REFLECTIVE OCEANIC-CRUST FORMED AT A FAST-SPREADING CENTER IN THE PACIFIC

We present new seismic images of Cretaceous crust formed at a fast-spr eading center in the Pacific. The high crustal reflectivity observed i n these data contradicts the conventional wisdom that accretionary str uctures formed at fast-spreading centers are not seismically detectabl e. Subhorizontal reflections can be traced at 600-800 ms two-way time below the top of basement for tens of kilometres, suggesting the prese nce of a widespread seismic boundary, possibly a structural discontinu ity related to the maximum depth of hydrothermal circulation at the sp reading center or the base of the sheeted dikes. Lower-crustal reflect ions, dipping dominantly toward the paleo-spreading center, may repres ent mafic-ultramafic banding similar to that observed in the lower cru st of reconstructed ophiolite sections.