Slow-spread ocean lithosphere accretes and evolves via temporally and spatially variable magmatic and tectonic processes (e.g., Bonatti and Honnorez, 1976; OTTER, 1984; Dick, 1989; Lin et al., 1990; Sinton and Detrick, 1992; Cannat, 1993; Lagabrielle et al., 1998). OCCs, in particular, mark significant periods (1–2 m.y.) when a distinct mode of rifting/accretion persists, in contrast to the more typical interplay between magma supply and faulting that generates the ubiquitous abyssal hills. Long-lived displacement along detachment faults active within the ~20 km wide axial zone of a slow-spreading center exhumes the characteristic domal cores of an OCC, which are often capped by spreading-parallel corrugations (e.g., Cann et al., 1997; Tucholke et al., 1998). Beneath this exposed fault zone, gabbroic rocks with lenses, or possibly greater volumes of mantle peridotite, are present, providing access to a major component of Earth’s deep lithosphere for detailed chemical and physical property investigations. Conditions of OCC development are documented by igneous and metamorphic assemblages, as well as by deformation recorded during evolution of the footwall.

Geological setting

Atlantis Massif is a young OCC where regional geophysical surveys and seafloor mapping and sampling coverage are good; major structural blocks within the faulted lithosphere have been identified (Fig. F1). The domal core of Atlantis Massif was unroofed via detachment faulting that occurred within the rift zone of the Mid-Atlantic Ridge at ~1.1–0.5 Ma (Blackman et al., 2011; Grimes et al., 2008). Atlantis Massif was initially hypothesized to be an OCC on the basis of morphologic and backscatter mapping and dredging results that documented the shallow, corrugated, and striated domal core underlain by mafic and ultramafic rocks (Cann et al., 1997). The spreading-parallel corrugations are equated with similar-scale features mapped on continental detachment faults (John, 1987) and suggest that the surface was a slip plane associated with the detachment fault that unroofed the dome. Schroeder and John (2004) and Karson et al. (2006) confirmed the existence of a long-lived normal fault at the top of the Southern Ridge by documenting deformation within a zone extending at least a few kilometers in length. The juxtaposition of volcanic eastern blocks against the corrugated dome, whose Southern Ridge samples include gabbroic rocks (~30%) and serpentinized peridotite (~70%), supports the OCC model. Gravity and seismic data indicate that significant portions of the footwall to the detachment contain rocks with anomalously high density (200–400 kg/m3 greater than surrounding rock) (Blackman et al., 2008) and velocity (4–6 km/s in the upper kilometer, compared to average Atlantic upper crust at ~3–5 km/s) (Canales et al., 2008; Collins et al., 2009). The active serpentinite-hosted Lost City hydrothermal vent field (Kelley et al., 2001; Früh-Green et al., 2003) is located just below the peak of the massif at the apex of the Southern Ridge. The Central Dome, extending smoothly to the north, is several hundred meters deeper; it is against only this part of the footwall that the juxtaposed volcanic hanging wall exists. It is assumed to overlie the detachment where it extends at depth.

Differences between the Central Dome and the domal Southern Ridge (Karson et al., 2006; Boschi et al., 2006; Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and the Expedition 304/305 Scientists, 2006; Blackman et al., 2011; Ildefonse et al., 2007; Canales et al., 2008) raise questions about how axial magmatism, the detachment system, and subseafloor alteration may have progressed in space and time as this core complex formed. If we can determine the geologic origin of reflectivity within the uplifted footwall to the detachment, future seismic imaging could provide definitive tests of models for along- and across-strike variation in the structure and development of oceanic core complexes. The availability of the 1415 m deep borehole at IODP Site U1309 provides a unique opportunity to groundtruth properties measured at seismic wavelengths.

Seismic studies/Site survey data

MCS data (Canales et al., 2004; Singh et al., 2004) show significant reflectivity throughout the Central Dome and Southern Ridge (Fig. F2), but the cause of this reflectivity is difficult to explain based on what is known about the dominantly gabbroic primary lithology at Site U1309. Results from Hole U1309D indicate that alteration varies quite rapidly downhole and there are a number of sharp changes in borehole resistivity, two of which coincide with the boundaries of several tens-of-meters thick, highly altered olivine-rich troctolite units (Fig. F3). The strong D-reflection, noted by Canales et al. (2004) to be pervasive throughout the dome and apparently an isolated event at 0.2–0.5 s two-way traveltime using initial processing, has been shown via a wide-angle reflection processing method (Masoomzadeh et al., 2005; Jones et al., 2007) to most likely be the first in a series of reflections (Fig. F2) (Singh et al., 2004). This reflective zone may be associated with altered olivine-rich troctolite units (Fig. F2C). However, this interpretation needs to be investigated more carefully using a better in situ velocity model and the best-possible ties to the core/borehole data.

Modeling of near-bottom explosive source (NOBEL) (Collins et al., 2009) and MCS streamer refraction traveltimes (Canales et al., 2008; Henig et al., 2010, in press) indicates that at least parts of the dome are capped by a 100–200 m thick low-velocity layer (<4 km/s) (Fig. F2D). Reliable first arrival times for vertical seismic profile (VSP) stations in the 50–200 m depth interval can provide groundtruth on this crucial interval, where imprints of detachment zone processes may extend beyond the very narrow, high-deformation interval documented by talc-schist fault rock sampled only in the upper several meters at Site U1309 (Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and the Expedition 304/305 Scientists, 2006; Blackman et al., 2011; McCaig et al., 2010). Sonic logging in the 800–1415 meters below seafloor (mbsf) interval can provide velocity constraints on the 1080–1200 mbsf altered olivine-rich troctolite interval (Fig. F3A). The VP/VS ratio of the ~350 mbsf olivine-rich units appears to be higher (~2.0) than average (~1.8), and the Expedition 340T data can show whether this is characteristic of these units.