The Demerara marginal Plateau, part of the French Guiana-Suriname passive margin, extends offshore the continental shelf down to ca. 3000 m water depth. This marginal plateau results from a polyphased rifting history with the successive formation of the western divergent margin during Jurassic times, followed during the Early Cretaceous by the eastern divergent and northern transform margins. Recent results from the DRADEM (dredging) and MARGATS (multitrace and wide angle seismics) cruises (2016) revealed the nature of the basement. These cruises highlighted that the Jurassic margin corresponds to a magmatic divergent margin and that the basement of the Demerara Plateau is made of more than 20 km pile-up of magmatic rocks. Sampling indicated that magmatism was related to an active hotspot at ca. 173 Ma. Kinematic reconstructions suggest that it was the same hotspot that was at the center of the Central Atlantic Magmatic Province (CAMP) at 200 Ma.

As an oceanic plateau, the crust of the Demerara marginal plateau build up by the accumulation of magmatic material, but not in the middle part of an ocean, as it rests at the continent-ocean transition of a divergent margin. This plateau also represents a very specific example as the same hotspot, at the origin of the CAMP, would have controlled both rifting stages at the end of Lias and during Early Cretaceous time.

The Demerara marginal plateau also presents specific seafloor surface processes. At the junction between the Equatorial and Central Atlantic Oceans, it forms a prominent bathymetric salient that is thought to guide and accelerate bottom currents as the North Atlantic Deep Water (NADW), and thus favor the development of a contourite depositional system. Bottom currents shape the seafloor in forming longitudinal sedimentary ridges and giant flute casts. The location of these structures seems to be controlled by the bathymetric expression of underlying mass transport deposits that develop down the main slope failure headscarp (ca. 350 km in length), and possibly by active fluid flows. Existing sedimentary records allow to reconstruct NADW intensity variations for the last 100 000 years, allowing to document oceanic circulation dynamics in this tropical domain at glacial-interglacial transitions.

The aim of the DIADEM (DIves At DEMerara) cruise is to test the working hypothesis linked to the formation of the Demerara Plateau and the formation of superficial structures by in-situ observations and sampling at the seafloor (Nautile and cores) or close to the seafloor (AUV). The main objectives are:

• to map and date magmatic rocks outcropping on the plateau slope, to replace these rocks within sedimentary series in order to confirm the evolution model related to the hotspot, to determine its influence on the vertical movements and on the successive stages of deformation.
• to combine high resolution mapping, hydrodynamic measurements and sampling to characterize the irregular seafloor morphologies that affect recent sedimentation, to better understand their repartition and the sedimentary processes that are related to strong bottom currents.
• to collect sedimentary and paleoceanographic records from before the last 100 000 years by acquiring new long cores located within the contouritic drift.

Data collected during this cruise will also help us to investigate fluid flows linked to a probable mud volcano, as well as presumed fluid flows associated with mass transport deposits and the formation of canyons near the slope.