Hydrate Ridge is part of the Cascadia accretionary complex, which is formed by the oblique subduction of the Juan de Fuca plate beneath the North America plate (Fig. F1A). The current subduction rate is ~4.5 cm/yr (Tréhu, Bohrmann, Rack, Torres, et al., 2003), with an age of <10 Ma identified for the Juan de Fuca slab (Jarrard, 1986). A deformation front is observed ~100 km west of the coast and is considered the seaward edge of the Cascadia subduction zone. Landward to the deformation front is the subduction complex, which is composed of highly deformed marine sediments of Pliocene to Miocene age (Snavely and Wells, 1996). The Siletz terrane, a basaltic formation of Paleocene to early Eocene age, forms the backstop to the accretionary complex and the basement to much of the Cascadia forearc. The boundary between the Siletz terrane and the accretionary complex is obscured by high-velocity rocks that appear in seismic surveys in the lower continental crust beneath the coastline and extend to the top of the descending slab ~20 km offshore (Parsons et al., 1998). The origin of this high-velocity body is unknown. Snavely (1987) mapped this contact and suggested that the western boundary of the Siletz terrane is an Eocene-age strike-slip fault 30 km west of the coastline, called the Fulmar fault, which coincides with the location of the edge of the high-velocity body. The Fulmar fault is also thought to be the western boundary of the marginal rift basin that is filled by a sequence of siltstone, turbidite sandstone, and conglomerate of early Eocene age. The bottom of the basin is mostly Paleocene to lower Eocene basalt. The Siletz volcanic highs and overlying marine sedimentary rocks were deformed by the subduction of the Kula-Farallon plate at ~52 Ma (Snavely and Wells, 1996). The complicated tectonic and depositional history of the Cascadia margin is reflected in numerous faults, mostly dipping from west to east (Snavely, 1987; Snavely and Wells, 1996). Some prominent faults dipping in the opposite direction have been identified in the western flank region of Hydrate Ridge, apparently related to the most recent tectonic events in this area (Johnson et al., 2003; Chevallier et al., this volume). Figure F1B is a highly simplified cross section through the Cascadia margin, illustrating schematically the age relations in this region (after Snavely, 1987; Snavely and Wells, 1996).
Hydrate Ridge itself is 25 km long and 15 km wide, trending roughly north–south, and is subdivided into the northern and southern summits. Water depths along the ridge are ~1000 m with the summits reaching depths of 600 and 800 m, respectively. The ridge is covered by sandy and silty marine sediments of Quaternary age. Gas hydrates are ubiquitous throughout the entire ridge, indicated by nearly continuous BSRs and the presence of massive hydrate deposits at the top of the southern summit (Tréhu et al., 1999; Tréhu, Bohrmann, Rack, Torres, et al., 2003). U-Th data of seafloor carbonates suggest that gas hydrates on the southern summit are somewhat younger than those on the northern summit (Teichert et al., 2003). The ridge has been the site of numerous investigations, the latest of which was Leg 204 (Tréhu, Bohrmann, Rack, Torres, et al., 2003), focusing on the southern summit. The nine drill sites (Fig. F2) reached depths between 150 and 440 meters below seafloor (mbsf). We report here results for all nine sites, which we have subdivided for further discussion into flank sites (1244–1248), summit sites (1249–1250), and slope basin sites (1251–1252). Based on the iodine results, we group Site 1244 with the flank sites, deviating slightly from the grouping suggested in Tréhu, Bohrmann, Rack, Torres, et al. (2003), where it is listed among the slope basin sites.