Geoscience Reference
In-Depth Information
10.5.2 Archaean tectonics and ocean crust
Thermal models are important in attempts to model Archaean tectonics. For the
modern Earth (see Section 7.4), about 65% of the heat loss results from the
creation and destruction of plates and about another 17% is from radioactive heat
produced in the crust. The heat flow from the mantle into the crust is about 29
×
10 3 Wm 2 . Most of the heat that is lost comes from the mantle as the Earth cools.
The Archaean Earth had much higher rates of radioactive heat generation than
does the modern Earth. At 3 Ga, the internal heat production was 2.5-3.0 times its
present value (see Table 7.2). It has been shown that, if plate tectonics had not been
operating in the Archaean, and if all this heat had been lost from the asthenosphere
and had flowed through the lithosphere by conduction, then the equilibrium heat
flow at the base of the lithosphere would be roughly 140
10 3 Wm 2 . When this
value is used to calculate temperatures in the Archaean continents, geothermal
gradients of about 50 Ckm 1 are obtained. The temperature at the base of the
crust would have been high enough to melt it. Indeed, if this model is correct,
at 3.5 Ga the heat flow into the base of the lithosphere would have been about
190
×
10 3 Wm 2 , with a temperature of 800 Cat10km depth. These results
are contrary to the metamorphic record of deep crustal rocks preserved from the
Archaean (Fig. 10.64(b)). The continental crust is clearly self-stabilizing: heat
production is moved to the surface by geochemical processes such as partial
melting. The problems remain, however, what to do with the heat, and how it was
dissipated.
Massive volcanism on a large scale - in other words, spreading centres or
mid-ocean ridges - could provide a solution. However, the heat problem is not
neatly resolved. To dissipate such large amounts of heat, spreading rates need to
be very high, which in turn means, assuming that the Earth did not expand, that
the destruction or subduction rates must also have been very high. The dilemma
is that young, hot oceanic lithosphere does not subduct easily. What would drive
the system? Would it not heat up until a different tectonic pattern was attained?
A possible model for Hadean tectonics is that mid-ocean ridges created komati-
itic crust. If so, then subduction might have taken place because a komatiitic
crust would be denser than a basaltic crust. Such a crust would have been con-
siderably thicker and denser (approximately 15 km and 3.23
×
10 3 kg m 3 ) than
modern oceanic crust. However, like the modern oceanic crust, the Archaean
oceanic crust would probably have had a layered structure with lavas overlying
dykes overlying cumulates. A thermal model for such early oceanic lithosphere,
which is based on the assumption that lithosphere is cooled mantle, is shown in
Fig. 10.65(b). A schematic representation of Archaean plate tectonics is shown
in Fig. 10.66.
One unresolved controversy about the structure of the Archaean mantle is
particularly interesting because it is in strong contrast to today's mantle. At depths
greater than 250 km, it is possible that olivine was less dense than melt. This has
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