New seismic and magnetic information from the Antilles Outer Ridge and Trench region has been combined with previous geophysical observations to provide an explanation for the origin of the crust there, in the framework of plate tectonics. The magnetic-anomaly pattern shows the east-west trending ridge to be on the boundary between two magnetic provinces, one north of the ridge trending N20°E and one south, trending N60°W. This boundary suggests that the ridge was formed at the junction of two ancient oceanic plates which accreted as Africa separated from North America and South America, respectively. Seismic profiles show the outer ridge to be underlain by a N80°E-trending mantle ridge or "welt" which coincides with a basement ridge and a positive free-air gravity anomaly. These features result from minor compressive motions acting normal to the trench region, which presently is dominated by strike-slip motion associated with the overthrusting of the present Atlantic plate by the Caribbean plate. The age and distribution of the acoustically transparent sediments relative to layered valleys of "turbidite" material on the ridge indicate that gradual uplift and compression have been active in the region since Eocene time.