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Heat flow and heat production in the Canadian Shield . Jean-Claude Mareschal, GEOTOP-UQAM-McGill, with a little help from my friends…

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## Heat flow and heat production in the Canadian Shield

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**Heat flow and heat production in the Canadian Shield**Jean-Claude Mareschal, GEOTOP-UQAM-McGill, with a little help from my friends… Claude Jaupart, Clement Gariepy, Christophe Pinet, Laurent Guillou-Frottier, Li Zhen Cheng, Frederique Rolandone, Claire Perry, Chloe Michaut, Gerard Bienfait, Raynald Lapointe, …**Measuring heat flow**• Canadian Shield • Heat flow in the Canadian Shield • Interpretation: crustal heat production, Moho, and basal heat flow • Sudbury site**In continents, radioactivity of crustis large component of**surface heat fluxHeat flux integrates total crustal heat production In steady state: Q0 = surface heat flow Qm = mantle heat flow (at depth of Moho) A(z) = crustal heat production Zm = Moho depth**Linear heat flow heat production relationship?Example of the**Trans Hudson Orogen • Model of crustal heat production based on linear relationship between heat flux and heat production • In Shield, heat flux and surface heat production data do not fit a linear relationship • No such relationship for the entire THO nor for individual belts. • No linear relationship for any province of the Canadian Shield.**Mantle heat flow in the Canadian Shield**• Qs = Qm + ∫ A dz with A(z) estimated from exposures of different crustal levels (i.e. Kapuskasing area) • Lowest values Qs = 22 mW m-2 => Qm < 18 mW m-2 • Exposed crustal section Kapuskasing Qs=33 mW m-2 => Qm =13 mW m-2 • Grenville <Qs> = 41 mW m-2 <A> =0.75 µW m-3 Qm = 13 mW m-2**Spatial variations in Moho heat flux?**• Downward continuation to base of lithosphere δQb = δQm exp(2πz/λ) => δQm < 3 mW/m2**Regional heat flow heat production relationship=>on regional**scale heat flux uniform below 10km**Archean cratons**<Q> = 42 mW m-2 zm = 38 km <Ac> = .75 μW m-3 Average continental crust <Q> = 55 mW m-2 ; zm = 40 km; <Ac> = 1. μW m-3 Heat productionof stable continental crust These estimates of average crustal heat production are slightly higher than those of Taylor and McLennan (1985).**Archean (>2.5Ga)**Slave Province 52mW m-2 Superior Province 41mW m-2 Proterozoic (0.6-2.5Ga) Wopmay orogen (reworked Archean) 90mW m-2 ? Trans Hudson orogen (juvenile crust only) 37mW m-2 Thompson Belt (reworked Archean in THO) 57mW m-2 Grenville Province 42mW m-2 Heat flow vs Age in the Shield? Appalachians (400Ma) have higher heat flow(55mW m-2) because of radioactive granitic intrusions**Sudbury sites**• Copper Cliff 51 mWm-2 3.2 µWm-3 • Falconbridge 46 mWm-2 0.8 µWm-3 • Lockerby 63 mWm-2 3.3 µWm-3 • Sudbury 1 47 mWm-2 1.4 µWm-3 • Elliott Lake (100km W) 60 mWm-2 • Systematic sampling for hpe (Schneider et al., Geophys. Res. Lett., 14, 264-267, 1987)**Conclusions**• Most of the heat flux in stable continents comes from crustal radioactivity • Important variations in crustal radioactivity (mostly in shallow part of the crust) • Crustal radio-activity relatively high and variable in Sudbury basin**Heat flow vs Age?**• No relationship between heat flow and age • Active belts with high heat flow (not steady state). • Wopmay ??? • At 2.5 Ga, Slave had very high heat flow**Differentiation index**• As = average surface heat production • Ac = average crustal heat production • Ac = (q0 – qm) / Zm • Zm = Moho depth • Usually Di > 1, but for Flin Flon belt Di=0.4**Differentiation index vs average crustal heat production**• Higher heat production leads to more differentiated crust.

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