Pattern of mantle convection and Pangaea break-up, as revealed by the evolution of the African plate

Abstract

A tensional stress regime has governed the tectonic evolution of most of the African continent. The long history of crustal extension is documented by the widespread occurrence of continued intraplate rifting and volcanism since Triassic times and by the large-scale plate movements following the break-up of Pangaea. A remarkable result of the present investigation is the recognition that the extension displays a continent-wide and plate-wide radial pattern, centred in equatorial Africa near 10°E/0°N, the African Centre A. The radial pattern is evident from the diverging block movements, documented by the intra-continental rifting in Africa, by the break-up of Pangaea and the subsequent, diverging drift of North America and the Gondwana fragments away from Africa, entailing a radial growth of the African plate. All these phenomena are manifestations of what may be referred to as the ‘African lithospheric divergence’. The African plate is surrounded, to 85%, by active oceanic ridges. Ridge-push forces exerted on the African plate have to be balanced by some other forces in order to explain the tensional stress regime which, even at present, dominates large parts of the African continent . Shear traction exerted by the convecting asthenosphere on the base of the African lithosphere offers an explanation for the observed phenomena. It is proposed that relatively warm and less dense mantle material rises from the deep mantle below the African plate. In the upper mantle and at the base of the African lithosphere, the ascending material diverges and flows radially away from the centre of ascent. Upper mantle flow below the sea-floor spreading axes is unidirectional and horizontal and is directed away from the African centre A. It is proposed that the ascending flow beneath Africa forms part of a very large, bicellular circulation system in the Earth9s mantle. The origin of geotectonic cycles is probably interrelated with the onset, build-up, main phase and decay of such large-scale convection systems.