Rifting in Hess Deep has exposed complete sections of young oceanic lithosphere generated at the East Pacific Rise. At Ocean Drilling Program Site 894, a 150-m section of gabbroic rocks from the upper part of the plutonic section was drilled. The rocks are not compositionally layered but do have l or l-s fabrics defined by the shape-preferred orientation of idiomorphic plagioclase and other phases, including magnetite. We show here that the fabrics formed as a result of viscous magmatic flow and that, in contrast to previously drilled sections of ocean-floor gabbro, evidence for solid-state deformation is entirely absent. Restoration of the fabrics to geographical coordinates, and accounting for subsequent tectonic rotations, shows that the magmatic foliation was originally parallel to the East Pacific Rise axis, steeply dipping, and with a strong, near-vertical flow lineation.

From compositional considerations the Site 894 section is thought to have crystallized at fairly high pseudostratigraphic levels within the plutonic portion of the crust, probably slightly below the level of the axial melt lens. This corresponds to a region of moderate seismic attenuation observed at the present-day East Pacific Rise axis, which is believed to represent a crystal mush zone but is otherwise poorly constrained. We discuss the possible implications of the observed fabrics for the mechanisms of melt transport through the mid-crust at fast-spreading ridges.

Date of initial receipt: 8 August 1994
Date of acceptance: 27 April 1995

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