Recognizing the structures of a basin at the crustal scale requires the realization of wide-angle seismic profiles which, by modeling seismic wave velocities and gravity data, allow to image the crust down to the Moho. Padron et al (2021) carried out this work from the three profiles GA01, GA02 and GA03 along which wide-angle seismic data were acquired. On the one hand, in the north (GA03), from the Aves ridge in the back arc to the front arc, we observe a thick crust of about 20 km without much lateral variation of structure, with typical seismic characteristics of a volcanic arc or oceanic plateau crust. On the other hand, towards the south (GA01) and in the center of the GA02 profile, we have highlighted a crust about 17 km thick with a velocity profile typical of an oceanic crust, confirming undoubtedly the oceanic nature of the southeastern Grenada Basin. The transition zone between the thick arc and the oceanic domain constitutes the passive margin of the Grenada Oceanic Basin and provides information on the modalities of oceanization of the basin. By comparing to the structures of typical cases of passive margins from other oceans we conclude that the Grenada back-arc basin opened obliquely to the direction of convergence along the subduction zone and in a moderately volcanic context. This result supports oblique opening models in the debate on how the Grenada back-arc basin was emplaced.

 Velocity model showing the crustal structure of the Lesser Antilles back-arc domain (Padron et al., 2021).

These results are complemented by seismic reflection data that provide a detailed picture of the sedimentary basins units, upper crustal units and faults that affect them. Using stratigraphic correlation across the basin to boreholes and based on our rock sampling that provided information on the lithology of the sedimentary units and ages to the formations, it was possible to reconstruct the tectonic history of the Grenada Basin and its relationship with the Aves ridge. Garrocq et al (2021) have thus accurately determined the extent of the oceanic domain of the Grenada Basin and proposed a first structural map and tectonic interpretation of the margins of this basin along the Aves ridge to the west and north. We show that the Grenada Basin opens in a back-arc context from the late Paleocene and that oceanization ends in the middle Eocene. The asymmetry of the oceanic basin shows that it must extend under the southern Lesser Antilles arc that emplace from the Upper Eocene or Oligocene. The vertical motions recorded by the ridge and the basin show syn-rift subsidence followed by a discrete phase of tectonic inversion in the Upper Oligocene - Lower Miocene, then a resumption of the subsidence amplified southward probably due to the load of sediments that originate from the South American continent.

 Garroq et al 2021, Reconstitution of the emplacement and evolution of the Grenada Basin in the back arc domain of the Lesser Antilles subduction following the results of the GARANTI cruise

In the northern Grenada Basin and Aves Ridge, our activities are concentrated around the Saba Bank and the Anguilla Plateau on either side of the volcanic arc of the northern Lesser Antilles. There, our work is complemented by our onshore work on the islands in the framework of the ANR GAARAnti and also complements our work in the ANTITHESIS program (Marcaillou, Klingelhoefer, Lebrun 2013-2017 https://doi.org/10.18142/242.

The GA15 MCS line, south of Saba Bank in the back-arc, revealed the presence of thrusts sealed by overlying basin sediments. Correlations from oil drilling data, conducted in the Saba Bank, allowed us to date this compressive event to the middle and upper Eocene. Philippon et al (2020) showed the concomitance of these offshore thrusts with a recognized thrust on the island of St Barthélémy belonging to the remnent volcanic arc. The study reveals that this event is responsible shortening perpendicular to the subduction trench and a regional surge at the end of the Eocene that led to the emersion of a whole territory extending from the Greater Antilles to the north of the Lesser Antilles, nicknamed GRANOLA (GReater Antilles and NOrthern Lesser Antilles). In the paper, we hypothesize that this compressive moment is related to the kinematic reorganization of the Caribbean plate following the collision of the Bahamas Bank with the margin and the development of the new transform plate boundary along the Greater Antilles.

Image of thrusting south of Saba Bank in the northern Lesser Antilles and correlation of the upper Eocene unconformity surface from oil drilling SB-2 (Philippon et al., 2020),

More regionally, extending from Anegada Passage and Anguille Bank in the north, to Saba Bank, until Antigua bank to the south, Cornée et al. (in revision) propose from land-sea correlations, paleogeographic maps since 40 Ma indicating the episodic appearance of mega-islands that once had a role in the dispersion of terrestrial faunas between the Greater Antilles and South America. The article is based on a thorough revision of the stratigraphy of the carbonate platforms of the islands of these banks and the chronology of their development (Anguilla, Tintamare, St Martin, St Barthelemy, Barbuda and Antigua). We propose a new regional stratigraphic setting at sea. The lithological control and the chronological setting at sea are brought by the new samples of the GARANTI campaign and the compilation of all the marine samplings carried out in the region. The compressive event dated at the beginning of the Lower Eocene related to the rearrangement of the North Caribbean plate boundary was followed by a generalized distensive tectonic regime alternating periods of subsidence and margin uplift during the acquisition of the curvature of the subduction zone. In this context, the Kalinago Basin opened intra-arc at the end of the Eocene and beginning of the Oligocene.

Interpretation of profile GA09 north of Saba Bank through the Kalinago Basin and paleogeographic reconstruction in the Lower Oligocene corresponding to the depositional period of the MS3 sedimentary unit (in purple on the profile) syntectonic to the formation of the Kalinago Basin. Cornée et al (in press).

Stratigraphic calibrations carried out jointly at sea and on land allow to constrain regionally and precisely the chronology of tectonic-sedimentary events on the entire Lesser Antilles margin. Thus, Boucard et al. 2021 showed that the opening of V-shaped basins, open to the trench, which develop in the Eocene to Oligocene fore-arc and which pre-date a paroxysm of surrection of the northern part of the Lesser Antilles margin in the Lower Miocene, was concomitant with the acquisition of the margin curvature following the collision of the Bahamas Bank and the reorganization of the North Caribbean plate boundary. We show that the margin front subsequently subsided rapidly in response to strong basal tectonic erosion, which helps to explain the westward (landward) retreat of the northern Lesser Antilles volcanic arc in the Late Miocene.

Reconstruction of the northern margin of the Lesser Antilles showing the development of V-shaped basins in response to the increase in trench curvature. Boucard et al 2021 (see also ANTITHESIS campaign)

Finally, an unexpected valorization of the GARANTI mission results was the discovery of a seafloor morphology of the Grenada basin made of polygonal structures. Gay et al (submitted) show that these structures cover the largest known surface in the world. This morphology is acquired by the formation in the underlying sedimentary formations of polygonal faults resulting from the contraction of sediments during their dehydration and compaction. In the Grenada Basin, we show that polygonal faults develop in the lower and middle Pliocene units and then propagate upward and downward, these sediments possessing a mineralogy rich in smectite that is very favorable to their contraction. Moreover, the morphological analysis of the polygons showed an evolution of their shape, towards an elongation oriented N40° in the north then progressively oriented N160° and more and more elongated towards the south of the basin. This analysis would reveal an active extension in the sedimentary cover of the Grenada basin, controlled in the north by regional tectonic fracturing suggesting a NE-SW oriented extension while in the south the reorientation of the elongation of the polygons follows the increased subsidence of the basin towards the south.

Left: bathymetric map showing polygonal structures in the Granada Basin and the edge of the Aves ridge. Right: against a regional map background, diagram showing elongation of polygon shape and reorientation along the Grenada Basin in response to regional tectonic stress in the north and basin subsidence in the south. Gay et al.

(submitted).