North-south variability in the history of deformation and fluid venting across Hydrate Ridge, Cascadia margin

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 1Johnson, J.E., Goldfinger, C., Trehu, A.M., Bangs, N.L.B., Torres, M.E., and Chevallier, J., 2006. North-south variability in the history of deformation and fluid venting across Hydrate Ridge, Cascadia margin. In Trehu, A.M., Bohrmann, G., Torres, M.E., and Colwell, F.S. (Eds.), Proc. ODP, Sci. Results, 204: College Station, TX (Ocean Drilling Program), 1–16. doi:10.2973/odp.proc.sr.204.125.2006 2Present address: University of New Hampshire, Department of Earth Sciences, 56 College Road, James Hall, Durham NH 03824, USA. joel.johnson@unh.edu 3College of Oceanic and Atmospheric Science, Oregon State University, Corvallis OR 97331, USA. 4University of Texas, Institute for Geophysics, 4412 Spicewood Springs Road, Austin TX 78759, USA. Initial receipt: 12 May 2005 Acceptance: 5 February 2006 Web publication: 20 October 2006 Ms 204SR-125 J.E. JOHNSON ET AL. HISTORY OF DEFORMATION AND FLUID VENTING 2 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. INTRODUCTION On the Cascadia continental margin offshore central Oregon, Hydrate Ridge (Figs. F1, F2) has been the focus of numerous geologic and geophysical investigations for nearly two decades. During the mid1980s, its location within the lower slope of the accretionary wedge initially prompted investigations of seafloor fluid flow and the dewatering processes associated with accretionary wedge deformation and development and resulted in one of the first discoveries of chemosynthetic biological communities (Suess et al., 1985; Kulm et al., 1986; Ritger et al., 1987). By the early 1990s this early work was supplemented by Ocean Drilling Program (ODP) drilling (Fig. F2; Sites 891 and 892), during which gas hydrates were first recovered (Westbrook, Carson, Musgrave, et al., 1994), and detailed structural investigation (MacKay et al., 1992; Goldfinger et al., 1992, 1996). Subsequent work, including numerous sea-floor observation and sampling expeditions since the late 1990s (e.g., Suess et al., 1999, 2001) and more recently a gas hydrate–dedicated ODP leg (Trehu, Bohrmann, Rack, Torres, et al., 2003; and Trehu et al., this volume) (Fig. F2; Sites 1244–1252), focused on the surface and shallow subsurface gas hydrate system, seeking to characterize the distribution, concentration, and behavior of gas hydrates in an active margin setting. In this paper we integrate our recent work on 1. The history of deformation, constrained by structures interpreted from seismic reflection data and core biostratigraphy; 2. The record of fluid venting, as imaged on sidescan sonar data; 3. The surface and subsurface gas hydrate distribution, constrained by seafloor observations and ODP coring; and 4. Records of slope failure, interpreted from piston cores and ODP cores in the Hydrate Ridge region. Together, these data suggest that variability in structural style may strongly influence the distribution and concentration of fluids and gas hydrates in the subsurface and the susceptibility of accretionary ridges to slope failure. GEOLOGIC SETTING 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; Trehu, 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 125° 123° 127° 129° 131°W 0 lanco Tnsform ault Juan de Fuca plate Ju an d e Fu ca R id ge