The Juan de Fuca plate is currently being subducted obliquely beneath North America at the Cascadia subduction zone. Much of the 3- to 4-km-thick sediment cover of the subducting plate is accreted to North America in the process, either by offscraping at the deformation front or by underplating beneath the accretionary complex some tens of kilometers east of the deformation front (MacKay et al., 1992; MacKay, 1995; Johnson et al., this volume), resulting in a broad fold and thrust belt on the continental slope. Hydrate Ridge, a north-south-trending peanut-shaped structure, is one of these ridges. Since discovery of cold seeps and associated vent fauna on Hydrate Ridge two decades ago (Kulm et al., 1986), it has been the focus of many studies. A nearly ubiquitous bottom-simulating reflector (BSR) suggests that gas hydrate is widespread in the sediments of Hydrate Ridge (Tréhu et al., 1999). Seafloor manifestations of gas hydrate and associated authigenic carbonate, however, are more limited (e.g., Johnson et al., 2003).
The northern summit of Hydrate Ridge (NHR) is affected by widespread venting, as indicated by an extensive carbonate carapace (Bohrmann et al., 1998; Greinert et al., 2001; Johnson et al., 2003; Teichert et al., 2005a) and numerous sites where bubbles emerge from the seafloor (Heeschen et al., 2003). ODP Site 892, drilled during Leg 146, was located on the northern summit of Hydrate Ridge where an upward deflection of the BSR is cut by a fault (Shipboard Scientific Party, 1994). At this site, massive gas hydrates were recovered from 2 to 19 mbsf (Kastner et al., 1995). No hydrate was recovered from near the BSR, but pore water geochemical and temperature anomalies suggested the presence of disseminated hydrate in the pore space to 68 mbsf (Kastner et al., 1995; Hovland et al., 1995). Vertical seismic profiles (VSPs) and seismic refraction data indicate the presence of an extensive free gas zone beneath the gas hydrate stability zone (MacKay et al., 1994; Tréhu and Flueh, 2001). Methane at Site 892 is primarily of biogenic origin (Kvenvolden, 1995), but higher-order hydrocarbons of thermogenic origin are also present (Hovland et al., 1995). In contrast, seafloor manifestations of venting at SHR are limited to a single, 50-m-high carbonate pinnacle 250 m southwest of the summit and to a massive hydrate deposit and a single, persistent bubble plume at the summit (Suess et al., 1999, 2001; Torres et al., 2002; Heeschen et al., 2003; Tréhu et al., 1999, 2004b; Johnson et al., 2003).
The geologic history of Hydrate Ridge, which provides constraints on the origin of methane for forming gas hydrates and on the temporal evolution of these deposits, can be reconstructed from a series of seismic imaging experiments conducted as site surveys for Legs 146 and 204. The Leg 146 site survey, which included a series of seismic profiles spaced 1–3 km apart across the subduction zone deformation front and accretionary complex between 44.5°N and 45.5°N, indicates that Hydrate Ridge is located where the dominant vergence of thrusting at the deformation front changes from seaward to landward (MacKay et al., 1992; MacKay, 1995; Johnson et al., this volume). This transition corresponds to a transition from accretion of all sediment on the incoming plate off northern Oregon and Washington (Fisher et al., 1999) to subduction of the lower half of the incoming sediment column.
Figure F3 shows the deep structure beneath SHR. The deformation front is characterized by a distinct landward-dipping thrust fault that can be followed intermittently to a depth of ~7 km beneath the summit of Hydrate Ridge. The amount of underthrusting shown in Figure F3B corresponds to ~0.3 m.y. of subduction at the current oblique subduction rate of ~4 cm/yr, which is consistent with the estimate of 0.3 Ma for reactivation of uplift of SHR derived by Chevallier et al. (this volume) based on 3-D seismic and biostratigraphic (Watanabe, this volume) data from Leg 204. The cross-sectional area of the accretionary complex sediments overlying the décollement represents approximately twice this amount of time, assuming constant incoming sediment thickness and subduction rate; this consistent with the estimate by Chevallier et al. (this volume) for the initiation of uplift of SHR at ~1 Ma.
Perhaps surprisingly, the thickness of the underthrust sediment package does not appear to decrease significantly arcward of the deformation front as would be expected because of compaction and sediment dewatering. In fact, it appears to thicken somewhat, suggesting either that the thickness of sediment on the subducting plate has decreased recently or that underthrust sediments are being tectonically thickened. Uplift of the accreted sediments by duplexing and underplating, and additional uplift of pore fluid through mechanical dewatering, contribute to gas hydrate formation because the solubility of methane near the base of the GHSZ in a water depth of 800 mbsl (i.e., at the summit of SHR) is ~50% of the solubility in a water depth of 3000 mbsl (i.e., in the sediments of the abyssal plain) (Claypool and Kaplan, 1974). DSDP Site 174 indicated that the trench sediments are gas-charged (Claypool and Kaplan, 1974), although the absence of a BSR in the abyssal plain or in the uplifted sediments of the first accretionary ridge suggests that the methane content of the trench sediments is below saturation in central Cascadia. This contrasts with southern Cascadia, where a BSR is observed to extend into the abyssal plain (Gulick et al., 1998), and with northern Cascadia, where gas hydrate is present in the first accretionary ridge (Expedition 311 Scientists, 2006). For a more general review of the multiple factors, including solubility, that affect gas hydrate stability in marine sediments, see Tréhu et al. (in press).
Differences have been documented in the composition of the clay fraction between the early Pleistocene to Holocene sediments that comprise the slope-basin deposits and the late Pliocene to early Pleistocene strata from the slope-basin deposits (Underwood and Torres, this volume; Gràcia et al., this volume). The slope basin sediments contain, on average, 29% smectite, 31% illite, and 40% chlorite (+ kaolinite), whereas the late Pliocene to early Pleistocene strata from the underlying accretionary prism contain moderately greater amounts of smectite, with average values of 38% smectite, 27% illite, and 35% chlorite (+ kaolinite). The moderate enrichment of expandable clay minerals in the accreted sediments is likely due to detrital point sources associated with the ancestral Columbia River, combined with south-directed transport of hemipelagic suspensions and turbidity currents on the floor of Cascadia Basin (Underwood and Torres, this volume).