Crustal structure of the middle American trench off Costa Rica from wide-angle seismic data

Off Costa Rica seismic wide-angle experiments were made using marine seismic sources of high shot density and ocean bottom hydrophones (OBH) and land stations for recording. Three profiles perpendicular to the Middle American Trench (MAT) and one parallel to it are presented here. The coverage from closely spaced OBH, and structural constraints from the coincident near-vertical reflection seismic survey provided detailed velocity depth distributions. Over 4.0 km/s velocities were found directly underlying the thick continental margin sediment in a margin wedge that extends from below the coastal area seaward within 10 km from the MAT axis. The interpretation of the wide-angle data required a low velocity zone (LVZ) between the upper and lower plate, indicating its nature as subducted and underplated sediment. The velocity depth distributions support the hypothesis that the continental margin wedge off Costa Pica is most likely an offshore extension of the Nicoya complex ophiolitic rocks, which are exposed along the coast. The subduction of the sediments seems very efficient and the accretionary prism appears to be confined to the lower slope.