Silicic magma reservoir anisotropy persists through protracted crystallization and low strain rates

Magma reservoir organization is hidden from view but important to systematic understanding of active volcanoes and intrusive igneous rocks. Seismic anisotropy in active silicic reservoirs suggests horizontally elongated melt sheets or sills, but it remains unclear if shallow silicic reservoirs in low strain rate settings without recent recharge maintain similar organization. Here we present new 3-D seismic tomography of the upper crustal reservoir beneath the tectonically quiescent Valles Caldera using dense nodal arrays. Results reveal >15% radial anisotropy consistent with up to 17-24% melt in horizontally elongated volumes interspersed with melt-poor layers, despite net crystallization since about 0.5 Ma and little contemporary strain. Structural similarity of Valles Caldera to more dynamic modern systems indicates that intrinsic reservoir processes like iterative magma injection, crystal settling, and melt compaction organize long-lived horizontally-layered structure, largely independent of tectonic setting and recharge rate. A partially molten sill-complex structure is found within the slowly crystallizing and tectonically quiet magma reservoir beneath Valles Caldera using anisotropic Rayleigh and Love wave tomography.