Crystallographic Preferred Orientation of Amphibole Experimentally Deformed in Simple Shear

Seismic anisotropy of the intermediate to lower crust can be attributed to the crystallographic preferred orientation (CPO) of deformed amphibole because of its high anisotropy and abundance in the deep crust. However, the lack of in-depth studies on simple-shear experiments of amphibole has obstructed the view of the CPO of amphibole to explain the intricate flow patterns in the crust. Therefore, we conducted simple-shear deformation experiments of amphibolite at the pressure of 1 GPa and at the temperatures of 500 - 700°C. Starting material is a fine-grained natural amphibolite which consists of hornblende (70 %), anorthite (30 %), and minor ilmenite and titanite (~2 %). Sample was deformed to large shear strain with a strain rate of 10^-5 to 10^-4 s^-1 by using a modified griggs apparatus installed at Seoul National University (SNU). Microstructure of the deformed amphibolite was observed by scanning electron microscope, and the CPO of hornblende was analyzed by electron backscatter diffraction (EBSD) technique with Channel 5 software. Deformed amphibolites showed strong grain-size reduction by fracturing with strain localization, suggesting that the dominant deformation mechanism was cataclastic flow accompanied by mechanical rotation of grains. We found three types of CPOs of hornblende: type-I, type-II, and type-III. All fabric types showed (100) plane aligning nearly parallel to the shear plane. Type-I fabric represented that [001] axes are aligned subparallel to the shear direction. Type-II fabric represented that (010) poles are aligned subparallel to the shear direction. Type-III fabric represented that (010) poles and [001] axes form a girdle nearly parallel to the shear plane. Type-I fabric was produced at low temperatures (500 - 550°C), but type-II and type-III fabrics were developed at higher temperatures (600 - 700°C) in a dry and wet condition, respectively. Seismic velocities and anisotropies corresponding to each CPO type were calculated. The P-wave velocity of amphibole was up to 7.24 km/s, and the P- and S-wave anisotropies were up to 14.6 and 13.0 %, respectively, indicating that the CPO of amphibole can be a major contributor for strong seismic anisotropy in the intermediate to lower crust as previous studies of naturally deformed amphibole suggested. The P-wave anisotropy depends on the fabric type of amphibole. For vertically propagating S-wave, the fast S-wave anisotropy with a dipping angle of 45° depicts that its polarization direction is nearly normal to the flow direction with much higher anisotropy (up to ~9 %) than that (less than 1 %) in horizontal shear. This result suggests that trench-parallel seismic anisotropy at the subduction channel where amphibole is available can be attributed to the CPO of amphibole.