Seamount chain–subduction zone interactions: Implications for accretionary and erosive subduction zone behavior

Sediment volume at the trench and topographic highs on the incoming plate are two of the main factors controlling whether a forearc will undergo subduction erosion or accretion. On oceanic plates, topographic highs such as large seamount complexes are commonly associated with significant volumes of flanking volcaniclastic sediments in the form of >100-km-wide debris aprons, with the largest deposits found in flexural moat basins. We propose that subduction of these sediment accumulations promotes localized frontal accretion, even in otherwise non-accretionary margins. The Osa mélange in southwestern Costa Rica is a field example that provides new insights into the nature and occurrence of this interaction. The southwestern margin of Central America is punctuated by accreted Late Cretaceous–middle Eocene seamounts that formed at the Galápagos hotspot and accreted throughout the late Miocene. In contrast to most accreted seamounts along this margin, which retained their overall structure, the Osa mélange is a chaotic mixture of seamount lithologies. It consists of basalt, chert, and carbonate blocks in a fine-grained pelitic matrix composed predominantly of feldspar and pyroxene grains with rare quartz. This lithology is consistent with sediment from a seamount chain’s debris apron, such as the Hawaiian moat sampled during Ocean Drilling Program (ODP) Leg 136 and the Canary Islands moat sampled by ODP Leg 157. Subduction of seamounts and their debris aprons promotes concurrent accretion and erosion over short distances along the trench. This introduces heterogeneity into the subduction channel, with implications for deformation within the subduction zone plate interface.