We should take in the two great fault zones,
the STZ and the GSLSZ. And now, after the gigantic fusion processes
of the Early Proterozoic, a much enlarged Canadian Shield entered
a long period of relative tectonic quiet.
The Completion of the Canadian Shield--1,000 Ma Ago
There was only one orogenic intermission, so to speak, in the ECSOOT
transect, which brought us the Labrador Orogen at about 1,700 to
1,650 Ma, before the supercontinent, amalgamated from microcontinents,
got set upon again by yet another squeeze from another geotectonic
plate. There were long millennia of relative tectonic peace and
quiet in North America, for at least 350 Ma, that is the time between
the formation of the Labrador and the later Grenville orogens. During
this period much anorogenic (non-orogenic) magmatism occurred.
But then, following this period of placid stability, the biggest
crunch ever came when the Grenville Orogen was formed by thrusting
of previously deeply buried rocks upon the supercontinent by continents
or microcontinents of uncertain affinity situated to the southeast.
The Grenville Orogen arguably was the largest mountain system earth
ever has seen. Its formation completed the main Proterozoic assembly
of the continent, really a supercontinent now known as Laurentia.
Think of how the Himalayas are being formed today as the old India
craton is pushing its way against the underbelly of Eurasia. That's
what happened then to what now is North America's easternmost Precambrian
mountain belt, the Grenville orogen. We don’t know the Precambrian
equivalent to today’s India, only that the resulting orogen
probably was even bigger than the Himalayas are today.
Have a look at the regions which are marked by the grass-green
"Grenville Orogen". They include southeastern Ontario
(places like Parry Sound, London, Kingston, Toronto or Ottawa),
much of Quebec (Montreal, Sept-Isle), and much of Labrador. And
note how long this orogen is, some 5,000 km long, starting in the
south in northern Mexico, through the United States, eastern Canada,
and, after leaving the Labrador coast, continuing into southern
Sweden, where it is known as the Sveconorwegian Province. The missing
piece in between is taken up by the Atlantic, which is a new rift.
Actually, before the Atlantic started to rift another ocean had
opened and closed, forming the Appalachian - Caledonide orogens
in the process - about which we shall hear shortly.
Think of the enormous height and width of the Grenville Orogen
or mountain system! Even the remnant which we can map from geological
and geophysical evidence today, is only part of the width the system
once occupied, before it was torn asunder by Earth's relentness
tectonic forces.
|
