Prior to acquisition of a 3-D high-resolution seismic site survey in 2000 (Tréhu and Bangs, 2001; Tréhu et al., 2002), the relationship between subsurface reflections and the summit vents was not known because no seismic profiles crossed the summit. The 3-D survey covers a 4 km x 10 km region that includes the southern summit and the slope basin to the east. Shots from two generator-injector guns fired simultaneously were recorded on the Lamont-Doherty Earth Observatory (LDEO) portable 600-m-long 48-channel towed streamer and on an array of 21 four-component OBSs (Institute for Geophysics, University of Texas). The locations of the ship and of the streamer were determined via the Differential Global Positioning System and four compasses, respectively, and data coverage was monitored during the cruise to identify locations where additional data were needed. Excellent data quality was obtained in spite of strong winds and high seas. The data contain frequencies up to ~250 Hz, providing considerable stratigraphic and structural resolution.
Figure F5 shows an east-west slice from the 3-D data. The profile is coincident with line 2 from the 1989 site survey (Fig. F2B). Locations of Sites 1244-1246 and 1252 are shown. Features in the data that were particular targets of Leg 204 are labeled. An upper facies of folded and uplifted sediments unconformably overlies a low-frequency incoherent facies interpreted to be highly deformed accretionary complex material. These two facies were sampled during Leg 204. Figure F6 shows seismic profiles that trend approximately north-south and illustrates the setting of Sites 1245, 1247, and 1248-1250, which form a transect from the flank to the summit.
The BSR is a negative polarity reflection generally believed to result from free gas underlying gas hydrate at the base of the GHSZ; a strong BSR is seen everywhere along the profiles of Figures F5 and F6, except for locally near Site 1252. The seismic data also show considerable stratigraphic and structural complexity both above and below the BSR. Certain reflective horizons are anomalously bright, and these amplitude anomalies are laterally continuous for hundreds of meters. Table T1 gives the depth to the BSR and other major reflections estimated prior to Leg 204 using seismic velocities obtained from inversion of first arrivals of data recorded on OBSs during the 3-D seismic survey (Arsenault et al., 2001). These estimated depths proved to be quite accurate. Some minor revisions to these depths, based on data from Leg 204, are also given.
The seismic reflection labeled "A" on Figure F5 has an amplitude that is ~10 times that of adjacent stratigraphic events and 10 times that of the BSR. Horizon A gets shallower and brighter toward the southern summit, as shown on relative true-amplitude seismic sections in Figure F6. Maps of the two-way traveltime (TWT) to this surface, of the time between this surface and the BSR, and of the amplitude of the seismic wavelet are also shown in Figure F6. These maps show that the amplitude of the Horizon A reflection is related to depth below sea level (or hydrostatic pressure) rather than depth below the seafloor. Speculation that this horizon is a major path transporting methane-rich fluids to the summit of Hydrate Ridge and that the change in amplitude results from the onset of pressure-dependent exsolution of methane from fluids rising along Horizon A (Tréhu et al., 2002) were tested during Leg 204 by drilling at Sites 1245, 1247, 1248, and 1250. Sediments that lap onto Horizon A suggest that it is an uncomformity, as are the overlying Horizons Y and Y´.
Figure F7 shows the characteristics of Horizon A and of overlying actively venting features in the immediate vicinity of the southern summit of Hydrate Ridge. Locations of sections are shown on a map of seafloor reflectivity obtained by a deep-towed side scan (Johnson et al., in press) to illustrate the relationship between seafloor manifestations of venting and subsurface reflectivity. Chaotic bright reflectivity is observed just beneath the seafloor at the summit (line 300) (Fig. F7B). This reflectivity pattern is observed only at the summit and is almost exactly coincident with the "tongue" of intermediate strength seafloor reflectivity northeast of the Pinnacle observed in the deep-towed side-scan data. This pattern also underlies the acoustic bubble plume that was observed each time the southern summit was crossed during the seismic data acquisition cruise. We speculated that this pattern indicates the depth extent of surface massive hydrate (Tréhu et al., 2002) and tested this speculation by drilling at Site 1249.
Horizon A is probably a primary source of fluids for the summit vents. The mechanism whereby methane migrates from Horizon A to the seafloor, however, is not imaged in the seismic data. The region between Horizon A and the seafloor may be broken by small faults too small to be resolved in the seismic data. Site 1250 was planned to test this hypothesis.
A pair of strong reflections, referred to as Horizons B and B´, are also observed east of the southern axis of Hydrate Ridge and appear to be associated with an active secondary anticline (Anticline A) (Fig. F5). In contrast to Horizon A, Horizons B and B´ are pervasively faulted, with offsets consistent with tensional cracking in response to uplift and folding. These reflections seem to originate at Reflection AC (Fig. F5), interpreted to represent the top of the accretionary complex. Tréhu et al. (2002) speculated that Horizons B and B´ might be permeable stratigraphic horizons transporting fluids from the accretionary complex into the GHSZ. Sites 1244 and 1246 were designed to test this hypothesis by sampling Horizons B and B´ above and below the BSR.
Site 1251 is located in the slope basin east of Hydrate Ridge. Sediments are accumulating rapidly in this basin, and the BSR is characterized by a change in amplitude of dipping stratigraphic horizons, with large amplitudes indicative of free gas below the BSR (Fig. F8). This site was chosen to provide a relative reference site where the processes controlling hydrate formation were hypothesized to be similar to those on the Blake Ridge of the Atlantic continental margin of the United States. Gas hydrates on the Blake Ridge were drilled during ODP Leg 164 (Paull, Matsumoto, Wallace, et al., 1996). A secondary objective was to sample a layer in the center of the basin (labeled DF1 in Fig. F8), which was interpreted to represent a massive debris flow based on the absence of internal reflectivity. This horizon is one of two such thick events that can be traced through much of the 3-D data set.
Site 1252 was chosen to sample the sediments underlying Horizon AC where they are uplifted to form Anticline B. Here the sediments appear to be less deformed than beneath the crest of Hydrate Ridge and retain some coherent internal structure. Another objective at Site 1252 was to determine the reason for the absence of a BSR at Site 1252, in contrast to the very strong BSR observed only ~300 m to the east in the core of Anticline B. A third objective was to sample the lower inferred debris flow, which appears to have been blocked by Anticline B. Anticline B must have represented a topographic high when these sediments were deposited.