Tectonic Implications of Seismic Anisotropy Layering Beneath the Southern Tibetan Plateau Revealed by Integrated Shear Wave Splitting and Receiver Function Analyses

Abstract

AbstractTo investigate continental dynamics underneath the south‐central Tibetan plateau, which composes the Himalayan, Lhasa, and Qiangtang blocks, we have conducted comprehensive examinations of seismic azimuthal anisotropy in the crust using receiver functions (RFs) and crustal and mantle anisotropy using teleseismic shear wave splitting (SWS) analysis. In the Qiangtang block, the observed predominantly E‐W fast orientations from RF and SWS analyses with similar magnitude are interpreted as resulting from eastward crustal flow with minor contributions from the mantle. In the Lhasa block, the crustal anisotropy is approximately N‐S oriented, which is parallel to the strike of rift basins and southward crustal flow. Anisotropy revealed by SWS demonstrates a rotation from E‐W in the north to NE‐SW in the south, which can be interpreted as reflecting mantle flow field induced by the northward movement of the subducting Indian plate. The addition of PKS and SKKS measurements and extension of epicentral distance range to 171.8° for SWS analysis revealed dominantly strong E‐W oriented anisotropy in most parts of the Himalayan block, where most previous studies reported pervasively null measurements. The absence of azimuthal anisotropy is observed in two regions in the Himalayan block which is attributable to mantle upwelling through a previously identified slab window. A two‐layered anisotropy structure with different fast orientations for the upper and lower layers can be constrained in the southern Qiangtang and the vicinity of the Main Boundary Thrust.