Geology Reference
In-Depth Information
been brought together by a process
of accretion, lends strong support to
the proposition that plate tectonic
processes similar to today's oper-
ated in much the same way as far
back as ~4000 Ma ago when the oldest
rocks we can now see were formed.
The contrasts between the Superior
and North Atlantic cratons may be
due mainly to the difference in level
of exposure, as the NAC forms the
upper plate of the collision with the
Superior Province, thus exposing its
lower crust, whereas the latter is sur-
rounded by continental plates that have
been thrust over it. The large granitic
plutons that are such a characteris-
tic feature of the granite-greenstone
terrains, for example, may be repre-
sented in the high-grade lower crust
by their much less prominent roots.
Although differences in detail cer-
tainly exist between Archaean and
post-Archaean orogenic belts, these
can probably be explained to some
extent by variations in the plate tectonic
mechanism arising from the greater
Archaean heat flow. It is difficult to
obtain enough information about the
extent or nature of orogenic belts or
orogenic processes further back than
the Neo-archaean because of the dis-
ruption of the pre-existing crust by the
pervasive late-Archaean orogenies.
