The influence of weak layers on thrust structure deformation: A finite element numerical simulation study

Objective As ubiquitous critical structural units in sedimentary basins, weak layers are characterized by low shear strength, low Young’s modulus, and pronounced plastic rheological behavior, playing a key role in stress accommodation and strain partitioning during tectonic deformation. Seismic reflection profiles from areas such as Liangcun, Jiaoshiba, and Changning in southeastern Sichuan reveal widespread regional weak layers within sequences overlying deep thrust fault systems. Methods To investigate the dynamic control mechanism of weak layers on thrust deformation, a typical bend fault was selected as the pre-existing fault structure, and comparative experiments with/without weak layers were designed. Finite element modeling was employed to conduct numerical simulations under lateral compression. A comparative analysis of the simulation results from the two model sets systematically examines how weak layers control structural deformation during tectonic movement, particularly the influence of weak layer thickness on upper and lower structural deformation. Results (1) The weak layer-free model demonstrates that, under lateral compression, the overlying strata undergo thrust-parallel slip and fold deformation along the pre-existing bend fault. The deformation exhibits remarkable coherence among strata, with no observable interlayer slip or stratified differential deformation. Meanwhile, the maximum principal stress field displays characteristic tectonic stress zoning, while plastic strain concentrates on both the forelimb and the backlimb with upward-decreasing intensity. (2) The weak layer-bearing model reveals that, under combined lateral compression and underlying structural uplift, the weak layer experiences plastic flow, manifesting as top-thinning and limb-thickening. This results in stratified deformation patterns bounded by the weak layer. Furthermore, the distribution of maximum principal stress and plastic strain shows distinct stressen–strain decoupling across the weak layer interface. Conclusion (1) The weak layer constitutes a critical factor in initiating structural stratification. Under lateral compression conditions, the weak layer undergoes plastic flow accompanied by localized thickening and thinning. It significantly accommodates underlying structural deformation and stress-strain, thereby generating differential deformation across the weak layer interface and producing distinct stress-strain decoupling between the upper and lower structural domains. (2) The thickness of weak layers constitutes a critical parameter controlling deformation styles. Thicker weak layers result in longer wavelengths and gentler limb dips with reduced uplift amplitudes for overlying folds; they also produce shorter wavelengths, steeper limb dips and greater uplift amplitudes for underlying folds, thereby enhancing deformation partitioning. Conversely, thinner weak layers lead to more coherent deformation between the upper and lower structural domains. [ Significance] The research findings regarding the influence of weak layers on thrust structural deformation revealed in this study can provide valuable references for structural deformation analysis and dynamic modeling in regions with similar stratigraphic characteristics, such as the Liangcun area, the Jiaoshiba block, and the Changning region in southeastern Sichuan.