NEW HINTS FOR SUBDUCTION EROSION ACTIVE AT THE PACIFIC MARGIN OFF COSTA RICA

Within a series of continuing studies BGR conducted a marine geophysical survey in the fall of 1999 on the convergent Pacific margin off Costa Rica. Profiles of 3,000 km total length have been acquired with multichannel seismic reflection, magnetic and gravity methods. Part of this study serves as pre-site survey for a proposed ODP drilling site. Focus is given to the internal structure and nature of the so-called margin wedge, the subduction front and the plate boundary. With the new data the landward portion of the margin wedge can be better related to geological units observed on the adjacent Nicoya and Osa Peninsulas. Moreover, it can be shown that this part is dissected by landward dipping faults with little offset. A major one represents the boundary of a frontal and lower unit with irregular reflectivity. This unit could represent a broken part of the former that collapses due to subduction erosion or a splinter of oceanic crust that was underplated from the west. The plate boundary shows the well-known low-frequency high-energy laminated reflection pattern. At some locations, however, strong upward directed bulges were detected riding on top of it. These observations along with the highly resolved near-coastal sedimentary cover at the upper slope support the hypothesis that at least intermittently basal erosion was active at the plate boundary. A particular role in this regard plays the Cocos Ridge that is being subducted beneath the Osa Peninsula causing its elevated position. A pronounced positive magnetic anomaly is observed parallel to the margin the relation of which to either the upper or the lower unit could not be unambiguously determined, yet.