Page 144 - Global Tectonics

130 CHAPTER 6

GDH1 PSM HS

(a)
3 HS
Depth
3
4
km
5 2

6 1
PSM GDH1
0

(b)
150
Heat flow HS PSM
m Wm 2 100 .2

50 .1
GDH1

0 0
0 50 100 150 Misfit
Age (Myr)

Fig. 6.9 Observed depth and heat flow data for oceanic ridges plotted as a function of lithospheric age, and
compared to the predictions of three thermal models: HS, half space model; PSM, model of Parsons, Sclater and

McKenzie; GDH1, global depth and heat flow model of Stein and Stein (redrawn from Stein & Stein, 1996, by permission
of American Geophysical Union. Copyright © 1996 American Geophysical Union).

−1/2
q = 510t for t ≤ 55 Ma Blanketing by sediment does not appear to be the cause
and q = 48 + 96exp(−t/36) for t > 55 Ma. of the low heat flow because the troughs are within the

least sedimented areas of the ridge and also the young-

The variation of heat flow with age predicted by all est and therefore hottest. To explain these phenomena

three thermal models is illustrated in Fig. 6.9b and com- it was proposed that the pattern of heat flow is con-
pared to observed heat flow values. It will be noted that trolled by the circulation of seawater through the rocks

observed values for younger lithosphere have not been of the oceanic crust.
plotted. This is because there are large variations in the Although the penetration of water through the hard

heat flux measured in young oceanic crust (Fig. 6.4). rock of the sea fl oor at first seems unlikely, it has been
The values obtained are typically less than those pre- shown that thermal contraction can induce suffi cient
dicted by the models and there is now thought to be permeability for efficient convective flow to exist. The

good reason for this. In particular, there is a large scatter cracks are predicted to advance rapidly and cool a large

in heat flow magnitude near the crests of ocean ridges. volume of rock in a relatively short time, so that intense
Thermal lows tend to occur in fl at-floored valleys and localized sources of heat are produced at the surface.

highs within areas of rugged topography (Lister, 1980). Active geothermal systems that are driven by water