Rayleigh wave tomography of the northeastern margin of the Tibetan Plateau

The convergence between the Indian continent and the southern margin of Eurasia has eventually led to the growth of the Tibetan Plateau eastward. Various models have suggested that a large-scale flow in the lower crust or/and the asthenosphere is responsible for the uplift of eastern Tibet. It has also been suggested that the continental flow may move around the thick lithosphere of the Ordos Plateau and the Sichuan Basin. In order to investigate the validity of the crust/mantle flow and its possible path, we have used Rayleigh wave tomography with sensitivity kernels to estimate both the phase velocity structure (20-143. s) and the shear wave velocity structure (0-200. km) across the northeastern margin of the Tibetan Plateau. From the surface to 100. km, our shear wave velocity model shows a prominent low-velocity anomaly beneath northeastern Tibet, suggestive of a warm lithosphere. Its boundary lies at ~. 105° E longitude which borders the western Ordos Plateau and Sichuan Basin. From 125. km to 200. km, our shear wave velocity model shows a low-velocity channel beneath the Qilian-Qinling Orogen that lies between the Ordos and Sichuan blocks. This channel may represent the asthenosphere, given the fact that the lithosphere is ~. 120-150. km thick in northeastern Tibet and the Qinling Orogen. We also inverted for the anisotropy parameters in the s tudy area simultaneously with the velocity parameters. The dominant fast direction is NWW-SEE, generally consistent with the SKS splitting results, the calculated shear-strain from GPS, and the fault slip rate measurements. Furthermore, the fast directions in our anisotropic model vary only slightly with periods and the anisotropy magnitudes at long periods (100-143. s) are large and exceed those at short-medium periods (20-80. s), indicating potentially (although not necessarily) vertically coherent deformation from the crust to the asthenosphere and possibly large deformation in the asthenosphere. The deformation seems to be linked to the southeastward extrusion tectonics caused by the collision boundary force.10 page(s