Résumé
The recent deployment of temporary seismic stations across northwestern Canada allows us to investigate the seismic velocity structure and dynamics of the crust and the uppermost mantle. We measure fundamental-mode Rayleigh-wave group velocity dispersion from regional earthquakes with magnitude > or = 4.5 that occurred between 1993 and 2019. We invert the dispersion data set using trans-dimensional Bayesian tomography in order to estimate azimuthally anisotropic group velocity maps at periods ranging from 10 to 60 s, as well as their associated uncertainties. Crustal seismic anisotropy (at periods < or = 20 s) shows a clear alignment (i.e., NW-SE direction) with the main tectonic structures throughout the northern Canadian Cordillera (NCC) with one noticeable exception in northern Yukon where the fast axis directions rotate to be oriented NE-SW. At periods predominantly sensitive to deep crustal and uppermost mantle structure (> 40 s), the NW-SE alignment of fast axis directions is still present in the southern region of the NCC and is overprinted by a NE-SW alignment in the northwestern region of the NCC. We subsequently invert our 2-dimensional group velocity maps to obtain the pseudo-3-dimensional shear-wave velocity (Vs) structure across northwestern Canada. Our results show that the NCC is underlain by a heterogeneous seismic velocity structure from the surface to 100 km depth. We observe two slow Vs anomalies at mid-to-lower crustal depths (20-30 km depth) beneath the northern region of the Mackenzie Mountains and south of the Denali Fault in the Gulf of Alaska. At uppermost mantle depths, we find a fast Vs anomaly across northern Yukon, potentially reflecting a mechanically strong, relatively cold and resistant lithospheric mantle in the region, which is consistent with the hypothesis of the buried Mackenzie craton (the westward extent of the North American craton in the NCC). These results have implications for the dynamics and history of the region.