HR: 1340h
AN: T43B-1319    [Abstracts]
TI: Scale and Amplitude of Heat Flow Variations at the Base of the Continental Lithosphere
AU: Mareschal, J
EM: jcm@olympus.geotop.uqam.ca
AF: GEOTOP-UQAM-McGill, University of Quebec at Montreal, P.O. Box 8888, sta. "downtown", Montreal, QC H3C3P8 Canada
AU: * Jaupart, C
EM: cj@ccr.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4. Pl. Jussieu, Paris, F75252 France
AB: Thermal conditions at the base of the continental lithosphere provide a constraint that models of mantle convection must satisfy. The scale of the surface heat flow variations controls how they affect the thermal structure and thickness of the lithosphere. In order to downward continue the heat flow to the base of the lithosphere, one must determine the average heat flow over a scale which is sufficiently large: the thicker the lithosphere, the larger the horizontal averaging scale. Thus, the determination of lithospheric thickness and average heat flow cannot be treated independently. Within a single craton, such as the North American craton for example, there are significant variations of surface heat flow at short spatial scales which bear no consequence for the deep lithosphere. 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 have analyzed the entire data set as well as data from five compositionally distinctive subprovinces ranging in age from Archean ($>$2.5Ga) to Mid-Proterozoic (1.1Ga). 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 ($\overline{Q}$) and heat production ($\overline{A}$) conform to the simple relationship $\overline{Q} = Q_o + H \overline{A}$, where $H \approx$ 9km and $Q_o $\approx 33 \mathrm{mW~m^{-2}}$. 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. For instance, the large heat flow step at the Grenville-Appalachian boundary ($\approx 16 \mathrm{mW~m^{-2}}$) may be accounted for by a change in crustal heat generation only. In that case, the lithosphere is $\approx$ 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 $\pm 2 \mathrm{mW~m^{-2}}$ , 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 $\approx 7 \mathrm{mW~m^{-2}}$. 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. Because of the thickness of the lithosphere, these low amplitude long wavelength variations do not include transient or short period (i.e. $<$ 500 Myears) thermal fluctuations at the lithosphere-asthenosphere boundary which have no effect on surface heat flow. Without these fluctuations, the heat flow over large cratonic areas remains low on very long time scale.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8130 Heat generation and transport
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting