THE EFFECT OF ORIENTED MICROCRACKS ON SEISMIC VELOCITIES IN AN ULTRAMYLONITE

The influence of oriented microcracks on the directional dependence of P- and S-wave velocities has been investigated in an epidote-muscovite-ultramylonite. Optical and SEM studies revealed that the fabric of the mylonite is characterized by oriented intragranular and grain-boundary cracks. Comparison of orientation patterns of cracks with calculated elastic properties (Voigt average) of a model rock (isotropic matrix, ultramylonite crack geometry) and laboratory seismic data indicates that wave propagation is particularly affected by oriented grain-boundary cracks. Microcracks are most effective at low pressure and largely closed above about 200-300 MPa. The residual part of velocity anisotropy observed above this crack-closing pressure is mainly due to preferred mineral orientation, in particular of mica. Maximum concentration of poles to (001) mica planes corresponds to the maximum of poles to oriented grain-boundary cracks so that the effects of texture and oriented cracks on wave velocities line up, giving rise to extreme values of velocity anisotropy and shear-wave splitting at low-pressure conditions.