REEVALUATING TERRESTRIAL HEAT FLOW IN MINNESOTA

Terrestrial heat flow observations in Minnesota are few with conventional borehole measurements were made in two holes at one site in the Duluth Complex, i.e., exposed mafic rocks of the Midcontinent Rift, with low radioactive heat production which is a surface expression, and three marine measurements in shallow lakes. The observed heat flow values average 39 ± 6 mWm^-2 and are similar to observations in the rest of the Western Superior Province of the Canadian Shield, 42 ± 8 mWm^-2. One-hundred-sixty-two marine measurements in Lake Superior also sample heat flow in the mafic rocks of the Midcontinent Rift and range from 19 mWm^-2 to 61 mWm^-2. We suggest that the low heat flow is biased toward low values for two reasons. First, all of the observations are sampled in rocks of the Midcontinent Rift which is characterized by low radioactive heat production. The only reported heat production measurement in the mafic Midcontinent Rift in Minnesota is 0.46 μW m^-3 at the Ely-3 heat flow site and yields a reduced heat flow of 29 mWm^-2. One observation of heat production in the granitic Vermillion Massif in northern Minnesota indicates radioactive heat production ranging from 2 μW m^-3 to 4 μW m^-3. This would suggest heat flow in the massif is higher and more on the order of 57 mWm^-2. Second, heat flow in Minnesota has not been corrected for the post-glacial climate signal that affects the gradient in the upper 2.5 km of the surface. Wells logged in the U.S. lack a climatic correction compared to those made in Canada. Heat flow data in the Canadian part of the Superior Province is corrected up to 30% from the model developed by Jessop, 1971. This correction indicates a maximum warming of 7-8˚C, when recent findings indicate warming may be 15˚C and would require a higher climatic correction for all measurements made in the Superior Province. These reasons suggest an average heat flow of 51-57 mWm^-2 in Minnesota is likely.