Based on an extensive seismic and multibeam dataset, 1-5 km wide giant polygons were identified at the bottom of the Grenada basin, covering a total area of ~55000 km². They represent the top part of an active underlying polygonal fault system due to the volumetric contraction of clay- and smectite-rich sediments during burial. To date, this is the widest area of outcropping polygonal faults ever found on Earth. The seabed polygons are bounded by rectilinear ~1000-1500 m wide and ~10-60 m deep furrows, depending on the location in the basin. They are relatively regular in the north Grenada Basin, whereas they are getting longer and more elongated in the south Grenada Basin. The polygonal faults consist in a set of discrete normal faults affecting a 700 to 1200 m thick interval, initiated in the shallow sub-surface at the transition between Early to Middle Pliocene and then having propagated both upward and downward during sedimentation. The centre-to-centre method has been applied to determine the local ellipse of strains, providing a major orientation for extension needed for polygons to initiate. In the north, the minor axes are oriented N40°, indicating a general NE-SW extension of the upper part of the sedimentary cover consistent with the forearc/backarc regional extension. In the south Grenada Basin, minor axes are progressively turning towards the south, pointing out the actual maximum subsidence point. This implies that seabed polygonal faults could thus be indicative of the present-day (or recent) strain state within the upper sedimentary column.

Oblique collision of buoyant provinces against subduction zones frequently results in individualizing and rotating regional-scale blocks. In contrast, the collision of the Bahamas Bank against the Northeastern Caribbean Plate increased the margin convexity triggering forearc fragmentation into small-scale blocks. This deformation results in a prominent >450-km-long sequence of V-shaped basins that widens trenchward separated by elevated spurs, in the Northern Lesser Antilles (NLA, i.e. Guadeloupe to Virgin Island). In absence of deep structure imaging, various competing models were proposed to account for this faults-bounded Basins-and-Spurs System. High-resolution bathymetric and deep multichannel seismic data acquired during cruises ANTITHESIS1-3, reveal a drastically different tectonic evolution of the NLA forearc. During Eocene-Oligocene time, the NLA margin accommodated the Bahamas Bank collision and the consecutive margin convex bending by trench-parallel extension along N40-90°-trending normal faults, opening V-shaped valleys in the forearc. Backarc spreading in the Kalinago Basin and block rotations went along with this tectonic phase, which ends up with tectonic uplifts and an earliest-middle Miocene regional emersion phase. Post middle Miocene, regional subsidence and tectonic extension in the forearc is partly accommodated along the newly-imaged N300°-trending, 200-km-long Tintamarre Normal Faults Zone. This drastic subsidence phase reveals vigorous margin basal erosion, which likely generated the synchronous westward migration of the volcanic arc. Thus, unlike widely-accepted previous theoretical models, the first deep seismic images in the NLA forearc show that the NE-SW faulting and the prominent V-Shaped valleys result from a past and sealed tectonic phase related to the margin bending and consecutive blocks rotation.