Shear velocity evidence of upper crustal magma storage beneath Valles Caldera

Abstract

Valles Caldera was formed by large rhyolitic eruptions at ˜1.6 and 1.23 Ma and it hosts post-caldera rhyolitic deposits as young as ˜70 ka, but the contemporary state of the magmatic system is unclear. Local seismicity beneath Valles Caldera is rare and shear-velocity (Vs) structure has not been previously imaged. Here, we present the first local Vs tomography beneath Valles Caldera using ambient noise Rayleigh dispersion from a ˜71 km transect of nodal seismographs with mean spacing of ˜750 m. An ˜6 km wide low-Vs anomaly (Vs<2.1 km/s) is located at ˜3-10 km depth within the 1.23 Ma caldera's ring fracture. Assuming magma in textural equilibrium, the new tomography suggests that melt fractions up to ˜17-22% may be present within the upper crustal depth range where previously erupted rhyolites were stored.