Extent of the Lower Crustal Flow Constrained by the Rheological Structure at the Eastern Boundary of the Chuan-Dian Rhombus Block

The Chuan-Dian Rhombus Block is the main channel for materials escaping southeastward from the Tibetan Plateau. Verifying the extent of the lower crustal flow and its impact on the lithospheric deformation, isostatic state, and seismicity in the Chuan-Dian Rhombus Block is of vital importance. Based on data from the China Seismic Experimental Site, the current study analyzes the rheological structure of the eastern boundary of the Chuan-Dian Rhombus Block (i.e., the Xianshuihe-Xiaojiang Fault System). The results reveal a low-viscosity zone of 1020 Paes, with a thickness of approximately 20 km in the middle-lower crust beneath the Xianshuihe fault. The low-viscosity zone extends southward and gradually thins until it basically disappears at the Xiaojiang fault. Our results indicate that the extent of the lower crustal flow is restricted in the Xianshuihe fault and the northernmost part of the Anninghe-Zemuhe fault and does not extend to the Xiaojiang fault. The crust of the Xianshuihe fault is notably thickened and in an isostatic disequilibrium state. The Xiaojiang fault maintains a relatively thick seismogenic layer and isostatic equilibrium.