Geology Reference
In-Depth Information
call a plate, and the term
lithosphere
, meaning strong layer, might not apply either.
If some of the thermal boundary layer can founder, then it can drive a mode of
mantle convection. There could then be a mobile upper thermal boundary layer
and therefore fairly efficient removal of mantle heat. If the mafic crust (formed
at the extensional zone over a mantle upwelling) were thin, as the models here
have suggested, it might simply be pulled down semi-continuously by boundary
layer drips. On the other hand, if the mafic crust were thick enough, it might resist
foundering and accumulate into thick piles. If such piles reached a thickness of
about 60 km or greater, the lower parts might transform to dense eclogite. These
would then tend to sink fairly rapidly, and they would pull shallower material after
them. Thus a mafic 'sinker' might founder into the mantle, and this might occur in
a very episodic manner.
It is not at all clear that such a regime ever operated on Earth, although it is
one way in which the Archaean nuclei, like the Pilbara in Figure 9.15, might have
formed. This scenario nevertheless serves to expand thinking about possibilities,
and to clarify what needs to be established before it can be concluded that plate
tectonics operated. Many of the structural features of such a compression zone
might resemble a modern subduction zone. Many of the petrological and geo-
chemical signatures might also resemble modern subduction zones, because the
primary petrological process in both scenarios may be the remelting of hydrated
mafic material at significant depth. One way in which petrology and geochemistry
might provide key evidence is if they revealed asymmetry of the overall structure,
the way the magmatic arcs do in modern subduction zones.
Apart from asymmetry, another key criterion for plate tectonics is the presence
of plates outside the active belts. This may be very difficult to demonstrate, because
of course there is little evidence generated outside of mobile belts.
To summarise this brief discussion of whether plate tectonics or some other
regime operated earlier in Earth history, it is plausible that sufficient evidence
will be accumulated to demonstrate the existence of something fairly close to a
modern asymmetric subduction zone. The asymmetry would imply a lithosphere
acting more as a brittle solid than as a malleable fluid, and it would suggest that
plates may be present. However, it would not directly demonstrate the presence
of such plates outside the mobile belt, nor indicate how large they might have
been.
Turning to plumes, arguments were given in Chapter 7 that plumes are to be
expected in the silicate mantles of Earth-like planets. The models shown here
indicate that plumes may have been at least as active as they are now. Some
versions of such thermal history calculations can yield considerably higher activity
in the past than now [65]. Plumes are secondary tectonic agents and their record
may be more ephemeral than that of subduction zones, both because much of