A model of the crust and mantle structure down to 700 km depth beneath the Lhasa to Golmud transect across the Tibetan plateau as derived from seismological data

Abstract

A 700 km deep seismic velocity cross-section beneath the Lhasa to Golmud transect across the Tibetan plateau is presented. In contrast to the first version of this cross-section, which comprised an 800 km wide swath centred on the Lhasa to Golmud transect, due to the recent proliferation of publications concerning the mantle structure beneath Tibet, this study is based only on seismic profiles which either run along or cross the transect and arrays or studies which at least partly cover the transect. The results from the recent INDEPTH IV project indicate that the crustal thickness change from 70 km beneath the Songpan–Ganzi terrane and Kunlun mountains to 54 km beneath the Qaidam basin is located about 100 km north of the Kunlun Fault and almost 45 km north of the North Kunlun Thrust. The Qaidam basin Moho is underlain by crustal velocity material for almost 45 km and the apparently overlapping crustal material may represent Songpan–Ganzi lower crust underthrusting or flowing northward beneath the Qaidam basin Moho. Thus the high Tibetan plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath the stronger Qaidam crust. Beneath the crust, high-velocity, dense, cold Indian lithospheric mantle extends northwards until about the Banggong–Nujiang suture. Northwards, Asian lithospheric mantle is overlain by a low-velocity, less dense, warm Tibetan plate consisting of an upper lithospheric and a lower asthenospheric part. The apparent northwards deepening of the 410 and 660 km discontinuities by about 20 km implies that the upper mantle beneath north Tibet is slower, less dense and warmer than under south Tibet, in agreement with the observed uppermost mantle velocities. This, in turn, could provide some of the isostatic support for the high elevations in the north where the crust is somewhat thinner than in the southern plateau.