The Shear Wave Splitting Beneath The Northern Segment Of The North-south Tectonic Belt And Its Adjacent Area And Its Implications

The northern segment of the north-south tectonic belt is located in the northeastern margin of the Tibetan Plateau and its adjacent areas.The lithospheric deformation is very important for exploring the deformation mechanism and its extension of the northeastern margin of the Tibet Plateau.Seismic wave anisotropy can well reflect the upper mantle deformation.Thus,we analyze shear-wave splitting at temporary seismic stations in the northern segment of the north-south tectonic belt and obtain the anisotropic image and deformation characteristics of the lithospheric mantle under the study area.The value of the delay time indicates that the anisotropy originates from the upper mantle beneath the study region.The shear-wave splitting results reveal obvious variations of the fast-wave directions in the different tectonic units.The dominant fast polarization direction(FPD)is NW-SE along the NE Tibetan Plateau,the western margin of the Ordos block and the northern Helan Mountain,which is consistent with the FPDs in the Yinchuan graben and the Helan Mountain revealed by the previous studies.The results show the coherent deformation of the lithospheric mantle under these regions,and it is the result of the subduction of the India-Eurasia plate and its continuous northward compression,so the result suggests that the northeastern extension of the Tibetan Plateau influenced the deformation of the lithospheric mantle beneath the northern segment of the Helan Mountain.Combined with geological research results,the NE-SW FPD in the Alashan block maybe represent the "fossil" anisotropy,which results from the deformation the lithospheric mantle related to the closure of the Paleo-Asian Ocean during the Late Paleozoic.The NE-SW FTD within the Ordos block may also represent the "fossil" anisotropy formed by the collision of the Tethys tectonic domain,the southward subduction of the Siberia block and the northwest moving of the Paleo-Pacific plate during the Middle-Late Jurassic,which result in the NE-SW trending thrust nappe structure.Combined with GPS results,S-wave splitting results,and other geophysical research results in the region,we think that the deformation of the crust and upper mantle is not coupled in the northeastern margin of the Tibetan Plateau.