The mechanism of deep material transport and seismogenic environment of the Xiaojiang fault system revealed by 3-D magnetotelluric study

The Xiaojiang fault system (XJFS), located to the southeast of the Tibetan Plateau, has a complicated tectonic history and is an ideal location to study the Tibetan Plateau in terms of its deep material transport mechanism and the effects of past tectonic events. In this study, broadband and long-period magnetotelluric data were collected above this fault system and inverted to build a 3-D resistivity model of the lithosphere. As shown in the model, at upper-middle crustal depths, three high-resistivity anomalies separate the strike-slip faults located in the study area, which may be the remnants of the Emeishan large igneous province that was destroyed and modified by Cenozoic crustal activity. The lower crust is characterized by significant low-resistivity anomalies that extend downward to the upper mantle. The low-resistivity anomalies in the upper crust may be caused by brines or/and conductive minerals (e.g., graphite and sulfides), and the possible reason for the low-resistivity anomalies that were imaged in the lower crust and upper mantle may be the presence of hydrogen in nominally anhydrous minerals and partial melts. According to the seismic activity distribution and resistivity structure, we propose dividing the seismic activity of the study area into three categories: tectonic earthquakes, earthquakes with no active faults on the surface, and other scattered earthquakes with no general features. Seismic activities are controlled by tectonic activities, fluid transportation, and the adjustment of the Earth's stress field. It is believed that there is a mutually reinforcing relationship between seismic activity and deep fluids. Fluids could lower the frictional force in faults, promote movement, and thus induce earthquakes; on the other hand, seismic activities and the long-term strike-slip movements of faults could generate heat and increase the connectivity of fluids, which decreases the strength of the crust and facilitates the flow of fluids. Based on the resistivity model, it is demonstrated that the present tectonic activity in the XJFS is complicated and characterized by rigid block extrusion along strike-slip faults in the upper crust, ductile deformation with channel flow in the lower crust, and the upwelling of mantle materials. In combination with previous studies, our results indicate that the weak crustal materials from the Tibetan Plateau are blocked by (1) the lithosphere modified by the Emeishan plume and (2) the South China block when flowing through the Sichuan-Yunnan block. Therefore, these weak materials turn to the southwest direction along the XJFS, then pass through the Red River fault and enter the Indochina block.

Automated summary

This study presents a 3-D resistivity model of the lithosphere in the Xiaojiang fault system (XJFS), which provides insights into the tectonic history and material transport mechanism in the Tibetan Plateau. The model reveals high-resistivity anomalies in the upper-middle crust that may be remnants of the Emeishan large igneous province. The lower crust and upper mantle display low-resistivity anomalies possibly due to the presence of hydrogen. The seismic activity in the area is divided into three categories based on the distribution and structure of resistivity. The tectonic activity in XJFS involves rigid block extrusion, ductile deformation with channel flow, and upwelling of mantle materials. Weak crustal materials from the Tibetan Plateau are blocked by the modified lithosphere and the South China block, flowing southwestward along the XJFS.