The Guinea-Demerara volcanic conjugate margins: comparisons of their rifted crustal structure and overlying passive margin stratigraphy

Abstract:

The Guinea Plateau extends over 300 km offshore from the present-day coastline of Senegal, Guinea Bissau, Guinea, and Sierra Leone and forms the Equatorial Atlantic conjugate margin to the Demerara Rise in northeastern South America. Due to a lack of deep-penetrating seismic reflection data, the underlying crustal structure of the Guinea Plateau and its continent-ocean transition (COT) remain poorly understood. We use 2D forward gravity modeling, seismic reflection data, and open-source gravity and magnetic field data to characterize the underlying crustal structure of the Guinea Plateau, map the COT, and identify and map a package of seaward dipping reflectors (SDRs) on the southwestern Guinea Plateau. Results from gravity modeling show that the Guinea Plateau is underlain by rifted continental crust, which thins from west to east and transitions to Jurassic-aged oceanic crust near its present-day, western shelf edge. The COT is more abrupt on the southern flank of the plateau, where the Guinea Fracture Zone juxtaposes continental crust with Cretaceous-aged oceanic crust. Late syn-rift SDRs fill rifts on the southwestern Guinea Plateau, covering an area of ~22,000 km². The location of SDRs is consistent with the paleogeographic location of the region during the Central Atlantic Magmatic Province (CAMP) event at 201 Ma. This magmatic event occurred shortly before the break-up of Pangea and is thought to be responsible for the emplacement of SDRs on the Demerara Rise and east coast of North America as well as widespread, radial dikes found across northeast South America, West Africa, and eastern North America. The CAMP-aged Freetown Layered Igneous Complex, which is exposed on land near Freetown, Sierra Leone, covers an area of ~20,000 km² offshore along the eastern Guinea Plateau. Comparisons of the crustal structure of the Guinea Plateau with the Demerara Rise show that rifted continental crust underlies the entire Guinea Plateau, while a significant portion of the Demerara Rise is underlain by igneous crust. The volume and extent of syn-rift volcanics is higher on the Demerara Rise than the Guinea Plateau, indicating that the mantle plume responsible for their emplacement was located under the South American plate during the late syn-rift phase of Central Atlantic opening.