Subsidence and extension at a convergent plate margin: evidence for subduction erosion off Costa Rica

Subduction erosion rather than subduction accretion is proposed as the domi nant process currently occurring at the Nicoya segment of the convergent pl ate margin off Costa Pica. Based on new results from ODP drilling cores and our interpretations of published seismic data we present a tectonic model of subsidence and extension due to tectonic erosion of the forearc wedge an d landward migration of the Nicoya coastline. High seismic velocities in th e outermost part of the forearc wedge off the Nicoya Peninsula below the BO SS (bottom-of-slope-sediment) reflector indicate the seaward continuation o f the Nicoya ophiolite complex into the basement of the forearc wedge. ODP Site 1042 revealed neritic sediment that points to strong subsidence of the forearc basement, progradation of the sedimentary succession, and landward migration of the coastline. Tilted block structures are explained by subst antial fore-are extension. In our interpretation, the removal of material f rom the base of the forearc wedge by the process of basal subduction erosio n leads to progressive subsidence and thus to landward migration of the coa stline. Landward shift of the active volcano chain in the same order of mag nitude as the coastline migration is consistent with this interpretation.