Shallow subduction of the Cocos Ridge beneath the Costa Rican island arc results in six major tectonic effects. These effects include a volcanic gap in the Costa Rican volcanic arc chain, a shallowing of the dip of the subducted Cocos plate beneath Costa Rica, forearc indentation of the Pacific margin of Costa Rica, structural inversion of forearc (Terraba) and backarc (Limon) basins, arching of on- and offshore acoustic basement in a direction parallel to plate convergence between Costa Rica and the Cocos plate, and a radial stress pattern around the underthrust area of the Cocos Ridge as inferred from earthquake and geologic indicators. Structures formed in forearc basin sedimentary and volcanic rocks of the Térraba belt above the subducted Cocos Ridge include major reverse faults that consistently place older lithologic units over younger lithologic units. One of these faults, the Ballena-Celmira fault zone, forms a prominent linear contact between Quaternary alluvium of the Pacific coastal plain and the Térraba belt. Bedding plane and fault data in the Térraba belt constrain a maximum shortening direction of N30-34°E for the central and eastern Térraba belt. This direction of maximum shortening corresponds closely to the N35°E direction of maximum shortening of Corrigan et al. (1990) from Plio-Pleistocene rocks of the outer forearc in the Burica/Osa area to the south and southeast of the Térraba belt. Assuming that the predicted plate convergence direction (N32°E) and the direction of maximum shortening in the forearc subparallel, thrusting and tilting in the forearc of westernmost Panama and eastern and central Costa Rica is interpreted as the result of regional northeast-southwest-oriented maximum compressive stresses exerted by post-Miocene shallow subduction of the Cocos Ridge.

Ignimbrites, as widespread sheets tens of meters thick, form the Central American Tertiary Ignimbrite Province. Geochemical data were collected from 99 Cenozoic marine ash layers within Caribbean Sea sediments, 76 vitrophyres, and 21 mafic lavas from Nicaragua and Honduras. Two major eruptive periods, one in the Eocene and one in the Miocene, have been broadly identified. 40 Ar/ 39 Ar laser fusion ages, determined from sanidine or plagioclase in 10 of the vitrophyre samples, have been interpreted to indicate that the bulk of the younger group of ignimbrites formed largely in the middle Miocene during a 3.5-m.y. period between 16.9 and 13.4 Ma. Modeling indicates that initial melts were from a normal mid-oceanic-ridge basalt (N-MORB)–type source, rather than the enriched mid-oceanic-ridge basalt (E-MORB)–type source postulated for the modern arc. All of the ignimbrites analyzed have 87 Sr/ 86 Sr isotope values (87 Sr/ 86 Sr = 0.7040–0.7069) within the range of continental crust. Trace element trends are similar to those estimated for lower continental crust. Assimilation– fractional crystallization and melt mixing models produce trends that are consistent with ignimbrite compositions. This evidence is consistent with a large influence of continental crust in the ignimbrite formation. In addition, the ignimbrite magmas, like those of the modern arc, have also been determined to have been contaminated by sediment-derived fluids. Abnormally rapid subduction of the Farallon-Cocos plate, which coincides with the formation of the ignimbrites, may have resulted in their generation. A slab gap that currently exists beneath the modern arc may be the cause of a change in source from N-MORB to E-MORB by allowing rising asthenospheric material to “recharge” the mantle wedge in trace elements that had been depleted prior to the gap formation.

The Tacaná Volcanic Complex represents the northernmost active volcano of the Central American Volcanic Arc. The genesis of this volcanic chain is related to the subduction of the Cocos plate beneath the Caribbean plate. The Tacaná Volcanic Complex is influenced by an important tectonic structure as it lies south of the active left-lateral strike-slip Motozintla fault related to the Motagua-Polochic fault zone. The geological evolution of the Tacaná Volcanic Complex and surrounding areas is grouped into six major sequences dating from the Mesozoic to Recent. The oldest basement rocks are Mesozoic schists and gneisses of low-grade metamorphism. These rocks are intruded by Tertiary granites, granodiorites, and tonalites ranging in age from 12 to 39 Ma, apparently separated by a gap of 9 m.y. The first intrusive phase occurred during late Eocene to early Oligocene, and the second during early to middle Miocene. These rocks are overlain by deposits from the Calderas San Rafael (ca. 2 Ma), Chanjale (ca. 1 Ma), and Sibinal (unknown age), grouped under the name Chanjale–San Rafael Sequence, of late Pliocene–Pleistocene age. The activity of these calderas produced thick block-and-ash flows, ignimbrites, lavas, and debris flows. The Tacaná Volcanic Complex began its formation during the late Pleistocene, nested in the preexisting San Rafael Caldera. The Tacaná Volcanic Complex formed through the emplacement of four volcanic centers. The first, Chichuj volcano, was formed by andesitic lava flows and pyroclastic deposits, after which it was destroyed by the collapse of the edifice. The second, Tacaná volcano, formed through the emission of basaltic-andesite lava flows, as well as andesitic and dacitic domes that produced extensive block-and-ash flows ∼38,000, 28,000, and 16,000 yr B.P. The Plan de las Ardillas structure (the third volcanic center) consists of an andesitic dome with two lava flows emplaced on the high slope of the Tacaná ∼30,000 yr B.P. Finally, the San Antonio volcanic center was built through the emission of lava flows, andesitic and dacitic domes, and it was destroyed by a Peléan eruption at 1950 yr B.P. that produced a block-and-ash flow deposit. The Tacaná Volcanic Complex was emplaced along a NE-SW trend beginning with Chichuj, followed by Tacaná, Las Ardillas, and San Antonio. This direction is roughly the same as the NE-SW Tacaná graben (as proposed in this work), together with other faults and fractures exposed in the region. The rocks of the Chanjale-San Rafael Sequence and the Tacaná Volcanic Complex have a calc-alkaline signature with medium K contents, negative anomalies of Nb, Ti, and P, and enrichment in light rare earth elements, typical of subduction zones.

The gaps of volcanic activity and the associated shallow-dipping seismicity in South America can be explained by the consumption of the thick-rooted, buoyant, aseismic Nazca and Juan Fernandez Ridges and perhaps also the Cocos Ridge. The ridges erase the trench where they collide with the overriding continent. The point of collision migrates north or south along the plate boundary, depending on the orientation of the ridge relative to the direction of plate motion. This migration leaves behind a zone in which subduction is temporarily stopped; lack of subduction leads to the cessation of volcanism, perhaps owing to lack of water needed for partial melting. Although the present aseismic ridges probably consist of basaltic cumulates, there is some indication that earlier-consumed parts of these ridges (or different, previously consumed ridges) contained continental fragments that are now embedded in the western coast of South America.