RELATIVE INFLUENCE OF QUARTZ MICROSTRUCTURE ON CRUSTAL SEISMIC ANISOTROPY

Deformation induced crystallographic preferred orientation (CPO) of anisotropic minerals is an important constraint on seismic anisotropy of the middle and lower crust. Understanding the mineralogical and deformational influence on seismic anisotropy may offer the potential to better interpret deep crustal structure. Experiments show the highest degree of seismic anisotropy in samples containing significant amounts of foliated mica, however, the contribution of other common crustal minerals is poorly understood. As quartz is one of the most common crustal minerals, and its contribution to seismic anisotropy is poorly constrained, I am investigating a suite of mylonitic micaceous quartzites to examine the influence of quartz microstructures on seismic anisotropy. Electron backscatter diffraction (EBSD) techniques can characterize not only the phases present in a sample, but also their complete 3-dimensional crystallographic orientations. This method provides a statistically robust dataset on which textural analysis is easily performed and from which seismic properties can be calculated. Using this method, the following hypotheses are being tested: 1) a quartz CPO, in the presence of aligned mica, will constructively interfere with bulk rock seismic anisotropy therefore increasing the overall magnitude of anisotropy and influencing the anisotropic orientation and 2) a quartz CPO, in the presence of aligned mica, will destructively interfere with bulk rock seismic anisotropy therefore decreasing the overall magnitude of anisotropy and altering the anisotropic orientation. Preliminary results from rocks in central Colorado and southern Wyoming suggest that a quartz CPO decreases the overall anisotropy compared to that solely due to mica and that the orientation of seismic anisotropy is affected by the presence of a quartz CPO. This supports our latter hypothesis and may have important implications when interpreting mid-crustal deformation via seismic data, although further investigation is required. Future research plans include the analysis of samples deformed at various metamorphic conditions in an effort to place constraints on the effect of quartz deformation mechanisms on seismic anisotropy.