Mantle upwellings and convective instabilities revealed by seismic tomography and helium isotope geochemistry beneath eastern Africa

Abstract : The relationship between intraplate volcanism and continental tectonics has been investigated for North and East Africa using a high resolution three-dimensional anisotropic tomographic model derived from seismic data of a French experiment ''Horn of Africa'' and existing broadband data. The joint inversion for seismic velocity and anisotropy of the upper 400 km of the mantle, and geochemical data reveals a complex interaction between mantle upwellings, and lithosphere. Two kinds of mantle upwellings can be distinguished: The first one, the Afar ''plume'' originates from deeper than 400 km and is characterized by enrichment in primordial 3 He and 3 He/ 4 He ratios higher than those along mid-ocean ridges (MOR). The second one, associated with other Cenozoic volcanic provinces (Darfur, Tibesti, Hoggar, Cameroon), with 3 He/ 4 He ratios similar to, or lower than MOR, is a consequence of shallower upwelling. The presumed asthenospheric convective instabilities are oriented in an east-west direction, resulting from interaction between south-north asthenospheric mantle flow, main plume head and topography on the base of lithosphere.