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Analysis of seismic refraction data at Atlantis Massif (Collins and Detrick, 1998) indicates that velocities of 8 km/s occur within several hundred meters of the seafloor in at least parts of the core of the massif (Figs. F2, F3A). The inferred gradient of seismic velocity in the central dome of Atlantis Massif is similar to that determined near Ocean Drilling Program (ODP) Site 920, where 100–200 m of serpentinized peridotite was drilled (Fig. F3B). This gradient is quite distinct from that characterizing gabbro-hosted Atlantis Bank (Southwest Indian Ridge) and other sections of the MAR. Multichannel seismic (MCS) reflection data show a major difference in the structure of the outside (conjugate) corner lithosphere versus that hosting Atlantis Massif (Canales et al., 2004). Although the seismic Layer 2a/2b boundary is well imaged on the eastern flank of the ridge axis, it is not evident on the western flank where the massif is located. A strong reflector is visible at 0.2–0.5 s two-way traveltime below much of the domal surface (Fig. F3C, F3D). Two interpretations have been posed: (1) an alteration/serpentinization front or (2) a structurally deeper detachment fault, subparallel to the seafloor beneath the dome (Canales et al., 2004).

Modeling of sea-surface (Fig. F4) and sparse seafloor gravity data (Blackman et al., 1998, 2004; Nooner et al., 2003) suggests that there is a wedge-shaped body in the domal core with density 200–400 kg/m3 greater than the surrounding rock. In the model, the footwall is overlain by tilted hanging wall blocks that are capped by material with density typical of upper crustal rock (2.5–2.7 kg/m3). The interface between the model blocks on the east is a gently inclined (15°–25°) boundary that dips more steeply than the exposed corrugated surface (~11°) where it meets that hanging wall. It is possible that the density interface coincides with the base of the detachment fault zone, a region inferred to be highly altered and therefore of lower density.

Rock samples collected by the manned submersible Alvin and a dredge from the central dome are mostly angular talus and rubble of metabasalt and limestone (Fig. F2) (Cann et al., 2001; Blackman et al., 2004). A few samples showing cataclastic deformation fabrics or highly serpentinized and metasomatically altered peridotite were also recovered. The protolith of most of the serpentinite sampled on the south wall of the massif is harzburgite (Fig. F2). These rocks are commonly cut by highly altered gabbroic veins composed dominantly of talc, tremolite, and chlorite (Früh-Green et al., 2001; Schroeder et al., 2001). Low-temperature overprinting, seafloor weathering, and carbonate vein formation mark the youngest phases of alteration.

Microstructural analysis of samples from the south wall indicates shear deformation and dilational fracturing at metamorphic conditions ranging from granulite to subgreenschist facies (Schroeder et al., 2001). Ductile fabrics in peridotite samples are overprinted by semibrittle and brittle deformation (Schroeder and John, 2004). Stable mineral assemblages of tremolite, chlorite, and chrysotile indicate that the latter process occurred at <400°C. The distribution of samples and their deformation characteristics suggest that strong semibrittle and brittle deformation is concentrated at shallow structural levels (<90 m beneath the domal surface) at the southern ridge (Schroeder and John, 2004). Outcrop mapping with the Alvin and photomosaics constructed from Argo digital still camera images show that this uppermost fault extends across much of the top of the southern ridge (J. Karson, pers. comm., 2005).

Only a few expeditions in the history of scientific ocean drilling have recovered lower crust and upper mantle rocks near a mid-ocean ridge axis (Fig. F5). During Deep Sea Drilling Project (DSDP) Leg 45 (Melson, Rabinowitz, et al., 1979) on the western flank of the MAR south of the Kane Fracture Zone, a 587.9 m deep hole was drilled into sediments and basaltic basement. A few gabbro cobbles were recovered from the top of the core, and two serpentinized harzburgite and lherzolite cobbles were trapped between two basaltic units. During DSDP Leg 82 (Bougault, Cande, et al., 1985), with a drilling plan designed to address regional variations in basalt chemistry along the ridge axis at three sites (DSDP Sites 556, 558, and 560) between 34°43'N and 38°56'N, a few tens of meters of metamorphosed gabbro and pervasively serpentinized peridotite represented the first in situ recovery of these lithologies. During ODP Leg 109 (Bryan, Juteau, et al., 1988), the first intentional drilling for mantle peridotites at a mid-ocean ridge at ODP Site 670 was completed with 7% recovery of sepentinized peridotite. Figure F5 shows all holes (recovery > 5%) in upper mantle and lower crustal rocks drilled to date at or near mid-ocean ridges during nine different ODP and IODP expeditions. ODP Leg 147 at Hess Deep (Gillis, Mével, Allan, et al., 1993) is the only one which took place in crust created at a fast-spreading ridge (Hess Deep, East Pacific Rise).

Atlantis Massif is the fourth location where drilling an inside corner high and/or a corrugated dome at a slow-spreading ridge has been attempted by ODP or IODP. A total of 11 holes (>10 m deep) were cored at 6 different sites in 3 different locations (Atlantis Bank, Southwest Indian Ridge, 57°16'W; MAR, 15°44'N; Kane Fracture Zone [MARK] area, MAR, 23°32'N) during ODP Legs 118, 153, 176, and 209 (Robinson, Von Herzen, et al., 1989; Cannat, Karson, Miller, et al., 1995; Dick, Natland, Miller, et al., 1999; Kelemen, Kikawa, Miller, et al., 2004). In all of these holes, the dominant rock type recovered is gabbro and ranges from diabase to troctolitic in composition. The maximum distance between two holes in each of these regions is 2.8 km (MARK), 1.3 km (Atlantis Bank), 900 m (MAR 15°44'N), and 20 m (Atlantis Massif). ODP Site 1270 is located on a crest of an axis-parallel ridge, inferred to be an OCC by Fujiwara et al. (2003), but with no obvious corrugation pattern on the available bathymetric data. ODP Site 1272 is located at an inside corner massif, which does not culminate higher than other hills in the 15°20'N Fracture Zone area.

The gabbroic and ultramafic rocks recovered during previous expeditions at slow-spreading ridges show a wide range of geochemical compositions (Fig. F6), reflecting a strong lithologic heterogeneity, from very primitive troctolitic assemblages to evolved, oxide-rich gabbros (ODP Leg 209; Kelemen, Kikawa, Miller, et al., 2004).

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