Drilling at Site 895 in the Hess Deep provided 200-m sections of interlayered harzburgites, dunites (containing interstitial clinopyroxene and plagioclase), and troctolites, similar in lithology and overall stratigraphic organization to Moho transition zones reported in ophiolite complexes such as Oman. Similar lithologies previously collected from the East Pacific Rise have been described as mantle rocks within which “impregnated” melt has become entrapped. Our textural studies of the Site 895 specimens confirm this interpretation, based upon the deformation and annealing of the peridotite framework contrasting with the lack of deformation in the interstitial phases. High dislocation densities in all the different lithologies emphasize the residual character of the olivine crystals.

For the first time ever in an in situ section of oceanic mantle peridotites, the orientations of the deformation fabrics in the peridotites, and hence the geometry of mantle flow, have been restored to geographical coordinates (with reference to stable magnetic remanence directions). The oriented fabrics show that mantle shear plane is moderately to steeply dipping and that it strikes parallel to the East Pacific Rise axis. This relationship is particularly consistent in the harzburgites but less so in the massive dunites from Hole 895E, within which the dip of the foliation decreases systematically toward the top of the section and the lineation plunge directions become scattered. We interpret this geometry as representing the fossilized top of a diapir that ascended beneath the EPR and, it would appear, started to overturn in a melt-charged transition zone at the crust/mantle boundary. Although such a flow geometry might normally be expected to be destroyed by continued upwelling and lithospheric expansion, we speculate that the structures observed were frozen in by rapid cooling and uplift of the EPR lithosphere in the Hess Deep rift valley by the propagation of the Cocos-Nazca Spreading Center.

Date of initial receipt: 4 August 1994
Date of acceptance: 5 January 1995

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