Heat flow in the Bushveld Complex, South Africa: implications for upper mantle structure

Abstract

Abstract Geothermal measurements in South Africa since 1939 have resulted in a good coverage of heat flow observations. The Archaean Kaapvaal Craton, in the central part of South Africa, is the best-studied tectonic domain, with nearly 150 heat flow measurements. The greatest density of heat flow sites is in the Witwatersrand Basin goldfields, where geothermal data are essential for determining refrigeration requirements of deep (up to 4 km) gold mines; the average heat flow is 51 ± 6mWm-2. The Bushveld Complex north of the Witwatersrand Basin is an extensive 2.06 Ga ultramafic-felsic intrusive complex that hosts the world’s largest reserves of platinum. The deepest platinum mines reach ~2 km and the need for thermal information for mine refrigeration engineering has led to the generation of a substantial geothermal database. Nearly 1000 thermal conductivity measurements have been made on rocks constituting the Bushveld Complex, and borehole temperature measurements have been made throughout the Complex. The temperature at maximum rock-breaking depth (~2.5 km) is 70°C, approximately 30°C higher than the temperature at equivalent depth in the Witwatersrand Basin; the thermal gradient in the Bushveld Complex is approximately double that in the Witwatersrand Basin. The main reason for this is the low thermal conductivity of rocks overlying platinum mines. The Bushveld data also resulted in 31 new estimates for the heat flux through the Earth’s crust. The overall average value for the Bushveld, 47 ± 7 mW m-2, is the same, to within statistical error, as the Witwatersrand Basin average. The heat flow for platinum mining areas (45 mW m-2) and the heat flux into the floor of the Witwatersrand Basin (43 mW m-2) are typical of Archaean cratons world-wide. The temperature structure of the Kaapvaal lithosphere calculated from the Witwatersrand geothermal data is essentially the same as that derived from thermobarometric studies of Cretaceous kimberlite xenoliths. Both lines of evidence lead to an estimated heat flux of ~17 mW m-2 for the mantle below the Kaapvaal Craton. The estimated thermal thickness of the Kaapvaal lithosphere (235 km) is similar to that defined on the basis of seismic tomography and magnetotelluric studies. The lithosphere below the Bushveld Complex is not significantly hotter than that below the Witwatersrand Basin. This favours a chemical origin rather than a thermal origin for the upper mantle anomaly below the Bushveld Complex that has been identified by seismic tomography studies and magnetotelluric soundings.