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.