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ABSTRACT: Seismic Reflection Images of the Accretionary Wedge off Costa Rica

Thomas H. Shipley, Kirk McIntosh, Paul L. Stoffa, Eli A. Silver

The large-scale structure of modern accretionary wedges is known almost entirely from seismic reflection investigations using single or grids of two-dimensional profiles. We will report on the first three-dimensional seismic reflection data volume collected of a wedge. This data set covers a 9-km-wide × 22-km-long × 6-km-thick volume of the accretionary wedge just arcward of the Middle America Trench off Costa Rica. The three-dimensional processing has improved the imaging ability of the multichannel data, and the data volume allows mapping of structures from a few hundred meters to kilometers in size.

These data illustrate the relationships between the basement, the wedge shape, and overlying slope sedimentary deposits. Reflections from within the wedge define the gross structural features and tectonic processes active along this particular convergent margin. So far, the analysis shows that the subdued basement relief (horst and graben strutures seldom have relief of more than a few hundred meters off Costa Rica) does affect the larger scale through going structural features within the wedge. The distribution of mud volcanoes and amplitude anomalies associated with the large-scale wedge structures suggests 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 at a distance of 50-100 km. Offscr ping of the uppermost (about 45 m) sediment occurs within 4 km of the trench, creating a small pile of sediments near the trench lower slope. Underplating of parts of the 400-m-thick subducted sedimentary section begins at a very shallow structural level, 4-10 km arcward of the trench. Volumetrically, the most important accretionary process is underplating.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990