Properties of the proterozoic geomagnetic field and geological applications of paleomagnetic data from rocks of the North American Midcontinent rift

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 the Proterozoic geomagnetic field and the history of the rift itself. Detailed paleomagnetic and paleointensity studies of different rock units associated with the MCR, including the 1108 Ma alkaline Coldwell Complex (Ontario, Canada), the basaltic lava flows of the Geordie Lake (Ontario, Canada) and Silver Mountain (Upper Michigan, USA) that are assumed to be 1107-1108 Ma, the ∼1095 Ma lava flows of the Portage Lake Volcanics (PLV) (Keweenaw Peninsula, Michigan), and the ∼1088 Ma flows of the Lake Shore Traps (LST) (Keweenaw Peninsula, Michigan) are presented. Paleomagnetic data from the Coldwell Complex indicate that the apparent asymmetry of geomagnetic reversal, recorded by many Keweenawan rocks is an artifact due to fast equator-ward motion of the North American plate during the MCR evolution. The Coldwell Complex data support the validity of the geocentric axial dipole assumption for the ∼1.1 Ga. Extrusive rocks exposed on the Keweenaw Peninsula reveal similar to that of the present day geomagnetic field paleosecular variation. Samples from the ∼1088 Ma Lake Shore Traps yielded consistent paleofield values with a mean value of 26.3 +/- 4.7 &mgr;T, which corresponds to a virtual dipole moment of 5.9 +/- 1.1 x 10 22 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 modern type 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 with 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.