South Atlantic continental drift and the Benue Trough

Opening of the South Atlantic was probably initiated by separation of Africa and South America along a fracture which opened from the south during the Late Jurassic and Lower Cretaceous, and reached Nigeria by Mid-Cretaceous times. There is evidence that the two continents at first remained joined along the present east-west coastal segment of Africa, however, so that the African continental plate was subjected to slight but increasingly significant distortion. The Benue Trough, now situated on the bend between the east-west and north-south trending coastlines, originated as a northeast-southwest tensional graben system. It is filled with several thousand feet of Middle Albian to Senonian sediments, which were folded in the Maastrichtian mainly about axes parallel to the trough. The trough and its sediment filling are believed to owe their origin to partial relief of the distorting stresses, which accumulated as the southern portions of the continents were wedged apart. When Africa and South America finally separated, the southern half of Africa tended to swing back, so that the hitherto stretched Benue Trough region was transformed into a minor compressional belt, folding the sediments within it. This interpretation requires that final separation of the two continents occurred in the Late Senonian, about 80 million years ago. Other fracture and flexure systems in Africa can be plausibly linked with the foregoing sequence of events, including the Cameroun rift valley, the middle Niger lineament, and the Chad and Congo basins, as well as the elongate domical uplift related to the extensive Cenozoic rifting and alkaline vulcanism of eastern Africa. If recent claims that there has been little drift since the Late Eocene have any validity, separation of Africa and South America 80 million years ago requires a Late Cretaceous-Early Tertiary sea floor spreading rate in the order of 6-8 cm/year. This approaches figures recently deduced for northward drift of India during the same period.