SEISMIC ANISOTROPY PATTERNS IN DEEP CONTINENTAL CRUST: EXAMPLES FROM AN EXHUMED HIGH PRESSURE TERRANE IN THE WESTERN CANADIAN SHIELD

Seismic anisotropy in deep continental crust is a complex product of mineralogy and deformation-induced crystallographic alignment. Commonly cited minerals that contribute to anisotropy in the crust include mica (muscovite and biotite) and hornblende. However, thermodynamic stability of these hydrous minerals is limited in many high-grade deep crustal environments. Furthermore, the mobility of hydrous fluids during the dynamic evolution of the crust in many orogens suggests that the preserved anisotropy state may vary widely in the crustal column.

We present examples of these interactions from the Athabasca granulite terrane, a laterally extensive exposure (>20,000 km²) of high-pressure tectonite in the western Canadian Shield. Particular focus will be on the nature of anisotropy in the lower crust and its evolution in localized km-scale shear zones during tectonic exhumation to the middle crust. Mylonitic lithologies inititially developed under relatively dry high-pressure granulite-facies (1.0 GPa, 800-900 °C) conditions and were locally converted to medium-pressure hydrous amphibolite-facies (0.5 GPa, 600-700 °C) tectonites during exhumation. Fabric studies of deformation mechanisms and CPO in sillimanite in aluminous felsic granulites and hornblende in charnockitic granitoids suggest that these minerals may be the primary contributors to anisotropy in these rocks under high-grade conditions. Both minerals have seismically fast directions subparallel to their crystallographic c axes, which commonly align in the extension direction in a strain field. Thus, the dominant symmetry of anisotropy in the deep crust may be orthorhombic. However, synkinematic hydrous retrogression of both of these lithologies during exhumation introduced mica to the assemblage, which is highly anisotropic with a fast plane perpendicular to the c axis. Modal increases in mica can induce a switch from orthorhombic symmetry to dominantly hexagonal, and the constructive nature of the aligned minerals results in increased anisotropy magnitudes from ~5% to as much as 15% or more in some lithologies. The interplay between deformation and metamorphic reactions, P-T conditions, and the availability of water all play critical roles in the degree of intrinsic seismic anisotropy that will be exhibited in the deep crust.