Confirmation of progressive plate motion during the Midcontinent Rift's early magmatic stage from the Osler Volcanic Group, Ontario, Canada

As the supercontinent Rodinia was assembling ca. 1.1 billion years ago, there was extensive magmatism on at least five Proterozoic continents including the development of the North American Midcontinent Rift. New paleomagnetic data from 84 lava flows of the Osler Volcanic Group of the Midcontinent Rift reveal that there was a significant and progressive decrease in inclination between the initiation of extrusive volcanism in the region (ca. 1110 Ma) and ca. 1105 ± 2 Ma (during the "early stage" of rift development). Paleomagnetic poles can be calculated for the lower portion of the reversed Osler Volcanic Group (40.9N, 218.6E, A₉₅ = 4.8, N = 30) and the upper portion of the reversed Osler Volcanic Group (42.5N, 201.6E, A₉₅ = 3.7, N = 59; this pole can be assigned the age of ca. 1105 ± 2 Ma). This result is a positive test of the hypothesis that there was significant plate motion during the early stage of rift development. In addition to being a time of widespread volcanism on Laurentia and other continents, this interval of the late Mesoproterozoic was characterized by rapid paleogeographic change. Key Points Paleomagnetic data reveal progressive directional change through the Osler Group Rapid plate motion was ongoing during the early magmatic stage of rifting Plume activity was ongoing in Laurentia and elsewhere