Off-ridge volcanism and seafloor spreading in the Shikoku Basin

THE nature of the crust and origin of marginal basins of the western Pacific continue to be of widespread interest to earth scientists. Recent advances in mapping magnetic anomalies and drilling basement rocks in these areas have suggested that marginal basins are floored by ocean crust 1 and formed by conventional seafloor spreading processes from a mid-basin ridge 2–4 . Our drilling results in the Shikoku Basin during Leg 58 of the Deep Sea Drilling Project reported here indicate that this basin formed as a consequence of back-arc spreading, but that the original simple spreading hypotheses need reappraisal because the crustal stratigraphy and so-called basement age relationships are more complex than expected.