Seismic Anisotropy Of The Helan-Liupan Tectonic Belt And Its Adjacent Regions

The growth of the Tibetan Plateau and its surrounding response have been scientific challenges to be solved in geoscience.The time,range and mechanism of the northeastward extension of the northeastern Tibetan Plateau are key issues on the geoscience study,and it is debated that the northeastward extension range of the northeastern Tibetan Plateau.Therefore,we select the Helan-Liupan tectonic belt as the study region,which is located in the front edge of the northesatward extension of the Tibetan Plateau.Applying the shear wave splitting method to the seismic data recorded by the broadband stations of the Northwest University,the seismic anisotropy is determined.Our results suggest that the anisotropy obtained mainly result from the lithospheric mantle.The dominant fast polarization direction(FPD)is NW-SE in the eastern Qilian Orogenic belt,Helan-Yinchuan tectonic belt and southern Alxa block,subparallel to the prominent orientation of tectonics in the NE Tibetan Plateau.The NW-SE FPDs may be originated from NE-SW compression induced from the northeastward extension of Tibetan Plateau due to the India?Eurasia collision.NE-SW FPDs are revealed in the interior Alxa block,which track correspondingly to the striking direction of Early-Middle Permian magmatic rocks,folds and thrusts,and are regarded as the "fossil" anisotropy formed during the subduction and cloison of the PaleoAsian oceanic.Within the western Ordos block,the obtained NEE-SWW FPDs may also reflect the "fossil" anisotropy,which indicate that the lithospheric mantle deformation under the southwestern Ordos block was incompletely modified by the far-field effect of the Indian-Eurasian collision,combined with the dominant NWW-SEE FPDs in the southwestern Ordos block revealed by previous studies.Combined with the results of other geophysical methods in this area,this paper suggests that the the northeastward extension of the northeastern Tibetan Plateau has extended to the eastern Qilian Orogenic belt,western Ordos block and southern Alxa block,and its further northward expansion and transformation is mainly along the Helan Yinchuan structural belt.