Upper mantle flow beneath - Pages perso de
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Upper mantle flow beneath - Pages perso de
1 Guilhem BARRUOL T23A-1201 I. UPPER MANTLE ANISOTROPY δt + φ (azimuth of the fast S wave) Upper mantle PS upper mantle (V1) ScS (V2) We use SKS and SKKS phases from distances in the range 85° to 145° to ensure no interference with other seismic phases. Only events with a signal-to-noise ratio > 3 were analyzed. We used the method of Silver and Chan (1991) and also the SpliLab software Where anisotropy is observed Seismic event Crust SS ScS2 S Lower mantle S Anisotropic SKS D" 180 (Wuestefeld et al., 2007) to compute the splitting parameters: the polarization of the fast shear-wave, φ and the delay time between the fast and slow shear wave, δt. 1 54˚ 90 HURE event 27/07/03 SKS HURE φ = -48°, δt = 1.4 s 60 100˚ fast axi s 0 ya 0 -30 -60 -5000 -20 0 2000 20 -1 40 5 10 15 20 W-E 25 1 BAYN event 27/07/03 SKS 0 -90 0 1 2 90 3 OKTB fast axi s ARSH KIRN 52˚ 0 -60 0 20 -1 40 0 5 10 15 20 25 -90 W-E 1 SKS 0 1 2 3 4sec 90 DALA event 27/07/03 DALA φ = -31°, δt = 1.2 s 60 0 110°E 115°E 60° 100° 120° Sa meters Bolnay tay Han gay Bogd GobiAltay 4 ika 0 Ba Hantay 2000 -90 0 45 90 135 180 225 Backazimuth 270 ULN, best 2 layers model: φupp = 72° δtupp = 1.0 s φlow = 136° δtlow = 0.8 s Mongolia 1000 330 0 30 60 300 China A 90 3s 2s 1s delay time 45 135 46˚ BUGA TSET ULN BUMB UULA DALA ALTA 44˚ 180 150 a 225 Backazimuth best φupp Gobi Alta i Individual splitting measurements at the MOBAL station together with the TLY and ULN IRIS permanent broad band station. Fair (grey) and good (black) quality are plottted. The splitting are characterized by their azimuths and the delay time (length of the segment). 200 270 315 360 d 250 e 200 150 ULN, best 150 1000 models 100 100 0 0 50 20 40 60 80 100 120 140 160 180 Upper layer fast direction 0 0 20 40 60 80 We performed direct modelling of 2 anisotropic layers that we compared with our observations. a: anisotropic parameters of the 50 good splitting measurements in a polar diagram. best φlow 120 210 180 Delay times 2 A H S 2.0 48 46 YN A B DR BA 44 B UM B TA L A TB OK 0. 5 52 50 48 46 Latitude 44 DALY DLY2 SHA2 Splitting parameters obtained at ULN (Ulaanbaatar, Mongolia, more than 10 years of data) display backazimutal variations that suggests the presence of 2 layers of anisotropy. 90 50 ( SKS, lithospheric thickness and volcanism Tomographies (Mordvinova et al., 2007) and petro-geochemical analyses of mantle xenolith (Ionov, 2002) suggest that the Siberian lithosphere is around 200 km thick whereas it is less than 100 km thick beneath central Mongolia. The lithospheric structures clearly wrapped around the Siberian platform that acted as a rigid nucleus during the Paleozoic block accretion. The NW-SE φ trend observed across the Hangay dome could then be partly related to old lithospheric deformation. Interestingly, the largest δt are observed above the low velocity anomalies in the upper mantle tomography, compatible with a strong asthenospheric participation in the signal. The anisotropy beneath the Hangay dome could therefore result from two lithospheric and asthenospheric anisotropies of similar orientations, their individual effects adding together. OVGO c 0 0 360 52 ) p up TLY TUSG ULN 50 240 0 315 U 1.5 BAYN Han gay dom e 1 b LN (l SHAR 2 -60 TLY ULN 30 O TA L A ) ow BADR 4sec 3 -30 Russia 3 108˚ Two layers of anisotropy beneath ULN TH MU ZI Al 45°N n Hövsgöl 50°N 3000 l ya 2 The two stations located in the Gobi-Altay range (ALTA and DALA) are characterized by intermediate δt (1.3 s) and statistically different φ directions (N030°W) than the Hangay stations. 60 140° 1 Delay times 20° 80° 0 Stations on the Hangay dome are characterized by homogeneously NW-SE trending φ and high δt (2.8 s at BAYN, 1.9 s at BADR, 2.3 s at BUMB), suggesting a strong and coherent mantle deformation beneath the stations. fast Azimuth 55°N -90 W-E 30 90 Siberian Craton 0° 20 TLY exhibits a complex anisotropy pattern which is not compatible with a single anisotropic layers but suggests strong mantle heterogeneities beneath this area. 40° 20° INDIA 10 Except at the northernmost stations where only few data were available, clear anisotropies have been detected in central Mongolia. 4000 EURASIA 0 Number of models 60°N 105°E -1 40 48˚ From April to October 2003, 18 three-component, broadband stations from the French Lithoscope program were deployed along a NS trending profile extending from the southern Siberian platform to the Gobi-Altay range, crossing the southwestern tip of the Baikal rift and the whole Hangay dome. This temporary seismic deployment took place in a larger project combining other observations such as geodesy and seismotectonics in order to constrain the crustal and mantle structures but also the past and present-day tectonic processes occurring in the Baikal-Mongolia system. 100°E 20 Examples of splitting measurements obtained for event 2003/208 (27-Jul-2003, 02:04, Mw=6.6, lat. -21.08°N, long. -176.59°E, depth 212km, backazimuth 110°) at stations HURE, BAYN and DALA. Mongolian-Baikal Lithosphere seismological Transect 95°E 50˚ -60 0 HURE 0 -30 -4000 -20 90°E fast axi s 0 S-N II. THE MOBAL NETWORK TORI -30 -3000 -20 106˚ KAIT 4sec 30 0 B T K 1.0 n BAYN φ = -58°, δt = 2.9 s 60 0 104˚ Siberian Platform Sa 30 0 102˚ 2.5 δt (s) Map of Eigenvalues Particle motion before (- -) & after (-) S-N Shear wave splitting analysis is a way to scan the active or frozen upper mantle flow. It is directly induced by olivine preferred orientation and its intrinsic anisotropy. In oceanic domains, upper mantle anisotropy is likely controlled by the asthenospheric flow gradually freezing at the bottom of the lithosphere. In continental domain, the deformation may be partly frozen within the lithosphere and related to ancient tectonic processes but also present in the underlying asthenosphere and related to present-day processes, such as the plate motion. 5000 Corrected Fast (-) & Slow(- -) 98˚ al Outer core UL 80 3.0 Original radial (- -) & transverse (-) components N 60 30 Mongolia represents the northernmost area affected by the Indian-Asia collision and is actively deformed along transpressive belts closely associated to strike-slip faults. In order to investigate the deep mantle deformation beneath central Mongolia and its relation with the surrounding major structures such as the Siberian craton, the Altay range and the Baikal rift, a NS trending profile of broadband seismic stations has been deployed in summer 2003 from the southern Siberian craton to the Gobi-Altay range, crossing the whole Hangay dome. Mantle flow is deduced from the splitting of teleseismic shear waves such as SKS phases. In eastern Mongolia, the permanent station ULN in Ulaanbaatar reveals the presence of two anisotropic layers, the upper one being oriented NE-SW, close to the trend of the lithospheric structures and the lower one NW-SE, close to the trend of the plate motion. Along the NS profile in central Mongolia, seismic anisotropy deduced from SKS splitting reveals a homogeneous NW-SE trending structure, fully consistent with the observations performed in the Altay-Sayan in western Mongolia. Since the observed delay times of 1.5 to more than 2.0 s suggest coherent mantle flow over large mantle thicknesses and since the observed fast directions are parallel to the trend of the lithospheric structures but also close to the trend of the plate motion, we propose that both the lithosphere and the asthenosphere may add their anisotropic effects beneath central Mongolia. In order to interpret the slight clockwise rotation of the fast directions relative to the plate motion vector, we propose that the root of the Siberian craton could deflect the asthenospheric flow around its southwestern side. GPS vectors and SKS splitting depicts a similar trend beneath central Mongolia, suggesting that the block “escaping” toward the east moves consistently with the lithospheric and asthenospheric mantle flow. A strikingly different behaviour is observed in western Mongolia: The GPS vectors trend NS whereas the fast SKS directions trend EW, suggesting that a decoupling occurs somewhere between the upper crust moving northwards and the mantle flowing eastwards. 120 V. CONCLUSION SOUTH Fast split directions ik Isotropic NORTH 140 100 Polarized incident SKS wave 4000 ABSTRACT 160 φ (°N) SKS wave splitting IV. DISCUSSION III. SHEAR WAVE SPLITTING MEASUREMENTS S-N (1) Géosciences Montpellier, CNRS, Université Montpellier II, France. [email protected] [email protected] (2) Géosciences Azur, UNSA/CNRS Valbonne, France. [email protected] (3) Université de Bretagne Occidentale, CNRS, Plouzané, France. [email protected] [email protected] (4) Institute of the Earth's Crust Irkutsk, Russia. [email protected] (5) Research Centre of Astronomy & Geophysics, Ulaanbaatar, Mongolia. [email protected] [email protected] Upper mantle flow beneath the Hangay dome, central Mongolia 3 Julie PERROT , 4 Alexandre ARTEMIEV , 5 Tundev DUGARMAA 1 Götz BOKELMANN Ba 2 Anne DESCHAMPS , 3 Jacques DEVERCHERES , 4 Valentina MORDVINOVA , 5 Munkhuu ULZIIBAT , 100 120 140 160 180 Lower layer fast direction b and c: apparent variations of φ and δt as a function of the backazimuth of the incoming wave, for the best two-layer model found and for our observations. d and e: histograms showing the distribution of the upper and lower fast directions of the 1000 best two-layers models amongst more than 1 million models tested. GPS and SKS : crust mantle (de)coupling This map of Mongolia and surrounding areas presents the mean SKS splitting measurements available in the region, together with the GPS vectors from Calais et al. (2003) and the absolute plate motion vectors calculated from HS3-Nuvel1A. In central Mongolia, the SKS fast directions and the GPS vectors are rather close from each other, and also to the present-day APM, suggesting that the crustal block escape is coherently accompanied by a present-day mantle flow. In western Mongolia, GPS vectors are close from NS and SKS φ close from EW, suggesting a complete decoupling between the crustal tectonics and the underlying mantle deformation. Siberian Platform APM 80 Km Ha B ng ai l ka ay ULN a a: At lithospheric depth (e.g., 80 km), the main lithospheric structures wrapped around the Siberian craton. The anisotropic upper layer at ULN trends NE-SW and NW-SE across the hangay dome. 86˚ 88˚ 90˚ 92˚ 94˚ 96˚ 98˚ 100˚ 102˚ 104˚ 106˚ 108˚ 110˚ 112˚ 54˚ 52˚ 50˚ 48˚ Acknowledgements This work was performed with funding from the French CNRS (Centre National de la Recherche Scientifique) PICS (International program for scientific collaboration) program 1251, and DYETI CNRS contract and Lithoscope French mobile instrumentation. We also thank the Institute of the Earth's Crust RSA, Irkutsk, RCAG, Ulan Bator and DASE/CEA, France for their support. The data will be available in 2008 at the site http://bdsis.obs.ujf-grenoble.fr/ maintained by C. Pequegnat. 46˚ 44˚ 42˚ The MOBAL temporary deployment across central Mongolia allowed us to characterize upper mantle anisotropy beneath most stations. The anisotropy pattern is clearly defined across the Hangay dome, with NW-SE trending φ and relatively large δt. Geophysical and geochemical arguments favors a lithosphere thinner than 90 km beneath central Mongolia, contrasting with a thick (at least 200 km) Siberian platform. This suggests that the splitting across the Hangay dome cannot be explained by a lithospheric deformation alone. We propose that the observed anisotropy reflects a combination of both lithospheric and asthenospheric deformation, the first resulting from the long-lasting geological evolution and recent (Cenozoic to present-day) deformation of the Mongolian lithosphere along active margins and along the present-day large-scale strike-slip faults and the second, asthenospheric, being related to the present-day Eurasian plate motion. Our preferred model suggests that the NW-SE trending φ observed on the Siberian platform could be dominated by an asthenospheric flow and that both the lithospheric and asthenospheric flow have similar orientations beneath central and western Mongolia and may add their effects to explain the strong delay times observed. On the other hand, beneath eastern Mongolia, the lithospheric structures clearly rotate to a NE-SW dominant direction inducing a two-layered structure that is well imaged beneath Ulaanbaatar. The very homogeneous anisotropy pattern characterizing the whole central and western Mongolia region contrasts with the different behavior of these two regions as seen by the GPS velocity field, suggesting a complete crust-mantle decoupling beneath western Mongolia whereas a better coupling seems to exists in central Mongolia. References Absolute plate velocity (HS3-Nuvel1A) GPS velocity (Calais etal., 2003) Shear wave splitting 1.0 s 2.0 s 5 mm/yr 10 mm/yr (Dricker et al., 2002) (Gao et al., 1994, 1997; Vinnik et al., 1992; Silver et al., 1991) Schematic presentation of the upper mantle flow that could explain the seismic anisotropy observed in Mongolia. In this model are summarized the SKS splitting, the absolute plate motion (APM) and the GPS vectors. Such model explains the NW-SE trending anisotropy and the large δt across the Hangay dome and the two layers of anisotropy beneath Ulaanbaatar. 150 Km Calais, E., Vergnolle, M., San'kov, V., Lukhnev, A., Miroshnitchentko, A., Amarjargal, S. and Déverchère, J., 2003. GPS measurements of crustal deformation in the Baikal-Mongolia area (1994-2002): implications for current kinematics of Asia. J. Geophys. Res., 108, doi:10.1029/2002JB002373 Dricker, I., Roecker, S., Vinnik, L., Rogozhin, E.A. and Makeyeva, L., 2002. Upper-mantle anisotropy beneath the Altai-Sayan region of central Asia. Phys. Earth Planet. Int., 131, 205-223. Gao, S., Davis, P.M., Liu, H., Slack, P.D., Rigor, A.W., Zorin, Y.A., Mordvinova, V.V., Kozhevnikov, V.M. and Logatchev, N.A., 1997. SKS splitting beneath continental rift zones. J. Geophys. Res., 102, 22781-22797. Siberian Platform APM B Ha a a ik Gao, S., Davis, P.M., Liu, H., Slack, P.D., Zorin, Y.A., Mordvinova, V.V., Kozhevnikov, V.M. and Meyer, R.P., 1994b. Seismic anisotropy and mantle flow beneath the Baikal rift zone. Nature, 371, 149-151. l Ionov, D., 2002. Mantle structure and rifting processes in the Baikal-Mongolia region: geophysical data and evidence from xenoliths in volcanic rocks. Tectonophysics, 351, 41-60. nga y Mordvinova, V.V., Deschamps, A., Dugarmaa, T., Déverchère, J., Ulziibat, M., Sankov, V.A., Artem'ev, A.A. and Perrot, J., 2007. Velocity structure of the lithosphere on the 2003 Mongolian-Baikal transect from SV waves. Izvestiya-Physics of the Solid Earth, 43, 119-129. ULN b b: At asthenospheric depth (e.g., 150 km for central Mongolia), the active mantle flow is likely deflected by the thick Siberian cratonic root. Such lower anisotropic layer trends homogeneously NW-SE. Silver, P.G. and Chan, W.W., 1991. Shear wave splitting and subcontinental mantle deformation. J. Geophys. Res., 96, 16429-16454 Vinnik, L.P., Makeyeva, L.I., Milev, A. and Usenko, A.Y., 1992. Global patterns of azimuthal anisotropy and deformations in the continental mantle. Geophys. J. Int., 111, 433-437 Wüstefeld, A. G.H.R. Bokelmann, C. Zaroli, G. Barruol, SplitLab: A shear-wave splitting environment in Matlab, Computer & Geosiences (2007) doi:10.1016/j.cageo.2007.1008.1002.