THE LAGO DEL LAJA FAULT SYSTEM: ACTIVE INTRA

UNIVER SIDAD DE CONCEPCIÓN
DEPARTAMENTO DE CIENCIAS DE LA TIERRA
10° CONGRESO GEOLÓGICO CHILENO 2003
THE LAGO DEL LAJA FAULT SYSTEM: ACTIVE INTRA-ARC
COLLAPSE IN THE SOUTHERN CENTRAL ANDES (37°15´S)
MELNICK, D.1, FOLGUERA, A.2, ECHTLER, H.1, CHARLET, F. 3, BÜTTNER, O.4,
CHAPRON, E.5, DE BATIST, M.3 , SHARF, B.4, VIETOR, T. 1
1
GeoForschungsZentrum Potsdam, Germany
[email protected], [email protected], [email protected], Telegrafenberg, D-14473 Potsdam
2
Laboratorio de Tectónica Andina, Universidad de Buenos Aires, Argentina
[email protected], Ciudad Universitaria, CPC1428EHA Buenos Aires
3
Renard Centre of Marine Geology, Geological Institute, University of Gent, Belgium
[email protected], [email protected], Krijgslaan 281 S8, B-9000 Gent
4
UFZ-Center for Enviromental Research Leipzig-Halle, Germany
[email protected], [email protected], Brückstraße 3a, D-39114 Magdeburg
5
ETH Zürich, Geological Institute, Switzerland
[email protected], Rämistrasse 101, CH-8006, Zürich
INTRODUCTION
The Andean Main Cordillera at 37°15´S was initially built during the late Miocene inversion of
the Oligo-Miocene Cura-Mallín basin (Niemeyer and Muñoz, 1983; Folguera et al., 2001; Radic
et al., 2002). This short compressive phase formed a mountain range of ~2,200 m average altitude
and ~100 km wide at this latitude. During the very Late Miocene-Early Pliocene, shortening
ceased in the internal domain and the Cola de Zorro volcanoclastic intra-arc basin developed
during extension until Early Pleistocene (Vergara and Muñoz, 1982; Folguera et al., 2003) when
the volcanic front narrowed and shifted towards the trench to its actual position (Stern, 1989;
Lara et al., 2001). The Lago del Laja fault system runs along the intra-arc axial zone for ~60 km
from the Antuco volcano through the bottom of the Laja Lake, Quique and Aguila rivers (Fig. 1).
Recent field observations, offshore lake reflection seismic, bathymetry, and air photo
interpretation allows to map and characterize the Lago del Laja as an active intra-arc extensional
fault system in a volcanic complex, bordered by compressive and shortening patterns.
REGIONAL TECTONIC SETTING
South of 38°S, the volcanic arc front and intra-arc tectonics are controlled by the Liquiñe-Ofqui
fault zone, an active dextral strike-slip system that runs for ~1,200 km from the Antarctic-PacificNazca triple point until the Copahue volcano area (Fig. 1) (Lavenu and Cembrano, 1999;
Folguera et al., 2001; Melnick et al., 2002; Rosenau et al., 2003). Neotectonic (Quaternary to
Recent) deformation north of the Copahue volcano area (37°55´S) shows contrasting kinematics:
(i) An orogenic front develops ~30 km to the east of the water divide in the back-arc region (Fig.
1) (Ramos and Folguera, 1998; Folguera et al., 2001; Iaffa et al., 2002; Melnick et al., 2002;
Folguera et al., 2003), folding and thrusting Plio-Pleistocene lavas and Quaternary fluvial and
lacustrine sediments; (ii) Intra-arc deformation is dominated by normal faults that affect the
Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad.
Quaternary sediments, Plio-Pleistocene Cola de Zorro Formation and older units (Fig. 2);
and (iii) at 37°20´S, the contact between the Central Valley and the Principal Cordillera
was observed, 10 km west of the Antuco town, as a WNW vergent reverse fault, that thrusts
Cura-Mallín Tertiary lavas on top of fluvial conglomerates of probable Late Pleistocene age
(Fig. 1).
Figure 1. Location and regional tectonic setting of the study area.
LLF: Lago del Laja fault system. CCM: Callaqui-CopahueMandolegüe transfer zone. LOFZ: Liquiñe-Ofqui fault zone. Black
triangles denote Holocene volcanoes. Topography from
GTOPO30.
THE LAGO DEL LAJA FAULT SYSTEM
The Lago del Laja is a narrow ~32 km long volcanic-dammed lake, located in the intra-arc
axial zone (71°15’W) of the Southern Central Andes. The 2,979 m high Antuco volcano,
located on the southwestern side of the lake (Fig. 2) suffered a major caldera collapse
event, the volcanic avalanche deposits from this event and later basaltic lavas originated the
Laja Lake. This collapse event has been dated 14C at 9,700±600 (Moreno et al., 1986) and
6,250±60 B.P. (Lohmar, 2000). The second age has less analytic error and was done on a
single piece of ‘Coihue’ Nothofagus dombeyi found two km northwest of the Abanico town
on the avalanche deposits.
Niemeyer and Muñoz (1983) in their regional study mapped a fault along the Quique river,
on the northern end of the Laja Lake (Fig. 2) but no kinematic information was given. A
regional tectonic survey (Melnick et al., 2002) between (37°-39°S) also mapped a fault
along the entire Laja Lake, although based on remote sensing interpretation.
At the crossing of the Aguila and Polcura rivers (Fig. 2), a west narrowing wedge-shaped
plateau formed by Pleistocene valley-confined probably subglacial lavas, outcrops on top of
folded Cura-Mallín strata. A steep topographic break marks the contact of the plateau with
the higher Cura-Mallín rocks to the east (Fig. 2). Preliminary computations indicate the
emission of ~1 km3 of lavas, probably during one event since there are no interbedded slope
deposits expected from the steep topographic change to the east. The plateau morphology,
valley-confinement, wedge shape, and alignment with the Quique River indicate that the
ascension of these flows was facilitated by the Lago del Laja fault system. Air photo
interpretation also show a scarp affecting these lavas, indicating that tectonic activity along
this segment of the fault system continued after the volcanic emissions.
Figure 2. Geological map of the Laja Lake (modified from Niemeyer and Muñoz, 1983) with shaded bathymetry.
Thick black lines: Quaternary faults. Thin lines: Late Miocene faults and folds. OMc: Cura-Mallín basin deposits.
Pcz: Cola de Zorro Formation. Qv: Quaternary volcanics. Qs: Quaternary sediments. Hv: Holocene volcanics. Tg:
Tertiary intrusions. Gray lines: Reflection seismic lines indicating Holocene faulting.
At a road-cut on the shore of the Polcura River, immediately south of the lava plateau, a set
of ten normal faults were recognized (Fig. 4). They affect post-glacial fallout tephra
deposits from the Chillán volcano, located 23 km to the north. The faults have displacement
of up to 3 m, showing horst-graben geometry with a general west-down polarity. Fallout
tephra deposits from the Chillán volcano have been regionally dated 14C between 9,300±70
and 2,270±60 B.P. (Dixon et al., 1999).
An air photo (SAF Geotec 1:70.000 N° 8483) of the Quique valley (Fig. 3) shows a N
trending fault-scarp, that forms a ~2 km long alignment of dense vegetation, about 20 to 25
m wide, probably concentrated due to the spring of water trough the densely fractured fault
zone. This lineament also marks a topographic break of the glacial valley bottom, from
where recent alluvial fans develop. Fluvial terraces along the Quique valley show
asymmetric topographic levels along an E-W profile, being the western at lower altitude,
due to normal faulting. These observations indicate post-glacial activity along the Lago del
Laja fault system in this area.
Within the framework of paleoclimatic research, high- reflection seismic data were
collected on Lago del Laja (Charlet et al., this volume). They were recorded using a
“Centipede” multi-electrode sparker, operated at 500 J with a signal frequency range of
around 100-1,500 Hz, and a subbottom-profiler (Transducer MountModel 132B, 3.5 kHz),
composed by a transmitter Model 5430A and a receiver Model 5210A. The pinger profile
Laja04 located in the central part of the lake (Fig. 2), show two hemi-graben structures with
associated antithetic faults that produced a horst pillar in the central part. These faults
affected up to the most recent sediments of the Lake-bottom (Fig. 5), generating relief on
the flat-lake morphology, product of active extensional deformation. The lake-bottom
sediments are younger than 6,250±60 B.P. (Lohmar, 2000) constraining the age of the
deformation.
Bathymetric profiles show steep-depth breaks in the lake’s mostly flat bottom (Fig. 2). A
steep linear topographic break, in the southern part of the lake along the eastern slope of the
Antuco volcano, indicates that this part of the lake is probably controlled by an active
normal fault (Fig. 7). This NNW trending fault produces the sinking of the Antuco eastern
slope lavas, probably allowing the lake to still exist.
REGIONAL QUATERNARY INTRA-ARC COLLAPSE
Normal faults with well-exposed conjugated riedel shears, and small rotated domino-like
blocks filled with syntectonic alluvial deposits, were identified on several cuts along the
road from the Antuco volcano toward the west. These normal faults affect post-glacial
pyroclastic flows from the Antuco volcano and created a dissected morphology for the
alluvium that filled the basin. East of the Quillailebu River, on the southern flank of the
Laja River, the contact between Plio-Pleistocene lavas and Miocene granitoides is
interpreted as a west-dipping normal fault, with a minor antithetic fault on the hanging wall
affecting the lavas.
Deep glacial erosion exposes the interiors of the Sierra Velluda volcano, a 0.6-0.3 Ma
(Moreno et al., 1986) 3,585 m high volcano, located immediately southwest of the Antuco
volcano. This abnormally enormous edifice is composed by ~2,500 m of lavas, pyroclastic
flows and breccias. On the uppermost Malalcura river, a N60°W/65°SW striking normal
fault was identified cutting a 2,000 m section of the volcano (Fig. 6A), with at least 500 m
of displacement. This fault juxtaposes, to the southwest, composed only by lavas,
corresponding to the upper terms of the Sierra Velluda volcano with, to the northeast, a
sequence of lavas with light gray interbedded pyroclastic flows, section (Fig. 6B). This
normal fault has several antithetic small faults on the hanging wall and a ~20 m wide
hydrothermal alteration zone along the fault plane (Fig. 6B). No caldera collapse events
have been described nor identified in the current survey for the Sierra Velluda volcano, so
we interpret this fault as an evidence for a Quaternary extensional collapse of the intra-arc
zone.
Figure 3. Left: SAF 1:70,000 Geotec air photo. Right: Map of the Lago del Laja fault in the
Quique river area, digital elevation model and river network from 1:50.000 topomaps.
Figure 4. View to the south of Holocene fallout tephra deposits, from the Chillán volcano,
affected by normal faults. Location on Figure 2.
Figure 5. Transversal Pinger profile Laja04 from the Laja Lake showing horst-graben structures affecting
the lake-bottom sediments. Location on figure 2.
Figure 6. A: View to the southeast of the Sierra Velluda volcano from the Laja valley. Black triangles
denote fault zone. B: Detail view of the fault zone from the uppermost Malalcura River. Black triangle
denotes hydrothermal alteration zone. Dipping varies at both sides of the fault.
Figure 7. Shaded image of merged bathymetric and 1:50.000 topographic data. Black triangles denote
fault scarp. White triangle denotes Antuco volcano caldera border. Bathymetric contour interval 20 m.
CONCLUSIONS
Field observations, seismic reflection profiles, bathymetric data, and air photo
interpretation indicate that the Lago del Laja fault system has been active during
Pleistocene and Holocene times. We characterize the Lago del Laja fault system as an
active intra-arc fault. Probably the activity of the adjacent Antuco volcano is somehow
related to the Lago del Laja fault system as observed further south for the 1989 Lonquimay
volcano (38°50’S) eruption and dextral activity of the Liquiñe-Ofqui fault zone (Barrientos
and Acevedo, 1992). The N trending axial intra-arc position of the Lago del Laja fault
system and the equivalent strike and extent with the back-arc thrust-front indicate that the
intra-arc between 37°S and 37°45’S is actively collapsing in response to shortening in the
back-arc. The Andean Main Cordillera, at these latitudes, seems to be growing like a
wedge, with opposite-vergent thrusts on each side, defining a pop-up structure with active
collapse in the axial central part.
ACKNOWLEDGMENTS
We would like to thank K. Bataille (U. de Concepción) for more than logistic support, E.
Bartulovic (Oleoducto Transandino S.A.) and R. Verdugo (CONAF) for allowing us to
access the Ñuble protected area, the staff of “Carabineros de Chile” who where at
“Avanzada Cuatro Juntas” during February 2003 for invaluable help, M. Pino (U. Austral,
Valdivia), R. Urrutia (EULA, U. de Concepción), R. Fuenzalida for his help in the field, C.
Muñoz, G. Hermosilla and M. Moreno. This work was supported by IQN “International
Quality Network” Universität Potsdam, GeoForchungZentrum Potsdam Southern Andes
Project, SFB 267 “Deformation Processes in the Andes” and PICT 06729/99 of “Agencia
Nacional de Promoción Científica y Tecnológica” to V.A. Ramos. The bathymetric survey
of Lake Laja took place within the chilean-german cooperation project “Water Resources
and Energy: Aquatic ecosystem responses to water level” (WTZ 00/002). Special thanks
apply for the staff of EULA Centre at the University of Concepción for support in field
work, particularly to A. Peña and H. Alonso as well as K. Rahn from UFZ Centre for
Environmental Research Leipzig Halle, Germany.
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