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Fig. 12.4
A more realistic continental geotherm, based on exponential decrease of heat production
H
with depth
the geotherm (
12.24
) to the lithospheric man-
tle would give anyway unrealistic temperatures
on the basis of the estimated values from kim-
berlites. The mineral compositions from garnet
peridotite nodules can be used to estimate both
depth and temperature at which a nodule reached
the equilibrium. These data can be used to con-
strain the geotherms of several continental re-
gions. McKenzie et al. (
2005
) built steady state
continental geotherms solving the equations sep-
arately for the crust, an underlying mechani-
cal boundary layer (MBL), and a lower thermal
boundary layer (TBL). In fact, as we mentioned
sidered as formed by an elastic-ductile upper
layer and a lower more viscous layer that are
separated by the 650
ı
C isotherm and a sharp cut-
off of seismicity. In their study, McKenzie et al.
(
2005
) assumed a constant thermal conductivity
k
D
2.5 Wm
1
K
1
for the crust, whereas the
radiogenic heat generation rate
H
was expressed
in terms of energy density and set to 1.12 Wm
-3
for the upper crust and 0.4 Wm
-3
in the lower
crust. The heat generation within the thermal
and mechanical boundary layers was set to zero,
thereby the steady state heat flux throughout these
layers was constant and coincident with the heat
flow through the Moho.
Figure
12.5
illustrates the model geotherm
proposed by McKenzie et al. (
2005
), which is in
agreement with heat flow measurements and fits
(
P
,
T
) estimates from kimberlite nodules. Differ-
ently from previous models, in this geotherm the
crustal contribution to the heat flow is increased,
while the mantle heat flow is decreased. The
surface heat flux determined from this geotherm
results to be
52 mWm
-2
.
12.3 Non-steady State Heat
Conduction
The steady-state equation of heat conduction
(
12.18
) can be used to model the temperature
distribution in old continental crust and in
the MBL. However, it cannot describe time-
dependent
processes
such
as
the
cooling
of
the
oceanic
lithosphere.
Usually,
radiogenic
