Two seismic reflection profiles of the southeastern end of the Puerto Rico trench show that the strata forming the ocean floor, including the oceanic basement, dip westward beneath the eastern flank of the Antilles arc. Farther south a profile across the eastern margin of the northern end of the Barbados ridge shows a similar westward dip of oceanic strata beneath the ridge and suggests that thrust faults dipping 20°–30° west cut the overlying sediments that form the east flank of the ridge. Similar thrust‐faults appear in a profile southeast of Barbados. The evidence from the reflection profiles is compatible with seismological and magnetic evidence that suggests that the ocean floor is spreading west from the mid‐Atlantic ridge and is being thrust westward beneath the Antilles. The Barbados ridge is composed of sedimentary material that formerly formed a continental rise overlying the oceanic crust along the northern margin of South America. Thrusting of oceanic crust beneath the Lesser Antilles arc and displacement of South America westward relative to the Antilles during the Cenozoic resulted in the crumpling, thickening, and accumulation of the material of the continental rise in front (east) of the Antilles arc to form the Barbados ridge.

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Continuous seismic profiling in the western Somali Basin reveals basement topography aligned approximately north to south. We interpret three buried ridges, approximately 50 km apart, approximately parallel to the Davie Ridge in the Mozambique Channel, as fracture zones along which Madagascar moved away from Africa. These data therefore support the northern paleopositions of Madagascar suggested by du Toit [1937] and Smith and Hallam [1970]. The ridges seem to provide an eastern barrier to sedimentation. The basement topography forming these fracture zones has a different origin from that of Chain Ridge.

The western Somali Basin in the northwestern Indian Ocean is covered by thick deposits of terrigenous sediments. Seismic reflection profiles show, however, the northern and southern parts to be very different. The northern sections is a deep basin filled with thick uniformly stratified sediments. It is enclosed by the continental margin to the west and north, Chain ridge to the east, and shallow basement structure to the south. A change in depth of basement occurs along an approximately east‐west line at latitude 3°30′N very near the southern end of Chain ridge. In the southern portion of the basin the basement is shallow, and, immediately south of latitude 3°30′N, it has high relief. Stratified flat‐lying sediments fill the basement depressions, and isolated hills formed of basement material rise above the abyssal plain deposits. Farther to the south the abyssal plain becomes very narrow. Gabbro dredged from the southeast slope of Chain ridge has been dated by the potassium‐argon method as 89.6±4.5 m.y., which should be considered a minimum age. The evidence suggests that the entire sediment sequence of the northern basin was deposited subsequent to the formation of the ridge. The thin sediment cover of the southern portion of the basin is probably no older than Tertiary.