The oldest rocks in Costa Rica, the Nicoya Complex, crop out in an arcuate belt along the Pacific coast and outside the mapped area. The complex is an ophiolite assemblage and indicates that a Late Cretaceous arc-trench system, probably a southerly extension of the Middle America Trench, lay along and west of the present Pacific coast. Polarity of the system is indicated by the presence of a volcanic olivine basalt arc east of the ophiolite belt and by a younger magmatic arc farther east. Relative lack of metamorphism of the ophiolite assemblage suggests that the rocks were obducted rather than subducted. Fossil evidence indicates that the complex was emplaced no later than the late Campanian. By the late Eocene both the ophiolite assemblage and the olivine basalt arc had been established as major sediment sources for basins that lay to the east, and they continued as such through the Oligocene and possibly into the middle Miocene. Widespread deposition of trachyandesite lava and volcaniclastic sediments, local marine planation, uplift, and renewed volcanism mark what is presumed to be the end of the Miocene. Extreme south-to-north coarsening of the unconformably superposed Doan Formation and the presence of corals in the shaly facies of the Doan in the southern part of the mapped areas suggest that marine sedimentation there was undisturbed by orogeny and that intrusion of the Talamancan magmatic rocks must have followed deposition of the Doan. Volcaniclastic block breccia, characteristic of northern exposures of the Doan, indicates that volcanism in the proto- Cordillera Central had begun by at least Doan time. Volcanism has been a dominant process in central Costa Rica since that time.
