We redescribed the~0.5-km gabbro section drilled in Hole 735B at the Ocean Drilling Program Gulf Coast Repository. Included in this work was a redivision and clarification of the location and nature of the major lithologic boundaries and a division of the major units into subunits. In all, we found 495 distinct lithologic intervals in the core. Most of the section consists of a single olivine gabbro body having only minor cryptic variations, which we think represents a small intrusion. At the top of the section, the olivine gabbro is intercalated with a medium-to coarse-grained gabbronorite, which we postulate was intruded by the olivine gabbro. The base of the olivine gabbro has been intruded by troctolites and troctolitic gabbros, which may be the precursors of a major troctolite intrusive body immediately below the base of the hole. This section is variously crosscut by small microgabbro bodies, which are the products of crystallization and wall-rock reaction of small magma bodies that migrated through the olivine gabbro prior to complete solidification. Overall, the plutonic section drilled in Hole 735B is unlike those found at layered intrusions as it lacks evidence for extensive magmatic sedimentation. Rather, it appears to represent a plutonic basement composed of small, relatively short-lived, rapidly crystallized intrusions. This is consistent with the ephemeral volcanism and low rates of magma supply postulated for very slow-spreading ocean ridges. This whole section underwent "syntectonic differentiation": a process in which deformation and compaction of a rigid, partially molten gabbro drove intercumulus melt out of the olivine gabbro into ductile shear zones. Chemical exchange, precipitation of oxides, and trapping of the migrating melt at the end of deformation altered the gabbro in the shear zones to ferrogabbro. These oxide-rich horizons have the potential to be major shallow-dipping seismic reflectors. The largest such zone is 103 m thick and consists of foliated disseminated oxide olivine and oxide olivine gabbros of lithologic Units III and IV. The last igneous event was back-intrusion of trondhjemite veins that formed either by fractional crystallization from the interstitial melt and/or by wall rock anatexis of intruded amphibolites. Alteration and relatively rapid cooling of the gabbro body occurred by penetration and circulation of seawater into the plutonic section caused by thermal contraction and cracking under tensile stress, much as envisaged by Lister (1970). Initially, this circulation was greatly enhanced tectonically by the tensile component provided by lithospheric necking and the formation of brittle-ductile faults beneath the median valley. This circulation was sufficiently pervasive to alter about 25% of all the matrix pyroxene in the body, mostly to amphibole, in the amphibolite facies. Alteration was heaviest in the vicinity of the brittle-ductile faults, where formation of crack networks, cataclasis, and granulation were ongoing processes continuously creating porosity and p...

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