An isostatic study of the Karoo basin and underlying lithosphere in 3-D

A 3-D density model of the crust and upper mantle beneath the Karoo basin is presented here. The model is constrained using potential field, borehole and seismic data. Uplift of the basin by the end of the Cretaceous has resulted in an unusually high plateau (> 1000 m) covering a large portion of South Africa. Isostatic studies show the topography is largely compensated by changes in Moho depths (similar to 35 km on-craton and > 45 km off-craton) and changes in lithospheric mantle densities between the Kaapvaal Craton and surrounding regions (similar to 50 kg m(-3) increase from on- to off-craton). This density contrast is determined by inverted satellite gravity and gravity gradient data. The highest topography along the edge of the plateau (> 1200 m) and a strong Bouguer gravity low over Lesotho, however, can only be explained by a buoyant asthenosphere with a density decrease of around 40 kg m(-3).