Scaled two-dimensional sandbox experiments are used to investigate the effect of (1) the location of erosion with respect to the convergence geometry and (2) two erosion modes, distributed or focused, which are thought to represent end members, on the distribution and propagation of deformation within bivergent orogens. This study applies to the brittle part of medium-sized, natural orogens. Particle image velocimetry (PIV) is used to analyze the experiments with respect to surface uplift, thrust displacement, and finite strain.

The experiments suggest that deformation responds immediately to erosion. Retro-wedge erosion amplifies the displacement of the basally accreted material, whereas pro-wedge erosion accelerates and additionally redirects the particle flux of the frontally accreted material. Pro- and retro-wedge erosion retards the propagation of deformation within the pro-wedge. This effect is stronger for pro-wedge erosion. Retro-wedge erosion amplifies vertical growth and leads to strain accumulation along the retro shear-zone and the mid-level detachment. Thus, during retro-wedge erosion, cause (erosion) and response (deformation) are significantly offset in space. Since pro-wedge erosion evokes a complete decoupling of the retro-wedge from the pro-wedge, cause and response are spatially more closely related. With respect to the erosion mode, we found that a more focused erosion leads to a more focused strain accumulation. Similarly, focused erosion applied at the pro-wedge deformation front prohibits accumulation of out-of-sequence displacement. In contrast, distributed pro-wedge erosion amplifies out-of-sequence displacement. Thus, a forward breaking or “piggyback” sequence of thrusting might involve considerable out-of-sequence displacement, which is strongly controlled by the erosion mode.