VELOCITY ANISOTROPY OF THE RUBY MOUNTAINS—EAST HUMBOLDT RANGE FROM ELECTRON BACK-SCATTER DIFFRACTION

The Ruby–East Humboldt Range consists of a metamorphic core formed at 9 kb and 800°C overprinted by a mylonitic carapace formed at 3 kb and <600°C [Hurlow et al., 1991; McGrew et al., 2000]. Velocities of samples from this range were measured by McDonough & Fountain [1993] using the pulse-transmission (PT) technique. We used EBSD to calculate the velocity anisotropy of the same specimens to assess i) anisotropy magnitude and symmetry, ii) the relationship between calculated and measured velocities, and iii) which minerals and slip systems are responsible for the anisotropy. The measured CPOs are taken to define the flow plane and flow direction, and suggest [001]{120} slip in hornblende and clinopyroxene, [001](010) slip in plagioclase and K-feldspar, [uv0](001) slip in mica, mixed slip in quartz, and [1000]{10-14} slip in dolomite. The CPOs are asymmetric with respect to the foliation and lineation by as much as 25°, implying locally noncoaxial deformation. The calculated VP anisotropy of individual samples varies from 2–20%—typically twice the measured VP anisotropy because the principal directions of the mineral CPOs deviate from the rock foliation and lineation. The upper few hundred meters of the mylonite zone have an anisotropy of 5% that is dominated by quartz; the flow direction is aligned with the slow VP direction and differs from the VS1 polarization planes by 60°. The lower 800–1800 meters of the mylonite zone have an anisotropy of 4% that is dominated by feldspar and mica, a VP anisotropy that is uniaxial slow and a VS anisotropy that is uniaxial fast; the flow direction is aligned with the fast VP direction and differs from the VS1 polarization planes by 75°. The underlying gneiss has an anisotropy of 10% that is dominated by mica, a VP anisotropy that is uniaxial slow and a VS anisotropy that is uniaxial fast; the flow direction has no simple relationship to the VP anisotropy and differs from the VS1 polarization planes by 45°. Thus, for this sample suite, the velocity anisotropies measured by the PT technique significantly underestimate velocity anisotropy. The lack of correspondence between the rock flow direction (inferred from CPOs) and the VS polarization planes means that, at least at the sample scale, it is difficult to use seismic anisotropy to infer the rock flow plane or flow direction.