Effects of Shear Zone Development on Seismic Anisotropy in the Lower Grenvillian Crust

Abstract:

Deep crustal structure, particularly the geometry of shear zones, affects the degree of crust-mantle coupling and the kinematics of crustal deformation. In principle, shear zones in the deep crust can be visible using seismic imaging due to the change in the orientations and modes of anisotropic minerals. However, matching the seismic signals to structures present remains a challenge. This work seeks to bridge some of that gap.

We utilize the Parry Sound domain in the western Central Gneiss Belt of the Grenville orogen, Ontario, Canada, to develop quantitative relationships between geologic structures and seismic anisotropy. This region provides excellent examples of granulite and amphibolite facies shear zones up to several km wide. We investigated three rock types: (1) regionally deformed mafic and felsic granulite facies orthogneiss, (2) granulite facies shear zones, and (3) amphibolite facies shear zones. Both of the latter two derived from (1). Using the numerical architecture of asymptotic expansion homogenization (which considers grain-scale elastic interactions), we computed much higher precision seismic velocities than is possible with conventional Voigt-Reuss-Hill algorithms. In all sheared felsic rocks, the dominant quartz slip system was prism <a> + rhomb <a> indicating slowest Vp direction paralleled lineation because in quartz a-axis is near the slowest direction. In contrast, in all sheared mafic rocks, the fastest amphibole direction is strongly parallel to the lineation. As a consequence of combining the quartz and amphibole deformation, rocks comprising felsic and mafic layers have a weak seismic anisotropy. In monolithological shear zones, anisotropy can exceed 10%. Despite the promise this work illustrates, we must continue to consider the influence of inherited fabrics in the host rock. In a second line of investigation, we explored how shear zone volume fraction affects seismic anisotropy.