Lithospheric anisotropy structure inferred from collocated teleseismic and magnetotelluric observations: Great Slave Lake shear zone, northern Canada

Abstract

Accurate interpretation of SKS shear-wave splitting observations requires inherently indeterminate depth information. Magnetotelluric electrical anisotropies are depth-constrained, and thereby offer possible resolution of the SKS conundrum. MT and teleseismic instruments, deployed across the Great Slave Lake shear zone, northern Canada, investigated lithospheric anisotropy and tested a hypothesis that seismic and electrical anisotropy obliquity can infer mantle strain shear-sense. Lithospheric mantle MT strike (N60°E) differs significantly from crustal MT strike (N30°E). SKS splitting vectors outside the shear zone exhibit single-layer anisotropy with fast axis parallel to upper-mantle MT strike and oblique to present-day plate motion (N135°W). Back-azimuth sensitivity at sites within the ∼ 30 km wide shear-zone imply more complex layering, with two-layer inversion yielding an upper layer of ∼N20°E and a lower layer of ∼N66°E. The MT data help to constrain the depth location of SKS anisotropy and, taken together, support a model of fossil lithospheric anisotropy. © 2004 by the American Geophysical Union.