Seismically anisotropic magma reservoirs underlying silicic calderas

Abstract

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 (V SH) and vertically (V SV) polarized shear wave velocities, was investigated beneath Yellowstone caldera (Wyoming, United States) and Long Valley caldera (California). Significant positive radial anisotropy indicating V SH > V SV 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. 1 GSA Data Repository item 2018264, details on data sources, analysis methods, and synthetic tests, including Figures DR1-DR10, and Tables DR1 and DR2

Magmatic and Sedimentary Structure beneath the Klyuchevskoy Volcanic Group, Kamchatka, From Ambient Noise Tomography

“…This hypothesis is supported by a cross section displayed in Figure 15, where the slow velocity anomaly in question is continuously linked with the slow velocities of Sedimentary Layer S2 in the CKD. Future radial anisotropy tomography studies might help to test this hypothesis and our magma storage interpretations (Figure 14), by distinguishing between anisotropic sill-like magmatic storage (Jaxybulatov et al, 2014;Jiang et al, 2018) and sedimentary material.…”

Electrically Anisotropic Crust From Three‐Dimensional Magnetotelluric Modeling in the Western Junggar, NW China

“…It is widely accepted that rising magma can modify channels and their surroundings, and produce metamorphic and/or metasomatic hydrous minerals in the crust expressed as subvertical low‐velocity bodies. These could result in negative radial anisotropy (Jiang et al, ; Luo et al, ) and also possibly in electrically azimuthal anisotropy (Figure b).…”

Weak B‐Type Olivine Fabric Induced by Fast Compaction of Crystal Mush in a Crustal Magma Reservoir

“…In conjunction with velocity anomalies, seismic anisotropy can provide further information on the architecture of crustal magmatic systems and the configuration of melts in mush. Based on the modeling of a sill‐stacked structure where the melt‐rich sills are emplaced between layers with fast seismic velocities, a positive radial seismic anisotropy ( V SH > V SV ) is suggested for melt‐rich layered sills, while negative anisotropy ( V SH < V SV ) for dyke networks containing significant amounts of melts (Jaxybulatov et al, ; Mordret et al, ; Spica et al, ; Godfrey et al, 2017; Jiang et al, ). However, there is a lot of evidence demonstrating that melt within deep trans‐crust mush only accounts for a small volume fraction, and is distributed either on a micros‐scale, or a mesoscale in the form of veins and pockets (Sparks et al, ).…”