DEFORMATION AND VELOCITY ANISOTROPY OF THE FUNERAL MOUNTAINS MIDDLE–LOWER CRUST FROM ELECTRON BACK-SCATTER DIFFRACTION

Advances in scanning electron microscopy and electron back-scatter diffraction (EBSD) now permit complete and rapid characterization of mineral compositions, abundances, and crystal preferred orientations at the submicron scale. When combined with single-crystal compliances, these data can be used to calculate the velocity anisotropy of rocks. We have made such measurements and calculations for a suite of crustal rocks from Monarch Canyon in the Funeral Mountains that were deformed at 8 kb and 650°C [Labotka, 1980; Mattinson et al., 2007]. The measured CPOs are taken to define the rock flow plane and flow direction, and suggest [001](010) slip in plagioclase, [uv0](001) slip in mica, [001](010) slip in sillimanite, and mixed slip in quartz. The CPOs are asymmetric with respect to the foliation and lineation by less than 5°, implying nearly coaxial deformation. The anisotropy of the main gneiss is dominated by mica; the bulk VP anisotropy is 14% and uniaxial slow, and the bulk VS anisotropy is 19% and uniaxial slow. The flow direction is aligned with the intermediate VP direction and differs from the VS1 polarization planes by <5°. The anisotropy of subordinate leucosomes (10 vol%), is dominated by K-feldspar. The bulk VP anisotropy is 6% and orthorhombic slow; the bulk VS anisotropy is 7% and orthorhombic slow; the flow direction is aligned with the slow VP direction and differs from the VS1 polarization planes by 45°. The anisotropy of minor garnet–sillimanite gneiss, is dominated by biotite and sillimanite. The bulk VP anisotropy is 13% and orthorhombic slow (1.02 : 1 : 0.91); the bulk VS anisotropy is 17% and orthorhombic slow; the flow direction is aligned with the slow VP direction and differs from the VS1 polarization planes by <5°. The entire measured section shows uniaxial slow VP with the unique axis perpendicular to the foliation, but no relationship to the flow direction; the VS1 polarization planes, however, differ from the flow direction by <10°.