Seismic Anisotropy Layering Beneath the South-central Tibetan Plateau Constrained by Shear Wave Splitting and Receiver Function Analyses

To investigate continental dynamics in the south-central Tibetan Plateau, which is composed of the Himalayan, Lhasa, and Qiangtang blocks, we measure seismic azimuthal anisotropy in the crust and upper mantle. Crustal anisotropy is determined at about 40 stations by fitting the sinusoidal variation of the Pms arrival times on the receiver functions. In the Qiangtang Block, the observed crustal anisotropy is related to eastward crustal flow. In the Lhasa Block, it is approximately N-S oriented and is attributable to upper-crustal rifts originating from E-W extension, and in the Himalayan Block, it is consistent with the orientation of horizontal compression stress. The shear wave splitting parameters measured at 141 stations demonstrate a 90-degree periodicity with respect to the back azimuth, which is consistent with a two-layered anisotropy model. Due to the well-known non-uniqueness of the two pairs of splitting parameters, we use the resulting crustal anisotropy as the upper layer anisotropy and grid-search for the splitting parameters of the lower layer. The results suggest that in the Himalayan Block, the E-W oriented lower layer anisotropy can be explained by mantle flow below the subducting plate. Two localized areas are characterized with pervasive null measurements which may indicate mantle upwelling. In the Lhasa Block, it can be interpreted as reflecting an approximately northward mantle flow system traversing a tear of the Indian slab. Weak lower layer anisotropy beneath the Qiangtang Block suggests that the northward mantle flow is greatly weakened or disappeared in this area.