Lithospheric structures across the Longmen Shan mountain range from seismologic and gravimetric data

Abstract

The Wenchuan earthquake was somehow unexpected because it occurred in a region where the present-day deformation was not considered to be very high, as attested by GPS measurements and the lack of a well- developed foreland basin to the East of the Longmen Shan range. These observations, together with the persistence of a high topographic border are at the heart of a wide variety of mechanical models, among which several go by the existence of a low viscosity channel at mid crustal depth. Thus, bringing strong constrains on the lithospheric structure and composition in this region is a key to firm up these models. This is what motivated a sino-french team to deploy a dense seismic network between November 2005 and April 2007 in the central part of the Longmen Shan belt, very close to where the Wenchuan earthquake occurred one year after the end of this experiment. Applying the receiver function method to the teleseismic earthquakes recorded by the 36 seismic stations, we obtain a detailed and continuous image of the crustal structure from the Sichuan basin up to the Xianshuhe fault. This image reveals a crustal discontinuity at about 15km depth which may connect with the ruptured zone of the Wenchuan earthquake. Moreover, our results highlight an abrupt 20 km Moho offset between the Sichuan Basin and the Tibetan plateau and this sharp Moho offset is consistent with regional gravity data and microgravity measurements acquired along the seismological profile. These observations indicate a major contrast in strength between the Tibetan plateau and the Yangtze craton, the latest acting as a rigid block resisting the eastward displacement of the Tibetan Plateau. However, the mean crustal Poisson ratio, deduced from the analysis of the receiver functions, is low to normal beneath all the stations located on the Songpan-Ganze terrane, which suggests the absence of a thick and extensive zone of partial melt within the crust of this region.