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.

We present on- and offshore structural data from the Nazca-Panama plate boundary zone in the Gulf of Chiriquí and surrounding onshore areas of southwest Panama. Major offshore structures interpreted on multichannel seismic profiles from the Gulf of Chiriquí include Cébaco basin complex, a series of northeast-striking, Plio-Pleistocene half-grabens, and Montuosa basin, an asymmetric Plio-Pleistocene sag basin associated with a major strike-slip fault. We interpret Cébaco basin complex as a pull-apart basin between two major, left-lateral strike-slip faults that accommodate oblique motion between the Nazca plate and the mainland of southwestern Panama. Interpretation of regional seismic stratigraphic data indicates that the Plio-Pleistocene extensional phase that produced the Cébaco basin complex extended the area by about 7%. We studied outcrop-scale, conjugate strike-slip fault systems exposed on landmasses surrounding the Gulf of Chiriquí in order to place kinematic and age constraints on large-scale faults mapped on seismic profiles. Fault systems deforming Eocene to Lower Miocene sedimentary rocks on Coiba Island and the Azuero and Soná Peninsulas suggest an approximately northwest-southeast orientation of maximum extensional strain in an area that encompasses the offshore Cébaco basin complex. We propose three possible models to explain the observed pattern of strike-slip deformation observed in the Gulf of Chiriquí: (1) Neogene oblique subduction of the Nazca plate beneath Panama produces left-lateral strike-slip faulting and related northwest-oriented extension within the forearc (Gulf of Chiriquí) (2) Plio-Pleistocene shallow subduction/collision between the Cocos ridge and Costa Rica produces southwestward motion or “escape” of a Gulf of Chiriquí block that is detached from the rest of Panama by left-lateral strike-slip faults, and (3) Neogene bending of the Panama island arc following collision with the South American continent is accommodated in part by strike-slip motion and underthrusting along the southwest margin of Panama. Observed deformation may be a composite effect of more than one of these tectonic mechanisms.

We correlate seismic map units identified on industry seismic lines in the Gulf of Chiriquí, southwestern Panama, with onland igneous rocks and sedimentary formations described in this chapter. We propose six principal stages in the stratigraphic development of southwestern Panama based on our results and the results of previous workers in Costa Rica, westernmost Panama, and the western Colombian basin. The first stage in southwestern Panama is represented by basaltic basement rocks of Jurassic?-Late Cretaceous age interbedded with Upper Cretaceous pelagic sedimentary rocks. Following previous workers and data presented here, we suggest that these rocks formed in an intraoceanic, oceanic plateau setting. A second stage is represented by a major stratigraphic hiatus inferred to represent an erosional event that affected the basaltic basement of Panama in Paleocene time. A third stage is represented by a widespread basal transgressive section of coarse clastic rocks and reefal carbonate rocks of early to middle Eocene age. This section records initiation of clastic sedimentation over much of southern Central America. A fourth stage is represented by a thick section of mainly marine turbidites that appears to represent continued erosion of the land areas in southern Central America and upward deepening of adjacent marine environments. A fifth stage is represented by a stratigraphic hiatus in middle Miocene to late Miocene time that may represent the “breakup” unconformity associated with initiation of strike-slip faulting and rifting in the Gulf of Chiriquí. A sixth stage is represented by early Pliocene to Pleistocene rifting and syn-rift sedimentation in the Gulf of Chiriqu&iacute. Thick sedimentary fill of rift basins may reflect accelerated uplift of southern Central America and increased activity of the Middle America arc. The regional extent of the stratigraphic record of several of these stages across large areas of southern Central America and the western Colombian basin supports the previously proposed hypothesis that the crust of southern Central America represents the western upturned edge of a Late Cretaceous Caribbean oceanic plateau known from deep-sea drilling and seismic stratigraphic studies in the Colombian and Venezuelan basins of the Caribbean Sea.

Shallow subduction of the Cocos Ridge beneath the Costa Rican island arc results in six major tectonic effects. These effects include a volcanic gap in the Costa Rican volcanic arc chain, a shallowing of the dip of the subducted Cocos plate beneath Costa Rica, forearc indentation of the Pacific margin of Costa Rica, structural inversion of forearc (Terraba) and backarc (Limon) basins, arching of on- and offshore acoustic basement in a direction parallel to plate convergence between Costa Rica and the Cocos plate, and a radial stress pattern around the underthrust area of the Cocos Ridge as inferred from earthquake and geologic indicators. Structures formed in forearc basin sedimentary and volcanic rocks of the Térraba belt above the subducted Cocos Ridge include major reverse faults that consistently place older lithologic units over younger lithologic units. One of these faults, the Ballena-Celmira fault zone, forms a prominent linear contact between Quaternary alluvium of the Pacific coastal plain and the Térraba belt. Bedding plane and fault data in the Térraba belt constrain a maximum shortening direction of N30-34°E for the central and eastern Térraba belt. This direction of maximum shortening corresponds closely to the N35°E direction of maximum shortening of Corrigan et al. (1990) from Plio-Pleistocene rocks of the outer forearc in the Burica/Osa area to the south and southeast of the Térraba belt. Assuming that the predicted plate convergence direction (N32°E) and the direction of maximum shortening in the forearc subparallel, thrusting and tilting in the forearc of westernmost Panama and eastern and central Costa Rica is interpreted as the result of regional northeast-southwest-oriented maximum compressive stresses exerted by post-Miocene shallow subduction of the Cocos Ridge.