Reflection profiling in and around the Puerto Rico Trench

Abstract

Continuous seismic reflection profiles were recorded across the Dominican trench (south of Puerto Rico and Hispaniola), the Puerto Rico trench, and the outer ridge. These profiles show details of the structure in the upper layers which could not be measured by the refraction techniques heretofore employed. In the Caribbean there are several layers (unconsolidated and consolidated sediments) which dip northward as though the crust were depressed by the weight of the island ridge. A very small abyssal plain in the Dominican trench suggests either a relatively recent date for the deformation or the absence of a good source of turbidity current sediments with access to the trench. The measurements in the Puerto Rico trench strongly suggest that the north wall is a fault scarp or a series of parallel fault scarps. Along a traverse extending 120 miles north from the shelf of Puerto Rico, the sediments are thin, ranging in thickness from zero to about 200 meters except under the narrow trench abyssal plain. Distortion of the sediments in the bottom of the trench suggests that they have tilted downward toward the south during deposition. The total accumulation of sediment is remarkably small. The measurements on the outer ridge show up to 1 km of low-velocity sediments, except in the region just north of the trench where there is some question of interpretation of the records. In some areas the sediments are markedly distorted, probably indicative of post-depositional tectonic activity. Along a traverse (SH-2B) extending about 200 miles northeast from 21°25′N, 69°50′W, near Silver Bank, the average thickness of sediments approaches 1 km. The southwest half of this traverse crosses an abyssal plain beneath which the sediments show stratification of a type hitherto found by us only near continents. The northwest half shows less stratification and a rough surface, apparently resulting from post-depositional deformation.