The Shear Wave Velocity And Radial Anisotropy Structure In The Crust And Upper Mantle Of The Northeastern Tibetan Plateau

The northeastern margin of the Tibetan Plateau(NETP)is adjacent to the stable Ordos Block,which is the leading edge of the entire plateau expanding into the interior of the continent.Owing to the eastward escaping of material beneath the Tibet Plateau under the north-south extrusion,the Ordos block,the Sichuan Basin,and the SichuanYunnan diamond-shaped block all underwent complex multiple deformations.As a result,the crust and mantle structure of this region is very complicated and the NETP has a variety of lithospheric tectonic patterns.The NETP has become an important place to study the tectonic deformation and deep dynamic process of continental lithosphere.The study of the deep structure beneath the NETP is helpful to reveal the mechanisms of plateau uplift and crustal thickening and to understand the interaction between the plateaus and the surrounding stable blocks.In this study,I collect three-component continuous waveform data recorded by the China National Digital Seismic Station Network,China Regional Seismic Station Network and IRIS from January 1,2007 to June 30,2012,which contain 875 broadband seismic stations deployed in China and adjacent regions.Furthermore,the C1 crosscorrelation function(CCF)is obtained from ambient noise cross-correlation and teleseismic cross-correlation,and the C2 CCF is obtained via the asynchronous twostation C2 method.The phase velocity maps of Rayleigh waves(8-100 s)and the Love waves(8-50 s)are obtained by the ambient noise finite-frequency tomography method.By jointly inverting the local dispersion curves of Rayleigh waves and Love waves,I construct the shear wave velocity and radial anisotropy model of the crust and upper mantle beneath the NETP.Based on these imaging results,I study a series of scientific issues of NETP including the boundary morphology between the NETP and the North China Block,the boundary morphology between the NETP and the Yangtze Craton,the connectivity of the crustal flow in the middle and lower crust beneath the NETP,and the upper mantle flow among the NETP,the Ordos block and the Qinling Orogenic Belt.The main work and results of this thesis are as follows:1)In this study,a two-station C2 method is proposed to increase the ray paths for ambient noise tomography to improve lateral resolution of the imaging result.The reliability of the two-station C2 method is verified by theoretical derivation and numerical calculation of the simulated wavefield data with uneven source.The results show that the CCF obtained from the two-station C2 method is consistent with the C1 CCF using a correction of pi/4 phase shift.2)The Rayleigh wave and Love wave ambient noise CCFs are obtained by the teleseismic cross-correlation and the two-station C2 method.The checkerboard tests show that the C2 method effectively increase ray paths and then improve the lateral resolution of the imaging results.3)Based on the dispersion curves extracted by frequency-time analysis(FTAN),the phase velocity maps of Rayleigh waves at 8-100 s and the phase velocity maps of Love waves at 8-50 s in the NETP were obtained by finite frequency ambient noise tomography.The phase velocity slices of Rayleigh waves have similar features with the phase velocity slices of Love waves.At the short period(<20s),the phase velocity maps of Rayleigh waves and Love waves are consistent with the surface geological formations.Low velocity anomalies are shown in the basin and high-speed anomalies are shown in the areas around the orogenic belts.The Tibetan Plateau generally has a low-speed structure inside the plateau and a high-speed structure at the edge.In the period of 30-100 s,the surface wave velocity is strongly affected by the thickness of the crust and the shear velocity of the lower crust and upper mantles.In 30-50 s,the phase velocity structure of the Rayleigh wave is generally negatively correlated with the change of the depth of Moho.Areas with thin crust(Sichuan Basin)have high speeds,and areas with thick crust(Tibetan Plateau areas)have low speeds.It corresponds well to the characteristics of the thickness of the crust of which the west is thick and east is thin of the study area.The Tibetan Plateau as a whole shows a significantly low speed.At the period of 60 s,the structure of the mantle lithosphere has a greater influence on the Rayleigh wave velocity.The most notable feature here is the high-speed block underground of the main basin of the Tibetan Plateau,indicating the cold and hard nature of its underground structure at this depth.Compared to the short period,the velocity perturbation of the Rayleigh wave phase velocity slice with a period of 70-100 s is relatively small,indicating that the deep structure is stable.4)Based on the finite frequency tomography method,the local dispersion of Rayleigh wave and Love wave are obtained.They are combined by Markov chain Monle Carlo(MCMC)method for jointly inversion which is used to build the shear wave velocity structure and radial anisotropy model.In the shear wave velocity transections,I found three main directions of crustal flow movement,including(1)from Qiangtang and Songpan-Ganzi to the southeast of Yangtze Block;(2)from Qiangtang and SongpanGanzi to the northeast of the Western Qinling Orogenic Belt and Kunlun Fault;and(3)from the Qiangtang block to the Lhasa block.5)The distribution and connectivity of crust flow in the NETP were analyzed by the constraints of horizontal slice of shear wave velocity and radial anisotropy.It is inferred that there is a short-distance connection between the crust flow in the NETP.By combining the shear wave velocity and the radial anisotropy profile,the asthenosphere layer is found at a depth of 200 km below the Ordos block.Mantle flow channels are found around the Qinling Orogenic Belt,which is located between the Ordos Block and the Sichuan Basin.This provides the evidence for the mantle flow channel connecting the northeastern margin of the Tibetan Plateau and the North China Craton.