Obliquity between seismic and electrical anisotropies as a potential indicator of movement sense for ductile shear zones in the upper mantle

Abstract

Teleseismic shear-wave splitting and magnetotelluric experiments across the Grenville front, between the Archean craton and the Proterozoic Grenville province in the regions of the Pontiac subprovince and northwestern Grenville province (Canada), show a consistent obliquity between the polarization direction of the fast split shear wave Φ and the most electrically conductive direction (ΦMT) in the upper mantle transcurrent shear zones. At all well-recorded stations, Φ is nearly N103°E, and ΦMT is approximately N80°E. The obliquity may be considered a potential kinematic indicator, because the seismic and electrical anisotropies are thought to be controlled by lattice-preferred and shape-preferred orientations of mantle minerals (mainly olivine), respectively. The dextral movement sense of the transcurrent shear zones in the mantle, inferred from the observed obliquity, is consistent with that inferred from surface geology of the crustal shear zones. This consistency implies that deformation of the crust and the subcrustal upper mantle in the lithosphere was largely coherent in the study region.