Early Tertiary volcanic clasts were collected from the streambed of the Rio Morti near the village of Morti, eastern Panama, as part of a reconnaissance study. The samples range from basalts to rhyolites. K-Ar dates cluster around 58 Ma. The phenocryst mineralogy of the samples is typical of that found in arc-related volcanics: plagioclase (the dominant phase), clinopyroxene, titanomagnetite, and minor orthopyroxene. The clinopyroxenes are augites that plot in the field of orogenic lavas. The geochemistry of the rocks—high ratios of large-ion lithophile elements to high-field strength elements, negative Nb and Ta anomalies, positive Ba anomalies, and relatively low Th to U values—confirms that they are arc related (specifically the calc-alkaline series) and strongly suggests that the samples are not cogenetic. Volcanic rocks with similar ages are exposed in several other localities throughout Panama and Costa Rica (e.g., the Azuero and Sona Peninsulas in Panama and the Nicoya Peninsula in Costa Rica as well as other areas in the Darien of eastern Panama). We suggest that there may have been a more or less continuous arc from South America to the Chortis block of Nicaragua during the Paleocene-Early Eocene. The presence of the arc would imply that the breakup of the Farallon plate and its subduction below the new Caribbean plate in present-day southern Central America probably started some time close to the Cretaceous-Tertiary boundary.

Shallow subduction of the Cocos Ridge below southeastern Costa Rica (a process that was initiated ca. 5 Ma) has elevated the central segment of the Chorotega arc (the Cordillera de Talamanca) to heights over 3,500 m. Rapid uplift and associated faulting exposed the remains of volcanic arcs predating the Quaternary volcanics that dominate in the northwestern and eastern segments of the Chorotega arc. Because of extensive cover by cloud forests and generally steep topography, collection of fresh samples remains problematic. Access has become easier, however, in the last few years because of development of a network of logging roads that enabled collection along several transects. The geochemical and isotope data indicate that at least three, possibly four, phases of igneous activity occurred in the Cordillera de Talamanca. Evidence for the earliest Paleocene-early Eocene arc is provided indirectly by the volcanic contents of both the forearc (Fila Costena) and back-arc (Limon Sur) basins. Steep subduction appears to have been responsible for formation of this presently severely segmented volcanic arc. Intermediate-angle subduction gave rise to volcanics arcs in the mid-Oligocene and mid-Miocene. Igneous rocks emplaced during these periods are similar, but their contents of the element Zr differ. On chondrite-normalized spider diagram, the Oligocene intrusives display a negative, the Miocene intrusive rocks (Talamanca suite sensu stricto) a positive Zr anomaly. Low-angle subduction characterized the Plio-Pleistocene. Associated volcanism is quite distinct from that in the Miocene and consists predominantly of intermediate to felsic magmas with geochemical signatures suggesting partial melting of a MORB source. The four volcanic phases are separated by periods of relative quiescence that correlate with periods during which the plate tectonic configuration in the east-central Pacific changed.