4. THE LESSER ANTILLES ISLAND ARC1

The Lesser Antilles island arc is composed of two Cenozoic volcanic arcs, the older (early Eocene to mid-Oligocene) and the Recent (early Miocene to Present) arcs, superimposed on a Meszoic proto-arc belonging to a larger geodynamic feature, the Mesozoic Caribbean arc (M.C.A.), initiated in the Early Cretaceous. The Lesser Antilles arc experienced two major volcanic interruptions of 8-to-10 m.y. duration each. The first interruption accompanied the Paleocene opening of the Grenada Basin, with oceanic crust forming in the southern part of the basin. The splitting of the central part of the M.C.A. (eastern Caribbean) isolated a remnant arc, the Aves Swell, from an active arc, the Lesser Antilles. The second volcanic interruption occurred in late Oligocene, as a consequence of a first-order tectonic event: the colli sion of a buoyant Atlantic aseismic ridge. Thereafter, the subduction geometry of the renewed volcanic line (Recent arc) was modified in the northern half by a westward jump from the outer arc to the inner arc. In the Neogene, the subduc tion of non-buoyant ridges had second-order, but nevertheless significant, effects on the tectonics and volcanism of the arc. The development pattern of the construction of the arc ridge by volcanic processes is described. Emphasis is placed on the role of subducted sediments in the generation of orogenic magmas and, conversely, on the role of subducted ridges in the local interruption of arc volcanism. The morphology of the underthrust oceanic crust controls the mag matic activity of the island arc, and particularly the development, in space and time, of "arc compartments.&quot