1. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
2. Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada
3. Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA
4. Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093, USA

Correspondence to: J. Pablo Canales¹ Correspondence and requests for materials should be addressed to J.P.C. (Email: jpcanales@whoi.edu).

The oceanic crust extends over two-thirds of the Earth's solid surface, and is generated along mid-ocean ridges from melts derived from the upwelling mantle¹. The upper and middle crust are constructed by dyking and sea-floor eruptions originating from magma accumulated in mid-crustal lenses at the spreading axis^{2, 3, 4, 5, 6}, but the style of accretion of the lower oceanic crust is actively debated⁷. Models based on geological and petrological data from ophiolites propose that the lower oceanic crust is accreted from melt sills intruded at multiple levels between the Moho transition zone (MTZ) and the mid-crustal lens^{8, 9, 10, 11}, consistent with geophysical studies that suggest the presence of melt within the lower crust^{12, 13, 14, 15, 16}. However, seismic images of molten sills within the lower crust have been elusive. Until now, only seismic reflections from mid-crustal melt lenses^{2, 17, 18} and sills within the MTZ have been described¹⁹, suggesting that melt is efficiently transported through the lower crust. Here we report deep crustal seismic reflections off the southern Juan de Fuca ridge that we interpret as originating from a molten sill at present accreting the lower oceanic crust. The sill sits 5–6 km beneath the sea floor and 850–900 m above the MTZ, and is located 1.4–3.2 km off the spreading axis. Our results provide evidence for the existence of low-permeability barriers to melt migration within the lower section of modern oceanic crust forming at intermediate-to-fast spreading rates, as inferred from ophiolite studies^{9, 10}.

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