The Guinea Plateau is a shallow-marine, flat-lying bathymetric province situated along the equatorial West African margin, offshore Republic of Guinea. The Guinea Plateau and the conjugate Demerara Plateau hold particular geologic significance, as they represent the final point of separation between the African and South American continents during Gondwana break-up. Recent interpretation of both 2-D and 3-D seismic surveys along the Guinean margin have illuminated subsurface features related to Early Cretaceous crustal extension. Seismic structural investigations on these datasets suggest that the majority of extension is accommodated along large-scale listric normal faults located on a relatively narrow (<50 km) continental slope (up to ~39% extension). Minimal faulting reveals that little upper-crustal extension has occurred on the Guinea Plateau. Additionally, multiple 2-D seismic profiles image the transition from continental crust on the plateau and slope, to oceanic crust in the deeper marine basin. This continent-ocean boundary is the most representative boundary when testing the accuracy of plate reconstructions. Mapping of both the continent-ocean boundary and fracture zones across the equatorial Atlantic suggests that the Demerara Plateau and the South American plate are too far south in previous pre-rift reconstructions. A revised model introduces 20 km of Early Cretaceous NNW-oriented contraction across the Amazon Basin; an area of relative weakness where both geologic and geophysical data support such accommodation. Sea-surface flow models, which used this revised reconstruction and interpreted paleo-bathymetric data, predict upwelling throughout the newly formed equatorial seaway, and later along the West African margin during periods of regional organic-rich black shale deposition. With reduced decomposition of organic matter strongly correlated to upwelling, being able to predict these zones is of particular significance to petroleum companies, who have recently started exploring both the equatorial South American and West African coastlines.

La marge NE Américaine est une des marges les mieux étudiées au monde, elle a fait l’objet de plusieurs études géophysiques. En comparaison, la marge africaine reste peu étudiée car uniquement deux campagnes océanographiques y ont été menées : la campagne Sismar (2001) au large de la Meseta et la campagne Dakhla (2002) au large du Sahara. La structure profonde de la marge canadienne est connue grâce aux profils de sismique grand-angle SMART-1, 2 et 3. Le premier objectif du projet MIRROR était d’acquérir des profils combinant sismique grand-angle et sismique réflexion sur un segment homologue au profil SMART-1. La comparaison entre les segments homologues de ces deux marges ayant pour but de mieux comprendre le mode d’ouverture de l'océan Atlantique Central. Une comparaison entre les modèles Sismar, Dakhla et Mirror montre que la croûte continentale est plus épaisse au nord et s'amincit vers le sud. La largeur de la zone de transition est plus étroite au sud et les profils Sismar sont localisés sur un bassin sédimentaire posé sur une croûte continentale très amincie. La comparaison avec la marge homologue montre que l'épaisseur, la structure de la croûte continentale et la zone d'amincissement sont très semblables. Par contre, il existe une zone de manteau exhumé et serpentinisé sur le profil Canadien qui n'a pas d’homologue sur la marge africaine. De plus, l'épaisseur de la croûte océanique est différente avec 8 km sur la côte africaine et seulement 3-4 km sur la marge canadienne. Plusieurs hypothèses ont été proposées pour expliquer cette différence (a) une différence d’âge entre les deux croûtes (b) un épaississement lié au passage du point chaud des Canaries (c) une accrétion asymétrique. / The NE American margin represents one of the best studied margins in the world, it was the subject of several scientific programs. In comparison, the conjugate NW African margin remains fairly unknown, only two deep seismic cruises were acquired: the SISMAR cruise (2001) offshore the Meseta and the DAKHLA cruise (2002) offshore the Sahara. The deep structure of the Canadian margin is known due to the SMART wide-angle seismic profiles 1, 2 and 3. The first objective of the MIRROR project was to acquire combined wide-angle and deep reflection seismic data offshore a segment conjugate to the SMART-1 profile. The comparison between the homologous segments of these two margins aimed to better understand the opening mechanism of the Central Atlantic Ocean. A comparison between Sismar, Dakhla and Mirror models shows that the continental crust is thicker in the north and thins toward the south. The width of the transition zone is narrower south and Sismar profiles are located on a sedimentary basin placed on a very thinned continental crust. Comparing the Mirror profile with that of the Canadian conjugate margin (Smart 1) shows that the thickness, the structure of the continental crust and the thinning is very similar. However, zones of exhumed and serpentinized mantle were imaged along the Canadian profile that have no conjugate on the African margin. Moreover, the thickness of the oceanic crust is variable with 8 km on the African side and only 3-4 km on the Canadian margin. Several hypotheses have been proposed to explain this difference (a) an age difference between the two types of crust (b) thickening associated with the passage of the Canary hotspot (c) an asymmetric accretion or (d) an accretion at slow to ultra-slow speading centers.