Characteristics Of Crustal And Mantle Anisotropy Beneath The Northeastern Margin Of Qinghai-Tibet Plateau

Seismic anisotropy is one of the basic characteristics for the interior earth,and the shear wave splitting is an intuitive and effective method to study seismic anisotropy.We can obtain mineral in character,internal structure of anisotropic medium and deformation characteristics for different layers by studying the seismic anisotropy.The northeastern margin of Tibetan Plateau,as the large-scale boundary of deformation zone in the northeastern of Tibet,is locates among the Tibetan plateau,Ordos block and Alxa block.It is relatively more active tectonic units since new tectonic period.There are multitudinous and large-scale faults,such as Arkin fault,Qilian Mountains fault,Haiyuan fault and so on.According to research of the crustal and mantle anisotropy beneath the northeastern margin of Tibetan Plateau,on the one hand,we can discuss the relationship between anisotropy and regional stress field,local structure,and the distribution of crustal stress.On the other hand,we also discuss and explore the deformation characteristics of lithosphere or asthenosphere,and seek the eastward flow channel for plateau.It is important significance for explaining the questions of the movement of plate and the flow of asthenosphere.In this study,the polarization of fast shear waves and time delay of low shear-wave beneath the 48 stations in this area were computed using shear-wave Splitting Analysis Method(SAM)based on the seismic data during January 2010 to December 2015,recorded by Gansu Regional Digital Seismic Network.Meanwhile,we also analysed and discussed the spatial distribution of the crust anisotropy characteristics in northeastern margin of Qinghai-Tibet plateau.Due to the impact of the regional principal compressive stress and local geological structure,the spatial distribution of polarization of the fast shear-wave shows partly complicated and localized characteristics.The second predominant polarization direction of fast shear-wave is consistent with strike direction of fault in local area,which indicates discreteness and differences about anisotropy.However,the principle polarization direction,which is near NE direction,is obvious in northeastern margin of Qinghai-Tibet plateau.Analysis suggests the anisotropic direction of Qilian Mountains-Liupan Mountains seismic belt is consistent with direction of tectonic stress field,while the crustal anisotropic distribution is affect by regional stress and active fault in Southeast of Gansu and its periphery.The anisotropy of individual regions is different and discrete.Based on teleseismic SKS phase recorded by 43 permanent broadband stations in Gansu province in northeastern margin of Qinghai-Tibet plateau,we obtains the splitting parameters of fast polarization direction and delay time at each station using the minimum energy method and the rotation-correlation method.The results can be summarized as follows.In Qilian-Hexi Corridor region,the fast wave polarization trends NW-SE,consistent with strike direction of the main faults in this region.The deformation model of the crust and upper mantle is possibly vertically coherent deformation by compare the fast wave direction with the direction of lithospheric extension and GPS observed velocity.In the northern fringe fault of western segment of Qilian mountain,we observed that slow wave time delay has a larger anisotropy in the vicinity.This phenomenon maybe hint the upper mantle deformation,which caused by both the deformation of the lithosphere and the flow of asthenosphere.In the Ordos block,the fast direction is NW-SE,which may suggest that the movement of deep material exists motion of flow around in Tibet Plateau.The collision between the plates made the crust of western China thickening and uplifting,and material eastwards extruding in the northeastern margin of the Qinghai Tibet Plateau.Different regional stations have different anisotropy in the northeastern margin of the Qinghai Tibet Plateau,which further proves the complexity of tectonic deformation in the northeastern margin of the Qinghai Tibet Plateau.According to the splitting parameters of SKS wave,anistropic charateristics in the study region are analyzed to investigate the dynamic process in the Earth.