Branched Crustal Flow and Its Dynamic Significance in Sanjiang Area, Eastern Tibetan Plateau--Insights From 3-D Magnetotelluric Imaging

The crustal material from central Tibet is extruded in a clockwise direction along a belt on the eastern plateau. In the inner arc region of the escaping belt, the absence of key and detailed 3-D crustal resistivity structure hinders a comprehensive understanding of the dynamic processes of material escape in both the inner and outer arc regions. Here, we conducted magnetotelluric imaging and obtained the crustal 3-D resistivity structure in Sanjiang area. The results reveal the presence of two branched high-conductivity anomaly belts in the middle crust. Combining with other resistivity and velocity models, we speculated that crustal flow is widely distributed in the middle crust of the Chuan-Dian block. The crustal flow in the Sanjiang area may connect to that in the outer arc region. The crustal flow in the eastern part is extensively continuous, causing decoupling and flowing that facilitate intense horizontal movements and deformation of the upper crust. In the western Sanjiang area, the upper crust is strongly coupled with the lithosphere beneath the decoupling layer, resulting in weaker horizontal deformation, and fewer larger earthquakes. The initially weak crustal zone in the eastern Tibet may have been caused by uplift of hot mantle material. The high heat flow associated with uplift of hot mantle material and the frictional heating caused by the horizontal movement of weakly coupled crust further facilitated the formation of crustal flow in the outer arc region. The branched crustal flow in the Sanjiang area may have flowed from the outer arc region of the escaping belt. The crustal flow model can well explain the expansion of the Tibetan Plateau in the eastern margin of the plateau, and the high-conductivity bodies in the middle-lower crust may reflect the morphology of crustal flow. In the Sanjiang area of eastern Tibet, we used magnetotelluric data from 41 sites to discover branched high-conductivity belts in the middle crust through 3-D imaging, which are different from the extensively continuous high-conductivity layer in the eastern part. They reveal the differences in the morphology of crustal flow between the inner and outer arc regions of the material escaping belt. The difference in the morphology of high-conductivity structures leads to different degrees of crustal mechanical coupling, which in turn is related to surface strain rate, topography, and seismic activities. We speculate that crustal flow initially originated from the outer arc region and gradually developed toward the inner arc region. 3-D magnetotelluric imaging confirmed the presence of branched crustal flow in Sanjiang area, eastern Tibetan Plateau Crustal flow is widely in the middle crust of the North Chuan-Dian block and its morphologies in eastern and western are different The branched crustal flow in the Sanjiang area may have flowed from the outer arc region of the escaping belt