SEISMIC ANISOTROPY IN THE MID- AND LOWER-CRUST

The field of crustal seismic anisotropy has been growing in recent years with advances in seismic techniques that allow for distinguishing mantle vs. crustal anisotropy. In order to understand the causes of seismic anisotropy in Archean Shields, the crustal contribution to anisotropy must also be considered. For this the seismic properties, including anisotropy, need to be known for the expected crustal rock types. A large body of work focusing on mantle materials has provided a database of this information for mantle rock types, but much less work has focused on the mid- and lower-crust. This study uses electron backscatter diffraction (EBSD) measurements of mineral crystallographic preferred orientations (CPOs) and published single-crystal elastic constants to calculate the seismic properties of mid- and lower-crustal rocks. We investigate several crustal rock types from various localities including the Basin and Range and Mojave Desert regions of the western US. Preliminary results indicate that anisotropy in crustal rocks typically ranges from ~3 – 20% in V_P and V_S and can be as high as ~30% in rocks containing abundant sheet silicates, and that crustal rocks with significant (>10 %) anisotropy can be approximated as uniaxial-slow. Currently used inversions of seismic data for anisotropy require simplification to hexagonal symmetry, a simplification which our results indicate is valid for crustal rocks if anisotropy is caused primarily by the crystallographic preferred orientation of mineral grains.