Jared W. Kluesner Eli Silver Rachel Lauer Nathan Bangs Brian Boston Joel Edwards 2021 <p><span>The Costa Rican convergent margin has been considered a type erosive margin, with erosional models suggesting average losses up to −153&nbsp;km</span>³<span>/km/m.y. However, three‐dimensional (3D) seismic reflection and Integrated Ocean Drilling Program data collected offshore the Osa Peninsula images accretionary structures and vertical motions that conflict with the forearc basal erosion model. Here we integrate such data to do an in‐situ accounting of material transfer at the plate boundary across the outermost 10&nbsp;km of the forearc, characterized by active and inactive megathrusts. Our in‐situ budget finds an approximate balance between sediment recycling via accretion and underplating, 0.7–2.3&nbsp;km</span>³<span>/km/m.y., and basal erosion, 0.7&nbsp;km</span>³<span>/km/m.y., while subducting sediment volumes, 7.8&nbsp;km</span>³<span>/km/m.y., greatly outpace either material transfer volumes. These budget results differ significantly from published estimates based on simple proxies of trench axis deflection and slope subsidence. These budget results are the summation of thin incoming hemipelagic sediments that variably accrete along the deformation front, underplating of hemipelagic sediments on the upthrown‐side and basal erosion on the downthrown‐side of active plate bending faulting landward of the trench axis, and sediment subduction primarily composed of pelagic sediments.</span></p> application/pdf 10.1029/2020GC009190 en American Geophysical Union In‐situ mass balance estimates offshore Costa Rica article