The geology, geochemistry and emplacement of the Cretaceous—Tertiary ophiolitic Nicoya Complex of the Osa Peninsula, southern Costa Rica

Abstract

The Nicoya Complex of the Osa Peninsula is essentially an obducted segment of oceanic crust comprising basaltic lavas and associated intrusive dolerite and gabbro, interstratified with lesser amounts of pelagic limestones, cherts and argillites. The sediments contain a minor clastic component and were deposited on an ocean floor of considerable relief and distant from a major landmass. The extrusive and intrusive basaltic rocks have geochemical affinities to large ion lithophile (LIL) element-enriched oceanic crust, and are interpreted to have formed in a back-arc basin analagous to the Mariana Trough, Lau Basin or Gulf of California. One sample has distinctly different geochemical characteristics and may represent a younger within-plate seamount. In the Late Cretaceous, an E-W-trending intra-oceanic trench/volcanic/back-arc system developed in association with an active southward-dipping subduction zone located south of the present-day southern Central American isthmus. Pelagic sediments and basaltic lavas accumulated in the back-arc over a period of at least 34 Ma spanning the Late Cretaceous and Early Tertiary. During this period there were three major volcanic events dated respectively as Santonian-Campanian (78.0 ± 2 Ma), Palaeocene (60.2 ± 7.6 Ma) and Middle Eocene (44.0 ± 4.4 Ma). Continuing northward movement of the southern plate caused overthrusting of the volcanic arc onto the northern plate and production of a thickened embryonic continental crust. Inferred reorganization of crustal stress in the Late Eocene caused fragmentation of the single ancestral plate into the Caribbean and “East Pacific” plates, with a flipping of the subduction zone accompanying development of the NE-dipping Middle America subduction zone and andesitic volcanism. During the Oligocene, the ancestral East Pacific plate split into the NE-moving Cocos plate and the eastward-moving Nazca plate, separated by the E-W-trending Colón spreading ridge and a series of N-S-trending transforms. The Cocos plate was subsequently split into two blocks separated by the Costa Rica Fracture Zone that extends northeastwards from the western end of the Colón Ridge to Costa Rica, which it divides into two distinctive volcanotectonic domains. To the north of the Costa Rica Fracture Zone, the Cocos plate is moving northeastwards and being consumed by the Middle America subduction zone, whereas the southern block is under the influence of the Colón spreading ridge with a N-S-oriented main stress axis. The N-S-trending Panama Fracture Zone can be extrapolated northwards via three submarine canyons on the continental slope to merge into a braided system of curved NW-trending coast-parallel wrench faults on which predominantly dextral strike-slip movement has produced pull-apart and tipped wedge basins with adjacent uplifted mini-horsts dating back to the Middle Pliocene. Therefore, although the Nicoya Complex of the Osa Peninsula was originally emplaced by accretion and thrusting related to pre-Oligocene plate movement, it owes its present-day exposure to post-Late Miocene wrench fault tectonics, with superimposed isostatic uplift.