VELOCITY ANISOTROPY IN BASIN AND RANGE LOWER CRUST FROM EBSD

Velocity anisotropy in the lower and middle crust results from layering, microcracks, and the preferred alignment of anisotropic minerals. This research investigates the effect of crystal preferred orientations (CPOs) on the velocity anisotropy of rocks. Recent advances in electron scanning microscopy and electron backscatter diffraction (EBSD) now permit the complete characterization of modal abundance and orientation within a sample. Combined with single-crystal compliances, this information may be used to calculate the 3-D velocity anisotropy of individual rocks [e.g. Mainprice, 1990]. Such measurements and calculations have been made on a suite of quartzofeldspathic and carbonate-rich crustal rocks from two Basin and Range locations: the Funeral Mountains in Death Valley, California and the Ruby–East Humboldt Range in northeastern Nevada.

Comparing these new calculations to previous results and those measured by McDonough and Fountain [1993], this research testifies to the necessity of ensuring the agreement between mineral identification and electron backscatter patterns. Preliminary results indicate differences average 18% between previous and re-evaluated Vp and Vs anisotropy calculations, and may be as high as 30%.

Our results suggest that the velocity anisotropy of the lower crust is transversely isotropic with a unique slow axis. The P-wave velocity anisotropy ranges from 6 to 9% with no simple relation to rock fabric in the Ruby–East Humboldt Range samples. The S-wave velocity anisotropy is 6–13%, with the fast-shear wave polarization plane parallel to the flow direction in the Funeral Mountain samples, but orthogonal to the flow direction in the Ruby–East Humboldt Range. The anisotropy is best correlated to the volume percent of mica in the sample. Additionally, relationships between CPO strength and the resulting velocity anisotropy magnitude and structure are discussed in order to elucidate the origin of seismic anisotropy and its geophysical expression in the lower crust.­