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International Ocean Discovery Program (IODP) Expedition 360 was the first leg of Phase I of the SloMo (shorthand for “The nature of the lower crust and Moho at slower spreading ridges”) Project, a multiphase drilling program that proposes to drill through the outermost of the global seismic velocity discontinuities, the Mohorovičić seismic discontinuity (Moho). The Moho corresponds to a compressional wave velocity increase, typically at ~7 km beneath the oceans, and has generally been regarded as the boundary between crust and mantle. An alternative model, that the Moho is a hydration front in the mantle, has recently gained credence upon the discovery of abundant partially serpentinized peridotite on the seafloor and on the walls of fracture zones, such as at Atlantis Bank, an 11–13 My old elevated oceanic core complex massif adjacent to the Atlantis II Transform on the Southwest Indian Ridge. Hole U1473A was drilled on the summit of Atlantis Bank during Expedition 360, 1–2 km away from two previous Ocean Drilling Program (ODP) holes: Hole 735B (drilled during ODP Leg 118 in 1987 and ODP Leg 176 in 1997) and Hole 1105A (drilled during ODP Leg 179 in 1998). A mantle peridotite/gabbro contact has been traced by dredging and diving along the transform wall for 40 km. The contact is located at ~4200 m depth on the transform wall below the drill sites but shoals considerably 20 km to the south, where it was observed in outcrop at 2563 m depth. Moho reflections, however, have been found at ~5–6 km beneath Atlantis Bank and <4 km beneath the transform wall, leading to the suggestion that the seismic discontinuity may not represent the crust/mantle boundary but rather an alteration (serpentinization) front. This in turn raises the interesting possibility that methanogenesis associated with serpentinization could support a whole new planetary biosphere deep in the oceanic basement. The SloMo Project seeks to test these hypotheses at Atlantis Bank and evaluate the processes of natural carbon sequestration in the lower crust and uppermost mantle. A primary objective of SloMo Leg 1 was to explore the lateral variability of the stratigraphy established in Hole 735B. Comparison of Hole U1473A with Holes 735B and 1105A allows us to demonstrate a continuity of process and complex interplay of magmatic accretion and steady-state detachment faulting over a time period of ~128 ky. Preliminary assessment indicates that these sections of lower crust are constructed by repeated cycles of intrusion, represented in Hole U1473A by approximately three upwardly differentiated hundreds of meter–scale bodies of olivine gabbro broadly similar to those encountered in the deeper parts of Hole 735B. Specific aims of Expedition 360 focused on gaining an understanding of how magmatism and tectonism interact in accommodating seafloor spreading, how magnetic reversal boundaries are expressed in the lower crust, assessing the role of the lower crust and shallow mantle in the global carbon cycle, and constraining the extent and nature of life at deep levels within the ocean lithosphere. 

International Ocean Discovery Program (IODP) Expedition 360 will form the first leg of a multiphase drilling project that aims to drill through the crust/mantle boundary at the ultraslow-spreading Southwest Indian Ridge and investigate the nature of the Mohorovičić seismic discontinuity (Moho). Expedition 360 is expected to drill ~1300 m into lower crustal gabbro and is unlikely to penetrate the crust–mantle transition or recover a significant amount of peridotite. Drilling will be sited at Atlantis Bank, on an elevated wave-cut platform on the east flank of the Atlantis II Transform. Previous drilling and mapping shows that Atlantis Bank is a large oceanic core complex, exposing a tectonic window of deep crustal and lithospheric mantle exhumed on the footwall of an oceanic detachment fault. The shallowest part of Atlantis Bank, at 700 m water depth, consists of a ~25 km2 wave-cut platform rimmed by a thin bioclastic limestone cap. The platform is part of a continuous gabbro massif ~40 km long by 30 km wide, overlying granular mantle peridotite that forms the lower slopes of the eastern wall of the Atlantis II Transform. Mapping shows that basement on the wave-cut platform consists largely of shallow-dipping amphibolitized gabbro mylonite generated by detachment faulting. This fault rooted near-continuously into partially crystalline gabbro for >4 million years. The mylonite exposed on the platform, and by cross-faulting and landslips on the sides of Atlantis Bank, both cut and are cut by steeply north dipping greenschist-facies diabase dikes. Thus, the gabbro crystallized at depth was uplifted into the zone of diking at the ridge axis, creating, in effect, the equivalent to the base of a dike–gabbro transition seen in many ophiolites. Previous Ocean Drilling Program (ODP) operations at Atlantis Bank drilled the 1508 m deep Hole 735B and 150 m deep Hole 1105A, both recovering long sections of gabbro. During Expedition 360, we propose to drill to a nominal depth of 1.3 km at a site on the northern edge of the Atlantis Bank platform, ~1 km north-northeast of Hole 1105A and ~2 km northeast of Hole 735B. A future drilling expedition, SloMo-Leg 2, aims to deepen the hole to ~3 km, with the overall goal of penetrating the crust–mantle transition, which is believed to be ~2.5 km above the seismically determined Moho. Specific objectives of Expedition 360 include establishing the lateral continuity of the igneous, metamorphic, and structural stratigraphies previously drilled to the southwest, testing the nature of a magnetic polarity transition, and investigating the biogeochemistry of the lower crust.