Effect of Backstop Position and Basal Weak Layer Viscosity on Along‐Strike Variations in Structural Vergence of FTBs: Insights From Analogue Modelling

ABSTRACT Fold‐and‐thrust belts (FTBs) form forward and backward thrusts and symmetrical pop‐up structures. However, some uncommon deformation features with landward vergent thrust wedges have not been adequately explained by the classic Coulomb wedge theory, such as the FTBs in the Cascadia convergent margin and Eastern Sichuan Basin, China. Based on previous studies, this study was conducted using six analogue models with different viscosities and backstop positions to understand the causes of these uncommon structures. The results showed that (1) structural style differences are primarily related to the viscosity of the detachment layers and the position of the piston. Wider deformation scopes of FTBs were formed by low viscosity and basal friction strengths, followed by low‐amplitude folds and low‐angle faults. (2) The symmetrical pop‐up structure and the forward and backward thrust structures of curved FTB's front edge were changed by the shear stress ratio of the brittle‒ductile layer. (3) The pure backward thrust fault of the Cascadia wedge was controlled by the low viscosity of the detachment layers, and the along‐strike variation in the structural vergence of the Eastern Sichuan Basin FTBs may have been caused by the lower basal weak detachment layer and different shear stresses.