Shear Wave Velocity Structures In The Northeastern Tibetan Plateau By Joint Inversion Of Receiver Function And Rayleigh Waves

The Northeastern Tibetan Plateau(NET) has been an important region to study the mechanism of continental collisions and is characterized by a series of parallel NW-SE-trending mountain ranges, including the Qilian orogen, the west Qinling orogen, the Songpan-Ganzi and Qiangtang terranes. The NET is surrounded by active thrusts and sinistral faults-- the Haiyuan fault system in the northeast, the eastern Kulun fault system in the southwest, and the Altyn Tagh fault system in the north. We collect the teleseismic earthquake data and continuous ambient noise seismic data from 223 seismic stations from 2007 to 2010. First we obtain the Moho variations in the NET by teleseismic P wave receiver functions. Receiver functions results show that the crustal thickness is about 50-55 km in the Qaidam basin, 60-65 km to ~50 km in the Songpan-Ganzi terrane, 60-65 km in the northwestern Qilian orogen, and 50-55 km in the southeastern Qilian orogen and the west Qinling orogen. The Vp/Vs ratio is about 1.8 in the west of Songpan-Ganzi and Qiangtang terranes and about 1.75 in other regions. Then Rayleigh wave phase velocities from 25 s to 111 s are computed by two-plane wave tomography and from 10 s to 40 s by ambient noise tomography.Finally the crustal and upper mantle shear wave velocity structures from the surface to150 km in the NET are obtained from joint inversion of receiver functions and Rayleigh wave dispersion curves derived from both teleseismic earthquake data and continuous ambient noise seismic data.S-wave velocity results show that:(1) a continuous low-velocity zone(LVZ) is found from the mid-to-lower crust down to 150 km beneath the eastern Kunlun fault.(2) The shear wave velocity of the Qilian orogen shows obvious heteogenities. The presence of a local crustal LVZ beneath the northwestern Qilian orogen suggests that the crustal LVZ is more likely related to crust thickening. No crustal LVZ is observed beneath the west Qinling and southeastern Qilian orogens, which are featured with a relatively thin(50-55 km) crust. These observations indicate that crustal channel flow is unlikely the primary mode by which the NET grows.