Heat flow in the Great Plains of the United States

Abstract

Anomalous high heat flow previously reported for the Great Plains is inconsistent with the tectonic setting and requires reexamination. Forty‐six new heat flow measurements, 12 revised heat flow values, and several hundred geothermal gradient measurements indicate extensive geothermal anomalies with heat flows ranging from 80 to 140 mW m−2 in the northern and central Great Plains. Heat flow in the Great Plains outside the geothermally anomalous regions ranges from 40 − 60 mW m−2. The heat flow anomalies result from the thermal effects of regional groundwater flow where it moves upward either within a dipping aquifer or by cross‐formational flow through fractures. The gravitational driving force for the groundwater flow derives from the eastward sloping surface of the Great Plains, and the locations of the geothermal amonalies are determined by the structures of the aquifers and the crystalline basement rocks. The most widespread and largest‐amplitude geothermal anomaly occurs in southern South Dakota and northern Nebraska. Another large anomaly occurs on the eastern flank of the Denver Basin, and small anomalies occur on structures such as the Billings and Nesson anticlines in the Williston Basin. Previous reports of high heat flow in the Great Plains generally are supported by the results of this study. However, the source of anomalous heat is shown to be nontectonic, and theoretical arguments for normal continental heat flow in the Great Plains are supported. Another difference from the results of previous heat flow studies is that the thermal conductivities of shales in the Mesozoic strata in the Great Plains are about 40% lower than the conductivities that commonly have been used for shales. This observation and recent studies which have suggested lower thermal conductivities for shales in the Great Plains are the reasons for revision of some previous heat flow calculations. A significant result of revising some of the previous heat flow values is that the high heat flow zone that previously has been shown to extend through North Dakota into Canada is a region of normal continental heat flow.