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 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 •eat flow-heat production relationship (Qr = 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 450øC isotherm, which controls the effective elastic thickness of the lithosphere, is very sensitive to crustal heat production.

[1] Measurements of heat flow and U, Th, K concentrations are used to determine the amount of heat generated in various belts of the Superior Province, the largest Archean craton on Earth. These data allow estimates of the average crustal heat production and indicate compositional differences between upper and lower crustal assemblages. The bulk average heat production of the Superior Province crust is 0.64 mW m À3 and is almost the same in different belts of slightly different ages, illustrating the remarkable uniformity of crust-building mechanisms. In the wider context of the North American continent, the bulk crustal heat production decreases from 1.0 mW m À3 in the oldest Slave Province to a minimum of 0.55 mW m À3 in the Paleo-Proterozoic Trans-Hudson Orogen. It increases in younger provinces, culminating with a high value of 1.05 mW m À3 in the Phanerozoic Appalachian Province. In all provinces, U and Th enrichment is systematically associated with sedimentary accumulations. A crustal differentiation index is obtained by calculating the ratio between the average values of heat production at the surface and in the bulk crust. The differentiation index is correlated with the bulk average heat production, which suggests that crustal differentiation processes are largely driven by internal radiogenic heat.

[1] Ten new heat flux determinations have been made using measurements in 22 mining exploration boreholes located at latitudes higher than 51°N in the Canadian Shield. They provide data in poorly sampled regions near the core of the North American craton where one expects the lithosphere to be thickest. The new heat flux values are all smaller than 34 mW m −2 and are among the lowest recorded so far in the shield. For all the new sites, there is no relationship between heat flux and heat production in surface rocks. In the Canadian Shield, heat flux variations occur at wavelengths <100 km and are mostly of crustal origin. Local averages in two 250 × 250 km windows located on Archean areas at high latitudes on either side of James Bay are 29 mW m −2 and 31 mW m −2 , the lowest values found so far at this scale in the Canadian Shield. S wave traveltime delays derived from tomographic models provide the additional constraints needed to resolve differences of deep lithospheric thermal structure. There is no significant correlation between average surface heat flux and traveltime delays within the Canadian Shield, confirming that variations of the surface heat flux are mostly of crustal origin. Traveltime delays cannot be explained by variations in crustal heat production only and require variations of heat supply to the lithosphere and/or radiogenic heat production in the lithospheric mantle. These variations are associated with changes of lithospheric thickness that may be as large as 80 km. The heat flux at the base of the Superior lithosphere is constrained to be 11 ± 2 mW m −2 .

The Paleo‐Proterozoic (1.8 Ga) Trans‐Hudson Orogen (THO), of intermediate age between the Superior (2.7 Ga) and Grenville (1.0 Ga) provinces, is located near the center of the Canadian Shield. We report on new measurements of heat flow and radiogenic heat production in 30 boreholes at 17 locations in this province. With these data, reliable values of heat flow and heat production are available at 45 sites in the THO. The mean and standard deviation of heat flow values are 42 ± 9 mW m−2. In this province, distinctive geological domains are associated with specific heat flow distributions. The heat flow pattern follows the surface geology with a central area of low values over an ancient back arc basin (Kisseynew) and an ancient island arc (Lynn Lake Belt) made of depleted juvenile rocks. Higher heat flow values found in peripheral belts are associated with recycled Archean crust. Within the Canadian Shield, there is no significant variation in heat flow as a function of age between provinces spanning about 2 Gyr. There is no geographic trend in heat flow across the Canadian Shield from the THO to the Labrador Sea. Low heat flow areas where the crustal structure is well‐known are used to determine an upper bound of 16 mW m−2 for the mantle heat flow. Present and paleogeotherms are calculated for a high heat flow area in the Thompson metasedimentary belt. The condition that melting temperatures were not reached in Proterozoic times yields a lower bound of 11–12 mW m−2 for the mantle heat flow.

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