Study On The Crust Shear Wave Velocity Structure Of Yunnan And Northern Vietnam With Ambient Noise

The extraction of surface wave from the ambient noise,which ensures the even distribution of ray paths in the station area,is an important method to study the underground structure.Surface wave dispersion,which contains the information of physical properties of the underground medium,is an effective data source for the inversion of shear wave velocity structure.Using the classical two-step inversion method,we can obtain the surface wave phase velocity map and the 3-D shear wave velocity model from the mixed-path dispersion between stations.When considering the anisotropy of the shear wave as a correction term of the isotropic structure,we can divide the 3-D shear wave velocity inversion into isotropic inversion and anisotropic inversion.In our study,we first get the phase velocity profile with surface wave tomography.Then,we carried out the isotropic inversion and the anisotropic inversion with the pure-path dispersion of every grid point.In the anisotropic inversion,we applied the direct search method based on neighborhood algorithm and the indirect inversion method based on linear algorithm.The final results of the two algorithms are consistent in general.In comparison with the neighborhood algorithm,the linear algorithm provides better computation efficiency and depth resolution,but are limited in stability and adaptabilityThe isotropic strcucture is the main feature of the seismic wave velocity model.It is interpreted as the inhomogeneity produced by the tectonic activity,so it is of great significance for us to understand the tectonic deformation and evolution of the study region.To describe the tectonic evolution of SE Tibet and its marginal areas,several models have been proposed.Hence,high-resolution crustal velocity models are essential to address this controversy.With waveform data from 73 broadband stations in southwest China and northern Vietnam,we invert for a 3-D shear wave velocity model of the crust and uppermost mantle from ambient noise tomography.Our model reveals that the mid-lower crustal low-velocity zone(LVZ)in the Xiaojiang Fault Zone extends farther southward across the Red River Fault to Vietnam and is approximately bounded by the Xiaojiang and Dien Bien Phu faults to the east.We suggest that the observed LVZ represents a mechanically weak zone in the mid-lower crust,which may serve as a channel for efficient southward material transport in SE Tibet.With our results and previous evidence,we propose a combined model that integrates rigid block extrusion and crustal channel flow to describe the large-scale material transport in SETibet.We further propose a two-phase material transport model in SE Tibet after the India-Eurasia plate collision:(I)rigid block extrusion between the right-lateral Sagaing Fault and left-lateral Red River Fault during the early Oligocene-early Miocene and(2)a combined model of rigid block extrusion and material channel flow in the mid-lower crust from the late Miocene to the present.The southward crustal material extrusion is likely to be diverted along two major channels around the more rigid crust beneath the inner zone of the Emeishan large igneous province(LIP).The anisotropy of the medium mainly comes from the preferred orientation of cracks and minerals underground,which always results from structural deformation and material transport.Therefore,the anisotropic structure plays an important role in our further understanding of the tectonic evolution of SE Tibet.Based on the data used in isotropic inversion,we added 55 permanent stations from Hainan and Guangxi to invert for the shear wave anisotropy of this region.The final results are consistent with previous studies.The fast axis in the middle crustal low-velocity zone(LVZ)is parallel to the spread direction of LVZ,providing strong support for our channel flow model.In addition,the anisotropy of Sichuan-Yunnan Block(SYB)shows a completely different depth dependence on both sides of the Luzhijiang faults(LZJF):the west part has nearly N-S trending anisotropy throughout the whole crust,the east part undergoes a relatively variation with depth.This inconsistency of depth dependence reflects the different tectonic activities on both sides of the LZJF.which is in accordance with the second phase we have proposed.