New Seismic Imaging of the Crustal Structure beneath the Eastern Sichuan and Wuling Mountains, South China: Insights into the Formation of Fold-and-Thrust Belts

Intracontinental deformation is out of the theory of conventional plate tectonics. It is widely recognized with deformation within the continental interior instead of the plate margin, yet its formation mechanism has long been controversial. The eastern Sichuan- Wuling mountains (ESWM) area is located similar to 1300 km away from the subduction plate boundary and had developed intracontinental deformations, including crustal shortening and fold-and-thrust (FAT) tectonics, making it an ideal place to understand the mechanism of intracontinental deformation. In this study, we obtain a new seismic image of the 3D crustal structure of the ESWM area using the continuous ambient noise data of 67 broadband seismic stations. We invert the Rayleigh-wave dispersions of 5-30 s derived from cross-correlating the Z-component of all station pairs and obtain the fine crustal VS model. Our new seismic image reveals distinct velocity characteristics between the thin-skinned chevron anticline FAT tectonics in the eastern Sichuan basin and the thick-skinned chevron syncline FAT tectonics in the Wuling mountains area. Specifically, a low-VS layer observed beneath the Wuling mountains area, together with the crystalline basement beneath the eastern Sichuan basin, marks the ductile decollements confining the folding and thrusting deformation. Based on our new VS model and some previous studies, we propose a geodynamic model, which is associated with the far-field effect of the westward paleo-Pacific subduction during the late Mesozoic. Our model meets all the structural investigations at surface and geophysical observations at depth, and is reliable and valuable for further studies on similar intracontinental deformation in other regions.