P-wave anisotropy of mylonitic and infrastructural rocks from a Cordilleran core complex: the Ruby-East Humboldt Range, Nevada

Abstract

In order to evaluate the possibility that compressional-wave anisotropy might characterize the upper to middle crust of the North American Cordillera, we examined the nature and degree of seismic anisotropy of rock samples collected from the mylonite zone and underlying infrastructure of the Ruby-East Humboldt metamorphic core complex in northeastern Nevada. Seismic anisotropy in quartzofeldspathic rocks from all structural levels is low and generally related to the volume of mica relative to feldspar. Upper amphibolite facies infrastructure quartzite has a stronger and different quartz crystallographic preferred orientation (CPO) and, thus, higher seismic anisotropy than the lower grade mylonitic quartzites, a feature related to the higher temperature deformation conditions that characterized the infrastructure. Schistose mylonite zone rocks, as a group, have the highest anisotropy. We infer that this is, in large part, due to mica CPO although the anisotropy patterns suggest that quartz CPO may be a factor. Anisotropy is higher in infrastructural quartzites, calc-silicates, and marbles than in mylonite-zone equivalents. Seismic anisotropy data for other Cordilleran core complex samples suggest that these patterns are common characteristics of Cordilleran core complex rocks.