Deep structure of the Demerara Plateau and its two-fold tectonic evolution: from a volcanic margin to a Transform Marginal Plateau, insights from the conjugate Guinea Plateau

Transform marginal Plateaus (TMPs) are large and flat structures commonly found in deep oceanic domains, but origin and relationship to adjacent oceanic lithosphere remain poorly understood. This paper focuses on two conjugate TMPs, the Demerara Plateau off Suriname and French Guiana and the Guinea Plateau, located at the junction of the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic Oceans. The study helps to understand (1) the tectonic history of both Demerara and Guinea Plateaus, (2) the relationship between the Demerara Plateau and the adjacent oceanic domains and finally, (3) to throw light on the formation of Transform Marginal Plateaus (TMPs). We analyze two existing wide-angle seismic derived velocity models from the MARGATS seismic experiment (Demerara Plateau), and adjacent composite industrial seismic lines covering the Demerara and Guinea Plateaus. The Demerara Plateau displays a 30 km thick crust, subdivided into 3 layers, including a high velocity lower crust (HVLC). The velocities and velocity gradients do not fit values of typical continental crust but instead correspond to volcanic margin or Large Igneous Province (LIP) type crusts. We propose that the, possibly continental, lower crust is intruded by magmatic material and that the upper crustal layer is made of extrusive volcanic rocks of the same magmatic origin, forming thick seaward (westward) dipping reflectors (SDRs) sequences. This SDR complex extends to the Guinea Plateau as well and was emplaced during hotspot (Sierra Leone)-related volcanic rifting preceding the Jurassic opening of the Central Atlantic and forming the western margin of the plateau. N-S composite lines linking Demerara and Guinea plateaus reveal the spatial extent of the SDR complex but also a preexisting basement ridge separating the two plateaus. The entire Demerara-Guinea margin would therefore be an inherited Jurassic volcanic margin bordering the Central Atlantic Ocean to the east, with as a possible