Abstract
Subduction erosion rather than subduction accretion is proposed as the dominant process currently occurring at the Nicoya segment of the convergent plate margin off Costa Rica. 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 and landward migration of the Nicoya coastline. High seismic velocities in the outermost part of the forearc wedge off the Nicoya Peninsula below the BOSS (bottom‐of‐slope‐sediment) reflector indicate the seaward continuation of 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 substantial fore‐arc extension. In our interpretation, the removal of material from the base of the forearc wedge by the process of basal subduction erosion leads to progressive subsidence and thus to landward migration of the coastline. Landward shift of the active volcano chain in the same order of magnitude as the coastline migration is consistent with this interpretation.
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