Fabric and seismic properties of Carrara marble mylonite

Shear deformation in calcite-rich rocks can produce strong lattice preferre d orientations (LPO), which result in a high anisotropy of bulk seismic pro perties because of the high elastic anisotropy of calcite (32% Vp anisotrop y). Deformed rocks often show also strong shape preferred orientations (SPO ). Theories for averaging the elastic properties have not yet satisfactoril y predicted the contribution to the seismic anisotropy caused by the SPO al one. A calcite mylonite from Carrara (Italy) was investigated, which is characte rised by a strong SPO and a weak LPO. It was composed of about 80% calcite, then white mica, quartz and hematite. Flattening of mica and of calcite gr ains defined the mylonitic foliation, and elongation of calcite grains defi ned the lineation. On average calcite grains have aspect ratios of about 2. 5:1.6:1, and grain sizes of about 10 mu m At 400 MPa confining pressure, th e measured Vp (km/s) parallel to the lineation (X direction) was highest (6 .63), lower in the intermediate Y direction (6.47); the Vp normal to the fo liation (Z direction) was lowest (6.30). This yielded a Vp anisotropy of 5% . The LPO, determined by automated electron backscatter diffraction (EBSD), was very weak (texture index 1.1), with intensities between 0.6 and 1.6 m. r.d. in the c-axis pole figure. Extrapolation of the texture index to an in finite number of orientation measurements indicated that the observed varia tions were mostly random noise in the orientation distributions and that th e bulk rock texture was random. The Vp anisotropy of the Voigt, Reuss and H ill averages calculated from this calcite LPO is predicted to be close to z ero. Adding 5% of muscovite with (001) perfectly aligned parallel to the fo liation, we calculated a total anisotropy of 2.8%. The anisotropy calculate d for the special directions X, Y and Z remained at 2.6% only. It was concluded that the measured seismic anisotropy cannot be explained b y the LPO of calcite and by 5% of mica alone. It is also attributed to the strong SPO and to further grain boundary effects. (C) 2000 Elsevier Science Ltd. All rights reserved.