LITHOSPHERIC AND UPPER-MANTLE FABRIC ACROSS THE CANADIAN HIGH ARCTIC: EVIDENCE FROM SHEAR-WAVE SPLITTING

The Canadian High Arctic is a complex region that preserves over 3 billion years of the geological record, from Archean cratons to Eocene orogenic belts. Significant episodes of deformation, including rifting, basin formation and mountain-building events, have affected the lithosphere, and resulting structural fabric is thought to be preserved as “fossil” seismic anisotropy. Present-day mantle dynamics such as basal shear of the lithosphere in response to plate motion, or active flow of the underlying convective mantle, may also contribute to measurable anisotropy.

To investigate seismic anisotropy beneath the Canadian High Arctic, we perform shear wave splitting measurements of core-refracted shear phases recorded at 11 seismic stations situated across the region. 5 of the stations are long-term deployments (5 to >20 years) of the Canadian National Seismograph Network (CNSN) and Global Seismographic Network (GSN), and the other 6 were deployed in a NS line along Ellesmere Island by the ELLITE (Ellesmere Island Lithosphere Experiment) project, recording data for periods of up to 2 years.

Inspection of the dominant fast-polarisation orientations estimated for the region shows a strong correlation with regional surface tectonic structures, suggesting that “fossil” lithospheric anisotropy, with vertically-coherent deformation between crust and lithospheric mantle, is a significant contributor to the measured seismic properties.

The longest-running seismic stations have a sufficiently large range of measurements (mainly from southerly and west-to-north directions) to show systematic back-azimuthal variation in splitting parameters. Although the gaps in azimuthal coverage preclude a detailed analysis, the data set provides clear evidence that there is more than one anisotropic layer beneath the High Arctic regions. A likely explanation is the combination of “fossil” lithospheric anisotropy with that caused by present-day sublithospheric shear and/or mantle flow.