Page 29 - Geothermal Energy Systems Exploration, Development, and Utilization

1.1 Expressions of Earth’s Heat Sources 5
Beneath continents, mantle heat flow variations do not follow such simple
physical consideration since large contrasts exist for both crustal heat production
and lithospheric thickness. However, at the scale of the mantle, heat loss is
mainly sensitive to large-scale thermal boundary conditions at the top of the
convecting system, and not to the detailed thermal structures of the overlying
lithospheres. Beneath continents, the earth’s mantle is not constrained by a fixed
temperature condition as is the case beneath oceanic lithosphere (see above), and
thus large-scale temperature and heat flow variations are expected at the top surface
of the subcontinental convecting system.
Surface heat flow measurements over continents and estimates of associated heat
production rates have shown that mantle heat flow values beneath thermally stable
(older than about 500 Myr) continental areas would be low, around 15 ± 3mWm −2
(Pinet et al., 1991; Guillou et al., 1994; Kukkonen and Peltonen, 1999; Mareschal
et al., 2000). On the contrary, mantle heat flow would be significantly enhanced
beneath continental margins (Goutorbe, Lucazeau, and Bonneville, 2007; Lucazeau
et al., 2008) where crustal thickness and heat production rates decrease. Old central
parts of continents would be associated with a low subcontinental mantle heat
flow while younger continental edges would receive more heat from the mantle.
The so-called ‘‘insulating effect’’ of continents is described here in terms of heat
transfer from the mantle to the upper surface, where most of mantle heat flow is
laterally evacuated toward continental margins and oceanic lithosphere. The term
insulating should in fact be replaced by blanketing since thermal conductivity values
of continental rocks are not lower than that of oceanic rocks (Clauser and Huenges,
1995).

1.1.3.2 Subcontinental Thermal Boundary Condition
A fixed temperature condition applies to the top of oceanic lithosphere while a
low subcontinental heat flow is inferred from surface heat flow data over stable
continental areas. As shown by laboratory experiments, this low mantle heat flow
beneath continents cannot be sustained if continental size is small (Guillou and
Jaupart, 1995). Indeed, a constant and low heat flux settles beneath a continental
area for continental sizes larger than two mantle thicknesses. For smaller sizes,
subcontinental heat flow is increased.
In the field, it was shown that mantle heat flow beneath stable continents may be
as low as 10 mW m −2 (Guillou-Frottier et al., 1995), whereas beneath continental
margins, values around 50 mW m −2 have been proposed (Goutorbe, Lucazeau, and
Bonneville, 2007; Lucazeau et al., 2008). Beneath young perturbed areas, similar
elevated values have been suggested, such as the mantle heat flow estimate of
60–70 mW m −2 beneath the French Massif Central (FMC) (Lucazeau, Vasseur,
and Bayer, 1984).
At large scale, one may infer a continuous increase of mantle heat flow from con-
tinental centers to continental margins, but laboratory and numerical simulations
of thermal interaction between a convecting mantle and an overlying conducting
continent have shown that the mantle heat flow increase is mainly focused on