A Numerical Simulation Of The Stress State Along The Eastern Boundary Of The Sichuan-Yunnan Block And Its Surrounding Area Considering Rheological And Locking Effects

The eastern boundary of sichuan-Yunnan block is one of the remarkable geological regions to study the deformation of different blocks in the Southeastern Tibet and their driven mechanism.The kinematic characteristics and crustal stress of the block boudary faults are key parameters for exploring the geodynamic processes that drive deformation of the overall study area.This study presents the current stress field pattern at the eastern boundary of the Sichuan-Yunnan block and its surrounding area at various scales using data from in-situ stress measurements,focal mechanisms,and anisotropy.This paper established a 3D viscoelastic FEM model incorporating the fault as a Coulomb-type friction zone by the stress field and deformation field observation data as constraints,considering the influence of lithospheric rheological structure and fault locking effect.The kinematic characteristics and three-dimensional stress state of the eastern boundary and adjacent areas of the Sichuan-Yunnan block are obtained by numerical simulation.The main achievements of this paper are as follows:(1)The viscosity of Southeastern Tibet and the fault depth of the Xianshuihe-Xiaojiang fault system(XXFS)were investigated and constrained by interseismic GPS observations.We derived the lower-crust viscosity under Southeastern Tibet,the upper-mantle viscosity beneath Southeastern Tibet,and the fault depth of the XXFS as 1×1018 Pa·s,5×1019 Pa·s,and 20 km,respectively.The interaction between the Daliangshan fault and the Anninghe-Zemuhe fault governs slip partitioning around the middle part of the XXFS.The XXFS’ overall mechanical efficiency is improved by the initiation and development of the Daliangshan fault.The interseismic strain buildup rate appears to be spatially heterogeneous,depending on the changes in slip rate along the strike of the XXFS.The accumulated strain energy in the Shimian-Miannning and Mianning-Xichang seismic gaps has enhanced the seismic hazard,which should be closely monitored.(2)The stress field on the eastern boundary of the Sichuan-Yunnan rhombic block has multi-scale characteristics.Generally,the direction of the maximum principal stress rotates clockwise along this boundary,which starts from the south end of the Xianshuihe Fault and ends at the Xiaojiang Fault,i.e.,the stress orientation varies from NW to NS.On a smaller scale,the direction of the maximum principal stress in the deep and shallow crust of the Zemuhe fault,xiaojiang fault and the northern section of the Daliangshan fault is not consistent and gradually turns clockwise with the increase of the depth.The characteristics of stress accumulation are as follows:The stress accumulation rate of the Sichuan-Yunnan block has a clear boundary,which is genearlly defined by the Lijiang-Xiaojinhe Fault,i.e.,the stress buildup is fast in the north and slow in the south.The maximum principal stress decreases gradually from north to south along the XXFS and reaches its lowest in the southern section of the Xiaojiang fault,which increases with the increase of depth.The high stress rate and differential stress mainly occur at the north segment of the Anninghe fault,the north segment of the Xiaojiang fault and the south segment of the Daliangshan fault.On the Anninghe-Zemuhe fault,the stress state is predominantly strike-slip,whereas the southern section of the Daliangshan fault and the northern section of the Xiaojiang fault are predominantly thrust and strike-slip.