Surface-Wave Tomography of the Northern Canadian Cordillera Using Earthquake Rayleigh Wave Group Velocities

We measure fundamental-mode Rayleigh wave group velocity dispersion from regional earthquakes and employ transdimensional Bayesian tomography to invert for azimuthally anisotropic group velocity maps at 10-60 s periods. Seismic azimuthal anisotropy within the crust and uppermost mantle shows fast-axis alignment with principal northern Canadian Cordillera (NCC) tectonic structures suggesting large-scale and coherent deformation across the crust and upper mantle. We extract group velocity dispersion curves at each grid point of the isotropic group velocity maps and invert them independently for V-S as a function of depth in a probabilistic framework. At mid-to-lower crustal depths, our pseudo 3-D V-S model reveals an anomalously low V-S structure (<3.8 km/s) across the NCC, which likely reflects elevated crustal temperatures that buoyantly support regional high elevations. Two high-velocity regions (>= 4.5 km/s) at uppermost mantle depths underpin the edges of the arcuate Mackenzie fold and thrust belt. We observe variations in the dip direction of the high-velocity craton edge in the upper mantle, from subvertical in the northern NCC to westward in the southern NCC. We interpret a low-velocity region (<= 4.3 km/s) between the Tintina and Denali faults at uppermost mantle depths to reflect upwelling asthenosphere, which likely postdates 430 km of previously estimated Eocene lithosphere-scale horizontal displacement along the Tintina fault. Our surface-wave model resolves a high-velocity region (>= 4.4 km/s) dipping north-northeast below the Wrangell volcanic field, which we interpret as a down-going segment of the Pacific slab and may contribute to the observed volcanism in the area.

Automated summary

This study measures fundamental-mode Rayleigh wave group velocity dispersion from regional earthquakes and reveals azimuthally anisotropic structures in the crust and upper mantle of the northern Canadian Cordillera (NCC) using transdimensional Bayesian tomography. The results show coherent deformation with fast-axis alignment along major tectonic structures in the NCC. Anomalously low V-S structures and high-velocity regions at different depths in the crust and uppermost mantle are observed, providing insights into the thermal and tectonic processes in the region. Variations in the dip direction of high-velocity regions and the presence of low-velocity regions between faults are interpreted as reflecting different geodynamic processes beneath the NCC.