Deformation of the lithosphere at the eastern boundary of the Sichuan-Yunnan Block and adjacent areas revealed by magnetotelluric data

ABSTRACT To analyze the deep deformation pattern in the eastern boundary of the Sichuan-Yunnan Block (SYB), four magnetotelluric profiles are arranged to establish a 3D electrical structure of the study area. The model reveals lithospheric-scale high resistivity along the Anninghe fault, likely representing a residue from the Late Permian Emeishan volcanic activity. The rock rheology of the crust in the study area is evaluated based on conductivity and average flow velocity/viscosity relationships. On this basis, a combined model is developed, which suggests that the upper and lower crusts are decoupled, with lateral extrusion in the upper crust and predominantly ductile deformation in the lower crust. A dynamical process is described in which the weak materials in the crust of the Tibetan Plateau flow southward into the SYB in a channel dominated by the Lijiang-Xiaojinhe fault (low resistivity and low velocity) to escape and are blocked by the rigid lithospheric barrier (high resistivity and high velocity) in the area of Panzhihua-Xichang-Mianning, and that the weak materials in the crust may not have flowed to the Xiaojiang fault (XJF). The low-resistivity low-velocity bodies near the XJF may originate from fault activity or mantle material upwelling.