Crustal and upper mantle structure beneath the NE Tibetan Plateau and its tectonic implication

Abstract:

The crustal and upper mantle velocity structures in the northeastern Tibetan Plateau are obtained from joint analysis of receiver functions and Rayleigh wave dispersion curves derived from teleseismic earthquake arrivals and ambient noise seismic data. The resulting velocity model reveals a close correlation between the thick (>60 km) crust and the presence of an intra-crustal low-velocity zone, which is detected beneath the Qiangtang and Songpan-Ganzi terranes as well as the northwestern Qilian orogen. However, the high Vp/Vs ratio is found only beneath the Qiangtang and Songpan-Ganzi terranes. The crustal low-velocity zone is not observed beneath the west Qinling and southeastern Qilian orogens, which have a relatively thin (~50 km) crust, indicating that crustal channel flow is not the primary mechanism by which the northeastern Tibetan plateau grows. In contrast to the widespread low velocities in the mid-to-lower crust beneath the Qiangtang and Songpan-Ganzi terranes, the upper mantle in these two regions shows alternating high and low velocity anomalies. A continuous low-velocity zone from the mid-to-lower crust down to 140 km beneath the eastern Kunlun fault suggests an induced local mantle upwelling after the delamination of the lithosphere.