The GLIMPCE transect is directed at investigating
deep geology beneath the Great Lakes. Lake Superior covers
the 1100 million
year old (Ma) Keweenawan Rift System, an arcuate structure
extending 2000 km from Kansas to central Michigan. This structure
is the deepest known rift on the planet, with up to 25 km of
syn-rift volcanics and interlayered sediments plus an additional
10 km of post-rift sediments. The crust was thinned to the
extent that North America was almost split into two separate
blocks. Beneath Lake Huron, the Grenville Front Tectonic Zone
(1300 - 1000 Ma) was shown to be the locus of an intercontinental
collision which formed a mountain chain, probably rivalling
that of today's Himalayas. These mountains were rapidly eroded
such that only the deep roots of the orogen are now exposed.
An evolutionary model is inferred:
- between 1800 and 1300 Ma,
rocks of the front were emplaced a shallow levels;
- about 1300 Ma, the outer margin was overridden by northwesterly
transported microterranes depressing rock units to
lower crustal levels;
- a later episode (~1200 Ma) of northwest-directed thrusting
ramped the buried rocks back up to the surface, forming
the mountain system.
Synthesis volume: Canadian Journal of Earth Science, V31,
#4, April, 1994
GLIMPCE Publications
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