Structural mapping (Goldfinger et al., 1992, 1996, 1997; MacKay et al., 1992; MacKay, 1995; Johnson, 2004) (Fig. F3) using multichannel seismic reflection profiles from an ODP site survey conducted in 1989 (seismic lines shown in Fig. F4) and insights from mapping within the three-dimensional (3-D) seismic survey on SHR (Tréhu, Bohrmann, Rack, Torres, et al., 2003; Chevallier, 2004) reveal the variability in structural styles across the Hydrate Ridge region (Fig. F3). Based on correlations between various structures and strata and their relative position in the wedge, the Hydrate Ridge region can be divided into three strike-parallel zones (I, II, and III) across the wedge from Hydrate Ridge to the deformation front (Fig. F3). A summary of the structural vergence variation and schematic cross sections along three dip transects (north of Hydrate Ridge, NHR, and SHR) are shown in Figure F4. Assuming a steady-state model of westward wedge growth through time, Zones I, II, and III also indicate the relative timing of major accretionary wedge growth (major deformation is oldest in Zone I and youngest in Zone III). However, because deformation on structures across the wedge has likely continued throughout the Pleistocene, we cannot assume that deformation across these three zones is completely steady state. For this reason, we use the geometries and timing constraints (from ODP biostratigraphy) of the major faults and the common stratigraphic packages associated with each fault to infer the relative order of thrusting through time.
Using the biostratigraphic results from ODP drilling at Sites 891 (Shipboard Scientific Party, 1994a; Zellers, 1995), 892 (Fourtanier and Caulet, 1995), 1244 (Shipboard Scientific Party, 2003a), and 1245 (Shipboard Scientific Party, 2003b) (Fig. F2), we can constrain the timing of deformation in the Hydrate Ridge region. These sites were chosen because they lie within the three different structural zones: Site 891 in Zone III, Site 892 and 1244 in Zone I, and Site 1245 in Zone II (Figs. F2, F3). The sediments at both Sites 891 and 892 represent an uplifted and accreted abyssal plain section (Shipboard Scientific Party, 1994b, 1994c). At Site 1244, sediment cored beneath the slope cover was also accreted material of similar age (1.7–1.6 Ma) as Site 892 and thus is likely the equivalent facies. Given the above, the folding and thrusting of the accreted material at Sites 892 and 1244 (Zone I) most likely occurred after the deposition of the youngest abyssal plain deposits recovered at these sites. Based on comparison of the core biostratigraphies at these sites, the youngest age of the abyssal plain sediments at Sites 892 and 1244 is 1.7–1.6 Ma. Therefore, deformation of Zone I postdates 1.7–1.6 Ma. Currently, uplift and erosion of NHR has exposed this 1.7- to 1.6-Ma and older abyssal plain stratigraphic package at the seafloor, whereas at SHR less uplift resulted in the burial of the accreted abyssal plain sediments beneath younger overlying slope basin sediments (Tréhu, Bohrmann, Rack, Torres, et al., 2003). Because the age of the youngest sediments deformed by the frontal thrust in Zone III cannot be well determined at Site 891, we use the range of 0.30–0.25 Ma given by Westbrook (1994) for the timing of uplift of the first accretionary ridge (Fig. F2).
The above age relationships imply that because Zone I of Hydrate Ridge lies farther east in the accretionary wedge and contains older deformed sediments than the first accretionary ridge (Zone III), the difference in age of deformed sediments at Sites 892 and 1244 (Zone I) and the timing of uplift of the first accretionary ridge at Site 891 (Zone III) can be used to determine the maximum time between accretion of Zones I and III. Thus, Zone I was incorporated into the wedge sometime after the early Pleistocene (1.7–1.6 Ma). Assuming steady-state growth of the wedge, the major period of uplift of Zone I was likely completed by the late Pleistocene (0.30–0.25 Ma), the earliest age of the first accretionary ridge uplift (Zone III deformation). Because the wedge normally builds westward with continued accretion, the uplift of the first accretionary ridge at the deformation front initiates a time when shortening, previously taken up on structures to the east, was beginning to be accommodated on the first accretionary ridge frontal thrust. For this reason, we suggest that most of the uplift of Hydrate Ridge (Zone I deformation) was completed by the time ridge one or Zone III deformation was initiated.
The above age constraints imply that the major period of uplift and SV slip along Zone I structures in the NHR and SHR regions occurred sometime after the early Pleistocene (1.7–1.6 Ma) and was mostly completed by the late Pleistocene (0.3–0.25 Ma). Because of the position of Zone II deformation within the wedge (between Zones I and III), the timing of Zone II accretion into the wedge (LV in the region north of Hydrate Ridge and at SHR and SV at NHR), must have occurred sometime within this same time window (1.7–1.6 to 0.3–0.25 Ma). Through sequential unfolding of biostratigraphically constrained horizons at Site 1245, Chevallier (2004) suggests that initiation of the most eastward LV fold within Zone II began at 1.2 Ma and was completed by 0.3 Ma. Johnson (2004) suggests that this period of LV at SHR was coincident with the period of LV in the region north of Hydrate Ridge and SV at NHR, as all three regions appear to lie within the same location along strike within the wedge (the Zone II region). With steady-state growth of the wedge, this implies that Zone II deformation throughout the region most likely occurred between 1.2 and 0.3 Ma.
These results suggest that the wedge in this region generally advanced westward in a series of three structural phases since the late Pliocene–early Pleistocene (1.7–1.6 Ma): a SV phase (1.7–1.2 Ma), a dominantly LV phase (1.2–0.3 Ma), and a SV phase (0.3 Ma to recent). Superimposed on this structural vergence variation with time is the influence of left-lateral strike-slip faulting (not discussed here), which appears to have resulted in the clockwise rotation of structures during their accretion (Johnson, 2004).