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doi:10.2204/iodp.sp.345.2012

Hess Deep Rift crustal section

Geological setting

The Hess Deep Rift is located in the vicinity of the Galapagos triple junction (ridge-ridge-ridge) at the intersection of the Cocos, Nazca, and Pacific plates (Fig. F3) (Hey et al., 1972, 1977; Holden and Dietz, 1972). The north-south–trending EPR is spreading at 130 mm/y, and the east-west–trending Cocos-Nazca rift is propagating westward toward the EPR at a rate of 42 mm/y. The major plate boundaries are marked by a series of minor ephemeral rifts to the north and south of the Cocos-Nazca spreading center (Schouten et al., 2008; Smith et al., 2011). These rifts formed in response to the regional stress fields associated with the westward propagation of the Cocos-Nazca spreading center, similar to extension associated with the propagation of a nonerupting dike (Schouten et al., 2008; Floyd et al., 2002). Reconstruction of the bathymetry in this region suggests that the current configuration of the Galapagos triple junction has been active for at least 10 m.y. (Smith et al., 2011). The Galapagos microplate was initiated at ~1.5 Ma. Although the cause is not known, it may be related to the formation of seamounts in the vicinity of Dietz Deep (Fig. F3)(Lonsdale, 1989; Smith et al., 2011; Schouten et al., 2008).

The Hess Deep Rift is a complex region that formed by extension in the wake of the westward propagating Cocos-Nazca spreading center. The surface expression of rifting is first evident ~30 km from the EPR, where two 5 km wide, east–west grabens expose ~0.5 Ma crust (Fig. F4). As the rift valley traces eastward, the small grabens merge and the rift broadens to ~20 km and deepens to >5400 meters below sea level (mbsl) at Hess Deep. An intrarift ridge rises to 3000 mbsl north of Hess Deep. Further to the east, the tip of the Cocos-Nazca spreading center starts to build a volcanic ridge that spreads at 55 mm/y. Uplifted rift escarpments rise to depths <2200 mbsl to the north and the south of the rift (Northern and Southern Escarpments), and their eastward extension defines the boundaries of the Hess Deep Rift and the “rough/smooth boundary” of the crust produced at the Cocos-Nazca spreading center, known as the Galapagos gore (Holden and Dietz, 1972) (Fig. F4).

The western end of the intrarift ridge and its southern slope, described in more detail below, has a coherent distribution of lithologies, with evolved gabbros capping the intrarift ridge and more primitive gabbro along its southern slope toward Hess Deep. Mantle peridotites crop out in the vicinity of Site 895 (Fig. F4) and along the western margin of the area surveyed for microbathymetry along the southern slope (Fig. F5). On-bottom seismic and gravity surveys indicate that the western end of the intrarift ridge and its southern slope are underlain by coherent crustal blocks (Ballu et al., 1999; Wiggins et al., 1996). These geophysical results, in combination with the regional geology, has led to a model whereby crustal blocks were unroofed by detachment faulting and block rotation on listric normal faults (Francheteau et al., 1990; Wiggins et al., 1996). This model is supported by structural and paleomagnetic data for ODP Site 894 that indicate that the intrarift ridge represents a large, intact crustal block that has been rotated along both horizontal and vertical axes (MacLeod et al., 1996b; Pariso et al., 1996). MacLeod et al. (1996b) conclude that emplacement of the intrarift ridge was accomplished by a combination of low-angle detachment and high-angle normal faulting. Refinement of this model is in progress, based on new bathymetric data collected as part of a site survey cruise for Expedition 345 (C. MacLeod, unpubl. data, 2008), that considers the role of dynamic uplift associated with the Cocos-Nazca spreading center (MacLeod et al., 2008) and gravitational collapse linked with rifting (Ferrini et al., unpubl. data). An earlier model, that calls for diapiric uplift of serpentinized mantle beneath the intrarift ridge uplift (Francheteau et al., 1990), has largely been abandoned.

The EPR at the latitude of the Hess Deep Rift is made up of many short first-order segments, separated by second-order overlapping spreading centers (Lonsdale 1988). Thus, the crust exposed at the Hess Deep Rift likely formed within a short segment, perhaps at a segment end (Lonsdale 1988). Reconstructions of the EPR flanks indicate several episodes of migrating offsets in the recent past and suggest that the crust now exposed in the Hess Deep Rift formed on the western margin of the EPR (Lonsdale, 1989; Smith et al., 2011; Mitchell et al., 2011). Multichannel seismic reflection profiling along the EPR flanks north of the Northern Escarpment indicates a crustal thickness of 5–5.5 km, with a seismic Layer 3 thickness of 3.0–3.5 km (Zonenshain et al., 1980).

Hess Deep Rift crustal section

The Hess Deep Rift exposes contiguous sections of the mid- to upper crust along the northern and southern bounding escarpments and sections of the mid- to lower crust along the rift valley floor. Extensive investigation of these crustal sections during three submersible cruises (Francheteau et al., 1992; Karson et al., 1992, 2002), a remotely operated vehicle (ROV) cruise (MacLeod et al., unpubl. cruise report, 2008), and Leg 147 (Gillis, Mével, Allan, et al., 1993), makes the Hess Deep Rift the best-studied tectonic window into fast-spreading crust. The geological relationships observed in the field, coupled with investigations of recovered samples, provide a comprehensive framework for the igneous, metamorphic, and tectonic processes active at the fast-spreading EPR. This geological framework is summarized below by lithologic unit and location; the reader is referred to the cited articles for more in depth review of our state of understanding of this region.

Upper crust

EPR upper crustal sections have been extensively studied along the northern rift escarpment ~29 km northeast of the intrarift ridge and, to a lesser degree, the escarpment south of Hess Deep (Fig. F4) (Francheteau et al., 1990; Karson et al., 1992, 2002). Lateral variation in the thickness and internal structure of the volcanic sequence and sheeted dike complex is attributed to temporal variations in magma supply (Karson et al., 2002). The lavas and dikes are normal depleted mid-ocean-ridge basalt (N-MORB) and fall into two compositional groups (Stewart et al., 2002). The main compositional group, which includes most of the dikes and some lavas, has Mg# = 52–66 [Mg# = Mg/(Mg + Fe) × 100]. Significant variations in the major and trace element concentrations and ratios of the dikes, on a scale of meters to kilometers, is interpreted to reflect the intercalation of dikes emanating from distinct magma reservoirs and transported along axis. A low-Fe group composed largely of lavas is attributed to the accumulation of plagioclase that acted to decrease magma bulk density and may enhance eruption potential, thus leading to the dominance of lavas in the low-Fe group (Stewart et al., 2005).

Hydrothermal alteration of the upper crust is largely focused in the sheeted dike complex, where the dikes are variably altered to amphibole-dominated or, locally, chlorite-dominated assemblages (Gillis, 1995; Gillis et al., 2001). No systematic variations are apparent in alteration assemblages or peak metamorphic temperatures with depth, similar to other sheeted dike sections (ODP Holes 504B [Alt et al., 1996] and 1256D [Alt et al., 2010]; Pito Deep [Heft et al., 2008]). The average minimum fluid/rock ratio and fluid flux for the sheeted dike complex, calculated by mass balance using whole-rock Sr isotope compositions, are 0.7 × 10⁶ and 1.5 × 10⁶ kg/m², respectively (Gillis et al., 2005), within the range for sheeted dike complexes from other fast- and intermediate-spreading ridges (Barker et al., 2008; Teagle et al., 2003).

Mid- to lower crust

EPR mid- to lower crust has been mapped and studied in three areas: the Northern Escarpment, subjacent to the sheeted dike complex described above; the western end of the intrarift ridge, including core recovered at Site 894; and the southern slope of the intrarift ridge between 4400 and 5400 mbsl (Fig. F4). The general characteristics of the plutonic and associated rocks for each of these areas is described, followed by a summary.

Northern Escarpment

Along the northern rift escarpment, the sheeted dike–plutonic transition is generally marked by a narrow zone of intercalated dikes and gabbros, indicative of a gradational boundary (Karson et al., 2002). In one location, the presence of thermally metamorphosed dikes suggests that locally the gabbros intruded into the base of the sheeted dike complex (Gillis, 2008). The plutonic sequence is dominated by gabbronorite, with lesser amounts of Fe-Ti oxide and amphibole gabbro, varitextured gabbro, olivine gabbronorite, Fe-Ti oxide gabbronorite, and rare tonalite (Hanna, 2004; Kirchner and Gillis, in press; Natland and Dick, 1996). Where the exposed plutonic sequence is thickest (~1000 m), a 150–200 m thick gabbro unit directly underlies the sheeted dike complex and overlies a >500 m thick gabbronorite unit. Elsewhere along the northern rift valley wall where plutonic exposures are much thinner (<140 m), the sheeted dike complex is directly underlain by either gabbronorite or gabbro.

The upper plutonic sequence has a wide range in Mg# (mean Mg# = 0.56; range = 0.76–0.30), with gabbronorite being the most primitive and Fe-Ti oxide ± amphibole gabbro being the most evolved end members (Hanna, 2004; Kirchner and Gillis, in press; Natland and Dick, 1996). The more evolved samples (Mg# < 0.6) are generally concentrated in the upper gabbro unit, with isolated samples within the underlying gabbronorite unit. In general, plutonic rocks from the Northern Escarpment are more evolved than those recovered from the intrarift ridge and Site 894 (Coogan et al., 2002a; Hékinian et al., 1993; Pedersen et al., 1996). Plagioclase displays extensive zoning and a large range in composition (An_35–70), and the Mg# of mafic phases is similarly broad (orthopyroxene Mg# = 56–68, and clinopyroxene Mg# = 30–80) (Gillis, 1995; Hanna, 2004). Geochemical evidence from magmatic amphibole (e.g., Cl content) hosted in the evolved gabbros, and whole rock Sr isotope compositions of relatively fresh gabbros (Kirchner and Gillis, in press), indicate that hydrothermally altered dikes were assimilated along the sheeted dike–plutonic transition, with the geochemical effects of assimilation reaching depths of at least 800 m into the gabbroic sequence (Gillis et al., 2003). Crystal-plastic deformation in the gabbroic sequence is rare, and weak to strong magmatic foliation is defined by the preferred orientation of plagioclase laths (Coogan et al., 2002a).

Gabbroic samples are variably altered by pervasive fluid flow along fracture networks to amphibole-dominated assemblages. The gabbroic rocks are significantly less altered (average = 11% hydrous phases) than the overlying sheeted dike complex (average = 24%), and the percentage of hydrous alteration diminishes with depth (Gillis, 1995; Kirchner and Gillis, in press). Incipient, pervasive fluid flow along microfractures occurred at amphibolite facies conditions (average temperature = 720°C), with slightly higher temperatures in the lower 500 m of the section (Coogan et al., 2002a; Kirchner and Gillis, in press; Manning et al., 1996). Fluid flow at lower temperatures leads to localized groundmass replacement by greenschist assemblages, as gabbroic outcrops display only minimal brittle deformation (Karson et al., 2002). Lower temperature alteration associated with tectonic unroofing is minimal and is manifest largely as oxidation halos and rare occurrences of clay minerals in the groundmass filling fractures.

Summit of intrarift ridge

Exposures of plutonic rocks at the summit of the western end of the intrarift ridge are located very near the sheeted dike–gabbro transition. This interpretation is based upon

1. The presence of dolerites with EPR compositions and regular north to north-northwest-trending, steeply westward-dipping joint patterns, interpreted as sheeted dike margins, intermixed with gabbroic rocks along the northern margin of the intrarift ridge (MacLeod et al., 2008);

2. The compositional overlap with the gabbroic sequence that underlies the sheeted dike complex exposed along the Northern Escarpment (Natland and Dick, 1996; Pedersen et al., 1996; Coogan et al., 2002a); and

3. Cooling rates comparable to the uppermost gabbros at the Northern Escarpment and Oman ophiolite (Faak et al., 2011; Coogan et al., 2007a).

The summit of the western end of the intrarift ridge is largely composed of olivine gabbro, gabbronorite, oxide gabbronorite, gabbro, and patches of pegmatitic amphibole gabbro (Fig. F6). These upper gabbroic rocks lack modal layering but locally contain lithologic boundaries identified by grain-size variation (Gillis, Mével, Allan, et al., 1993; Hékinian et al., 1993). The most chemically evolved gabbroic rocks (Mg# mean = 0.58; range = 0.35–0.68) are found along the northern margin of the intrarift ridge, north of proposed Site HD-04B and Site 894 (Fig. F7). The compositional range is slightly narrower at Hole 894G (Mg# mean = 0.65; range = 0.51–0.68) and becomes slightly more mafic along the slope immediately south of the summit (mean Mg# = 0.74; range = 0.65–0.85). Similar to the Northern Escarpment, plagioclase displays extensive zoning and a large range in composition (An_44–87), whereas the range of Mg # for mafic phases is less broad (olivine Fo_61–71, orthopyroxene Mg# = 64–74, and clinopyroxene Mg# = 62–81) (Pedersen et al., 1996; Natland and Dick, 1996; Kelley and Malpas, 1996).

The gabbroic rocks at Site 894 show no systematic variation in mineral or bulk rock compositions downhole but exhibit small-scale (1–5 m) excursions in elemental abundances (Natland and Dick, 1996; Pedersen et al., 1996). The parental melts are highly depleted (Pedersen et al., 1996), consistent with the refractory harzburgites recovered at Site 895 (Dick and Natland, 1996; Arai et al., 1997). Strongly zoned plagioclases and unusually high Cr contents of clinopyroxenes indicate early crystallization from a primitive magma (Natland and Dick, 1996). Local enrichment in incompatible elements in rocks that are relatively enriched in compatible elements indicates interactions of a partly crystallized matrix with a high abundance of interstitial melt (Pedersen et al., 1996). Alternatively, using a different geochemical approach that invokes a more evolved parental magma, Natland and Dick (1996) predict that a partly crystallized matrix interacted with very low abundances of interstitial melt.

Crystal-plastic deformation is rare in the gabbroic rocks at the summit of the intrarift ridge, with minor undulose extinction and very rare deformation twins in plagioclase. Similar to the Northern Escarpment gabbroic rocks, weak to strong magmatic foliation is defined by the preferred orientation of plagioclase laths (Hékinian et al., 1993; Coogan et al., 2002a; Gillis, Mével, Allan, et al., 1993; MacLeod et al., 1996a). Reorientation of some sections of the Hole 894G core to geographical coordinates shows that the foliations are steeply dipping (mean dip = 69°) with a nearly north–south orientation, parallel to the EPR, and that there is steeply plunging lineation (MacLeod et al., 1996a). Measurements of the anisotropy of magnetic susceptibility in samples show that there is also a magnetic fabric parallel to the plagioclase fabric (Richter et al., 1996).

Similar to the upper gabbroic sequence at the Northern Escarpment, incipient fluid flow was initiated along microfractures at high temperatures (Manning et al., 1996; Manning and MacLeod, 1996). When this upper gabbroic section had cooled to a temperature of ~450°C, it became influenced by the effects of Cocos-Nazca rifting, creating a dense array of east–west tensile fractures filled with greenschist to zeolite facies assemblages (Früh-Green et al., 1996; Manning and MacLeod, 1996). Away from the late-stage brittle fractures, gabbroic rocks are variably altered to amphibole-dominated assemblages (Früh-Green et al., 1996). Whole-rock samples unaffected by this lower temperature stage of brittle deformation are depleted in δ¹⁸O relative to fresh values (Agrinier et al., 1995; Lécuyer and Reynard, 1996) and show minor enrichment in ⁸⁷Sr/⁸⁶Sr (Lécuyer and Grau, 1996). Calculated fluid/rock ratios using both isotopic systems range from 0.1 to 1 (Lécuyer and Grau, 1996; Lécuyer and Reynard, 1996).

Mantle peridotites are exposed in the vicinity of Site 895, southeast of Site 894 (Fig. F4). Clinopyroxene-poor harzburgites, at the most depleted end of the range for abyssal peridotites, are interpreted to be the residues of melting of an N-MORB source (Dick and Natland, 1996). The association of dunite-troctolite-olivine gabbro with depleted harzburgite records the interaction of migrating melt with depleted harzburgite wall rock in the shallowest mantle (Dick and Natland, 1996; Arai and Matsukage, 1996; Arai et al., 1997). The geometry of these associations suggests that at least some melt transport was fracture controlled. Significant modal layering in the gabbroic rocks, a feature of the Oman mantle transition zone (Korenga and Kelemen, 1997), was not observed.

Lower slope of intrarift ridge

Plutonic rocks dominate the central and eastern region of the southern slope of the intrarift ridge, between 4400 and 5400 mbsl (Fig. F5). Plutonic rocks range in composition with gabbros, gabbronorites, and olivine gabbros that are, on average, more primitive than the summit of the intrarift ridge (mean Mg# = 0.71; range 0.39–0.85) (Fig. F7) (Blum, 1991; C. MacLeod, unpubl. data, 2009). Plagioclase is less zoned and on average is more anorthitic, and the Mg# of mafic phases have a more restricted range than the upper gabbros (plagioclase An_87–48; olivine Fo_72–88, orthopyroxene Mg# = 67–88, and clinopyroxene Mg# = 65–89) (Hékinian et al., 1993). Unlike shallower level gabbros, the Mg# and compatible elements in olivine and clinopyroxene co-vary, such that decreases in Mg# and compatible elements in the melts and rapid decreases in the most compatible elements can be largely explained by fractional crystallization processes (Coogan et al., 2002a; Hékinian et al., 1993). Layering was locally observed in outcrop, and many samples display weak to strong magmatic foliation.

Similar to the upper gabbroic sequence, incipient fluid flow was initiated along microfractures at high temperatures (Agrinier et al., 1995; Coogan et al., 2002a) and was later influenced by lower temperature hydrothermal alteration associated with Cocos-Nazca rifting (Agrinier et al., 1995).

Basaltic rocks associated with the intrarift ridge

Basaltic rocks, distributed throughout the intrarift ridge (labeled dolerite and basalt in Fig. F5), have two possible origins: (1) the EPR or (2) more recent intrusion of Cocos-Nazca magmas into rifted EPR crust, in advance of full Cocos-Nazca spreading. Although unequivocal chemical criteria to distinguish between EPR and Cocos-Nazca spreading center origin have not yet been established, basaltic samples that are more depleted in incompatible trace elements than EPR basalts recovered along the Northern Escarpment of the Hess Deep Rift (Stewart et al., 2002) or the equatorial EPR (Petrological Database of the Ocean Floor, www.petdb.org/) are commonly attributed to the Cocos-Nazca spreading center.

At the summit of the intrarift ridge, basaltic rocks along the northern margin of the intrarift ridge have EPR compositions and regular north- to north-northwest-trending, steeply westward-dipping joint patterns indicative of a sheeted dike complex (MacLeod et al., 2008). Rare basaltic dikes recovered from Hole 894G have more equivocal origins, being interpreted as originating at the EPR (Pedersen et al., 1996) or from a Cocos-Nazca spreading center source (Allan et al., 1996).

Along the southern slope of the intrarift ridge, basaltic rocks dominate between 4000 and 4400 mbsl, are intermixed with gabbroic rocks and peridotites in the central and eastern regions between 4400 and 5400 mbsl, and dominate in the western region between 4400 and 5400 mbsl (note that the western region lacks comprehensive coverage) (Fig. F5). The majority of the basaltic rocks (60%) have compositions that fall within the range of EPR sheeted dikes and lavas exposed along the northern rift valley wall (Stewart et al., 2002; Hékinian et al., 1993). The remaining samples are generally more depleted and primitive than the EPR dikes and lavas and thus may be associated with Cocos-Nazca rifting.

Summary

It is not possible to place the southern slope of the intrarift ridge into a precise structural context, as the nature of intermediate to lower plutonic crust formed at fast-spreading ridges is largely unknown. What we know very clearly is that the primitive gabbros structurally underlie regions where evolved gabbros dominate at the Hess Deep Rift. The geological relationships along the Northern Escarpment provide the best evidence for this, as evolved gabbros dominate the entire 800 m of the gabbro exposures that extend downward from the base of the sheeted dike complex. In this way, the southern slope of the intrarift ridge between 4400 and 5400 mbsl is likely analogous to the intermediate to lower plutonic rocks of the Oman ophiolite.

Gabbroic rocks from the Hess Deep Rift have a broad range in composition, from primitive troctolites to evolved Fe-Ti ± amphibole gabbros. In general, the plutonic sequence is dominated by cumulates, with the lower gabbros being more primitive and less enriched in incompatible elements than the upper gabbros. The nature of mantle-crossing melts and their evolution within the crust is still being debated. The geochemical trends constrained to date, from the Moho to the base of the sheeted dike complex, suggest the plutonic crust is built by fractional crystallization from primitive melts, with greater amounts of reaction with interstitial melt in the upper plutonics or crystallization of the upper plutonics from highly fractioned melts.

Gabbros from the Hess Deep Rift provide the first constraints on fracture formation and metamorphism in the root zones of hydrothermal systems at the EPR. Seawater influx into Layer 3 commenced soon after crystallization, with pervasive influx of water along randomly oriented microfracture networks and grain boundaries, mostly at temperatures of 600°–750°C. This permeability was probably created by tensile brittle failure upon subsolidus cooling and thermal contraction of the gabbro. When the upper plutonic section had cooled to a temperature of ~450°C, it became influenced by the effects of Cocos-Nazca rifting, and a dense array of east-west tensile fractures developed.

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