Heat flow in the COSC-1scientific borehole, implications for the Caledonian paleothermal state

The scientific drilling project “Collisional Orogeny in the Scandinavian Caledonides” (COSC), supported by ICDP and the Swedish Research Council, involved the drilling of two vertical boreholes through carefully selected sections of the Paleozoic Caledonian orogen in Central Sweden. The main objectives of the COSC geothermal team are: a) to determine the vertical variation of the geothermal gradient, heat flow and thermal properties, and to determine the required corrections for shallow (< 1 km) heat flow data; b) to advance basic knowledge about the thermal regime of Palaeozoic orogenic belts, ancient shield areas and high heat-producing plutons; c) to improve understanding of climate change at high latitudes (i.e. Scandinavia), including historical global changes and recent palaeoclimate development (since last ice age); d) to explore the geothermal potential of the Åre-Järpen area; e) to assess to what degree the conductive heat transfer is affected by groundwater flow in the uppermost crust, and f) to determine the heat generation input and impact from the basement and the alum shales.

The present contribution focuses on themes “b” and “f” and evaluates the likely paleothermal state of the lithosphere of Baltica, in the region of the COSC boreholes, at the onset of the Caledonian orogeny. We concentrated on the results obtained from COSC-1, which was drilled, fully cored and repeatedly logged for temperature down to ~2.5 km depth. Average heat generation of the penetrated Caledonian metamorphic rocks was derived from the spectral gamma ray logs. The analysis yields a low average value of 0.8 µW/m³. Thermal conductivities were determined from 105 core samples. On average, thermal conductivity equals 2.8±0.4 W/(m K), down to ~2 km depth, and increases to 4.1±1 W/(m K) in the lowermost section of the borehole. The thermal gradient shows obvious paleoclimatic disturbances but seems largely unaffected below ~2 km depth and no advective signal is detected. The calculated heat flow for the deepest section of the well amounts to ~82 mW/m². This unusually high heat flow value for cratonic lithosphere reflects, most likely, dominant input from the underlying highly radioactive Transscandinavian Igneous Belt (TIB), which is Late Proterozoic in age. We therefore propose that the lithosphere of Baltica involving the TIB was relatively warm at the time of the Caledonian orogeny. We anticipate that the relatively high temperatures of the margin of Baltica strongly influenced deformation style.