Geology Reference
In-Depth Information
Fig. 12.10
Heat flux as a function of the age of the ocean
floor. Black dots are observed data included in the global
data base maintained by the International Heat Flow Com-
mission
The theoretical trend based on the HSC model is displayed
as a
red line
. This trend line was determined using (
12.44
)
and assuming ›
1mm
2
s
-1
,
T
0
D
10
ı
C,
T
a
D
1,280
ı
C,
D
3.3 W m
1
K
1
(see
and
k
D
To estimate the suitability of the half-space
cooling (HSC) model, we can compare the pre-
dicted surface heat flux
q
0
at any sea floor age
t
with observed values. Figure
12.10
shows the
distribution of global marine heat flow data on
oceanic crust provided by the
International Heat
Flow Commission
and the predicted HSC heat
flux given by (
12.44
). We note that there is
considerable scatter in the observed data, even
taking into account that they have been extracted
from a global catalogue. A major source of scat-
tering is associated with hydrothermal circulation
through the oceanic crust and the consequent heat
loss. However, the agreement between the heat
flow data and the HSC trend is acceptable after
60 Ma, although the theoretical trend appears
to decrease a little bit more rapidly. In fact, in old
oceanic basins the observed heat flow apparently
levels at
53 mW m
-2
. A comparison with mea-
surements performed only in thick sedimentary
successions, which possibly block hydrothermal
circulation and heat loss, would confirm more
clearly this misfit (Turcotte and Schubert
2002
).
This suggests that beyond some sea floor age
the rate of conductive cooling is to some ex-
tent compensated by additional heat production.
However, a more serious problem in Fig.
12.10
is represented by the apparent misfit between the
observed heat flow and the theoretical trend on
young oceanic lithosphere. In fact, the former ap-
pears to be significantly lower than that predicted
by the HSC model.
An improvement over the HSC model, which
was originally developed by Davis and Lister
(
1974
), can be obtained taking into account that
the lithosphere is heated from below as a conse-
quence of mantle convection. In the plate cooling
model (PCM) of Parsons and Sclater (
1977
),
the oceanic lithosphere forms by cooling of an
asthenospheric plate having finite thickness
h
and an isothermal lower boundary. Such a lower
isothermal boundary prevents continued cooling
and thickening of the oceanic lithosphere for
older ages. The parameter
h
is fixed and chosen
