Three Dimensional Gravity Modelling of Southern African Crust and Upper Mantle

Susan J. Webb¹ and David E. James^2
1School of Geosciences, University of the Witwatersrand, Wits, South Africa
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC

By combining crustal thickness data with delay time tomography results we have developed a three-dimensional model of the long wavelength gravity field of southern Africa. The gravity field was calculated using sources of three-dimensional spheres as an approximation to spherical prisms. The results are determined on a spherical Earth surface. An examination of the Bouguer gravity and elevation data demonstrates that some recently eroded areas are isostatically overcompensated, while several locations in the Namaqua-Natal Mobile belt are isostatically undercompensated. From the combined modelling of the crustal and mantle components, the region of the Bushveld Complex and Limpopo Belt appear to have higher than average crustal densities. The Kaapvaal Craton is underlain by a thick, seismically fast keel of low-density depleted mantle material that counterbalances the gravity signal produced by regions of thinner crust. This delicate balance appears to explain a significant proportion of the long wavelength gravity signal in southern Africa. This forward modelling method can be expanded to large (plate sized) regions where spherical geometry is required, without encountering the localization effects that are common with spherical harmonic formulations.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery