Prior to acquisition of a 3-D high-resolution seismic site survey in 2000 (Trehu and Bangs, 2001), the relationship between subsurface reflections and the summit vents was not known because no profiles crossed the summit. The 3-D survey covers a 4 km x 10 km region that includes the southern summit and an adjacent slope basin. Shots from two generator-injector guns fired simultaneously were recorded on the Lamont portable 600-m-long 48-channel towed streamer and on an array of 21 University of Texas Institute for Geophysics four-component ocean-bottom seismometers. The locations of the ship and of the streamer were determined via differential global positioning satellites and four compasses, respectively, and 3-D fold 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 EW Line 230 from the data volume. The profile is coincident with Line 2 from the 1989 site survey (Fig. F2B). Locations of Sites 1244, 1245, 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 stratigraphic sequence in which layering is less pronounced. This facies, in turn, overlies a low-frequency incoherent zone interpreted to be highly deformed accretionary complex material. The upper two facies were sampled during Leg 204. Figure F6 shows seismic profiles that trend approximately north-south and illustrate the setting of Sites 1245, 1247, 1248, 1249, and 1250, which form a transect from the flank to the summit.
A strong BSR, which is a negative-polarity reflection generally thought to result from free gas underlying gas hydrate at the base of the GHSZ, is seen everywhere along the profiles of Figures F5 and F6, except for locally near Site 1252. The 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 consistent for hundreds of meters.
In particular, we point out the event labeled "A" on Figure F5. This reflection has an amplitude that is ~10 times greater than that of adjacent stratigraphic events and 2 times that of the BSR. Horizon A gets shallower and brightens toward the 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 and dominant frequency of the seismic wavelet are also shown in Figure F6. These maps show that the updip change in amplitude follows depth beneath sea level (or hydrostatic pressure) rather than depth beneath the seafloor. Speculations 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 release degassing of fluids migrating along Horizon A (Trehu et al., 2002) were tested during Leg 204 by drilling at Sites 1245, 1247, 1248, and 1250.
Figure F7 shows the characteristics of Reflection A and of overlying actively venting features near the southern summit of Hydrate Ridge. Locations of sections are shown on a map of seafloor reflectivity obtained by deep-towed side scan (Johnson and Goldfinger, pers. comm., 2002) 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 (Trehu et al., 2002) and tested this speculation by drilling at Site 1249. It appears that Reflection A is a primary source of fluids for the summit vents (the mechanism whereby methane migrates to the seafloor), is not imaged in the seismic data. We speculated that the region between Reflection A and the seafloor is broken by small faults that are not well resolved in the seismic data but that permit methane-rich fluids to rise vertically from Reflection A to the seafloor. This speculation was tested by drilling at Site 1250.
Complicated reflectivity patterns are also observed east of the southern Hydrate Ridge axis and are associated with an active secondary anticline (Anticline A) (Fig. F5). The "double BSR" originally identified on Line 2 from the 1989 site survey (Fig. F5C) shallows to the south and merges with the BSR along 3-D Line 274. It is continuous with a package of bright regionally extensive reflections that cut across the BSR (Fig. F5). Reflections B and B' are pervasively faulted, with offsets consistent with tensional cracking in response to uplift and folding. We speculated that Reflection C represents the top of the accretionary complex and that Horizons B and B' might be permeable stratigraphic horizons transporting fluids from the accretionary complex into the GHSZ. The lack of consistent amplitude and polarity changes across the BSR, however, makes this interpretation quite uncertain. During Leg 204, Sites 1244 and 1246 were designed to sample Horizons B and B' above and below the BSR.
Site 1251 is located in the slope basin east of Hyrate Ridge. The seismic setting of the BSR in this basin is similar to that of the BSR beneath the Blake 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 beneath the BSR (Fig. F8). This site was, therefore, chosen for drilling to provide a relative reference site where the processes controlling hydrate formation were hypothesized to be similar to those on the Blake Ridge. A secondary objective was to sample a layer in the center of the basin (labeled DF1 in Fig. F8) that was interpreted to represent a massive debris flow based on the basis of 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 underlying seismically chaotic sediments 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.
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