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doi:10.2204/iodp.proc.314315316.101.2009

Site descriptions and results from three tectonic domains

In this section, the principal results of drilling in each of the three tectonic regions are synthesized, beginning with the two sites drilled in the frontal thrust region, moving to the four sites in the mid-slope megasplay fault zone region, and finally moving to the single Kumano Basin site.

Frontal thrust region (Sites C0006 and C0007)

Drilling at Sites C0006 (Expeditions 314 and 316) and C0007 (Expedition 316 only) allowed examination of the frontal thrust region (Fig. F4). In Hole C0006B, LWD operations successfully drilled and logged to 885 m LSF, crossing the probable frontal thrust zone in the interval between 657 and 711 m LSF. The ambiguity exists because of the complex seismic reflectivity character, including apparent trench-fill channel deposit packages, and the lack of coring to this depth to reveal the detailed structure. The 657 m interval is a zone with an apparent high degree of fracturing/​brecciation based on LWD resistivity images and may be a candidate for the frontal thrust. During Expedition 316, drilling in Holes C0006E and C0006F did not penetrate to this depth before drilling was stopped because of poor conditions and extremely poor core recovery. In the interval beneath 711 m LSF, however, gamma and resistivity logs indicate that the section is sand dominated. The apparently equivalent units at Site C0007 exhibit massive coarse dark gray sands.

The plate boundary frontal thrust was successfully cored nearby at Site C0007, and thrust fault material ranging from breccia to fault gouge was recovered. The lowermost part of fault Zone 3 at 438 m core depth below seafloor (CSF) is intensely brecciated into fragments ~1–10 mm in size (Fig. F5). This 29 cm thick breccia shows a foliated aspect from an anastomosing network of polished and striated surfaces. At the base of this zone, a 2 mm thick dark layer sharply separates intensely brecciated hemipelagic mudstone above from unbroken hemipelagic mudstone and ash below. There is a biostratigraphic age reversal as well. These features indicate that the thin dark layer most likely represents extreme localization of slip associated with thrust faulting.

As a result of coring at the two sites, several new aspects of the accretion/​subduction system were revealed. The upper accretionary prism is composed of coarse terrigenous sediments, including gravel-dominated and mud-dominated lithologies in the upper and lower parts of the accretionary wedge, respectively. They likely represent trench wedge to slope sediments and hemipelagic Shikoku Basin sediments, respectively; however, that interpretation must be confirmed by more detailed onshore analysis and results of planned drilling of sediments on the incoming plate. The modern trench wedge is composed of channel-filling deposits that are currently underthrusting beneath the toe of the accretionary prism. The plate boundary frontal thrust zone at 418–438 m CSF is located in a stratigraphic package of upper Miocene mud, which appears to be similar to the Shikoku Basin facies that hosts the décollement in the Muroto and Ashizuri transects (Moore, Taira, Klaus, et al., 2001).

Lithologic Unit I sediments at Site C0006 are interpreted to represent a transition from trench to slope deposition; thus, the Unit I/II boundary records the uplift of trench material into the prism. Unit I sediments are younger than or the same age as sediments filling the basin behind the thrust. The age of the boundary is ~0.9 Ma. Taking into account the relative plate motion velocity between the overriding Japanese islands and the Philippine Sea plate (~4 cm/y [Seno et al., 1993]) the relative slip distance has to be ~40 km. No frontal accretion during this period means that the plate boundary frontal thrust has large displacement (total relative motion minus horizontal shortening of the accretionary prism). How such a large amount of slip has concentrated within the fault zone and how the evolution of the fault zone has affected the characteristic features in this region are two of the important issues of postexpedition research.

The accretionary prism in the frontal thrust region is deformed by thrusting, as visible on the seismic profiles (Figs. F3, F4; also Moore et al.). Most of the thrusts inferred from the seismic profiles and LWD data were confirmed through age reversals, fault zones sampled in cores, and repetition of specific strata, but some additional faults were also defined during Expedition 316 drilling. Chemical analysis of interstitial fluid and microbial habitat around most of these “intraprism” thrusts do not indicate any signal of active fluid flow.

In contrast to such intraprism thrusts, many normal faults are developed at the core scale and appear to be the youngest deformation feature. Clear slope-parallel mass sliding is observed from the submarine topography, seismic profiles, and shallow cores. These facts suggest that the taper angle of the prism is presently above the critical wedge taper angle and is unstable; there is evidence that the system is currently in a period of collapse.

Porosity data suggest that a considerable thickness of material has been eroded or otherwise removed from the surface at Sites C0006 and C0007. Porosity is quite low at shallow depths below the surface, reaching 48% at 5 and 34 m CSF at Sites C0006 and C0007, respectively. In contrast, porosity does not decrease to <50% until ~150 or ~200 m CSF, respectively, at other sites on the Stage 1 transect.

Megasplay fault region (Sites C0001, C0003, C0004, and C0008)

The OOS megasplay fault zone branches into a number of individual splays as it enters the upper few kilometers of the subsurface (Fig. F3; also Moore et al.). A series of sites targeted the thrust sheet, fault zone, and footwall of the lower of these branches, which was judged to be the most active and most significant of the seismically imaged splays (Tobin and Kinoshita, 2006b; Moore et al.). Site C0001 was chronologically first in drilling and targeted only the thrust sheet in the most landward position, not the underlying fault itself. The fault zone was judged too deep (~2000 mbsf) to access with riserless drilling at this site.

Site C0001

For NanTroSEIZE Stage 2, 3.5 km riser drilling was planned at Site C0001, located at a small bench on the hanging wall of the main branch of the megasplay fault (Fig. F6) where a small slope basin with coherent layered reflectors has developed, overlying a more seismically chaotic thrust sheet above the splay fault. In Stage 1, pilot-scale drilling of the uppermost 1000 m at this site was executed to test conditions for riser drilling and set the surface casing for later efforts. We first drilled a LWD-only hole at this site, achieving 976 m of penetration (only 24 m short of the planned depth and sufficient to achieve all logging objectives). Logging results suggest that the sediments of the hanging wall thrust sheet are primarily muds and mudstones with some silty to sandy (or ash bearing) sediments overlain by ~200 m of hemipelagic slope deposits. Resistivity imaging suggests widespread fracturing and variable bedding orientation, which in turn suggests strong deformation. Subsequent coring at this site during Expedition 315 confirmed and greatly added to these observations. The cored interval extends to 458 m CSF including the slope basin (lithologic Unit I) and the top ~250 m of the underlying accretionary prism (Unit II). The slope basin is composed mainly of Quaternary to late Pliocene silty clay and clayey silt with intercalations of volcanic ash. The boundary between Units I and II, identified at 207.17 m CSF, is an unconformity located immediately below a thick sand layer. Unit II is composed of mud-dominated sediments of late Pliocene to late Miocene age. Structural style and inferred stress state vary widely across a deformed zone between 220 and 230 m CSF. Normal faults indicating northeast–southwest extension are dominant above this zone; however, a few thrust faults dipping at 50° were encountered just above the deformed zone. These thrust faults are consistent with a northwest–southeast shortening subparallel to the direction of plate convergence. On the other hand, many thrust and strike-slip faults and a normal fault are found below the 220 m CSF deformed zone. The geometry and kinematics of planar structures display great variation. Kinematic solutions computed from normal and thrust faults are consistent with northeast–southwest extension and northwest–southeast shortening, respectively. Interstitial water geochemistry data show interesting trends for most elements; however, changes in these trends do not necessarily correspond to unit boundaries. Methane and ethane concentrations and their ratio (C₁/C₂) decrease with depth to 100 m CSF and remain constant to the base of lithologic Unit I. The increase of methane concentrations and C₁/C₂ ratios in Unit II indicate the contribution of biogenic methane. Total organic carbon and calcium carbonate decrease monotonously to the base of Unit I and remain low throughout Unit II. Physical properties also show a clear break at the boundary between Units I and II. Porosity generally decreases downhole within each unit; however, there is a step across the unit boundary with higher porosity below the boundary. Below this, the trend of decreasing porosity with depth resumes.

Site C0003

This site was intended to begin the downdip transect of the megasplay fault system by sampling a relatively shallow, presumably aseismogenic point on the fault zone at ~800 mbsf. Unfortunately, we encountered very difficult drilling conditions at Site C0003, such as especially pronounced caving and washout of likely fault zones and possibly sandy intervals, which caused the drill string to become irretrievably stuck before reaching the primary objective. We were unable to recover the bottom-hole assembly with the LWD tool string and ~200 m of drill collars at this site. Nevertheless, real-time data transmission provided substantial logging data from the seafloor to ~530 m LWD depth below seafloor (LSF). No further drilling or coring was attempted here; we instead moved to Site C0004 (see next section).

Consistent with the other mid-slope sites, the lithology at Site C0003 is apparently composed of relatively coarse silt-sand and hemipelagic mud in the shallowest portion underlain by more clay rich, generally homogeneous muddy deposits interrupted by prominent zones of washouts interpreted as brecciated intervals, indicating likely faults. The seismic reflection imaging shows that this site was drilled into a series of at least three individual thrust sheets within the hanging wall of the megasplay fault system (Fig. F7). Therefore, major washout zones identified in logs at ~240 and 420–450 m LSF are likely to be strongly brecciated damage zones from subsidiary thrusts, analogous to fault breccia recovered in core at other sites (see for example the “Expedition 316 Site C0004” and “Expedition 316 Site C0007” chapters).

The seismic interpretation is consistent with substantial uplift of the thrust sheet bringing older and more deeply buried accretionary prism rocks to within 500 m of the seafloor. Log density data show relatively constant and high values for this depth, suggesting that anomalously well-indurated rocks make up the thrust sheet. When the broken section of drill pipe was recovered to the rig floor after loss of the LWD string, it was plugged with numerous large chunks (as large as 5–8 cm in diameter) of cavings that had come from an unknown position in the hole. This material had a nannofossil age of late Miocene (5.5–7.2 Ma), bulk density of 2.1 g/cm³, and P-wave velocity of ~2.1 km/s, also consistent with the thrust sheet rocks having been uplifted from greater depth within the accretionary complex.

Site C0004

Site C0004 is located seaward of Sites C0001 and C0003 (Fig. F6) and targeted the prominent splay fault reflector at ~300 mbsf. The overlying thrust sheet, megasplay fault zone, and ~100 m of the footwall sediments were successfully drilled with both LWD logging (Expedition 314) and coring (Expedition 316). Drilling at this site also examined the youngest sediments on the slope overlying the accretionary prism, which consist of slowly deposited hemipelagic marine sediments and redeposited material from upslope. This redeposited material provides information about past slope failures, which may be related to past megasplay movement, earthquakes, and tsunamigenesis. The top of the prism corresponds to a prominent unconformity (age gap = ~1 m.y.) that contains pyrite and other mineralization. Structural observations of core material from the fault zone and two age reversals suggested by nannofossils indicate a complex history of deformation. Sediments under the fault zone were sampled to understand their deformation, consolidation, and fluid flow history. Further results and discussion of Site C0004 are synthesized in “Synthesis of key results.”

Site C0008

Drilling at Site C0008 targeted the slope basin seaward of the megasplay fault (Fig. F6) as a complement to Site C0004, which is ~1 km farther landward. This basin records the history of fault movement. Sediments are Pleistocene to late Pliocene hemipelagic silts and clays with ashes. Several gravelly sequences interpreted as indicative of mass transport complexes were identified. 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 beneath the splay fault, driving fluids out along permeable sandy layers.

Kumano forearc basin region (Site C0002)

Site C0002 sampled the Kumano forearc basin and underlying old accretionary prism material. LWD drilling was very successful here, with a single hole achieving 1401 m penetration, making it the deepest LWD hole in scientific ocean drilling to date. Approximately 940 m of drilling was through the sediments of the Kumano forearc basin, and the lower ~460 m accessed the underlying rocks interpreted to be the accretionary prism formed through earlier frontal accretion.

This site is slated to be the centerpiece of the NanTroSEIZE project, with 6 km or more drilling planned to access the plate boundary at seismogenic zone depths in a later stage. Understanding the physical properties and documenting tectonic processes and lithology in the hanging wall of the plate boundary here were therefore important goals of Stage 1 drilling, not least because of the need for high-quality borehole engineering parameters to plan riser drilling. Scientifically, the hole records the timing and history of the megasplay system in the formation and filling of the Kumano Basin and was used to document the present-day stress regime in this portion of the margin (see “Synthesis of key results”).

During Expedition 315, Site C0002 was drilled again in two holes to 1057 m CSF with coring of the intervals from 0 to 204 m CSF and 475 to 1057 m CSF. We cored across the basal unconformity of the Kumano forearc basin at ~922 m CSF and cored another 135 m into the accretionary prism. The forearc basin sequence is divided into two units based on lithofacies. All units are dominated by mud and mudstone; however, lithologic Units I and II contain more sand and silt intercalation and have a much faster sedimentation rate. The age ranges from Quaternary to late Miocene. Underlying accretionary prism materials contain moderately more lithified and much more deformed sediments.

The Kumano Basin is a young feature (mostly Quaternary) with a high sedimentation rate (>800 m/m.y.) overlying a late Miocene (5–6 Ma) accretionary prism, much younger than most of the Tertiary Shimanto belt outcropping onland. Biostratigraphic data show that the transition from Pliocene to late Miocene strata occurs as a marked age gap around 922 m CSF, ~15 m above the log-lithostratigraphic unit boundary defined during Expedition 314 based on LWD data.

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