Magnetic signature of upper plate structures and subducting seamounts at the convergent margin off Costa Rica

Offshore the Pacific margin of Costa Pica the Caribbean Plate converges with the subducting Cocos Plate along the Middle America Trench. The tectonics of both plates have been studied on the basis of reflection seismic, gravimetric, and magnetic data and swath-mapping. The magnetic data were acquired by GEOMAR, Kiel, Germany, during cruise SO-76 of R/V Sonne off Costs Pica in 1992. Two different magnetic anomaly patterns are observed. Off Nicoya Peninsula the linear magnetic anomalies trend about N50 degrees E and can be traced from the ocean basin to the landward end of the profiles. To the southeast of a major structural break trending parallel to Fisher Seamount and the associated ridge the strike of the magnetic anomalies is N70 degrees E. In the area southeast of Fisher Seamount the magnetic signatures of numerous seamounts are superimposed upon the linear anomalies. We interpret the magnetic lineations as chrons 5C to 6B (16-22.5 Ma). Magnetic modeling demonstrates a significant magnetization (similar to 1 A/m) of the margin wedge in the continental slope, a structural unit known from seismic studies. This is not compatible with the previously discussed origin of the continental slope as an accretionary wedge. A composition of the margin wedge similar to the ophiolitic rocks found onshore on Nicoya Peninsula appears likely. A number of morphological furrows observed in the landward trench slope opposite the seamount covered domain of the oceanic crust coincide with magnetic anomalies. Three-dimensional magnetic modeling demonstrates that these anomalies can be explained well by subducting seamounts. This implies that the seamounts remain intact, at least magnetically, for a long time interval during the subduction process. Thus they may indeed be important seismogenic asperities.