Terrestrial Heat Flow and Thermal Structure of the Lithosphere in Southern Africa (Botswana, Namibia, South Africa, Zambia).

Abstract

I report terrestrial heat flow measurements from 25 new sites in Botswana and Namibia. Combining these data with previously published results from South Africa and Zambia there are now 84 heat flow observations from southern Africa which together reveal a contrast in surface heat flow of about 25 mW m$\\sp{-2}$ between the southern African Archean craton and the younger surrounding mobile belts. I use a multi-dimensional numerical heat transfer model to investigate two possible contributions to this contrast: (1) a shallow geochemical component, comprising a difference in crustal heat production between the two terrains, and (2) a deeper geodynamical component, arising from the existence of a thick lithospheric root beneath the Archean craton which diverts heat away from the craton into the thinner surrounding lithosphere. Results suggest that a lithospheric root extending to depths of 200 to 400 km beneath the Archean craton can divert enough heat to account for 50 to 100% of the observed contrast in surface heat flow with the remainder resulting from a difference in crustal heat production between the craton and the surrounding mobile belts. Estimates of temperatures in Archean cratons during the Archean suggest that thermal conditions within the cratonic lithosphere shortly after stabilization were the same as they are today, even though heat production in the Earth and the mean global heat flow were substantially higher in the Archean. Extrapolation of present-day models for southern Africa to thermal conditions appropriate for the Archean is inadequate to explain the similarity of present and Archean temperatures in the cratonic root. Reconciliation of the modern and ancient temperature estimates requires either relaxation of the constraints that the cratonic crustal heat production and /or the earth's mean mantle temperature were higher in the Archean than they are today, or that substantial 'erosion' of the lithosphere comprising the cratonic root has occurred since the Archean. The latter possibility could perhaps result from revolatilization of the cratonic root in association with thermal perturbations in the mantle, for which there is evidence in southern Africa in the form of post-Archean tectonic and igneous activity.