The seismological structure of the Tibetan Plateau crust and mantle down to 700 km depth

Abstract

AbstractA seismic velocity cross-section down to 700 km depth beneath the Tibetan Plateau has been constructed. Beneath the cover layer, felsic rocks rich in α quartz exist down to 15–25 km depth. Beneath these depths, temperatures are probably high enough for ductile flow and partial melting to occur. The velocity increase across the boundary at 30–40 km depth marks the interface between felsic upper crust and more mafic lower crust. Crustal thickness is greatest (c. 74 km) south ofc. 31.5°N, where Indian lower crust forms the basal layer. Northwards, crustal thickness decreases toc. 66 km around 33°N, before increasing toc. 70 km beneath northern Tibet. Crossing the Kunlun, the crust thins toc. 54 km beneath the Qaidam basin. High-velocity, dense, cold Indian lithospheric mantle extends northwards until about the Banggong-Nujiang suture, where it downwells to 350–400 km depth. The lithosphere–asthenosphere boundary occurs at 160–225 km depth. The apparent northwards deepening of the 410 and 660 km discontinuities implies that the upper mantle beneath northern Tibet is slower, less dense and warmer than under southern Tibet which, in turn, could provide some of the isostatic support for the high elevations in northern Tibet where the crust is thinner than under southern Tibet.