Study On Deep And Shallow Tectonic Deformation Of Lithosphere In The Northeast Margin Of The Tibetan Plateau And Southwestern Yunnan Based On GNSS,Focal Mechanism And Seismic Anisotropic Data

Seismic geodesy has opened a new chapter in the accurate measurement and study of the evolution of continental deformation dynamic system and its seismic behavior.GNSS is more and more applied to the monitoring of crustal movement and deformation.The study of crustal movement and deformation is of great significance to understand the characteristics and dynamic mechanism of tectonic activity in a region.However,only relying on GNSS observation data to study the deformation characteristics of the region is not enough.Combined with the stress field and anisotropy data to clarify the deformation characteristics of the deep and shallow lithosphere,we can more clearly understand the regional dynamic mechanism and its seismogenic environment.From the perspective of comparative analysis of strain rate field and stress field,combined with the data of crustal anisotropy,this paper studies the deep and shallow tectonic deformation characteristics of the lithosphere in the northeast edge of the Tibetan Plateau and Southwest Yunnan.In the northeast edge of the Tibetan Plateau,we have processed the GNSS data of Crustal Movement Observation Network of China to obtain a new velocity field.Through data fusion of our GNSS velocity and the velocity field published by prior researchers,we present a combined GNSS velocity field with higher spatial resolution.The multi-scale spherical wavelet method is used to calculate the strain rate tensors,and the spatial distribution characteristics of principal strain rate,surface strain rate and maximum shear strain rate are analyzed.The strain rates show that most areas in the northeast edge of the Tibetan Plateau is characterized by slight surface compression,and the surface compression rate is less than 15 nstrain/yr.The areas with higher surface compression rate are concentrated in the northeast edge of the Tibetan Plateau,among which the surface compression in the Qilian Mountain Fault Zone and Haiyuan fault zone is greater than 20 nstrain/yr.There are higher shear strain rates in the eastern Kunlun fault zone,Qilian Mountain Fault Zone,Haiyuan fault zone and the southern section of Liupanshan fault zone,with a maximum of about 40 nstrain/yr.The focal mechanism solutions from 1904 to 2021 on the northeast edge of the Tibetan Plateau are also collected.The stress field in the region is obtained by the method of regional damping stress inversion.The direction of the maximum principal stress is basically consistent with the direction of the principal strain rate,showing that the area in the south of 35 ° N is featured by EW compression,and the area in the north of 35 ° N is characterized by northeast compression.The study also classifies the study area as normal,thrust and strike slip zones according to the dip angles of the maximum and minimum principal stresses.Our findings show that most zones within the Tibetan Plateau and the northern section of Liupanshan fault are characterized by strike slip,while the Qilian Mountain Fault zone,Haiyuan fault zone and the south of West Qinling fault zone show compressive thrust movement,coinciding with the areas of high value in surface compression.Comparing the directions of principal stress axes and principal strain rate axes,we find that the deformation of the upper and middle crust in the northeast margin of the Tibetan Plateau is vertically coherent,while the directions of principal stress and principal strain rates in the areas around the Tarim Basin,Alxa block and South China block are pretty different,likely related to the weak zones in the crust and local surface structures.The intense deformation and strong seismicity in these areas are possibly ascribed to the northeastward expansion of the Tibetan Plateau impeded by the relatively rigid the Alxa block,Ordos block and South China block.In Southwest Yunnan.Firstly,the observation data of 19 new GNSS stations in the Honghe fault from 2018 to 2021 are calculated with high precision.Combined with the existing GNSS data in this area,the latest GNSS velocity field in Southwest Yunnan is presented.Then the multi-scale spherical wavelet method is used to calculate the strain rate tensor,and the spatial distribution characteristics of principal strain rate,surface strain rate and maximum shear strain rate are analyzed.The results show that the southwest of Yunnan is under compression in the direction of NNE,and the surface compressibility is ~10 nstrain/yr.The near east-west-striking fault is characterized by left-lateral strike slip,and the near north-south-trending fault is characterized by rightlateral strike slip.There is an obvious transformation between surface compression and surface expansion in the area between Heihe fault and Wuliangshan fault.The area around Wanding fault,east of Longling Ruili fault,east of Nanting River fault,Heihe fault,Lancang fault and Xiaojiang Fault,from Honghe fault to Menglian-Mengxing fault has high shear strain rate,up to 50 nstrain/yr.Based on the GNSS velocity profile,the characteristics of Honghe fault activity are analyzed.The results show that the strike slip rates of the middle and South sections of Honghe fault are 2.45 ± 0.46 mm/yr and0.72 ± 0.24 mm/yr,respectively,with a small amount of compression and tension movement,respectively.The focal mechanism solutions of the study area from 1976 to2020 are collected,the regional damping stress inversion method is used to invert the stress tensor,and the R-value distribution reflecting the relative size of the principal stress is solved.The inversion results show that the southwest of Yunnan is characterized by the principal compressive stress in the NNE direction,and the area is featured by strike slip stress as a whole,The North-South fault near Tengchong volcano in the east of Dayingjiang fault has the characteristics of normal fault and oblique slip.The magnitude of principal stress in Southwest Yunnan is obviously different in space.The south of Nanting River fault has the largest principal stress,and its strike slip characteristics are more obvious.In addition,this study also collects seismic wave anisotropy data in the study area,compares the angle between the direction of the principal stress axis and the principal strain rate axis,the angle between the direction of the principal stress axis and the polarization direction of PMS fast waves,the angle between the direction of the principal stress axis and the polarization direction of XKS fast waves,and obtains the structural deformation differences between the surface and middle upper crust,middle upper crust and lower crust,lower crust and upper mantle respectively.It shows that the lower crust and upper mantle are decoupled in Southwest Yunnan.At the same time,we find that there are differences in deformation between the lower crust and the middle upper crust near Longling-Ruili fault,Nanting River fault and Menglian fault,indicating that the lower crust and the middle upper crust are decoupled in this area,and there may be a hot material flow channel in the lower crust.