We present new seismic images of Cretaceous crust formed at a fast-spreading center in the Pacific. The high crustal reflectivity observed in these data contradicts the conventional wisdom that accretionary structures formed at fast-spreading centers are not seismically detectable. Subhorizontal reflections can be traced at 600–800 ms two-way time below the top of basement for tens of kilometres, suggesting the presence of a widespread seismic boundary, possibly a structural discontinuity related to the maximum depth of hydrothermal circulation at the spreading center or the base of the sheeted dikes. Lower-crustal reflections, dipping dominantly toward the paleo–spreading center, may represent mafic-ultramafic banding similar to that observed in the lower crust of reconstructed ophiolite sections.

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