Heat-flow measurements in Late Palaeoproterozoic to Permian geological provinces in south and central Norway and a new heat-flow map of Fennoscandia and the Norwegian–Greenland Sea

Abstract

New heat-flow determinations from 9 sites in south and central Norway, situated in Late Palaeoproterozoic to Permian geological provinces, show that topographically and palaeoclimatically corrected surface heat flow generally varies between 50 and 60 mW m − 2 . The palaeoclimatic corrections are significant, typically constituting 10–20% of the final heat-flow values. Higher heat flow was encountered at 3 sites, however, only at the Fredrikstad site, situated in the high heat-producing Iddefjord granite, could this elevated heat flow be related to high heat production. A compilation of new and previously published heat-flow data from Norway show that on the scale of major geological provinces, heat flow varies significantly and is generally overlapping. The lowest heat-flow values (ca. 30 mW m − 2 ) come from the Early Palaeoproterozoic Karasjok–Kautokeino Greenstone Belt and the Early Neoproterozoic Egersund anorthosite complex, both of which are dominated by low heat-producing rocks. The highest heat-flow values (ca. 60–80 mW m − 2 ) come from Early Neoproterozoic, high heat-producing post-orogenic granites in south Norway. These geological provinces display relatively little variation in lithology and heat production, which allows such relationships to be made. In general, however, significant lithological variation within geological provinces makes relationships between heat flow and geological factors such as tectonometamorphic age, lithology and heat production on a country scale obscure at best. In contrast, a compilation of almost 1000 heat-flow measurements from Fennoscandia and the Norwegian–Greenland Sea shows a distinct correlation between heat flow and tectonic age. The Archaean provinces in NE Fennoscandia (Karelia) display low heat flow of ca. 35–40 mW m − 2 , increasing gradually to ca. 50–55 mW m − 2 southwestwards in Palaeoproterozoic and Mesoproterozoic provinces. The Transscandinavian Igneous Belt, which consists largely of relatively high heat-producing granites, breaks this trend with a heat flow of ca. 60–65 mW m − 2 . On the Norwegian continental margin, the heat flow is rather constant at ca. 60 mW m − 2 , and the oceanic crust shows an exponential increase in heat flow from ca. 60 mW m − 2 near the continent–ocean boundary (oceanic crust ca. 55 million years old) to > 200 mW m − 2 near the mid-Atlantic spreading ridge, similar to heat flow determined from oceanic crust elsewhere as well as to theoretical models of oceanic heat flow. Although a number of factors may render individual heat-flow measurements uncertain, the abundance of available data obtained by different methods, and the clear correlation between heat flow and geology, provide confidence as to the significance of these heat-flow values.