Upper-Mantle Velocity Heterogeneity of Eastern Tibetan Plateau from Teleseismic P-Wave Tomography and Its Tectonic Implications

An attempt has been made to reveal the upper mantle velocity structure of the eastern Tibetan Plateau using 628 teleseismic events recorded from 2003 to 2009 at 95 stations.A total of 8 532 Pwave arrival time residuals were inverted by using the FMTOMO(fast marching tomography) software package.Tomographic results show upper mantle velocity heterogeneity in many aspects.In the southern part visible high velocity anomaly is denoted as the Indian lithosphere.This part seems to be affected by slab tearing at 940E longitude as it is located on the eastern Himalayan syntaxis(EHS).The high velocity zone down to 500 km depth in the northern part could be the Asian lithosphere.At the central part some high velocity anomalies can be identified as detached patches of the lithosphere,surrounded by low velocity anomalies.These anomalies are the potential to create thermal convection and trigger plateau uplift or plateau growth.Sudden velocity change occurs on both sides of patches where low velocity anomaly is visible in between patches and Bangong-Nujiang suture even in between Songpan-Ganzi terrain and Asian Plate.In both cases intense low velocity zone spread down to 500 km.The depth range of low velocity anomalies in between two plates observed from 200 to-500 km.Hence the low velocity anomalies detected in our results may reflect either the hot asthenosphere upwelling or the mantle wedge due to the presence of the cold lithosphere.