Depth-dependent Pn velocities and configuration of Indian and Asian lithosphere beneath the Tibetan Plateau

Abstract

Using Pn-wave traveltimes from three regional distance ranges we generated Pn tomography models to investigate the 3-D nature of the uppermost mantle lid P-wave velocity structure beneath the Tibetan Plateau and surrounding regions. Significant velocity variations spatially and with depth are observed. High-velocity regions are found beneath the Himalayas and most parts of southern Tibet. These high-velocity regions can be interpreted as subducting Indian continental mantle lithosphere, accreted terranes and a cold, non-convective mantle wedge beneath central Plateau. They are disjointed suggesting that the subducting Indian lithosphere is fragmented laterally. In the western Tibetan Plateau, the high-velocity region extends northwards to the middle of Qiangtang Terrane. In the central Plateau, the highvelocity region reaches near the Bangong-Nujiang suture. The northern extent of the subducted Indian continent cannot be determined uniquely from Pn models because the Indian plate dips moderately beneath southeastern Tibet. Around the Plateau, Tarim, Qaidam, Gonghe and Sichuan basins are floored by high-velocity Asian continental blocks that keep the elevation of these basins lower than surrounding regions. We found no evidence of an ongoing southward subduction of Asian lithosphere beneath central to northeastern Tibetan Plateau. Two major Pn low-velocity anomalies are found beneath northern and northeastern Tibet, primarily within the Qiangtang and Songpan-Ganzi Terranes. The northern Tibet low-velocity mantle is situated in a continental backarc of the India-Asia continental subduction zone, and hence we interpreted this region as thin thermal lithosphere with upwelling mantle driving by the subducting Indian lithosphere. The Qilianshan is also underlain by a low-velocity mantle structure and could be related to the same upwelling in this continental backarc. A narrow N-S trending low-velocity anomaly is found beneath the Yadong-Gulu Rift and confirmed that the Lhasa Terrane is not limited to the crust, but involves the entire lithosphere. Another pronounced low Pn velocity region is observed in the southeastern Tibetan Plateau, southern Yunnan and northeastern Myanmar. This feature is probably related to backarc convection associated with the subduction and rollback of Indian oceanic slab beneath Myanmar and Yunnan province, China.International Seismological Centre, On-line Bulletin, http://www.isc.ac.uk, Internatl. Seismol. Cent., Thatcham, United Kingdom, 2016.