Heat Flow in the Canadian Shield and Its Relation to Other Geophysical Parameters

Abstract

The Canadian Shield, the exposed part of the vast assemblage of Precambrian terrains in North America, consists of several distinct subprovinces of different ages and tectonic styles. The mean heat flow for 54 borehole sites from five of the subprovinces is 44 mWm−2, with a standard deviation of 8 mWm−2. There are some differences in mean heat flow among the different provinces, but they are not large. There is no indication of a decrease in heat flow with tectonic age, even though the sites occur in terrains ranging in age from approximately 1 to 3 Ga. Nor is there an unambiguous relation between heat flow and crustal thickness, in spite of a wide variation in the latter, from approximately 30 to 50 km. Variations in heat flow are principally the result of variations in heat production in the upper 15 to 20 km of crust.Some variations in heat flow can be ascribed directly to major tectonic features. In the Superior Province, in which mean heat flow is 42 ± 8 mWm−2, heat flow varies from 33 to 59 mWm−2 across the Kapuskasing Structural Zone, an exposed oblique section through the upper two-thirds of crust. One-dimensional calculations of heat flow across the zone as modelled from geological and geophysical techniques are in excellent agreement with measurement of heat flow observations. In the Southern Province, a heat flow profile across the Mid-Continent Rift System in eastern Lake Superior is inconsistent with an interpretation of seismic reflection data. The heat flow data do not support the proposition that the rift contains significant thicknesses, ~ 15 km, of volcanics.The comparison of heat flow results with other geophysical methods for studies of major tectonic features shows that measurement of heat flow is an essential part of multidisciplinary geophysical studies.