EVIDENCE AND MECHANISMS FOR FORE-ARC EXTENSION AT THE ACCRETIONARY COSTA-RICA CONVERGENT MARGIN

Seismic reflection data across the upper trench slope off the Nicoya Peninsula, Costa Rica, reveal a wide zone of nearly trench-parallel normal faults. Although work in the last decade has shown that normal faults are present at many convergent margins, most examples (e.g., Japan, Peru-Chile, and Guatemala) have been associated with margins experiencing subduction erosion or non-accretion. In contrast, extension in the Costa Rica study area apparently is coeval with frontal accretion and underplating. The normal faults across the Costa Rica forearc are striking in seismic section due to the well-layered, 2-km-thick upper slope apron. Fault plane reflections and reflector terminations show that the faults extend through the sedimentary apron and apparently into the underlying accretionary prism, indicating a deep-seated deformation process. The zone of extension is from the midslope area to within 10 km of the shelf edge, a minimum width of about 20 km; the estimated extension across the zone is at least 1.5 to 3 km. Within the apron section, spacing between the faults is generally 200-500 m, and nominal fault dip is 20-degrees-40-degrees and predominantly landward. Activity on the normal faults appears to have occurred over a significant period of time based on increased displacement with depth and on fault-controlled sedimentary thickening. At least some of the faults may be presently active; shallow reflectors and possibly the seafloor are displaced by faulting. Contemporary sediment accretion is documented by the same seismic reflection profiles showing offscraping and underplating near the toe of the wedge and out-of-sequence thrusting primarily below the midslope area. The consistent landward normal fault dip may be influenced by structural anisotropy in the prism and possible extensional reactivation of earlier thrust faults associated with accretion processes. With the available data it is not possible to conclusively determine the cause of the stress field leading to the upper prism and apron extension. However, the three most likely causes are underplating, changes in basal shear stress, or a brief episode of subduction erosion.