Abstract The seismic section across the Pacific margin of Central America off Costa Rica provides a structural cross section of the southern terminus of the Middle America trench. The section begins less than 10 km from the shoreline of the Nicoya Peninsula and extends for 70 km across the trench slope and trench, and into the ocean basin. Regionally, the trench shallows toward the southeast because of a large sea-floor rise, the Cocos Ridge, at the southern boundary of the Middle America trench. The Cocos and Caribbean plates are converging at about 9 cm/yr nearly normal to the margin (McNally and Minster, 1981), with earthquake activity defining a subduction zone dipping to the northeast (Burbach et al., 1984). Old oceanic crust is exposed on the Nicoya Peninsula in the form of Late Jurassic and Cretaceous volcanic and sedimentary rocks. The structural relationships within these rocks have been reported by a number of different investigators, but their exact origin remains unresolved (Dengo, 1962; Schmidt-Effing, 1979; Lundberg, 1982). The presently subducting ocean floor is of early Miocene age (Hey, 1977).