In this section we describe the characteristics of the seismic units and relate them to the lithologic units described by Leg 204 shipboard sedimentologists (Shipboard Scientific Party, 2003a). We illustrate our conclusions by showing several interpreted transects through the seismic volume (Fig. F4A–F4D), selected details of the seismic data, and maps of key stratigraphic horizons or fault surfaces. Further details are discussed in Chevallier (2004).
Although velocity constraints are available from ocean-bottom seismometers deployed during the 3-D seismic survey (Arsenault et al., 2001), from sonic logs during Leg 204 (Lee and Collett, this volume; Guerin et al., this volume), and from vertical seismic profiles (VSPs) during Leg 204 (Tréhu et al., this volume), velocities within the strata imaged by the 3-D survey and discussed in this paper are generally <1800 m/s and the interval velocity above the BSR is <1600 m/s. Because of the difficulty of interpolating the velocity field to define velocity at all data points in the 3-D data set and degradation of data quality because of variable stretch due to local velocity variations (some of which are real and some of which may be artifacts), data are presented here with a vertical axis of two-way traveltime. Because the average velocities are uniformly low, the effect on stratigraphic and structural patterns is small. A constant velocity of 1600 m/s in the subsurface is adequate to estimate horizon depths and isopach thicknesses throughout the 3-D data volume.
The seismic units fall into two different groups: sediments deposited on the abyssal plain and transferred to the accretionary complex and sediment deposited in slope basins overlying the accreted sediments.
Unit S.VII represents deep-sea fan sediments older than 1.6 Ma (late Pliocene) and Unit S.V represents younger deep-sea fan sediments of age 1.6–1.1 Ma (Fig. F2). Unit S.VII is characterized by a high degree of lithification and microfracturing (Shipboard Scientific Party, 2003b, 2003d, 2003e), consistent with its long history of burial and deformation. Internal structures in these units are poorly preserved. Glauconite-rich layers near the top of this sequence indicate extended periods of exposure on the seafloor (Shipboard Scientific Party, 2003b, 2003c, 2003d, 2003e).
Unit S.V is composed of nannofossil-rich silty claystone interspersed with thick turbidites containing wood fragments that was deposited rapidly (~70 cm/k.y.) (Shipboard Scientific Party, 2003c) in the trench as part of the Astoria Fan. The top of this seismic unit is defined by Horizon A, a 2- to 4-m-thick, coarse-grained, volcanic-ash-rich turbidite layer that appears as an bright reflection in the 3-D data. Drilling revealed that Horizon A contains abundant free gas and plays a critical role in transporting methane to feed methane vents and formation of massive gas hydrate at the summit (Tréhu et al., 2004b).
At Sites 1244 and 1252, Unit S.VII is overlain by sediments with an age that overlaps Unit S.V but have characteristics that are similar to Unit S.VII. We call this Unit S.VI. Seismically, it is difficult to distinguish Unit S.VI from Unit S.VII. Sediments recovered from this zone are relatively lithified and fractured and contain abundant glauconite similar to Unit S.VII, reflecting exposure on the seafloor and a very low sedimentation rate. Combining biostratigraphic data with the structural reconstruction presented in "Tectonic History of Southern Hydrate Ridge," we conclude that these sediments were deposited on the lower slope of the overriding plate near a seaward-vergent frontal thrust fault.
Slope-basin sediments younger than ~1.0 Ma overlie Units S.VII–S.V (Figs. F2, F4). These sediments (Units S.IV to S.IA) filled a migrating series of depocenters created by temporal and spatial variations in the locus of uplift. They are composed mostly of turbidites interlayered with hemipelagic silty clays. The main distinction between the lithologic units is related to the frequency and thickness of turbidites. Units S.III and S.II in the western part of the survey were deposited in the lower slope basin formed by Fold F (Fig. F4). Distinct reflections B and B´ (Fig. F4A), found at the base of and within Unit S.II, result from coarse-grained gas-rich horizons similar to Horizon A. Horizons B and B´ host gas hydrate or free gas, depending on whether they were sampled within or beneath the GHSZ (Tréhu et al., 2004a; Guerin et al., this volume). Units S.III and S.II are thickest in the saddle between the northern and southern summits of Hydrate Ridge (Fig. F5).
In the eastern part of the survey region, Unit S.IV is a locally thick deposit that developed contemporaneous with Units S.III and S.II in association with Anticline B (see "The Dome and Anticlines A and B"). It is separated by an unconformity (U in Figs. F2, F4) from Units S.IB and S.IA, which fill a regional basin that has been deepening to the south with time as SHR has been uplifted (Fig. F4). The lithologic unit corresponding to Unit S.IV is distinguished from Units S.IA and S.IB by a decrease in biogenic elements and an increase in the terrigeneous material. It is also characterized by rapid deposition (~160 cm/k.y.) and a high frequency of locally derived turbidites generated by rapid changes in seafloor topography, which resulted in slope instability.
The primary structural features in the study area (shown on Fig. F4) are (1) Fault/Fold F, which represents landward-vergent deformation front during an earlier stage of development of Hydrate Ridge; (2) the "Dome," a broad uplift of Units S.VI and S.VII that has been intermittently active and now forms the core of Hydrate Ridge; (3) Anticlines A and B, which are localized uplifts of Units S.VI and S.VII; and (4) Fault System E, which forms the eastern boundary of the "Dome." In this section, we reconstruct the history of activity of these features through detailed examination of the topography of key unconformities and seismic stratigraphic patterns such as those shown in Figure F3.
The most prominent fault-related fold (Fold F) deforms Unit S.IV and controls the geometry of Unit S.IV and S.III strata in the western part of the survey region (Fig. F4). The fold is a north-northeast-striking, doubly plunging fold (Fig. F6A), which may be cored by Fault F1 and probably developed because of drag along Fault F2 (Fig. F4A). The seismic survey encompasses only its northern half, which is plunging by ~3°. The interpretation of Fold F as a thrust-cored, landward-vergent fold formed at the deformation front is based on (1) reconstruction of the fold's evolution inferred from the geometry of the strata (Fig. F6B), which suggests a thrust fold in the hanging wall and formation of a basin overlying the footwall of Fault F2, and (2) similarity to the imbricated, landward-vergent thrust faults studied by Flueh et al. (1998) on the accretionary prism offshore Washington, which have similar amplitude and wavelength to Fold F. Two splays (F1 and F2) of the deeper-rooted thrust fault system could be mapped. Splay F1 was inferred from the small interlimb angle of Fold F (Fig. F4A). Splay F2 is a conspicuous termination surface in the deeper stratigraphy (Fig. F4). It disrupts younger strata than does Splay F1, suggesting more recent activity than Splay F1.
Middle to late Pleistocene slope-basin strata lap onto a large dome-shaped feature (which we call the Dome) composed of Units S.VII and S.VI. The top of this feature was mapped and is referred to as Unconformity K (Figs. F2, F4, F7). The relief on Unconformity K is much greater than the present-day seafloor relief, and the shape of the Dome is nearly circular, centered on the present-day southern summit (Fig. F7). The 2-D lines suggest an even more pronounced bulge beneath northern Hydrate Ridge. A secondary elongated bulge on the eastern flank of the Dome is referred to as Anticline A. Anticline A has a distinct topographic signature at present. Both the Dome and Anticline A are characterized by onlapping of the overlying strata against the paleorelief (Fig. F8E). Relatively rapid growth of the Dome at ~1.0 Ma is inferred from the abrupt pinching-out of Unit S.III to the east (Fig. F4, F8A). Units S.VI and S.VII are too deformed to reconstruct the early evolution of the Dome.
Anticline B (Fig. F4A–F4B), in the northeast part of the survey region, has only a slight signature in the topography but represents dramatic topography on Unconformity K (Fig. F7). Progressive fanning of the strata of Unit S.IV on the eastern flank of Anticline B suggests that the major growth period of the feature occurred at 1.0 to 0.3 Ma (Fig. F2). A sharp unconformity at the top of Unit S.IV indicates that Anticline B represented a bathymetric high ~0.2 m.y. ago, which is the minimum age of Unit S.IB. The progressive tilting and bending of Units S.II and S.IB and onlap of these onto the flanks of Anticline B suggest progressive subsidence of the feature relative to the Dome and eventual burial by Unit S.IA since ~0.2 Ma. This is probably a result of reinitiation of uplift of the Dome at ~0.3 Ma. This stage of uplift probably continues at present.
Contours on the time map of Horizon B, located within Unit S.II, show an anticline plunging to the north-northeast; the fold is slightly asymmetrical, with the northeast limb dipping more steeply than the western limb (Fig. F9A). The hinge line of the fold corresponds to the crest of SHR and lies slightly to the east of and approximately parallel to Fault F2 where defined by Horizon A, suggesting continuing thrusting on Fault F2 through at least the time of deposition of Unit S.II. The eastern limb of the fold is more prominent and is disrupted by clear north-south-striking normal faults that extend to the seafloor (Fig. F10B). Thickening of the interval between the stratigraphic Horizons B and B´ on the downdropped side of the faults (Fig. F9B) suggests synsedimentary faulting.
A deeply buried normal fault system accommodates the subsidence of the eastern basin relative to SHR. Two north-south-striking fault-splays are shown on Figure F4C and F4D. The offsets of the faults average ~0.2 s and sum to a total offset of 0.5 s (~387 m) in Unit S.VI in the southern part of the survey. Offset decreases to zero to the north, where these faults are no longer imaged (Fig. F4A). It is possible, however, that the vertical offsets observed in Figure F4C and F4D are the result of wrench faulting along a strike-slip fault system. The maximum offset of 600 m recorded during the 3-D survey precludes imaging of a steeply-dipping strike-slip fault where there is no significant component of dip-slip displacement.
The geometry of Units S.IA and S.IB, which are draped over Fault System E, suggests syndeformation sedimentation accompanying uplift of the Dome. This can be interpreted to result from either rapid uplift of the Dome since 0.3 Ma in response to underplating and duplexing of oceanic crust and sediments at depth (Tréhu et al., this volume), from clockwise block rotation of Hydrate Ridge, which would result in downdrop of the basin relative to the Dome (Johnson et al., this volume), or from a a combination of these mechanisms.