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doi:10.2204/iodp.sp.340T.2011

Background

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). Oceanic core complexes (OCCs), in particular, mark significant periods (1–2 m.y.) where 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 spreading center exhume the characteristic domal cores of an OCC, 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, and possibly more significant 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 contextual data from regional geophysical surveys, as well as seafloor mapping and sampling, is good and 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 intrusive 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 it was a slip surface associated with the detachment fault that unroofed the dome. Schroeder and John (2004) and Karson et al. (2006) document deformation within a zone that confirms the existence of a long-lived normal fault at the top of the Southern Ridge, with at least a few kilometers extent. The juxtaposition of volcanic eastern blocks against the corrugated dome, where 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; Nooner et al., 2003) and velocities (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, the apex of the Southern Ridge. The Central Dome extending smoothly to the north is several hundred meters deeper, and 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 et al., 2006, 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 cause(s?) 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/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

Multichannel seismic (MCS) data (Canales et al., 2004; Singh et al., 2004) shows significant reflectivity throughout the Central Dome and Southern Ridge (Fig. F2), but its cause 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 tens-of-meters thick, highly altered olivine-rich 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 (TWTT) 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 units (Fig. F2C). However, this 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., 2009, 2010) indicates that at least parts of the dome are capped by a 100–200 m thick low-velocity layer (< 4 km/s; Fig. F2D). Obtaining reliable first arrival times for VSP stations in the 50–200 m depth interval would provide groundtruth in this crucial interval, where imprints of detachment zone processes may extend beyond the very narrow, high deformation documented by talc-schist fault rock sampled only in the upper few meters at Site U1309 (Blackman et al., 2006, 2011; McCaig et al., 2010). Sonic logging in the 800–1415 mbsf interval will provide velocity constraints on the 1080–1200 mbsf highly altered olivine-rich troctolite interval (Fig. F3A). The V_P/V_S ratio of the ~350 mbsf olivine-rich units appears to be higher (~2.0) than average (~1.8) and the new data will show whether this is characteristic of these units. Also, the velocity of the 1100 mbsf fault zone, which is marked by a density low (Fig. F3C), will be measured for the first time.

Previous drilling at Site U1309

IODP Expeditions 304 and 305 cored and logged a 1.4 km, dominantly gabbroic section at Site U1309 (Fig. F3A). The presence of many thin interfingered petrologic units (Blackman et al., 2006; John et al., 2009), together with age dating (Grimes et al., 2008), indicates that the intrusions forming the domal core were emplaced over a minimum of 100–220 k.y. and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the uppermost 100 m but decreases in intensity with depth (Blackman et al., 2006; Nozaka et al., 2008; McCaig et al., 2010). Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, and igneous contacts and to an interval of locally intense serpentinization in olivine-rich troctolite (Beard et al., 2009; Nozaka and Fryer, 2011). Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized (Blackman et al., 2006; Michibayashi et al., 2008; Hirose and Hayman, 2008), despite paleomagnetic indications that the dome has undergone at least 45° rotation (Morris et al., 2009), presumably during unroofing via detachment faulting.

The main geochemical characteristics of Site U1309 rocks are consistent with formation as a cumulate sequence built from a series of parental mid-ocean-ridge basaltic (MORB) melt injections (Godard et al., 2009). Self-intrusion of cooling, partially crystallized magma likely occurred, and infiltration of evolved melt from a given intrusion into preexisting mafic cumulate rock certainly occurred. The age of zircon-bearing core samples (Grimes et al., 2008) is consistent with formation in the axial zone and a period of asymmetric spreading, with the footwall to a detachment fault moving at or near the full spreading rate for the segment. The few thin peridotite intervals transected at Site U1309 are residual, but petrographic and geochemical evidence indicate that later-formed or injected melts fluxed the residuum (Godard et al., 2009) or infiltrated it as dikelets (Tamura et al., 2008). Olivine-rich troctolites are the product of intense melt-rock interactions between an olivine-rich protolith (either ultramafic cumulate or mantle peridotite) and basaltic melt (Suhr et al., 2008; Drouin et al., 2009, 2010). They cannot simply be the primitive, first-crystallized cumulate within cooling magma. Such melt-rock interaction processes are expected to play a significant role in crustal accretion at slow-spreading ridges and to contribute through melt-rock interactions to MORB chemistry (Lissenberg and Dick, 2008; Drouin et al., 2010).

Alteration, via reaction with seawater, is pervasive in the upper few hundred meters of the core, but the lower part of the section, particularly at depths below 800 meters below seafloor (mbsf), has several intervals with very little alteration (Fig. F3B). Instances of alteration of the recovered core being 50% or greater are very rare below 750 mbsf (except in the 1080–1200 mbsf interval) but are common at shallower depths. By depths of 800 mbsf, instances of 40% or higher overall alteration are uncommon. By 850 mbsf, many instances of <10% alteration are reported (although less commonly in the 1080–1200 mbsf interval). Throughout, intervals with higher olivine content (e.g., olivine-rich troctolites) show greater overall alteration than surrounding lithologies (gabbro and less common diabase).

Wall rock density and resistivity were logged throughout the hole, and seismic data (check shot with ~50 m station spacing and sonic logging) were obtained in the uppermost 800 m of the borehole (Fig. F3C–F3E). Compressional velocity averages 5.62 ± 0.03 km/s in the 272–477 mbsf interval and 6.01 km/s in the 522–792 mbsf interval, with check shot–determined velocities tracking the average logged wall rock and core sample velocities (Collins et al., 2009).

A temperature log at the end of Expedition 305 shows small dips (a few degrees) in each of the 170, 750, and 1100 mbsf fault zones (Blackman et al., 2006), but given the disturbed condition immediately following drilling, these dips could simply reflect pooling of cool flushing fluid in local breakouts. Obtaining a (relatively) undisturbed temperature log will allow confident interpretation of any such dips near fault zones that are measured and provide a reliable indication of general borehole temperature at depth (previous maximum of 119°C was measured by the Temperature/Acceleration/Pressure [TAP] tool at 1400 mbsf).

Supporting site survey data for Expedition 340T are archived at the IODP Site Survey Data Bank and at the Marine Geoscience Data Center (www.marine-geo.org/​tools/​search/​entry.php?id=MAR:30N_Blackman) and the Academic Seismic Portal at the University of Texas at Austin’s Institute for Geophysics (www.ig.utexas.edu/​sdc/cruise.php?cruiseIn=ew0102; www.ig.utexas.edu/​sdc/​cruise.php?cruiseIn=ew9704).

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