Effects of backstop vertical position on the evolution of a backthrust dominated fold-and-thrust belt: Insights from viscous/brittle analogue models and comparison with cascadia

The most common features of a fold-and-thrust belt (FTB) are forward and backward thrusts, the distribution of which depends on several parameters that have been tested extensively. To further understand the development of FTBs in which an incompetent (viscous) detachment layer exists between an overlying deformed sedimentary pack (reverse faulting) and underlying undeformed basement, this study comprises four different viscous(bottom)/brittle(top) analogue models to test the effects of a new parameter, the vertical position of the piston's base relative to the viscous/brittle interface. Given that the models include a basal viscous layer, we also tested the effects of variable strain rate. The results indicate that both tested parameters significantly influence the internal structure of the sand wedge regarding wedge slope, structural style and thrust vergence. Finally, we compare the experimental results with the formation of the Cascadia FTB, and conclude that the model with shallow piston and constant piston velocity reproduces all the critical features of the composite structure of the Cascadia FTB, i.e. a foreland domain with dominant landward vergent thrusts and a shallow taper angle, a hinterland domain with dominant oceanward vergent thrusts and a flat topographic surface (taper angle = 0°), and a triangular zone separating the two domains with opposite vergences.