Upper Mantle Velocity Structure and Its Dynamic Implications beneath the Tibetan Plateau from Triplicated Seismic P Waveforms

Due to its complex structure and evolution history, Tibetan plateau has always been a focused region for study continental collision, tectonic movement and its geodynamics. Based on the the vertical P wave records from a selected earthquake recorded by CDSN (Chinese Digital Seismic Network) and temporary seismic stations, the high resolution upper mantle structures beneath the Tibetan Plateau including the Lhasa Terrane,south Qiangtang terrane and Songpan-Ganzi terrane are obtained by triplication waveform fitting technique. Our results show that: (1) The high velocity anomaly observed at the bottom of MTZ (mantle transition zone) beneath Lhasa and south Qiangtang terranes are interpreted as the remnants of the subducted Indian slab,and these cold material depressed the post-spinel phase transition of 660km discontinuity by 3~8km. The high velocity anomaly can also be observed beneath Songpan-Ganzi terrane, which may be caused by the delamination of thickened Eurasian lithosphere. It suggests that the influence of Indian slab subduction has reached the MTZ, and the subduction front beneath the Tibetan Plateau is located near the Bangong-Nujiang suture. (2) From Lhasa, south Qiangtang to Songpan-Ganzi terranes, the upper-mantle velocities over 200km decreases gradually along the N-S profile, and the high-velocity lid even disappears in Songpan-Ganzi terrane. Influenced by the small-scale mantle convection or thermal instability, the thickened Eurasian lithosphere beneath south Qiangtang and Songpan-Ganzi terranes was delaminated, and the lithosphere was thinned and weakened, resulting in the low velocity of upper mantle in Qiangtang and the absence of high-velocity lid in Songpan-Ganzi terrane. Some of the delaminated lithosphere maybe now sitting atop of the 410km discontinuity, causing the uplift of 410km discontinuity by about 10km and others may have penetrated into the MTZ, characterized by the high velocity in the lower MTZ beneath Songpan-Ganzi Terrane. Low temperature causes the depression of 660km discontinuity by about 8km, resulting in the thickening of the MTZ.