Contributions to the Gravity Field of Southern Africa from Crustal Thickness and Seismic Velocity Variations

As a result of the Kaapvaal Project we now have unprecedented coverage of the crustal thickness and details of the seismic velocity variations in the tectosphere beneath southern Africa. By using these data and reasonable density relationships we can calculate the contribution of each of these components to the overall gravity field of southern Africa. These gravity modeling results confirm the suggestion that the high seismic velocity keel beneath southern Africa has a lower density than surrounding mantle material and is consequently highly depleted; a result in agreement with the xenolith data obtained from kimberlites. We estimate that the 1% seismic velocity variation observed between on and off craton leads to a 1% variation in mantle density due to compositional variation. This low density keel is vital to counterbalance the effect of the thinner than average crust that is observed beneath the Kaapvaal and Zimbabwe cratons. Thus in regions where the Archaean craton boundaries may not be apparent from geological mapping, seismic studies and gravity modelling could assist with delineating potentially cratonic regions with diamond potential.