Tectonic feature and layered anisotropy on the northeastern margin of the Qinghai-Xizang Plateau

In this paper, we synthesize the seismic anisotropy results of the Qinghai-Xizang Plateau obtained by various methods and data from previous studies, and analyze the tectonic features and the layered anisotropy of the northeastern margin of the Qinghai-Xizang Plateau. The results of S wave splitting in the upper crust show that the fast-wave direction changes significantly in the Yinchuan graben as well as in the Haiyuan fault. The dominant direction of the second polarization in the Hexi Corridor and Qilian block is consistent with the strike of the active faults, especially in the domain of the Haiyuan fault, which exhibits an obvious along-strike rotation. The higher delay time at block junctions and block margin may reflect the increased strength of the upper crustal medium in these areas. Receiver function results often used to reflect the anisotropic characteristics of the whole crust. Previous studies have revealed that the Qilian-Haiyuan faultand the West Qinling Northern margin fault and other deep and large fault cut through the crust, and the fast-wave directions show an obvious rotation along the strike of the fault. Among them, the fast-wave direction of the Haiyuan-Liupanshan faultshows obvious decoupling of the upper and lower crust, and the upper crust tends to be more aligned with the direction of the compressive stresses. Different anisotropy results between the Yinchuan graben and the northern part of the Ordos block, which may come from complex tectonic activities reflect the weaker anisotropy of the block. The XKS （SKS, PKS and SKKS shortly named XKS） wave splitting can clearly reflect the anisotropic characteristics of the lithosphere under the surface. The results show that the main fast-wave polarization directions in the studied area are NW and WNW, and the Haiyuan fault and the Longmenshan fault are NS and NE, respectively, which are different from the average directions in the studied area.. Combined with the receiver function and XKS results, there are deviations of nearly 90° in the fast-wave direction in the eastern Alax block, the western Ordos block, the northern part of the Yinchuan graben, and the Longmenshan fault, and the deformation modes of the crust and the upper mantle in these regions may be different. The phase velocity results indicate that the causes of the large-scale low velocity anomalies are different for the Qilian and West Qinling. The low velocity zones in the Qilian orogen may be caused by crustal shortening, whereas for the low velocity zone in the Songpan-Garze area, large scale crustal channel flow does not seem to explain the lower v_P/v_S ratios and the more fragmented low-velocity layers in the receiver function results. Low velocity anomalies in the shallow part of the Ordos block may be related to surface sedimentation, and smaller delay time may indicate a steady state of lithospheric structure, but the northern part may have been influenced by tectonic activity of the Yinchuan graben. Summarizing the results, it is found that the fast wave directions of multiple methods in the Qilian orogen are close to each other, the S-wave splitting is more consistent with the upper layer fitted by the layered anisotropy, and the lower layer is more consistent with the anisotropic character of the upper mantle as reflected by the XKS results and the long-period surface wave results. The upper crust of the Haiyuan fault is more affected by the compressive stress, and the fast-wave direction is in the NE direction, but in the lower-middle crust and upper mantle, the long-period surface waves, the receiver functions, and the fast-wave directions of XKS turn out to be approximately parallel, and the directions of lattice-aligned dominance in the lower and middle crust and upper mantle may be the same. The difference in anisotropy between the crust and upper mantle in the Yinchuan graben reflects the NW- and WNW-oriented lattice orientations of the mantle lithosphere in the Qinghai-Xizang Plateau, while the NE-oriented crustal fast-wave direction may reflect simple shearing of faults in the Yinchuan graben, and there may be a difference in crust-mantle deformation. The receiver function and ambient noise results suggest that with the NE-directed thrusting of the Qinghai-Xizang Plateau in the Western Qinling, the middle and lower crust may have been heated by mantle material and produced NE-directed channel flow or detachment, but the more similar splitting parameter of XKS and Pms and the lower v_P/v_S are contrary to the idea of large-scale crustal channel flow, and other scholars have suggested that the layered phenomenon found is a transition from brittle deformation of upper crust to ductile deformation of the middle and lower crust. Therefore, the layered characteristics of the West Qinling and Songpan-Garze blocks are still inconclusive. The crust and upper mantle of the Qinling orogen are in continuous deformation motion, and the mantle material is transported along the southern margin of the Ordos block and the middle of the northern margin of the South China block. The fast-wave directions shown by XKS and Pms indicate that the direction of crustal-mantle lattice dominance is aligned along the direction of material flow, and that the crust and mantle are strong coupled. Controlled by the complex tectonic features and the constraints of observatory station density, the exploration of shell-mantle deformation characteristics, shell-mantle coupling problems, anisotropic layering and dynamical features in the studied area still needs the incorporation of higher-quality and higher-density data as well as more accurate analytical methods.