Geoscience Reference
In-Depth Information
(a)
(b)
150
(c)
2
3
4
5
GDH1
PSM
HS
100
3
50
4
6
0
5
0 50 100 150
Age (Ma)
0 50 100 150
Age (Ma)
6
0
5
10
1/2
Square root of age (Ma )
Figure 7.7.
Mean oceanic depth (a) and oceanic heat flow (b) with standard
deviations plotted every 10 Ma against age. The data are from the north Pacific and
northwest Atlantic. These global depths exclude data from the hotspot swells. The
three model predictions for ocean depth and heat flow are shown as solid and
dashed lines. The plate model GDH1 fits both data sets overall better than does
either the half-space model HS or the alternative plate model PSM. Data shown in
black were used to determine GDH1. Heat flow data at
50 Ma are shown in grey -
these are affected by hydrothermal circulation and were not used to determine
GDH1.(c) Mean oceanic depth plotted against the square root of the age of the
lithosphere (
√
t
). The solid line is the best-fitting half-space model:
d
= 2.607 +
0.344
t
1/2
.(After Stein and Stein (1992) Thermo-mechanical evolution of oceanic
lithosphere: implications for the subduction process and deep earthquakes
(overview), Geophysical Monograph 96, 1-17, 1996. Copyright 1996 American
Geophysical Union. Modified by Permission of American Geophysical Union; and
Carlson and Johnson, On modeling the thermal evolution of the oceanic upper
mantle: an assessment of the cooling plate model,
J. Geophys
.
Res
.,
99
, 3201-14,
1994. Copyright 1994 American Geophysical Union. Modified by permission of
American Geophysical Union.)
<
age and approximates a negative exponential:
d
=
5
.
65
−
2
.
47e
−
t
/
36
(7.57b)
Figure 7.7(b) shows the measured heat flow plotted against age. A simple
relationship, linked to that for ocean depth, between heat flow
Q
(10
−
3
Wm
−
2
)
and lithosphere age
t
(Ma) is predicted for crust younger than 55 Ma:
510
t
−
1
/
2
Q
=
(7.58a)
Heat flow decreases linearly with the inverse square root of age. For ages greater
than 55 Ma this simple relation does not hold; heat flow decreases more slowly
with increasing age and follows a negative exponential:
Q
=
48
+
96e
−
t
/
36
(7.58b)
7.5.2 Models of plate formation and cooling
The creation of a lithospheric plate at the axis of a mid-ocean ridge and the
subsequent cooling of the plate as it moves away from the ridge axis give rise to
