Serpentinized ultramafic rocks were recovered at Ocean Drilling Program (ODP) Sites 897, 899, and 900, which are inferred to represent upper mantle material exposed at the seafloor during the latter stages of continental rifting. At all three Leg 149 sites, the ultramafic sequence shows varying degrees of foliation, discoloration, veining, and inferred fluid alteration (Sawyer, Whitmarsh, Klaus, et al., 1994). The presence of both plagioclase feldspar and spinel in some rocks suggests that these rocks last equilibrated at shallow depths (~30 km), so the deformation features must have developed during or after the latest stages of extension.
A similar sequence of ultramafic rocks was recovered on ODP Leg 103 on the Galicia margin (Boillot, Winterer, Meyer, et al., 1987). The Leg 103 rocks show clear indications of high-temperature ductile deformation, and a strong pervasive foliation. The ductile deformation and foliation were interpreted as having developed during continental rifting along an east-dipping ductile shear zone (Girardeau et al., 1988; Boillot et al., 1989; Beslier et al., 1990). The Leg 149 ultramafic rocks are somewhat different from the Leg 103 rocks in that the foliation is not pervasive over large depth intervals and there is little indication of high-temperature ductile deformation throughout most of the recovered section. Nevertheless, the foliations and low-temperature ductile deformation features present suggest a stress history similar to that of the ultramafic rocks on the Galicia Bank, in which the foliation and deformation textures developed during late-stage shearing and exhumation of the upper mantle rocks.
One important result from Leg 103 is the observation that the foliated ultramafic rocks exhibit pronounced acoustic anisotropy, with wave propagation faster in the direction parallel to the foliations (Boillot, Winterer, Meyer, et al., 1987). The anisotropy was attributed to compositional banding related to the foliations and preferred mineral-grain orientation developed during the foliation event. Thus, anisotropy on the Galicia margin may be a good indicator of the intensity and direction of simple shear deformation during continental rifting. The ultramafic rocks recovered on Leg 149 do not display the well-developed indicators of shear that were observed in the Leg 103 samples. It is possible that this is a result of less intense shear deformation on the Iberia Abyssal Plain segment of the margin, a different style of deformation on the margin, or a different uplift history of the shallow ultramafic rocks that were recovered during the two legs. The prevalence of late-stage brecciation and multistage veining and fracture filling suggests that the rocks recovered during Leg 149 have suffered a more complex postextensional strain history than the rocks on the Galicia margin. If so, the less brecciated samples from Leg 149 may be more representative of extensional deformation.
If the relation between extensional deformation and acoustic anisotropy observed on the Galicia margin is applicable to the Iberian Abyssal Plain, then it is desirable to test for a relationship between foliation and anisotropy in the Leg 149 ultramafic rocks. We selected nine samples from Sites 897, 899, and 900 to test for acoustic anisotropy. The samples from each site represent varying degrees of foliation and intensity of veining. Acoustic anisotropy was determined by measuring the traveltime of shear waves propagating in the horizontal direction, with particle displacements oriented in 10° increments between vertical and horizontal. The traveltimes were used to deduce shear-wave velocity with various particle displacement azimuths and its relationship to veining and foliation.