Varying deformation patterns in central Tibet revealed by radial anisotropy tomography

Abstract

P wave radial anisotropy tomography of the upper mantle beneath the central Tibetan plateau is derived using a large number of traveltime data of teleseismic events. Our results show a prominent high-velocity (high-V) zone which is separated by a low-velocity (low-V) anomaly beneath the Himalayan block, extending to ~33°N, and a low-V zone down to ~300 km depth beneath the northern Qiangtang block. The upper mantle beneath northern Qiangtang exhibits a significant positive radial anisotropy (i.e., horizontal Vp > vertical Vp), implying lateral material flow rather than material upwelling in the upper mantle. In the central part of the study region, a prominent high-V zone exists at depths of 150 to 300 km and exhibits a negative radial anisotropy (i.e., horizontal Vp < vertical Vp), reflecting the subducting Indian lithospheric mantle. Variations of shear wave splitting in this region revealed by previous studies may be attributed to the low-V layer above the Indian lithospheric mantle. A high-V zone with a positive radial anisotropy is located beneath the southern Lhasa block, which is interpreted to be the northward underthrusting Indian lithospheric mantle. Multilayer radial anisotropy is detected beneath southern Tibet, which may reflect a complex deformation pattern there.