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

PUBLICATIONS Geochemistry, Geophysics, Geosystems RESEARCH ARTICLE 10.1002/2013GC005180 Special Section: Magnetism From Atomic to Planetary Scales: Physical Principles and Interdisciplinary Applications in Geo- and Planetary Sciences 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 Supporting Information: Readme IPython notebook with data and analysis Correspondence to: N. L. Swanson-Hysell, swanson-hysell@berkeley.edu Citation: Swanson-Hysell, N. L., A. A. Vaughan, M. R. Mustain, and K. E. Asp (2014), Conﬁrmation of progressive plate motion during the Midcontinent Rift’s early magmatic stage from the Osler Volcanic Group, Ontario, Canada, Geochem. Geophys. Geosyst., 15, 2039– 2047, doi:10.1002/2013GC005180. Received 3 DEC 2013 Accepted 23 APR 2014 Accepted article online 27 APR 2014 Published online 30 MAY 2014 Confirmation of progressive plate motion during the Midconti- nent Rift’s early magmatic stage from the Osler Volcanic Group, Ontario, Canada Nicholas L. Swanson-Hysell 1,2 , Angus A. Vaughan 1,3 , Monica R. Mustain 1,4 , and Kristofer E. Asp 1,5 Institute for Rock Magnetism, Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota, USA, Department of Earth and Planetary Science, University of California, Berkeley, California, USA, 3 Department of Geology, Carleton College, Northﬁeld, Minnesota, USA, 4 Department of Geology and Geography, Illinois State University, Normal, Illinois, USA, 5 Department of Geological Sciences, University of Minnesota, Duluth, Minnesota, USA Abstract 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 Mid- continent 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 6 2 Ma (during the ‘‘early stage’’ of rift develop- ment). Paleomagnetic poles can be calculated for the lower portion of the reversed Osler Volcanic Group (40.9 N, 218.6 E, A 95 5 4.8 , N 5 30) and the upper portion of the reversed Osler Volcanic Group (42.5 N, 201.6 E, A 95 5 3.7 , N 5 59; this pole can be assigned the age of ca. 1105 6 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. 1. Introduction Despite being active for more than 20 million years [Davis and Green, 1997] and resulting in the thinning of prerift crust to less than 10 km [Cannon, 1992], the 1.1 Ga Midcontinent Rift failed to dismember Laurentia (cratonic North America). This failure resulted in the preservation of a thick record of rift-related volcanic and sedimentary rocks that gives geoscientists insight into the development of this ancient rift. Most mod- els for the development of the Midcontinent Rift attribute its origin to the upwelling and decompression melting of a mantle plume [Shirey, 1997]. On the basis of the great volume of generated magma and inter- pretation of geochemical data, it is argued that the early stage plateau ﬂood basalts of the rift (ca. 1110– 1105 Ma) and the main stage volcanics that erupted into the central basin (ca. 1100–1095 Ma) were both dominated by plume-sourced melts. This deep-plume origin for the rift needs to be considered in conjunc- tion with paleogeographic change that has been inferred to have been ongoing throughout rift develop- ment. Fully constraining this paleogeographic change is essential for understanding rift development and for constraining late Mesoproterozoic paleogeographic reconstructions given the centrality of Laurentia’s apparent polar wander path to such efforts. It has long been noted that there is a signiﬁcant difference in paleomagnetic inclination between the steep (dominantly reversed polarity) magnetizations of the oldest volcanics and intrusives of the Midcontinent Rift and the shallower (dominantly normal polarity) magnetizations from the younger main stage volcanics and intrusives [Halls and Pesonen, 1982]. This inclination change has been interpreted either as resulting from rapid plate motion [Robertson and Fahrig, 1971; Davis and Green, 1997] or as being the result of large nondipolar contributions to the late Mesoproterozoic geomagnetic ﬁeld that led to asymmetry across rever- sals [Pesonen and Nevanlinna, 1981]. The interpretation of the record as recording stepwise inclination change across reversals associated with a signiﬁcant sustained departure from a geocentric axial dipole (GAD) dominated ﬁeld was challenged by the observation of a progressive decrease in paleomagnetic incli- nation across multiple geomagnetic reversals up through the succession of Midcontinent Rift lavas at Mamainse Point, Ontario [Swanson-Hysell et al., 2009, 2014]. This progressive decrease in inclination leads to the interpretation of multiple symmetric reversals when the data are considered in stratigraphic context SWANSON-HYSELL ET AL. C 2014. American Geophysical Union. All Rights Reserved. V