Lithospheric variations across the Superior Province, Ontario, Canada: Evidence from tomography and shear wave splitting

Résumé 0

The Superior Province of the Canadian Shield is the largest contiguous region of the Archean crust. A combination of data from multiple experiments is used to obtain shear wave splitting parameters and a three-dimensional tomographic velocity model beneath a large portion of the Superior, corresponding approximately to the province of Ontario. Shear wave splits are obtained at 24 sites across the Superior, displaying strong (averaging 1.34 s) ENE-WSW splitting at stations west of 86°W and weaker (0.67 s) E-W splitting in the east. The fast direction is subparallel to both absolute plate motion and tectonic belt boundaries. The recovered tomographic velocity model shows a major boundary oriented NNW-SSE, separating high velocities in the western Superior (WS) from low velocities in the east and coinciding with the divide between weak and strong shear wave splits. Other features include a 200-km-thick low-velocity anomaly corresponding to the Nipigon Embayment, a 1.0-Ga failed-rift branch; and a linear low-velocity anomaly in the east, attributed to the Great Meteor hot spot track. The Nipigon anomaly, in the western portion of the model, is probably in situ, while the Great Meteor track is displaced from crustal features associated with the hot spot. We interpret this displacement as evidence that the eastern lithosphere has been deformed by basal drag, while the western lithosphere has remained stable, and propose that the east-west lithospheric boundary we have detected represents a change in mechanical properties, between stronger, higher velocity western material with consistent anisotropic fabric, and weaker eastern material with more variable fabric.