The Pacific margin of Costa Rica is a very deformed basement terrane of ophiolite composed of pillowed and massive basalt, mafic and ultramafic plutonic rocks, volcanic breccia, hyaloclastite, radiolarian chert, and limestone. The ophiolite underwent changes through burial metamorphism after subduction of the Cocos plate under the southwestern margin of the Caribbean plate. In some areas the ophiolite is a mélange. While the age of the emplacement of the ophiolite in northwestern Costa Rica is late Santonian to early Campanian, the period of accumulation of the ophiolite seems to be very long, possibly extending from middle Tithonian to late Santonian. New age determinations based on foraminifera and radiolaria support the previous dating. A relative scarcity of turbidite in the ophiolite of Costa Rica, compared to other similar terranes of the Pacific margin, has been associated with the intra-oceanic origin of the southern Central American arc.

The island-arc suite clastic rocks first unconformably covered the ophiolite in Costa Rica in early Campanian time. Their origin is closely related to the intrusive and volcanic activity of a plutonic-volcanic arc located between South and North America since early Campanian time. The clastic rocks of the island-arc suite are low-porosity volcanogenic types. Limestone has accumulated from Cretaceous time to the present, in some areas forming porous biohermal bodies. Vertical tectonics originated marginal- and intra-arc sedimentary basins intermittently throughout the evolution of the arc. A fourfold geotectonic division of Costa Rica is proposed.

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