RELATING DEFORMATION FABRICS AND TECTONICS TO SEISMIC ANISOTROPY IN CONTINENTAL CRUST

Seismic anisotropy originating within the continental crust is used increasingly to determine kinematic flow lines within active mountain belts and is widely attributed to regionally aligned mineral fabrics. Although many common crustal minerals (e.g. plagioclase, quartz) are significantly anisotropic on the grain-scale, when found in deformed aggregates they produce complex patterns that appear statistically isotropic. Exceptions are amphibole and, especially, mica. However, the orientations of these minerals have different kinematic significance. Here, these aspects are examined using exhumed ductile deformation fabrics within well constrained outcrop settings that provide analogues for modern continental crust at depth. We compare the behavior of deformed granitic gneiss, where fabrics are dominated by mica, to deformed metabasic rocks dominated by amphibole. Seismic responses are modeled using crystallographic preferred orientations for the polymineralic rocks, measured using Electron Back-Scatter Diffraction. The sample results are generalized by modeling the effects of variations in modal composition and the relative importance of deformation fabrics of variable orientation, so-called rock and fabric recipes. Micaceous tectonites show radically different fabrics depending on their kinematics (e.g. S-C mylonites vs pure shear flattening) and these contrast in turn with amphibole-dominated fabrics. For example, in high simple shear deformations, deformed amphibolites are predicted to show maximum Vp along the tectonic transport axis with no well-defined orientation for minimum Vp. For S-C fabrics in sheared micaceous rocks, the maximum Vp direction is perpendicular to the tectonic transport direction. These effects, coupled with the geographical orientation of fabrics in nature, can generate substantial variations in the orientation and magnitude of seismic anisotropy (especially for s-waves) as measured for the continental crust using existing receiver function and teleseismic near-vertical incidence methods. Thus, maps of seismic anisotropy varying with depth in deforming continents need not imply necessarily depth-varying deformation kinematics and tectonic decoupling.