Abstract
Abstract : The cosmogenic radionuclide (10)Be is a unique tracer of shallow sediment subduction in volcanic arcs. The range in (10)Be enrichment in the Central American Volcanic Arc between Guatemala and Costa Rica is not controlled by variations in (10)Be concentrations in subducting sediment seaward of the Middle America Trench. Grain size is the principal control on the sedimentary (10)Be concentrations. (10)Be in fine-grained, terrigenous sediments is diluted by larger grained volcanogenic material. The sharp decrease in (10)Be enrichment in the Central American Volcanic Arc between southeastern Nicaragua and northwestern Costa Rica correlates with fault structure in the subducting Cocos plate. Offshore of Nicaragua, extensional faults associated with plate bending have throw equal to or greater than the overlying subducting sediment thickness. These faults enable efficient subduction of the entire sediment package by preventing relocation of the decollement within the downgoing sediments. Offshore of Costa Rica, the reduction of fault relief results in basement faults that do not penetrate the overlying sediment. A conceptual model is proposed in which the absence of significant basement roughness allows the decollement to descend into the subducting sediment column, leading to subsequent underplating and therefore removal of the bulk of the sediment layer that contains (10)Be.
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