Figure F10 summarizes the tectonic history of SHR as inferred from the stratigraphic and structural interpretations presented in "Interpretation of Seismic Units and Structures." This reconstruction was achieved by sequential removal of the effects of sedimentation and deformation. Structures of interest were unfolded by sequential flattening of key horizons, resulting in the geometric adjustment of overlying layers. The changes in geometry of the overlying layers through time were then used to indicate the timing of various folds throughout the region. Plane strain was assumed, allowing application of the constant bed length and the constant area rules. Some unit boundaries had to be approximated, for they are poorly constrained by the data. Horizontal original bedding was assumed except in cases of clear buttress unconformities associated with Anticline B and Fault System E. We also interpreted the absence of depositional sequences to be evidence for paleobathymetric highs that either impeded the deposition or caused slope failure events. The cross sections correspond to east-west Lines 230 (Fig. F4A) and 300 (Fig. F4C) from the 3-D survey. These two lines cut across most of the ODP sites, maximizing temporal constraints. Line 230 represents the northern part of the survey area where the slope basins formed by Units S.III, S.II, and S.IB are prominent and provides detailed information on the late Pleistocene geology. Line 300 crosses the shallowest part of Fold F and the Dome and provides better resolution for the tectonic reconstruction of early Pleistocene events. Comparison of the sections illustrates the important north-south variations within the survey area.
Late Pliocene sediments originally deposited as part of the deep-sea Astoria abyssal fan occur in the core of the proto-Dome (Unit S.VII), which was probably formed at the toe of the accretionary complex overlying a seaward-verging frontal thrust (Johnson et al., this volume). Rapid deposition of Astoria Fan sediments at the base of the slope continued in the early Pleistocene with deposition of Unit S.V. Simultaneously, sediments of Unit S.VI were deposited on the lower slope, albeit at a slow rate. Glauconite-rich layers recovered at Sites 1251 and 1252 near the top of Unit S.VII and within Unit S.VI support the inference of very slow sedimentation in the eastern part of the study region during this time period (Shipboard Scientific Party, 2003e). The wavy and chaotic seismic character of Unit S.VI suggests pervasive compressional deformation throughout the proto-Dome during this time period, but the details are not well determined.
Sometime between the deposition of Horizon A´ and Horizon A, (~1.2 m.y., based on biostratigraphy), the vergence of thrusting at the deformation front changed to landward, which thrust sediments of Unit S.V over the older sediments of Unit S.VII along Fault F2. The upper panels of Figure F10 depict this transition. Development of Fault F2 was accompanied by development of a drag fold, Fold F, creating a basin that was filled by sediments of Unit S.III. This model is supported by the linear character and north-south strike of the hinge line of Fold F, indicating east-west strain.
Fold F was active throughout deposition of Unit S.III, as indicated by the divergence of the strata on the flanks of the fold (Figs. F4, F6). Analysis of the internal geometry of onlap and truncation of strata within Unit S.III indicates that three paleobathymetric highs controlled the distribution of Unit S.III: Fold F to the west, the Dome to the south, and Anticline A to the east. This episode of sedimentation extended from ~1.0 to ~0.5 Ma, although timing of the transition from Unit S.III to S.II is poorly constrained. Inconsistencies between mapping of coherent seismic sequences and biostratigraphic constraints during this time period may be due to redeposition of sediments shed from the topographic highs because of slope instability. This pattern of sedimentation continued from ~0.5 to ~0.3 Ma with deposition of Unit S.II. As Anticline B grew relative to Anticline A, it became the eastern boundary structure of the basin in which Unit S.II was deposited.
Biostratigraphic data indicate that sediments of Unit S.IV, which are found on the eastern and southern flanks of Anticline B, are coeval with Units S.III and S.II. The strata of Unit S.IV are divergent as the result of the synsedimentary activity of Anticline B and unconformably overlie S.VII strata at a very small angle (Fig. F4), indicating low relief of Anticline B at 1.0 Ma and major uplift soon after 1.0 Ma, during the deposition of Unit S.IV. Uplift of Anticline B appears to have stopped relative to the Dome since 0.3 Ma, as the eastern basin was filled with Units S.IA and S.IB. The lapping onto and thinning of Unit S.II on the western flank of Anticline B and the rapid deposition of coeval Unit S.IV on the eastern flank suggest that either there was a continuous high between Anticline A and Anticline B that did not provide any accommodation space for sediment deposition in this region or that one or more major slope collapses occurred between 1.0 and 0.3 Ma along Horizon K on the northern flank of Anticline A. The latter would also explain the sudden truncation of Unit S.VI strata to the north as observed on 2-D Line NS3 (Fig. F5).
Unit S.IV strata are truncated at the erosion Surface U (Fig. F4), indicating another major erosion event at ~0.3 Ma. Because the erosion surface is currently dipping to the south and there is no indication for a tilt of the strata to the south, we suggest that the Event U corresponds to a slope failure along the southern flank of Anticline B that truncated Unit S.IV. This event possibly occurred during the final stage of uplift of Anticline B. That Anticline B has been inactive since ~0.2 Ma is indicated by onlap of Unit S.IA.
The northeastern migration of uplift from Anticline A to Anticline B is enigmatic, since the deformation front was probably migrating west during this time as sediment was accreted. The abrupt southern termination of the Anticline B uplift is also difficult to explain in the context of a north-south-trending subduction zone. We speculate that uplift followed by subsidence may have been due to subduction of a topographic feature riding on the Juan de Fuca plate. Flemings and Tréhu (1999) inferred the presence of a deeply subducted ridge or chain of seamounts beneath the upper slope of the Cascadia subduction zone from 42° to 44°N based on modeling of gravity and magnetic data. If this buried structure is attached to the subducting plate, it would have been beneath the crest of Hydrate Ridge at ~1 Ma and would have moved northeast relative to the upper plate. Subduction of seamounts is known to have a impact on the structure of the continental margin offshore Central America (e.g., Ranero and von Huene, 2000; von Huene et al., 2000) and Nankai (e.g., Park et al., 2004). In this model, the first episode of uplift of the Dome (~1 Ma) and the uplift of Anticline B are attributed to passage of the subducted ridge. Both of these episodes of uplift were followed by sedimentation over the crest of the former topographic high, consistent with collapse of the margin in the wake of the subducted ridge.
Reactivation of uplift of Unit S.VII along the present-day axis of Hydrate Ridge since ~0.3 Ma is inferred based on eastward migration of the depocenter for Units S.II and S.IB (Fig. F8). This uplift along a north-south axis was accompanied by normal faulting in Unit S.II. We interpret this to be due to a reinitiation of seaward-vergent thrusting farther to the west accompanied by underthrusting of sediment and formation of deeply buried sediment duplexes at depth beneath Hydrate Ridge (Tréhu et al., this volume).
The development of Fault System E, which disrupts Unit S.VI beneath the southeastern flank of SHR, is the most recent structural feature we discuss. Offlapping of the Unit S.IB strata shows an increase in the uplift rate relative to the sedimentation rate since ~0.2 Ma. Within Unit S.IB, the increased abundance of the pelagic sediments with time is observed (Shipboard Scientific Party, 2003b). This is interpreted to result from the isolation of the top of the ridge as it is uplifted out of the region of turbidity current deposition and into a regime of pelagic drape sedimentation. An increase in dip of the strata within Unit S.IA with time and the two debris flow deposits (DBF1 and DBF2 on Fig. F4), as well as the thinning of the beds against the flanks of the Dome provide evidence for the ongoing relative subsidence of the eastern basin during the deposition of Unit S.IA. This may be due to rotation of Hydrate Ridge in a clockwise direction, with extension in the basin adjacent to southern end of the ridge, as discussed by Johnson et al. (this volume).