Abstract

We examine upper-mantle seismic anisotropy beneath the cordillera and craton of northwestern Canada using the multi-event station averages of shear-wave splitting of SKS, SKKS, and sSKS phases recorded during the Canadian Northwest Experiment (CANOE). Splitting times derived from multi-event averaging at each station range from 0 to 1.4 s, with an array average of ∼0.65 s. Over broad portions of the array, fast directions are coherent and roughly consistent with the direction of absolute plate motion in a hotspot reference frame, suggesting that coherent asthenospheric fabric underlies the plate in much of the region. Within this broad framework, fast directions and splitting times show marked variability over length scales of 50–200km, and are generally correlated with surface and/or crustal tectonics. Anomalous splitting is observed across an ancient suture within the cratonic lithosphere, apparently associated with complex dipping fabric produced during continental assembly. The deformation front of the Canadian Rockies correlates with a significant change in splitting behavior, consistent with the front range demarking the craton-cordillera transition within the mantle. Splitting times are small across much of the cordillera, indicating that lithospheric and/or asthenospheric fabric is weaker or less coherent than beneath the craton. In the western cordillera, fast directions rotate abruptly to parallel the plate boundary, implying that fabric associated with plate-boundary deformation extends ∼200km into the North American continent.

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