Upper mantle Vp/Vs image beneath the Central Tibet from Receiver Function Tomography

Although numerous researches have already provided basic information on the deeper lithospheric structure under the Tibet, the northern foremost edge and geometry of the northwards underthrusting Indian lithosphere under the Asia plate are yet to be understood. A combination of V_p and Vs (V_p/V_s) could provide more insight into mantle physical state than either V_p and Vs alone, owing to their different sensitivities to partial melting, temperature, and composition.

By means of the strong dependence of the travel-time of the converted wave at the 410-km discontinuity on the V_p/V_s in the upper mantle, we developed a new mapping method to constrain the V_p/V_s in the upper mantle. Using the teleseismic records of 158 broadband stations deployed at the Central Tibet, which is consisted of 53 stations of SANDWICH array across Bangong-Nujiang suture, 48 stations east-west trending array in Lhasa block and 57 stations INDEPTH array, we obtained a high-resolution velocity ratios image of the upper mantle. A striking difference is shown between the western and eastern Tibet. A distinct high-velocity ratio zone exists at relatively shallow depth (<160 km) in the western Tibet, while in the eastern Tibet there is an east-west trending un-continuous high-velocity ratios at greater depth (270 340 km). We propose that this difference reflects different geometries of the underthrusting Indian lithospheric slab from west to east. A slab tearing under eastern Tibet and lithospheric delamination under western Tibet are likely a good candidate to explain the observation.