Radiolarian chert, serpentinized peridotite, pillow lava, and metamorphosed tholeiitic rocks form an ophiolite association in southwestern Puerto Rico. These are the oldest rocks in Puerto Rico and apparently represent exposed oceanic crust. Relations between the lithologies are complex; there is an older complex of serpentinized peridotite and metatholeiite 110 m.y. old or older, an intermediate pillow lava and felsite association, and a younger radiolarian chert formation of middle Cretaceous (Albian) or older age. A gravitational nappe structure, showing northward movement probably of Albian or earlier age, is composed of a disharmonically deformed sheet of radiolarian chert. Serpentinite occurs in small lenses along the sole of the nappe and probably served as a lubricant for movement of the nappe.

A second nappe is suggested, on less extensive data, to have moved Campanian-Maestrichtian pelagic limestones northward over the older chert nappe. This movement occurred in Maestrichtian or later time and is perhaps synchronous with smaller northward slides of the same formation during the Maestrichtian about 20 km to the north. Other northward slides occurred in early Tertiary time in several places in southern Puerto Rico.

Northward gravity transport of the nappes and other slides is opposite to the sense of tectonic transport indicated by northward-dipping axial planes in the volcanic and volcano-sedimentary rocks in central Puerto Rico. Similar relations exist in Cuba where thrust and gravity slides show northward motion toward and into the volcanic pile. Gravity transport may have been from an oceanic ridge northward toward a trench, with volcanism developing between the two structures. Early buckling of lithosphere plates into an oceanic ridge and complementary trench, before rupture and subduction, may provide the necessary relief for gravitational trench-ward slides.

A speculative geologic history of the Greater Antilles contains three stages: (1) pre-Albian buckling of the lithosphere into ridge and trench, with gravitational sliding of crustal rocks toward the trench; (2) rupture of the lithosphere and Albian-Campanian (or Maestrichtian) subduction, forming volcanism between ridge and trench; (3) choking of the Benioff zone by continental lithosphere of the northern plate, and further Maestrichtian to Eocene movement resolved along lateral faults with fault-controlled volcanism along an ancestral Cayman trough–Puerto Rico trench trend.

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