Joint seismic–geodynamic-mineral physical modelling of African geodynamics: A reconciliation of deep-mantle convection with surface geophysical constraints

Abstract

Recent progress in seismic tomography provides the first complete 3-D images of the combined thermal and chemical anomalies that characterise the unique deep-mantle structure below the African continent. We present a tomography-based model of mantle convection that provides an excellent match to fundamental surface geodynamic constraints on internal density heterogeneity that includes both compositional and thermal contributions, where the latter are constrained by mineral physics. The application of this thermochemical convection model to the problem of African mantle dynamics yields a reconciliation of both surface gravity and topography anomalies to deep-seated mantle flow under the African plate, over a wider range of wavelengths than has been possible to date. On the basis of these results, we predict flow in the African asthenosphere characterised by a clear pattern of focussed upwellings below the major centres of late Cenozoic volcanism, including the Kenya domes, Hoggar massif, Cameroon volcanicline, Cape Verde and Canary Islands. The flow predictions also reveal a deep-seated, large-scale, active hot upwelling below the western margin of Africa under the Cape Verde Islands that extends down to the core–mantle boundary. The scale and dynamical intensity of this ‘West African Superplume’ is comparable to the ‘South African Superplume’ that has long been assumed to dominate the large-scale flow dynamics in the deep-mantle under Africa. We evaluate the plausibility of the predicted asthenospheric flow patterns through a comparison with seismic azimuthal anisotropy derived from independent analyses of African shear wave splitting data.