Speaker: Assistant Professor Nick Swanson-Hysel from University of California, Berkeley

Title: "Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis"

Abstract: The late Mesoproterozoic was a time of large-scale tectonic activity both in the interior and on the margins of Laurentia—most notably the development of the Midcontinent Rift and the Grenvillian orogeny. The development of the North American Midcontinent Rift between ca. 1110 and 1080 million years ago has provided an opportunity to develop extensive paleomagnetic data sets spanning this time period. These data result in an apparent polar wander path for Laurentia that goes from a high latitude apex known as the Logan Loop into a swath known as the Keweenawan Track. A long-standing challenge of these data was the appearance of asymmetry between relatively steep reversed polarity directions from older rift rocks and relatively shallow normal polarity directions from younger rift rocks. This asymmetry was used to support an interpretation that there were large non-dipolar components to the geomagnetic field at the time. Recent data sets support the interpretation that this directional change was progressive and therefore a result of very rapid motion of Laurentia from high to low latitudes rather than a stepwise change across non-dipolar reversals. New high precision U-Pb dates and paleomagnetic data from Midcontinent Rift volcanics combined in a volcanostratigraphic context allows us to constrain the rate of implied plate motion more precisely than has previously been possible. We apply a novel Bayesian approach to assess the rate of implied plate motion through inverting for paleomagnetic Euler poles. If the path is to be explained by a single Euler pole these inversions reveal that motion of the continent exceeded 27 cm/year (for reference, the most rapid motion of a plate with substantial continental lithosphere in more recent Earth history is India's motion of ~17 cm/year). The path is particularly well-explained by a model wherein there is continuous true polar wander in addition to rapid plate motion that changes direction and slows at ca. 1096 Ma. Laurentia's movement from high to low latitudes resulted in collisional tectonics on its leading margin which could be associated with such a change in plate motion. We propose that upwelling of the Keweenawan mantle plume was associated with an avalanche of subducted slab material with downwelling that drove fast plate motion. This fast plate motion was followed by the Grenvillian orogeny from ca. 1090 to 980 million years ago. Prolonged collisional orogenesis could have been sustained due to this strong convective cell that therefore played an integral role in the assembly of the supercontinent Rodinia.[Background Reading]