Seismically anisotropic magma reservoirs underlying silicic calderas

Seismic anisotropy can illuminate structural fabrics or layering with length scales too fine to be resolved as distinct features in most seismic tomography. Radial anisotropy, which detects differences between horizontally (VSH) and vertically (VSV) polarized shear wave velocities, was investigated beneath Yellowstone caldera (Wyoming, United States) and Long Valley caldera (California). Significant positive radial anisotropy indicating VSH > VSV and low isotropic velocities, were found beneath both calderas at ~5-18 km depths. The positive radial anisotropy ( > 8%) volumes beneath the calderas are anomalously strong compared to the surrounding areas. The absence of a similar anisotropic signal in the wake of the propagating Yellowstone hotspot indicates that the radial anisotropy diminishes after the locus of voluminous silicic magmatism moves. We propose that the anisotropic volumes represent sill complexes of compositionally evolved magma, and the magma's seismic contrast with the crust would largely fade upon crystallization. The similarity of magma reservoir anisotropy in varied tectonic settings suggests that such mid-crustal sill complexes may be ubiquitous features of silicic caldera-forming magmatic systems, and that anisotropy should be considered to seismically estimate melt content and mobility. The absence of similar radial anisotropy in the lower crust beneath the calderas suggests lower melt fractions or a transition in the geometry of magma pathways.