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Results of Sea-Surface Mapping, Seismic, and Submersible Studies

The Atlantis Massif formed within the past 1.5–2 m.y., and it currently bounds the median valley of the Mid-Atlantic Ridge (MAR) on the west side. The corrugated, striated central portion of this domal massif displays morphologic and geophysical characteristics inferred to be representative of an oceanic core complex (OCC) exposed via long-lived detachment faulting (Fig. F1) (Cann et al., 1997; Blackman et al., 1998, 2004; Collins et al., 2001). The adjacent basaltic block to the east is interpreted as the hanging wall to the detachment fault. A thin cover of lithified sediment and rubble on the dome of the massif impedes seafloor mapping and sampling of the fault surface. Evolution of the southern portion of the massif differs somewhat from that of the central portion. The southern ridge (Fig. F1) has experienced greater uplift, shoaling to 700 m below sea level. At the southern ridge 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. The serpentinization-driven Lost City hydrothermal vent field is located just below the summit of the southern ridge (Kelley et al., 2001, 2003; Früh-Green et al., 2003).

The results of seismic refraction experiments at Atlantis Massif (Fig. F2) (Detrick and Collins, 1998) indicate that velocities of 8 km/s occur within several hundred meters of the seafloor in at least parts of the core of the massif. The 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. The gradient is quite distinct from that characterizing gabbro-hosted Atlantis Bank (Southwest Indian Ridge) and other sections of the MAR.

Multichannel seismic reflection data (Fig. F3) show a major difference in structure of the outside (conjugate) corner lithosphere versus that hosting Atlantis Massif (Canales et al., 2004). The seismic Layer 2a/2b boundary is quite clear on the eastern flank of the ridge axis but it is not evident on the western flank where the massif occurs. A strong reflector is visible at 0.2–0.5 s below much of the domal surface (Fig. F4) and coincides with the depth below which mantle velocities (~8 km/s) are deduced. One interpretation suggests that the reflector marks an alteration front within the peridotite-dominated massif.

Modeling of sea-surface and sparse seafloor gravity data (Fig. F5) (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 likely 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. Sea-surface gravity data indicate that the extent of alteration within the southern ridge is greater than that of the central dome (Blackman et al., 2004).

In situ rock samples, side-scan imagery, and gravity data suggest that the majority of the hanging wall block comprises erupted basalt. Seismic data show a discontinuous but persistent reflector at 0.2–0.5 s beneath the seafloor (Fig. F6), which, according to Canales et al. (2004), coincides with the projection of the corrugated slope beneath the western edge of the hanging wall block. They interpret this reflector to be the unexposed detachment fault. Assuming an average velocity of 4 km/s in fractured basalt, the detachment is predicted to occur at 200–300 m depth below seafloor at the hanging wall drill site.

Rock samples collected by the manned submersible Alvin from the central dome are mostly angular talus and rubble of peridotite, metabasalt, and limestone (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. 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, 2003). Stable mineral assemblages of tremolite, chlorite, and chrysotile indicate that the latter process occurred at <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) at the southern ridge (Schroeder, 2003). 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 (Karson, 2003).

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