Crustal Anisotropy And Stress/strain Field In The Eastern Margin Of The Tibetan Plateau

The Tibetan Plateau,known as the highest and largest plateau in the world,has been created by the continuous collision between the Indian and the Eurasian plate.However,the deep structure and the deformation mechanism of the plateau are still unclear.Using unprecedented data recorded by dense seismic array,I applied three seismological methods,including focal mechanism solution and stress field inversion,crustal anisotropy inversion by receiver function,and the surface-wave anisotropic tomography to study the detailed structure and deformation at different depths in the eastern margin of the Tibetan Plateau.Compared with previous studies,these results may provide important constraints on the rheological properties of different blocks,and reveal the uplift and deformation mechanism.Firstly,I use the focal mechanism solution and stress field inversion to obtain the tectonic stress field in the upper crust of the whole plateau,especially for its northeastern margin.The results show that the maximum principal stress over the plateau is fan-shaped.In the NE Tibet,the crust is generally under compression faulting environment but locally under strike-slip faulting environment.The maximum principal stresses rotate clockwise from NE-SW to E-W.Considering the surface strain field and the seismic anisotropy of the upper mantle,we infer that the lithosphere of the northeastern Tibetan plateau may prefer the vertically coherent deformation mode.Secondly,I use the receiver function harmonic decomposition and Markov-chain Monte-Carlo inversion algorithm to study the layered anisotropy of the crust in the NE and SE Tibetan plateau.The results show that the upper-crustal anisotropy in the eatern Tibet is relatively weak,and the fast polarization directions(FPDs)are consistent with the fault,or controlled by the regional stress field.The lateral difference is small,indicating dominant diffuse deformation of the upper crust.The anisotropy of the midlower crust presents significant lateral variation.The FPDs are parallel to the extrusion direction beneath the relatively fragile block,but turn to be parallel to the plateau margin along the Lijiang-Xiaojinhe Fault and Qinling orogen,indicating that the crustal flow is localized.Finally,I use the surface-wave eikonal-tomography and further conduct the inversion to determine the 3-D anisotropic S-wave velocity structure in the SE Tibetan plateau.The results show that there are two dis-connected low-velocity zones in the mid-lower crust.The FPDs in the mid-lower crust align with the plateau boundary beneath the Lijiang-Xiaojinhe fault zone.We infer that the potential crustal flow extruded from the plateau in the north have not spread widely into the Dianzhong block.The low-velocity anomaly beneath the Xiaojiang fault zone may be caused by the thickening of the igneous rock.At the same time,considering the upper mantle anisotropy obtained by previous studies,we favor the coupled crust-mantle deformation in the SE Tibetan plateau,but also suggest that there may exist asthenosphere flow beneath the plateau.In summary,I use three seismological methods to study the fine crust structure and deformation characteristics of the eastern margin of the Tibetan Plateau from shallow to deep.We prefer that the plateau deformation is generally distributed thickened,which is also regulated by intracontinental subduction and lateral extrusion as well as the midlower crustal flow.These results provide important enlightenment for us to understand the deformation mechanism of the continental lithosphere.