4: The pre-Keweenawan tectonic history of southern Canadian Shield and its influence on formation of the Midcontinent Rift
J. S. Klasner, W. F. Cannon, W. R. Van Schmus, 1982. "4: The pre-Keweenawan tectonic history of southern Canadian Shield and its influence on formation of the Midcontinent Rift", Geology and Tectonics of the Lake Superior Basin, Richard J. Wold, William J. Hinze
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The Midcontinent Rift System is a major tectonic feature that formed about 1.1 to 1.2 b.y. ago in Precambrian crust that had a long and complex history but that was a rigid crustal block for several hundred million years prior to rifting. The rift formed at a high angle to the generally east-northeast-trending structural grain of the older rocks and crossed the boundary between two very different geologic provinces. The northern part of the rift, including all the rift in the immediate vicinity of Lake Superior, formed on rocks of the Superior Province or where Superior Province rocks are basement for Proterozoic supracrustal rocks. Available geochronologic data suggest that these rocks are not older than 3.1 b.y. and that they have remained a rigid crustal block since 2.6 b.y. ago. To the south, the rift developed in rocks that may be as old as 3.8 b.y. and that have since undergone repeated orogenesis at 2.6, 1.85, and 1.76 b.y. ago. Intrusion of the 1,5-b.y.-old, non-orogenic Wolf River Batholith in central Wisconsin marks the last major event prior to Keweenawan rifting 1.1 b.y. ago.
The boundary between the Superior Province and rocks to the south seems to have had a pronounced effect on the nature of the Midcontinent Rift System. Where the rift occurs in rocks of the Superior Province, it is about 150 km wide and is complex. Where it is in rocks of the more complex terrane to the south, it is much narrower (about 90 km wide) and has a simpler shape. The width of the rift changes abruptly at the contact of the two terranes.
Although orientation of the rift does not seem to be strongly influenced by the older northeast-trending fabric that it crosscuts, the orientation and location of the rift seem to have been controlled by pre-Keweenawan faults and fractures and a major intrusive body in central Wisconsin. Lineament analysis of gravity and magnetic data and Landsat imagery, as well as orientation of major dike swarms and mapped faults show that individual rift segments are either parallel to or colinear with these features. A major elliptical gravity low in central Wisconsin suggests the presence of a large body of subjacent granite. The arcuate shape of the rift encircles this gravity low, suggesting that the inferred granite served as a tectonic guide during rift formation.
Some important constraints can be placed on the width and direction of rifting, even though a unique solution for the width of the rift is not available from gravity and magnetic data. Geometric limitations suggest that the rift formed by roughly north-south separation parallel to a major preexisting tectonic zone, herein named the Trans-Superior Tectonic Zone, which bisects the arc formed by the two arms of the rift. Correlation of pre-Keweenawan features across the rift suggests that maximum separation was about 50 km in a north-south direction. But the presence of compressional features along the rift, such as the Keweenawan and Isle Royale faults, makes a unique solution impossible.