Geology Reference
In-Depth Information
The Grenville province of North America is a well-recognised late Proterozoic
example of continental collision, and is characterised by high metamorphic grades
indicative of deep burial followed by erosional exhumation [158]. Collisions must
certainly have contributed to the episodicity of the record, but how much is not
so obvious for earlier times. For example, the greatest concentration of ages is in
late Archaean (around 2.7 Ga), and the episode seems to have extended at least
over a large region, if not globally. However, the terrains from this time are mostly
relatively low grade and much new crust seems to have formed, and neither of these
features is characteristic of collisions.
9.5 Intermittent plate tectonics
A final potential source of complication is that plate tectonics may have ceased for
significant periods [147, 159]. In this case the episodic behaviour might be due to
complex rheology rather than to compositional buoyancy.
The re-establishment of subduction after a hiatus might have generated substan-
tial tectonic and magmatic signatures if mantle heat had accumulated significantly
during a quiescence. Models have been developed that exhibit something like this
kind of behaviour, and an example is shown in Figure 9.14. In this case the episode
is due to the accumulation of negative thermal buoyancy in the top boundary layer
combined with a nonlinear rheology, which allows strain to become concentrated
and the lithosphere eventually to 'break'.
9.6 Implications for tectonics
The goal of this topic is to bring readers to the point where they can bring a
well-informed general understanding of mantle convection, and mantle convection
models, to their own specialty, and thus partake more usefully in continuing discus-
sions and debates. Therefore the implications of the above modelling will not be
pursued in great detail, but some general directions of current thought will be briefly
surveyed. Thus, for example, there is no attempt here to review comprehensively
the substantial literature relating model studies with many kinds of observation.
The near-simultaneous recognition of plate tectonics and mantle plumes pro-
vided geology for the first time with well-defined fundamental driving mecha-
nisms for tectonics and the many subsidiary geological processes that accompany
tectonic activity. There was, indeed, a revolution in our understanding of the geo-
logical Earth. However, it is evident from the geological record that the Earth
has not always behaved the way it has for the past 500 million years, or per-
haps one or two billion years. This is evident even from a glance at a satellite
image of the 3.5 Ga Pilbara block in northwest Australia (Figure 9.15), where the
