Geology of the Lesser Antilles
The Lesser Antilles and the South Sandwich archipelagoes (Scotia arc) are the only two active island arcs of the Atlantic Ocean. The Lesser Antilles arc (Fig. 1) is on the eastern margin of the Caribbean plate, at the site where the Caribbean plate is underthrust by the subducted Atlantic oceanic crust. The rate of convergence is low compared to other subduction zones; it is generally assumed to be about 2 cm/yr (Jordan, 1975; Minster and Jordan, 1978; Tovish and Schubert, 1978). The convergence vector is thought to be westward oriented, although Sykes and others (1982) and McCann and Sykes (1984) suggest a westsouthwest direction. The tip of the Wadati-Benioff zone reaches about 200 km depth: the average dip is 50Â° to 60Â° (Girardin and Gaulon, 1983), but it is vertical south of Grenada (Fig. lc); beneath the present volcanic line, the depth of the subducted slab is 120 to 180 km (Tomblin, 1975; Dorel, 1978, 1981; Stein and others, 1982; Wadge and Shepherd, 1984). As shown in Figure lc, there is a kink in the subduction zone between Martinique and St. Lucia; this kink may represent the deformation of a single subducting plate or two separate North and South American plates subducting beneath the Caribbean plate (Wadge and Shepherd, 1984).
The Lesser Antilles arc is composed of a score of major islands; it is about 850 km long, and has a radius of curvature of about 450 km. It stretches from the South American continental margin (eastern Venezuela) to the