Heat flow and heat production in the Namaqua Mobile Belt, South Africa

Abstract

Results of heat flow and heat production surveys at 13 localities in the Namaqua mobile belt, southern Africa, are presented. Twelve stations traverse the 1200–1100 m.y. old gneissic and metamorphic Bushmanland and Gordonia subprovinces. One station is situated in the Richtersveld subprovince, a 2000–1730 m.y. old volcano‐sedimentary domain mildly affected by the 1200–1100 m.y. old orogeny. Surface heat flow in the mobile belt ranges from 39 to 81 mW m−2 and averages at 61 ± 11 mW m−2. Surface heat production ranges from 0.8 μW m−3 in mafic rocks to extreme values of 6.9–12.3 μW m−3 in the Springbok district, western Namaqualand. High heat production in the Springbok district is associated with a thin, tabular granitic intrusive and is not representative of the basement. The mean heat production of the remaining stations in the mobile belt is 2.3 ± 1.1 μW m−3. Linear regression analysis of the heat flow‐heat production data yields a poor fit (correlation coefficient = 0.62) and imprecise estimates for the intercept (34 ± 6 mW m−2) and slope (11 ± 3 km). The eastern margin of the mobile belt has relatively low heat flow and heat production; geothermal, geological, and gravity data suggest a relative depletion of crustal radiogenic heat production in the region. However, the average Namaqua heat flow is higher than worldwide averages for Proterozoic terrains. There is no geological or geophysical evidence for recent mantle heating below the Namaqua mobile belt, suggesting that an elevated crustal radioactive component is largely responsible for the elevated heat flow. In this respect, the mobile belt is similar to several mid‐late Proterozoic heat flow provinces that are characterized by high average surface heat flow and probably have elevated crustal radiogenic heat input. These observations, and seismic data, suggest that the Namaqua crust consists of an upper layer that contributes 40–50%, on average, to the heat flow and a depleted lower layer; the inferred heat flow at midcrustal levels is 50–60% of the average surface heat flow. The mobile belt's average surface heat flow is 40–85% higher than that from the basement of the adjacent Archaean Kaapvaal craton, and its inferred midcrustal heat flow is similar to the lowest surface values in the craton. Heat production data from the Kaapvaal craton indicate that its upper crust is not depleted of heat sources. While enhanced radiogenic heat input from the Namaqua upper crust may account for part of the higher‐than‐craton heat flow, it cannot account for the full excess. Consequently, a larger equilibrium mantle heat flow, related to a reduced lithospheric thickness below Namaqualand, seems to be required by the new data.