Serpentinized ultramafic rocks and chloritized mafic rocks recovered during ODP Leg 149 from the Iberia Abyssal Plain were examined for shear-wave anisotropy by measuring the difference in traveltime of shear waves propagating with different particle displacement orientations. The rocks are thought to represent upper mantle through lower crust exhumed during the late stages of rifting. A series of measurements was taken for shear waves propagating in the horizontal direction, with particle displacement directions varying in 10° increments from 0° to 360° in the vertical plane. Shear-wave velocity in dry samples at atmospheric pressure ranges from about 1320 to 1800 m/s in serpentinized peridotite and from about 1625 to 2000 m/s in metamorphosed mafic rocks. Shear-wave anisotropy ranges from <1% to >25% and shows a systematic increase with the intensity of veining and foliation. The greatest anisotropy occurs in strongly veined samples, with little difference observed between the amount of anisotropy in strongly foliated ultramafic rocks and moderately veined ultramafic rocks. A single sample of strongly foliated mafic rock was found to be <2% anisotropic. In the anisotropic samples, the slowest velocities were measured for waves with particle displacement perpendicular to the dominant foliation or vein direction. The fastest velocities were measured in waves with particle displacement parallel to the vein or foliation orientation. No evidence was found for anisotropy resulting from preferred grain orientation within the rock matrix, although mineral grain orientation parallel to vein or foliation orientation can not be ruled out as a possible contributor to anisotropy. In samples containing both foliation and veining, both structures contribute to anisotropy. However, the relative importance of foliation and vein orientation to anisotropic properties varied with different samples, with no obvious systematic relation between the dominant fabric and macroscopic features or composition.
Date of initial receipt: 1 December 1994
Date of acceptance: 22 May 1995
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