Geologic context

IODP Site U1364 lies roughly 20 km landward of the toe of the Cascadia subduction zone accretionary prism, where much of the thick section of turbidite and hemipelagic sediments deposited on the eastern flank of the Juan de Fuca Ridge are scraped off the underthrusting oceanic crust (Fig. F1). Convergence of the Juan de Fuca oceanic plate relative to the North American continental plate occurs in a direction roughly normal to the continental margin and at a rate of roughly 42 mm/y (DeMets et al., 1990). A topographic trench at this subduction zone is absent as a consequence of the extremely high rate of glacial sediment supply from the continent during the Pleistocene. The majority of the supply has been impounded by the elevated igneous crustal topography of the Juan de Fuca Ridge to form what is called Cascadia Basin. At the accretionary prism toe (also referred to as the deformation front, where the thrust-fault interplate interface intersects the seafloor), the sediments of Cascadia Basin are >2 km thick; at Site U1364 the accreted sedimentary section is roughly twice this thickness (Fig. F2). With tectonic thickening and compaction, pore fluids are expelled, and gas—primarily biogenic methane—is transported upward to contribute to the formation of gas hydrates in the upper few hundred meters of the sediment section. The location for Site U1364 was chosen on the same basis as that for Site 889. It lies at a position landward of the prism toe where the fluid expulsion rate, estimated on the basis of the rates of compaction and vertical growth of the prism, reaches a cross-margin maximum and where a clearly developed bottom simulating reflector marks the base of the gas hydrate stability field (Davis et al., 1990). Other holes drilled earlier during ODP Leg 146 and IODP Expedition 311 (Westbrook, Carson, Musgrave, et al., 1994; Riedel, Collett, Malone, et al., 2006) documented the nature of the incoming undeformed sediments, the compaction history during accretion, the details of the sediment lithology, and the distribution and composition of gas hydrates across the area. This information, along with extensive site survey studies (Schwerwath et al., 2006; Riedel et al., 2010) provided an excellent basis for planning the depth, location, and other details of the ACORK installation at Site U1364 and will provide a valuable context for anticipated ACORK observations.

Seafloor morphology in the vicinity of Site U1364 (Fig. F3) reveals an active and heterogeneous accretional, depositional, and erosional geologic regime. The deformation front/prism toe is marked by uplifted anticlinal ridges, thrust fault traces, and local slope failures with accompanying debris deposits on the adjacent Cascadia Basin abyssal plain. In several places, the uplifted frontal structures are cut by canyons that funnel gravity-driven sediment transport from sources higher on the continental slope and outer shelf. Large scours and sand waves can be seen where these empty into Cascadia Basin. Headless canyons are common high on the slope, terminating at the shelf-slope break.

A closer look in the area of Site U1364 (Fig. F4) shows seafloor manifestations of focused fluid discharge. Seafloor seeps and vents discharge water expelled from the thickening prism, and in many instances free gas is transported across the 250 m thick zone of gas hydrate stability by relatively rapid focused upward flow. No vents have been observed in the immediate vicinity of Site U1364, but those that are present in the area (e.g., "Bullseye vent" and other locations where pock-marks, carbonate mounds, and gas hydrates are present at the seafloor) demonstrate that super-hydrostatic pore-fluid pressure and free gas are present beneath the seafloor.

A simplified schematic cross section in Figure F5 illustrates one way gas hydrates are believed to accumulate in accretionary prisms. Pore-fluid expulsion, driven by tectonic thickening and consolidation, is rapid near the prism toe and diminishes landward. Vertical migration of water from the prism delivers small amounts of dissolved methane produced in the sediment by biological CO2 reduction to the level of gas hydrate stability (a weak function of pressure and strong function of temperature), where gas hydrates accumulate primarily in permeable fractures and coarse-grained layers. The boundary between sediments containing free gas and sediments containing gas hydrate in the sediment pore volume may be enhanced as a result of methane recycling at the phase boundary from vertical tectonic motion, sedimentation, and erosion (Haacke et al., 2007). This boundary is seen in seismic profiles throughout the Site U1364 area as a bright reflection (the bottom-simulating reflector) at a generally uniform depth below the seafloor (a consequence of its depth being primarily temperature controlled), with a polarity opposite to that from the seafloor (Fig. F6).

Seismic reflection profiles crossing Site U1364 (Fig. F6) (Riedel, 2001) also provide a clear image of the local sediment structure, which comprises a gently deformed sequence of slope-basin deposits draped over highly deformed accretionary prism sediments. Accreted and deformed sediment occurs close to the seafloor or in outcrop in the high-standing area southeast of Site U1364 (Fig. F4); this lithology plunges northwest and is buried by a veneer of gently deformed locally deposited slope-basin sediments roughly 100 m thick at the location of Site U1364. These lithologic units have been characterized in detail by coring and logging at the numerous holes drilled in the immediate vicinity of Site U1364 (Fig. F4) (Westbrook, Carson, Musgrave, et al., 1994; Riedel, Collett, Malone, et al., 2006; Riedel et al., 2010; see next section). The gas/gas hydrate interface responsible for the extensive bottom-simulating reflector was intersected by these holes at a depth of roughly 225 meters below seafloor (mbsf), well within the deformed prism unit. Gas hydrates above the interface appear to be virtually absent in fine-grained sediments; most of the gas hydrate is concentrated in permeable coarse-grained units and massive gas hydrate lenses that mark present or past pathways of focused fluid flow.