A multi-sill magma plumbing system beneath the axis of the East Pacific Rise

Abstract

Upper oceanic crust at fast- to intermediate-spreading mid-ocean ridges is thought to form from the intrusion and eruption of magma accumulated within a mid-crustal reservoir present beneath the ridge axis^1,2,3. However, the mechanisms for formation of the lower crust are debated^4,5,6,7,8. Observations from pieces of ancient oceanic crust exposed on land — ophiolites — imply that multiple small magma lenses exist throughout the lower crust at mid-ocean ridges and help form the crust^4,6,7, yet seismic data have imaged only a single lens beneath the innermost axial zones of various mid-ocean ridges^1,2,3. Here we use high-fidelity seismic data to image the crust beneath the East Pacific Rise. We identify a series of reflections below the axial magma lens that we interpret as magma lenses in the upper part of the lower crust. These reflections are present between 9° 20′ and 9° 57′ N and are located up to 1.5 km below the axial magma lens. From the geometry and amplitude of the reflections in a zone beneath a recent volcanic eruption⁹, we infer that magma drained from a lower lens helped replenish the axial magma lens above and, perhaps, contributed to the eruption. Our data indicate that a multi-level complex of magma lenses is present beneath the East Pacific Rise and probably contributes to the formation of both the upper and lower crust.