G. Bienfait

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 μW 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 (Qr = 30 ± 2 in 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°C isotherm, which controls the effective elastic thickness of the lithosphere, is very sensitive to crustal heat production.

Crustal heat production in the Superior Province, Canadian Shield, and in North America inferred from heat flow data

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 A3 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 A3 in the oldest Slave Province to a minimum of 0.55 mW m A3 in the Paleo-Proterozoic Trans-Hudson Orogen. It increases in younger provinces, culminating with a high value of 1.05 mW m A3 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.

Surface heat flow, crustal temperatures and mantle heat flow in the Proterozoic Trans-Hudson Orogen, Canadian Shield

The mean and standard deviation of heat flow values are 42 ± 9 mW m � 2 .I n 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. INDEX TERMS: 8130 Tectonophysics: Evolution of the Earth: Heat generation and transport; 8120 Tectonophysics: Dynamics of lithosphere and mantle—general; 1020 Geochemistry: Composition of the crust; 8015 Structural Geology: Local crustal structure; KEYWORDS: heat flow measurements, crustal thermal structure, mantle heat flow

An empirical relationship between thermal conductivity and elastic wave velocities in sandstone

Measurements in three samples of very clean quartz sandstone in the porosity range 4–16 %, under dry and 100 % water-saturated conditions, show that P- and S-wave velocities are linearly correlated with thermal conductivity. The experimental results agree with the theoretical relation between seismic velocities (predicted by the Kuster and Toksoz model (1974)) and thermal conductivity (predicted by weighted geometric mean).

Low mantle heat flow at the edge of the North American Continent, Voisey Bay, Labrador

Heat flow measurements in 4 deep drillholes near Voisey Bay, Labrador, have yielded the lowest value ever reported in the Canadian Shield, 22 mW m−2. This very reliable estimate is also one of the lowest continental heat flow values world wide. It requires the crust to be very poor in radioelements in this part of the Archean Nain Province. It also strongly supports the view that mantle heat flow is low (<15 mW m−2) throughout the Canadian Shield, with no trend of increasing mantle heat flow near the edges of the continent. It also raises questions about the controlling mechanism for rifting and the opening of the Labrador Sea at 100 Ma.

Heat flow variations in the Grenville Province, Canada

Heat flow and heat production data provide strong constraints on the composition and evolution of the continental crust. Four new heat flow and heat production data from the Grenville Province in Canada are presented and included in local and regional analyses of heat flow variations. Thirty heat flow data are now available in the Late Proterozoic Grenville Province, where the mean heat flow (41 ± 11 mW · m−2) does not differ from that in the Archean Superior Province (41 ± 9 mW · m−2). It is shown that varying crustal heat production or heat refraction effects account for most local heat flow variations. The average crustal heat production is lower than in the other Late Proterozoic provinces because the tectonic evolution of the Grenville promoted the emplacement of large mafic bodies. Statistical analyses of heat flow and heat production data in the Grenville Province, in the surrounding provinces (Canadian Appalachians and Superior Province) and in the Norwegian Shield indicate that, within each province, the range of heat flow and heat production variations attains 40 mW · m−2 and 2 μW · m−3 respectively. However, the patterns on the histograms are distinctive and are shown to be regulated by the vertical distribution of heat production. Crustal structure and composition of the Grenville Province and the Norwegian Shield account for similarities and differences in the heat flow and heat production patterns of the two areas. Mantle heat flow in the Grenville Province lies between 9 and 16 mW · m−2, as in the Superior Province and in the Norwegian Shield.

High heat flow in the trans‐Hudson Orogen, Central Canadian Shield

Nine new heat flow and heat production measurements are presented for the Trans-Hudson Orogen, an Early Proterozoic Province between the Archean Superior and Hearne Provinces of the Canadian Shield. Seventeen heat flow values now available in this region range between 23 and 81 mW.m−2. The highest value is anomalous and is due to heat refraction effects in and around highly conducting quartzite formations. When refraction effects are accounted for, the Flin Flon and Thompson belts appear as distinct crustal blocks with different mean heat flow values of 42±3(s.d.) mW.m−2 and 54±8(s.d.) mW.m−2. Such differences can be accounted for by variations in the amount of felsic rocks in the uppermost 15 km of the crust. Comparison between heat flow and elastic thickness studies show that variations in elastic thickness of the lithosphere are not thermally induced.

Heat flow variations in a deep borehole near Sept-Iles, Québec, Canada : Paleoclimatic interpretation and implications for regional heat flow estimates

A deep (> 2000m) borehole in the Sept-Iles intrusion, on the north shore of the Saint Lawrence River, in Quebec, Canada, was repeatedly logged for temperature. Systematic variations of the temperature gradient with depth are not correlated with the thermal conductivity. We interpreted the temperature profile as follows: (1) During the last glacial maximum, the temperature at the base of the ice sheet was cold (≈−5°C); (2) When the region was below sea level, between 10 and 5ky B.P., the ground surface temperature was warm (≈ 6°C); (3) The average ground surface temperature dropped to ≈ 2°C at 5ky B.P. when the region rebounded above sea level; (4) The long time averaged ground surface temperature before the last glacial maximum was ≈0 - 1°C; (5) The reference heat flow (36 – 37mW m−2) is 4–5mW m−2 higher than estimated from the upper 1000m of the heat flow profile. This interpretation can not be extrapolated to the entire region covered by the Laurentide ice sheet. Except for extremely deep (> 1500m) boreholes, the small uncertainty (< 15%) affecting heat flow estimates can not be eliminated.

Heat flow in the Nipigon arm of the Keweenawan rift, northwestern Ontario, Canada

[1] In the Archean Superior Province, the Nipigon Embayment, in the area of Lake Nipigon north of Lake Superior, is covered by MidProterozoic sediments intruded by Keweenawan diabase sills. It has been interpreted as a failed arm of the ca. 1100 Ma Keweenawan rift. Six new heat flow values in this area show that the region of low heat flow associated with the Keweenawan rift in Lake Superior extends northwards along the western margin of the Nipigon Embayment. The average heat flow in the Nipigon area (39 ± 5 mWm � 2 ) is only slightly lower than