The role of hornblende in deep crustal seismic anisotropy: an investigation of the importance of deformation mechanisms

Abstract:

Mineral deformation plays a key role in the creation of seismic anisotropy, an important tool used to understand the structure and nature of deep continental crust. Hornblende is a common anisotropic mineral that may account for much of the seismic anisotropy observed at deep crustal levels. Recent studies demonstrate a large range in the amount of seismic anisotropy produced by amphibole-rich tectonites (P-wave anisotropy values from 3-17%) without a systematic correlation between amphibole mode and anisotropy magnitude. We investigate this variation by studying two exhumed deep crustal shear zones where tectonites have similar mineralogy and foliation strength but contrasting anisotropy magnitude.

The structures are the km-scale Grease River shear zone (SZ1) in the Athabasca granulite terrane in northern Saskatchewan and a series of cm-scale shear zones (SZ2) in and near Gallatin Canyon in SW Montana. SZ1 reworked a deep crustal meta-granodiorite at conditions of 0.7-0.5 GPa and 600-700 °C, overprinting an earlier developed granulite facies assemblage and deformation fabric. SZ2 structures are characterized by strain gradients from undeformed meta-gabbro with igneous textures to a laminated ultramylonite at metamorphic conditions of uppermost amphibolite facies (1.1 GPa, 800° C). Both structures developed during fluid infiltration and involved significant growth and subsequent recrystallization of new hornblende, from less than 10% in the host rock to near 50% in the mylonite, at the expense of earlier pyroxene-rich assemblages. SZ1 mylonite has a strongly developed hornblende crystallographically preferred orientation (CPO) resulting in a moderately high P-wave anisotropy of 6.7%, whereas SZ2 ultramylonite exhibits a weak CPO and a low P-wave anisotropy of 2.1%, despite containing a well-developed hornblende shape-preferred orientation. We hypothesize that these contrasting microstructural and anisotropy patterns are due to deformation achieved through different hornblende deformation mechanisms, with dislocation creep dominant in SZ1 and dissolution-precipitation creep dominant in SZ2. We investigate factors influencing the dominant deformation mechanism including fluid composition, metamorphic grade, amphibole chemistry, strain rate, and kinematic regime.