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ABSTRACT: Accretionary Processes along the Middle America Trench off Costa Rica

T. H. Shipley, K. McIntosh, P. L. Stoffa, E. A. Silver

The geometry of large-scale structures within modern accretionary prisms is known entirely from seismic reflection studies using single or grids of two-dimensional profiles. Off Costa Rica we collected a three-dimensional reflection data set covering a 9 km wide × 22 km long × 6 km thick volume of the accretionary prism just arcward of the Middle America Trench. The three-dimensional processing and ability to examine the prism as a volume has provided the means to map structures from a few hundred meters to kilometers in size with confidence.

Reflections from within the prism define the gross structural features and tectonic processes active along this particular portion of the Middle America Trench. So far in the analysis, these data illustrate the relationships between the basement, the prism shape, and overlying slope sedimentary deposits. For instance, the subducted basement relief (of several hundred meters amplitude) does seem to affect the larger scale through-going faults within the prism. Offscraping of the

uppermost 45 m of sediments occurs within 4 km of the trench creating a small pile of sediments at the base of the trench. How this offscraped sediment is incorporated into the prism is still being investigated. Underplating of parts of the 400 m thick subducted section begins at a very shallow structural level, 4 to 10 km arcward of the trench. Amplitude anomalies associated with some of the larger arcward dipping structures in the prism and surface mud volcanoes suggest that efficient fluid migration paths may extend from the top of the downgoing slab at the shelf edge out into the lower and middle slope region, a distance of 50 to 100 km.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990