Abstract

The geology and stratigraphic relationships of the Cayman Ridge, Nicaraguan Plateau, and mid-Cayman spreading center, which have been determined from the succession of rocks recovered at 80 dredge stations in the Cayman Trench, are consistent with the inferred crustal structure of the region deduced from published geophysical data. The Cayman Ridge and Nicaraguan Plateau are composed of metamorphic, plutonic, volcanic, sedimentary, and carbonate rock units that typically crop out along continental margins and in island arcs. In contrast, the trench floor is composed of mafic and ultramafic rocks identical to those recovered from the major ocean basins.

Our petrographic, radiometric, and paleontologic data are correlated with the regional geology of Central America and the Greater Antilles and suggest that the Cayman Ridge and Nicaraguan Plateau developed as a single, broad island arc during the Laramide Orogeny. By late Eocene time, volcanism had greatly diminished, uplift and erosion had exposed the underlying igneous rock, thick clastic sequences had been deposited in newly formed grabens, and a zone of east-west axial accretion had been created between the rifted ridge and plateau. An average spreading rate of .4 cm/yr across the mid-Cayman spreading center since the Eocene accounts for approximately 200 km of left-lateral displacement between the Cayman Ridge and Nicaraguan Plateau. The geologic histories of the ridge and plateau differ slightly after the Eocene, but general subsidence, at an average rate of 6 cm/1,000 yr, caused progressive restriction of carbonate banks and reefs to a few isolated islands and algal pinnacles.

The pre-Cretaceous history of the Cayman Trench has been extrapolated from the Paleozoic-Mesozoic geology and structure of Central America and Cuba, while the Cretaceous to Holocene evolution of the trench has been related to the relative motions between the North American, South American, and Caribbean plates. Plate convergence during the Cretaceous caused southerly subduction of oceanic lithosphere beneath the ancestral Cayman Ridge–Nicaraguan Plateau, which subsequently led to the formation of a chain of volcanic islands to the south, along the North American–Caribbean plate boundary. Numerous ophiolite-like outcrops that fringe the north Caribbean margin are believed to reflect a Late Cretaceous closure of this subduction zone. Early Tertiary left-lateral shear and tensional stresses along this plate boundary split the Cayman Ridge from the Nicaraguan Plateau, and a small spreading center was created in the rift. Declining isotherms beneath the ridge and plateau caused general subsidence and local tectonic re-equilibration in late Tertiary time.

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