Upper mantle anisotropy in the southeastern margin of the Tibetan plateau and geodynamic implications

Abstract

Continued northeastward compression owing to the colliding Indian subcontinent has led to pronounced surface uplift and significant crustal and mantle deformation in the Sichuan–Yunnan region along the southeastern margin of the Tibetan Plateau. Seismic anisotropy is an effective physical property for characterising crustal and upper-mantle deformation, as these deformation mechanisms can be investigated by analysing the splitting of compressional-to-shear-wave (P-to-S) converted phases (XKS, which includes the SKS, SKKS, PKS and PKKS phases) at the core–mantle boundary. However, we cannot distinguish any crustal and/or mantle anisotropy variations owing to the relatively low vertical resolution of XKS splitting. Here we compute the P-wave receiver functions from teleseismic events recorded at 90 permanent and 350 mobile broadband seismic stations that were established by the ChinArray project in the Sichuan–Yunnan region and analyse the P-to-S converted phases at the Moho interface (Pms) to measure the crustal anisotropic parameters. The Pms splitting results are then regarded as the crustal anisotropic parameters, and the anisotropic parameters in the upper mantle beneath 347 stations are extracted from the XKS splitting parameters by removing this crustal anisotropy. The results indicate that an approximately E–W anisotropic fast-wave polarisation direction (FPD) exists in the upper mantle beneath the southeastern margin of the Tibetan Plateau, and that the splitting times range from 0.06 to 1.39 s, with an average splitting time of 0.93 s. The approximately E–W anisotropic FPD in the upper mantle is mainly due to asthenospheric flow induced by the eastward subduction of the Indian Plate beneath Myanmar. The small splitting times within the southern Sichuan–Yunnan diamond-shaped block (SYDSB) may be attributed to vertical growth of the lithosphere induced by mantle-plume-related magmatic uplift.