THE GEOMETRY OF THE WADATI-BENIOFF ZONE UNDER SOUTHERN CENTRAL-AMERICA AND ITS TECTONIC SIGNIFICANCE - RESULTS FROM A HIGH-RESOLUTION LOCALSEISMOGRAPHIC NETWORK

We present here a detailed geometry of the Wadati-Benioff zone under C osta Rica, obtained from seismicity recorded by a dense local seismogr aphic network jointly operated by the Costa Rica Volcanological and Se ismological Observatory, National University, and the Charles F. Richt er Seismological Laboratory, University of California, Santa Cruz. Und erneath the Nicaragua-Costa Rica border the Wadati-Benioff zone smooth ly contorts (from steep to shallow dip angles, NW to SE), but does not show evidence of a brittle tear, as postulated by others. However, fu rther to the SE, NE of Puerto Caldera, Costa Rica, the Wadati-Benioff zone does show a segmentation (the Quesada Sharp Contortion) at interm ediate depths (h > 70 km). NW of this sharp contortion the deepest por tion of the seismically active slab dips at about 80-degrees and reach es maximum depths ranging from 200 km, near the Nicaragua-Costa Rica b order, to 135 km under Ciudad Quesada. To the SE the deeper portion of the Wadati-Benioff zone dips at about 60-degrees and the seismicity d oes not extend below depths ranging from 125 km, behind the volcanic a rc, to 50 km, east of Quepos. In southern Costa Rica, east of 83-degre es 55'W, we find no evidence of the Wadati-Benioff zone deeper than 50 km. The obtained geometry and other known tectonic features related t o the subduction of the Cocos plate under the Caribbean plate along th e southern terminus of the Middle America Trench (Nicaragua and Costa Rica) correlate well with along-trench variations in age of the subduc ted Cocos plate. Some of these tectonic features are: (1) the shallowi ng of Middle America Trench bathymetry from NW to SE; (2) variations i n the energy release within the subducted slab; (3) differences in cou pling between Cocos and Caribbean plates; (4) the termination of the C entral America Volcanic Chain in central Costa Rica; (5) distinct stre ss field variations on the overriding Caribbean plate. The subduction of the Cocos Ridge under southern Costa Rica is partially responsible for some of these features (shallowing of the Middle America Trench in southern Costa Rica) and for the high uplift rates of the outer arc. However, as the presence of the Panama Fracture Zone limits the subduc ted extension of the Cocos Ridge to less than 100 km from the trench, we propose that the overall geometry of the Wadati-Benioff zone is con trolled by abrupt along-trench changes in the age of the subducted Coc os plate.