| Abstract |
Low heat flow has been previously reported in Minnesota (40±6 mWm-2) and the Superior Province of the Canadian Shield (42 ± 8 mWm-2). These heat flow averages are significantly lower than the rest of the eastern U.S. and lower than shields in the southern hemisphere. We find that the low heat flow in Minnesota is due to heat flow measurements only in the mafic rocks of the Mid-Continent rift system, which have low radiogenic heat production, unlike the granitic and gneissic rocks that make up the Canadian Shield; and post-glacial climatic changes thataffect temperature in the upper two kilometers of the crust, which has not been accounted for in previously published heat flow values. We have obtained new heat flow measurements, and collected over two hundred rock samples, obtained from cores and outcrops, for measurement of 238 U, 232Th, and 40K concentrations and thermal conductivity in Minnesota. Based on corrections for postglacial warming, we find the average heat flow in Minnesota to be 44.8± 8 mWm-2 which is similar to heat flow in the Quetico, Wawa, and Wabigoon Superior Subprovinces. Previously published maps of the U.S. suggest that because of low heat flow values the potential for developing Enhanced Geothermal Systems (EGS) in Minnesota may be small. We find that the newly acquired data on heat flow, thermal conductivity, and radiogenic heat production, allows us to give an assessment of the EGS resource, at depths from 3 to 10 kilometers, for the state. This new data and correcting regional heat flow values for climatic changes has reduced the depth to the 150?C isotherm, making EGS applications viable in Minnesota at a more reasonable depth. |