Characteristics of deep crustal seismic anisotropy from a compilation of rock elasticity tensors and their expression in receiver functions

Rocks in the continental crust are long lived and have the potential to record a wide span of tectonic history in rock fabric. Mapping rock fabric in situ at depth requires the application of seismic methods. Below depths of microcrack closure seismic anisotropy presumably reflects the shape and crystallographic preferred orientations influenced by deformation processes. Interpretation of seismic observables relevant for anisotropy requires assumptions on the symmetry and orientation of the bulk elastic tensor. We compare commonly made assumptions against a compilation of 95 bulk elastic tensors from laboratory measurements, including electron backscatter diffraction and ultrasound, on crustal rocks. The majority of samples developed fabric at pressures corresponding to middle to lower crustal condition. Tensor symmetry is a function of mineral modal composition, with mica-rich samples trending toward hexagonal symmetry, amphibole-rich samples trending toward an increased orthorhombic symmetry component, and quartz-feldspar-rich samples showing a larger component of lower symmetries. Seventy-seven percent of samples have a best fit hexagonal tensor with slow-axis symmetry, as opposed to mantle deformation fabric that usually has fast-axis symmetry. The best fit hexagonal approximation for crustal tensors is not elliptical but deviates systematically from elliptical symmetry with increasing anisotropy, an observation that affects the magnitude and orientation of anisotropy inferred from receiver function and surface wave observations. We present empirical linear relationships between anisotropy and ellipticity for crustal rocks. The maximum out-of-plane conversion amplitudes in receiver functions scale linearly with degree of anisotropy for nonelliptical symmetry. The elliptical assumption results in a bias of up to 1.4 times true anisotropy.