Accelerated subduction erosion opposite the Cocos Ridge: Implications for the initiation of ridge subduction

Subduction erosion has been invoked as the dominant process controlling the tectonic evolution of the Pacific margin of Costa Rica. In an erosive subduction margin, the upper plate material is incorporated into the subduction channel due to basal erosion, resulting in long term subsidence of the upper plate. Estimation of mass removal from the upper plate based on subsidence profiles of slope sediments is crucial to assessing the thickness of the subduction channel. CRISP (Costa Rica Seismogenesis Project) is designed to understand the processes that control fault zone behavior during earthquake nucleation and rupture propagation at erosional subduction zones. IODP Expedition 334 successfully recovered slope sediments offshore the Osa Peninsula where the Cocos Ridge is subducting beneath the Caribbean plate. Preliminary results from biostratigraphic dating obtained from two slope sites indicate high sediment accumulation rates in the terrestrially-sourced slope sequence, ranging from 516-236 m/my at Site U1378 in the middle slope to 1035-160 m/my at Site U1379 in the upper slope. In particular, the accumulation rate of the slope sediments, mainly composed of clayey silt/silty clay, at Site U1379 is 1035 m/my, much higher than that of slope sediments offshore Nicoya (38-99 m/my) (Kimura et al., 1997). The remarkably high accumulation rate offshore Osa could be derived from subaerial uplift triggered by the subduction of the Cocos Ridge. Concomitant with uplift, the subduction of such a bathymetric high likely accelerates the basal erosion of the upper plate. The subsidence/uplift profiles of the slope sediments offshore Osa are controlled by the high sediment accumulation rate and the basal erosion; the subsidence of the upper plate likely occurred when the rate of subduction erosion was exceeded the sediment accumulation rate on the slope. Detailed studies of sedimentary facies and benthic foraminifera faunal in slope sediments at Sites U1378 and U1379 are keys to estimate the mass removal associated with basal erosion and the thickness of the subduction channel that may elucidate earthquake processes. Kimura, G., Silver, E., et al., 1997, Proceedings of the Ocean Drilling Program Initial reports, Volume 170: College Station, Texas, Ocean Drilling Program, 458 p.