INFLUENCE OF METAMORPHIC AND DEFORMATION PROCESSES ON THE EVOLUTION OF CRUSTAL SEISMIC ANISOTROPY

Detection of middle- and deep-crustal seismic anisotropy is potentially one of the most powerful emerging techniques in 3-dimensional crustal-scale tectonic analysis. However, full realization of the utility of these techniques will require a better understanding of both the intrinsic nature of seismic anisotropy in crustal rocks and the crustal processes that govern its development and preservation. Some important but incompletely resolved issues include (i) the effects of the thermodynamic stability of anisotropic phases, including mica and amphibole, during prograde and retrograde metamorphism, (ii) the role of CPO of other common silicates and the effects of their constructive and/or destructive interference on bulk anisotropy, and (iii) the contribution of fabric components other than the commonly assumed single foliation, such as lineations and composite shear fabrics, and (iv) variability in deformation mechanisms. Exhumed lower crust in the western Canadian Shield records the transition of anhydrous felsic granulite with less than 5% Vp anisotropy (AVp) to a mica-rich mid-crustal tectonite with AVp likely exceeding 15% during exhumation along a major shear zone. Lower crustal xenoliths from southern Wyoming experienced a major hydration event that converted pyroxene-rich granulites to amphibole-rich tectonites with AVp > 10%. A similar evolution in a prograde sense (conversion of detrital K-feldspar to muscovite) is preserved in nearby exhumed mid-crustal quartzite mylonites. These and other similar factors are significant for recognizing spatial variations in the strength and symmetry of anisotropic signals within actively deforming regions, and may also prove useful for distinguishing “active” from “ancient” structures in seismic observations. The purpose of this presentation will be to illustrate potential evolutionary pathways for crustal seismic anisotropy as a function of some common crustal processes. The implications are relevant to interpretations of seismic anisotropy in these regions, as well as in the Himalayan, New Zealand Alpine, and other modern orogens.