Significant and vertically coherent seismic anisotropy beneath eastern Tibet

Shear wave splitting measurements using teleseismic SKS and SKKS phases recorded by the INDEPTH-IV arrays has revealed a strong upper mantle anisotropic fabric in northeastern Tibet with large delay times of up to 2.2s, suggesting that anisotropy exists in both the lithospheric and asthenospheric mantle. The coherence among fast polarization orientations of split core phases and the left-lateral slip on eastern-striking, southern-striking faults in eastern Tibet and the surface deformation fields calculated from both GPS observations and Quaternary fault slip rates support the idea that left-lateral shear strain is the predominant cause of the orientation of the upper mantle petrofabrics. We suggest the bending of the Eastern Himalayan Syntaxis around the foundering Burma-Andaman-Sumatra slab also contributes to the observed seismic anisotropy in the Eastern Himalayan Syntaxis region. Two plausible competing processes are proposed for the flow of asthenosphere in eastern Tibet. In the first, the deforming lithosphere glides over the passive asthenosphere inducing flow in the asthenospheric mantle. In the second, the asthenosphere beneath northeastern Tibet is squeezed between the advancing Indian continental lithosphere and the thick Tarim and Qaidam lithospheric blocks to the north. A westward retreat of the Burma slab from Eurasia may induce flow that is toroidal and located exclusively around the northern edge of the slab. The rotation of fast orientations for stations in the Eastern Himalayan Syntaxis region are consistent with the toroidal flow pattern as well as the rotational deformation of the overlying lithosphere.