Clay-Mineral Assemblages Across the Nankai-Shikoku Subduction System, Offshore Japan: a Synthesis of Results from GEOSPHERE; V

Research Paper THEMED ISSUE: Subduction Top to Bottom 2

GEOSPHERE Clay-mineral assemblages across the Nankai-Shikoku subduction system, offshore Japan: A synthesis of results from GEOSPHERE; v. 14, no. 5 the NanTroSEIZE project https://doi.org/10.1130/GES01626.1 Michael B. Underwood1 and Junhua Guo2 15 figures; 1 table 1Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA 2Department of Geological Sciences, California State University Bakersfield, Bakersfield, California 93311, USA

CORRESPONDENCE: underwoodm@missouri.edu

CITATION: Underwood, M.B., and Guo, J., Clay- ABSTRACT more illitic clays moving from offshore Taiwan through the Okinawa Trough, mineral assemblages across the Nankai-Shikoku subduction system, offshore Japan: A synthesis of although its compositional signal is masked by simultaneous enrichment of results from the NanTroSEIZE project: Geosphere, The Integrated Ocean Drilling Program, as part of the Nankai Trough illite from the Outer Zone sources. Accreted trench-wedge deposits in the v. 14, no. 5, p. 2009–2043, https://doi.org/10.1130 Seismogenic Zone Experiment, recovered samples of mud and mudstone from frontal prism (Sites C0006 and C0007) originated mostly from the Izu-Honshu /GES01626.1. the Kumano forearc basin, the inner and outer accretionary prisms, the over collision zone, and most were transported down the axis of the trench by sedi lying slope apron, and the Shikoku Basin (subduction inputs). This unprece ment gravity flow. Hemipelagic deposits in the Kumano Basin (Site C0002) Science Editor: Shanaka de Silva Guest Associate Editor: Gray Bebout dented suite of cores and cuttings captures an unusually complicated history were homogenized from a combination of transverse gravity flows, north of subduction-zone tectonics, erosion and dispersal of suspended sediment east-directed surface current, and thermohaline bottom currents. Slope-apron Received 23 October 2017 from multiple sources, and sedimentation in diverse environments. Our X-ray and slope-basin deposits (including mass-transport deposits) likewise show Revision received 1 March 2018 diffraction analyses of 1567 samples show that clay-mineral assemblages uniform clay-mineral assemblages indicative of northeast-directed transport Accepted 22 June 2018 shifted gradually throughout the subduction system, from a smectite-rich as by the Kuroshio Current, transverse resedimentation, and bottom-water cir Published online 26 July 2018 semblage during the Miocene to a more illite- and chlorite-rich assemblage culation. Collectively, these differences in sediment composition in both time during the Pliocene and Quaternary. Miocene muds in the Shikoku Basin and space set the Nankai-Shikoku depositional system apart from other sub (Sites C0011 and C0012) originated primarily from weathering of anomalous, duction zones, and they are important to consider when assessing the mar near-trench felsic-volcanic rocks along a broad swath of the Outer Zone of gin’s hydrogeology and frictional and/or geotechnical properties. Japan. The middle Miocene, however, was also a time of sediment transport into the Shikoku Basin by turbidity currents emanating from the East China INTRODUCTION Sea (Kyushu Fan). Interfingering of clays from those two sources resulted in considerable compositional scatter. Our results also reveal large discrepancies Clay-mineral assemblages in marine environments are widely known to in contents of smectite between Miocene mudstones from the inner accre change in response to the types of detrital source rocks, tectonic influences on tionary prism (Sites C0001 and C0002) and coeval mudstones from the Shi the distribution of such sources, the conditions of weathering and climate in koku Basin. We suggest that frontal accretion during the early-late Miocene the source areas, and the physical mechanisms and special patterns of trans- OLD G was a product of Pacific plate subduction rather than subduction of the Phil port and dispersal once suspended sediments enter the ocean (e.g., Naidu and ippine Sea plate. Routing of sand through the East China Sea was effectively Mowatt, 1983; Petschick et al., 1996; Fagel et al., 2001). As general rules, smec- cut off by ca. 7 Ma due to rifting of the Okinawa Trough and the buildup of tite tends to track volcanic sources, chlorite tends to increase toward higher topography along the Ryukyu arc-trench system. Subduction of Shikoku Basin latitudes, kaolinite is more prevalent in tropical latitudes (where chemical OPEN ACCESS restarted at ca. 6 Ma, and denudation of the Outer Zone continued through the weathering is more intense), and illite is a typical product of physical weather- Pliocene and Quaternary. Those adjustments in weathering, from volcanoes ing on continental landmasses (e.g., Biscaye, 1965; Thiry, 2000). To a first ap- to exposures of plutons and metasedimentary rocks, gradually increased the proximation, those general rules for clay provenance also apply to suspended concentrations of illite and chlorite. By the late Pliocene, multiple sources, sediments in subduction zones. including the rapidly uplifted Izu-Honshu collision zone, supplied suspended Some subduction zones (mostly intraoceanic examples) display relatively sediment through a combination of transverse and trench-parallel (axial) rout simple histories and spatial patterns of sedimentation dominated by steady This paper is published under the terms of the ing. At the same time, the northeast-directed Kuroshio Current intensified at pelagic to hemipelagic settling (Underwood and Moore, 1995; Spinelli and CC‑BY-NC license. ca. 3.5 Ma. That regional-scale oceanographic transition probably resulted in Underwood, 2004; Underwood, 2007; Kimura et al., 2012; Kameda et al., 2015a,

2015b). The majority of regional-scale dispersal systems, however, are much The Nankai-Shikoku system provides an ideal natural laboratory to study more complicated and diverse. That diversity is caused by different levels subduction-zone sedimentation and tectonics; it is arguably the best studied of influence among several governing factors: distribution of widely spaced example in the world. Located offshore south-central to southwest Japan point sources (i.e., submarine canyons), diffuse transverse delivery through (Fig. 1), this particular margin has received considerable attention over five slope gullies and by unconfined gravity flows, margin-parallel routing by both decades of scientific ocean drilling, largely because of its 1300-year historical ocean currents and gravity flows, and progradation of large submarine fans record of great earthquakes and tsunamis (Ando, 1975; Cummins and Kaneda, or channel-levee complexes onto adjacent (subducting) abyssal plains (e.g., 2000; Cummins et al., 2001). With that history in mind, the Nankai Trough Seis- Karlin, 1980; Emmel and Curray, 1985; Kolla and Coumes, 1985; Underwood mogenic Zone Experiment (NanTroSEIZE) was designed and implemented to and Moore, 1995; Spinelli and Field, 2001; Noda and TuZino, 2007; Noda et al., improve our collective understanding of Nankai fault-slip phenomena (Tobin 2008; Alexander et al., 2010; Gulick et al., 2015; Meridth et al., 2017). In fact, and Kinoshita, 2006). Twelve sites have been cored along the NanTroSEIZE imbalanced competition between transverse and margin-parallel routing can transect by the Integrated Ocean Drilling Program (IODP). Most of those sites result in compositional decoupling between coarser gravity-flow deposits and are within the confines of a 3D seismic-reflection survey (Moore et al., 2009). finer-grained hemipelagic muds (e.g., Underwood, 1986; Hathon and Under The tectonostratigraphic environments sampled along the transect include the wood, 1991). Essentially, the time-honored paradigm of a predictable up- Kumano forearc basin, the inner and outer accretionary prisms, and subduc- ward-thickening and upward-coarsening stratigraphic succession (Piper et al., tion inputs that were deposited seaward of the trench in the Shikoku Basin 1973) is usually violated by, rather than supported by, individual case studies. (Ashi et al., 2009; Screaton et al., 2009a; Expedition 319 Scientists, 2010; Under- The connections of cause-and-effect between subduction-zone sedimen- wood et al., 2010; Expedition 333 Scientists, 2012a; Strasser et al., 2014a; Tobin tation and tectonics are also multifaceted, and they work in both directions. et al., 2015a; Kopf et al., 2017). Proportions of common clay minerals are known to affect tectonic processes This paper provides a synthesis of X-ray diffraction (XRD) results from 1567 in several fundamental ways (Underwood, 2007). Mineralogy of the clay-size specimens of mud and mudstone, covering virtually all of the Nankai-Shikoku fraction, for example, often dictates the shear strength and permeability of depositional environments. Our data compilation demonstrates that clay-min- mudstone (Saffer et al., 2001; Brown et al., 2003; Kopf and Brown, 2003; Saffer eral assemblages fluctuated over time throughout the regional system, largely and Marone, 2003; Ikari and Saffer, 2011; Ikari et al., 2013). Expandable clays in response to: (1) gradual changes in proportions of parent rocks exposed to of the smectite group are mechanically weaker and often dominate mineral weathering within detrital source areas; and (2) reorganizations of the plate assemblages in subduction-zone sediments because of their proximity to boundaries. Understanding how, when, and where the budget of suspended volcanic sources (Vrolijk, 1990), although some notable exceptions to that sediment changed carries important implications relative to the specific goals standard do exist (Naidu and Mowatt, 1983; Hathon and Underwood, 1991). of NanTroSEIZE (Tobin and Kinoshita, 2006), especially with regard to predic- Unusually high concentrations of smectite are thought to control the strati- tions and modeling of such phenomena as fault-slip behavior and intraprism graphic position of the frontal décollement in some systems (e.g., Deng and hydrogeology. We believe those fundamental connections between cause and Underwood, 2001; Kameda et al., 2015b). Flips in fault vergence within some effect are also exportable to other subduction zones around the world. accretionary prisms have been attributed to along-strike changes in clay mineralogy and clay diagenesis, and their effects on frictional properties and GEOLOGIC SETTING shear strength (Underwood, 2002). Release of interlayer water and silica, as reaction products of smectite-to-illite diagenesis (Bruce, 1984; Colten-Bradley, Structure and Bathymetry 1987), modulate the freshening of pore water and increase the likelihood of having pore-fluid pressures rise above hydrostatic, especially in areas where The Nankai Trough currently forms the boundary between the subducting drainage is retarded (Bekins et al., 1995; Saffer and Bekins, 1998, 2006; Saffer Philippine Sea plate and the overriding Eurasia plate (Fig. 1). The trench axis is and McKiernan, 2009; Saffer and Tobin, 2011). Accordingly, the dehydration of <5000 m below sea level, and the gradient of the trench-floor dips gently to the smectite, if the minerals are volumetrically significant, probably helps regulate southwest (Fig. 2). Suruga Trough (Fig. 2) is the largest sediment conduit from the down-dip transition from aseismic (stable sliding) to seismogenic (stick shoreline to the trench floor, extending from the west side of the Izu Peninsula slip) behavior along megathrust faults (Hyndman et al., 1995; Wang et al., into the northeast end of the Nankai Trough (Le Pichon et al., 1987; Nakamura 1995; Oleskevich et al., 1999; Moore and Saffer, 2001; Spinelli and Saffer, 2004). et al., 1987). The Nankai deep-sea channel meanders down the axis of Nankai Other scientists have suggested that zones of fluid overpressure contribute Trough to a termination point offshore from the Kii Peninsula (Shimamura, to occurrences of slow-slip events along some plate interfaces (Kodaira et al., 1989). Several more through-going submarine canyons (e.g., Tenryu Canyon) 2004; Schwartz and Rokosky, 2007; Bell et al., 2010; Saffer and Wallace, 2015). are incised into the accretionary prism offshore Shikoku Island, the Kii Penin- Thus, we recognize many ancillary reasons to scrutinize the spatial and tempo- sula, and central Honshu (e.g., Soh and Tokuyama, 2002; Underwood et al., ral changes in clay mineralogy within subduction zones. 2003a; Kawamura et al., 2009; Tsuji et al., 2014).

Korea Sea of Japan Honshu Figure 2 Sagami Japanrench Suruga T Tsushima Trough N 34°N East China Sea Trough Shikoku 34°N Trough Izu-Bonin Nankai11 73 C0011 400 km Kyushu C0012 1177 Eurasia Izu-B Kuroshio Shikoku T rench Pacific plate plate Basin 30° Trough o Kinan seamounts (spreading ax Kyushu-Palau Ridge nin China

CurrentOkinawa Trench 26° Kuroshio Ryukyu lowstand SL Mariana

aiwan T West Philippine Basin is ) Tr ench 22° Philippine Sea plate Parece Vela Basin

120°E124°128°132°136° 140° 144° 148°

Figure 1. Index map of the northern Philippine Sea region and Japanese Islands with locations of key tectonic elements. Generalized pathways for the Kuroshio Current include conditions of sea level (SL) highstand (blue arrow) and lowstand (green arrow). See Figure 2 for more details of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) study area (white box).

Located immediately seaward of the trench, the Shikoku Basin (Fig. 1) ini- The lower trench slope of Nankai Trough (outer or frontal accretionary tially formed behind the Izu-Bonin island arc during a phase of backarc spread- prism) is dominated by a fault-controlled ridge-and-trough landscape (Ashi ing that persisted from 26 Ma to 15 Ma (Chamot-Rooke et al., 1987; Kobayashi and Taira, 1992; Okino and Kato, 1995; Moore et al., 2001, 2011; Bangs et al., et al., 1995; Okino et al., 1994, 1999; Sdrolias et al., 2004). The Kyushu-Palau 2004; Gulick et al., 2004). The outer accretionary prism is characterized by Ridge on the basin’s western edge (Fig. 1) represents the remnant arc (Ishizuka a critical taper that varies along strike, by internal deformation via folds et al., 2011). The northern (proximal) part of the basin includes several bathy- and in-sequence, seaward-vergent thrusts, and by an aseismic décollement metric complexities (Fig. 2): four cross chains of en echelon seamounts (in (Kimura et al., 2007). In the vicinity of the NanTroSEIZE transect, frontal im- the vicinity of IODP site U1437); Zenisu Ridge and associated fault fragments; brication appears to have been dormant until recently, and the frontal fault a large canyon-channel complex (Zenisu channel); and the Kinan seamount can be traced landward (down-dip) from the base-of-slope to its intersection chain, which runs down the axis of the basin (Lallemant et al., 1989; Ishii et al., with the basal décollement (Moore et al., 2009; Screaton et al., 2009b). The 2000; Sato et al., 2002; Wu et al., 2005; Machida et al., 2008). Kashinosaki taper angle of the prism is unusually large in that area (Kimura et al., 2007), Knoll, another prominent bathymetric high within the NanTroSEIZE transect and the anomalous structural architecture was probably induced by recent area (Figs. 2 and 3), is thought to have formed by reorientation of the back- subduction of a seamount (Moore et al., 2009). Integration of seismic, log- arc spreading center and off-axis volcanism during the Miocene (Ike et al., ging, and coring results confirms the existence of many subparallel thrusts 2008a). Total sediment thickness across the Shikoku Basin ranges from 300 m within the frontal prism, as evidenced by age reversals, repetition of distinc- to 2200 m, with pronounced thinning toward the south and above the larger tive stratigraphic intervals, and localized core-scale deformation (Screaton basement highs (Higuchi et al., 2007; Ike et al., 2008b). et al., 2009b).

133°E 134° 135° 136° 137° 138°

Honshu Suruga Trough Tenryu a Canyon m

Kii Peninsul l 34° e N ial channe ax Kumano Basin Accretionary pris Shikoku Zenisu Ridg C0001 Tenryu C0009 Fan Zenisu C0002 channel Muroto Basin C0004/C0010 C0018/C0021 33° C0008 C0006 C0011 sa Basin C0007 To C0012 Trough Kashinosaki NanTroSEIZE Knoll transect m 1173 32° Muroto en echelon Accretionary prisNankai transect seamount Shikoku chains U1437 1177 Ashizuri Kinan N transect Basin 100 km seamounts 31°

Figure 2. Enlarged map of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) transect area with locations of important bathymetric features and Inte grated Ocean Drilling Program (IODP) drill sites. For reference, ODP Sites 1177 and 1173 are shown to identify where subduction inputs were cored along the Ashizuri and Muroto transects. IODP Site U1437 was drilled in the Izu-Bonin rear arc. See Figure 1 for regional context.

Moving farther landward, the most prominent structural element is an out- a seismogenic megathrust along its base (Kimura et al., 2007). The prism’s of-sequence thrust (Fig. 3), usually referred to as the megasplay fault (Park geologic makeup is poorly documented but thought to consist of old accreted et al., 2000, 2002; Moore et al., 2007; Strasser et al., 2009; Kimura et al., 2011; sediments (Nakanishi et al., 2002). Analysis of 3D seismic data provides clear Yamada et al., 2013). Similar faults exist along many subduction margins (e.g., evidence of thrust-fault reactivation after the overlying forearc basin formed Barnes et al., 2002; Collot et al., 2008; Hsu et al., 2013; Lauer and Saffer, 2015), (Boston et al., 2016). In addition, deformation of the inner prism is hetero and some such faults are thought to be responsible for destructive earth- geneous and locally intense, and the prism’s semitransparent acoustic char- quakes and tsunamis (e.g., Sibuet et al., 2007; Waldhauser et al., 2012; Melnick acter is indicative of a generally uniform lithology with steep bedding dips et al., 2012). The Nankai megasplay marks the boundary between the inner (Boston et al., 2016). Slip along the Nankai megasplay started ca. 1.95 Ma, and accretionary prism and the outer prism (Fig. 3). The inner prism is character- the thrust has remained active in the eastern domain of the 3D survey since ized by a consistently narrow taper, weakly deformed internal structure, and 1.24 Ma (Kimura et al., 2011). Subsurface fault traces correlate with clusters

C0004 Frontal (outer) Kashinosaki C0009 Kumano Basin C0002 C0001 C0008 accretionary Knoll 2 C0018 prism Shikoku Basin C0006 C0012 C0007 Trench C0011 wedge 4

Inner accretionary prism Shikoku Basin sediments 6 Frontal thrust Top of Depth (km) ts igneous crust 8 Megasplay fault derthrust sedimen Un Top of igneous crust 0 km 10

Figure 3. Composite seismic-reflection profile crossing the NanTroSEIZE transect with locations of Integrated Ocean Drilling Program (IODP) drill sites. See Figure 2 for geographic context.

of low-frequency tremor, very low frequency earthquakes, and co-seismic slip (including mass-transport complexes); (4) the shallow hanging wall to the during great earthquakes (Cummins and Kaneda, 2000; Cummins et al., 2001; megasplay fault (inner accretionary prism); (5) deeper (intermediate) lev- Ito and Obara, 2006; Obana and Kodaira, 2009). els of the inner accretionary prism; and (6) the Kumano forearc basin. This The seaward edges of several forearc basins (e.g., Kumano Basin, Muroto grouping into domains is a modification of the Stage 1 summary by Under- Basin) are positioned close to the landward edge of the transition zone be- wood and Moore (2012); it is more fundamental in terms of the depositional tween the inner and outer prisms (Fig. 2) (Kimura et al., 2007). Accordingly, environments and takes into account the most recent drilling results (Under many workers have suggested that initial formation and subsequent infilling wood et al., 2010; Expedition 333 Scientists, 2012a; Strasser et al., 2014a; of the basins by turbidites were triggered by offset along the megasplay (e.g., Tobin et al., 2015a). Gulick et al., 2010; Hayman et al., 2012; Moore et al., 2015; Ramirez et al., 2015). On the other hand, numerical modeling indicates that infilling of such basins results in emergence or reactivation of out-of-sequence thrusts (Mannu et al., Subduction Inputs 2016), rather than the other way around; so the links between cause and effect are probably more complicated. Subduction inputs were cored at Sites C0011 and C0012 (Underwood et al., As documented along many other subduction zones (Moore and Karig, 2010; Expedition 333 Scientists, 2012a). The two sites are located seaward of 1976; Underwood and Moore, 1995), slope-apron and slope-basin environ- the trench on the northwest flank and near the summit of Kashinosaki Knoll ments are ubiquitous across the landward slope of Nankai Trough. The sedi (Figs. 2 and 3). Basaltic basement at Site C0012 is older than 18.9 Ma (Under- ment carapace overlies the accretionary prism and records complicated local wood et al., 2010) and overlain by a thin interval of pelagic claystone (Fig. 4). histories of sedimentation that have been tempered by changes in sediment A unit of tuff mixed with volcaniclastic-siliciclastic turbidites overlies the clay- supply, mass wasting, and bathymetric adjustments to accretion-related defor- stone and grades upsection into a silty turbidite facies. Pickering et al. (2013) mation (Underwood et al., 2003a; Strasser et al., 2011). Slope basins within the named those turbidite deposits the Kyushu Fan (Figs. 4 and 5). Turbidite depo- NanTroSEIZE transect area host several prominent mass-transport complexes sition near Kashinosaki Knoll ceased ca. 12.5 Ma to 12.2 Ma, after which hemi- (MTCs), as well as erosional features that have been linked to episodes of off- pelagic mud dominated until 9.2 Ma to 9.1 Ma (Underwood et al., 2010). An set along the megasplay fault (Strasser et al., 2011; Alves et al., 2014; Kana- overlying volcanic turbidite facies comprises interbeds of hemipelagic mud- matsu et al., 2014; Moore and Sawyer, 2016; Laberg et al., 2017). stone, volcaniclastic and siliciclastic sandstone, ignimbrites, mud turbidites, and mass-transport deposits (Schindlbeck et al., 2013; Kutterolf et al., 2014). Pickering et al. (2013) named that interval the Zenisu Fans (Figs. 4 and 5). The Lithostratigraphy of the NanTroSEIZE Transect youngest lithologic unit in Shikoku Basin is a hemipelagic-pyroclastic facies that ranges from late Miocene (7.6 Ma to 7.8 Ma) to Holocene in age (Under- In this paper, we group the many lithologic units and subunits recognized wood et al., 2010; Expedition 333 Scientists, 2012a). As analyzed in more detail by NanTroSEIZE shipboard scientists into six tectonostratigraphic domains: (i.e., basin-wide) by Underwood and Pickering (2018), the unit boundaries de- (1) subduction inputs of Shikoku Basin; (2) the frontal thrust zone of the ac- scribed above are diachronous, and some units cannot be correlated across cretionary prism; (3) the carapace of slope-apron and slope-basin deposits the width of the basin.

1600 1900 1800 1000 1500 130 0 1200 11 20 0 70 0 600 800 100 400 900 500

300 00 Kumano Basin / Inner Site C0002 inner accretionary prism starved forearc basin turbidites 1.24 Ma 0.44 Ma 1.60 Ma 1.46 Ma Ma 0.90 1.67 Ma To >5.9 Ma 5.04 Ma 3.79 Ma tal depth = 3058 mbsf 20 0 10 0 400 30 0 unconformity Site C0001 inner accretionary prism slope apron 1.24 Ma 0.90 Ma 5.04 Ma 5.32 Ma unconformity 3.79 Ma 2.06 Ma Depth (meters below seafloor) Accretionary Prism 70 0 20 0 600 40 0 80 0 30 0 10 0 500 Site C001 20 0 100 mixed turbidite hemipelagic volc. turbidite pyroclastic-hemipelagic 30 0 core no subduction inputs Shikoku Basin Site C0004 fault inner prism MTC 12.3 Ma 7.6 Ma Fa n Kyushu slope basin slope 9.1 Ma Fans Zenisu @ 1040 m Acoustic basement 1 0.90 Ma 1.60 Ma 1.67 Ma 3.65 Ma 2.39–4.13 Ma 2.87 Ma 1.60 Ma 2.06–2.87 Ma 200 10 0 50 0 30 0 40 0 3.65 Ma Site C001 2 unconformity mixed turbidite hemipel. volc. pyro-hemi. basalt 9.4 Ma 7.8 Ma 12.7 Ma Fa n Kyushu unconformity unconformity

Depth (meters below seafloor) Slope with Mass-T 20 0 10 0 30 0

accreted MTC slope apron Depth (meters below seafloor) 20 0 10 0 Site C000 8 1.60 Ma 1.46 Ma 1.24 Ma 3.65 Ma Depth (meters below seafloor) 5.59 Ma 20 0 50 0 10 0 40 0 30 0 Site C001 8 1.67–2.87 Ma

Site C000 6 slope with MTCs basi n slope sandy

pyro./hemipelagic accretionary prism - trench wedge Frontal thrust zone 10 0 1.46 Ma 1.24 Ma frontal thrus t 1.08 Ma 0.90 Ma 0.44 Ma Ma 1.46 1.24 Ma 5.32 Ma deposit Shikoku Basi 0.44 Ma 3.79 Ma 0.90 Ma 2.87 Ma slope apron ransport Complexe MT C s 1.46 Ma 1.24 Ma 10 0 20 0 10 0 30 0 Ma 1.67 Site C002 1 n Site C000 7 basi n slope sandy slope with MTCs

accreted accreted trench wedge core d Not 1.46 Ma 1.24 Ma 0.90 Ma 5.32 Ma 3.65 Ma frontal thrus t slope apron 0.44 Ma unconformity s

GEOSPHERE | Volume 14 | Number 5 Underwood and Guo | Nankai-Shikoku clay-mineral assemblages 2014 Research Paper

Subduction Inputs - Shikoku Basin Chlorite + kaolinite Total clay minerals in Smectite in clay-size Illite in clay-size in clay-size Illite crystallinity Illite/smectite bulk mudstone (wt%) fraction (wt%) fraction (wt%) fraction (wt%) index (∆°2θ) expandability (%) Site C0011 20 40 60 80 20 40 60 80 20 40 60 20 40 60 0.30.5 0.70.9 50 70 90 Quat .

100 Pliocene 200 )

300 Hemipelagic-pyroclastic facies

400 Zenisu lcanic

facies Fans turbidite Vo late Miocene

500 s Anchizone Depth (meters below seafloor zone Epizone Diagenetic 600 Hemipelagic facie s 700

Kyushu

middle Miocene Fan 800 not cored Mixed turbidite facie

Acoustic basement @ ~1050 mbsf Mass-transport deposit Hemipelagic mud(stone) Tuffaceous/volcaniclastic sandstone Volcanic ash/tuff Siliciclastic sandstone Sandy clay Siltstone

Figure 5. X-ray diffraction (XRD) results for subduction inputs at Site C0011, Shikoku Basin. See Underwood et al. (2010) for stratigraphic overview. Data are from Expedition 322 Scientists (2010a), Expedition 333 Scientists (2012b), and Underwood and Guo (2013, 2017).

Frontal Accretionary Prism and upper Shikoku Basin deposits into a complicated system of imbricate thrust slices (Moore et al., 2009; Screaton et al., 2009b). Within the hanging Drilling at Sites C0006 and C0007 (Figs. 2 and 3) showed that the frontal wall to the frontal thrust, an unconformity separates the hemipelagic-pyroclas- thrust domain comprises three stratigraphic components: accreted Shikoku tic facies (maximum age of 5.32 Ma) from Quaternary gravity-flow deposits Basin deposits (hemipelagic-pyroclastic facies), accreted trench-wedge facies, (Screaton et al., 2009a). The associated hiatus spans from 2.87 Ma to 1.46 Ma and slope-apron deposits (Fig. 4). Frontal accretion molded the trench-wedge at Site C0006 and from 3.65 Ma to either 2.06 Ma or 1.46 Ma at Site C0007

(Screaton et al., 2009a) (Fig. 4). The accreted trench-wedge deposits range Farther upslope at Site C0001 (Fig. 3), accreted Pliocene mudstone yields a in age from 1.46 Ma to 0.44 Ma and display an overall upward coarsening maximum age of 5.32 Ma (Fig. 4) (Ashi et al., 2009). An angular unconformity trend with abundant sand and gravel (Screaton et al., 2009b; Underwood and at the top of the accretionary prism has a time gap that extends from 3.79 Ma Moore, 2012). to 2.06 Ma (Ashi et al., 2009). Acoustic character below the unconformity is semi-transparent, which is indicative of relatively homogeneous lithologies. Trench-Slope Deposits Cores display abundant small faults, shear zones, vein structures, and steep- ened bedding dips (Ashi et al., 2009). Shipboard scientists during Expedition Slope-apron and slope-basin deposits were recovered from seven sites 315 were perplexed by the enigmatic, mudstone-dominated character of the (Fig. 3). Above the frontal accretionary prism, a thin carapace of fine-grained accretionary prism, which is in stark contrast to the coarse-grained Quaternary slope-apron facies is less than 0.44 Ma in age (Screaton et al., 2009a) (Fig. 4). trench-wedge deposits in the frontal prism (Screaton et al., 2009b; Underwood The equivalent facies at Site C0001 (Fig. 3) is ~200 m thick and consists of and Moore, 2012); they speculated that the trench (and/or spillover into Shi- hemipelagic mud with scattered interbeds of fine sand, silt, and volcanic ash; koku Basin) during latest Miocene to early Pliocene time was isolated for some the maximum age there is 2.06 Ma (Ashi et al., 2009). Matsuzaki et al. (2014) unspecified reason from influx by sandy sediment gravity flows (Ashi et al., compiled a detailed age-depth model for those slope deposits using radiolar- 2009). The lithologic match to coeval deposits within Shikoku Basin (i.e., the ian biostratigraphy and oxygen-isotope stratigraphy. Comparable recoveries hemipelagic-pyroclastic facies) is dubious, however, judging from the ubiquity from Site C0008, located just seaward of the megasplay, include a lower sand- of discrete volcanic ash beds in one (C0011 and C0012) and the paucity of rich unit (accreted trench-wedge facies), a mass transport complex (MTC) with similar beds in the other (C0001). Furthermore, correlation between accreted an age range of 2.9 Ma to 1.6 Ma, and Quaternary slope-basin deposits (Expe- mudstones and the older hemipelagic facies at Sites C0011 and C0012 fails dition 333 Scientists, 2012a; Strasser et al., 2014a). completely because of their disparate ages (Underwood et al., 2010). Thus, Sites C0018 and C0021 (Fig. 4) were drilled in a nearby slope basin (Fig. 3) facies relations documented at Site C0001 point to a depositional environment to investigate the timing and emplacement mechanisms of MTCs. Hemipelagic quite unlike the setting for coeval subduction inputs on the Philippine Sea plate mud is the dominant lithology, but thin interbeds of silt- to sand-size volcanic (Underwood, 2018). ash are abundant, as are silty to fine-sand turbidites (Fig. 4). The ages of depo- Farther landward, the uppermost accretionary prism at Site C0002 (Fig. 3) sition, based on nannofossil and paleomagnetic datums, are less than 1.46 Ma also contains deformed Pliocene–Miocene mudstone with scattered beds of (Expedition 333 Scientists, 2012a; Strasser et al., 2014a). Mesoscale evidence sandstone and siltstone, yielding a maximum age of 5.9 Ma (Ashi et al., 2009). for mass transport (i.e., debris flow and submarine slide mechanisms) includes Those strata are similar to core recoveries of the accretionary prism at Site convolute and tilted strata, intervals with remobilized mud clasts overlying C0001. The hiatus along the angular unconformity at the top of the prism ex- tilted strata, and discrete shear zones. Thicker MTCs comprise a mixture of tends from 5.0 Ma to 3.8 Ma (Fig. 4) (Ashi et al., 2009). The basal unconformity mud-rich debris-flow deposits, intact hemipelagic beds, and contorted lay- is covered by mudstone deposits (starved slope or slope-basin facies) that are ers that evidently resulted from extensive deformation within the interior of Pliocene to Pleistocene in age (3.8 Ma to 1.6 Ma) (Ashi et al., 2009). At Site the failure. C0009 to the north (Fig. 3), the boundary between slope deposits and the accretionary prism is more complex, as is the transition from trench-slope to tur- Inner Accretionary Prism bidite facies within the overlying Kumano forearc basin (Hayman et al., 2012; Ramirez et al., 2015; Boston et al., 2016). Shallow portions of the inner accretionary prism (hanging wall to the mega Interpreting facies relations within older and deeper portions of the inner splay) were sampled at three sites. Coring successfully penetrated the megasplay prism at Site C0002 (Fig. 3) is hampered by the paucity of core and the reliance fault at Site C0004 (Fig. 3). The footwall to the thrust is typical slope-apron facies on analysis of cuttings that were sampled during riser drilling (Strasser et al., (Fig. 4), as described above, with an age of ca. 1.6 Ma (Screaton et al., 2009a). 2014a; Tobin et al., 2015a). A unit boundary at ~1665 mbsf (Hole C0002N) is The fault zone, itself, contains a thin sliver of Pliocene mudstone with numer- based on a significant down-hole reduction in the proportion of sandstone to ous interbeds of volcanic ash (age 3.65 Ma) (Screaton et al., 2009a), superfi- mudstone in the cuttings (Fig. 4). The underlying, mudstone-dominant unit cially similar to the hemipelagic-pyroclastic facies of Shikoku Basin (Underwood displays increases in clay-size content with depth. Age control is sparse, but et al., 2010). That fault sliver, however, is not present at Site C0010, which was nannofossil datums range from >5.59 Ma to <10.73 Ma (Strasser et al., 2014a; drilled along strike for installation of a borehole observatory (Kopf et al., 2017). Tobin et al., 2015a). The hanging wall at Site C0004 includes a Pliocene subunit of deformed mud- The cored interval in Hole C0002P provides a better indicator of the finer- stone and a Pliocene–Pleistocene subunit of synsedimentary mudstone breccia scale lithologic diversity within the accretionary prism. The most common lith (Screaton et al., 2009a). An unconformity separates the breccia from overlying ology is hemipelagic mudstone, with thin interbeds of siltstone to fine sand- slope-apron deposits (age <1.6 Ma) (Screaton et al., 2009a). stone and scattered laminae of pyrite. Bedding dips are steep, with angles of 60°

to 90° (Tobin et al., 2015b). Correlations between these fine-grained rocks and singular value decomposition (SVD). We used the saddle/peak method (Rettke, lithofacies in the Shikoku Basin are questionable (Underwood, 2018). The oldest 1981) to calculate the percent expandability of smectite and illite/smectite (I/S) nannofossil datum from the inner prism (ca. 10.7 Ma) (Tobin et al., 2015b) falls mixed-layer clay. The proportion of illite in the I/S mixed layer is based on within the age brackets of the mudstone-dominant hemipelagic facies of Shi- the position (d-value) of the (002/003) peak (Moore and Reynolds, 1989). To koku Basin (12.5 Ma to 9.1 Ma) (Underwood et al., 2010). The overlying accreted discriminate between dioctahedral and trioctahedral varieties of smectite, we sandstone, however, is quartzose, with relatively few lithic fragments or volcanic determined the d-value of the (060) reflection using randomly oriented pow- glass shards (Tobin et al., 2015b), in stark contrast to the distinctive volcanic and ders of the <2 μm size fraction (Brindley, 1980). Values of illite crystallinity index tuffaceous sandstones that typify the Zenisu Fans at Sites C0011 and C0012 are based on peak width at half-height for the illite (001) reflection (Blenkinsop, (Pickering et al., 2013; Kutterolf et al., 2014). Thus, facies relations for accreted 1988; Kisch, 1990). sediments at Sites C0002 and C0001 both point to a depositional environment dissimilar to the setting for coeval subduction inputs (Underwood, 2018). RESULTS

Forearc-Basin Deposits Shikoku Basin Subduction Inputs

At Site C0002, rapid accumulation of silty and sandy turbidites in the Among the tectonostratigraphic domains sampled during this study, forearc basin commenced at ca. 1.6 Ma, with an overall trend of thickening Sites C0011 and C0012 (Figs. 3 and 4) display the largest amounts of com- and coarsening upward (Ashi et al., 2009; Strasser et al., 2014a). Kumano Ba- positional variability over the longest range of depositional ages. The depth sin shifted to that phase of rapid sedimentation in response to a combination distributions of mineral abundances define a general trend of decreasing of uplift at the basin’s seaward edge, which created more accommodation smectite and increasing illite and chlorite upsection, particularly within the space (Strasser et al., 2009; Gulick et al., 2010), and incision of slope gullies hemipelagic-pyroclastic facies (Figs. 5 and 6). The relative abundance of and submarine canyons near the shoreline, which enhanced the sediment smectite within that uppermost facies ranges from 12 wt% to 81 wt%, with a supply to that growing accommodation space (Guo et al., 2013). The basin mean value (μ) of 44.6 wt% and a standard deviation (s) of 13.2 (Table 1). The traps sediment gravity flows emanating from small canyons and slope gullies proportion of illite in the clay-size fraction ranges from 18 wt% to 49 wt% (μ = along the upper slope, even during the Holocene highstand of sea level (Blum 34.7; s = 6.3). Percentages of chlorite range from 2 wt% to 29 wt% (μ = 13.5; and Okamura, 1992; Omura and Ikehara, 2010; Shirai et al., 2010; Shirai and s = 5.8). Percentages of clay-size kaolinite and quartz average 4.2 wt% (s = Hayashizaki, 2013). Proximal areas of the basin also contain thick deposits of 3.4) and 3.0 wt% (s = 3.5), respectively. For the older lithologic units (depo- channelized turbidite sand and sheet-like sand that act as reservoirs for gas sitional ages >7.2 Ma to 7.4 Ma), the relative abundance of smectite ranges hydrates (Noguchi et al., 2011; Egawa et al., 2013). Deformation of those sand from 23 wt% to 100 wt% (μ = 64.2; s = 13.6); 41 of those samples are benton- bodies attests to complex tectonic reorganization of the forearc, perhaps in ites, with smectite contents greater than or equal to 80 wt%. The scatter of response to subduction of seamounts. In addition, 3D seismic-reflection data values is particularly large within the Kyushu Fan and Zenisu Fan intervals from the distal basin show evidence of buried landslides, rotational slumps, (Figs. 5 and 6). The specific variety of smectite is consistently dioctahedral, and disintegrative slides (Moore and Strasser, 2016). probably montmorillonite. The proportion of illite ranges from 0 to 50 wt% (μ = 23.7; s = 7.9), and percentages of chlorite range from 0 to 18 wt% (μ = MATERIALS AND METHODS 6.3; s = 3.4). Average values for kaolinite and clay-size quartz are 1.8 wt% and 4.3 wt%, respectively. Sample preparation, XRD protocols, and error analysis have been thor- Figure 7A shows 502 values of %-smectite within the clay-size fraction oughly described in a series of IODP data reports (Underwood et al., 2003b; plotted as a function of depositional age, up to a maximum of 15 Ma. The Guo and Underwood, 2012; Underwood and Guo, 2013, 2017; Underwood and ages were extrapolated from each sample’s depth position on the inte- Song, 2016a, 2016b; Underwood, 2017a, 2017b, 2017c). The vast majority of grated age-depth models for Sites C0011 and C0012 (Expedition 322 Sci- our specimens are from “clusters” of collocated specimens used to charac- entists, 2010a, 2010b; Expedition 333 Scientists, 2012b, 2012c). Linear re- terize whole-round intervals that were extracted from the dominant lithology gression yields a correlation coefficient of 0.68 for the entire data set, and by other shipboard scientists, ostensibly for coordinated studies of interstitial the slope of the regression line indicates ~2.8 wt% decrease in the amount water geochemistry, frictional and/or geotechnical properties, and hydrogeo- of smectite for each 1 m.y. of younging. The slope of the regression line logical properties. Accordingly, our data set targets the dominant background is steeper when data are limited to the past 9 m.y., with a decrease of 4.1 lithology rather than interbeds of volcanic ash or turbidites. wt% for each 1 m.y. (Fig. 7B). Underwood and Pickering (2018) presented Our calculated values of mineral abundance in the glycol-saturated clay- similar plots for each site using calculated values of %-smectite within the size fraction (<2 μm) utilized a matrix of normalization factors derived from bulk mud or mudstone.

Subduction Inputs - Shikoku Basin Chlorite + kaolinite Total clay minerals in Smectite in Illite in clay-size in clay-size Illite crystallinity Illite/smectite bulk sediment (wt%) clay-size fraction (wt%) fraction (wt%) fraction (wt%) index (∆°2θ) expandability (%) Site C0012 20 40 60 80 20 40 60 80 20 40 60 20 40 60 0.30.5 0.70.9 50 70 90 Q

Hemipelagic- Plioc . pyroclastic Slumping 100 facies

) Volc. Zenisu turbidite Fans 200 facies late Miocene Affected by Hemipelagihemipelagic c slumping facies facies zone

300 Epizone Diagenetic

Mixed turbidite Kyushu 400 facies Fan Depth (meters below seafloor middle Miocene

500 Pelagic clay facies Anchi- Hemipelagic mud(stone) Siliciclastic sandstone zone Volcanic ash/tuff Tuffaceous/volcaniclastic sandstone Siltstone Mass-transport deposit Basalt early Miocene

Figure 6. X-ray diffraction (XRD) results for subduction inputs at Site C0012, Shikoku Basin. See Underwood et al. (2010) for stratigraphic overview. Data are from Expedition 322 Scientists (2010b), Expedition 333 Scientists (2012c), and Underwood and Guo (2013, 2017).

TABLE 1. STATISTICAL COMPARISON OF CLAY MINERAL ASSEMBLAGES FROM NANKAI TROUGH AND SHIKOKU BASIN Relative mineral abundance in clay-size fraction (wt%)^Bulk sediment (wt%) Smectite Illite Chlorite Kaolinite Quartz Total clay minerals# Age Number Tectonostratigraphic setting IODP sites (Ma) analyzed Mean S.D. Mean S.D. Mean S.D. MeanS.D. MeanS.D.MeanS.D. Slope apron, slope basin, MTDs C0001, C0004, C0008, C0018, C0021, C0006, C0007 0.0–2.87 384 29.0 6.936.44.2 20.0 3.65.1 3.09.5 4.046.16.5 Forearc basin C0002 0.0–3.79 88 26.9 6.935.62.9 20.8 4.14.7 2.812.03.1 46.2 5.9 Frontal (outer) prism C0006, C0007 0.90–5.32 206 29.7 8.235.44.3 22.9 5.64.5 2.37.5 3.349.411.2 Inner prism (shallow) C0001, C0002, C0004 3.65–5.9 123 35.7 6.835.93.3 16.5 3.76.5 2.35.4 3.159.85.4 Inner prism deep (unit IV)C0002 (cuttings) 5.9–7.3 53 41.7 9.234.24.4 17.5 5.33.2 3.63.4 4.156.15.1 Inner prism deep (unit V) C0002 (above 2220 m) 7.3–10.7 90 34.5 10.6 35.6 6.519.35.7 5.03.7 5.74.5 58.4 4.9 Inner prism (overprinted) C0002 (below 2200 m) 7.3–10.7 93 20.6 9.237.46.9 24.6 6.27.3 3.410.06.0 58.0 3.4 Subduction inputs (unit I) C0011 0.0–7.6 210 44.6 13.2 34.7 6.313.55.8 4.23.4 3.03.5 63.3 8.7 C0012 0.0–7.8 Subduction inputs (units II–V)C0011 7.6–14.1 298 64.2 13.6 23.7 7.96.3 3.41.8 1.64.3 5.368.09.6 C0012 7.8–18.7 ^Smectite + illite + chlorite + kaolinite + quartz = 100%. #Total clay minerals + quartz + feldspar + calcite = 100%. Abbreviations: IODP—Integrated Ocean Drilling Program; S.D.—standard deviation; MTD—mass transport deposit.

) 100 C0012 C0011 y = 30.0 + 4.16x 80 y = 32.9 + 2.82x r = 0.68 r = 0.68 60

20 Zenisu Kyushu C0012 Fans Fan C0011 Relative abundance clay-size smectite (wt% 0510 15 2468 Age (Ma) Age (Ma)

Figure 7. Temporal changes in relative abundance of smectite within hemipelagic deposits at Integrated Ocean Drilling Program (IODP) Sites C0011 and C0012, expressed as wt% of the clay-size fraction. See Underwood and Pickering (2018) for age-depth models. X-ray diffraction data are from Underwood and Guo (2013, 2017). Linear regression for the entire data set yields a correlation coefficient of r = 0.68 for a population of n = 502. Dashed lines define windows of 10 wt% above and 10 wt% below the regression line. The slope of the regression line increases from 2.82 to 4.16 when the data set is limited to ages younger than 9 Ma.

Values of the illite crystallinity (Kubler) index for the subduction inputs trench-wedge facies and the inferred Shikoku Basin hemipelagic-pyroclastic range from 0.84 Δ°2θ to 0.27 Δ°2θ, with a majority of results falling within the facies (Fig. 8). Contents of smectite below the unconformity match favorably confines of anchizone metamorphism; as a frame of reference, we follow Warr with the results from coeval strata at Sites C0011 and C0012 (Figs. 5 and 6); that and Mahlmann (2015) on the anchizone limits of 0.32 Δ°2θ and 0.52 Δ°2θ. The close correspondence reinforces the interpretation of thrust slices displacing scatter of values is unusually large, and a significant number of data plot in the subducted Shikoku Basin sediments (e.g., Screaton et al., 2009b). Values of diagenetic zone (Figs. 5 and 6). We see no systematic gradient in illite crystal- illite crystallinity consistently straddle the boundary between anchizone and linity as a function of burial depth, although the upper portion of the section epizone metamorphism (Fig. 8), and values of I/S expandability are uniformly shows less scatter. The values of expandability for illite/smectite mixed-layer between 60% and 70%. clays also display an unusually large range, from 49% to 100%. The average value for Site C0011 is 72.1% (s = 8.3), and the average for Site C0012 is 73.8% Inner Accretionary Prism (s = 7.8). In addition, I/S expandability decreases upsection by an average of ~10% within the hemipelagic-pyroclastic facies (Fig. 5 and 6), in the sense op- Shallow levels of the inner accretionary prism (hanging wall to the mega posite to normal burial diagenesis. splay) were sampled at Sites C0004, C0001, and C0002 (Figs. 3 and 9). The most abundant clay mineral at Site C0004 is illite (μ = 38 wt%) followed by smectite Frontal (Outer) Accretionary Prism (μ = 28 wt%) and chlorite (μ = 21 wt%). Mudstones from the fault-bounded package (2.87 Ma to 3.65 Ma) (Screaton et al., 2009a) contain more smectite The accreted trench-wedge facies at Sites C0006 and C0007 (Fig. 3) display (μ = 36 wt%) and are similar in composition to the hemipelagic-pyroclastic fa- an overall upward-coarsening trend accentuated by progressive increases in cies of the Shikoku Basin (Figs. 5 and 6). At Site C0001, accreted strata consist silt and sand turbidites. Contents of total clay minerals in the bulk mud tend mostly of deformed mudstone, with ages ranging from 3.79 Ma to 5.32 Ma to decrease upsection in response to those changes in texture (Fig. 8). Overall, (Ashi et al., 2009). Proportions of illite and smectite there are roughly equal, at illite is the dominant clay mineral (μ = 35.4 wt%, s = 4.3), followed by smectite ~35 wt% each (Fig. 9). The top of the accretionary prism is slightly older at Site (μ = 29.7 wt%, s = 8.2), chlorite (μ = 22.9 wt%, s = 5.6), kaolinite (μ = 4.5 wt%, C0002, ranging in age from 5.0 Ma to 5.9 Ma (Ashi et al., 2009), and detrital s = 2.3), and quartz (μ = 7.5 wt%, s = 3.32) (Table 1). We see a decrease in smectite increases by a few wt% relative to Sites C0001 and C0004, with a smectite content upsection, especially across the unconformity between the maximum value of 60 wt% (Fig. 9); scatter for those abundances also increases.

Frontal accretionary prism

Total clay minerals in Smectite in clay-size Illite in clay-size Chlorite + kaolinite in Illite/smectite Illite crystallinity bulk sediment (wt%) fraction (wt%) fraction (wt%) clay-size fraction (wt%) expandability (%) index (∆°2θ) 20 40 60 80 20 40 60 20 40 60 20 40 60 40 60 80 0.20.4 0.6

100 C0006 C0007

C0006 C0007 C6 C7 200

C0006 C0007 C0006 C0007

300 e Epizon

Unconformity Depth (meters below seafloor)

400 Anchizone Diagenesis

Unconformity

500 Accreted Shikoku Basin deposits

Figure 8. X-ray diffraction (XRD) results for accreted units at Sites C0006 and C0007, frontal accretionary prism of Nankai Trough. See Screaton et al. (2009a) for stratigraphic and structural overview. Data are from Expedition 316 Scientists (2009b, 2009c) and Guo and Underwood (2012).

Inner accretionary prism Our XRD results from deeper levels of the inner accretionary prism come mostly from analysis of cuttings, and we see clear evidence of a progressive (shallow hanging wall to megasplay fault) smectite-to-illite diagenetic overprint with depth. Diagenetic reaction progress Smectite in Illite in Chlorite + kaolinite below the cored interval of Hole C0002P (2163 mbsf to 2216 mbsf) is consistent Total clay minerals in clay-size clay-size in clay-size with temperature gradients derived from borehole and observatory measure- bulk sediment (wt%) fraction (wt%) fraction (wt%) fraction (wt%) ments (e.g., Sugihara et al., 2014), with temperatures documented elsewhere for 20 40 60 80 20 40 60 20 40 20 40 0 the onset of illitization (e.g., Freed and Peacor, 1989), and with kinetic modeling (e.g., Pytte and Reynolds, 1989). Below 2220 mbsf, proportions of smectite in the clay-size fraction steadily decrease with depth, balanced by steady increases in C0004 C0004 the proportion of illite (Fig. 10). In addition, the proportion of illite in I/S mixed- C0004 C0004 layer clays increases monotonically with depth to a maximum of 68%, and the 200 scatter among those values shrinks (Fig. 10). Values of illite crystallinity increase over the same depth range (i.e., the peaks broaden) due to enrichment of the Fault poorly crystalline diagenetic illite. Thus, we see four interrelated indicators of pro-

) package gressive smectite-to-illite conversion over one common depth interval (Fig. 10). Data from above 2220 mbsf retain the sediment’s primary detrital signatures 400 C0001 (Fig. 10), and the proportion of smectite in the clay-size fraction varies from an average of 41.7 wt% in unit IV to an average of 34.5 wt% in unit V (Table 1). Aver- C0001 C0001 C0001 age values for the other minerals in units IV and V, respectively, are: illite = 34.2 wt% and 35.6 wt%; chlorite = 17.5 wt% and 19.3 wt%; kaolinite = 3.2 wt% and 5.0 wt%; and quartz = 3.4 wt% and 5.7 wt%. We note a small but progressive decrease in smectite down-hole between 1100 and 2200 mbsf (Fig. 10). Values 600 of smectite within the cored interval (Underwood and Song, 2016b) show con- Depth (meters below seafloor siderably more variability (11 wt% to 98 wt%); the larger range is due partly to differences in total clay minerals within the bulk sediment (38 wt% to 73 wt%) among thin interbeds of mudstone, siltstone, and fine sandstone, together with scattered occurrences of altered volcanic ash (bentonite). Mean values for the 800 core samples, however, are nearly identical to the cuttings results from Hole C0002N over the same depth interval. As expected, the mixing of cuttings tends to homogenize small-scale compositional differences among interbeds. The depth interval between 2220 mbsf and 1100 mbsf (i.e., above the dia- C0002 C0002 C0002 C0002 genetic overprint) coincides with a span of depositional ages between 10.5 Ma 1000 and 6 Ma (Fig. 4). In the Shikoku Basin, coeval Miocene mudstones contain dramatically higher percentages of smectite, typically between 50% and 62% Figure 9. X-ray diffraction (XRD) results for strata in the shallow hanging wall to the megasplay of the clay-size fraction (Fig. 7). Those compositional contrasts, moreover, in- at Sites C0004, C0001, and C0002, Nankai Trough. See Ashi et al. (2009) and Screaton et al. flate steadily as the depositional ages get older; they are too large (especially (2009a) for stratigraphic and structural overview. Data are from Expedition 315 Scientists (2009a, 2009b), Expedition 316 Scientists (2009a), and Guo and Underwood (2012). for unit V) for us to explain in any way other than a difference in detrital provenance and depositional setting (Underwood, 2018).

Kumano Basin When results from the three sites are grouped (Table 1), the shallow hanging wall contains roughly equal proportions of smectite (μ = 35.7 wt%, s = 6.8) The inner accretionary prism at Site C0002 is buried beneath forearc-basin and illite (μ = 35.9 wt%, s = 3.3), followed in rank order by chlorite (μ = 16.5 deposits (Figs. 3 and 4). Figure 11 shows that illite in the fill of Kumano Basin wt%, s = 3.7), kaolinite (μ = 6.5 wt%, s = 2.3), and clay-size quartz (μ = 5.4 wt%, is consistently the most abundant clay-size mineral (μ = 35.6 wt%, s = 2.9), s = 3.1). Values of illite crystallinity index fall consistently near the anchizone- followed by smectite (μ = 26.9 wt%, s = 6.9), chlorite (μ = 20.8 wt%, s = 4.1), epizone boundary, and values of I/S expandability are consistently between quartz (μ = 12.0 wt%, s = 3.1), and kaolinite (μ = 4.7 wt%, s = 2.8). The aver- 60% and 70% (Fig. 9). age proportion of total clay minerals in the bulk sediment is 46 wt% (Table 1).

Inner accretionary prism - deep riser drilling

Chlorite + kaolinite Illite (%) in Total clay minerals in Smectite in Illite in clay-size in clay-size illite-smectite Illite crystallinity bulk sediment (wt%) clay-size fraction (wt%) fraction (wt%) fraction (wt%) mixed-layer clay index (∆°2θ) 020406080 20 40 60 80 20 40 60 20 40 60 20 40 60 0.20.4 0.6

Hole C0002F Epizone Anchizone Hole C0002N Diagenesis Hole C0002P C0002N C0002P core C0002P 1500

Unit IV Unit V )

2000

Cored interval

Depth (meters below seafloor C0002F C0002N C0002P Core

2500

Smectite-to-illite C0002F reaction C0002N C0002P

3000

Figure 10. X-ray diffraction (XRD) results for strata from intermediate levels of the hanging wall to the megasplay at Site C0002, inner accretionary prism of Nankai Trough. Most of the sam ples were recovered as cuttings during riser drilling. See Strasser et al. (2014a) and Tobin et al. (2015a) for stratigraphic and structural overview. Data are from Strasser et al. (2014b), Tobin et al. (2015b), Underwood and Song (2016a, 2016b), and Underwood (2017b, 2017c).

Kumano forearc basin at Sites C0018 and C0021 (Fig. 12). The relative abundance of illite throughout the slope cover ranges from 32 wt% to 52 wt%, with a mean value of 36.4 wt% Smectite in Illite in Chlorite + kaolinite and a standard deviation of 4.2 (Table 1). The proportion of smectite in the Total clay minerals in clay-size clay-size in clay-size clay-size fraction ranges from 8 wt% to 39 wt% (μ = 29.0; s = 6.9). Percentages bulk sediment (wt%) fraction (wt%) fraction (wt%) fraction (wt%) 20 40 60 80 20 40 60 20 40 60 20 40 60 of chlorite range from 9 wt% to 36 wt% (μ = 20.0; s = 3.6). Clay-size kaolinite 0 and quartz are subordinate, averaging 5.1 wt% (s = 3.0) and 9.5 wt% (s = 4.0), respectively. Values of the illite crystallinity index range from 0.21 Δ°2θ to 0.42 Δ°2θ, straddling the anchizone-epizone boundary (Fig. 11). I/S expandability values display a considerable amount of scatter, from 48% to 68%, usually with no systematic changes as a function of burial depth. At Site C0001, however, 200 I/S expandability increases downsection within the slope apron (Fig. 12), which is opposite to the trend expected with conventional burial diagenesis.

400 DISCUSSION

Our interpretations of hemipelagic sediment dispersal and paleogeography for the Nankai-Shikoku depositional system have been influenced heavily by refinements to the region’s reconstructed history of plate interactions (e.g., 600 Depth (meters below seafloor) Wu et al., 2016). Our considerations of specific detrital sources for suspended sediment entering the Nankai-Shikoku system also include localities both close to and outside of the immediate geographic area of the NanTroSEIZE transect (e.g., Clift et al., 2013; Pickering et al., 2013). Transport of suspended 800 sediment in most deep-marine environments is governed by a dynamic com- Starved bination of surface currents, thermohaline bottom currents, sediment gravity basin facies flows, and biologic resuspension (e.g., Gorsline, 1984). Therefore, the likely impact of ocean circulation on sedimentation across the Nankai-Shikoku sys- Figure 11. X-ray diffraction (XRD) results for deposits in the Kumano forearc basin, Site C0002. tem also needs to be incorporated into the interpretations. To set the stage, we See Ashi et al. (2009) for stratigraphic overview. Data are from Expedition 315 Scientists (2009b) summarize below some of the key studies to date. and Guo and Underwood (2012).

The starved-basin facies at the base of the forearc section contains proportions Plate Boundary Reconstructions of smectite modestly higher than the overlying turbidites (Fig. 11), with small corresponding reductions of illite and chlorite. All of those values are similar to The consensus among recent plate-tectonic reconstructions (e.g., Hall, the results from broadly coeval slope-apron and slope-basin deposits (see be- 2012; Seton et al., 2012; von Hagke et al., 2016; Wu et al., 2016) is that sub- low). The expandability of I/S mixed-layer clay minerals does not change sig- duction of the Pacific plate dominated across southwest Japan prior to ca. nificantly downsection, averaging ~60%, and values of illite crystallinity index 16 Ma, creating a trench that extended from Hokkaido in the north to Kyushu are mostly between 0.30 Δ°2θ and 0.40 Δ°2θ (Fig. 11), consistent with detrital and beyond to the southwest (Fig. 13A). The Shikoku Basin during that time source rocks exposed to anchizone-to-epizone metamorphism. probably was separated from Japanese landmasses by a salient of the Pacific plate (Hibbard and Karig, 1990). The Philippine Sea plate evidently initiated its descent beneath the proto–Nankai Trough just before the Shikoku Basin Slope-Apron and Slope-Basin Deposits spreading center went extinct at ca. 15 Ma. The most dramatic response to subduction of young and unusually warm As we discovered in the Kumano Basin, detrital illite is the most abundant lithosphere (including the extinct spreading center) was anomalous near-trench clay mineral in the slope-apron and slope-basin deposits at all sites; detrital magmatism across the Outer Zone of Japan (Kimura et al., 2005) (Fig. 13A). The smectite is consistently second in abundance (Fig. 12). There are no obvious compilation of age data by Kimura et al. (2014) shows that the oldest of those differences in composition between the MTCs and intact hemipelagic deposits magma bodies is 16.8 ± 0.8 Ma, and most radiometric ages cluster around 15 Ma.

GEOSPHERE | Volume 14 | Number 5 Underwood and Guo | Nankai-Shikoku clay-mineral assemblages Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/5/2009/4338211/2009.pdf 2023 by guest on 26 September 2021 on 26 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/5/2009/4338211/2009.pdf Research Paper A mega- MTCs—mass - transport complexes. (2009a), Expedition 316 Scientists (2009a, 2009b, 2009c, 2009d), Expedition 333 Scientists (2012d), Strasser et al. (2014c), Guo and Underwood (2012), and Underwood (2017a). (2009), ScreatonSee Ashi et al. (2009a), Expedition 333 Scientists (2012a), et al. and Strasser (2014) et al. for stratigraphic overview. Data are from Expedition 315 Scientists Figure 12. X - ray diffraction (XRD) results for Quaternary slope - apron and slope - basin deposits at Sites C0001, C0007, C0006, C0004, C0018, C0008, and C0021, Nankai Trough. splay basin MTC s Slope bove and Depth (meters below seafloor) Depth (meters below seafloor) 40 0 30 0 20 0 10 0 0 30 0 20 0 10 0 Above frontal accretionary 20 To bulk sediment (wt% prism 20 tal clay minerals in To bulk sediment (wt% tal clay minerals in 40 40 C000 C000 Overthrust by 60 megaspla 4 1 60 50 0 80 C0021 C0018

C000 ) 80

y Slope apron, slope basin, and mass-transport deposit ) To 20 bulk sediment (wt%

8 tal clay minerals in Smectite in clay-size Smectite in clay-size 40 fraction (wt% 20 fraction (wt% ) 20 60 40 C000 C000 40 4 1 C0007 C0006 80 60 ) C0018 C0021 C000 )

Illite in clay-size Smectite in clay-size fraction (wt% ) 20 Illite in clay-size fraction (wt% ) fraction (wt% ) 20 20 C000 C000 40 40 40 4 1 60

60 C0007 C0006 C0018

C0021 C00 60

clay-size fraction (wt% Chlorite + kaolinite in

08 clay-size fraction (wt% Chlorite + kaolinite in 20 20 C000 C000 Illite in clay-size C000 C000 40 fraction (wt% ) 20 40 4 1 6 7 60

C0021 C0018 C0008 40 60

)

60 ) 50 expandability (%

40 expandability (%) Illite/smectit e Illite/smectit e clay-size fraction (wt% s Chlorite + kaolinite in 60 70 C0001 20 80 90 C08 C1 8 C2 1 ) 40 0. 20 0. 20 Illite crystallinit C000 C000 Illite crystallinity index ( ∆ °2 θ )

index ( ∆ °2 θ ) Epizone 60 Epizone 7 6 .4

.4 ) Anchizone Anchizone C2 1 C1 8 C08 0. 6 0. 6 Diagenesis Diagenesis y

GEOSPHERE | Volume 14 | Number 5 Underwood and Guo | Nankai-Shikoku clay-mineral assemblages 2024 Research Paper

Figure 13. Paleogeographic reconstructions for the northern Shikoku Basin and surrounding Jurassic prism Collision Japan regions during three stages of tectonic evolution, with major bedrock sediment sources of the Sanbagawa belt Sea of Japan N zone Trench Outer Zone as summarized by Fergusson (2003). Median Tectonic Line (MTL) marks the bound Shimanto belt ary between the Outer Zone to the south and the Inner Zone to the north. Keys to symbols apply Accreted Izu arc Honshu to all three time stages: (A) 17–12 Ma; (B) 12–4 Ma; and (C) 4–0 Ma. Distributions of igneous rocks that formed within each time period are modified from Kimura et al. (2005). Paths for Tectonic Line Kuroshio Current depict both large-meander and straight modes based on present-day observa Median tions and may be inaccurate for times prior to ca. 3.5 Ma. MORB—mid-ocean ridge basalt. Red Current hexagon depicts approximate location of study area.

Kuroshio Izu-Bonin Kyushu

The near-trench volcano-plutonic complex included I-type granites, S-type Trough

granites, rhyolitic lavas, and rhyolitic ignimbrites. Intrusions are surrounded Ar by accreted sedimentary rocks (Shimanto Belt) and associated forearc-basin Kuroshio Large c Curren meander Proto- Nankai deposits. Overprints of paleothermal structure within those country rocks have mode t been documented using such techniques as vitrinite reflectance (e.g., Chijiwa, 17–12 Ma 1988; Underwood et al., 1992). A 100 km By 14 Ma to 13 Ma, magmatism shifted north of the Median Tectonic Line (Fig. 13A) and became better organized along the Setouchi Belt of high-mag- Sub-alkali Alkali Japan Sea of Japan I-type felsic Setouchi Trench nesium andesites (Kimura et al., 2005; Tatsumi, 2006). Overlapping that time S-type felsic high-Mg period (17 Ma to 11 Ma), the triple junction among the Pacific, Philippine MORB Honshu Sea, and Eurasia plates appears to have migrated toward the northeast, as indicated by a crude spatial pattern for the ages of anomalous near-trench MTL magma bodies (Kimura et al., 2014). Once that phase concluded, Kyushu and Shikoku experienced a long period of quiescence in subduction-related vol-

canism (Fig. 13B); the absence of magmatism from 10 Ma to 6 Ma has been Izu-Bonin attributed to either sinistral strike-slip motion or pronounced deceleration of Shikoku gh subduction due to highly oblique plate convergence along the proto-Nankai Trou margin (Kamata and Kodama, 1994; Mahony et al., 2011). How far the sinistral ai Arc Kyushu ank N boundary extended to the northeast remains uncertain, but the entire strike- Kuros roto- length of the proto-Nankai margin was probably in transcurrent slip from P hio 10 Ma to 6 Ma (Fig. 13B). 100 km 12–4 Ma Motion of the Philippine Sea plate changed dramatically ca. 6 Ma, when B subduction rates increased and/or the convergence vector reestablished its direction to roughly trench-normal (e.g., Wu et al., 2016). Studies of mantle C Japan Trench tomography reinforce this interpretation; seismic slab-subduction depths for Sea of Japan Mt. Fuji the Philippine Sea plate extend only to ~60 km, whereas the Pacific plate slab can be traced to depths of ~500 km beneath Hokkaido and northern Honshu Honshu (Honda and Nakanishi, 2003; Nakajima and Hasegawa, 2007; Zhao, 2012; Zhao MTL et al., 2012). Re-establishment of near-orthogonal convergence along the Kii proto–Nankai Trough resulted in a series of collisions between bathymetric nsula

Peni Izu-Bonin highs along the Izu-Bonin arc and the Honshu arc (Niitsuma, 1989; Amano, oku Shik 1991). In addition, scattered andesitic volcanism returned to Kyushu by ca. rough 5 Ma (Kamata, 1989; Mahony et al., 2011) (Fig. 13C). The convergence vec- T

tor for the Philippine Sea plate then rotated to the northwest at ca. 2 Ma (Wu Arc Kyushu Kuroshio et al., 2016). Across the Kyushu arc, explosive calderas became fully estab- Nankai lished during that time (Mahony et al., 2011), and incipient collision between Ryukyu 100 km the Kyushu forearc and the Kyushu-Palau Ridge (Fig. 1) resulted in widespread Arc 4–0 Ma crustal deformation (Kamata and Kodama, 1994; Wallace et al., 2009).

At the present time, the Ryukyu Trench, islands of the Ryukyu volcanic arc, Zone ignimbrites and welded tuffs, and the Setouchi high-Mg andesites (Fig. and the Okinawa Trough separate the East China Sea from the northeastern 13A) (Kimura et al., 2005). In addition, the Honshu Arc to the northeast (Figs. edge of the West Philippine Basin and the northwest corner of the Shikoku 13B and 13C), a product of Pacific plate subduction, has been erupting nearly Basin (Fig. 1). Those bathymetric obstructions, however, did not exist during continuously since the late Miocene (Cambray et al., 1995; Kimura et al., 2005; the early to middle Miocene (e.g., Letouzey and Kimura, 1986; Sibuet et al., Acocella et al., 2008; Uto and Tatsumi, 1996). On Kyushu, the Hohi volcanic 1995). To the north of the Okinawa Trough, the continental margin of the East zone initiated at 6 Ma to 5 Ma, whereas calderas and ignimbrites dominated China Sea shows widespread evidence for extensional faulting (Cukur et al., after 2 Ma (Figs. 13B and 13C); active volcanoes from that system also ex- 2011; Gungor et al., 2012). Rifting of the shelf began in the Late Cretaceous tended westward into the islands of the Ryukyu Arc (Kamata and Kodama, and resulted in the creation of several sedimentary basins separated by buried 1999; Mahony et al., 2011). continental ridges or rifted remnants of proto-Ryukyu arc basement (Sibuet The Izu-Bonin Arc (Honza and Tamaki, 1985) borders Shikoku Basin to the et al., 1987). Multiple phases of uplift (basin inversion) also affected the shelf northeast (Fig. 1) and provides another potential source of both coarser volcani (Yang et al., 2004; Lee et al., 2006; Cukur et al., 2011; Yang et al., 2011). The clastic sediment (Marsaglia, 1992) and smectite-rich clays. The initial phases of timing of initial rifting and seafloor spreading in the Okinawa Trough remains backarc spreading in the Shikoku Basin were accompanied by reduced arc vol- a point of debate. Estimates vary considerably from <2.6 Ma (Kimura, 1985; canism, but bimodal lava flows and explosive silicic eruptions have continued Park et al., 1998), 7 Ma to 2 Ma (Sibuet et al., 1998), 10 Ma to 6 Ma (Miki, 1995), along the island arc from the middle Miocene to Holocene (Taylor, 1992; Gill and 13 Ma to 7 Ma (Sibuet et al., 1987). Gungor et al. (2012) described the et al., 1994; Bryant et al., 2003; Straub, 2003; Tamura et al., 2009). Ash layers formation of a small rift basin (Ho Basin) between a basement high (Longwan recovered from the Shikoku Basin (distal tephra fallout) record several discrete Ridge) and the proto–Ryukyu Trench during the middle Miocene (15.9 Ma to pulses of explosive Izu-Bonin volcanism (Cambray et al., 1995; Straub, 2003). 11.6 Ma); according to their reconstructions, the Okinawa Trough did not open Incipient collisions between bathymetric highs along the Izu-Bonin Arc and and the Ryukyu islands did not grow to elevations above sea level until the late the Honshu Arc (Fig. 13B) evidently started ca. 12 Ma, with three subsequent Miocene (ca. 7 Ma). phases of collision-accretion from 9 Ma to 7 Ma, 5 Ma to 3 Ma, and during Ryukyu volcanoes (Kimura et al., 2005) erupted above a basement of ac- the past 1 m.y. (Niitsuma, 1989; Amano, 1991). The Quaternary phase of col- creted sedimentary and metamorphic rocks that are broadly correlative with lision has been especially important because it dramatically changed the re- the Shimanto Belt of southwest Japan (Kizaki, 1986; Ujiie, 1997; Schoonover gional paleogeography (Kitazato, 1997; Hashima et al., 2016). The transformed and Osozawa, 2004). Detailed bathymetry and analysis of magnetic anomalies topography rerouted the primary pathways for sediment gravity flows into indicate that the volcanic front shifted toward the west ca. 2.1 Ma (Sato et al., the Nankai Trough, to a system dominated by southwest-directed funneling 2014), which is approximately the same time as the change in subduction vec- through Suruga Trough (Fig. 2). The watersheds of the collision zone are un- tor for the Philippine Sea plate (Wu et al., 2016). Currently, the deep Ryukyu usually diverse in their bedrock and surficial geology (Fig. 13A), with older Trench floor and the seaward slope of the trench (Hsu et al., 2013) provide Cenozoic and Cretaceous accretionary complexes, Neogene plutonic, volcanic, additional impediments to sediment transport from the East China Sea all the and sedimentary rocks, and a cover of active Pleistocene–Holocene volcanoes way into the Shikoku Basin. that include Mount Fuji (e.g., Ogawa et al., 1985; Sato and Amano, 1991; Soh et al., 1991, 1998; Kitazato, 1997; Saito et al., 1997, 2007). Exposures of Miocene tonalites (Fig. 13A) are proof of rapid uplift from intermediate levels of the crust Prospective Parent Rocks on the Japanese Islands (Kawate and Arima, 1998). We infer that individual watersheds such as the Fuji River have probably changed significantly through time, in terms of their aerial The onland bedrock geology of central and southwest Japan comprises extent, their geometry, and sediment composition as they adjusted to uplift, a wide variety of Mesozoic and early Cenozoic accretionary complexes denudation, and the growth of large stratovolcanoes. (Maruyama et al., 1997; Taira, 2001; Isozaki et al., 2010). The Inner Zone to the north of the Median Tectonic Line (Fig. 13A) contains low-pressure, high-temperature metamorphic rocks of the Ryoke Belt and related granitic rocks (Naka Ocean Currents jima, 1997). The Outer Zone, to the south (Fig. 13A), consists of parallel bands of high-pressure metamorphic rocks and low-grade metasedimentary strata— A warm-water western boundary current, the Kuroshio Current (Fig. 1), the Sanbagawa, Chichibu, and Shimanto Belts (Toriumi and Teruya, 1988; dominates surface-water circulation off the coast of central Japan (Taft, 1972; Taira et al., 1988; Higashino, 1990). All of those accreted terranes are potential Andres et al., 2015). The Kuroshio Current flows northward past the Philippines, sources for detrital illite and chlorite. enters Okinawa Trough through the Yonaguni Depression east of Taiwan (Fig. 1), We need to critique several volcanic arcs around the rim of Shikoku Ba- bifurcates to feed the Tsushima Current into the Sea of Japan (Moriyasu, 1972), sin as potential sources for detrital smectite, including the anomalous Outer and returns to the Philippine Sea through the Tokara Strait south of Kyushu

(Ichikawa and Beardsley, 2002; Andres et al., 2015; Rudnick et al., 2015). The The main effects of lowered sea level near the Japanese Islands have been current has profoundly affected shipboard operations at all of the NanTroSEIZE to impede ventilation of the Okinawa Trough, to focus Kuroshio circulation drill sites (Fig. 2), reaching speeds as high as 5 knots (~250 cm/s) (Nagata et al., outboard of the Ryukyu Islands (Fig. 1), and to shift the balance of sediment 1999). The circulation system is quite complicated over short periods of time, delivery into the Okinawa Trough toward continental sources bordering the with alternating phases of large meanders and straighter-path courses (Fig. 13), East China Sea. Conversely, the core of the Kuroshio Current migrates into generation of mesoscale eddies, countercurrents, interaction with seamounts, the Okinawa Trough during interglacial highstands (Fig. 1), which increases and significant subannual changes in both speed and direction (Worthington the contribution of sediment from Taiwan into the Ryukyu backarc (Ujiié and and Kawai, 1972; White and McCreary, 1976; Taira and Teramoto, 1981; Nagata Ujiié, 1999; Kao et al., 2006; Lee et al., 2013; Shi et al., 2014; Amano and Itaki, et al., 1999; Ebuchi and Hanawa, 2000; Kimura and Sugimoto, 2000; Endoh 2016; Nishina et al., 2016). Under both sets of eustatic circumstances, however, et al., 2011; Shen et al., 2014; Katsumata, 2016). When averaged over scales of the dominant direction of surface-water transport across the Nankai Trough geologic time, however, sediment suspended in the surface water off central and the northern Shikoku Basin should have been toward the northeast. Our Japan follows a net transport direction toward the northeast (Fig. 13). interpretations of detrital provenance and routing have taken that dominant At water depths greater than ~500 m, thermohaline circulation involves pathway into account (Fig. 13). south-directed North Pacific Deep Water (NPDW), which enters the Shikoku Temporal and spatial changes in the behavior of thermohaline bottom Basin through gaps in the Izu-Bonin Arc (Fig. 1). More buoyant north-directed currents are more difficult to evaluate than surface currents, but studies of Antarctic Intermediate Water (AAIW) enters the Philippine Sea through the Yap middle to late Miocene and Plio-Pleistocene sedimentary rocks bordering the gateway south of the Mariana Arc. Based on sparse measurements along the northeast side of the Izu-Honshu collision zone (Fig. 13B) show clear evidence Izu-Bonin Ridge, the boundary between the two water masses is positioned at of strong bottom-water circulation within forearc-basin and slope-basin envi- a water depth of ~2000 m (Lee and Ogawa, 1998). Local ageostrophic pertur- ronments (Ito, 1996, 1997; Ito and Horikawa, 2000; Stow et al., 2002). The sandy bations are created by internal tides and steep seafloor topographies associ- contourites were deposited at paleowater depths of 400 m to 1500 m; their ated with submarine canyons and seamounts (Okada and Ohta, 1993; Nagano paleocurrent indicators are variable in orientation but dominated by slope-par- et al., 2013). The bathymetric obstructions that surround Shikoku Basin on allel and upslope directions (Ito, 2002). Movement of sand toward the south all sides (Fig. 1) allow for very limited exchange with abyssal water masses. and southwest was probably induced by circulation of NPDW through the Abyssal circulation is largely inferred, but nepheloid suspensions evidently ro- Izu-Bonin forearc, and those currents likely contributed to the downstream tate counterclockwise at speeds of 5 cm/s to 10 cm/s, with southwest-directed transport of suspended sediment in the Shikoku Basin. We surmise that the transport along the margins of the Nankai Trough (Fukasawa et al., 1986; Lee main role of bottom currents was to homogenize suspended sediment enter- and Ogawa, 1998). The deep currents, however, are also sensitive to depth- ing the Nankai-Shikoku system from multiple sources. dependent variations and the straight versus large-meander cycles that de- velop in the overlying Kuroshio Current (Fukasawa and Teramoto, 1986). We expect this deep-water circulation to enhance sustained suspension, repeated Regional Trends in Clay Composition resuspension, and long-distance transport of fine-grained sediment within the near-bottom nepheloid layer, thereby homogenizing the compositional signals Our interpretations of sediment dispersal for the NanTroSEIZE transect of multiple potential sources. area build upon an extensive foundation of compositional data generated The oceanographic behaviors outlined above have, of course, changed by previous workers. Those earlier results from Nankai Trough and Shikoku over time. One particularly transformative event occurred ca. 3.5 Ma, when the Basin include Leg 31 of the Deep Sea Drilling Project (DSDP) (Cook et al., 1975; Central America Seaway closed at the Isthmus of Panama (Maier-Reimer et al., Underwood et al., 2003b), DSDP Legs 58 and 87 (Chamley, 1980; Chamley 1990; Coates et al., 1992; Ibaraki, 1997; Molina-Cruz, 1997; Tsuchi, 1997). The et al., 1986), Leg 131 of the Ocean Drilling Program (ODP) (Underwood et al., Kuroshio Current intensified at that time as part of the overall strengthening of 1993a, 1993b; Underwood and Pickering, 1996; Masuda et al., 1996, 2001), and the North Pacific subtropical gyre. The Tsushima Current likewise strengthened ODP Leg 190 (Steurer and Underwood, 2003; Underwood and Steurer, 2003; at the end of the Pliocene, as evidenced by increases of continental (illite-rich) Underwood and Fergusson, 2005). Direct quantitative comparisons among sediment passing from the East China Sea and Taiwan through the Korea Strait those data sets are unreliable because of differences in instrumentation, ana into the Sea of Japan (Fagel et al., 1992). There is also considerable evidence lytical methods, and weighting factors. All of the results, however, reinforce for increased sensitivity of the western Pacific and Nankai Trough to obliquity one first-order tendency: proportions of detrital smectite decrease significantly forcing of climate change during the Plio-Pleistocene (Ito and Horikawa, 2000; in younger strata, regardless of the site of deposition, and those decreases Chiyonobu et al., 2012; Venti et al., 2013; Matsuzaki et al., 2015; Iwatani et al., are balanced by increases in detrital illite and chlorite. Below, we explore the 2012, 2016; Ujiié et al., 2016). The 41 k.y. obliquity cycles have amplified the reasons behind that important temporal trend, and we consider the geologic, eustatic responses to glacial-interglacial oscillations. tectonic, and oceanographic causes for a list of second-order complications.

Composition and Provenance of Subduction Inputs deeper-seated granitic rocks now exposed across the Outer Zone (Fig. 13A) experienced rapid cooling, uplift, and exhumation, as indicated by apatite fis- X-ray diffraction data from Sites C0011 and C0012 (Figs. 5–7) provide an un- sion-track ages (Hasebe and Hoshino, 2003; Hasebe et al., 2000). We suggest precedented record of the gradual temporal shifts in clay-mineral assemblages that 15 m.y. of progressive denudation stripped away the weathered volcanic entering the Shikoku Basin. Minor inputs of eolian dust are to be expected cover and gradually exposed more of the deeper-seated intrusions and sur- (e.g., Miyazaki et al., 2016) as documented by others in the pelagic realm of rounding country rocks. the central Philippine Sea (Mahoney, 2005; Wan et al., 2012; Seo et al., 2014; Denudation across the Outer Zone of Japan was uneven and has resulted Xu et al., 2015). The rates of sedimentation and proximity of Sites C0011 and in diverse exposures of the parent-rock lithologies. Fission-track studies have C0012 to continental watersheds, however, both point to marine processes highlighted several examples of local differential uplift and exhumation along (surface currents, bottom currents, turbidity currents, etc.) as the dominant the strike-length of the Shimanto Belt, with younger (ca. 10 Ma) cooling ages mechanisms of suspended sediment influx and resedimentation. in the Muroto Peninsula (Hasebe et al., 1993; Hasebe and Tagami, 2001; Hasebe Underwood and Fergusson (2005) recognized from studies associated and Hoshino, 2003; Hasebe and Watanabe, 2004). The Izu-Honshu collision with ODP Leg 190 that fine-grained suspended sediments had been eroded zone (Fig. 13) followed a similar history, with rapid exhumation bringing and transported from multiple sources; competition among those sources tonalities to the surface from midcrustal (7 km to 12 km) depths (Yamada and and dispersal routes had changed steadily over time. They attributed the de- Tagami, 2008). Even though uplift and weathering trends were not entirely pletion of detrital smectite, with commensurate increases in illite and chlorite, uniform along strike, the first-order response was consistent: progressive ex- to four factors: (1) intensification of the northeast-directed Kuroshio Current posure of more granitic plutons, accreted sedimentary rocks, and associated after closure of the Central America seaway ca. 3.5 Ma (e.g., Molina-Cruz, metasedimentary rocks to chemical and physical weathering. The weathering 1997); (2) strengthening of ocean bottom currents ca. 6 Ma, in response to products, therefore, shifted gradually toward a clay-mineral assemblage that buildup of Antarctic ice and enhanced circulation of Antarctic Bottom Water became progressively more enriched in illite and chlorite. (AABW) (e.g., Lee and Ogawa, 1998); (3) shifts in centers of active volcanism, Another important observation from Shikoku Basin is the unusual amount as well localities with widespread exposures of weathered volcanic rock (e.g., of scatter in the compositional trend, especially for intervals older than 7 Ma Cambray et al., 1995); and (4) progressive uplift and erosion of accreted sedi where the hemipelagic deposits are interbedded with numerous sand and silt mentary and metasedimentary rocks across the Outer Zone of Japan (e.g., turbidites (Fig. 7). Underwood and Pickering (2018) discussed the evidence for Hasebe and Tagami, 2001). Our refinements of those interpretations have entrance of sandy sediment gravity flows into Shikoku Basin from the East been influenced by improved reconstructions of regional plate-tectonic his- China Sea during Kyushu Fan deposition. Their reconstruction is consistent tory (e.g., Wu et al., 2016), better mapping and dating of time-space patterns with the U-Pb dates of detrital zircons (Clift et al., 2013), which correlate with for magma bodies (e.g., Kimura et al., 2014), expanded records of regional a Yangtze River watershed. If the coarser components of the basin’s sediment and local denudation history (e.g., Yamada and Tagami, 2008), consideration budget followed that dispersal route from 16 Ma to 7 Ma, then some clay-size of some new geochemical indicators of detrital provenance for coeval sand suspended sediment must have tracked the same path within the entrained deposits in the Shikoku Basin (e.g., Clift et al., 2013), and superior reconstruc- layers of turbidity currents. Thus, the likelihood of finding interlayered muds tions of regional paleoceanography (e.g., Diekmann et al., 2008). We argue derived from disparate sources seems strong. The best direct test of that idea below that the effects of tectonics and bedrock geology have dominated over would have been to sample the fine-grained tops of Shikoku Basin turbidites the effects of oceanography. and compare their clay-mineral assemblages with overlying and underlying The most likely source of the detrital smectite that dominated sedimenta- hemipelagic muds. The sampling strategy during NanTroSEIZE did not have tion during the Miocene was the broad swath of anomalous near-trench mag- that goal in mind. matic centers that migrated across the Outer Zone of Japan from 16 Ma to A more indirect way to fingerprint the East China Sea source is through 12 Ma (Kimura et al., 2005; Kimura et al., 2014) (Fig. 13A). The felsic caldera clay mineral abundances in near-surface muds. Numerous studies show those eruptions left regionally extensive ignimbrite deposits, with abundant, chem- muddy sediments to be highly enriched in detrital illite (>50%) with relatively ically unstable volcanic glass (e.g., Danhara et al., 2007; Iwano et al., 2007; low percentages of smectite, typically <30% of the total clay (Dou et al., 2010; Miura and Wada, 2007). Studies completed elsewhere lead us to suggest that Xu et al., 2014, 2017; Wang et al., 2015). Diekmann et al. (2008) showed a com- subtropical chemical weathering of the ignimbrites and associated lava flows parable dominance by detrital illite in Quaternary deposits at ODP Site 1202, produced soils with high concentrations of smectite (e.g., Parra et al., 1985; which is located closer to Taiwan on the western edge of southern Okinawa Hodder et al., 1990; Fagel et al., 2001; Wan et al., 2012). In addition, sub-sea- Trough (Fig. 1). Deposits to the south of Taiwan are similarly dominated by floor alteration of the marine ash layers and dispersed volcanic glass prob- illite (Liu et al., 2008). ably resulted in the development of interbedded bentonites (e.g., Hein and If we assume that bulk-sediment compositions derived from kindred Scholl, 1978; Hodder et al., 1993; Naish et al., 1993). At the same time, the sources were similar during the Miocene, then any pulses of suspended

sediment emanating from mainland China (via East China Sea) and/or far- of water masses and their dilute concentrations of suspended sediment (Ujiie ther west from Taiwan should have been distinctly illite-rich. Their contrast and Ujiie, 1999). to documented concentrations of smectite within the Shikoku Basin is enor- Complementary evidence for a mixed mud provenance comes from Sr- mous; thus we conclude that the dominant route for suspended sediment Nd-Pb isotope compositions of upper Shikoku Basin sediments (i.e., the hemi- during the Miocene was probably not through the East China Sea. Instead, pelagic-pyroclastic facies) (Saitoh et al., 2015). Figure 14 illustrates convinc- we surmise that the steady background of smectite-rich hemipelagic trans- ingly that mixing occurred between two sources over the past 7 m.y.—the East port from the Outer Zone sources was interrupted periodically by sediment China Sea source and a Japan margin source (Saitoh et al., 2015). The mixing gravity flows that contained higher concentrations of detrital illite in their en- line shows progressive temporal decreases in the influx of sediment from trained layers. Our XRD evidence to support these conclusions comes mostly continental China, especially between 4.4 Ma and 2.9 Ma (Fig. 14). In addition from the scatter of %-smectite values in deposits older than ca. 7 Ma (Fig. 7). to the role of bathymetric and/or topographic obstructions mentioned above, We see roughly two times as many examples of smectite-depleted speci reductions of sediment routing toward the south from the shallow shelf of mens (relative to the regression line) over the time span of 7 Ma to 15 Ma East China Sea were probably enhanced by intensification of the Kuroshio and as compared to smectite-enriched specimens. A similar increase in scatter Tsushima Currents, which now push surface water into the Okinawa Trough is also evident through the Kyushu Fan interval at ODP Site 1177, Ashizuri and the Sea of Japan (Fig. 1) (Diekmann et al., 2008; Chen et al., 2011; Xu et al., transect (Fig. 2) (Steurer and Underwood, 2003; Underwood and Pickering, 2014, 2017; Dou et al., 2010, 2012, 2016). Migration of the paleoposition of depo 2018). The route for those sandy turbidity currents from China was termi- sitional sites (C0011 and C0012) relative to the core of the current also needs to nated at ca. 7 Ma (Underwood and Pickering, 2018), but trapping of gravity be considered (Saitoh et al., 2015). flows behind the Ryukyu Arc did not necessarily force a permanent and total To complicate matters, we emphasize that the mixing trend of Saitoh et al. cutoff of suspended-sediment exchange. Some exchange of surface water (2015) should have been accompanied in the clay-mineral budget by contem- probably persisted through gaps and sills between the volcanic islands (e.g., poraneous decreases in the proportion of detrital illite relative to smectite (i.e., Nishina et al., 2016), especially during highstands of sea level. During low- from 4.4 Ma to present); instead, we see the opposite trend in XRD results stands, a nearly continuous land bridge evidently impeded such exchanges from the Shikoku Basin (Fig. 7). Moreover, the inverted trend for I/S expand-

Site C0011: Shimajiri Group Japan margin sediments <2.9 Ma Central Okinawa Trough 2.9–4.4 Ma Miyazaki Group SW Okinawa Trough >4.4 Ma

–3 15.71 39.4

East 90% China Japan margin East China Sea Sea Eolian dust 04 Eolian dust 04 Nd 10% 10% ε Pb /

50% Pb / 20 72 50% 50% 20 82 Eolian dust 90% 10% 90% East Japan margin Japan margin China Sea –13 15.59 38.6 0.728 18.4 206204 19.1 18.4 206204 19.1 0.708 87 86 Sr / Sr Pb / Pb Pb / Pb

Figure 14. Plots of Nd-Sr-Pb isotope ratios for mud deposits in the upper Shikoku Basin (hemipelagic-pyroclastic facies), modified from Saitoh et al. (2015). See Figure 5 for stratigraphic context. Mixing lines illustrate progressive temporal shifts from dominant sources in the East China Sea to dominant sources along the Japan margin (Saitoh et al., 2015). These results are consistent with the notion of having turbidity-current influx to the Shikoku Basin terminated ca. 7 Ma by rifting in the Okinawa Trough and build-up of relief along the Ryukyu arc-trench system (Underwood and Pickering, 2018). Some exchange of fine-grained suspended sediment continued into the Pliocene but was gradually dominated by influx from the Japanese Islands.

ability (i.e., less expandable I/S moving upsection) is consistent with the idea ternary analogs fall apart because of glaring mismatches in age, facies, and/or of tapping into higher levels of diagenesis and metamorphism over time in the sediment composition. The most striking discrepancy occurs between pro- sedimentary and metasedimentary parent rocks. Thus, while endorsing the portions of smectite in bulk mudstones from the inner prism (smectite poor) idea of increasing proportions of suspended-sediment influx from the Japan compared to coeval deposits in the Shikoku Basin (smectite rich). Within the margin over time (Fig. 14), we argue that those increases must have been ac- Miocene part of the record, such numerical differences in bulk mudstone com- companied by gradual changes in the proportions of parent rock types within position increase over time from ~10 wt% to 13 wt% (at 6 Ma) to ~25 wt% to 27 detrital source areas across the Outer Zone, particularly the sources of smec- wt% (at 10.5 Ma). From a sedimentological perspective, it stands to reason that tite (Figs. 13B and 13C). accreted Miocene mudstones were initially deposited somewhere other than To summarize, the time period of ca. 9 Ma marks the onset of gradual the proto–Nankai Trough or the Shikoku Basin (Underwood, 2018). In addition, and irreversible smectite depletion in the suspended sediments entering Shi- seismic and logging data (Boston et al., 2016) show evidence for large-scale koku Basin, particularly in the vicinity of the NanTroSEIZE transect (Fig. 7). reactivation of thrust faults, steep rotation of older folds and faults, and mul- That episode in the margin’s geologic history represents the turning point tiple fracture populations within the inner prism. We further suggest that the when widespread exposures of weathered Miocene ignimbrites across the younger structural fabric is a product of Philippine Sea plate subduction, but it Outer Zone began to shrink at the expense of uplifted granitic intrusions and is superimposed on an older fabric imparted by Pacific plate subduction. metasedimentary rocks. Several subordinate sources, including a small com- If plate-tectonic reconstructions for the Philippine Sea region (Mahony ponent of eolian-dust input (e.g., Mahoney, 2005; Xu et al., 2015), certainly et al., 2011; Hall, 2012; Wu et al., 2016) are correct in paleogeographic detail, complicated the picture. Sporadic delivery of illitic clays by turbidity currents then the subduction zone at 15 Ma offshore central Honshu (Kii Peninsula) was moving through the East China Sea (Kyushu Fan) helps explain the scatter in one of convergence between the Pacific plate and the Eurasia plate (Fig. 15A). %-smectite values (Fig. 7). Similarly, tuffaceous gravity flows (Zenisu Fans) It follows that deposition of inner-prism strata during the Miocene must have likely resulted in punctuated increases in the delivery of detrital smectite occurred on the Pacific plate rather than the Philippine Sea plate (Underwood, from the Izu-Honshu collision zone (Kutterolf et al., 2014) during the late Mio- 2018). A rigorous regional-scale test of this hypothesis would require strategic cene (Fig. 7). sampling of Pacific plate deposits to the north of Sagami Trough. The closest As time progressed into the late Pliocene and early Pleistocene, strength- drilling transect across the Japan Trench (DSDP Legs 56 and 57) is located ening of the Kuroshio Current would logically have dampened transport of near 40°N latitude (Mann and Muller, 1980). Sediments in the forearc of the smectite from weathered volcanic islands of the Izu-Bonin chain (Fig. 13C). Japan Trench contain higher percentages of biogenic silica; so proportions of Stronger Kuroshio circulation reinforced the evolving Outer Zone illite-chlorite total clay within the bulk sediment are much smaller than what we find in the assemblage by adding more illite-rich mud from Taiwan and East China Sea Shikoku Basin. Accepting those caveats, however, we note that proportions sources. To support that idea, we note that IODP Site U1437, at the northeast of smectite within the clay-size fraction are roughly equal to the proportions edge of the Shikoku Basin (Fig. 2), yields evidence for unusually high rates of illite (~40:40), and the ratio does not change significantly over time (Mann of sedimentation for tuffaceous mud and/or mudstone deposits, consistent and Muller, 1980). Compositional uniformity over time is consistent with the with long-term delivery of mud by a strong northeast-directed current (Tamura relatively slow rates of erosion in the Tohoku catchments of northern Hon- et al., 2015). Our NanTroSEIZE results, however, indicate that the oceano- shu (Korup et al., 2014). More to the point, smectite-to-illite ratios in the Japan graphic effect remained subsidiary to the effects of the evolving source areas Trench (Mann and Muller, 1980) are roughly equal to those in the deep inner on the Japanese Islands. accretionary prism of Nankai Trough (Fig. 10). We therefore contend that a shared detrital provenance is more than just a remote possibility. By ca. 10 Ma, sinistral slip along the transcurrent margin of southwest Ja- Provenance of the Inner Accretionary Prism pan appears to have brought the triple junction between Pacific, Eurasia, and Philippine Sea plates closer to the geographic corridor of the NanTroSEIZE Several uncertainties surround geologic interpretations of the inner Nan- transect (Fig. 15B). The inferred paleolocation of Site C0002 deposition, how- kai prism (e.g., Tsuji et al., 2015). The seaward extent of Miocene magmatic ever, was still to the northeast of the Izu-Bonin volcanic arc; so accretion of bodies, for example, remains poorly constrained (Kimura et al., 2014), as does Shikoku Basin sediments seems unlikely at that time. Instead, late Miocene the distribution of coeval volcaniclastic deposits and/or cemented sandstone sedimentary rocks currently within the inner prism probably were deposited (Tsuji et al., 2015). The paleogeography and depositional environment for near the triple junction, in a setting that drew most of its suspended sediment strata at Sites C0001 and C0002 remain cryptic. Shipboard scientists (Ashi from watersheds in northern to central Honshu rather than from Shikoku and et al., 2009; Strasser et al., 2014b; Tobin et al., 2015b) drew vague references Kyushu (i.e., similar to present-day Suruga Trough). That pattern of sediment to the trench or the Shikoku Basin as sites of deposition for inner-prism strata. routing seems to have held until 6 Ma, when near-orthogonal subduction As scrutinized by Underwood (2018), however, those links to modern or Qua- reinitiated along the proto–Nankai Trough (Fig. 15B).

15 Ma Basin Philippine We Kyushu-Palau Ridg Kyushu-Palau 200 km ough st

N Ryukyu st Kina

200 km

Basi n We n Kyushu-Pal

c Trench Kyushu e seamount

st Philippine Sea of Japan chai seamount Kinan Kyushu Kyushu Shikoku

Basin au Ridg au 132° Kyushu-Palau Ridge Kyushu-Palau

Shikok e

Sea of Japa n

h Sea of Japan n ai Sea plat e Philippine Shikok u Shikoku

Basi n e Shikoku

Nankai u

plat e Sea Philippin n plate Sea Philippine 136°

Honshu s Kinan

Honshu z

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Shikok u Troug

eamount chai eamount

Basi n Izu-Bonin

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c Ar Arc

h n plate Pacifi c 140°

plate Pacifi c

n Izu-Bo

in in plat e Pacifi c c Ar 200 km C B A sect is highlighted by the red pentagons. mogenic Zone Experiment (NanTroSEIZE) tran Approximate position of the Nankai Trough Seis relative to present - day geographic coordinates. For simplicity, the Japanese Islands are held fixed plate renewed ca. (see also Wu 6 Ma et al., 2016). trench - normal subduction of the Philippine Sea and to 6 Ma, oblique subduction from 10 Ma margin experienced sinistral slip or highly (2011)et al. and Hall (2012). The proto– (B) 10 Ma, and (C) 5 Ma, adapted from Mahony tions for the Philippine Sea region at (A) 15 Ma, Figure 15. Simplified plate - boundary reconstruc Nankai

GEOSPHERE | Volume 14 | Number 5 Underwood and Guo | Nankai-Shikoku clay-mineral assemblages 2031 Research Paper

The unconformity between the accretionary prism and slope sediments at anism, with a primary source in the Izu-Honshu collision zone (i.e., funneling Sites C0001 and C0002 (Fig. 3) reveals a long hiatus that could be a mechanical through Suruga Trough and Tenryu Canyon). response to cessation and renewal of subduction (e.g., Tsuji et al., 2015). Alter- Most previous workers have argued likewise that sand-rich deposits in natively, we suggest that erosion or nondeposition may be symptoms of more the Quaternary trench wedge originated mostly from the Izu-Honshu collision complicated interactions between the upper plate and the migrating triple zone (De Rosa et al., 1986; Taira and Niitsuma, 1986; Marsaglia et al., 1992; junction or to a shift from Pacific plate convergence to renewed Shikoku Basin Underwood et al., 1993a; Fergusson, 2003; Underwood and Fergusson, 2005; subduction. As summarized previously, mud composition within the Shikoku Usman et al., 2014). Clift et al. (2013) provided a contrary and provocative in- Basin changed gradually due to two major factors: (1) uplift and denudation terpretation in which dispersal and along-strike (axial) transport of sediment of deeper parent rocks across the Outer Zone of Japan, which increased pro- from the collision zone has been subdued throughout the margin’s history, portions of illite and chlorite, and (2) intensification of the northeast-directed with little or no effect on trench-floor sedimentation offshore Shikoku and the Kuroshio Current, which brought more illitic mud from Taiwan and the East Kii Peninsula (Fig. 13); they further claimed that no sediment from the collision China Sea (Underwood and Pickering, 2018). As time progressed from the late zone can be found offshore Kyushu, a distance of 350–500 km along the strike Miocene into the early Pliocene (Fig. 13C), we suggest that sediment composi- of the margin. The rationale behind their assertion lies mostly in an apparent tion on the southwest side of the triple junction became increasingly depleted mismatch between the U-Pb ages of detrital zircons from the trench-wedge in detrital smectite and, thus, increasingly similar to trench and abyssal-plain deposits versus zircons from the present-day watershed of the Fuji River (Clift deposits on the northeast side. Watersheds on Honshu to the northeast were et al., 2013), which drains into Suruga Trough (Fig. 2). Those assertions may evidently more stable over the same time period than watersheds within and be correct for some of the older (Miocene to early Pleistocene) turbidites that southwest of the triple junction; thus Japan Trench clay-mineral assemblages were accreted at ODP Sites 1178 and 1176 (see also, Fergusson, 2003; Under- remained relatively static through Miocene–Pliocene time (e.g., Mann and wood and Fergusson, 2005), but we refute the interpretation for the Quater- Muller, 1980). We contend that the combined effect of both influences was to nary trench wedge for a variety of reasons. First, the mismatch in U-Pb dates shrink the provenance contrasts at Site C0001 (i.e., between younger inner- versus the modern Fuji River drainage (Clift et al., 2013) can be explained by prism strata and coeval deposits in the Shikoku Basin) relative to similar con- the recent construction of the active Mount Fuji (Fig. 13C), a famous stratovol- trasts between older accreted strata at Site C0002 versus Shikoku Basin. cano that must have changed both topography and ground-surface geology Another factor to consider in the paleogeography is the topographic re- dramatically as it grew. The modern Fuji watershed probably did not exist sponse to Izu-Honshu collision after 6 Ma. Sediment gravity flows eroded when the turbidites sampled at Sites C0006 and C0007 were deposited. large submarine canyons on either side of the collision zone (Sagami Trough, Beyond cogent arguments based on sand petrography (e.g., Marsaglia Suruga Trough, and Tenryu Canyon), but those incisions required time to reach et al., 1992; Fergusson, 2003; Usman et al., 2014), we reiterate the following their impressive present-day dimensions (e.g., Kawamura et al., 2009). With- lines of evidence for a dominant source of sediment in the collision zone (Ja- out large, through-going submarine canyons, and with less-frequent sediment pan Alps) during the Quaternary: (1) erosion rates and sediment yield from the gravity flows, muddy suspended sediment probably dominated in both the Japan Alps are the highest among catchments draining Japan’s eastern sea- Japan and Nankai trenches during the early stages of renewed Philippine Sea board, higher than all of the Nankai inner forearc segments combined (Korup subduction (Figs. 13C and 15C). The sluggish response of bathymetry and et al., 2014); (2) the inferred Izu-Honshu source area (e.g., Tanzawa Mountains) coarse-sediment routing to reorganized plate interactions helps explain why has experienced a 7 m.y. history of rapid uplift, characterized by high rates of the youngest strata recovered from the inner accretionary prism (3.8 Ma to denudation (up to 2 mm/yr) and resulting in exhumation of tonalities from 5.3 Ma) are unusually fine grained when compared to the sand and gravel midcrustal depths (Yamada and Tagami, 2008; Tani et al., 2010; Hashima et al., deposits of the Quaternary trench-wedge facies (Screaton et al., 2009b; Under- 2016); (3) bedrock uplift rates in the nearby Kiso Range of central Japan equal 3 wood and Moore, 2012). mm/yr to 6 mm/yr, with denudation rates of 1 mm/yr to 4 mm/yr (Sueoka et al., 2012); (4) an exceptionally large submarine canyon (Suruga Trough) has been deeply incised from the mouth of the Fuji River into the northeastern end of Provenance of the Frontal Accretionary Prism Nankai Trough (Fig. 2) (Nakamura et al., 1987); (5) three subaqueous slope-type fan deltas have prograded from the head of Suruga Trough, and all are aligned The clay-mineral assemblages at Sites C0006 and C0007 (Fig. 8) are indic- with the mouths of high-gradient rivers draining the Japan Alps (Soh et al., ative of a mixed provenance that included roughly equal proportions of sedi 1995); (6) typhoon-induced sediment-transport events are frequent and flush mentary, metasedimentary, and igneous rocks, much the same as interbeds sediment from the head of Suruga Trough, even during the Holocene highstand of turbidite sand (Milliken et al., 2012). All of those rock types are widespread of sea level (Yoshikawa and Nemoto, 2010); (7) the gradient of the Nankai trench within the Izu-Honshu collision zone (Fig. 13A). We assert that axial (mar- floor dips to the southwest, which favors transport in that direction after sedi- gin-parallel) turbidity currents have provided the dominant transport mech- ment gravity flows exit Suruga Trough; (8) high-resolution bathymetric records

highlight a prominent axial channel on the Nankai trench floor; the channel strike movement of illitic suspended sediment in the surface waters toward begins at a point source (mouth of the Fuji River) and extends to a position the northeast from Kyushu and more distal sources (Fig. 13C); some backflow offshore the Kii Peninsula (Shimamura, 1989); (9) a second large submarine toward the southwest by comparatively sluggish bottom currents is also likely. canyon (Tenryu Canyon) was incised immediately seaward of the Tenryu The net effect of ocean circulation, however, was to homogenize the composi- River mouth (Fig. 2), and erosion there is deep enough to expose metamor- tional fingerprints from multiple point sources of mud. phosed Miocene accretionary prism along the canyon walls (Kawamura et al., Results from the lower trench slope are a bit more complicated to interpret. 2009); (10) erosion in Tenryu Canyon was rejuvenated following the collision Mass-transport deposits at Site C0018 (Fig. 3) are indistinguishable in clay com- of proto–Zenisu Ridge with the accretionary prism (Soh and Tokuyama, 2002); position from intact intervals of hemipelagic sediment (Fig. 12). That similarity (11) Tenryu Fan prograded onto the trench floor at the mouth of Tenryu Canyon indicates shallow remobilization of the slope apron rather than having sub (Fig. 2), causing southward deflection of the axial channel (Soh et al., 1991); marine slides cut down into the underlying accretionary prism. On the other (12) deeper seismic-reflection character of the trench wedge (i.e., high-ampli- hand, younger fine-sand deposits (<1 Ma) in the slope basin are significantly tude reflections) shows temporal continuity of axial-channel migration, extend- more enriched in volcaniclastic materials than their older counterparts, similar ing back at least into the late Pleistocene (Shimamura, 1989); (13) paleocurrent to the Quaternary trench-wedge facies at Sites C0006 and C0007; that shift evidence from DSDP Site 808 (Muroto transect; Fig. 2) is consistent with reflec- has been explained by tapping into longitudinal (axial) flows that emanated tion or deflection of turbidity currents off the seaward slope (Pickering et al., from the Izu-Honshu collision zone (Usman et al., 2014). The re-routing inter- 1992); (14) magnetic fabric in the distal trench turbidites of Nankai Trough at pretation raises additional questions because of the slope basin’s elevation DSDP Sites 582 and 583 (Ashizuri transect; Fig. 2) is oriented parallel to the above the trench floor (Fig. 3). Re-routing among nearby submarine canyons axis of the trench (Taira and Niitsuma, 1986). These observations collectively and their associated watersheds (i.e., Tenryu River and Tenryu Canyon ver- point to a long-lasting detrital source of sediment in the Izu-Honshu collision sus Fuji River and Suruga Trough), in response to seamount subduction and zone, combined with sustained, long-distance axial transport of sand, gravel, forearc uplift, is one viable scenario (Usman et al., 2014). Another possible con- and suspended sediment down the Nankai trench floor by turbidity currents. tribution, however, is expansion of the frequency and dimensions of turbidity currents moving into the trench through Suruga Trough (Fig. 2) as uplift and denudation in the Izu-Honshu collision zone accelerated (Fig. 13C). We suspect Provenance of Forearc-Basin and Trench-Slope Sediments that many of those axial flows were thick enough (i.e., several hundred meters) to lap significantly onto the lower slope of the trench. The chances of upslope Interpretations of sediment dispersal for the Kumano Basin and coeval deposition obviously increase as one moves downslope from Site C0018 to- slope-apron sediments are more straightforward (compared to Shikoku Ba- ward Sites C0006 and C0007, which currently rest <350 m above the trench sin) because of the fixed position of depositional sites on the upper plate. In floor (Fig. 3). In any case, when the routing of sand to Site C0018 changed, the addition, most sediment gravity flows across a forearc maintain predictable assemblages of clay minerals did not. We attribute that decoupling between downslope trajectories toward the trench. The clay-mineral assemblages are, sand and clay budgets to the homogenization of fine-grained suspended sedi consequently, more homogeneous both within and among individual sites. Al- ment in the nepheloid layer, in concert with multiple entry mechanisms and though there is good evidence for sensitivity of zooplankton to climate change initial transport directions (i.e., turbidity currents, vertical settling from surface at the mid-Pleistocene transition (Matsuzaki et al., 2015), we see no such ad- currents, plus resuspension by thermohaline bottom currents). justments in the clay mineral assemblages. We attribute that behavior to the dominance of geologic and tectonic forcing relative to oceanographic forcing. Older starved-basin deposits at Site C0002 (3.8 Ma to 1.6 Ma) (Ashi et al., 2009) CONCLUSIONS are modestly enriched in smectite relative to overlying forearc-basin turbidites, consistent with the long-term temporal trend in the Shikoku Basin (Fig. 7). Our synthesis of XRD results from the NanTroSEIZE transect vividly We reiterate that the Outer Zone of Japan served as the main source for demonstrates how histories of sedimentation and accretion along subduc- transverse routing across the forearc over the past 1.5 m.y. to 2 m.y. (Fig. 13C). tion margins can be punctuated by unexpected complications. The first-order That view is consistent with multiple provenance indicators (e.g., heavy-min- temporal trend for the regional system is relatively straightforward: detrital eral assemblages and pyroxene geochemistry) from interbedded sand tur- clay-mineral assemblages shifted gradually from a smectite-rich assemblage bidites in the Kumano Basin (Usman et al., 2014; Buchs et al., 2015). Just as during the Miocene to more illite- and chlorite-rich assemblages during the with our interpretation for Sites C0011 and C0012, the inverted trend for I/S Pliocene and Quaternary. Most of the detrital smectite probably originated as expandability at Site C0001 (i.e., less expandable I/S moving upsection) is weathering products of anomalous near-trench silicic volcanism that covered explained best by deeper erosion over time into higher levels of parent-rock much of the Outer Zone of central Japan, whereas sources of detrital illite and diagenesis. In addition, the swift Kuroshio Current probably drove some along- chlorite included both low-grade metamorphic rocks and sedimentary rocks

with moderate levels of diagenesis (e.g., Shimanto Belt). Gradual uplift and un- accretionary prism (Sites C0006 and C0007) have been dominated by roofing of the anomalous Miocene plutons and the surrounding country rocks axial transport from sources in the collision zone, including the Mount resulted in gradual increases of detrital illite and chlorite. We also recognize Fuji stratovolcano. several interesting complications to the first-order trends in clay mineralogy, (7) Quaternary muds in the Kumano Basin (Site C0002) and the slope- listed below in order of occurrence. apron/slope-basin facies above the accretionary prism (Sites C0001, C0004, C0006, C0007, C0008, C0018, and C0021) show the greatest (1) Sandy turbidity currents evidently entered the Shikoku Basin from the compositional uniformity among all of the depositional domains. We East China Sea during the middle Miocene (Kyushu Fan) and carried attribute that homogeneity to balanced competition among multiple higher concentrations of illite in their suspended loads. That mixing entry mechanisms and their associated transport directions: (1) trans- among sources resulted in greater scatter of clay-mineral abundances, verse (trench-directed) re-sedimentation by unconfined gravity flows; but it is important to note that we have not sampled and analyzed the (2) onlap of axial turbidity currents (southwest-directed) onto the lower- turbidite clays directly. most trench slope; (3) steady movement of surface-water suspensions (2) Routing of coarser sediment from the East China Sea was terminated by the northeast-directed Kuroshio Current; and (4) retention or resus- ca. 7 Ma because of rifting in the Okinawa Trough and growth of topog pension of mud in the bottom nepheloid layer by thermohaline bottom raphy along the adjacent Ryukyu arc-trench system. The sand influx to currents. Shikoku Basin associated with the younger Zenisu Fans had a smaller effect on clay-mineral budgets because those flows tapped into vol The unusual complexity of Nankai-Shikoku tectonic history probably canic and sedimentary sources (smectite-rich) near the Izu-Honshu means that the depositional system should not be held up as the “type local- collision zone. That provenance domain was broadly similar to parent ity” for subduction zones, worldwide. When compared to such noteworthy ex- rocks across the Outer Zone during that same time period. amples as the Japan Trench (e.g., Kimura et al., 2012), Costa Rica (e.g., Spinelli (3) Comparisons between early Pliocene and late Miocene strata from the and Underwood, 2004), and Sumatra (e.g., Hüpers et al., 2017; McNeill et al., inner accretionary prism (Sites C0001 and C0002) and coeval (5 Ma to 2017), the facies architecture of Nankai subduction inputs (including the trench 11 Ma) mudstones from the Shikoku Basin (Sites C0011 and C0012) re- wedge) is much more diverse; the lithofacies types and sediment composition veal stark contrasts in clay mineralogy, particularly in their concentra- have changed much more in three dimensions over time. tions of smectite. We reiterate the suggestion (Underwood, 2018) that The significance of our exhaustive case study also extends beyond sedi the older accreted sediments (more illite- and chlorite-rich) were ini- mentology. For example, with regard to the Nankai subduction interface tially deposited on the Pacific plate rather than the Philippine Sea plate, (megathrust), we recognize predictable temporal changes in the composition to the north of a migrating trench-trench-transform triple junction. of both hanging-wall and footwall successions that carry important ramifica- Their mineral assemblages are similar to those in the Japan Trench. tions for understanding deformation and fault-slip mechanisms. We expect (4) Renewal of Philippine Sea plate subduction ca. 6 Ma coincided with the volumetric fluid production from clay dehydration (Saffer et al., 2008) to be sig- re-establishment of more conventional source-to-sink linkages from nificantly greater in the smectite-rich footwall of the megathrust (i.e., subduct- the Outer Zone provenance to sites of Philippine Sea deposition to the ing Shikoku Basin sediments of Miocene age) as compared to the illitic base tectonic domain of frontal accretion. That frontal part of the Nankai ac- of the hanging wall. Those contrasts in fluid production probably contribute to cretionary prism (Sites C0006 and C0007) shares detrital provenance heterogeneous buildups of fluid overpressure, thereby modulating downdip traits with coeval hemipelagic deposits in the Shikoku Basin. transitions among stable sliding, stick-slip, and slow-slip events (Moore and (5) Intensification of the northeast-directed Kuroshio surface current ca. Saffer, 2001; Saffer and Wallace, 2015). The same may be true for deeper levels 3.5 Ma probably brought more illite toward the trench and the Shikoku of the megasplay. Basin from distal sources (Taiwan, Okinawa Trough). The XRD signal for that oceanographic shift is weak, however, because the current merely reinforced the compositional changes that were caused by progressive ACKNOWLEDGMENTS weathering of uplifted Outer Zone parent rocks. Dampened transport of This research used samples provided by the Integrated Ocean Drilling Program (IODP). We thank smectite from the Izu-Bonin volcanic islands is also likely. the drilling crew, laboratory technicians, and fellow scientists aboard D/V Chikyu for their as- sistance with sampling during IODP Expeditions 315, 316, 322, 333, 338, and 348. Funding was (6) Accelerated collision between the Izu-Bonin Arc and the Honshu Arc granted by the Consortium for Ocean Leadership, U.S. Science Support Program (grants T315B58, ca. 1.5 Ma resulted in high rates of uplift and erosion, rapid incision T322A58, T333A58, T338A58, and T348A58 to Underwood, and grant T315C58 to support Guo) of large submarine canyons (Suruga Trough, Tenryu Canyon), and fre- and the National Science Foundation (grant OCE-07518190). U. Shriniwar, H. Banno, D.M. Hillix, M. Rader, H. Wu, K. Ekinci, C. Song, and N. Walla assisted with sample preparation and data reduc- quent funneling of large, energetic turbidity currents into the Nankai tion at the University of Missouri. Two anonymous reviewers provided thoughtful critiques that Trough. Consequently, Quaternary trench-wedge deposits in the frontal improved the quality of the manuscript.

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