ORIGIN OF CURVATURE IN THE MID-CONTINENT RIFT: PALEOMAGNETISM OF THE PORTAGE LAKE VOLCANICS IN MICHIGAN’S KEWEENAW PENINSULA

The origin of curved features on the Earth's surface remains poorly understood. The Keweenaw Peninsula displays a change in structural trend from east to west, varying in strike from 100^o in the east to 35^o further west before returning to a more east-west trend of 80^o in the far west. The origin of curvature can be described either as primary or secondary; primary arcs develop in their present curved state and secondary arcs form in an initially straight geometry and subsequently become curved. A powerful tool in evaluating the origin of curvature is paleomagnetism where coincident change in declination and strike is considered evidence for a secondary curvature.

Several paleomagnetic studies have been completed on rift-sequence lithologies from the Mid-Continent Rift (MCR) that are exposed in the Lake Superior region. The sediments overlying the Portage Lake Volcanics vary much more in declination than in inclination. One possible explanation is that some of these MCR sediments in the Lake Superior Region have undergone secondary rotations. If secondary rotation has occurred in the sediments, the underlying volcanics should demonstrate a similar rotation.

Thirty-one sites were collected from the Portage Lake Volcanics in the curved portions of the Keweenaw Peninsula in the Upper Peninsula of Michigan to explore the occurrence of rotation through paleomagnetic analysis. Sites were chosen to maximize the variation in structural trend. Thermal demagnetization was carried out on all samples, displaying two components in many sites, both a lower temperature component (<580^oC) and a higher temperature component (>580^oC). Both components were tested for primary remanence through a conglomerate test. The lower temperature component, believed to be carried by magnetite, passed the conglomerate test at a 95% confidence level, and is therefore assumed a primary remanence. The higher temperature component, carried by hematite, failed at the 95% level and is considered a remagnetization. Declination of the primary magnetization between sites shows no correlation with strike, demonstrating that vertical axis rotations cannot explain the curvature of the Mid-Continent Rift. We conclude that curvature in the Lake Superior Region is primary, possibly reflecting a pre-existing zone of weakness exploited during rifting.