The Grenada back-arc basin is located between the Aves Ridge, which hosted the remnant Early Paleogene “Great Caribbean Arc”, and the Eocene to Present Lesser Antilles Arc. Several earlier studies have proposed different modes of back-arc opening for this basin, including N-S and E-W directions. The main aim of this study is to constrain the circumstances leading to the early opening of the basin. Three combined wide-angle and reflection seismic profiles were acquired in the Grenada basin. The final velocity models from forward and gravity modeling image variations in thickness and velocity structure of the sedimentary and crustal layers. The sedimentary cover has a variable thickness between one kilometer on top of the ridges to ~10 km in the basin. North of Guadeloupe Island, the crust is ~20 km thick without significant changes between the Aves Ridge, the Eocene and present Lesser Antilles volcanic arc. South of Guadeloupe Island the Grenada basin is underlain by a 6.5-7 km thick oceanic crust of mainly magmatic origin over a width of ~80 km. Here,the western flank of the Lesser Antilles Arc, the crust is 17.5-km thick. The velocity structure of the Lesser  Antilles Arc is typical of volcanic arcs or oceanic plateaus. West of the basin, the crust thickens to 25 km at Aves Ridge in a 80-100 km wide arc-ocean transition zone. The fact that this  transition zone is narrow suggests that opening might have proceeded in a direction oblique to the main convergence. Opening probaly was accompanied by moderate volcanism.