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Accueil du site > Les thèmes scientifiques > Chaînes de montagnes et bassins > Chantiers > Longmen Shan-Tibetan Plateau-Longriba

Contacts : Julia de Sigoyer, Manu Pubellier, Nicolas Chamot-Rooke

Longmen Shan-Tibetan Plateau-Longriba

Géodynamique

GIF - 4.6 ko

Our current investigation is founded by the ANR "jeunes Chercheurs" on a program call "The Longriba fault : the key to unravel the tectonics of the Eastern Tibetan Margin"

With 3 million of Km2 at 5km of elevation the Tibetan plateau represents the biggest and highest geomorphic continental structure on Earth surface. Its uplift is assumed to be related to the collision of India with Eurasia as first suggested by Argand, 1920, since 55-50 Ma ago (de Sigoyer et al., 2000). This uplift is either related to the thickening of the crust that is about 70 km (Braintenberg et al., 1999) or to removal of its lithospheric mantle that leads to the thinning of the lithosphere (Molnar et al., 1993). Beside the Tibetan plateau has been formed by the accretion of different terranes since the end of the Paleozoic. It comprises from the South to the North the Himalayan block, the Lhasa block, the Qiangtang block, the Songpan Garze block (Pubellier et al., 2008). Hereafter are presents our mains results and projects on the eastern Tibetan plateau and Longmen Shan belt (Sichuan China).


 DE SIGOYER Julia (MCF)ENS Paris UMR 8538-Geodynamic-metamorphic petrology

 GODARD Vincent (MCF) Univ. Aix en Provence-Geomorphology modelling

 BELLIER Olivier (Prof) Univ. Aix en Provence-Neotectonics

 BOSSE Valérie (MCF) Univ. Clermont Ferrant-U-Th-Pb in situ dating by La-ICPMS

 CATTIN Rodolphe (Prof) Univ. Montpellier- Seismic cycle modelling.

 CHAMOT-ROOKE Nicolas (CR1) ENS Paris UMR 8538-Strain rate modelling

 DOIN Marie-Pierre (CR1) ENS Paris UMR 8538-INSAR

 DUCHENE Stéphanie (Prof.) Univ. Toulouse-Isotopic geochemistry

 LASSERRE Cécile (CR1) Grenoble Univ.-INSAR

 PUBELLIER Manuel (DR2) ENS Paris UMR 8538-Neoctectonic Geodynamic

 DENSMORE Alexander(Associate Professor), Durham University UK

 LI Yong (Prof)Chengdu Univ. Of Technology CH

PNG - 4.7 Mo
carte asie

The Longmen Shan represents the border of the eastern Tibetan plateau against the Yangtze craton. Our group has shown that under this belt a sharp Moho step of about 20km high between the western thick Tibetan crust (with a thickness of about 65km) against the resistant 45km-thick lithosphere of the Yangtze craton (Robert et al., 2010b ; Robert et al., 2010a). The Longmen Shan presents very high topographic gradient whereas the convergence rate deduced by geodetic data is low : 3 +/-3mm/yr (Gan et al., 2007, Shen et al 2004, Chen et al., 2010). The formation of the high topography in the area, is recent from 8 to 11Ma as shown by thermochronological studies on the exhumed basement (Godard et al. 2010, Kirby et al., 2002, 2010).  : the paradox of high eastern Tibetan plateau with very low convergence rate (Shen et al., 2005 ; Gan et al., 2007) that led to an underestimation of the seismic hazard in the Longmen Shan area prior to the May 12th 2008, Mw 7.9 Wenchuan earthquake. The Wenchuan earthquake struck Sichuan Province (China) causing one of the worst natural disaster in China for 50 years. The earthquake killed more than 80,000 people and more than four million inhabitants were left homeless. The rupture occurred in a region of no prior large historical earthquakes. GPS results have repeatedly shown no significant convergence across the Longmen Shan fault system, and for decades, many scientists have considered that only minor recent deformation had occurred in the area. The Wenchuan earthquake has cruelly challenged this belief. In 2004, based on geomorphic and long term geological observations, our team claimed that it was important to improve the seismic hazard assessment in the Longmen Shan area and obtained ANR JC funding in 2006, headed by R. Cattin and entitled “Importance and coupling of processes responsible for the patterns of continental dynamics and geomorphic evolution”. This prior work was motivated by a crucial problem in our understanding of the seismic hazard in Eastern Tibet : it is difficult to reconcile long-term geological processes (several million years), which have led to the thick and steep Tibetan margin, and short-term geophysical studies (seconds to decades) that show little evidence for shortening across this area.

We have obtain an ANR funding in 2011, headed by Julia de Sigoyer to investigate this paradox, focusing our study on the Longriba fault located 150 km west of the Longmen Shan front (Tibetan margin). This fault is a key structure that is rarely taken into account in conceptual frameworks for the geodynamics and seismo-tectonics of this area, although it seams to accommodate a large part of the present-day relative movement (6-8 mm/yr) between the Songpan block (Bayar Han block) and the south China block (Thatcher, 2005, Shen et al 2005). It seems also to partition the movement between dextral strike-slip movement along the fault itself and residual E-W convergence that is taken on the Longmen Shan. The interaction between the Longriba fault and the faults in the Longmen Shan is critical, because its estimated Quaternary slip rate is an order of magnitude higher than that of the Longmen Shan faults ; it may thus account for a large part of the present-day deformation, and must be accounted for in any regional assessment of seismic hazard.

Bibliographie
 Cavalié 0., C. Lasserre, M.-P. Doin, G. Peltzer, J. Sun, X. Xu and Z.-K. Shen (2008), Measurement of interseismic strain across the Haiyuan fault (Gansu, China), by InSAR, Earth and Planetary Science Letters, 275, p 246-257, doi:10.1016/j.epsl.2008.07.057

 De Michele, M., Raucoules, D., de Sigoyer, J., Pubellier, M., and Chamot-Rooke, N., 2010, Three-dimensional surface displacement of the 2008 May 12 Sichuan earthquake (China) derived from Synthetic Aperture Radar : evidence for rupture on a blind thrust : Geophysical Journal International, v. 183, p. 1097-1103.

 De Michele M., Raucoules D., Lasserre C., Klinger Y., Van Der Woerd J., de Sigoyer J., Xu X., Aochi H., 2010, The Sichuan Earthquake Rupture Measured by Synthetic Aperture Radar, EPS

 Godard, V., Cattin, R., and Lave, J., 2009, Erosional control on the dynamics of low-convergence rate continental plateau margins : Geophysical Journal International, v. 179, p. 763-777.

 Godard, V., Lave, J., Carcaillet, J., Cattin, R., Bourles, D., and Zhu, J., 2010, Spatial distribution of denudation in Eastern Tibet and regressive erosion of plateau margins : Tectonophysics, v. 491, p. 253-274.

 Godard, V., Pik, R., Lave, J., Cattin, R., Tibari, B., de Sigoyer, J., Pubellier, M., and Zhu, J., 2009, Late Cenozoic evolution of the`centRal!Longmen Shan, eastern Tibet : Insight from (U-Th)/He thermochronometry : Tectonics, v. 28.

 Raucoules, D., and de Michele, M., 2010, Assessing Ionospheric Influence on L-Band SAR Data : Implications on Coseismic Displacement Measurements of the 2008 Sichuan Earthquake : Ieee Geoscience and Remote Sensing Letters, v. 7, p. 286-290.

 Robert, A., Zhu, J., Vergne, J., Cattin, R., Chan, L.S., Wittlinger, G., Herquel, G., de Sigoyer, J., Pubellier, M., and Zhu, L.D., 2010a, Crustal structures in the area of the 2008 Sichuan earthquake from seismologic and gravimetric data : Tectonophysics, v. 491, p. 205-210.

 Robert, A., Pubellier, M., de Sigoyer, J., Vergne, J., Lahfid, A., Cattin, R., Findling, N., and Zhu, J., 2010b, Structural and thermal characters of the Longmen Shan (Sichuan, China) : Tectonophysics, v. 491, p. 165-173.

Mots-clés

Longmen Shan