Effective elastic thickness of the continental lithosphere in South Africa

Abstract

We have estimated the effective elastic thickness of the continental lithosphere beneath South Africa using the coherence technique. This involves (1) estimating the coherence between Bouguer gravity anomalies and topography in the spectral domain and (2) comparing the coherence with that predicted by an elastic plate model that flexes under loads placed on, within and beneath the lithosphere. The depth to the base of this elastic layer which gives the best root‐mean‐square fit between estimated and predicted coherence is the effective elastic thickness (Te). Two major tectonic provinces, namely, the Archean Kaapvaal Craton and the Mesoproterozoic Namaqua‐Natal Mobile Belt (which together form the Kalahari Craton in southern Africa), are found to have Te values of 72 km and 38 to 48 km, respectively. There is indication from the coherence data that over the Kalahari Craton, topographic features with equivalent wavelengths less than ∼200–300 km are supported by the rigidity of the lithosphere while features with wavelengths greater than 700 km are compensated. It is implied from the present findings that each of the two South African tectonic provinces can be considered as separate coherent domains. On the basis of geotherms from both provinces, the effective elastic thicknesses obtained point to a lithospheric basal temperature of about 600°C. Geologic and geophysical considerations suggest that the contrast in flexural rigidity of the lithosphere between the two provinces can be attributed to the combined effects of compositional and thickness differences of the lithosphere, as well as variation in present‐day asthenospheric heat flow. Our Te estimates of the Namaqua‐Natal Mobile Belt are similar to those of the Mesoproterozoic Grenville belt, corroborating tectonic models which suggest that these two belts may once have been part of the same mobile belt. In contrast, the Te estimate of the Kaapvaal Craton fall below average global Archean cratonic values; the reasons for this are not clear.