Abstract:
The Pacific face of Costa Rica and western Panama has been extensively studied because of the wide occurrence
of oceanic assemblages. In Northern Costa Rica, the Santa Elena Nappe made by ultramafic and mafic
associations overthrusts the Santa Rosa Accretionary Complex. The Nicoya Complex corresponds to a preCampanian
oceanic plateau association, cropping out in the Nicoya Peninsula and the outer Herradura Block.
The 89 Ma high MgO Tortugal Komatiitic Suite corresponds to 14-km long, 1.5-km wide body, with no clear
relation with to the Nicoya Complex. The Tulín Formation (Maastrichtian to Lower Eocene) forms the main
edifice of an accreted ancient oceanic island of the Herradura Block. The Quepos Block was formed by the
accretion of a late Cretaceous-Paleocene seamount. In the Osa and Burica peninsulas, Caño Island and Golfito
area, a series of Upper Cretaceous to Eocene accreted plateau and seamount blocks crop out. In western Panama,
the oceanic assemblages range from Upper Cretaceous to Miocene, and their geochemical signature show
their oceanic plateau association. The Costa Rica and western Panama oceanic assemblages correspond to a
fragmentary and disrupted Jurassic to Miocene sequences with a very complicated geological and geotectonic
history. Their presence could be interpreted as a result of accretionary processes rather than tectonic erosion;
despite this last process is nowadays active in the Middle American Trench. The whole picture has not been
completed yet, but apparently, most of the igneous rocks have a geochemical signature similar to the Galapagos
mantle plume. The later has been acting in pulses, or otherwise the outcropping occurrences could be part of
several plateaus somehow diachronically formed in the Pacific basin.
