PATTERN OF MANTLE CONVECTION AND PANGAEA BREAK-UP, AS REVEALED BY THEEVOLUTION OF THE AFRICAN PLATE

A tensional streSS regime has governed the tectonic evolution Of Most of the African continent. The long history of crustal extension is doc umented by the widespread occurrence of continued intraplate rifting a nd volcanism since Triassic times and by the large-scale plate movemen ts following the break-up of Pangaea. A remarkable result of the prese nt investigation is the recognition that the extension displays a cont inent-wide and plate-wide radial pattern, centred in equatorial Africa near 10-degrees-E/0-degrees-N, the African Centre A. The radial patte rn is evident from the diverging block movements, documented by the in tra-continental rifting in Africa, by the break-up of Pangaea and the subsequent, diverging drift of North America and the Gondwana fragment s away from Africa, entailing a radial growth of the African plate. Al l these phenomena are manifestations of what may be referred to as the 'African lithospheric divergence'. The African plate is surrounded, t o 85%, by active oceanic ridges. Ridge-push forces exerted on the Afri can plate have to be balanced by some other forces in order to explain the tensional stress regime which, even at present, dominates large p arts of the African continent . Shear traction exerted by the convecti ng asthenosphere on the base of the African lithosphere offers an expl anation for the observed phenomena. It is proposed that relatively war m 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 lith osphere, 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 Af rican centre A. It is proposed that the ascending flow beneath Africa forms part of a very large, bicellular circulation system in the Earth 's mantle. The origin of geotectonic cycles is probably interrelated w ith the onset, build-up, main phase and decay of such large-scale conv ection systems.