Subduction of Trench-Fill Sediments Beneath an Accretionary Wedge: Insights from Sandbox Analogue Experiments GEOSPHERE, V

Research Paper THEMED ISSUE: Subduction Top to Bottom 2

GEOSPHERE Subduction of trench-fill sediments beneath an accretionary wedge: Insights from sandbox analogue experiments GEOSPHERE, v. 16, no. 4 Atsushi Noda1, Hiroaki Koge2,*, Yasuhiro Yamada3,4,5, Ayumu Miyakawa1, and Juichiro Ashi2 1Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan https://doi.org/10.1130/GES02212.1 2Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan 3Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3173-25 Showa-machi Kanazawa, Yokohama 236-0001, Japan 11 figures; 1 table; 1 set of external supplemental 4Graduate School of Integrated Arts and Sciences, Kochi University, 2-5-1 Akebono-cho, Kochi 780-8520, Japan files (hosted by authors on Figshare) 5Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK

CORRESPONDENCE: [email protected] ABSTRACT southwestern Japan contains HP-LT psammitic and even conglomeratic schists, CITATION: Noda, A., Koge, H., Yamada, Y., Miyakawa, and the depositional ages and geochemical characteristics of the protolith are A., and Ashi, J., 2020, Subduction of trench-fill sedi- Sandy trench-fill sediments at accretionary margins are commonly scraped almost identical to those of sandstone from the low-grade Shimanto accretion- ments beneath an accretionary wedge: Insights from off at the frontal wedge and rarely subducted to the depth of high-pressure ary complex (Kiminami et al., 1999; Shibata et al., 2008; Aoki et al., 2012) and sandbox analogue experiments: Geosphere, v. 16, no. 4, p. 953–968, https://doi.org/10.1130 /GES02212.1 . (HP) metamorphism. However, some ancient exhumed accretionary complexes submarine fan turbidites deposited in the associated forearc basin (Fig. 1; Noda are associated with high-pressure–low-temperature (HP-LT) metamorphic and Sato, 2018). These observations indicate that terrigenous trench-fill sedi- Science Editor: Shanaka de Silva rocks, such as psammitic schists, which are derived from sandy trench-fill ments were accreted in a shallow subduction zone and were also subducted Guest Associate Editor: Philippe Agard sediments. This study used sandbox analogue experiments to investigate the to >20 km depth. Other examples of such subduction-accretion–related HP-LT role of seafloor topography in the transport of trench-fill sediments to depth metamorphic rocks can be seen in the Franciscan complex in California (e.g., Received 11 November 2019 during subduction. We conducted two different types of experiments, with Hsü, 1974; Ernst, 2011; Jacobson et al., 2011; Dumitru et al., 2015; Raymond, Revision received 21 April 2020 Accepted 13 May 2020 or without a rigid topographic high (representing a seamount). We used an 2018), the Chugach terrane in Alaska (Plafker et al., 1994), the Central Pontides undeformable backstop that was unfixed to the side wall of the apparatus to in Turkey (Okay et al., 2006), and the Coastal Cordillera in Chile (Glodny et al., Published online 17 June 2020 allow a seamount to be subducted beneath the overriding plate. In experi- 2005; Willner et al., 2004; Angiboust et al., 2018). ments without a seamount, progressive thickening of the accretionary wedge At typical sedimentary accretion zones, such as those in Cascadia (Gulick pushed the backstop down, leading to a stepping down of the décollement, et al., 1998; Booth-Rea et al., 2008; Calvert et al., 2011), Alaska (Moore et al., narrowing of the subduction channel, and underplating of the wedge with 1991; Ye et al., 1997), Java (Kopp et al., 2009), southern Chile (Glodny et al., 2005; subducting sediment. In contrast, in experiments with a topographic high, the Melnick et al., 2006), Sumatra (Singh et al., 2008; Huot and Singh, 2018), and subduction of the topographic high raised the backstop, leading to a stepping Japan (Park et al., 2002b; Kimura et al., 2010), terrigenous trench-fill sediments up of the décollement and widening of the subduction channel. These results are generally scraped off at the frontal wedge, whereas hemipelagic-to- pelagic suggest that the subduction of stiff topographic relief beneath an inflexible sediments underplate the base of the accretionary wedge (e.g., Scholl, 2020). overriding plate might enable trench-fill sediments to be deeply subducted This may be because the increased structural thickness of the wedge and pro- and to become the protoliths of HP-LT metamorphic rocks. gressive dewatering of subducting sediment caused the upper boundary of the subduction channel to step down (e.g., Sample and Moore, 1987; Vannucchi et al., 2012). This suggests that the growth of the accretionary wedge might ■■ INTRODUCTION inhibit the subduction of terrigenous sediment beyond the wedge through the subduction channel. However, occurrences of HP-LT metasandstone at some High-pressure–low-temperature (HP-LT) metamorphic rocks derived from accretionary margins demonstrate that terrigenous sediment can be subducted terrigenous sedimentary rocks are known to occur at subduction margins beneath the wedge. One hypothesis is that a stiff topographic high enables (e.g., Agard et al., 2009; Guillot et al., 2009). Such metamorphic rocks are trench-fill sediments to be subducted under the wedge (Fig. 2). In fact, sub- exposed alongside low-grade accretionary rocks and forearc basin strata that ducting seamounts followed by subducting material can be observed beneath include coarse-grained sandy deposits with the same depositional ages as the the wedge along accretionary margins in southwestern Japan (Moore et al., metamorphic rocks. For example, the Sanbagawa metamorphic complex in 2014), Alaska (Li et al., 2018), Barbados (Moore et al., 1995), and Hikurangi (Barker et al., 2009; Bell et al., 2010).

This paper is published under the terms of the *Current address: Geological Survey of Japan, National Institute of Advanced Industrial Science The subduction of terrigenous material associated with the rough topogra- CC‑BY-NC license. and Technology (AIST), Central 7, Higashi 1-1-1, Tsukuba, Ibaraki 305-8567, Japan. phy of a subducting oceanic plate has been proposed to explain tectonic erosion

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X C A N. American X B 300 km (Okhotsk) 34.5°N Plate Cretaceous igneous and 40˚N Eurasia low-P/T metamorphic complex Plate (Ryoke pultono- metamorphic complex) B Pacific Cretaceous forearc basin Plate deposits (Izumi Group) 30˚N Philippine Sea 81 Cretaceous high-P/T MC Plate 78 78 79 (Sanbagawa MC) 130˚E 140˚E 81 75 34.0°N 74 71 82 Pre-Cretaceous AC & Paleogene covering sediments (Chichibu AC) Accretionary complex 88

114 Cretaceous AC Cretaceous 106 (Northern Shimanto AC) Sedimentary rocks 103 82 74 Cretaceous AC (sandstone and mudstone) 73 73 61 Paleogene AC Igneous (granite and rhyolite) 64 63 (Southern Shimanto) and low P/T metamorphic rocks 65

Accretionary complex 68 Moho 61 High P/T metamorphic rocks 33.5°N (psammitic and conglomeratic schist)

Paleogene AC High P/T metamorphic rocks Oceanic crust (pelitic schist, siliceous schist, mafic schist)

Pre-Cretaceous

Accretionary complex V.E. = 1 0 20 km Miocene–present AC 0 20 km Y Sea level 133.5°E 134.0°E 134.5°E Y Figure 1. (A) Index map for the location of eastern Shikoku. (B) Generalized geological map of eastern Shikoku, southwestern Japan, reproduced from the Seamless Digital Geological Map of Japan (Geological Survey of Japan, AIST, 2015). Black dots are labeled with detrital zircon U-Pb ages (Ma) of felsic tuff beds in the Izumi Group, composed mainly of sandy turbidites and mudstone (Noda et al., 2017, 2020a), the psammitic schist of the Sanbagawa metamorphic complex (Aoki et al., 2007; Nagata et al., 2019; Otoh et al., 2010), and sandy turbidites in the northern Shimanto accretionary complex (Hara et al., 2017; Hara and Hara, 2019; Shibata et al., 2008). MC and AC—metamorphic and accretionary complex, respectively; P/T—pressure/temperature. (C) Geological cross section along the line X-Y in B, modified from Ito et al. (2009). V.E.—vertical exaggeration.

of the wedge (e.g., von Huene and Culotta, 1989; Lallemand et al., 1994; von However, the influence of a subducting seamount beneath an accretionary Huene et al., 2004). Sandbox analogue experiments (Lallemand et al., 1992; wedge on subduction and accretion fluxes is not well understood. In particular, Dominguez et al., 2000) and numerical simulations (Ruh, 2016; Morgan and the role of topographic highs and backstops in modifying the décollement Bangs, 2017) have shown the potential for sediment transport below the frontal level and in maintaining or rejuvenating the subduction channel as a conduit wedge behind a subducting topographic high. In addition, recent seismic pro- for sediment subduction needs to be explored. The purpose of this study was files across the accretionary margins of the Nankai Trough (Bangs et al., 2006) (1) to investigate how the topographic roughness of the subducting plate and the Hikurangi Trench (Bell et al., 2010) indicate that subducting seamounts interface influences material fluxes, including the accretion of sediment to the or ridges and the surrounding sediment are accommodated by a step up in wedge and the subduction of sediment along the subduction channel, and the décollement, and the surrounding sediment is being transported to depth. (2) to propose a model that explains how terrigenous trench-fill sediments

can be transported to depth. We performed two types of sandbox analogue 2 Forearc basin fill A experiment, one with and one without a topographic high. Although there are several experiments focusing on seamount subduction 4 Older accretionary prism (Lallemand et al., 1992; Dominguez et al., 1998, 2000) and material transfer Décollement beneath the wedge (Kukowski et al., 1994; Gutscher et al., 1998; Lohrmann et al., 6 2006; Albert et al., 2018), experiments with a combination of these two factors Subducting sediment Topographic high have not yet been reported to date. In order to reproduce the subduction of a rigid Depth (km) 8 topographic high beneath an accretionary wedge and its associated underplat- Subduction channel ing of sediment, we used an unfixed backstop to the side wall of the apparatus, Top oceanic crust 10 which is another novelty of this study. We also inserted two weak layers within the 5 km sand, to reproduce the situation where the subducting sediment includes several Forearc basin fill potential slip surfaces. Multiple weak layers are considered to affect the evolu- B tion of tectonic underplating and related antiformal stacking (cf. Ruh et al., 2012). 7

9 ■■ SUBDUCTION CHANNEL Depth (km) Subduction channel 11 Here, for the sake of clarity of the terminology, we briefly review the term Topographic high 5 km “subduction channel.” A subduction channel model (Shreve and Cloos, 1986; 4 C Cloos and Shreve, 1988a, 1988b) was originally introduced from hydrodynamic theory of lubrication in the slip zone between overriding (accretionary wedge 6 or continental crust) and subducting (oceanic) plates (Jischke, 1975; Sorokhtin 5 km Depth (km) Subduction channel and Lobkovskiy, 1976). Flow of sediments in this zone is formed by the super- Topographic high Oceanic crust Subducting sediment position of a pure shear flow arising as a result of the relative movement of the (Subduction channel) surfaces bounding the channel on the flow (Sorokhtin and Lobkovskiy, 1976). Accretionary prism Forearc and slope basins V.E. = 2.5 Because the shear flow is governed by the pressure in this model, material Figure 2. Representative cross sections of accretionary margins with topographic highs. within the subduction channel is modeled by a viscous rheology with constant (A) Nankai Trough (Moore et al., 2014). (B) Southwestern Alaskan margin (Li et al., density, uniform viscosity, and with a permeability depending on porosity 2018). (C) Northern Barbados margin (Moore et al., 1995). All drawings come from (Shreve and Cloos, 1986; Cloos and Shreve, 1988a). Sediment subduction seismic profile interpretations. Vertical exaggeration (V.E.) is 2.5 for all cross sections. occurs through the channel when the downward-directed shearing overcomes the buoyancy and the opposite-directed pressure gradient. bottom, leading to upward migration of the roof décollement (cf. von Huene Cloos and Shreve (1988a) introduced the concept of “inlet,” which means et al., 2004). On the other hand, dewatering from the upper boundary of the the opening between the deformation front and the toe of the overriding plate subduction channel through fracture networks related to seamount subduction or the accretionary wedge, and its capacity controls the subduction channel (e.g., Dominguez et al., 2000; Ruh et al., 2016) can reduce the fluid pressure and thickness. When the inlet capacity is lower than the incoming sediment on increase the strength of the sediment below the upper boundary, which results the subducting oceanic plate, the excess will accrete to the overriding plate. in underplating of the dewatered channel material (Shreve and Cloos, 1986; This idea of capacity and the subduction channel model were revised by von Cloos and Shreve, 1988a) and downward migration of the upper décollement. Huene et al. (2009) and Vannucchi et al. (2012) to explain tectonic erosion from The concept of the subduction channel has been used to explain exhuma- the bottom of the overriding plate or accretionary wedge based on field and tion of HP and ultrahigh-pressure (UHP) metamorphic rocks due to buoyancy seismic observations of localized shear zones with fault-like slip at both top and returning flow within the subduction channel filled with mechanically and bottom boundaries of the subduction channels (cf. Kitamura et al., 2005; weak sediments and sedimentary rocks (England and Holland, 1979; Gerya Sage et al., 2006; Vannucchi et al., 2008; Collot et al., 2011). In their models, et al., 2002; Jolivet et al., 2003; Agard et al., 2009; Guillot et al., 2009). Such the subduction channel can change its upper and lower boundaries with time rheological behavior within the channel would be expected for exhumation and space depending on material supply, fluid content, and heterogeneity of of HP metamorphic rocks from deep (>15 km) environments. deformation. For example, rheological effects from the progressive dewatering In this study, we use the term “subduction channel” to mean a zone bounded and consolidation of channel material can deactivate the basal décollement by the upper bounding fault (décollement) and the top of the oceanic plate. In and weaken the upper part of the subduction channel with respect to the other words, it represents the capacity of the subduction gap through which

sediments are transported beneath the accretionary wedge and the backstop rig with internal dimensions of 100 cm × 30 cm × 20 cm was used (Fig. 3). (cf. Cloos and Shreve, 1988a). Both upper and lower boundaries can shift their A steel plate was positioned at one end as a fixed wall with a small open positions with time and space depending on the topography of the overriding window (1.6 cm in height) at the bottom. A rigid wooden protowedge was and subducting plates and thicknesses and properties of the incoming sediment. placed next to the steel plate but was not fixed to it; it was just loosely lain Similar meanings of this term can be found in previous studies of analogue atop of the sand layer. The wedge was designed to have a higher mechanical experiments (Kukowski et al., 1994; Gutscher et al., 1998; Lohrmann et al., 2006). strength than the accretionary wedge. The reason we used a rigid wooden The subduction channel here does not presume any effects of buoyancy or backstop was to ensure stability during the experiments and for repeatabil- returning flow of the sediment within the channel. In this sense, our “subduc- ity. The mobility of the backstop helped to replicate the movable nature of tion channel” is comparable to the “plate interface” of Agard et al. (2018) and equivalent structures in natural geological systems, and to allow topographic different from some interpretative meanings of subduction channel such as relief to be subducted. The backstop had a surface slope that dipped at 30° the idea representing forced return flow in the channel (e.g., Gerya et al., 2002). and was covered by sandpaper. A plastic (Mylar®) sheet was placed over the rig’s base plate and fixed to a roll that pulled the sheet using a stepper motor (on the left side in Fig. 3). The sheet was pulled beneath the rigid backstop at ■■ METHODS a rate of 0.5 cm/min, thereby compressing the experimental material above. Two types of granular material were used for the experiments: Toyoura sand Experimental Apparatus and Materials and glass microbeads. Dry granular materials like these are widely used as analogue materials to simulate the nonlinear deformation of crustal rocks under A scaled two-dimensional (2-D) analogue model was established for this brittle condition, because they display an elastic/frictional plastic behavior with study so that the results could be compared with naturally occurring geological transient strain hardening prior to failure and subsequent strain softening structures (e.g., Graveleau et al., 2012). A glass-sided rectangular deformation until the onset of stable sliding at constant friction (Lohrmann et al., 2003).

100 cm A Sand input Unfixed Glass side wall

Area of sand supply

Steel wall Rigid backstop Palastic sheet Movable 30º Influx Outflux Subduction channel Rigid base Sand (4 mm) Sand (3 mm) B Sand input Unfixed Glass side wall Sand (5 mm)

Sand (3 mm) Seamount Area of sand supply Sand (5 mm) Glass beads (3 mm) Rigid backstop Steel wall Sand (4 mm) Plastic sheet Movable 30º Glass beads (3 mm)

Sand (4 mm) Subduction channel Rigid base Wooden board Rotation 18 cm

Figure 3. Experimental apparatus. The labels “Influx” and “Outflux” indicate the heights at which we calculated sediment influx and outflux, respectively.

Toyoura sand, a standard testing material commonly used by Japanese civil surfaces of accretionary wedges. A total amount of 910 g of sand was added engineers, is a spherical quartz-rich sand with a particle size of 0.14–0.26 mm over the course of experiment A and 1129 g during experiment B. The volumes −3 3 (D50 = 0.2 mm), a density of ~1600 kg m , an internal coefficient of friction, μ, of sand added during experiments A and B were 569 and 706 cm , respectively. of 0.59–0.68, and a cohesion, C, of 105–127 Pa (Yamada et al., 2006; Dotare In addition to investigating wedge morphology, we studied temporal vari- et al., 2016). The glass microbeads are spherical and 0.045–0.063 mm in diam- ations in sediment influx/outflux. The sediment influx and outflux2 (cm ) were eter, have a low internal coefficient of friction μ( = 0.47) and low cohesion (C = calculated using the thicknesses (cm) of the trench-fill sediments (influx) and 40 Pa), and are considered to be a suitable analogue for weaker layers (Yamada the subduction channel underneath the backstop (outflux), which were mul- et al., 2006, 2014). The theoretical minimum critical taper angle of the wedge tiplied by the rig width (30 cm) and divided by the length of shortening (cm). ranges in between 8.7° and 10.6° (cf. Dahlen, 1984). Input and output (cm3) are here defined to be the integrals of influx and out- Layers of sand and glass microbeads with a total thickness of 3.4 cm were flux, respectively, with respect to shortening length (cm). Time-lapse digital used in the experiments. The sand and glass were sprinkled into the rig from images were taken through the transparent side glass at 5 s intervals using a a height of ~30 cm above the rig floor (Fig. 3). Alternating layers of blue, red, personal computer–based controller. The images were later analyzed to calcu- and black sand were laid down to help visualize the cross-sectional geometry late sediment influx/outflux values and to study the cross-sectional geometry of the models, without influencing the mechanical homogeneity. of the wedges. The experiments did not account for the effects of isostatic Mechanically, weak layers were created by adding two thin layers of glass compensation and erosion, which would have contributed to the differences microbeads, each 3 mm thick. These glass-bead layers mimicked multiple between our models and natural examples (e.g., Schellart and Strak, 2016). décollement layers with a lower Mohr-Coulomb failure criterion than the Toy- oura sand of the wedge-forming material. The reason we used two weak layers was to reproduce an occurrence of underplating or duplex structures during Scaling subduction, which have been investigated by analogue (Konstantinovskaya and Malavieille, 2011; Pla et al., 2019) and numerical studies (Ruh et al., 2012). Models used in laboratory experiments should be properly scaled so that Such multiple décollements are commonly found within subducting sediments the results can be considered true analogues of geological processes (e.g., or at the top of the oceanic crust, including at the Nankai (Moore et al., 2001; Hubbert, 1937). It is assumed that brittle deformation will obey frictional Park et al., 2002b), Hikurangi (Ghisetti et al., 2016; Plaza-Faverola et al., 2016), Mohr-Coulomb–type laws. The basic scaling relationship between the physical and Barbados (Saffer, 2003) accretion zones. properties of a model and those in nature, which relates the stress, σ, density, ρ, gravity, g, and length, l (Hubbert, 1937; Schellart, 2000), is

σ l g ρ Experimental Setup g =×g g × g , (1) σm lm gm ρm Experiment A investigated the subduction of a smooth oceanic plate beneath where the subscripts m and g indicate model and geological values, respec- a static backstop (Fig. 3A). Experiment B investigated the subduction of topo- tively. The cohesion C can substitute for stress, σ (Schellart, 2000; Graveleau

graphic relief (e.g., a seamount), using a 5-cm-width and 1.6-cm-height wooden et al., 2012), and the experiments were performed under normal gravity (gm ⁄ gg block that was attached to the plastic sheet (Fig. 3B). The height of the relief was = 1); consequently, Equation 1 can be modified to give approximately half of the total thickness of the sediment, which was chosen to l C ρ avoid drastic deformation of the accretionary wedge. The surface of the topo- g = g × m . (2) l C ρ graphic relief was covered by a Teflon® sheet. The total amount of horizontal m m g shortening was 30 cm for experiment A and 35 cm for experiment B. For mean bulk density values of 2000–2500 kg m−3 and cohesion values After each 2 cm increment of shortening (every 4 min), we sprinkled dry of 5–20 MPa, which are typical of sedimentary rocks in accretionary wedges sand from at least 10 cm above the surface of the accretionary wedge to fill (Schumann et al., 2014), the length scale ratio ranges from ~3 × 104 to 1.5 × 105. the topographic lows that had developed. The amount of sand input at each Details of the scaling for the materials used in this study can be found in Noda supply was varied with repetitions in a cycle of 70, 9, 9, 70, 131, and 131 g et al. (2020b). in order. Total numbers of sediment supply were 13 and 17 times, meaning The 5 cm width and 1.6 cm height of the topographic relief used in exper- repetitions of 2 and 3 cycles for experiment A and experiment B, respectively. iment B can be scaled to 1.5–7.5 km and 0.5–2.4 km, respectively. The scaled When we supplied sand, we first filled the retroside of the wedge while it dimensions of the topographic relief are comparable to many seamounts on was underfilled, and then the rest of the sand was sifted in the wedge top. If the Pacific plate. However, the height-to-radius ratio of 0.64 in the model is sand still remained, the rest was spread at the front of the wedge. This sand higher than that of 0.21 for natural seamounts (Jordan et al., 1983; Smith, 1988). was used to replicate sedimentation in forearc/slope basins that form on the This high ratio was used to enhance the effects of topography. The total amount

0 cm 8 cm 16 cm T 24 cm Exp. A without seamount 5 T6 T5 T T 6 T7 5 T7 T8

2 cm 10 cm 18 cm T 26 cm 5 T6 T5 T6 T7 T5 T6 T7 T8

4 cm 12 cm 20 cm T5 28 cm T5 T6 T T6 T7 5 T7 T6 T8

6 cm 14 cm 22 cm T5 30 cm T5 T6 T5 T6 T7 T6 T7 T8 T7 T9

10 cm

Figure 4. Representative images of experiment A. Length in cm at the upper-right corner in each frame indicates shortening length. T5–T9 are thrusts that formed during the experiment.

of shortening during the experiments was 30–35 cm, which is equivalent to in stage 1, and thus the slope increased rapidly, exceeding 12° by the end of

9–53 km of displacement. Assuming a plate convergence rate of 5 cm/yr, this stage 1 (Fig. 5C). After the emergence of T6 (stage 2), the frequency of fore- in turn corresponds to 0.2–1.1 m.y. The sediment supply to the topographic thrust initiation and the uplift rate of the wedge (0.10 cm/cm) were lower than lows of 910–1129 g for 1 m.y. is equivalent to a sediment budget on the order during stage 1. The slope of the wedge surface ranged from 8.5° to 13° and of 106 t/yr. This calculated sediment budget is the same order of magnitude was 9.5° at the end of the experiment (Fig. 5C). as the sediment load in many mountainous rivers in Japan and New Zealand Deformation was concentrated in the upper layer of glass beads, which (Milliman and Syvitski, 1992), and the sedimentary influx into the Kumano acted as a décollement, until 16 cm of shortening (Figs. 4 and 6A). At around Basin (50 km × 70 km × 2 km) during the last 4 m.y. 18 cm of shortening, the décollement stepped down to the lower layer of glass beads as the toe of the backstop subsided below the upper layer of glass beads

(Figs. 6A and 7). During this stage, the footwall of forethrust T7 underthrust the ■■ RESULTS wedge, and the sand layer between the two layers of glass beads underplated the wedge, creating a duplex structure (18–24 cm of shortening in Figs. 4 and

Experiment A: Subduction without a Seamount 6A). This underthrusting raised the hanging wall of T7 and created a steep (25°) slope and a wedge-top basin (trench-slope basin) on top of the wedge (22 cm

During the first ~9 cm of shortening, high-frequency, low-amplitude fore- of shortening in Fig. 6A). After the activation of T8, with the lower layer of glass thrusts developed in front of the backstop (8 cm of shortening in Figs. 4 and 5A; beads acting as a décollement, subducting sediment was accreted to both the supporting information1). We defined this term as stage 1 and the following term frontal and basal parts of the wedge, with an increasing amount of underplat-

as stage 2, which was characterized by low-frequency, high-amplitude forethrust ing and thickness of the forethrust sheet of T8. The final forethrust, T9, was activity (Fig. 5A). The rate of wedge widening (wedge width divided by the length of initiated with the upper layer of glass beads acting as the detachment (30 cm shortening) was nearly constant through stages 1 and 2 (0.21–0.22 cm/cm; Fig. 5B). of shortening), but the lower layer of glass beads and even the surface of the 1 Supporting information for this paper can be found on Figshare at https://doi.org/10.6084/m9.figshare The wedge uplifting rate (uplift rate, which was calculated from height of plastic sheet were used as the sliding planes below the middle-inner wedges .10263674.v3. the wedge divided by the length of shortening; Fig. 5D) was large (0.34 cm/cm) (Fig. 6A). The final wedge was nearly 30 cm in length and had a constant slope

9 30 T T9 9 0.10 8 A B 0.22 Stage 1 T 25 T8 7 8 T7 T7 Stage 1 Stage 2 6 20 T7 T7 T6 T6 T6 5 T5 15 4 T5 Stage 2 T 3 10 0.21 6 Wedge width (cm)

Number of forethrusts 2 5 Exp. A Figure 5. Geomorphic parameters of the wedges. 1 Exp. A Exp. B Exp. B (A) Number of forethrusts. (B) Wedge width. Dashed 0 0 lines are labeled with wedge progradation rates 0 4 8 12 16 20 24 28 32 0 4 8 12 16 20 24 28 32 calculated from the amount of progradation (cm) di- Shortening (cm) Shortening (cm) vided by the amount of shortening (cm). (C) Wedge 8 slope angle. (D) Wedge height. Dashed lines are la- C D 0.29 0.06 7 beled with uplift rates calculated from the amount of Stage 1 15 Stage 1 Stage 2 uplift (cm) divided by the amount of shortening (cm). 6 T –T are thrusts that formed during the experiment. 0.10 5 9 5

10 4 0.06 3 Uplift due to

Slope (degree) seamount subduction Stage 2 Uplift due to 0.34 5 seamount subduction Wedge height (cm) 2 Exp. A 1 Exp. A Exp. B Exp. B 0 0 0 4 8 12 16 20 24 28 32 0 4 8 12 16 20 24 28 32 Shortening (cm) Shortening (cm)

Figure 6. Line drawings of selective images of the experiments. (A) Experiment A, showing the slid- A Exp. A without seamount 10 cm B Exp. B with seamount ing surface that existed in the upper glass bead 14 cm Wedge-top basin layer and the frontal thrust during the early stage 14 cm Underthrusting of the experiments (~14 cm of the shortening). 8º Frontal accretion 4º 30º The wedge surface slope was consistent with the T6 T7 T5 critical taper angle. The décollement layer stepped down due to subsidence of the rigid backstop, and a branched back thrust was activated (22 cm of the shortening). The frontal thrust (T ) was used 7 Décollement Décollement as a roof thrust for underthrusting of the incoming Branched back thrust Wedge-top basin 22 cm 22 cm layer, leading to a steep slope of the wedge front 6º 5º (25°). The sediment layer within the two glass 25º Underthrusting Frontal accretion bead layers underplated the inner wedge with 36º duplex structures. In the final phase, the sliding T7 T6 Subsidence Uplift surface further migrated downward into the low- est sand layer or possibly along the surface of the plastic sheet (30 cm of the shortening). The Décollement Décollement branched back thrust continued to accumulate its Underplating 30 cm 30 cm displacement. (B) Experiment B, showing defor- 7º 7º mation concentrated in the upper glass bead layer Frontal accretion Frontal accretion 9.5º and the frontal thrust T5 as a roof thrust after the 12º seamount subducted beneath the wedge (14 cm T9 T7 of the shortening). The subducting seamount en- Subsidence abled the incoming layer to underthrust beneath the inner wedge. This underthrusting led to uplift of the outer wedge and the steep slope at the Subduction channel Décollement Subduction channel Décollement wedge front (30°). After the seamount collided with the backstop, it raised the backstop and increased the wedge surface slope up to the angle of repose (22 cm of the shortening). The upper glass bead layer was used as a décollement below the accretionary wedge, while the upper boundary of the subduction channel was located within the sand layer above the upper glass bead layer (30 cm of the shortening).

0.4 a wedge-top basin (Fig. 6B). This also steepened the frontal edge up to 30° Exp. A (Fig. 6B) and the averaged forearc slope to 17.7° (19 cm of shortening in Fig. 5C). Exp. B The wedge progradation rate during stage 2 was 0.10 cm/cm, i.e., nearly half 25 20 0.2 27 that of experiment A (Fig. 5A). The uplift rate varied from 0.06 to 0.29 cm/cm, but the mean rate was the same as in experiment A (Fig. 5B). Sand 29 19.5 5 The wedge deformation process during stage 1 of experiment B was similar 0.0 0 30 0 to that in experiment A (0–6 cm of shortening in Fig. 8). However, at 7 cm of 5 31 10 19 shortening, the seamount triggered the first forethrust of stage 2 at 10 cm from Upper glass beads 32 10 the toe of the wedge (T5 in Fig. 8). The subduction of the seamount led to an 18.75 −0.2 15 33 undeformed layer underthrusting the wedge, and then uplifted the hanging 16 17 wall as a trench-slope basin to create accommodation space (10–16 cm of 34 18.5 15 shortening in Figs. 6B and 8). 18 −0.4 19 A décollement was formed in the upper layer of glass beads on the land-

Y (cm) 35 20 18 ward side of the seamount and in T5 on the trenchward side during the period 21 Sand between the initiation of T5 and collision of the seamount with the backstop −0.6 22 (8–12 cm of shortening in Fig. 8). Just prior to the collision (12–18 cm of shorten- 23 24 ing), both the upper and lower layers of glass beads were sliding, and the sand 25 layer between two layers of glass beads was underplated and injected into T5. −0.8 26 The collision of the seamount with the backstop stepped up the décollement 27 Lower glass beads to the same height as the top of the backstop, leading to thickening of the sub- 28 duction channel (>20 cm of shortening in Figs. 6B, 7, and 8). This also uplifted 29 −1.0 the accretionary wedge and increased the slope up to the angle of repose (36°;

30 Fig. 6B). The subsequent forethrusts, T6 and T7, were rooted in a décollement in Sand the upper layer of glass beads beneath the wedge. After passing through the

−1.2 seamount, a certain range of shear zone was created just below the backstop. −0.40 −0.30 −0.20 −0.10 0.00 Finally, the toe of the backstop subsided slightly (Fig. 7), causing the lower X (cm) layer of glass beads to act as a décollement (35 cm of shortening in Fig. 8). Figure 7. Location of the position of the backstop toe. Red solid line is experiment A, and blue dashed line is experiment B. Values beside the lines indicate length of shortening. Sand and glass beads indicate the setup of the layered incoming sediment (Fig. 3). The backstop Sediment Flux in experiment A continuously subsided with a changing rate, while the backstop in experiment B rapidly uplifted during 18–20 cm of shortening, when the seamount collided with the backstop. After Experiment A: Subduction without a Seamount passing the seamount, the backstop returned to subsidence again. The outflux from the subduction channel (sediment subduction) gradually decreased, but its rate of change increased as the backstop progressively sub- of 9.5° (Fig. 5). The toe of the backstop further subsided (Fig. 7) to the lower sided (Figs. 7 and 9A). In particular, after the backstop position was lower than layer of glass beads (30 cm of shortening in Fig. 4). the upper glass bead layer (17 cm of shortening in Figs. 7 and 9A), correspond- ing to a step down of the décollement, the outflux dropped rapidly. Influx to the accretionary wedge (dashed line in Fig. 9A) increased to balance the total Experiment B: Subduction with a Seamount sediment influx. The output-to-input ratio of the experiment was 0.36 (Table 1).

Stage 1 of experiment B was almost identical to that of experiment A in terms of wedge progradation and the widening and uplift rates of the wedge Experiment B: Subduction with a Seamount (Fig. 5; supporting information [footnote 1]). Stage 2 started after the initiation

of forethrust T5, earlier than in experiment A. T5 was active for over 12.8 cm Sediment outflux gradually decreased (blue line in Fig. 9B), as it did during of shortening, exceeding that of any other forethrust in either experiment experiment A, until the seamount reached the backstop. After the seamount raised (Fig. 5A). This long activity acted to reduce the width of the wedge and created the backstop, at around 18–20 cm of shortening (Figs. 7 and 9B), sediment outflux

Exp. B with seamount 0 cm 10 cm 20 cm 30 cm T5 T T6 T5 5 T7

2 cm 12 cm 22 cm 32 cm T5 T6 T5 T5 T6 T7

4 cm 14 cm 34 cm 24 cm T5 T5 T6 T5 T6 T7

6 cm 35 cm 16 cm 26 cm T T5 5 T 6 T T5 T6 7

8 cm 18 cm 28 cm T5 10 cm T5 T5 T6 T7

Figure 8. Representative images of experiment B. Length in cm at the upper-right corner in each frame indicates shortening length. T5–T9 are thrusts that formed during the experiment.

100 A 100 B

Stage 1 Stage 2 Stage 1 Stage 2 /cm) /cm)

3 80 3 80 Seamount subduction Figure 9. (A) Sediment influx and outflux for 60 60 experiment A (without seamount). (B) Sed- Décollement iment influx and outflux for experiment B step down (with seamount). Asterisk (*) and dagger (†) 40 40 indicate outfluxes including and excluding Influx the volume of the seamount, respectively. Influx Outflux* Sediment flux (cm Sediment flux (cm 20 20 Outflux Outflux† Influx – Outflux Exp. A Influx – Outflux* Exp. B 0 0 0 4 8 12 16 20 24 28 32 0 4 8 12 16 20 24 28 32 Shortening (cm) Shortening (cm)

fully recovered and even exceeded its initial rate (Fig. 9B). Outflux soon decreased TABE 1. TOTA SEDIMENT INPT AND OTPT AND THEIR RATIO again as the seamount subducted farther landward and the backstop subsided Experiment Displacement Input Output Accretion Output/input (Figs. 7 and 9B). The output-to-input ratio of the experiment was 0.46 (Table 1). (cm) (cm3) (cm3) (cm3) ratio A (without seamount) 30.0 2912 1052 1860 0.36 B (with seamount) 35.0 3315 1527 1788 0.46 ■■ DISCUSSION Output includes the volume of the seamount. Décollement Step Down and Underplating If we assume that sand above the upper layer of glass beads is terrigenous The gradual decrease of the outflux in experiment A (Fig. 9A) increased the sediment, and that sand below this layer is hemipelagic-pelagic sediment, the influx to the accretionary wedge, which raised its growth rate. During the time former can be scraped off at the wedge front, and the latter may be under- the upper layer of glass beads acted as a décollement, the sediment above it was plated below the wedge. This occurs because terrigenous and hemipelagic accreted to the wedge front. As the slip switched to the lower layer of glass beads, sediments tend to be detached as a result of variations in diagenetic alteration the sediment between the two layers of glass beads underplated the wedge, (Moore, 1975) or smectite content (Vrolijk, 1990; Deng and Underwood, 2001), and frontal accretion continued. Similar results have been reported in previous or existence of weak smectitic pelagic clay (Moore et al., 2015). This can be analogue experiments (Bonnet et al., 2007; Konstantinovskaya and Malavieille, observed in the Nankai Trough, where there is a step down in the décollement 2011) and numerical experiments (Ruh et al., 2012); i.e., underplating becomes at 3–5 km depth, in the transitional region between the aseismic and seismic significant when the outflux from the subduction channel (sediment subduction) zones (Fig. 10A; Park et al., 2002b; Kimura et al., 2007). This can be ascribed is smaller than the influx (Kukowski et al., 1994). The results of our experiment to lithification that concurrently occurred with deformation across the upper support the conclusion that a narrowing of the subduction channel and a decrease boundary of the subduction channel, stepping down the décollement to the in outflux can lead to sediment underplating the wedge and faster wedge growth. lower boundary of the subduction channel (Kimura et al., 2007).

0 A 0 C

5 5

Subduction channel

Depth (km) Décollement step-down 10 V.E. = 2.5 Depth (km) HKB 10 MES Seamount Underplating 10 km

0 V.E. = 2.5 B Décollement step-downSubduction channel 10 km D 0 5

Splay fault Knoll 5

Depth (km) Seamount / ridge 10 HKB Seamount Subduction channel Depth (km) V.E. = 2.5 V.E. = 2.5 10 10 km Subduction channel 10 km 15 Underplating

Figure 10. Examples of interpretations from published seismic profiles including a subduction channel. (A) Nankai Trough, southwestern Japan (Line KR9806–02; Park et al., 2002a; Kimura et al., 2007). (B) Ecuador-Colombia margin (Line SIS-44; Collot et al., 2008). (C) Central Hikurangi margin, New Zealand (Line 05CM-38; Plaza-Faverola et al., 2016). MES and HKB—Mesozoic sediments and the Hikurangi Plateau oceanic volcanic sequence, respectively (Davy et al., 2008). (D) Northern Hikurangi margin, New Zealand (Line 05CM-04; Barker et al., 2018). V.E.—vertical exaggeration.

The stepping down of the décollement in this study was associated with subduction developed (1) back thrusts in the landward side of the subduct- subsidence of the backstop, which was probably linked to increased overbur- ing seamount, (2) dense networking of faults within the overriding wedge, den stress caused by thickening of the wedge. Increased overburden stress may (3) normal faults in the trenchward side, (4) thickening and shortening of the inhibit the subduction of terrigenous sediment to great depth. An example from wedge with a slope break, and (5) a shadow zone behind the seamount trailing the Ecuador-Colombia margin (Fig. 10B) shows that underplating and progressive slope. In particular, item 5, the shadow zone, will be important to transport seaward migration of duplexes are considered to be related to the splay faults trench-fill sediments and parts of the wedge front beneath the wedge (Domin- that force the underlying décollement thrust to step down (Collot et al., 2008). guez et al., 2000). Numerical simulations also suggest fracturing or faulting atop of the seamount and the presence of a shadow zone in the wake of the seamount (Ruh, 2016; Ruh et al., 2016). In addition, a subducting seamount can Décollement Step Up and Sediment Subduction generate a large overpressure above the landward flank and underpressure above the trenchward flank, producing a drop in vertical compressional stress In experiment B, the subduction of the seamount shifted the décollement from on the trenchward side of the seamount (Baba et al., 2001; Ruh et al., 2016).

the glass bead layers into forethrust T5 along the leading flank of the seamount. The major difference in our study from these previous experiments of sea-

While T5 was active as a “top décollement” (cf. Lallemand et al., 1994), incoming mount subduction is that we used a rigid but unfixed backstop. Dominguez et al. undeformed layered sand in the wake of the seamount was underthrust below the (2000) used a cohesive (deformable) backstop that allowed the décollement to accretionary wedge. This is similar to what is seen in seismic profiles in Figure 2, return to its original position just after the seamount passed due to the overbur- which show a décollement with a step up caused by seamount subduction. den stress. This indicates that it is difficult for the flexible backstop to maintain Another effect of seamount subduction in experiment B is that raising the subduction channel, if the overriding plate readily collapses gravitationally the wedge and the backstop widened the subduction channel, allowing thick along the trenchward flank of the seamount. The rigid backstop in our study layers of sediment to subduct below the backstop through the subduction kept a certain thickness of the subduction channel during the passage of the channel (Figs. 10B–10D). In nature, if an oceanic plate with sufficiently large seamount beneath the backstop, and this enabled the sediment to be trans- and rigid topographic highs subducts under a static backstop (cf. Tsuji et al., ported more than the deformable backstop. It is also suggested that the strength 2015), trench-fill terrigenous sediment accompanying the highs will be trans- of the seamount must be high enough to support the weight of the backstop. ported through the subduction channel to a higher-pressure environment than Another difference in our study is multiple décollements within the incom- sediment on a smooth oceanic plate. Experiment B may be analogous to the ing layer. Due to progressive subsidence of the backstop (Fig. 7), sediment transport mechanism of the protolith of the ancient Sanbagawa metamorphic underplating with duplex structures was successfully reproduced within the and Shimanto accretionary complexes of southwestern Japan. two glass bead layers in experiment A without a seamount (Fig. 6A). The effect Where the trench-fill sediments are insufficient to fully cover the topo- of decreasing outflux relative to influx for underplating has been indicated graphic relief of the subducting oceanic crust, tectonic erosion may dominate, by previous experiments. Kukowski et al. (1994) pointed out that a required and the accretionary wedge cannot grow, as seen in northeastern Japan, Costa condition for underplating is a ratio of incoming to outgoing material that is Rica, and Ecuador (von Huene et al., 2004; Collot et al., 2011) and suggested significantly larger than unity. Our results agree with this point (Fig. 9). by other experiments (Dominguez et al., 1998; Ruh, 2016). The field example In addition, experiments presented here simultaneously reproduced fron- from southwestern Japan shows that terrigenous sediments can be trans- tal accretion and underplating by using multiple décollements in experiment ported from shallow depths (e.g., the Shimanto accretionary complex) to the A. This suggests that a stepping down of the décollement is a key factor for depth of HP metamorphism (e.g., the Sanbagawa metamorphic complex) underplating (cf. Malavieille, 2010). On the other hand, experiment B with a almost coincidentally. The subducting seamount in our experiment, which seamount mostly used the upper glass bead layer and the sand layer at the was totally buried by trench-fill sediments, reproduced frontal accretion and same height as the top of the seamount as the décollement, because the sediment subduction with the seamount at about the same time. Therefore, height of the seamount was larger than the height of the upper glass bead a sediment-rich subduction zone may be required for terrigenous sediments layer. Therefore, if the height of the topographic relief is higher than any weak to accrete and underplate at different depths. layers within the incoming sediment, the upper boundary of the subduction channel will be determined by the height of the topographic relief.

Comparison with Previous Experiments Subduction of Trench-Fill Sediments to Depth Previous analogue experiments of seamount subduction showed significant impacts on the geometry and deformation of accretionary wedges (Lallemand Here, we propose a schematic model for the subduction of terrigenous et al., 1992; Dominguez et al., 1998, 2000). Those impacts include that seamount sediment to depth (Fig. 11). A progressive thickening of the wedge increases

A Transverse sediment supply C Axial sediment supply

Seamount

Forearc basin

Trench Trench

Slope basin Slope Slope basin Slope Deformation front

Frontal accretion Trench-fill sediments Proto-thrusts Pelagic sediments

Arc crust or Accretionary wedge Push down Consolidated wedge Weak layer Push up Underplating Décollement Topographic high Oceanic crust Subduction channel Décollement step down Subduction Décollement step up Sediment subduction through subduction channel

B Subduction channel Compaction & fluid release from sediments opened High excess pore pressure & weak coupling at décollement

Shallow seismogenic zone Transition zone Aseismic zone > 5 km depth 3–5 km depth 0–3 km depth T > 150ºC T = 90– 150ºC T = 0–90ºC Dewatering Décollement step down Décollement step up

Figure 11. Schematic model of sediment subduction through a subduction channel beneath an Underplating accretionary wedge. (A) Subduction of a topographic high raises the décollement to accommo- Push down date the high and the following trench-fill sediments. (B) An increase in overburden gravitational Subduction of undeformed force under the inner wedge shifts the décollement downward and facilitates underplating. In the terrigenous trench-fill sediments wake of the subducting seamount, terrigenous sediment is transported beneath the accretionary Compression of underplated sediments wedge. (C) The seamount raises the backstop, enabling the subduction of terrigenous sediment. Subduction channel closed Fluid escape through thrusts After the passage of the seamount, the décollement returns to the original, lower position, and Reducing excess pore pressure from the upper décollement the subduction channel closes, resulting in underplating beneath the wedge.

the overburden on the décollement, which develops along weak layers in the a high-pressure environment (Fig. 11C). After the topographic high passes cover sediment deposited on the subducting oceanic plate. This overburden the inner wedge or backstop, the décollement under the accretionary wedge results in dewatering and diagenetic alteration of the subducting sediment, returns to the plate boundary or a weak layer within the trench-fill sediments. which increases its mechanical strength, leading to a step down in the décol- We speculate about the ways in which the trench-fill sediments can be lement. The reduction of sediment outflux due to narrowing of the subduction subducted deeper (>15 km of burial) to be protolith of HP-LT metamorphic channel increases the mass of sediment underplated beneath the wedge and rocks. As shown in this study (experiment B), the strength of the subducting the rate of frontal accretion. When a topographic high (e.g., a seamount or an seamount is crucial to maintain the subduction channel for sediment trans- aseismic ridge) subducts under the wedge, the décollement steps up to the fer to the deep interior. Although common isolated seamounts composed of forethrust along the leading flank of the seamount (Figs. 11A and 11B). This volcaniclastic rocks and hyaloclastite with various degreed of alteration can likely enables the subduction of terrigenous sediment beneath the wedge. be shred off during the shallow stage of subduction, some of them can be Further subduction of the topographic high would raise the backstop and subducted into the HP environment (Ueda, 2005; John et al., 2010; Bonnet open the subduction channel for terrigenous sediment to be subducted into et al., 2019a, 2019b). Alternatively, a large-scale aseismic ridge and horst

composed of oceanic crust may have sufficient mechanical strength against ■■ CONCLUSIONS the overriding plate and open the subduction channel for sediments. In such a situation, trench-fill sediments will be transported deeper than the updip We conducted a series of analogue experiments to investigate how ter- limit of the seismogenic zone (zone 4, 5–15 km of vertical burial; Vannucchi rigenous sediment is subducted under an accretionary wedge. The results et al., 2012). yielded the following conclusions. However, at some depth, when the shear force along the décollement (1) An increase in overburden stress due to progressive thickening of the has increased and reached the mechanical strength of the topographic highs accretionary wedge leads the décollement to step down and narrows (i.e., seamount), the topographic highs and the sediments will be sheared off, the subduction channel. This accelerates the growth of the wedge stacked, and underplated. Although surface erosion is one of the major driv- through underplating and frontal accretion. ing forces for exhumation of HP metamorphic rocks (e.g., Malavieille, 2010; (2) When a topographic high subducts under the wedge, the décollement Malavieille and Konstantinovskaya, 2010), the underplating of a seamount may steps up from a weak detachment layer within the incoming sediment be another trigger. There is an ancient example of stacking of seamounts and to the forethrust along the landward flank of the seamount. This enables clastic sequences under the HP-LT environment in the Lower Cretaceous Iwashi- terrigenous sediment in the wake of the seamount to be underthrust mizu accretionary complex in the Kamuikotan HP-LT zone, Hokkaido, Japan beneath the wedge. (Ueda, 2005). The metavolcanic sequence, composed of hyaloclastite, pillow (3) If a topographic high is rigid enough to uplift the backstop, and the breccia, and pillow lava (Pirashuke Unit), is considered to have been subducted backstop is mechanically stable, it can widen the subduction channel under a pressure of 0.5–0.6 GPa and temperature of 200 °C, suggesting depths to transport the terrigenous sediment that follows toward a deeper of ~20 km. This unit is associated with a clastic sequence consisting of sandy environment. turbidite and mudstone, and even conglomerate (Shizunai Unit). Based on (4) A sufficient sediment supply to the trench and a rough oceanic crust the downward progression of underplating accretion and higher pressure in surface are necessary for simultaneous shallow accretion, underplating the structurally upper unit as compared to the structurally lower units, Ueda of the wedge, and transportation of sediment to deeper settings as the (2005) suggested that structurally upper units were exhumed synchronously protolith of HP-LT metamorphic rocks. with growth of the structurally lower units.

ACKNOWLEDGMENTS Limitations We are grateful to Takato Takemura for useful suggestions regarding experimental material and to Yujiro Ogawa for discussions about sediment subduction and accretion. This work was funded by a Grant-in-Aid from the Japan Society for the Promotion of Science (17K05687). This research Our experiments did not incorporate effects of pore pressure, but it is was also supported by the Cooperative Program (no. 143, 2017; no. 147, 2018; no. 151, 2019) of the important in maintaining subduction channels along the plate interface (e.g., Atmosphere and Ocean Research Institute, The University of Tokyo. Relevant multimedia data files for this study are available on Figshare (https://doi.org/10.6084/m9.figshare.10263674.v3). Saffer and Bekins, 2006). As the fluids escape from the subducting material Constructive comments from the associate editor, Philippe Agard, and journal reviewers, Jones in the subduction channel, a significant part of the overburden load is trans- Ruh and Stephan Dominguez, are greatly appreciated. Managing Editor Gina Harlow is thanked ferred to the solid fraction of the subducting material, and the mechanical for facilitating revisions and final publication. behavior gradually changes to that of a consolidated media with a continuous solid matrix (cf. Nankai and Barbados; Saffer, 2003; Calahorrano et al., 2008). REFERENCES CITED This probably accelerates both the stepping down of the décollement and Agard, P., Yamato, P., Jolivet, L., and Burov, E., 2009, Exhumation of oceanic blueschists and eclog- underplating (Strasser et al., 2009; Kimura et al., 2011). In contrast, numerical ites in subduction zones: Timing and mechanisms: Earth-Science Reviews, v. 92, p. 53–79, simulations predict that the raising of the wedge due to the subduction of a https://doi.org/10.1016/j.earscirev.2008.11.002. seamount could delay the release of fluid from subducting sediment (Baba Agard, P., Plunder, A., Angiboust, S., Bonnet, G., and Ruh, J., 2018, The subduction plate interface: Rock record and mechanical coupling (from long to short timescales): Lithos, v. 320–321, et al., 2001; Ruh, 2016). Low-velocity layers observed in the wake of subducting p. 537–566, https://doi.org/10.1016/j.lithos.2018.09.029. seamounts could provide evidence of undercompacted sediment with poten- Albert, F., Kukowski, N., Tassara, A., and Oncken, O., 2018, Material transfer and subduction channel tially high pore pressures (e.g., Sage et al., 2006). Furthermore, the seismic segmentation at erosive continental margins: Insights from scaled analogue experiments: reflection characteristics of the Hikurangi subduction margin also suggest Tectonophysics, v. 749, p. 46–61, https://doi.org/10.1016/j.tecto.2018.10.019. Angiboust, S., Cambeses, A., Hyppolito, T., Glodny, J., Monié, P., Calderón, M., and Juliani, C., localized reductions in effective stress associated with seamount subduction 2018, A 100-m.y.-long window onto mass-flow processes in the Patagonian Mesozoic sub- (Bell et al., 2010). In addition to topographic relief, pore pressure can allow duction zone (Diego de Almagro Island, Chile): Geological Society of America Bulletin, v. 130, subduction channels to persist for longer than would otherwise be possible. p. 1439–1456, https://doi.org/10.1130/B31891.1. Aoki, K., Iizuka, T., Hirata, T., Maruyama, S., and Terabayashi, M., 2007, Tectonic boundary between Our experiments cannot currently incorporate such effects of pore pressure; the Sanbagawa belt and the Shimanto belt in central Shikoku, Japan: Journal of the Geological consequently, we need to consider ways to include these effects. Society of Japan, v. 113, p. 171–183, https://doi.org/10.5575/geosoc.113.171.

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