This record is preserved by many rocks from the time of their formation. The paleomagnetic data have played an instrumental role in deciphering the history of our planet including a decisive evidence for continental drift and global plate tectonics.
The data have also been crucial for better understanding the problems of regional and local tectonics, geodynamics, and thermal history of our planet. The rifting began during an interval of reversed polarity of geomagnetic field. The reversely magnetized lavas the Siemens Creek formation of Powder Mill group, the lowermost part of North Shore volcanics, Osler volcanics, and the lower part of Mamainse Point formation are found in many locations around Lake Superior see figure from Nicholson et al.
This early stage magmatism occurred from to approximately million years ago. The period of active magmatism was followed by a quiescence period when a geomagnetic field reversal took place.
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Magmatism renewed by Ma Ojakangas et al. During this interval, a sequence of Portage Lake lava flows erupted within a two to three million year interval around million years ago.
These rocks represent the main stage of the rift-related magmatism. All younger sedimentary and igneous suites exposed on the Keweenaw peninsula the Copper Harbor conglomerate, LST, etc have normal polarity magnetization. However, the geomagnetic field reversal mentioned above is characterized by an asymmetry, manifested in natural magnetization recorded by Keweenawan rocks that crop out around the Lake Superior e.
The two most favored hypotheses for this reversal asymmetry are either apparent polar wander during Keweenawan times Davis and Green, ; Schmidt and Williams, or the presence of a persistent non-dipole field causing the geomagnetic field to depart from a geocentric axial dipole geometry Pesonen and Nevanlinna, ; Halls and Pesonen, ; Nevanlinna and Pesonen, ; Pesonen and Halls, The recent study of this problem Swanson-Hysell et al.
The other study by Kern et al, on rocks of the alkaline Coldwell Complex Ontario, Canada also suggests no asymmetry in geomagnetic reversal during Keweenawan time.