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doi:10.2204/iodp.proc.304305.101.2006

Background

Atlantis Massif formed within the past 1.5–2 m.y., and it currently bounds the median valley on the western flank of the Mid-Atlantic Ridge (MAR) at 30°N (Fig. F1). The corrugated, striated central portion of the domal massif displays morphologic and geophysical characteristics inferred to be representative of a structural component of an oceanic core complex (OCC) exposed via long-lived detachment faulting (e.g., Cann et al., 1997; Blackman et al., 1998, 2004; Collins et al., 2001). The “core” of the complex comprises crust and possibly upper mantle rocks, denuded by a detachment fault exposed over an 8–10 km wide, 15 km long area that forms the elongate, doubly plunging domal seafloor morphology of Atlantis Massif. An adjacent basaltic block to the east is interpreted as the hanging wall to the detachment fault. A thin cover of variably lithified sediment, volcanic deposits, and rubble on the dome of the massif impedes seafloor mapping and sampling of the fault surface. The sediment-draped volcanic morphology and basalt sampled from scarps in the eastern block show its general character; the location of its contact with the dome can only be inferred from the break in slope. The detachment is inferred to dip beneath the seafloor at the base of the dome and to continue at a shallow angle (<15°) beneath the eastern block toward the present-day ridge axis.

Atlantis Massif shows significant along-axis variations in morphology; evolution of the southern portion of the massif may differ from that of the central dome. The Southern Ridge (Fig. F1) is shallower than the central dome, shoaling to 700 m below sea level (mbsl). There, the corrugated surface extends eastward to the top of the median valley wall. Exposures along the south face of the massif represent a cross section through the core complex and display a fault system as thick as ~100 m (Schroeder and John, 2004). The serpentinization-driven Lost City hydrothermal vent field is just below the summit of the Southern Ridge (Kelley et al., 2001, 2005; Früh-Green et al., 2003).

Analysis of seismic refraction data (Fig. F2) at Atlantis Massif (Fig. F3A, F3B) (Collins and Detrick, 1998; Collins et al., 2002) indicates that velocities of ~7.5–8 km/s may occur within several hundred meters below seafloor at least locally in parts of the core of the massif. The gradient of seismic velocity in at least parts of the central dome of Atlantis Massif is shown to be similar to that determined (Collins et al., 2001) near Ocean Drilling Program (ODP) Site 920, where 100–200 m of serpentinized peridotite was drilled. The determined gradient is quite distinct from that characterizing gabbro-hosted Atlantis Bank (Southwest Indian Ridge) and other sections of the MAR.

Interpretation of multichannel seismic (MCS) reflection data (Fig. F2) suggests a major difference in structure between the outside (conjugate) corner lithosphere versus that hosting Atlantis Massif (Canales et al., 2004). The Layer 2a/2b boundary is quite clear on the eastern flank of the ridge axis, but it is not evident on the western flank across the massif. A strong reflector is visible at 0.2–0.5 s below much of the domal surface (Fig. F3C, F3D) and coincides roughly with the depth below which mantle velocities (~8 km/s) are inferred from the seismic refraction data. One interpretation suggests that the reflector marks an alteration front within the peridotite-dominated massif (Canales et al., 2004).

Modeling of sea-surface gravity (Fig. F4) and sparse seafloor data (Blackman et al., 1998, 2004; Nooner et al., 2003) suggest that rocks beneath the central and southern dome have densities 200–400 kg/m³ greater than the surrounding rock. Results from a two-dimensional model support the interpretation that the footwall is overlain by tilted hanging wall blocks capped by rocks with density typical of the upper crust (2.5–2.7 kg/m³). The interface between the model blocks in 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, possibly coinciding with the base of a fractured, highly altered detachment fault zone.

In situ rock samples (Fig. F2) from scarps, side-scan imagery, and gravity data suggest that the majority of the hanging wall block is composed of erupted basalt. Seismic data show a discontinuous but persistent reflector 0.2–0.5 s beneath the seafloor, which Canales et al. (2004) suggest coincides with the projection of the corrugated slope beneath the western edge of the hanging wall block. Canales et al. (2004) interpret this reflector to be the unexposed detachment fault. Assuming an average velocity of 4 km/s in fractured basalt, the reflector is predicted to occur at 200–300 meters below seafloor (mbsf) at the hanging wall drill site.

Rock samples collected by the manned submersible Alvin from the central dome are dominated by angular talus and rubble of serpentinized peridotite, metabasalt, and limestone (Cann et al., 2001; Blackman et al., 2004). A few samples from the central dome show cataclastic deformation or are highly serpentinized and/or metasomatically altered peridotite. The protolith of most of the serpentinite sampled on the south wall of the Southern Ridge is harzburgite. Highly altered gabbroic veins composed dominantly of talc, tremolite, and chlorite commonly cut these rocks (Früh-Green et al., 2001; Schroeder et al., 2001). Talc-rich fault rocks preserve textural and geochemical characteristics of their ultramafic protoliths (Boschi et al., 2006). Low-temperature seafloor weathering and carbonate vein formation mark the youngest phases of alteration in these samples.

Microstructural analysis of samples from the south wall of the Southern Ridge 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 processes occurred at temperatures <400°C. The distribution of samples suggests that strong semibrittle and brittle deformation is concentrated at shallow structural levels (<90 m beneath the domal surface) along 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 surface extends across much of the top of the Southern Ridge (Karson, 2003).

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 37 (Aumento, Melson, et al., 1977), ~10 m of interlayered gabbro, olivine gabbro, and serpentinized peridotite were recovered at Site 1334 (37°N; FAMOUS area) During DSDP Leg 45 (Melson, Rabinowitz, et al., 1979) on the western flank of the MAR south of the Kane Fracture Zone (MARK), 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 basaltic units. The drilling plan for DSDP Leg 82 (Bougault, Cande, et al., 1985) was designed to address regional variations in basalt chemistry along the ridge axis at three sites (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 were recovered. During ODP Leg 109 (Detrick, Honnorez, 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 Integrated Ocean Drilling Program (IODP) expeditions. ODP Leg 147 (Gillis, Mével, Allan, et al., 1993) is the only deep crustal drilling program that 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 16 holes (>10 m deep) were cored at 8 different sites in 3 different locations—Atlantis Bank, Southwest Indian Ridge, 57°16′W; MARK, 23°32′N; and MAR, 15°44′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 gabbroic 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).

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