Dynamic Evolution of Forearc Subsidence Controlled by Slab Geometry

Abstract

Several subduction zones worldwide present an enigmatic forearc topography and strain characterized by extension at a forearc ridge near the trench and shortening within a forearc valley located between the ridge and the magmatic arc. The development of this topography has been linked to progressive slab steepening, while strain patterns emerge from lithospheric bending. However, the sensitivity of forearc deformation to subduction parameters remains poorly constrained. Here, we present results from 4D buoyancy‐driven analogue experiments investigating how overriding plate thickness, subducting plate thickness, far‐field boundary conditions, and subduction interface rheology control the development of this forearc deformation, particularly during the free sinking stage. The parametric analysis shows that thinner overriding plates develop narrower, more curved forearc valleys, with greater trench‐normal shortening. Subducting plate thickness has a non‐linear effect on valley evolution, while laterally fixed plates reduce surface deflections and shortening by enhancing suction forces unrelated to slab steepening. Finally, stronger interface rheologies promote deeper, more curved valleys with higher shortening. Our experiments demonstrate that forearc deformation is governed primarily by slab geometry (which is affected by initial slab dip angle), with surface curvature and shortening correlating with the slab bending radius. We also obtain a strong positive spatio‐temporal correlation () between surface curvature and shortening, largely independent of the parameters tested, confirming that forearc strain arises from lithospheric bending. Our results provide explanations for the forearc topography and strain observed at the Chile, Alaska and Cascadia subduction zones, including the trench‐parallel variation in forearc basin sediment thickness observed in Chile.