The variation in shear-wave velocity with the azimuth of particle displacement (in the vertical plane) indicates that both foliation and veining contribute substantially to acoustic anisotropy in mafic and ultramafic rocks recovered from beneath the Iberia Abyssal Plain. Little evidence was found for anisotropy attributable to preferred mineral orientation in rocks that do not show significant foliation. However, it is possible that anisotropy in samples with significant veining may partially result from preferred mineral orientation parallel to the vein orientation. The vein and mineral orientations would produce identical azimuthal variations in velocity in this case. Therefore, we were not able to use the experiments to distinguish between the two if they have the same orientation. Similarly, we were unable to determine from the experiments the extent to which microcracks in the samples contribute to the observed anisotropy. Microcracks develop as a result of dilatation when the sample is decompressed during recovery. Pressure-dependent velocity changes attributed to closure of microcracks are commonly observed in crystalline rocks at pressure below about 100 MPa (Christensen and Wepfer, 1989). Because microcrack orientation may be controlled by mineral-grain orientation and/or foliation and vein fabric, it is not possible to distinguish between anisotropy arising from microcracks and anisotropy produced by structural fabric.
At Hole 897D, the amount of anisotropy observed in horizontally propagating shear waves increases in a systematic way with increasing degree of veining, from less than 5% in rocks containing only a few thin veins to 25% in strongly veined rocks. The fast direction of propagation is in directions in which particle displacement is oriented parallel to veining. At Hole 899B, anisotropy also increases systematically with the intensity of veining, ranging from less than 1% in vein-poor clasts from the brecciated peridotite unit to 6% in rocks with minor fracturing and vein filling and in mylonitized clasts. Little difference is seen between the degree of anisotropy in mylonitized rocks and those with minor veining. At Hole 900A, the degree of anisotropy also varies systematically with the degree of foliation or veining, but seems to depend strongly on the nature of the vein material, the type of foliation, and the sequence of polyphase deformation common in the recovered rocks. A strongly foliated chloritized breccia clast (Sample 149-900A-85R-4 [Piece 10]) shows little evidence of significant anisotropy. Contrarily, a moderately foliated clast (mafic protolith) containing abundant calcite veins shows a strong anisotropy (6%). The direction of fastest wave propagation appears to be controlled by the foliation orientation, not the vein orientation. However, rays with particle motion at a high angle to the vein orientation are substantially delayed, indicating that vein orientation may more strongly affect the propagation speed than the foliation in this narrow range of azimuths. Finally, a sample with similar intensity of foliation (Sample 149-900A-85R-6 [Piece 8]), but with two generations of epidote veins, shows a somewhat larger degree of anisotropy (12%), but in this sample the vein orientation appears to play a stronger role than the foliation in determining the propagation speed.
The samples examined in this study are a limited representation of the crystalline rocks beneath the Iberia Abyssal Plain. Furthermore, anisotropy measured at surface pressures is probably much more pronounced than anisotropy at in situ pressures (Christensen and Wepfer, 1989). Nevertheless, the experiments suggest that the structural history of the basement rocks recovered on Leg 149 can be at least partially deduced from the pattern of anisotropic wave propagation. However, the degree of anisotropy is not simply related to foliation direction or veining; it depends upon the vein composition, the polyphase deformation history of the rock, and the azimuth of particle displacement relative to foliation and vein orientation. Thus, although a careful seismic study of anisotropy beneath the Iberia Abyssal Plain may reveal information about the direction and nature of continental extension, it is important to first understand the complicated emplacement and alteration history of the rocks recovered on Leg 149 before such seismic studies can by fully understood.