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Geological setting

The Nankai accretionary prism along the Kumano Transect

Along the Nankai Trough subduction system, a large accretionary complex off the coast of southwest Japan has been accumulated as a result of the Philippine Sea plate being subducted beneath the Eurasian plate at a rate of ~4 mm/y (Fig. F1A). The several hundred kilometer wide accretionary wedge consists of mainly trench-fill turbidites and incoming Shikoku Basin hemipelagic sediments that were offscraped from the downward moving plate. This study covers the central portion of the Kumano transect (Fig. F1B), which was recently extended both landward and seaward during NanTroSEIZE Stage 2 drilling (Expedition 319 Scientists, 2010; Underwood et al., 2010). The transect is divided into six main morphotectonic zones, which are, from southeast to northwest, trench, frontal thrust zone (FTZ), imbricated thrust zone (ITZ), megasplay fault zone (MSFZ), Kumano Basin edge fault zone (KBEFZ), and the Kumano forearc basin (Moore et al., 2009).

In the trench zone, younger trench deposits, the so-called trench wedge facies, overlie the oceanic crust and Shikoku Basin sediments (Fig. F1B). Farther landward, a well-developed protothrust zone has developed; however, it is overlain by a slice of trench strata previously accreted into the prism and emplaced over the trench strata by an OOST (see the “Expedition 316 summary” chapter [Screaton et al., 2009a]; Moore et al., 2009; Screaton et al., 2009b). The FTZ appears highly complex with a very steep slope (~10°) and a large embayment interpreted as slump scar possibly reflecting indentation by a recently subducted seamount (Screaton et al., 2009b). The deposits of this prism-toe collapse are found as irregular hummocky bathymetry seaward of the embayment, where the trench channel has been deflected significantly, likely caused by blockage of axial flow by mass-wasting deposits (Kawamura et al., 2010).

Landward of the prism front is the ITZ a series of thrust packages reflecting past in-sequence thrusting and accretion. The ITZ is overlain by slope sediments deposited in slope basins within the ridge-basin topography typical of fold-and-thrust belts developed in many accretionary prisms (e.g., Morley, 2009). The thickness of these basins generally increases landward from southeast to northwest (Fig. F1B). Beneath the upper slope and Kumano Basin, a regional splay fault system, first recognized by Park et al. (2002) and later termed “megasplay” by Tobin and Kinoshita (2006), discontinuously cuts across the older part of the accretionary prism and intersects its shallow landward edge (Moore et al., 2007; Moore et al., 2009). The shallow part of the MSFZ is a complex thrust system with backward breaking branches that truncate the imbricate thrust faults within the accretionary prism and override younger slope basin sediments (Moore et al., 2007; Strasser et al., 2009). Landward of the MSFZ, along the forearc high, the Kumano forearc basin is bounded on the southeast by a topographic valley. Beneath the valley is a complex fault zone that may comprise a combination of normal and strike-slip faults (KBEFZ; Martin et al., 2010). More than 2 km of sediment is imaged in the Kumano forearc basin. The seaward portion of the basin section is progressively tilted and dips landward, likely because of repeated motion on the megasplay fault (Park et al., 2002). New data by Gulick et al. (2010) point toward a major phase of landward tilting of sediment packages and inferred megasplay activity between 1.3 and 1 Ma that postdates an earlier phase of asymmetric forearc high uplift (more uplift in southwestern part of the Kumano transect) that may have occurred in concert with splay fault steepening and underthrusting of a large volume of sediment beneath the thrust (Bangs et al., 2009). In the shallow subsurface deposits, however, evidence for soft-sediment deformation and submarine landslides exists in the steeper portion of the slope (Kawamura et al., 2009; Strasser et al., 2011).

Lithostratigraphic summary of Sites C0001–C0008

During IODP Expedition 315, coring at Sites, C0001 and C0002 took place (Fig. F1B). Drilling at Site C0001 cored to 458 meters below seafloor (mbsf) and 60 cores, 32 with the hydraulic piston coring system (HPCS), 2 with the extended shoe coring system (ESCS), and 26 with the rotary core barrel (RCB), were collected from five holes covering the slope basin (lithologic Unit I) and the uppermost 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 was identified at 207 mbsf and represents an unconformity located immediately below a thick sand layer (Fig. F2; left column). Unit II is composed of mud-dominated sediments of late Pliocene to late Miocene age (see the “Expedition 315 summary” chapter [Ashi et al., 2009]).

At Site C0002, a total of depth of 1057 mbsf was reached, with cored intervals from 0 to 204 and 475 to 1057 mbsf and 86 cores cut, 18 cored with the HPCS, 2 with the ESCS, and 66 with the RCB, from three holes. The basal unconformity of the Kumano forearc basin was drilled at ~922 mbsf and cored another 135 m into the accretionary prism. The forearc basin sequence was divided into two lithologic units based on lithofacies. The boundary core between lithologic Units I and II was not recovered; hence, the logging unit boundary defined by logging-while-drilling (LWD) during IODP Expedition 314 was applied for this boundary. All units (i.e., Units I–IV) are dominated by mud and mudstone; however, 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. Units I and II are Quaternary deposits, whereas the underlying basal basin units are comprised of hemipelagic mud of Pliocene age (Fig. F3; left column). These sediments are underlain by accretionary prism materials containing moderately more lithified and much more deformed sediments (Unit IV). Biostratigraphic data show that the transition from Pliocene to late Miocene strata occurs as a gap around 922 mbsf, ~15 m above the lithologic unit boundary defined during Expedition 314 based on LWD data (see the “Expedition 314 summary” chapter [Tobin et al., 2009b]). Faults and shear zones are frequently observed in the core and clustered at several depths around 700, 920–950, and 1000–1050 mbsf.

IODP Expedition 316 was designed to evaluate the deformation, inferred depth of detachment, structural partitioning, fault zone physical characteristics, and fluid flow at the frontal thrust and at the shallow portion of the megasplay system. To accomplish these objectives, drilling was conducted at two sites in the megasplay region (Sites C0004 and C0008), one within the fault zone and one in the slope basin seaward of the MSFZ. Two sites (C0006 and C0007) were also drilled within the FTZ and protothrust zone (PTZ) (Fig. F1B) (see the “Expedition 316 summary” chapter [Screaton et al., 2009a]).

Site C0004 is located along the slope of the accretionary prism landward of the tip of the megasplay fault zone. Drilling at this site examined the youngest hemipelagic sediments on the slope (interpreted to be early to late Pleistocene) overlying the accretionary prism. These accreted sediments consist of slowly deposited marine sediments and redeposited material from upslope and were deposited during the late to middle Pliocene (Fig. F4; left column). This redeposited material consists mainly of sedimentary breccia and was interpreted as a mass transport complex (MTC) with silty clay clasts that most likely result from deposition of slumps along an unstable slope. The dominant lithology here is dark greenish gray silty clay. The accretionary prism (middle Pliocene) was sampled, and the MSFZ was successfully drilled. The top of the prism corresponds to a prominent unconformity (age gap = ~1 m.y.) that displays mineralization by pyrite and other minerals. Structural observations of core material from the fault zone and two age reversals suggested by nannofossils indicate a complex history of deformation and the presence of multiple imbricated sediment packages. Sediments under the fault zone were sampled to understand their deformation, consolidation, and fluid flow history. This succession (lithologic Unit IV; 307 mbsf to the bottom of Hole C0004D at 403 mbsf) (Fig. F4) is early Pleistocene in age and consists of dark olive-gray silty clay with a moderate amount of calcareous nannofossils and a lesser amount of calcareous and siliceous microfossils. Thin sand and silt beds are common in this unit, particularly in the uppermost part. It is interpreted to have formed in a lower trench-slope basin, dominated by fine-grained hemipelagic deposition with relatively minor sand input (for details, see the “Expedition 316 Site C0004” chapter [Expedition 316 Scientists, 2009a]).

Drilling at Site C0008 targeted the slope basin seaward of the megasplay fault. This basin records the history of megasplay fault movement. In addition, sediment layers within this basin provide a reference for sediment underthrusting the splay fault zone at Site C0004. Two lithologic units were identified at Site C0008. The uppermost unit (lithologic Unit I) consists of a 272 m (in Hole C0008A) succession of hemipelagic silty clay with thin sand beds and volcanic ash layers. In addition to the discrete ash layers, volcanic glass and pumice are disseminated as a significant component within the sediments. At the base of Unit I, a 40 m section of clayey gravel containing rounded clasts of mudstone and pumice constitutes Subunit IB. This subunit is interpreted as a MTC accumulated in the lower slope basin, possibly during an early stage of basin formation. The Pleistocene/Pliocene boundary is found within Subunit IB. Unit II includes ~57 m of sand-rich sediment for which there was very limited recovery. This sand, along with a minor gravel component, contains a diverse detrital grain assemblage that includes clasts of sedimentary, metasedimentary, plutonic, and volcanic rocks (see the “Expedition 316 Site C0008” chapter [Expedition 316 Scientists, 2009d]).

Drilling at Sites C0006 and C0007 allowed examination of the frontal thrust region. At Site C0006, coring was completed to 603 mbsf before poor hole conditions stopped drilling prior to reaching the frontal thrust. Three lithologic units were recognized: Unit I is Pleistocene–Holocene in age; extends from the seafloor to 27 mbsf; and consists of a fining-upward succession of silty clay, sand, silty sand, and rare volcanic ash layers. Unit II (27–450 mbsf) is Pleistocene in age and is interpreted as having been deposited in a trench setting. It contains dark gray to black fine-grained sand consisting dominantly of metamorphic and volcanic lithic fragments with secondary quartz and feldspar but also finer sand, silty sand, and silty clay. According to shipboard core description, individual sand beds (~1–7 m thick) typically grade into silt and occasionally silty clay with indistinct boundaries between the different lithologies (see the “Expedition 316 Site C0006” chapter [Expedition 316 Scientists, 2009b]). Unit III is late Miocene–early Pliocene in age and consists of greenish gray silty clay with some interbedded volcanic ash, including dolomite- and calcite-cemented ash. Unit III has overall increased clay content and decreased quartz and feldspar contents compared to the overlying sediments. Unit III was deposited by hemipelagic settling along with accumulation of volcanic ash. The Miocene–early Pliocene age and lithologic content of Unit III are similar to the Shikoku Basin facies documented at Ocean Drilling Program (ODP) Sites 1173 and 1174 in the Muroto transect more than 100 km west-southwest along the Nankai Trough (e.g., Moore et al., 2001). A broad fractured/brecciated zone extending from 230 to 545 mbsf and commonly strongly fractured, striated, or with polished planes suggests that several fault strands within the prism were penetrated before drilling had to be stopped because of unfavorable hole conditions (see the “Expedition 316 Site C0006” chapter [Expedition 316 Scientists, 2009b]).

The plate boundary frontal thrust was successfully drilled, and thrust fault material ranging from breccia to fault gouge was successfully recovered at Site C0007. In summary, four lithologic units were identified. The uppermost unit (lithologic Unit I; 0–33.94 mbsf) consists of hemipelagic silty clay with interbedded sand. Deposition of this unit is interpreted to have occurred on the lowermost slope above the trench floor by hemipelagic settling, turbidite deposition, and possibly subsequent soft-sediment slumping on an oversteepened slope. Unit II extends to ~362 mbsf and constitutes a coarsening-upward succession from fine-grained mud to sand- and gravel-rich deposits. Biostratigraphy suggests a possible age reversal below 135 mbsf in Hole C0007C (within Unit II) and a significant age gap between Units II and III (see the “Expedition 316 Site C0007” chapter [Expedition 316 Scientists, 2009c]). Unit III (362–439 mbsf) is a Pliocene succession of green bioturbated fine-grained hemipelagic sediments. Thin (<1 cm) greenish layers, reworked glauconite, and pervasive burrowing suggest that low sedimentation rates accompanied the deposition of this lowermost mud. Underneath, the significantly younger deposits (Pleistocene) of Unit IV may be correlated on seismic profiles with the active trench wedge of the Nankai Trough further southwest, an observation that is supported by enhanced drilling rates in the fine- to medium-grained sands. Fault zones at 237–259 mbsf (Fault Zone 1) and 341–362 mbsf (Fault Zone 2) are located at depths of lithologic changes. For detailed description, see the “Expedition 316 Site C0007” chapter (Expedition 316 Scientists, 2009c).