Pn wave tomography of eastern Tibetan plateau

Data from recent INDEPTH IV and other broadband deployments in Tibet are used to invert for crustal delays and Pn velocities beneath the eastern Tibetan Plateau and surrounding regions. The average Pn velocity for the region is 8.1 km/s but varies from 7.8 to over 8.3 km/s. Generally low Pn velocities are found in the northeastern Tibet in the Qiangtang and Songpan-Garze terrains. This includes a zone of very low Pn velocity northwest of the Longmen Shan thrust, along the eastern Kunlun fault, and beneath the eastern Qilian Shan. A region of high Pn velocity underlies the eastern end of the Bangong-Nujiang Suture (bounded by 31-33 degrees N; 90-98 degrees E). It could represent part of the underthrusting Indian shield. The region between the Qilian Shan and Kunlun Shan is also characterized by high Pn velocity with several zones of extremely high velocity. This includes two high velocity features beneath Qaidam basin and Gonghe basin. These features may correspond to cratonic fragments that accreted during the closure of the Tethys Ocean and have impeded, but not stopped, the northward growth of the plateau. The station delays show the thickest crust is beneath Tanggula Shan in central Tibet and should be the result of the internal deformation within Qiangtang terrain due to the collision of Indian and Eurasia plates. There is a significant decrease of crustal thickness in the northeast plateau and lateral variation also exists within the region.