Variations of surface heat flow and lithospheric thermal structure beneath the North American craton

Abstract

Two end-member models have been proposed to explain the variations in surface heat flow in stable continents, calling for changes of either crustal heat production or heat flow at the base of the lithosphere. The scale of the surface heat flow variations controls how these variations affect the thermal structure and thickness of the lithosphere and provides constraints on these models. Data in the Canadian Shield and the Appalachians are now extensive enough to address problems of scale and relationship between average heat flow and heat production. We analyze the global data set as well as data from five compositionally distinctive subprovinces. Within each province, on scales <500 km, observed heat flow variations are linked to changes of local crustal structure. For the five subprovinces, the average values of heat flow ( Q̄) and heat production ( Ā) conform to the simple relationship Q̄= Q o+ HĀ, where H≈9 km and Q o≈33 mW m −2. This shows that, on scales larger than the dimensions of these provinces (>500 km), variations in crustal heat production dominate and hence that variations of mantle (Moho) heat flow must be small. The large heat flow step at the Grenville–Appalachian boundary (≈16 mW m −2) may be accounted for by a change in crustal heat generation only. In that case, the lithosphere is ≈40 km thinner in the Appalachians than in the Shield. At wavelengths of 500 km or more, mantle (Moho) heat flow variations are constrained to be smaller than the detection limit of heat flow studies, or about ±2 mW m −2, and may not be correlated with surface geology. Downward continued to the base of the lithosphere, the amplitude of these variations depends on wavelength and must be smaller than ±7 mW m −2. Such variations imply that temperature differences must be smaller than 400 K at 150 km depth. These bounds are consistent with seismic shear wave velocity variations and geothermobarometry studies on mantle xenoliths.