Tectonic evolution of the Sichuan Basin, Southwest China

Surface deformation in continental interiors is typically coupled to processes at depth. However, in the Sichuan basin, east of the Tibetan Plateau, recent seismic data detect Neoproterozoic subduction and rifts, indicating long-term tectonic quiescence of the basement. The Sichuan basin is characterized by two episodes of weak extension and compression which influenced the architecture of the marine platform during Palaeozoic to Middle Triassic times. The weak extensional episodes occurred in the early Cambrian and late Permian, characterized by formation of intracratonic sags oriented at high angles to the basin margins, the early Cambrian Mianyang-Changning sag and the late Permian Kaijiang-Liangping sag. The formation of these sags was followed by weak compressional episodes that formed the Caledonian (Early Palaeozoic) Leshan-Longnvshi palaeo-uplift and the Mid. to Late Triassic Luzhou and Kaijiang palaeo-uplifts. These intracratonic sags and palaeo-uplifts show no genetic correlation in terms of geometry or inherited deformation. Subsequent evolution of the Sichuan basin was characterised by foreland basins controlled by the Longmenshan and Dabashan fold-and-thrust belts along the western and northeastern margins of the basin respectively. A large wedge-shaped body of strata was subsequently eroded, removing of 2.0-3.0 km of rock in the Cenozoic. The subsidence history of the Sichuan basin is characterized by an increase in subsidence rate due to the formation of the foreland basins and by a subsequent period of uplift and exhumation. We argue that structural decoupling occurred both between the sedimentary cover and the basement and between the multiple sedimentary sequences in the Sichuan basin. The Sichuan basin may thus represent a unique type of superimposed basin, as its evolution appears to have been controlled by deformation propagated from its peripheral orogens. This suggests a mechanism of basin architecture controlled by crustal deformation, rather than by deep lithospheric and mantle processes.