HEAT-FLOW MAP OF NORTHERN AND CENTRAL PARTS OF THE FENNOSCANDIAN SHIELD BASED ON GEOCHEMICAL SURVEYS OF HEAT PRODUCING ELEMENTS

The heat flow-heat production (Q-A) relationship is a useful tool in g eothermal research, and it has been widely used for delineating geothe rmal provinces and determining characteristic parameters of heat produ ction in the continental crust. In this study, a simple but rarely use d 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 gl acial till with a sampling density of 1 sample/300 km2. Heat productio n values determined from U, Th and K concentrations in these samples w ere used to calculate a map of heat flow density. A previously determi ned Q-A relationship, Q = 15.8 + 10.8 . A, was applied. The compiled m ap 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 w ere controlled with 12 drill hole measurements not used in calculating the applied Q-A regression line. Nine of them are from previously unp ublished data from the Finnish part of the Shield. The agreement with drill hole measurements and the geochemical estimate is reasonable, al though not perfect in all cases. The differences can be attributed to anomalous vertical variation in heat production, reliability of the ap plied Q-A plot, reliability of till geochemistry in bedrock studies, c onvective groundwater disturbances or local structural effects. The ca lculated heat flow map can be used as a data set supplementing drill h ole measurements to determine representative values of heat flow densi ty in areas with low numbers of drill hole measurements.