Crust And Mantle Structures Beneath The Northeastern Tibetan Plateau Revealed By Eikonal Eauation-Based Traveltime Tomography

The uplift of the Tibetan Plateau has been caused by the continuous collision between the Indian plate and the Eurasian plate during the past ～55 Ma.Due to the subduction of the Indian plate,the strong compression stress between the plates and the material movement of the Tibetan Plateau,the high altitude(～4500 m)and complex and intense tectonic activity are formed in the inner and surrounding areas of the Tibetan Plateau.Shallow structures of Tibetan Plateau may provide direct evidence for the study of Tibetan plateau continental-continental collision model and uplift deformation mechanism.However,deep structure of the Tibetan plateau is still controversial.Therefore,detecting the crust and mantle structure beneath the Tibetan plateau,will provide new constraints for studying dynamic process of crust and mantle.Taveltime tomography methods,which are based on traditional ray theory,and obtain the velocity structure of earth by inverse traveltime data.Solving the eikonal equation can yield accurate ray paths in complex heterogeneous media,and hence eikonal equation-based traveltime tomography achieves high stability.Therefore,we conduct equation-based traveltime tomography of the velocity and anisotropy structures beneath the northeastern Tibetan Plateau.The northeastern Tibetan Plateau serves as the frontier of the northeastward expansion of the Tibetan Plateau.In this area,the Tibetan Plateau interacts with the surrounding blocks,such as the Alxa block,the Ordos block,the Kunlun-West Qinling block and the Sichuan basin.Because of the expansion and interaction,this area suffers from intense deformation.At present,the evolution and deformation mechanisms of the northeastern Tibetan Plateau are hot debated.To provide new insight into the mechanisms,in this study we conduct tomography of the velocity structure and radial anisotropy beneath the northeastern Tibetan Plateau.We choose the waveforms of the teleseismic earthquakes recorded by broadband seismic stations in the northeastern Tibetan Plateau and surrounding areas.We first perform waveform cross-correlation between adjacent stations,and obtain P-wave traveltime differences.After that,we invert the differential traveltime data by using an eikonal equation-based teleseismic tomography method.Finally,the P-wave velocity structure and radial anisotropy at depths from 30 km to 800 km below the northeastern Tibetan Plateau are obtained.Our tomography results show clear low-velocity anomalies and positive radial anisotropy in the lower crust under the northeastern Qilian orogen,the northeastern Songpan-Garze belt and the western Qinling fold zone.These features are integrated to demonstrate the existence of lower crustal flow in the study area.Prominent low-velocity anomalies and positive radial anisotropy are found in the uppermost mantle beneath the Qilian orogen,the northeastern SongpanGanzi belt and the western Qinling fold zone.These characteristics are combined to reveal the weak lithosphere and horizontal mantle flow under these tectonic units.Both the Ordos block and the Sichuan basin exhibit clear high-velocity anomalies and negative radial anisotropy in the uppermost mantle,which indicates the high mechanical strength of the lithosphere of the Ordos block and the Sichuan basin.High-velocity anomalies are in the upper mantle under the northern Chuandian block,and they may imply northward subduction of the Indian plate.The Qinling belt is located in the central China,a juncture region of the North China Block,the South China Block and northeast Tibet Plateau.Therefore,the deformation mechanism of Qinling belt is complex.It is worth studying that the detailed crustal structure of this area for provide new insight into the mechanisms that the detailed crustal structure of this area for provide new insight into the mechanisms.We manually pick Pwave first arrivals of the regional earthquakes recorded by broadband seismic stations in the northeastern Tibetan Plateau and Qinling belt.Finally,we utilize eikonal equationbased seismic traveltime tomography to obtain the P-wave velocity and azimuthal anisotropy structures of crust and uppermost mantle beneath the Qinling belt and adjacent areas by the inversion of first P-wave arrivals.Our tomography results show clear lowvelocity anomalies and the fast velocity directions of anisotropy are nearly E-W in the middle-lower crust beneath the west western Qinling.These features demonstrate the existence of lower crustal flow in this area.The east western Qinling and west eastern Qinling exhibit significant low-velocity anomalies and fast velocity directions of anisotropy are nearly NNE-SSW or N-S.These characteristics reveal that the deformation of this area is N-S oriented convergence of the North China Block,the South China Block and Qinling microblocks.Significant high-velocity anomalies in the upper crust beneath the HNMC and the SNHL domes,suggesting that arc-shaped deformaton of Dabashan was constructed from the convergence of its surrounding blocks and the HNMC and the SNHL domes serve as indenters.