Proceedings of the Integrated Ocean Drilling Program volume 304/305

Integrated Ocean Drilling Program Expeditions 304 and 305 composed a two-expedition program at Atlantis Massif, Mid-Atlantic Ridge 30°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 (~3%) and gabbroic (~91%). A series of olivine-rich rocks (~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 deeprooted 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

IODP Expeditions 304 & 305 Characterize the Lithology,Structure, and Alteration of an Oceanic Core Complex

More than forty years after the Mohole Project (Bascom,1961), the goal of drilling a complete section through in situoceanic crust remains unachieved. Deep Sea Drilling Project– Ocean Drilling Program (DSDP-ODP) Hole 504B withinthe eastern Pacifi c (Alt et al., 1993) is the deepest hole everdrilled into ocean crust (2111 mbsf), but it failed to reachlower crustal plutonic rocks below the pillow basalts andsheeted dikes. IODP Expeditions 309 and 312 eventuallyrecovered the long-sought transition from sheeted dikes intounderlying gabbros by drilling into very fast-spreadingPacifi c crust (Wilson et al., 2006). The lithology and structureof oceanic crust produced at slow-spreading ridges areheterogeneous (e.g., Cannat et al., 1997) and offer uniquedrilling access to lower crust and upper mantle rocks. AfterODP Hole 735B penetrated 1500 m of gabbro at the SouthwestIndian Ridge (Dick et al., 2000), IODP Expeditions 304 and305 recently recovered just over 1400 m of little-deformed,gabbroic lower crust from a tectonic window along the slowspreading Mid-Atlantic Ridge

Oceanic core complexes and crustal accretion at slow-spreading ridges

Oceanic core complexes expose gabbroic rocks on the seafloor via detachment faulting, often associated with serpentinized peridotite. The thickness of these serpentinite units is unknown. Assuming that the steep slopes that typically surround these core complexes provide a cross section through the structure, it has been inferred that serpentinites compose much of the section to depths of at least several hundred meters. However, deep drilling at oceanic core complexes has recovered gabbroic sequences with virtually no serpentinized peridotite. We propose a revised model for oceanic core complex development based on consideration of the rheological differences between gabbro and serpentinized peridotite: emplacement of a large intrusive gabbro body into a predominantly peridotite host is followed by localization of strain around the margins of the pluton, eventually resulting in an uplifted gabbroic core surrounded by deformed serpentinite. Oceanic core complexes may therefore reflect processes associated with relatively enhanced periods of mafic intrusion within overall magma-poor regions of slow- and ultra-slow-spreading ridges