The Earth's oceanic crust crystallizes from magmatic systems generated at mid-ocean ridges. Whereas a single magma body residing within the mid-crust is thought to be responsible for the generation of the upper oceanic crust, it remains unclear if the lower crust is formed from the same magma body, or if it mainly crystallizes from magma lenses located at the base of the crust^{1, 2, 3}. Thermal modelling^{4, 5, 6}, tomography⁷, compliance⁸ and wide-angle seismic studies⁹, supported by geological evidence^{3, 10, 11, 12, 13, 14, 15, 16, 17, 18}, suggest the presence of gabbroic-melt accumulations within the Moho transition zone in the vicinity of fast- to intermediate-spreading centres. Until now, however, no reflection images have been obtained of such a structure within the Moho transition zone. Here we show images of groups of Moho transition zone reflection events that resulted from the analysis of 1,500 km of multichannel seismic data collected across the intermediate-spreading-rate¹⁹ Juan de Fuca ridge. From our observations we suggest that gabbro lenses and melt accumulations embedded within dunite or residual mantle peridotite are the most probable cause for the observed reflectivity, thus providing support for the hypothesis that the crust is generated from multiple magma bodies.

1. Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, PO Box 1000, Palisades, New York 10964-8000, USA
2. Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0225, USA
3. Woods Hole Oceanographic Institution, 360 Woods Hole Rd, Woods Hole, Massachusetts 02543-1542, USA

Correspondence to: Mladen R. Nedimovi¹ Correspondence and requests for materials should be addressed to M.R.N. (Email: mladen@ldeo.columbia.edu).

Received 13 January 2005; Accepted 15 June 2005

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