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The Eocene tectonic evolution of the easternmost Caribbean Plate (CP) boundary, i.e. the Lesser Antilles subduction zone (LASZ), is debated. Recents works shed light on a peculiar period of tectonic duality in the arc/back-arc regions. A compressional-to-transpressional regime occurred in the north, while rifting and seafloor spreading occurred in Grenada basin to the south. The mechanism for this strong spatial variation and its evolution through time has yet to be established. Here, using 3-D subduction mechanical models, we evaluate whether the change in the trench-curvature radius at the northeast corner of the CP could have modulated the duality. We assume asymmetrical CP boundaries at the north (from east to west: oblique subduction to strike-slip) and at the south (subduction-transform edge propagator-like behavior). Regardless of the imposed trench curvature, the southern half of our modeled CP undergoes a NW-to-W-oriented extension due to the tendency of the southernmost part of the South-America oceanic slab to rollback. In contrast, the tectonic regime in the northeast corner of the CP depends on the trench-curvature radius. A low radius promotes transtension-to-transpression, with a NE-oriented compressive component of the principal stress. A high radius largely reduces the compressive component and promotes an extensional regime similar to that in the south. We thus propose that an initially low-curvature radius of the NE-LASZ triggered the tectonic N-S duality in the Eocene and led to an ephemeral period of transpression/compression at the north. However, an additional mechanism might have been required to locally enhance compression.

Based on 2D seismic profiles, multibeam and seabed grab cores acquired during the Garanti cruise in 2017, 1-5 km wide seabed giant polygons were identified in the Grenada basin, covering a total area of ∼55000 km2, which is the largest area of outcropping polygonal faults (PF) ever found on Earth so far. They represent the top part of an active 700-1200 m thick underlying polygonal fault system (PFS) formed due to the volumetric contraction of clay- and smectite-rich sediments, initiated in the sub-surface at the transition between the Early to Middle Pliocene. The short axes of the best-fit ellipses obtained from a graphical centre-to-centre method were interpreted as the local orientation of a preferential contraction perpendicular to the creep deformation of slope sediments. In the North Grenada Basin, the polygons are relatively regular, but their short axes seem to be parallel to a N40°E extension recently evidenced in the forearc, possibly extending in the backarc, but not shown in the study area. They are most probably related to a progressive burial due to a homogeneous subsidence. In the South Grenada Basin, the polygons are more elongated and their axes are progressively rotating southeastward towards the depocenter, indicating a creep deformation towards the center of the basin created by a differential subsidence. Seabed polygons and underlying PF could thus be indicative of the deformation regime of shallow sediments related to main slopes controlled by two different basin architectures.

Strain partitioning related to oblique plate convergence has long been debated in Northern Lesser Antilles. Geophysical data acquired during the ANTITHESIS cruises highlight that the sinistral strike‐slip Bunce Fault develops along the vertical, long, and linear discontinuity between the sedimentary wedge and a more rigid backstop. The narrowness of the 20‐ to 30‐km‐wide accretionary wedge and its continuity over ~850 km is remarkable. The Bunce Fault extends as far south as 18.5°N where it anastomoses within the accretionary prism where the sharp increase in convergence obliquity possibly acts as a mechanical threshold. Surface traces related to subducting seamounts suggest that 80% of the lateral component of the convergent motion is taken up by internal deformation within the accretionary prism and by the Bunce Fault. The absence of crustal‐scale, long‐term tectonic system south of the Anegada Passage casts doubt upon the degree of strain partitioning in the Northern Lesser Antilles. Plain Language Summary Lithospheric plates are frequently bounded by subduction zones where oceanic plates underthrust overriding plates. In most cases, this convergence is oblique to the margin, its resulting tectonic deformation is generally due to margin‐normal and margin‐parallel components of the plate convergence vector. At the Northern Lesser Antilles, the North American Plate subducts beneath the Caribbean Plate with oblique convergence increasing from Guadeloupe to Virgin Islands. This study aims to analyze and resolve the tectonic deformation along this margin. We acquired marine geophysical data during ANTITHESIS cruises (2014–2016) to image the seafloor and the crustal structure. We place a particular emphasis on the strike‐slip Bunce Fault, which extends over ~850 km, including a newly discovered 350‐km segment, 20–30 km landward from the trench. Although long strike‐slip faults have already been observed at oblique subduction zones, the proximity of the Bunce Fault to t