Biogenic and thermogenic gases within the pore fluids throughout the accretionary wedge are transported into the gas hydrate stability zone (GHSZ) via faults and fractures, dipping stratigraphic horizons, or during diffuse intergranular fluid flow. Given sufficient gas saturation and sediment and/or fracture porosity and permeability, these fluids, if present within the GHSZ, will precipitate gas hydrate. Tréhu et al. (2004b) document the distribution and concentration of gas hydrate within the GHSZ across SHR and show that it is highest at the southern summit, near the location of the largest authigenic carbonate occurrence. Previous seafloor observations and sampling at SHR (e.g., Suess et al., 2001) also confirm abundant seafloor gas hydrate present at this location, which is consistent with the simple model of updip migration and anticlinal focusing of fluids near the crest of Hydrate Ridge discussed in Johnson et al. (2003) and more recently in Weinberger et al. (2005). The implications of this model and the observations at SHR would imply that most anticlinal ridges within the GHSZ across the margin would contain abundant gas hydrate near their crests. However, comparison between the history of deformation and fluid venting at SHR and NHR reveals that the duration and intensity of fluid migration, gas hydrate formation, and authigenic carbonate precipitation can vary along strike even within one accretionary ridge.
Because accretionary wedge dewatering and fluid migration are generally more intense near the deformation front and decrease with distance back into the wedge, Tréhu et al. (1999) suggested Hydrate Ridge represents an intermediate stage in the temporal evolution of gas hydrate systems within accretionary ridges across the margin. Although this is a likely model for the fluid venting and gas hydrate forming window that accretionary ridges pass into and out of with continued accretion to the margin, our recent structural work suggests that along-strike variations in structural style (primarily thrust vergence) control uplift, and thus indirectly influence the history of fluid venting and gas hydrate development within the same accretionary ridge system.
As Hydrate Ridge exists at the transition zone between a dominantly LV portion of the wedge to the north and a SV wedge to the south, it contains both SV and LV structures. NHR however, contains all SV structures, whereas SHR consists of two LV thrust folds juxtaposed against an older SV core (Fig. F4, inset C). As described above, greater uplift is expected in SV portions of the wedge because of thrust duplexing and wedge thickening from the base, as observed at NHR, and less uplift is likely in LV-dominated portions of the wedge, as observed at SHR. Thrust duplexing, wedge thickening, and uplift in SV portions of the wedge are likely to have a larger effect on fluid focusing, and thus gas hydrate formation and authigenic carbonate precipitation, compared to LV portions of the wedge. Intense focusing of fluids in SV portions of the wedge is also likely to have the effect of making thrust ridges in those regions more susceptible to slope failure. The records of slope failure in the adjoining slope basins on each flank of Hydrate Ridge provide evidence that the cap of younger slope basin sediments once preserved at NHR was eroded during Holocene and mostly late Pleistocene sediment failures (Johnson, 2004; Tréhu, Bohrmann, Rack, Torres, et al., 2003; Watanabe, this volume). In addition, the largest SV portion of the Cascadia accretionary wedge, from just south of Hydrate Ridge to the Rogue Canyon, catastrophically failed at least three times during the last ~1.2 m.y. (Goldfinger, 2000), whereas the LV-dominated wedge of Northern Oregon and Washington is well organized into elongate thrust ridges with only minor slope failure scars observed on their flanks.
Based on the above arguments, it is possible that SV portions of the wedge may be more susceptible to intense fluid focusing, gas hydrate formation, and slope failure than LV portions of the wedge. The interplay between all of these effects as the wedge develops through time, and the timescale at which gas hydrates remain stable within the sediment column, however, is difficult to reconstruct or predict. Additional investigations, focused on the interplay between structure in the wedge, the subsurface gas hydrate distribution, and the frequency of slope failure in the Cascadia wedge or in other gas hydrate–bearing accretionary settings, are needed to address these issues further.