A series of deep imbricate thrust structures associated with a period of margin accretion, have been interpreted in seismic reflection data offshore Costa Rica. This interpretation for accretion is inconsistent with the long-term margin subsidence recorded in that setting. Using Discrete Element Method simulations, we test the idea that thinning of the incoming sedimentary blanket exposed underlying seamounts that interacted with the overriding plate, and drove a change in tectonic regime from margin accretion to erosion. Our modeling results show seamounts can enhance the rates of wedge growth, at least transiently, even as they cause erosion during later stages of forearc evolution. Seamounts also impart considerable changes to forearc structure as they pass by, rotating faults, initiating a cross-cutting backthrusts, activating landslides, and steepening the forearc slopes. We also show seamounts partitioning strain within the wedge by landward reactivation of structures, which may have implications to seismogenesis.