THE PHASE-VELOCITY SIGNATURE OF LITHOSPHERIC DEFORMATION IN CENTRAL CANADA AND THE NORTH-CENTRAL UNITED STATES

The Superior province is composed of Archean craton bounded by orogens, including the Trans-Hudson to the north and west, the Penokean to the south, and the Grenville and later Appalachian-forming orogenies to the east. Additional modification resulted from the Mid-Continent Rift (MCR) and Great Meteor Hotspot. We examine the effects of this old deformation on the continental lithosphere by taking advantage of the available seismic networks, including the USArray Transportable Array stations to the south, permanent and temporary Canadian stations to the north, and SPREE stations, to do a new regional surface-wave study. We select 80 stations with roughly 200 km spacing, resulting in dense path coverage of a broad area centered on the Great Lakes. We use teleseismic data for all earthquakes from January 2005-August 2016 with a magnitude greater than 6.0, amounting to over 1200 events, and we make Rayleigh wave two-station dispersion measurements for all station pairs with suitable event-station geometry. We invert these measurements for anisotropic phase-velocity maps at discrete periods between 20-200 s, yielding information not only on the wave speed but also the current fabric of the lithosphere, a complicated record of strain from formation, through modification from orogeny, attempted rifting, and hotspot interaction, to present day plate motion.

At shorter periods, we observe slow velocities in the areas of the Trans-Hudson and Appalachian orogenies, in sharp contrast with the fast velocities of the Superior province. We observe a clear signature of the MCR, with the slowest phase-velocity anomaly in the region aligning with the strongest gravity anomaly. At increasing periods, and thus greater depths, this slowest anomaly shifts to beneath the center of Lake Superior (30-40 s), and then further north to the Nipigon Embayment. Next to this strong anomaly, the Penokean does not show a definitive signature. At longer periods, low velocities are observed in the eastern/Appalachian area, and high velocities are found to the northwest, particularly under the western Superior, where lithospheric thickness is thought to be up to 200-250 km. In addition to these large-scale features, we observe complex small-scale structure in isotropic and anisotropic phase velocity throughout the region.