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doi:10.2204/iodp.sp.327.2010

Background

Geological setting and earlier work

Many studies summarize the geology, geophysics, and basement-fluid chemistry and hydrogeology of young seafloor on the eastern flank of the Endeavour segment of the Juan de Fuca Ridge (JFR) (e.g., Davis et al., 1989, 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 JFR axis and abyssal hill bathymetry on the ridge flank have helped trap turbidites flowing west from the continental margin (Fig. F1). This 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 located as far as 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 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.

Leg 168 completed a transect of eight sites across 0.9–3.6 Ma seafloor, collecting sediment, rock, and fluid samples; determining thermal, geochemical, and hydrogeologic conditions in basement; and installing a series of CORK observatories 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 (Fig. F1). 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, et al., 2005).

Before Leg 168 there was a largely two-dimensional view of the dominant fluid circulation pathways across the eastern flank of the JFR, with recharge occurring across large areas of basement exposure close to the ridge (near the western end of the Leg 168 transect) and then 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 farther to the east along the drilling transect (Davis et al., 1999; Elderfield et al., 1999; Stein and Fisher, 2003). However, Leg 168 results also revealed inconsistencies with this conceptual model of large-scale hydrogeologic flow. Although basement fluids warm and age along the western end of the Leg 168 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 and collection of additional bathymetric data along the western end of the Leg 168 transect show that basement outcrops to the north and south could allow hydrothermal fluids to recharge and discharge, with flow occurring largely perpendicular to the transect (Hutnak et al., 2006). Leg 168 results also present the vexing problem of explaining where fluids flowing toward the east at the western end of the Leg 168 transect might exit the crust (Davis et al., 1999).

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 that allow hydrothermal flows to bypass generally thick and impermeable sediments (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 Leg 168 transect are consistent with a significant component of north–south fluid flow in basement in this area (Hutnak et al., 2006). Numerical models created to simulate outcrop-to-outcrop hydrothermal circulation between the 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 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), 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.

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. Results from both sets of measurements, as well as 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. The hypothesis that basement permeability is anisotropic is also consistent with preferential flow in the north–south direction at both ends of the Leg 168 transect, based mainly on thermal and chemical observations, and will be tested directly during and after Expedition 327, when multidirectional cross-hole experiments are run 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 ImageFlux survey was completed with the R/V Sonne, including collection of nearly 500 lines of seismic data and extensive hydrosweep coverage (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; Wheat et al., 2000). Finally, a 2002 expedition of the R/V Maurice Ewing collected multichannel seismic (MCS) data mainly across the JFR, with one line positioned to cross Leg 168 and Expedition 301/327 drilling sites (Carbotte et al., 2008; Nedimovic et al., 2008). This seismic line also crosses the secondary Deep Ridge (DR) sites. Collectively, these data provide clear drilling targets for Expedition 327 operations.

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 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 target goals from seismic data comes from picking targets on a narrow basement peak where the upper basement surface is somewhat irregular. However, experience from Leg 168 and Expedition 301 shows that these picks have uncertainties equivalent to ±5 m to basement.

The supporting site survey data for Expedition 327 are archived at the IODP Site Survey Data Bank.

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