Joint Oceanographic Institutions, United States

Hydrate Ridge is an accretionary thrust ridge located on the lower slope of the central Cascadia convergent margin. Structural mapping based on two-dimensional and three-dimensional multichannel seismic reflection profiles and gridded bathymetry coupled with deep-towed sidescan sonar data and Ocean Drilling Program (ODP) biostratigraphy suggests that seafloor fluid venting patterns are likely controlled by the seaward-vergent (SV) structural style at northern Hydrate Ridge (NHR) and by the dominantly landward-vergent (LV) structural style at southern Hydrate Ridge (SHR). North-south structural variability across Hydrate Ridge is coincident with the seafloor authigenic carbonate distribution, which varies from aerially extensive authigenic carbonate crusts at NHR to a minor focused occurrence of authigenic carbonate at SHR. The older stratigraphy exposed at the seafloor at NHR (>1.6-1.7 Ma) has likely been subjected to a longer history of sediment compaction, dewatering, and deformation than the younger slope basin strata preserved at SHR (1.7 Ma to recent), suggesting the extent of carbonates at NHR may result from a longer history of fluid flow and/or more intense venting through a more uplifted, lithified, and fractured NHR sequence. Furthermore, recent work at SHR shows that the major seafloor fluid venting site there is fed by fluid flow through a volcanic ash-bearing turbidite sequence, suggesting stratigraphic conduits for fluid flow may be important in less uplifted, LV-dominated portions of Hydrate Ridge. In addition, the variability in structural style observed at Hydrate Ridge may have implications for the distributions and concentrations of fluids and gas hydrates in other accretionary settings and play a role in the susceptibility of accretionary ridges to slope failure.