Tectonic controls on magma storage beneath a distributed volcanic field

Distributed volcanic fields occur globally, but the processes that control volcanic composition and behavior remain uncertain. To investigate the controls on eruptive style, melt storage, and hazards in a system with voluminous mafic and intermediate and/or silicic lavas, we seismically imaged the subsurface beneath the San Francisco volcanic field in northern Arizona (USA). Results reveal two partial melt zones and regional changes in crustal thickness, related to lower crustal removal. San Francisco Mountain, a felsic stratovolcano, is located atop a boundary between intact and removed crust. This lateral boundary in lithospheric properties concentrates melt into mid- and lower-crustal reservoirs, enabling felsic volcanism in this dominantly basaltic field. This demonstrates how lateral lithospheric gradients focus melt in distributed volcanic fields, with removal playing a key role in creating these gradients.