Drowned 14-m.y.-old Galápagos archipelago off the coast of Costa Rica: Implications for tectonic and evolutionary models

Abstract

Volcanic rocks were dredged from the Cocos and Fisher ridges and seamounts along a 250 km profile parallel to the Pacific coast of Costa Rica. The composition and laser 40Ar/39Ar ages of the Cocos Ridge and Seamounts are consistent with their formation above the Galapagos hotspot 13.0–14.5 Ma. The reconstructed paleoenvironment and chemistry of the Fisher Ridge are consistent with it having originated at a mid-oceanic ridge system. Laser 40Ar/39Ar dating of fresh basalt glass from the Fisher Ridge yielded isochron ages of 19.2 ± 0.3 Ma and 30.0 ± 0.5 Ma. The Fisher Ridge is along a lithospheric fault that may represent an extensional fracture formed when the oceanic floor rode over the Galapagos hotspot. Even though the younger structures are currently at water depths of >1000 m, volcanological, geochemical, and geophysical observations indicate that they once formed an emerged archipelago very similar in morphology to the Galapagos islands. The diversity of the biota on the isolated Galapagos islands, as first described by Charles Darwin, has had an important influence on the development of the theory of evolution. The existence of a now-drowned Galapagos archipelago 14.5 Ma considerably increases speciation times for the Galapagos biota and provides a complete solution to a long-standing controversy concerning the divergence of the Galapagos marine and land iguanas from a single ancestral species.