The island of Puerto Rico, in the northeast Caribbean, lies within a broad deformation zone between the Caribbean and North American plates. The simplest model for the tectonic setting of Puerto Rico has major strike-slip movement on nearly east-west lines in the vicinity of the Puerto Rico Trench coupled to a small counterclockwise rotation of a Puerto Rico block within the broader plate boundary zone. This simple model is attractive because it predicts the tectonic regime south of Puerto Rico, and provides an explanation for a possible component of extension across the Puerto Rico Trench west of 65.5°W. GLORIA long-range sidescan sonar data and seismic reflection profiles have been used to test this model by mapping the major tectonic features across the plate boundary north and south of Puerto Rico. To the north, the new data help to resolve between conflicting models, of underthrusting or strike-slip motion at the Puerto Rico Trench. No direct evidence of compression is seen, although evidence for normal and strike-slip movement is abundant. This, combined with regional considerations, leads us to conclude that the main east-west-trending part of the Puerto Rico Trench between 65.5°W and 68°W lies within a strike-slip regime, although oblique convergence occurs both to the east and west where the plate boundary trends east-southeast. To the south of Puerto Rico, underthrusting of the Caribbean plate beneath the island decreases from west to east, and it is ultimately replaced by extension in the Virgin Islands Basin east of 65°W.
