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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.

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