Heat flow map of northern and central parts of the Fennoscandian Shield based on geochemical surveys of heat producing elements

Abstract

The heat flow-heat production ( Q- A) relationship is a useful tool in geothermal research, and it has been widely used for delineating geothermal provinces and determining characteristic parameters of heat production in the continental crust. In this study, a simple but rarely used technique of utilizing the heat flow-heat production relationship is discussed. In central and northern parts of the Fennoscandian Shield extensive geochemical surveys have produced 1483 samples taken from glacial till with a sampling density of 1 sample/300 km 2. Heat production values determined from U, Th and K concentrations in these samples were used to calculate a map of heat flow density. A previously determined Q- A relationship, Q = 15.8 + 10.8 · A, was applied. The compiled map covers all Finland, northern Sweden and northern Norway, about 35% of the exposed shield area in Fennoscandia. Heat flow density and heat production increase with decreasing geological age and correlate with granitoid types. The calculated heat flow density values on the map were controlled with 12 drill hole measurements not used in calculating the applied Q- A regression line. Nine of them are from previously unpublished data from the Finnish part of the Shield. The agreement with drill hole measurements and the geochemical estimate is reasonable, although not perfect in all cases. The differences can be attributed to anomalous vertical variation in heat production, reliability of the applied Q- A plot, reliability of till geochemistry in bedrock studies, convective groundwater disturbances or local structural effects. The calculated heat flow map can be used as a data set supplementing drill hole measurements to determine representative values of heat flow density in areas with low numbers of drill hole measurements.