S receiver function analysis of the crustal and lithospheric structures beneath eastern Tibet

S receiver functions from 51 permanent broad-band stations are used to investigate the structures of the crust and lithosphere beneath eastern Tibet and Sichuan Basin. By stacking the S receiver functions, the Sp converted phases from the Moho and the lithosphere–asthenosphere boundary (LAB) are clearly visible. The Moho depth ranges from 40 to 50 km beneath Sichuan Basin and from 55 to 60 km on the west side of the Longmenshan (LMS) fault. The largest depth is 70 km, occurring at the northern Sichuan–Yunnan terrane. The LAB depth beneath Sichuan Basin ranges from 130 to 170 km, while that on the west side of LMS ranges from 100 to 120 km. From the east to the west across LMS, there is a sudden rise of the LAB, followed by a gradual dip. Western Sichuan Basin is found to be surrounded by thinner lithosphere, extending northeastward along the LMS and southward to Indochina. The thinner lithosphere may indicate that the lower part of eastern Tibet lithosphere is heated and delaminated by the underlying hot asthenospheric flow, thus reflecting the size and direction of the flow. It appears that asthenospheric flow from Tibet is resisted by the cold rigid Sichuan Basin, thus branching into northeastward and southeastward parts. The complicated topography of the lithosphere likely results from this dynamic process.