NEW GEOCHRONOLOGIC AND PALEOMAGNETIC CONSTRAINTS ON MIDCONTINENT RIFT DEVELOPMENT

There is a storied and fruitful history both of paleomagnetic and geochronological study of Keweenawan magmatic rocks from the Midcontinent Rift of North America. This work has provided the foundation for lithostratigraphic and chronostratigraphic correlation of units across the rift and has formed the central record for reconstructions of late Mesoproterozoic paleogeography during assembly of the supercontinent Rodinia. Paleomagnetic data from the rift have led to a series of paleomagnetic poles that form an apparent polar wander path known as the "Logan Loop'' at its apex and the "Keweenawan Track'' as it continues to younger poles. These data indicate that Midcontinent Rift development spanned an interval of more than 25 million years during which Laurentia was rapidly moving towards the equator. We will present new high-precision U-Pb geochronology based on weighted mean ²⁰⁶Pb/²³⁸U zircon dates from the Osler Volcanic Group, the North Shore Volcanic Group, and the Portage Lake Volcanics. The precision on these U-Pb dates gives a fresh perspective on the correlation of successions of extrusive volcanics across the rift as well as on the timing of geomagnetic reversals. Together with compiled and newly developed paleomagnetic data, these dates allow for tighter constraints on the rate of Laurentia's motion. A novel Bayesian approach for fitting these data with an Euler pole, which utilizes both the uncertainty on pole position and age, leads to interpreted plate rates of 21.6 cm/year (95% credible interval of 19.3 to 24.2 cm/year) throughout the time interval of rift development. In total, the thick magmatic products of the rift reveal that Laurentia traversed more than 40º of latitude throughout rift development.