Mantle dynamics in the SE Tibetan Plateau revealed by teleseismic shear-wave splitting analysis

Abstract

The tectonic evolution of the southeastern Tibetan Plateau is key to understanding the mechanism of growth of the whole Tibetan Plateau. Its imprint on lithospheric and asthenospheric mantle rock fabrics is expected to have a strong signature on seismic anisotropy. In this study, we measure the splitting delays and fast polarization directions (FPD) with the minimum-transverse-energy and the splitting intensity methods using core-refracted phases (e.g., SKS, SKKS, PKS) recorded by the ChinArray experiment. The potential complex anisotropic structures, i.e., the frequency-dependent splitting and two-layer anisotropy, are investigated in particular. The FPDs are consistent in different frequency bands, but the delay times measured at low frequency (8–20 s) are often larger than those measured at high frequency (2–8 s). The measurements can be better explained by a two-layer anisotropy model at many stations. Splitting intensities are also inverted to constrain the depth distribution of seismic anisotropy in the upper mantle. Combined with the two-layer anisotropy models, these results suggest different patterns of seismic anisotropy in the lithosphere and asthenosphere. In the lithosphere, anisotropy shows distinct patterns in the different tectonic blocks, while the FPDs are oriented along the major tectonic boundaries. In the asthenosphere, continuous NW-SE FPDs spread from the Tibetan plateau to the Youjiang Orogen along the southwestern margin of the Yangtze craton, implying asthenospheric extrusion from the high plateau. In the northern Indochina block, the dominant E-W FPDs may indicate asthenospheric flow driven by the eastern subduction and subsequent rollback of the Indian plate.