from the melt, based on glass inclusion and matrix glass analyses (Goss, 2003), whereas the mean RFE flux provides a rock dissolution rate of ~2 105 m3/decade (Varekamp et al., 2001). The rock dissolution processes together with the summit fractures with active cones probably led to periodic flank collapses, which, both with glacial processes, has given Copahue its rounded shape and modest elevation of 3000 m. Discussion The origin of the hyper-acid brines is fairly well agreed upon and best explained by the capture of magmatic waters, SO2, HCl, and HF in a meteoric cell of water above the degassing magma at ~ 1500 m depth below the summit. The resulting acid brine reacts with surrounding rock, creating high concentrations of the common rock-forming elements (Varekamp et al., 2009). Precipitation of secondary minerals like Alunite, Jarosite, Anhydrite and Silica fractionate the fluids over time, and dissolution of these mineral phases during later stages may enhance the element fluxes again. The Tromen lakes are most likely fed by high temperature steam with H2S, and the sulphuric acid that forms from the oxidation of H2S then dissolves the surrounding rocks. The neutral Cl brines at Domuyo are relatively uncommon volcanic fluids, and their origin is enigmatic. Similar fluids occur as fluid inclusions in minerals associated with porphyry copper deposits (Heinrich, 2007). The high density of these fluids prevents them from rising into the surface environments where they could mix with shallow waters. Intrusive igneous masses may retain the exsolved brines as small blebs of NaCl-rich fluids, which may be incorporated into circulating meteoric fluids. Stable isotope systematic of the Domuyo fluids suggests a mixing line between a possible saline magmatic end member with ~ 2 % NaCl and local meteoric water (Unpublished data).This Cl-rich fluid may also be a residual fluid from a boiling geothermal aquifer associated with the vapour dominated geothermal system at Domuyo. A third pos60 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 Table 1 - Chemical composition of hydrothermal fluids (ppm). Sample pH F Cl SO4 HCO3 Al Fe Mg Ca K Na Domuyo Agua Blanca 6 7.4 2.2 915 93 96 2 22.6 40 550 Los Tachos 90C 8.1 4.8 1834 180 155 1 52.3 69 1055 Los Tachos 1 8.0 4.8 1786 182 138 1 53.1 69 1051 Covunco 3A 7.9 1.5 512 215 95 6 84.1 36 320 Las Olletas 5 8.1 4.0 1515 154 106 1 40.5 57 931 Aguas Calientes 8.0 2.7 1011 101 78 1 25.5 39 601 3.0 4.1 4 3246 130 2.2 162 520 0.5 51 Hot spring 2006 1.2 601 6770 14,213 2022 1111 867 805 240 921 Crater Lake 2006 0.7 541 6967 15,766 1135 515 402 965 156 376 Tromen Laguna Rojiza Copahue sibility is that these saline fluids have absorbed salts from Jurassic evaporites in the subsoil of Domuyo, and then would be non-volcanic in origin. Clear differences exist in major element chemistry between the different fluid types: the Na/K ratios are fixed in the Domuyo hot springs (Na=20 K) whereas the Copahue fluids have highly variable Na/K values, with large K excesses relative to the neutral saline fluids. The variations in Na/K may stem from the precipitation and redissolution of Alunite/Jarosite together with the dissolution of volcanic glass inside the volcano. The hyper-acid fluids have high Mg concentrations from olivine and pyroxene dissolution, whereas the neutral saline fluids lack Mg and have much higher alkalinities (mainly HCO3-). The Ca concentrations in the acid fluids (several 100 ppm of Ca) are possibly buffered by CaSO4 precipitation. The acid fluids carry relative high concentrations of Al and Fe, elements that are almost insoluble in the neutral brines. The concentrations of toxic elements in the acid fluids are up to 14 ppm As, 3 ppm Pb, and 0.16 ppm Cd. Such element concentrations are all very low in the neutral and more dilute acid fluids, possibly the result of adsorption on precipitated hydrous Fe-oxides. The Copahue acid fluids are mixtures between acid volcanic brines (condensed volcanic gases) and local meteoric waters, modified by evaporation for the crater lake (Varekamp et al., 2004). The Tromen lake consist largely of meteoric water modified by evaporation at temperatures slightly above ambient. The Domuyo brines have isotopic compositions very close to local meteoric waters despite their high Cl contents, which suggest that they are mixtures of very saline source fluids and meteoric waters. Conclusions Volcanic fluids emitted at the surface in the southern Andes thus cover the full range from acid sulphate brines to neutral saline fluids, with the Tromen crater lake fluids as intermediate type-2 acid fluids. REFERENCES • Bermúdez A Delpino D., and López Escobar. L., 2002. Caracterización geoquímica de lavas y piroclastos holocenos del volcán Copahue, incluyendo los originados en la erupción del año 2000. Actas de XV Congreso Geológico Argentino. El Calafate, Tomo I, pp.: 377 – 382. • Bermúdez A., Delpino D. and Loscerbo C., 2006. Anomalía termal en la cima del Volcán Tromen (37ºS - 70ºO), Provincia del Neuquén, Argentina Geoacta (Revista de la Asociación Argentina de Geofísicos y Geodestas) Vol. 31:pp133 – 141. • Delpino D. and Bermúdez A 2002. La Erupción del Volcán Copahue del año 2000. Impacto social y al medio natural. Neuquen. Argentina, Actas de XV Congreso Geológico Argentino. El Calafate, Tomo III, pp: 365 - 370 Heinrich, C.H., 2007, Fluid-Fluid Interactions in Magmatic-Hydrothermal Ore Formation, Mineralogical Society of America, Reviews in Mineralogy & Geochemistry, 65, pp: 363-387 • Varekamp, J.C., Ouimette, A.P., Herman, S.W., Bermudez, A., and Delpino, D., 2001. Hydrothermal element fluxes from Copahue, Argentina: a “beehive” volcano in turmoil. Geology: 29, pp1059-1062. 61 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Varekamp, J.C., Ouimette, A. and Kreulen. R., 2004. The magmato-hydrothermal system of Copahue volcano, Argentina. Proceedings of the 11th Water Rock Interaction Symposium, (Eds Wanty, R.B. and Seal, R.R.) V.1, pp. 215-218, Balkema Publishers, Leiden. • Varekamp, J. C., 2008, The acidification of glacial Lake Caviahue, Province of Neuquén, Argentina. Special Issue Volcanic Lakes, J, Volcanology and Geothermal Research, 178, pp.184-196. • Varekamp, J.C., Herman, S., Ouimette, A., Flynn, K., Bermudez, A., and Delpino, D., 2009, Naturally acid waters from Copahue volcano, Argentina. Applied Geochemistry, Spec. Issue on ‘Geogenic acid fluids’, 24 pp. 208–220 CERRO NEGRO DEL GHÍO: MAGMATISM IN-BETWEEN SOUTHERN PATAGONIAN BATHOLITH AND LAGO BUENOS AIRES PLATEAU LAVAS 2-02 Castro,J.1*, Sánchez, A.1 , Hervé, F.1, De Saint Blaquat M.2, Polvé M.2 (1) Universidad de Chile,Plaza Ercilla #803, Santiago Centro (2) LMTG/Observatoire Midi-Pyrénées, Université de Toulouse, 14 av. Edouard-Belin, 31400 Toulouse, France * Presenting author’s e-mail: [email protected] Introduction In Patagonia, east of the South America-Nazca-Antarctica triple joint, there are several magmatic units, being the Patagonian batholith, and plateau lavas the most important of them. Nevertheless, in the region south of the Lago General Carrera/Buenos Aires, there are several satellite intrusive bodies, Fig. 1 - Regional geological map. After Espinoza 2007 and Lagabrielle et al., 2007. 62 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA with Pliocene to Miocene radiometric ages (Ramos, 2002). Several works relate the Pliocene magmatism to an astenospheric window generated by the subduction of the Chile Rise beneath the South American plate (e.g. Gorring et al., 1997). We present new petrographic and geochemical data of Cerro Negro del Ghío diorites, located between the Chile Chico and Lago Buenos Aires plateaus (10 km westward meseta del Lago Buenos Aires (MLBA), emplaced in volcanic rocks of Jurassic Ibañez Group (Fig. 1). It has a K/Ar determination in hornblende of 15.8 ± 0.6 Ma (Ramos, 2002). The outcrops of the Miocene Cerro Negro diorite are split in two areas, separated by a little valley. The larger outcrop is round and flat of 2 km in diameter and with variable thickness, with a maximum of ca. 100 m, it has a tabular sill shape. The other outcrop is in the top off a cliff just in face the sill and it has a lesser outcrop area and is highly affected by weathering. Fig. 2 - Geochemical diagrams. Showing in A: TAS classification (after Cox et al., 1979, subalkaline-alkaline divide after Irvine and Baragar (1971)) open symbols correspond to sill samples, filled symbol correspond to the cliff sample.; B: trace element spidergram (Sun and McDonough, 1989) and C: 87Sr/86Sr vs ÂNd diagram. In all diagrams Southern Patagonian Batholith (SPB) projection data is shown (from Hervé et al., 2007) for comparison. In A and B the analysed samples clearly differ from the SPB by their more alkaline nature. Petrographic features The igneous bodies consist of grey porphyritic rocks mainly of pyroxene diorite. Four samples were studied. The samples have porphyritic textures with a 40% of microphaneritic mass composed of plagioclase, biotite, piroxene, hornblende, magnetite, calcite and glass. The phenocrysts are mainly of euhedral zoned plagiocase with 2 mm mean size. Some of them have inclusions of fundamental mass and re-absorption textures with glass inclusions in the core of the crystal. Also clinopyroxene is a common mineral, they range in size being bigger northward up to 4 mm. They have a reaction border composed of magnetite 63 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA and biotite. Locally they have ophitic texture. Biotite occurs as crystals less than 0.5 mm. Cumulophyric texture is usual for biotite and plagioclase. Also green hornblende with size up to 5 mm are common. They are zoned and/or in cumulophyric textures. Most of them have a biotite rim as a reaction border. The hornblende together with the plagioclase present in some areas glomeroporphyric textures. Chemical features The same samples described before, were used for geochemical analyses. They are chemically classified as syenodiorites (classification of Cox et al., 1979). They range in SiO2 from 58 to 60 wt.% they are in the limit of alkaline/subalkaline fields (Fig. 2A) Trace element diagrams (Fig. 2B) in general terms show similar features with the SPB including negative anomaly in Nb and Ti, although not so strong. Furthermore Cerro Negro is richer in LILE and LREE than SPB; the Ba/La ratio is >10 which is a characteristic signature of intraplate magmatism. Isotopic ratios show positive ÂNd (+2-+3), near the mantle array, similar to MLBA transitional basalts (Gorring et al., 1997) and also similar to the projection area of the Neogene SPB samples (Fig. 2C). Discussion As seen before trace elements pattern of Cerro Negro syenodiorites shows some differences with both: those of MLBA main plateau lavas, which have OIB signature. (Guivel et al., 2006) and with BSP which are arc derived (Hervé et al., 2007). Nevertheless they share chemical properties with both magmatic units (e.g. Nb and Ti anomaly, La/Nb >1 TiO2 wt% <2 and positive ÂNd –SPB shows a Jurassic to Neogene trend of <0 to >0 ÂNd). This allows us to interpret the Cerro Negro magmatism as originated in a transition magmatic event, so much geochemical as temporarily, between the two main igneous units. Acknowledgements We thank “Becas de Doctorado” Conicyt (A.Sánchez), Anillo Project ARTG-04, ECOS-CONICYT C05U02 grant and the Ea. Sol de Mayo owner. Bruno Scalabrino field assistance is also appreciated. REFERENCES • Espinoza, F.; 2007: Evolución Magmática de la Región de Trasarco de Patagonia Central (47º S) durante el Mio-Plioceno. Ph.D Thesis, Universidad de Chile, 195 p. • Gorring. M., Kay, S., Zeitler, P., Ramos, V., Rubiolo, D., Fernandez, M., Panza, J.; 1997: Neogene Patagonian plateau lavas: Continental magmas associated with ridge collision at the Chile triple junction. Tectonics, 16, 1-17. • Guivel, C., Morata, D., Pelleter, E., Espinoza, F., Maury, R., Lagabrielle, Y., Polve, M., Bellon, H., Cotten, J., Benoit, M., Suárez, M. and de la Cruz, R.; 2006: Miocene to Late Quaternary Patagonian basalts (46-47º S): Geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction. Journal of Volcanology and Geothermal Research 149, 346-370 • Hervé, F., Pankhurst, R.J., Fanning, C.M., Calderón, M. and Yaxley, G.M.; 2007: The South Patagonian batholith: 150 my of granite magmatism on a plate margin. Lithos, 97, 373-394 • Irvine, T. N. and Baragar, W. R. A.; 1971: A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Earth Sciences 8, 523–548. • Lagabrielle, Y., et al.; 2007: Pliocene extensional tectonics in the eastern central Patagonian Cordillera: Geochronological constraints and new field evidence. Terra Nova, 19, 413 – 424. • Ramos, V.A.; 2002: El magmatismo neógeno de la Cordillera Patagónica. In M.J. Haller (ed.) Geología y recursos naturales de Santa Cruz. XV Congreso Geológico Argentino (El Calafate) Relatorio I (13): 187-200, Buenos Aires. • Sun, S. and McDonough, W.F.; 1989: Chemical and isotopic systematics of oceanic basalts; implication for mantle composition and processes. Magmatism in the ocean basins, Spec Pub. Geol. Soc. London, 42, 313-45. 64 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA MAGMATIC ACTIVITY AND STRIKE SLIP TECTONICS IN THE SOUTHERNMOST ANDES: KRANCK PLUTON, CHARACTERIZATION AND PRELIMINARY AMS SURVEY 2-03 Cerredo, M.E.1, Remesal, M. B.1, Tassone, A. A.2, Peroni, J. I.2, Menichetti, M.3, Lippai, H.2 (1) CONICET-Dpto. Geología, FCEN, UBA, Ciudad Universitaria, C1428EHA, Bs Aires, Argentina (2) CONICET-INGEODAV,Dpto. Geología, FCEN, UBA, Ciudad Universitaria, C1428EHA, Bs Aires, Argentina (3) Istituto di Scienze della Terra, Università di Urbino, Campus Scientifico Universitario, 61029, Urbino, Italy Three major strike-slip structures of dominant WNW-ESE trend characterize the southernmost Andes, from N to S: the Magallanes-Fagnano Fault System (which represents the onland boundary between the South America and Scotia plates), the Carbajal Valley Fault System in central Tierra del Fuego and the Beagle Channel Fault System which separates de Tierra del Fuego Island from the southern archipelago. The Neogene cinematic evolution of the Magallanes-Fagnano Fault System (MFS) is characterized by its transtensive nature with associated pull-apart basins both in onland and onshore areas (Lodolo et al., 2003). These major fault systems are near parallel to the main lineaments of the Late Cretaceous to Tertiary Fuegian fold and thrust belt which suggests that the transtensional structures may have developed along preexisting zones of weakness formed by crustal shortening (Klepeis and Austin, 1997). The Jeujepen and Kranck plutons are located along the strike of MFS (Fig. 1A), the former at the western termination of the Río Turbio fault segment and the latter at the eastern tip of the M. HopeCatamarca-fault segment (Fig. 1B). The alignment of intrusive bodies along the several transforming Fig. 1 - A) General map showing the tract of Magallanes-Fagnano Fault System (MFS) in central Tierra del Fuego; stars indicate the plutonic bodies located along the strike of MFS. B) Geological sketch depicting the main structures and units in the area of Kranck Pluton (KP) within the fold-and-thrust Fuegian belt overprinted by strike-slip structures. 65 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA structures of the southernmost Andes has already been reported as a causal relationship (i.e. the Palaeocene Yamana Granite Suite in Chilean Tierra del Fuego, Cunningham 1993; the Ushuaia Pluton, Peroni et al., 2009; the Jeu Jepen pluton, Cerredo et al., 2000). The Jeujepen and Kranck plutons are hosted in the pelite/mudstone sequence of the Lower Cretaceous Beauvoir Formation which was involved in the Andean compression. Presently available radiometric data indicate a Late Cretaceous age (93 ± 4 Ma, K/Ar whole rock age) for the Jeujepen Pluton (Acevedo et al. 2000). The Kranck Pluton (KP) is located along the WNW-ESE left-lateral M. Hope-Catamarca fault, the western strand of Magallanes Fagnano Fault system in Argentine Tierra del Fuego (Menichetti et al., 2008). KP is affected by a set of transtensive faults which have down-thrown (with offsets of several hundreds meters) the southern outcrops the intrusive body. These transtensional structures exhibit many fault planes dipping at low angle and are superimposed on the north-verging thrust slices. Although KP outcrops are restricted and largely covered by forest, aeromagnetic surveys have revealed an outstanding anomaly with a subcircular pattern and average diameter of around 6 km. Modeling of the aeromagnetic anomaly related to KP yielded a laccolithic body (Peroni et al., 2008 a and b). The KP is an epizonal intrusive body with a large compositional variation (cumular ultrabasic facies, gabbros, monzodiorite and monzonite facies and late stage syenite veins and dykes). Monzodiorite to monzonite facies are generally heterogeneous, they host mafic microgranular enclaves (several cm to dm in size) either with typical crenulate outlines or as diffuse ghosts parallel to magmatic banding. Preliminary chemical data indicates that KP shares with other Fuegian intrusions a dominant shoshonitic nature. The magmatic series of amphibole ± clinopyroxene ± biotite-gabbro to syenite is metaluminous, evolved from a highly hydrous melt. Characteristic Nb-Ta troughs on multi-element plots point to a subduction-related component in the petrogenesis of KP. The anticorrelation (Dy/Yb)N vs silica attests to significant amphibole fractionation along evolution. Accessory phases also played a role in liquid evolution, i.e. apatite fractionation resulted in typical P troughs which is associated with a slight decrease in LREE content along evolution. The characteristic Sr spyke on multi-element plots along with lack of significant Eu anomaly for all lithologies suggests that no plagioclase fractionation has controlled significantly the series evolution. This agrees with the H2O-rich nature of liquids which precludes plagioclase stability, and restricts its crystallization to shallow upper crustal levels. Deformation/crystallization relationships indicate a dynamic scenario for the emplacement and cooling evolution; synmagmatic foliations were recognized both at the meso- and microscales; not penetrative subsolidus medium- T microstructures parallel the submagmatic ones and both are variably overprinted by brittle deformation. A pilot AMS (anisotropy of magnetic susceptibility) survey of the different petrographic facies of KP and its host was carried out. It comprised 11 sampling sites (eighty three cores), 7 within the central part of the intrusive body and the remainder in the sedimentary host. The intrusive rocks show magnetic susceptibility (k) values falling in the ferromagnetic field (Tarling and Hrouda, 1993), between 2.7* 10-4 and 3.7 * 10-1 SI units, whereas the host rocks have k values in the range 9.1* 10-5 to 4.5* 10-4 SI units, in the paramagnetic field. Both groups display distinct trends in the k vs. P´ plot (Fig. 2). The intrusive rocks show fairly well clustered k values over a restricted P´ range (1.03-1.15) and the host rocks display a trend toward lower k values within a similar range of anisotropy degree (1.03-1.13). The AMS ellipsoid is oblate (T>0) to neutral both in the intrusive as in the host rocks. Regardless of the involved lithology, both in the marginal areas of the intrusion as Fig. 2 - Î bulk (site average mean bulk susceptibility) vs P´ well as in the thermal aureole within the (anisotropy degree) plot for each AMS sampling site. 66 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA host the magnetic fabric is dominated by WNW/NW-ESE/SE vertical to steeply dipping (either to the NE or the SW) foliations. When magnetic lineations are considered two populations emerge. One shows K1 clusters along foliation strike with characteristic horizontal to very shallow plunge. The other, restricted to the KP sites, displays magnetic lineations of high N-NE plunge (50-70º) clustered in the down dip direction of foliation planes. Within the central areas of KP a distinct magnetic fabric was acquired, it is characterized by subhorizontal foliation and no clustering of magnetic lineations. This fabric pattern corresponds to a monzodiorite facies rich in mafic (ultramafic) enclaves. Preliminary AMS data on KP suggest a probable layered nature for the central part of the intrusion. The prevailing WNW-ESE oriented, vertical magnetic foliations mimic the extensional component inferred for the transtentive movements along the M. Hope-Catamarca fault, with horizontal magnetic lineations pointing to the stretching direction whereas the steep lineations might signal upsurges of fresh magma. We preliminary infer that there would be some competence between the internal dynamics of pluton building and assembly and the externally imposed stress field. REFERENCES • Acevedo, R D., Roig, C.E., Linares, E., Ostera, H, Valín-Alberdi, M. and Queiroga-Mafra, Z.M. 2000 La intrusión plutónica del Cerro Jeu-Jepén. Isla Grande de Tierra del Fuego, República Argentina. Cadernos Laboratorio Xeolóxico de Laxe, 25: 357-359 • Cerredo; M. E., Tassone, A. A., Coren, F., Lodolo, E. y Lippai, H. 2000. Postorogenic, alkaline magmatism in the Fuegian Andes: The Hewhoepen intrusive (Tierra del Fuego Island). IX Congreso Geológico Chileno, Actas, (2): 192-196, Puerto Varas. • Cunningham, W. D., (1993) Strike-slip faults in the southernmost Andes and the development the Patagonian orocline. Tectonics, v. 12 (1), 169-186 • Klepeis, K. A., and J. A. Austin Jr., 1997. Contrasting styles of superposed deformation in the southernmost Andes, Tectonics, 16: 755 – 776, • Lodolo E., Menichetti M., Bartole R., Ben Avram Z., Tassone A. and Lippai H. 2003. Magallanes-Fagnano continental transform fault (Tierra del Fuego, Southernmost South America). Tectonics 22 (6), 1076, doi:10.1029/2003TC001500 • Menichetti, M., Lodolo, E., Tassone, A. 2008. Structural geology of the Fuegian Andes and Magallanes fold and thrust belt: a reappraisal. GeoSur Special Issue. Geologica Acta. Vol. 6 (1), 19-42. • Peroni, J. I.; Tassone, A.; Lippai, H.; Menichetti, M.; Lodolo, E.; Vilas, J. F. 2008a. Estudio Geofísico Del Plutón Kranck. Tierra Del Fuego. Argentina . Actas del XVII Congreso Geológico Argentino • Peroni, J.; Tassone, A.; Menichetti, M; Lippai, H.; Lodolo, E.; Vilas, J.F. 2008b. Geologia e geofisica del plutone Kranck. Rendiconti online della Societá Geologica Italiana. Roma: Societá Geologica Italiana, vol. 1: 132-136 • Peroni, J: I.; Tassone; A. A., Menichetti,M. and Cerredo, M. E. 2009. Geophysical modeling and structure of Ushuaia Pluton, Fuegian Andes, Argentina. Tectonophysics, 476,(3-4), 25:436-449 • Tarling, D.H.; Hrouda, F. 1993. The Magnetic Anisotropy of Rocks. Chapman and Hall: 217 p. London. CHARACTERIZATION OF AN ULTRABASIC LAMPROPHYRE (EVOLVED DAMTJERNITE) IN THE TANDILIA BASEMENT, SOUTHERNMOST RÍO DE LA PLATA CRATON, ARGENTINA 2-04 Dristas, J.A.1,2*, Martínez, J.C.1, Massonne, H.J.3, Wemmer K.4 (1) Universidad Nacional del Sur, Departamento de Geología and Ingeosur-CONICET (2) CIC de la provincia de Buenos Aires, Argentina (3) Institut für Mineralogie und Kristallchemie, Universität Stuttgart (4) Geozentrum Göttingen, Universität Göttingen * Presenting author’s e-mail: [email protected] A small dyke of an ultrabasic lamprophyre was recently found in the Sierra Alta de Vela (SAV), east of the town of Tandil, where basement of the southernmost Río de la Plata Craton is exposed. The wall rock of this dyke is a variably deformed meta-igneous rock of granodioritic composition. The ultrabasic dyke, which is also partially deformed, consists mainly of zoned clinopyroxene (Cr-free diopside) and phlogopite (Cr-free, Ti-poor and Al-rich) phenochrysts. A scarce oxide phase is chromite. The three mentioned minerals are considered to be primary. The groundmass is also dominated by Cr67 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA free diopside, which occurs as micro-phenochrysts with aegerine-augite rims, and subordinate interstitial phlogopite altered to chlorite. Additional phases are calcite, chlorite, sericite, andradite-rich garnet, pure albite, epidote and apatite forming a partial “ocellar” interstitial texture. According to the bulk rock chemistry, the dyke is a damtjernite (SiO2: 44.70- 46.45 wt. %). The Cr (550 ppm) and Ni (160 ppm) contents as well the Mg number (64) point to a moderately evolved rock, though the Ce to Pb ratio was found to be 44 (>25), but the contents of this rather mobile elements could have been changed during the secondary processes. The Ti and K contents of the SAV damtjernite are low compared to typical damtjernites, but CO2, Ca, Al and LaN/YbN fit well world-wide occurrences of damtjernites (see Tappe et al., 2006 Journal of Petrology, 47 (7): 1261-1315). Also the Zr/Nb ratio of 7 satisfies the higher values for typical damtjernites compared to other ultrabasic dykes. The positive anomaly of Dy in the REE pattern of the SAV-damtjernite may be due to the dominant presence of clinopyroxene in the rock, which enhances the fractionation (higher mineral/melt partition coefficient) of Dy from basic melts according to Fujimaki et al. (1984, J. Geophys. Res., 89, suppl. B662-B672). The oscillatory zoning of the andradite-rich garnets reflects mainly the Fe3+-Al3+ substitution. This phase probably formed at upper crustal levels. The corrosion of garnet (atoll texture) is also due to late-stage processes. The absence of large quantities of Ti in andradite also points to a secondary origin of this phase. We will continue our work in the Sierra Alta de Vela area by studying the probable presence of a other lithological variety of this type of diatremites outcropping there and in other areas of Tandilia. CONSTRAINTS ON OIB-TYPE PREMA AND EM1 MANTLE SOURCES FROM TRACE ELEMENT AND PB, SR, AND ND ISOTOPIC RATIOS OF PRIMITIVE EOCENE TO RECENT BACKARC PATAGONIAN BASALTS 2-05 Mahlburg K.S.1*, Jones, H,1, Gorring, M.L.2 (1) Dept of Earth Atmospheric Sciences, Cornell University, Ithaca, NY, 14853 USA (2) Earth Studies, Montclair State Univ., Upper Montclair, NJ, 07043, USA * Presenting author’s e-mail: [email protected] The South America Patagonian region between 36° and 52°S is the host to extensive and voluminous Eocene to Recent backarc mafic volcanic flows that have erupted under neutral to mildly extensional stress conditions. This widespread mafic magmatism is best attributed to a mantle that has been continuously on the verge of melting since before the Eocene. Major backarc melting events can be correlated with tectonic perturbations that vary in space and time. These perturbations include late Miocene to Pliocene steepening of a shallow subduction zone in the north, Oligocene to early Miocene trench roll-back in the middle and asthenospheric windows related to the Miocene to Recent collision of the Chile Ridge with the Chile Trench in the south. Clues to the general nature of the backarc Patagonian mantle come from trace element and isotopic signatures on Eocene to Recent primitive to near primitive alkaline to sub-alkaline lavas (44-53% SiO2) chosen from across the region based on their high Cr (>200 ppm), Ni (>130 ppm) and MgO (>7%) contents. Evidence that crustal contamination plays little or no role in perturbing their mantle signatures comes from trace element analyses, as plots of Th/La versus Ta/U and Ce/Pb versus Nb/U show the samples falling in or near fields for MORB and OIB lavas. Overall, their trace elements show a generally OIB-like character with little to no slab influence as indicated by relatively low ratios of La/Ta (7.7 and 21.5), Sr/Ta (180 to 800), Ba/Ta (60-560) and Th/Ta (0.8-3.6). Comparisons of Nd, Sr and Pb isotopic measurements of the most primitive samples with globally defined mantle reservoirs on diagrams from the W.M. White website (Cornell Univ.) show that Patagonian basalts dominantly have OIB type signatures that generally fall between those of PREMA (prevalent mantle) and EMI (enriched mantle with lithospheric and lower crust type tendencies) man68 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA tle reservoirs. Measurements on 30 samples yield 87Sr/86Sr from 0.7031 to 0.7049 and 143Nd/144Nd from 0.51288 to 0.51254 (?Nd = -0.2 to +5) with measurements on 19 yielding 206Pb/204Pb from 18.2 to 19.1, 207Pb/204Pb from 15.5 to 15.7 and 208Pb/204Pb from 38.2 to 39.3. The ratios plot between PREMA and EMI mantle reservoirs on ?Nd-87Sr/86Sr and 87Sr/86Sr-206Pb/204Pb diagrams, falling on the low 87Sr/86Sr side of South Atlantic Tristan/Walvus/Gough and Indian Kerguelen oceanic basalt fields. Pb ratios plot in the regions of these same oceanic islands, overlapping Indian MORB in the PREMA 206Pb/204Pb range. Both 207Pb/204Pb and 208Pb/204Pb generally plot above the Pacific MORB field consistent with a role for recycled continental material. Overall, a general PREMA-like mantle source with variable tendencies towards EM1 in the Patagonian mantle is consistent with the consequences of ridge collision, contributions from depleted backarc mantle above a subducting slab, recycled oceanic slabs from the long subduction history along the Patagonian margin and recycling of continental lithosphere at the South American margin by subduction erosion. The same processes have played a role in shaping the chemical signatures of South Atlantic and Indian Ocean hotspot magma mantle sources. SERRA GERAL VOLCANISM IN THE PROVINCE OF MISIONES (ARGENTINA): 2-06 GEOCHEMICAL ASPECTS AND INTERPRETATION OF ITS GENESIS IN THE CONTEXT OF THE LARGE IGNEOUS PROVINCE PARANÁ-ETENDEKA-ANGOLA. ITS RELATION WITH THE ALKALINE VOLCANISM OF CÓRDOBA PROVINCE Lagorio, S.L.1*, Vizán, H.2 (1) Servicio Geológico Minero Argentino (SEGEMAR). J. Roca 651, piso 10 – Ciudad Autónoma de Buenos Aires (2) Departamento de Ciencias Geológicas – Facultad de Ciencias Exactas y Naturales (UBA – CONICET). Ciudad Universitaria, Pab. II. Ciudad Autónoma de Buenos Aires (*) Presenting author’s e-mail: [email protected] New geochemical data from Serra Geral basalts of Misiones Province (Argentina) are complementary of the already published data of the Large Igneous Province (LIP) Paraná-Etendeka-Angola (PEA; e.g. Piccirillo and Melfi, 1988; Peate, 1997; Marzoli et al., 1999; see Fig. 1a and b), displaying the typical tholeiitic nature. The volcanic rocks from Misiones belong either to the high- or low-Ti varieties; the coexistence of both types in the present sampling, agrees with the fact that this area belongs to the central and southern regions of the Paraná Magmatic Province. Paranapanema, Ribeira, Gramado, Pitanga and Urubici varieties were recognized, being the former the most abundant in the collected rocks. Urubici type sample, from San Ignacio area, is the westernmost occurrence of this variety at this latitude. Chemical data point out that magmas of high- and low-Ti were originated from different sources, and evolved through fractional crystallization under low pressure conditions, involving significant crustal contamination only in the Gramado magma type. Heterogeneity in the magma, on small and large scales, is in agreement with a subcontinental lithospheric mantle source. Geochemical features, particularly the Nb-Ta negative anomaly in the multi-elemental diagram normalized to primitive mantle as well as some ratios (e.g. La/Nb and Zr/Ta) point out significant differences in relation to alkaline volcanic rocks from the called plume Tristan da Cunha, as mentioned also by other authors (e.g. Ernesto et al., 2002). Geochemical data, particularly the Nb-Ta negative anomaly shown by tholeiites from Misiones and from the whole PEA LIP, would be related to ancient subduction processes (e.g. Transamazonian and Brasiliano Precambrian Events) that metasomatized the mantle source as mentioned by other authors also considering isotopic data (e.g. Iacumin et al., 2003). This is also supported by Nd model ages (e.g. Comin Chiaramonti and Gomes, 2005) and Hf model ages (Santos et al., 2008) also involving Grenvillian Events. The great heat budget involved in mantle melting might correspond to a thermal blanketing caused by 69 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - a) Sketch-map showing the Early Cretaceous tholeiitic magmatism of Large Igneous Province (LIP) ParanáEtendeka-Angola Province (PEA). b) Sketch-map of PEA and Sierra Chica de Córdoba (SCC) alkaline locality in a reconstruction at nearly 130 Ma (after Marzoli et al., 1999, Lagorio, 2008 and therein references). RPC = Río de la Plata Craton, SFC = San Francisco Craton, AC = Amazonia Craton, CC = Congo Craton, RAC = Río Apa Craton, MB CB = Córdoba Mobile Belt, MB FM = Dom Feliciano Mobile Belt, MB R-SI = Rondonia-San Ignacio Mobile Belt. ancient Pangaea. The location of the main melting zone in the lithospheric mantle must have been determined by the effects of the hot uprising limb of a possible large scale convection that affected zones of crustal weakness (e.g. sutures between former cratons). On the other hand, volcanism of the sierra Chica de Córdoba (SCC, Fig. 1a and b), nearly coeval with that of the PEA LIP, is alkaline of high-Ti (Lagorio, 2008) and display a peripheral location in relation with the great heat source mentioned above. This is consistent with the involved lower melting degrees of the volcanism of SCC in relation to those of PEA LIP. Melting could have been triggered by a small scale process like edge-driven convection resulting from the thickness contrast between Rio de la Plata Craton and Pampia terrane, as is shown in Fig. 2 of Vizán and Lagorio, this volume. REFERENCES • Comin-Chiaramonti, P. and C.B. Gomes (Eds), 2005. Mesozoic to Cenozoic alkaline magmatism in the Brazilian Platform. Edusp/Fapesp, San Pablo. pp 750. • Ernesto, M., L.S. Marques, E.M. Piccirillo, E.C. Molina, N. Ussami, P. Comin-Chiaramonti and G. Bellieni, 2002. Paraná Magmatic Province-Tristan da Cunha plume system: fixed versus mobile plume, petrogenetic considerations and alternative heat sources. Journal of Volcanology and Geothermal Research, 118: 15-36. • Iacumin, M., A. De Min, E.M. Piccirillo and G. Bellieni, 2003. Source mantle heterogeneity and its role in the genesis of Late Archean-Proterozoic (2.7-1.0 Ga) and Meozoic (200 and 130 Ma) tholeiitic magmatism in the South American Platform. EarthScience Reviews, 62: 365-397. • Lagorio, S.L. 2008. Early Cretacous alkaline volcanism of the Sierra Chica de Córdoba (Argentina): Mineralogy, gochemistry and petrogenesis. Journal of South American Earth Sciences 26(2): 152-171. • Marzoli, A., L. Melluso, V. Morra, P.R. Renne, I. Sgrosso, M. D’Antonio, L. Duarte Morais, E.A.A. Morais and G. Ricci, 1999. Geochronology and petrology of Cretaceous basaltic magmatism in the Kwanza basin (western Angola), and relationships with the Paraná-Etendeka continental flood basalt province. Journal of Geodynamics, 28: 341-356. • Peate, D.W., 1997. The Paraná-Etendeka Province. In: Large Igneous Provinces: Continental Oceanic and Planetary Flood Volcanism. Mahoney, J.J. and M.F. Coffin (Eds). Geophysical Monograph, 100. American Geophysical Union. 215-245. • Piccirillo, E.M. and A.J. Melfi (Eds), 1988. The Mesozoic Flood Volcanism from the Paraná Basin (Brazil): Petrogenetic and Geophysical Aspects. Universidad de San Pablo, San Pablo. pp 600. • Santos, J.O.S., W. Wildner, L.A. Hartmann, W.L. Griffin y N.J. McNauthton, 2008. Lower Cretaceous U-Pb age and Grenvillian Hf model-age of the large Serra Geral magmatism of Paraná basin, South America. 6° South American Symposium on Isotope Geology. Abstracts: 90, Bariloche. 70 GEOSUR2010 LITHOLOGY AND AGE OF THE CUSHAMEN FORMATION. DEVONIAN MAGMATISM IN THE WESTERN NORTH PATAGONIAN MASSIF. ARGENTINA 22-23 NOVEMBER 2010 – MAR DEL PLATA 2-07 López de Luchi, M.G.1*, Cerredo, M.E.2, Martínez Dopico, C.1 (1) CONICET-INGEIS, Pabellón INGEIS, Ciudad Universitaria, C1428EHA, Bs Aires, Argentina (2) CONICET-Dpto. Geología, FCEN, UBA, Ciudad Universitaria, C1428EHA, Bs Aires, Argentina * Presenting author’s e-mail: [email protected] Introduction Accurate temporal constraints for the metamorphic peak conditions, granitoid emplacement, deformational events and cooling paths are a key issue for unraveling the tectono-metamorphic history of basement complexes. One of the greatest difficulties in Rb-Sr dating on metamorphic rocks is the lack of evidence of complete re-homogenization and the assumption of a common initial Sr ratio. Magmatic units that were emplaced at different stages of the metamorphic evolution as indicated by deformation features could be satisfactorily dated with this method. The pre Jurassic metamorphic basement of the eastern foothills of the Main Cordillera along the western border of the North Patagonian Massif (NPM) is composed of metamorphic and igneous complexes. The former is represented by the Cushamen Formation (CF) which forms a poorly exposed N-S trending belt from Catan-Lil River (39º 45´S/ 70º 36´W, Neuquén province) in the North down to Leleque in the South. The general tracks of the regional metamorphism in CF had been reported for some areas (Volkheimer 1964, Varela et al. 1991, Cerredo 1997, Franzese et al. 1992, Cerredo and López de Luchi 1998, Giacosa et al. 2004, Lucassen et al. 2004, López de Luchi et al. 2005, Von Gosen 2009). Ostera et al. (2001 and references therein) proposed an Early- Middle Devonian metamorphic event probably associated with a collisional-accretional episode for rocks located west of Río Chico and in Colonia Cushamen (Fig. 1) whereas Hervé et al. (2005) proposed a 335 Ma -earliest Permian metamorphism based on U-Pb SHRIMP dating of magmatic rims in zircon from an inferred medium grade metaclastic unit located in the Puesto Miranda area (Fig. 1). U-Pb SHRIMP dating of amphibolite grade paragneiss from south of El Maitén showed zircons with metamorphic overgrowths at 330, 340 and 365 Ma, and zircon cores with a major provenance at 440 Ma (Pankhurst et al. 2006). These authors considered that the protoliths were continental margin sediments that hosted their inferred Carboniferous arc. In more regional perspective a migmatite from San Martín de los Andes (40º09´S-71º21´W) yielded an internal Rb–Sr isochron of 368± 9 Ma whereas U–Pb age determinations of four concordant fractions of titanite of a calc-silicate rock from a gneiss/migmatite sequence near Piedra del Aguila (40º02´S-70º04´W) indicate 375 ± 15 Ma (207Pb/235U) and 380± 2 Ma (206Pb/238U) (Lucassen et al. 2004). This age of ca. 380 Ma is interpreted as the age of crystallization of titanite, close to the metamorphic peak. This paper focuses on the recalculation of whole rock Rb/Sr isochrons based on lithological and chemical analysis of the different lithologies that make up the CF at the Río Chico-Cushamen and El Maitén-Leleque areas (Fig. 1). These data are used to constrain the metamorphic evolution and the emplacement of the granitoids in relation with deformational phases at the SW corner of the North Patagonian Massif. Geological background The metamorphic series of CF is made up of metasedimentary and metaigneous rocks; the former includes metapelitic, metagreywackes, minor quartz-rich sandstones, metaconglomerates, calcsilicates and sparsely interbedded tourmaline bearing schists. Magmatic additions accompanied the entire CF evolution since the pre-metamorphic stage. Near Río Chico (Fig. 1) bimodal volcanism emplaced in the CF sedimentary basin is represented by rare thin dikes and layers of dacitic and basaltic composition (López de Luchi et al. 2002). Variably foliated tonalitic to leucogranitic rocks (ranging from pegmatite through aplite textures) occurring as sheets, veins and dykes within the metamorphic series have been reported for the whole CF belt. The metamorphic series display a complex structural evolution characterized by several deformation phases (Franzese et al., 1992; Cerredo, 1997; Cerredo et al., 2002; Giacosa et al., 2004; von Gosen, 2009). The two older deformations, D1 71 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 Table 1 - Lithologies and representative mineral assemblages of Cushamen Formation (summary from Cerredo, 1997; Giacosa et al., 2004; von Gosen, 2009). Mineral symbols after Kretz, 1983. Mineral assemblagles Metapsammites/m etapelites Chlorite zone Chlorite-Garnet zone Muscovite-Garnet zone AluminosilicatePlagioclase zone Calcsilicate TourmalineMetavolcanic Metagranitoid bearing schists rocks s Chl-Ms-Ab Chl-Ms-Ab Chl-Ms-Bt-Grt-Olig Tr-Ep-Chl Ms-Bt-Grt-Olig Cc-Scp-Di-Qtz Tur-Bt-Pl Pl-Bt Sil-Pl-Tur Hb-Bt-Sph Bt-And-Qtz and D2, produced foliations and have been recognized in the whole CF belt and interpreted as part of the prograde metamorphic evolution, whereas the later D3 and D4 phases have only been reported for the northern areas (from Comallo to Río Chico, Fig. 1) and interpreted to accompany the retrograde, uplift path of CF (Cerredo and López de Luchi, 1998; von Gosen, 2009). Several episodes of synkynematic and intertectonic magmatic additions accompanied the D1-D3 evolution of CF (Giacosa et al. 2004, López de Luchi and Cerredo, 2007, Von Gosen, 2009). The metamorphic series of CF underwent a medium-pressure regional metamorphism ranging from low-greenschist facies to upper-amphibolite facies. Table 1 summarizes the mineral assemblages. Major element chemical data point to an active continental margin as a likely setting for the sedimentary protoliths of CF, which bear compositional signatures suggesting their provenance from felsicacid plutonic and volcanic detritus as well as recycled mature polycyclic quartzose detritus (unpublished data). Metagranitoids are calc-alkaline, peraluminous Bt-tonalites to Ms (± Bt ±Grt)-granites to leucogranites. Geochronological constraints Metasedimentary rocks (Ms-Grt schists) did not produce isochrons since MSWDs are higher than 10 and errors than 20% which indicate the lack of re-homogenization for the Rb-Sr system. Metagranitoids were separated in groups on the basis of their chemical composition and relationship with the prograde deformation phases D1-D2. A main group mostly represented at Cañadón La Angostura and at Pto. Miranda areas (Fig. 1) is made up by foliated Bt-tonalite to granite which underwent D1-D2 and are characterized by 87Sr/86Sri > 0.710 either at 370 or 330 Ma. These rocks yielded a Rb-Sr WR isochron of 373±15 Ma, 87Sr/86Sri= 0.71130, MSWD = 4.6. If only the isotopic results for Cañadón La Angostura area are considered a Rb-Sr WR isochron of 374.4±7.3 Ma, 87Sr/86Sri=0.71129, MSWD = 0.76 is obtained. If granites are separated from tonalite-granodiorite the error increase and no isochron is obtained. Rocks with calculated 87Sr/86Sri < 0.710 either at 370 or 330 Ma from Leleque and Pto. Miranda area yielded a Rb-Sr WR isochron of 323±26 Ma, 87Sr/86Sri=0.71079, MSWD=2. Discussion The obtained isochrons correspond to igneous rocks therefore the age of the metamorphic peak has to be interpreted based on the analysis of the timing of the emplacement of the magmatic units in relation with the metamorphic evolution. A minimum age for the metamorphism ca 370 Ma could be inferred based on the isochron for the granitoids the area Río-Chico-Cushamen which underwent the D1-D2 deformation. In the area of Laguna del Toro the 372 Ma Cáceres orthogneiss (Pankhurst et al., 2006) is emplaced in a metamorphic pile which exhibit similar lithologies than the CF. (Fig. 1). Recalculation of the U-Pb data for metamorphic rims of zircons of El Maitén paragneiss (Pankhurst et al., 2006) indicate a peak between 360-380 Ma and subordinate peak at 330-340 Ma. Results extracted from the analysis of the relativi72 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Geological sketch of the main outcrops of the Cushamen Formation ty probability vs. age plot of the zircons of the Pto. Miranda area (Hervé et al., 2005) indicate that only three data have 206Pb/238U ages younger than 350 Ma. In this connection, we want to highlight that our new isochron enlarges the group of Late Devonian intrusions occurring along the western border of the NPM. From San Martin de los Andes in the north to the area of Colonia Cushamen in the south, several ~400-360 Ma (Varela et al., 2005; Pankhurst et al., 2006;) deformed tonalitic to granitic intrusions show isotopic signatures indicating to crustal sources (Sr/Sri > 0.710, eNd < -4). Such a widespread episode of crustal melting might result either within a thickened chemically closed crust or within a crust of normal thickness with advective heat contributions in the form of mantle derived melts. Current scenarios for the Devonian in the western NPM propose a magmatic arc setting (Pankhurst et al., 2006) whereas the Devonian tectonomagmatic evolution has been also related to the Chanic movements of Precordillera (Varela et al 2005). The lack of complete chemical data is a severe restriction to properly constrain the meaning of this magmatism. Our younger Rb-Sr WR isochron age ca 320 Ma is close to the reported U-Pb SHRIMP 329 Ma age for intermediate rocks of the Tonalite El Platero and Cordón El Serrucho (Pankhurst et al. 2006). Tonalite El Platero intrudes CF south of Pto Miranda. Giacosa et al. (2004) mentioned that the NW planar fabric of the tonalite is subvertical whereas the remaining igneous and clastic rocks of CF exhibit a shallower dipping parallel NW fabric. Rocks at Cordón el Serrucho belong to a belt of foliated calc-alkaline rocks which were interpreted as syn to late tectonic regarding the deformation of its host metamorphic rocks. On the contrary Pankhurst et al. (2006) proposed that the ca. 330 Ma represent a subduction event developed along the southwestern margin of the NPM. Furthermore in the area of Chacay Huarruca, immediately north of Cañadón La Angostura (Fig. 1) Varela et al. (2005) obtained a 206Pb/238U lower intercept at 302±39 Ma for a leucogranite which intrudes the CF. Pankhurst et al. (2006) reported U-Pb SHRIMP ages for S-type peraluminous Grtbearing leucogranites, 314±2Ma from Paso del Sapo and 318± 2Ma from Sierra de Pichiñanes south of the studied areas. Both were interpreted as crustal granitoids emplaced in a syn-collisional setting after the ca. 330 Ma subduction. Therefore we propose that at least a part of CF underwent a Devonian metamorphic event and was 73 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA intruded by a series of granitoids with different chemical signatures probably emplaced in different tectonic settings. Accurate interpretation of the timing and meaning of penetrative fabric in these granitoids needs further study. REFERENCES • Cerredo, M.E., López de Luchi, M.G., 1998. Mamil Choique Granitoids, southwestern North Patagonian Massif, Argentina: magmatism and metamorphism associated with a polyphasic evolution. J. South Amer Earth Sciences, 11(5): 499-515 • Cerredo, M.E., 1997. The metamorphism of Cushamen Formation, Río Chico area. North Patagonian Massif. Argentina. 8ºCong. Geol. Chileno, Actas 2:1236-1240, Antofagasta. • Cerredo, M E.; Parica, C.A., Remesal, M.B. 2002. Facies de alto grado de la Formación Cushamen en Aguada del Pajarito, Macizo Norpatagónico. Chubut. 6º MinMet, Actas: 85-88, Buenos Aires • Dalla Salda, L., Varela, R., Cingolani, C., Aragón, E., 1994. The Río Chico Paleozoic Crystalline Complex and the evolution of Northern Patagonia. J. South American Earth Sciences 7 (3-4), 377-386 • Franzese, J., Días, G., Dalla Salda, L., 1992. Las estructuras de las Metamorfitas Cushamen, Provincia de Chubut. VI Reunión Microtectónica. Acad. Nac. Cs Ex, Físicas y Naturales. Monografías, 8: 27-30. • Giacosa, R., Márquez, M., Nillni, A., Fernández, M., Fracchia, D., Parisi, C., Afonso, J., Paredes, J., Sciutto, J. 2004. Litología y estructura del basamento ígneo-metamórfico del borde SO del Macizo Nordpatagónico al oeste del río Chico. Revista de la Asociación Geológica Argentina, 59 (4): 569-577 • Hervé, F., Haller, M.J., Duhart, P., Fanning, C.M. 2005. SHRIMP U-Pb ages of detrital zircons from Cushamen and Esquel Fms, North Patagonian Massif, Argentina, 16º Cong Geol Arg, Actas I: 309-314 • Kretz, R. 1983. Symbols for rock-forming minerals. American Mineralogist, 68: 277-279 • López de Luchi, M.G., Cerredo, M.E., 1997. Paleozoic basement of the southwestern corner of the North Patagonian Massif: an overview, 8º Cong. Geol. Chileno, Actas 3: 1674-1678, Antofagasta • López de Luchi, M.G., Ostera, H., Cerredo, M.E., Cagnoni, M., Linares, E. 2000. Permian magmatism in Sierra de Mamil Choique, North Patagonian Massif. 9º Cong. Geol. Chileno, Actas 2: 750-754, P Varas. • López de Luchi, M.G, Ostera, H., Cagnoni, M., Cerredo, M.E., Linares, E., 2002. Geodynamic setting for the western border of the North Patagonian Massif: Cushamen Formation at Río Chico, Río Negro. In: Cabaleri, N., Linares, E., López de Luchi, M.G., Ostera, H., Panarello, H. (Eds). 15º Cong. Geol. Arg., Actas 2: 210-216. • López de Luchi, M.G., Cerredo, M.E., Wemmer, K. 2006. Time constraints for the tectonic evolution of the SW corner of the North Patagonian Massif. 5º S Amer Symp Isotope Geol, Actas: 114-118, Pta del Este. • López de Luchi, M. G., Cerredo, M. E. 2008. Geochemistry of the Mamil Choique granitoids at Rio Chico, Río Negro, Argentina: Late Paleozoic crustal melting in the North Patagonian Massif, J. South Amer Earth Sciences, 25, (4): 526-546 • Lucassen, F., Trumbull, R., Franz, G., Creixel, C., Vázquez, P., Romer, R., Figueroa, O., 2004. Distinguishing crustal recycling and juvenile additions at active continental margins: the Paleozoic to Recent evolution of the Chilean Pacific margin (36º-41ºS). J. South Amer Earth Sciences 17, 103–119. • Ostera, H.A., Linares E., Haller, M.J., Cagnoni, M.C., López de Luchi, M.G., 2001. A widespread Devonian metamorphic episode in Northern Patagonian. In: Tomlinson, A. (Ed.), 3º S Amer Symp. Isotope Geol., Abb. Abstr. Vol, Edición Especial Revista Comunicaciones, 52 and CD edition. Pucon • Pankhurst, R.J.; Rapela, C.W., Fanning, C.M., Márquez, M. 2006. Gondwanide continental collision and the origin of Patagonia, Earth-Science Reviews 76: 235–257 • Varela, R., Dalla Salda, L.H., Cingolani, C., Gómez, V. 1991. Estructura, petrología y geocronología del basamento de la Región del Limay, provincias de Río Negro y Neuquén. Rev Geol. Chile, 18: 147-163. • Varela, R., Basei, M., Cingolani, C.A., Siga, O., Passarelli, C., 2005. El basamento cristalino de los Andes Norpatagónicos en Argentina: Geocronología e interpretación tectónica. Rev Geol. Chile 32: 167–187. • Volkheimer, W., 1964. Estratigrafía de la zona extrandina del Dpto. Cushamen (Chubut) entre los paralelos 42º y 42º 30´ y los meridianos 70º y 71º. Revista de la Asociación Geológica Argentina, 19 (2): 85-107 • von Gosen, W. 2009. Stages of Late Paleozoic deformation and intrusive activity in the western part of the North Patagonian Massif (S. Argentina) and their geotectonic implication. Geol Mag. 146: 48-71 74 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA METAMORPHIC EVOLUTION OF THE CINCO CERROS AREA, SIERRA DE TANDIL, ARGENTINA 2-08 Massonne, H.J.1*, Dristas, J.2, Martinez J.C.2 (1) Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, D-70174 Stuttgart, Germany (2) Departamento de Geología, Universidad Nacional del Sur, Bahia Blanca, Argentina * Presenting author’s e-mail: [email protected] The metamorphic basement of the WNW-ESE-trending Tandilia range in the Buenos Aires Province represents the southernmost extension of the Rio de la Plata craton and consists mainly of metaigneous rocks. However, garnet-bearing metapelites occasionally crop out as well. These rocks have a much higher potential for the derivation of metamorphic P-T conditions than the garnet-absent, metaigneous rocks. For this reason, we have studied metapelites of the Cinco Cerros area in the northeastern Tandilia range. In this area various lithologies occur which partially show migmatitic structures. However, gneiss sample 06181109 does not show any evidence for partial melting. Garnet in this sample is slightly zoned with core and rim compositions of pyrop17gross6spess2alm75 and pyrop13gross5.5spess1.5alm80, respectively. The Mg to Mg+Fe ratio of biotite is around 0.49. The plagioclase composition is Ab65An34Kf1. No cordierite, staurolite, and Al2SiO5-phase were noted to occur. We used the PERPLE_X computer software package to calculate a P-T pseudosection for metapelite 06181109. The calculated pseudosection was contoured by isopleths of various parameters especially molar fractions of garnet components. According to the latter calculation results we derived P-T conditions of 6.5 kbar and 670°C for an early metamorphic stage. Subsequently, a pressure release occurred at decreasing temperatures. The final metamorphic P-T conditions recorded by the studied rock are 4.5 kbar and 600°C. The corresponding P-T path is compatible with the observed mineral assemblage and the fact that in rocks adjacent to that of sample 06181109 partial melting occurred. We achieved 16 analyses of monazite in this sample with the electron microprobe for age dating. The obtained ages from 11 analyses of Th-bearing monazite scatter around 2.02 Ga, whereas 5 almost Thfree monazites gave ages around 1.82 Ga. Thus, the metamorphic event can be in any case related to the Transamazonian cycle. As our study area is close to the margin of the Rio de la Plata Craton, where abundant magmatic-arc derived plutonic rocks outcrop, we interpret the derived P-T data as follows: An early metamorphic event (6.5 kbar, 670°C) of the Transamazonian cycle could have resulted from underplating of magmas of a magmatic arc. This event was followed by relatively slow exhumation, as we observed a significant cooling and, thus, thermal relaxation during a pressure release of about 2 kbar. Possibly, only superficial erosion caused the exhumation of the studied rocks at this metamorphic stage. On the basis of a study of metamorphic rocks from an area nearby our study area, Delpino and Dristas proposed thinning of the crust possibly by the formation of a marginal back-arc basin. However, these authors deduced a somewhat different P-T path starting at 750-800°C and 5-6 kbar and ending at 450-500°C and 5.5-6.5 kbar. This demonstrates the necessity to derive P-T paths as precise as possible in order to relate metamorphic processes of a study area to the right geodynamic model. For this reason, we will continue our work in the Cinco Cerros area by studying the complete lithological variety outcropping there. 75 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 PALEOMAGNETIC STUDIES OF CENOZOIC BASALTS FROM NORTHERN NEUQUÉN AND SOUTHERN MENDOZA PROVINCES: STRATIGRAPHIC IMPLICATIONS 2-09 Re, G.H(*)1, Vilas, J.F.2 (1) INGEODAV (Instituto de Geofísica Daniel A. Valencio) Dpto. Ciencias Geológicas - FCEyN (UBA). (2) CONICET - INGEODAV (Instituto de Geofísica Daniel A. Valencio) Dpto. Ciencias Geológicas FCEyN (UBA. *Presenting author’s e-mail: [email protected] Introduction The Cenozoic basaltic sequences of Neuquén have been thoroughly studied, in terms of their regional geological, paleomagnetic and geochronological features. The distribution and chemical composition of the Paleogene and Neogene volcanism in north western Neuquén and southern Mendoza are controlled by several factors, such as: i- changes in the speed and direction of convergence between the oceanic plate (the Farallón and later Nazca plate) and the continental plate (South American plate); ii- the inclination change in the Wadati-Benioff zone; iii- the age of the subducted oceanic plate; and iv- the thickness of the overriding plate (Kay et al., 1988; Ramos and Barbieri, 1988). Noteworthy, the variation of such parameters produced different magmatic arcs, which formed during the following time intervals: Paleocene-Eocene, Oligocene, middle to late Miocene, late Miocene - Pliocene, and Fig. 1 - a) Left: satellite image, b) Right: map, northwestern Mendoza and southern Neuquén map with the location of the sites that were studied. A B C Fig. 2 - Photographs of chalcographic polished: A) titanomagnetite with exsolution to ilmenite, B) titanomagnetite altered to hematite, C) a: titanomagnetite to hematite at the edges, b: and c: hematite to pseudobrookite 76 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Pleistocene-Holocene. The volcanic centers are located between the precordilleran blocks which are separated by structures feature an extensional intra-arc setting during the Pliocene and Quaternary. The volcanism in the back-arc area, on the other hand, consists of extensive basaltic mantels, with volcanic cones that set either along fractures or dispersed with no apparent structural control (Bermudez et al., 1993). In general, they present low grade of chemical differentiation and significant compositional homogeneity. Fig. 3 - Results of the IRM studies and back-field sample locations of Arroyo Rucachoroy (Ar), Junin de los Andes (Ja) and Puesto Quiroga (Pq). Studies of basalt in northern Neuquen and southern Mendoza Systematic paleomagnetic samplings were carried out in various volcanic units cropping in north western Neuquén and southern Mendoza (Fig. 1), in order to establish a detailed chronology of the volcanic activity from the late Oligocene to the Pleistocene. The study area is located to the east of the current volcanic front, characterized by intra-arc and back-arc volcanism. Geochronological and paleomagnetic results from 11 localities are presented, which comprise the Rancahué Michacheo and Hueyeltué Formations, the informally “Basalto Zapala”, Basalto Macho Viejo, Basalto Los Mellizos and the “Chapualitense inferior”. These correspond to alkaline-type basalts, conformed by olivine and clinopyroxene phenocrysts immersed in a groundmass composed by plagioclase, clinopyroxene, olivine and opaque minerals (e.g. magnetite and hematite) microlites. In general, they present porphyric to sub-ophytic textures in intergranular groundmass, with olivine phenocrysts. The groundmass is mainly composed by euhedral plagioclase microlites arranged in a fluid way, together with small crystals of clinopyroxene somewhat altered to chlorites and opaque minerals, and scarce olivine crystals. Occasionally, plagioclase 77 GEOSUR2010 Fig. 4 - Hysteresis loops of samples from the localities of Arroyo Rucachoroy (Ar), Junín de los Andes (Ja) and Puesto Quiroga (Pq). HYS = without eliminating the contribution of paramagnetic minerals, SLO = with elimination of the contribution of paramagnetic minerals 22-23 NOVEMBER 2010 – MAR DEL PLATA cores look propylitized altered to chlorites and oxides. Polished sections studies allowed establishing the occurrence of titanomagnetites with exsolution to illmenite and in some cases to hematite. Likewise, hematite can be occasionally dismissal to illmenite, pseudobrookite and titanohematite (Fig. 2). From studies of acquisition of Isothermal Remanent Magnetization (IRM) and back-field of the IRM (Fig. 3), it was interpreted that the main carrier would be mainly magnetite and secondly, hematite. Hysteresis loops (Fig. 4) in ambient temperature and low temperatures (190°C) established on the other hand, that occasionally, samples yield Bcr (magnetic coercivity remanence) / Bc (coercivity field) ratios of ~ 4 that is indicative of mixtures, and relatively high Jr/Js rations =0.3, suggestive of a distributed mixture of MD (multi-domain) and PSD (pseudo-singledomain). In other cases, the Fig. 5 - Theoretical Day graph, (modified by Dunlop, 2002) for magnetite. References: SD = single domain, MD = Multiple Domain, SP = Super-paramagnetic. Mrs = saturation remanent magnetization, Ms = saturation magnetization; Hcr = remnant coercivity force, Hc = coercivity force. 78 GEOSUR2010 Fig. 6 - Magnetostratigraphic scheme, and geochronological correlation, between the studied area and the Geomagnetic Time Scale (Cande and Kent, 1995) 22-23 NOVEMBER 2010 – MAR DEL PLATA Jr/Js increases to 0,4 and up to 0,6 at low temperatures, which could indicate that particles would be in the PSD size range or correspond to a mixture of SD with superparamagnetic particles (Fig. 5). On the basis of magnetostratigraphic and geochronological correlations (Fig. 6), we interpret that the Rancahué Formation that yield a radiometric age of 13±1 Ma (39Ar/40Ar), recorded normal and reverse polarities (Re, 2008; Re et al, 2000). The phonobasalts of the Michacheo Fm on the other hand, bear a radiometric age of 17.7 ± 0.8 Ma,. The Tipilihuque Formation has a wide regional distribution. However, the Lonco Luan and Rahue (6.2±0.3Ma) sequences are not coeval with the Zapala Basalt that bears only normal polarity with a radiometric of 4.8±0.7 Ma in Cañadón Santo Domingo (Re, 2008; Re, et al. 2000). The Chapua Formation, assigned to the lower Chapualitense like the Zapala Basalt, bear a radiometric age of 2.8±0.6 Ma in the Barranca River, southern Mendoza (Re, 2008). The basalts cropping out in Cerro Bandera and Primeros Pinos that are assigned to the informal “Basalto Macho Viejo” unit, traditionally assigned to the Chapualitense cycle, are however upper Pliocene (1.7±0.3Ma) (Re, 2008). On the other hand, the basalts in Arroyo Covunco, are assigned to the Pleistocene based on their position between the Macho Viejo Basalt and the Los Mellizos Basalt cropping out in Portada Covunco and in Estancia Llamuco. Therefore, our results allow interpreting that the Michacheo and Rancahué Formations constituted the middle to late Miocene volcanic arc, and are the result of a change in the convergence angle with the South American margin after carrying from significantly oblique to practically orthogonal to it. This convergence style continued, with minor changes, in the Mio-Pliocene. During this time interval, many basaltic extrusions took place as part of this arc, such as the Tipilihuque Formation, Zapala, Macho Viejo and Los Mellizos Basalts. REFERENCES • Bermudez, A.; Delpino, D.; Frey, F.; Saal, A.; 1993. Los Basaltos de Retorarco Extrandinos. XII Cong. Geol. Arg. Geología y Recursos Naturales de Mendoza V. Ramos (Ed.), Relatorio, 1(13):161 172. • Dunlop, J.; Özdemir, Ö; 2002. Rock Magnetism: Fundamentals and forntiers. Cambridge University Press. , pp:573. UK. • Kay, S. M.; Maksaev, V.; Mpodozis, C.; Moscoso, R.; Nasi, D. C.; Gordillo, C. E.; 1988. Tertiary Andean magmatism in Argentina y Chile between 28-33 S: Correlation of magmatic chemistry with a changing Benioff zone. J. South American Earth Sciences 1:2138. 79 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA • Leanza, H.A.; Hugo, C., Repol, D.; González, R.; Daniela, J.; Lizuain, A.; 2005. Hoja Geológica 3969-I, Zapala. Instituto de Geología y Recursos Naturales, SEGEMAR, Bol. 275:1 128. Buenos Aires. • Ramos, V. A.: Barbieri, M.; 1988. El volcanismo Cenozoico de Huantraico: edad y dataciones isotópicas iniciales, provincia del Neuquén. Asociación Geológica Argentina, Rev. 43(2): 210-223. Buenos Aires. • Re, G. H.; 2008. Magnetoestratigrafías del NO argentino (entre 27° y 31°s) aplicadas al análisis de la deformación andina, y su relación con la subducción de la placa de nazca durante el cenozoico tardio). Doctoral Thesis (FCEyN-UBA) pp:278. • Re, G.H.; Geuna, S.E.; Lopez Martinez, M.; 2000. Geoquímica y geocronología de los basaltos neógenos de la región de Aluminé (Neuquén- Argentina). XI Congreso Geológico Chileno, Actas 2:62-66. Puerto Varas - Chile. AMPHIBOLE MEGACRYSTS OF THE CERRO JEU-JEPÉN PLUTON: NEW CONSTRAINTS ON MAGMA SOURCE AND EVOLUTION (FUEGIAN ANDES, ARGENTINA) 2-10 Ridolfi, F.1*, Renzulli, A.1, Cerredo, M.E.2, Oberti, R.3, Boiocchi, M.4, Bellatreccia, F.5, Della Ventura, G.5, Menichetti, M.1, Tassone, A.2 (1) Università degli Studi di Urbino “Carlo Bo”, Dipartimento di Scienze Geologiche, Tecnologie Chimiche e Ambientali, 61029 Urbino (PU), Italy (2) Universidad de Buenos Aires, Dpto. de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales. Ciudad Universitaria. Pabellón 2. CP - C1428EHA- Buenos Aires. Argentina (3) CNR-Istituto di Geoscienze e Georisorse, UOS Pavia, 27100 Pavia, Italy (4) Centro Grandi Strumenti, Università di Pavia, 27100 Pavia, Italy (5) Università degli Studi Roma Tre, Dipartimento di Scienze Geologiche, 00146 Roma, Italy. * Presenting author’s e-mail: [email protected] The Cretaceous plutons of Argentine Tierra del Fuego Island are characterized by mildly alkaline (shoshonitic) to high-K affinity (hereafter Fuegian Potassic Magmatism; González Guillot et al., 2009), with igneous intrusives ranging from ultramafic rocks to monzogabbro and monzonite/syenite (Cerredo et al., 2000, 2005, 2007; Peroni et al., 2009). By contrast the coeval intrusions of the South Patagonian Batholith (Nelson et al., 1988; Hervé et al., 2007) and the Beagle Channel Plutonic Group of the southern archipelago (Hervé et al., 1984) are characterized by a dominant subalkaline gabbro to granite trend. Among the intrusive bodies belonging to the Fuegian Potassic Magmatism, the Cerro Jeu-Jepén (CJJ) pluton is a small (< 10 km2), Late Cretaceous (93±4 Ma, Acevedo et al., 2000) intrusive body locat- Fig. 1 - (a) Pseudo-secondary fluid inclusions filling cracks and cleavage and (b) a multi-phase primary inclusion within amphibole megacrysts. 80 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - (a) K2O-SiO2 diagram showing the composition (and uncertainty bars) of the estimated melt in equilibrium with amphibole megacrysts (filled circles) and the shoshonitic trend of the CJJ whole-rocks (red arrow). (b) P-T plots for the various thermobarometric constraints (see text); uncertainty bars of amphibole (black) and clinopyroxene-liquid (red) thermobarometric methods are also given. ed at the SE tip of the Lago Fagnano which is stretched out along the inferred strike of the MagallanesFagnano Fault (MFF) system (Cerredo et al., 2005). Magnetic and gravimetric data suggest a prominent crystalline body in the subsurface (roots at depth of about 8 km; Cerredo et al. 2000), partially exposed at CJJ (ca. 55.6°S - 67.2°W). The shape and position of the CJJ pluton suggest that its emplacement was localized in a releasing bend of the MFF system (Tassone et al., 2005). CJJ is a composite intrusive plug made of basic (monzogabbro) to intermediate (monzodiorite, monzonite and syenite) rocks showing a shoshonitic affinity (Cerredo et al., 2000, 2005). Monzonite is the dominant lithology and shows poikilitic and patchy (perthitic) alkali feldspars (Or 8-88%) enclosing highlyzoned plagioclases (An 56-23%) and Fe-diopside clinopyroxenes (with aegirine molecula up to 4% at their rims), phlogopite and minor amounts of magnetite, titanite and apatite. A remarkable peculiarity of the studied monzonite is the widespread occurrence of amphibole megacrysts (up to 5 cm long) often showing corona reactions (made of phlogopite and clinopyroxene crystals) and cracks filled with crystalline materials. The megacryst core displays abundant pseudo-secondary and primary fluid inclusions (Fig. 1a and 1b, respectively), which also occur within larger Fe-diospide crystals. In order to unravel the physical-chemical conditions of crystallization of the CJJ amphibole megacryst and the whole evolution of the monzonite body, a comprehensive study based on EMP, single-crystal XRD, SREF, FTIR and thermobarometry, was carried out. A compositional and thermobarometric picture of the genesis and evolution of CJJ magmas was obtained with the application of several phase equilibria methods. EMP analyses and structure refinement of a portion of an amphibole megacryst show a magnesio-hastingsite composition, with a partial but significant oxocomponent (0.45 O2- apfu), which is completely balanced by the occurrence of Fig. 3 - Fourier transform infrared (FTIR) spectrum for a fluid inclusion in an Ti at M(1), a typical feature amphibole megacryst, showing the occurrence of CO2. 81 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA of mantle-megacrysts (e.g. Oberti et al., 2007; King et al., 1999). Single-crystal amphibole thermobarometry (Ridolfi et al., 2010) has been extended to high P-T with the supplemental collection of literature experimental amphiboles in equilibrium with both subalkaline and alkaline melts, at conditions up to 2200 MPa and 1100°C. A series of formulations to estimate P, T, H2Omelt (total content of H2O and CO2 dissolved in the melt), fO2 and the major anhydrous element oxide composition (i.e. SiO2, TiO2, Al2O3, FeO, MnO, MgO, CaO, Na2O, K2O) of the melt in equilibrium with amphibole, gives reasonably low uncertainties: 11% for P, 24°C for T, 0.5 wt% for H2Omelt, 0.17 log units for NNO (i.e. logfO2 – Ni-NiO buffer) and errors within 2.0-0.05 wt% for all the major oxides. Fig. 2 shows (a) the K2O-SiO2 diagram for the calculated basic liquids in equilibrium with the amphiboles and (b) the P-T conditions of amphibole crystallization. The H2Omelt was possibly very high and preliminary FTIR, EMP and textural analyses of the primary fluid inclusions indicate the presence of CO2 species (Fig. 3) and tiny (<3 Ìm) magnetite crystals. Oxygen fugacity was slightly above the NNO buffer and the application of GFluid (Zhang and Duan 2010) to the P-T-fO2 conditions obtained for the amphibole megacrysts (i.e. 959-1029°C, 1.2-2.5 GPa, ∆NNO 0-0.8), confirms the presence of CO2 and strongly indicates the stability of graphite which, together with magnetite, has been possibly entrapped within the tiny (<30 Ìm; Fig. 1) amphibole fluid inclusions. Figure 2b suggests that the crystallization of amphibole megacrysts occurred at mantle conditions, at the upper experimental limit of amphibole stability, during the uprising of the shoshonitic magma. The equilibrium between the melt (estimated from amphibole thermobarometry) and the core composition of fluid inclusion-bearing clinopyroxenes was tested by the thermobarometric equations of Putirka (2008). P-T conditions indicated by the clinopyroxene-liquid pairs (955-1019°C, 1.7-2.7 GPa) are consistent with those obtained by amphibole thermobarometry (Fig. 2b). Petrogenesis of CJJ intrusives involved fractional crystallization and magma mixing and EMP results of plagioclase and alkali feldspar pairs allowed to estimate, according to the two-feldspar thermometer of Putirka et al. (2008), the late-stage magmatic temperature of crystallization (665-750°C; Fig. 2b) of the shallow pluton. This study emphasizes the mantle P-T condition of crystallization of the CJJ magnesio-hastingsite megacrysts, most likely after small degree of partial melting of the peridotite source. In addition, the strong-zoning of monzonite plagioclase and clinopyroxene phenocrysts, and the disequilibrium corona textures around amphibole megacrysts in the studied CJJ monzonite suggest that the crystallization of the shoshonitic magma (which finally gave rise to the pluton at shallow crustal levels) followed a decreasing P-T path just above the upper limit of amphibole stability, probably promoted by transtensive movements along the MFF system. REFERENCES • Acevedo, R D., Roig, C.E., Linares, E., Ostera, H, Valín-Alberdi, M., Queiroga-Mafra, Z.M. 2000 La intrusión plutónica del Cerro Jeu-Jepén. Isla Grande de Tierra del Fuego, República Argentina. Cadernos Laboratorio Xeolóxico de Laxe, 25: 357-359. • Cerredo, M.E., Remesal, M.B., Tassone, A., Menichetti, M., Peroni, J.I., 2007. Ushuaia pluton: petrographic facies and geochemical signature. Tierra del Fuego Andes. International Geological Congress on the Southern Hemisfere (Geosur 2). Santiago. Chile. Libro de Resúmenes, p. 31. • Cerredo, M.E., Remesal, M.B., Tassone, A., Lippai, H. (2005). The shoshonitic suite of Hewhoepen pluton, Tierra del Fuego, Argentina. In: XVI Congreso Geológico Argentino, La Plata, Actas I, 539-544. • Cerredo, M.E., Tassone, A., Coren, F., Lodolo, E., Lippai, H. (2000). Postorogenic, alkaline magmatism in the Fuegian Andes: the Hewhoepen intrusive (Tierra del Fuego Island). In: IX Congreso Geológico Chileno, Puerto Varas, Actas 2, Simposio Nacional 2, 192-196. • González Guillot, M., Escayola, M., Acevedo, R., Pimentel, M., Seraphim, G., Proenza, J., Schalamuk, I. (2009). The Plutón Diorítico Moat: Mildly alkaline monzonitic magmatism in the Fuegian Andes of Argentina. Journal of South American Earth Sciences 28, 345-359. • Hervé F., Pankhurst R.J., Fanning C.M., Calderón M., Yaxley G.M.; 2007. The South Patagonian Batholith: 150 My of Granite Magmatism on a plate margin. Lithos 97, 373–394. • Hervé, M, Suárez, M. and Puig, A. 1984 The Patagonian Batholith S of Tierra del Fuego, Chile: timing and tectonic implications. Journal Geological Society, London, 141: 909-917 • King, P.L., Hervig, R.L., Holloway, J.R., Vennemann, T.W., Righter, K. (1999). Oxy-substitution and dehydrogenation in mantlederived amphibole megacrysts. Geochimica et Cosmochimica Acta 63, 3635-3651. • Nelson, E., Bruce, B., Elthon, D., Kammer, D., Weaver, S., 1988. Regional lithologic variations in the Patagonian Batholith. Journal of South American Earth Sciences 1, 239-247. • Oberti, R., Hawthorne, F.C., Cannillo, E., Cámara, F. (2007). Long-Range Order in Amphiboles. Reviews in Mineralogy and Geochemistry 67, 125-171. • Peroni, J: I.; Tassone; A. A., Menichetti,M., Cerredo, M. E. 2009. Geophysical modeling and structure of Ushuaia Pluton, Fuegian Andes, Argentina. Tectonophysics, 476,(3-4), 25:436-449 82 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES • Putirka, K.D. (2008). Thermometers and Barometers for Volcanic Systems. Reviews in Mineralogy and Geochemistry 69, 61-120. • Ridolfi, F., Renzulli, A., Puerini, M. (2010). Stability and chemical equilibrium of amphibole in calc-alkaline magmas: an overview, new thermobarometric formulations and application to subduction-related volcanoes. Contributions to Mineralogy and Petrology 160, 45-66. • Tassone, A.,. Lippai, H., Lodolo, E., Menichetti, M, Comba, A., Hormaechea, J.L., Vilas, J.F. (2005). A geological and geophysical crustal section across the Magallanes–Fagnano fault in Tierra del Fuego. Journal of South American Earth Sciences 19, 99-109. • Zhang, C., Duan, Z. (2010). GFluid: An Excel spreadsheet for investigating C–O–H fluid composition under high temperatures and pressures. Computers and Geosciences 36, 569-572. “CIRCULAR FEATURES” ON OLD SOLIDIFIED LAVA FLOW FIELDS ASSOCIATED WITH SOME YOUNG SCORIA CONES FROM LLANCANELO AND PAYÚN MATRU VOLCANIC FIELDS, MENDOZA PROVINCE, ARGENTINA 2-11 Risso, C.1*, Nemeth, K.2, Nullo, F. 3, Inbar, M.4 (1) Dept. de Ciencias Geológicas-Universidad de Buenos Aires (2) Massey University, New Zealand (3) CONICET-SEGEMAR (4) Dept. of Geography, University of Haifa, Israel * Presenting author’s e-mail: [email protected] The Plio-Pleistocene Llancanelo and Payún Matru Volcanic Fields (LPMVF) are two broad back-arc lava plateau with hundreds (more than 800) of monogenetic scoria cones that cover an extensive area behind the active Andean volcanic arc. Volcanic activity of these fields started at the beginning of the Pliocene, and probably continued until the last millennium with three main peaks of volcanic activity occurring at 3.6-1.7 Ma, c. 450 Ka, and in the Holocene; equivalents to the Chapúa, Puente and Tromen Formations/Groups. Basalts of Chapúa Formation (Nullo, 1985), or Chapúa Group (Bermúdez et al., 1993) are of a glossy black color, with texture that goes from vesicular on top to dense, non-vesicular at it’s base. Extensive, fresh-looking lava flows are predominantly pahoehoe type with subordinate “aa” type, and in the western part of the volcanic field with tumuli, ropy lava and occasional lava tubes and skylights. The source of these extensive lava flows is unknown. They have been formed by successive overlapping (37-40 meters depth) of individual flow units and form the depositional and palaeo surface upon which most of the new and younger volcanoes (Pencoso, Colorado, etc.) formed and deposited their eruptive products. Overlying the previous rock units is the Puente Formation (Nullo, 1985) or Puente Group (Bermúdez et al., 1993), which also consists of extensive lava flows related to volcanic cones still recognizable. Volcanic activity in the LPMVF was primarily of Strombolian and Hawaiian type, resulting in scoria and/or lava spatter cones. Cone deposits are coarse-grained and commonly consist of red, scoriaceous lapilli beds with meter-sized ballistic bombs and blocks. Large vesicular, spindle shaped lava bombs and blocks as well as bread crusted bombs and blocks up to 3.5 m in diameter, are common. In both lava fields, we observe some special “circular pattern” on the present surface. It seems they formed in the previous lava flows and surrounding the younger volcanic cones (Fig. 1a,b,c,d,e,f, and g in white) like: Colorado, las Bombas, Pencoso, etc. The diameter of these “circular feature” is around 1000 m (Table 1). A possible explanation about this curious feature is given in Fig. 2. In Fig. 2.1 we can presume the Hawaiian-Strombolian style explosive eruptions that occurred between 3.6-1.7 Ma with the formation of pyroclastic cones and successive lava flows. During a long period of erosion (Fig. 2.2) intense eolian and fluvial processes removed significant portion of these cones. Circa 450.000 years BP a new batch of magma was rising, causing the fracturation of the older solidified lava succession (Fig. 2.3). Fig. 2.4 and 5 mark the explosive disruption that we infer to have 83 GEOSUR2010 Fig. 1 - Circular feature in different volcanoes of LPMVF. a: unnamed volcano with “circular feature” in Payun Matru Volcanic Field. b: Pencoso volcano in Llancanelo Volcanic Field. c: unnamde volcano feature south of Colorado volcano in Llancanelo Volcanic Field. d: Colorado volcano in Llancanelo Volcanic Field. e: Las Bombas volcano in Llancanelo Volcanic Field. f: Same as c. g: Front view of oldest lava flows in Colorado volcano. 22-23 NOVEMBER 2010 – BUENOS AIRES been occurred due to the sudden degassing of the rising magma in near surface which potentially opened up new fractures and gravitationally destabilised the older solidified lava flows. Perhaps we cannot rule out an initial phreatomagmatic explosive event triggered by the explosive interaction between magma and water that was trapped between solidified lava flow units. Such process has been interpreted to be the mechanism of the formation of few phreatomagmatic volcanoes like Carapacho volcano in the LPMVF (Risso et al., 2008). Unfortunately there is no direct evidence such as preserved phreatomagmatic pyroclastic deposit that could support to constrain better such model. When the fracturing, decompression and sudden discharge of fractured blocks due to explosion ended, the new magma has built a new cone in the newly formed depression on the solidified lava surface (Fig. 2.6). The deposits produced prior the magmatic eruptions are not preserved in the circular zones around the cone and the old solid flows, perhaps due to extreme arid and windy conditions. A new period of degradation of the cone by climatic forces take place (Fig. 2.7) leading to the present day scenario with a scoria cone with a reduced size of cone heights range between 20 and 100 meters and slope angles much lower (Table 1) than young scoria cones of 32-35º. The modification of the cone geometry (height and width) over time opened a gap between the cone and the old lava flows allowing to function as a small sedimentary basin around the cone collecting aeolian as well Fig. 2 - Geological sketch (out of scale) about development of “circular features” during time. 84 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 Name Alco (m) Hco (m) Wco (m) Hco/ Wco Slope (degrees) Dcr (m) Circular feature (m) Pencoso 1505 53 552 0.09 19 235 1123 Las Bombas 1485 33 330 0.1 10 129 1120 Colorado 1550 38 520 0.07 12 — 1341 Jarilloso 1558 97 1059 0.09 20 240 2670 186-15 1575 60 550 0.11 19 125 1134 186-40 1810 113 700 0.16 10 164 1227 n/n 1 1524 21 280 0.07 11 — 570 n/n 2 1475 20 350 0.05 12 170 1180 as sheet wash deposits from the new cone (Fig. 2.8). From the air, these depressions look circular and surrounding relatively young scoria cones as it can be seen in Fig. 1. REFERENCES • Nullo, F., (1985). Hoja Geológica Cerro Campanario, SEGEMAR. 1:250.000. Unpublished. • Bermúdez, A., Delpino, D., Frey, F. and Saal, A., (1993). Los basaltos de retroarco extraandinos. In:Ramos, V., (Ed.) Geología y Recursos Naturales de Mendoza. XIIº Congreso Geológico Argentino y IIº Congreso de Exploración de Hidrocarburos, Mendoza. Relatorio, I (13), 161-172. • Risso,C., Németh, K., Combina, A.M., Nullo and F., Drosina, M., (2008). The role of phreatomagmatism in a Plio-Pleistocene highdensity scoria cone field: Llancanelo Volcanic Field (Mendoza), Argentina. Journal of Volcanology and Geothermal Research, 169 (1-2): 61-86. THE NEOGENE BARRIL NIYEU VOLCANIC COMPLEX. SOMÚN CURÁ MAGMATIC PROVINCE. NORTHERN EXTRA ANDEAN PATAGONIA. ARGENTINA 2-12 Salani, F.M.*, Remesal, M., Cerredo, M.E. CONICET – Universidad de Buenos Aires * Presenting author’s e-mail: [email protected] The mainly Oligocene (25-28Ma) Somún Curá basaltic plateau covers about 25000 km2 in the north of Extra Andean Patagonia (Argentina, Fig. 1). Several eruptive centers of different complexities were built over the basaltic plateau during Late Oligocene-(Early) Miocene which, according to the wide compositional and eruptive diversity may be grouped in two main types: a) large bimodal complexes and b) monogenetic eruptive centers. The former are characterized by their alkaline nature associated with a typical gap in the ~ 53-60% SiO2 range. Most of the large bimodal complexes are aligned along a WNW-ESE trending belt (Fig. 1). A prominent transcurrent fault of roughly WNW-ESE orientation was identified offshore the northern Extra Andean Patagonia (Urien and Zambrano, 1996), where it bounds the Cenozoic Valdez and Rawson basins. On land, the Telsen fault (Ciciarelli, 1990), and further W the alignment of large bimodal complexes, might represent the continuation of this major onshore-offshore wrench structure. From NW to SE: Agua de la Piedra, Pire Mahuida, Barril Niyeu, Talagapa, Apas and Telsen volcanic complexes (Salani et al, 2008) form a fairly well defined mountain chain which cuts across the main Somún Curá plateau (Fig. 1). We interpret that the emplacement of these bimodal complexes was controlled by transtensive movements along the Telsen-Valdez fault system. 85 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Location map and geological sketch displaying Somún Curá plateau and post-plateau bimodal complexes; box highlights the Barril Niyeu Volcanic Complex. The most voluminous post-plateau volcanic field is the Lower Miocene (20-18 Ma) Barril Niyeu Volcanic Complex (BNVC; Remesal et al, 2001), which was built through several stages of both effusive and explosive activity outpoured from at least five emission centres (presently cauldrons, ranging from 2 to 5 km in diameter) and of distinct trachyte/rhyolite and basaltic compositions. The final basaltic stage is dominated by lavas, although minor breccia and clastogenic facies and spatter cones also occur. Local variations from this general stratigraphic scheme exist, which suggest a certain synchronism of different composition events related to distinct vents. The bulk of the complex is formed by porphyric trachyte lavas (bearing sanidine and aegirine, aegirine/augite phenocrysts set in a trachytic groundmass) and comendite or quartz-trachyte lava domes within the cauldrons. Although several trachyte groups were dintinguished (Salani et al., 2006), all share a peralkaline character, as is common in other post- Somun Curá plateau Neogene complexes. They show > 10% alkali content (Fig. 2), high Nb/Y = 2.6 -4.2, (La/Yb)N (~15-16) and Eu negative anomaly mirroring feldspar fractionation. The explosive volcanic facies is represented by two mesosiliceous to acid, and subordinate basic pyroclastic episodes. The earlier one includes fall-out mainly plinian (lesser strombolian) and pyroclastic flow deposits. This episode was partially contemporaneous with the deposition of primary and reworked Andean-related pyroclastics of the Miocene Sarmiento Group. Sometimes these deposits are interlayered with trachyte and basaltic flows. The later pyroclastic stage is characterized dominantly by ignimbrites that commonly overlay trachyte lavas and, to a smaller extent, the older pyroclastics flows. Rocks are composed of crystals (30-20%) of sanidine, embayed quartz, with subordinated zircon and opaque minerals; lithic fragments (<10%) of trachyte and basaltic volcanics. Vitroclastic components are pumice and devitrified glass shards. Trydimite occurs as vapour phase crystallization. The deposits show different welding degrees which, in many cases, produce eutaxitic textures. Strombolian deposits are lapilli and block size, mainly composed of brown glassy patches, porfiritic pumices with olivine crystals, and basaltic lithic fragments. The pyroclastic rocks show alkaline and subalkalic affinities, (La/Yb)N ratios (6-13) and strong negative Ba, Sr, P, Eu and Ti anomalies. Basaltic lavas mainly cover the northern area of the Complex and a minor facies appear in the southwest side. Petrographic and geochemical characteristics distinguish three main basaltic groups: transitional basalts, alkaline basalts and trachybasalts (Fig. 2); the Fig. 2 - SiO2 vs. total alcalis (TAS) Le Maitre et former correspond to porphyric lavas with olivine al. (1989). Data from Remesal et al (submitted), and plagioclase phenocrysts which display the lowest Salani et al. (2006) and Remesal et al. (2008). 86 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA (La/Yb)N ratio (9.6). The alkaline basalts are mainly porphyritic lavas bearing plagioclase, olivine and titanoaugite phenocrysts; intermediate (La/Yb)N ratios (12-14) characterize this group. The last eruptive stage of the BNVC corresponds to the trachybasalts which appear either as lava flows or as spatter cones; rocks display aphanitic textures and the highest (La/Yb)N ratios (14-18) of the basic rocks. According to Th/Yb vs. Ta/Yb ratios, Fig. 3 - 143Nd/144Nd vs. 87Sr/86Sr of CVBN. DM and EM1 the basaltic rocks show an intraplate components are from Faure (2001); solid squares: new data for character (Pearce, 1982, 1983). The BNVC, hollow squares: data of trachybasalts likely forming part of differentiated rocks also fit within BNVC (from Kay et al. 2007). plate field in the Gorton and Schandl (2000) diagram with low ratios Th/Ta (1-6). Nevertheless, some interelemental ratios (i.e. high Ba/La > 20) in the basaltic rocks suggest the contribution of a subduction zone component. Simple mixing models that combine 90–85% of a depleted mantle end-member (DM) with 5–10% of a type 1 enriched mantle (EM1) approach the 87Sr/86Sr 20 Ma ratios (0.704085-0.704384) and 143Nd/144Nd 20 Ma (0.512733-0.512652), eNd 20 Ma (0.8 to 2.4) of the BNVC basalts. A recent seismic tomography survey carried on across the North Patagonian Massif revealed a Paleogene subduction gap due to Aluk plate breakoff (Aragón et al., 2009). The pacific margin of the South American plate was dominated by transcurrent motion during Paleogene times (Somoza and Ghidella, 2005) until ~25 Ma when the breakup of the Farallón plate caused a major change from highly oblique to near-normal convergence along the Andean margin (Somoza, 1998).The Somún Curá magmatic province evolved within this regional geodynamic scenario. The BNVC (as well as other Early Miocene bimodal complexes) was built when a dramatic plate reorganization process had taken place along the pacific margin. Although the subduction regime was restablished by Late Oligocene times, the tectono-thermal flux in the supra-subduction zone was active only from Middle Miocene onwards as indicated by magmatic activity in the Patagonian Batholith (Aragón et al., 2009). We propose that the partial melting of the Patagonian asthenosphere was triggered both by this drastic geodynamic change as well as by the dehydration/partial melting processes underwent by foundered Aluk plate. Therefore, the arc-like signatures of some inter-elemental ratios would be related to the detached slab and not to the influence of a contemporaneous arc. This contribution was possible due to the financial support of Universidad de Buenos Aires (UBACYT-X185). REFERENCES • Aragón, E., Spakman, W. Brunelli, D., Rivalenti, G., D`Eramo G. D`Eramo, F., Pinotti, L., Rabbia , O. Cavarozzi, C., Aguilera, and Ribot, A., Mazzucchelli, M., 2009. El gap de subducción Paleógeno del segmento patagónico 35º-44º. XIV Reunión de tectónica. Abstracts: 53, Río Cuarto. Córdoba, Argentina. • Ciciarelli, M. 1990. Análisis estructural del sector oriental del Macizo Nordpatagónico y sus significado metalogenético. Tesis doctoral, Facultad de Ciencias Naturales y Museo. Universidad Nacional de La Plata. 155 pp. • Faure, G. 2001. Origin of Igneous Rocks. The isotopic Evidence. Springer. 496 p. • Gorton, M. P., Schandl, E. S., 2000. From Continents to Island Arcs: A Geochemical Index of Tectonic Setting for Arc-Related and Within-Plate Felsic to Intermediate Volcanic Rocks. The Canadian Mineralogist, 38: 1065-1073. • Kay, S. M., Ardolino, A. A., Gorring, M.L. and Ramos, V. 2007. The Somuncura Large Igneous Province in Patagonia: interaction of a transient mantle thermal anomaly with a subducting slab. Journal of Petrology, 48 (1): 43-77. • Le Maitre, R.W, Bateman, P., Dudek, A., Keller. J., Lameyre, J., Le Bas, M.J., Sabine, P.A., Schmid, R., Sorensen, H., Strekeisen A., Woolley, A.R, Zanettin, B. 1989. A classification of igneous rocks and glossary of terms. Blackwell, Oxford. • Pearce, J.A. 1982. Trace element characteristics of lavas from destructive plate boundaries. En Andesites: Orogenic Andesites and Related Rocks (R.S. Thorpe, ed.). John Wiley and Sons, Chichester, U.K, 525-548. • Pearce, J.A. 1983. Role of the sub-continental lithosphere in magma genesis at active continental margins. En Continental Basalts 87 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA and Mantle Xenoliths (C.J. Hawkesworth and M.J. Norry, eds.). Shiva Press, Nantwich, U.K, 230-249. • Somoza, R. 1998. Updated Nazca (Farallón)–South America relative motions during the last 49 m.y.; implications for mountain building in the Central Andean region. Journal of South American Earth Sciences 11, 3211–3215. • Somoza, R., M.E. Ghidella, 2005. Convergencia en el margen occidental de América del Sur durante el Cenozoico: subducción de las placas de Nazca, Farallón y Aluk, Revista de la Asociación Geológica Argentina, 60 (4): 797-809. • Remesal, M., F. Salani, M. Franchi and A. Ardolino, 2001, Hoja Geológica 4169-IV, Maquinchao. Provincia de Río Negro. Instituto de Geología y Recursos Minerales, Servicio Geológico Minero Argentino. Boletín Nº 312: 1- 72. Buenos Aires. • Remesal, M, Salani, F. M. and Cerredo, M. E., 2008. Los basaltos del Complejo Volcánico Barril Niyeu, en 9 º Congreso de Mineralogía y Metalogenia, San Salvador de Jujuy, Argentina, Asociación Mineralógica Argentina, 265-270. • Remesal, M. , Salani, F. and Cerredo, M.E. Submitted. Petrología y Evolución Del Complejo Volcánico Barril Niyeu, Patagonia Argentina. Revista Mexicana de Ciencias Geológicas . • Salani, F.; M. Remesal and M. E. Cerredo. 2008. Somun Curá post- plateau stage: large bimodal complexes, northern Patagonia. Argentina. IAVCEI. Reykjavík, Iceland. • Salani, F. M.; M. B Remesal and M. E. Cerredo, 2006. Las Rocas Traquíticas del Complejo Volcánico Barril Niyeu. . 8º Congreso de Mineralogía y Metalogenia. Buenos Aires, Acta: 427-434. • Urien, C. M. and Zambrano, J. J. 1996. Estructura del margen continental. XIII Congreso Geológico Argentino and III Congreso de Exploración de Hidrocarburos. Geología y Recursos Naturales de la Plataforma Continental Argentina. Ramos, V. and Turic, M. eds.: 29-65. THE MAGNETIC SUSCEPTIBILITY OF IGNIMBRITES FROM 2-13 THE ALTIPLANO- PUNA VOLCANIC COMPLEX, CENTRAL ANDES: A USEFUL TOOL TO DISTINGUISH LITHOMAGNETIC DOMAINS ACROSS THE ARC Singer S.E.* INGEODAV (Instituto de Geofísica Daniel A. Valencio), Dpto. Ciencias Geológicas, FCEyN (UBA) Ciudad Universitaria, Pab. II.1428 Buenos Aires, Argentina. * Presenting author´s e-mail: [email protected] Silicic volcanism in the Andean Central Volcanic Zone originated one of the world´s largest Neogene ignimbrite provinces. Between 21° and 24° S, the Altiplano - Puna Plateau shows a concentration of predominantly dacitic ignimbrites that constitute the Altiplano- Puna Volcanic Complex. Their compositions and huge erupted volumes suggest they originated from large scale crustal melting. Moreover, geophysical evidence presently indicates the occurrence of partial melting zones in the middle crust beneath the plateau and very high heat flow values. On the other hand, the bulk susceptibility of a rock represents the addition of the susceptibilities of all minerals that are present in a sample, although the magnetite is present even in small amounts it will control the magnetic properties due to its high magnetic susceptibility. As a consequence, bulk susceptibility of a rock gives us a very first idea of the amount of magnetite which is in that rock. In agreement, susceptibilities of different rock types give rise to the concept of lithomagnetic domains. For our study, measurements of magnetic susceptibility were carried out, together with microscopic observations of Fe-Ti oxides of Late Miocene ignimbrites in the Central Andes (22°S - 23°S) located in the forearc and retroarc. The main goal of this project is thus to study the variations in susceptibilitiy values as well as the creation, alteration and breakdown of the magnetite in these rocks and subsequently, to establish a probable link between susceptibility and process. The analysis of the data shows two clearly defined lithomagnetic domains, one paramagnetic located in the retroarc and the other ferromagnetic situated in the forearc whose susceptibility values vary in an order of magnitude. These results indicate different contents of magnetite in the ignimbrites that, combined with the different mineralogical assemblages, suggest that such variations could be explained by two types of magmas. On the basis of these results, some interesting challenges arise: Study the contribution from the serpentinized forearc mantle to the composition of the magmas in the forearc, taking into account that quantitative models based on gravity and aeromagnetic anomalies 88 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA suggest that the magnetic mantle may be common in forearc settings. Although it is still hot matter of debate, arc basin magmas are produced by partial melting of an asthenospheric mantle source that has been metasomatized by the addition of one or more slab components. The key topics are the nature of the metasomatizing agent(s), their detailed chemical composition and mass fraction, and the relative proportion of subducted sediment and altered oceanic crust in the source of the metasomatizing agent. On the other hand, models of the thermal structure of the mantle wedge and subducting slab generally conclude that slab-surface temperatures are too low for melting to occur, except in cases where the subducting slab is exceptionally young and warm. However, new models predict in slab-surface, temperatures that are 100°C – 300°C hotter, with the implication of slab melting. Thus the main challenge that remains is to test the contributions from the slab, particularly if we consider the anomalous thermal state of the region. Finally, to integrate the “exotic” iron-oxide rich lava flows at El Laco (23°48’S, 67°30’W) within the regional tectonic framework moreover taking also into account the temporal coincidence with the dacitic ignimbrite Cerro Galán that heralds the beginning of a new flare-up in the Southern Puna. In other words, we argue that these exotic lava flows could actually be one of the key issues to the understanding of the magnetic magmatic processes in the region. FIRST KIMBERLITE PIPE IN CENTRAL YAKUTIA (RUSSIA): 2-14 MINERAL COMPOSITION AND THICKNESS OF LITHOSPHERIC MANTLE AND AGE Smelov, A.P.1*, Zaitsev, A.I.2, Ashchepkov, I.V.3 (1) Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, Yakutsk, Russian Federation (2) Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, Yakutsk, Russian Federation (3) Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russian Federation * Presenting author´s e-mail: [email protected] Introduction Active diamond prospecting, which was initiated by scientific forecasts (Sobolev, 1951) and done within the Siberian platform in the late 1940s and 1950s, revealed commercial diamondiferous kimberlites in Western Yakutia. Comprehensive studies of kimberlites, their mantle xenoliths and diamonds, yielded important information on the origin of kimberlites, the structure and composition of the lithospheric mantle parageneses and formation conditions of natural diamonds. In Central Yakutia, kimberlite and diamond prospecting has become more active since 2000. That year chromian pyropes were found in a locality with known “pipelike” geochemical anomalies (Protopopov, 1993) in the modern alluvium of the Kengkeme and Chakyya Rivers (Lena Basin) and high-chromium spinellids of diamond association, with up to 68 wt % Cr2O3 (Izbekov et al., 2006; Okrugin et al., 2007), were found in the Menda Basin. These findings, combined with structural data, allowed to suggest kimberlite magmatism in that area. Drilling of the geophysical anomalies carried out by geologists from Yakutskgeologiya in 2007 – 2008 revealed the first kimberlite pipe (Fig. 1). We studied the composition of the rocks that conform the Manchary pipe and the mantle-derived minerals contained in them (Smelov et al., 2009, 2010). The focus of study was to determine the characteristics of the kimberlites, to estimate their diamond potential and to obtain preliminary information on the composition of the lithospheric mantle substrate. Indicator minerals composition from the pipe under study were compared with those of alluvium samples from some of streams of the area. This comparative analysis provides reasons to estimate objectively the possibility of the occurrence of other kimberlites in the area. 89 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Tectonic sketch of the North Asian craton (Smelov and Timofeev, 2007) showing the kimberlite magmatism. 1- Siberian platform; 2 - subsided craton margin—fold and thrust belts (ST, South Taimyr; EA, East Angara; BP, Baikal–Patom; VR, Verkhoyansk); 3 - Precambrian basement (shields and uplifts); 4 - Mesozoic volcano-plutonic belt; 5 - kimberlite fields; 6 - Manchary kimberlite pipe; 7 - sites with kimberlites minerals (Izbekov et al., 2006; Okrugin et al., 2007); 8 - “pipelike” geophysical anomalies (Protopopov, 1993). Close up of Yakutsk area. 1-2 deposits: 1 Cenozoic; 2 - Mesozoic. Geological setting The Manchary pipe is located in the Tamma basin (the right tributary of the Lena) 100 km southward of Yakutsk. It was explored by three drill holes up to 150 - 170 m depth. It breaks through Upper Cambrian carbonate deposits and is overlain by Jurassic terrigenous masses. Mineral composition The pipe consists of a greenish-gray kimberlite breccia with a massive serpentine-micaceous cement. The rocks in the upper parts are mudded to varying degrees. Kimberlite breccia typically hosts serpentinites, micaites and micaceous and garnet serpentinites up to 25 cm in size. They display porphyric texture due to the presence of serpentinized olivine, phlogopite, picroilmenite, and garnet phenocrysts (macrocrystals). Serpentinite inclusions and macrocrystals are highly resorbed. Phlogopite and picroilmenite macrocrystals often occur as fragments with fine-grained serpentinemagnetite rims. The rock texture in these areas resembles autolithic texture. Both the chemical analysis of the least altered rocks from the pipe without xenogenic matter and the petrographic and mineralogical results confirm their kimberlite nature. Most of the samples are typical noncontaminated kimberlites, according to their SiO2 (20-35 wt.%) and Al2O3 (5 wt.%) contents. Based on the ratio of CaO/(CaO + MgO) to SiO2/MgO and depending on the degree of secondary alteration, the pipe rocks fall into three petrochemical categories: magnesian kimberlites, kimberlites s.s., and carbonatite kimberlites. 90 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA The main kimberlite-forming minerals are serpentine, carbonates, and phlogopite. The groundmass has a significant content of ore minerals – aggregates of ferro- and chromian spinels, perovskite, magnetite and minor magnesian chromian magnetite. Serpentine forms greenish-gray ribbon-like lamellar aggregates or fibrous foliated aggregates after olivine. Relictic olivine is recognized in the least altered kimberlite at 141146 m depth. Porphyry segregations of serpentinized olivine content vary from 20% in fine-grained kimberlites up to 45% in coarse-grained ones. Phlogopite macrocrystals (8x?6 mm), which conform up to 5-10% of the rock occur as nodules or xenomorphic Fig. 2 - TP estimates for the mantle culumn beneath the Manchary segregations with traces of deformation kimberlite pipe. 1-2 Gr thermobarometry (1 - Ashepkov et al., 2008; and resorption. Phlogopite crystals are Krogh, 1998. 2 - Ashepkov et al., 2008; McCammon et al., 2001); 3 chloritized and surrounded by Ilm thermobarometry (Taylor et al., 1998); 4 - Sp thermobarometry reaction rims composed by fine (Ashepkov et al., 2008; O’Neill and Wall,1987). aggregates of ore minerals. They host idiomorphic grains of perovskite and titanomagnetite. It is rich in MgO and Al2O3, and almost depleted in TiO2. According to the Fe-Ti ratio, the macrocrystals and groundmass phlogopites belong to the reaction type of micas from metasomatized garnet-spinel peridotites, as well as from metasomatized ilmenite peridotites and pyroxenites. The micas of the binder mass show BaO contents up to 8.0 wt.%. Spinelloid macrocrystals include both high-titanium (TiO2 >1.0 wt.%) and low-titanium (TiO2 < 1.0 wt.%) types. According to the ratio of Fe2/(Fe2 + Mg) to Cr/(Cr + Al), two groups of spinelloids form a kimberlite trend. Picroilmenite macrocrystals, 2 to 5 mm in size, have irregular, baylike boundaries. They have high MgO contents (8.2 to 11.5 wt.%) and reaction rims consisting of fine grains of perovskite, ferrospinels, and magnetite. According to their MgO/TiO2 ratios they plot along the “kimberlite trend” of ilmenites. Pyropes show uneven distribution in kimberlite breccia. Most pyrope grains are sharp-edged fragments. Chemically, they are of lherzolite, wehrlite, or nondiamondiferous dunite-harzburgite parageneses. We did not find pyropes typical of deep-seated xenoliths of deformed peridotites from the Udachnaya pipe. Neither we did find high-chromium subcalcic pyropes analogous to diamond inclusions, which could show the diamond potential of those kimberlites. Garnets corresponding to lherzolites of anomalous composition (Tychkov et al., 2008) conform up to 8% of the rock; this is close to the garnet content of Middle Paleozoic kimberlites from the Yakutian province. However, eclogitic garnets were not found in the Manchary pipe, and this is not typical of the kimberlite breccias. Thickness of lithospheric mantle P-T crystallization parameters were estimated using monomineralic garnet thermometers as well as choromite and ilmenite thermobarometers (Fig. 2). From chemical composition of garnets we established a conductive geotherm (35 mw/m2) typical of the mantle beneath the Paleozoic pipes in Yakutia, and a layered structure of the mantle column to a depth of 230 km (70 kbar) consisting of 8 intervals. In the middle part, they are separated by a 50-40 km thick horizon (35-65 kbar) with little garnet. The horizon is supposed to have a pyroxenite composition. That structure of lithosphere is 91 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA typical of the Arkhangelsk diamondiferous province and the Nakyn kimberlite field. P-T parameters estimated by chromite and ilmenite geothermobarometry display a high-temperature line which is due to the heating of mantle rocks by protokimberlitic melts. In general, the estimated P-T parameters and geotherms show that lithosphere in Central Yakutia has thicknesses of a mantle root corresponding to the diamond stability field. Conclusions Petrographic, mineralogical, and geochemical studies confirm the presence of a kimberlite breccia pipe near Yakutsk. It resembles Middle Paleozoic kimberlite breccias from the Yakutian province in geological position and the composition of pyropes. Chemical composition of pyropes and chrome spinellids from Manchary pipe kimberlites is similar to that from marginally diamond - bearing Middle Paleozoic kimberlites in the north of the Yakutian province, for example, in Middle Olenek area (Smelov et al., 2010). The first Rb-Sr isochrone of unaltered kimberlites yielded an age of 359±50 Ma (Io=0.7052), which is close to that of diamond - bearing kimberlite pipes from the Yakutian province. A comparative analysis revealed a significant compositional difference between pyropes from the Manchary pipe and those from modern fluvial alluvium. Consequently, the rocks of the pipe could not be a source of pyropes for this alluvium. Many “pipelike” geophysical anomalies allow forecasting a new kimberlite field in Central Yakutia. REFERENCES. • Ashchepkov I. V., Pokhilenko N. P., Vladykin N.V. et al.; 2008: Reconstruction of mantle sections beneath Yakutian kimberlite pipes using monomineral thermobarometry. In: Geological Society, London, Special Publications, 293, pp.335-352. • Izbekov E.D., Pod’yachev B.P. and Afanas’ev V.P.; 2006: Signs of symmetric diamond concentration in the eastern Siberian Platform (relative to the Vilyui syneclise axis) - Dokl. Earth Sci., 411A (9), 1339-1340. • Krogh E.J.; 1988: The garnet-clinopyroxene Fe-Mg geothermometer a reinterpretation of existing experimental data - Contrib. Minera. Petrol., 99, 44-48. • McCammon C.A., Griffin W.L., Shee S.R. et al.; 2001: Oxidation during metasomatism in ultramafic xenoliths from the Wesselton kimberlite, South Africa: implications for the survival of diamond - Contrib. Mineral. Petrol., 141( 3), 287-296. • Okrugin A.V., Belolyubskii I.N., Oleinikov O.B., et al.; 2007: Kimberlitic indicator minerals from Kengkeme River alluvium within the Yakut uplift - Nauka i Obrazovanie, 4, 17-23. • O’Neill, H. St. C. and Wall, V. J.; 1987: The olivine orthopyroxene-spinel oxygen geobarometer, the nickel precipitation curve, and the oxygen fugacity of the Earth’s upper mantle - Journal of Petrology, 28, 1169-1191. • Protopopov Yu.Kh.; 1993: Tectonic Complexes of the Platform Cover of the Vilyui Syneclise [in Russian]. Izd. Yakut. Nauchn. Tsentr, Sib. Otd. Ross. Akad. Nauk, Yakutsk. • Smelov A.P., Timofeev V.F.; 2007: The age of the North Asian Cratonic basement: An overview - Gondwana Res., 12, 279-288. • Smelov A.P., Ashchepkov I.V., Oleinikov O.B. et al.; 2009: Chemical composition and P-T conditions of the formation of barophilic minerals from Manchary kimberlitic pipe (Central Yakutia) - Otechestvennaya Geologiya, 5, 27-30. • Smelov A.P., Andreev A.P., Altukhova Z.A. et al.; 2010: Kimberlites of the Manchary pipe: a new kimberlite field in Central Yakutia - Russian Geology and Geophysics (Geologiya i Geofizika) 51 (1), 121-126. • Sobolev V.S.; 1951: Geology of Diamond Deposits of Africa, Australia, Borneo Island, and North America [in Russian]. Gosgeoltekhizdat, Moscow. • Taylor W.L., Kamperman M. and Hamilton R.;1998: New thermometer and oxygen fugacity sensor calibration for ilmenite amd Crspinel- bearing peridotite assemblage. In: 7th IKC Extended abstracts. pp. 891. • Tychkov N.S., Pokhilenko N.P., Kuligin S.S. et al.; 2008: Composition and origin of peculiar pyropes from lherzolites: evidence for the evolution of the lithospheric mantle of the Siberian Platform - Russian Geology and Geophysics (Geologiya i Geofizika) 49 (4), pp. 225-239 (302-318). 92 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA DEPOSITION AND REWORKING OF PRIMARY PYROCLASTIC DETRITUS IN PUESTO LA PALOMA MEMBER, CRETACEOUS CERRO BARCINO FORMATION, SOMUNCURÁ-CAÑADÓN ASFALTO BASIN, PATAGONIA, ARGENTINA 2-15 Umazano, A.M.* INCITAP (CONICET-UNLPam); Av. Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina * Presenting author’s e-mail: [email protected] The Puesto La Paloma Member (PLPM) constitutes the basal section of the Cretaceous Cerro Barcino Formation (Chubut Group), which was accumulated during sag conditions in the Somuncurá-Cañadón Asfalto Basin, Patagonia, Argentina. The PLPM overlies Los Adobes Formation and is covered by Cerro Castaño Member. It is composed of greenish, pyroclastic-rich strata, which were interpreted as continental deposits without further details. The aim of this contribution is to analyze the sedimentary paleoenvironments of the unit with emphasis in the recognition of reworking processes of the primary pyroclastic deposits. The PLPM was studied in four localities of the western sector of the basin within the Chubut province; from NW to SE they are: Cerro Los Chivos (43º13’10’’ S, 68º56’43’’ W; 42 m thick), Estancia La Payanca (43º29’43’’ S, 68º52’31’’ W; 20 m thick), Estancia La Madrugada (43º35’20’’S, 68º56’32’’W; 19 m thick) and Estancia La Juanita (43º48’2’’ S, 68º52’15’’W; 18 m thick). In all localities, the PLPM is mainly constituted of fine-grained tuffs and tuffaceous sandstones with minor amounts of tuffaceous mudstones, conglomerates and chert. The recognized facies associations (FA) include sub-aerial primary pyroclastic deposits (FA1) and epiclastic deposits partially originated by reworking of the former (FA2 to FA5). FA1 (ash-falls) comprises massive or plane parallel laminated, accretionary lapilli-rich, fine-grained tuffs with mantle bedding and non erosive bases; intercalations of apedal paleosoils and burrowing levels are common. FA2 (unconfined fluvial flows) includes stacked and amalgamated sheet-like bodies composed of fine-grained tuffaceous sandstones; individual bodies have irregular bases and commonly show massive aspect in the base and lamination and roots in the top. FA3 (permanent fluvial channel belts) is mainly composed of tuffaceous sandstones with irregular and erosive bases and plane tops; internally, the bodies present laterally stacked large-scale inclined surfaces. FA4 (aeolian system) is represented by a single body composed of medium-grained tuffaceous sandstones; the lower part (dunes) shows planar tabular cross-beddings with average foresets dip 25°; the upper sector (interdunes) exhibits low-angle cross-bedding with intralamina inverse grading. FA5 (ponds) is formed by tabular tuffaceous mudstones with massive aspect in the base then passing upward to levels with plane parallel lamination or asymmetrical ripples; coarse to fine-grained chert intercalations are relatively common. FA1 and FA2 occur in all localities, the first with less occurrence (<10%) and the later constituting always more than 50% of the sections. FA3 only occurred in Estancia La Payanca (? 40%) and FA4 and FA5 in Cerro Los Chivos (7% and 8%, respectively). Facies relationship suggests accumulation of sub-aerial ash-falls (FA1) and frequent reworking of the tephra by unconfined fluvial flows (FA2); locally, primary pyroclastic substrates were also reworked by channelled fluvial flows (FA3), aeolian processes (FA4) and shallow lacustrine sedimentation (FA5). 93 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA GEODYNAMIC THAT GENERATED THE CRETACEOUS VOLCANISM OF CÓRDOBA AND THE LARGE IGNEOUS PROVINCE OF PARANÁ, INCLUDING THE ORIGIN OF THE TRISTAN “PLUME” 2-16 Vizán, H.1*, Lagorio, S.L.2 (1) Departamento de Ciencias Geológicas – Facultad de Ciencias Exactas y Naturales (UBA – CONICET). Ciudad Universitaria, Pab. II. Ciudad Autónoma de Buenos Aires (2) Servicio Geológico Minero Argentino (SEGEMAR). J. Roca 651, piso 10 – Ciudad Autónoma de Buenos Aires * Presenting author e-mail: [email protected] The aim of this work is to present a geodynamic model coherent with geochemical data pertaining to two contemporaneous volcanic events occurred in Córdoba and in Misiones (Argentina). In this model Fig. 1 - Palaeoreconstruction of Pangaea for 130 Ma. Cratons and terranes: AM (Amazonia), RA (Río Apa), CN (Congo), K (Kalahari), OAC (Oriental Africa Craton), A (Arequipa), PA (Pampia), C (Cuyania). The black point inside a circle represents the main centre of the eruptive pipe of the Large Igneous Province Paraná-Etendeka-Angola. The other black points represent the location of Misiones and Córdoba Cretaceous volcanisms. In colours: seismic velocity anomalies (S wave) in the lower mantle. High (low) velocities anomalies are interpreted as high (low) temperatures. Fig. 2 - East-West section of Western Gondwana (South America and Africa) for 130 Ma. The thikness of the cratons are assumed according to the thikness determined for San Francisco craton (200300 km). (1) Large-scale convection roll induced by subduction lateral cooling. (2) Edge-driven convection determined between Río de la Plata Craton and the terrane Pampia. 94 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 3a and b - Temporal evolution of the different geodynamic processes that occurred between 125 Ma and 80 Ma. The break-up of Western Gondwana and the drift of South America respect to Africa are represented. Astenospheremesosphere boundary at about 650 km; core-mantle boundary at about 2,900 km. Orange triangles: volcanic sea mounts caused by a conduit of magma outpouring that evacuated energy insulated by Pangaea; grey triangle: central oceanic ridge; dark blue: subducted slab; different orange tones: Pangaea thermal blanketing; black closed circuits: edge-driven convection; purple closed circuit: large-scale lateral convection induced by subduction; green closed circuit: convective cell that cause the South America drift; curve arrows in blue: downwelling currents in the lower mantle; curve arrows in red: upwelling currents in the lower mantle. a) 125 Ma; b) 80 Ma. Fig. 3c and d - Temporal evolution of the different geodynamic processes that occurred between 40 Ma and 10 Ma. See references of Fig. 3a and b. c) 40 Ma; d) 10 Ma. 95 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA it is considered what is known for the convection processes that happen in the mantle. It is suggested that the alkaline Cretaceous volcanism of Córdoba was basically generated by edge-driven convection (King and Anderson, 1998) between the Río de La Plata craton and the Pampia terrane or the ancient mobile belt (suture zone) that separates both geologic elements. On the other hand, it is considered that the origin of the tolheiitic basalts of the Large Igneous Province of Paraná and its African counterpart was controlled by a large-scale lateral convection induced by subduction (Nataf et al., 1981), stimulated also by the energy insulated by Pangaea (Anderson, 1982), (Fig. 1 and Fig. 2). It is suggested, in addition, that the Tristán “plume” was actually a conduit of magma outpouring caused by lithospheric discontinuities due to the distribution of mobile belts surrounded by old cratons. The pipe for carrying the magma would have being set up by the mobile belts where the lithosphere was weaker (Fig. 2). This pipe would have evacuated the caloric energy insulated by Pangaea and would not correspond to a plume generated by a thermal discontinuity at the core-mantle boundary (Fig. 3a, b, c and d). This conduit could have acted as a guide for deep currents that transported chemical elements to the surface, previously carried to deep zones of the mantle in former subducted slabs (Hofman, 1997). A geodynamic evolution from 125 Ma to 10 Ma is shown in Fig. 3a, b, c and d. It is suggested that the processes of convection in the mantle would be determined by the architecture and dynamics of the lithosphere. REFERENCES • • • • Anderson, D.L., 1982. Hotspots, polar wander, Mesozoic convection and the geoid. Nature, 297: 391-393. Hofman, A.W., 1997. Mantle geochemistry: the message from oceanic volcanism. Nature, 385: 219-229. King, S.D. and D.L.Anderson, 1998. Edge-driven convection. Earth and Planetary Science Letters, 160: 289-296. Nataf, H.C., C. Froidevaux, J.L. Levrat and M. Rabinowicz, 1981. Laboratory convection experiments: Effect of lateral cooling and generation of instabilities in the horizontal boundary layers. Journal of Geophysical Research, 86: 6143-6154. MAFIC MICROGRANULAR ENCLAVE SWARMS IN GRANITIC PLUTONS OF GASTRE, CENTRAL PATAGONIA 2-17 Zaffarana C.B.1*, Somoza R,1 (1) Departamento de Ciencias Geológicas, Universidad de Buenos Aires. Intendente Güiraldes 2160. Buenos Aires C1428EGA * Presenting author’s e-mail: [email protected] In the Gastre region, Central Patagonia, the Late Triassic-Early Jurassic Central Patagonian Batholith, (Rapela et al., 1991) crops out. These authors recognized two superunits, according to field and petrographic characteristics: the Gastre Superunit, composed of biotite-hornblende granitoids, porphyritic and equigranular biotite granitoids and the younger Lipetrén Superunit, mostly composed of biotitic granitoids. One outstanding difference between them is that the Gastre Superunit granitoids host mafic microgranular enclaves (MME), mafic dikes and mafic stocks. In particular, in one of the localities where the porphyritic biotitic granitoids crop out, spectacular occurrences of homogeneous enclave swarms have been found. Mafic microgranular enclaves are interpreted as hybrids of acidic and basic magmas (Didier and Barbarin, 1991). In this contribution we describe field and petrographic characteristics of enclave swarms recognized in the Gastre superunit. Field appearance of the enclave swarms Enclave swarms have a NW-SE strike and extend along a 2,5 km corridor (Fig. 1A). The area covered by the enclave swarms has an approximated surface of 1, 24 km2. The geometry of the swarms is mostly tabular (Fig. 1C). Host granitoid is a coarse-grained porphyritic amphibole-biotitic granodiorite with up to 2-3 cm long microcline megacrysts. 96 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Field appearance of the enclave swarms of the Gastre area A) General view of a swarm with a NW-SE strike B) Microcline megacrysts mechanically intruded in an MME C) Ovoid to ellipsoidal MME shapes defining a planar enclave swarm geometry D) MME appearing as dismembered mafic dikes E) MME appearing with irregular, angular shapes F) More irregular, lenticular shapes G) Diffuse borders in an MME. Note that this MME has microcline megacrysts inside. MME are ovoidal to ellipsoidal-lenticular (Fig. 1C), but also irregular (subangular to angular) shapes can be observed (Fig. 1D, F). The MME size is variable, they are within the common reported size range of 1 cm to 1 m in diameter (Vernon, 1983). MME have sharp boundaries with the granitoid, although sometimes the borders may look more diffuse (Fig. 1G). In several places, microcline megacrysts of the granodiorite impinge the enclave border (Fig. 1B). Sometime a dike appears dismembered in several MME (Fig. 1E). The fabric of individual MME inside the swarms is magmatic and parallel to the NW-SE strike – 30/40º SW dip magmatic fabric of the hosting granitoid and parallel to the strike of the swarm. Although in many plutons the presence of MME swarms increases towards contacts with wall rocks (Vernon, 1983), in the Gastre area this relationship can not be controlled because plutons are nested in plutons, and their contacts are difficult to identify. Petrography Petrographic analysis was made on the base of 10 thin sections. The MME have magmatic fabric (Fig. 97 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES Fig. 2 - Petrography of the MME A) Magmatic texture defined mostly by plagioclase and biotite, and a plagioclase xenocryst with numerous inclusions of hornblende, opaque minerals and apatite B) Plagioclase nuclei amalgamated and surrounded by acidic borders. Note the difference in grain size with the groundmass material composed of smaller plagioclase and biotite C) Anhedral poikilitic amphibole with pyroxne nueclei and anhedral to subhedral biotite, photograph taken with parallel nichols D) Anhedral microcline poikilitically enclosing hornblende, biotite and plagioclase (oikocrysts) 2A) and are composed of plagioclase (60-15%), hornblende (35-10%), biotite (25-10%), quartz, microcline, apatite, titanite and opaque minerals. They are classified as quartzose diorites transitional to monzonites and quartzose monzonites. Plagioclase is euhedral and has calcic nuclei and acidic borders. Some euhedral, early-formed nuclei are amalgamated and surrounded by plagioclase of late crystallization (Fig. 2B). Some plagioclase individuals have inclusions of hornblende, acicular apatite, titanite and opaque minerals (Fig. 2A). Hornblende is subhedral and can occasionally have pyroxene nuclei (Fig. 2C). Acicular apatite presence indicates a magmatic quench origin (Vernon, 1983, Wyllie et al., 1962) when hotter mafic magma gets in contact with the cooler felsic magma. Quartz and microcline content is generally scarce and interstitial, except where they poikilitically enclose plagioclase and mafic grains conforming oikocrysts, (Fig. 2D) common in monzodioritic enclaves (Janousek et al., 2000). The Gastre enclave swarms are a field evidence that juvenile, mafic magmas have been added to the central Patagonian crust in late Triassic - early Jurassic times. REFERENCES • Didier, J. and Barbarin, B.; 1991: The different types of enclaves in granites - Nomenclature. In: B.B. J. Didier (ed), Enclaves and Granite Petrology, Orsay, France, pp. 19-23. • Janousek, V., Bowes, D. R., Braithwaite, C. J. R. and Rogers, G.; 2000: Microstructural and mineralogical evidence for limited involvement of magma mixing in the petrogenesis of a Hercynian high-K calc-alkaline intrusion: the Kozárovice granodiorite, Central Bohemian Pluton, Czech Republic. Edinb. Roy. Soc. Trans., Earth Sciences, 91, 15-26. • Rapela, C. W., Dias, G. F., Franzese, J. R., Alonso, G. and Benvenuto, A. R.; 1991: El Batolito de la Patagonia central: evidencias de un magmatismo triásico-jurásico asociado a fallas transcurrentes. Rev. Geol. Chile, 18, 121-138. • Vernon, R. H.; 1983: Restite, xenoliths and microgranitoid enclaves in granites. Journal and Proceedings, Roy. Soc. N. S. Wales, 116, 77103. • Wyllie, P. J., Cox, K. G. and Biggar, G. M.; 1962: The habit of apatite in synthetic systems and igneous rocks. J. Petrol., 3, 238-243. 98 Session 3 GEOPHYSICAL PROSPECTING AND GEODESY GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA ANALYSIS OF A PRECISE REGIONAL GEOID MODEL IN BUENOS AIRES PROVINCE COMPUTED BY LEAST SQUARES COLLOCATION 3-01 Bagú, D.1*, Del Cogliano, D.1,2, Scheinert, M.3, Dietrich, R.3, Schwabe, J.3, Mendoza, L.1,2 (1) Grupo Geodesia Espacial, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, La Plata, Argentina (2) Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina (3) Institut fur Planetare Geodasie, Technische Universitat Dresden, Dresden, Germany * Presenting author’s e-mail: [email protected] An analysis of a precise regional geoid model in Buenos Aires province computed by Least Squares Collocation (LSC) is shown. The study zone (lat 35°S to 38.5°S, long 63°W to 58.5°W) is mainly flat with two zones to the south with rough topography. To achieve the goal, terrestrial and satellite data were combined in the well known Remove-ComputeRestore technique. The terrestrial data consisted in gravity observations, levelling heights and GPS on benchmarks. The long-wavelengths components of the Earth’s gravity field were modelled by the EGM2008 global gravity model. The SRTM3v.4 was used to compute the topographic effects in the Helmert’s second method of condensation scheme (short-wavelengths). Taking into account this reduction, gravity anomalies were computed. After removing the long and short wavelengths from the gravity anomalies, the residual data (over 9462 gravity stations) was used to compute 156 residual geoid heights by LSC (Nlsc) using the GravSoft package. The differences with respect to the GPS/levelling heights observations (Nobs) were estimated (“only gravity solution”). The results show that 47% of the absolute differences between Nlsc and Nobs are below 5cm, and 83% below 10cm. Other solutions using the 9462 residual gravity anomalies but now also GPS/levelling as input observations were computed, showing improvements with respect to the “only gravity solution”. GEOPHYSICAL SURVEY IN THE NORTHERN REGION OF CUYO BASIN 3-02 García, M.1*, Luna, E.1, Alvarez, O.1, Spagnotto, S.2, Nacif, S.2, Martínez, P.3, Gimenez, M.3 (1) Instituto Geofísico Sismológico Volponi – Univ. Nac. de San Juan. Ignacio de La Roza y Meglioli S/N. Rivadavia, San Juan (5400) (2) CONICET (3) CONICET. Instituto Geofísico Sismológico Volponi – Univ. Nac. de San Juan * Presenting author’s e-mail: [email protected] A seismic-gravimetric survey was made in the northern area of the Cuyo basin, between the cities of San Juan and Mendoza. From the processing of gravimetric data a Bouguer map of anomalies was obtained, which was adequately filtered to the purpose of obtaining residual anomalies linked to the upper crust. The analysis of these Bouguer residual anomalies allowed to identify the Jocolí graben on the base of the fold and thrust belt of Precordillera. Anomaly enhancement techniques such as analytical signal, tilt and phase of tilt were done to highlight different wavelength anomalies. These gravimetric gradients match with the eastern edge of the suture area of the Pie de Palo range. Once the morphology of serious anomalies was determined, the depths and distribution of generating sources of said anomalies through the Werner and Euler deconvolutions were analyzed as well as the average of power spectrum for the structures of the survey area. From the analysis of seismic data, combined with well logs and gravimetric results, it was possi101 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA ble to interpret three horizons: Paleozoic, Triassic and Tertiary-Quaternary. With the obtained results, one upper crust models were done in two sections where there is more seismic coverage. The models proved the Bouguer residual anomaly and the interpretation of the seismic horizons. In the model located to the South of the survey area, a sedimentary body of around 7 kilometers deep filled by Triassic and Tertiary-Quaternary sediments is interpreted. This indicates one tectonic clash area of thin skinned and thick-skinned corresponding to Cuyania and Pampia terrane. In the section located to the north of the survey area, a small inverted depocenter was interpreted through seismic and below it the Paleozoic lies unconformably. The sedimentary layers that were confirmed by gravity and seismic, generate prospective expectation of hydrocarbon in the area. MONITORING AND FORECASTING THE STATE OF THE SOUTH ATLANTIC MAGNETIC ANOMALY 3-03 Gianibelli, J.C.* Departamento de Geomagnetismo y Aeronomía, Facultad de ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Argentina * Presenting author’s e-mail: [email protected]. Introduction The models of Geomagnetic Field of the Earth (IGRF: International Geomagnetic Reference Field), are published from 1900 up to 2010 at intervals of 5 years. Barraclough (1978) published and normalized all available models coefficients of the spherical harmonics analysis for the magnetic field of the earth from 1550 up to 1978. From this models, Gianibelli (2006) calculated the energy evolution for dipole and cuadrupole effects at the surface of the Earth, from 1550 to 2005 and made a forecast for the period 2010-2500. The energy of cuadrupole with respect of the dipole energy is less than 2% at the present. The evaluation of the energies from IGRF coefficients, corresponding to the orders 1,2,3 and 4, and the prediction of them up to 2500, is presented in Fig. 1. This result was Fig. 1 - Energy relationship versus dipole energy W1. 102 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 Table 1 - Results obtained from annual mean values analysis. OBS: YEAR HUA F:nT LQA F:nT VSS F:nT PIL F:nT HER F:nT LAS F:nT TRW F:nT ORC F:nT AIA F:nT DRV F:nT 2000 26290 23790 23300 23470 26230 23840 26480 33090 39430 69380 2050 23850 21150 21820 20620 20870 21150 23080 27370 34480 67200 2100 21100 18860 20050 17770 15450 18910 19680 21650 29530 64980 ¢F 1: -5190 -4930 -3250 -5700 -10780 -4930 -6800 -11440 -9900 -4400 calculated by a linear model of each coefficient of spherical harmonics analysis (SHA) of orders 1,2,3 and 4, and the most important result is that order 2 of the SHA represents the effects of magnetic field at the surface of the Earth and the evolution of the South Atlantic Magnetic Anomaly (SAMA). The Magnetic Observatories in the SAMA region, and in the South Hemisphere are (Fig. 2): Huancayo (HUA), La Quiaca (LQA), Pilar (PIL), Vassouras (VSS), Hermanus (HER), Las Acacias (LAS), Trelew (TRW), Islas Orcadas del Sur (ORC), and Vernadsky (AIA). The Magnetic Observatory of Dumond D’Urville (DRV) is also plotted because is close to the Geomagnetic South Pole region and is monitoring the change of intensity. The annual mean values of HUA are important because they allow to monitor the possible future Fig. 2 - Distribution of Magnetic Observatories. 103 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 Table 2 - Results obtained from the IGRF model. OBS: YEAR HUA F:nT LQA F:nT VSS F:nT PIL F:nT HER F:nT LAS F:nT TRW F:nT ORC F:nT AIA F:nT DRV F:nT 2000 26069 23507 23431 23622 26358 23593 26844 33630 39889 66693 2050 24138 21382 22388 20962 21273 20990 22906 28526 34385 65587 2100 22208 19257 21345 18301 16188 18387 18968 23421 28882 64480 F 2: -3861 -4250 -2086 -5321 -10171 -5207 -7875 -10209 -11007 -2213 Table 3 - Discrepancies of estimated values for 2100 OBS: HUA F:nT LQA F:nT VSS F:nT PIL F:nT HER F:nT LAS F:nT TRW F:nT ORC F:nT AIA F:nT DRV F:nT F 1: -5190 4930 -3250 -5700 -10780 -4930 -6800 -11440 -9900 -4400 F 2: 3861 4250 -2086 5321 10171 5207 7875 10209 11007 -2213 DIFF: -1329 -679 -1164 -379 -609 277 1075 -1231 1107 -2188 changes of magnetic equator position. Data analysis and results The annual mean values of total magnetic intensity (F) of each observatory is analized and the tendence through time is calculated. The objetive of this paper is to obtain the values of F from this linear time series model for 2000 and to predict the tendency for 2050 and 2100 for each observatory and compare the results obtained by the IGRF model for the same observatories in the same date. From this comparison it is possible to predict the state of SAMA in the future from two different elementary methods. The results starting from annual mean values analysis of each observatory is presented in Table 1. Table 2 shows the values obtained from the IGRF model. The ¢F value changes from 2000 to 2100 is shown in each table. In Tables 1 and 2, ¢F values are different, and in the case of DRV Observatory, ¢F is 50 % greater than the IGRF estimation from the annual mean values. Table 3 is a resume of the discrepancy, wich is the difference DIFF=¢F1 – ¢F2 calculated for each observatory. Conclusion With this two methods, a continuous magnification of the total intensity of magnetic field depression of the SAMA region is predicted. The estimation of the annual mean changes for the time interval 2000-2100 in some observatories is less than the estimation from the IGRF change for the same epoch (LAS, TRW and AIA). The values of DIFF show a change of sign and magnitude in nT. A great difference occurs in the region of south pole (DRV), and in the region of magnetic equator (HUA), and this is a possible situation is the fitting with a cut up of degree 10 in the model of the IGRF. In this scenario, changes in the solar wind that interacts with the magnetic field of the Earth in the SAMA region are expected, with an important deformation of the inner radiation belts to ionospheric levels according with SAMA evolution, the total magnetic field intensity reduction and the effects in the ionospheric pattern equivalent current systems. This possible situation is evaluated by the estimation of changes of the energy of the dipole and cuadrupole fields effects at different ionospheric heights for 2000, 2050 and 2100. 104 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 3 - Energy variations for year 2000 and predicted for years 2050 and 2100. This result is presented in Fig. 3. In conclusion, the cuadrupole energy from 2050 to 2100 is increasing with a magnitude greater than the increment from 2000 to 2050. Possibly, a change of solar wind-Earth magnetic field interactions may account for this trend. The relatively low number of magnetic observatories in the South Hemisphere for monitoring the evolution of SAMA is a problem that should be considered in the near future. It is suggested to build in the future an improved network of observatories that monitor the absolute values of the geomagnetic elements at the surface of the Earth to model the IGRF with greater detail. REFERENCES • Barraclough, D. R. 1978, Spherical Harmonic Models of the Geomagnetic Field. Institute of Geological Sciences. Geomagnetic Bulletin 8. 1-66. • Gianibelli, J. C. 2006. Sobre la Evolución temporal del Dipolo y Cuadrupolo del Campo Geomagnético. Geoacta, vol 31, 175-181. 105 GEOSUR2010 IMPORTANCE AND FUTURE OF THE MAGNETIC OBSERVATORY NETWORK IN SOUTH AMERICA 22-23 NOVEMBER 2010 – MAR DEL PLATA 3-04 Gianibelli, J.C.1*, Sánchez Bettucci, L.2, García, R.E.3, Rodriguez, G.D.3, Quaglino, N.1, Novo, R.4, Tancredi, G.5 (1) Departamento de Geomagnetismo y Aeronomía, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata Paseo del Bosque S/N. 1900 La Plata, Argentina.TE: (054)0221-4236594 ext 132 (2) Área Geofísica-Geotectónica, Departamento de Geología, Facultad de Ciencias, Universidad de la República, Uruguay (3) Departamento de Electrónica. Facultad de Ciencias Astronomicas y Geofísicas Universidad Nacional de La Plata. Paseo del Bosque S/N 1900 La Plata Argentina (4) Instituto de Ciencias Geológicas, Facultad de Ciencias, Universidad de la República, Uruguay (5) Instituto de Ciencias Físicas, Universidad de la República, Uruguay * Presenting author’s e-mail: [email protected] Introduction The Earth’s magnetic field is known in South America since the beginning of twentieth century. The first Magnetic Observatory was Pilar (PIL), and started operations in 1904 with the absolute determination of Declination, Horizontal and Vertical components and their variations with a photographic recorder. Following PIL, the other observatories were Huancayo (HUA, started in 1922), Vassouras (VSS, started in 1915) La Quiaca (LQA, started in 1920 and stopped in 1992), Islas Orcadas del Sur (ORC, started in 1905, before the International Geophysical Year (IGY) in 1957). After the IGY, followed the installations of the observatories of Tatuoca (TTB, started in 1957), Trelew (TRW, started in 1957), Las Acacias (LAS, started in 1961) and Port Stanley (PST, started in 1994). Now, the Goverment of Uruguay is constructing the non-magnetic houses to install a new observatory at a place named “Casa de Aigua” (Fig. 1). The future Observatory of Aigua (ODA), and the others observatories locations are shown in Fig. 2. The objective of this paper is to give a brief overview of the state of the Total Magnetic Intensity for ODA, and comparatively for the others observatories, using the IGRF-11 model. Fig. 1 - Residence for the magnetic installation at “Casa de Aigua”. 106 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - Location of the Magnetic observatoires in South America. Data analysis and results The proton precession magnetometer installed at the “Casa de Aigua” was designed in the Faculty of Astronomical and Geophysical Sciences (UNLP) by Ing. Ezequiel Garcia and Ing. Guillermo Fig. 3 - Comparative IGRF values of Total Magnetic Intensity F for PIL, ODA, LAS and VSS time series. 107 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 4 - The total magnetic intensity changes with latitude determined for HUA, LQA, PIL, TRW, PST and ORC Rodriguez. The system is similar to the instrument wich operates at LAS Magnetic Observatory (Garcia et al., 2007). For analyzing the site of ODA, we evaluated and compared all the elements of the geomagnetic field, obtained by the model of the International Geomagnetic Reference Field (IGRF-11) from 1900 to 2015. This model is based in the Spherical Harmonic Analysis up to degree 10 of the all absolute values obtained by the World Magnetic Observatory Network (Lanza and Meloni, 2006; Merrill et al., 1996). The results of IGRF evaluation for each observatories show important values for the magnetic total Fig. 5 - Secular variations characteristic for HUA, LQA, PIL, TRW, PST and ORC. 108 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA intensity secular change in the time interval 1900-2010: TTB=-44.3 nT/year; HUA=-41.4 nT/year; LQA=-43.2 nT/year; VSS=-20.1 nT/year; PIL=-52.7 nT/year; ODA=-46.2 nT/year; LAS=-50.9 nT/year; TRW=-77.1 nT/year; PST=-87.5 nT/year; ORC=99.5 nT/year. These values demonstrate that this region has the minimum change of the secular variation in the region of the VSS Observatory. LAS, PIL, and ODA sites have similar values. The comparative IGRF values of Total Magnetic Intensity F for PIL, ODA, LAS and VSS time series is shown in Fig. 3. The total magnetic intensity changes with latitude determined for HUA, LQA, PIL, TRW, PST and ORC, is shown in Fig. 4. The minimum of F is seen in the region of PIL. The secular variations characteristic for HUA, LQA, PIL, corresponding to continental environment aspects, and TRW, PST and LQA, corresponding to oceanic environment, are shown in Fig. 5. REFERENCES. • García R. E., Gianibelli J. C., Solans J. H. y Quaglino N., 2007. Ampliación de la capacidad de memoria en los magnetómetros de precesión protónica. Geoacta 32: pp.207-212. • IGRF-11 (2010). http://ngdc.noaa.gov/geomagmodels/IGRFWMM.jsp • Lanza R. and Meloni A (2006). The Earth’s Magnetism, pp 1-66. Springer, Berlin. • Merrill R.T., McElhinny M.W.and McFadden P.L. (1996). The magnetic field of the Earth: Paleomagnetism, the core and the deep mantle. Academic Press, San Diego, California, 531 pp.• IGMAS+ A NEW 3D MODELLING TOOL FOR GRAVMAG FIELDS AND GRADIOMETRY 3-05 Goetze, H.J.*, Schmidt S., Institute of Earth Sciences, University Kiel, Otto-Hahn-Platz 1, 24118 Kiel, Germany * Presenting author’s e-mail: hajo@geophysik. uni-kiel.de It is well known that 3D gravity and magnetic modelling appreciably improves the results of distinct depth imaging projects. This regards especially to areas of strong lateral velocity and density contrasts and corresponding imaging problems. Typical areas where grav/mag modelling has been successfully used are sub-salt and sub-basalt. The interactive 3D gravity and magnetic application IGMAS+ (Interactive Gravity and Magnetic Application System) has been around for more than 20 years. Being initially developed on a mainframe and then transferred to the first DOS PCs, it was in the 90s adapted to Linux PCs. The program has proven to be very fast, accurate and easy to use once a model has been established. The analytical solution of the volume integral for the gravity and magnetic effect of a homogeneous body is based on the reduction of the volume integral to an integral over the bounding polyhedrons; triangles in the case of IGMAS+. Later the algorithm has been extended to cover all elements of the gravity tensor as well. In the modelling interface, after geometry changes the gravity effect of the model can quickly be updated because only the changed triangles have to be recalculated. Optimized storage enables very fast inversion of densities. Changes of the model geometry are restricted to predefined parallel vertical sections. This is a small restriction to the flexibility but makes geometry changes easy. No complex 3D editor is needed. The vertical sections are displayed together with the measured and calculated gravity fields. The geometry is updated and the gravity recalculated immediately after each change. Because of the triangular model structure IGMAS can handle complex structures (multi Z surfaces) like the overhangs of salt domes very well. Drawbacks are the lack of integration with seismic interpretation systems and the fairly complex and slow model building of the start model. The software development was directed towards scientific usage at universities with frequent, often experimental changes. The integration of the workflow and the tools is important to meet the needs of today’s more 109 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA interactive and interpretative depth imaging workflows. For the integration of gravity and magnetics IGMAS+ can play an important role in this workflow. ADVANCES IN THE DETERMINATION OF A HEIGHT REFERENCE SURFACE FOR TIERRA DEL FUEGO 3-06 Gomez,M.E.1,2*, Perdomo, R.1,2, Del Cogliano, D.1,2, Hormaechea, J.L.1,2,3 (1) Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, La Plata, Argentina (2) Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina (3) Estación Astronómica Río Grande, Acceso Aeropuerto, V9420EAR Río Grande, Argentina * Presenting author’s e-mail: [email protected] From 1998 to the present, several efforts from different scientific institutions have contributed to collect gravity data, as well as the establishment of leveling lines including GPS coordinates in Tierra del Fuego province. This work is devoted to the use of the Equivalence Source Technique (EST) as a tool for combining different kind of data, in order to obtain a geoid model for Tierra del Fuego. The first attempts to build a geoid model in the region were purely numeric, interpolating the geoid undulations from a grid of measured differences between ellipsoidal and orthometric heights Many experiences have been done using EST in order to estimate the impact of gravity contribution in contrast with these numeric models, obtaining successful results from the combined data. Besides, the possibility of including GPS measurements on the surface of the Lago Fagnano (assuming that it is a geopotential surface) is also analyzed. The main results show that it is possible to get a geoid model with a 1Û error of the order of 0.07 m for a big portion of the island. This represents a significant improvement for the region where the recent global geopotential models do not fit satisfactorily. FIRST MAGNETOMETRIC SURVEY IN THE ZAPICÁN AND NICO PÉREZ AREA (URUGUAY) 3-07 Novo, R.1, Seluchi, N.1, Suarez, I.1, Sánchez Bettucci, L.1*, Gianibelli, J.2 (1) Area Geofísica-Geotectónica, Instituto de Ciencias Geológicas, Facultad de Ciencias-UdelaR, Montevideo, Uruguay (2) Departamento de Geomagnetismo y Aeronomía de la Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata. Paseo del Bosque S/N, CP 1900, La Plata. Argentina * Presenting author’s e-mail: [email protected] A magnetometer survey in the Zapicán and Nico Pérez region was performed, Department of Lavalleja, Uruguay. The area is characterized by a deformed granitic basement intruded by tholeiitic basaltic and basalts andesite dikes (age 581 ± 13 Ma). These dikes have an approximate E-W trend in this area specifically, with few outcrops. These dikes present porphyric texture with plagioclase and clynopiroxene. Recent studies suggest an important contribution of magnetite and/or titanomagnetite. A transect in a N-S direction was realized (6 kilometers long) with the purpose of corroborate the existence of intrusive bodies and locate other buried bodies. The instrument used is a proton 110 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA precession magnetometer from Geometrics brand, model G-856. The magnetic profiles obtained show evidence of significant magnetic anomalies that testify the presence of these dikes. MAPS OF ABSOLUTE GRAVITY, GRAVITY ANOMALY, AND TOTAL MAGNETIC FIELD ANOMALY OF VENEZUELA FROM SATELLITE DATA 3-08 Guevara, N.O.1*, Reyes, A.G.2, Tabare T.2 (1) Universidad Central de Venezuela, Escuela de Geología, Minas y Geofísica. Caracas, Venezuela (2) Agencia Bolivariana para Actividades Espaciales, MCTI. Caracas, Venezuela * Presenting author’s e-mail: [email protected] Abstract We present three maps (scale 1:2.000.000) of absolute gravity, Bouguer anomaly and total magnetic field anomaly of Venezuela, in the geographic window 0º to 13º latitude N and 58º to 74º longitude W. The absolute gravity and Bouguer anomaly are projected on the mean sea level, while the total magnetic field anomaly is projected to 4000 meters above the mean sea level. The database used for the construction of each map has 188.072 sampling points with spacing of 3.7 km. Absolute gravity was obtained from the Venezuelan gravity satellital database (Orihuela and Garcia, 2009). The calculation of the Bouguer anomaly was made from the absolute gravity and ETOPO2v2 terrain model (NGDC, 2006). The total magnetic field anomaly was calculated using the combined model EMAG2 (Maus et al., 2009) which was developed from satellite data of the CHAMP mission and land and marine data from the global network. This paper presents a general review of the absolute gravity, Bouguer anomaly and total magnetic field anomalies of the major geological structural features of Venezuela and it classifies the Venezuelan territory gravitationally and magnetically. Introduction There are relatively few direct measurements of absolute gravity and total magnetic field at the surface of the Earth due to the difficulty of transportation and handling of instruments for this purpose. However, a large amount of absolute gravity data and total magnetic field data from satellites are now available. Regular grids of absolute gravity and total magnetic field measurements represent the necessary condition to generate gravity anomaly and total magnetic field anomalies maps that can represent the distribution of changes in density and magnetization of the subsurface. In addition to the existing studies, this work presents total magnetic field anomaly map of Venezuela derived from satellite (magnetic anomaly EMAG2 model) projected 4000 m above the mean sea level, allowing regional geological interpretations of the subsurface of our territory without disturbance for cultural noise or superficial geological features. Methodology The absolute gravity values available in the Venezuelan gravity satellital database (Orihuela and García, 2009) were reduced to estimate Bouguer anomaly, the digital terrain model selected was ETOPO2v2. The density of reduction was 2.67 g/cm3 and the theoretical gravity estimation was made from the 1967 International gravity equation. This allowed the comparison with the terrestrial data. The topographic correction was applied using the Oasis Montaj algorithm (Geosoft, 2007). The free air correction was applied with the assumption that gravity varies 0.3086 mGal/m. The total magnetic field anomaly was calculated from the Earth Magnetic Anomaly Grid (EMAG2). The magnetic anomalies are associated with magnetic highs and lows that complement positively or negatively depending on the lateral contrast of magnetic susceptibility and the relative position of the bodies with respect to the orientation of the total field vector in the study area. 111 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Venezuelan absolute gravity map, units in mGals (a); Venezuelan gravity anomaly map, units in mGals (b); Venezuelan total magnetic field anomaly map, units in nT (c); Venezuela geographical ubication (d). For all these maps the projection system is WGS84. Longitude and latitude are in degrees. Results Absolute gravity map (Fig. 1a). From north to south: Gravity high at the southern end of the Lesser Antilles and the extinct Aves arcs, with a clear continuity of the Lesser Antilles Arc with the Margarita Island platform, and of the Aves Arc with the Blanquilla Island platform (A), the gravity highs associated with the Netherlands and Venezuelan islands (B), a relative gravity low associated with Bonaire Basin (C) with a high gravimetric value to the east that seems to have continuity with the gravity high associated with the previously mentioned arcs. In the Maracaibo Lake Basin the map highlights a pronounced gravity low, which is sub-parallel to the Andean Chain (D) and the expression of Icotea Fault (E). Among the Barinas-Apure basin and Eastern basin, the map shows the gravitational expression of the Baúl High (F), which is evidenced by a significant gravity high that breaks the continuity between the basins cited above, it extends to the northeast in the basement of Anzoátegui State (G) marking the northern flank of the Espino Graben (H). In the Maturín Subbasin, there is a gravity low associated with the depocenter of the basin (I) which is expressed as a discontinuous low, that represents the expression of the faults present in the zone (Urica, San Francisco, etc), as well as the prolongation in the basin (J). At the eastern end of the map, a gravity high in the south limit of the Orinoco Delta is present (K), with a NW-SE direction, defining the southwestern termination of a wide channel of low gravity that extends sub-parallel to the northern South America Atlantic coast (L) and changes direction between the parallels 10° and 11° N to join the low gravity belt and associated to the accretionary prism of the Lesser Antilles (M). Gravity anomaly map (Fig. 1b). From north to south: Positive gravity anomalies range from 269 to 86 mGal in the area, bounded by the southern end of the Lesser Antilles Arc and Venezuelan and Netherlands Antilles, the southern end of Venezuela and Grenada Basins, the eastern end of the northcentral coastal platform. The distribution of the contours reflect a semicircular geometry, with concavity to the north, and may indicate the expression of a tectonic stress pattern (transpressional plate 112 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA boundary), with a component of convergence at the extremities in the western and eastern boundaries. The gravity anomalies in the physiographic provinces are distributed, in general, with negative intervals, as is characteristic of continental areas. This trend is broken in the central plains region where a strong positive anomaly is present. In the low gravity belt of the Andean region, the contours are interrupted by a short wavelength gravity signatures associated with high density outcrops. In the eastern part of Anzoátegui State and the Monagas State, a negative gravity anomaly is associated with the Venezuela eastern basin with a minimum of -269 mGal. The Guiana Shield is characterized by mostly negative gravity anomalies, distributed between 0 and -120 mGal, with smaller areas of sparse positive gravity anomalies. The northeast side of the shield and the Orinoco delta front is characterized by a positive gravity anomaly that extends east to reach the domain of positive anomalies characteristic of the shelf in the Atlantic Ocean. Total magnetic field anomaly map (Fig. 1c). Magnetic anomalies are associated to the presence of magnetic susceptibility contrasts and the relative position of the bodies with respect to the total field vector orientation in the study area. The islands present magnetic anomalies in the range of -132 to 175 nT. Magnetic anomalies to the north of the Aves Island and the Los Roques Archipelago present values which range from -130 to -35 nT with preferential direction N78°W, sub-parallel to the Curaçao high. The center of this anomaly is slightly shifted southward with respect to the main axis of the Curaçao high. The Coastal Range is characterized by the presence of values in the range -22 to 36 nT, with preferred orientation N94°W. The Interior Ridge is characterized by magnetic contours that range from -3 to 34 nT, with preferred orientation E-W. The southern limit of the Araya Peninsula has very well-defined anomaly contours with a broad E-W direction; its range is between 49 to 60 nT and follows the preferential orientation of the El Pilar fault. The eastern part of the Cariaco Trench is distinguished by a high magnetic anomaly of the Araya Peninsula; the length of this high is about 40 km and presents values in the range of 18 to -28 nT. The geographic window between the parallels 4º and 6º N and the meridians 61º and 64º W has no magnetic data available. The magnetic response is mainly positive in the Guiana Craton. Along the Orinoco River, stands a magnetic corridor, with a preferential direction N59ºE, whose values range from -371 to -56 nT, representing the area of most relevant and extensive low magnetic anomalies in the Venezuelan territory. In the Barinas-Apure Basin and its eastern alignment, a low magnetic anomaly in the direction N70°E with values in the range of -56 to -155 nT is found. This area represents a strip of magnetic anomalies of great importance that divides the territory in a NE-SW direction. No information are available to extend the analysis of the magnetic anomalies in the region of the Andean mountain system. Acknowledgements The authors appreciate the facilities of access to geophysical information in the databases of the NOAA and ICGEM service, and the support provided by Franz Barthelmes, Nikolaos Pavlis, and Stefan Maus. REFERENCES • Maus, S., Barckhausen, U., Berkenbosch, H., Bournas, N., Brozena, J., Childers, V., Dostaler, F., Fairhead, J., Finn, C., Von Frese, R., Gaina, C., Golynsky, S., Kucks, R., Lühr, H., Milligan, P., Mogren, S., Müller, R., Olesen, O., Pilkington, M., Saltus, R., Schreckenberger, B., Thébault, E. AND F. Caratori. (2009) EMAG2: A 2-arc-minute resolution Earth Magnetic Anomaly Grid compiled from satellite, airborne and marine measurements. Journal of Geophysical Research. Estados Unidos. DOI: 10.1029, 30 pp. • National Geophysical Data Center (2006) ETOPO2 V.2 2-Minute Gridded Global Relief Data. Nacional Geophysical Data Center. Estados Unidos. • ORIHUELA, N & A. GARCÍA (2009) Mapas de anomalías gravimétricas y magnéticas de Venezuela generados a partir de datos satelitales. Thesis. Universidad Central de Venezuela. Facultad de Ingeniería. Escuela de Geología, Minas y Geofísica. Caracas. Venezuela. 205 pp. • Pavlis, N., Holmes, S., Kenyou, S. and J. Factor (2008) An Earth Gravitational Model to Degree 2160. EGM2008. National Geospatial Intelligence Agency. EGU General Assembly, Vienna, Austria. 113 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOPHYSICAL INVESTIGATION OF THE NAVARINO ISLAND PLUTONS (BEAGLE CHANNEL, CHILE) 3-09 Peroni, J.I.1*, Tassone, A.1, Lippai, H.1, Hervé, F.2, Menichetti, M.3, Lodolo, E.4 (1) CONICET-INGEODAV. Dpto. de Ciencias Geológicas. Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires. Argentina (2) Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile (3) Istituto di Scienze della Terra. Università di Urbino. Campus Scientifico Universitario.- 61029 Urbino. Italy (4) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy * Presenting author’s e-mail: [email protected] Geological setting The Canal de Beagle Plutonic Group (BCPG) is a complex of granitoids exposed along the margins of the Beagle channel, in the Gordon, Londonderry, Hoste and Navarino islands (Suárez et al., 1985a). The BCPG belongs to the Fuegian Batholith (Figure 1c), the southernmost segment of the three that compose the Patagonian Batholith (Hervé et al. 2007). Along the northern shore of the Beagle Channel, in the Argentine part of Isla Grande de Tierra del Fuego, there are small outcrops, less than 20 km2 in size (Fig. 1b) of the Ushuaia Pluton (Menichetti et al., 2007; Tassone et al., 2007; Peroni et al., 2008; Peroni et al., 2009a) and Trapecio Pluton (Peroni et al., 2009b), while in the southern sector of the channel, in the Navarino island, larger plutonic bodies outcrop (Fig. 1b). The upper Cretaceous (83-93 Ma, hornblende K/Ar age, Suárez et al., 1985a) Castores and Santa Rosa plutons plutons hosted in the Yahgán Formation are mainly composed of tonalite/granodiorite facies, with common dismembered melanocratic synplutonic dykes (Suárez et al., 1985b). The Castores Pluton shows a system of radial and concentric fractures, with an unfoliated granitoid core, interpreted as a diapiric intrusion (Suárez et al., 1985a). The Beagle Channel area and the Navarino island are dominated by a system of E-W sinistral strikeslip faults (Figure 1b) which belong to the Beagle Chanel fault system (Cuningham 1993) and are associated with several normal faults (Menichetti et al., 2008). These transtensional structures show many fault planes dipping at low angle and are superimposed on the north-verging thrust slices. Sampling and data processing In order to determine the geometry in depth of the Castores and Santa Rosa plutons, three magnetic, lithologic, and structural data campaigns were carried out between 2008 and 2010 in the area of Navarino and Hoste islands and in the Beagle Channel. Magnetic measurements were acquired with two magnetometer (EG&G Geometrics and a Scintrex Envi Grad). 72 hand-oriented samples were obtained in 13 sites for lithologic, paleomagnetic, magnetic susceptibility and AMS studies. A DC-squid cryogenic magnetometer (2G-750R) was used in the laboratory to verify the absence of remanent magnetization. This information is essential to generate the magnetic model presented in this work. The Koenigsberger coefficient (Q), which compares the value of the remanent magnetization (Jr) and the induced magnetization (Ji), is Q << 1 for the studied intrusive bodies. The susceptibility values used for the model are shown in Table 1. Modeling The first step to make de model was to generate a grid from the 4070 measured stations which cover an area of 430 km2, including both onland (along the northern margins of Hoste and Navarino islands) and offshore (in the Beagle channel) data. Based on this grid, an E-W-trending, 21-km-long magnetic profile (profile A-B in Fig. 1) was generated which contains the highest density of measured data both in the Navarino Island as in Beagle Channel offshore. The profile cuts across the central part of the Santa Rosa Pluton and a small part of the northern sector of the Castores Pluton. The magnetic profile (Fig. 2) shows two main anomalies. The easternmost one corresponds to the outcrops of the Castores Pluton and reaches a value of +1020 nT, with a secondary maximum of +840 nT located 1 km to the west. The westernmost maximum (+1090 nT) occurs in correspondence with 114 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 Fig. 1 - A) Location of the area of Fig. 1B within the southern tip of South America. B) Geologic map of the Navarino and Hoste islands (modified from Suárez et al., 1985a). BCFS: Beagle Channel Fault System. Segment A-B: Magnetic profile employed for the model. 1: Quaternary sediments 2: Yahgán Fm. 3: Ushuaia Pluton 4: Castores Pluton 5: Santa Rosa Pluton 6: Other plutons belonging to the Canal Beagle Plutonic Group 7: Tortuga Complex. C) The three segments that compose the Patagonian Batholith. BPN: North Patagonian Batholith. BPA: South Patagonian Batholith. BF: Fuegian Batholith outcrops of ultrabasic rocks on Calete Fique and Segura islands (Fig. 3C), which are very similar to those observed in the Ushuaia pluton in the northern margin of the Beagle Channel (Fig. 1 of Peroni, et al., 2009a). Except for this maximum, the value of magnetic anomaly remain around +400 nT for the profile segment corresponding to the outcrops of Santa Rosa Pluton. Geological and geophysical interpretation Two intrusive bodies were modeled along the magnetic profile using the Encom ModelVision Pro 7.0 software (Encom Technology, 2002). The obtained model for Castores Pluton (Body 1, Figure 2) yielded an ellipsoidal body with a 2 km. vertical axis, E-W horizontal axis of 4.4 km. and N-S horizontal axis of 8 km. Given that the magnetic profile cuts across the northern margin of the Castores Pluton, the obtained modeled body represents only a lateral image of the whole intrusive body. The magnetic modeling of the Santa Rosa Pluton (Body 2, Figure 2) produced an overall oblated body with an average thickness of 1 km., with horizontal axis of 7.1 (E-W) and 4 km. (N-S). Table 1 - Values of the magnetic susceptibility measured in laboratory, for the different lithologies employed for the magnetic modeling. (*) susceptibility from Peroni et al. (2009). Unit Susceptibility [SI] Santa Rosa Pluton 30 x 10-3 Ultrabasic Facies SR 62 x 10-3 (*) Castores Pluton 45 x 10-3 Yahgán Formation 6 x 10-5 115 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - Magnetic model profile A-B. Location in Fig. 1b. Fig. 3 - A: Block diagram composed by combining a MrSid satellite image , a digital elevation model (DEM), and the magnetic model profile A-B located in Fig. 1; view from the north. Dashed lines indicate the limits of the outcrops of both plutons. B: Panoramic view of the Castores pluton outcrops. C: Photography of the outcrops of the ultrabasic rocks of the Caleta Fique and Segura island, where a magnetic anomaly maximum (1090 nT) was measured. 116 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA The A-B magnetic section with the obtained modeled plutonic bodies was combined with a DEM (SRTM30) and a MrSid satellite image (GeoCover Landsat – NASA) in Figure 3a which clearly shows the topography and the outcrops of the studied plutons. The obtained modeled section provides a 2D picture of the uppermost 2 km of the crust of Navarino island viewed from the north. REFERENCES • Cuningham; W. D. 1993. Strike-slip faults in the Southernmost Andes and the development of the Patagonian Orocline. Tectonics 12(1): 169-186 • Encom Technology, 2002. ModelVision Pro v.7.0. Encom Technology, Sydney, Australia. • GeoCover Landsat – NASA <https://zulu.ssc.nasa.gov/mrsid/> • Hervé F., Pankhurst R.J., Fanning C.M., Calderón M., Yaxley G.M.; 2007. The South Patagonian Batholith: 150 My of Granite Magmatism on a plate margin. Lithos 97, 373–394. • Menichetti, M., Tassone, A., Peroni, J.I., Gonzàlez Guillot, M., Cerredo M.E.; 2007: Assetto strutturale, petrografia e geofisica della Bahía Ushuaia – Argentina. Rend. Soc. Geol. It., 4 (2007), Nuova Serie, 259-262, 3 ff. • Menichetti M., Lodolo, E., Tassone A.; 2008: Structural geology of the Fuegian Andes and Magallanes fold-and-thrust belt – Tierra del Fuego Island. Geologica Acta, 6, 1. • Peroni, J. I., Tassone, A., Cerredo, M., Lippai, H., Menichetti, M., Lodolo, E., Esteban, F.,Vilas, J. F.; 2008: 3D Geophysic model of Ushuaia Pluton. Tierra del Fuego. Argentina. GeoMod 2008 Bollettino di Geofísica teorica ed applicata. Nº2 supplement. Extended Abstract. pp: 263-267 • Peroni, J.I., Tassone, A., Menichetti, M., Cerredo M.E.; 2009a: Geophysical modeling and structure of Ushuaia Pluton, Fuegian Andes, Argentina, Tectonophysics doi:10.1016/j.tecto.2009.07.016 • Peroni, J. I., Tassone, A., Menichetti, M., Lippai, H., F.,Vilas, J. F.; 2009b: Geologia e geofisica del plutone del Cerro Trapecio Tierra del Fuego – Argentina. Rendiconti online Soc. Geol. It., Vol. 5: 160-163 • Suárez, M., Hervé, M. and Puig, A.; 1985a: Hoja Isla Hoste e islas adyacentes, XII Región. Carta Geológica de Chile No. 65, 113 pp. Servicio Nacional de Geología y Minería de Chile.113 pp • Suárez, M., Hervé, M. and Puig, A.; 1985b: Plutonismo diapírico del Cretácico en Isla Navarino. IV Congreso Geológico Chileno, Actas (4): 549-563 • SRTM30 Digital Elevation Map (DEM) <http://dds.cr.usgs.gov/srtm/version2_1/SRTM30/> • Tassone, A., Peroni, J. I., Cerredo, M. E., Lippai, H., Vilas, J. F.; 2006: Estudio geofísico del cuerpo intrusivo Ushuaia. Margen norte del Canal de Beagle, Argentina. XI Congreso Geológico Chileno. 7-11 de agosto, Antofagasta, Chile. Actas. GEOPHYSICAL CHARACTERIZATION OF FILLED ZONES ALONG THE COAST OF BUENOS AIRES 3-10 Prezzi, C.1*, López, R.2, Vásquez, C.1, Marcomini, S.2, Fazzito S.1 (1) CONICET – Universidad de Buenos Aires. INGEODAV, Dpto. Cs. Geológicas, FCEyN, UBA, Ciudad Universitaria, Pabellón 2, Buenos Aires, Argentina (2) Universidad de Buenos Aires. Dpto. Cs. Geológicas, FCEyN, UBA, Ciudad Universitaria, Pabellón 2, Buenos Aires, Argentina * Presenting author’s e-mail: [email protected] The coast of Buenos Aires (Argentina) is located on the southern margin of La Plata river estuary. The original morphology of this area was completely changed by fill works. Such works began in 1836 and continue until today. However, fill was carried out mostly between 1964 and 1991. The total filled surface is of approximately 2054 hectares, with a coast advance ranging between 400 and 1000 m (Marcomini and López, 2004) (Fig. 1). The anthropogenic changes imposed on the coast line configuration have generated a great variety of problems in building foundations due to the heterogeneous composition of the fill materials. 117 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Buenos Aires coastline in 1836 (Natural coast line), in 1964 (Artificial coast line) and nowadays. Modified from Marcomini and López (2004). Standard drilling methods do not provide the level of stratigraphic detail necessary to accurately describe the geology and hydrogeology of a site and to evaluate the true distribution of fill materials. Such lack of detail may result in construction worker health and safety issues, difficult excavation conditions, and off-site waste disposal requirements. Consequently, it could generate project delays, cost overruns and, in some cases, the termination of the project (Green et al., 1999; Byer and Mundell, 2004). On the other hand, near surface geophysical surveys allow for a very detailed and rapid characterization of subsurface materials at relatively low cost (Byer and Mundell, 2004; Prezzi et al., 2005). Taking into account the ongoing urban development and the absence of information about the nature of the filled zones in La Plata river, we try to determine the variation in the subsurface materials by identifying areas of similar and dissimilar properties. The type, homogeneity, spatial distribution and thickness of the fill materials will be investigated by means of ground geophysical surveys. Such information is vital for the adequate evaluation of the coastal sectors’ development options, for the corresponding environmental impact assessment and for a proper planning of urban expansion. Our approach consists of a multi-faceted geophysical survey conducted applying three types of geophysical methods: magnetic, ground penetrating radar (GPR) and electrical resistivity imaging (ERI). These methods are sensitive to: 1) metallic/conductive objects such as reinforced concrete, structural steel, and metal-bearing fill materials, and: 2) variations in soil and fill types based on subtle changes in soil moisture, porosity, and chemistry across the site. We surveyed different filled sectors (A and B) in Ciudad Universitaria (Fig. 2) with the aim of calibrate and test the suitability of the distinct geophysical methods for the characterization of the diverse fill materials. In sector A short wave-length, high amplitude, conspicuous and localized circular magnetic anomalies ranging between -700 and 500 nT were detected (Fig. 3). Such anomalies suggest the presence of demolition materials (beams, concrete blocks with iron rods, etc.) at shallow depths (fill). Maximum fill depths of approximately 10 m were estimated applying Euler deconvolution. In sector B, the magnetic survey detected a pattern which indicated the presence of an underground canal. Such pattern showed an elongated positive anomaly of 1900 nT. In this sector, fill depths of approximately 5 m were calculated through Euler deconvolution. GPR profiles did not generate good results due to high signal attenuation. The presence of clayey wet soil would be responsible of the very limited GPR signal penetration. Only a couple of diffraction hyperbolae were observed. In sector A, two ERI surveys were carried out using different electrode spacing and spread lengths. At the top of both sections a 5m thick layer was observed, which showed a patchy resistivity pattern with values ranging between 35 and 65 ohm.m (Fig. 4). This layer would indicate the existence of demolition material fill. 118 GEOSUR2010 A 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - Satellite images showing: A) the location of Buenos Aires and Ciudad Universitaria, B) the location of the sectors (A and B) surveyed in Ciudad Universitaria. Continuous white lines: magnetic profiles, dashed white lines: GPR profiles, black line: ERIs. B Fig. 3 - Magnetic anomalies detected in sector A. Open circles: measured magnetic stations. Short wavelength, high amplitude, circular anomalies ranging between -700 and 500 nT were detected. 119 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 4 - One of the ERI surveys conducted in sector A. Electrode spacing: 3m, spread length: 141 m. Below this layer, an important resistivity reduction is registered (down to 7 ohm.m), probably related to the phreatic zone. Below 20 m depth, resistivity increases up to 50 ohm.m (Fig. 4). Although GPR profiles did not provide good results, ERI and magnetic methods proved to be useful techniques, appropriate to characterize the filled zones along the coast of Buenos Aires. The obtained results allowed the determination of the thickness, homogeneity and type of the fill material in the studied areas. To further investigate the coastal filled zones in Buenos Aires, new magnetic, GPR, ERI and microgravity surveys will be conducted. REFERENCES • Byer, G. B. and Mundell, J. A., 2004. Use of Geophysical Surveys for Fill Characterization and Quantity Estimation at Brownfield Sites – A Case History. Proceedings: SAGEEP 2004, Environmental and Engineering Geophysical Society, Colorado Springs, Colorado, United States. • Green, A., Lanz, E., Maurer, H., and Boerner, D., 1999. A template for geophysical investigations of small landfills. The Leading Edge, 2: 248-254. • Marcomini, S.C. and López, R.A., 2004. Generación de nuevos ecosistemas por albardones de relleno en la costa de la ciudad de Buenos Aires. Revista de la Asociación Geológica Argentina, 59(2):261-272. • Prezzi, C., Orgeira, M.J., Ostera, H. and Vásquez, C.A., 2005. Ground magnetic survey of a municipal solid waste landfill: pilot study in Argentina. Environmental Geology, 47(7): 889-897. doi: 10.1007/s00254-004-1198-6. 120 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA BAJADA DEL DIABLO IMPACT CRATER-STREWN FIELD (ARGENTINA): GROUND MAGNETIC AND ELECTROMAGNETIC SURVEYS 3-11 Prezzi, C.B.1*, Orgeira, M.J.1, Acevedo, R.2, Ponce, F.2, Martínez, O.3, Vásquez, C.1, Corbella, H.4, González, M.2, Rabassa, J.2 (1) CONICET – Universidad de Buenos Aires. INGEODAV, Dpto. Cs. Geológicas, FCEyN, UBA, Ciudad Universitaria, Pabellón 2, Buenos Aires, Argentina (2) CADIC – CONICET. Ushuaia, Tierra del Fuego, Argentina (3) Universidad Nacional de la Patagonia San Juan Bosco. Esquel, Chubut, Argentina (4) CONICET – Museo Argentino de Ciencias Naturales Bernardino Rivadavia. Buenos Aires, Argentina * Presenting author’s e-mail: [email protected] Bajada del Diablo impact crater field is located in the Northern Patagonian Massif, Chubut, Argentina (Fig. 1). Impact craters have been identified on two rock types: the Quiñelaf Eruptive Complex and Pampa Sastre Formation (Acevedo et al., 2009). Most of the rocks forming the Quiñelaf Eruptive Complex have been classified as trachytes, but other rocks are present, such as rhyolites, trachyandesites, trachybasalts, and pyroclastic rocks. Pampa Sastre Formation corresponds to conglomerate layers with basalt clasts boulder and blocks in size (up to 50 cm in diameter) in a coarse sandy matrix. The study area (Fig. 1) includes at least 66 impact craters found in Miocene olivine basalts of the Quiñelaf Eruptive Complex and in the Late Pliocene/Early Pleistocene Pampa Sastre conglomerate (Acevedo et al., 2009). It is widely accepted that a key tool in the initial recognition and characterization of terrestrial impact craters is geophysics (e.g. Pilkington and Grieve, 1992; Hawke, 2004). The magnetic signature of craters varies considerably (Pilkington and Grieve, 1992), but an overall circular magnetic low due to demagnetization of the target rocks and reduction in susceptibility is expected (Pilkington and Grieve, 1992; Hawke, 2004). Pilkington and Grieve (1992) established a set of general criteria that correspond to the geophysical signature of impact craters. These criteria can be used to evaluate the hypothesis of impact origin of circular structures. However, such origin can only be confirmed on the basis of geologic evidence. With the aim of further investigate the proposed impact origin of the circular structures identified in Fig. 1 - Location map. The inset shows a satellite image of the study area. Modified from Acevedo et al. (2009). 121 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 Detailed topography of crater 8. Black dots: magnetic stations measured in and out crater 8. Bajada del Diablo (Acevedo et al., 2009), we carried out detailed topographic, magnetic and electromagnetic ground surveys in two craters (8 and A) found in Pampa Sastre conglomerates. Both craters are simple, bowl-shaped structures with rim diameters of 300 m and maximum depths of 10 m (Figs. 2 and 3). They have been partially filled in by debris flows from the rims and wind-blown sands (Acevedo et al., 2009). Total magnetic field was measured at 1563 stations located in and out of craters A and 8, using a Geometrics 856 proton precession magnetometer (Figs. 2 and 3). The obtained data were corrected for the diurnal variations in the Earth’s magnetic field and the IGRF value was subtracted. Basalts boulders, sandy matrix and infilling sediments were collected, and the corresponding magnetic susceptibilities were measured; the intensity of the Fig. 3 - Detailed topography of crater A. Black dots: magnetic remanent magnetization of basalt stations measured in and out crater A. boulders was also measured. 20 profiles were surveyed at crater 8 with a GEM-2 small broadband electromagnetic sensor using 5 different frequencies. Detailed crater topography was determined using a total station. 726 topographic points were surveyed in craters A and 8. The magnetic anomalies show a circular pattern with magnetic lows (-100 to -200 nT) in the crater’s floors, characteristic of impact structures. Furthermore, in the crater’s rims, high-amplitude, conspicuous and localized (short wavelength) anomalies, ranging between 2000 and -1500 nT, are observed (Figs. 4 and 5). Such large amplitude and short wavelength anomalies are not detected out of the craters. Euler’s deconvolution was applied in order to estimate the depth of the sources. The first and the second vertical derivatives, the analytic signal and the curvature attributes of the residual magnetic field, were also calculated with the aim of sharpening and further analyse the detected anomalies. 2.5 and 3D modelling were carried out, considering the existence of induced and remanent magnetizations. The parameters used for each modeled body (i.e. susceptibility and remanent 122 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 4 - Magnetic anomalies detected in and out crater 8. Diamonds: magnetic stations. magnetization intensity) were measured in the laboratory and/or estimated taking into account previously published data. For all used frequencies, the electromagnetic profiles show lower apparent electrical conductivities in the crater’s floor, while the rims present notably higher values (Fig. 6). Our results suggest that in the crater’s floors Pampa Sastre conglomerate would be absent or deeply buried. On the contrary, the crater’s rims exhibit high-amplitude, localized magnetic anomalies and higher apparent electrical conductivities, which would be related to the anomalous accumulation of basalt boulders and blocks remanently magnetized (probably due to shock and heat effects). The fact that such highamplitude anomalies are not present out of Fig. 5 - Magnetic anomalies detected in and out crater A. Diamonds: magnetic stations. Fig. 6 - Apparent electrical conductivity registered in crater 8 using 3950 Hz. Grey dashed lines: contour lines showing crater 8’s detailed topography. 123 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 the surveyed craters, supports this hypothesis. The morphological, geological and geophysical features of the studied circular structures could only be satisfactorily explained assuming an extra-terrestrial projectile impact. REFERENCES • Acevedo, R., Ponce, J., Rocca, M., Rabassa, J. and Corbella, H., 2009. Bajada del Diablo impact crater-strewn field: The largest crater field in the Southern Hemisphere. Geomorphology, 110: 58-67. • Hawke, P., 2004. The geophysical signatures and exploration potential of Australia’s meteorite impact structures. PhD Thesis, The University of Western Australia, 314 pp. • Pilkington, M. and Grieve, R., 1992. The geophysical signature of terrestrial impact craters. Reviews of Geophysics, 30: 161-181. EARTH TIDE OBSERVATIONS IN TIERRA DEL FUEGO (ARGENTINA) A.1*, R.1, Richter, Perdomo, Hormaechea, J. Fritsche, M.3, Scheinert, M.3, Dietrich, R.3 L.2, Mendoza, L.1, Del Cogliano, 3-12 D.1, (1) Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque, 1900 La Plata, Argentina (2) Estación Astronómica Río Grande, Acceso Aeropuerto, V9420EAR Río Grande, Argentina (3) Institut für Planetare Geodäsie, Technische Universität Dresden, 01062 Dresden, Germany * Presenting author’s e-mail: [email protected] The solid earth tides, originating from luni-solar forcing, manifest themselves in different effects, such as gravity variations, deformations of the earth crust, and variations in the tilt of the earth surface with respect to an equipotential surface. The ocean tidal loading generates an additional contribution to these effects. The load tides depend on the ocean tide signal and the elastic-rheological properties of the earth crust. These tidal effects are contained in geodetic (e.g. GPS), astronomical (e.g. zenith tube) and geophysical (e.g. gravity metre) observations and exceed nowadays achievable measurement uncertainties. They must therefore be accounted for in high-accuracy observations of different geophysical phenomena. On the other hand, such observations provide the opportunity to gain new insights into the response of the solid earth to the tidal forcing. The first systematic investigation of tidal effects in Tierra del Fuego was based on lake-level observations in Lago Fagnano. Based on pressure tide gauge records at three sites in the lake the amplitudes, phase angles, and circulation patterns for the four main tidal waves were determined. In this way we utilized the lake as a 100 km long tidal tilt sensor. The comparison of the observed lake tide signal with a model accounting for both solid earth tides and load tides revealed a significant deviation of the observations from the theoretical prediction. It suggests that this difference is due to an anomalous amplification of the load tides in the order of 20%. One possible explanation of this anomaly consists in a deviation of the elastic crustal properties in the Lago Fagnano region from the global earth model assumed in our load tide model. With the aim to shed light onto the cause of the detected anomaly additional earth tide observations were commenced at a number of sites in the Argentine part of Tierra del Fuego main island in November 2009. Here we present the continuous tidal gravity metre and GPS records obtained so far. The results of a preliminary tidal analysis of these records are compared to model predictions. They are discussed with an emphasis on their possible implications for the effective elastic crustal rheology in the region under investigation. 124 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA MORPHO-BATHYMETRIC SURVEY OF LAGO ROCA (TIERRA DEL FUEGO) 3-13 Lodolo, E.1*, Tassone, A.2, Baradello, L.1, Lippai, H.2, Grossi, M.1 (1) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy (2) CONICET-INGEODAV. Dpto. de Ciencias Geológicas. Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires, Argentina * Presenting Author’s email: [email protected] Lago Roca occupies a secondary tectonic lineament pertaining to the main Beagle Channel fault system (Tassone et al., 2005). It is located within the Lapataia Natural Park, 25 km to the west of the city of Ushuaia (Fig. 1). The lake, trending broadly NW-SE, is about 10.5-km-long, with average width of about 0.7 km. In order to analyze the main morphological and shallow structural setting of this basin, an extensive high-resolution seismic survey was carried out on November 2009, in the frame of an Italian-Argentinean cooperative research study funded by the Italian Foreign Ministry. These data have permitted to derive for the first time a bathymetric map of the lake (through the conversion of seismic arrival times in water depths), and image morphologies and depositional architecture of the glacial and glacio-lacustrine deposits filling the basin. These information, combined with analysis of structural and geomorphological features of the surrounding areas, will aid to reconstruct the Roca basin origin and identify the Late Quaternary inter-glacial episodes responsible of the deposition of the sedimentary sequences. The glacial activity, in combination with the sea level variations and tectonic activity, has played an important role in shaping the morphology of the Lago Roca basin. In fact, the area lies at the foot of the Cordillera Darwin, where a large ice-sheet is still present above the higher peaks of this mountain chain (Gordillo et al., 1993). Morphological and sedimentological evidence, mostly represented by raised beaches and deposits rich in marine organisms, testify that the northern coast of the Beagle Channel has undergone a general drop of the sea level during Holocene (Rabassa et al., 1986). This progressive marine regression has modified the coastal landscape of the area and in some cases has severely changed the morphological environment of the surrounding areas (Borromei and Quattrocchio, 2007). Moreover, the general morphology of Lago Roca is clearly controlled by the tectonic activity along the Beagle Channel fault system and its geometry most probably reflects its sub-bottom structure. Fig. 1 - Left: Bathymetric map of Lago Roca (maximum water depth is 85 m), and Digital Elevation Model of the surrounding areas. Box indicates the location (light grey star points to Lago Roca). Right: Position map of the highresolution seismic profiles acquired in Lago Roca. 125 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA REFERENCES • Borromei, A. M. and Quattrocchio, M. (2007). Holocene sea-level change inferred from palynological data in the Beagle Channel, southern Tierra del Fuego, Argentina. Ameghiniana, 44, (1). Buenos Aires. Jan./Mar. Print version. • Gordillo, S., Coronato, A. and Rabassa, J. (1993). Late Quaternary evolution of a sub-antarctic paleofjord, Tierra del Fuego. Quat. Sci. Rev., 12, 889-897. • Rabassa, J., Heusser, C. and Stuckenrath, R. (1986). New data on Holocene sea transgression in the Beagle Channel: Tierra del Fuego, Argentina. Quat. S. Am. Ant. Pen., 4, 291-309. • Tassone, A., Lippai, H., Lodolo, E., Menichetti, M., Comba, A., Hormaechea, J. L. and Vilas, J.F. (2005). A geological and geophysical crustal section across the Magallanes-Fagnano fault in Tierra del Fuego and associated asymmetric basins formation. Jour. South Am. Earth Sci., 19, 99-109. 126 Session 4 TECTONIC PROCESSES AND SEISMOLOGY GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA NEW STRUCTURAL MAPS AND CROSS-SECTIONS OF THE PATAGONIAN FOLD-THRUST BELT NEAR SENO OTWAY, SENO MARTÍNEZ AND PENINSULA BRUNSWICK, SOUTHERN CHILE 4-01 Betka, P.1*, Klepeis, K.2, Mosher, S.1 (1) Jackson School of Geosciences, The University of Texas at Austin, 1 University Station C1100, Austin, Texas 78712, USA (2) Geology Department, The University of Vermont, 180 Colchester Ave, Burlington, VT 05405, USA * Presenting author’s e-mail: [email protected] We present preliminary results, maps and cross-sections from two strike-perpendicular transects across the Patagonian retroarc fold-thrust belt (FTB) near: A) Seno Otway; B) Peninsula Brunswick and Seno Martínez. The onset of the Andean orogeny between 50°-56°S is marked by the LateCretaceous closure and inversion of the Rocas Verdes back-arc basin and the development of the Patagonian retroarc FTB. The nearly 90° bend in the trend of the orogen in this location is commonly attributed to significant along-strike variation in structural-style and tectonic shortening during the development of the FTB. Despite studies elsewhere in southern Patagonian, relatively little is known about the FTB in a ~150 km2 area south of 53.0°S and west of 70.5°W. Near the southwestern regions of Seno Otway chlorite-grade pelitic schists (basement) are imbricated with massive basalt, gabbro, chert and quartzite of the Rocas Verdes basin floor and mudstone (Zapata Fm.) representing the basin fill. Farther to the north, basement involved thrust sheets are structurally above the Zapata Fm. and foreland basin strata (Punta Barrosa and Cerro Torro Fms.) along an outof-sequence thrust (OOST). Below the OOST overturned-to-the north, northwest plunging tight folds thicken the Zapata Fm. Foreland basin strata display decameter-scale overturned folds. At least one main thrust imbricates the foreland basin strata. Several late, brittle strike slip faults cross cut contractional structures in this area. Near Seno Martínez, south of the Magallanes-Fangano fault zone (MFFZ) mafic, chloritic schsits interpreted as Rocas Verdes infill are structurally above a ~25 km exposure of chorlite- and garnetgrade basement schists of the Darwin Complex. An ~8 km wide ductile shear zone characterized by a SW-plunging down-dip quartz lineation, quartz rods up to 50 cm long, and sheath folds that have southwest-plunging long-axes internally thickens the basement schists. Structurally below this shear zone, chlorite-grade phyllitic shists of the Darwin Complex are in startigraphic contact with the Tobifera Formation. North of the MFFZ, the Tobifera Fm. outcrops structurally above turbiditic rocks of the Zapata Fm. that are folded by tight overturned folds with a top-to-the-northeast vergence. The Zapata Fm. is cut by a south-dipping OOST that places deformed rocks from the Zapata Fm., and Tobifera Fm. above weakly deformed foreland basin strata (Punta Barrosa, Cerro Torro and Tres Pasos Fms.). Deformation propagated at least 25 km northward into the foreland and at least three thrust faults cut up-section into foreland basin strata. Two large (>5m wide) strike-slip fault zones cut contractional structures along the eastern shore of Peninsula Brunswick. Our preliminary results provide new structural constraints on the development of the Patagonian FTB and help to link structures previously described in the Ultima Esperanza and Tierra del Fuego regions of southern Patagonia. 129 GEOSUR2010 TECTONIC EVOLUTION OF THE BERMEJO BASIN FROM BROKEN PLATE FLUXURAL MODEL (PRELIMINARY STUDY) 22-23 NOVEMBER 2010 – MAR DEL PLATA 4-02 Carugati, G.1*, Novara, I.L.1, Gimenez, M.E.2, Introcaso, A.1 (1) CONICET. Grupo de geofísica del Instituto de Física Rosario – UNR, Av. Pellegrini 250. Rosario. (2) CONICET. Instituto Geofísico Sismológico “Ing. Fernando Séptimo Volponi”. Facultad de Ciencias Exactas, Físicas y Naturales, UNSJ, Av. Jose I. de la Rosa y Meglioli. San Juan. 5400 * Presenting author's e-mail: [email protected] In this paper an isostatic flexural elastic model is proposed as the main mechanism in Bermejo foreland basin structures formation. Two periods can be described since the structures formation, or at least since the Neogene’s time, in which tectonic phase has canghed. A first one includes an interval since the formation of Andean foreland until the beginning of Western Pampean Ranges uplift. In this period the main mechanism of elastic flexure in continuous plate was suggested. A second and later period, includes the Desaguadero-Bermejo megafault reactivation when the continuous plate elastic mechanism ends, giving place to a broken plate flexural mechanism. Flexural responses have been obtained by considering a 7 km equivalent elastic thickness crust. Sedimentary thickness and shallow basin features obtained from residual Bouguer anomalies is consistent with previous models TECTONIC IMPLICATIONS OF A PALEOMAGNETIC STUDY OF MESOZOIC 4-03 MAGMATIC ARC ROCKS IN CIERVA POINT, NORTHWEST ANTARCTIC PENINSULA Cosentino, N.J.1*, Tassone, A.A.1,2, Lippai, H.F.1,2, Vilas, J.F.A.1,2 (1) Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires - Pabellón II, Ciudad Universitaria, Ciudad de Buenos Aires (Argentina) (2) Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) * Presenting author’s e-mail: [email protected] Introduction Antarctic Peninsula’s paleogeographic evolution since Gondwana’s fragmentation is still a subject of debate. This is so for two main reasons: the fact that the Scotia plate got in the way between the South America and Antarctica plates many millions of years after Gondwana’s break-up, destroying the ocean floor’s magnetic anomalies between these two plates in the process, and the fact that only a reduced amount of paleomagnetic data exists for Antarctic Peninsula (AP). A paleomagnetic sampling of Cierva Point’s Late Jurassic – Early Cretaceous magmatic arc rocks has been carried out; the studied area is located in the Danco coast northwest of the peninsula, at coordinates 64°09’S and 60°57’W (Fig. 1), within a protected area (ZAEP 134) which also includes the argentinian base Primavera. The outcrops consist of plutonic rocks (granodiorites, tonalites and granites) intruded in acid volcaniclastic host rocks (cristaline and vitreous tuff). This predominantly acidic volcanism belongs to the Antarctic Peninsula Volcanic Group (APVG), defined in other parts of northern AP. The age of the Fig. 1 - (a) Regional map volcaniclastic rocks is assigned to the interval 162-153 My, showing the sampling area in the based on a correlation with similar outcrops of that age, in an context of Antarctic Peninsula. area close to Cierva Point (Pankhurst et al., 2000). Available (b) A local map showing the K/Ar whole rock dating of the intrusive rocks yielded a Late sampling location in detail 130 GEOSUR2010 a 22-23 NOVEMBER 2010 – MAR DEL PLATA b Fig. 2 - (a) IRM adquisition curve showing a quick saturation attained at values of ~ 200 mT. (b) curves numerically fitted according to two mineralogical phases with defined coercitivity intervals. The number of phases is iteratively calculated using the maximization-expectation algorithm. Cretaceous age (95 Ma, Codignotto et al., 1978). Magnetic mineralogy The magnetic mineralogy of the sampled rocks (including the plutonic body and its host) was studied with different techniques. Isothermal remanent magnetization (IRM) was acquired by samples, obtaining curves which were then modelled into their individual components of coercitivity using the maximization-expectation algorithm (Fig. 2). Also, Lowrie demagnetization experiments were carried out in order to analyse different magnetic mineralogy according to coercitivity (Fig. 3). Finally, thermorremanent experiments allowed determination of magnetic domains (Fig. 4). These studies were carried out with the objective of defining the magnetic remanence carriers. The results show a generalized dominance of MD or PSD magnetite with coercitivities in the range of 15-54 mT among the magnetic mineralogy. Fig. 3 - Lowrie thermal demagnetization curves. The x, y and z curves represent mineralogical populations with coercitivities of 0-120, 120-400 and 400-1000 mT, respectively. The most important contributor is the first of these populations, which is in accordance with the IRM acquisition curves (Fig. 2 (a)). Plotting normalized values of IRM allow a ~ 580°C visual estimation of the Curie temperature for all populations. Paleomagnetism Samples were demagnetized by the AF method in almost all cases, successfully Fig. 4 - Thermorremanent curves showing the relationship between mass-normalized magnetic susceptibility and defining characteristic remanence temperature. The continuous curve shows the heating cycle (in magnetizations by principal component Ar atmosphere) and the discontinuous one the cooling cycle. The analysis (Fig. 5). Two populations were Verwey transition is identified, as well as a drastic Km drop at ~ defined according to the strength of the 580°C. Also, an irreversible local maximum at ~ 300°C in Km is remanence, measured by the field by observed during the heating cycle. which half of the original NRM vanished (Fig. 6). The hard and soft remanence populations strongly coincide with the volcaniclastic host rocks and the plutonic rocks, respectively. Previous sampling of the former in this same area (Valencio et al., 1979) defined five sites with similar magnetic behaviour which group with high precision parameter with 131 GEOSUR2010 a 22-23 NOVEMBER 2010 – MAR DEL PLATA b Fig. 5 - NRM demagnetization curve and vector components diagrams of a sample corresponding to the (a) hard and (b) soft magnetization populations. Solid and open data points indicate vector end points projected onto the horizontal and vertical plane, respectively. the hard population defined in this study. Hence, a unique population was defined in this case. Two average poles were calculated from these two populations: 76.5°S, 49.2°E, dp = 7.6°, dm =8.7°, N = 11 (hard population) and 82.4°S, 148.3°E, dp = 3.6°, dm = 4.1°, N = 19 (soft population). VGP dispersion values of S ~ 14.3° and ~ 9.6° for the hard and soft populations, respectively, indicate an adequate paleosecular variation sampling in the first case. In the second case, the extremely low value of S can be explained either by an inadequate sampling of the paleosecular variation or by an adequate sampling of it at in intra-site level. All of the sites corresponding to the soft population have normal polarity directions, while some of the hard population sites are of reverse polarity. However, the Fig. 6 - Equal-area projection of the 26 sitemean directions. The directions in black correspond to the hard remanence directions, while the white ones correspond to the soft remanence directions. The latter’s average is distinct from the GAD direction in the area to the 95% significance. latter consist of too few specimens to define well-grouped site-mean directions. Fig. 7 - South America’s reference paleomagnetic poles (Somoza & Zaffarana, 2008; Besse & Courtillot, 2002) were rotated using the SAM with respect to AP Euler pole (of appropriate age) and marked with their respective ages. Also shown are the average poles obtained in this study (1: hard population, 2: soft population) and Antarctica in today’s coordinates. 132 Discussion A number of different rigid-plate cinematic models have been proposed for Antarctica since Gondwana’s breakup. One of these (Ghidella et al., 2002) calculates its rotation poles from four interval poles between 160 and 83 My, which are obtained from magnetic and gravimetric lineations in western Weddell under the assumption that they represented movement between Antarctica and South America (SAM). This model does not take into consideration relative movement between AP and SAM, and does not 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 Fig. 8 - Equal-area projection showing the average poles obtained in this study (black: hard population, grey: soft population) and the EANT reference paleopoles between 150 and 90 Ma (Torsvik et al., 2008).on, 2: soft population) and Antarctica in today’s coordinates. generate any overlapping between these continental masses. Considering this model to be correct, the comparison between SAM’s reference poles and this study’s average poles (Fig. 7) suggests a net local counter-clockwise rotation with respect to a vertical axis of 61-47° between 150-120 My and 100-90 My. If, on the other hand, the poles obtained in this study are considered trustful paleomagnetic poles, the comparison between these and the reference East Antarctica (EANT) paleopoles of the same age (Fig. 8) suggests a regional counter-clockwise rotation of AP with respect to EANT between 150-120 My and 100 My, after which no more relative motion takes place. Paleogeographic reconstructions according to the Konig and Jokat (2006) plate model give credence to these results. REFERENCES • Besse, J., Courtillot, V. 2002. Apparent and true polar wander and the geometry of the geomagnetic field over the last 200 Myr. Journal of Geophysical Research, vol. 107, no. B11. • Codignotto, J. O., Llorente, R. A., Mendía, J. E., Olivero, E., Spikermann, J. P. 1978. Geología del Cabo Spring y de las islas Leopardo, Pingüino y César. Contribución del Instituto Antártico Argentino Nº 216, Buenos Aires • Ghidella, M. E., Yaniez, G., LaBrecque, J. L. 2002. Revised tectonic implications for the magnetic anomalies of the western Weddell Sea. Tectonophysics 6528. • Konig, M., Jokat, W. 2006. The Mesozoic breakup of the Weddell Sea. Journal Geophysical Research 111 (B12): 102, doi: 10.1029/2005JB004035. • Pankhurst, R.J., Riley, R. R., Fanning, C.M., Kelley, S. P. 2000. Episodic silicic volcanism in Patagonia and the Antarctic Peninsula: Chronology of magmatism associated with the break-up of Gondwana. Journal of Petrology: 41(5): 605-625. • Somoza, R., Zaffarana, C. B. 2008. Mid-Cretaceous polar standstill of South America, motion of the Atlantic hotspots and the birth of the Andean cordillera. Earth and Planetary Science Letters 271, 167-277. • Torsvik, T., Gaina, C., Redfield, T. 2008. Antarctica and Global Paleogeography: From Rodinia, Through Gondwanaland and Pangea, to the Birth of the Southern Ocean and the Opening of Gateways. • Valencio, D. A., Mendía, J. E., Vilas, J. F. 1979. Palaeomagnetism and K-Ar Age of Mesozoic and Cenozoic Igneous Rocks From Antarctica Earth and Planetary Science Letters, 45 61-68. PALAEOTECTONIC SETTING OF PRECUYANO GROUP. UPPER TRIASSIC- LOWER JURASSIC VOLCANIC DEPOSITS OF THE NEUQUEN BASIN (37º- 39º 30´LS). ARGENTINA 4-04 Delpino, D.1*, Bermudez A.2 (1) YPF, E and D, Talero 360, Neuquén, Argentina (2) CONICET, National University of Comahue, Neuquén, Argentina * Presenting author’s e-mail: [email protected] The Upper Triassic–Lower Jurassic volcanic continental sedimentary sequences contained within the Precuyano Group are considered to be related to the beginning of the structural evolution of the Neuquén Basin, one of the most productive hydrocarbon basins of Argentina. 133 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Plate reconstruction for Pangaea after Vaughan and Storey (2007). Areas affected by deformation (dark grey). Precuyano depocentres: L (Lapa), CB (C° Bandera), CM (Cupen Mahuida), LN-LY (Loma Negra-La Yesera), M-PA (Medanito - Portezuelo Alto), LA (Loma Amarilla), CC (Cara Cura), Z (Zapala). The purpose of this paper is to define the palaeotectonic setting of this particular unit using a geochemical data set of 150 samples obtained from different depocentres within the Neuquen Basin, (Fig 1). The Basin is located adjacent to the western palaeomargin of the Pangaea supercontinent, and developed with a regional arc/backarc extensional event that generated dozens of half-graben on Paleozoic and lower Triassic age “basement”. The typical half-graben has an elongate shape with an axial length from 15 to 30 km. Today some half-grabens Fig. 2 - Winchester and Floyd (1977) Nb/Y vs Zr/Ti diagram. are now partially or totally exposed due Field A,120 acid rocks. Field B 30 intermediate and basic rocks. to Andean orogenesis, while others remain covered by Mesozoic and Tertiary sequences. The half-graben evolution coeval with the development of a Upper Triassic-Lower Jurassic eruptive period are referred to as “Precuyanolitense”. A large volume of volcanic rocks was erupted within a short geologic time period (213-198 Ma). Tectonic-volcanic activities generated sedimentary-volcanic sequences up to 2000 m thick. Spatial and volumetric thickness distributions were controlled primarily by Fig. 3 - Sun and McDonough (1989) chondrite Normalized diagram. Open boxes average acid rocks pattern. faults, and associated local syn-deposiDotted line basic rocks pattern. tional structures. As a consequence, volcanic and sedimentary deposits show abrupt thickness changes and local discordant relationships. Typical Precuyano deposits are formed by alternating sequences of acid pyroclastic flow, tuff fallout, volcaniclastic and continental sedimentary rocks, as well as basic and intermediate lava flows. Dacitic and Rhyolitic (SiO2 66-74%) pyro134 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA clastic rocks generally predominate. Acid tuffs have crystal-rich and vitriclastic porphyritic textures with phenocrysts mineralogy: plagioclase + quartz + anfíbol +/- biotite +/- opaque minerals. Intermediate rocks (SiO2 56-60 %) have porphyritic textures with phenocrysts of plagioclase and amphibole. Porphyritic texture, with plagioclase as phenocrystal, characterize basic rocks (SiO2 47- 53 %). Using the Nb/Y vs Zr/Ti diagram for identifying rock type samples classified as Dacites, Rhyodacites, Andesites, Basaltic Andesites and Fig. 4 - Basic (dotted line) and acid (open squares) rocks patterns on Basalts. Nb/Y ratios are typical MORB-normalized diagram with trace elements placed in order of calc-alkaline series (Fig. 2). Rare increasing incompatibilty from Sr to Yb. Earth Elements abundances normalized to Chondrite values, shows different patterns in basic and acid rocks. Basaltic rocks normalized to Chondrite show patterns with low La/Yb(n)=4.4, La/Sm(n)=1.87 Sm/Yb(n)= 2.35 and lack Eu anomaly (Fig. 3). Low La/Yb and Sm/Yb ratios indicate relative high percentage melts without residual garnet in the source. Dacites and Rhyolites rocks pattern are distinguished by higher La/Yb(n)= 10, La/Sm (n) = 5, but lower Sm/Yb(n)= 1.8 and negative Eu/Eu*= 0.57 anomalies (Fig. 3). High La/Yb ratios in silicic magmas can be a sign of magmas equilibrated with amphibole or accessory mineral-bearing residual mineral assemblages. Incompatible LREEs enrichment and negative Eu anomaly indicate plagioclase subtraction or residual plagioclase in a crustal source region or both. HREE straight pattern shows that amphibole could have played an important part during the crystallization processes. Basic and acid rocks MORB-normalized show patterns of calc-alkaline series and the key feature of volcanic arc rocks which is Nb-Ta negative anomaly. (Fig. 4). Another feature of basic calc-alkaline rocks formed in continental arcs is the similar abundances of Ti and Y than N-MORB and the relatively higher than MORB Nb and Zr contents. Acid rocks patterns on MORB-normalized diagrams show negative anomalies of Ti, P and Ba and low values of Sr. These anomalies were probably controlled by fractionation of Titanite (Ti), Apatite (P), feldspar (Ba) a b Fig. 5 - 5a. Discrimination diagram (Wood, 1980). Field A Basic lavas. Field B Acid lavas. 5b. Discrimination diagram for acid rocks (after Pearce et al, 1984). All samples plott in volcanic arc field. 135 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 6 - Schematic cartoon showing the proposed tectonic setting of the Precuyano Group. and plagioclase (Relative low Sr and Eu negative anomaly) crystallizing phases. Ratios of basic rocks La/Ta=44, Ba/La=33 and Ba/Ta=726 reflect slab related processes. Low Ta/Hf=0.11 ratio indicate depleted MORB or arc mantle sources and Th/Hf=0.61 are typical of calcalkaline arc sources with components from a subducting oceanic slab. Acid rocks ratios La/Ta=34, Ba/Ta=906 and Ba/La=27 are also typical of arc magmas. Low Ta/Hf=0.16 are characteristic of depleted mantle and low Th/Hf=0.25 is distinctive of source components from a subducting slab. On Th-Hf-Nb discrimination diagram that can be applied to intermediate and silicic lavas as well as to basalts and it is particularly good at identifying volcanic-arc basalts. Basic and acid rocks plot in the field of calc-alkaline arc rocks (Figs. 5a and b). On tectonic discrimination diagram based on Ta vs Yb, Precuyano samples plot within the field of acid rocks originated from an igneous source (Type I) characteristic of rocks originated in active Andean Type convergent margins. Conclusions The interpretation of the geological and geochemical data indicates that between latidudes 37°-39°LS the western margin of the Pangaea supercontinent during the Upper Triassic-Low Jurassic times was strongly influenced by the subduction of an oceanic plate underneath a continental plate (Fig. 6). This collisional zone has an evident continuity with similar structures postulated by different authors for the same times for Pangea margin south of 40° LS extending the paleosubduction continuity northward to 37º LS. According to rock geochemical characteristics this active continental margin could be classified as an “Andean” type, with patterns displaying negative anomalies of Nb-Ta, normalized values of some trace elements La, Ta, Hf, Th and low Y values. This indicates that these rocks belong to the calc-alkaline series. Basic rocks do not show evidence of important fractionation processes. On the other hand, acid rocks show evidence of magmatic disequilibrium or fractional crystallization processes developed inside the magmatic chambers. Half-graben volcanic sequences have notable regional compositional homogeneity suggesting that its formation was controlled by major geotectonic processes, influenced by subducting oceanic plate with associated intra-arc to back arc extension. Two principal features strongly influenced depocenters fill histories: 1) volume of volcanic rocks, generate during the maxima volcanic activity and 2) coeval tectonic evolution of the half-grabens. REFERENCES • Pearce J., Harris N. and Tindle A., 1984. Trace element discrimination diagrams for the tectonic interpretacion of granitic rocks. 136 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES Journal of Petrology, V. 25, pp. 956-983. • Sun,S. and McDonough,W.,1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geological Society of London, Special Publications, V.42, pp. 313-345. • Vaughan,A and Storey,B, 2007. A new supercontinet self-destruct mechanism: evidence from the Late Triassic-Early Jurassic. Journal of the Geological Society of London, V.164, pp. 383-392. • Winchester, J. A. and Floyd, P. A., 1977. Geochemical discrimination of different magma series and their differentiation products using immobile elements. Chemical Geology, V 20, pp. 325–343. • Wood, D., 1980. The application of a Th–Hf–Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary volcanic province. Earth and Planetary Science Letters V. 50, pp. 11–30. PRELIMINARY RESULTS OF A PALEOMAGNETIC STUDY ON THE ORDOVICIAN CALMAYO GRANITOID, SIERRAS DE CÓRDOBA, ARGENTINA 4-05 Geuna, S.1,2*, D’Eramo, F.1,3, Pinotti, L.1,3, Di Marco, A.2, Mutti, D.2, Escosteguy, L.4 (1) CONICET (2) Departamento de Ciencias Geológicas, FCEyN, Universidad de Buenos Aires. Ciudad Universitaria Pab. 2, 1874 CABA, Argentina (3) Departamento de Geología, FCEFQyN, Universidad Nacional de Río Cuarto (4) Instituto de Geología y Recursos Minerales, Servicio Geológico Minero Argentino * Presenting author’s e-mail: [email protected] Introduction The South American margin is characterized by the successive accretion of terranes (Pampia, Precordillera-Cuyania, and Chilenia) during the Early-Middle Palaeozoic; the evolution of the associated magmatic arcs and the collisional settings can be studied through the presently outcropping related granitoids. The Sierras de Córdoba are an outstanding place to study the Pampean, Famatinian and Achalian magmatism, linked to the aforementioned accretions in the Cambrian, Ordovician and Devonian, respectively. This contribution presents preliminary results of the paleomagnetic study of the Calmayo tonalite, an Ordovician pluton outcropping in the Sierra Chica de Córdoba (Fig. 1). The results allowed establishing several working hypotheses involving the Pampean terrane history, and the way it was affected by later deformation events. Geological Background The Ordovician magmatism in the Eastern Sierras de Córdoba is coeval with Famatinian arc magmatism located to the west. They are a dozen of small, discordant plutons, interpreted as emplaced at shallow depths in a rigid basement (Bonalumi and Baldo, 2002). The Calmayo tonalitetrondhjemite (32o S, 64o30’ W, Córdoba, Argentina) is an elliptical pluton that trends NE, with a maximum extension of 4.5 x 2.5 km (Fig. 1). It is a zoned leucotonalite which locally shows fragile deformation overprinted. A crystallization age of 490 Ma was obtained by D’Eramo (2003). The north-eastern corner of the tonalite is affected by the Soconcho shear belt, imposing a mylonitic foliation to the area (Martino 2003). The belt has inverse cinematic with a dextral component, and it was reactivated later in a fragile regime. The age for the shear belt is unknown, though it was interpreted as a contractional Achalian (Devonian) belt related to the collision of Cuyania/Chilenia (Martino, 2003). Paleomagnetic Study Palaeomagnetic sampling was carried out on twenty-eight sites widely distributed in the Calmayo tonalite (Fig. 1). 137 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES Fig. 1 - Geological map of Calmayo area in the Sierra Chica de Córdoba. Location of other trondhjemite plutons of similar age is also shown. Fig. 2 - Typical magnetic behavior. (left) Vector end-point diagrams and (right) normalized intensity plots for thermal demagnetization from 0 to 680oC. A) Most of the sites show hematite with a small content of ilmenite as the main magnetic carrier. The high coercivity shown by AF demagnetization (inner inset) is characteristic of hematite, while the unblocking temperature (higher than magnetite -580oCand lower than pure hematite 680oC-) indicates a small amount of Ti in the structure. B) The magnetic component erased at lower temperatures and lower alternate fields points to magnetite accompanying hematite in a few sites (an AF of 20 mT was applied previous to thermal demagnetization). On the demagnetization diagram open (solid) symbols indicate projection onto the vertical (horizontal) plane. Insets in normalized intensity plots show normalized alternating field demagnetization on the same samples. 138 GEOSUR2010 Magnetite is present only in the north-eastern area of the pluton, the rest of it being weakly magnetic, with a mean magnetic susceptibility of 25 x 10-5 (SI), mainly due to the content of ilmeno-hematite and biotite. Five sites were discarded due either to intra-site inconsistency or very weak magnetization. The remaining 23 sites have ilmeno-hematite as the main magnetic carrier of a stable remanence, as identified by thermal and alternating field (AF) demagnetization procedures (Fig. 2). Magnetite may appear in subordinate proportions. The ilmenohematite is an abundant accessory mineral, which shows as exsolved intergrowths with (hemo)ilmenite. The hematite-rich member is usually the host of discshaped rods of exsolved ilmenite-rich member. Thermal demagnetization up to 620-640oC isolated steeply dipping, normal polarity remanence directions (Fig. 3). The preliminary mean direction is Dec. 270o, Inc. -73o, a95 6.4o. The palaeomagnetic pole is Lat. 27oS, Long. 330oE (Fig. 4). 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 3 - Mean directions for the magnetic remanence in Calmayo sites. All directions with negative (up) inclination. The 95% confidence cone for the mean is drawn around the cross. Discussion The eutectoid of the hematite-ilmenite system has been estimated at 390oC (Robinson et al., 2004) or 520oC (McEnroe et al., 2007). It means the magnetic carrier of the remanence was formed at high temperature in the pluton, and the remanence became locked at the moment of ilmeno-hematite exsolution. It is expected that the remanence direction records the paleomagnetic field at the moment the pluton was still at high temperatures, possibly near the crystallisation age. The paleomagnetic pole calculated from the in situ remanence directions carried by the Calmayo tonalite is not consistent with the 500-470 Ma segment of the Gondwana apparent polar wander path (APWP) proposed by McElhinny et al. (2003), but it coincides with an approximate 350 Ma-position (Fig. 4). The discrepancy of the pole with the APWP accepts several hypotheses which must be Fig. 4 - Paleomagnetic poles in the southern hemisphere, Schmidt projection, in present African coordinates (parameter reconstructions after Lawver and Scotese, 1987). The Pampia terrane poles (ACH Achala batholith, Late Devonian, Geuna et al. 2008; CMY Calmayo tonalite, Early Ordovician, this work; CMP Campanario Fm., Late Cambrian, Spagnuolo et al. 2008a) are compared with the Gondwana APWP proposed by McElhinny et al. (2003). Numbers note age of the mean APWP poles in Ma. 139 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA explored in the light of further geological-petrological evidence: a) Hematite-ilmenite is a magmatic mineral which retains a primary magnetisation acquired on cooling, at ~490 Ma. The discrepancy is due to not having restored the pluton to its palaeohorizontal position in the Ordovician, and it could be accounted if the pluton axis, supposed originally vertical, would now plunge 50o to the SW. b) Hematite-ilmenite formed (or recrystallized) in response to high-temperature deformation affecting the pluton. The coincidence of the paleomagnetic pole with the 350 Ma- segment of the APWP would imply that high-temperature deformation along the Soconcho shear belt continued until the Devonian (Achalian orogeny). Calmayo would not have experienced significant net tilting since the Devonian. c) Should the pluton acquired the remanence at its present position, and the remanence was primary (i.e. Calmayo pluton shows no significant net tilting since the Ordovician), then the discrepancy can be analized together with discrepancies noted for other Pampean poles (Spagnuolo et al., 2008 a, b, Geuna et al., 2008; see Fig. 4). All these poles could be reconciled with the Gondwana APWP by moving Pampia in a way consistent with current geological models of dextral displacement relative to Gondwana (Rapela et al., 2007, Spagnuolo et al., 2008 b). Further petrological – geophysical – structural studies will allow establishing the preference for a), b) or c), either of them having important implications to elucidate the mechanisms of Pampia accretion to the South American margin and its late history of deformation. Acknowledgements This work was partially financed with research grants by UBACyT (X156, X442), ANPCyT (PICT 1074 y 02266/06), and CONICET (PIP 1502). SuperIAPD and GMAP programs were utilized in the analysis of palaeomagnetic data and palaeoreconstructions, respectively. REFERENCES • Bonalumi, A., Baldo, E., 2002. Ordovician magmatism in the Sierras Pampeanas of Córdoba. En: Aceñolaza, F.G. (Ed.): Aspects of the Ordovician System in Argentina. INSUGEO, Serie Correlación Geológica 16, p. 243-256. San Miguel de Tucumán. • D’Eramo, F., 2003. Petrología y emplazamiento de los plutones El Hongo y Calmayo, y su relación con la evolución de la Sierra Chica de Córdoba. Tesis Doctoral, UNRC, 200 pp. • Geuna, S.E., Escosteguy, L.D., Miró, R. 2008. Palaeomagnetism of the Late Devonian - Early Carboniferous Achala Batholith, Córdoba, central Argentina: implications for the apparent polar wander path of Gondwana. Gondwana Research, Special Issue “The Western Gondwana Margin: Proterozoic to Mesozoic”, 13: 227-237. • Lawver, L.A., Scotese, C.R., 1987. A revised reconstruction of Gondwana. In: McKenzie, G.D. (Ed.), Gondwana Six: structure, tectonics, and geophysics. American Geophysical Union, Monographs, 40: 17-23. • Martino, R.D., 2003. Las fajas de deformación dúctil de las Sierras Pampeanas de Córdoba: Una reseña general. Revista de la Asociación Geológica Argentina, 58 (4): 549-571. • McElhinny, M.W., Powell, Ch.McA., Pisarevsky, S.A., 2003. Paleozoic terranes of eastern Australia and the drift history of Gondwana. Tectonophysics, 362: 41-65. • McEnroe, S.A., Robinson, P., Langenhorst, F., Frandsen, C., Terry, M.P., Boffa Ballaran, T., 2007. Magnetization of exsolution intergrowths of hematite and ilmenite: Mineral chemistry, phase relations, and magnetic properties of hemo-ilmenite ores with micron- to nanometer-scale lamellae from Allard Lake, Quebec. Journal of Geophysical Research 112 (B10103), doi:10.1029/2007JB004973. • Rapela, C.W., Pankhurst, R.J., Casquet, C., Fanning, C.M., Baldo, E.G., González-Casado, J.M., Galindo, G., Dahlquist, J., 2007. The Río de la Plata craton and the assembly of SW Gondwana. Earth-Science Reviews, 83: 49-82. • Robinson, P., Harrison, R.J., McEnroe, S.A., Hargraves, R.B., 2004. Nature and origin of lamellar magnetism in the hematiteilmenite series. American Mineralogist 89, 725-747. • Spagnuolo, C.M., Rapalini, A.E., Astini, R.A., 2008 a. Paleogeographic and tectonic implications of the first paleomagnetic results from the Middle–Late Cambrian Mesón Group: NW Argentina. Journal of South American Earth Sciences, 25: 86-99. • Spagnuolo, C.M., Rapalini, A.E., Astini, R.A., 2008 b. Palaeomagnetic confirmation of Palaeozoic clockwise rotation of the Famatina Ranges (NW Argentina): implications for the evolution of the SW margin of Gondwana. Geophysical Journal International, 173: 63-78. 140 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 NORTH-SOUTH VARIATIONS IN PROVENANCE IN THE LATE PALEOZOIC ACCRETIONARY COMPLEX OF CENTRAL CHILE (34º – 40º LAT. S) AS INDICATED BY SHRIMP DETRITAL ZIRCON U-TH-PB AGES 4-06 .Hervé,F1*, Calderon, M.1, Fanning, C.M.2, Godoy, E.1 (1) Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile (2) Research School of Earth Sciences, Australian National University, Canberra, Australia * Presenting author’s e-mail: [email protected] A late Paleozoic to Early Mesozoic fossil accretionary complex extends from 34º S. Lat to the extreme south of the continent along coastal Chile. Previous detrital zircon studies in the southern part of this complex (44º - 55º S) have shown that Permian to Jurassic detrital rocks have been incorporated into the complex where they have been metamorphosed under high P/T conditions. New detrital zircon ages have been obtained from the northern (34 – 42º S) segment of the accretionary complex, area in which a well established paired metamorphic belt system with the high P/T Western Series (WS) - interpreted to represent basally accreted rocks - lies outboard of the Eastern Series (ES) - interpreted to represent frontally accreted rocks. The data reveals that they have Permian or older maximum possible sedimentation ages, with no Mesozoic detrital zircons present. In the northernmost section, (34 – 36 ºS) the detrital zircon age spectra show more than 50 % of Proterozoic zircon grains in contrast with less than 10% further south. Also, the younger igneous detrital zircons, in both the WS (330 Ma) and the ES (345 Ma) are older than the Pennsylvanian (310 Ma) intrusion of the Coast Range Batholith. In contrast, detrital zircons of Early Permian (285 – 295 Ma) age are present in the Western Series of the southernmost (40º S) exposures, together with even younger zircons of late Permian ages. These results suggest that the cratonic source areas of South America, probably the Pampia and maybe even the Rio de la Plata cratons, were able to shed detritus to the paleo-Pacific coast prior to the inception of subduction as indicated by the establishment of the Pennsylvanian magmatic arc near the continental margin. It is speculated here that the sedimentary protolith of the northern portion of the accretionary complex was probably deposited in a passive margin setting, and that they were incorporated later into de accretionary wedge. The data also suggest that the accretionary processes, or at least the preservation of the subducted rocks, were diachronous along the considered segment of the belt, as the sedimentary protoliths become younger southwards. This research is supported by FONDECYT Project 1095099. THE ROLE OF TRUE POLAR WANDER IN THE JURASSIC Iglesia Llanos, M.P.1*, Prezzi, 4-07 C.B.1 (1) INGEODAV, Depto. Ciencias Geológicas, Fac. Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pab. 2, Ciudad Universitaria, C1428EHA, Buenos Aires, Argentina * Presenting author’s e-mail: [email protected] It is generally accepted that Gondwana remained in a present-day latitudes during most of the Mesozoic and even part of the Palaeozoic. This “stationary” geodynamic model is recurrently shown in the literature, and is based on the fact that the South American Jurassic palaeomagnetic poles clustered around the southern geographic pole (e.g. Valencio et al., 1983; Oviedo and Vilas, 1984; Rapalini et al., 1993; Beck, 1999). In more recent years however, it has been demonstrated that some of the palaeopoles of this age fell far away from the geographic pole, particularly during the Early Jurassic. As a consequence, it was mandatory to test the former geodynamical model, which is what we present here, i.e. a substantially different model for Pangea during the Jurassic. We used other continents that by then were part of Pangea yielding reliable palaeomagnetic data, i.e. Eurasia, North America and Africa, and constructed the corresponding apparent polar wander (APW) 141 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Master Jurassic APW path derived from those of North America, Eurasia, Africa and South America in South American present-day coordinates. paths, which we compared with the one we obtained in South America (Iglesia Llanos et al., 2006). We observed that all four curves were fully consistent (Fig. 1, in South American present-day coordinates). The rotation to South America was performed after testing several well-known palaeogeographical reconstructions for Pangea, and decided on those which provided the best palaeomagnetic fit. We established that such fit varied with pole ages, and thus used different kinematic models for palaeopoles bearing ages between ~215 and 193 Ma, and those between ~ 192155 Ma. According to our palaeomagnetic data, some sort of geodynamic event might have taken place around 192 Ma. The cusp in the path implies a substantial change in pole positions between 197 and 185 Ma (Early Jurassic). Typically, such change can be attributed to some sort of geodynamical phenomenon, such as lithospheric motion and/or true polar wander (TPW), which is defined as the drift of the spin axis relative to the rigid Earth. Precisely, one of the goals of this study is throwing some light regarding the primary cause of the polar shift. On the basis of palaeomagnetic and hotspots data, we performed palaeolongitude-controlled or “absolute” palaeogeographical reconstructions. We used Morgan’s (1983) “fixed” grid of hotspots (HS) that goes back to 200 Ma to compensate for the motion of South America in relation with the Atlantic Ocean hotspots. From the resulting palaeoreconstructions, changes in latitude and orientation in the Early Jurassic look more conspicuous. Given that these palaeolatitudinal shifts might have occured at a global scale, we investigated whether there was a correlation with reported geological and/or palaeocological major changes/turnovers during this time from both hemispheres, and found that indeed there was. The applicability of completely different disciplines is maybe the most convincing argument to sustain or discard the methodologies used. REFERENCES • Beck Jr., M.E., 1999. Jurassic and Cretaceous apparent polar wander relative to South America: Some tectonic implications. J. Geophys. Res. 104: 5063-5067. • Creer, K. M., Irving, E., and Runcorn, S. K., 1954. The direction of the geomagnetic field in remote epochs in Great Britain. J. Geomag. and Geoelect. 6 163-168. • Iglesia Llanos, Riccardi, A.C., Singer, S.E., 2006. Palaeomagnetic study of Lower Jurassic marine strata from the Neuquén Basin, Argentina: A new Jurassic Apparent Polar Wander Path for South America. Earth Planet. Sci. Lett. 252: 379-397. 142 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 • Morgan, W.J., 1983. Hotspot tracks and the early rifting of the Atlantic. Tectonophysics 94: 123-139. Oviedo, E., Vilas, J.F., 1984. Movimientos recurrentes en el Permo-Triásico entre el Gondwana Occidental y el Oriental. Actas 9º Congreso Geológico Argentino 3, Buenos Aires, 97-114. • Rapalini, A.E., Abdeldayem, A.L., Tarling, D.H., 1993. Intracontinental movements in Western Gondwanaland: a palaeomagnetic test. Tectonophysics 220: 127-139. • Valencio, D.A., Vilas, J.F., Pacca, I.G., 1983. The significance of the palaeomagnetism of Jurassic-Cretaceous rocks from South America: predrift movements, hairpins and Magnetostratigraphy. Geophys. J. R. Astron. Soc. 73: 135-151. THE WEDDELL SEA REVISITED 4-08 Lawver, L.A.1*, Ghidella, M.E.2 (1) The Jackson School of Geosciences, Institute for Geophysics, University of Texas at Austin, 10100 Burnet Rd. – R2200, Austin, TX 78758-4445, U.S.A. (2) Instituto Antártico Argentino, Cerrito 1248, C1010AAZ, Buenos Aires, Argentina * Presenting author’s e-mail: [email protected] The satellite derived gravity map of the Weddell Sea allows new interpretations of the early tectonic evolution of the area. Many presentations of identified magnetic anomalies in the Weddell Sea have been made with new data being recently added by König and Jokat (2006). Unfortunately, most all interpretations showing magnetic anomaly identifications in the Weddell Sea fail when the isochrons are put into a system that utilizes major plate motions. It is known that there was virtually no motion between South America and Africa prior to 132 Ma, the time of the eruption of the Parana-Edenteka mantle plume at 132 Ma, or about magnetic anomaly M10. The earliest identified magnetic anomalies in the South Atlantic are variously M9 or M7 depending on the anomalies used. Consequently any anomalies older than M10 found in the Weddell Sea need to be the product of a simple two-plate breakup between East (East Antarctica, India, Australia and Madagascar) and West (Africa and South America) Gondwana. Also, any identified anomalies in the Weddell Sea need to be able to be reconstructed with room for their conjugate half available. In addition, the Cretaceous Normal Superchron or Cretaceous magnetic quiet zone is exceptionally noisy in the Weddell Sea with even the identification by many authors of an “isochron” at 93 Ma. All of these problems will be examined and a tectonic evolution scenario will be presented that may reduce many of these discrepancies. CRUSTAL STRUCTURE AND TECTONICS OF THE EAST ANTARCTIC PASSIVE MARGIN Leychenkov, G.L.1*, Guseva, 4-09 G.L.2 (1) Institute for Geology and Mineral Resources of the World Ocean. 1, Angliysky Ave. 190121, St.Petersburg, Russia (2) Polar Marine Geosurvey Expedition. 24, Pobeda St., 189510, Lomonosov, St.-Petersburg, Russia * Presenting author’s e-mail: [email protected] The East Antarctic margin (EAM) formed as a result of Gondwana break-up and is of a rifted (passive) type. Its different sectors developed due as a consequence of the successive separation of Africa, India and Australia from the Antarctic. The best studied part of the EAM is the 6500 km-long southern Indian Ocean between 5E and 150E (which is mostly non-volcanic margin) where c. 150 000 km of 143 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA multichannel seismic data, c. 300 refraction seismic data and more than 250 000 km of potential field data have been acquired by different countries and organizations (Fig. 1). All these numerous data sets were integrated with use of the international ADMAP (Antarctic Digital Magnetic Anomaly Map), SDLS (Seismic Data Library System) and other data bases. Conducted investigations enabled us to define crustal structure/characteristics of this region, to map principal tectonic provinces and features and to develop models of its geodynamic evolution. The most complicated and controversial scientific problem in study of the EAM tectonics (as well as other passive margins) is the identification of the boundary between rifted continental and oceanic crust (continent-to-ocean boundary, COB) that is the locus of continental break-up. The rifted continental crust is generally characterized by a complex structure with development of extensional features while the oceanic crust is usually undisturbed. However these differences are rarely recorded in seismic data especially close to COB where crustal parameters of stretched continental and oceanic domains are very similar. Various geophysical criteria are applied to recognize the COB on the EAM. Magnetic data are generally reliable source of information to identify the oceanic crust by appearance of spreading-related linear anomalies however in Antarctica magnetic measurements are still sparse enough to map magnetic lineations. Gravity information has a limited capability in COB identification but nevertheless, free-air anomaly field Fig. 1 - Tectonic sketch of the EAM (Indian Ocean part) derived from satellite altimetry data 1 - magnetic leniation (with number of chron polarity) and fracture shows in many places position of zones; 2 - extinct ridges; 3 – COB; 4 – volcanic margins; 5 - volcanic fracture zones (paleotransform edifices; 6 - zones of mantle unroofing; 7 - thickness of sediments. faults) which are recognized only within the oceanic crust and fades close to the COB. Using the different geophysical and structural approaches the COB has been mapped (with different reliability) around most of the APM and this study shows that the width the 144 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA rifted (stretched) continental crust around East Antarctica ranges from 200 to 400 km. One of the basic criteria for identification of crustal types on the ACM is believed to be the differences in basement velocities which are changed across the COB from 5.9–6.2 km/s (typical to continental crust) to 4.9-5.8 km/s (typical to oceanic crust; layers 2A or 2B). In many cases seismic reflection pattern and morphology of a basement surface show marked changes across the margin/abyssal plane and this information is often decisive for identification and areal mapping of the COB. Following are brief overview of three sectors of the EAM: eastern Dronning Maud Land margin, Enderby–Queen Mary Land margin and Wilkes Land–George V Land margin, which are conjugate to south-east Africa/Madagascar, India and Australia, respectively (Fig. 1). The Dronning Maud Land margin is the oldest one. Rifting and sea-floor spreading started here about 170–180 Ma and 160 Ma ago respectively (Fig. 1). This margin has contrasting basement morphologies and crustal thicknesses. The crust ranges in thickness from about 35 km under the shelf, 26–28 km under the Gunnerus Ridge (500 km-long projection of unstretched continental crust toward the ocean), 12–17 km under the Astrid Ridge, and 9.5–10 km under the deep-water basin. A 50-kmwide block with increased density and magnetization is modeled from potential field data in the upper crust of the inshore zone and is interpreted as being representative of mafic intrusions. The succession of linear magnetic anomalies from M0 to M24 is identified in the western part of this area and from M2 to M16 in the eastern part. Within the southwestern continental rise, a symmetric succession from M24B to 24n with the central anomaly M23 is recognized. This succession is obliquely truncated by younger lineation M22–M22n. It is proposed that seafloor spreading stopped at about M23 time and reoriented to the M22 opening direction. The Enderby Land–Queen Mary Land margin is characterized by complex Late Jurassic to Early Cretaceous extensional evolution. The eastern part of this area (between 65E and 95E) has been studied in more details owing to the joint IPY 2007-2008 Russian-German Project (Fig. 1). The COB in the study area is defined by a set of structural and geophysical features, the major of which are distinct differences in seismic pattern of crustal structure and peculiar high-amplitude positive linear magnetic anomalies. The outer part of the marginal rift is proposed to be saturated by mantle rocks and its differentiates. A sequence of E-W oriented magnetic lineations from M11An to M2 is well identified to the west and east of Kerguelen Plateau. Oceanic crust around the southern Kerguelen Plateau is thicker than normal due to excessive magmatic production in spreading ridges (as a response to Kerguelen Plume). The southern Kerguelen Plateau itself is thought to be underlain by a block of stretched and modified continental crust. This block was probably isolated from the India margin at about 129 Ma due to ridge jump(s). The Wilkes Land–George V Land margin developed as a result of extreme crustal extension and synrift mantle unroofing culminating in the formation of peridotites/ gabbro ridges. The COB is defined mostly by the changes in acoustic basement morphology and crustal reflection pattern and is interpreted to be located at a distance of 300–500 km from the middle shelf where the inboard rift boundary is suggested. The oceanic crust of the study region is characterized by sea-floor spreading lineations from 33 to 18 with a spreading half-rate ranging between 2.5 and 11 mm/yr. Revised identification suggests that break-up between Australia and Antarctica commenced within anomaly 33, i.e. at about 79–81 Ma ago. Previously recognized anomaly 34 is situated within the zone of mantle unroofing (Fig. 1). Our research shows that the width of marginal rift (zone of crustal stretching) varies significantly along the studied margin and amounts c. 250 km on the eastern Droning Maud Land, 200–350 km on the Enderby Land–Queen Mary Land and 300–500 km on the Wilkes Land–George V Land. 145 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA STRUCTURE AND TECTONIC DEVELOPMENT OF THE SOUTHERN MARGIN OF THE SCOTIA SEA 4-10 Lodolo, E.1*, Civile, D.1, Tassone, A.2 (1) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy (2) CONICET - Instituto de Geofísica “Daniel Valencio”, Univ. de Buenos Aires, Argentina * Presenting author’s e-mail: [email protected] The southern margin of the Scotia Sea corresponds to the Scotia-Antarctica plate boundary, along which the relative motion between the two plates is mostly left-lateral. Available multichannel seismic reflection profiles (see Fig. 1), integrated with earthquake data, and literature information, have allowed to define the general structural architecture of the margin, and propose a tectonic evolutionary history. Three main tectonic segments have been identified along the margin, from the Elephant Island to the Herdman Bank. Along the arcuate western segment (from the Elephant Island to the South Orkney microcontinent), seismic data have shown the presence of a buried, scarcely developed accretionary body, and an evident deepening of the oceanic crust beneath the crustal blocks forming the South Scotia Ridge. Along this segment of the margin, the transition between the continental elevated blocks and the oceanic crust is abrupt. The central part of the south Scotia margin is occupied by the northern margin of the South Orkney microcontinent, where a quite developed S-verging subduction zone of the Scotia sea oceanic crust beneath the continental block, is present. The sector to the east of the South Orkney microcontinent till the Herdman Bank shows a very complex structural assemblage, due to the presence of several bathymetric continental highs separated by deep troughs and restricted oceanic basins. An ENE-oriented basin (the Bruce Deep) was found to the E of the South Orkney microcontinent. To the south of the Bruce Deep, a wide deformation zone with N-verging folds and thrusts (here named Jane Thrust Belt), has been identified. The easternmost segment of the plate boundary is structurally the less constrained, and may be composed by a series of tectonic lineaments of different lengths. Analyzed data in general have shown that in the western sector of the southern Scotia margin, the Scotia oceanic crust seems to have subducted beneath the Antarctic plate, whereas the Weddell Sea subducted beneath Scotia plate, in the eastern sector. The activation of left-lateral transtensional strike–slip lineaments generated narrow pull-apart basins in the eastern sector in correspondence of Fig. 1 - Seismic profiles position map for the southern margin of the Scotia Sea (bathymetry based on satellite-derived data). Data are available at the web site: http://snap.ogs.trieste.it/SDLS/. It works under the auspices of Scientific Committee on Antarctic Research (SCAR) to provide open access to all multichannel seismic reflection data collected south of 60°S. SSR = South Scotia Ridge; EI = Elephant Island; SOM = South Orkney microcontinent. 146 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA the fore-arc of the convergent zones. These evidence suggest that the southern Scotia margin may represent an example of an opposite subduction polarity environment, for many aspects similar to the tectonic setting of the central sector of the northern Carribbean margin where a double subduction with N- and S-vergence is documented in correspondence of the Hispaniola Island. Three main tectonic phases have been recognized in the deformational history of the Scotia-Antarctica plate boundary. The first phase (Lower Miocene) was characterized by a north-directed convergence of the Weddell Sea beneath the series of bathymetric highs now distributed along the south-eastern part of the Scotia Sea. A second phase (about 12 Ma) was characterized by the presence of the two possibly coeval and opposite-vergence subduction zones of the Scotia Sea and Weddell Sea oceanic crust. The subduction of the Scotia Sea is testified by the presence of some small and scarcely developed accretionary prisms. In the eastern zone the system of NNE-trending dextral transform faults was active and separated zones with different direction or rate of movement. These faults dismembered and partly deactivated the subduction zone and facilitated the process of fragmentation and dispersion of the crustal blocks. During this phase, an accretionary prism (Jane Thrust Belt), formed to the south of the Bruce Bank due to a presence of a S-verging subduction zone. The final phase was characterized by the deactivation of the subduction zones and by the activation of leftlateral strike-slip regional lineaments. These structures generated several narrow pull-apart basins in the eastern sector of the Scotia margin, while an abrupt contact between continental and oceanic crust, without the presence of a transitional crust, is observed along the western margin. 3D DENSITY MODEL OF THE CENTRAL AMERICAN SUBDUCTION ZONE FROM SATELLITE GRAVITY DATA INTERPRETATION 4-11 Lücke, O.H.1*, Götze H.J.1 (1) Christian-Albrechts-Universität, Kiel Germany. * Presenting author’s e-mail: [email protected] The Central American convergent margin poses several challenges regarding the availability and quality of potential field data. Satellite gravity data and subsequent combined geopotential models provide a homogenous database with global coverage which is suitable for geophysical applications. The aim of this work is to model in 3D the lithospheric structure along the Middle American trench and assess the scale in which available satellite derived gravity data provides input for the modelling of the solid Earth. The EGM2008 combined geopotential model is being used for the density modelling on the regional scale and the density model was constrained by available and previously published seismic velocity models, magnetotelluric profiles and receiver function data. The 3D density model shows the overall geometry and density distribution of the subducting Cocos Plate and the overriding Caribbean Plate. Thickening of the oceanic crust by the influence of the Galapagos hotspot was also modelled and the structure was carried into the slab outlining the effects of ridge subduction. The structure of the overriding plate is heavily influenced by the tectonic evolution of the Caribbean region and its crust presents a patchwork of tectonic blocks with crustal basements of contrasting densities such as the continental Chortis Block, the mainly ultramafic Mesquito Composite Oceanic Terrain and a basaltic unit part of the Caribbean Large Igneous Province. 147 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA ITALY-ARGENTINA COOPERATION IN THE FIELD OF SEISMOLOGY: THE ASAIN. HISTORICAL REVIEW AND RECENT PROGRESS 4-12 Russi, M.*, Cravos, C., Plasencia Linares, M.P. Istituto nazionale di oceanografia e di Geofisica Sperimentale OGS. Borgo Grotta Gigante 42/c, 34010 Sgonico (TRIESTE), Italia * Presenting author’s e-mail: [email protected] On August 4th, 2003 a major earthquake of 7.5 Ms known as “Centenary Earthquake”, struck the Scotia Sea region. The earthquake epicentre was located on the ocean bottom in the proximity of the South Orkney Islands /Islas Orcadas archipelagus and the strong shaking caused some damages (no casualties) to the structures of the permanent Argentinean base Orcadas, located about 70 km to the South-West from the epicentre, also determining extensive cracks in the ice pack all along the shores of the island (Laurie Island) hosting the base. During one year after the main event several thousands aftershocks with magnitudes up to 5.8 and epicenters spanning a ~150x50 km2 East-West elongated area followed the main shock. At that time the Antarctic Seismographic Argentinean Italian Network ASAIN already consisted of four broad-band seismographic stations installed in Antarctica (Base Orcadas, ORCD and Base Jubany, JUBA) and Argentinean Tierra del Fuego (Estancia Despedida, DSPA and Ushuaia, USHU). The whole network recorded both the main shock and all the stronger aftershocks while station ORCD, due to its favourable location, also recorded thousands of minor shocks belonging to the “Centenary Earthquake” series. August 4th, 2003 has been a fundamental date for the progress of seismometry and seismological studies in the Scotia Sea providing a practical demonstration of the usefulness of a permanent regional seismographic network in the area. During the nineties and the past ten years several seismological research groups have been working in the Scotia sea and neighbouring areas but the realization of a permanent seismographic network is mainly the result of the joint effort of the Italian Programma Nazionale di Ricerche in Antartide (PNRA) / Ist. Naz di Oceanografia e di Geofisica Sperimentale–OGS and the Argentinean Dirección Nacional del Antártico (DNA) / Instituto Antártico Argentino (IAA) groups which installed the first ASAIN station at Base Esperanza at the beginning of 1992. Today the ASAIN consists of seven broadband stations sending their recordings using internet and satellite links provided by Argentina to the OGS and the IAA. ASAIN real time data are also transmitted to the ORFEUS Data Centre as a contribution to the Virtual European Broadband Seismographic network (VEBSN). A review of the network activity during its 20 years life and some information about the on going work will be presented. The most exciting planned activity is represented by the installation at Belgrano II station during the 2010-2011 Antarctic campaign of a polar seismometer which can properly operate at temperatures reaching -50° C. OROGENESIS REFLECTED IN THE TRANSITION FROM EXTENSIONAL RIFT BASIN TO COMPRESSIONAL FORELAND BASIN IN THE SOUTHERNMOST ANDES (54.5°S): NEW PROVENANCE DATA FROM BAHÍA BROOKES AND SENO OTWAY 4-13 McAtamney, J.1*, Klepeis, K.1, Mehrtens, C.1, Thomson, S.2 (1) University of Vermont, Dept. of Geology, 180 Colchester Ave, Burlington VT 05401 (2) University of Arizona, Dept. of Geosciences, 1040 E. 4th St, Tucson AZ 85721 * Presenting author’s e-mail: [email protected] South of 51ºS latitude, in the southernmost Andes, the Cretaceous inversion of the Late Jurassic Rocas Verdes rift basin created the Cretaceous-Neogene Magallanes foreland basin between an active volcanic arc and the South American craton. We present stratigraphic and sedimentary provenance data from the Lower and Upper Cretaceous sedimentary units, known as the Zapata Formation and 148 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Punta Barrosa Formation, that record this tectonic transition to test previous models of rift basin inversion and foreland sedimentation patterns. We present the results from sandstone petrography, an analysis of sandstone detrital modes and mudstone Rare Earth Element geochemistry, and U/Pb detrital zircon ages from pre- and post-inversion sediments within the Zapata and Punta Barrosa Formations in two little studied parts of the Magallanes foreland basin: Bahía Brookes and Seno Otway. The results constrain the timing of uplift and deundation of source terrane in the internal part of the orogen and characterize the depositional setting and provenance during the transition from rift to foreland basin sedimentation in the southernmost Andes. Three kilometers of measured section record the stratigraphic transition from the Zapata Formation to the Punta Barrosa Formation. Thinly bedded shallow marine mud and incomplete Bouma sequences characterize the Zapata Formation. Fining-upward packages of thickly bedded coarse-grained sand mark the onset of deposition of the Punta Barrosa Formation. Complex paleocurrent patterns from both units support a sedimentation model of multiple back-arc submarine fans during the initiation of the foreland basin. By Late Cretaceous time paleocurrent data indicate a north-south trending axial channel transport system parallel to the orogen. Modal analysis and petrography of sandstone from both units shows sediments are compositionally immature, highly feldspathic, and derived from a volcanic arc. Detrital modes record the transition from dominantly volcanic lithic fragments in the Zapata Formation to dominantly metamorphic lithic fragments in the Punta Barrosa Formation. Rare Earth Element fractionation shows compositional overlap between the two formations in the southern basin, and compositional distinction between formations in the northern basin. Detrital zircon age spectra yielded maximum depositional ages between 89-88 Ma for the base of the Punta Barrosa Formation and 81-80 Ma for the top of the Punta Barrosa Formation. Andean orogenesis began as a narrow submarine thrust wedge behind a Late Cretaceous volcanic arc. The Late Cretaceous sedimentary fill of the Magallanes foreland basin was sourced from uplifted horst-and-graben style blocks proximal to an active volcanic arc. Preferential uplift and erosion of pre-rift basement schists and Upper Jurassic volcanic rocks occurred after about ~82 Ma. The depositional environment during initiation of the Magallanes fold-thrust belt included multiple apron style submarine fans prograding from the rising Cordillera. CRUSTAL DEFORMATIONS ASSOCIATED TO THE M 8.8 MAULE EARTHQUAKE IN CENTRAL CHILE, 27 FEBRUARY 2010, DETECTED BY PERMANENT GPS STATIONS IN ARGENTINA 4-14 Mendoza, L.1,2*, Vasquez, J.1,2, Del Cogliano, D.1,2 (1) Grupo de Geodesia Espacial, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque, B1900FWA La Plata, Argentina (2) CONICET, Argentina * Presenting author’s e-mail: [email protected] The analysis of records from permanent GPS stations registered before, during and after an earthquake provides direct measurements of the co-seismic offsets and the post-seismic deformation associated to the seismic event. The magnitude and distribution of the observed deformation may help to infer the nature of the fault at depths; on the long term the post-seismic relaxation observed by GPS could be useful to other techniques (e.g. fit together InSAR scenes). On the other hand, the co-seismic displacements and the post-seismic velocities of the GPS stations are required in order to evaluate the impact of the earthquake on the regional geodetic infrastructure itself and to assure, if necessary, a smooth redefinition of the affected geodetic networks; this situation necessarily concerns to not scientific applications including the surveying, the engineering and the land registry. We present results from a consistent processing of GPS records, spanning 16 months and centred 149 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 around the M 8.8 Maule earthquake in central Chile (27 February 2010), involving permanent GPS stations from the RAMSAC network (Argentina) and GPS stations from the SIRGAS-CON and the IGS tracking networks. The magnitude and spatial distribution of the observed deformation are presented and discussed, with special attention to the implication for the geodetic infrastructure of the country. STRUCTURAL GEOLOGY OF THE EASTERN TIERRA DEL FUEGO ISLAND Menichetti, M.1*, Tassone, A.2, Lippai, 4-15 H.2 (1) Dipartimento di Scienze [email protected] (2) CONICET-INGEODAV. Dpto. de Ciencias Geológicas. Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires. Ciudad Universitaria. Pabellón 2. CP - C1428EHA- Buenos Aires. Argentina * Presenting author’s e-mail: [email protected] The eastern part of Tierra del Fuego, along the Atlantic coast, is characterized by continuous outcrops of Cenozoic rocks pertaining to the Magallanes perisutural basins developed in front of the external margin of the southernmost Andean Cordillera. This basin is located on continental crust, extends to the Atlantic on-and-off-shore and is filled by a 5-km-thick siliciclastic sedimentary succession spanning in age from Cretaceous to Holocene. The basin is located in front of the Cordillera, has a concave arcuate shape and a depocenter in southern Patagonia. On the western side of the Tierra del Fuego Island the basin is oriented NW-SE with a width of many hundreds km while toward the east, along the Atlantic coast, it strikes roughly E-W, following the Fuegian Andes curvature, and is restraining for about one hundred km. Since the Late Cretaceous its southern margin underwent significant compressional deformation that originated the present-day Magallanes fold-and-thrust belt. The geometry of the basin is controlled by northward thrust propagation involving the foreland in the deformational process. The stratigraphic succession from the Late Cretaceous to the Miocene can be subdivided into four stratigraphic units separated by syn-tectonic angular unconformities of the Late Cretaceous, Palaeocene, Eocene and the Lower Miocene. Marine sandstones and mudstones of Andean provenance were deposited in a system of clastic wedges progressively shifted toward the foreland. The syn-tectonic angular and progressive unconformities constrain the timing of the thrusts propagation in the frontal part of the chain. In the last years, several kinematic models have been proposed based mainly on stratigraphic and sedimentary methodologies, in several cases not supported by field evidence. A simple fold-and-thrust belt geometry with a basal detachment level in the Tertiary mudstone has been proposed for the area without consideration of the structural complexity of the region affected by at least two tectonic phases: compressional, from late Cretaceous to Oligocene, and transtensional, from Late Oligocene on. The basins subsidence, at least until the Late-Middle Eocene unconformity, has been mainly controlled by several transtensional faults running broadly parallel to the compressional front. Since the late Oligocene, a left-lateral strike slip fault system has been overprinting the thrust structures and partitioning the deformation along different fault arrays of the Atlantic Coast, from Cabo S. Diego to Cabo S. Pablo. The main compressional structures mapped along the coast present a northward verging fault-bend fold geometry, with an amplitude of a few km, a detachment level located in the marls layers, low angle thrust planes and a shortening of about 30 %. The external compressional front of the orogen can be located at Punta Gruesa, where the anticline thrusts over the slightly deformed structures of Cabo S.Pablo. The main E-W left lateral strike-slip faults are located at Cabo Leticia, Capo Campo del Medio and Cabo Irigoyen, and are associated with a set of N-S and NE-SE extensional faults. These later fault systems with offsets of many hundreds of meters represent the fault array associated with the Magallanes-Fagnano transform fault. Many seismically-triggered sand 150 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES intrusions are present throughout the stratigraphic succession in the Late Cretaceous, the Late Palaeocene and the Middle Miocene marking important tectonic events. Several of these sedimentary and structural features can be easily correlated with the structures observable in the off-shore seismic reflection profiles. The joint analysis of these data, document a multi-stage evolution of the basin with the presence of five seismic units, separated by angular unconformities recording different tectonosedimentary events. Several pull-apart basins are located along the off-shore alignment of the principal deformation zone of the Magallanes-Fagnano fault and have their on-shore counterparts in the Cabo Malenguena basin and western Lago Fagnano. THE PRE-FARELLONES DEFORMATION (PEHUENCHE PHASE), CORDILLERA PRINCIPAL AND FRONTAL (31°45’LS), SAN JUAN PROVINCE, ARGENTINA 4-16 Pérez D.J.1*, Sanchez Magariños J.M.1 (1)Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II (1428), Buenos Aires, Argentina * Presenting author’s e-mail: [email protected] Introduction The objective of this study is to analyze the volcanic rocks and structure of the Mondaca river region and their relationship within Mesozoic sedimentary deposits. New age data for Los Pelambres Formation, and field structural data from these volcanic deposits, indicate an Oligocene and Miocene deformation related to the Pehuenche phase. The study area is located in the Frontal and Principal Cordillera at 31º45’S and 70°15’W, on the boundary with Chile, in the San Juan province, Argentina. This region corresponds to the southern part of the non-volcanic “flat-slab” region betwen 28°S and 33°S (Cahill e Isacks, 1992) under which the Nazca plate forms a broad sub-horizontal bench between about 100 and 150 km. The first studies in the region were done by Groeber (1951), Polanski (1964), Olivares Morales (1985), Rivano and Sepulveda (1991), and more recently, by Alvarez (1996), Pérez (2001), Ramos et al. (1998). Geology and structure The stratigraphic sequences of the region begin with Permo-Triassic rhyolitic and rhyodacitic rocks of the Choiyoi Group; then continue with Triassic rocks of the Rancho de Lata Formation and Jurassic sequences of the Los Patillos, La Manga and Tordillo Formations. Then, follows the Auquilco formation (gypsum and diapirs), but without stratigraphic relationships, and a sequence of volcanic rocks follows, defined in the Chile region as Los Pelambres Formation (Rivano and Sepúlveda, 1991) and Alitre and Mondaca Pass in Argentina. These same rocks in the La Ramada, located immediately to the south of the study area, were related to the Juncal Formation (Ramos et al., 1990); these same volcanic rocks immediately to the north of the study region, upper Oligocene and lower Miocene in age, are assigned to the Pelambres and Pachón Formations (Fernández et al., 1974; Mpodozis et al., 2009). These are correlated with the Abanico Formation. Unconformable above these rocks, are the volcanic sequences of Farellones Formation, of Miocene age. Quaternary deposits cover all the units. The study region presents two structural styles: a thin skinned and a thick skinned styles, which affected different rocks and in different periods of time. The first style can be recognized in the Mondaca and Carnicerias River, where the low-angle thrust of Los Pelambres is uplifting the volcanic sequences of upper Oligocene and lower Miocene over the Jurassic Los Patillos Formation. Toward the west and in the Chilean territory, another thrust of low angle would be the responsible of uplifting of the Cretaceous deposits above the Tertiary volcanic rocks. These events 151 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES are attributed to different deformation phases (out of sequences thrust) of Miocene times. Similar structure have been already describes to the south of the study region which affected OligoMiocene volcanic rocks of Los Pelambres Formation. Conclusions The volcanic deposits of the Los Pelambres Formation, previously assigned to the Cretaceous, were redefined in age to the Oligocene, 33.4-25.2 Ma. The volcanic and volcaniclastic deposits of the Pachón (Abanico) Formation were recognized; the age of this Formation, previously assigned to the Cretaceous, was redefined to the late Oligocene - early Miocene, 25-21 Ma. The volcanic deposits of Los Pelambres and Pachón Formations (Oligocene-Miocene) that constitute the Abanico basin, are highly deformed, and are underlying the Farellones Formation of middle Miocene (18,3 Ma). This unconformable relationship would be indicating a very important deformation phases occurred during the upper Miocene (~21-18 Ma) and corresponding to the Pehuenche phase. REFERENCES • Alvarez, P.P., 1996. Los depósitos triásicos y jurásicos de la Alta cordillera de San Juan. En V.A. Ramos (ed). Geología de la región del Aconcagua, provincias de San Juan y Mendoza. Subsecretaría de Minería de la Nación. Dirección Nacional del Servicio Geológico. Anales 24 (5): 59-137, Buenos Aires. • Cahill, T. y Isacks, B.L., 1992. Seismicity and shape of the subducted Nazca plate. Journal of Geophysical Research . Nº97, p. 17503-17529. • Fernández, R.R., Brown, R.F., y Lencinas, A.N., 1974. Pachón un nuevo pórfido cuprífero argentino, Dto. de Calingasta, prov. de San Juan, República Argentina. 5º Congreso Geológico Argentino, Actas II: 77-89, Buenos Aires. • Groeber, P., 1951. La Alta Cordillera entre las latitudes 34° y 29°30’. Instituto Investigaciones de las Ciencias Naturales. Museo Argentino de Ciencias Naturales B. Rivadavia, Revista (Ciencias Geológicas) I(5): 1-352, láminas I-XXI, Buenos Aires. • Mpodozis, C., Brockway, H., Marquardt, C. y Perelló, J., 2009. Geocronología U/Pb y tectónica de la región de Los PelambresCerro Mercedario: Implicancias para la evolución cenozoica de los Andes del centro de Chile y Argentina. XII Congreso Geológico Chileno, Actas S9_059, 22-26, Santiago • Olivares Morales América Patricia, 1985. Geología de la Alta Cordillera de Illapel entre los 31°30 y 32° Latitud Sur. Tesis de Grado, Universidad de Chile, Facultad de Ciencias Físicas y Matematicas Departamento de Geología y Geofísica. • Pérez, D.J., 2001. El volcanismo neógeno de la cordillera de las Yaretas, Cordillera Frontal (34°S) Mendoza. Revista de la Asociación Geológica Argentina, 56 (2):221-23, Buenos Aires. • Polanski, J., 1964. Descripción geológica de la hoja 25a Volcán San José, provincia de Mendoza, Dirección Nacional de Geología y Minería, Boletín 98: 1- 94, Buenos Aires. • Ramos, V.A., Rivano, S., Aguirre-Urreta M.B., Godoy, E. y Lo Forte, G.L., 1990. El Mesozoico del Corcón del Límite entre Portezuelo Navarro y Monos de Agua (Chile-Argentina). XI Congreso Geológico Argentino, Actas II: 43-46, San Juan. • Rivano, G. y Sepulveda, H.,1991. Hoja Illapel Región de Coquimbo, Servicio Nacional de Geología y Minería, Carta Geológica de Chile. Nº69. 132pp.. . CORRELATIONS OF TECTONO-MAGMATIC EVENTS IN THE SOUTH VERKHOYANSK OROGENIC BELT (EASTERN SIBERIA, NORTHEAST ASIA) 4-17 Prokopiev A.V.1* (1) Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, 39, Lenin Avenue, Yakutsk, 677980, Russia * Presenting author’s e-mail: [email protected] The South Verkhoyansk orogenic belt extends submeridionally for 850 km along the southeastern margin of the North Asian craton. Three tectonic zones are recognized there from west to east: Kyllakh (frontal), Sette-Daban, and Allakh-Yun’; in the hinterland of the belt there is the Okhotsk cratonal terrane (Fig. 1). The history of the belt includes the following major geodynamic events (Fig. 2): 1. Within the Kyllakh and Sette-Daban tectonic zones, the Riphean rifting processes were followed by pre-Vendian folding and thrusting. Here the folds and thrusts deforming the Riphean rocks are 152 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA overlain, with an angular unconformity, by Vendian deposits. Within the Okhotsk terrane, these events are recorded by unconformities at the base of Middle Riphean, Vendian, and Cambrian sections. 2. The earliest Phanerozoic subduction events are reported from the eastern (hereafter in present-day coordinates) margin of the Okhotsk terrane, which occurred in the Middle-Late Ordovician. At that time, the granitoid complex of a continental-marginal arc was forming there. Synchronous with the crustal extension, within the Kyllakh and, likely, the Sette-Daban tectonic zones, dikes and sills of basic composition were emplaced. Their age was first determined as Late Ordovician (451±24, 450±12 Ma) from U-Pb dating of baddeleyite. 3. Next pulse of subduction events in the eastern Okhotsk terrane (North Okhotsk magmatic arc) is confidently dated as late as the Late Devonian on the basis of subduction-related granitoids (U-Pb, 375.3±2.3 Ma, zircon) and calc-alkaline volcanics present there. Concurrent with this, continental rifting processes occurred within the Sette-Daban and AllakhYun’ tectonic zones, which manifested themselves as normal faulting and eruptions of alkali basalts of Devonian-Early Carboniferous age. Within the Kyllakh zone this time interval was marked by the Fig. 1 - Structural setting of the South Verkhoyansk orogenic belt. emplacement of basic dikes and sills. 4. The Middle-Late Triassic subduction in the eastern Okhotsk terrane is evidenced by calc-alkaline volcanics and exhumation of the crystalline basement as seen from (U-Th)/ He low-temperature thermochronometry data. No evidence of this event is found within the tectonic zones in the west. 5. The Late Mesozoic geodynamic events were related to the subduction of the paleo-Pacific beneath the eastern margin of the Okhotsk terrane and the formation of the Uda (Late Jurrasic-Neocomian) and the Okhotsk-Chukotka (Albian-Late Cretaceous) active continental margins. The first pulse of Late Mesozoic thrusting and dislocation metamorphism occurred within the Sette-Daban tectonic zone in the latest Late Jurassic (40Ar/39Ar, 160±1 Ma). This metamorphic event marks the initiation of folding and the onset of the formation of a metamorphic belt in South Verkhoyansk region. These deformations are supposed to be the result of accretion processes that occurred along the subduction zone of the Uda active continental margin. In the Allakh-Yun’ zone, the folding and metamorphic processes occurred in the Late Neocomian and Aptian (40Ar/39Ar, 119±0.5 Ma). On the southern flank and in the rear part of the Okhotsk terrane, deformation processes related to subduction along the Uda magmatic arc continued. It is established that folding was initiated in the Sette-Daban zone much earlier than in the neighboring Allakh-Yun’ zone. The timing of dislocation metamorphism in the 153 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - Schematic correlation of tectonomagmatic events in the south Verkhoyansk orogenic belt. latter zone coincides with the crystallization age of major granite plutons (U-Pb, 120-123 Ma, zircon). In the Late Cretaceous, synchronous with the formation of volcanics of the Okhotsk-Chukotka volcanogenic belt above the subduction zone, emplacement of granitoids aged at 95 Ma (U-Pb, zircons) and left-lateral transpression reverse and strike-slip motions occurred within the Allakh-Yun’ zone. THE LATE OLIGOCENE-MIOCENE ÑIRIHUAU FORMATION INTERPRETED AS A FORELAND BASIN IN THE NORTHERN PATAGONIAN ANDES 4-18 Ramos, M.E.*, Orts, D., Calatayud, F., Folguera, A., Ramos, V.A. Laboratorio de Tectónica Andina, Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires. Ciudad Universitaria, Pabellón II, Ciudad de Buenos Aires, 1428, Argentina. CONICET * Presenting author’s e-mail: [email protected] A field study of the Ñirihuau basin southwest region, nearby the Maitén range, between the 42º00´S and the 42º20´S, has revealed new stratigraphic relationships. The structural analysis of this Andean segment, allowed a new interpretation of the tectonic setting of these Patagonian foothills. The stratigraphic sequence consists of four units: The base is represented by the Oligocene Ventana Formation, formed mainly by andesitic volcanic and pyroclastic rocks defining the Maitén eruptive belt (Rapela et al., 1988). Based on chemical analyses, these authors recognized this sequence as a typical arc succession. The shales, sandstones and coal beds of the Ñirihuau Formation of ~22-17 Ma rest in general conformably on the previously described volcanic piles (González Bonorino, 1973; Cazau, 1980). More recently, Paredes et al. (2009) made a detailed sedimentary analysis in the northern part of the Ñirihuao basin. Here, they differentiated a series of lithofacies associations in strati154 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA graphical order: deep lacustrine, volcaniclastic flows, shallow lacustrine interbedded with Gilberttype deltas, volcaniclastic flows, fluvial channels, and volcanics at the top. We identified a series of sections, circumscribed to the southern part of the basin, that can be compared to the ones described by Paredes et al. (2009) to the north. These start with a thin carbonaceous section that could be associated with a relatively deep lacustrine association 10 meter thick, constituting an excellent stratigraphic marker through the basin. This is followed by a 50 meter thick volcaniclastic sandtstones. The upper section begins with liquefact levels, associated with Gilbert-type deltas, and floodplain deposits 350 meter thick. Finally a fluvial system progrades on top of the deltaic section represented by a series of channels and associated flood plain deposits. These sequences are exhumed at the eastern slope of the Maitén range which is interpreted as an eastverging thick-skinned structure dismembered by a series of synthetic-to the main thrust front structures (eg. El Pantano thrust), affected by a series of west-verging backthrusts developed in the back limb (e.g. El Maitén thrusts). Basement shortening produced by these structures was transmitted to the upper sedimentary cover, where a frontal monoclinal ramp was formed (Giacosa and Heredia, 2004a), absorbing discrete amounts of thin-skinned deformation related to fault bend folds associated with flexural slip (Fig. 1). In the eastern sector, four sets of progressive unconformities from the base to the top of the Ñirihuau Formation and the base of the Collón Cura Formation were recognized. These were explained applying the concept of growth strata bed-by-bed by kink-band migration (Suppe et al., 1997). These find- Fig. 1 - Structural cross section and corresponding palinspastic restoration. Note a thick skinned domain in the westernmost part and a thin-skinned deformation at the eastern section associated with flexural slip 155 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA ings imply that this basement structure was created at the time of both Ñirihuao and Collón Curá sedimentation at the foothills and therefore that these constitute remnants of a proximal foreland basin. These progressive unconformities are typically found at the fold and thrust belt top wedge (DeCelles et al., 1996), implying that the early to late Miocene orogenic front was located in the Maitén range. Based on these findings, four pulses of contractional deformation are proposed for the fold and thrust belt at these latitudes coetaneous to the sedimentation of the Ñirihuao and the base of the Collón Cura Formations (>22 to ~15 Ma), implying a mechanism of subsidence associated with orogenic loading of the El Maiten range. REFERENCES • Cazau, L.B.; 1980. Cuenca de Ñirihuau-Ñorquinco-Cushamen Leanza, A. (ed.), Geología Regional Argentina 2, Academia Nacional de Ciencias de Córdoba, Córdoba, Argentina, pp. 1149-1171. • DeCelles, P.G., Giles K.A.; 1996. Foreland basin systems. Basin Research Vol.8, pp. 105-123. • Giacosa, R.E., Heredia N.; 2004a. Structure of the North Patagonian thick-skinned fold-and-thrust belt, southern central Andes, Argentina (41°–42°S) Journal of South American Earth Sciences, Vol.18, pp. 61–72 • Giacosa, R., Heredia, N.;2004b. Estructura de los Andes Nordpatagónicos en los cordones Piltriquitrón y Serrucho y en el valle de El Bolsón (41 30‘–42 00‘ S), Río Negro. Revista Asociación Geológica Argentina, Nº59 Vol.1,pp. 91–102. • González Bonorino, F.;1973. Geología del área entre San Carlos de Bariloche y Llao Llao. Fundación Bariloche, Dpto. Rec. Nat. Energ. Buenos Aires, Publ. 16, pp. 1-53. • Paredes, J.M., Giacosa, R.E., Heredia, N.; 2009. Sedimentary evolution of Neogene continental deposits (Ñirihuau Formation) along the Ñirihuau River, North Patagonian Andes of Argentina. Journal of South American Earth Sciences, Vol. 28, pp. 74-88. • Rapela, C., Spalletti, L., Merodio, J., and Aragón, E.; 1988. Temporal evolution and spatial variation of early Tertiary volcanism in the Patagonian Andes (40° S - 42°30’ S): Journal of South American Earth Sciences, Vol. 1, pp. 75-88.• • Suppe, J.; 1997. Bed-by-bed fold growth by kink-band migration: Sant Llorenq de Morunys, Eastern Pyrenees. Journal of Structural Geology, Vol. 19 Nos. 3-4pp, 443-461. A CASE OF PALEOHORIZONTAL RESTORATION OF PLUTONIC BODIES USING PALEOMAGNETIC DATA: THE SIERRA DE VALLE FÉRTIL MAGMATIC COMPLEX, WESTERN ARGENTINA 4-19 Rapalini, A1*, Pinotti, L.2, D’Eramo, F.2, Otamendi, J.2, Vegas, N.3, Tubía, J.3, Singer, S.1, Vujovich, G.4 (1) Institutode Geofísica Daniel A. Valencio (INGEODAV)- Departamento de Ciencias Geológicas FCEN- Universidad de Buenos Aires. CONICET (2) Departamento de Geología, FCEFQyN, Universidad Nacional de Río Cuarto.CONICET (3) Departamento de Geodinámica- Universidad del País Vasco-España (4) Departamento de Ciencias Geológicas - FCEN- Universidad de Buenos Aires. CONICET * Presenting author’s e-mail: [email protected] Introduction Reliable paleohorizontal control is essential in any tectonic or paleogeographic application of paleomagnetic studies. As such, the use of paleomagnetic data from plutonic bodies, for which paleohorizontal control is frequently lacking, is severely restricted. However, if magnetization age can be determined or constrained and the expected paleomagnetic direction is already known, paleomagnetic information can be very useful in reconstructing the original position of plutonic bodies, providing important information for the understanding of emplacement processes. Recently Castro et al. (2008) proposed that the magmatic structures related to the mechanical interaction between mafic magmas and granitoids in the Early Ordovician Sierra del Valle Fértil magmatic complex are the result of top-to-down intrusions of a mafic magma into a granodioritetonalite mass. These sinking structures would be the result of a reverselly stratified magma chamber with gabbros and diorites at the top and granodiorite-tonalite at the bottom. This interpretation would not be valid if the plutons have been subsequently tilted so that sub-vertical to high inclination pipelike structures were originally subhorizontal, reflecting a different layering process. 156 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA A paleomagnetic study was carried out on fifty-one oriented cores from the Sierra de Valle Fértil magmatic complex in order to evaluate to which extent these plutonic rocks have been subsequently tilted, and in such way to provide independent information to improve the petrological model of Castro et al. (2008). Geological Setting The magmatic complex of Sierra de Valle Fértil (San Juan province, Argentina) consists of igneous and metamorphic rocks. This complex formed at low- to middle-crustal paleodepths within the Early Ordovician subduction-related magmatic belt from central- and northwestern Argentina, which is known as Famatinian arc. The age of magmatism in this sector has been well constrained between 490 and 465 Ma (Pontoriero and Castro de Machuca, 1999; Pankhurst et al., 2000). Recent ages of igneous plutonic rocks yielded ages between 469 to 477 Ma (Ducea et al., 2010). Within the western belt of the currently exposed Fig. 1 - Main geological units of central western Argentina and Famatinian magmatic arc, the Sierras location of the study area. de Valle Fértil contain fairly wellexposed sections showing the transition between lower-crustal and upper-crustal levels (Otamendi et al., 2008). The reconstruction of the Famatinian arc suggests that this deep seated crustal sequence formed in an outboard belt (e.g. Quenardelle and Ramos, 1999). Tilting and uplifting of the studied crustal section during emplacement in the upper crust might be primarily related to the collision between the Laurentiaderived terrane of Cuyania (or “Precordillea”) and the proto-Pacific margin of Gondwana (Thomas and Astini, 1996; Ramos et al., 1996). From west to east, the lithologic units display a progression towards more evolved igneous compositions. In general, the stratigraphy may be described considering four units: (Mirré, 1976; Vujovich et al., 1996) : (1) A layered mafic unit including Ultramafic, Ol-rich and Px-rich cumulates. (2) A tonalite-dominated igneous unit comprising coarse grained biotite tonalites and extremely heterogeneous rocks with mafic enclaves. (3) A felsic igneous unit making up a batholith-scale Bt ± Amph granodiorite which hosts chilled mafic dikes, enclaves, and blocks of Amph-bearing gabbros. (4) Migmatites (metatexite to diatexite) appearing as kilometric strips interlayered with igneous mafic, intermediate and felsic rocks. The two silicic units, tonalites and granodiorites, contain abundant mafic microgranular enclaves (Castro et al., 2009). Paleomagnetic Study and Results Sixty-two specimes were selected to perform stepwise AF (47) and thermal (15) demagnetization. Both methods proved to be equally efficient to isolate the characteristic remanence. Most samples are carrier of a stable magnetic remanence which may be determined with the standard techniques. Each magnetic component was defined by principal component analysis (Kirschvink, 1980). Remanece coercivity and unblocking temperatures suggest magnetite as the main carrier in most samples. The 157 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - a) Paleomagnetic pole position for the Sierra de Valle Fértil complex from “in situ” mean remanence direction (A) and after untilting into “paleohorizontal” (B) and “paleovertical” (C) positions for magmatic structures. b) Comparsion of pole positions A and C with other poles from the Sierras Pampeanas, Famatina system, Eastern Puna and Early Ordovician reference pole for Gondwana. More references in the text. occasional occurrence of pyrrothite is suspected. After erasing a very soft and randomly directed component, a consistent characteristic remanence was defined in 50 specimens. Mean remanence direction is Dec: 14°, Inc: -73°, ·95: 5°. Only three samples showed a reverse polarity. This direction is 23° apart from the geocentric axial dipole direction and even farther from the present-day Earth magnetic field direction at the sampling locality, nor is it coincident with any expected direction for the last 200 m.y. A paleomagnetic pole was computed from this mean direction. This is presented in African coordinates according to a Gondwana reconstruction (Reeves et al., 2004) in Fig. 2 (A). In order to evaluate possible tilting of the intrusive bodies, two alternative paleomagnetic poles were computed. The first (B) was obtained after untilting of 50° around a horizontal axis of Az 165° (right–hand rule). This rotation turns most structures related to mechanical interaction between mafic magmas and granitoids subhorizontal. The other (C) was the product of untilting of 40° around a subhorizontal axis trending 345° (right - hand rule). This would turn the same structures subvertical. According to geologial information in the area, tilting around a NNW trending axis is likely due to the Andean orogeny in the Tertiary. Comparison of three alternative poles with reference poles clearly indicates that rotation into subhorizontal (B) produces a pole position incompatible with any Phanerozoic pole position. Alternative A produces a pole position roughly consistent with Devonian paleomagnetic poles from the Devonian Achala Batholith (Geuna et al., 2009) and Baritú Fm. (Spagnuolo, 2009). This option may be valid if the studied rocks were remagnetized in Devonian times and no significant tilting due to Andean or other orogenesis occurred since then. In the third case (C) rotation into paleovertical position for most of the already mentioned magmatic structures produces a pole position consistent with previous paleomagnetic poles from Early Ordovician plutons and lavas from the Sierra de Famatina and Eastern Puna (Conti et al., 1996, Spagnuolo et al., 2008), coeval with the studied rocks. These results strongly support the third alternative suggesting that the described magmatic structures were produced in a subvertical position which is consistent with the petrological model of Castro et al. (2008). Neogene sediments in the area show minor to moderate dips towards the ENE, consistent with tilting indicated by the paleomagnetic data. REFERENCES • Castro, A., Martino, R., Vujovich, G., Otamendi, J., Pinotti, L., D’Eramo, F., , Tibaldi, A., Viñao, A., 2008. Top-down structures of mafic enclaves within the Valle Fértil magmatic complex (Early Ordovician, San Juan, Argentina). Geologica Acta: Vol. 6, 217-229. • Conti, C.M., Rapalini, A.E., Coira, B. y Koukharsky, M. 1996. Paleomagnetic evidence of an early Paleozoic rotated terrane in 158 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Northwestern Argentina, a clue for Gondwana-Laurentia interaction?. Geology 24: 953-956. • Ducea, M.N., Otamendi, J.E., Bergantz, G., Stair, K., Valencia, V., and Gehrels, G., 2010. Timing constraints on building an intermediate plutonic arc crustal section: U-Pb zircon geochronology of the Sierra Valle Fértil, Famatinian Arc, Argentina. Tectonics, in press. • Geuna, S.E. , Escosteguy, L.D., Miró, R., 2008. Palaeomagnetism of the Late Devonian–Early Carboniferous Achala Batholith, Córdoba, central Argentina: Implications for the apparent polar wander path of Gondwana. Gondwana Research, 13, 227-237. • Kirschvink, J.L. 1980. The least - squares and plane and the analysis of paleomagnetic data. Geophysical Journal of the Royal Astronomical Society, vol. 67, p. 699-718. • Mirré, J.C., 1976. Descripción geológica de la Hoja 19e, Valle Fértil, Provincias de San Juan y La Rioja. Servicio Geológico Nacional, Boletín N° 147. Ministerio de Economía, Buenos Aires, 70 pp. • Otamendi, J.E., Tibaldi, A.M., Vujovich, G.I., Viñao, G.A., 2008. Metamorphic evolution of migmatites from the deep Famatinian arc crust exposed in Sierras Valle Fértil-La Huerta, San Juan, Argentina. Journal of South American Earth Sciences 25, 313–335. • Pankhurst, R.J., Rapela, C.W., Fanning, C.M., 2000. Age and origin of coeval TTG, I- and S-type granites in the Famatinian belt of NW Argentina. Transaction Royal Society of Edinburgh: Earth Sciences, 91, 151-168. • Pankhurst, R.J., Rapela, C.W., Saavedra, J., Baldo, E., Dahlquist, J., Pascua, I., Fanning, C.M., 1998. The Famatinian magmatic arc in the central Sierras Pampeanas: an Early to Mid-Ordovician continental arc on the Gondwana margin. In: Pankhurst, R.J., Rapela, C.W. (eds.). The Proto-Andean Margin of Gondwana. Special Publication. Geological Society, London, 343-368. • Pontoriero, S., Castro de Machuca, B., 1999. Contribution to the age of the igneous-metamorphic basement of La Huerta range, province of San Juan, Argentina, II South American Symposium of Isotopic Geology, Proceedings, 101-104. • Quenardelle, S., Ramos, V., 1999. The ordovician western Sierras Pampeanas magmatic belt: record of Argentine Precordillera accretion. In: Ramos, V.A., Keppie, D. (Eds.), Laurentia Gondwana Connections before Pangea, vol. 336. Geological Society of America, Boulder, pp. 63–86. Special Paper. • Ramos, V.A., Vujovich, G.I., Dallmeyer, R.D., 1996. Los klippes y ventanas tectónicas de la estructura preándica de la Sierra de Pie de Palo (San Juan): edad e implicanciones tectónicas. In: Proceedings of the Actas XIII Congreso Geológico Argentino y III Congreso de Exploración de Hidrocarburos, vol. 5. pp. 377–392. • Reeves, C.V., de Wit, M.J., Sahu, B.K. 2004. Tight reassembly of Gondwana exposes Phanerozoic shears in Africa as global tectonic players, Gondwana Res. 7 ; 7–19 • Spagnuolo CM (2009). Evolución paleogeográfica del Noroeste Argentino en el Paleozoico temprano en base a estudios paleomagnéticos. PhD Thesys (unpublished). Buenos Aires Uiversity, Argentina. 325 pp. • Spagnuolo C.M., Rapalini, A.E. y Astini R.A. 2008. Palaeomagnetic confirmation of Palaeozoic clockwise rotation of the Famatina Ranges (NW Argentina): implications for the evolution of the SW margin of Gondwana. Geophysical Journal International 173 (1): 63. • Thomas, W.A., Astini, R.A., 1996. The Argentine precordillera: a traveler from the ouachita embayment of North American Laurentia. Science 273, 752–757. • Vujovich, G.I., Godeas, M., Marín, G., Pezzutti, N., 1996. El complejo magmático de la Sierra de La Huerta, provincia de San Juan, Actas XIII Congreso Geológico Argentino y III Congreso de Exploración de Hidrocarburos, Argentina, 465-475. SEISMICITY AND EARTHQUAKE HAZARD IN TIERRA DEL FUEGO PROVINCE, ARGENTINA 4-20 Sabbione, N.C.1*, Buffoni, C.1,2, Barbosa, N.1, Badi, G.1, Connon, G.3, Hormaechea, J.L.1,3 (1) Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Argentina (2) CONICET, Argentina (3) Estación Astronómica Río Grande, Tierra del Fuego. CONICET and Universidad Nacional de La Plata, Argentina * Presenting author’s e-mail: [email protected] Since the 1990s a local seismometric network installed in Tierra del Fuego, including permanent broadband and short period stations, provides the seismological information required to afford different kind of studies. This network also allows proper monitoring of Tierra del Fuego seismicity for civil protection purposes considering the high risk of heavy damages in the towns of Ushuaia and Rio Grande in the eventuality of a major earthquake such the event occurred in 1949. Seismicity has been studied and records have been processed by Seisan Software, from Bergen University (Norway); a significant level of low to medium magnitude earthquakes with epicentre in Tierra del Fuego continental region and oceanic surrounding areas was found. The obtained seismicity map shows that beyond rather dispersed seismicity related to the Magallanes-Fagnano fault system (MFFS), a concentration of epicentres is found in the Darwin Cordillera and in the margins of 159 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA the Scotia plate. It is well known that the region has a complex tectonic setting: the island is crossed by the MFFS which divides Tierra del Fuego in two continental blocks. This MFFS constitutes the major continental segment of the South America-Scotia plate’s border. Results obtained are then used to calculate shear wave attenuation parameters with the spectral technique. Adjustment of the Q spectral S wave is made in the frequency band between 1 and 9 Hz, which results in quality factor values of about 100 for short distances and 300-400 for more distances, according to a region with an important tectonic activity where earthquakes of magnitude 2-3.5 occur. An earthquake hazard map for Tierra del Fuego Province, Argentina is here presented. Among the important concerns are the completeness of the catalogues and whether sufficient studies have been conducted to estimate the earthquake sizes and locations in order to assess the implications of a modern recurrence. The most fundamental information for a hazard assessment is the record of past earthquakes. The island has an important seismological history which includes an event of magnitude 7.8 occurred on December 1949. Reports of earthquakes occurred in 1929, 1930, 1944, 1949 and 1970 are known by a study of historical seismicity. SYNOROGENIC SEQUENCES ASSOCIATED WITH THE ANDEAN FRONT AT 37º S AS A CLUE FOR AGE EXHUMATION AND STRUCTURATION OF THE FORELAND AREA 4-21 Sagripanti, L.*, Naipauer, M., Folguera, A., Ramos, V.A. Laboratorio de Tectónica Andina, Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires Ciudad Universitaria, Pabellón II, Ciudad de Buenos Aires, 1428, Argentina. CONICET * Presenting author’s e-mail: [email protected] The eastern Andean slope at 37º S was formed by the Malargüe fold and thrust belt (Kozlowski et al., 1993), whose orogenic front exhumed a lower angular unconformity between Eocene terms of the Pircala Formation and late Oligocene to early Miocene rocks of the Palauco Formation and an upper uncorformity between Palauco Formation and a Neogene sedimentary cover (Tristeza Formation) and Pliocene to Quaternary volcanic beds of the Payenia volcanic field. In order to constrain the relative importance between these different contractional pulses that led to the formation of the orogenic front at these latitudes, a sedimentological and petrographical study was carried out through profiles in the Neogene sequences (Tristeza Formation) of the Pampa de Carrizalito syncline eastward the Sierra de Reyes anticline (Fig. 1) plus a series of U/Pb datings in detrital zircons. The Sierra de Reyes anticline is a basement structure that was produced by tectonic inversion of Late Triassic normal faults (Kozlowski et al., 1993; Zamora and Zapata, 2005; Giambiagi et al., 2009). Three contractional stages have been related to exhumation in the area (Late Cretaceous, late Eocene and late Miocene), based on different studies (Cobbold and Rosello 2003; Orts and Ramos 2006; Tunik et. al., 2010). The Upper Cretaceous contractional stage affected just the westernmost zone and probably did not reach the present orogenic front where this study is hosted. The second contractional event took place at the present orogenic front, although this event is just associated with a 50 – 100 m thick shales and sandstones of the Pircala Formation (Kozlowski et al., 1987). The third mountain building episode registered at the orogenic front developed in late Miocene times and is associated with synorogenic sedimentation. We have focussed our attention in this last episode to compare this sedimentary record and evaluate total denudation. Historically the Tristeza Formation hosted in the Pampa de Carrizalito depocenter was assigned to the late Miocene, based on their occurrence on top of the Palauco Formation whose upper part was dated in 18,12 ± 0,24 Ma (Ar-Ar; Silvestro and Atencio, 2009). Two profiles located in both syncline flanks (A – A’ and B – B’; see Fig. 1 for location) were done in order to constrain the geometry, thickness variation next to the orogenic front, and detrital compositional changes, of main sequences. Based on 160 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Geological map of the Neogene synorogenic sequences developed on top of the orogenic wedge front, at the Sierra de Reyes latitudes; a-f are the defined units in the sedimentary and petrologic analyses performed in the Neogene synorogenic deposits (see Fig. 2). 161 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - Progressive exhumation of the eastern flank of the Sierra de Reyes is recognized and correlated through the Neogene Pampa de Carrizalito foreland basin. Pampa de Carrizalito depocenter represents a clear unroofing sequence. Total exhumation of this system is recorded in Neogene times which show that older deformations have not produced significant topography. the sedimentological and petrographical analysis plus the results of U/Pb dating, 6 different units (a,b,c,d,e and f in A – A’ profile) could be identified in a series of terms characterized by contrasting sedimentary sources. In detail basal sections (a+b units) are mainly composed of ignimbrites and volcaniclastic beds containing basalt clasts coming from the underlying Palauco Formation; then there are sandstone clasts on top of the previous units (c unit), followed by beds with limestone detritus (d unit), then gypsum clasts (e unit) and finally shale clasts (f unit) (Fig. 2). The uppermost section contains clasts of polycrystalline quartz associated with a lithic metamorphic source. This same compositional variations are also recognized in the eastern profile (B-B’; see Fig. 2), with the only exception of the lowest volcaniclastic and the gypsum clast horizons. The lower sandstones are derived from the erosion of the Neuquén Group, while the limestone fragments are related to the erosion of the Mendoza Group, and the gypsum clasts to the Auquilco Formation. Finally, the shales are related to the Bardas Blancas Formation. All these rocks are in a reverse order represented in the eastern Sierra de Reyes flank. This implies that the succession hosted in the Pampa de Carrizalito syncline constitutes a typical unroofing sequence: At the time when the Sierra de Reyes was uplifted its exhumation led to the deposition of a sequence characterized by a variable clastic composition. Finally, metamorphic clasts indicate that the foreland area was uplifted and exhumed at this time, implying the cannibalization of the foreland basin (Fig. 2). Since the geochronological dating of detrital zircons is a valuable tool in provenance analysis of sedimentary basins, we have obtained several U-Pb (LA-ICP-MS) ages from six samples in the late Miocene Tristeza Formation. Analyzed detrital zircons in unit b yielded prominent U/Pb age peaks for the early Jurassic (ca. 190-180 Ma) and Permo-Triassic (ca. 270-260 Ma). In addition, minor peaks appear with Paleozoic ages (Devonian and Carboniferous). Equivalently, the detrital grains from units d to e are characterized for prominent peaks located in the early Jurassic and Permo-Triassic, but in addition they have an important group of early Cretaceous ages (ca. 98 Ma). Subordinate peaks appear in the Carboniferous, Devonian, and Neoproterozoic-Paleozoic. The U-Pb age spectra of the younger unit (f) showed significant changes in the pattern of detrital zircon ages respect to the previously described units. In the former, the early Cretaceous zircons are absent, and the Jurassic and Permo-Triassic zircons become less important, although several populations of zircons of Mesoproterozoic, Neoproterozoic and early Paleozoic ages begin to dominate, with subordinate Paleoproterozoic ages (ca. 2200 Ma). 162 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA The obtained detrital zircon ages are clearly in line with the results of the microscopic and macroscopic detrital analyses. The main characteristics can be summarized as follows: An igneous source (volcanic and plutonic) in the basal half of the sequence that is probably coming from the Jurassic to Cretaceous Andean arc roots, located to the west in the main Andes. Additionally a volcanic component is associated with the Permian to Triassic Choiyoi Group. The origin of the Carboniferous and Devonian zircons without a clear neighbor source remains more controversial. Finally, is worth noting that at the top of the sequence microscopic evidence of a metamorphic source appears. Equivalently, the ages obtained are consistent with a source area formed during the Grenville, Pampean, and Famatinian cycles. The San Rafael-Las Matras block, located about 200 km east of this Neogene synorogenic depocenter is the best candidate as a source area. The fact that an entire unroofing sequence is registered in a Neogene depocenter next to the orogenic front implies that complete exhumation of the Sierra de Reyes started about this time, and therefore that previous phases are negligible in the study region. We can conclude that the Neogene contractional stage was the responsible for the exhumation of the orogenic front at these latitudes and created the present topography. These provenance studies show that the wedge top foreland basin was cannibalized during the exhumation of the last stages of the sedimentary record when the San Rafael-Las Matras block was exhumed to the east in the broken foreland final stage. REFERENCES • Cobbold, P. and Rossello, E.; 2003: Aptian to Recent compressional deformation in the foothills of the Neuquén basin Argentina. Marine and Petroleum Geology, 20, 429-443. • Giambiagi, L., Ghiglione, M., Cristallini, E. and Bottesi, G.; 2009: Caracteristicas estructurales del sector sur de la faja plegada y corrida de Malargüe (35º - 36º): Distribución del acortamiento e influencia de estructuras previas. Revista de la Asociación Geológica Argentina, 65 (1), 140-153. • Kozlowski, E., Cruz, C. and Rebay, G.; 1987: El Terciario volcaniclástico de la zona Puntilla del Huincan. Mendoza. X Congreso Geológico Argentino, Actas 4, 229-232. Tucumán. • Kozlowski, E., Manceda, R. and Ramos, V.A.; 1993: Estructura. In: Ramos V.A. (ed), Geología y Recursos Naturales de Mendoza. XII Congreso Geológico Argentino and II Congreso de Exploración de Hidrocarburos. Relatorio 1, 18, 235-256. • Orts, S. and Ramos, V.A.; 2006: Evidence of middle to late Cretaceous compressive deformation in the high Andes of Mendoza, Argentina. Backbone of the Americas, abstract with Programs 5, 65, Mendoza. • Silvestro, J. and Atencio, M.; 2009: La cuenca Cenozoica del Río Grande y Palauco: edad, evolución y control estructural. Faja plegada de Malargüe (36°S). Revista de la Asociación Geológica Argentina. 65 (1), 154-169. • Tunik M., Folguera A., Naipauer M., Pimente M. and Ramos V.A.; 2010: Early uplift and orogenic deformation in the neuquén basin: constraints on the Andean uplift from U – Pb and Hf isotopic data of detrital zircons. Tectonophysics, 489, 258-273. • Zamora Valcarce G. and Zapata T.R.; 2005: Estilo estructural del frente de la faja plegada neuquina a los 37º S. VI Congreso de exploración y desarrollo de Hidrocarburos. Electronic files. Mar del Plata. FURTHER EVIDENCE OF LOWER PERMIAN REMAGNETIZATION IN THE NORTH PATAGONIAN MASSIF, ARGENTINA 4-22 Tomezzoli, R.1*, Rapalini, A.E.1, Lopez de Luchi, M.G.2 (1) Instituto de Geofísica “Daniel A. Valencio (INGEODAV)”, Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos – CONICET (2) Instituto Nacional de Geocronología y Geología Isotópica (INGEIS), CONICET-Universidad de Buenos Aires * Presenting author’s e-mail: [email protected] The origin of Patagonia has long called the attention of South American earth scientists. In recent years a dispute over whether it is an accreted crustal block that collided with Gondwana in Late Paleozoic times or an autochthonous part of South America has taken place. Scarce paleomagnetic data mostly younger than Devonian, preclude a definite paleomagnetic test on the origin of this terrane. The presence of well-dated and undeformed Ordovician granitoids discordantly covered by the Silurian-Devonian Sierra Grande Fm in the northeastern corner of the North Patagonian Massif 163 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA (NPM) are a suitable target to undertake such test. The plutonic rocks belong to the Punta Sierra and related granitoids which have recently yielded U-Pb crystallization ages between 472-476 Ma (Pankhurst et al., 2006, Varela et al., 2009 and references therein). A paleomagnetic study of these intrusives (41.5°S, 65.0°W) was carried out. As part of a multidisciplinary study, anisotropy of magnetic susceptibility (AMS) measurements, systematic analyses of petrographic thin sections and rock magnetic analyses, have also been performed. About one hundred specimens were processed, comprising ten sites on the granite and two sites on the quartzites and sandstones of the Early Devonian Sierra Grande Formation, with five cores each. Demagnetization at high temperatures isolated a reverse characteristic remanent magnetization, suggesting of being acquired during the Kiaman reverse superchron. Structural correction of paleomagnetic data from plutons was available only at a few sites from bedding attitudes of the Devonian or Tertiary sedimentary rocks. Seven out of twelve sites, all in the granites, provided consistent remanence directions. Structural correction could be applied at a single site worsening the statistical parameters of the mean site direction and suggesting a secondary magnetization. A paleomagnetic pole (PP) was computed from the mean of the seven site directions. The position of this PP on the apparent polar wander path of South America is at: 11.5°E, 65.0°S; A95=12°, K=24.5 suggesting that magnetization was acquired during the Early Permian, being this pole consistent with previous poles of that age from South America (Tomezzoli, 2009). A remagnetization during the late Early Permian has been already reported on some outcrops of the Devonian Sierra Grande Fm. in the same area (Rapalini and Vilas, 1991). Our data suggests that the remagnetization was pervasive and affected the Ordovician granitoids as well. Whether this remagnetization is due to the widespread Permian magmatism that affected the NPM or to the deformational phase ascribed in some models to the collision of Patagonia against the Gondwana margin is to be determined. However, it is significant that several South American Lower Permian paleomagnetic poles have been computed from syntectonic magnetizations (e.g. Ponón Trehue; Tunas I PP; Cochico PP; Río Curacó PP; and Sierra Chica PP, among others). All these PPs come from localities along a 500 km long WNW-ESE orogenic belt that extends from the San Rafael block in the province of Mendoza to the Ventana System in the province of Buenos Aires. Deformation along it has been assigned to the San Rafaelic orogenic phase and dated at approximately 290 Ma. This phase has been recognized mainly in the western areas of Argentina and has been linked to remagnetization of a regional scale (Rapalini and Astini, 2005). Time coincidence of remagnetizations suggests that causal links for all of them including those in the NPM are likely. Regional remagnetization associated to this major orogenic phase and its geotectonic framework should be explored. REFERENCES • Pankhurst, R.J., Rapela, C.W., Fanning, C.M. and Márquez, M. 2006. Gondwanide continental collision and the origin of Patagonia. Earth-Science Reviews 76(3-4): 235-257. • Rapalini, A.E. and Vilas, J.F., 1991. Preliminary paleomagnetic data from the Sierra Grande Formation: Tectonic consequences of the first mid-Paleozoic paleopoles from Patagonia. J. South Am. Earth Sci. 4(1-2): 25-41. • Rapalini, A.E. and Astini, R.A.. La remagnetización Sanrafaélica de la Precordillera en el Pérmico: Nuevas evidencias. Revista de la Asociación geológica Argentina: 60(2): 290-300. • Tomezzoli, R.N., 2009. The Apparent Polar Wander Path for South America During the Permian-Triassic. Gondwana Research, 15: 209 – 215. • Varela, R., Sato, K., González, G.P., Sato, A.M., and Basei, M.A.S, 2009. Revista de la Asociación geológica Argentina: 64(2): 275284. 164 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA TECTONIC CONTROL ON THE EVOLUTION OF MAASTRICHTIAN-PALEOGENE SYNOROGENIC SEQUENCES OF THE FUEGIAN THRUST FOLD BELT, ARGENTINA 4-23 Torres Carbonell, P.J.1*, Olivero, E.B.1, Dimieri, L.V.2 (1) Centro Austral de Investigaciones Científicas (CADIC-CONICET). B. A. Houssay 200, 9410 Ushuaia, Tierra del Fuego, Argentina (2) Instituto Geológico del Sur (INGEOSUR-CONICET), Departamento de Geología, Universidad Nacional del Sur. San Juan 670, 8000 Bahía Blanca, Buenos Aires, Argentina * Presenting author’s e-mail: [email protected] A geological study of the Atlantic coast of Tierra del Fuego between the punta Gruesa (54º 21’ S; 66° 38.5’ W) and the río Policarpo (54º 39’ S; 65º 30’ W) (Fig. 1), and other sectors of the Fuegian Andes, Fig. 1 - Geologic map of the study area. Trace of the composite cross-section of Fig. 2 is shown. 165 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - Kinematic evolution of the studied portion of the Fuegian thrust fold belt, with the contractional stages depicted. PT: Policarpo thrust, CLT: Cabo Leticia thrust, PT: Punta Torcida, PAT: Punta Ancla thrust, CDMB: Campo del Medio backthrust, MB: Malengüena backthrust, PGIS: Punta Gruesa imbricate system, LCT: La Chaira thrust, FTS: Fagnano transform system. Stratigraphic units: 1, Upper Jurassic; 2, Lower Cretaceous; 3, Upper Cretaceous-Danian; 4, Paleocene; 5, Ypresian; 6, Paleocene-Ypresian; 7, Lutetian; 8, upper Lutetian-Priabonian; 9, Oligocene; 10, uppermost OligoceneMiocene; 11, upper Miocene-?Pliocene. The segments of this cross-section are located in Fig. 1. 166 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA allowed us to define the stratigraphy and sedimentology of synorogenic successions from the Austral basin, and their genetic relations with the geometry and kinematics of the Fuegian thrust fold belt. We define seven sequences between the Maastrichtian and the Miocene, bounded by syntectonic unconformities: Maastrichtian-Danian (180 to 800 m), Paleocene (50 to 370 m), Ypresian (450 to 650 m), Lutetian (80 m), upper Lutetian-Priabonian (1200 m), Oligocene (1600 to 200 m) and uppermost Oligocene-Miocene (200 m). These successions are composed of marine sedimentites, mostly deposited by gravity flows below the storm-wave base. The paleocurrent directions and the petrography indicate sediment provenance areas in the volcanic arc along the Pacific margin of the Andes, and in the core of the Fuegian Andes, the former dominant between the Maastrichtian and the Lutetian, and the latter since the late Lutetian. The detailed mapping of the Fuegian thrust fold belt structures allowed us to construct two balanced cross-sections that depict their subsurface geometries. The southern cross-section shows main décollements at the base of the Cretaceous and above the Maastrichtian-Danian, and thrust-related folding of the Cretaceous-Miocene sedimentary cover. The total shortening in that cross-section is of 41.8 km. The northern cross-section has a main décollement at the base of the Paleocene-Ypresian rocks, and minor ones in Bartonian-Priabonian levels. This section shows thrust-related folding of the Paleocene-Miocene sedimentary cover, with a total shortening of 17.8 km. By combining both cross-sections, estimating their location before the development of the Neogene Fagnano transform system (Fig. 2), six contractional stages are defined in relation to the evolution of the thrust-fold belt, age-constrained by the biostratigraphy of the synorogenic successions recognized: Df1 (Danian) with low percentages of layer parallel shortening in the foreland; Df2 (Ypresian) with development of thrust-related folding in a forward thrust-sequence and shortenings between 7 and 18.8 km (21%); Df3 (Lutetian) with development of out-of-sequence structures and a shortening of 6.6 km (7.3%); Df4 and Df5 (Oligocene) related to backthrusting with a shortening of 13.6 km (15.2%); Df6LC and Df6PG (latest Oligocene-Miocene), the last contractional stages recorded in the eastern Fuegian thrust-fold belt, comprising thrust-related folding within the belt with a shortening of 2.8 km (3.1%), and in the leading edge of deformation with a shortening of 10.5 km (11.6%). The thrust-fold belt reveals an episodic evolution that can be analyzed in terms of the Coulomb wedge theory, obtaining a model with three main stages: a critical wedge during the Danian to Ypresian (stages Df1 and Df2) with forward directed thrusting and a progressively diminishing taper angle, a subcritical wedge between the Lutetian and the ‘mid’ Oligocene (Df3 a Df5) with development of outof-sequence structures and backthrusts and a tendence to attain a critical taper, and a critical wedge during the latest Oligocene to early Miocene (Df6PG) with foreland displacement of the thrust wedge and propagation of the basal décollement to shallower levels. The Austral foreland basin system evolved as a single depocenter (foredeep) during deposition of the Maastrichtian-Danian, Paleocene and Ypresian sequences, the latter posibly also accumulated in depocenters atop active structures (wedge-top). Between the Lutetian and the Miocene, sedimentation occured in two depocentres: the wedge-top and the foredeep. During the Oligocene this segmentation of the basin resulted in a thicker succession within the wedge-top, which distinguishes it from classic tectonostratigraphic models. During the Miocene, cessation of contractional deformation in the thrustfold belt was simultaneous with the development of the last foredeep of the basin. 167 Session 5 STRATIGRAPHY AND SEDIMENTOLOGY GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA PRESERVATION OF TOTAL ORGANIC CARBON AND EVALUATION OF CORG/NTOT ATOMIC RATIO IN A SEDIMENT OUTCROP LOCATED S-E OF THE LAGO FAGNANO (TIERRA DEL FUEGO, ARGENTINA) 5-01 Caffau, M.1*, Comici, C.2, Zecchin, M.1, Presti, M.1, Lodolo, E.1, Tassone, A.3, Lippai, H.3, Menichetti, M.4 (1) Istituto Nazionale di Oceanografia e Geofisica Sperimentale (OGS) – Trieste, Italy (2) Dipartimento di Biologia e Oceanografia (OGS) – Trieste, Italy (3) Dept. de Geofisica, Universidad de Buenos Aires, Argentina (4) Dipartimento di Scienze Geologiche - Università di Urbino, Italy * Presenting author’s e-mail: [email protected] This study evaluates the content of the Total Organic Carbon (TOC), the Total Nitrogen (TN) and the Corg/Ntot atomic ratio in the sediments outcropping at a cliff in the south-eastern corner of the Lago Fig. 1 - Map of Tierra del Fuego, showing the location of the studied sequence. 171 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - Sand-silt-clay distribution in the sequence. (a) Detail of wavy and flaser bedding commonly found in the upper part of the sequence. (b) Detail of planar lamination that is dominant in the lower part of the sequence. Fagnano. It consists of muddy to sandy deposits arranged to form a succession about 8 m thick. The studied deposits are part of a larger depositional system referred here as the Lago Fagnano Gilbert-type delta (Fig. 1), which has been interpreted as a proglacial delta (Bujalesky et al., 1997); this sedimentary body is interbedded between levels of basal till (Bujalesky et al., 1997). The latter is composed of heterogeneous conglomerate-size clasts immersed in a muddy to sandy matrix and containing vegetable remnants (Bujalesky et al., 1997; Coronato et al., 2009). The Lago Fagnano delta has prograded on the eastern margin of the lake, and is composed of finegrained bottomsets, gravelly foresets, and fine to coarse-grained topsets. Delta foresets are inclined up to 30°, near the angle of repose, and rapidly wedge-out toward the north-east from about 20 to 0 m of thickness. They rest abruptly on the bottomset strata. The delta topset, consisting of peaty lacustrine and fluvio-glacial deposits (Bujalesky et al., 1997), is overlain by younger till deposits. 172 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA The examined sequence consists of fine- to medium-grained sands that are interlaminated with light silt and clay, forming planar to ondulate lamination, wavy and flaser bedding. In detail, planar lamination is dominant in the lower part (Figs. 2, 2b), whereas wavy to flaser bedding is common in the upper part (Figs. 2, 2a). This feature allows to subdivide the bottomset sediments into an upper, mostly rippled unit (Unit 1, from cm 325 to the core top), a middle, mostly planar laminated unit (Unit 2, from cm 532 to cm 325 depth) (Fig. 2) and a lower, muddy and planar thin laminated unit (Unit 3, from cm 532 to the core bottom). Sand and mud laminae of Unit 3 are 1 to 3.5 mm thick. Sand contents average 28,4%, ranging from 56,9% (sample at cm 688) to 9,7% (cm 560). Silt contents are inversely correlated to sand. Clay contents are low, varying from 1.8 % (cm 580) to 0.4% (cm 684). Sand and mud laminae of Unit 2 are 1,5 to 6 mm thick (Fig. 2b). Sand contents average 41.9%, ranging from 70.9% (sample at cm 461) to 18.1% (cm 401). Silt contents are inversely correlated to sand. Clay contents are very low, varying from 1.3 % (cm 483) to 0.1% (cm 524). Ripples of Unit 1 are 0.5 to 2 cm thick, are typically symmetrical to weakly asymmetrical with rounded crests, and locally show a triangular shape and sharp crests (Fig. 2a). Foreset laminae are poorly visible yet they may be evidenced by mud drapes, showing local sigmoidal forms and variable accretion patterns. Also ripple profiles are highlighted by mud drapes, which may be overlain by interlaminated sand and mud forming climbing sets (Fig. 2a). Rare coarser-grained lenses 1 cm thick are present in the upper part of the bottomset deposits. In Unit 1, Sand contents average 49.7%, ranging from 87.5% (sample at cm 141) to 8.1% (cm 284). Silt content is inversely correlated to sand. Clay contents are very low also within this unit, ranging from 1.3 % (cm 202) to almost 0 (cm 180). Samples displaying sand contents above 75% (highlighted in Fig. 2) are those corresponding to the lower and coarser parts of the ripples. Preliminary analyses highlight that the sediments contain very poor Total Organic Carbon (TOC) and Total Nitrogen (TN); similar contents of TOC were found by Harvard et al. (1999) in the southern Taymir Peninsula (Central Siberia) and Waldmann et al. (2009) from Lago Fagnano bottom sediments. The TOC content shows initial very low values with an increase in the central part and a subsequent decrease at the top. The TN content shows constant low values throughout the sequence. These low values are probably due to the coarse grain size of sediments, the age of the glaciolacustrine sediments, and to processes that impact the organic matter in the relatively short time between its synthesis and burial, such as the degradation during sinking, the bioturbation of bottom sediments that causes oxidation of the organic matter and the alteration by anaerobic bacteria (Meyers and Ishiwatri, 1993; Meyers, 1994; Waldmann et al., 2009). In agreement with the sedimentological description, the TOC and TN values identify three units. In Unit 1 the content of TOC ranges from 0.13% dwt (dry weight) and 0.04% dwt with an average value of 0.07% dwt. The average TOC in Unit 2 is 0.21% dwt an it ranges from 0.31% dwt to 0.10% dwt. Unit 3 is characterized by an average TOC of 0.07% dwt and ranges from 0.08 to 0.05 % dwt. The Corg/Ntot atomic ratio provides information on the origin of the organic matter of the lake sediments (Meyers, 1994; Meyers and Lallier-Vergès, 1999). The Corg/Ntot atomic ratios in the studied sequence show generally low values with a little increase in the central part. In Unit 1 the average Corg/Ntot atomic ratio is 3.03 and it ranges from 16.16 to 4.19, in Unit 2 the average Corg/Ntot atomic ratio is 8.93 and the maximum and minimum values are 16.16 and 4.19 respectively; in Unit 3 the average Corg/Ntot atomic ratio is 2.87 and it varies from 4.37 to 2.06. Probably these data indicate an autochtonous origin of the organic matter in the sediments. Process interpretation of bottomset deposits and TOC and Corg/Ntot contents The sand-mud interlamination of bottomset sediments testifies phases of active hydrodinamics and sediment availability that alternate with quieter periods characterized by the settling of mud by suspension. The abundance of symmetrical ripple profiles in the upper part of the bottomset (Unit 1), the presence of sharp and rounded crests, sigmoidal foresets and of local migration suggest the action of waves combined with currents producing traction on the bed (Yokokawa et al., 1995; Myrow et al., 2002). These bedforms, therefore, can be referred as wave and combined-flow ripples. Climbing patterns are referred to high rates of sediment supply. The observed alternation between high- and 173 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA low-energy phases may reflect seasonal flood events, probably associated to ice melting, that were also responsible for the avalancing of the large-scale delta foresets. Floods triggered density flows along the delta foresets transporting sandy sediment in the bottomset area. The low average values of TOC (0.07%) and organic matter (3.03) are in accordance with this depositional context, mainly characterized by transport of sediments due to active hydrodinamics, a high sedimentation rate and a high average sand value (49.7%). The vertical sequence from the planar laminated Unit 3 to the rippled Unit 1 suggests a shallowingupward trend, with the superposition of more energetic facies including traction structures on more distal and deeper facies accumulated below wave base. The described facies show some resemblance with those illustrated by Myrow et al. (2008) in prodeltaic systems. In Unit 2, the TOC average value is 0.21% and the organic matter average value is 8.93. These values testify a quieter period with settling of mud by suspension. The low values of TOC (0.07%) and organic matter (2.87) found in Unit 3 are probably correlated with an alteration of the organic matter content in a post diagenetic phase, as testified by the abundant oxidation found in the sand levels that are present in this unit. REFERENCES • Bujalesky G., Heusser C., Coronato A., Roig C., Rabassa J. (1997): Pleistocene glaciolacustrine sedimentation at Lago Fagnano, Andes of Tierra del Fuego, Southernmost South America. Quaternary Science Review 16, 767-778 • VCoronato A., Seppälä M., Ponce J.F., Rabassa J. (2009): Glacial geomorphology of the Pleistocene Lake Fagnano ice lobe, Tierra del Fuego, southern South America. Geomorphology 112, 67-81. • Harvart S., Hagedorn B., Melles M., Wand U. (1999): Lithological and biochemical properties in seiments of Laa Lake as indicators for the Late Pleistocene and Holocene ecosystem development of the southern Taymir Peninsula, Central Siberia. Boreas 28, 167180. • Meyers P.A., Ishiwatari R. (1993): Lacustrine organic geochemistry-an overview o indicators of organic matter sources and diagenesis in lake sediments. Organic Geochemistry 20/7, 867-900. • Meyers P.A. (1994): Preservation of elemental and isotopic source identification of sedimentary organc matter. Chemical Geology 114, 289-302. • Meyers P.A., Lallier-Vergès E. (1999): Lacustrine sedimentary organic matter records of Late Quaternary paleoclimates. Journal of Paleolimnology 21, 345-372. • Waldmann N., Aritzegui D., Anselmetti F.S., Austin Jr. J. A., Moy C. M., Stern C., Recasens C., Dunbar R.B. (2009): Holocene climatic fluctuations and positioning of the Southern Hemisphere westerlies in Tierra del Fuego (54° S), Patagonia. Journal of Quaternary Science. DOI: 10.1002/jqs.1263. • Myrow P.M., Fischer W., Goodge J.W. (2002): Wave-modifiedturbidites: Combined-flow shoreline and shelf deposits, Cambrian, Antarctica. Journal of Sedimentary Research 72, 641-656. • Myrow P.M., Lukens C., Lamb M.P., Houck K., Strauss, J. (2008): Dynamics of a transgressive prodeltaic system: Implications for geography and climate within a Pennsylvanian intracratonic basin, Colorado, U.S.A. Journal of Sedimentary Research 78, 512-528. • Yokokawa M., Masuda F., Endo N. (1995): Sand particle movement on migrating combined-flow ripples. Journal of Sedimentary Research A65, 40-44. STRATIGRAPHIC AND STRUCTURAL REVIEW OF CAÑADÓN ASFALTO BASIN, CHUBUT PROVINCE, ARGENTINA 5-02 Figari, E.1*, Ramos, V.A.2 (1) Repsol Exploracion S.A., Dirección General Exploración Upstream, Madrid, Spain (2) Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Argentina * Presenting author’s e-mail: [email protected] The Cañadón Asfalto Formation is a succession of volcanic, biochemical, pyroclastic and epiclastic deposits which are exposed in the central and northern regions of the province of Chubut close to the limit with the Río Negro Province. These levels were originally described only in the Middle Valley of the Chubut River and for several years the main uncertainty was the age, extension, and genesis of the related basin, because classically the area was known as the North Patagonian Massif (Fig. 1). Detailed field studies suggest that under the denomination of the Cañadón Asfalto Formation (sensu lato) different litostratigraphic units have been grouped or misidentified, such as Cañadón Calcáreo Formation, the Almada Beds, and inclusively the Chubut Group generating an extended confusion and 174 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Geologic Map of Cañadón Asfalto Basin in the area of the Middle Valley of Chubut River. 175 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - Stratigrahic column of Cañadón Asfalto Basin (based on outcrop information). Fig. 3 - Cross section showing the dramatic lateral thickness and facies changes of Sierra de La Manea Fm. and the unconformable relationship over C. Asfalto Fm. 176 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA severe problems in the stratigraphic correlation and tectonic interpretation over the whole region (Fig. 2). This reconsideration allows recognizing, essentially with outcrop data, a north-western trending rift system that occurred between the Middle Jurassic up to the Early Cretaceous, formed by a series of halfgrabens and linked among them by accommodation zones. In the North-eastern part these outcrops disappear below the Somuncura Volcanic Plateau. We suggest the use of a more dynamic nomenclature such as Megasequences in order to describe these sedimentary units related with rifting evolution. However, from the litostratigraphic point of view and mapping we propose to keep the name Cañadón Asfalto Fm. exclusively for the older unit (OxfordianCallovian), and to introduce the name Sierra de la Manea Fm. for the younger (Titho-Neocomian?) and more extended unit that covers the previous one with a regional unconformity (Fig. 3). The main orientation of the extensional stress field that created the available space for generating this basin could be related to the opening of the Weddell Sea during Middle Jurassic times and with the Gondwana break-up in the Early Cretaceous. Compressive east-west stresses (related with the Andean Orogeny in an internal intraplate setting) were responsible for the partial tectonic inversion of the basin, mainly along its westernmost margin (Agnia and Lonco Trapial Hills). THE CONTROVERSY ABOUT MIOCENE MARINE SEDIMENTATION ALONG THE FORELAND OF ANDES, SOUTH AMERICA: THE CASE OF SANTA MARIA GROUP, ARGENTINA 5-03 Gavriloff, I.J.C.1*, Arce, M.N.1 (1) Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucumán, Miguel Lillo 205, 4000 San Miguel de Tucumán, Tucumán, Argentina * Presenting author’s e-mail: [email protected] The middle/upper Miocene was characterized by large changes in the geodynamic processes of the planet. Some of these changes were: the accelerated uplift of large mountain ranges in almost all continents, the drastic changes in global sea level and shifts in oceanic circulation, and the transition from an optimum climatic during the middle Miocene to the development and growth of the ice sheet in the East and West Antarctica during the upper Miocene. In this global scenario, the uplift of the Andes in South America formed several foreland basins that have been interpreted as marine during the Miocene, but with considerable controversy. For example, in the Amazonia Basin, the Pebas, Solimôes and others correlated formations are assigned to either a tidal environment by some authors as a fluvial environment by others. Recently, in the Chaco-Parana Basin, the Ituzaingo Formation (upper Miocene-Pliocene), historically considered fluvial, has been assigned to tidal environment. This formation is located immediatly above the marine Parana Formation. The “Paranaense Sea” (middle/upper Miocene) in Argentina, shows a clearly marine gradient from the East to the West into the Chaco-Parana basin. To the West, near the Andes, several formations present paranaense’s foraminifera together with groups of brackish or fresh-water macro and microfauna. In this region, the Santa Maria Group (Sierras Pampeanas, Tucuman and Catamarca provinces), includes two formations with thick lacustrine sequences, the San Jose Formation (middle Miocene) and the Chiquimil Formation (upper Miocene). Since the foraminifera discovery in this group in the ’80 decade, the sedimentary rocks that hold them were in general assigned to the San Jose Formation, even though without a precise stratigraphic control. In this first stage of our research, the goals are: the sistematic identification of the foraminifera fauna, the stratigraphic identification into lithostratigraphic units of the layers containing them and the proposition of hypotheses explaining the 177 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA presence of foraminifera in lacustrine facies. Metodologically we have made facial and sistematic field sampling of layers assigned as fertile in foraminifera; samples supplied by YPF petroleum company have also been studied. From a total of 35 samples, 14 were positive containing foraminifera. Four samples correspond to Chiquimil Formation with the following species: Ammonia parkinsoniana (d’Orbigny) and Lippsina demens (Bik), forma santamariana Zabert. In this formation is noteworthy the joint presence of Characids with Ammonia parkinsoniana (d’Orbigny) in the same stratigraphic level. Ten samples of San Jose Formation gave the following species: Ammonia parkinsoniana (d’Orbigny) and Nonion sp. We conclude that both lacustrine formations of Santa Maria Group are carrier of a foraminiferal microfauna. The direct relationship of the lakes with the Paranense Sea,or the possible sowing of foraminifera in the coastal lakes due to the migration of birds, are the two hypothesesthat must be considered to explain the presence of foraminifera in the Santa Maria Group. GEOLOGY OF THE LAGO FAGNANO AREA (FUEGIAN ANDES, TIERRA DEL FUEGO ISLAND) 5-04 Menichetti, M.1*, Tassone, A.2, Lippai, H.2, Lodolo, E.3 (1) Dipartimento di Scienze Geologiche, Tecnologie Chimiche Ambientali - Università di Urbino, Italy (2) CONICET-INGEODAV. Dpto. de Ciencias Geológicas. Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires, Argentina (3) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy * Presenting author’s e-mail: [email protected] Lago Fagnano is the southernmost ice-free lake on Earth. It is elongated in an E-W direction for 105 km and has an average width of 8 km and a maximum depth of 206 m. Its basin covers an area of approximately 1900 km2 and has an irregular shape, especially in its eastern part, where is the dividing line between the Atlantic and Pacific/Magallanes Strait. The morphostructural evolutionary history of the region is complex, particularly in the Quaternary, which has strongly influenced the regional water divide and drainage of the main tributaries that run along the N-S and E-W directions. To the east, the border river runs close to the shore of the lake, while in other areas it is difficult to identify because of flat topography with large ponds and bogs. Along the southern and western lake shores, Upper Jurassic volcanoclastic metasediment of the Rocas Verdes marginal basin with basalts of the Lemaire Fm. outcrop. Rhyolites of the M.te Buckland Fm. in the Sierra de Alvear are present. Andesitic volcanoclastic turbidites of the Lower Cretaceous Yahgán Fm. outcrop in the east side of the Lago Fagnano in the Sierra Lucas Bridge. Sediments pertaining to the Cenozoic Magallanes foreland basin outcrop in the north side of the basin. In the Sierra Inju Gooiyin (Sierra de Beauvoir), metasediments of dark slates, marls and tuffs of the Beauvoir Fm. are present, while to the east, in the Sierra de las Pinturas, outcrops of marls, sandstones and siltstone of Upper Cretaceous of the Cerro Matrero Fm. can be seen. Two Upper Cretaceous intrusive plutons are located south of Tolhuin (Cerro Jeujepen), and in the southern slope of Sierra de Inju Gooiyin (Cerro Krank). Glacio-fluvial and Quaternary sediments with moraine, glacio-fluvial and glacio-lacustrine facies cover all the south-eastern sectors of the Lago Fagnano area. Tens of meters-thick glacial moraines, recoding different ice advances linked to the Cordillera Darwin ice cap that flowed eastward, are exposed along the river incisions and in the south-eastern shore of the lake. The sedimentary cover of Lago Fagnano, surveyed by geophysical measurements and core samplings, is composed of at least two glacial and glacio-lacustrine stratigraphic units, resting on a deformed basement. Lago Fagnano is located in the frontal part of the main stack of the Fuegian Cordillera basement sole thrust front, at the boundary with the Magallanes foreland. A system of low angle, NNE-verging thrusts constitute the main compressional structures related to the Upper Cretaceous Andean collisional tectonic phase. A thick-skinned tectonic geometry with arrangements of imbricate fan 178 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES thrust systems characterize the internal zones of the Sierra del Alvear and Sierra Valdivieso, where basement sole thrusts can be observed. Thin-skinned tectonics with thrust duplex geometries prevail in the foreland north of the lake. Several Oligocene E-W left-lateral strike-slip faults that define the principal deformation zone of the Magallanes-Fagnano transform faults, superimpose and possibly reactivate older compressive structures. The most prominent structure of the area is the Co. HopeCatamarca-Knokeke fault, running E-W through the lake area over a distance of more than 100 km. Several outcrops allow the characterization of the geometries and the kinematics of this left-lateral, transtensional, sub-vertical, south-dipping structure. Furthermore, the fault trace from the eastern arms of the Magallanes Strait to the Atlantic offshore shows strong morphostructural evidence. Two other faults, along both the north and the south lake edges, the Rio Turbio-Las Pinturas and the S. Rafael, respectively, form a releasing step-over within the principal deformation zone. The kinematics of these sub-vertical, E-W faults is mainly transcurrent with an important extensional component. The fault arrays form complex structures that could derive from deformational partitioning involving the distribution of strain orientation or intensity in various domains. Reactivations of oldest faults and preexisting structural weaknesses are common especially where the thrusts and wrench faults strikes intersect at an angle of a few tenths of degrees. The geodynamic evolution of the Lago Fagnano pull-apart basin can be related to the westward migration of several step-overs along the Co. Hope-Catamarca-Knokeke strike-slip fault. The age of the fault activity can be inferred by regional geological data since the Lower Miocene, while the geomorphological evidence and the seismicity of the area indicate a still active tectonic activity. A set of extensional faults NW-SE oriented, intercept and in same case cut the E-W strike-slip faults. THE AGE OF DINOSAURS IN SOUTH AMERICA 5-05 Novas F.* Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina * Presenting author’s e-mail: [email protected] Since dinosaurs were first documented in South America in 1890, the knowledge of their evolutionary history remained fragmentary and restricted to a handful of poorly understood species. However, work carried on since the end of the 1970´s in Mesozoic outcrops in Argentina, resulted in a remarkable string of fossil findings, revealing that a rich and complex evolutionary history took place in this southern continent. The fossil evidence amassed in this continent (consisting in skeletons, footprints, eggs, nests and skin impressions belonging to the main evolutionary streams of Saurischia and Ornithischia) sheds lights on a variety of interesting paleobiological aspects, such as the rise of dinosaurs, the effects of continental break-up on their evolutionary history, the emergence of birds and their flight, the development of the largest land vertebrates, etc. A brief review of the available information will be offered in this talk. 179 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES SEDIMENTATION ENVIRONMENTS AND FACIES DEVELOPED 5-06 DURING THE MIOCENE TO EARLY PLIOCENE IN THE EASTERN BASIN OF FALCON, WESTERN VENEZUELA Romero, B.F1*, Bastos, P.1, Strakos, K.2, Baquero, M.1 (1) PDVSA EXPLORATION, PUERTO LA CRUZ, VENEZUELA (2) BEICIP FRANLAB, PUERTO LA CRUZ, VENEZUELA * Presenting author’s e-mail: [email protected];[email protected] A combined fieldwork and well data and biostratigraphic analysis on Cenozoic rocks from the eastern Falcón Basin, Western Venezuela was conducted. The results are used to characterize the different sediment types and to investigate the depositional processes and facies in the basin from Lower Miocene to Lower Pliocene. These data allowed us to analyze the distribution of integrated paleofacies in the basin. The deposits are organized into four sequences (SM1, SM2, SM3 and SM4) defined by lithofacies and bounded by unconformities. Several maps were drawn up for each of the identified sequences. The deposition of the Sequence SM1 ranges from the Upper Aquitanian to Lower Burdigalian, it begins with neritic deposits, with the calcareous facies of the Member Cauderalito (Agua Clara Formation) sandy facies of the Pedregoso Formation, and conglomeratic facies of the Guarabal Formation, in the west of the area. In the southeast part of the basin, the SM1 sequence is represented by the sandy-clay facies of the San Lorenzo and Agua Linda formations. The clay facies of the Agua Clara and San Lorenzo formations are bathyal. The deposition of the sequence SM2 starts in the Burdigalian, in the south of the Cumarebo structure, with sandy-clay facies of the Cerro Pelado Formation in fluvial environments, with marine influence. During the Langhian, in the northern areas the sequence is represented by a succession of inland and coastal conglomerates, sandstones, delta and lagoons clays and platform limestones of the Cantaure Formation. The maximum of the transgression is represented by the bathyal clays of the Querales Formation. The Sequence SM3, preceded by a period of erosion, as shown by the absence of late Langhian in the zone north, began with the Socorro Formation (Late Langhian-Serravallian) which contains two facies; the lower clay facies corresponding to a transgressive event and the upper clay-sandy facies, the retrograde part of the SM3. The depositional environment was middle-outer neritic in the structure south of the La Vela Tierra. In the offshore, from north and east of the La Vela Tierra facies indicate bathyal environments, as evidenced by the decrease in the sandy layers. Toward the southeast the facies gradationally become more argillaceous, indicating the deposition in a bathyal environment (Pozón Formation), and the Capadare Formation of internal neritic setting, gradually pass to coastal deposits of the Agua Linda Formation The abrupt contact between the bathyal facies (Pozón Formation) and inner neritic to continental facies (Upper Agua Linda Formation) is seen in this southeast part. The boundary of the Middle-Late Miocene (SM4) corresponds to a tectonic event that is manifested by erosion at the top of Socorro Formation (incised valleys). The trend is regressive during this period. Bathyal sediments are located in the southeast, at the base of the sequence with the Pozón Fomation, grading into coastal to shallow marine facies of the Ojo de Agua Formation. In the north, the Caujarao Formation is middle neritic with more marine influence towards the top. In the La Vela Tierra east nine rhythmic sequences of limestone, sand and clay are recognized in outcrop representing the lower part of the formation, whereas clay facies dominate the top of the formation coming to settle in at lower Pliocene. Sedimentation is slightly influenced by eustatic cycles in the area of study and according to the conception of this study, the limits of the sequences are regional unconformities of tectonic origin: extensional during the base of the Aquitanian and compressive since the Langhian to the Pleistocene. 180 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA AN EXAMPLE OF COMPLEX FLUVIO-AEOLIAN SEDIMENTATION: THE UPPER MEMBER OF THE MIOCENE-PLIOCENE RÍO NEGRO FORMATION, NORTHERN PATAGONIA, ARGENTINA 5-07 Umazano, A.M.1*, Visconti, G.2, Pérez, M.2 (1) INCITAP (CONICET-UNLPam); Av. Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina (2) Fac. de Cs. Exactas y Naturales (UNLPam); Av. Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina * Presenting author’s e-mail: [email protected] The Miocene-Pliocene Río Negro Formation is a sandstone-dominated unit that outcrops in several parts of northern Patagonia from the Andean foothill to Atlantic coast. The formation includes three members denominated lower (aeolian), middle (marine) and upper (fluvio-aeolian). Two basal members crops out in coastal cliffs between 41° 09’- 41° 07’latitude, whereas the upper member is mainly exposed in both margins of the Negro river valley. Goal of this contribution is to analyze the complex fluvio-aeolian interaction recorded in the upper member of the unit from three-dimensional sections exposed at Carmen de Patagones (40°47’41’’ S, 63°0’4’’ W; Buenos Aires province). The studied sedimentary succession has a maximum thickness of 12 m and a measured lateral extension of 121.5 m. Stratigraphically, the succession overlies to middle member, although the contact is not exposed, and is covered by the Rodados Patagónicos. It is mostly composed by volcaniclastic, medium to fine-grained sandstones with minor occurrence of mudstones, conglomerates and vitric, fine-grained tuffs. The succession was studied in both sides of a NNE oriented route-cut and other two near orthogonal exposures. Methodology included the measurement of nine detailed sedimentary logs, as well as facies and architectural analysis, the later using four photomosaics. Six facies associations (FA) were distinguished: sandstone dominated aeolian deposits, including dune and dry interdune zones (FA1); sandstone-loessic aeolian deposits with water reworking and soil development, deposited in relatively flat areas (FA2); intermittent mudstone fluvial channel-belt deposits (FA3); permanent sandstone fluvial channel-belt deposits (FA4); pyroclastic deposits reworked by unconfined fluvial flows and later subjected to pedogenesis (FA5) and shallow lacustrine deposits (FA6). Most of the paleosoils did not display differentiation of horizons nor pedic structure. Complex spatial arrangement of FA can be summarized as follows: 1) a lower sector dominated by FA1 deposits, which displays abundant simple and complex fluvial channel-belt deposits of FA3 in the upper part; 2) a middle sector composed of FA2 deposits with scarce amount of simple fluvial channel-belt deposits of FA4; 3) an upper sector constituted by deposits of FA5 in the base and FA6 in the top. The boundary between middle and upper sectors is pointed by the best developed paleosoil of the succession, which shows two horizons separated by a diffuse limit, as well as microscopic evidences of clay lixiviation. Vertical distribution of the FA suggests an increment in water availability probably linked to wetter climatic condition. THE BRUNHES/MATUYAMA BOUNDARY AND ROCK MAGNETIC PARAMETERS IN PLEISTOCENE LOESS DEPOSITS OF CAMET, MAR DEL PLATA (ARGENTINA) 5-08 Bidegain, J.C.1*, Gomez Samus, M.1 (1) Laboratorio de Entrenamiento Multidisciplinario para la Investigación Tecnológica-CIC Calle 52 e/ 121 y 122, La Plata, Buenos Aires. Argentina * P resenting author’s e-mail: [email protected] The Pleistocene-Holocene loess/paleosoils sequences exposed at the north of Mar del Plata have been studied by several researchers from the beginning of the last century (Ameghino, 1908), more recently by Schnack et al., (1982) and Fassano (1991). The present contribution refers to the sedimentary 181 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - The Camet stratal sequence; main units (U1-U5) identified, lithostratigraphic characteristics, positions of paleomagnetic samples and inferred magnetostratigraphy. sequence exposed at Camet (37º 53´39,11´´S and 57º 31´17,8´´W) which contents records of normal and reverse polarity that can be assigned to Brunhes and Matuyama Polarity Chrons. First paleomagnetic studies were carried out in the sector Santa Clara del Mar and Arroyo La Tapera by Bidegain et al. (2005). Paleomagnetic directions and magnetic parameters were measured on samples collected from a section of five units separated by discontinuities. The present contribution confirm previous ones in the concern of paleomagnetic zonation i.e. the unit labeled U1 at the base of the profile (Fig. 1) presents reverse polarity levels while the units U2 to U5 present normal polarity. The former is assigned to upper Matuyama (> 0.78 Ma) and the latter to Brunhes Normal Polarity Chronozone (< 0.78 Ma). According to the records of magnetic parameters the pattern of behavior follows that obtained in the north of the Buenos Aires province. The less pedogenized materials show the higher LF susceptibility (330 m3/kg), B and BC horizons show values ranging between 80 and 200 m3/kg and the gley horizons 20 and 90 m3/kg. Frequency dependent susceptibility (¯fd) defined as ¯fd= (¯ 470 Hz - ¯4700 / ¯ 470 Hz ) x 100 was employed to estimate the superparamagnetic contribution. This contribution is low along the profile, it increases in the pedogenized horizons (5%) and decreases noteworthy in the loess layers (0.6 %). REFERENCES • Ameghino, F. 1908. Las formaciónes sedimentarias de la región litoral de Mar del Plata y Chapadmalal. Anales Museo Nacional de Buenos Aires. Serie 3ª, X, 843-428. • Bidegain, J.C., Osterrieth, M.L., Van Velzen, A.J., Rico, Y. 2005. Geología y registros magnéticos entre arroyo La Tapera y Santa 182 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Clara del Mar, Mar del Plata. Rev. Asoc. Geol. Arg., 60 (3): 599-604. • Fasano, J.L. 1991. Geología y Geomorfología. Región III Faro Querandí-Mar de Cobo, Provincia de Buenos Aires. Informe Final. Convenio de cooperación horizontal entre el Consejo Federal de Inversiones y la Universidad Nacional de Mar del Plata, 118p. • Schnack, E.J., Fasano, J.L. Isla, F.I. 1982. The evolution of Mar Chiquita lagoon coast, Buenos Aires province, Argentina. En Colquhom, D.J. (Ed.), Holocene Sea Level Fluctuations, Magnitude and Causes. IGCP-INQUA, Colombia S.C. U.S.A.: 143-155. THE LATE CENOZOIC SEDIMENTARY SEQUENCES IN THE CHAPADMALAL AREA (BUENOS AIRES). POLARITY CHANGES AND MAGNETOCLIMATOLOGY 5-09 Bidegain J.C.1, Rico Y.1 (1) Laboratorio de Entrenamiento Multidisciplinario para la Investigación Tecnológica (LEMIT-CIC). Calle 52 e/ 121 y 122, Provincia de Buenos Aires (Argentina) * Presenting author’s e-mail: [email protected]; [email protected] The present contribution is focused on the polarity changes and magnetic parameters obtained in cliffs of Chapadmalal area, Buenos Aires province. The sedimentary section comprises the Vorué, San Andres, Miramar and Arroyo Seco Formations. The Vorohué Formation and lower part of San Andrés Formation content records of normal polarity and both were assigned to the Gauss Chron (> 2.6 Ma). The main section of San Andrés Formation appears to have been deposited during lower and middle Matuyama including Olduvai (1.9 Ma). The boundary between Miramar and San Andrés Formations coincides with a discordance and a new polarity change attributed to Middle Matuyama/Jaramillo (0.99-1.05 Ma). The Miramar Formation shows normal polarity at the base (Jaramillo) and reverse polarity at the top, those reverse levels (Upper Matuyama) were also recorded in the lower part of the onlying Arroyo Seco Formation. Finally, the upper part of Arroyo Seco Formation was assigned to Brunhes normal polarity chron (< 0.78 Ma). Paleomagnetic data are in agreement – to some extend- with the previous ones carried out in the area by Orgeira, 1988, 1990, and Ruocco,1990, however, it is arising some differences as regarding older geological units which should encourage deeper investigations and further discussion. The magnetic susceptibility as environmental proxy data in San Andrés shows a clear alternated sequence of highs and lows that may be referred to changes in environmental conditions (Bidegain et al., 2005, 2007). The major contrast in susceptibility values among layers is “enhanced” by the presence of calcrete layers producing the almost total depletion of records. Both concentration parameters (susceptibility and SIRM ) confirm the pattern of magnetic behavior of loess studied in the north of the Buenos Aires province but there are also some differences in this concern. Highest susceptibility values are > 300x 10-8 m3/kg in contrast with the similar ones of La Plata-Baradero area, usualy between 100-200 x10-8 m3/kg (Bidegain et al., 2009). Paleosols and less pedogenized materials do not show so sharp differences as regarding IRM, the SIRM values range between 22 A/m to 28 A/m in both materials. Due to the low content of magnetic minerals the calcrete layers show very low SIRM values (1.35 to 7.1 A/m). The saturation (SIRM) is reached at low field (< 0.5 T) in all samples and the coercitivity of remanence (Bcr) varied between 29.5- 34 mT which corresponds to low coercitivity magnetites. The influence of pedogenesis in the enhancement of F factor seems not to have been relevant - as in other localities studied until now- being the higest value around 7% . REFERENCES • Orgeira, M.J. Estudio Paleomagnético de los Sedimentos del Cenozoico Tardío en la Costa Atlántica Bonaerense. RAGA 1988. XLII (3-4): 362-376. • Orgeira, M.J. Paleomagnetism of late Cenozoic fossiliferous sediments from Barranca de Los Lobos (Buenos Aires,Argentina). The magentic age of the South American land –mammal ages. Physics of the Earth and Planetary Interiors 1990. 64 :121-132. • Ruocco, M. A 3 Ma paleomagnetic record of coastal continental deposits in Argentina.Palaeogeogr. Palaeoclimat. Palaeoecol 1989. 72: 105-113. • Bidegain, J.C, M.E. Evans, A.J. van Velzen A magnetoclimatological investigation of Pampean loess, Argentina. Geophs., J.Int. 2005. 160: 55-62. 183 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA • Bidegain, A.J. van Velzen, Y. Rico The Brunhes/Matuyama boundary and magnetics parameters related to climatic changes in Quaternary sediment of Argentina, South Am. Earth Sciences 2007. 23: 17-29. • Bidegain, J.C., Y. Rico, A. Bartel, M. Chaparro, S. Jurado. Magnetic Parameters Reflecting Pedogenesis in Pleistocene Loess Deposits of Argentina. Quaternary International 2009. 209: 175-186. 184 Session 6 SURFACE PROCESSES AND PALEOCLIMATE GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA RECONSTRUCTION OF LATE-GLACIAL TO HOLOCENE CLIMATE AND EARTHQUAKE HISTORIES ACROSS SOUTHERN CHILE BASED ON THE SEDIMENTARY RECORD OF 21 LAKES 6-01 De Batist, M.1*, Moernaut, J.1, Heirman, K.1, Van Daele, M.1, Bertrand, S.1, Abarzua-Vasquez,A.M.1, Pino, M.2, Brümmer, R.2, Urruti, R.3, Vila, R.4, Roberts, S.5, Kilian, R.6, Verleyen, E.7, Vyverman, W.7, Keppens, E.8, Fagel, N.9, Gieles, R.10, Sinninghe Damsté, J.10, Hebbeln, D.11, Gilli, A.12, Brauer, A.13, the ENSO-CHILE, CHILT Project Members (1) Renard Centre of Marine Geology (RCMG), Universiteit Gent, Gent, Belgium (2) Instituto de Geociencias, Universidad Austral de Chile, Valdivia, Chile (3) Centro EULA, Universidad de Concepción, Concepción, Chile. (4) Laboratorio de Palinología, Facultad de Ciencias, Universidad de Chile, Santiago, Chile (5) British Antarctic Survey (BAS), Cambridge, United Kingdom (6) Department of Geology, University of Trier, Trier, Germany (7) Protistology & Aquatic Ecology, Universiteit Gent, Gent, Belgium (8) Department of Geology, Free University of Brussels, Brussels, Belgium (9) Department of Geology, University of Liège, Liège, Belgium (10) Royal NIOZ, Den Burg, Texel, The Netherlands (11) MARUM, University of Bremen, Bremen, Germany (12) ETH Zürich, Zürich, Switzerland (13) GFZ, Potsdam, Germany * Presenting author’s e-mail: [email protected] During the past years and in the context of a succession of international research initiatives (o.a. the ENSO-CHILE and CHILT Projects), the sedimentary infill of 21 lakes in southern Chile, extending from 37°20’ S in the north to 53°35’ S in the south, was investigated by means of dense grids of highresolution reflection seismic profiles and of multi-proxy analyses of multiple short gravity cores and long piston cores. This combined data set represents a vast sedimentary “library”, from which the history of climate change, seismicity, volcanic activity and human impact in South-Central and South Chile can be reconstructed, including its regional and latitudinal variability. As most of these lakes are glacial in origin, the time window of investigation is in most cases effectively limited to the last ~1214 ka. However, some of the studied lakes contain unique, continuous sediment records that extend much further in time, down to ~20 ka or even to ~40 ka. Moreover, many of these records are characterized by high sedimentation rates and are annually laminated and they can thus potentially produce very detailed paleo-reconstructions at annual resolution. In this presentation, an overview will be given of recent research results obtained from the study of these lake records. Emphasis will be given on the potential (advantages and disadvantages) of these records for paleoclimate studies (at high resolution) and for paleoseismology. RAPID CRUSTAL UPLIFT IN PATAGONIA AS A CONSEQUENCE OF INCREASED ICE LOSS 6-02 Dietrich, R.1*, Ivins, E.R.2, Casassa, G.3, Lange, H.4, Wendt, J.†3, Fritsche, M.1 (1) Institut fuer Planetare Geoda¨sie, Technische Universitaet Dresden, 01069 Dresden, Germany (2) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA (3) Centro de Estudios Cientificos, Arturo Prat 514, Valdivia, Chile (4) TERRASAT S.A., Av. Eliodoro Yañez 2050, Santiago de Chile, Chile * Presenting author’s e-mail: [email protected] GPS observations were carried out between 2003 and 2006 at the northeastern edge of the Southern Patagonian Icefield. The data analysis was performed with the Bernese Software and revealed uplift 187 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA rates of up to 39 mm/yr. For the region an accelerated glacier wasting has been observed since the termination of the Little Ice Age. This increasing ice loss continues up to present time. Advanced modeling shows that the rapid ice melting in combination with relatively low viscosity of the Earth’s mantle caused by the unique regional slab-window tectonics is central for the interpretation of the results. The profile of GPS observations link ice loss to the soft viscoelastic isostatic flow response over the time-scale of the Little Ice Age (LIA), including ice loss in the period of observation. IMPLEMENTATION OF AQUIFER PROTECTION ZONING 6-03 Dustay, S.1*, Nel J.1, Xu, Y.1, Massone, H.2 (1) UNESCO Chair Centre, University of the Western Cape, Bellville, South Africa (2) Centro de Geología de Costas y del Cuaternario, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata, Funes 3350, 7600 Mar del Plata, Argentina * Presenting author’s e-mail: [email protected] Introduction Two thirds of South Africa’s and Argentina’s population depends on groundwater for their domestic water needs. Currently limited progress is made in South Africa (and Africa) and Argentina on the protection of water. To achieve the objective of water for growth and development and to provide socio-economic and environmental benefits of the communities, significant aquifers and well fields must be adequately protected. Implementation of groundwater protection zones is seen as an important step in this regard. From initial literature review the implementation of protection zoning in developing countries like Africa is new or non-existent. Methodology Initial protection-zone delineation will be made using published reports and database data. This initial delineation will probably be based on a simplified 2D model created with the US-Environmental Protection Agency wellhead-delineation software, “WhEAM”. From these capture zones the planning of the detailed study will be conducted. An inventory of the activities that can potentially impact water quantity will be made and ranked according to their degree of risk for impacting water sources. This information will be used to prioritize areas where more data is needed and where additional data gathering is required. This data can be collected through a hydro census and through aquifer tests. Aquifer tests that will be conducted include tracer tests, Flowing FEC and constant discharge tests. This improved information can be used to build the conceptual model and implement a trustworthy groundwater protection plan. Expected Results Development of protection zones and protection plans should lead to sustainable water use that include environmental and ecosystem health as well as provision of water for human needs. The expected results will have applicability to groundwater management in general. The methodology and guidelines developed for the purpose of this project will be used to update and improve policy implementation. The protection of these groundwater resources will ensure its sustainability from a quality perspective for future generations. 188 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA TERRESTRIAL AND LACUSTRINE EVIDENCE OF HOLOCENE GLACIER ACTIVITY IN TIERRA DEL FUEGO (SOUTHERNMOST SOUTH AMERICA) 6-04 Maurer, M.1, Menounos, B.1, Clague, J.J.2, Osborn, G.3, Rabassa, J.(*)4,5, Ponce, J.F.4, Bujalesky, G.4,5, Fernández, M.4, Coronato, A.4,5 (1) Geography and Natural Resources and Environmental Studies Institute, University of Northern British Columbia, Prince George, BC, Canada, V2N 4Z9 (2) Department of Earth Sciences, Simon Fraser University, Burnaby, BC, Canada, V5A 1S6 (3) Department of Geosciences, University of Calgary, Calgary, AB, Canada, T2N 1N4 (4) CADIC-CONICET, Ushuaia, Tierra del Fuego, Argentina (5) Universidad Nacional de la Patagonia-San Juan Bosco, Sede Ushuaia * Presenting author's e-mail: [email protected] The synchronicity of Holocene glacier fluctuations in the northern and southern hemispheres is a subject of debate. Our research addresses this issue by examining the lacustrine and terrestrial evidence of glacier activity in Tierra del Fuego. In April 2009, we performed a bathymetric survey of glacier-fed Lago Roca (S 54º 48’, W 68º 38’) and recovered four percussion cores from the lake floor. The cores are 1-2 m in length and consist of inorganic, rhythmically laminated silt and clay. Laminae are 1-2 mm thick, silt-clay couplets that appear to be clastic varves. Magnetic susceptibility generally increases upward to a peak 1.5 m below the top of the longer cores. The uppermost 20 cm of the cored sediments are the least clastic-rich and have the lowest magnetic susceptibility. Organic content is inversely related to magnetic susceptibility, with low values near the top and bottom of the cores. A field study at Stoppani Glacier, 25 km northeast of Lago Roca, in December 2009 provided evidence of several advances of the glacier during the past 4000 years. The left lateral moraine of the glacier is composed of multiple tills separated by glaciofluvial and glaciolacustrine sediments. Radiocarbon ages of stumps in growth position, detrital wood in till units, and vegetation mats indicate that the glacier repeatedly advanced between 3510 ± 15 and 184 ± 15 14C yr BP (3830–150 cal yr BP). The advances broadly coincide with documented intervals of glacier expansion in the Northern Hemisphere. Additional work on the Lago Roca sediment record is underway. Those data will supplement the discontinuous terrestrial record preserved at Stoppani Glacier. The work includes analysis of pollen, diatom, and phytoliths recovered from the sediment cores. The results will be compared with palynologic records from Holocene peat bogs, which are abundant on Tierra del Fuego. These new lake and terrestrial records will help constrain the age and magnitude of past glacier advances in Patagonia, allowing an assessment of inter-hemispheric synchronicity of climate events during the Holocene. ROCK MAGNETISM STUDY ON SEDIMENTS FROM STREAMS OF THE PARANÁ DELTA (ARGENTINA) 6-05 Mena, M.*, Dupuy, J.L. CONICET-INGEODAV, Dpto. Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina * Presenting author’s e-mail: [email protected] We present a rock magnetism study performed on bed sediments from Paraná Delta, Buenos Aires, Argentina. The sampling comprises eighty sites located along 30 km of streams. At each GPS-located site, 300 g of bottom sediments were extracted using a dredge sampler. Four specimens were taken from each sample, previously dried at room temperature. The magnetic susceptibility at three frequencies was measured for all specimens. For each site, mean and standard deviations for mass magnetic susceptibility (¯) and frequency-dependent susceptibility factor (FDF) were calculated. 189 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Measurements of temperature dependence and field dependence of magnetic susceptibility were performed on one specimen per site using a MFK1-FA + CS4 Kappabridge susceptibilimeter. Stepwise acquisition of isothermal remnant magnetization (IRM) was performed on another previously consolidated specimen from each site. The ¯ presents an increasing tendency from rural to urbanized areas. The FDF is relatively low (<10%) in all sites. The statistical analysis of IRM acquisition curves allowed defining four magnetic components with different coercivity ranges. The absolute contributions of the biggest coercivity component (400-600 mT) and the intermediate coercitivity component (100-300 mT) vary moderately among the samples. This suggests that these magnetic phases should be representative for the detrital input into the river. The two components with smaller coercivity (soft component: 5-20 mT) and (moderately soft component: 20-70 mT) have low values of their coercivity dispersion parameters. The absolute contributions of these components are widely variable among the samples. Their coercivities are comparable to those expected for magnetic grains of authigenic chemical and bacterial origin. A direct relationship among the magnetic enhancement and the location of the alleviation channel mouths, access creeks to nautical neighbourhoods and more urbanized areas was found. Many of these ¯ peaks are associated to FDF minima, and all of them coincide with major contributions of the low coercivity components, especially the moderately soft. The presence of mainly biogenetic ferrimagnetic minerals may be related to decreased pH by the presence of industrial effluents and sewage. A ¯ peak associated with a local maximum FDF and local increase of the contribution of both low coercivity phases is located in front of a nautical fuel dispenser. These magnetic properties may be due to both biogenic and chemical formation of ferrimagnetic minerals, probably by the combined effects of small fuel spills which increase the acidity of the medium and generate reductive environments, of internal combustion engines and of urbanization. The drops in the moderately soft component contribution match the local ¯ minima located at the exit of streams in less populated zones. These minima could be associated with more oxygenated local environments. The presented results support the usefulness of employing magnetic properties of bed load sediments to monitor the environmental evolution of riverbeds in the area. CHARACTERIZATION OF COMPLEXITY OF FRACTURED ROCK AQUIFERS 6-06 Nel, J.1*, Xu, Y.1, Batelaan, O.2,3 (1) UNESCO Chair Centre, University of the Western Cape, Bellville, South Africa (2) Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium (3) Department of Earth and Environmental Sciences, Katholieke Universiteit Leuven, Celestijnenlaan 200e - bus 2410, 3001 Heverlee, Belgium * Presenting author’s e-mail: [email protected] Characterization of fractured rock aquifers using borehole logging tools like calliper, acoustic televiewer, neutron and gamma are well established in the oil and water industry to determine geological zones of suitably high oil or water yields. Fractures identified with these methods are however not necessarily active and would therefore not all contribute to flow and transport. Methods like packer testing and borehole flow meters have been used by some researchers to identify hydraulic active fractures in boreholes and to identify fracture zones connecting different boreholes. These types of equipment are very expensive and not commercially available in South Africa, requiring the import of custom built setups for working around 200 m depths. Alternative cheaper methods with simple field application were therefore investigated to identify fracture positions and hydraulic properties. Borehole temperature data and electrical conductivity logging data is used in this paper to identify fracture positions and the change in flow conditions under different pumping conditions. Borehole temperature profiles and Fluid Electrical Conductivity (FEC) 190 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA logs were measured using a YSI 6600 multi-parameter sonde with specific conductance, temperature and depth logging capabilities. The specific conductance in the borehole was slightly increased by dispersing table salt through the borehole column using an injection sock. As soon as possible after the salt is added to the borehole the specific conductance of the entire borehole column was logged. This logging of the borehole column is then repeated over time. Any flow through the borehole will dilute the salt concentration back to natural conditions. This data of salt dilution at specific points gives an indication of flow zones in the borehole. The data obtained from the FEC logs were be used to simulate flow rates of individual fractures. The 2-dimensional finite difference model BORE II software was used to simultaneously solve for flow and mass transport on the borehole scale. Initial concentrations are provided as input, with fracture flow rates and quality simulated as the borehole water is diluted. Calibration is done against the measured data using a trial-and-error approach. Both temperature and FEC logging was successfully employed to determine fracture positions and ambient flow conditions in the aquifer. ENVIRONMENTAL MAGNETISM STUDY OF A HOLOCENE EOLIAN SEDIMENTS AND PALEOSOLS SEQUENCE IN THE NORTH OF TIERRA DEL FUEGO (ARGENTINA) 6-07 Orgeira, M.J.1,2*, Coronato, A.3,4, Vásquez, C.A.2,5, Ponce, A.3, Moretto, A.3, Egli, R.6, Onorato, M.R.7 (1) Depto. Cs. Geológicas-FCEN, UBA. Cdad Universitaria Pab II, 1428 Buenos Aires, Argentina (2) INGEODAV/UBA-CONICET, Buenos Aires, Argentina (3) CADIC/CONICET, Ushuaia, Argentina (4) FHCS-UNPSJB, Ushuaia, Argentina (5) CBC-UBA, Buenos Aires, Argentina (6) Dept. of Earth and Environmental Sciences, Ludwig-Maximilians University, Munich, Germany. (7) FCEFN-UNSJ, San Juan, Argentina * Presenting author’s e-mail: [email protected] The studied sequence is located in the northern region of Isla de Tierra del Fuego, Argentina (53° 42’ 48.6’’S, 68° 18’ 20.3’’W, Fig. 1), altitude 71 m asl (Coronato et al, 2009). The present mean annual rainfall in the area is around 380 mm and the mean annual temperature is 5.2° C. The sequence is located in a large area of low pressure under the effect of both the westerlies and the Polar Front. Wind frequency is daily, with and average rate of 25 km/h, with frequent periods of higher wind speeds. The influence of Antarctic air produces short periods of colder and drier climate. The studied sequence is located in the semiarid Fuegian steppe where Festuca gracillima (coirón) is the dominating specie. The sequence comprises a succession of 20 m of eolian silty-fine sand-clay sediments with 8 paleosols layers interbedded. Paleosols can be clearly identified on the basis of typical pedofeatures such as, coal grains, clay coatings, organic content and dark colour (10Y/R to 5YR). The paleosols layers contain abundant fossil record represented by Lama guanicoe and Ctenomys sp, and three or four different horizons have been identified in each paleosol. Radiometric data of the intermediate paleosoil 4 (5800 yr) and its overlying tephra layer (ca 3200 yr) allow to infer that the eolian deposition and the edaphic processes started during the Early/middle Holocene or earlier. The origin of the deposits is not clear yet. Two hypothesis are being analyzing: a) they could be the result of deflation during dissication periods of a shallow lake located near the eolic deposit, or b) they could be deposited forming perched dunes, ciclically eroded, edaphized and buried by new deposits. 191 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Overview of the studied Holocene eolian sequence. Fig. 2 - Details of sampling. View of the paleosol 5 located in the middle of the profile 192 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 3 - W values (moisture ratio) calculated for the studied area in Tierra del Fuego, Argentina (a) and Alaska, USA (b). In case a) the source of the sediments must be the lake bottom formed by weathered marine Tertiary rocks and probably till; in case b) the source must be the weathered silty-sand bedrock which form the cliff over which de deposits lay. On going geomorphological studies will help to explain this depositional sequence. Additionally, this sequence is interesting due to archeological record. Abundant lithic and faunal archeological materials, formely buried are exposed by present eolian erosion. This contribution includes preliminary results, mainly focused in environmental magnetism measurements of 93 samples collected along the sequence and present soils of the same area (Fig. 2). The magnetic results include magnetic susceptibility measurements at room temperature and two frequencies (470 and 4700 Hz) (Bartington MS2), Vibrating samples magnetometer (Molspin VSM) measurements at room temperature and some measurements of susceptibility at high and low temperatures (Kappa bridge) in selected samples. The concentration of ferrimagnetic mineral recorded along the sequence (mass specific magnetic susceptibility, c between 17 and 7 E-7 m3/Kg) is similar to those of Pampean loess sequences. This fact allows to compare magnetic results from both areas in order to evaluate wind impact in the magnetic signal. The obtained magnetic results are used to prove, for high-mid latitudes (Tierra del Fuego and Alaska), the quantitative model for magnetic signal proposed by Orgeira et al (2010). Fundamentally, the model is based on the hypothesis of ultrafine magnetite precipitation during altenating wetting and drying cycles in the soil micropores. The rate at which this occurs depends on the frequency of drying/wetting cycles, and on the average moisture of the soil. In order to do the comparison cited above, the W values (moisture ratio = precipitation/evapotranspiration, Orgeira et al., 2010) were calculate for both areas (Figs. 3a and 3b). Therefore, the magnetic signal of the area and its W value were compared with those obtained in a loess deposit from Alaska (Lagroix and Banerjee, 2002). After the comparison above mentioned, the magnetic results obtained in the paleosols can be transformed in paleo-average moisture. Finally, these magnetic proxi plus the other multiproxi data obtained in the studied sequence define the Holocene climatic pattern for the area, wich is characterized by an important climate variability occurred in the southern extremity of the Americas. Based on this study we hope to contribute to the knowledge of the variability of the southern atmospheric circulation for the most recent geological times. REFERENCES • Lagroix F. and Banerjee S.K.; 2002: Palaeowind directions from the magnetic fabric of loess in central Alaska, Earth Planet. Sci. Lett., 195, 99-112. • Orgeira M.J., Egli R. and Compagnucci R.H.; 2010: A quantitative model of magnetic enhancement in loessic soils. Chapter in Earth magnetic Interior (IAGA Special Sopron Book series), Springer; in press. • Coronato A., Fanning P., Saleme M., Oría J. and Pickard J., 2009: Aeolian paleosoils and the archeological record at Lake Arturo, Nothern Tierra del Fuego, Argentina. IV Cong. De Cuaternario y Geomorfología. XII Congresso da Associacaode Studos Do Cuaternario. Abstracts: 234. 193 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA MANAGEMENT AND CONTROL OF THE WATER RESOURCES OF SAN LUIS PROVINCE (ARGENTINA) 6-08 Pedersen*, O.A. San Luis Agua S.E. Justo Daract 1695 San Luis Capital * Presenting author’s e-mail: [email protected] To effectively manage the water resources of the Province of San Luis, at the end of 2009 the San Luis Agua SE company was created, based on high standards of a private management company. Plan, regulate, control and manage resources in a given territory, constitute a complex task. Part of the problem is to identify the variables involved in the process of management of water resources. It is also important to understand and analyze the interactions among these variables. From those premises, it is possible to build a digital model to simulate possible behaviors in order to drive efficient use of natural resources of the province, and to be able to react quickly to unforeseen situations. The GIS (Geographic Information System) uses a set of graphical information in the form of geo-referenced maps and methods of management of databases. On this basis, the company uses the GIS as a tool for information management and geospatial data for an ideal use of its capital. The company has also created, within the GIS, a system where users of the province have access to information of public interest like water wells, waterworks, etc. The goal of San Luis Agua SE is to maximize its resources, both human and infrastructure, result in a product economically viable over time, reducing operating costs and management. The GIS provides the technical tools for these purposes and provides an environment for work and consultation that best suits the present needs. SEDIMENTARY IMPRINT OF THE 2007 AYSÉN EARTHQUAKE AND TSUNAMI IN AYSÉN FJORD (CHILEAN PATAGONIA) 6-09 Van Daele, M.1, De Batist, M.1*, Versteeg, W.1, De Rycker, K.1, Cnudde, V.2, Gieles, R.3, Duyck, P.4, Pino, M.5, Urrutia, R.6 (1) Renard Centre of Marine Geology (RCMG), Ghent University, Krijgslaan 281/S8, B-9000 Gent, Belgium (2) Department of Geology and Soil Science, Ghent University, Krijgslaan 281/S8, B-9000 Gent, Belgium (3) Royal Netherlands Institute for Sea Research (NIOZ), Landsdiep 4, 1797 SZ’t Horntje (Texel), Netherlands (4) Department of Radiology and Medical Imaging, Ghent University Hospital, De Pintelaan 185, B9000 Gent, Belgium (5) Instituto de Geociencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile (6) Centro EULA, Universidad de Concepción, Casilla 160-C, Concepción, Chile * Presenting Author e-mail: [email protected] On 21 April 2007, the Mw 6.2 Aysén earthquake caused several subaerial mass movements (landslides, rockfalls) along the slopes of Aysén fjord (Fig .1). The three most voluminous of these triggered several tsunamis. These landslide-induced tsunamis not only destroyed small villages, houses and salmon farms along the shores of the fjord, but also resulted in 3 casualties and 7 missing persons. The earthquake had its epicentre at a depth of < 9 km below the fjord. It was the most important earthquake of a seismic swarm, which had lasted 3 months (starting on January 22) and in which more than 7000 earthquakes were registered. Intensities as high as VIII to IX were recorded around the epicentral zone, i.e. where the landslides occurred (Fig. 1). In Puerto Chacabuco and Puerto Aysén (at the end of the fjord) intensities of VII were recorded (Naranjo et al., 2009; Sepulveda and Serey, 2009). 194 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Location map of the study area, with indication of the epicentral zone of the earthquake swarm, location of landslides and cores studied. This type of mass movements and their related tsunamis can leave an eventdeposit in the fjord’s sedimentary record. Such deposits and the processes forming them have been described by several authors. However, coupling the nature of these deposits to ‘what occurred on which slope’ and to earthquake intensity is a complicated assignment. Historical records are often incomplete and only range back in time a few hundred years, depending on the region. As such, examples where the original processes are well documented are scarce. In order to gain a better insight in the sedimentological characteristics of event deposits caused by landslide-induced tsunamis, we conducted a multidisciplinary study of the recent sedimentary infill of Aysén fjord. By studying the 2007 event-deposit with multibeam bathymetry, highresolution reflection seismics and a multiproxy analysis on 20 short gravity cores (Fig. 1), we aim to fingerprint this deposit in the highest detail. Multibeam bathymetry and reflection seismics are used to map out the occurrence, morphology and thickness of the deposit throughout the fjord, and gravity cores are taken to ground-truth the geophysical data. Sedimentological characterisation of the event-deposit is achieved by combining CT-scans, high-resolution (1 mm) grain-size analyses, XRF-scanning (1 mm) and magnetic-susceptibility measurements (2.5 mm) of the sediment cores. The deposit is very heterogeneous in space and has a varying thickness (centimetre- to metrescale). The internal structure varies between parallel laminations, fine cross-bedding, ripples and homogeneous, with grain sizes ranging from fine clay to gravel. Different phases in the deposition can be correlated between most of the cores and thereby allow us to gain an insight into the evolution of this deposit. Comparing this complex sedimentary imprint with eye-witness reports, field observations, records of seismic-shaking and macro-intensities allows us to better understand the processes forming these deposits. 195 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA REFERENCES • Naranjo, J.A., Arenas, M., Clavero, J. and Munoz, O., 2009. Mass movement-induced tsunamis: main effects during the Patagonian Fjordland seismic crisis in Aisen (45 degrees 25’S), Chile. Andean Geology, 36(1): 137-145. • Sepulveda, S.A. and Serey, A., 2009. Tsunamigenic, earthquake-triggered rock slope failures during the April 21, 2007 Aisen earthquake, southern Chile (45.5 degrees S). Andean Geology, 36(1): 131-136. RECONSTRUCTION OF THE EVOLUTIVE STAGES OF LLANCANELO LAKE 6-10 AND SURROUNDINGS (SOUTHERN MENDOZA PROVINCE, WESTERN ARGENTINA) Rovere, E.I.1, Violante, R.A.2*, Osella, A.3, De la Vega, M.3, López, E.3 (1) Dirección de Geología Regional, Servicio Geológico Minero Argentino SEGEMAR, Av. Julio A Roca 651. Buenos Aires C1067ABB, Argentina (2) Servicio de Hidrografía Naval, Departamento Oceanografía, División Geología y Geofísica Marina. Av. Montes de Oca 2124, Buenos Aires C1271ABV, Argentina (3) Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Buenos Aires C1428EGA, Argentina * Presenting author’s e-mail: Roberto A. Violante, [email protected] Introduction Llancanelo Lake (southern Mendoza province, western Argentina, 35º35´S-69º09´W) is located at the foot of the Andes cordillera in a transitional region between northwestern Patagonia and western dry Pampas. It occupies a tectonic depression surrounded by extensive volcanic fields largely composed of volcanic cones and basaltic flows. As a result it behaved as the main regional depocenter through its evolutionary history, which has been affected by diverse intra and extra basinal sedimentation through different sedimentary processes. Consequently, thick sedimentary and volcaniclastic sequences were deposited in the basin, controlled by tectonic and volcanic factors under fluctuating climatic changes. As the last stages of evolution were dominated by arid conditions, the lake progressively changed from an ancient, largely extended water body, to a presently smaller, endorreic, shallow and highly saline lacustrine environment. Therefore, in the lake and its basin the records of the regional evolution are preserved, what makes the area a key region for paleoenvironmental, paleoclimatic and paleovolcanic reconstructions. Geophysical and geological surveys have been performed in order to: 1) define the depocenter geometry and its substratum; 2) evaluate the extension and characteristics of the sedimentary sequences; 3) recognize the volcanic events that occurred during its evolution; 4) reconstruct the evolutionary history of the lake. This contribution synthesizes the present knowledge on aspects related to the conditioning factors that influenced the evolution of the region. Geological setting Llancanelo Lake occupies the southern extreme of the so-call Huarpes Depression, a tectonic basin extended from N to S between the Andes cordillera to the W and the San Rafael Block to the E. The southern boundary of the basin is the Payenia Volcanic Field. Thickness of the alluvial sedimentary sequences filling the basin where the lake is contained ranges between 500 and 1000 m (Ostera and Dapeña, 2003). Although the present lake covers an area of around 370 km2 with an average depth not exceeding 1 m, evidence arising from regional geological, morphological, faunistic and archeological records (Groeber, 1939; Delpino, 1993; Dieguez et al., 2004; Gil et al., 2005; Guerci et al., 2006; among others) suggest that an ancient “big” lake containing not only the present Llancanelo Lake but also the present El Nihuil Lake, covered in the past an area of around 5000 km2 reaching a level of +50 m above the present lake, although no evidence exist about the depth of that ancient lake. The Payenia Volcanic Field located immediately south of (and partially surrounding) Llancanelo Lake, is a Tertiary-Quaternary back-arc basaltic province (Delpino, 1993; Bermúdez et al., 1993) in 196 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA which volcanism has been very intense during the Cenozoic, when some 800 volcanoes have been active at different times. Basaltic lava flows were the main volcanic products, mostly originated in complex volcanoes and monogenic cones, some of them hydroclastic and maars (Delpino, 1993; Bermúdez et al., 1993; Risso et al., 2008; among others). On the other hand, the neighboring Andean arc contains -in the 300 km long stretch of mountains that extend west of Llancanelo Lake and Payenia- around 12 volcanoes, some of them active up to very recent times (eg.: Quizapu, Peteroa); dominantly acid explosive eruptions occurred there during most of the Cenozoic under the effect of strong westerly winds, therefore they contributed with huge volumes of tephras to the regions located to the east, which were partially accumulated in the Llancanelo Lake depression. Results The lake substratum and surroundings are mainly formed by basaltic lava flows. Geophysical surveys (geoelectric and electromagnetic induction) performed on the lacustrine plains and beyond in a W-E transect in the middle part of the lake, detected both very high and very low resistivity layers, which were respectively associated to unconsolidated sedimentary sequences and basaltic rocks. In the western lacustrine plain, the basaltic substratum crops out several km to the W of the shoreline and progressively dips to the E reaching depths of at least 30 m at the lake´s shore. In the eastern lacustrine plain the basaltic substratum (found with geoelectric soundings at depths deeper than 100 m) rapidly rises to the E in a stepping geometry, and crops out close to the eastern lake shoreline. This stepping and rapid decreasing in depth of the substratum is considered as an evidence of the Llancanelo fault, which is a major N-S-trending normal fault that marks the eastern tectonic boundary of the basin (Ramos and Folguera, 2005; among others). The geoelectric surveys also allowed to recognize layers of low resistivity sequences indicative of soft, probably lacustrine sediments, interbedded between high resistivity layers associated to basaltic flows, what preliminary indicates that the lake evolution could have been sometimes interrupted by volcanic episodes. The uppermost subsoil sedimentary sequences were recognized along the W-E transect by both shallow geophysical surveys and drillings. Mainly lacustrine, paludal and eolian (most of the times volcaniclastic) sediments including levels of tephras, paleosoils and evaporites (gypsum) constitute the sequences. Sedimentological and microfaunistical analysis reveal the larger extension of the lake in the past as well as fluctuations in its size and depth. Volcanism was defined as a major conditioning factor in the regional evolution. Besides the evident surface records of volcanic activity, main subsurface volcanic evidence are represented by geoelectrical/sedimentological anomalies preliminary hypothesized as representing buried volcanic edifices (possible maars and diatremes). On the other hand, at least one discrete although highly significant ash layer was identified at the top of the sedimentary sequences at a regional level, which is the product of the Quizapú volcano eruption occurred in 1932. Conclusions The evolution of the region was conditioned by tectonic, volcanic and climatic factors that acted together, although three main stages dominated by each of them can be synthesized. Stage 1: Tectonic (Pre-Pliocene): Prior to the Pliocene, a complex tectonic history dominated, which varied from extensional phases to contractional deformation, development of the Malargüe fold and thrust belt and subsidence of the Rio Grande foreland basin (Ramos and Folguera, 2005). Faulting is evidenced by the Llancanelo fault in the eastern margin of the lake. Stage 2: Volcanic (Pliocene / Early Quaternary): Payenia volcanism has been active since the middle Pliocene, around 3.4 Ma ago, with significant reactivations at 1.7 and 1.2 Ma. In the late Pleistocene (after 450 Ka) several large volcanic events that gave origin to some of the more extensive lava flows took place (Germa et al., 2007; Quidellieur et al., 2008; Pasquaré et al., 2008). Stage 3: Climatic (late Pleistocene / Holocene): During these times, climatic changes were the most important factor involved in the regional evolution, associated to alternating cold (glacial) and warm (interglacial) periods related to the large Quaternary glaciations. However, no evidences of glacial activity in the study area have been found up to the present. Despite the relative predominance of climate variability, volcanism was still somehow relevant as evidenced by some records of volcanic 197 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA events, like that occurred around 7 Ka (Germa et al., 2007; Quidellieur et al., 2008). REFERENCES • Bermúdez, A., Delpino, D., Frey, F. and Saal, A. (1993). Los basaltos de retroarco extraandinos. 12º Congreso Geológico Argentino and 21º Congreso de Exploración de Hidrocarburos, Mendoza. Relatorio: 161–172. • Delpino, D.H. (1993). Fue el sur Mendocino similar a Hawaii? Evidencias del Pasado para entender el Presente. Primeras Jornadas Nacionales de Vulcanismo, Medio Ambiente y Defensa Civil, Asociación Geológica de Mendoza-Subsecretaría de Ciencia y Técnica, Malargüe (1992): 67–77. • Dieguez, S., De Francesco, C., Páez, M., Navarro, D., Quintana, F., Guerci, A., Zárate, M., Giardina, M., Neme, G. and Gil, A. (2004). Paleoambiente y ocupación humana en el valle del río Atuel: trabajos recientes. Resúmenes del 15º Congreso Nacional de Arqueología Argentina (Río Cuarto, Córdoba): 39–49. • Germa, A., Quidellieur, X, Gillot, P.Y. and Tchilinguirian, P. (2007). Volcanic evolution of the back-arc complex of Payun Matru (Argentina) and its geodynamic implications for caldera forming eruption in a complex-slab geometry setting. IUGG, Perugia. Abstract: 10028. • Gil, A., Zarate, M. and Neme, G. (2005). Mid-Holocene paleoenvironments and the archeological record of southern Mendoza, Argentina. Quaternary Internacional, 132: 81-94. • Groeber, P. (1939). Informe Geológico sobre la Zona de Embalse del Proyectado Dique en Nihuil (Provincia de Mendoza). Dirección de Minas y Geología, 53 p. • Guerci, A., Paez, M., Dieguez, S., De Francesco, C., Gil, A., Neme, G. and Polimeni, M. (2006). Estudios paleoambientales al sur del río Atuel durante el Holoceno. Resúmenes del 13er. Simposio Argentino de Paleobotánica y Palinología (Bahía Blanca): 126135. • Ostera, H. and Dapeña, C. (2003). Environmental isotopes and geochemistry of Bañado Carilauquen, Mendoza, Argentina. In: IV South American Symposium on Isotope, Geology, Short Papers: 461-464. • Pasquaré, G., Bistacchi, A., Francalanci, L., Bertotto, G.W., Boari, E., Massironi, M. and Rossotti, A. (2008). Very long Pahoehoe inflated basaltic lava flows in the Payenia volcanic province (Mendoza and La Pampa, Argentina). Asociación Geológica Argentina, 63: 131-149. • Quidellieur X., Carlut, J., Tchilinguirian, P., Germa A. and Gillot, P.Y. (2008). Paleomagnetic directions from mid-latitudes sites in the southern hemisphere (Argentina): Contributions to Time Averaged Field models. Physics of the Earth and Planetary Interiors, 172 (3-4): 199-209. • Ramos, V.A. and Folguera, A. (2005). Los Andes Australes: una evolución tectónica excepcional entre el sur de Mendoza y el Norte de Neuquén. In: VI Congreso de Exploración y Desarrollo de Hidrocarburos, Mar del Plata, Actas: 75-83. • Risso, C., Nemeth, K., Combina, A.M., Nullo, F. and Drosina, M. (2008). The role of phreatomagmatism in a Plio-Pleistocene highdensity scoria cone field: Llancanelo Volcanic Field (Mendoza) Argentina. Journal of Volcanology and Geothermal Research, 169: 61–86. ROCK-MAGNETISM CHARACTERIZATION OF A LATE QUATERNARY SOIL HORIZON (SAN SEBASTIÁN BAY, ISLA GRANDE OF TIERRA DEL FUEGO) 6-11 Walther, A.M.1-2*, Raposo, M.I.B.3, Vilas, J.F.1-2 (1) Departamento de Ciencias Geológicas Fac. Cs. Exactas y Naturales, Universidad de Buenos Aires. Ciudad Universitaria Pab II, 1428 Buenos Aires (2) Consejo Nacional de Investigaciones Científicas y Técnicas, Instituto de Geofísica Daniel Valencio (INGEODAV) Depto. de Ciencias Geológicas FCEN UBA (3) Laboratorio de Anisotropía Magnéticas y Magnetismo de Roca del Instituto de Geociencias de la Universidad de San Pablo * Presenting author’s e-mail: [email protected] Introduction An environmental magnetism study was performed on Quaternary sediments from a pedogenetic horizon that lies on the lateral moraines of the Río Cullen glaciation (Cabo Vírgenes drift) and of the San Sebastián glaciation (Punta Delgada drift), in San Sebastián Bay, Tierra del Fuego, Argentina Rabassa et al. (2005). The main objectives of the study were two-fold: (1) to perform a detailed determination of the nondirectional magnetic parameters in the analyzed profile sediments; (2) to perform a comparison of the recorded magnetic signal against previous studies carried on paleosoils from the Pampean plain 198 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 developed on sediments of different origins and ages. Fig. 1- Location Map in San Sebastián Bay, Tierra del Fuego, Argentina. Stratigraphy In the San Sebastián Bay area Fig 1, located north of Río Grande, two very well-defined alignments of moraines outcrop on both margins of the depression formed by Inútil and San Sebastian bays, corresponding to the Punta Delgada and Cabo Virgenes drifts Coronato et al. (2004). A pedogenic horizon aged post-main Pleistocene develops above both drifts. This horizon is a sedimentary sequence of variable thickness of 1.15 m in the studied area. Four informal sedimentary units were differentiated through a macroscopic description of the profile and the corresponding sampling from base to top. Unit A (Fig. 2) lies unconformably on a gravel and coarse sands deposit, presenting a thickness of 0.52m. It is a poorly sorted silt-sandy sediment, friable, which presents some poorlydefined prismatic structures (samples 1-4). The upper limit is transitional. Unit B (Fig. 2) has a thickness of 0.82 m, is friable and is constituted by light brown-colored argillaceous silts. It presents prismatic block structure with abundant organic matter, and root molds and cutans are common. Root molds appear coated with argillo and ferrocutans. Edaphic processes intensify towards the unit top (samples 5-8). The upper limit is transitional. Unit C (Fig. 2), with a thickness of 0.36 m, is composed of light brown-gray silty argillous sandy sediments, unconsolidated, with abundant root molds. The upper limit is clearer and smooth (samples 9-14). This unit is overlaid with dark grey silt-sandy sediments where the present soil is developing. Fig. 2 - Geological profile (left side) and susceptibility, Ms and Mr values as a function of depth Rock magnetism study Weather influence is a first order factor in the formation of a soil. Although the magnetic minerals constitute a minor fraction within the rocks and soils, their sensibility to chemical changes make them excellent detectors of environmental changes (Verusob, 1995). Fourteen successive levels were sampled from the base of sedimentary unit A to the top of sedimentary unit C. The distance between levels is 10 to 15 cm. Bulk magnetic susceptibility measurements at two frequencies, hysteresis loops and isothermal remanent magnetization (IRM) acquisition experiments were 199 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA performed. Results The interpretation of rock magnetism parameters provides information about the concentration, grain size and magnetic mineralogy of the analyzed sedimentary material. Initial specific magnetic susceptibility (X) was measured in low and high frequencies (470 Hz and 4700 Hz), recording values of XfD% (susc 470 Hz susc 4.700 Hz / susc 470 Hz) lower than 5% in all samples. These suggest the existence of a non significant superparamagnetic (SP) fraction in the sequence. Fig. 2 represents the fluctuations of total magnetic X in two frequencies, as function of the stratigraphic position of the analyzed samples throughout the profile. The values for this parameter oscillate between 0.4 and 1.2 x 10-6 m3/kg. A significant feature observed in Fig. 3 - Magnetic grain size characteristics from hysteresis properties of the samples shown in a modified bivariate Day plot. (Day et al., the X’s profile is the presence of a 1977; Dunlop, 2002). decreasing tendency from base to top in unit B, which intensifies in unit C. The observed minimum X corresponds with the more edaphic sectors of the studied units. Saturation magnetization (Ms) and remanent saturation magnetization (Mrs) values from hysteresis loop (Fig. 2) show that both parameters present a decrease in the samples from unit C. This behavior, similar in the three parameters, can be attributed to a lower amount of ferrimagnetic minerals. Through the analysis of variations in the parameters of coercivity (Hc) and remanence coercivity (Hcr) it was found that Hc values increases in unit C, while Hcr values increase in units B and C. The fluctuations of Hc and Hcr in the studied material are those expected for magnetite and/or titanomagnetite Dekkers (1988); Roberts et al. (1995). Hysteresis loops and IRM acquisition curves, performed up to 1 T fields, display different characteristics, which can be divided in two groups: Those that form very narrow loop, typical of minerals with low coercivity and multidomain (MD) or pseudo simple domain (PSD) magnetic grain size. This behavior appears in levels 1 to 11. Loop corresponding to minerals with low coercivity and elevated content of paramagnetic minerals, belonging to samples 12, 13 and 14. The IRM curves done up to 1 T present a typical behavior of low coercivity minerals such as magnetite and titanomagnetite, which saturate at less than 300 mT. Neither the hysteresis loops nor the IRM curves detect the presence of antiferrimagnetic minerals in the edaphic levels of the sequence. Since the ferrimagnetic minerals are dominant in the sequence, it would be valid to use the Mrs/Ms vs. Hcr/Hc ratios in the graph by Day et al. (1977) modified by Dunlop (2002) for a magnetite unimodal aggregate. Fig. 3 displays the Mrs/Ms vs. Hcr/Hc relations. It can be observed that the corresponding values fall predominantly in the pseudo single domain (PSD) field. Discussion The presence of organic matter, molds coated by cutans and prismatic structures observed throughout the profile and more intensely in unit C, indicate the action of edaphic processes which would be 200 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA linked to the existence of a benign (temperate???) climate which would have allowed the development of vegetation. The decrease of magnetic parameters (X, Ms and Mrs) in the profile can be attributed in the first instance to a process of gradual decrease of the amount of detritic ferrimagnetic particles, and in the second instance to a mineralogical change. The decrease of these parameters in the profile matches the intensification of pedogenetic features observed in the field. The rise in edaphic activity at these levels would be associated to the fall in the amount of ferrimagnetic particles and mineralogical changes on the surface of the magnetic grain. Bidegain et al. (2001) arrive to similar conclusions regarding the magnetic behavior of loessical sediments studied near the La Plata city (Buenos Aires, Argentina), suggesting “dissolution” of magnetite as a consequence of the weathering phenomena. On the other hand, the presence of high coercivity magnetic minerals is not detected by the analysis of hysteresis cycles. Then, the pedogenetic processes that affected the sequence studied here from the point of view of magnetic behavior could be reflecting a decline of the detritic ferrimagnetic fraction (magnetite and/or titanomagnetite) in the paleosol. The registered behavior in the analyzed paleosol is similar to the one occurring in the Pampean plain. Moreover, there is no detection of neoformation of high coercivity minerals (hematite and/or goethite) or of low coercivity ones (ultra fine magnetite), a very conspicuous process in the Pampean plain paleosols developed under weathers with a pronounced dry season (Walther et al., 2004). Although the average temperatures during the development of this paleosol and the Pampean plain ones are different, the benign climatic events that generated them should have affected the magnetic minerals in the same way. The glacial advance of Cabo Virgenes drift, occurred during Stage 12 and the Punta Delgada drift advance, happened during Stage 10, are prior to this unit’s deposition. In the study area, the next glacial advance is the Primera Angostura drift, which was estimated to have occurred during Stage 6. Therefore it is probable that these sediments were deposited during this period or later, and then were edaphized during the warmer Isotope Stage 5 (125 ky-75 ky B.P.), during the substage 5e (the warmest and longest), or after it. Conclusions The magnetic parameters in the studied paleosol indicate a depletion of ferromagnetic minerals in the edaphic levels compared with the inferior levels. This loss would be due to the edaphic processes that acted during the soil formation. Similar behavior was observed both in eolic sediments as well as in fluvial sediments of different ages in the Pampean plain. Neoformation of magnetite and/or maghemite (SP magnetic grain size) was not detected. There was no manifestation of neoformation of antiferromagnetic minerals by dehydration of amorphous gels created during edaphic-synthesis processes. The absence of these minerals is associated with periods of warm weather, with no dry season. This soil is probably reflecting the climate change occurred in the area during Isotope substage 5e. REFERENCES • Bidegain, J. C.and van Velzen, A. J.,Rico, Y.; 2001: Parámetros magnéticos en una secuencia de loess y paleosuelos del Cenozoico tardío en la cantera de Gorina, La Plata: su relevancia en el estudio de los cambios paleoclimáticos y paleoambientales. Revista de la Asociación geológica Argentina. 56(4): 503-516. • Day, R., Fuller, M. and Schmidt, V. A., 1977: Hysteresis properties of titanomagnetites: grain-size and compositional dependence. Physics of the Earth and Planetary Interiors, 13: 260-267. • Dekkers, M. J., 1988: Some rock magnetic parameters for natural goethite, pyrrhotite and fine grained hematite. Geologica Ultraiectina, N 51 Ph. D. Thesis, University of Utrecht, 231, p. Utrecht • Coronato, A., Meglioli, A., and Rabassa, J. 2004: Glaciations in the Magellan Straits and Tierra del Fuego, southernmost South America. Quaternary Glaciations- Extent and Chronology, Part III Editors Ehlers and P L Gibbard. Elsevier. • Dunlop, D. J., 2002. Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 2.Aplication to data for rocks, sediments, and soils. Journal of Geophysical Research vol 107 (B3) 10.1029-2001 JB000487 • Rabassa, J., Coronato, A.M., and Salemme, M. 2005: Chronology of the late Cenozoic Patagonian glaciations and their correlation with biostratigraphic units of the Pampean region (Argentina).Journal of South American Earth Sciences 20 81-103 Elsevier • Roberts, A. P, Yulong Cui Y., and Verosub, K.L., 1995 : Wasp-waisted hysteresis loops: Mineral magnetic characteristics and discrimination of components in mixed magnetic systems. Journal of Geophysical Research, vol. 1000. N0 B9 17909-17924. Washington. 201 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA • Verosub, K. L. and Roberts, A. P. 1995: Environmental magnetism: Past, present, and future. Journals Geophysics Research 100: 2175-2192. • Walther, A. M., Orgeira, M. J., and Lippai H. F., 2004: Magnetismo de rocas en sedimentos cenozoicos tardíos en San Antonio de Areco provincia de Buenos Aires. Revista de la Asociación Geológica Argentina Vol. 59 (3): 433-442. 202 Session 7 MARINE GEOLOGY AND GEOPHYSICS GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA CHARACTERIZATION OF THE MAGNETIC RESPONSE OF 7-01 THE NORTHERN ARGENTINE CONTINENTAL MARGIN (SOUTH ATLANTIC OCEAN) Abraham, D.A.1,2*, Ghidella, M.3, Paterlini, M.1,4, Schreckenberger, B.5 (1) COPLA. Comisión Nacional para la Determinación del Límite Exterior Argentino (2) Escuela de Ciencias del Mar, Instituto Universitario Naval, Argentina (3) Instituto Antártico Argentino, Argentina (4) Servicio de Hidrografía Naval, Argentina (5) Federal Institute for Geosciences and Natural Resources (BGR), Germany * Presenting author's e-mail: [email protected] Introduction We have compiled a magnetic anomaly grid by using several data sets collected along the Argentine continental margin in the sector comprised between 35º S and 48º S. We have calculated the anomaly field’s numerical curvature along specific directions. The results were then overlaid on the original anomaly grid with a transparency factor, and this resulted in the enhancement of the magnetic alignments and the improvement of their outline, thus allowing a better mapping of these features. We have identified both numerous anomaly alignments and their fragmented character. We have also improved the definition of magnetic provinces as a result of a further qualitative analysis. These results were correlated with the proposed segmentation of this sector of the Argentine continental margin at the time of the ocean opening, as asserted by several authors. Margin structure The sector under study is a typical extensional volcanic margin. One of the main characteristics of this type of margin is the presence of volcanic wedges, recognized from multichannel seismic reflection as seaward dipping reflector sequences (SDRS). They are mainly represented by buried basaltic flows. Based on seismic data analysis, Franke et al. (2007) have identified four segments in the Argentine margin, being their boundaries interpreted as transfer zones. These are: the Malvinas, Colorado, Ventana, and Salado transfers. The main criterion to map the abovementioned segments was the presence of great lateral displacements in the seismic reflector wedges as well as abrupt changes in their architecture. The transfer zones may represent old weakness areas which date from the beginning of the oceanic opening in the Late Cretaceous. The first margin segment (segment I), located between the Malvinas fracture zone and the Colorado transfer, comprises the transition between the sheared margin and the typical volcanic margin. Segment II is between the Colorado transfer and the Ventana transfer. The Colorado transfer area is characterized by a first-order magnetic anomaly (Ghidella et al., 1995). This prominent anomaly was interpreted as the limit between the Rio de la Plata and the Patagonian cratons (Max et al., 1999). The well developed SDRS disappear south of the magnetic discontinuity. Segment III corresponds to a volcanic margin with multiple seismic reflector wedges, and Segment IV is the area to the north of the Salado transfer (Franke et al., 2007). We can see those segments in Figs. 3 and 4. Magnetic anomalies The Colorado discontinuity divides two sectors, north and south, with different magnetic responses. The northern sector belongs to the extensional margin of the Río de la Plata craton. The southern one extends until the Malvinas fracture zone. This is the extensional Patagonian continental margin. From the coastline to the east, the predominant trend of the magnetic anomalies becomes parallel to the continental slope. Here there are significant SW-NE alignments attributed to the volcanic activity when the margin was formed. South of the Colorado Basin there is a highly intense magnetic anomaly (Tona anomaly) that disappears abruptly at the Colorado discontinuity. The magnetized bodies that are sources for this anomaly do not show important density contrasts in the free air anomaly map. According to Max et al. (1999), the Tona sources seem to be too deep to correspond to volcanic intrusions and too shallow to be represented by basaltic underplating. Along the Atlantic volcanic margins in general and in the Argentine margin in particular there are large 205 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 Fig. 1 - Data coverage. Survey track lines over a bathymetric grid. Fig. 2 - Magnetic provinces over the anomaly magnetic model. G: G anomaly area. G1: magnetic province shifted eastward with respect to anomaly G. CO_M: Oceanic region where the M series magnetic alignments can be identified. CO_M1: here there are oceanic magnetic alignments, but not as sharp as in CO_M. LPD: Río de la Plata craton domain. ZQ: Cretaceous quiet magnetic zone. PMPP: magnetic province of the Patagonian shelf. ZP: porphyry zone. deep igneous bodies located in the lower crust just above the Moho discontinuity that have been detected and recognized by seismic refraction experiments (“high velocity bodies”). In the Argentine margin those bodies appear below the volcanic wedges (Neben et al., 2002); they are also present in the Namibia margin. The magnetic response of those lower crustal bodies is not clear. They are very deep in transitional or oceanic crust and their temperatures may be above the Curie point. Furthermore two dimensional magnetic modeling (Ghidella, et al, 2005, http://dna.gov.ar/mararg/pictr2002/Interpretacion/tona/index.html) can reproduce the anomalies without including such a body. The prominent anomaly G is an isostatic gravity anomaly with associated magnetic expression which was determined using magnetic and gravity data and interpreted as marking the transition between oceanic and continental crust (Rabinowitz and LaBrecque, 1979). The G anomaly is related to the presence of SDRS (Hinz et al., 1999). With the available magnetic data (Fig. 1) we developed a model for total field anomalies where magnetic provinces are distinguished (Fig. 2). Areas not surveyed were completed with the WDMAM (World Digital Magnetic Anomaly Map) compilation (http://ftp.gtk.fi/WDMAM2007/WDMAM_1.0_DVD_2007_Edition/). We distinguish four areas north of the Colorado discontinuity: G: G anomaly area. This zone reflects the SDRS magnetic expression. The zone exhibits fragmentations, some of which correspond to the transfer zones determined by Franke et al. (2002, 2007). The variable width of the segments becoming narrow northward supports the inference of a diachronic opening of the margin from south to north, posed by several authors. The western edge of this zone largely corresponds to the 2000 m isobath. CO_M: This is the oceanic region where the M series magnetic alignments can be identified. In order to improve their definition, we resorted to the numerical curvature of the total anomaly field with which we performed a semi-transparent mask and overlaid it on the anomaly map. The anomaly field curvature was calculated according to its definition: K= d 2 ÄT dl 2 3 ⎛ ⎛ dÄT ⎞ 2 ⎞ 2 ⎜ 1+ ⎜ ⎟ ⎟ ⎝ ⎝ dl ⎠ ⎠ 206 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 Fig. 3 - Magnetic anomaly map with the curvature image superimposed with transparency. Magnetic alignments are enhanced. Fig. 4 - Magnetic provinces. Sea-floor magnetic lineaments in the CO_M and CO_M1 zones. Magnetic discontinuities. Transfer zones from Franke et al. (2007). Where: K: curvature of the magnetic anomaly field calculated in a specific direction. ¢T: magnetic anomaly field. The first and second derivatives are directional. This direction can be characterized by an angle ·, which is the angle between the E-W direction and the derivative direction measured counterclockwise. The expression of the parameterized curvature is as follows: K= ∂2 ÄT ∂2 ÄT ∂2 ÄT 2 2 sin á cos sin + cos +2 á á á ∂x 2 ∂x ∂y ∂y 2 3 2 ⎛ ⎛ ∂ÄT ⎞ ⎞2 ∂ÄT sin á ⎟ ⎟ cos á + ⎜ 1+ ⎜ ⎠ ⎠ ∂y ⎝ ⎝ ∂x We found that an angular parameter of 40° highlighted the seafloor magnetic lineaments, identified from M0r to M5n (Fig. 3), according to the Gradstein et al. (2004) Reversal Polarity Time Scale. LPD: Río de la Plata craton domain which extends from anomaly G to the coastline. The Tona anomaly stands out with its prominent amplitude and three-dimensional morphology. ZQ: Cretaceous quiet magnetic zone. From the Colorado discontinuity to the Malvinas transfer zone, extends the volcanic passive Patagonian margin. A flat low-amplitude anomaly field characterizes the magnetic province of the Patagonian shelf (PMPP). To the west of this area next to the coastline we identified a porphyry zone (ZP), with a high-frequency and medium-amplitude magnetic response. This region has no alignments in any predominant direction. Area G1, centered at about 46º S and 58° W, is a magnetic province shifted eastward with respect to anomaly G. This offset discontinuity occurs in the Colorado transfer and may be interpreted as an early continuation of the anomaly G zone southward. Adjacent to this area, we identified the CO_M1, centered on 47º S and 56º W, where there are oceanic magnetic alignments, but not as sharp as in CO_M. The lineaments observed here have a predominant N-S direction. We have identified magnetic alignment fragmentations in the G area and between CO_M1 and CO_M zones partially corresponding to the transfer zones identified by Franke et al. (2007). We note that the Colorado discontinuity plotted on the magnetic anomaly map is similar to the Colorado transfer as determined from seismic work, although there are some differences. The abrupt vanishing of the anomalies on this line probably marks the end of their causative bodies, thus defining a first order discontinuity, although the geological history of their emplacement cannot be explained from the magnetic point of view only. Further magneto-gravimetric modeling, based on the geometry 207 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA imaged by seismic data, may improve significantly the geological knowledge of this margin. Conclusions This paper attempts to contribute to the knowledge of the Argentine continental margin, based on magnetic data interpretation. We have developed a method that improves the definition of structural alignments in the oceanic crust. We have also characterized extensional margin volcanic provinces and identified magnetic alignments although the classification of those areas does not allow a univocal determination of the intra-basement bodies that produce the magnetic signal. REFERENCES • Franke, D., Neben, S., Hinz, K., Meyer, H., and Schreckenberger, B., 2002. Deep crustal structure of the Argentine continental margin from seismic wide-angle and multichannel reflection seismic data. AAPG Hedberg Conference, Hydrocarbon Habitat of Volcanic Rifted Passive Margins, September 8-11, 2002, Stavanger, Norway. • Franke, D., Neben, S., Ladage, S., Schreckenberger, B. and Hinz, K., 2007. Margin segmentation and volcano-tectonic architecture along the volcanic margin off Argentina/Uruguay, South Atlantic. Marine Geology 244, 46-67. • Ghidella, M.E., Paterlini, M., Kovacs, L.C. and Rodríguez, G., 1995. Magnetic anomalies on the Argentina Continental Shelf. 4th International Congress of the Brazilian Geophysical Society and 1st Latin American Geophysical Conference. Expanded abstracts, 269–272, Río de Janeiro. • Gradstein, F.M., Ogg, J.G., Smith, A.G., Agterberg, F.P., Bleeker, W., Cooper, R.A., Davydov, V., Gibbard, P., Hinnov, L.A., House, M.R., Lourens, L., Luterbacher, H.P., McArthur, J., Melchin, M.J., Robb, L.J., Shergold, J. and Villeneuve, 2004. A Geologic Time Scale 2004. Cambridge University Press, 589. • Max, M.D., Ghidella, M., Kovacs, L., Paterlini, M. and Valladares, J.A., 1999. Geology of the Argentine continental shelf and margin from aeromagnetic survey. Marine and Petroleum Geology 16, 41–64 • Neben, S., Franke, D., Hinz, K., Schreckenberger, B., Meyer, H. and Roeser, H.A., 2002. Early Opening of the South Atlantic: PreRift Extension and Episodicity of Seaward Dipping Reflector Sequence (SDRS) Emplacement on the Conjugate Argentine and Namibia Continental Margins. AAPG Hedberg Conference, Hydrocarbon Habitat of Volcanic Rifted Passive Margins, September 8-11, 2002, Stavanger, Norway. American Association of Petroleum Geologists. LINKING SEAFLOOR MORPHOLOGY, HYDROSEDIMENTARY PROCESSES AND LIVING RESOURCES IN SUBMARINE CANYONS OF THE NW MEDITERRANEAN SEA: A UNIQUE STUDY CASE 7-02 Canals, M.* GRC Geociències Marines - Universitat de Barcelona * Presenting author's e-mail: [email protected] Several submarine canyons deeply dissect the continental shelf and slope of the Gulf of Lion and the northern Catalan margin in the NW Mediterranean Sea. These canyons are efficient conduits from the continental shelf to the deep basin, as they are able to rapidly transport large amounts of dense water, sedimentary particles, organic matter and pollutants when specific environmental conditions occur. Two main modes of massive transport events from shallow to deep have been identified in the last few years: i) Dense Shelf Water Cascading (DSWC) that is triggered by persistent, cold and dry northerly winds blowing mainly in winter months, and ii) highly energetic coastal storms that may occur at any time during the year. The occurrence of DSWC is further favoured when low river discharge has been low during the previous months, as this diminishes the buoyancy of upper waters that, therefore, may easily reach the critical densities required to initiate cascading flows. Near-bottom speeds of 1 m s-1 and sand contents as high as 80% have been measured in situ during cascading episodes. Furthermore, canyon floors and flanks display large-scale furrows that are attributed to the abrading effect of the sand load involved in cascading events. These canyons are preferred habitats for cold-water corals in the warm Mediterranean Sea. It has been also demonstrated that DSWC causes the temporary collapse of the most valuable fishery in the area, the rose shrimp Aristeus antennatus one, which was previously attributed to overfishing and pollution. Highly energetic coastal storms erode the beaches and sand bodies on the inner shelf therefore easing 208 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA the trapping of coarse sediment into canyon heads. Furthermore, such storms create a generalised resuspension of fine sediment over most of the continental shelf, which is subsequently exported to other areas by currents and down canyon flows. A succession of large coastal storms occurred from December 2008 to March 2009 for which the most outstanding in situ observations will be presented. High concentrations of organic pollutants have been found in both deep slope and rise sediments and in organisms living there, which is likely related to transfers from shallow to deep due to DSWC and large coastal storms. The occurrence and impact of similar processes in other margin areas, such as the Patagonian margin, is certainly worth investigating. MORPHOSTRUCTURE OF THE WESTERN SECTOR OF THE NORTH SCOTIA RIDGE 7-03 Esteban, F.D.1*, Tassone, A.1, Lodolo, E.2, Menichetti, M.3 (1) CONICET-INGEODAV. Dpto. de Ciencias Geológicas. Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires. Argentina (2) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) – Trieste. IItaly (3) Istituto di Scienze della Terra, Università di Urbino – Campus Universitario. Urbino. Italy * Presenting author’s email: [email protected] Introduction The North Scotia Ridge (NSR) is the submerged morpho-structural expression of the Scotia plate northern edge. It is constituted by the Tierra del Fuego continental margin, isla de Los Estados, Burdwood (BB), Davis and Aurora banks, and the Georgias islands shelf (Parker et al., 1996; Barker, 2001; Giner-Robles et al., 2003; Pandey et al., 2010). About 40 Ma these blocks were grouped forming a continental link between Tierra del Fuego and Antarctica. Afterwards, with the development of the Scotia plate, the blocks drifted towards the east to their actual position (Barker, 2001; Pandey et al., 2010). Several authors have established that the actual movement of the South America - Scotia plate boundary is left-lateral (Forsyth, 1975; Pelayo and Wiens, 1989; Giner-Robles et al., 2003; Thomas et al., 2003; Smalley et al., 2007). In the Tierra del Fuego region, the plate boundary is represented by a mostly transtensional fault system known as Magallanes-Fagnano (Lodolo et al., 2002, 2003, 2006, Tassone et al., 2008; Menichetti et al., 2008). Towards east, the boundary is located in the Malvinas trough, at the north of the BB and it would be transpressive (Cunningham et al., 1998; Giner-Robles et al., 2003; Bry et al., 2004). The change of the tectonic regime (transtensional to the W to transpressional to the E) would occur at 63.5 ºW (Lodolo et al., 2003; Yagupsky et al., 2003). As part of a study of the evolution of the SW Atlantic continental margin, we analyze and describe the morpho-structure of the western sector of the North Scotia Ridge (Figs. 1 and 2). Sources and methods One hundred sixty eight unpublished multi-channel seismic lines were integrated with seismic sections taken from literature (Platt and Philip, 1995; Yagupsky et al., 2003; Bry et al, 2004; Lodolo et al., 2006; Tassone et al., 2008). In addition, single-channel seismic lines available on the web (GeoMapApp, GeoDas), bathymetric (GEBCO) and gravimetric data (Sandwell and Smith, 2009) were also used. The metodology consisted in the recognition of the acoustic basement in the seismic lines. Then, the points of the top of the acoustic basement between 0.5 and 3.5 seconds two-way travel time (twtt) every 0.5 seconds were drawn in the map. The points with the same twtt were connected in lines (isobaths). The bathymetric data was used to assist the interpolation between lines, specially when the distance between two nearest lines was considerable.The lines were interpreted by using the Kingdom software suite (version 8.2). The lines taken from the published data and from the Web (in image 209 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - A) Map with the location of the main tectonic plates. B) Bathymetric map of the studied region with the contours lines of the gravimetric anomalies every 50 mGal (dotted lines), and the twt isobaths of the acoustic basement every 0.5 seconds (solid lines). Plate boundary (segmented lines) from PLATES Project (www.ig.utexas.edu/research/projects/plates/). A-B segment indicates the location of the seismic section of Figure 2. Used abbreviations: AB, Austral (Magallanes) Basin; BB, Burdwood Bank; EFZ, Endurance Fracture Zone; IE, isla de Los Estados; MB, Malvinas Basin; MT, Malvinas Trough; QFZ, Quest Fracture Zone; SAM, South American plate; SCO, Scotia plate; SMB, South Malvinas Basin; TdF, Isla Grande de Tierra del Fuego. format) were converted into Segy format with Image2segy software (Farran, 2006), which runs under MatLab. The seismostratigraphic units recognized in the foredeep (Fig. 2) were obtained by correlating seismic lines and published results (Yagupsky et al., 2003; Tassone et al., 2008). Western edge of the North Scotia Ridge. Geophysical interpretation. In Fig. 1, the distribution of the isobaths of the top of the basement clearly defines the structural highs that constitute the NSR. These highs are oriented WSW-ENE to W-E (with a break at ~62 °W), and have sharp limits as indicated by the proximity of the isobaths. This is particularly evident between 60 °W and 66 °W, where these highs border the Austral and Malvinas basins. More gentle slopes are located in the extreme eastern of the BB. The 0.5 seconds isobaths, unlike the others, allow recognizing two units. The first, located to the east, extends from the isla Grande de Tierra del Fuego to the isla de Los Estados. The other unit, located to the west, corresponds to the BB. In the seismic lines over the structural highs of the NSR a notable contrast in the sedimentation infill has been recongnized (Fig. 2, see also Fig. 2 of Kimbell and Richards, 2008). Over the southern flank of these structural highs, the sedimentation is very reduced. This can be visualized by the good correlation between the isobaths and the bathymetry (Fig. 1). On the other hand, the important sedimentary infill of the Austral and Malvinas basins (Galeazzi, 1996; Tassone et al., 2008) has partially covered the northern flank of these structural highs (Fig. 2). This can be seen specially near the northern shore of isla de Los Estados where there is no correlation between the isobaths and the bathymetry (Fig. 1). The gravimetric anomalies (Fig. 1; Smith and Sandwell, 2009) are distributed along E-W-trending belts and range from -150 mGal to 150 mGal. The distribution is characterized by a positive belt over the southern edge of the NSR, and a parallel negative belt which coincides with the Malvinas Trough and the northern edge of the NSR. In the eastern sector, over the BB, Bry et al. (2004) and Kimbell and Richards (2008) correlated this high-low pattern with the overthrust of the BB over the South America plate southern edge. The relative high level of the BB generates a positive anomaly, while the downwards flexure of the South America plate explain the negative anomaly. Instead, offshore of Tierra del Fuego, Lodolo et al. (2003) correlated the gravimetric minima with the sedimentary fill of pull-apart basins (Tassone et al., 2008; Esteban et al., 2009) developed along the Magallanes-Fagnano system fault. 210 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - S-N multichannel seismic section. Location in Fig. 1. NSR: North Scotia Ridge. FD: Malvinas Foredeep. Nomenclature of the seismic units recognized in the foredeep follows Yagupsky et al. (2003) and Tassone et al. (2008). The N-S seismic section in Fig. 2 is located in the central part (~62 ºW) of the studied area (Fig. 1). In general, the bathymetry is flat (less than 600 m), except for the southern edge, where there is a steep slope (up to 2000 m). In the seismic section, two sectors can be distinguished. The first, from the SPs 16800 to the north, is characterized by a shallow acoustic basement (less than 2 s twt), a minimal sedimentary fill, strong multiples and numerous vertical faults (some exceed 2 s twt). The basement is seismically homogeneous, with poor lateral continuity and moderate amplitude, resulting in a chaotic arrangement. Between SPs 18000 and 18500, a small basin limited by faults can be recognized. To the north, levels of folded reflector can be seen and would correspond to the fold-andthrust belt (SPs 17500 to 17000; Lodolo et al., 2003; Bry et al., 2004; Fish, 2005; Tassone et al., 2008). In contrast, the northern sector (SP 16800 to the south) corresponds to the Malvinas basin foredeep and the important sedimentary fill observed was correlated with published seismic lines (Yagupsky et al., 2003; Tassone et al, 2008). The five seismic units (units 2, 3, 4, 5a and 5b of Tassone et al., 2008) span from Middle-Late Jurassic to Holocene, and include syn rift (unit 2), sag and foredeep (units 3 and 4), and foreland (5a and 5b) tectonic phases. REFERENCES • Barker, P.F., 2001. Scotia Sea regional tectonics evolution: Implications for mantle flow and paleocirculation. Earth Science Reviews, 55, 1-39. • Bry, M., White, N., Singh, S., England, R., Trowell, C., 2004. Anatomy and formation of oblique continental collision: South Falkland basin. Tectonics, 23, TC4011, doi: 10.1029/2002TC001482. • Cunningham, A.P., Barker, P.F., Tomlinson, J.S., 1998. Tectonics and sedimentary environment of the North Scotia Ridge region revealed by side-scan sonar. Journal of Geological Society, 154, 849-862. Esteban, F., Tassone, A., Menichetti, M., Lodolo, E., 2009. Morfoestructuras de las cuencas sedimentarias asociadas al límite de placa Sudamérica-Scotia Atlántico SW. VII Jornadas Nacionales de Ciencias del Mar, Bahía Blanca 30 Nov. – 4 Dec. 2009, ISBN 978987-25479-0-5. • Farran, M., 2006. IMAGE2SEGY: Una aplicación informática para la conversión de imágenes de perfiles sísmicos a ficheros en 211 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA formato SEGY. Geo-Temas 10, pp. 1215-1218. ISSN: 1567-5172. • Fish, P., 2005. Frontier South, East Falkland basins reveal important exploration potential. Oil and Gas Journal, 103, 34–40. • Forsyth, D.W., 1975. Fault plane solutions and tectonics of the South Atlantic and Scotia Sea. Journal Geophysical Research. 80, 1429–1443. • Galeazzi, J.S., 1996. Cuenca de Malvinas. Geología y Recursos Naturales de la Plataforma Continental Argentina, Ramos, V. A., Turic, M. A. (Eds.). XIII Congreso Geológico Argentino y III Congreso de Exploración de Hidrocarburos vol. 15: 273-309. • GEBCO_08 Grid, version 20091120, http://www.gebco.net • Giner-Robles, J.L., González-Casado, J.M., Gumiel, P., Martín-Velázquez, García-Cuevas, C. 2003. A kinematic model of the Scotia plate (SW Atlantic Ocean). Journal of South America Earth Science, 16, 179-191. • Lodolo, E., Menichetti, M., Tassone, A., Sterzai, P., Lippai, H., Hormaechea, J.L., 2002. Researchers Target a continental transform Fault in Tierra del Fuego. EOS Transactions AGU, 83, 1, 1-6. • Lodolo, E., Menichetti, M., Bartole, R., Ben-Avraham, Z. Tassone, A. y Lippai, H., 2003. Magallanes-Fagnano continental transform fault, (Tierra del Fuego, southermost South America). Tectonics, 22(6): 1076, doi: 10.1029/2003TC001500. • Lodolo, E., Donda, F., Tassone, A. 2006. Western Scotia Sea margins: improved constraints on the opening of the Drake Passage. Journal Geophysical Research, 111, B06101, doi:10.1029/2006JB004361. • Kimbell, G.S., Richards, P.C., 2008. The three-dimensional lithospheric structure of the Falkland Plateau region based on gravity modelling. Journal of Geological Society, 165, 4, 795-806. • Menichetti, M., Lodolo, E., Tassone, A., 2008. Structural geology of the Fuegian Andes and Magallanes fold-and-thrust belt – Tierra del Fuego Island. Geologica Acta, 6, 1, 19-42. • Pandey, A., Parson, L., Milton, A., 2010. Geochemistry of the Davis and Aurora Banks: Possible implications on evolution of the North Scotia Ridge. Magine Geology, 26, 1-4, 106-114. • Parker, G., Violante, R.A., Paterlini, M.C., 1996. Fisiografía de la plataforma continental. Geología y Recursos Naturales de la Plataforma Continental Argentina, Ramos, V. A., Turic, M. A. (Eds.). XIII Congreso Geológico Argentino y III Congreso de Exploración de Hidrocarburos vol. 15: 1-16.. • Pelayo, A. M., Wiens, D.A., 1989. Seismotectonics and relative plate motions in the Scotia Arc region, Journal Geophysical Research, 94, 7293 – 7320. • Platt, N.H., Phillip, P.R., 1995.Structure of the southern Falkland Islands continental shelf: Initial results from new seismic data. Marine and Petroleum Geology. 12, 759-771. • Sandwell, D.T., Smith, W.H.F, 2009. Global marine gravity from retracked Geosat and ERS-1 altimetry: Ridge segmentation versus spreading rate. Journal of Geophysical Research, 114, B01411, doi:10.1029/2008JB006008. • Smalley, R., Dalziel, I.W.D., Bevis, M.G., Kendrick E., Stamps, D.S., King, E.C., Taylor, F.W., Lauría, E., Zakrajsek, A., Parra, H., 2007. Scotia arc kinematics from GPS geodesy, Geophysical Research Letters, 34, L21308, doi:10.1029/2007GL031699. • Tassone, A., Lodolo, E., Menichetti, M., Yagupsky, D., Caffau, M., Vilas, J.F., 2008. Seismostratigraphic and structural setting of the Malvinas Basin and its southern margin (Tierra del Fuego Atlantic offshore). Geologica Acta, 6, 1, 55-67. • Thomas, C., Livermore, R., Pollitz, F., 2003. Motion of the Scotia Sea plates. Geophyical Journal International. 155, 789–804. • Yagupsky, D., Tassone, A., Lodolo, E., Vilas, J.F., Lippai, H. 2003. Estudio sismoestratigráfico del sector sudoccidental de la cuenca de antepaís de Malvinas. Margen continental atlántico. Argentina. 10° Congreso Geológico Chileno. 6-10 de octubre. Concepción. Chile: 10 p. En CD de Actas del Congreso. 212 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES GIANT MOUNDED DRIFTS IN THE ARGENTINE CONTINENTAL MARGIN 7-04 Hernández-Molina, F.J.1*, Paterlini, M.2, Somoza, L.3, Violante, R.2, Arecco, M.A.4, De Isasi, M.4, Rebesco, M.5, Uenzelmann-Neben, G.6, Marshall, P.4 (1) Facultad de Ciencias del Mar, Universidad de Vigo, 36200 Vigo, Spain (2) División Geología y Geofísica Marina, Servicio de Hidrografía Naval (SHN), Montes de Oca 2124, Buenos Aires, C1270ABV, Argentina (3) Instituto Geológico y Minero de España (IGME), c/ Ríos Rosas, 23, 28003 Madrid, Spain (4) Argentine National Commission of the Outer Limit of the Continental Shelf (COPLA), Montes de Oca 2124, Buenos Aires, C1270ABV, Argentina (5) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/C, 34010 Sgonico, Italy (6) Alfred Wegener Institute (AWI), Foundation for Polar and Marine Research Geophysics, P.O. Box 12 01 61, Am Alten Hafen 26, 27515 Bremerhaven, Germany * Presenting author’s e-mail: [email protected] Introduction Whilst interaction between down-slope and along-slope sedimentary processes is common over continental margins when along-slope processes dominate, a Contourite Depositional System (CDS) may develop as an association of various depositional features (drifts) and erosive features (Rebesco and Camerlenghi, 2008). The interaction of one or more water masses with a smooth-morphology margin may cause large drifts, but a complex physiography can create multiple vortices associated with each water mass, and both the erosive and depositional features can become difficult to decipher. Some contourite deposits represent giant, mounded elongated contourite drifts (hereafter, giant drifts) extending along large distances, such as those described in the Weddell and Scotia Basins, south-westernmost Indian Ocean and Greenland margin (Rebesco and Camerlenghi, 2008). These giant drifts are generated during a long period of relatively stable hydrological conditions that lead to long-term bottom water flows. Onset of many of these drifts is related to a gateway opening or deepening, owing to long-term plate-tectonic evolution, and/or Fig. 1 - Location of the Argentine Margin, with the regional bathymetric map, study area and the general circulation of surface- and deep-water masses indicated. Simplified hydrographic sections are shown below (modified from Hernández-Molina et al., 2010). 213 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES Fig. 2 - Multichannel seismic reflection profile across the buried giant-drifts, where morphosedimentary features, seismic units (LU, IU & UU) and seismic facies are shown. large-scale palaeoceanographic changes associated with climatic changes. Therefore, giant drifts are essential to understanding major tectonic or palaeoceanographic changes, and consequently, to understand how the bottom circulation and climate were in the past. Moreover, they have great potential for mineral and energy resource exploration, since they are often amenable to generation of poly-metallic nodules and to accumulation of hydrates and free gas (Rebesco and Camerlenghi, 2008). This abstract is base on the recent contribution by Hernández-Molina et al. (2010) and describes the partially buried giant drifts located on the extensional Argentine Continental Margin (Fig. 1), in the southern portion of the South Atlantic Ocean, in an area crucial for geologic and palaeoceanographic reconstruction between the Atlantic and Antarctica. Its genesis and evolution in the Argentine Basin are explained, and its global implications are discussed, primarily based on the bathymetric, multichannel seismic reflections profiles (MCS) and gravimetric broad database (Fig. 1). This margin encompasses the Brazil/Malvinas Confluence (BMC), as well as the interaction of Antarctic water masses (Antarctic Intermediate Water [AAIW], Circumpolar Deep Water [CDW] and Antarctic Bottom Water [AABW]), with the Brazil Current, re-circulated AAIW and North Atlantic Deep Water (NADW) (Piola and Matano, 2001; Carter et al., 2009) at different depths (Fig. 1). The surface circulation around the Argentine margin results from interaction of the Malvinas Current toward the north-northeast with the Brazil Current toward the south-southwest, both of which determine the BMC. The intermediate water masses circulation south of this confluence is conditioned by the circulation toward the north of the Antarctic Intermediate Water (AAIW), and of the two CDW fractions: Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW). Northward of the confluence, apart from the aforementioned water masses, the NADW develops, circulating toward the south (Fig. 1). The deep circulation is caused by the displacement of AABW (Fig. 1), which is partially trapped in the basin, generating a large cyclonic gyre, the influence of which is felt at depths greater than 3500 to 4000 m. This oceanographic regime is clearly significant in controlling sedimentary processes across the entire ocean basin, and particularly on the Argentine margin (Hernández-Molina et al., 2010; Violante et al., 2010). The giant drifts There are two buried giant-drifts within the Argentine CDS, in the transition between the base of the slope and the abyssal plain, at 5300 to 5400 m water depth, south and north of a large seamount (The Austral Seamount). The southern drift is the biggest (ca. 40 to 50 km wide and 250 to 300 km long) and has a sedimentary thickness of 830 to 950 m. The asymmetrical external shape is characterised by a steep west side and a gently-dipping smooth east side, and its local internal reflections prograde eastward (Fig. 2). In contrast, the northern drift is ca. 35 km wide and has a sedimentary thickness of 767 m. Its geometry is opposite to that of the southern zone: its west side is smooth and its east side is steep. Likewise, its internal reflections prograde westward. Three seismic units have been defined and correlated regionally: the Lower Unit (LU, EoceneOligocene boundary / early middle Miocene), Intermediate Unit (IU, middle Miocene) and Upper Unit (UU, late middle Miocene / present day). The giant-drifts developed essentially during deposition of the LU, which exhibits the major progradation stage and local maximum of thickness, and are 214 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA buried by the UU (Fig. 2). The seismic facies of the giant-drifts shows very weak to transparent acoustic response, with discontinuous reflections and abrupt changes in the acoustic facies, indicating a fluidified seismic signature. Some high amplitude reflections with greater lateral continuation reveal that long-term cycles of drape deposition and erosion have combined to form the giant-drifts (Fig. 2). Final considerations Buried, asymmetrical, mounded, elongated contourite drifts are located in the southernmost sector of the extensional Argentine margin, below the presently active Contourite Depositional System (CDS). These giant-drifts have a northerly trend at the base of the slope, at water depths of 5300 to 5400 m. Their summit outcrops at present seafloor, generating a bathymetric jump that represents an important change in the slope gradient trend at the base of the slope. Based on its position, morphology and internal Fig. 3 - Evolutionary sketches for: A) development of the giant drifts characteristics, it has been deduced (LU) and B) the present scenario. Major tectonic and that these giant-drifts were morphosedimentary features as well as the seismic units and generated, in an open deep marine discontinuities are shown. Water masses distribution along the Segment environment, by the AABW, from I and southernmost part of Segment II are indicated in every sketch. UU = upper unit; IU = intermediate unit; and LU = lower unit (from the Eocene-Oligocene boundary Hernández-Molina et al., 2010). until the middle Miocene. The drifts were inferred to record a major palaeoceanographic change between the middle to very late Miocene (Hernández-Molina et al., 2010), when a new oceanographic scenario was established, starting to develop the present day morphosedimentary features of the CDS as a result of the northward flow of the Antarctic water masses. The results presented in this contribution are testament to how large contourite drifts in deep marine environments can yield evidence for reconstructing palaeoceanographic changes, and help to explain Thermohaline Circulation and climate in the past. Acknowledgements This work has been partially funded by the Mobility Award of the Spanish Ministry of Education and Science (PR2007-0138). This work was supported through projects CTM 2008-06399-C04/MAR, CTM2008-06386-C02/ANT, and ANPCyT – PICT 2003 Nº 07-14417. The authors thank COPLA (Argentina) and the BGR (Germany) for allowing us to use their bathymetric and multichannel seismic (MCS) reflections profiles database. 215 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 REFERENCES • Hernández-Molina, M. Paterlini, M., Somoza, L., Violante, R., Arecco, M.A., de Isasi, M., Rebesco, M., Uenzelmann-Neben, G., Neben, S., Marshall, P., 2010. Giant mounded drifts in the Argentine Continental Margin: Origins, and global implications for the history of thermohaline circulation. Marine and Petroulem Geology. , 27: 1508-1530. • Violante, R., Paterlini, C.M., Costa, I.P., Hernández-Molina, F.J., Segovia, L.M., Cavallotto, J.L., Marcolini, S., Bozzano, G., Laprida, C., García Chapori, N., Bickert, T. and Spieß, V. (2010). Sismoestratigrafía y evolución geomorfológica del talud continental adyacente al litoral del este bonaerense. Latin American Journal of Sedimentology and Basin Analysis. In press. • Piola A.R., Matano, R.P., 2001. Brazil and Falklands (Malvinas) Currents. In: Steele, J.H., Thorpe, S.A., and Turekian, K.K. (Eds), Encyclopedia of Ocean Sciences, London, Academic Press, 1: 340 - 349. • Carter, L., McCave, I.N., Williams, M.J.M., 2009. Circulation and Water Masses of the Southern Ocean: A Review. In: Fabio Florindo and Martin Siegert (Eds.), Developments in Earth and Environmental Sciences. Antarctic Climate Evolution, The Netherlands: Elsevier, vol 8: 85–114. PLIOCENE SUBMERGED CRATERS AT THE UPPER CONTINENTAL SLOPE OF MAR DEL PLATA (ARGENTINA) 7-05 Isla, F.1*, Madirolas A.2 (1) Instituto de Geología de Costas y del Cuaternario, CONICET-UNMD, Funes 3350, Mar del Plata, Argentina (2) INIDEP, Escollera Norte s/n, Mar del Plata, Argentina * Presenting author’s e-mail: [email protected] Introduction Bolides, including meteoroids, asteroids or comets (Greeley, 1994; Poag, 1997), used to strike on Earth episodically, although some recurrence has been suggested (Torbett, 989). Considering the present distribution of continents and oceans, there is only 38% of probability to leave a record on a continent. Notwithstanding this probability, many of the craters that stroke on Earth may have disappeared due to erosion, plate consumption at convergent margins, or they can be buried by sediments. In this regard, comparative planetology proposed to search former bolide rains in the closer planets or moons without plate tectonics, or without a liquid cover. On the Earth, crater erosion is more expected at the continents, and sediment burial at the ocean. The continental slope is part of the plate margin dominated by gravity-transport phenomena. Erosion is restricted to the location of submarine canyons where turbidity currents may occur frequently. Sedimentation is more frequent at the bottom of these canyons, and dominantly at the foot of the continental slope. The upper slope (between 100 and 500 m depth) is therefore dominated by sediment transport; and can be considered with less effects of erosion or deposition. a b Fig. 1 - a) MBES record of the major crater found at about 350 m water depth. b) two profiles from the same feature. 216 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA In this paper, we present the morphological evidence supported by swath bathymetry surveys from the upper slope of the continental margin of Mar del Plata, Argentina. These evidences are related to previous geological studies suggesting an impact that have occurred about 3.3 million years ago (Ma). Geological setting The continental shelf of Buenos Aires has been surveyed using highresolution seismic methods and sampling piston cores. The more Fig. 2 - Minor craters detected 10 km to the south of the major crater. recent siliciclastic deposits have been discriminated into 5 depositional sequences, 4 of them were assigned to the Plio-Pleistocene “pampean” series, and the uppermost to the Holocene transgression-regression cycle (Parker et al., 2008). Methods Several legs were performed using a SIMRAD EM1002 multibeam echosounder (MBES) mounted on the R/V Cap. Oca Balda (INIDEP). These legs were composed into detailed bathymetric charts with different colours either for different depths and sound dispersion. One major crater was found with two small craters close to it. From these data, bathymetry allows to analyze the slopes of the walls of the major crater. Results The larger crater is located on the continental slope at 38º 14.7´S and 55º 19.1’W, between 347 and 377 m water depth (Fig. 1a). The wall located at a shallower depth (N-W side) has a maximum slope of 30%, the wall towards the slope (S-E side) is also steep, about 20% (Fig. 2b). The hollow is 30 m depth. The 500 m diameter rim is more evident to the down-slope border. At the bottom of the crater, the diameter is about 50 m. Some km to the south (38º 20´S and 55º 18’ 30”W), other two small craters were surveyed at a greater depth (at about 400 m water depth; Fig. 2). During many years the PlioPleistocene sequence outcropping at the coastal cliffs south of Mar del Plata was known by their escorias content and reddish bricklike remains (Fig. 3). Today, these remains were known as tektites related to an impact that have occurred about 3.3 Ma (Schultz et al., 1998). As the crater has never been found, it was assumed that it might have fallen on the present Fig. 3 - Dispersed escorias, bulk of 0.70 x 0.40 m, with pieces of continental shelf and eroded during reddish bricklike interfingered remains (courtesy L. Cortizo) 217 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA the Quaternary marine transgressions. Discussion and conclusions Although the sizes of these craters could not explain the extension (hundreds of km) of tektite remains (escorias), they are probing a rain of bolides in the region. However, these are the only features of impacts on the region. In this sense, the dating of the tektite layer allows to assign an Upper Pliocene age to these hollows on the upper portion of the Argentine continental slope. We can conclude that: A hollow of 500 m diameter and 30 m depth is found at the upper slope offshore Mar del Plata. The presence of other smaller hollows at the surroundings forced to consider a bolide rains. The evidence of Upper Pliocene tektites in the region led to consider this age for these meteoric impacts. REFERENCES • Greele, R., 1994. Planetary landscapes. Chapman and Hall, New York, 286 pp. • Parker, G., Violante, R. A., Paterlini, C. M., Marcolini, S., Costa, I. P. And Cavallotto, J. L., 2008. Las secuencias sismoestratigráficas del Plioceno-Cuaternario en la plataforma submarina adyacente al litoral del Este Bonaerense. Latin American Journal of Sedimentology and Basin Analysis 15, 2, 105-124. • Poag, C. W., 1997. The Chesapeake Bay bolide impact: a convulsive event in Atlantic Coastal Plain evolution. Sedimentary Geology 108, 45-90. • Schultz, P. H., Zárate, M., Hames, W., Camilión, C. and King, J., 1998. A 3.3-Ma impact in Argentina and possible consequences. Science 282, 2061-2063. • Torbett, M. V., 1989. Solar system and galactic influences on the stability of the Earth. Global and Planetary Change 75, 3-33. THE 2005-2006 EXPERIMENT IN ANTARCTICA WITH MABEL SEAFLOOR MULTIDISCIPLINARY OBSERVATORY 7-06 Marinaro, G.*, Falcone G., Frugoni, F., Favali, P. Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy * Presenting author’s e-mail: [email protected] MABEL (Multidisciplinary Antarctic BEnthic Laboratory) is an Italian PNRA (Programma Nazionale di Ricerche in Antartide) project performed by INGV (Istituto Nazionale di Geofisica e Vulcanologia) in collaboration with AWI (Alfred Wegener Institut, Bremerhaven). MABEL is a deepsea multidisciplinary observatory for long-term autonomous observations. It was deployed by R/V Polarstern on December 5th, 2005 on the seafloor of Weddell Sea (69° 24, 29 S and 5° 32,2 W) at 1884 m w.d. MABEL is able to measure and record autonomously and automatically data for one year with the following instruments: three component broad band seismometer (100 Hz per 3 channel); conductivity, pressure and temperature (CDT, 1 sample hour); light transmissometer (1 data/hour); All these instruments are time-referenced with a high precision rubidium clock. Data acquisition started on December 6th, 2005 and lasted until December 31st, 2006, when the observatory automatically ended acquisition and all instruments were switched off. On December 16th, 2008 MABEL was recovered always using R/V Polarstern and using MODUS (MObile Docker for Underwater Sciences) of the BEUTH (former TFH) and TUB. The status of MABEL after about three years in deep-sea environment was very satisfactory with no significant corrosion. There was no water intrusion inside these vessels and the most important part of MABEL, which is the DACS (Data Acquisition and Control Unit) was found in very good conditions. The data acquisition covered the whole period of the mission, from December 6th 2005 to 218 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Seismograms recorded by Mabel OBS station. Teleseismic event (May 3, 2006; Tonga, Mw=8.0): a) unfiltered raw data; b) bandpass filtered data (0.5-2 Hz) and c) corresponding spectrogram. Ice quake (April 7, 2006) d) unfiltered raw data; e) bandpass filtered data (2-8 Hz) and f) corresponding spectrogram December 31st 2006, as we expected, with a total amount of data of more than 13 GBytes. The continuous recording and acquisition of data from seismometer allows to focus on local activity, also with the integration of the data collected on Neumayer “on-land” seismic network. The teleseismic waveform monitoring will be integrated with the records collected by ASAIN (Antarctic Seismographic Argentinean Italian Network) of the OGS (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale). Acknowledgments We thank PNRA (Programma Nazionale di Ricerche in Antartide) for the support to the project. A particular thank to the other Institutes and Companies participated to the activities: Tecnomare S.p.A. (Francesco Gasparoni, Flavio Furlan) for the engineering of the system; Beuth Hochschule für Technik (BEUTH), Berlin (Hans W.Gerber) for deployment/recovery activities; 219 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Technische Universität Berlin (TUB) (Haiko de Vries) for deployment/recovery activities; Alfred Wegener Institute for Polar Sciences, Bremerhaven (Wilfried Jokat); Istituto Nazionale di Oceanografia e Geofisica Sperimentale (OGS) (Marino Russi); The Captain and the crew of R/V Polarstern for their precious and valuable support; Massimo Calcara, Nadia Lo Bue, Capt. Emanuele Gentile and Marco Lagalante for participation to the cruises. COASTAL EROSION IN MAR DE COBO (BUENOS AIRES PROVINCE) 7-07 San Martín, L.*, Marcomini, S.C., López, R.A. Dpto. de Geología, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina * Presenting author’s e-mail: [email protected] Introduction Mar de Cobo, located in Buenos Aires Province (Fig. 1), is considered to have a high rate of coastal recession, ranging from 3.5 to 6 m/yr according to different authors (Schnack et al., 1983; López and Marcomini, 2002). Values calculated for Mar Chiquita, located 4 km northwards, include 6 m/yr (Isla and Villar, 1992), 7 m/yr (Isla, 1997) and 5.16 m/yr (Merlotto and Bértola, 2007 and 2008). This area is naturally erosive, fact that is strongly reinforced by human activities (Isla and Bértola, 2005; Merlotto and Bértola, 2008). The aim of this study is to characterize the erosion acting in this area trough beach parameters (width and slope) and statistical parameters (average, selection and asymmetry), together with geomorphologic features from beach profiles (abrasion platform, marine abrasion terrace, conservation degree of the coastal ridge, presence of berms or submerged bars). Regarding beach parameters, a wider beach indicates lower vulnerability to erosion, the same as active coastal dunes (Marcomini et al., 2007). What is more, an increase in beach slope also points to beach erosive conditions (Marcomini and López, 1995). Relationships between grain size and erosive or aggrading beaches were considered by Mazzoni and Spalletti (1980), who related a bigger grain size average and a lower selection value to coastal areas in erosive stages. At the same time, they consider this study area to be within a zone with backward movement of the coast line, high energy and strong mechanical activity. In addition to the natural erosive characteristics of this area, already determined by Teruggi et al. (1959) and Spalletti and Mazzoni (1979), protection structures for coastal moderation increases rates of erosion. This structures, although causing the recovery in volume of beach sediments, also generate a loss of saturation downwater in the litoral drift which in turn increases rates of erosion (López and Marcomini, 2002; Isla and Bértola, 2005). At the same time, protection structures located in Mar del Plata have played a unique role in causing induced erosion in this study area (Isla and Villar, 1992; Schnack et al., 1983). Methodology Six beach profiles were analyzed along Mar de Cobo´s shore, and numbered from south to north. The distance between them is about 400 meters. The equipment used for this task was Total Station, which assembles transverse beach profiles from a fixed point, located in the dune, up to the low tide. This profiles provided the information to calculate beach parameters. Parameters used were total beach width and slope and backshore width. Furthermore, we collected samples of sand from the different beach environments in three of those profiles (MC1, MC3 and MC5) in order to perform grain size analysis. The samples were sifted with a Ro-Tap, using half phi divisions. Granus software (Perillo et al., 1985) was used to calculate statistical values (average, selection and asymmetry). 220 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Location map showing the six beach profiles. Results The study of Mar de Cobo site presents both natural and man-made unique characteristics, which made the comparison between profiles difficult. We selected total beach width and back shore width, since they are affected mostly by long term dynamics. Results are shown in Fig. 2, with an important decrease in total beach width from south to north and a local increase in MC4, which is located between two jetties. There is also a slight increase in beach width in MC6 that could be related to the end of the urban section and stabilized dunes. Lower values point towards profiles MC3 and MC5. Backshore width displays much more variability, with null values for MC3 and MC5, therefore represent the most erosive ones. Globally, data follows the same pattern as total beach width. Total beach slope was also taken into account, and it shows the same pattern as total beach width (Fig. 2). That would be an increase in slope values from south to north, showing an increase in erosion, except for MC4 and MC6. Beach slope data also indicates MC3 and MC5 as the most erosive profiles. Regarding grain size, samples from backshore and foreshore are strongly affected by jetties and geomorphologic features in this study area, so they were discarded. Instead, we selected the fore dune ridge since it nourishes from the beach and it would be the less affected. Concerning coastal dune sediment average, there is an important increase in grain size from south to north, from fine to medium sand. Selection is similar in the first two profiles, being moderately good to good, but it turns into poor on the northern profile. In addition, the first two profiles present almost symmetric distributions, but the northern one shows negative asymmetry (Fig. 3). In conclusion, analyzed parameters showed an increase in erosion towards the north. As for geomorphologic features, we studied the presence or absence of different features in the beach. We observed there are no berms in any profile and only a submerged bar in one of them. These characteristics are an erosion-sensitive feature, and their absence indicates erosive conditions in many beach models (López and Marcomini, 2002). Study area has natural erosive features, including a marine abrasion terrace and an abrasion platform. The first one is approximately 400 meters long, has an irregular edge and locates in the foreshore of MC3, and the second one is situated in MC6, is 50 to 60 meters wide and generates a “cape” type irregularity situated in the north extreme of the city. 221 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - Variation in beach parameters through the coast line, from south to north, along the six profiles registered in Mar de Cobo. There is no back shore development in MC3 and MC5. Fig. 3 - Variation of statistical parameters along the beach line: average, selection and asymmetry. Coastal dune is registered in all profiles, even if it is only preserved as a relict nucleus or incipient dune. Every studied profile presents some kind of alteration, which differs from each other. Mostly, the dune is fixated by Carpobrotus edulis and is scarped. In every dune there is also some kind of autochthonous flora, mainly Panicum racemosum, which allows incipient dunes to form (Table 1). MC1 shows a particular feature, with blow out dunes over the coastal dune caused by vehicles. Since fixed dunes increase coastal recession speed compared to active dunes (López and Marcomini, 2002), MC5 would be the most erosive profile because the coastal dune is only composed by isolated Table 1 - Coastal dune characteristics in Mar de Cobo. 222 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA relict nucleus with Carpobrotus edulis and scattered incipient dunes with Panicum racemosum. Conclusions Coastal parameters showed a strong erosive trend that increases to the north in the study area. Geoindicators best reflecting erosion for this area were total beach and backshore width, average and selection. Protective structures for coastal moderation along the shore caused local modifications in erosion and accumulation rates for this village. Higher erosion risk zones are near De Las Torres St. and Del Trabajo St. REFERENCES • Isla, F., 1990. Tendencias litorales y transversales de transporte en playas y boca de marea: Mar Chiquita, Buenos Aires. Revista de la Asociación Argentina de Mineralogía, Petrología y Sedimentología, 21(1): 75-87. • Isla, F., 1997. Seasonal behaviour of Mar Chiquita tidal inlet in relation to adjacent beaches, Argentina. Journal of Coastal Research, 13(4): 1221-1232. • Isla, F. y Bértola, M., 2005. Litoral bonaerense. Relatorio del XVI Congreso Geológico Argentino: Geología y Recursos Minerales de la Provincia de Buenos Aires. pp. 265 – 276. • Isla, F. y Villar, M., 1992. Ambiente Costero. Pacto ecológico. Universidad Nacional de Mar del Plata – Senado de la Provincia de Buenos Aires, La Plata. • López, R. y Marcomini, S., 2002. Pautas para el manejo en costas acantiladas y de dunas, Provincia de Buenos Aires. Revista de Geología Aplicada a la Ingeniería y al Ambiente,18: 59–68. • Marcomini, S. y López, R., 1995. Strategies for the coastal management of Villa Gesell, Argentina. Proc. Int. conf. “Coastal Change 95´ Bordomer-IOC, Bordeaux, p. 819–831. • Marcomini, S., López, R. y Spinoglio, A., 2007. Uso de la morfología costera como geoindicador de susceptibilidad a la erosión en costas cohesivas, Necochea, Buenos Aires. Revista de la Asociación Geológica Argentina, 62(3): 396–404. • Mazzoni, M. y Spalletti, L., 1980. Características sedimentológicas de playas en erosión y en agradación. Revista de la Asociación Geológica Argentina, 23(3): 355–363. • Merlotto, A. Y Bértola, G., 2007. Consecuencias socio-económicas asociadas a la erosión costera en el Balneario Parque Mar Chiquita, Argentina. Investigaciones Geográficas (Esp), Núm. 43, sin mes, 2007, pp. 143-160. Universidad de Alicante, España. • Merlotto, A., Bértola, G., 2008. Evolución urbana y su influencia en la erosión costera en el balneario parque Mar Chiquita, Argentina. Papeles de Geografía, Núm. 47-48, enero-diciembre, 2008, pp. 143-158 Universidad de Murcia España. • Merlotto, A., Verón., E., Sabula, F., 2008. Riesgo de erosión costera en el Balneario Parque de Mar Chiquita. Párrafos Geográficos (7)1: 103-121. • Perillo, G., Gómez, E., Aliotta, S. y Galíndez, D. 1985. Granus: un programa FORTRAN para el análisis estadístico y gráfico de muestras de sedimentos. Revista Asociación Argentina Mineralogía, Petrología y Sedimentología, 16(1-4): 1-5. • Schnack, E., Álvarez, J. y Cionchi, J., 1983. El carácter erosivo de la línea de costa entre Mar Chiquita y Miramar, Provincia de Buenos Aires. Simposio Oscilaciones del nivel del mar durante el último hemiciclo deglaciar en la Argentina, INQUA, Mar del Plata. Actas pp. 118–130. • Spalletti, L. y Mazzoni, M., 1979. Caracteres granulométricos de arenas de playa frontal, playa distal y médano del litoral bonaerense. Revista de la Asociación Geológica Argentina, 34(1): 12 – 30 • Teruggi, M., Chaar, E., Remiro, J. y Limousin, T., 1959. Las arenas de la costa de la provincia de Buenos Aires entre Cabo San Antonio y Bahía Blanca. LEMIT serie II, 77, 1-54. CONDITIONING FACTORS AND RESULTING MORPHOSEDIMENTARY FEATURES IN THE UPPER-MIDDLE CONTINENTAL SLOPE OFFSHORE EASTERN BUENOS AIRES PROVINCE, ARGENTINA 7-08 Violante, R.A.1*, Paterlini, C.M.1, Hernández Molina, F.J.2, Bozzano, G.1, Pastor Costa, I.1, Marcolini, S.1 (1) División Geología y Geofísica Marina, Departamento Oceanografía, Servicio de Hidrografía Naval, Av. Montes de Oca 2124, Buenos Aires C1271ABV, Argentina (2) Facultad de Ciencias del Mar, Universidad de Vigo. 36200, Vigo, Spain * Presenting author’s e-mail: Roberto A. Violante, [email protected]. Introduction The morphosedimentary characteristics of the Argentine Continental Margin are dominated by interaction between alongslope and downslope processes, which gave rise to a complex sedimentary 223 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA system composed of contourites, turbidites and debris flows deposits. The main sedimentary body was defined as a Contourite Depositional System (CDS, Hernández Molina et al., 2009), genetically related to termohaline circulation, which affects sedimentation and induces both erosive and depositional action on the slope surface associated to the interfaces of the Antarctic water masses, particularly the Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW). The CDS is better developed offshore the Patagonian region where the unidirectional south-to-north oceanic circulation is clearly manifested. In the northern part of the slope adjacent to eastern Buenos Aires province, the progressive northward decreasing transport capacity of the Antarctic-sourced waters, together with their complex interaction with North Atlantic water masses and the oceanic dynamic imposed by the Confluence Zone, influences the regional circulation. In addition, local downslope processes associated to shelf-originated sediment supply and transport across the shelfslope transition, as well as slope-originated gravitational processes, are masking the regional alongslope processes. As a result, the CDS and the related features loose there their typical configuration. Recent research was focused on the eastern Buenos Aires continental slope between 37-39ºS, where detailed bathymetric as well as mono and multichannel high resolution seismic surveys were performed, and sediment cores were recovered, during two campaigns carried out in 2009: M78/3 (R/V Meteor, Universities of Bremen and Kiel, Germany) and Litoral Bonaerense IV (LBIV, R/V Puerto Deseado, Argentina Hydrographic Survey). Complementary data were gathered from former cruises (R/V Meteor: M29/2 1994, M46/3 2000 and M49/2 2001; and R/V Puerto Deseado: Fisiografía 1996, Litoral Bonaerense III 2000, Prueba de Coring 2001, Coring 2002 and Pistón II 2004). Additionally, seismic data from Lamont-Doherty Earth Observatory (Columbia, USA) and Federal Institute of Geosciences and Natural Research (Hannover, Germany) were used. The analyzed information, which reaches a total of around 6600 km of seismic and bathymetric lines, allowed to define the margin architecture, major sedimentary structures and sedimentary bodies’ configuration. Around 90 piston and gravity cores were available for studing the sedimentary characteristics. This contribution deals with the description of the eastern Buenos Aires province continental slope as well as the definition of the conditioning factors involved in the evolution of the main features and depositional processes. Major morphosedimentary features The major morphosedimentary features are (Fig. 1): continental shelf, continental slope, continental rise and Mar del Plata submarine canyon (Hernández Molina et al., 2009; Violante et al., 2010). The Continental slope extends between 120 m and 3500 m water depth (shelf break and slope-rise transition respectively). It is constituted by three lower-order features: upper, middle and lower slope. The upper slope extends between the shelf break and 700-800 m depth and is characterized by a steep gradient. Upslope the Mar del Plata canyon´s head, the upper slope reaches depths not deeper than 500 m as its foot is locally affected by gravitational erosive processes and consequent erosional retreat. The middle slope extends downslope reaching around 1300 m depth and is regionally formed by a low-gradient terraced surface named Ewing Terrace (ET), which is the morphological expression of the CDS and associated erosive processes resulting from the dominating oceanographic conditions; it is characterized by a sub-horizontal, slightly concave configuration in a transverse (WNW-SSE) section with a longitudinal, alongslope mounded fringe at its outermost (seaward) side. A particular feature in the region is another terrace, or wedge-shaped fan-like body (WF), which reaches depths of 1100-1200 m and partially covers and distorts the western side of the Ewing Terrace. WF is preliminary associated to deposition from the shelf as it is closely related to an incision that cuts the shelf-upper slope at 38º50´S, although later redistribution by alongshore currents is also evident; however, this feature is not well known yet and needs more detailed studies in order to define its origin and evolution. Along the boundary between ET and WF, an alongslope, northwards flowing longitudinal channel extends, which begins at around 900 m depth, flows towards the Mar del Plata canyon and meets it at around 1300 m depth. At depths below 1300 m, the lower slope extends with a steep slope reaching around 3500-3700 m depth from where it grades offshore to the continental rise. The Mar del Plata submarine canyon dissects the continental slope at around 38ºS and 224 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Location map, detailed bathymetry and seismic example of part of the offshore area of the Buenos Aires province. represents the major sediment pathway towards the base of the slope. Sedimentary processes and deposits The dynamic and oceanographic complexity of the region produces diverse sedimentary processes which interact each other, giving rise to a variety of morphological features and sedimentary deposits. The dominant regional sedimentary body in the middle slope is the CDS, which constitutes most of the Ewing Terrace and is evidenced in the seismic records by agradational and progradational configurations, usually mounded-shaped, up to 300-400 m thick, with high amplitude internal reflections. Several minor internal unconformities are present, what allows to differentiate several evolutive stages. These contouritic bodies are being formed since middle Miocene (Hernández Molina et al., 2009, Violante et al., 2010), according to the age of the basal seismic horizon AR5 (Hinz et al., 1999). Contouritic sediments recognized in cores have thicknesses up to 7 m and are mainly composed of dark olive gray very fine sands with a “wet” appearance (Bozzano et al., 2010). Turbidites and debris flows deposits constitute very important components of the sedimentary sequences. Seismic records show large bodies of these deposits up to several tens thick, constituted by stratified configurations for the turbidites and chaotic for the debris flows. Sedimentary records observed in cores usually do not exceed a thickness of 10 cm for each single turbidite unit, which are represented by well defined layers of black fine sands showing a marked stratified and upward decreasing grain-size distribution, most of the times with sharp basal contacts with underlying sediments and gradual transition to the overlying sediments. On the other hand, debris flows are represented by chaotic accumulations of muddy pebbles included in a sandy-muddy matrix. It is 225 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA common to find a sequence of basal debris flows grading to turbidites in an upward transition. The presence of large blocks of rocks on the seabed at the canyon´s head and main channel reveals the occurrence of active high-energy processes, sometimes related to collapse of the canyon´s walls. In this sense, the structural, dynamic, sedimentary and oceanographic scenario around the canyon is compatible with the occurrence of significant bottom instabilities manifested by submarine landslides and other gravitational processes (Krastel et al., 2010). A significant, locally extended terraced feature is the wedge-shaped fan-like body, which is preliminary considered as formed by sediment transport from the shelf via a channel incised at the shelf-slope transition and consequent deposition at the foot of the upper slope. It develops above the Ewing Terrace and partially covers its western part, showing a marked asymmetrical, northwardelongated shape (Fig. 1). This geometry indicates a net alongshore drifting to the north, suggesting that sediments are being largely incorporated to the contourite system as shown by the contouritic character found in sediments contained in cores obtained on the fan. Although further detailed studies are necessary on the WF, preliminary interpretation of seismic sections indicates that it could have begun to be formed since Pliocene-Quaternary times, probably associated to higher sediment supply from the shelf during lowstands periods. Final remarks Three main sedimentary processes are significant in the shaping of the region: 1) those related to contouritic (alongslope) deposition/erosion on the middle slope driven by contourite currents originated by the activity of the AAIW, which is the major morphosedimentary factor involved in the formation and evolution of the Ewing Terrace; 2) those related to turbiditic-debris flows-submarine landslides (downslope) deposition which are important in the steeper slopes, like the upper and lower slope as well as the surroundings of the Mar del Plata submarine canyon; 3) those related to the formation of a wedged fan body originated at the shelf break that was accumulated above the western part of the Ewing Terrace primarily by gravitational sediment deposition and later reworking by contour currents. The interaction among the regional contouritic processes and the local gravitational processes occurring in the shelf, slope and submarine canyon, allowed the region to acquire very complex morphosedimentary configurations. REFERENCES • Bozzano, G., Violante, R.A., Paterlini, C.M., Hernández-Molina, F.J, Hanebuth, T., Huppertz, T., Orgeira, M.J. and Krastel, S. (2010). Depositional pattern of contourite facies on the terraced slope off Buenos Aires province (NE Argentina, SW Atlantic): a sedimentological approach. International Congress “Deep-Water Circulation: Processes and Products”, Baiona, Pontevedra, Spain, Abstracts: 41-42. • Hernández-Molina, F.J., Paterlini, C.M., Violante, R.A., Marshall, P., de Isasi, M., Somoza, L. and Rebesco, M. (2009). Contourite Depositional System in the Argentine Margin: an Exceptional Record of the Influence and Global Implications of Antarctic Water Masses. Geology, 37 (6): 507-510. • Hinz, K., Neben, S., Schreckenberger, B., Roeser, H.A., Block, M., Goncalves de Souza, K. and Meyer, H. (1999) The Argentine continental margin north of 48º S: sedimentary successions, volcanic activity during breakup: Marine and Petroleum Geology, 16: 1-25. • Krastel, S., Freudenthal., T., Hanebuth, T., Preu, B., Schwenk, T., Strasser, M., Violante, R.A., Wefer, G. and M78-3 shipboard scientific party (2010). Sediment Dynamics and Geohazards offshore Uruguay and Northern Argentina: first results from the multidisciplinary Meteor-Cruise M78-3. 18th Meeting of Swiss Sedimentologists, Fribourg, Swiss, Abstract volume. • Violante, R.A, Paterlini, C.M., Costa, I.P., Hernández-Molina, F.J., Segovia, L.M., Cavallotto, J.L., Marcolini, S., Bozzano, G., Laprida, C., García Chapori, N., Bickert, T. and Spieß, V. (2010). Sismoestratigrafía y evolución geomorfológica del talud continental adyacente al litoral del este bonaerense. Latin American Journal of Sedimentology and Basin Analysis, 17 (1), MS188. . 226 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA CENOZOIC GROWTH PATTERNS AND PALEOCEANOGRAPHY OF THE OCEAN BASINS NEAR THE SCOTIA-ANTARCTIC PLATE BOUNDARY 7-09 Maldonado, A.1*, Bohoyo, F.2, Galindo-Zaldívar, J.1,3, Hernández-Molina, F.J.4, Lobo, F.J.1, Martos-Martin, Y.1, Schreider, A.A.5 (1) Instituto Andaluz Ciencias de la Tierra (IACT). CSIC/Universidad Granada. 18002 Granada, Spain (2) Instituto Geológico y Minero de España (IGME), Ríos Rosas, 23, 28003 Madrid, Spain (3) Departamento de Geodinámica, Universidad de Granada. 18071 Granada, Spain (4) Facultad de Ciencias del Mar, Universidad de Vigo, 36200 Vigo, Spain (5) P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences. 23 Krasikova 117218 Moscow, Russia *Presenting author’s email: [email protected] The tectonics and distribution of seismic units of the southwestern Scotia Sea are described based on multichannel seismic profiles, swath bathymetry and magnetic anomalies. Recently acquired profiles suggest that spreading of the Drake Passage was active prior to the Eocene/Oligocene boundary and that gateways may have connected the Pacific and Atlantic oceans allowing a restricted circumpolar current. After the initial breakup the Scotia Sea resulted from several spreading centers that developed deep oceanic basins. The three youngest units identified in the Cenozoic deposits exhibit similar seismic facies and are correlated at regional scale. The deposits show a variety of contourite drifts that resulted from the interplay between the northeastward flows of Weddell Sea Deep Water (WSDW), the Antarctic Circumpolar Current (ACC) and the complex bathymetry. Introduction A host of ocean basins and migrating spreading centers, with intervening banks of continental slivers, were active along the Antarctic and the South American plates between the Oligocene and the present (Fig. 1). The result of this tectonic evolution was the opening of the Drake Passage and the creation Fig. 1 - Regional map showing the location o the BIO HESPERIDES profiles collected in the Scotia Sea and Weddell Sea during the SCAN cruises. 227 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 2 - Map showing the tectonics and the location o the BIO HESPERIDES profiles of the southern Drake Passage (modified from Aldaya and Maldonado, 1996). of the Scotia Sea, one of the most important Cenozoic features of the Southern Ocean (Bohoyo et al., 2007).The opening of Drake Passage during the Cenozoic created the final gateway for a continuous deep-water circulation around Antarctica (Livermore et al., 2004; Maldonado et al., 2006). The gateway allowed the instauration of the Antarctic Circumpolar Current (ACC), which is proposed to have profound effects on paleoceanography, the evolution of the Antarctic climate and the beginning of the north-south ocean circulation patterns. We describe the correlation of the basin depositional units and show how their evolution was influenced by tectonics, which controlled the opening and closing of gateways modifying global paleoceanographic events with remark impact on erosion and depositional processes. Methods Multichannel seismic reflection profiles and magnetic anomalies, complemented with high resolution TOPAS profiles, swath bathymetry and gravity data that we collected in the area during seven oceanographic cruises with the BIO HESPERIDES allow us to establish the main tectonic events and the geodynamics of the area (Figs. 1, 2). High resolution subbottom profiles were obtained with a Topographic Parametric Sonar (TOPAS) Konsberg Simrad PS018. The swath bathymetric data were obtained with a SIMRAD EM 12 system and post-processed with NEPTUNE software and FLEDERMAUS for visualization. Total intensity magnetic field data were recorded every 5 s with a Geometrics G-876 proton precession magnetometer along the ship track lines. Gravity data were acquired with a Bell Aerospace TEXTRON BGM-3 marine gravimeter. The ship tracks cover the study area reasonably well, although the profiles are widely spaced. The stratigraphic analysis and the regional distribution of depositional units and discontinuities in the area were complemented, however, with additional MCS profiles acquired in previous cruises by Italian, 228 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 3 - Representative multichannel seismic profile, and line drawing interpretation with the magnetic anomalies across the southwestern Scotia Sea Russian and Spanish institutions. The age of the main seismic units was tentatively calculated on the basis of: (a) the age of the igneous basement provided by the magnetic anomalies, (b) the total thickness of the depositional sequence for selected stratigraphic sections, (c) the sedimentation rate of surface sediment cores, and (d) the results of ODP borehole sites in the area. Basin age and seismic stratigraphy The oldest magnetic anomaly previously reported in the southwestern Scotia Sea is chron C10, (Lodolo et al., 1997), whereas our recently acquired magnetic profiles in the area show up to chron C12n (30.5-31 Ma). Spreading of the Drake Passage was active prior to 31 Ma, although older magnetic anomalies were, moreover, identified during the SCAN 2008 cruise southwestward of Terror Rise. The distribution of deep basins in the southwestern Scotia Sea suggests an initial phase of diffuse spreading and that rifting of the margins and shallow seaways between the Antarctic Peninsula and South America existed prior to the Eocene/Oligocene boundary (Fig. 2). The development of Protector Basin is well constrained by the seafloor magnetic anomalies (14.0-14.4 to 17.6 Ma), whereas the magnetic anomalies of the central Scotia Sea indicate an age of spreading between 20.7 and 14.2 Ma (Maldonado et al., 2003, 2006; Bohoyo et al., 2007). Six main seismic units are identified regionally, although locally older units may exist. The distribution and seismic features of these deposits vary in relation to the bottom topography, which significantly influenced the distribution of bottom flows (Maldonado et al., 2003, 2006). In the unconfined setting of the abyssal plain, the types of contourite drifts are determined by the interplay of strong currents shearing along the margins of submarine banks and the basement disruptions of the sea floor. The sheeted drifts dominate in the abyssal plain, but along the margins of the banks slope plastered and giant elongatedmounded drifts are developed by the currents. The units are bounded by high-amplitude continuous reflectors, named a to d from top to bottom. The three older units are of different age and seismic facies in each basin and they generally correspond to the syn-drift deposits. The three youngest units (3 to 1) exhibit, in contrast, rather similar seismic facies and can be correlated at a regional scale. The contourite drifts that resulted from the interplay between the northeastward flows of the Weddell Sea Deep Water (WSDW), the Antarctic Circumpolar Current (ACC) and the complex bathymetry. Reflector c (~12.1-12.6 Ma) can be correlated basin.wide and it is coeaval with the timing of 229 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA connection between the Scotia Sea and the Weddell Sea (Maldonado et al., 2006). Unit 3 (~Middle to Late Miocene) shows the initial incursions of the WSDW into the Scotia Sea, which influenced a northward progradational pattern, contrary to the underlying deposits. The tentative age calculated for Reflector b is coincident with the end of spreading in the West Scotia Ridge (~6.4 Ma). The deposits of Unit 2 (~Late Miocene to Early Pliocene) have abundant high-energy, sheeted deposits in the northern Weddell Sea, which may reflect a higher production of WSDW as result of the advance of the West Antarctic ice-sheet onto the continental shelf. Reflector a represents the last major regional paleoceanographic change. The timing of this event (~3.5-3.8 Ma) coincides with the end of spreading in the Phoenix-Antarctic ridge, but it may be also correlated with global events such as the initiation of the permanent northern Hemisphere ice-sheet and a major sea level drop. Unit 1 (~Late Pliocene to Recent) is characterized by high-energy contourite deposits, which suggest intensified deep water production. Units 1 and 2 show, in addition, a cyclic pattern, more abundant wavy deposits and the development of internal unconformities, all of which attest to alternative periods of increased bottom current energy. The older units of the isolated basins are generally affected by a northward thrusting below the margin of the South Scotia Ridge, and a portion of the oldest crust was probably consumed by the subduction processes (Fig. 3). These data bear evidences for an earlier than previously postulated opening of a full circum-Antarctic gateway through Drake Passage, which at least allowed an eastward circulation throughout the gateway of the superficial and intermediate water masses. Discussion and conclusions The Earth’s climate experienced a major change near the Eocene-Oligocene boundary, but whether it can be explained strictly as a result of the opening of southern latitude oceanic gateways, or attributed to changes in atmospheric CO2 concentrations, or is in fact the result of multiple causes, is a subject of debate (Lawver and Gahagan, 2003; DeConto and Pollard, 2003; Livermore et al., 2004). The new magnetic anomalies indicate the development of oceanic crust in Drake Passage and that an oceanic gateway existed between South America and the Antarctic Peninsula prior to 30.9 Ma. Taking into consideration the timing for breakup and the tectonics of the area, a gateway may have developed in Drake Passage before the Eocene/Oligocene boundary. The ridges and basins that were active during the early stages in the evolution of the Scotia Sea controlled the development of the Antarctic Circumpolar Current and the deep water flows (Maldonado et al., 2003). The major regional unconformity represented by Reflector c seems coeval with a major Miocene glaciation (Mi4), a lowering of sea level (Ser3) and with the initiation of the permanent East Antarctic ice-sheet. This reflector suggests a major event in the dynamics of bottom water circulation, which would represent the connection between the Scotia Sea and the Weddell Sea across the South Scotia Ridge. The Oligocene glaciers of Antarctica were isolated and a West Antarctic ice-sheet that advanced onto the continental shelf did not develop until the Late Miocene (Miller et al., 2009), which seems to be recorded by Reflector b and may also be coincident with the end of spreading at the West Scotia Ridge. It has been proposed that a major factor for the present global ocean circulation and the Late Pliocene formation of the West Antarctic and Northern Hemisphere ice-sheets was the closure of the Isthmus of Panama, at about 3.7-3.0 Ma ago (Lawver and Gahagan, 2003). The coincidence between the ages calculated for Reflector a and the timing of closure of the Panamanian seaway is remarkable. Spreading ended almost coetaneous at the Phoenix Ridge, however, which may also had a more significant influence on the paleoceanography of the area by modifying the deep water flows through significant changes in the sea bottom topography of Drake Passage. The ridges and basins that were active during the early stages in the evolution of the Scotia Sea controlled the development of the Antarctic Circumpolar Current and the deep water flows and they have, hence, a profound influence on paleoceanography and climate. Acknowledgements Spanish Comisión Interministerial de Ciencia y Tecnología (CICYT) supported this research through Projects CGL2004-05646; POL2006 13836/CGL and CTM2008-06386-C02/ANT.. 230 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA REFERENCES • Aldaya, F. and Maldonado, A.(1996): Tectonics of the triple junction at the southern end of the Shackleton Fracture Zone (Antarctic Peninsula). Geo-Marine Letters, 16: 279-286 • Bohoyo, F., Galindo-Zaldívar, J., Jabaloy, A., Maldonado, A., Rodríguez-Fernández, J., Schreider, A. and Suriñach, E. (2007): Extensional deformation and development of deep basins associated with the sinistral transcurrent fault zone of the Scotia-Antarctic plate boundary.. Geological Society, London, Sp. Pub, 290: 203-217. • DeConto, R. M., and D. Pollard (2003), Rapid Cenozoic glaciation of Antarctica induced by declining atmospheric CO2, Nature, 421, 245–249. • Lawver, L. A., and L. M. Gahagan (2003), Evolution of Cenozoic seaways in the circum-Antarctic region, Palaeogeogr., Palaeoclim., Palaeoecol., 198, 11–37. • Livermore, R., Eagles, G., Morris, P. and Maldonado, A. (2004): Shackleton Fracture Zone: No barrier to early circumpolar ocean circulation. Geology, 32: 797-800. • Lodolo, E., Coren, F., Schreider, A.A. and Ceccone, G., (1997): Geophysical evidence of a relict oceanic crust in the South-western Scotia Sea. Mar. Geophys. Res. 19, 439-450. • Maldonado, A., A. Barnolas, F. Bohoyo, J. Galindo-Zaldívar, J. Hernández-Molina, F. Lobo, J. Rodríguez-Fernández, L. Somoza, and J. T. Vázquez (2003): Contourite deposits in the central Scotia Sea: the importance of the Antarctic Circumpolar Current and the Weddell Gyre flows, Palaeogeogr., Palaeoclim., Palaeoecol., 198, 187–221. • Maldonado A., Bohoyo F., Galindo-Zaldívar J., Hernández-Molina F.J., Javaloy A., Lobo F.J., Rodríguez-Fernández J., Suriñach E. and Vázquez J.T. (2006): Ocean basins near the Scotia–Antarctic plate boundary: Influence of tectonics and paleoceanography on the Cenozoic deposits. Marine Geophysical Researches, 27 (2): 83-107. • Miller, K.G., Wright, J.D., Katz, M.E., Browning, J.V., Cramer, B.S., Wade, B.S. and Mizintseva, S.F. (2008): A view of Antarctic ice-sheet evolution from sea-level and deep-sea isotope changes during the Late Cretaceous-Cenozoic. In: Cooper, A. K., P. J. Barrett, H. Stagg, B. Storey, E. Stump, W. Wise, and the 10th ISAES editorial team, eds. (2008). Antarctica: A Keystone in a Changing World. Proceedings of the 10th International Symposium on Antarctic Earth Sciences. Washington, DC: The National Academies Press, pp: 55-70. 231 Session 8 OIL AND MINERAL RESOURCES GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA OCCURRENCE OF SHALLOW GAS IN THE EASTERNMOST LAGO FAGNANO (TIERRA DEL FUEGO) 8-01 Darbo, A.1, Baradello, L.1, Lodolo, E.1, Grossi, M.1, Tassone, A.2, Lippai, H.2 (1) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy (2) Instituto de Geofísica “D. Valencio”, Dpto. de Geologia, Universidad de Buenos Aires, Argentina High-resolution seismic profiles acquired on November 2009 in the Lago Fagnano (Tierra del Fuego) have shown the presence of shallow gas layers in the south-easternmost sector of this basin. Data have been acquired in the frame of an Italian-Argentinean scientific project funded by the Italian Foreign Ministry. The survey consisted of single-channel seismic reflection profiles acquired using a Boomer source and a single-channel streamer (10-hydrophones selectable array). These data complement and complete the bathymetric and seismic surveys carried out previously in the Lago Fagnano (Lodolo et al., 2007; Waldmann et al., 2010). Moreover, during this Campaign, some gravity piston cores have been collected to analyze the stratigraphy of the most recent (Middle to Late Holocene) sedimentary cover in the eastern sector of the basin. The high-resolution seismic investigation has revealed an extensive area marked by poor seismic penetration that is caused by the presence of shallow gas (Fig. 1). The gas-related features observed on the seismic profiles include typical acoustic turbidity with a strong phase reversal reflector on top that creates multiple reflections (Best et al, 2004) . The gassy sediments exhibit high attenuation (blanking) that hide geological sub-surface structures. The lake-floor morphology does not reveal any evidence of clear gas escape from the floor. The top of the acoustically turbid layer is located between 0-1 and 7-10 m below the lake-floor surface. It generally forms a sharp boundary, often marked by a varying offset probably due to different levels of gas penetration which could be related to the lithology (poorly consolidated muddy layers) of the overlying sediments. Shallow gas horizons are all located in the south-eastern sector of Lago Fagnano where water depths vary from 20 to 50 m and where a presence of a ground moraine in the vicinity of the lake shore is reported (Coronato et al., 2009). This geographical distribution may be in some ways conditioned by shallow structural lineaments associated to the left-lateral transform system which separates the continental South American plate from the Scotia plate (Lodolo et al., 2003; Menichetti et al., 2008). Lago Fagnano itself occupies a segment of the transform system, and is considered an example of pull-apart basin developed in a series of graben-shaped, asymmetrical tectonic sinks disposed in an en-echelon arrangement along the transform boundary (Lodolo et al., 2002; Lodolo et al., 2003). The tectonic structure of the Lago Fagnano formed presumably during the Paleogene and was subsequently modified by glacial erosion, especially during Late Quaternary (Menichetti et al., 2007). Seismic characteristics of the profiles where shallow gas layers have been individuated seem to suggest a low concentration of gas, most likely less than 1%. To confirm the actual presence of gas, some gravity coring have been performed in correspondence of the high-resolution seismic profiles Fig. 1 - Lago Fagnano location map (left); map of the shallow gas occurrence with the grid of the high-resolution seismic profiles acquired in the easternmost sector of the basin (middle);example of a high-resolution line showing the presence of a shallow gas layer characterized by a blanking effect (right). 235 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA in both places where the blanking effect was most relevant, and in areas where shallow gas was not evident. These cores were then analyzed in laboratory in order to quantify and define the type of gas. Gas was not sampled from the cores, mostly because it was almost completely released in the lake water during the core recovery. It can be thought of as the reduction of hydrostatic pressure from 40 m depth at which the samples have been retrieved to atmospheric pressure, has favoured the immediate volatilization of gases in the water column. Further laboratory analyses will be carried on the recovered sediments to eventually detect the presence of heavier gases. As a preliminary interpretation, we may assume that the main origin of the gas could to some extent be linked to the presence of a shallow, thin peat-rich layer of Middle-Late Holocene age. To date, this is the first evidence of shallow gas layers in Tierra del Fuego lakes. REFERENCES • Best, A.I., Tuffin, M.D.J., Dix, J.K. and Bull, J.M. (2004). Tidal height and frequency dependence of acoustic velocity and attenuation in shallow gassy marine sediments. Journal of Geophysical Research, 109, (B8), B08101. • Coronato, A., Seppala, M., Ponce, J.F., Rabassa, J. (2009). Glacial geomorphology of the Pleistocene Lake Fagnano ice lobe, Tierra del Fuego, southern South America. Geomorphology, 112, 67-81. • Lodolo, E., Menichetti, M., Tassone, A., Geletti, R., Sterzai, P., Lippai, H. and Hormaechea, H-L. (2002). Researchers target a continental transform fault in Tierra del Fuego. EOS, Trans., AGU, 83, 1-5. • Lodolo, E., Lippai, H., Tassone, A., Zanolla, C., Menichetti, M., Hormaechea, J. L. (2007). Gravity map of the Isla Grande de Tierra del Fuego, and morphology of Lago Fagnano. Geologica Acta, 4, 307-314. • Lodolo, E., Menichetti, M., Bartole, R., Ben-Avraham, Z., Tassone, A., Lippai, H. (2003). Magallanes-Fagnano continental transfom fault ( Tierra del Fuego, southernmost South America). Tectonics, 6, 1076. • Menichetti, M., Lodolo, E., Tassone, A., Hormaechea, J. L., Lippai, H. (2007). Geologia dell’area del Lago Fagnano in Terra del Fuoco (Sud America). Rend. Soc. Geol. It., 4, 251-254. • Menichetti, M., Lodolo, E., Tassone, A. (2008). Structural geology of the Fuegian Andes and Magellanes fold-and-thrust beld Tierra del Fuego Island. Geologica Acta, 1, 19-42. • Waldmann, N., Ariztegui, D., Anselmetti, F.S., Austin, J.A., Moy, C.M., Stern, C., Recasens, C., Dunbar, R.B. (2009). Holocene climatic fluctuations and positioning of the Southern Hemisphere westerlies in Tierra del Fuego (54°S), Patagonia. Journal of Quaternary Science, doi: 10.1002/jqs.1263. GEOLOGY OF THE SAN PEDRO MINING DISTRICT, SAN RAFAEL MASSIF (ARGENTINA) 8-02 Gómez, A.1*, Rubinstein, N.2 (1) CONICET. Departamento de Geología, Ciudad Universitaria, Buenos Aires (2) CONICET-Universidad de Buenos Aires. * Presenting author’s email: [email protected] Introduction The San Pedro mining district is located in the central part of the San Rafael Massif (35º 21’ 58.6” S; 68º 23’ 22” W), province of Mendoza, Argentina. The San Rafael Massif is characterized by widespread volcanic and pyroclastic rocks of Gondwanian age, known as Choiyoi Magmatic Cycle. Two different suites can be distinguished within this volcanic sequence (Llambías et al., 1993). The Lower Permian suite (lower section) has geochemical characteristics that indicate a subduction setting and a transpressional deformation style. The Upper Permian suite (upper section) has a geochemical signature which can be interpreted as transitional between subduction and continental intraplate settings and a structural style typical of an extensional regime (Kleiman and Japas, 2009). New information provided by fieldwork and petrographic studies allow redefining the hydrothermal alteration assemblages and mapping the alteration zones. Geological setting In the studied area the Choiyoi Magmatic Cycle is represented by the lower section which includes 236 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Geology of the San Pedro mining district, showing the location of alteration zones and the polymetallic veins outcropping in the area. pyroclastic rocks intruded by a sub-volcanic intrusive (San Pedro Hill) cut by andesitic dykes. The whole Permian sequence is overlain by Quaternary alkaline basalts (Fig. 1). The pyroclastic rocks correspond mainly to a moderately welded massive ignimbrite of dacitic composition that in the upper part of the deposit shows fine lamination and strong oxidation. It is composed of quartz, feldspars and micas crystaloclasts and accessory and cognates lithic fragments in a felsitic matrix with recrystallized shards and fiamme and scarce disseminated pyrite. In the northwestern part of the studied zone a pyroclastic breccia (probably a “block and ash” deposit) crops out, and is composed of andesitic clasts up to 10 cm in length bounded by curviplanar surfaces immersed in a very fine andesitic matrix. The sub-volcanic intrusive has a porphyritic to granular texture and quartz-dioritic composition. It is composed of plagioclase and minor clinopyroxene with scarce reddish brown biotite, pale green amphibole and interstitial K-feldspar and quartz (occasionally conforming graphic intergrowth) and abundant disseminated magnetite and pyrite crystals. In the western contact with the dacitic ignimbrite it produced a poorly sorted volcanic breccia composed of ignimbritic clasts with ameboid shape. 237 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 Fig. 2 - Potassic alteration veins: a) Quartz veins with K-feldspar alteration halos. b) Quartz -magnetite veins. The San Pedro Mining district This mining district is hosted by the Permian pyroclastic and volcanic rocks of the area and consists of an alteration zone with disseminated copper mineralization and a group of polymetallic veins that were partially mined during the end of the XIX century and the beginning of the XX century (Salazar, 1974). It was preliminarily classified as a porphyry copper deposit (Delpino et al., 1993; Rubinstein et al., 2002). Later fluid inclusions studies (Korseniewsky and Rubinstein, 2005) reveal the presence of high salinity and high temperature inclusions with daughter crystal (including hematite and opaque minerals) supporting the proposed genetic model. Based on stratigraphic constraints, Rubinstein et al., (2002) suggested that the polymetallic veins are genetically related to the porphyry copper system. Lead isotopes analyses confirmed the genetic link between this mining district and the lower section of the Choiyoi Magmatic Cycle (Rubinstein et al., Table 1 - Ore paragenesis and hydrothermal alteration assemblage for the principal veins of Cerro San Pedro (modified from Rubinstein et al., 2002). ag: silver; ap: apatite; az: azurite; bn: bornite; cb: carbonate mineral; cerussite: cer; cct: chalcocite; ccp: chalcopyrite; chal: chalcanthite; chl: chlorite; ccl: chrysocolla; cv: covellite; gn: galena; gp: gypsum; hem: hematite; ilt: illite; lm: limonites; mlc: malaquita; mol: molibdenita; py: pyrite; qtz: quartz; ser: sericite; rt: rutilo; sp: sphalerite; str: stromeyerite tnt: tennantite. Veins Trend and dip Ore paragenesis Gangue Supergene paragenesis Hydrothermal alteration La Julia N25ºE/72º SE N35ºW/ vertical N70ºW/ vertical N42ºW/ 72ºSW py- ccp - gn- molbn qtz cv - cc- lm -mal ser (ilt)-qtz(rt- ap - chl) La Margarita N74º W/ vertical py – sp - ccp -(gn) qtz lm - gp - mlc -az – ccl – chal ser (ilt) – qtz (rt -ap) cb - qtz San Pedro N70ºE/ 80ºSE qtz lm - az – mlc ser - qtz Santo Tomás N80º E/ vertical; 75º SE/ vertical N51ºE/ vertical N75ºW/ vertical py qtz lm – mlc ser (ilt) - qtz Sin Nombre 65º/subvertical hem- py - (ccp) qtz qtz - ser - (chl) San Eduardo N75ºE gn - (sp - ccp - py) qtz ser –qtz cb– (qtz) Juanita 290/ 68º S gn - py qtz La Salvadora N13ºE/ N25ºW 47ºSW gn - cct - (ag - cp qtz - cb bn-str-tnt-sp- hem) 238 lm ser – qtz cb - qtz lm - mlc- cer ser – qtz cb - qtz GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA 2004). Outwards, the alteration zone consists of pervasive propylitization homogeneously distributed in the dioritic intrusive; locally chlorite-epidote veinlets are observed (Fig. 1). The alteration assemblage consists of chlorite, epidote, carbonate, tremolite, sericite and minor albite. The potassic alteration is irregularly distributed within the intrusive (Fig. 1) and has an assemblage of K-feldspar, biotite, magnetite and quartz. It occurs pervasively and in veinlets with parallel walls suggesting that they were formed under a brittle regime. Three types of veins have been preliminary recognized based on their morphology, mineralogy and character of the halos: barren K-feldspar veins without alteration halo; quartz veins with no or very scarce pyrite and chalcopyrite with or without K-feldspar alteration halos (Fig. 2a) and quartz - magnetite veinlets (Fig. 2b). About 3 km south-east to San Pedro Hill, in La Totora River, there is a small outcrop (about 250 x 15 m) similar in composition to the quartz-dioritic intrusive (Fig. 1). It shows intense pervasive potassic alteration with an assemblage of K-feldspar and a quartz stockwork structure with chalcopyrite and pyrite. Veinlets with oxidation minerals (gypsum, malachite and azurite) are frequently observed. Geochemical analyses carried out by Portal Resources (Davicino, 2008) returned values that reach 2.5% Cu and 542 ppm Mo. Geophysical surveys (Johanis, 2003) show a Th/K and U/K low and a positive magnetometric anomaly in centre of San Pedro Hill coinciding with the central potassic zone. Weak phyllic alteration is irregularly distributed in the quartz-dioritic intrusive and the dacitic ignimbrite (Fig. 1). It bears pervasive silicification and sericitization (illite determined by short wave infrared reflectance spectrometry, SWIR) with scarce disseminated pyrite and quartz-pyrite stockwork structure with minor chalcopyrite, sphalerite and galena. A late carbonatization process, pervasive and also present in veins with minor quartz, overprints both the potassic and phyllic alteration. The main characteristics of the polymetallic veins of the area (Fig. 1) are summarized in Table 1. They crop out within or close to the alteration zone and are controlled by N-S, NW-SW and NW-SE regional structures. Close to the contact with the veins the host rocks show pervasive and vein-type silicification and pervasive sericitization (Rubinstein et al., 2002). Conclusion Studies carried out in the San Pedro mining district allow mapping the geological units and also discriminating the alteration assemblages and their field distribution. In this way the new information will contribute to characterize the ore deposits of the area and hence to establish its genetic model. Acknowledgments This study was financially supported by UBACyT X485 project (Universidad de Buenos Aires). We thank the Servicio Geológico Minero Argentino (SEGEMAR) for supporting the field work. REFERENCES • Davicino, R.; 2008. A review of the Anchoris proyect, Argentina. Unpublished report, 39 p. • Delpino, D., Pezzutti, N., Godeas, M., Donnari, E., Carullo, M., Núñez, E., 1993. Un cobre porfírico paleozoico superior en el centro volcánico San Pedro, distrito minero El Nevado, Provincia de Mendoza. Comptes Rendus XII ICC-P, 1: 477-490. Buenos Aires. • Johanis, P., 2003. Informe geofísico San Pedro – Las Chilcas. Unpublished report, 2p. Servicio Geológico Minero Argentino, Buenos Aires. • Kleiman, L.E., Japas, M.S.; 2009. The Choiyoi volcanic province at 34°S – 36°S (San Rafael, Mendoza, Argentina): Implications for the Late Palaeozoic evolution of the southwestern margin of Gondwana. Tectonophysics, 473, (3-4):283 – 299. • Korseniewsky L.I., Rubinstein, N., 2005. Estudio de inclusiones fluidas en la veta La Julia, Cerro San Pedro, provincia de Mendoza. Congreso de Geología Económica, 1:171-174. Buenos Aires. • Llambías, E .J., Kleiman, L. E. and Salvarredi, J. A., 1993. El magmatismo gondwánico. En: Geología y Recursos Naturales de Mendoza, (Ed: Ramos, V.A.), Relatorio 12° Congreso Geológico Argentino: 53-64. (Mendoza). • Rubinstein, N., Carpio, F., Mallimacci, H., 2002. Las vetas polimetálicas del área del Cerro San Pedro, provincia de Mendoza, Argentina. 15° Congreso Geológico Argentino, 2: 263 – 266. Calafate. • Rubinstein, N., Ostera, H., Mallimacci, H., Carpio, F., 2004. Lead isotopes from gondwanic ore polymetallic vein deposits, San Rafael Massif, Argentina. Journal of South American Earth Science 16 (7): 595 – 602. • Salazar, L., 1974. Distrito mineralizado “Costa del Nevado”, Cu, Pb, Zn, Ag. Unpublished report, Los Chalanes S.A., 20p. Servicio Geológico Minero Argentino, Buenos Aires. 239 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA INDUCED POLARIZATION–RESISTIVITY EXPLORATION IN THE POLYMETALLIC PURÍSIMA-RUMICRUZ DISTRICT, JUJUY PROVINCE (ARGENTINA) 8-03 López, L.1,2*, Echeveste, H.1, Tessone, M.1 (1) INREMI. Facultad de Ciencias naturales y Museo (2) CONICET. Consejo Nacional de Investigaciones Científicas y Técnicas * Presenting author’s e-mail: [email protected] Introduction The studied area is located at the edge between the Puna and Cordillera Oriental geological provinces, in the north-western Argentina. The district is 25 km east of Abra Pampa town, 1800 km north of Buenos Aires. North-western Argentina has a long mining history. Aguilar is the most important mine in the area and produces a Pb-Ag-Zn concentrate from an Ordovician SEDEX deposit type, with a later remobilization event produced by intrusion-related metamorphism of Mesozoic granitic plutons (Sureda, 1999). Purísima-Rumicruz has anomalous contents of Ag, As, Ni, Co, Cu and Pb, and together with La Esperanza and La Niquelina, represents a small but uncommon metallogenic province in Argentina. This province gathers together deposits of Puna and Cordillera Oriental classified as five element deposit by Lurgo Mayón (1999). Induced Polarization (IP) - Resistivity study is a well known geophysical method to determine the presence of sulfide ores, particularly hosted in vein system. IP anomalies in sulfide-rich vein deposits are considered excellent exploration guides to determining future targets for exploration. Regional and structural geology Purísima-Rumicruz veins are hosted in the Acoite Formation, a low-grade metamorphosed sedimentary rocks composed by sandstones and dark shales that were deposited in a shallow wavedomain platform. A progradation sequence can be distinguished from base to top. Ortho-quartzite and grainstones beds are interbedded in the sandstone/siltstone sequence. Profuse biostratigraphic studies have been carried out on the Acoite Formation defining a late Tremadocian to Arenigian age for these rocks. The deformation history of the region can be summarized into three main episodes: (1) a late Ordovician deformation (Ocloyic orogeny); (2) an early Cretaceous to low Tertiary rifting process; and (3) the Eocene to Present Andean orogeny, characterized by deformation of variable intensity produced by subduction and generation of a magmatic arc. Mineralization Purísima-Rumicruz deposit has been mined on the sixties and seventies decades. The ore was mined with 450 meters of underground galleries manually worked. Those galleries are actually inaccessible because flooded and/or collapsed. Veins at surface are narrow, from 0.2 to 1 m wide, and have no topographic contrast with the host rocks. They are mostly covered by Quaternary sediments and their length is difficult to determine. Veins are characterized by brecciated textures, with sub-angular to round-shaped clasts. At least five events of brecciation and stockwork can be defined in the veins. First, a chalcocite, galena and chalcopyrite-bearing pulse, then a second pulse with coarse quartz, an extensive third infill of calcite, barite-rich pulse, and finally a limonite, malaquite and azurite stockwork. Alteration is subtle to weak, and it is mainly represented by oxidation of the vein and a few centimeters on the sides of it. There is also some restricted argillic alteration in the margins of the veins. Spatially related to this mineralization, another paragenetic association was described, consisting in narrow veinlets of nickeline, amorphous uraninite, rammelsbergite, gersdorffite, covellite, and sphalerite (Brodtkorb, 1973). Considering textures, structures, clast shape and size distribution, Purísima-Rumicruz veins were interpreted as infill of dilatational fault zones with mechanical brecciation produced by shear stress and minor chemical corrosion. A strong structural control was defined in the area (López et al., 2008). Sulfide veins have an E-W trend (PurísimaRumicruz, La Nueva) barite-bearing veins a WNW-ESE trend (El Brechón) and quartz veins are characterized by random orientations. Exploration of the area consisted in a lithological and structural mapping at 1:20,000 scale, followed by a detailed mapping of the host rock, veins, structure and alteration performed at 1:5,000 scale. In addition, geochemical data of the dump piles was performed 240 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - A. IP (left) and Resistivity (right) plan maps (-57 m) of Purísima-Rumicruz, La Nueva and Brechón veins. B. Geology, IP and Resistivity cross sections at line 600 in Purísima-Rumicruz veins. on 19 rock chip samples, and a 7,300 m of IP-Resistivity survey was completed. Methodology The IP techniques are based on the study of secondary electric fields generated in the ground by electric currents and is one of the most widely used techniques in ore deposit prospecting. Geoelectrical works were focused on the central portion of the District, considering the location of the veins. Three areas (Fig. 1) were defined “Purísima” (P), “La Nueva” (N) and “El Brechón” (B). A total of 17 lines were realized to perform this study. The orientation of these lines was normal to the average orientation of the veins. The length was variable; Purísima has all 500 m long lines, La Nueva 300 m lines and in Brechón the length varies between 500 m and 200 m. Additionally, a topographic survey along each line was realized with a hand clinometer (Abney Level) every 25 m, with GPS (Garmin eTrex) control points at the beginning and the end of each line. The methodology of the survey was carried out in a linear “multielectrodic” design, with a dipole-dipole configuration. The distance between the electrodes “a” was 25 m and on each station depth level (n) vary from n=1 to n=8. The duration of each cycle was two seconds. The receptor was a IPR-12 Time Domain IP/Resistivity Receiver and the energization was performed with a IPC-9/200W, both made by Scintrex Company. Apparent IP and resistivity data were processed with the RES3DINV 2.14 version software. Data inversion was made with the Gauss-Newton (Loke and Dahlin, 2002) method, because the amplitude of the resistivity data. The final result of the inversion shows a model of true (corrected) IP and resistivity. The Root Mean Square (RMS) was always less than 10%. True IP and resistivity of each line was presented in a geological section. Maps showing values of IP and resistivity results at 57m underneath surface were plotted. Results The IP anomalies are interpreted as due to the presence of polarizable minerals, and according to outcropping vein composition and structure, we assume the presence of sulfide-rich veins beneath the 241 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES surface. According to IP data, areas with values higher than 50 mv/v are considered anomalies. Range between intervals is 10mv/v. Three categories of IP anomalies were defined in decreasing order of importance, according to intensity, vertical and lateral continuity, and spatial relationship with outcropping mineralization. Despite this, caution must be taken in the interpretation because of presence of disseminated pyrite in the Acoite Formation siltstone. Nevertheless morphology of anomalies did not match with the strike of stratification, which could be interpreted as a pyrite rich bed, and correspond with orientation of veins on surface. Similar categories were established in the polymetallic vein of the Pingüino area, in the Santa Cruz Province. There, the most important anomaly was drilled and one of the holes becomes the most important base metal intersection of the entire project (Guido et al., 2009). Resistivity interpretation is more difficult than IP data. Usually high values of resistivity could be related to the presence of low porosity of host rock, quartz veins or silicified levels. Low values of resistivity could be associated to fault zones, sulfide-rich veins and/or the presence of water in pores or fractures. By the above, sulfide rich veins could be represented by high IP and low resistivity. In the area, some low values of resistivity fit with high values of IP, showing a negative correlation between these two parameters. This correlation is evident in the Brechón and in the La Nueva areas, but not very clear in Purísima-Rumicruz area. Conclusions A negative correlation between IP and Resistivity values was observed in the geophysical survey at Purísima Rumicruz veins. This can be related to minor presence of quartz or the brecciated texture of the veins, together with the presence of patches of massive sulfides. Purísima Rumicruz is a small but rare deposit with an unusual paragenetic association, and in spite of been mined in the past, there is still a potential ore deposit beneath surface that has the potential to be explored. Geoelectric (IP - Resistivity) shows to be an excellent methodology in areas covered with Quaternary sedimentation like this. Although the northern Argentina have the longest history of mining, an updating of the exploration procedures is necessary to achieve a more predictive method for discovering hidden ore bodies. REFERENCES • Brodtkorb de, M K.; 1973: Estudio de la mineralización del yacimiento “La Niquelina”, provincia de Salta y un análisis .comparativo de sus posibles relaciones con los depósitos “Rumicruz” y “Esperanza”. Revista de la Asociación Geológica Argentina, 27, 4: 364-368. • Guido D. M., Jovic S. M., Echeveste H., Tessone M. O., Ramayo Cortes L., Schalamuk I. B.; 2009: Descubrimiento y modelización de clavos mineralizados en vetas polimetálicas a partir de exploración geoeléctrica, proyecto Pingüino, Macizo del Deseado. Revista de la Asociación Geológica Argentina 64 (3): 203 - 210 (2009). • Loke, M.H. y Dahlin, T.;2002: A comparison of the Gauss-Newton and quasi-Newton methods in resistivity imaging inversion. Journal of Applied Geophysics 49: 149-162. • López L., Echeveste H., Schalamuk I. B.; 2008: Nuevos aportes en el distrito minero Purísima Rumicruz, provincia de Jujuy. XVII Congreso Geológico Argentino. San Salvador de Jujuy. Actas (II): 607-608. Jujuy, Argentina. ISBN 978-987-22403-1-8. • Lurgo Mayón, C. S.; 1999: Depósitos polimetálicos ricos en níquel, cobalto y arsénico de la Cordillera Oriental, Jujuy y Salta. In: Recursos Minerales de la República Argentina (Ed. E. O. Zappettini), Instituto de Geología y Recursos Minerales SEGEMAR, Anales 35: 999-1004, Buenos Aires. • Sureda, J. R.; 1999: Los yacimientos sedex de plomo y zinc en la Sierra de Aguilar, Jujuy. En: Recursos Minerales de la República Argentina (Ed. E. O. Zappettini), Instituto de Geología y Recursos Minerales SEGEMAR, Anales 35: 459-485, Buenos Aires. 242 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES ROCK-MAGNETISM PROPERTIES FROM DRILL CUTTING AND THEIR RELATION WITH HYDROCARBON PRESENCE AND PETROPHYSICAL PARAMETERS 8-04 Mena, M.*, Walther, A.M. CONICET-INGEODAV, Dpto. Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina * Presenting author’s e-mail: [email protected] We present a study of the vertical variations of magnetic properties, performed on cutting samples from an exploratory hydrocarbon well drilled in the Golfo San Jorge basin, Argentina. The studied stratigraphic section is 248 m long. The sampled levels correspond to tuffaceous sandstones and sillstones, and silty to sandy tuffs. A total of 108 samples of drill cutting were used, each one representative of a level 2 m to 3 m thick. The mass magnetic susceptibility was measured at three different frequencies for five specimens per sample. The related frequency-dependent susceptibility factor (FDF), the mean mass susceptibility (X) and the standard deviation for each level were calculated. Different magnetic phases were identified on the basis of detailed measurement of isothermal remanent magnetization (IRM) performed on previously consolidated specimen per each sample. The different coercivity ranges defined suggest the important presence of titanomagnetite, magnetite and magnetic pyrrhotite, and oxidized magnetite in lower proportion. Concentration indexes (CI) for the identified magnetic minerals were calculated. Curves of X, FDF, saturation of IRM (SIRM) and CI were drawn and analyzed together with lithological information and geophysical logs. The three parts of the profile where the cutting descriptions were performed indicate the presence of hydrocarbon-impregnated tuffaceous sandstone levels, which coincide with higher X and SIRM values. However, the correlation coefficients between the percent contribution of those lithologies and the susceptibility and SIRM values are not statistically significant. The profile depths with hydrocarbon-impregnated rocks that present certain productive interest are correlated with levels in which the magnetite CI shows relative increases. But in general, the areas with higher porosity, defined from density logs (DPHI) but especially those defined from sonic (SPHI) and neutronic logs (NPHI) match with areas with higher magnetite CI. On the contrary, the peaks of titanomagnetite CI coincide with areas of relative decrease of NPHI, DPHI and SPHI. The levels where gas was detected coincide with the biggest magnetite CI values. In those sectors, the levels of higher magnetite and pyrrhotite concentrations agree with the depths where higher porosity values (NPHI, DPHI and SPHI) were registered. Although the gas peaks are located in levels where the relative content of magnetite is higher, the immediately superior levels, about 3m above them, are the ones that present a larger growth of magnetite and also of pyrrhotite content. This could indicate the authigenic formation of magnetic minerals influenced by hydrocarbon microseepage from lower strata. Magnetic susceptibility values and quantitative mineralogical information derived from rock magnetism studies can provide accurate data concerning to lithological variations. The qualitative correlations between magnetic data and key petrophysical parameters such as porosity, joined to the association of magnetic mineralogy with hydrocarbon presence or migration, show the potential utility of these techniques for subsurface exploration. 243 GEOSUR2010 22-23 NOVEMBER 2010 – BUENOS AIRES USE OF ASTER IMAGERY TO IDENTIFY MINERALIZATION IN THE ANDEAN CORDILLERA FRONTAL (31º45´S), SAN JUAN PROVINCE (ARGENTINA) 8-05 Pérez, D.J.1*, D’Odorico, P.2 (1) Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II (1428), Buenos Aires, Argentina (2) ArPetrol Argentina S.A. * Presenting author’s e-mail: [email protected] Introduction The main aim of this study was to verify the potential uses of multispectral imagery for geologic/mineral mapping, and show a methodology for search of mineral resources. We used ASTER (Advanced Space borne Thermal Emission and Reflection Radiometer) data to detect the presence of altered rocks associated with hydrothermal alteration of porphyry system. In arid environments, the spectral signatures of diagnostic minerals are often not masked by water, vegetation, or superficial materials. The ASTER data characteristics were originally selected with the purpose of achieving remote mineralogical identifications and thus it became a power tool to apply in geology. Digital image processing techniques were used with ASTER data to enhance lithologies and to detect alteration associated with possible mineral deposits. ASTER imagery was combined with field mapping and PIMA (Portable Mineral Infrared Analyzer) field data, into a geographic information system; and was integrated in order to establish the relationship with a structural model of the mineralized bodies. The study region is located at 31°45’ S and 70°00’ W, in the Frontal Cordillera of San Juan Province, in the south end of flat slab subduction segment. Climate is generally arid, and vegetation and soil cover are poorly developed (Fig. 1). Geology and structure The stratigraphic sequence of the region is as follows (from bottom to top): A Carboniferous and Permo-Triassic basement (Choiyoi Group), composed of rhyolites and granites. Mesozoic deposits represented by the volcaniclastic, pyroclastic and sedimentary rocks of the Rancho de Lata Formation (Triassic-Jurassic), interpreted as syn-rift deposits. Jurassic marine deposits of the Los Patillos and La Manga Formations (Lias-Dogger), interpreted as a sag phase deposits, and continental sequences of the Tordillo Formation (Malm). Without stratigraphic relationship is the Auquilco Formation, formed by gypsum. Overlaying the latter sequence, continental sedimentary and volcaniclastic Cretaceous sequences of the Diamante and Cristo Redentor Formations are developed. These deposits are overlaid by the volcanic rocks of the Farellones formation, being this volcanism responsible for the development of the hydrothermal alteration areas like Los Pelambres, Pachón, Carnicerías, La Coipa and Altar. The structure of the region has two distinct styles, one of thick skinned, with tectonic inversion evidence that involves the Permo-Triassic basement rock; and a thin skinned style, that affects the Cenozoic and Mesozoic sediments. Remote Sensing Aerial photography has been the most commonly used type of remote sensing data, but satellite data have advanced applications because of the larger number of image wavelengths. There are several satellite platforms, many of them containing more than one sensor on board. ASTER is one of such instruments currently available. The ASTER is a multispectral instrument mounted on the Earth Observing System (EOS), TERRA. ASTER has 14 bands in 3 regions subsystems; three bands on visible and near infrared (VNIR, 0.52-0.86 Ìm), six bands on short-wave infrared (SWIR, 1.60-2.43 Ìm), and five bands on thermal infrared (TIR, 8.125-11.65 Ìm); which have 15, 30, and 90-m spatial resolution, respectively. It has also a stereo mode by the nadir looking band 3N and backward-looking band 3B of VNIR. Digital image processing techniques were used with ASTER data to enhance lithologies and to detect alteration associated with mineral deposits. ASTER imagery was combined with field mapping and PIMA data field, into a geographic information system. This data-set was integrated in order to 244 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 1 - Location map for the presented figures. 245 GEOSUR2010 Fig. 2 - Color composite, RGB 641. 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 3 - Band ratios 4/5, 4/6, 4/7 (RGB) establish the relationship with a geological model of the mineralized bodies. Processing, analysis and methodology Analysis of ASTER spectral reflectance data provides a basis for geological mapping and distinguishes hydrothermal alteration zones. Different image processing techniques were applied in order to extract the information. Samples were collected to be analyzed with a PIMA spectrometer in order to determine what alteration could be distinguished by spectral reflectance alone. Individual and combination minerals could be separated in terms of their reflectance. The integrated approach to this study aided geologists to identify several mine prospects such as Carnicería, La Coipa, Yunque and Yeguas Heladas. These deposits were found by using the ASTER imagery to identify areas with Fig. 4 - Index OHI (a) 246 Fig. 5 - Index OHI (b). 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 Fig. 6 - SAM classification Fig. 7 - SAM classification characteristics similar to known prospects such as Pachon, Los Pelambre and Altares. An ASTER L1B image of January 21 2001 was used, and the following pre-processing were applied: (1) correction of the effect Crosstalk; (2) correction for radiance; (3) atmospheric correction to obtain data in apparent reflectance. Then, the following processing were carried out: color composites, color ratios composites, Spectral Angle Mapper classification, and Ninomiya index. Band combinations The band combinations allow getting a first geological interpretation of the region. The band combination 321 (RGB) allowed to identify and correlate different lithologies with olds maps. The band combination 654 (RGB), allows a first regional identification of possible areas of hydrothermal alteration (Fig. 2). Band ratios Because in many cases the spectral characteristic of the rocks are similar, is not possible to discriminate different lithologies with imagery interpretation, from color bands combinations. For these reason band ratios combinations in the SWIR were used for discriminating areas of hydrothermal alteration, since these present picks of absorption and of reflectance characteristic in this region of the electromagnetic spectrum. The band ratios combination of 4/5, 4/6, 4/7 (RGB), allowed identify areas of hydrothermal alteration (Fig. 3). Ninomiya – SWIR index Minerals as montmorillonite and sericite present picks of absorption in band 6 of ASTER, while the pirofilite presents a characteristic pick of absorption in band 5. Beside that, the caolinite and alunite present characteristic picks of absorption in bands 5 and 6 respectively. Considering these parameters, the qualitative estimate of the presence of these minerals was achieved, using the indexes. • OHI • OHI • ALI: • CI: (a): (b): (band 4 * band 7) / (band 6 * band 6); (band 4 * band 7) / (band 5 * band 5); (band 7 * band 7) / (band 5 * band 8). (band 6 * band 9) / (band 8 * band 8). The OHI(a) index identifies minerals that present picks of absorption in the band 6 while the OHI(b) index allows the identification of minerals that present picks of absorption in the band 5; the ALI index allows to distinguish alunite for its pick of absorption in the band 8. Applying these indexes it is possible to distinguis minerals or mineral groups with alteration, based on their respective spectral 247 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA characteristics (Fig. 4 and Fig. 5). Spectral Angle Mapper Classification – SAM The supervised classification is an automated method for comparing image spectra to individual spectra or a spectral library. SAM assumes that the data have been reduced to apparent reflectance (true reflectance multiplied by some unknown gained factor controlled by topography and shadows). The algorithm determines the similarity between two spectra by calculating the “spectral angle” between them, treating them as vectors in a space with dimensionality equal to the number of bands (nb). A simplified explanation of this can be given by considering a reference spectrum and an unknown spectrum from two-band data. The two different materials will be represented in the 2-D scatter plot by a point for each given illumination, or as a line (vector) for all possible illuminations. For the present SAM classification the end-members from the ASTER image were collected (see Fig. 4). In the classification, various types of data were used, as the spectra database of the USGS (speclib4). Another used database was those derived from samples collected in the field, and analyzed with a spectrometer PIMA. These samples allowed to determine the present alteration minerals in the region. With these spectra, and taking into account the spectra base of the USGS (Speclib4), the following mixture of minerals was determined: illite, caolinite, jarosite, quartz and clorite. This process allowed to identify those minerals in the regions of Altar, The Coipa, Pachón and Los Pelambres (Figs. 6 and 7). Summary Analysis of ASTER spectral reflectance data provides a basis for geological mapping and allowed to identify and classify several areas that present processes of hydrothermal alteration. Two of these areas correspond to Pachón and Los Pelambres locations; a third area of hydrothermal alteration is the Altar prospect; and a fourth area is La Coipa, which would be temporally linked before to the mentioned deposits. These results show the potential uses of multispectral ASTER for geologic mapping in regions where superficial exposure of rocks are limited and for mapping areas of potential mining. SEISMIC EVIDENCE OF A GAS HYDRATE SYSTEM IN 8-06 THE WESTERN ROSS SEA (ANTARCTICA) BY TOMOGRAPHY, AVO ANALYSIS AND PRESTACK DEPTH MIGRATION Picotti, S.1, Geletti, R.1*, Gei, D.1, Mocnik, A.2, Carcione, J.M.1 (1) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy (2) Dept. of Geoscience, University of Trieste, Italy * Presenting author’s e-mail: [email protected] We present a seismic evidence of the presence of gas hydrates on the Lee Arch in the Terror Rift (western Ross Sea - Antarctica) (Fig. 1). The presence of gas hydrates is inferred from a bottom simulating reflection (BSR), the first identified in the Ross Sea (Geletti et al., 2008; Geletti and Busetti, 2009 and 2010). The BSR (Fig. 2) was identified and analysed through targeted reprocessing of the multichannel seismic reflection data (3000 m streamer, 120 channels, 60 fold) acquired in 1990 by the Italian research vessel OGS Explora. The BSR is characterised by high amplitudes of reverse polarity, above interval velocities as low as 1.4 km/s, consistent with the presence of free gas; a second reflection of normal polarity below (about 100 ms below the BSR) and parallel to the BSR is interpreted to mark the base of the free gas zone (Bottom of free Gas Reflector - BGR). The BSR cross-cuts stratal reflections of the Terror Rift and it is locally offset across faults of a positive flower structure along its eastern flank (Lee Arch). The multichannel seismic data also reveal the presence 248 22-23 NOVEMBER 2010 – MAR DEL PLATA GEOSUR2010 Fig. 1 - a) the map of Antarctica shows the location of the study area in the western Ross Sea; b) tectonic sketch-map and bathymetry of the western Ross Sea showing the position of the studied seismic profile (Figures 2 and 3). a b Fig. 2 - a) seismic profile where a gas hydrate related BSR (Base of Gas Hydrate Stability Zone - BGHSZ) and the Bottom of free Gas Reflector (BGR) are evident; b) the highlights from section in a) show the seismic character of the BSR and BGR compared to the seafloor reflection (modified after Geletti and Busetti, 2010). 249 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Fig. 3 - P- (a) and S-wave (b) velocity reflectivity sections. along the eastern flank of the rift of seabed elevations and depressions, some of which coincide with irregularities in the BSR that are interpreted to indicate an upward migration of gas/fluid toward seabed. Multibeam data acquired in 2006 by the R/V OGS Explora in the frame of the Italian Programma Nazionale di Ricerche in Antartide (PNRA) confirm the seabed elevations to be mud volcanoes (up to 80 m high and 1-2 km wide) in water depths of 450-750 m. The faults, which control the migration of gas toward the sea-floor, could connect the free gas zone below the BSR and the mud volcanoes on the sea-floor (Geletti and Busetti, 2010). The seismic data were reprocessed in order to increase the signal/noise ratio by adopting a ‘trueamplitude’ approach which preserve the real amplitudes of the reflection signals, allowing a successive AVO (Amplitude Variations with Offset) and tomographic analysis. 250 GEOSUR2010 22-23 NOVEMBER 2010 – MAR DEL PLATA Seismic tomography and AVO gave a fundamental contribution to characterise the geometry of the subsurface structures and the seismic properties of the sediments. Besides obtaining more detailed velocity fields, both for the P and S waves (Fig. 3), the travel-time tomographic procedure can determine the depths of horizons associated with the reflected and refracted arrivals. The adopted tomographic software CAT3D, based on the SIRT method and the minimum time ray tracing (Böhm et al. 1999), estimates the velocity field and the reflector structure in sequence, from the upper to the deeper horizon. Thus, using the tomographic velocity model as input for the pre-stack depth migration, we obtained a detailed imaging of the gas seeping features, the BSR and the overlying and underlying structures. The AVO analysis allows us to extract information on both P- and S-wave velocity reflectivity. The ratios between the P- and S-waves reflectivity are related to the Poisson’s ratio that supplies important information on the fluid content within a porous medium and on the rigidity of the solid matrix (Carcione and Tinivella, 2000). Figure 3 shows that the BSR is identified both in the P- and S-wave velocity reflectivity sections, which denotes that there is a change both in the properties of the solid matrix and in the pore fluids across the interface. In addition, the combined use of travel-times and attenuation in the tomographic inversion (Rossi et al., 2007; Picotti and Carcione, 2006) provides a multi-parameter velocity-Q (quality factor) model which, using rock-physics theories and AVO analysis, allowed to map the spatial distribution of gashydrate and free gas bearing sediments. Finally, using the obtained petrophysical model and adopting a numerical wave-modelling algorithm, we reproduced the real seismic section of the study area. REFERENCES • Böhm G., Rossi G. and Vesnaver A.; 1999: Minimum time ray-tracing for 3-D irregular grids. J. of Seism. Expl., 8, 117-131. • Carcione J. M. and Tinivella U.; 2000: Bottom-simulating reflectors: seismic velocities and AVO effects. Geophysics, 65, 54-67. • Geletti R. and Busetti M.; 2009: Evidenze di bottom simulating reflector (BSR) e gas seep nel Mare di Ross occidentale (Antartide). In: 28° National Meeting of GNGTS, Geofisica di esplorazione e produzione. Trieste, Italy, 16-19 novembre. Expanded Abstract, 644 – 648. • Geletti R. and Busetti M.; 2010: Bottom Simulating Reflector (BSR) and Base of the free-Gas Reflector (BGR) linked to gas hydrate and gas seepage in the western Ross Sea (Antarctica). J. Geophys. Res., submitted. • Geletti R., Praeg D. and Busetti M.; 2008: Evidence of gas hydrates and mud volcanoes in the western Ross Sea, Antarctica. In: J. Mienert, G. Westbrook, C. Paull, H. Haflidason (convenors), Gas hydrates in oceanic and permafrost environments - importance for energy, climate and geohazards - Session GAH-01, 33rd International Geological Congress, Oslo, 6-14 August, abstract 1352489. • Picotti S. and Carcione J. M.; 2006: Estimating seismic attenuation (Q) in the presence of random noise. Journal of Seismic Exploration, 15, 165-181. • Rossi G., Gei D., Böhm G., Madrussani G. and Carcione J. M.; 2007: Attenuation tomography: An application to gas-hydrate and free-gas detection. Geophysical Prospecting, 55, 655-669. 251 AUTHOR INDEX The code following the name indicates the session and the location in the book. Abarzua-Vasquez A.M. Abraham D.A. Acevedo R. Adams C.J. Ader M. Alvarez O. Amarasinghe U. Arecco M.A. Ashchepkov I.V. Avdeev D.V. Badi G. Bagú D. Baldo, E.G Baquero M. Baradello L. Barbosa N. Basei M.A.S. Bastos P. Batelaan O. Bellatreccia F. Bergantz G. Bermudez A. Bertrand S. Betka P. Bidegain J.C. Bingen B. Boedo F.L. Bohoyo F. Boiocchi M. Bozzano G. Brauer A. Brümmer R. Buffoni C. Bujalesky G. Caffau M. Calatayud F. Calderon M. Canals M. 6-01 7-01 3-11 1-01 1-22 3-02 1-05 7-04 2-14 1-08 4-20 3-01 1-20, 1-25 5-06 3-13, 8-01 4-20 1-17, 1-21 5-06 6-06 2-10 1-16 2-01, 4-04 6-01 4-01 5-08, 5-09 1-11 1-02, 1-27 7-09 2-10 7-08 6-01 6-01 4-20 6-04 5-01 4-18 4-06 7-02 Carcione J.M. 8-06 Cardona A. 1-10 Carugati G. 4-02 Casquet C. 1-20 Casassa G. 6-02 Castro J. 2-02 Cerredo M.E. 2-03, 2-07, 2-10, 2-12 Chernicoff C.J. 1-03 CHILT Project Members 6-01 Cingolani C. 1-04 Civile D. 4-10 Clague J.J. 6-04 Clark C. 1-05 Cnudde V. 6-09 Collins A.S. 1-05 Comici C. 5-01 Connon G. 4-20 Corbella H. 3-11 Coronato A. 6-04, 6-07 Cosentino N.J. 4-03 Costa P. 7-08 Cravos C. 4-12 Currie K. 1-24 Dalziel I.W.D. 1-06 Darbo A. 8-01 De Batist M. 6-01, 6-09 DeCelles P. 1-10 D’Eramo F. 4-05, 4-19 De Isasi M. 7-04 Del Cogliano D. 3-01, 3-06, 3-12, 4-14 De la Vega M. 6-10 Della Ventura, G. 2-10 Delpino G.D. 2-01, 4-04 De Saint Blaquat M. 2-02 De Rycker K. 6-09 Dickerson P.W. 1-07 Didenko A.N. 1-08 Dietrich R. 3-01, 3-12, 6-02 253 AUTHOR INDEX Dimieri L.V. Di Marco A. D´Odorico P. Dristas J.A. Drobe M. Ducea M.N. Dupuy J.L. Dustay S. Duyck P. Echeveste H. Egli R. Emmel B. Engvik A. Escayola M. Escosteguy L. Esteban F.D. Fagel N. Falcone G. Fanning C.M. Favali P. Fazzito S. Fernández M. Figari E. Folguera A. Fritsche M. Frugoni F. Galindo-Zaldivar García Morabito E. García M. García R.E. Gavriloff I.J.C. Gei D. Geletti R. Gehrels G. Geuna S. Ghidella M. Ghidella M.E. Gianibelli J.C. Gieles R. Gilli A. Gimenez M. Gimenez M.E. Godoy E. Goetze H.J. 254 4-23 4-05 8-05 2-04, 2-07 1-12 1-16 6-05 6-03 6-09 8-03 6-07 1-11 1-11 1-09 4-05 7-03 6-01 7-06 1-07, 1-18 1-20, 4-06 7-06 3-10 6-04 5-02 4-18, 4-21 3-12, 6-02 7-06 7-09 1-18 3-02 3-04 5-03 8-06 8-06 1-10 4-05 7-01 4-08 3-03, 3-04. 3-07 6-01, 6-09 6-01 3-02 4-02 4-06 3-05 Gómez A. 8-02 Gomez M.E. 3-06 Gomez S.M. 5-08 González M. 3-11 Götze H.J. 4-11 Griffin W. 1-04 Grossi M. 3-13, 8-01 Guevara N.O. 3-08 Guryano V.A. 1-08 Guseva G.L. 4-09 Hanson R.E. 1-07 Hebbeln D. 6-01 Helen J. 2-05 Hernández-Molina F.J. 7-04, 7-08, 7-09 Heirman K. 6-01 Hervé F. 1-18, 2-02, 3-09, 4-06 Hormaechea J. L. 3-06, 3-12, 4-20 Ibanez-Mejia M. 1-10 Inbar M. 2-11 Introcaso A. 4-02 Isla F. 7-05 Ivins E.R. 6-02 Jacobs J. 1-11 Kading T. 2-01 Kleinhanns I. 1-11 Klepeis K. 4-01, 4-13 Keppens E. 6-01 Kilian R. 6-01 Kinny P.D. 1-05 Kumar R. 1-11 Lagorio S.L. 2-06, 2-16 Lange H. 6-02 Lawver L.A. 4-08 Leychenkov G.L. 4-09 Linares E. 1-14 Lippai H. 2-03, 3-09, 3-13, 4-03, 4-15, 5-01, 5-04, 8-01 Llanos M.P.I. 4-07 Lobo F.J. 7-09 Lodolo E. 3-09, 3-13, 4-10, 5-01, 5-04, 7-03, 8-01 López E. 6-10 López L. 8-03 AUTHOR INDEX López R. López de Luchi M.G. Lossada A.L. Lücke O.H. Luna E. Madirolas A. Mahlburg K.S. Maldonado A. Marcolini S. Marcomini S.C. Marinaro G. Martínez J.C. Martínez O. Martínez P. Martínez Dopico C.I. Martos-Martin Y. Matthew G.L. Marshall P. Marvin B. McAtamney J. Massonne H.J. Maurer M. Mehrtens C. Mena M. Mendoza L. Menounos B. Menichetti M. Mocnik A. Moernaut J. Mora A. Moretto A. Mosher S. Mutti D. Nacif S. Naipauer M. Nel J. Nemeth K. Nogueira A.C.R. Noelia A.M. Novara I.L. Novas F. Novo R. Nullo F. 3-10, 7-07 1-12, 1-14, 2-07, 4-22 1-13 4-11 3-02 7-05 2-05 7-09 7-08 3-10, 7-07 7-06 2-04, 2-07 3-11 3-02 1-12, 1-14, 2-07 7-09 2-05 7-04 5-06 4-13 2-04, 2-07, 6-03 6-04 4-13 6-05, 8-04 3-01, 3-12, 4-14 6-04 2-03, 2-10, 3-09 4-15, 5-01, 5-04, 7-03 8-06 6-01 1-10 6-07 4-01 4-05 3-02 1-24, 4-21 6-03, 6-06 2-11 1-22, 1-23 5-03 4-02 5-05 3-04, 3-07 2-11 O’Brien B.H. 1-15 Oberti R. 2-10 Olivero E.B. 4-23 Onorato M.R. 6-07 Orgeira M.J. 3-11, 6-07 Orts D. 4-18 Osella A. 6-10 Osborn G. 6-04 Otamendi J.E. 1-16, 4-19 Pankhurst R.J. 1-20 Paterlini M. 7-01, 7-04, 7-08 Pedersen O.A. 6-08 Peel E. 1-17, 1-21 Perdomo R. 3-06, 3-12 Perestoronin A.N. 1-08 Pérez D.J. 4-16, 8-05 Pérez M. 5-07 Peroni J.I. 2-03, 3-09 Peskov A.Y. 1-08 Pino M. 6-01, 6-09 PicottI S. 8-06 Pinotti L. 4-05, 4-19 Plasencia Linares M. P. 4-12 Plavsa D. 1-05 Poiré D. 1-20 Polvé M. 2-02 Ponce A. 6-07 Ponce J.F. 3-11, 6-04 Presti M. 5-01 Prezzi C. 3-10, 3-11, 4-07 Prokopiev A.V. 4-17 Quaglino N. 3-04 Rabassa J. 3-11, 6-04 Ramos M.E. 4-18 Ramos V.A. 1-18, 4-18, 4-21, 5-02 Rapalini A.E. 1-13, 1-14 1-19, 4-19, 4-22 Rapela C.W. 1-20 Raposo M.I.B. 6-11 Rebesco M. 7-04 Re G.H. 2-09 Remesal M.B. 2-03, 2-12 Renzulli N.A. 2-10 Reyes A.G. 3-08 255 AUTHOR INDEX Riccomini C. Richter A. Rico Y. Ridolfi F. Risso C. Roberts J.M. Roberts S. Rodriguez G.D. Romero B.F. Rovere E.I. Rubinstein N. Russi M. Ruiz J. Sabbione N.C. Sagripanti L. Salani F.M. Sánchez A. Sanchez Bettucci L. Sanchez Magariños J.M. San Martín L. Sansjofre P. Santos J.O.S. Scheinert M. Schmidt S. Schreckenberger B. Schreider A.A. Schwabe J. Seluchi N. Selway K. Siegesmund S. Singer S.E. Sinninghe Damsté J. Smelov A.P. Soares J.L. Somoza L. Somoza R. Spagnotto S. Spalletti L.A Steenken A. Strakos K. Suarez I. Tabare T. Tancredi G. 256 1-23 3-12 5-09 2-10 2-11 1-07 6-01 3-04 5-06 6-10 8-02 4-12 1-10 4-20 4-21 2-12 2-02 1-13, 1-17, 1-21 3-04, 3-07 4-16 7-07 1-22 1-03, 1-04 3-01, 3-12 3-05 7-01 7-09 3-01 3-07 1-05 1-12 2-13, 4-19 6-01 2-14 1-22 7-04 2-17 3-02 1-20 1-12 5-06 3-07 3-08 3-04 Tassone A.A. 2-03, 2-10, 3-09, 3-13 4-03, 4-10, 4-15, 5-01 5-04, 7-03, 8-01 Tessone M. 8-03 Thomas R.T. 1-11 Thomson S. 4-13 Tohver E. 1-23 Tomezzoli R. 4-22 Torres Carbonell P.J. 4-23 Trindade R.I.F. 1-22, 1-23 Tubía J. 4-19 Ueda K. 1-11 Uenzelmann-Neben G. 7-04 Umazano A.M. 2-15, 5-07 Urruti R. 6-01, 6-09 Valencia V. 1-11 Van Daele M. 6-01, 6-09 Van Staal C. 1-09, 1-23 Varekamp J. C. 2-01 Vásquez C.A. 3-10, 3-11, 6-07 Vasquez J. 4-14 Vegas N. 4-19 Verdecchia S.O. 1-25 Verleyen E. 6-01 Vernikovskaya A.E. 1-26 Vernikovsky V.A. 1-26 Versteeg W. 6-09 Vila R. 6-01 Vilas J.F. 2-09, 4-03, 6-11 Violante R.A. 6-10, 7-04, 7-08 Visconti G. 5-07 Vizán H. 2-06, 2-16 Vujovich G.I. 1-02, 1-23, 1-27, 4-19 Vyverman W. 6-01 Walther A.M. 6-11, 8-04 Wemmer K. 1-12, 1-14, 2-04 Wendt J. 6-02 Willner A.P. 1-27 Xu Y. 6-03, 6-06 Zaffarana C.B. 2-17 Zaitsev A.I. 2-14 Zecchin M. 5-01 Zappettini E.O. 1-03