| The northeastern margin of Tibetan plateau, as the large-scale boundary of deformation zone in the northeastern of Tibet, is located among the Tibetan plateau, Ordos block and Alxa block. It is relatively more active tectonic units since new tectonic period, and is caused by continuous collision between the Indian and Eurasia plates. However, due to absence of the structure information about the subsurface, crust and upper mantle, the specific mechnisms of large-scale of crust movement and deformation in the crust and the upper mantle, driven by the collision, are still debated, which also is the hot topics in study of the seismology and geodynamics.We analyzed a large amount of three component broadband seismic data recorded by 204 broadband stations (Gansu, Henan, Hubei, Inner Mongolia, Ningxia, Qinghai, Sichuan, Shanxi and Shaanxi) from August of 2007 to October of 2013, using receiver function to measure the crustal thickness and average Vp/Vs ratio and to employ a joint inversion scheme to determine azimuthal anisotropy of the crust beneath each station. This method includes an estimate of fast polarization direction and splitting time by a joint analysis of radial and transverse receiver function data, and an evaluation of measurement reliability by statistical and harmonic analysis. The results show that there is significant azimuthal anisotropy within the crust beneath the margin, with a splitting time between 0.28s-1.14s. According to comparing with the delay time (0.1s-0.12s) calculated from local S wave, it infers that the primary source for the observed anisotropy should lie in the middle and lower crust. The observed crustal thickness exhibits large variations across the study area, varying from 31.3km to 71.3km, with average thickness of 45.9km. The northeast part of Tibet plateau is much thicker than other area, and the shallowest moho is observed at LINT at Weihe graben in the southern edge of the Ordos block, while the thickest crust is located beneath the stations QML in the central part of Tibetan plateau. Meanwhile, the Moho also appears to follow the changes in surface topography, and shows gradually deepening from east to west. The northeastern part of Tibetan plateau has a low Vp/Vs ratio, which means the mineral in this area is more felsic, while the Ordos block has a high Vp/Vs ratio, which means mafic rock is in this region.In order to understand the structure of the crust and upper mantle more clearly, we apply the ambient noise tomography method to study the velocity and azimuthal anisotropy. We collected the continuous ambient noise recorded by 118 broadband stations (Gansu, Ningxia, Qinghai, Sichuan, Xinjiang, Xizang and Mongolia) from 2011 to 2012. Using cross-correlation, we obtain 5773 Rayleigh wave dispersion curves in the period 5s to 38s, and then, invert the phase velocity and azimuthal anisotropy by 1°×1° grid. Finally, according to pure path dispersion inversion, we also obtain the shear wave velocity in each grid node in the northeastern Tibetan plateau. The results of azimuthal anisotropy show that, at 8-12s, the fast polarization has a good correlation with the strike of the regional structures. At 18-35s, the fast polarization in the Tibet displays a clockwise rotation. It is basically consistent with other research about the phase velocity and shear wave velocity, such as low velocity existing in the Songpan-Garze block and Qilian block. Furthmore, north Kunlun fault may be the north boundary of the low velocity zone in the Songpan-Garze block. Meanwhile, the reason of the low velocity in Qilian block may have relationship with crustal shortening caused by collision between Tibet and other block.With the upwards systemic studies, as well as the geology, geotectonics, geodynomics, petrology and geophysics, we get a comprehensive understanding the crust and upper mantle in the northeastern Tibetan plateau. All these observations suggest that the lithospheric deformation is primarily driven by vertically coherent shortening associated with the India-Asia collision, which cause continuous movement towards northeast, and at the same time, blocked by the strong Siberian platform. |
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