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Date of Award


Document Type


Degree Name

Doctor of Philosophy in Geology (PhD)

College, School or Department Name

Department of Civil and Environmental Engineering

First Advisor

Aleksey V. Smirnov


Rocks of the North American Midcontinent rift (MCR) exposed in the Lake Superior area provide an excellent opportunity to use paleomagnetism as a means of studying the characteristics of geomagnetic field in Proterozoic and the history of the rift itself. The detailed paleomagnetic and paleointensity studies of different rock units associated with the MCR, including the 1108 Ma alkaline Coldwell Complex (Ontario, Canada), presumably 1107-1108 Ma basaltic flows of the Geordie Lake (Ontario, Canada) and Silver Mountain (Upper Michigan, USA), ~1095 Ma lava flows of the Portage Lake Volcanics (PLV) (Keweenaw Peninsula, Michigan), and ~1088 Ma lava flows of the Lake Shore Traps (LST) (Keweenaw Peninsula, Michigan) are presented. Paleomagnetic study of the Coldwell Complex indicates that the apparent asymmetry of geomagnetic reversal, recorded by many Keweenawan rocks is an artifact due to missing paleomagnetic record of an equator-ward plate motion in most MCR sequences. The result supports the validity of the geocentric axial dipole assumption for the ~1.1 Ga. Extrusive sequences exposed on the Keweenaw Peninsula revealed lower paleosecular variation than that recorded by 0-5 Ma lava flows, indicating that the Proterozoic field was more stable. Samples from the ~1088 Ma Lake Shore Traps yielded consistent paleofield values with a mean value of 26.3 ± 4.7 µ T, which corresponds to a virtual dipole moment of 5.9 ± 1.1 × 1022 Am2. The mean and range of paleofield values are similar to those of the recent Earth’s magnetic field and incompatible with a “Proterozoic dipole low”. These results are consistent with a stable compositionally-driven geodynamo operating by the end of Mesoproterozoic. New high-quality paleomagnetic poles calculated for the ~1108 Ma Coldwell Complex and coeval extrusive rocks, and ~ 1094 Ma PLV indicate that North America was moving directly equator-ward with an approximately 20-25 cm/year rate between 1108 and 1094 Ma. However, the paleomagnetic data indicates a significant slowdown in motion between 1094 and 1088 Ma. This change in the plate tectonics regime coincides with a cessation of the MCR magmatism, which may indicate the beginning of renewed compression from the Grenville Front caused by continent-continent collision during the assemblage of the supercontinent Rodinia. New paleomagnetic data from the LST flows further confirm the idea of a primary nature of the MCR curvature in the Lake Superior area.