Tectonic Implications and Influence of the Midcontinent Rift System in Nebraska and Adjoining Areas

southeastern Nebraska is underlain by Early Proterozoic rocks of the Central Plains (1600–1800 Ma) and Penokean (1830–1890Ma) Provinces. These rocks were intruded by Middle Proterozoic epizonal plutons (1340–1480 Ma). The Midcontinent rift System was imposed upon and reflects the pre-existing tectonic grain of the older basement. This correspondence of the trends of the Rift segments with older tectonic zones has been noted by Klasner et al. (1982), Carlson (1988a and 1988b) and Sims (1990). On the western limb of the Rift, the Lake Superior segment lies along the Great Lakes Tectonic Zone. The Minnesota and Iowa segments sub-parallel the Penokean Suture; the offset between these segments crosses this Suture. The Nebraska segment is an extension of the southwestward-trending faults within the Penokean of Wisconsin which in turn are reflected in the Paleozoic rocks across Iowa. The Kansas segment resulted from preexisting tectonics within the Central Plains Province. The distinct separation and difference in trends between the Nebraska and Kansas segments are probably indicative of the boundary between the Central Plains and Penokean Provinces.