Shear Wave Velocity And Anisotropy Structure In The Crust And Upper Mantle Of The Eastern Tibetan Pleateau

The Tibetan Plateau is the largest,highest,and youngest plateau on the world.The collision of the Indian plate and Eurasian plate,which began approximately 50 Ma ago,results in the formation of the well-known Himalayan and Tibetan Plateau.It also leads to significant crustal shortening and uplift,as well as widespread deformation in both crust and upper mantle in this region.In the last few decades,researchers have conducted varied geophysical,geological,and geochemical studies in the Tibetan Plateau.These results improve our understanding on the uplift and the mechanism for the deformation of the Tibetan Plateau.However,they proposed different hypothesis and there exist many debates.The transport of material from the Tibetan Plateau mainly concentrate in the eastern part.There are also many large strike-slip faults and sutures developed in this region,which play key roles in the evolution of the Tibet Plateau and the transport of material.Therefore,investigations on seismic structure in the crust and upper mantle in the eastern Tibetan Plateau are essential to understand the tectonics and evolution of the Tibetan Plateau.Surface wave tomography is one of the most powerful tools to constrain shear wave velocity structure and deformation mechanism in the crust and upper mantle.In the last decade,ambient noise cross-correlation technique has developed rapidly.Utilizing this technique,we can measure short-to-intermediate-period surface wave dispersion curves.The classic earthquake-based method can extract intermediate-to-long-period surface wave dispersion curves.Combination of these two methods is able to better constrain the velocity model in the crust and upper mantle.This study mainly focuses on the 3D velocity structure and deformation features in the eastern Tibetan Plateau.We use ambient noise-based and earthquake-based surface wave tomography to resolve 3D isotropic wavespeed variations and anisotropic structure in Northeast and Southeast Tibet.Our study sheds new insights into the deformation history and geodynamic processes in the eastern Tibetan Plateau.First,we combine surface wave dispersion curve measurements recovered from earthquake and ambient noise data collected in Northeast Tibet and utilize both isotropic and azimuthally anisotropic surface wave direct inversion methods.As a result,detailed 3D isotropic shear wave velocity variations and azimuthally anisotropic structures in the crust and uppermost mantle are resolved in the northeastern Tibetan Plateau.Our results show widespread low-velocity zones(LVZs)in the middle and lower crust beneath the Qiangtang Block and Songpan-Ganzi Block of the Tibetan Plateau,which indicate partial melting of deep crustal materals.There exist LVZs in the uppermost mantle beneath the northern Songpan-Ganzi Block and Qiangtang Block,which probably result from the upwelling of asthenospheric material after the lithosphere delamination.In Northeast Tibet,fast axes of azimuthal anisotropy in the crust are dominant by the movement of large strike-slip faults and the regional stress field,while azimuthal anisotropy in the upper mantle is influenced by the transport of material in upper mantle asthenosphere.We think the northeastward crustal flow across the Kunlun fault is not likely to occur in the northeastern margin of the Tibetan Plateau.Secondly,utilizing three-component continuous waveforms recorded by a dense array within and around the Xiaojiang Fault zone of the southeastern Tibetan Plateau,we measure Rayleigh and Love phase velocity dispersion curves within the period band from 5s to 35s based on ambient noise cross-correlation.We apply a new 3D direct joint inversion method using both Rayleigh and Love wave dispersion data to obtain the isotropic shear wavespeed variations and radially anisotropic structure in the crust of the study region.Our results show that the upper crust and middle-to-lower crust are dominant by negative and positive radial anisotropy,respectively,which suggest that the deformation is depth-dependent.Along the Xiaojiang Fault zone,the negative radial anisotropy in the upper crust is associated with the relatively high fault slip rate in the north comparing to the south.In the middle crust,the Lvzhijiang Fault zone appears to be the boundary that separates the rigid block in the west and the relative weak block in the east.To the west of Lvzhijiang Fault zone,we observe the high-velocity anomaly and positive radial anisotropy in the middle-to-lower crust.Such feature may be related to the cooling remnants in the crust of the underplated magma from upper mantle during the period when the large volcanic province in Emeishan was generated in the Permian.Our new model constrains the detailed geometry of those mechanically weak zones in the crust of Xiaojiang Fault zone,which provides new insights in understanding the role of major faults in the regional tectonic evolution.Finally,utilizing the Rayleigh wave phase velocity dispersion curves measured from the data recorded by the dense array in the Xiaojiang Fault zone,we also resolve 3D isotropic shear wave velocity variations and azimuthally anisotropic structure in the region.We observe prominent LVZs in the middle crust beneath the Xiaojiang Fault zone and they pass through the Red River Fault zone as well.Within the LVZs,azimuthal anisotropy strikes NW-SE in the upper crust and NE-SW in the middle-to-lower crust,respectively.Such features may be associated with the existence of a middle-crustal channel flow dominant by low-velocity material.On the opposite sides of the Lvzhijiang Fault zone,velocity structure and fast axis of the azimuthal anisotropy in the middle-to-lower crust are significantly different,which is probably associated with the formation of the Permian Emeishan Large Igneous Province.In summary,our studies investigate the wavespeed variations and deformation patterns in the crust and upper mantle in the northeastern and southeastern Tibetan Plateau.It provides important seismological constraints for understanding the tectonic evolution in the eastern Tibetan Plateau.It also offers important reference models for future studies in the Tibetan Plateau.