Before looking at the largest recorded rift on
earth, the Keweenawan Rift System or KRS (marked on this map in
striped green as KR within the oddly shaped transect marked GL)
we should have another look at the heartland of the Canadian Shield.
This is the old craton of the Superior Province, which also is the
largest of the world's Archean cratons (1.6 million km2). The reason
is that we want to know more about the earliest stages of the formation
of the first cratons which form the nuclei of the North American
continent.
The Superior Province offers an unrivalled perspective on early
crustal genesis. It received additions of juvenile material (new
rock formed from magma) to Earth's lithosphere during the Mid to
Late Archean (approximately 3,100 to 2,600 Ma). Farther on we shall
discuss the Abitibi subprovince, which is the largest building block
of this juvenile crust. Geochemical data indicate that it was derived
directly from the Late Archean mantle.
Given the occurrence of these various, and very old, terranes within
the most prominent old craton invites the study of their internal
makeup, of how these various types of terranes came to be together,
or accreted. Was the mechanism the same which we have been observing
in more recent plate-tectonic processes?
This is the purpose of a new study area or transect now being activated
by LITHOPROBE, the Western Superior Transect. It follows a detailed
study of the Kapuskasing Structural Zone, a transect for one of
the earliest LITHOPROBE studies. Note the transect areas for each
as outlined on the map (slide # 37) .
|
