Crustal Structures underneath the Longmen Shan mountain range from passive seismological data

 A. Robert1, J. Vergne2, G. Herquel2, R. Cattin1, J. Zhu3

1. Laboratoire de géologie de l'ENS, 75231 Paris Cedex 5
2. E.O.S.T ., 67000  Strasbourg
3. Institute of Technology of Chengdu, China

 

Located between the eastern margin of the Tibetan plateau and the Yang Tse craton, the Longmen Shan mountains range is a key area for understanding mechanisms that control the deformation of the Tibetan plateau and its extrusion to the east.
Similarly to the Himalayas, this mountain range is characterised by a sharp geomorphic expression, and important seismicity along the front range.  However, the lack of a well-developed foreland basin east of the Longmen Shan and low shortening rate through the range as deduced from GPS (~ 4 mm/yr for the Longmen Shan vs ~20 mm/yr for the Himalayas of central Nepal) , tend to indicate different processes and/or history between these two regions.

The presence of a low viscosity channel flow in the middle/lower crust (Burchfiel et al. (1995), Clark and Royden (2000))  is often invoked to explain the occurrence of a major topographic step without horizontal shortening in the Longmen Shan. However these features can also be explained by a recent arrest of the convergence across the range (Arne et al., 2004) which is still dynamically maintained (Godard et al., 2005).

In order to bring new information on the lithospheric structure of this area,  36 seismic stations were deployed across the Longmen Shan, from November 2005 to April 2007, in the framework of a collaboration between several French institutes and the University of Chengdu.

Here we present the first results from this experiment based on receiver functions and SKS anisotropy which lead to a new, high resolution, image of the major interfaces within the lithosphere and insights into its dynamic. Combined with gravity anomalies, these information bring new arguments to the actual debate and  improve our understanding of this area.