WHAT CAUSES A HIGH TOPOGRAPHY OF THE KALAHARI CRATON?

We present a comparison of regional topography in different Archean and Proterozoic craton worldwide and demonstrate that only two cratons, the Kalahari in southern Africa and the Tanzanian craton, have the topography 500-700 m higher than in any other craton. The causes for an unusually high topography may be either the dynamic support of the mantle (below the lithosphere base), or by a low density (high depletion) of the cratonic lithospheric mantle, or a combination of both.

We calculate the relative contributions of the both factors to surface topography in the cratons of southern Africa and present the model of density structure of the lithospheric mantle in southern Africa. The results indicate that 0.5-1.0 km of topography cannot be explained by the lithosphere density structure within the petrologically permitted range of mantle densities. This unexplained topography requires the dynamic contribution from the sublithospheric mantle.

The amplitude of regional free air gravity anomalies is ca. +20 mGal on average, suggesting that the region is nearly isostatically compensated. We therefore propose that dynamic topography may be associated with the low-density region below the depth of isostatic compensation, which we assume to be at the lithosphere base. A likely candidate is the low velocity layer between the LAB and the top of the mantle transition zone at 410 km. The reduction in Vs velocity reported earlier in seismic studies of the Kaapvaal craton in a ca. 100-150 km thick layer may explain a ca. 0.5-1.0 km to the regional topographic uplift in case a temperature anomaly is ca. 100-200 deg.

We discuss the density structure of the lithospheric mantle in different tectonic blocks of the Kalahari craton and show that it has an overall agreement with xenolith-based data for lithospheric terranes of different ages. Density anomalies are associated with the Eastern and Western Cape Fold Belt, the Bushveld Intrusion complex, and the Limpopo belt. We link these and other mantle density anomalies to regional tectonic evolution and to regional variations in the crustal structure.