HEAT-FLOW AND STRUCTURE OF THE LITHOSPHERE IN THE EASTERN CANADIAN SHIELD

Twenty-two new determinations of heat flow and radiogenic heat production in the Superior and Grenville provinces of the Canadian Shield are presented. The new data and previously published measurements strongly constrain the thermal structure of the eastern Canadian Shield. In the Abitibi greenstone belt, heat flow gradually increases from 29 mW m-2 near the Grenville Front to 44 mW m-2 east of the Kapuskasing uplift. This heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1.1-mu-W m-3. This interpretation, based on estimated heat production of major rock types in the region, is consistent with crustal models derived from recent seismic reflection and refraction studies. It also leads to an estimate of about 12 mW m-2 for the mantle heat flow beneath the area. The average heat flow in the Grenville Province, 41 +/- 10 mW m-2, is the same as that of the Superior Province. This similarity and the lack of significant variation of heat flow across the Grenville Front indicate that the crust on both sides of the front has similar heat production and thus composition. In the western part of the Grenville Province, heat flow reaches high values in the vicinity of the boundary between the Allochtonous Polycyclic and Monocyclic belts where enriched granitic plutons are found. In the crystalline terranes in the central part of the Grenville Province, heat flow and heat production are related to each other. The parameters of the linear heat flow-heat production relationship (Q(r) = 30 +/-2 mW m-2 and D = 7.1 +/- 1.7 km) are close to those of the much younger Appalachian Province, implying that the higher Appalachian heat flow is due solely to higher heat production in the upper crust. The data provide no evidence for variation of mantle heat flow between the Superior, Grenville, and Appalachian provinces, whose tectonic ages range between 2700 and 400 Ma. The small value of the mantle heat flow, about 12 mW m-2, implies that the depth to the 450-degrees-C isotherm, which controls the effective elastic thickness of the lithosphere, is very sensitive to crustal heat production.