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

Robert Alexandra, arobert@geologie.ens.fr

Laboratoire de géologie de l’ENS - 24, rue Lhomond – 75231 Paris Cedex 5

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.

Similarly to the Himalaya, this mountains range is characterised by a sharp geomorphic expression, and is carried over a major thrust characterized by important seismicity. Several major differences, nevertheless, exist between these two mountain ranges, in particular the Longmen Shan is characterized by the lack of a well-developed foreland basin and GPS data document low shortening rate through the range (~ 4 mm/yr for the Longmen Shan vs ~20 mm/yr for the Himalayas of central Nepal) (Zhang et al., 2004).

A first model proposed that the presence of a channel flow in the ductile lower crust explain the presence of this topographic step (Burchfiel et al. (1995), Clark and Royden (2000)) whereas Arne et al. (2004) and Godard et al. (2005) propose that the strong topographic gradient is a result of a Tertiary orogenese.

In order to bring new information on this area, 36 seismic stations were deployed from November 2005 until April 2007, along a profile through the Longmen Shan area.

Using receiver function method and teleseismic tomography, a detailed image of the structures through the Longmen Shan Thrust belt has been produced. Gravity anomalies of this region coupled with the precise crustal structure given by seismological studies provided new information and then improve the understanding of this area.