Seismic anisotropy in the southeastern margin of the Tibetan Plateau and its deep tectonic significances

The southeastern (SE) margin of the Tibetan Plateau is a channel of the eastward escaping material flow of the Tibetan Plateau. In order to fully understand the complex lithosphere structure and strong deformation characteristics, this study introduces the anisotropic pattern in the SE margin of the Tibetan Plateau. Meanwhile, on the basis of other anisotropic studies in the crust and the upper mantle, increasing some of new data, we update the image of the lithospheric azimuthal anisotropy and discuss the significance of regional deep tectonics in the SE margin of the Tibetan Plateau. Based on the results from local seismic waveforms, teleseismic waveforms and ambient noise data, the seismic anisotropy shows the unique regional spatial distribution and the vertical hierarchical distribution, which reveals three main characteristics in the SE margin of the Tibetan Plateau. (1) The seismic anisotropy in the upper crust coincides with the measured surface deformation. The polarization direction of the fast shear-wave exhibits divergence, consistent with the crustal surface motion. It is consistent with the direction of the main compressive stress, but influenced by the geological tectonics. (2) The azimuthal anisotropy in the lower crust shows better directional consistency, but the degree of azimuthal anisotropy is relatively weak. There are two zones of low velocity beneath the northwest end of the Honghe fault belt and the Xiaojiang fault belt, where the degree of azimuthal anisotropy is as strong as that in the upper crust. (3) The lithospheric azimuthal anisotropy is characterized by two subzones, the south part and the north part. The dividing line is approximately 26°20'N between two subzones. The fast direction of S wave is approximately in NS in the north part and in EW in the south part. It is inferred that in the upper mantle on the north side of 26°20'N there is a thick high-velocity body, which southern edge presents an upright wall structure striking in nearly EW, so that in the upper mantle on the south side the material flows along EW direction. It induces the sudden spatial change of the azimuthal anisotropic characteristics in the lithosphere, the fast direction changes from NS in the north into EW in the south. This study considers that the Xiaojiang fault belt is the western boundary of the crust of the South China block. However the lithospheric azimuthal anisotropy displays that the upper mantle material of the South China block goes across the Xiaojiang fault belt to its west side and indicates that deep images of lithospheric deformation and upper mantle asthenospheric material movement are caused by collision between the South China block and the Tibetan Plateau. Seismic anisotropy reveals information on regional deep tectonics and medium deformation, and comprehensive analysis of different observations are helpful to obtain clearer 3D anisotropic images. © 2020, Science Press. All right reserved.