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doi:10.2204/iodp.proc.314315316.101.2009

Synthesis of key results

Lithology, structure, and recent activity of megasplay fault and associated thrust sheet (Sites C0001, C0004, and C0008)

Based on seismic and tsunami inversion studies combined with seismic reflection research, various theories have suggested that the splay fault system in this area slipped during the 1944 Tonankai M 8.2 earthquake to generate the observed tsunami (Baba et al., 2006; Park et al., 2002). Moore et al. (2007) also suggest recent historical and geological accumulation of displacement along the fault on the basis of three-dimensional (3-D) seismic reflection investigation. The age reversal from Pliocene to Pleistocene documented beneath the splay fault during Expedition 316 is consistent with geologically recent activity. However, age resolution is of course insufficient to document historical-scale fault activity.

The “Expedition 316 Site C0004” chapter discusses the evidence for recent activity of the splay fault. To summarize: the splay fault clearly thrusts the hanging wall prism over younger slope sediments in the footwall; however, the youngest slope sediments that cover the fault appear not to be cut by the fault. In addition, the lack of a slope break on the seafloor above the fault might also suggest that this splay fault is presently not active but ceased activity in the recent past. However, shipboard results provide support for the alternate interpretation that the splay fault is active as a blind thrust, in which the tip of the fault has not propagated to the surface but remains buried. Slip on deeper levels of the fault zone are expressed by some combination of folding and layer-parallel slip in the shallow slope sediments draping the uppermost portion of the megasplay, as well as the possible triggering of marine slump or slide deposits. Drilling results at Site C0004 indicate that the shallowest cover sediments above the hanging wall wedge are composed of repeated mass transport complexes associated with repeated slope collapses and rip-up debris generation. Pleistocene cover sediments dip steeply approximately parallel to the slope and are cut by numerous normal faults.

Despite inferences of recent activity on the splay fault system, no porosity inversion is observed beneath the splay fault; this contrasts with previous results from the décollement of the Muroto transect (Screaton et al., 2002), in which a clear porosity inversion across the fault likely reflects fluid overpressure. Unlike the Muroto transect décollement, the splay fault system observed at Site C0004 has permeable pathways for dewatering provided by the observed sand and coarse ash layers.

Sediments of the slope basin at Site C0008 provide a “reference site” for the sediments underthrust beneath the megasplay fault. Comparison of the interval 190–200 m CSF in Hole C0008A with an average porosity of 50% and the correlated interval 320–330 m CSF in Hole C0004D with an average porosity of 43% suggests the sediments are dewatering during underthrusting. Evidence for lateral flow is provided by C₁/C₂ ratios at Site C0008 that are slightly lower than expected for biogenic production at the estimated in situ temperature. Lateral flow along sand layers could transmit fluids from where the temperature is higher because of greater burial underneath the splay fault.

These sand layers are truncated at a normal fault drilled in Hole C0008C, where surficial material has slid downward. Hole C0008C structural descriptions document normal faults within the sediments at ~40 m CSF. Lateral transmission of fluids from areas with thicker to thinner overburden has previously been suggested as a mechanism for enhancing slope failure (Dugan and Flemings, 2000). As a result, splay fault movement could produce slope failures through seaward propagation of pore pressures from the footwall in addition to oversteepening of the hanging wall. Postcruise examination of lithologic evidence, structural data, and physical properties will help assess the interaction between splay fault movement and slope failures.

Two steps of age reversal are tentatively recognized across the splay fault zone; this evidence suggests that fault-bounded lithologic Unit III at Site C0004 is a sliverlike unit coming up from a much deeper setting. The lithology of the slope sediments and the old accretionary prism in Hole C0008A suggests that one of the possible sources for the Unit III “sliver” at Site C0004 is the lowermost slope sediments beneath the Pleistocene and late Pliocene slope sediments. In this case, displacement along the splay fault, especially the lower boundary fault beneath Unit III, might be more than a couple of kilometers. Interestingly, porosity within this lithologic unit is slightly higher than expected relative to trends observed in overlying material. If these materials have been brought up from depth, they either never had an opportunity to consolidate or have subsequently had considerable opening of porosity by microfractures in this fault sliver.

Age of cover sequence and uplifted accretionary prism units

Rocks of the thrust sheet below 500 m LSF at Site C0001 and also of the cavings sampled at Site C0003 are anomalously dense relative to their present depth of burial and show high sonic/​seismic velocity, indicating relatively advanced lithification. This suggests significant uplift and exhumation along the splay thrust. These inferences are consistent with the age determinations made on cores, showing that this thrust sheet contains rocks several millions of years old, in contrast to the immediately overlying slope deposits. Notably, thrust sheet material drilled at Site C0004 is substantially younger than the analogous material at Site C0001, C0002, or C0003 (Fig. F6), suggesting that the megasplay thrust sheet contains internal structural imbrication and has incorporated material progressively as it advanced.

At all of the sites landward of the frontal thrust area, units interpreted to be uplifted section that was frontally accreted to the prism are covered by varying amounts of slope drape. In the case of Site C0002, this slope drape is in turn covered with the thick, dominantly Pleistocene Kumano forearc basin section. Biostratigraphy (predominantly by nannofossil zones) and magnetostratigraphy define the ages of sediments above and below these boundaries, which also record time gaps of various durations between the prism section and cover.

In the frontal thrust region at Site C0006, the transition from uplifted trench sediments into the overlying slope apron cover sediments is dated to 0.436–0.78 Ma, presumably recording the timing of initial frontal thrust activity. Only 27 m of younger slope cover overlies this boundary, implying potential removal of young deposits through slumping or mass wasting.

Moving to the mid-slope megasplay fault region, the oldest sediment at the base of the slope cover at Site C0004 was dated at 1.46 Ma, unconformably resting on sediments that are 1.1. m.y. older than that. Just a short distance landward at Site C0001, in the same apparent overall thrust complex (Figs. F3, F6), the age of the base of the slope apron is 2.0 Ma, which is substantially older than at Site C0004, and rests on a nearly 4 Ma accreted section. Ten kilometers further landward in the Kumano Basin at Site C0002, the slope apron lies on top of the accreted complex at a depositional age of 3.79 Ma, resting on >5 Ma sediments in the accreted section. This landward progression of successively older dates marking the uplift and surface exposure of Shikoku Basin sediments is consistent with progressive growth of the accretionary prism through late Miocene to Quaternary time.

Furthermore, the slope apron at Site C0002 accumulated very slowly until ~1.5 Ma, then >800 m of forearc basin turbidites accumulated rapidly. The implication of this latter observation is that the onset of splay fault uplift of a pronounced outer arc high and/or capture of a significant turbidite sediment source for the basin was abrupt in the early Pleistocene. The youngest dated sediment in the uppermost 10 m at Site C0004 is several hundred thousand years old, and nannofossil Zone NN21 was not identified, raising the possibility that more recent sediments have been removed through slumping and mass transport processes.

Indicators of stress regime

Borehole breakouts and present-day stress orientations

Borehole breakouts were observed at all four sites for which we have imaging data and show very systematic orientations (Fig. F8). In a vertical borehole, the orientation of breakouts, or borehole wall failures upon drilling, is a well-established indicator of the orientation of the horizontal maximum principal stress in the present-day stress field extant at the location of the hole (Zoback et al., 2003). Less frequently, drilling-induced tensile fractures were observed, primarily at Site C0001 in the splay fault thrust sheet. Breakout orientations at Sites C0001, C0004, and C0006 all indicate northwest–southeast azimuths of the maximum horizontal principal stress (S_Hmax) (Fig. F9). At Site C0001, S_Hmax = 336°; at Site C0004, S_Hmax = 320°; and at Site C0006, S_Hmax = 330°. In the thrust-dominated tectonic environment, this is consistent with trench-normal shortening, although strike-slip and/or normal faulting stress states are also permissible (see next section).

Variations of the breakout orientation among these three sites of as much as 10°–16° are statistically significant and may be caused by local structure influencing local stress orientation. In particular, surface slope direction varies and slope related gravitational stresses might influence these values. Nevertheless, the overall consistency among these three distinct sites suggests that these stresses record the regional tectonic control. In addition, all of these S_Hmax orientations deviate from the far-field plate motion vectors based on Global Positioning System results (Heki, 2007) and global plate motion models (Seno et al., 1993), implying some strain partitioning between convergence and strike-slip motions.

In contrast, at Site C0002 in the Kumano Basin the orientation of S_Hmax is northeast–southwest at 134°, or very nearly perpendicular to that in the more trenchward sites (Figs. F8, F9). This is consistent with a normal faulting stress state that extends through the basin section, which in fact exhibits numerous normal faults, and also in at least the upper 400 m of the underlying older accretionary prism section. This contrast between the outer accretionary prism and the forearc basin region, including the buried prism rocks beneath, suggests that the tectonic stress magnitudes differ markedly in the upper part of the prism at sites just a few kilometers apart along the transect. The stress regimes are distinct. Furthermore, gravitationally driven extension may be important above the megasplay where the outer arc high has been uplifted. Implications of these data will be explored in postexpedition research to understand the mechanical state of the prism and basins.

Paleostress from core-based structural data

Core-scale structures can be used to infer paleostress regimes. Cores drilled during Expedition 315 in Holes C0002C and C0002D (Kumano Basin site) show small-scale structures consistent with the interpretation that the present-day stress field is extensional and directed northeast–southwest. This agrees very well with the interpretation of breakout data for the present-day stress field (see the “Expedition 315 Site C0002” chapter). Fault analyses from that site show the following time evolution of the stress field:

1. First phase of northwest–southeast shortening by thrust faulting and possibly strike-slip faulting,
2. Second phase of northeast–southwest extension by normal faulting, and
3. Third phase of north–south extension by normal faulting consistent with the main normal faults seen in the 3-D seismic lines. This last phase correlates with the borehole breakout observations.

In the minor structures observed in the megasplay thrust sheet at Site C0001 and at the frontal thrust at Site C0006, both reverse and normal faults were documented, with minor strike-slip as well. In general, there is an overall indication at both of these sites that the youngest and/or most numerous small faults are normal and record extensional conditions (see the “Expedition 315 Site C0001” and “Expedition 316 Site C0006” chapters). In the case of Site C0006, this was interpreted by the shipboard party as evidence of geologically recent collapse of an oversteepened frontal wedge (see the “Expedition 316 Site C0006” chapter). At Site C0004 in the front of the megasplay thrust sheet, by contrast, normal faults were not prominent except in a thin and shallow slope cover sequence from 0 to 78 mbsf, and reverse faults dominate, consistent in orientation with the breakout data for a reverse faulting stress state.

Site C0001 shows reverse faulting beneath the slope cover, but this is overprinted by normal faults recording northeast–southwest directed extension, even in the thrust sheet. The paleostress regime inferred from these small faults at Site C0001 is consistent with a maximum horizontal stress perpendicular to the margin as inferred during Expedition 314 from borehole breakouts but shows a permutation of the maximum principal stress between the upper 200 m (extension parallel to the margin, σ₁ vertical) and the deeper section (compression perpendicular to the margin, σ₁ horizontal). This can be explained by changes in the relative magnitude of the principal stresses without change of their orientations.

These results suggest an alternative interpretation of the borehole breakout data. S_Hmax at Sites C0001 and C0006 could represent the intermediate principal stress, consistent with an extensional stress state oriented perpendicular to the inferred compressional state. However, the Site C0004 observations favor the thrust state of stress. These inferences may not be in conflict if the three principal stresses differ only modestly at these shallow depths, and small changes in their relative magnitude can effect a “flip” in stress/​faulting regime. Further analysis awaits postcruise efforts.

Thermal regime

Acquisition of a transect of good quality downhole temperature profiles and thermal conductivity data was an important part of Stage 1 drilling. Thermal regime has been hypothesized to be closely tied to fault stability criteria (e.g., Hyndman et al., 1995), and the prediction of temperature at plate boundary fault zone depths is dependent on well-defined heat flow and thermal properties. Prediction of temperature at deeper borehole depths is also crucial for future deep-well planning and the mechanical specifications of long-term borehole tools to be installed during NanTroSEIZE Stage 4 because operating temperature placed severe constraints on available tools.

Downhole temperature measurement using the advanced piston corer temperature tool and Davis-Villinger Temperature Probe was successfully conducted across the Stage 1 transect. For details of the data from each site, see the respective site chapters in the Expedition 315 and Expedition 316 reports. In general, good linear gradients indicative of predominantly conductive heat flow were found at all sites, with the exception of some depths at Site C0006.

From southeast to northwest (i.e., seaward to landward along the transect), the results are as follows. In the frontal thrust region at Site C0007, the thermal gradient is 42°C/km (heat flow = 53 mW/m²). At the nearby Site C0006, a thermal gradient of 27°C/km (heat flow = 33 mW/m²) was computed from six measurements for anomalously low values of both gradient and heat flow, relative to regional data and the other sites. Moving to the megasplay sites, the best fitting thermal gradients were 51°C/km and 57°C/km in Holes C0008A and C0008C, respectively. Very close by at Site C0004 the measured gradient was 52°C/km. At Site C0001 the measured gradient was 44°C/km (heat flow = 47 mW/m²). Moving into Kumano Basin, the thermal gradient at Site C0002 was 43°C/km.

All of these gradients are consistently lower than those estimated from surface measurements (Kinoshita et al., 2008). The extremely low heat flow observed at frontal thrust Sites C0006 and C0007 might be related to stratigraphic or structural fluid pathways developed in this region, perhaps facilitating circulation of seawater down into the thrust sheet, though geochemical evidence of seawater circulation was not identified in pore water analyses at those sites.

Gas hydrates and bottom-simulating reflector

At Site C0002, a well-developed bottom-simulating reflector (BSR) is imaged in the seismic data, and LWD logs recorded comprehensive in situ information about the nature of this BSR (see the “Expedition 314 Site C0002” chapter). Resistivity logs showed a pattern of elevated resistivity background and spikes for ~200 m above the BSR depth at ~400 m LSF. The gamma response indicated that the spikes are in especially sandy intervals as thick as 1–2 m, interpreted as the coarse basal beds in turbidite deposits. Sonic and resistivity responses are consistent with pore space in these sands being partially filled and “cemented” with gas hydrate. In contrast, similar sands below the BSR depth show no elevated resistivity response. In a zone 80 m deeper and ~70 m thick, a low resistivity response in the sandy beds suggests a potential gas-charged interval beneath the gas hydrate stability field. The logging data indicate that the BSR is a response to both a small velocity high from hydrate cement in the hydrate stability zone and a more significant velocity low caused by the presence of uncemented sediments and/or free gas below the stability field.

Ongoing log analysis will quantify the amount of pore space charged with hydrate in the zone above the BSR and, integrated with 3-D seismic analysis, the total amount of gas in this region of the Kumano Basin section. During Expedition 316, Site C0002 was cored late in the expedition as a contingency operation. The BSR interval and overlying apparent gas hydrate–rich zone was deliberately not cored in order to allow time for deeper objectives, so core-based sampling of the zone awaits future drilling.

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