Geoscience Reference
In-Depth Information
Figure 7.13.
Reduced
heat flow
Q
r
versus time
since the last
tectono-thermal event for
the continental heat flow
provinces. The error bars
represent the
uncertainties in the data.
The solid lines show the
reduced heat flows
predicted by the plate
model. BR and BR', Basin
and Range; SEA,
southeast Appalachians;
SN, Sierra Nevada; EUS,
eastern U.S.A.; SP
1
and
SP
2
, Superior Province;
Bz, Brazilian coastal
shield; B, Baltic shield;
BM, Bohemian massif; U,
the Ukraine; EW, England
and Wales; N, Niger; Z
and Z', Zambia; WA,
western Australia; CA,
central Australia; EA,
eastern Australia; I
1
and
I
'
1
Indian shield; and I
2
,
Archaean Indian shield.
(After Morgan (1984) and
Stein (1995),
Heat Flow of
the Earth
, AGU Reference
Shelf 1, 144-58, 1995.
Copyright 1995 American
Geophysical Union.
Modified by permission of
American Geophysical
Union.)
120
100
80
BR
BR
EA
BM
60
Z
40
Z
I
1
WA
I
2
EUS
CA
I
1
SP
1
Bz
U
20
SP
2
B
SN
N
0
0 1 2 3 4
Tectono-thermal age (Ga)
the base of the crust (because
z
*isprobably approximately the thickness of the
crust).
Neither of these models of the distribution of heat generation within the crust
allows for different vertical distributions among the various radioactive isotopes.
There is some evidence for such variation. Nevertheless, it is clear that much
of the variation in measured surface heat flow is caused by the radioactive heat
generation in the crust and that the reduced heat flow
Q
r
is a reasonable estimate of
the heat flow into the base of the crust. Figure 7.13 shows this reduced heat flow
plotted against age. After about 300 Ma since the last tectonic/thermal event,
the reduced heat flow exhibits no variation and attains a value of (25
×
10
−
3
Wm
−
2
. This is within experimental error of the value predicted by the plate
model of the oceanic lithosphere and suggests that there should be no significant
difference between models of the thermal structures of the oceanic and continental
lithospheres. The present-day thermal differences are primarily a consequence of
the age disparity between oceanic and continental lithospheres.
±
8)
7.6.2 The temperature structure of the
continental lithosphere
Figure 7.14 shows two extreme temperature models of the equilibrium oceanic
lithosphere, O
1
and O
2
, and two extreme models of the old stable continental
