BACKGROUND

Geological Setting

The Endeavour segment of the Juan de Fuca Ridge (JFR) generates lithosphere west of North America (Fig. F1). Topographic relief produces barriers to turbidites from the continental margin, resulting in the accumulation of sediment and burial of the eastern flank of the JFR within Cascadia Basin. Oceanic basement is exposed to the west, where the crust is young, and the sedimented seafloor is relatively flat to the east, except over seamounts. 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. Basement relief is relatively low near the ridge (–200 m) and higher (–700 m) to the east. Low-permeability sediment limits advective heat loss across most of the ridge flank, leading to strong thermal, chemical, and alteration gradients in basement.

The study area contains structural features common to most ridge flanks: extrusive igneous basement overlain by sediments, abyssal hill topography, high-angle faulting, and basement outcrops. Whereas the work sites may not be typical of all ridge-flank settings (higher than average sedimentation rate, younger buried basement, and stronger lateral gradients in temperature), the field area is ideal, in part because of these extreme conditions. High gradients result in strong signals that rise above natural and experimental noise. The high sedimentation rate allows us to work on crust that is much younger than we could study otherwise, providing indications of ridge-crest as well as ridge-flank properties and allowing study of sites in different hydrologic settings that are close together. Because many experiments have been completed in this area (seismic, thermal, geochemical, and surface/borehole), we can "calibrate" and compare interpretations based on different methods.

During ODP Leg 168, an 80 km transect comprising 10 sites was drilled on the eastern flank of the Juan de Fuca Ridge (Davis, Fisher, Firth, et al., 1997). Borehole (Cork) observatories were installed at Sites 1024 and 1025, near the western end of the drilling transect, and Sites 1026 and 1027 (Figs. F2, F3, F4, F5, F6, F7, F8), at the eastern end of the transect (Davis and Becker, 2002; Davis et al., 2000). Hydrogeologic results from Leg 168 include documentation of large lateral gradients in fluid temperatures, compositions, and ages in upper basement with distance from the ridge axis; local, thermal, and chemical homogeneity between adjacent sites; and rapid fluid flow rates and very small driving forces. Extensive, across-strike advection in the upper crust may be inferred from these overall trends (e.g., Davis et al., 1997, 1999; Elderfield et al., 1999; Spinelli and Fisher, 2004; Stein and Fisher, 2003), which requires very high formation permeability. This interpretation is consistent with Cork observations indicating extreme transport properties and high rates of fluid flow (Davis and Becker, 2002, in press). Packer and open hole experiments also indicate high permeabilities in uppermost crust and a systematic evolution in crustal permeability with age (Becker and Davis, 2003; Becker and Fisher, 2000). However, there are large differences in permeability estimated with different methods, likely resulting from either differences in interpretive assumptions or a scale effect (Becker and Davis, 2003; Becker and Fisher, 2000; Fisher and Becker, 2000; Fisher, 1998; Fisher et al., 1997).

There is also geochemical and thermal evidence for along-strike fluid flow, perhaps related to abyssal hill topography and associated faulting. Site 1026 is located along a buried basement ridge (Second Ridge; SR) and ~8 km from Baby Bare and Mama Bare outcrops, the tops of which are exposed to the south and north, respectively (Figs. F2, F3). Fluids collected from shallow sediments, Baby Bare springs, and Hole 1026B are similar in bulk composition (Mottl et al., 1998; Wheat and Mottl, 2000), but subtle differences in fluid chemistry indicate increased water-rock interaction from south to north. The young ¹⁴C age of Hole 1026B water (4–5 ka) suggests that it is chemically distinct from older fluid to the west (Elderfield et al., 1999). Geochemical and thermal data suggest that Baby Bare springs recharge through basement more than 50 km south at Grizzly Bare outcrop (Fisher et al., 2003a). Although basement fluids from Sites 1030 and 1031 (First Ridge; FR) are also young (Fig. F2), they interacted chemically with basement as extensively as much warmer fluids to the east and are chemically distinct from crustal fluids to the west (Davis, Fisher, Firth, et al., 1997; Elderfield et al., 1999; Wheat et al., 2000). There are also important differences in composition between fluids from Baby Bare and Site 1027 (Wheat et al., submitted [N1]). Thus, there is geochemical evidence for the presence of distinct hydrologic systems within shallow basement in this area.

ODP Hole 1026B yielded some of the first direct microbiological observations of ridge-flank fluids. Rock and fluid samples collected during Leg 168 indicated the possible presence of microbes (Bach and Edwards, in press; Fisk et al., 2000), and a "biocolumn" experiment assessed microbial biomass and diversity in fluids venting from a subseafloor observatory. Cells collected from the biocolumn included bacteria and archaea, comprising nitrate reducers, thermophilic sulfate reducers, and thermophilic fermentative heterotrophs (Cowen et al., 2003).

The First Ridge area on 1.4 m.y. old crust (Fig. F7, F8) is known to experience upward fluid seepage through shallow sediments at a few millimeters per year (Spinelli et al., 2004; Wheat and Mottl, 1994). Sites 1030 and 1031 were positioned above a high-angle normal fault on a buried basement high. Heat flow above this ridge is elevated locally over that through surrounding crust of the same age (Davis et al., 1999; Davis, Fisher, Firth, et al., 1997), indicating that basement hosts vigorous convection. Measurements and sampling during Leg 168 revealed altered basal sediments and upper basement temperatures on the order of 40°C, but basement fluid composition is consistent with water-rock interaction at ~65°–70°C (Davis, Fisher, Firth, et al., 1997; Elderfield et al., 1999). In fact, basement fluid chemistry in this area is very similar to that at Baby Bare springs and Site 1026, 60 km to the east (Elderfield et al., 1999; Mottl et al., 1998; Wheat et al., 2000). Surprisingly, the ¹⁴C age of Site 1030/1031 fluids is considerably younger than fluids at Site 1025 to the west.

Several studies have noted seismic anomalies (places where lateral continuity of seismic layers is disrupted) (Zühlsdorff and Spiess, 2001; Zühlsdorff et al., 1999). Seismic anomalies are commonly associated with areas of seafloor seepage. Sediments from Sites 1030 and 1031 have porosities and permeabilities significantly greater than those of the surrounding sediments, but these properties are consistent with deposition of normal hemipelagic material over local basement highs (Giambalvo et al., 2000). The distribution of seafloor seepage may be explained largely by a combination of basement relief, differential sediment thickness, heating from below, and variations in sediment properties (Spinelli et al., 2004).

Site 1030/1031 pore fluid may be upwelling through shallow sediments from a hydrothermal system deeper than that inferred at nearby drilling sites. Greater fluid alteration probably indicates a higher maximum temperature, followed by conductive cooling during fluid ascent. Observed ¹⁴C dates preclude an alternative interpretation that Site 1030/1031 basement fluids are simply older. These fluids could not have recharged from areas of exposed basement 20 km to the west, since their age and composition is inconsistent with flow along this path. An alternative hypothesis is that these fluids recharged through seamounts (e.g., Fisher et al., 2003a, 2003b) north or south of the Leg 168 transect, perhaps flowing in basement through permeability enhanced by an along-strike crustal fabric.

Marine geophysical surveys in this region began in the 1950s and 1960s, but the first detailed studies of this ridge flank intended to resolve the existence and influence of hydrothermal circulation were completed in the mid- to late-1980s (Davis et al., 1989; Rohr, 1994). These studies included single-channel and multichannel seismic (MCS), gravity, magnetics, and heat flow, along with coring (with associated sediment and pore fluid analyses). Becker et al. (2000) show results from a 1992 John P. Tully survey that collected seismic data in the Second Ridge area, with an emphasis on nearby basement outcrops. Seismic results from two later surveys are summarized by Rosenberger et al. (2000).

Two additional site surveys were completed in 2000. The ImageFlux survey was completed with the Sonne (SO149; Chief Scientist: V. Spiess; seismic acquisition and processing: L. Zühlsdorff, both from University of Bremen, Germany), including collection of nearly 500 lines of seismic data and extensive hydrosweep coverage. The 2000 RetroFlux expedition was completed with the Thomas G. Thompson (Co-Chief Scientists: A. Fisher, University of California, Santa Cruz, USA; E. Davis, Pacific Geoscience Center, Geological Survey of Canada; M. Mottl, University of Hawaii, USA; and C.G. Wheat, University of Alaska, USA), with a focus on coring and heat flow (and limited acquisition of hydrosweep data). Finally, a 2002 expedition of the Maurice Ewing (Chief Scientist: S. Carbotte) collected MCS data mainly across the Juan de Fuca Ridge, but one line crossed the Expedition 301 area. Collectively, these data provide clear drilling targets at depth.

Conversions from two-way traveltime (TWT) 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-to-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 301, the greatest uncertainty in estimated depths to drilling targets results from picking targets on a narrow basement peak where the upper basement surface is somewhat irregular, but confidence in these picks is relatively high where previous drilling helped confirm the nature of prominent subseafloor reflectors (SR and FR areas).