LAUSR

Abstract The Costa Rican convergent margin is a typical erosive margin where a low-angle subduction of the Cocos Ridge (CR) beneath the Caribbean Plate takes place and subduction erosion of the basal and forearc area in the overlying plate occurs. Because subduction erosion is destructive to rock records, subduction erosion processes can only be understood by indirect means such as investigating the deformation of the forearc sediments of the overlying plate. To explore the deformation characteristics and their relationships to subduction erosion process, we conducted paleomagnetic and sedimentologic investigations on sedimentary strata of Site U1413, recovered by the International Ocean Discovery Program (IODP) Expedition 344 in the upper slope of the forearc area of the Costa Rican convergent margin. The magnetic fabric results show that the degree of offset of the Kmin axis from the vertical, as measured by the “90°-Kmin”, increases with depth in the strata below ~380 mbsf (meters below seafloor), indicating a deformation zone below ~380 mbsf. The age of the uppermost deformation zone of Site U1413 is similar to those at the upper slope Site U1379 and the middle slope Site U1380, suggesting that widespread deformation occurred in the forearc area since ~1.8 Ma as a result of the near-flat subduction of the CR. In addition, from the middle slope to the upper slope, the dip angle of the strata in the deformation zone decreases, suggesting that the degree of deformation decreases progressively landward. Furthermore, the sudden changes in depositional environments recorded in different sites spanning ~2.0 Ma to ~1.9 Ma, likely indicate small-scale rapid uplift and subsidence in the forearc in response to seamount subduction in the context of km-scale uplift and subsequent subsidence associated with the initiation of the CR subduction at ~2.0 Ma.