Shear wave splitting observations in the Archean craton of Western Superior

Shear wave splitting observations iri the Archean Superior Province of the Canadian Shield show moderate to large delay times (1.1-2.1 s) with azimuths suggesting upper-mantle anisotropy is subparallel to the structural grain of the craton. Two regions with uniform anisotropy azimuth (62 degrees +/- 7 degrees and 87 degrees +/- 3 degrees) are Separated by a transitional zone showing strong dependence of observed azimuth on source direction, suggesting that lateral structure is being observed. No splitting was observed at stations near the coast of Hudson Bay, continuing a pattern of weak splitting in surrounding Proterozoic orogenic belts. Current absolute plate motion is consistent with the direction of anisotropy, but would not explain the regional contrasts. Instead, the anisotropy appears to be related to Archean structure and tectonic history.