Layered Anisotropy In The Northeastern Tibetan Plateau And Its Implication For The Tectonic Deformation

The India-Asian collision induced the uplift of the Tibetan Plateau,which is one of the most significant geological events in the Cenozoic.The mechanism of the uplift of Tibetan Plateau attracts considerable interest.The northeastern Tibetan Plateau,adjacent to stable structures such as the Ordos block and Alxa block,is undergoing active deformation,and is the frontier of the plateau expansion.Thus,it is an ideal place to study the interaction between the expanding Tibetan Plateau and surrounding blocks.Rock's anisotropy is used to reveal the deformation process and thus helps to understand the deformation mechanism of northeastern Tibetan Plateau.In this work,the Rayleigh wave data recorded at the Chin Array II stations are used to measure the layered lithospheric anisotropy in the northeastern Tibetan Plateau.Rayleigh wave phase velocity tomography is performed by both ray-theory and Eikonal-equation methods.The measurements of both methods are generally comparable,while the phase velocity anomalies in the Eikonal tomography match the tectonic units better,indicating that the Eikonal tomography has more advantage in revealing the detailed structures.In the upper crust(< 20 km),the measured low velocities are notable in the Yinchuan-Hetao Graben and the northwestern Ordos block,indicating thick sedimentary layer.The highest velocities are present in the upper crust beneath Qinling Orogen,consistent with the orogenic belt.The shear wave fast directions of the upper crust are remarkably consistent with the strikes of major strike-slip faults: near the Kunlun Fault,the fast directions are NWW-SEE;from the Qilian Fault to the Haiyuan Fault,the fast directions change from NWW-SEE to E-W;in the Yinchuan-Hetao Graben,the fast directions change from NE-SW to E-W.We thought that the faultparallel anisotropy is caused by shear deformation along the fault.In the mid-lower crust,the northeastern Tibetan Plateau presents significant low velocity and complex layered anisotropy.Near the margin of the plateau,the fast directions in the mid-lower crust are parallel to the plateau margin,i.e.,N-S at the margin of Songpan-Ganzi Block,NW-SE at the margin of Qilian Block.Toward the interior of the high plateau,the fast directions of mid crust are also parallel to the plateau margin,whereas the fast directions of lower crust are perpendicular to the plateau margin.In the Yinchuan-Hetao Graben,the fast directions in the lower crust are NWSE,and the shear wave velocities in the lower crust and upper mantle are relatively low.This thesis constrains the crustal deformation of the northeastern Tibetan Plateau.The fast directions parallel to elevation gradient of the plateau are mainly present in the lower crust,consistent with the crustal flow model.However,different deformation patterns are present in the middle and upper crust.It is proposed that expansion of the northeastern Tibetan Plateau is resisted by surrounding stable blocks,and the crust of plateau margin has thickened by pure shear,producing margin-parallel fast directions.The Yinchuan-Hetao Graben may have weak lower crust.The expansion of Tibetan Plateau interacts with surrounding blocks,leading to the NW-SE extension of the Yinchuan-Hetao Graben,resulting in the corresponding lower crustal anisotropy.