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.

Abstract The margins of the Caribbean plate are characterized by varying amounts of strike-slip faulting and compressional folding, thrusting, warping, and extensional faulting. The Cayman Trough is a predominantly strike-slip transform boundary except for a short segment of a spreading ridge (Macdonald and Holcombe, 1978; Holcombe and Sharman, 1983). The Barbados Ridge- Lesser Antilles Arc system and the Middle America Trench-Central America Arc system are predominantly compressional, convergent boundaries. The other boundaries have experienced Neogene strike-slip faulting, compression, and extension across a broad plate boundary zone. Because we are dealing with several rigid plates in relative motion with respect to each other, we believe that the Neogene pattern has undergone slow secondorder changes with time (Dewey, 1975). The Neogene and Quaternary pattern has been quite different from Paleogene and Cretaceous patterns of plate boundary organization and deformation (Ladd, 1976; Pindell and Dewey, 1982; Pindell and Barrett, this volume). In this chapter we will review the northern, southern, and eastern boundaries of the Caribbean; the western boundary with the Cocos plate is reviewed only briefly here and more fully in the eastern Pacific volume (von Huene, 1989). Place names referred to in this chapter can be found in Plate 1.