S-Wave Velocity Structure and Seismic Anisotropy beneath Tibetan Plateau

Recent studies on Tibet have proposed that the uplift of the plateau is related to the removal of lithospheric mantle and its replacement by the hotter and less dense asthenospheric material. Conversely, other studies have proposed that the Tibetan plateau has been uplifted and the crust thickened through a process of lateral crustal flow. Some authors have attributed the geophysical properties of upper mantle beneath the Tibetan Plateau either to a delamination of the thickened lithosphere or an asthenospheric counterflow associated with subduction of Indian continental lithosphere. In order to test these models and determine the crustal and upper mantle shear wave velocity structure of lithosphere beneath the Tibetan plateau, our research focuses on investigating the dispersion of surface waves across the region. To determine the shear wave velocities and seismic anisotropy, we have deployed 74 broadband seismic stations throughout the northeastern Tibetan plateau, and we obtained Rayleigh wave phase velocities for the anisotropic media using dispersion data. Our preliminary tomographic models show an uppermost mantle low velocity zone north of the Bangong-Nujiang Suture (BNS) in northern Tibet, and a high velocity anomaly that extends at least to 200 km centered on the BNS. We suspect that the low velocity zone north of the BNS is due to a warm and possibly thin lithosphere. Our measurements indicate that there is significant azimuthal anisotropy (>2%) throughout the upper mantle to depths greater than 250 km. We also find evidence for north-south fast directions. On the northeastern Tibet (GanSu Providence) margin, we observe a velocity boundary between the low-velocity Tibet and the high-velocity Ordos and Sichuan blocks, with NNW-SSE fast directions. The north-south fast directions in the northern Tibet might be pointing to a more complex flow beneath the region considering east-west anisotropy which is generally dominant in central Tibet.