Seismic reflection analysis of the Western Sichuan Basin: Implications for the decoupling relationship between deep and shallow structures

The structural relationship between the deep and shallow structures of the Western Sichuan Basin (WSB) holds significant information for comprehending the tectonic framework of the basin and the eastward expansion of the Tibetan Plateau. Although the regional structural geometries in the WSB have been extensively studied, the kinematics and controlling factors are still debated. Field observations, seismic reflection profiles, and balanced sections hereby were used to quantitatively analyze the structural geometry and kinematic characteristics of the WSB and to estimate the structural shortening of major tectonic units, respectively. Combined with near-surface geological data, the seismic reflection profiles reveal a set of tectonic patterns decoupled vertically. The deep layer is characterized by stable marine sediments and Neoproterozoic basement rifts bounded by deep faults, while the shallow layer primarily comprises a distinct foreland basin consisting of a series of anticlines and thrust faults. The Middle Triassic evaporites, as the decollement layer of the decoupled tectonic patterns, regulated the deformation of the shallow asymmetrical anticlines and long-distance thrust faults, forming a typical thin-skinned structure. The balanced sections have confirmed a pattern of structural wedges with thrust faults in the shallow layers of the WSB, revealing that deformation propagation attenuates from the Longmenshan Thrust Belt (LMSTB) to the eastern part of Longquanshan (with a shortening rate < 5 %). The primary contribution to this deformation is attributed to the LMSTB and the Xiongpo anticline. The joint seismic sections show that the distribution of the Triassic decollement layer gradually contracts from southwest to northeast, indicating a decrease in the intensity of stress regulation. On the basis of the spatio-temporal framework, we argue that the mechanism of the complex structural configuration in the WSB is controlled by the Middle Triassic decollement layer and exhibits different characteristics in deep and shallow structures.