The Cascadia accretionary wedge evolved in response to the oblique subduction of the Juan de Fuca–Gorda plate system (Fig. F1) and is composed of folded and faulted abyssal plain turbidites and hemipelagic clays as well as the recycled products of these uplifted sediments as slope basin fills (Kulm and Fowler, 1974; Westbrook, Carson, Musgrave, et al., 1994; Tréhu, Bohrmann, Rack, Torres et al., 2003). The Quaternary portion of the accretionary wedge is widest off the Washington and northern Oregon margins, coincident with accretion of the thick Pleistocene Astoria and Nitinat Fans (Carlson and Nelson, 1987), and narrows to the south. The active accretionary thrust faults and folds of the lower slope are characterized by mostly landward-vergent (LV) thrusts on the Washington and northern Oregon margins and seaward-vergent (SV) thrusts on the central and southern Oregon margin (Seely, 1977; MacKay et al., 1992; Goldfinger et al., 1992, 1997; MacKay, 1995). Virtually all of the incoming section in the LV province is accreted to the margin above a deep décollement, whereas a shallower décollement in the SV portion of the margin results in accretion of the upper two-thirds of the incoming stratigraphic section and subduction and/or underplating of the lower one-third (MacKay et al., 1992). In addition to the SV and LV thrust faults and folds that comprise the Cascadia accretionary wedge, nine west-northwest-striking left-lateral strike-slip faults also cut across the lower slope of the wedge (Goldfinger et al., 1997). These faults form as antithetic Riedel shears (R´) in the lower plate as a result of dextral shear of the forearc during oblique subduction and propagate upward into the accretionary wedge through time. The outermost accretionary wedge abuts a steep slope break that separates it from the Eocene oceanic basalt Siletz Terrane that underlies the continental shelf off the central Oregon to southern Washington margins (Snavely, 1987; Tréhu et al., 1994). Above this oceanic basement terrane is a moderately deformed Eocene through Holocene forearc basin sequence (Snavely, 1987; McNeill et al., 2000).
In the central Oregon portion of the margin discussed here (the Hydrate Ridge region), a transition zone exists between the northern LV province and the southern SV province, yielding a narrow zone of mixed vergence, both along and across strike, which is coincident with the location of two of the nine left-lateral strike-slip faults (Goldfinger et al., 1996; Johnson, 2004) (Fig. F3). Structurally, Hydrate Ridge is a composite thrust ridge, formed from both SV and LV structures (discussed below). The morphologic expression of Hydrate Ridge on the seafloor is asymmetric along its length; northern Hydrate Ridge (NHR) lies at a shallower water depth (~600 m) than southern Hydrate Ridge (SHR) (~800 m).