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Key results and implications

Geomechanics: structures and stress state

We collected several data sets at Sites C0009 and C0010 that provide constraints on present-day in situ stress orientation and magnitude, as well as on past deformation. At Site C0009, borehole breakouts inferred from wireline calipers in Unit IV (1285–1580 m WMSF) indicate that SHmax is oriented 136°–316° (northwest–southeast). DITF tentatively interpreted from borehole resistivity images in Unit II (~800–1000 m WMSF) are also compatible with a northwest–southeast oriented SHmax. This orientation is ~90° to that at Site C0002, located ~20 km seaward in the Kumano Basin (Fig. F7). At Site C0010, breakouts observed in RAB images indicate a SHmax orientation of 145°–325° (northwest–southeast), consistent with that seen at previously drilled Sites C0001, C0004, and C0006 on the outer continental slope between the seaward edge of the Kumano Basin and the subduction trench (Tobin et al., 2009). The emerging picture of stress conditions (see also Kinoshita et al., 2008; Tobin et al., 2009) across the margin from borehole observations and fault patterns observed in seismic reflection data is one in which SHmax is slightly oblique to the plate convergence direction (but approximately perpendicular to the trench) in the outer accretionary wedge (Fig. F7). Near the seaward portion of the Kumano Basin, there is active northwest–southeast extension approximately perpendicular to the trench but limited to an ~15–20 km wide zone. Landward of this, SHmax rotates back to an orientation nearly perpendicular to the trench and similar to that on the outer slope.

We also obtained two independent direct measurements of σ3 magnitude at Site C0009 from an MDT hydraulic fracturing test at 879 m WMSF and a LOT at 704 m DSF (Fig. F9). In both cases, σv is greater than the measured value of σ3. Under the assumption that the principal stresses are horizontal and vertical, we conclude that σ3 is horizontal (σhmin = σ3) and the vertical stress is either the maximum or intermediate principal stress (σv = σ1 or σ2). Taken together, the results from resistivity imaging (breakouts and DITF) and direct stress measurement (MDT and LOT) at Site C0009 indicate either a normal or strike-slip faulting regime in which SHmax is oriented northwest–southeast at Site C0009. In the case of normal faults, the dominant fault strike would be northwest–southeast. The effective stress ratio (σ′hmin/σ′v, where effective stress is given by the total stress minus the pore fluid pressure, assumed to be hydrostatic) is significantly greater for the MDT measurement (σ′hmin/σ′v = 0.82) than for the LOT (σ′hmin/σ′v = 0.44) (Fig. F9A). We consider the MDT measurement to be slightly more reliable (see "Downhole measurements"). The effective stress ratio obtained in the LOT is consistent with active normal faulting for a friction coefficient of µ = ~0.4, whereas the differential stress obtained from the MDT measurement is considerably smaller and insufficient to drive active normal faulting unless µ < ~0.15 (e.g., Zoback, 2007).

In addition to in situ stress indicators, we documented fault types and orientations from resistivity images, seismic reflection data, and cores at Site C0009 to gain insight into deformation history and the associated stress conditions when the structures were active. The relative timing of different phases of faulting can be determined in some cases, but recent fault activity cannot be confirmed except where faults cut the seafloor in seismic reflection data. However, it is important to note that these still may not reflect present-day (instantaneous) stress conditions. Fractures in resistivity images, including one documented normal fault, strike northeast–southwest, and most dip to the northwest (average dip = 45°; modal dip = 60°–70°). Cores exhibit a range of fault types, crosscutting relationships, and orientations. In seismic reflection data close to Site C0002, recently active normal faults striking northeast–southwest and a second, less prevalent set striking northwest–southeast are observed (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009). In the landward part of the Kumano Basin near Site C0009, normal faults are sparse overall, and the northwest–southeast striking set is slightly more common relative to the northeast–southwest striking set. The presence and orientation of normal faults in the seismic reflection data (northwest–southeast strike most common) are generally consistent with in situ stress magnitude and orientation data. However, the predominant northeast–southwest strike of faults and fractures documented in borehole FMI images at Site C0009 is not; if they are normal faults, they are inconsistent with the orientation of SHmax, whereas if they are thrusts they are inconsistent with the fact that σhmin < σv.

There are several potential explanations for this range of observations. One possibility is that the structures identified in FMI images are not active and therefore should not be compared with the present-day stress regime at Site C0009. This hypothesis is consistent with the fact that the faults and fractures measured by FMI data are resistive, suggesting that they may not be currently active (e.g., Barton et al., 1995). The discrepancy between past and in situ stress states could be related to variations in stress during the earthquake cycle or to longer term processes related to the migration of deformation patterns within the basin. At Site C0002, however, fault orientations and present-day stress indicators are in agreement and indicate northwest–southeast extension normal to the margin (Tobin et al., 2009). This state of stress could have existed at Site C0009 previously and could explain the fault orientations in FMI data.

Alternatively, it is possible that measurements of stress state and those of long-term strain taken at different depths in the borehole represent changes in the stress regime with depth. In this case, the stress regime would be consistent with normal faulting in the upper ~900–1200 mbsf and transition to one of lateral compression below this, perhaps across the boundary into Unit IV. A third possibility is that the two horizontal principal stresses are close in magnitude in the landward portion of the basin (i.e., near Site C0009), such that σhmin ≈ σHmax < σv (i.e., σ3 ≈ σ2 < σ1). This stress state would permit normal faulting on structures as observed in seismic reflection data and borehole FMI images, while honoring the MDT and LOT stress measurements indicating σhmin < σv. However, the latter hypothesis does not explain the observation of only one dominant (northeast–southwest) strike for structures in the FMI resistivity data.

Forearc basin development and correlation with Site C0002: depositional and tectonic environment

Our interpretation of new data from Site C0009, evaluated in the context of previous results from drilling in the Kumano Basin (Ashi et al., 2008; Kinoshita et al., 2008), parallels the interpretation of geological and tectonic evolution initiated by the Expedition 314 and 315 scientists from drilling data at Site C0002 (Expedition 314 Scientists, 2009; Expedition 315 Scientists, 2009). At both Sites C0009 and C0002, drilling penetrated through Pliocene–Quaternary aged basin fill and across a basin-wide unconformity identified in seismic reflection data into finer grained, tilted, and variably deformed late Miocene sediments below. Four lithologic units were described at both drill sites. These units (Units I and II taken together, Unit III, and Unit IV) comprise three depositional sequences (Fig. F6).

At both sites, the lowest stratigraphic unit (Unit IV) is composed of mudstone with thin-bedded fine-grained turbidites. At Site C0002, sediments of this lower unit are significantly deformed and have undergone moderate carbonate dissolution, interpreted as deposition below or near the CCD. This unit was interpreted as accretionary prism material by Expedition 314 and Expedition 315 scientists (Ashi et al., 2008; Tobin et al., 2009). At Site C0009, Unit IV is weakly deformed and shows the presence of carbonate, indicating deposition above the CCD. From the data collected at Site C0009 and comparison with previous drilling results at Site C0002, we interpret Unit IV as weakly deformed accreted trench sediments, as trench-slope deposits overlying accreted trench sediments, or as sediments deposited within the earliest Kumano forearc basin. Above Unit IV, Unconformity UC2 exhibits more than 1000 m of relief between Sites C0009 and C0002 and marks a hiatus of approximately equal age and duration at both sites (~5.6–3.8 Ma) (Fig. F6). This suggests a tectonic event of regional significance, possibly related to the onset of out-of-sequence thrusting in the prism or to basement ridge subduction or underplating.

The lower part of Unit III at Site C0009 is approximately coeval with Unit III at Site C0002 but is considerably thicker at Site C0009; this part of the forearc basin sequence is highly variable in thickness throughout the basin (Fig. F6B). At Site C0002, ~20 km farther seaward in the basin, it is interpreted as a condensed section of early forearc basin or slope basin/apron mudstones (Ashi et al., 2008). At Site C0009, this unit is distinguished from its lateral equivalent at Site C0002 by the presence of silt and ash beds and by abundant terrigenous input including wood and lignite (Figs. F5A, F8). We suggest that Unit III at Site C0009 represents early (unconformable) forearc basin or slope deposits, some of which may be associated with transport from out of the plane of the cross section shown in Figure F6B and F6C.

Seismic Surface S2 separates laterally continuous high-amplitude reflections above from a more acoustically transparent unit of variable thickness below and is interpreted as a time transgressive surface based on biostratigraphic constraints at both Sites C0002 and C0009 (Fig. F6B, F6C). Seismic Reflectors S-A and S-B downlap onto this surface near Site C0009 (Fig. F6B, F6C). Units I and II are a conformable package of sediments grading upward from mud (Unit II) to interbedded mud and sand (Unit I). This conformable succession was deposited at both Sites C0009 and C0002. However, it is greatly expanded at Site C0009, with sedimentation rates of ~700–800 m/m.y. We suggest that these strata record infilling of the Kumano Basin and the progressive landward (northwestward) migration of the depocenter, likely caused by underplating and/or slip on the megasplay with resulting uplift of the seaward edge of the basin (e.g., Moore et al., 2007).

Plate boundary structure from walkaway VSP experiment

A collaborative effort between IODP-Center for Deep Earth Exploration and JAMSTEC enabled a long-offset two-ship walkaway VSP experiment using an air gun source towed by the Kairei, along with a zero-offset VSP using a source at the drillship, in both cases shooting to receivers within the borehole. The walkaway VSP tracklines included a single line crossing over the location of the borehole with offsets up to 30 km and a circular trackline around the borehole with ~3.5 km radius to investigate anisotropy. The long offsets allowed refractions and reflections from the accretionary wedge, plate boundary, and subducting plate to be recorded at the wireline tool within the borehole to ~1200 mbsf. Recording arrivals in the borehole environment provides a higher resolution image than surface ship acquisition or OBS because the seismometers are coupled to the stiff and less attenuative formation; this configuration also allows high-fidelity measurement of shear waves converted from P-waves at subsurface boundaries. The data will allow seismic analyses of the velocity structure of the subduction zone forearc and the seismic attributes of the plate boundary in the region beneath the borehole at ~10–12 km.

Architecture and along-strike variation of the megasplay fault

Although we drilled Site C0010 with only a limited suite of LWD/MWD tools, the resistivity and gamma ray data sets provide a useful basis for comparison with nearby Site C0004, located ~3.5 km along strike to the northeast. Based on the two penetrations of the thrust wedge, along with observations from 3-D seismic reflection data, it is clear that the character and physical properties of the megasplay fault system vary markedly along strike (Fig. F11).

At Site C0010, both gamma ray values and resistivity are higher in the thrust wedge than in the slope sediment above and below (Figs. F10, F11). In contrast, at Site C0004 gamma ray values and resistivity within the thrust wedge are only very slightly higher than in the overlying and underlying units and are considerably lower than in the thrust wedge at Site C0010 (Kinoshita et al., 2008). Both gamma ray and resistivity logs are also characterized by fluctuations to lower values in the thrust wedge at Site C0010 that are not observed at Site C0004. The values for the minima in gamma ray and resistivity at Site C0010 are similar to those for the entire thrust wedge at Site C0004.

The base of the thrust wedge at Site C0010 is marked by a negative polarity seismic reflection. In contrast, the base of the thrust wedge at Site C0004 is marked by a positive polarity reflection, consistent with an increase in impedance as expected from observed P-wave velocity and bulk density from LWD and core data at this site (Kinoshita et al., 2008; Kimura et al., 2008). The thrust wedge in the vicinity of Site C0004 is seismically transparent in character, whereas at Site C0010 there are several clear reflectors, which likely correlate with the observed variations in gamma ray and resistivity (Fig. F11). From both LWD azimuthal resistivity images and seismic data, the base of the thrust wedge is sharper at Site C0010 than at Site C0004, where coring of the latter documented a ~50 m thick fault zone (or "fault-bounded package") (Kimura et al., 2008). This is consistent with the observation that at Site C0010 the mean borehole breakout orientation changes abruptly by ~20°–30° across the base of the thrust wedge, whereas at Site C0004 it changes gradually (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009).

We suggest that the overall higher values of gamma ray and resistivity reflect increased compaction in the thrust wedge at Site C0010 relative to the sediment above and below and relative to the thrust wedge at Site C0004, although it is also possible that these data could reflect a higher clay content. In the latter case, resistivity would be higher due to increased tortuosity associated with fine grain size and surface area. Similarly, the fluctuations in gamma ray and resistivity in the thrust wedge at Site C0004 could reflect variations in porosity or fracture density (with lower values associated with zones of increased fracturing or lower porosity), compositional layering, or a combination of the two.

The negative polarity reflection at the base of the thrust wedge at Site C0010 also suggests that it has a lower porosity than the overridden slope sediments below. However, based on the compaction trend for the slope sediments above and below the thrust wedge inferred from resistivity data (Conin et al., 2008), the overridden slope sediments do not appear to be underconsolidated, as might be the case for compaction disequilibrium (Hart et al., 1995; Saffer, 2003). Thus we conclude that the thrust wedge at Site C0010 is overcompacted, meaning that its porosity is anomalously low for its burial depth. This could result from increased mean effective stresses in the thrust wedge or from uplift of the wedge along the megasplay from greater depth.

At Site C0004, the thrust wedge exhibits porosity similar to the slope sediments and there is no evidence for enhanced compaction. Downdip from Site C0004, the seismic reflection polarity at the base of the thrust wedge becomes negative, most likely indicating increased compaction of the thrust wedge. Overall, we suggest that in the area of Site C0010 the thrust wedge comprises a consolidated and fractured package that probably originated at greater depth than the thrust wedge sampled at Site C0004. In contrast, the thrust wedge at Site C0004 may primarily be composed of reworked and deformed slope deposits. This interpretation is consistent with the location of Site C0010 on the flank of a lateral ramp on the megasplay fault (Fig. F11).