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doi:10.2204/iodp.pr.327.2010

Introduction and background

Scientific background

Fluid flow within the volcanic oceanic crust influences the thermal and chemical state and evolution of oceanic lithosphere and lithospheric fluids; the establishment and maintenance of subseafloor microbial ecosystems; the diagenetic, seismic, and magmatic activity along plate-boundary faults; the creation of ore and hydrate deposits both on and below the seafloor; and the exchange of fluids and solutes across continental margins (e.g., Alt, 1995; Huber et al., 2006; Parsons and Sclater, 1977; Peacock and Wang, 1999). The global hydrothermal fluid mass flux through the upper oceanic crust rivals the global riverine fluid flux to the ocean and effectively passes the volume of the oceans through the crust once every 10⁵–10⁶ y (Elderfield and Schultz, 1996; Johnson and Pruis, 2003; Mottl, 2003). Most of this flow occurs at relatively low temperatures, far from volcanically active seafloor-spreading centers where new ocean floor is created. This "ridge-flank" circulation can be influenced by off-axis volcanic or tectonic activity but is driven mainly by the transport of lithospheric heat from below the crust. Although the average maximum age at which measurable heat is lost advectively from oceanic lithosphere is 65 Ma (Parsons and Sclater, 1977), many sites remain hydrologically active for tens of millions of years beyond this age, with circulation largely confined to basement rocks that redistribute heat below thick sediments (Fisher and Von Herzen, 2005; Von Herzen, 2004).

Despite the importance of fluid–rock interactions in the crust, little is known about the magnitude and distribution of critical hydrologic properties; the extent to which crustal compartments are well connected or isolated (laterally and with depth); the rates and spatial extent of ridge-flank fluid circulation; or the links between ridge-flank circulation, crustal alteration, and geomicrobial processes. Integrated Ocean Drilling Program (IODP) Expedition 327 is part of a long-term experimental program that began nearly two decades ago and has included multiple survey, drilling, submersible, and remotely operated vehicle (ROV) expeditions; observatory and laboratory testing, sampling, and monitoring; and modeling of coupled fluid-thermal-chemical-microbial processes. Expedition 327 builds on the technical and scientific achievements and lessons learned during Ocean Drilling Program (ODP) Leg 168 (Davis, Fisher, Firth, et al., 1997), which focused on hydrothermal processes within uppermost basement rocks and sediments along an age transect (Fig. F1), and IODP Expedition 301 (Fisher, Urabe, Klaus, and the Expedition 301 Scientists, 2005), which penetrated deeper into the crust at the eastern end of the Leg 168 transect (Fig. F2). During both expeditions, subseafloor borehole observatories ("CORKs") were installed in basement holes to allow borehole conditions to recover to a more natural state after the dissipation of disturbances caused by drilling, casing, and other operations; to provide a long-term monitoring and sampling presence for determining fluid pressure, temperature, composition, and microbiology; and to facilitate the completion of active experiments to resolve crustal hydrogeologic conditions and processes (Fisher et al., 2005). During subsequent ROV and submersible expeditions, data were downloaded from the Leg 168 and Expedition 301 CORKs, and batteries, data loggers, and sampling systems at the seafloor and downhole were replaced.

The primary goals of Expedition 327 were to (1) drill two new basement holes, core and wireline log one of these holes across a depth range of 100–360 meters subbasement (msb), conduct a 24 h pumping and tracer injection test, and install a multilevel CORK in each of the new holes; (2) recover an existing CORK installed in a shallow basement hole during Leg 168, deepen the hole by ~40 m, and install a new multilevel CORK with instrumentation; and (3) recover and replace an instrument string deployed in one of the Expedition 301 CORKs.

Geological setting and earlier work

Many studies have summarized geology, geophysics, and basement-fluid chemistry and hydrogeology within young seafloor on the eastern flank of the Endeavour segment of the Juan de Fuca Ridge (e.g., Davis et al., 1992; Elderfield et al., 1999; Fisher et al., 2003; Hutnak et al., 2006; Mottl et al., 1998; Stein and Fisher, 2003; Wheat and Mottl, 1994; Wheat et al., 2000, 2003, 2004). Topographic relief associated with the Juan de Fuca Ridge axis and abyssal hill bathymetry on the ridge flank has helped trap turbidites flowing west from the continental margin (Fig. F1), which has resulted in the burial of young oceanic basement rocks under thick sediments. Sediment cover is regionally thicker and more continuous to the east, but there are seamounts and smaller basement outcrops up to 100 km east of the spreading center, north and south of the Expedition 327 work area. Regional basement relief is dominated by linear ridges and troughs (Fig. F3) oriented subparallel to the spreading center and produced mainly by faulting, variations in magmatic supply at the ridge, and off-axis volcanism. Low-permeability sediment limits advective heat loss across most of the ridge flank, resulting in strong thermal, chemical, and alteration gradients in basement.

During Leg 168, a transect of eight sites was drilled across 0.9–3.6 Ma seafloor; sediment, rock, and fluid samples were collected; thermal, geochemical, and hydrogeologic conditions in basement were determined; and a series of CORKs was installed in the upper crust (Davis, Fisher, Firth, et al., 1997). Two of the Leg 168 observatories were placed in 3.5–3.6 Ma seafloor in Holes 1026B and 1027C, near the eastern end of the drilling transect (Figs. F2, F3). Expedition 301 returned to this area and drilled deeper into basement; sampled additional sediment, basalt, and microbiological materials; replaced the borehole observatory in Hole 1026B; and established two additional CORK observatories at Site U1301 for use in long-term, three-dimensional hydrogeologic experiments (Fisher, Urabe, Klaus, and the Expedition 301 Scientists, 2005).

Before Leg 168 there was a largely two-dimensional view of the dominant fluid circulation pathways across the eastern flank of the Juan de Fuca Ridge, with recharge occurring across areas of basement exposure close to the ridge (near the western end of the Leg 168 transect) and flowing toward the east. Some results from Leg 168 are consistent with this view, including seafloor heat flow and basement temperatures that increase and basement fluids that are warmer and more altered with progression from west to east along the drilling transect (Davis et al., 1999; Elderfield et al., 1999; Stein and Fisher, 2003). However, Leg 168 results and subsequent surveys revealed inconsistencies with this simple model of large-scale hydrogeologic flow. Although basement fluids are warmer and older along the western end of the drilling transect with increasing distance from the ridge (from Sites 1023 to 1025), fluids are younger with respect to ¹⁴C at the next nearest site to the east (Site 1031) and even younger farther to the east (Site 1026), despite being warmer and more altered (Elderfield et al., 1999; Walker et al., 2007). In addition, reexamination of existing bathymetric data and collection of new data near Sites 1023 to 1025 show that basement outcrops to the north and south could allow hydrothermal fluids to recharge and discharge, with flow occurring largely perpendicular to the Leg 168 transect (Hutnak et al., 2006). It is also difficult to understand how basement fluids flowing from west to east at the western end of the Leg 168 transect might exit the crust where sediment cover is thick and continuous and there are no known outcrops (Davis et al., 1999; Hutnak et al., 2006).

Regional site surveys in preparation for Expedition 301 focused on and near basement outcrops that could be fluid entry and exit points to and from the crust (Fisher et al., 2003; Hutnak et al., 2006; Zühlsdorff et al., 2005). Thermal data suggest a significant component of south–north (ridge parallel, along strike) fluid flow in basement at the eastern end of the Leg 168 transect, an interpretation consistent with geochemical studies (Walker et al., 2007; Wheat et al., 2000). Bathymetric, sediment thickness, and heat flow data near the western end of the transect are also consistent with a significant component of north–south fluid flow in basement (Hutnak et al., 2006). Numerical models were created to simulate single-outcrop and outcrop-to-outcrop hydrothermal circulation between Grizzly Bare and Baby Bare outcrops (separated by 52 km in the along-strike direction) and to estimate the nature of basement properties that would allow these inferred patterns and rates of fluid circulation (Fig. F4). These studies show that outcrop-to-outcrop hydrothermal circulation can be sustained when basement permeability is ≥10^–12 m². At lower permeabilities, too much energy is lost during lateral fluid transport for circulation to continue without forcing, given the limited driving pressure difference at the base of recharging and discharging fluid columns (Hutnak et al., 2006). In addition, fluid temperatures in upper basement are highly sensitive to modeled permeability. When basement permeability is too high (10^–10 to 10^–9 m²), as interpreted from analyses of formation responses to tidal and tectonic perturbations, fluid circulation is so rapid that basement is chilled to temperatures below those seen regionally (modeled values of 20°–50°C). A good match is achieved to observed upper basement temperatures of 60°–65°C when lateral basement permeability is 10^–11 m² (Fig. F4).

Drill string packer experiments in upper basement during Expedition 301 indicate a layered crustal structure with permeabilities of 10^–12 to 10^–11 m² (Becker and Fisher, 2008). Additional hydrogeologic analyses completed using the formation pressure response to the long-term flow of cold bottom seawater into basement at Site U1301 in the 13 months after drilling, as observed at Site 1027 (2.4 km away) (Fisher et al., 2008), suggest large-scale permeability at the low end of or below values indicated by packer testing (Fig. F5). Results from borehole testing during and after Expedition 301 are broadly consistent with the global ensemble of measurements, but cross-hole tests indicate lower crustal permeability than do single-hole tests (Fig. F6). This result was unexpected because the former method is generally thought to test a much larger crustal volume than does the latter. The results from both sets of measurements and the difference between these permeability estimates and others based on modeling and analyses of formation responses to tidal and tectonic perturbations may be reconciled by azimuthal anisotropy in basement hydrogeologic properties. Azimuthal permeability anisotropy is also consistent with preferential flow in the north–south direction at both ends of the Leg 168 transect, inferred mainly from thermal and chemical observations, and with the highly faulted nature of the upper crust in the Expedition 327 field area (Fig. F3). Experiments conducted during and planned for after Expedition 327 will provide a direct test of permeability anisotropy using a network of sealed borehole observatories.

Seismic studies/Site survey data

Two site surveys were completed in 2000 in support of Expeditions 301 and 327. The Image-Flux survey was completed with the R/V Sonne and included nearly 500 lines of seismic data and extensive hydrosweep coverage (Zühlsdorff et al., 2005; Zühlsdorff and Spiess, 2006). The RetroFlux expedition was completed on the R/V Thomas G. Thompson with a focus on coring and heat flow and limited acquisition of hydrosweep data (Fisher et al., 2003; Hutnak et al., 2006). Finally, a 2002 expedition of the R/V Maurice Ewing collected multichannel seismic (MCS) data mainly across the Juan de Fuca Ridge, with one line positioned to cross Leg 168 and Expedition 301/327 drill sites and additional secondary sites (Carbotte et al., 2008; Nedimovic et al., 2008). Collectively, these data provided clear drilling targets for Expedition 327 operations (Fig. F3).

Conversions from two-way traveltime between the seafloor and top of basement to sediment thickness were developed by Davis et al. (1999) using drilling results from Leg 168 (Davis, Fisher, Firth, et al., 1997). Shipboard P-wave velocity measurements made on recovered sediments were combined to generate an equation for time–depth conversion. This conversion was shifted linearly to force a fit through basement depths determined during drilling, with a resulting sediment velocity range of 1500–1700 m/s. For Expedition 327, the greatest uncertainty in estimating depths for drilling goals from seismic data lay in picking targets where the upper basement surface is sloped and irregular. Experience from Leg 168 and Expedition 301 suggested that these picks have uncertainties of ±5–10 m, which is consistent with our experience during this expedition.

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