Résumé
Integrated Ocean Drilling Program Expeditions 304 and 305 composed a two-expedition program at Atlantis Massif, Mid-Atlantic Ridge 30 degrees N, designed to investigate the processes that control oceanic core complex formation and the exposure of ultramafic rocks in young oceanic lithosphere. Geophysical interpretations of unaltered mantle rock occurring <1 km below the seafloor suggested we could drill through an alteration front in ultramafic rocks and obtain fresh mantle peridotite. In order to accomplish these objectives, we drilled at two sites: one in the footwall through an exposed detachment fault and one in the hanging wall. Drilling the fractured basalt in the hanging wall was thwarted by difficulties, whereas drilling in the footwall was very successful. The main hole at Site U1309 penetrated 1415.5 meters below seafloor, and recovery averaged 75%. Igneous rocks recovered from the hole are the most primitive ever cored in slow-spreading ocean lithosphere and provide an exceptional record of magmatic and tectonic accommodation to extension in this environment. The core recovered was dominantly crustal rock types: basalt ( approximately 3%) and gabbroic ( approximately 91%). A series of olivine-rich rocks ( approximately 5%; dunites, wehrlites, troctolites), grouped as olivine-rich troctolites, part of which likely represent primitive cumulates, are interlayered with gabbroic rocks. A few thin mantle peridotite intervals are recognized in the upper 180 m of the section. Overall, the section is moderately altered at conditions ranging from granulite to zeolite facies. The rocks record initial alteration at low strains under granulite- and amphibolite-facies conditions and subsequent, more fully developed, lower temperature alteration (greenschist facies). Fault zone(s) comprising the detachment system must be highly localized to within tens of meters of the present-day seafloor. The existence of a fault at the top of the domal surface is supported by fragments of brecciated talc-tremolite fault schist and fractured metadiabase recovered in Hole U1309B and during the series of shallow cores drilled. Extensive amphibolite facies deformation is lacking, and high-strain ductile shear zones are rare. The absence of a thick, high-temperature ductile deformation zone in the footwall, and the apparent tectonic history (less rotation in the upper 180 m and variable rotations between several distinct, few-hundred meter sections downhole) suggested by paleomagnetic inclination measurements indicate complexity in structural evolution that differs from the simplest models of deep-rooted detachment faults, predicted to be associated with high-temperature deformation, and with constant or monotonically varying footwall rotation with depth. Another challenge is that the central dome is clearly not an uplifted dominantly upper mantle section, as had been inferred prior to drilling. The exposures of peridotite along the southern wall of Atlantis Massif, the geophysical results suggesting at least portions of the dome contain fresh olivine-rich rock, and the downhole variability at Site U1309 all likely indicate significant lateral heterogeneity over short distances across the footwall.