Serpentinization, deformation, and seismic anisotropy in subducted terranes

Antigorite is a hydrous layer silicate with strongly anisotropic seismic and rheological properties. Hydrous minerals such as antigorite have been widely invoked to explain numerous geologic observations within subduction zones including: intermediate-depth earthquakes, arc volcanism, the persistent weakness of the subduction interface, trench-parallel S-wave splitting, and episodic tremors and slip. To understand how the presence of antigorite-bearing rocks affect observations of seismic anisotropy, three mylonites from the Kohistan paleo-island arc in northern Pakistan were analysed using electron backscatter diffraction (EBSD) and the resulting data were used to model seismic anisotropy. A fourth sample, which displayed optical evidence for crystallographically controlled replacements of olivine, was also investigated using EBSD to identify potential topotactic relationships. The resulting data were used to model how seismic anisotropy in antigorite-rich rocks influences bulk seismic properties. The mylonite samples exhibit incredibly strong bulk anisotropy (10-20% for the antigorite + olivine). Within the nominally undeformed protomylonite, two topotactic relationships were observed: 1. (010)ant//(100)ol with [100]ant//[001]ol and 2. (010)ant//(100)ol with [100]ant//[010]ol. However, the strength of a texture formed by topotactic replacement is markedly weaker than the strength of the textures observed in mylonitic samples. Since antigorite is thought to be rheologically weak, it is hypothesized that microstructures formed from topotactic reactions will become progressively overprinted as deformation is localised in regions with greater volume fractions of serpentine. Regions of highly sheared serpentine have a significant potential to influence seismic wave speeds in subduction settings. The presence of deformed antigorite in a dipping structure is one explanation for observations of both the magnitude and splitting pattern of seismic waves in subduction zones