Shear‐wave splitting observations in the lower Great Lakes region: Evidence for regional anisotropic domains and keel‐modified asthenospheric flow

Fast‐axis directions obtained from shear‐wave splitting analysis of core‐refracted shear waves reveal several distinct anisotropic domains in the lower Great Lakes region. We used data from 27 broadband seismograph stations extending across a low‐velocity anomaly in the lithospheric mantle. Observed splitting times vary from 0.4 to 1.4 s, with a mean of 0.7 s. A subset of the splitting vectors across a failed rift are oriented parallel to one arm of the rift zone; most others show a coherent pattern of fast directions close to the direction of plate motion, but oblique to surface tectonic belts. Within the area of the low‐velocity anomaly, our observations are most simply explained by single‐layer anisotropy induced by asthenospheric flow. Spatial variability in the direction and magnitude of splitting are consistent with a flow regime influenced by basal topology of the lithospheric keel.