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Keywords:

  • continental lithosphere;
  • polarization;
  • Rayleigh waves;
  • seismic anisotropy;
  • shear wave splitting;
  • Slave craton

SUMMARY

Teleseismic events recorded at a 25-element array in NW Canada between 2001 and 2006 provided sufficient distribution in back azimuth to demonstrate birefringence in SKS and SKKS waves as well as directional dependence of Rayleigh-wave phase velocities. Typical delays between orthogonally polarized SKS waves are 0.8–1.2 s, and modelling of azimuthal dependence indicates two nearly horizontal layers of anisotropy within the mantle. Anisotropy of Rayleigh waves is generally consistent with models of layered Vs anisotropies that increase with depth from 1 per cent at the Moho to 9 per cent at 200 km but vary between subarrays. Consistency between the SKS and Rayleigh wave anisotropies in one subarray suggests that the assumption of symmetry about a horizontal axis is valid there but is not fully valid in other parts of the craton. The upper layer of anisotropy occupies approximately the uppermost 120 km in which the fast polarization direction strikes generally north–south, coinciding with regional-scale fold axes mapped at the surface. The fast polarization direction of the deeper layer aligns with current North America plate motion, but its correlation with trends of coeval kimberlite eruptions within the Lac de Gras field suggests it can be at least partly attributed to structural preferred orientation of vertical dykes inferred to exist to depths of 200 km.