Seismic velocity and anisotropy in mylonites and the reflectivity of deep crystal fault zones

Compressional wave-velocity anisotropy has been measured to 1 kb confining pressure in several mylonites, in one cataclasite, and in their protoliths. No significant difference in the velocity or density was found between the protolith and the tectonically altered rock. At 1 kb, strong seismic anisotropy (greater than 10%) was present in only two of nine mylonites measured. The anisotropy was due to their aphanitic texture, high content of layered silicates, and pervasive foliation. Most mylonitic rocks studied were not strongly anisotropic despite a well-developed macroscopic foliation. Where anisotropy was present, velocities usually were slowest perpendicular to the foliation and fastest parallel to both foliation and lineation. These preliminary results suggest that ductile fault zones in the crust are not strong seismic reflectors unless mylonites in the fault zone are aphanitic and contain large amounts of foliated phyllosilicates. It is also possible that anomalous environmental conditions such as high pore pressure in the fault zone in situ may explain observed seismic reflectivity.