Confronting Solid-State Shear Bias: Magmatic Fabric Contribution to Crustal Seismic Anisotropy

Seismic anisotropy is controlled by aligned rock-forming minerals, which most studies attribute to solid-state shear with less consideration for magmatic fabric in plutonic rocks (rigid-body rotation of crystals in the presence of melt). Our study counters this traditional solid-state bias by evaluating contributions from fossil magmatic fabric. We collected samples from various tectonic settings, identified mineral orientations with electron backscatter diffraction and neutron diffraction, and calculated their bulk rock elastic properties. Results indicate that magmatic fabric may lead to moderate to strong anisotropy (3%-9%), comparable to solid-state deformation. Also, magmatically aligned feldspar may cause foliation-perpendicular fast velocity, a unique orientation that contrasts with a fast foliation typical of solid-state deformation. Therefore, magmatic fabric may be more relevant to seismic anisotropy than previously recognized. Accordingly, increased considerations of magmatic fabric in arcs, batholiths, and other tectonic settings can change and potentially improve the prediction, observation, and interpretation of crustal seismic anisotropy.