Crustal Anisotropy Beneath Northeastern Margin Of The Tibet Plateau By The Data From A High-density Seismic Array And The Significance Of Geodynamics

Since 50 to 70 million years ago,the continuous compression and collision of the India-Eurasia plate has created the Tibet Plateau,which has the most severe tectonic activity,the largest crustal deformation and the highest elevation on the earth.The Gansu southeast active tectonic zone is located at the northeastern margin of the Tibet Plateau.It is also the junction of the Qinghai-Tibet block,the Ordos block and the Alax block,including the margin of the West Qinling fault and the margin of the East Kunlun fault.This area is a high-incidence area of inland strong earthquakes.Historically,there ara a large amount of earthquakes with 7 magnitude or above,and it is also a region with strong tectonic activities,with many deep and large faults.The special geographical location and structural characteristics make the study of crustal anisotropy in this area more important.Compared with the shear wave splitting method,using receiver functions method to study seismic anisotropy has a good constraint on the depth of anisotropy.In the structure with an anisotropic layer near the Moho surface,the radial Pms phase of the receiver function changes periodically with the azimuth angle,and the tangential component exhibits a polarity reversal in the fast wave direction.According to these features,the anisotropy parameters can be determined,and the joint constraint of the R and T components can enhance the reliability of the anisotropic parameter.The results show that the depth,fast axis direction and intensity of the anisotropic layer have important reference value for analyzing the anisotropy and crustal movement characteristics of the study area.This paper collects and organizes the Himalayan Phase I project in the so utheastern Gansu area'stelemeismic data recorded of 141 stations,it is crossed t-he margin of the western Qinling northern faultLintan-Dangchang fault,Guang kaishan-Dieshan fault,Diebu Bailong River fault,Tazang fault,Minjiang fault,Xu-eshan fault,Huya fault et.al.Make the data to rotated,de-equalized,de-trended,a-nd deconvolved to obtain the receiver function.A low-pass Gaussian filter wi th half-width constant of 2.0 was applied to regulate the deconvolution.Finally,t here are 8954 waves are clearly visible.Moveout correction is then applied for a reference slowness of 6.4 s.we constructed binning stacks of RRFs and TR Fs of each station in an azimuth interval of 6°.The RRFs arrival time and theTRFs polarity reversal position jointly determine the crustal anisotropy parame ters of the study area.The results of the study area show that the anisotropy of the southeastern Gansu Plateau in the northeastern margin of the Tibet Plateau generally presents NWW-SEE,near EW and NNE-SSW directions.According to the delay time and depth model,the anisotropy of this region should come from the middle and lower crust.Deep and large faults occur frequently at the edge of the plate,and the lower anisotropy is mainly controlled by the regional tectonic stress and the large fault zone,while the anisotropy within the more stable plate is mainly related to the orientation of the crystals of the lower crust rock-forming minerals.The anisotropic characteristics at the intersection of plates are consistent with the crust flow model made by predecessors.According to the analysis,the direction of tectonic stress field in the Qinghai-Tibet Plateau is transformed from NE-SW to NWW-SEE in the southeastern Gansu,and the crust deformation should be coupled.In th V-shaped zone who formed by the intersection of the Qinghai-Tibet block and the Yangtze block the crustal pipeline flow may occur and the direction of movement changes from the near EW direction of the Qinling orogenic belt to the northward movement.