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

3. Determining continental heat flow

4. Measuring heat flow

7. Heat flow map of the southern Canadian Shield

8. In continents, radioactivity of crust is large component of surface heat flux Heat flux integrates total crustal heat production In steady state: Q 0 = surface heat flow Q m = mantle heat flow (at depth of Moho) A(z) = crustal heat production Z m = Moho depth

9. Linear heat flow heat production relationship? Example of the Trans Hudson Orogen 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 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 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 such relationship for the entire THO nor for individual belts. No linear relationship for any province of the Canadian Shield. No linear relationship for any province of the Canadian Shield.

10. Mantle heat flow in the Canadian Shield Q s = Q m + ∫ A dz with A(z) estimated from exposures of different crustal levels (i.e. Kapuskasing area) Q s = Q m + ∫ A dz with A(z) estimated from exposures of different crustal levels (i.e. Kapuskasing area) Lowest values Q s = 22 mW m -2 => Q m Q m < 18 mW m -2 Exposed crustal section Kapuskasing Q s =33 mW m -2 => Q m =13 mW m -2 Exposed crustal section Kapuskasing Q s =33 mW m -2 => Q m =13 mW m -2 Grenville = 41 mW m -2 =0.75 µW m -3 Q m = 13 mW m -2 Grenville = 41 mW m -2 =0.75 µW m -3 Q m = 13 mW m -2

12. Crustal models

13. Heat Flow and Gravity profiles

14. Gravity and Heat flow profiles Monte-Carlo inversion

15. Spatial variations in Moho heat flux? Downward continuation to base of lithosphere Downward continuation to base of lithosphere δQ b = δQ m exp(2πz/λ) => δQ m δQ m < 3 mW/m 2

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

17. Heat production of stable continental crust Heat production of stable continental crust Archean cratons Archean cratons = 42 mW m -2 = 42 mW m -2 z m = 38 km z m = 38 km =.75 μW m -3 =.75 μW m -3 Average continental crust Average continental crust = 55 mW m -2 ; = 55 mW m -2 ; z m = 40 km; z m = 40 km; = 1. μW m -3 = 1. μW m -3 These estimates of average crustal heat production are slightly higher than those of Taylor and McLennan (1985).

18. Heat flow vs Age in the Shield? Archean (>2.5Ga) Archean (>2.5Ga) Slave Province 52mW m -2 Slave Province 52mW m -2 Superior Province 41mW m -2 Superior Province 41mW m -2 Proterozoic (0.6-2.5Ga) Proterozoic (0.6-2.5Ga) Wopmay orogen (reworked Archean) 90mW m -2 ? Wopmay orogen (reworked Archean) 90mW m -2 ? Trans Hudson orogen (juvenile crust only) 37mW m -2 Trans Hudson orogen (juvenile crust only) 37mW m -2 Thompson Belt (reworked Archean in THO) 57mW m -2 Thompson Belt (reworked Archean in THO) 57mW m -2 Grenville Province 42mW m -2 Grenville Province 42mW m -2 Appalachians (400Ma) have higher heat flow(55mW m -2 ) because of radioactive granitic intrusions

20. Sudbury sites Copper Cliff 51 mWm -2 3.2 µWm -3 Copper Cliff 51 mWm -2 3.2 µWm -3 Falconbridge 46 mWm -2 0.8 µWm -3 Falconbridge 46 mWm -2 0.8 µWm -3 Lockerby 63 mWm -2 3.3 µWm -3 Lockerby 63 mWm -2 3.3 µWm -3 Sudbury 1 47 mWm -2 1.4 µWm -3 Sudbury 1 47 mWm -2 1.4 µWm -3 Elliott Lake (100km W) 60 mWm -2 Elliott Lake (100km W) 60 mWm -2 Systematic sampling for hpe (Schneider et al., Geophys. Res. Lett., 14, 264-267, 1987) Systematic sampling for hpe (Schneider et al., Geophys. Res. Lett., 14, 264-267, 1987)

23. Conclusions Most of the heat flux in stable continents comes from crustal radioactivity Most of the heat flux in stable continents comes from crustal radioactivity Important variations in crustal radioactivity (mostly in shallow part of the crust) Important variations in crustal radioactivity (mostly in shallow part of the crust) Crustal radio-activity relatively high and variable in Sudbury basin Crustal radio-activity relatively high and variable in Sudbury basin

25. Heat flow vs Age? No relationship between heat flow and age No relationship between heat flow and age Active belts with high heat flow (not steady state). Active belts with high heat flow (not steady state). Wopmay ??? Wopmay ??? At 2.5 Ga, Slave had very high heat flow At 2.5 Ga, Slave had very high heat flow

26. Differentiation index A s = average surface heat production A s = average surface heat production A c = average crustal heat production A c = average crustal heat production A c = (q 0 – q m ) / Z m A c = (q 0 – q m ) / Z m Z m = Moho depth Z m = Moho depth Usually D i > 1, but for Flin Flon belt D i =0.4 Usually D i > 1, but for Flin Flon belt D i =0.4

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