Abstract:

The NE-trending Qiyueshan Fault divides the Xuefengshan foreland fold-and-thrust belt into the northwestern Eastern Sichuan fold-and-thrust belt and the southeastern Western Hunan fold-and-thrust belt. However, the nature and formation process of the Qiyueshan Fault remain debated, resulting in significant discrepancies in understanding the formation and evolution mechanisms of the Xuefengshan foreland fold-and-thrust belt. To investigate the formation of the Qiyueshan Fault and its control on differential deformation on both sides, seismic reflection profiles were interpreted, and five discrete element numerical simulation models were designed for experimental analysis. The results reveal that pre-existing faults localize deformation and influence the sequence of fault generation. In contrast, without pre-existing faults, deformation predominantly propagates along the lower detachment layer under weak cohesion. When the thickness of the middle detachment layer exceeds that of the lower detachment layer, deformation is governed by the middle detachment layer, leading to partial decoupling of the competent layers above and below it. Based on a comparison with seismic reflection profiles, the Xuefengshan foreland fold-and-thrust belt is characterized as a “double-step fault-bend fold system.” The Qiyueshan Fault formed during progressive deformation of the foreland belt, and the structural differences across the fault are primarily controlled by the distribution of the basal detachment layer and the mechanical properties of the Cambrian detachment layer. The Western Hunan fold-and-thrust belt is governed by deep-seated fault-bend folding, while the Eastern Sichuan fold-and-thrust belt is controlled by shallow fault-bend folding.

Key words: Xuefengshan, foreland fold-and-thrust belt, Qiyueshan Fault, discrete element method, multiple-detachment