EFFECTIVE ELASTIC THICKNESS OF THE CONTINENTAL LITHOSPHERE IN SOUTH-AFRICA

We have estimated the effective elastic thickness of the continental l ithosphere beneath South Africa using the coherence technique. This in volves (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 o f this elastic layer which gives the best root-mean-square fit between estimated and predicted coherence is the effective elastic thickness (T-e). Two major tectonic provinces, namely, the Archean Kaapvaal Crat on and the Mesoproterozoic Namaqua-Natal Mobile Belt (which together f orm the Kalahari Craton in southern Africa), are found to have T-e val ues of 72 km and 38 to 48 lan, respectively. There is indication from the coherence data that over the Kalahari Craton, topographic features with equivalent wavelengths less than similar to 200-300 lan are supp orted by the rigidity of the lithosphere while features with wavelengt hs 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 fr om both provinces, the effective elastic thicknesses obtained point to a lithospheric basal temperature of about 600 degrees C. Geologic and geophysical considerations suggest that the contrast in flexural rigi dity of the lithosphere between the two provinces can be attributed to the combined effects of compositional and thickness differences of th e lithosphere, as well as variation in present-day asthenospheric heat flow. Our T-e 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 T-e estimate of the Kaapvaal Cr aton fall below average global Archean cratonic values; the reasons fo r this are not clear.