African lithospheric structure, volcanism, and topography

In Africa volumetrically minor, mid-plate volcanic rocks of Cenozoic age are concentrated in areas affected by Pan-African (500 _+ 150 Ma) crustal reactivation, and are virtually absent from cratonic areas. We interpret this as indicating that Pan-African areas are underlain by fertile lithospheric mantle and cratons by depleted lithospheric mantle, and propose a model to explain both the distribution of volcanism and the two kinds of African mantle lithosphere: (1) Cratons were formed by the assembly of collided island arcs, and for this reason are underlain by depleted, sub-oceanic-type mantle lithosphere. (2) Comparable depleted mantle lithosphere has been delaminated from beneath areas thickened during Tibetan-style continental collision (especially in Pan-African times) and replaced by fertile material which now forms the lithospheric mantle below the Pan-African reactivated crust. (3) When the African plate came to rest with respect to sub-lithospheric mantle circulation patterns at about 30 Ma, heating from below extracted magmas from the fertile but not from the depleted mantle lithosphere. (4) As a result of interaction with an underlying convective pattern for the last 30 Ma, Pan-African reactivated areas display both Neogene elevations and volcanism but cratonic areas display only elevations.