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

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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 © 2020 The Authors GEOSPHERE | Volume 16 | Number 4 Noda et al. | Subduction of trench-fill sediments Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/4/953/5144954/953.pdf 953 by guest on 30 September 2021 Research Paper 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 GEOSPHERE | Volume 16 | Number 4 Noda et al. | Subduction of trench-fill sediments Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/4/953/5144954/953.pdf 954 by guest on 30 September 2021 Research Paper can be transported to depth.
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