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Seafloor drilling during Integrated Ocean Drilling Program Expedition 304 was designed to investigate the processes that control formation of oceanic core complexes (OCCs), as well as the exposure of ultramafic rocks in very young oceanic lithosphere. Prior studies indicated that two main drill sites on a 0.5–2 m.y. old OCC on the western rift flank of the Mid-Atlantic Ridge (MAR), 30°N, could provide key constraints on the structure of the detachment fault zone, rock types exposed at shallow structural levels in the footwall, and their alteration history, as well as that of the volcanic succession in the hanging wall. Site U1309 is located in the footwall of the central dome of Atlantis Massif, where drilling was quite successful. The goal of establishing a reentry hole for subsequent deep penetration on Expedition 305 was accomplished, and coring during Expedition 304 went well beyond the initial plan to reach ~120 mbsf. In the 400 m penetrated at Hole U1309D during Expedition 304, a series of interfingered gabbroic intrusions were distinguished based on variation in olivine content, the presence of intercumulus phases, the extent of late magmatic dikes, and the presence of oxide gabbro. The boundaries of these lithologic zones commonly coincide with structurally defined boundaries.

Overall, the composition of the gabbroic rocks from Hole U1309D is among the most primitive known along the MAR. Several meter-scale intervals of serpentinized peridotite were recovered, comprising ~5% of the total recovery. A few of these ultramafic intervals may represent residual mantle harzburgite, but many have a low Mg number (88.9–89.6), indicating they are cumulates and/or have been impregnated by later melts. Diabase and basalt are present only in the upper 130 m of the footwall and are tholeiitic in composition.

Drilling in the hanging wall, Sites U1310 and U1311, was only marginally successful. The operational approaches used in our attempts to establish a reentry hole did not work. The main result from the limited recovery at these sites is the finding that relatively fresh basalt is present near the boundary of the central dome and adjacent volcanic block. The composition of these basalts is primitive tholeiite.

Shallow penetration holes through the sediment carapace on the footwall and into basement provided fossiliferous ooze, hyaloclasite, and fragments of fault rock and metabasalt/diabase. These samples provide initial confirmation that the corrugated dome coincides with an exposed detachment fault. If dating of the sedimentary deposits is possible, some constraints on the timing of its exposure at the seafloor may be obtained.

The lack of widespread deformation in the upper 400 m sampled at Site U1309 suggests that strain is concentrated in a small number of very localized zones. Several aspects of the drill core change across these faulted zones: the nature of the intrusive sequence and its alteration history, the intensity of deformation and abundance of veins, and average paleomagnetic inclination angle. A monotonic decrease in deformation intensity and/or systematic rotation of the footwall, such as predicted by the rolling hinge model, is not recorded in the recovered core.

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