Lithospheric rheological structure and dynamic mechanism in the Sichuan–Yunnan region

SUMMARY The Sichuan–Yunnan region is a crucial area for studying the deformation and tectonic evolution of the lithosphere within the Tibetan Plateau. However, a significant controversy exists about the spatial distribution of the low-viscosity zones in its mid-lower crust. Herein, we utilized a combination of topography, geoid height, surface heat flow and Rayleigh wave phase velocity dispersion curves to ascertain the lithospheric temperature, seismic wave velocity and density structure in this region. By correlating the inverted velocity and density structures with laboratory measurements of rock velocity and density, we further deduced the lithospheric lithology structure and rheological property of the Sichuan–Yunnan region. Our findings suggest that the lower crust of this region is predominantly composed of felsic granulite. The lower crust of the Qiangtang Block, the Chuan-Xi Block, the Dian-Zhong Block west of the Lvzhijiang fault and the IndoChina Block exhibit extensive areas with low-viscosity characteristics (<1021 Pa·s). In contrast, the Sichuan Basin, the Eastern Himalayan Syntaxis and the central region of the Emeishan Igneous Province are characterized by high strength. We argue that the collision between the Indian and Eurasian plates led to the thickening of the Qiangtang Block's crust, producing a large low-viscosity area within the mid-lower crust. The delamination of the IndoChina lithosphere may cause the upwelling of mantle material, thereby weakening the lithosphere of the Dian-Zhong Block west of the Lvzhijiang fault and the IndoChina Block. This study delineates the spatial distribution of low-viscosity zones within the mid-lower crust of the Sichuan–Yunnan region, offering a foundational rheological model that can be instrumental for subsequent seismological and dynamic analyses.