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In deformed peridotites and other olivine-rich rocks, the olivine grains commonly show preferential crystallographic orientation. Of six or more fabric models reported in naturally occurring peridotite tectonites, two are consistent with experimentally determined slip systems; one is consistent with experimentally verified recrystallization under a known stress system. Because of a strong anisotropy in compressional wave velocity in olivine crystals, seismic velocities in deformed peridotites are significantly higher in the direction of [100] maxima and lower in the direction of [010] maxima.

Recent refraction seismic work in the northeast Pacific (Raitt and others, 1969; Morris and others, 1969) has demonstrated anisotropies ranging from 0.3 to 0.6 km/sec. Small anisotropies can be explained by numerous combinations of fabrics and fabric orientations. Consider, for example, a common model having a [100] maximum horizontal, and [010] and [001] evenly distributed in a girdle normal to it. Using this model, an anisotropy of 0.3 km/sec is consistent with deformation of a peridotitic mantle which is about 17 percent serpentinized and in which 21 percent of the olivine grains are preferentially oriented. The model would require [100] to be subparallel to the direction of sea-floor spreading.

An anisotropy as great as 0.6 km/sec imposes more severe constraints. The model described above would require 42 percent of the olivine crystals to be oriented. Many other models have insufficient anisotropy and would not therefore be acceptable. Among them is a commonly encountered one having mutually perpendicular [100], [010], and [001] maxima.

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