Structure and fabric of the crust and uppermost mantle in the northern Canadian Cordillera from Rayleigh-wave tomography

The seismic structure and fabric of the lithosphere and underlying mantle beneath the northern Canadian Cordillera provides important constraints on its evolution and current tectonics; however, it is poorly characterized due to historically sparse networks of seismic instruments. We use data from past and recently deployed networks of broadband seismic stations in northwestern Canada and measure Rayleigh waves propagating between all available pairs of seismic stations using two complementary techniques: ambient noise and teleseismic two-station interferometry. The Rayleigh-wave data are processed to obtain phase velocity dispersion curves that are inverted for phase velocity maps at periods between 8 and 80 s. To first order these maps show high velocity anomalies within the Canadian Shield and low velocity anomalies within the Cordillera at all periods. At short periods (<30 s; mostly sensitive to crustal depths), we observe high velocity anomalies within the allochtonous accreted terranes, and low velocity anomalies bounded by autochtonous sedimentary rocks forming the fold-and-thrust belt. At longer periods (>30 s; mostly sensitive to uppermost mantle depths), high velocity anomalies of the Canadian Shield extend west past the Cordilleran Deformation Front and suggest the presence of cratonic lithosphere beneath the Cordillera, whereas the lowest velocities underlie the allochtonous terranes. Anisotropy within the crust and uppermost mantle exhibits fast-axis orientations aligned with the major faults and fabric of the Cordillera, and show evidence for vertical changes in anisotropy. These results provide new constraints on geodynamic models proposed to explain neotectonic deformation in this area.