Abstract of the interior of the Caribbean Plate as well as that of the Yucatán Basin. Proceeding from northwest to southeast, the basins and ridges of the Caribbean, exclusive of active plate margins, are the Yucatán Basin and the Cayman Ridge, part of the North American Plate; and the Nicaraguan Rise, Colombian Basin, Beata Ridge, Venezuelan Basin, Aves Ridge, and Grenada Basin, which make up part of the Caribbean Plate. Geologic history of the subject areas is limited to Mesozoic and Cenozoic time, with the possible exception of the Upper Nicaraguan Rise, which may be partly underlain by a core of pre-Mesozoic rocks. History of crustal formation, probably occurring in the Cretaceous or the Jurassic for most of the Caribbean sea floor, has not been well established because drillholes encountered a basaltic sill/flow sequence, which may postdate initial crustal formation, and because the Caribbean interior is isolated structurally by plate boundaries, relict and active. Identification of magnetic anomaly sequences has been speculative. Time of formation and structural development of the Yucatán and Grenada Basins are as yet also speculative; the basins possibly formed in early Cenozoic time. Evolution of the Caribbean interior is largely that of accumulation of sediments through the Late Cretaceous and Cenozoic, and structural response to stresses applied to existing crust and lithosphere. Some lithosphere was probably consumed along relict subduction zones (upper Nicaraguan Rise, Cayman Ridge, Beata Ridge?, and Aves Ridge) during Late Cretaceous and early Cenozoic time. Apparently, no major plate boundary has extended through the

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.

Seismic sections across the southern margin of the Caribbean reveal structures related to the convergence of the Caribbean and South American plates. The South Caribbean Deformed Belt and its eastward extension, the Curacao Ridge, is a zone of intensely deformed Cretaceous and Tertiary sediments that lies along the southern edge of the Colombia and Venezuela Basins. Undeformed sediments of the Caribbean basins abut the deformed belt abruptly to the north. To the south, the South Caribbean Deformed Belt gives way to older deformed belts of the Netherlands and Venezuelan Antilles Ridge and to the continental margin of Colombia and Venezuela containing pre-Tertiary structures. Along most of the South Caribbean Deformed Belt an apron of sediments progrades northward across the deformed belt suggesting active deformation at the northern edge of the belt and progressively older Tertiary deformation to the south. Caribbean oceanic crust extends southward beneath the deformed belt and southward-dipping reflections occur within the deformed belt possibly indicating slices of oceanic crust incorporated within it. Bottom simulating reflectors along parts of the deformed belt indicate the presence of gas hydrates. The chemical phase relationships of gas hydrates and the depth of the bottom simulating reflections indicate a thermal gradient of approximately 0.04 degrees/meter.

Abstract Multichannel seismic reflection records from the northern and southern margins of the Venezuela Basin reveal compressional structures analogous to structures in active Pacific trench-arc systems and indicate that these Caribbean margins have probably evolved during a Tertiary period of north-south convergence and consequent compression across the Venezuela Basin. Seismic records show evidence for underthrusting beneath and sediment accretion against the margins of the basin. A trench-slope break anticline has formed on the landward slope of both the Muertos Trench and the Venezuela Trench. This anticline has ponded thick sediment accumulations behind it in sediment basins homologous to forearc basins in other active trench-arc complexes. Progressively steeper landward dips deeper in the section on midslope terraces and in the forearc basin of the Muertos Trench suggest syndepositional tectonic rotations of the landward slopes. Caribbean convergence rates calculated from published plate tectonic models are consistent with convergence rates calculated from considerations of sediment volume within the landward slopes of Venezuela Basin trenches. These sediment calculations suggest that over half of the accreted sediment beneath the landward slopes may be terrigenous material slumped down the landward slope or trapped as turbidite fill in the trench axis and later incorporated into the inner slope.

Abstract Multifold seismic reflection investigations have provided data pertinent to the problem of origin and mode of deformation of salt in the Gulf of Mexico. The Challenger seismic unit which contains Jurassic salt covered the Jurassic abyssal basin and onlapped the Campeche and West Florida continental margins; it is thought to have on lapped the Texas-Louisiana, RioGrande,and East Mexican margins as well. The unit has an estimated average thickness of 1.5 km and a maximum thickness of at least2.5km.On lap and pinch out of the Challenge run it and isostatic considerations suggest that the unit was deposit edona sea floor several thousand meters below sea level.The data are inconclusive with respect to the question of whether the salt was deposited indeep water or whether the surfaceof the Gulf was greatly lowered. The principal dissimilarity of the Challenger from overlying units derives from mobilization of salt within the Challenger.The causes of localization of salt mobilization are not clear,but salt mobility is developed best in are as where the lower,seismically transparent part of the Challenger is thickest. The volume of salt with in the Challenger equals an estimated 20 to 50 times the volume of salt inpresent Gulf waters.An accumulation of this magnitude required continuous replacement from the world ocean and could not have resulted from a single episode of drying-up of the Gulf of Mexico. Four modes of salt mobilization and emplacement can be recognized:(1)geographically random diapirism continuously active from Jurassic to present in the Texas-Louisiana Shelf and upperslope, Campeche Knolls,and Sigsbee Knolls;(2)formation of sinuous,sub parallel ridges beneath the Mississippi cone,probably due to differential sediment loading of the prograding delta;(3)Pleistocene over thrusting of a salt“tongue”in the central Sigsbee Scarp;and(4)late Miocene-early Pliocene mobilization of Jurassic stratiformsalt in the Campeche Knolls province. The Mexican Ridges,which some investigators have suggested are cored by salt, appear to be cored with shale.