The role of crystallographic fabric in the generation of seismic anisotropy and reflectivity of high strain zones in calcite rocks

For a relatively pure calcite ultramylonite (grain size: 5 μm) from the Ivrea-Verbano zone of the the N.W. Italian Alps, the crystallographic preferred orientation was measured by X-ray texture goniometry. The fabric is characterized by a 3 times uniform, axisymmetric c-axis maximum, normal to the mesoscopic foliation. From this and available single crystal elastic properties of calcite, the expected variation of acoustic (P-wave) velocity with orientation can be calculated. This is compared with high pressure (up to 520 MPa at 20 °C) ultrasonic velocity measurements made on seven different core orientations from the same rock, and good agreement obtained between the calculated and measured velocity distribution. Seismic velocity contrasts range up to 3.3% relative to an isotropically-textured protolith, sufficient to give rise to detectable seismic reflectivity. For normal incidence P-waves on a horizontal shear zone, crystallographic texture of the type observed maximises the obtainable seismic reflectivity. For a c-maximum fabric oriented obliquely to the plane of the high strain zone (as is observed in some other natural shear zones), or for a gently dipping shear zone, reflectivity may be either enhanced or weakened. Thus, visibility may be very sensitive to the local dip of a shear zone and the fabric orientation and intensity within it.