Abstract Heat flow and heat generation data from the Superior, Churchill and Wyoming Provinces of western North America contribute to our understanding of the nature of the crust beneath the sedimentary cover of the Great Plains. A major structural boundary defined by gravity gradient data (Thomas et al., 1987) splits the terrains between the cratons into two distinct geothermal provinces. The Wyoming and Churchill Provinces are characterised by higher heat flow than the Superior Province and the eastern part of the intervening Hudsonian terrain, the Proterozoic Mobile Belt (PMB). Heat flow in the PMB is typical of that of the Superior craton. This suggests, on the basis of a model by Ballard and Pollack (1987), that the PMB is underlain by Archaean cratonic lithosphere. The similarity in heat flow between the PMB and the Superior craton suggests that the crust of the PMB is akin to that of the Superior craton rather than to that of the more radiogenic Wyoming and Churchill cratons. Geochronologic, gravity, aeromagnetic and geothermal data suggest that the structural boundary defining the western boundary of the PMB is also the eastern boundary of the Wyoming craton. Geothermal data alone suggest that the Wyoming craton might extend as far north as the cratonic segment of the Churchill Province, or that the Wyoming and Churchill Provinces may gradually merge together.