Accretion of sediments to the North American plate in response to oblique subduction of the Juan de Fuca plate beneath North America at a rate of ~4 cm/yr has formed a large accretionary prism composed of Pliocene and Pleistocene strata deformed into north–south-striking folded thrust slices (e.g., Seely et al., 1974; Snavely, 1987; MacKay et al., 1992; Goldfinger et al., 1996; McNeill et al., 2000). Hydrate Ridge, a 10-km-wide, 15-km-long ridge located 17.5 km east of the Cascadia deformation front ~100 km offshore Newport, Oregon, is one of these thrust slices (Fig. F1). Deep-penetration seismic-reflection data indicate that it is composed of several juxtaposed thrust folds and faults and has a steep western flank that has repeatedly been subjected to slope instability (Tréhu et al., 1999, Tréhu et al., this volume; Johnson et al., this volume).
Gas hydrates are present in the shallow sediments beneath Hydrate Ridge, as indicated by a widespread bottom-simulating reflection (BSR) (e.g., Tréhu et al., 1999) and by observations of gas hydrates, methane bubbles, and hydrate-related authigenic carbonates near the northern and southern summits (e.g., Bohrmann et al., 1998; Tréhu et al., 1999; Suess et al., 2001; Torres et al., 2002; Tryon et al., 2002; Johnson et al., 2003). Recent studies have focused on determining the amount of gas hydrate present (Milkov et al., 2003; Tréhu et al. 2004a), the dynamics of gas hydrate formation near the summit (Milkov et al., 2004; Torres et al., 2004a), the plumbing system feeding the summit deposits (Tréhu et al., 2004b), and the response of the gas hydrates to changes in sea level and water temperature (Bangs et al., 2005). Logging data and core recovery from Ocean Drilling Program (ODP) Leg 204 show that gas hydrate deposits within the gas hydrate stability zone (GHSZ) range from massive to patchy to absent, with hydrate generally filling, on average, <5% of the pore space but comprising as much as 25% of the sediment volume in a focused deposit at the southern summit (Tréhu et al., 2004a).
The primary objective of this paper is to reconstruct the stratigraphic and structural development of southern Hydrate Ridge (SHR) through a detailed analysis of three-dimensional (3-D) seismic data that were collected as a site survey for Leg 204. The lithologic characteristics of seismic facies and the timing of events are constrained using sedimentological and biostratigraphic data from ODP Leg 204 (Tréhu, Bohrmann, Rack, Torres, et al., 2003). The influence of structure and lithology on the free gas and gas hydrate distribution is also discussed.