CONFRONTING SHEAR BIAS: MAGMATIC FABRIC INFLUENCE ON CRUSTAL SEISMIC ANISOTROPY

The directional dependence of seismic wave velocity (seismic anisotropy) is influenced by the alignment of rock forming minerals. Most studies attribute strong crystallographic preferred orientations (CPOs) that generate seismic anisotropy to solid-state deformation (i.e., crystal-plastic slip along lattice planes). However, while much of continental crust also contains plutonic rocks with magmatically-aligned minerals (i.e., rigid-body crystal rotation in the presence of melt), such magmatic fabrics are underrepresented in seismic anisotropy research. Our study counters this traditional bias of solid-state deformation. We collect samples of magmatic fabrics from various tectonic settings, identify their constituent mineral CPOs with Electron Backscattered Diffraction and Neutron Diffraction, and calculate their seismic properties. Results indicate that: A) magmatic fabrics may lead to moderate (>5%) to strong (>10%) P-wave anisotropy, comparable to solid-state deformation; B) magmatically-aligned tabular feldspar may cause foliation-perpendicular fast P-wave velocity, a unique orientation compared to solid-state deformation. These findings suggest that magmatic fabrics may be much more important contributors to continental crustal seismic anisotropy than previously recognized. Accordingly, future work on predicting crustal seismic properties, modeling pluton geometry, and interpreting subsurface geology from geophysical data can each benefit from also considering magmatic fabrics.