AN EXPANDED STRATIGRAPHIC RECORD OF NORTH AMERICAN PLATE MOTION DURING THE EARLY STAGES OF THE MID-CONTINENT RIFT

The Osler Volcanic Group represents the extrusive component of the early magmatic stage of the Midcontinent Rift in the northern Lake Superior region. In order to test the hypothesis that there was significant equatorward motion of North America during the early stage of Midcontinent Rift development, we are conducting a paleomagnetic study in stratigraphic context through the Osler Volcanic Group. Along the eastern and south shores of Simpson Island in the Lake Superior Archipelago, ~2500 meters of relatively continuous tholeiitic basalt flows, with minor siltstone and conglomerate interbeds are exposed. The flows range in thickness from 5 cm to about 40 m thick; individual flows are recognized by a transition from massive basalt at the base of flows (occasionally with pipe vesicles in the basal 10-20 cm) to vesicular basalt towards the top, and by occasionally well-exposed pahoehoe flow tops. A rhyolite, interpreted as extrusive, occurs near the top of the Osler Volcanic Group basalt flows at Agate Point (stratigraphically higher than the highest flow on Simpson Island) and its U-Pb zircon age of 1105 ± 2 Ma provides a minimum age for the studied flows on Simpson Island. Two field seasons have been conducted on Simpson Island with oriented cores being collected from 45 flows within 5 stratigraphic sections in 2011 and 45 flows within 5 stratigraphic sections in 2012. These field seasons and resulting data build on the work of Halls (1974) in the Nipigon Stait region. The Simpson Island flows are all of reversed polarity and therefore are correlative to the lower reversed polarity zone of the more complete stratigraphy at Mamainse Point (Swanson-Hysell et al., 2009). Preliminary results from the flows indicate a significant decrease in inclination from the base to the top of the Simpson Island stratigraphy consistent with motion of North America to lower latitudes. These results promise to add to the database of paleomagnetic poles from Keweenawan volcanics that together imply rapid plate velocities (upwards of 20 cm/yr) for Laurentia during the development of the Midcontinent Rift.