Anisotropy of the crust and upper mantle beneath the central Tibetan plateau

Abstract

This paper analyzes S-wave splitting analysis in terms of P-wave receiver functions and the SKS shear waves from the INDEPTH-III profile which consists of 41 seismic stations deployed in central Tibet. Using the weight average technique with a weighting factor of the measuring error, the direction and extent of seismic polarization anisotropy in the crust and upper mantle beneath central Tibet, from the central Lhasa terrane, across the Karakoram-Jiali fault and Bangong-Nujiang suture to the central Qiangtang terrane, were imaged. Substantial splitting of Pms phase with delay time 0.3 similar to 0.5 s, and fast directions varying from NE-SW in the Lhasa terrane to EW in the Qiangtang terrane, was observed. The delay time of SKS for stations in the northernmost Lhasa terrane and Qiangtang terrane is 1 similar to 2 s which increases near the Bangong-Nujiang suture and Jiali fault, and the fast direction of SKS is similar to the Pms. No detectable SKS splitting was observed for stations located farther south in the central Lhasa terrane, which is related to a model with two-layer anisotropy of perpendicular fast direction. In combination with previous geological and geophysical studies, we infer that the Lhasa crust possesses high rigidity and has not experienced rheological deformation, so that the fast direction of the anisotropy layer in the crust is consistent with the direction of the India-Eurasia convergence. However, the Qiangtang terrane is possibly weaker and more likely has been rheologically deformed during the convergence process, so that the fast direction of the anisotropy layer in the crust and upper mantle is coincident with escaping direction of material of the Tibetan plateau.