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.

The structure and the tectonic development of the southern Panama plate boundary have been derived from an interpretation of marine geophysical data, including GLORIA (Geological LOng Range Inclined Asdic) long-range side-scan sonar and seismic reflection profiles, most of which were acquired in 1989 from a cruise of the RRS Charles Darwin. The northern boundary of the Nazca plate runs within the continental margin of southern Panama. South of the Gulf of Panama this boundary, the Southern Panama fault zone, is predominantly left-lateral strike-slip and occupies an elongate sedimentary basin. South and southwest of the Azuero Peninsula the boundary becomes one of oblique subduction, with active formation of an accretionary complex. The eastern part of this accretionary complex slips around the bend in the overriding Panama block to the purely strike-slip portion of the plate boundary, where it ceases to accrete sediment. South of the Gulf of Panama, the fault zone is flanked on its southern side by a bathymetrie ridge, containing rocks of a high density, that was once a part of the Panamanian continental margin. The ridge has been displaced 140 km eastward by the motion between the Panama block and the Nazca plate. The eastern end of this ridge is being subducted beneath South America, and at the ridge crest, the deformation front of the Colombian accretionary complex meets the Southern Panama fault zone. The inactive trench, filled with sediment, that lies at the foot of the Southern Panama continental margin owes its existence, in the west, to the downward flexure of the Nazca plate beneath the overriding Panama block at the oblique subduction boundary, and in the east, where the lithosphere of the Nazca plate is “broken” along its transform boundary, to the flexural load of the displaced basement ridge. A continuation of the Southern Panama fault zone runs southeastward behind the Colombian accretionary complex, separating it from forearc basin sediments deposited on the “continental” basement of northernmost Colombia. Deformed mud diapirs indicate a component of left-lateral strike-slip motion on the fault zone. This pattern of tectonics around this northernmost corner of the Nazca plate has probably been active since the collision of Panama and South America about 3.5 Ma.

The East Panama deformed belt is defined here as a seismically active, on- and offshore belt of deformed Late Cretaceous-Neogene rocks that forms a diffuse tectonic boundary between the eastern Isthmus of Panama and South America. Satellite imagery and geologic maps of eastern Panama compiled from previous studies indicate that northwest-striking, left-lateral strike-slip faults form the major Neogene tectonic structures in the onshore part of the deformed belt. The best studied onland strike-slip fault is characterized by left-stepping, en echelon folds that deform rocks ranging in age from Cretaceous to late Miocene and terminates at a high angle of intersection on north-striking reverse faults in the Pirre Hills on the Panama-Colombia border. North- to north-east-striking normal faults appear to terminate left-lateral strike-slip motion in the area of the Panama Canal at the northwestern edge of the deformed belt. Interpretation of marine multichannel seismic profiles and two well logs from the Gulf of Panama suggests that east-dipping reverse faults of middle Miocene through Plio-Pleistocene age form a largely buried, 90-km-wide thrust belt beneath the broad shelf area of the eastern half of the Gulf of Panama. We cannot prove or disprove a left-slip component on offshore reverse faults mapped on widely spaced seismic profiles. Exploration wells document the presence of an eastward-thickening Plio-Pleistocene foredeep basin overlying a middle Eocene section in the east-central Gulf of Panama. Post-middle Eocene sedimentary units were deposited in marine environments, range in age from middle Miocene to Plio-Pleistocene, and are locally deformed by east-dipping thrust faults. The previously mapped geology of the Pearl Islands at the eastern edge of the Panama deformed belt suggests that the islands are the exposed axis of a hanging-wall anticline associated with an east-dipping reverse fault zone. We correlate Plio-Pleistocene strike-slip faulting of the onshore Panama deformed belt and reverse faulting in the offshore Panama deformed belt and Pearl Islands with increasingly restricted surface water exchange and eventual Pliocene closure of the Caribbean-Pacific seaway in Panama, determined from micropaleontological studies by previous workers of Pacific and Caribbean sedimentary sections.