Seismic reflection data indicate that the Atlantic plate has been underthrust beneath the Caribbean plate east of the Lesser Antilles. The data further reveal that the transition from underthrust to strike-slip plate motion occurs near lat 19.3° N. and long 62° W. in alinement with the Anegada Trough. Oceanic basement and reflectors above basement have not been detected beneath the landward wall of the Puerto Rico Trench west of the transition zone. Southeast of the zone, the horizontal distance over which the reflectors and basement can be traced beneath the landward wall (outer part of the Lesser Antilles rise) systematically increases from a few kilometers in the north to as much as 37 km near lat 16.5° N. If a thrust fault emerges at the landward trench wall, the data suggest that a decollement exists between reflectors above basement and an overlying acoustically transparent zone. Gravity and seismic refraction data indicate a thick accumulation of low-density, low-velocity material beneath the Lesser Antilles rise, possibly sediments scraped off the underthrusting Atlantic plate, although massive slumping might cause a similar low-density configuration. The magmatic history of the Lesser Antilles arc suggests two periods of underthrusting, one in the Mesozoic or early Cenozoic and a second in the late Cenozoic. Although the magmatic history of the Lesser Antilles apparently began in the Mesozoic, its Cenozoic history is remarkably similar to the Cenozoic histories of circum-Pacific arcs. Magmatic histories in both areas suggest that island arc evolution in the Cenozoic may have worldwide synchroneity.
