Structural Style and Evolution of Fold-and-thrust Belts Associated with Multiple Detachments in the Eastern Sichuan Basin, South China

The Sichuan Basin is located in the northwest of the Yangtze Block, South China, and the eastern Sichuan Basin is bounded by the Huayingshan fault to the west and the Qiyueshan fault to the east, which is characterized by approximately parallel thin-skinned fold-and-thrust belts. The geometry and kinematics, however, have not been fully understood due to the previous poor seismic data. In this study, we have applied the new seismic data and constructed a 2D- and 3D seismic cross section which is around 300-km long perpendicular to the fold-and-thrust belts. The stratigraphic succession in the study area is composed of competent horizons interbedded with three incompetent horizons, which are the Middle-Lower Cambrian evaporates, Lower Silurian shales, and Middle-Lower Triassic evaporates, respectively. The mechanical anisotropic properties between the competent and incompetent horizons played a dominant role in the structural deformation, especially the lowest Lower-Middle Cambrian evaporates, above which the fold-and-thrust belts were generated. The main structural styles are detachment folds, fault propagation folds, and back thrusts developed associated with the multiple detachments. The kinematic analyses indicate that the fold-and-thrust belts had the similar evolution processes. The detachment fold originated from the Cambrian evaporates as the compression took place. As the compression increased, one limb of the detachment fold was broken and the fault propagation fold was formed. Afterward, another limb of the propagation fold was also broken and the back thrust was formed. The formation of the fold-and-thrust belts was due to the shortening from the hinterland in the Xuefengshan during the period of the Later Jurassic to Cretaceous. The structural deformation was decreased from the hinterland to the foreland and terminated in the central basin with the absence of the Cambrian evaporates.