The use of potential field and seismological data to analyze the structure of the lithosphere beneath southern Africa

Abstract

The Bouguer gravity anomaly of southern Africa ranges from low values of approximately -200 mGal in the Kaapvaal Craton to over 50 mGal in the Lebombo near the Mozambique border. Three major contributions to the long wavelength Bouguer gravity along a profile extending from Cape Town, South Africa to near Masvingo, Zimbabwe have been investigated: (1) large scale crustal features such as the Bushveld Complex and the Limpopo Belt (2) variations in the depth to the Mohorovičić discontinuity (Moho) and (3) upper mantle seismic velocity perturbations. Crustal thickness determinations from receiver function analysis from 82 sites were used to forward model the gravitational response due to Moho depth variations. These crustal thickness variations display little correlation with surface topography. The variations in the sharpness of phase weighted stacks of the receiver function results suggests regional varying density contrasts at the Moho correlating with geological terrain or crustal thickness. A change in crustal thickness of over 10 km results in a gravity anomaly amplitude at surface of ~120 mGal for a density contrast of 300 kg/m3 at the transition between the Namaqua Natal Mobile belt and the Kaapvaal craton, which is not observed in the measured gravity data. It appears probable that the ~350 km wide Bushveld Complex is connected laterally at depth within the crust. However, this dense layered mafic intrusion has a limited long wavelength gravity signal as it is compensated by significantly thickened underlying crust.