The dominant features of the geology in the Klerksdorp-Carletonville area in South Africa are a thick sequence of Precambrian stratified rocks of the Witwatersrand basin and the underlying Archean basement granite. The total crustal thickness is ~36 km, and the average measured heat flow is 46 mW m-2. At Vredefort, 50 km to the south of the Klerksdorp-Carletonville area, Archean granite forms the core of an updomed and overturned sequence of strata. Recent geochemical, geophysical, and geological studies provide evidence that the Vredefort basement granite has also been overturned, exposing a ~15 km thick section of the Archean crystalline crust. Profiles across the Vredefort basement, together with boreholes through the overlying stratified rocks, provide a unique opportunity of measuring the contribution of crustal radioactive heat production to surface heat flow. Adopting two extreme models for heat production in the lowermost crust, the total contribution to surface heat flow from radioactive heat generation in the crust is calculated at between 29 and 34 mW m-2. Heat flow from the mantle in the southwestern Transvaal sector of the Kaapvaal craton is estimated at between 17 and 12 mW m-2. This low uppermost mantle heat flux is about half of that estimated for an oceanic lithosphere at equilibrium. Accordingly, the low flux provides evidence for a lithosphere thickness about double that of an old oceanic region. The unusually high estimate of crustal heat production and the corresponding low mantle heat flow are due mainly to the occurrence of a heat production hump beneath an upper zone of exponential decline. Published statements that the reduced heat flow is an acceptable approximation to heat flow at the base of the stable continental crust are not supported by the present work.