Seismic-refraction results and gravity data have been used to deduce the crustal structure from the ocean basin north of the Puerto Rico trench to the Caribbean Sea. It is concluded that the Mohorovičić discontinuity (characterized by compressional-wave velocities of about 8 km/sec) lies at 9 km. below sea level under the ocean basin, 12 km under the Caribbean Sea, at about 16 km under the trench, and at slightly shallower depth under Puerto Rico. The large negative gravity anomaly is attributed to a great thickness of sediments in the trench rather than to a “sialic root” due to a down-buckle of the crust under the trench, as formerly thought.

Turbidity currents are assigned an important role in the accumulation of the sediments. It is suggested that a trench formed in an unspecified way quickly collects sediments, largely by turbidity currents. When granitized and uplifted the sediments form an island arc like the West Indies. Contamination of basaltic lavas by the sediments can account for andesitic lavas, and the accompanying water, rather than being juvenile, is derived from sea water. The trenches at or near the continental margins confine continental debris to the continental margins and collect oceanic debris. The basaltic crust and this debris are first formed into an island arc and later into a continental addition.

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