The Keweenawan Rift System is an Early Proterozoic, tectonic
event that is very popular for geologic studies. Nearly 1.109 billion
years from the present, a basaltic magma upwelling began to rift the Lake
Superior Craton. This magmatic upwelling or hotspot is very similar
to what is found at Yellowstone National Park. The hot spot created
a dome that covered all of the Lake Superior region. A linear belt
of subsidence was produced along the rift valley and in turn 25 million
years of volcanism followed. Many basalt flows erupted from the central
rift axis, similar to the expansion of the mid-ocean ridge in the Atlantic
Ocean. Early basaltic flows were found to be pillowed and displayed
evidence for the rift being once underwater. Subsequent flood basalts cover
lower pillow basalts and ranged in thickness up to 3 miles thick.
Maximum total thickness of the basalt flows ranged from 2 to12 miles in
thickness, while the rift itself extended over 100 miles on either side.
It is also noteworthy that these early flows exhibit a paleomagnetic signature
indicating a magnetic reversal of the Earth’s poles.
Rifting continued and eventually caused the thick basaltic units
to sink. The majority of the subsidence occurred after the volcanics
stopped (~1084ma.). Sedimentation and erosion quickly followed and
began to fill the rift valley. After deposition of the Oronto Group,
a thick sedimentary package, the region underwent a closure of the once
extensional rift valley. The failed continental break-up was signified
by compressional forces that caused normal fault blocks in the rift valley
to reverse and uplift. Large horsts of volcanic rock were thrusted
on top of the sedimentary rocks (Oronto Group) that were formally deposited
on top of the volcanics. These features were indicators of the compressional
forces subjected to the Lake Superior region and marked the end of the
once extensional Keweenawan Rift System.