Deformation and Anisotropy of Hydrous Silicates

Abstract:

Hydrous silicates such as talc and antigorite have highly anisotropic seismic and rheological properties. It is thought that these minerals may influence several geologic phenomena of subduction zones, including shear localization, intermediate-depth earthquakes, and shear-wave splitting. To understand the feedbacks between serpentinization, deformation, and seismic anisotropy, we analysed microstructures from a suite of antigorite-bearing samples from the Kohistan paleo-island arc in Pakistan. Weakly deformed samples display evidence for crystallographically-controlled growth of antigorite after olivine. Two distinct relationships were found: (1) (010)ant//(100)ol with [100]ant//[001]ol and (2) (010)ant//(100)ol with [100]ant//[010]ol. However, this topotactic replacement produces a bulk texture with only modest seismic anisotropy. In contrast, highly deformed samples, in which the serpentine was sheared to produce a strong LPO, were found to possess extremely strong bulk anisotropy (P-wave and S-wave anisotropy up to ~25% and 31%, respectively). Moreover, these microstructures would be capable of generating strong trench-parallel S-wave splitting when the sheared interface is dipping, or otherwise oriented at a high angle to an incident S-wave. Given the apparent rheological weakness of antigorite, we hypothesise that progressive deformation will localise in areas with higher modal percentages of antigorite, thus overprinting any original topotactic signature in the rocks. These regions of highly deformed antigorite then have the capability to substantially affect seismic wave patterns in a subduction zone. Mylonitic antigorite in a dipping structure could explain some of the observations of large-magnitude S wave splitting in subduction zones.