Deep structure of the Grenada Basin from wide-angle seismic, bathymetric and gravity data

The crescent-shaped Grenada back-arc basin is located between the Aves Ridge, which hosted the remnant Early Paleogene arc, and the Eocene to Present Lesser Antilles arc. During the 2017 GARANTI cruise (ANR GAARAnti program),three combined wide-angle and coincident reflection seismic profiles, together with gravity data, were acquired in the Grenada Basin. We focus here on one of them spanning the complete basin in its southern part. Forty ocean-bottom seismometers were deployed along the transect. Forward as well as tomographic approaches were applied to model the wide-angle seismic data. The velocity models show an asymmetric basin with a basement's top deepening from 7 km in the west to over 10 km in the east, with sediment thickness increasing from 4 to more than 7 km accordingly, beneath a flat seafloor. The velocity gradients and layer thicknesses indicate that the eastern part of the basin is underlain by a 6.5-7 km thick crust of oceanic origin over about 80 km.

The eastward deepening of the basement raises the question of its cause: sediment vs arc loading effect? On the eastern flank of the Aves ridge, the sedimentary layers thin to only 3-4 km. The underlying 25 km-thick crust is divided into two layers with a heavily faulted top of the basement. The seismic velocities along the Aves Ridge are compatible with an arc origin of the ridge. Crustal thinning takes place in a 100-125 km wide transition zone between the eastern flank of the Aves Ridge and the oceanic part of the Grenada basin.

The western flank of the Lesser Antilles Arc is characterized by a crustal thickness of at least 17.5 km, about 5 km less than the crustal thickness north of Martinique island. However, we cannot exclude a thicker arc further east. The crust of the eastern margin of the Grenada basin thins from 17.5 to 6.5 km over a distance of 45 km, while it varies from 25 to 7 km over 125 km across the eastern flank of Aves Ridge. The seismic velocities allow to distinguish an upper crust (5.75-6.35 km/s) and a lower crust (6.35-6.85 km/s). The upper crust represents one third of the crustal thickness on both Aves Ridge and Grenada Basin, while it represents two third of it in the active arc. This difference in the velocity gradient between Aves Ridge and the active arc supports a different origin.