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Pre-upper Senonian basement of Costa Rica crops out in the Santa Elena and Nicoya peninsulas. From south to north and from base to top the basement includes the Esperanza, Matapalo, and Santa Elena units. The Esperanza unit is Albian-Santonian in age and consists mainly of pillow basalt and massive basalt flows. The Matapalo unit includes Callovian-Cenomanian radiolarite and exhibits a massive basalt flow, pillow basalt, and dolerite basement. The Santa Elena unit contains ultramafic and mafic rocks in which harzburgite is the major component. The most important tectonic features of the Nicoya Complex are the large Santa Elena and Matapalo nappes. Nappe emplacement was from north to south during late Santonian time. The sedimentary cover of the Nicoya Complex comprises (1) the Campanian El Viejo Formation, which consists of shallow-water sediments to the north on Santa Elena Peninsula and the Campanian-Maestrichtian Sabana Grande Formation of deep-water origin to the south on Nicoya Peninsula; (2) Paleocene sediments, which indicate deposition in a deep-water environment and which comprise the Rivas, Las Palmas, and Samara formations; and (3) post-upper Eocene(?) sediments, which consist of the shallow-water Barra Honda and Montezuma formations. Two unconformities are the major geologic features of the upper Senonian-Tertiary sequence of Costa Rica. The lower one is at the base of the Sabana Grande Formation and marks a major change in the geologic conditions (basalt is scarce in the Campanian-Tertiary series); the upper unconformity at the base of the Barra Honda and Montezuma formations is not as pronounced as the lower one. During post-Campanian time, normal faulting occurred in two stages separated by a phase of strong erosion. The geology of the landward slope of the adjacent Middle America Trench is outlined in this chapter through interpretation of multifold seismic reflection records taken off the west coast of Costa Rica and from the DSDP Legs 67 and 84 transects off Guatemala. The western Caribbean Plate boundary may have been under extensional stress for the last 75 m.y. The strong landward-dipping reflectors of the landward slope of the Middle America Trench off Guatemala could be equivalent to the on-land pre-Campanian overthrusts of Costa Rica. The available data are consistent with the convergent extensional margin concept.

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A Tectonic Cross-Section of the Costa Rican Pacific Littoral as a Key to the Structure of the Landward Slope of the Middle America Trench off Guatemala