Heat flow and deep thermal structure near the edge of the Canadian Shield
Résumé
Five new heat-flow and heat-production measurements in the Archean Superior Province are presented. These measurements include the first heat-flow values to be reported for the Opatica subprovince and the Otish basin. These new data complete the data set acquired in the eastern Canadian Shield during the Abitibi-Grenville Lithoprobe transect. The data set now available in eastern Canada, covering geological provinces ranging in age from 2700 to 400 Ma, achieves sufficient sampling to define the deep thermal structure of a continent near the edge of the craton. It shows that, for the Canadian Shield, there is no simple relation between heat flow and the age of tectonic provinces. The map of heat flow in eastern Canada demonstrates that there is no significant difference in heat flow between the Abitibi subprovince and the Grenville Province (including the Adirondacks) where the area-weighted average heat flow is the same (39 vs. 38 mW*m-2, respectively). Outside the Abitibi, the Superior Province is characterized by a higher heat flow (45 mW*m-2). Heat-flow and gravity data are used together to determine changes in crustal composition and thickness. The analysis of these data and constraints from seismology support the view that the variations in surface heat flow can be entirely accounted for by changes in crustal composition. Heat-flow variations across the Abitibi subprovince indicate that there are significant differences in crustal composition that reflect the complex assemblages that make up the Archean crust. The heat-flow map shows a sharp transition between the Grenville Province and the Appalachians, where the average heat flow is significantly higher (57 mW*m-2). This difference is due to higher heat production in the Appalachian upper crust with the same mantle heat flow as in the shield (-12 mW*m-2 throughout eastern Canada). Lower crustal and upper mantle temperatures are typically low, which might explain the preservation of irregular crustal thickness over several billion years.