We present a magnetic fabric study of mafic and ultramafic rocks recovered at Ocean Drilling Program Sites 894 and 895 from Hess Deep, a tectonic rift in the equatorial East Pacific (2×15'N, 101×30'W). We demonstrate, using thermomagnetic curves and high-field (up to 1.5 T) susceptibility measurements, that the magnetite contributes >95% of magnetic susceptibility of these rocks. Bulk susceptibilities for the Site 894 gabbroic rocks are on average 1.72 × 10^–2 and for the Site 895 peridotites 4.29 × 10^–2 (SI volume units). The anisotropy of magnetic susceptibility is moderate to high and average k_max/k_min is 1.15 (k_max greater than or equal to k_int greater than or equal to k_min are the principal susceptibilities) at Site 894 and 1.11 at Site 895. Because of the pervasive serpentinization and the formation of randomly distributed secondary magnetite, the magnetic fabrics of the Site 895 peridotites show no apparent relationship to structural features. In the foliated gabbroic rocks from Site 894, we observe a close relationship between magmatic flow fabrics (defined by the preferred orientation of plagioclase) and magnetic fabrics: k_min is perpendicular to the macroscopic foliation and k_max dips steeply within the foliation plane and is parallel to the magmatic mineral lineation. Macroscopically isotropic gabbros yield the same principal susceptibility directions as the foliated gabbros. We interpret magnetic fabrics in these rocks to represent a weakly developed rock fabrics. We argue that the process for the development of the AMS is a distribution anisotropy which is caused by the growth of equant, irregular, or skeletal magnetite grains into a preferredly oriented (by magmatic flow) plagioclase "template." Magnetic fabric data record a mineral preferred orientation with a north–south, East Pacific Rise parallel strike and a near-vertical inclination, and can be interpreted to record the upward flow of melt at the top of an axial magma chamber into the base of an overlying sheeted dike complex.

Date of initial receipt: 1 August 1994
Date of acceptance: 12 December 1994

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