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Heat flow and heat production in the Canadian Shield

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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  1. 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, …

  2. Measuring heat flow • Canadian Shield • Heat flow in the Canadian Shield • Interpretation: crustal heat production, Moho, and basal heat flow • Sudbury site

  3. Determining continental heat flow

  4. Measuring heat flow

  5. Heat flow map of the southern Canadian Shield

  6. 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

  7. 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.

  8. 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

  9. Crustal models

  10. Heat Flow and Gravity profiles

  11. Gravity and Heat flow profilesMonte-Carlo inversion

  12. Spatial variations in Moho heat flux? • Downward continuation to base of lithosphere δQb = δQm exp(2πz/λ) => δQm < 3 mW/m2

  13. Regional heat flow heat production relationship=>on regional scale heat flux uniform below 10km

  14. 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).

  15. 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

  16. 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)

  17. 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

  18. 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

  19. 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

  20. Differentiation index vs average crustal heat production • Higher heat production leads to more differentiated crust.

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