Radiolarian and U–Pb zircon dating of Late Cretaceous and Paleogene Shimanto accretionary complexes, Southwest Japan: Temporal variations in provenance and offset across an out-of-sequence thrust

Journal of Asian Earth Sciences 170 (2019) 29–44

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Radiolarian and U–Pb zircon dating of Late Cretaceous and Paleogene Shimanto accretionary complexes, Southwest Japan: Temporal variations in provenance and offset across an out-of-sequence thrust

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Hidetoshi Haraa, , Kousuke Harab,1 a b

Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan

A R T I C LE I N FO

A B S T R A C T

Keywords: Radiolarian Zircon U–Pb age Accretionary complex Out-of-sequence thrust Late Cretaceous Eocene

In this study, we describe radiolarian fossils, analyze sandstone petrography and geochemistry, and perform detrital zircon U–Pb dating on the Late Cretaceous and Paleogene Shimanto accretionary complexes. These data are used to constrain temporal variations in provenance, which are related to volcanic arc activity. We also determine the behavior of the Aki Tectonic Line, which represents an out-of-sequence thrust. Based on lithology and radiolarian assemblages, we subdivide the Late Cretaceous to early Paleocene Mugi Unit into the Mg1, Mg2, and Mg 3 subunits, and the Eocene Naharigawa Unit into the Nh1 and Nh2 subunits. Detrital zircon U–Pb ages indicate that sandstones of the Mugi Unit were sourced mainly from Late Cretaceous to Paleocene igneous rocks. Nh1 sandstone records a single peak age in the Late Cretaceous, whereas Nh2 sandstone preserves multiple peaks from the pre-Jurassic to early Paleocene. The Naharigawa Unit lacks syn-depositional detrital zircons; however, felsic tuff within Nh1 records an age of 48.7 Ma, which is consistent with radiolarian ages. The Nh1 and Nh2 sandstones contain slightly higher Ba and Rb concentrations than those of the Mugi Unit, suggesting that basement had been uplifted and eroded in the source region, prior to deposition. We conclude that temporal variations in sandstone composition within the Late Cretaceous to early Paleocene Shimanto accretionary complex resulted from tectonic events that occurred in response to syn-depositional igneous activity. We infer that the evolution of the Eocene Shimanto accretionary complex was influenced by uplift and erosion of preJurassic basement. Late Cretaceous radiolarians were identified in subunit Mg3 that forms the footwall of the Aki Tectonic Line, which was previously interpreted as a boundary fault between the Late Cretaceous and Paleogene Shimanto accretionary complexes. The Aki Tectonic Line is re-interpreted as an out-of-sequence thrust (OST) that was active under low-grade metamorphic conditions. We classify several OSTs close to the Late Cretaceous and Paleogene Shimanto accretionary complexes, based on contrasts in metamorphic grades across the faults. Within the study area, the Aki Tectonic Line is classified as a Type-2 OST, indicating that it developed within the Cretaceous Shimanto accretionary complex at temperatures of 250–270 °C.

1. Introduction Southwest Japan is characterized by an arc–trench system, comprising a volcanic arc, forearc basin, and accretionary complex, which developed in response to the Pacific orogeny and associated subduction along the eastern margin of the Asian continent from the Paleozoic to the present day (e.g., Maruyama et al., 1997; Isozaki et al., 2010; Wakita, 2013). The Late Cretaceous to Paleocene arc–trench system records various geological events that resulted from the subduction of a hot and young slab produced at a nearby active spreading ridge (e.g.,

Kiminami et al., 1994; Sakaguchi, 1996; Iwamori, 2000; Kinoshita, 2002; Aoki et al., 2012; Aoya et al., 2013). From the Late Cretaceous to Paleocene, abundant volcanic rocks with caldera and cauldron structures, and numerous granites were produced (e.g., Iizumi et al., 2000; Imaoka et al., 2011; Iida et al., 2015; Sato et al., 2016), which formed the Ryoke, Sanyo, and Sanin batholith belts (Fig. 1). Sandstone petrography and geochemistry, and detrital zircon ages indicate that the Late Cretaceous Shimanto accretionary complex was sourced from these batholith belts (Kiminami et al., 1998; Hara et al., 2017). In addition, the high-P Sanbagawa metamorphic rocks, which are paired with the

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Corresponding author. E-mail address: [email protected] (H. Hara). 1 Present address: Kunimine Industries CO., Ltd., Iwaki, Fukushima 972-8312, Japan. https://doi.org/10.1016/j.jseaes.2018.10.016 Received 23 March 2018; Received in revised form 5 October 2018; Accepted 23 October 2018 Available online 25 October 2018 1367-9120/ © 2018 Elsevier Ltd. All rights reserved.

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Fig. 1. Distribution of the Cretaceous to Paleogene Shimanto accretionary complex and igneous rocks in Southwest Japan. The geological map is based on the 1:2,000,000 seamless digital geological map of Japan (Geological Survey and of Japan, AIST, 2018). The major tectonic lines in Southwest Japan are the Median Tectonic Line (MTL), Itoigawa–Shizuoka Tectonic Line (ISTL), and Usuki–Yatsushiro Tectonic Line (UYTL). The boundary between the Cretaceous and Paleogene Shimanto accretionary complexes is represented by the Nobeoka Thrust (NT) in Kyushu, Aki Tectonic Line (ATL) in Shikoku, Gobo–Hagi Tectonic Line (GHTL) in the Kii Peninsula, and the Matsuhime Fault (MF) in the Kanto Mountains.

(Long et al., 2012; Chen et al., 2016). Similar studies have used such techniques to constrain the tectonic evolution of the Japanese islandarc system (e.g., Joo et al., 2007; Kiminami, 2010; Aoki et al., 2014; Fujisaki et al., 2014). Combining with these techniques, we reconstruct the tectonic evolution of the Shimanto accretionary complex from the Late Cretaceous to Paleogene. In addition, we also discuss the significance of the Aki Tectonic Line as out-of-sequence thrust that developed within the Shimanto accretionary complex.

high-T Ryoke metamorphic rocks, formed deep within the accretionary complex and mantle wedge during the Late Cretaceous (e.g., Wallis and Okudaira, 2016). Recent studies have ascribed the origin and protoliths of the Sanbagawa metamorphic rocks to the deep portion of the Late Cretaceous Shimanto accretionary complex (Kiminami et al., 1999; Terabayashi et al., 2005; Aoki et al., 2008; Tsutsumi et al., 2009; Knittel et al., 2014). Thus, the evolution of the Late Cretaceous to Paleocene Shimanto accretionary complex was associated with the geological events by active subduction of a hot slab. Subsequence to the Paleocene, igneous activity was reduced with a hiatus in the early Eocene (Imaoka et al., 2011) or late Paleocene to early Eocene (Iida et al., 2015). In addition, Eocene igneous rocks are distributed in a small region within the Sanin batholith belt (Fig. 1). The Eocene Shimanto accretionary complex comprises thick coherent sequences with subordinate mélange and is widely exposed in Southwest Japan (e.g., Taira et al., 1980, 1988). The formation of accretionary complex required a large amount of sediment to be supplied to the trench from an arc or continent (von Huene and Scholl, 1991; Taira and Ogawa, 1991). The development of the Eocene Shimanto accretionary complex is inferred to have been triggered by a geological event after the Late Cretaceous to Paleocene intensive igneous activity. The Shimanto accretionary complex, in Shikoku Island, Southwest Japan, preserves a complete geological record from the Cretaceous to early Miocene (Fig. 1). In eastern Shikoku, Cretaceous and Paleogene accretionary complexes are juxtaposed across the Aki Tectonic Line, which is interpreted as an out-of-sequence thrust and is characterized by a clear change in metamorphic grades across the thrust (Mori and Taguchi, 1988; Hara et al., 2017). However, Cretaceous radiolarian fauna have been identified in the footwall of the Aki Tectonic Line, which was previously thought to have an Eocene age (Hara and Hara, 2016). Therefore, the boundary between the Cretaceous and Paleogene accretionary complexes in eastern Shikoku remains unclear. We have previously used provenance analysis to determine the tectonic evolution of the late Early to Late Cretaceous Shimanto accretionary complex (Hara et al., 2017). In the present study, we aim to extend this history from the Late Cretaceous to the Eocene. First, to constrain the relationship between the Cretaceous and Paleogene accretionary complexes, we use radiolarian biostratigraphy to determine the depositional ages of units adjacent to the Aki Tectonic Line. Spatiotemporal variations in provenance are determined by combining sandstone petrography and geochemistry with detrital zircon U–Pb age

2. Geological outline In the present study area of eastern Shikoku, the Cretaceous Shimanto accretionary complex comprises the Hiwasa and Mugi units, and the Paleogene Shimanto accretionary complex is represented by the Naharigawa Unit (Fig. 2). To determine changes in provenance during the Late Cretaceous to Paleogene, and to characterize the Aki Tectonic Line and associated faults, we focus on the Mugi and Naharigawa units. Based on lithology and radiolarian assemblages, we subdivide the Mugi and Naharigawa units into the Mg1, Mg2, and Mg3, and Nh1 and Nh2 subunits, respectively. The Aki Tectonic Line is an out-of-sequence thrust that is associated with a clear change in metamorphic grades between subunits Mg2 and Mg3 (Fig. 3a). The Inubo Fault, which is newly named in the present study, forms a boundary between subunits Mg3 and Nh1 (Fig. 3b), and therefore represents the boundary between the Cretaceous and Paleogene Shimanto accretionary complexes. In addition, the Kuki Fault was assigned as a boundary fault between subunits Nh1 and Nh2 (Kochi Prefecture, 1961). Photographs of representative outcrops are shown in Figs. 3 and 4. 2.1. Mugi Unit Mg1 represents the major subunit of the Mugi Unit, and comprises mélange that contains blocks of sandstone, basalt, and chert in an argillaceous matrix (Fig. 3c and d). Mélanges are generally subjected to an intensively shear deformation with scaly and foliated argillaceous matrixes (Onishi and Kimura, 1995). The chert is typically red and contains late Campanian radiolarians (Ishida and Hashimoto, 1998). Basaltic rocks have an N-MORB composition and are interpreted to have been erupted close to a trench (Kiminami et al., 1994). Previous studies have suggested that the mélange represents a plate boundary décollement (Matsumura et al., 2003; Ikesawa et al., 2005; Kitamura 30

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Fig. 2. Geological map of the Cretaceous and Paleogene Shimanto accretionary complexes in inland eastern Shikoku. Locations of samples discussed in the text are indicated on the map.

radiolarian zonation for Japan proposed by Hollis and Kimura (2001) and Hashimoto et al. (2015), and the Paleogene radiolarian zonation of Nigrini et al. (2005). The Paleogene radiolarian zonation of Nigrini and Sanfilippo (2001) and Nigrini et al. (2005) has been used to establish the biostratigraphy of the Paleogene Shimanto accretionary complex (Oyaizu et al., 2002; Suzuki and Fukuda, 2012), although the zonation was determined for the equatorial Pacific. Furthermore, Suzuki and Fukuda (2012) reported that radiolarian assemblages PR8–12 and RP16–19 (following the zonation of Nigrini et al., 2005) are dominant within the Paleogene Shimanto accretionary complex on the Kii Peninsula.

et al., 2005; Ujiie et al., 2010; Kimura et al., 2012; Yamaguchi et al., 2012). Subunit Mg2 comprises phyllitic shale with minor sandstone (Fig. 3e). The phyllitic shales contain aligned clay minerals, dark pressure solution seams, and silt-sized detrital quartz grains. They yield illite K–Ar ages of 42.4 ± 0.9 Ma and 48.4 ± 1.1 Ma, which are thought to represent the timing of low-grade metamorphism (Hara et al., 2017). Subunit Mg3 is defined in the present study and was previously assigned to the Naharigawa Unit. This subunit comprises mudstone (Fig. 3f), broken beds of sandstone and mudstone, and minor felsic tuff (Fig. 3g). Pebbly mudstones are observed in fresh stream outcrops, and soft-sediment deformation features, such as mud injection, are well preserved (Fig. 3h). This subunit represents a dismembered turbiditic sequence and lacks clasts of oceanic crust material (e.g., chert and basalt).

3.1. Subunit Mg1 Late Campanian to early Maastrichtian radiolarian assemblages, including Archaeodictyomitra lamellicostata and Pseudotheocampe abschnitta, have been identified in the argillaceous matrix of a mélange within Mg1 (Hara et al., 2017; see R1 in Fig. 2). In the present study, we also identified Amphipyndax tylotus, A. lamellicostata, Carpocanopsis costatum, Dictyomitra koslovae, Foremanina schona, and Myllocercion acineton in a red mudstone at locality R2 (Fig. 2). Based on the zonation of Hashimoto et al. (2015), A. tylotus, A. lamellicostata, and M. acineton are assigned to the Al to Co, Al to Sa, and Ma to Co zones, respectively. Following the zonation of Hollis and Kimura (2001), A. tylotus and A. lamellicostata are assigned to the Al to Cg and Dk2 to Pa zones, respectively. These assemblages suggest a late Campanian to early Maastrichtian age. Radiolarian assemblages with similar ages have been reported from several localities in eastern Shikoku (Yamazaki et al., 1993; Ishida and Hashimoto, 1998; Kiminami et al., 1998). We therefore infer that subunit Mg1 presents the radiolarian age in the late Campanian to early Maastrichtian.

2.2. Naharigawa Unit Subunit Nh1 represents the upper section of the Naharigawa Unit and comprises broken interbedded sandstone and mudstone (Fig. 4a and b), with minor felsic tuff (Fig. 4c). This subunit is a similar lithology to the subunit Mg3, but contains a greater proportion of sandstone and tuff. Subunit Nh2 comprises massively bedded sandstone (Fig. 4d), interbedded sandstone and mudstone (Fig. 4e), and minor mudstone (Fig. 4f). This subunit is interpreted as a coherent turbidite unit, contains abundant trace fossils (Nara and Ikari, 2011), and preserves sedimentary structures such as parallel and convolute laminations, ripple marks, and rip-up clasts (Taira et al., 1980). 3. Radiolarian assemblages and ages

3.2. Subunit Mg3 To determine the depositional age of the Mugi and Naharigawa units, we collected more than 200 mudstone samples. After standard hydrofluoric acid (HF) etching, well-preserved radiolarians were recovered from 12 samples. Selected age-diagnostic radiolarians are shown in Fig. 5 (Mugi Unit) and Fig. 6 (Naharigawa Unit). As Mg2 underwent low-grade metamorphism, radiolarians could not be recovered from this subunit. The occurrences of each identified species are listed in Tables S1 and S2. We follow the Late Cretaceous

We identified radiolarian assemblages in mudstones at three localities within the subunit Mg3 (R3–R5 in Fig. 2). Sample R3 is a tuffaceous mudstone and yields a radiolarian assemblage containing Alievium superbum, Amphipyndax stocki, Archaeodictyomitra simplex, Archaeospongoprunum hueyi, Artostrobium tina (Theocampe tina), Artostrobium urna (Theocampe urna), Cornutella californica, Cryptamphorella sphaerica, Dictyomitra formosa, D. koslovae, Immersothorax cyclops, 31

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Fig. 3. Photographs of faults and outcrops of the Mugi Unit in the study area. (a) The Aki Tectonic Line at outcrop F1 in Fig. 2. (b) The Inubo Fault at outcrop F2 in Fig. 2. (c) Tectonic mélange containing clasts of basalt and chert within subunit Mg1. (d) Tectonic mélange containing clasts of sandstone within subunit Mg1. (e) Phyllitic shale of subunit Mg2. (e) Mudstone of subunit Mg3. (f) Mudstone associated with tuff and sandstone of subunit Mg3. (g) Pebbly mudstone with sandstone conglomerate of subunit Mg3. Arrowheads indicate locations of mud injection into sandstone. ATL = Aki Tectonic Line, F = falut, Phy. sh = phyllitic shale, Ss = sandstone, Ms = mudstone, Br = broken beds of sandstone and mudstone, Ba = basalt, Ch = chert.

lamellicostata, Archaeodictyomitra sliteri, C. costatum, C. sphaerica, D. koslovae, F. schona, M. acineton, Rhopalosyringium magnificum, and Theocampe altamonetensis. The co-occurrence of A. pseudoconulus, A. tylotus, D. koslovae, and R. magnificum was assigned to the late Campanian (At zone) by Hollis and Kimura (2001). Furthermore, A. lamellicostata and A. tylotus occurred until the early Maastrichtian (Hollis and Kimura, 2001; Hashimoto et al., 2015). The radiolarian assemblage of sample R5 therefore suggests a late Campanian to early Maastrichtian age. Consequently, radiolarian ages of the subunit Mg3 was likely in the Santonian to early Maastrichtian and includes older fauna than Mg1.

Patellula euessceei, Rhopalosyringium kleinum, Stichomitra compsa, and Stichomitra manifesta. The co-occurrence of A. tina, A. urna, and D. formosa is characteristic of the Santonian to early Campanian (Dk1 to Dk2 zones; Hollis and Kimura, 2001). However, the first occurrence of A. hueyi is correlated to the base of Dk2 zone (Hollis and Kimura, 2001). The radiolarian assemblage of sample R3 therefore indicates an early Campanian age. Sample R4 is a mudstone that contains A. superbum, Amphipyndax ellipticus, A. stocki, Cryptamphorella macropora, Cryptamphorella sphaerica, D. formosa, D. koslovae, I. cyclops, Praeconocaryomma universa, Pseudoaulophacus praeflorensis, S. compsa, S. manifesta, and Theocampe salillum. C. sphaerica and P. praeflorensis occur from the middle Santonian to early Maastrichtian (Hollis and Kimura, 2001; Hashimoto et al., 2015). Furthermore, the last occurrences of D. formosa were in the late Campanian (Hashimoto et al., 2015). The radiolarian assemblage of sample R4 therefore indicates a middle Santonian to Campanian age. Sample R5 is a tuffaceous mudstone that contains Afens liriodes, Alievium gallowayi, Amphipyndax pseudoconulus, A. tylotus, A. stocki, A.

3.3. Subunit Nh1 We identified radiolarian fossils in Nh1 mudstone samples from three localities (R6, R7, and R8 in Fig. 2). Sample R6 is a mudstone containing Amphisphaera coronata, Bathropyramis magnifica, Calocycloma castum, Clathrocyclas universa, Dorcadospyris confluens, Dorcadospyris pentas, Lychnocanoma babylonis, Phormocyrtis striata striata, 32

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Fig. 4. Outcrop photographs of the Naharigawa Unit. (a) Interbedded sandstone and mudstone of subunit Nh1. (b) Broken beds of sandstone and mudstone of subunit Nh1. (c) Felsic tuff of subunit Nh 1 (N1-t). (d) Massive bedded sandstone of subunit Nh2. (e) Interbedded sandstone and mudstone in subunit Nh2. (f) Mudstone of subunit Nh2.

collected samples (R9 to R13) contain similar radiolarian assemblages, including Calocyclas turris, L. babylonis, T. mongolfieri, Theocampe ovata, and L. babylonis. Calocyclas turris is characteristic of the latest middle to late Eocene (Nigrini et al., 2005). Samples R9, R11, and R12 contain Cryptocarpium azyx, which was assigned to RP17 and RP19 zones by Nigrini et al. (2005), corresponding to c. 38.5–34.5 Ma. This subunit was therefore deposited in the Priabonian, in the late Eocene.

Rhopalocanium pyramis, and Theocotyle cf. venezuelensis. Phormocyrtis striata striata and C. castum are characteristic of the early to middle Eocene (Nigrini and Sanfilippo, 2001; Nigrini et al., 2005). This sample is assigned an age based on the last occurrence of P. striata striata and the first occurrence of L. babylonis, which corresponds to RP8 to RP11 zones of Nigrini et al. (2005). Sample R7 is a mudstone that contains B. magnifica, Lithochytris vespertilio, Lithomitra micropore, L. babylonis, Lychnocanoma bellum, Periphaena heliasteriscus, Podocyrtis (Lampterium) mirabilis, Stylosphaera minor, Tessarospyris (?) bicaudalis, Theocampe mongolfieri, Thyrsocyrtis (Pentalacorys) tensa, and Thyrsocyrtis (Pentalacorys) triacantha. The occurrence of T. (P.) triacantha is characteristic of the lower RP12 zone, and the last occurrence of T. (P.) tensa also occurred within PR12 zone (Nigrini et al., 2005). The radiolarian assemblage of this sample is therefore correlated with PR12 zone, indicating an early middle Eocene age, which is slightly younger than the age of R6. Sample R8 is a mudstone that contains B. magnifica, P. heliasteriscus, S. minor, and Theocotyle cryptocephala, which is a similar assemblage to sample R7. The occurrence of T. cryptocephala is consistent with RP9 to RP12 zones. Based on the observed radiolarian assemblages, we conclude that the depositional age of Nh1 ranges from c. 51.5 to 46 Ma (zones RP8 to RP12), which corresponds with the late Ypresian to early Lutetian in the early to middle Eocene.

4. Analytical methods For geochemical analysis, we crushed 10 samples (G01–G10; Fig. 2) of sandstone from the Mugi and Naharigawa units of the Shimanto accretionary complex. Major and minor element concentrations were measured by X-ray fluorescence (XRF; Rigaku RIX3000) on glass beads at Niigata University, Japan. Total Fe content is reported as Fe2O3. Loss on ignition (LOI) values were measured by weighing the samples before and after heating for 2 h at 850 °C. Zircon U–Pb analyses were conducted on four samples (M3 from the Mugi Unit and N1, N2, and N1-t from the Naharigawa Unit; Fig. 2). Zircon grains were separated from sandstone and tuff samples using standard magnetic and heavy liquid separation techniques, mounted on a Teflon sheet, and polished using diamond paste. The internal structure and zonation of the polished zircons were determined using cathodoluminescence (CL) imaging (JEOL JSM-6610 LV SEM equipped with a Gatan Mini CL system) and scanning electron microscope energy dispersive spectroscopy (SEM–EDS) at the Geological Survey of Japan, Tsukuba, Japan. Most zircons exhibit clear oscillatory to faint zoning in the CL image (Fig. S1), indicative of magmatic zircon growth (Corfu et al., 2003). Zircon U–Th–Pb isotopic analyses were performed using a quadrupole laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) system that comprised either a Nu Instruments

3.4. Subunit Nh2 Previous studies of subunit Nh2 have reported middle to late Eocene radiolarians, Eocene bivalves, and middle Eocene calcareous nannofossils (Katto and Tashiro, 1979; Okada and Okamura, 1980; Taira et al., 1980; Suyari and Yamazaki, 1988). We identified new radiolarian fossils in Nh2 mudstones at five localities (R9 to R13 in Fig. 2). The five 33

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Fig. 5. Selected age-diagnostic Late Cretaceous radiolarians recovered from mudstone of the Mugi Unit. Scale bar is 100 μm. 1: Archaeodictyomitra lamellicostata Foreman, R5. 2: Archaeodictyomitra sliteri Pessagno, R5. 3, 4, 5: Dictyomitra koslovae Foreman, R5, R2, and R4. 6: Amphipyndax pseudoconulus (Pessagno), R5. 7, 8: Amphipyndax tylotus Foreman, R5. 9, 10: Amphipyndax stocki Campbell and Clark, R3. 11: Stichomitra compsa Foreman, R4. 12: Stichomitra manifesta Foreman, R4. 13: Foremanina schona Empson-Morin, R5. 14: Immersothorax cyclops Dumitrica, R4. 15: Rhopalosyringium magnificum Campbell and Clark, R5. 16: Carpocanopsis costatum Nakaseko and Nishimura, R5. 17, 18: Myllocercion acineton Foreman, R2 and R5. 19: Cryptamphorella sphaerica (White), R5. 20: Cryptamphorella macropora Dumitrica, R4. 21: Artostrobium tina Foreman, R3. 22: Artostrobium urna Foreman, R3. 23: Theocampe salillum Foreman, R3. 24: Theocampe altamonetensis (Campbell and Clark), R5. 25: Cornutella californica Campbell and Clark, R3. 26, 27: Afens liriodes Riedel and Sanfilippo, R5. 28: Alievium gallowayi (White), R5. 29: Pseudoaulophacus praeflorensis Pessagno, R4. 30: Praeconocaryomma universa Pessagno, R4.

5. Sandstone petrography and geochemistry

AttoM or Thermo Fisher Scientific iCAP-Qc, equipped with a New Wave Research NWR-193 or Type-C Ti:S femtosecond laser at Kyoto University, Kyoto, Japan (now relocated to the University of Tokyo, Tokyo, Japan). The experimental conditions during analysis are summarized in Table S3. A Nancy 91,500 zircon standard (1062.4 Ma; Wiedenbeck et al., 1995) was used to correct mass bias effects on 206Pb/238U ratios during U–Pb analysis. Secondary zircon standards OD-3 (33 Ma; Iwano et al., 2013), Plešovice (337.1 Ma; Sláma et al., 2008), and GJ-1 (610.0 Ma; Jackson et al., 2004) were simultaneously analyzed for quality control. All U–Pb ages and concordia diagrams were obtained using the Isoplot v.3.75 software package (Ludwig, 2012).

Twenty sandstone samples were obtained from the Mugi and Naharigawa units (Fig. 2), and 11 samples (G01–10) were selected for XRF analysis. We also include data for four sandstone samples from the Mg1 subunit (S18, S25, S26, and S28), which were reported by Hara et al. (2017). Representative photomicrographs of the sandstone samples are shown in Fig. 7, the results of modal analyses (500 points per thin section) are shown in Fig. 8, and geochemical data are listed in Table S4. Sandstone samples from subunits Mg1, Mg2, and Mg3 (referred to as the Mg1, Mg2, and Mg3 sandstones, respectively) display similar characteristics and contain abundant quartz and felsic volcanic 34

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Fig. 6. Selected age-diagnostic Eocene radiolarians recovered from mudstone of the Naharigawa Unit. Scale bar is 100 μm. 1: Bathropyramis magnifica (Clark and Campbell), R6. 2, 3: Calocyclas turris Ehrenberg, R9 and R11. 4: Calocycloma castum (Haeckel), R6. 5: Clathrocyclas universa Clark and Campbell, R6. 6, 7: Cryptocarpium azyx (Sanfilippo and Riedel), R12 and R9. 8: Dorcadospyris confluens (Ehrenberg), R6. 9, 10: Dorcadospyris pentas Ehrenberg, R6. 11: Lithochytris vespertilio Ehrenberg, R7. 12: Lithomitra micropore Shilov, R13. 13: Lychnocanoma babylonis (Clark and Campbell), R7. 14: Lychnocanoma bellum (Clark and Campbell), R7. 15, 16: Phormocyrtis striata striata Brandt, R6. 17: Podocyrtis (Lampterium) mirabilis Sugiyama and Saito, R7. 18: Podocyrtis (Podocyrtis) papalis Ehrenberg, R13. 19, 20: Rhopalocanium pyramis (Heackel), R6. 21: Tessarospyris (?) bicaudalis Clark and Campbell, 21, R7. 22, 23: Theocampe mongolfieri (Ehrenberg), R13. 24: Theocampe ovata (Haeckel), R9. 25, 26: Theocorys spongoconum King, R13. 27: Theocotyle cf. venezuelensis Riedel and Sanfilippo, R6. 28: Thyrsocyrtis (Pentalacorys) tensa Foreman, R7. 29: Thyrsocyrtis (Pentalacorys) triacantha (Ehrenberg), R7. 30: Thyrsocyrtis (Thyrsocyrtis) rhizodon Ehrenberg, R9. 31: Periphaena heliasteriscus (Clark and Campbell), R7. 32: Amphisphaera coronata (Ehrenberg), R6. 33: Stylosphaera minor Clark and Campbell, R13.

lithic fragments (Fig. 7a). The samples plot primarily within the ‘dissected arc’ and ‘mixed’ fields on a monocrystalline quartz–feldspar–lithic fragment (Qm–F–Lt) diagram (Fig. 8). The modal compositions of sandstone samples from the Mugi Unit are similar to those reported by Kumon and Inouchi (1976) and Kiminami et al. (1998). Nh1 sandstones are typically poorly sorted and contain a large proportion of matrix (Fig. 7b). Rock fragments and matrix cannot be easily distinguished. Nh1 sandstones plot within the ‘dissected arc’ and ‘basement uplift’ fields in the Qm–F–Lt diagram, suggesting a minor

enrichment in plagioclase (Fig. 8). Nh2 sandstones also comprise quartz and felsic volcanic lithic fragments, as well as polycrystalline quartz, metamorphic lithic fragments, and detrital mica (Fig. 7c). Nh2 sandstones plot within the ‘mixed’ field in the Qm–F–Lt diagram, suggesting an enrichment in quartz. Kumon and Inouchi (1976) reported similar sandstone compositions from the Naharigawa Unit, although their data indicated a feldspathic arenite composition. All of the samples have similar compositions, characterized by high SiO2 concentrations (74%–85% wt.%). Fig. 9a, b shows trace element 35

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Fig. 7. Photomicrographs of sandstone and felsic tuff used for geochemical analysis and zircon U–Pb dating. (a) Sandstone from the subunit Mg3 (G02, M3). (b) Sandstone from the subunit Nh1 (G07, N1). (c) Sandstone from the subunit Nh2 (G9, N2). Dotted outlines indicate polycrystalline quartz. (d) Felsic tuff from the subunit Nh1 (N1-t). Qz = quartz, Pl = plagioclase, Qp = polycrystalline quartz, Lv = lithic volcanic fragment, Lm = lithic metamorphic fragment, Hlb = hornblende. a–c: crossed polarized light; d: plane polarized light.

positive correlation is observed between TiO2 and Rb, with samples Nh1 and Nh2 yielding the higher Rb concentrations (Fig. 9c). Ishihama and Kiminami (2000) used a Sr versus Fe2O3/MgO2 diagram to distinguish different sandstones in the Cretaceous Shimanto accretionary complex. However, our analyzed samples all plot close to the ‘KS2 field’ (Fig. 9d), with no clear difference between the Mugi and Naharigawa units.

6. Comparison between U–Pb zircon and radiolarian ages Using geological and radiolarian ages, we identified several tectonostratigraphic units that range in age from Cretaceous to Eocene. Here we compare youngest zircon U–Pb ages with radiolarian ages to acquire precise depositional ages for each unit. We use two values to determine youngest detrital zircon U–Pb ages: (1) youngest single zircon grain ages with a 1σ uncertainty (YSG); and (2) weighted mean ages for the youngest cluster of ages that overlap within 1σ uncertainty (YC). We also include data published by Hara et al. (2017) for samples KS2-Mg1 and KS2-Mg2 (corresponding to M1 and M2 in the present study) from subunits Mg1 and Mg2. U–Pb dating results are presented as age probability diagrams (Fig. 10) and concordia diagrams (Fig. S2), and are summarized in Tables 1 and S5. A summary of zircon U–Pb and mudstone radiolarian ages is presented in Fig. 11. The M1, M2, and M3 sandstones from the Mugi Unit are characterized by a weak peak in YC ages in the latest Cretaceous to earliest Paleocene (68–64 Ma), a strong peak in ages in the Campanian (Late Cretaceous; 82–77 Ma), and minor Jurassic and Triassic ages. Paleozoic and Proterozoic zircon grains were not observed in these samples. The samples yield YSG and YC ages ranging from the latest Maastrichtian to early Paleocene, which are younger than their radiolarian ages of Santonian to early Maastrichtian (Fig. 11). Hara et al. (2017) suggested

Fig. 8. Qm–F–Lt diagram for sandstone from the Mugi and Naharigawa units, with tectonic fields from Dickinson et al. (1983). Qm = monocrystalline quartz, F = feldspar (plagioclase and K-feldspar), Lt = lithic fragment and polycrystalline quartz.

compositions normalized to Post-Archean Australian Shale (PAAS; McLennan, 1989; Taylor & McLennan, 1985). All samples yield similar concentrations and are depleted relative to PAAS. Nh1 and Nh2 sandstones contain slightly higher concentrations of large-ion lithophile elements (LILE; Ba and Rb) than Mg1, Mg2, and Mg3 sandstones. A 36

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Fig. 9. Variations in major and trace elements in sandstone samples from the Mugi and Naharigawa units. (a, b) Trace element concentrations normalized to PostArchean Australian Shale (PAAS). (c) TiO2 versus Rb concentrations. (d) Sr versus Fe2O3/MgO2 diagram used to determine sandstone type (after Ishihama and Kiminami, 2000).

age is treated as a component of the YC age, as together they form a clear peak. The sample yielded YSG and YC ages of 55 and 61 Ma, respectively, older than the late Eocene radiolarian age for the subunit. Syn-depositional zircon grains are not observed in the N2 sandstone, and therefore the depositional age for subunit Nh2 is taken to be late Eocene (Priabonian), based on the radiolarian age.

that mudstones within mélanges may yield older depositional ages than sandstone blocks, following the scheme of Ocean Plate Stratigraphy (OPS; Matsuda and Isozaki, 1991, Wakita, 2015). Furthermore, radiolarian zonation has a low resolution at the Cretaceous–Paleocene boundary, except for well-studied biostratigraphic successions such as those in New Zealand (Hollis, 1997). Thus, comparison between sandstone U–Pb and mudstone radiolarian ages is problematic. Therefore, we follow Hara et al. (2017) in assigning a late Campanian to early Paleocene depositional age to the Mugi Unit. The Nh1 sandstone (sample N1) of the Naharigawa Unit yielded zircon U–Pb ages with a single clear youngest peak in the Campanian (81 Ma) and a YSG age of 72 Ma, which correlates with the second youngest age cluster obtained from the Mugi Unit. These ages are clearly older than the early to middle Eocene radiolarian age acquired from the mudstone (Fig. 11), indicating that the U–Pb ages cannot be used to determine the age of deposition. We conducted zircon dating on a sample of tuff (N1-t) from subunit Nh1. The tuff comprises 60-cm-thick beds, interlayered with sandstone and mudstone (Fig. 4c). It is fine-grained, vitric, and comprises quartz, plagioclase, heavy minerals, volcanic clasts, pumice, volcanic glass within clay minerals, and microcrystalline quartz (Fig. 7d). The tuff sample yielded YSG and YC ages of 47 and 49 Ma, respectively, consistent with the radiolarian age. Therefore, subunit Nh1 is inferred to have been deposited in the late Ypresian to early Lutetian (early to middle Eocene; Fig. 11). Zircon grains from the tuff sample also yielded a peak in U–Pb ages at 80 Ma, and are interpreted as detrital. Nh2 sandstone (sample N2) yielded peaks in U–Pb ages in the Paleocene to Late Cretaceous, Jurassic, Triassic, Permian, and Proterozoic. The YSG age for this sample does not overlap with the second youngest age within a 1σ uncertainty. However, the youngest

7. Temporal variations in provenance from the Late Cretaceous to Eocene Temporal variations in the provenance of the Cretaceous Shimanto accretionary complex in eastern Shikoku have been constrained through studies of sandstone petrography and geochemistry, and detrital zircon geochronology (Kiminami et al., 1998; Hara et al., 2017). Here, we extend this history from the Late Cretaceous to the Eocene. We focus on the distribution of detrital zircon U–Pb ages from the Cretaceous Shimanto accretionary complex reported by Hara et al. (2017), and from the Eocene Shimanto accretionary complex presented in the present study. We identified six U–Pb age clusters: pre-Jurassic (Archean to Proterozoic and Permian to Jurassic), Early Cretaceous, Late Cretaceous, latest Cretaceous to early Paleocene, middle to late Paleocene, and early Eocene. The number of zircon ages assigned to each age cluster is summarized in Table 2. Data from subunits Mg1, Mg2, and Mg3 are integrated to provide a representation of the Mugi Unit. Sandstone geochemistry indicates that the Hiwasa Unit, which is interpreted as a coherent and rhythmic turbiditic unit, was sourced from primarily Late Cretaceous (Campanian) felsic volcanic rocks (Hara et al., 2017). Intense igneous activity in the region is known to have occurred during this period, and Late Cretaceous felsic volcanic rocks (85–65 Ma) infilled caldera and cauldron structures in the Sanyo and 37

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Fig. 10. Probability distribution plots and histograms of acquired

206

Pb/238U ages.

Cretaceous (76%) and Paleocene (13%) (Fig. 12-2). This observation is consistent with widespread exposure of igneous rocks following intense igneous activity in the Late Cretaceous. There are few syn-depositional zircon grains in the Mugi Unit, as Paleocene igneous rocks were less abundant than those of Late Cretaceous age. Furthermore, the early Paleocene was associated with a period of intense igneous activity prior to a magmatic hiatus during the early Eocene (Imaoka et al., 2011), or the late Paleocene to the early Eocene (Iida et al., 2015). Based on recent plate reconstruction models (Whittakeret al., 2007; Seton et al., 2012, 2015; Domeier et al., 2017), the Izanagi–Pacific ridge was spreading close to trench during 55–50 Ma around the western Pacific region without the Kula plate. Hara et al. (2017) suggested that the young and hot Kula or Pacific plates was subducted and caused

Sanin belts in Southwest Japan (Terakado and Nohda, 1993; Imaoka et al., 1994; Yamamoto, 2003; Kishi et al., 2007; Yoshida et al., 2009; Sato et al., 2016). Furthermore, granite intruded the Ryoke, Sanyo, and Sanin belts at 95–60 Ma (Iijima et al., 1985; Kagami et al., 1992; Iida et al., 2015). Detrital zircons in the Hiwasa Unit indicate that the sediment source was 27% Late Cretaceous igneous rocks and 69% preJurassic basement (Fig. 12-1). We therefore infer that during this period, pre-Jurassic basement rocks were widely exposed, despite the onset of igneous activity in the region. The Mugi Unit, which is interpreted as a mélange, was sourced primarily from Late Cretaceous and early Paleocene (late Campanian to Danian) felsic volcanic rocks (Hara et al., 2017). Detrital zircons indicate that the sediment was sourced from igneous rocks of the Late Table 1 Summary of radiolarian and zircon U–Pb ages. Unit Subunit Samples used for U–Pb dating Mudstone radiolarian age

Mugi Unit Mg1 M1 78–69 Ma

Mugi Unit Mg2 M2 n.d.

Mugi Unit Mg3 M3 86–69 Ma

Naharigawa Unit Nh1 N1 51–45 Ma

Naharigawa Unit Nh1 N1-t 51–45 Ma

Naharigawa Unit Nh2 N2 39–35 Ma

Detrital zircon U-Pb data Number of concordant data Number of concordant data < 300 Ma Youngest single zircon grain age (1σ) Youngest cluster (YC) age (1σ) 2nd youngest cluster (1σ) Number of grains selected for YC age

56 52 62.7 ± 1.7 64.1 ± 2.1 77.2 ± 1.6 3

60 58 62.0 ± 1.3 65.2 ± 1.6 82.0 ± 1.5 13

60 56 65.7 ± 1.4 67.5 ± 1.0 78.6 ± 0.9 6

64 61 72.2 ± 2.4 80.5 ± 1.3 no peak 59

43 43 46.8 ± 2.7 48.7 ± 0.4 80.1 ± 1.2 31

58 40 54.5 ± 0.9 60.6 ± 4.0 140.1 ± 4.8 8

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n

Sanyo and Sanin belts after the magmatic hiatus in late Paleocene to early Eocene (Iida et al., 2015). However, syn-depositional zircon grains derived from Eocene igneous rocks were not recovered from sandstone of subunit Nh1. In addition, the Paleocene zircon grains were also lack from the Nh1 sandstone, provided from the Paleocene igneous rocks also only exposed in the Sanyo and Sanin belts. This may be due to: (1) a narrow distribution of Paleocene and Eocene igneous rocks; or (2) the abundance of Late Cretaceous igneous rocks that acted as a “sedimentary barrier” that restricted the supply of syn-depositional zircons. In contrast, zircon ages from a felsic tuff sample (N1-t) correlate with the depositional age of the subunit. This tuff likely fell directly in the forearc region, across the “sedimentary barrier”. Compared with the Mugi Unit, subunit Nh1 yields higher concentrations of LILE such as Ba and Rb (Fig. 9). The concentration of trace elements in clastic rocks can be used to constrain the nature of their source rocks. For example, incompatible trace elements are enriched in felsic volcanic rocks, LILE are concentrated in continental crust, and compatible elements are abundant in mafic and ultramafic rocks (Feng and Kerrich, 1990; McLennan et al., 1990). Sandstone compositions are therefore consistent with a contribution from Pre-Jurassic basement based on rich LILE; however, detrital zircon ages indicate a source comprising dominantly Cretaceous igneous rocks. Subunit Nh2 was deposited in the late Eocene (Priabonian). Detrital zircons were sourced dominantly from pre-Jurassic basement (71%), with minor contributions from Early Cretaceous (7%), Late Cretaceous (9%), and Paleocene (14%) units (Fig. 12-4). However, syn-depositional zircons, sourced from minor igneous rocks within the Sanin belt (Imaoka et al., 2011), were not observed. The sandstones in this subunit are also enriched in LILE, suggesting a component of continental-crustderived material. The proportion of Late Cretaceous zircon grains decreases from 92% to 14% from the Nh1 to Nh2 subunits. These observations suggest that pre-Jurassic basement was exposed through rapid uplift, and that Late Cretaceous igneous rocks that covered the basement were eroded in the source region. During the middle Eocene, motion of the Pacific plate subduction was changed from north to northwest at 47 Ma (Domeier et al., 2017). The Nh1 subunit was formed by northerly (strike-slip) subduction by the Pacific plate, based on zircon age of 48.7 Ma from felsic tuff. Rapid uplift of provenance for the Nh2 subunit was possibly generated by a change in plate motion to northwesterly (normal) subduction. The Naharigawa Unit is characterized by occurrence of conglomerates containing quartz, chert, felsic volcanic and igneous rocks, and orthoquartzite (Kumon and Inouchi, 1976). The conglomerates were also sourced from both of preJurassic basement and Late Cretaceous igneous rocks, particularly orthoquartzite was possibly originated from the North China block (Kumon et al., 2012). In addition, uplift and erosion of the Sanbagawa metamorphic rocks, which were originated from the Cretaceous Shimanto accretionary complex, began in the Eocene (Isozaki and Itaya, 1990; Aoki et al., 2008), as indicated by an unconformity between

Fig. 11. Stratigraphic summary of mudstone radiolarian and detrital zircon U–Pb ages. Periods of igneous activity are after Yamamoto (2003), Nishida et al. (2005), and Sato et al. (2016) for Late Cretaceous volcanic rocks, Iida et al. (2015) for Late Cretaceous to Paleogene granitic rocks, and Imaoka et al. (2011) for Paleogene volcanic rocks, with clarified U–Pb ages of igneous rocks. Subducted plates are based on Domeier et al (2017) with our interpretation. YSG age = Youngest single grain age, TC = Youngest cluster age. Geological timescale is after Gradstein et al. (2012).

intensive igneous activity in Southwest Japan during the Late Cretaceous. Here, we reinterpreted the young and hot plate deduced to intensive igneous activity was possibly the young Izanagi plate close to the Izanagi–Pacific ridge, based on plate reconstruction proposed by Domeier et al. (2017). In addition, magmatic hiatus during early Eocene corresponded to a period of the Izanagi–Pacific ridge subduction. Following the magmatic hiatus, subunit Nh1 of the Naharigawa Unit was deposited in the early to middle Eocene (late Ypresian to early Lutetian). Zircon U–Pb ages from a tuff sample (N1-t) correlate with the initiation of middle Eocene to early Oligocene magmatism, although radiolarian ages overlap with the magmatic hiatus. Detrital zircons indicate that 92% of the sediment was sourced from Late Cretaceous igneous rocks. Therefore, we infer that during this period, Late Cretaceous igneous rocks were widely exposed in the source region (Fig. 12-3). In contrast, Eocene igneous rocks were only exposed in the Table 2 Percentage rate of analyzed zircon ages, grouped into geological periods. Unit Subunit Samples of U–Pb dating Depositional age

Hiwasa Unit KS2-Hw 84–72 Ma

Mugi Unit Mg1 + Mg2 + Mg3 M1 + M2 + M3 86–69 Ma

Naharigawa Unit Nh1 N1 51–45 Ma

Naharigawa Unit Nh1 N1-t 51–45 Ma

Naharigawa Unit Nh2 N2 39–35 Ma

Early Eocene (51–47 Ma) Middle to late Paleocene (62–56 Ma) Latest Cretaceous to early Paleocene (70–62 Ma) Late Cretaceous (100–70 Ma) Early Cretaceous (145–100 Ma) Jurassic (201–145 Ma) Triassic (252–201 Ma) Permian (299–252 Ma) Proterozoic to Archean (2692–541 Ma)

– – 0 27.3 3.6 14.5 14.5 1.8 38.2

– – 12.5 76.1 4.0 0.6 1.1 0.0 5.7

0 0 0 92.2 3.1 0 0 0 4.7

72.1 4.7 0 23.3 0 0 0 0 0

0 8.5 5.1 8.5 6.8 18.6 13.6 8.5 30.5

Total (%)

100

100

100

100

100

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8. Significance of out-of-sequence thrusts within the Cretaceous and Paleogene accretionary complexes In eastern Shikoku, the Cretaceous and Paleogene Shimanto accretionary complexes are in fault contact along the Aki Tectonic Line (e.g., Suyari and Yamazaki, 1987), which is interpreted as an out-of-sequence thrust (OST). Consequently, the Inubo Fault is coincident with a boundary fault between the Cretaceous and Paleogene Shimanto accretionary complexes (i.e., a C–P Fault) in the present study. Furthermore, vitrinite reflectance, Raman spectrometry of carbonaceous material (CM), and illite crystallinity indicate that the paleo-temperature of the hanging wall of the Aki Tectonic Line increases toward the fault (Mori and Taguchi, 1988; Hara et al., 2017). In addition, the Aki Tectonic Line represents a clear change in the temperature of low-grade metamorphism within the Cretaceous accretionary complex (Fig. 13a, Type 2). Raman spectrometry of CM in subunit Mg2 yielded hanging wall temperatures of 266 °C and 273 °C (Hara et al., 2017). Samples from the footwall of the Aki Tectonic Line yield Raman spectra full width at half maximum (FWHM) values for the D1 band of 108.8, 109.2, and 111.3 cm−1, corresponding to temperatures of 239–244 °C using the geothermometer proposed by Kouketsu et al. (2014). The Aki Tectonic Line is associated with a temperature difference of 30 °C between the hanging wall and footwall. Next, we will review the relationship between OST associated with a clear change in metamorphic grades and C–P faults in the Shimanto accretionary complex from southwest to central Japan, and compare the temperature in the handing wall and footwall of the OSTs (Fig. 13). Along the coast of eastern Shikoku, the Mugi Mélange has been well studied and is interpreted to represent an underplated unit that formed along a plate boundary décollement (Matsumura et al., 2003; Ikesawa et al., 2005; Kitamura et al., 2005; Kimura et al., 2012). The Mizoochi Fault, which is interpreted as an OST, has also been identified within the Mugi Unit (Ikesawa et al., 2005). Ikesawa et al. (2005) performed a vitrinite reflectance study on the Mizoochi Fault and reported a hanging wall temperature of 170–200 °C and a footwall temperature of 130–150 °C. No variations in temperature were observed along the Aki Tectonic Line of a C–P fault (Ikesawa et al., 2005). This structure is classified the Type 2 OST (Fig. 13a, Type 2). In central Kyushu, the Nobeoka Thrust separates the Cretaceous and Paleogene accretionary complexes (Imai et al., 1971). The hanging wall (Morotsuka Group) underwent greenschist facies metamorphism (Toriumi and Teruya, 1988), and an increase in temperature is observed approaching the fault (Hara and Kimura, 2008). The footwall (Hyuga Group) underwent prehnite–pumpellyite facies metamorphism (Toriumi and Teruya, 1988). Using illite crystallinity data, Hara and Kimura (2008) estimated that the hanging wall and footwall were metamorphosed at ∼ 300 °C–310 °C and 260 °C–300 °C, respectively. We infer that in central Kyushu, the Nobeoka Thrust represents both an OST and a C–P Fault (Fig. 13a, Type 1). A similar interpretation has been proposed for the Matsuhime Fault in the Kanto Mountains, which separates the Cretaceous Kobotoke Group and the Paleogene Sagamiko Group (Hara and Kurihara, 2010). Using illite crystallinity, Hara and Kurihara (2010) constrained the temperature in the hanging wall to 290 °C–310 °C, and provided mean footwall temperatures of 245 °C and 285 °C in the western and eastern Kanto Mountains, respectively. In contrast, in coastal eastern Kyushu, the Nobeoka Thrust developed within the Paleogene accretionary complex and likely represents a seismogenic OST (Kondo et al., 2005; Mukoyoshi et al., 2009; Kimura et al., 2013; Fukuchi et al., 2014; Hamahashi et al., 2015). The Nobeoka Thrust in this area separates the Kitagawa Group in the hanging wall and the Hyuga Group in the footwall (Imai et al., 1971; Ogawauchi et al., 1984). The Furue Thrust represents the C–P fault in this region (Ogawauchi et al., 1984). Metamorphic temperature is observed to increase from the Cretaceous (Morotsuka Group) to Paleogene (Kitagawa Group) accretionary complexes (Mukoyoshi et al., 2007). Kondo et al. (2005) used vitrinite reflectance to determine hanging wall and

Fig. 12. Temporal variations in provenance in the Shimanto accretionary complex from the Late Cretaceous to Eocene, with pie charts showing the distribution of detrital zircon ages. See text for details. Creta = Cretaceous.

terrestrial sediments (the Eocene Hiwadatoge Formation and Miocene Kuma Group) and the Sanbagawa metamorphic rocks, and a conglomerate containing Sanbagawa schist within the Hiwadatoge Formation in western Shikoku (Ochi et al., 2014). We conclude that temporal variations in provenance within the Shimanto accretionary complex record Late Cretaceous and Paleocene igneous activity, erosion and uplift of pre-Jurassic basement, and a pause in igneous activity in the Eocene associated with plate motion. The evolution of the Eocene Shimanto accretionary complex was influenced by the uplift and erosion of pre-Jurassic basement.

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Fig. 13. Tectonic relationships among out-of-sequence thrusts developed within the Cretaceous to Paleogene accretionary complexes. (a) Classification of OSTs. (b) OST development at various depths. C–P fault: boundary fault between the Cretaceous and Paleogene Shimanto accretionary complexes; OST: out-of-sequence thrust; high and low: temperature conditions in the hanging wall and footwall. See text for details.

observed Type 3 OST, which developed within the Paleogene accretionary complex. Hanging wall temperatures were 320 °C–330 °C, consistent with those of Type 1 OSTs. We infer that Type 3 OSTs propagated toward the trench at a temperature of ∼300 °C (Fig. 13b). Type 2 OSTs developed within the Cretaceous accretionary complex and are represented by the Aki Tectonic Line in inland Shikoku and the Mizoochi Fault in coastal eastern Shikoku. Type 2 OSTs preserve a wide range of temperatures, from 170 °C to 270 °C, and temperatures associated with the Aki Tectonic Line are much higher than those of the Mizoochi Fault. Type 2 OSTs developed along a new detachment within the Cretaceous accretionary complex, which extended to shallow levels of the accretionary prism (Fig. 13b). Type 4 OSTs are characterized by the development of a C–P Fault, and a continuous metamorphic gradient, as represented by the Aki Tectonic Line in western Shikoku and the Gobo–Hagi Tectonic Line in the Kii Peninsula. These C–P faults were active under temperatures of 200 °C–250 °C, and therefore likely developed at similar depth as Type 2 OSTs in eastern Shikoku. These observations suggest that OSTs at this depth level developed within the Cretaceous accretionary complex, without the involvement of the C–P fault (Fig. 13b). The development of OSTs in the Shimanto accretionary complex was spatially heterogeneous, with four characteristic OST types recognized in the present study. These variations are explained by the development of OSTs at various depths at temperatures of 150 °C–330 °C. In the study area, the Aki Tectonic Line is interpreted to have formed at a moderate depth at 240 °C–270 °C. Based on illite K–Ar ages, OSTs was active during 48 to 40 Ma for the Nobeoka Thrust in Kyushu (Types 1 and 3 OSTs; Hara and Kimura, 2008), after 48–40 Ma for the Aki Tectonic Line in eastern Shikoku (Type 2 OST; Hara et al., 2017), after 40 Ma for the Matsuhime Fault in the Kanto Mountains (Type 1 OST; Hara and Kurihara, 2010). Almost all of OSTs were developed after the middle Eocene, and its timing of thrusting correlated

footwall temperatures of 320 °C–330 °C and 250 °C–270 °C, respectively. These observations are consistent with the Type 3 OST setting shown in Fig. 13a. In coastal western Shikoku, Sakaguchi (1999) used vitrinite reflection data to identify a continuous temperature gradient through the Cretaceous and Paleogene accretionary complexes (Taisho and Hata groups), without a break across the Aki Tectonic Line. Temperatures in the hanging wall and footwall range from 195 °C to 255 °C, based on the method of Kondo et al. (2005). The Aki Tectonic Line in western Shikoku therefore represents a C–P fault that lacks a break in metamorphic grade (Fig. 13a, Type 4). In addition, the Aki Tectonic Line was covered by latest Cretaceous to early Paleocene slope–basin sediments within the Nakasuji Graben, inland western Shikoku (Kano et al., 2003). A similar tectonic setting has been observed in the western Kii Peninsula where the Gobo–Hagi Tectonic Line separates the Cretaceous Nyunokawa Formation and Paleogene Otonashigawa Group (Awan and Kimura, 1996). Awan and Kimura (1996) used illite crystallinity data to estimate the temperature in the hanging wall and footwall of the Gobo–Hagi Tectonic Line as 235 °C, following the method of Hara and Kurihara (2010). We identified four characteristic relationships between OST and C–P faults in the present study (Fig. 13a). To constrain the amount of offset across the OSTs, we focus on the temperature associated with low-grade metamorphism in the hanging wall and footwall. Previously determined temperatures are listed in Table 3. We define Type 1 OST as being equivalent to the C–P Fault, as observed for the Nobeoka Thrust in central Kyushu and the Matsuhime Fault in the Kanto Mountains. Hanging wall temperatures in both of these OSTs were in excess of 300 °C, the highest recorded in the examined OSTs. We therefore infer that the OST developed at deep levels of accretionary prism along the C–P Fault at temperatures of ∼300 °C (Fig. 13b). The Nobeoka Thrust in eastern Kyushu represents the only 41

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Table 3 Summary of calculated temperatures in the hanging walls and footwalls of several OSTs. Area

central Kyushu eastern Kyushu western Shikoku eastern Shikoku (inland area) eastern Shikoku (coastal area) western Kii Peninsula Kanto Mountains

Fault

Nobeoka Thrust Nobeoka Thrust Aki Tectonic Line Aki Tectonic Line Mizoochi Fault Gobo–Hagi Tectonic Line Matsuhime Fault

Temperature (°C) Hanging wall

Footwall

300–319 320–330 195–255 266–273 170–200 235 290–310

260–300 250–270 195–255 239–244 130–150 235 245–285

OST classification

Reference

Type Type Type Type Type Type Type

Hara and Kimura (2008) Kondo et al. (2005) Sakaguchi (1999) Hara et al. (2017) Ikesawa et al. (2005) Awan and Kimura (1996) Hara and Kurihara (2010)

1 3 4 2 2 4 1

Y. Isozaki, Dr. A. Yamaguchi and Dr. T. Tokiwa for their constructive and valuable comments of the manuscript. Ternary plots were constructed using the freely distributed CoDaPack 2.0 software package developed by Thió-Henestrosa and Martín-Fernández (2005).

with a period of rapid uplift of provenance due to northwesterly motion of the Pacific plate. 9. Conclusions

Appendix A. Supplementary material

Based on radiolarian assemblages, sandstone petrography and geochemistry, and detrital zircon U–Pb age, we have reconstructed temporal variations in provenance that resulted from changes in volcanic arc activity. We interpret the Aki Tectonic Line as an out-of-sequence thrust that developed within the Cretaceous to Paleogene Shimanto accretionary complexes. We divided the Late Cretaceous to early Paleocene Mugi Unit into the Mg1, Mg2, and Mg 3 subunits, and the Eocene Naharigawa Unit into the Nh1 and Nh2 subunits. Detrital zircon U–Pb data indicate that sandstones of the Mugi Unit were sourced mainly from Late Cretaceous to Paleocene igneous rocks. Within the Naharigawa Unit, Nh1 sandstone yielded a single Late Cretaceous age peak, whereas Nh2 sandstone yielded multiple peaks with the youngest peak of the latest Cretaceous to early Paleocene, and older grains being pre-Jurassic. Syn-depositional zircons are common in the Mugi Unit but are not observed in the Naharigawa Unit. However, felsic tuff within the Nh1 subunit was dated to 48.7 Ma, corresponding to the depositional age of the unit, as determined by radiolarian biostratigraphy. The Nh1 and Nh2 sandstones yielded slightly higher concentrations of Ba and Rb than those of the Mugi Unit, suggesting that the basement was uplifted and eroded in the source region. Temporal variations in the development of the Cretaceous Shimanto accretionary complex resulted from changes in Late Cretaceous and Paleocene igneous activity, and the evolution of the Eocene Shimanto accretionary complex was influenced by the uplift and erosion of pre-Jurassic basement. The Aki Tectonic Line was previously described as a boundary fault between the Cretaceous and Paleogene Shimanto accretionary complexes. However, we conclude that the Aki Tectonic Line does not represent a boundary between Cretaceous and Paleogene units, instead we reinterpreted as an out-of-sequence thrust associated with a break in metamorphic grade. We classified the OSTs that have developed within the Cretaceous and Paleogene Shimanto accretionary complexes, based on contrasting metamorphic grades across the faults. In the study area, the Aki Tectonic Line is classified as a Type-2 OST and is inferred to have developed within the Cretaceous Shimanto accretionary complex at temperatures of 240 °C–270 °C.

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jseaes.2018.10.016. References Aoki, K., Itaya, T., Shibuya, T., Masago, H., Kon, Y., Terabayashi, M., Kaneko, Y., Kawai, T., Maruyama, S., 2008. The youngest blueschist belt in SW Japan: implication for the exhumation of the Cretaceous Sanbagawa high-P/T metamorphic belt. J. Metamorph. Geol. 26, 583–602. Aoki, K., Isozaki, Y., Yamamoto, S., Maki, K., Yokoyama, T., Hirata, T., 2012. Tectonic erosion in a Pacific-type orogen: detrital zircon response to Cretaceous tectonics in Japan. Geology 40, 1087–1090. Aoki, K., Isozaki, Y., Kokufuda, D., Sato, T., Yamamoto, A., Maki, K., Sakata, S., Hirata, T., 2014. Provenance diversification within an arc-trench system induced by batholith development: the Cretaceous Japan case. Terra Nova 26, 139–149. Aoya, M., Endo, S., Mizukami, T., Wallis, S.R., 2013. Paleo-mantle wedge preserved in the Sambagawa high-pressure metamorphic belt and the thickness of forearc continental crust. Geology 41, 451–454. Awan, M.A., Kimura, K., 1996. Thermal structure and uplift of the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan: an illite crystallinity and illite b0 lattice spacing study. Isl. Arc 5, 69–88. Chen, M., Sun, M., Cai, K., Buslov, M.M., Zhao, G., Jiang, Y., Rubanova, E.S., Kulikova, A.V., Voytishek, E.E., 2016. The early Paleozoic tectonic evolution of the Russian Altai: implications from geochemical and detrital zircon U-Pb and Hf isotopic studies of meta-sedimentary complexes in the Charysh–Terekta–Ulagan–Sayan suture zone. Gondwana Res. 34, 1–15. Corfu, F., Hanchar, J.M., Hoskin, P.W.O., Kinny, P., 2003. Atlas of zircon textures. In: Manchar, J.M., Hoskin, P.W.O., (Eds.), Zircon, Reviews of Mineralogy and Geochemistry, vol. 53, pp. 469–500. Domeier, M., Shephard, G.E., Jakob, J., Gaina, C., Doubrovine, P.V., Torsvik, T.H., 2017. Intraoceanic subduction spanned the Pacific in the Late Cretaceous-Paleocene. Sci. Adv. 3, eaa02303. Dickinson, W.R., Beard, L.S., Brakenridge, G.R., Erjavec, J.L., Ferguson, R.C., Inman, K.F., Knepp, R.A., Lindberg, F.A., Ryberg, P.T., 1983. Provenance of North American Phanerozoic sandstones in relation to tectonic setting. Geol. Soc. Am. Bull. 94, 222–235. Feng, R., Kerrich, R., 1990. Geochemistry of fine-grained elastic sediments in the Archean Abitibi greenstone belt, Canada: implications for provenance and tectonic setting. Geochim. Cosmochim. Acta 54, 1061–1081. Fujisaki, W., Isozaki, Y., Maki, K., Sakata, S., Hirata, T., Maruyama, S., 2014. Age spectra of detrital zircon of the Jurassic clastic rocks of the Mino-Tanba AC belt in SW Japan: constraints to the provenance of the mid-Mesozoic trench in East Asia. J. Asian Earth Sci. 88, 62–73. Fukuchi, R., Fujimoto, K., Kameda, J., Hamahashi, M., Yamaguchi, A., Kimura, G., Hamada, Y., Hashimoto, Y., Kitamura, Y., Saito, S., 2014. Changes in illite crystallinity within an ancient tectonic boundary thrust caused by thermal, mechanical, and hydrothermal effects: an example from the Nobeoka Thrust, southwest Japan. Earth Planets Space 66. https://doi.org/10.1186/1880-5981-66-116. Geological Survey of Japan, AIST, 2018. Seamless digital geological map of Japan 1: 200, 000. Jan 10, 2018 version. Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology. Gradstein, F.M., Ogg, J.G., Schmitz, M.D., Ogg, G.M., 2012. A Geologic Time Scale 2012. Elsevier, Amsterdam. Hamahashi, M., Hamada, Y., Yamaguchi, A., Kimura, G., Fukuchi, R., Saito, S., Kameda, J., Kitamura, Y., Fujimoto, K., Hashimoto, Y., 2015. Multiple damage zone structure of an exhumed seismogenic megasplay fault in a subduction zone – a study from the Nobeoka Thrust Drilling Project. Earth Planets Space 67. https://doi.org/10.1186/ s40623-015-0186-2.

Acknowledgments We would like to thank T. Danhara and H. Iwano of Kyoto Fission Track Co. Ltd, Japan, and T. Hirata of the University of Tokyo for support during zircon U–Pb analyses; T. Kurihara for support during radiolarian identification; R. Nohara for support during geochemical analyses; Y. Nakamura and S. Mitsuhashi for assistance during fieldwork and sampling; K. Tominaga for support during sandstone observations; and the laboratory for thin section preparation at the Geological Survey of Japan for their expertise. Thanks are due to Prof. 42

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Sanbagawa psammitic schists (Oboke unit) and the Cretaceous Shimanto sandstones in Shikoku, Southwest Japan and its geologic significance. Isl. Arc 8, 373–382. Kiminami, K., Matsuura, T., Iwata, T., Miura, K., 1998. Relationship between sandstone composition in the Cretaceous Shimanto Supergroup, eastern Shikoku, and Cretaceous arc volcanism. J. Geol. Soc. Jpn. 104, 314–326 (in Japanese with English abstract). Kiminami, K., 2010. Parentage of low-grade metasediments in the Sanbagawa belt, eastern Shikoku, Southwest Japan, and its geotectonic implications. Isl. Arc 19, 530–545. Kimura, G., Yamaguchi, A., Hojo, M., Kitamura, Y., Kameda, J., Ujiie, K., Hamada, Y., Hamahashi, M., Hina, S., 2012. Tectonic mélange as fault rock of subduction plate boundary. Tectonophysics 568–569, 25–38. Kimura, G., Hamahashi, M., Okamoto, S., Yamaguchi, A., Kameda, J., Raimbourg, H., Hamada, Y., Yamaguchi, H., Shibata, T., 2013. Hanging wall deformation of a seismogenic megasplay fault in an accretionary prism: the Nobeoka Thrust in southwestern Japan. J. Struct. Geol. 52, 136–147. Kinoshita, O., 2002. Possible manifestations of slab window magmatisms in Cretaceous southwest Japan. Tectonophysics 344, 1–13. Kishi, T., Imaoka, T., Kochihira, H., Nishimura, Y., Itaya, T., 2007. K-Ar ages of Cretaceous Kibe cauldron and related rocks in Yamaguchi Prefecture; spatiotemporal variation of Cretaceous volcano-plutonism in western Chugoku district, SW Japan. J. Geol. Soc. Jpn. 113, 479–491 (in Japanese with English abstract). Kitamura, Y., Sato, K., Ikesawa, E., Ohmori-Ikehara, K., Kimura, G., Kondo, H., Ujiie, K., Onishi, C.T., Kawabata, K., Hashimoto, Y., Mukoyoshi, H., Masago, H., 2005. Mélange and its seismogenic roof décollement: a plate boundary fault rock in the subduction zone – an example from the Shimanto Belt, Japan. Tectonics 24, TC5012. https://doi.org/10.1029/2004TC001635. Knittel, U., Suzuki, S., Nishizawa, N., Kimura, K., Tsai, W.-L., Lu, H.-Y., Ishikawa, Y., Ohno, Y., Yanagida, M., Lee, Y.-H., 2014. U-Pb ages of detrital zircons from the Sanbagawa Belt in western Shikoku: additional evidence for the prevalence of Kate Cretaceous protoliths of the Sanbagawa Metamorphics. J. Asian Earth Sci. 96, 148–161. Kochi Prefecture, 1961. Geological and Mineral Resources Map of Kochi Prefecture, 1:200, 000 with Explanatory Text (in Japanese). Kondo, H., Kimura, G., Masago, H., Ohmori-Ikesawa, K., Kitamura, Y., Ikesawa, E., Sakaguchi, A., Yamaguchi, A., Okamoto, S., 2005. Deformation and fluid flow of a major out-of-sequence thrust located at seismogenic depth in an accretionary complex: Nobeoka Thrust in the Shimanto Belt, Kyushu, Japan. Tectonics 24, TC6008. https://doi.org/10.1029/2004TC001655. Kouketsu, Y., Mizukami, T., Mori, H., Endo, S., Aoya, M., Hara, H., Nakamura, D., Wallis, S., 2014. A new approach to develop the Raman carbonaceous material geothermometer for low-grade metamorphism using peak width. Isl. Arc 23, 33–50. Kumon, F., Inouchi, Y., 1976. Stratigraphy and sedimentological studies of the Paleogene system of the Shimanto complex in the Shishikui-cho area in Tokushima Prefecture, the northeastern part of the Muroto Peninsula. J. Geol. Soc. Jpn 82, 383–394 (in Japanese with English abstract). Kumon, F., Bessho, T., Roser, B.P., 2012. Modal and chemical characteristics of the coarse clastic rocks from the Shimanto terrane in Kii Peninsula, Southwest Japan. Assoc. Geol. Collab. Japan, Monogr. 59, 193–216 (in Japanese with English abstract). Long, X., Yuan, C., Sun, M., Safonova, I., Xiao, W., Wang, Y., 2012. Geochemistry and UPb detrital zircon dating of Paleozoic graywackes in East Junggar, NW China: insights into subduction–accretion processes in the southern Central Asian Orogenic Belt. Gondwana Res. 21, 637–653. Ludwig, K.R., 2012. User's Manual for Isoplot 3.75. Special Publication. no. 5, Berkeley Geochronology Center, Berkeley, CA. Maruyama, S., Isozaki, Y., Kimura, G., Terabayashi, M., 1997. Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present. Isl. Arc 6, 121–142. Matsuda, T., Isozaki, Y., 1991. Well-documented travel history of Mesozoic pelagic chert in Japan: from remote ocean to subduction zone. Tectonics 10, 475–499. Matsumura, M., Hashimoto, Y., Kimura, G., Ohmori-Ikehara, K., Enjohji, M., Ikesawa, E., 2003. Depth of oceanic-crust underplating in a subduction zone: inferences from fluid-inclusion analyses of crack-seal veins. Geology 31, 1005–1008. McLennan, S.M., 1989. Rare earth elements in sedimentary rocks: influence of provenance and sedimentary processes. In: Lipinand, B.R., Mckay, G.A. (Eds.), Geochemistry and Mineralogy of Rare Earth Elements. Reviews in Mineralogy 21, Mineralogical Society of America, pp.169–200. McLennan, S.M., Taylor, S.R., Mcculloch, M.T., Maynard, J.B., 1990. Geochemical and Nd-Sr isotopic composition of deep-sea turbidites: crustal evolution and plate tectonic associations. Geochim. Cosmochim. Acta 54, 2015–2050. Mori, K., Taguchi, K., 1988. Examination of the low-grade metamorphism in the Shimanto Belt by vitrinite reflectance. Modern Geol. 12, 325–339. Mukoyoshi, H., Hara, H., Ohmori-Ikehara, K., 2007. Quantitative estimation of temperature conditions for illite crystallinity: comparison to vitrinite reflectance from the Chichibu and Shimanto accretionary complexes, eastern Kyushu, Southwest Japan. Bull. Geol. Survey Japan 58, 23–31 (in Japanese with English abstract). Mukoyoshi, H., Hirono, T., Hara, H., Sekine, K., Tsuchiya, N., Sakaguchi, A., Soh, W., 2009. Style of fluid flow and deformation in and around an ancient out-of-sequence thrust: an example from the Nobeoka Tectonic Line in the Shimanto accretionary complex, Southwest Japan. Isl. Arc 18, 331–351. Nara, M., Ikari, Y., 2011. “Deep-sea bivalvian highways”: an ethological interpretation of branched Protovirgularia of the Palaeogene Muroto-Hanto Group, southwestern Japan. Palaeogeogr. Palaeoclimatol. Palaeoecol. 305, 250–255. Nigrini, C., Sanfilippo, A., 2001. Cenozoic radiolarian stratigraphy for low and middle latitudes. ODP Technical Note 27. Nigrini, C., Sanfilippo, A., Moore, T.J.Jr., 2005. Cenozoic radiolarian biostratigraphy: a

Hara, H., Kimura, K., 2008. Metamorphic and cooling history of the Shimanto accretionary complex, Kyushu, Southwest Japan: implications for the timing of out-ofsequence thrusting. Isl. Arc 17, 546–559. Hara, H., Kurihara, T., 2010. Tectonic evolution of low-grade metamorphosed rocks of the Cretaceous Shimanto accretionary complex, Central Japan. Tectonophysics 485, 52–61. Hara, K., Hara, H., 2016. Late Cretaceous and Eocene radiolarians from the Shimanto accretionary complex distributed around the Aki Tectonic Line. In: Abstract Volume of the 123rd Annual Meeting of the Geological Society of Japan, p. 255 (in Japanese). Hara, H., Nakamura, Y., Hara, K., Kurihara, T., Mori, H., Iwano, H., Danhara, T., Sakata, S., Hirata, T., 2017. Detrital zircon multi-chronology, provenance, and low-grade metamorphism of the Cretaceous Shimanto accretionary complex, eastern Shikoku, Southwest Japan: tectonic evolution in response to igneous activity within a subduction zone. Isl. Arc 26, e12218. https://doi.org/10.1111/iar.12218. Hashimoto, H., Ishida, K., Yamazaki, T., Tsujino, Y., Kozai, T., 2015. Revised radiolarian zonation of the Upper Cretaceous Izumi inter-arc basin (SW Japan). Rev. Micropaléontol. 58, 29–50. Hollis, C.J., 1997. Cretaceous-Paleocene radiolarian from eastern Marlborough, New Zealand. Institute of Geological and Nuclear Sciences, Monograph 17, Lower Hutt. 152 pp. Hollis, C.J., Kimura, K., 2001. A unified radiolarian zonation for the Late Cretaceous and Paleocene of Japan. Micropaleontology 47, 235–255. Iida, K., Iwamori, H., Orihashi, Y., Park, T., Jwa, Y.J., Kwon, S.T., Danhara, T., Iwano, H., 2015. Tectonic reconstruction of batholith formation based on the spatiotemporal distribution of Cretaceous-Paleogene granitic rocks in southwestern Japan. Isl. Arc 24, 205–220. Iijima, S., Sawada, Y., Sakimiya, T., Imaoka, T., 1985. Cretaceous to Paleogene Magmatism in the Chugoku and Shikoku Districts, Japan Earth Science (Chikyu Kagaku), 39, 372 384 (in Japanese with English abstract). Iizumi, S., Imaoka, T., Kagami, H., 2000. Sr–Nd isotope ratios of gabbroic and dioritic rocks in a Cretaceous-Paleogene granite terrain, Southwest Japan. Isl. Arc 9, 113–127. Ikesawa, E., Kimura, G., Sato, K., Ikehara-Ohmori, K., Kitamura, Y., Yamaguchi, A., Ujiie, K., Hashimoto, Y., 2005. Tectonic incorporation of the upper part of oceanic crust to overriding plate of a convergent margin: An example from the Cretaceous–early Tertiary Mugi Mélange, the Shimanto Belt Japan. Tectonophysics 401, 217–230. Imai, I., Teraoka, Y., Okumura, K., 1971. Geologic structure and metamorphic zonation of the northeastern part of the Shimanto terrane in Kyushu, Japan. J. Geol. Soc. Japan 77, 207–220 (in Japanese with English abstract). Imaoka, T., Ohira, T., Sawada, Y., Itaya, T., 1994. Radiometric ages of Cretaceous to Tertiary igneous rocks from Chugoku and Shikoku districts, Southwest Japan. The bulletin of Research Institute of Natural Sciences. Okayama Univ. 20, 3–57 (in Japanese with English abstract). Imaoka, T., Kiminami, K., Nishida, K., Takemoto, M., Ikawa, T., Itaya, T., Kagami, H., Iizumi, S., 2011. K-Ar age and geochemistry of the SW Japan Paleogene cauldron cluster: implications for Eocene-Oligocene thermo-tectonic reactivation. J. Asian Earth Sci. 40, 509–533. Ishida, K., Hashimoto, H., 1998. Upper Cretaceous radiolarian biostratigraphy in selected chert-clastic sequences of the North Shimanto Terrane, East Shikoku. News Osaka Micropaleontol. Spec. Vol. 11, 211–225. Ishihama, S., Kiminami, K., 2000. Petrofacies units on the basis of chemical composition of sandstone and shale in the Southern Chichibu Belt and the Northern Shimanto Belt, eastern Shikoku, Japan. Mem. Geol. Soc. Japan 57, 97–106 (in Japanese with English abstract). Isozaki, Y., Itaya, T., 1990. Chronology of Sanbagawa metamorphism. J. Metamorph. Geol. 8, 401–411. Isozaki, Y., Aoki, K., Nakama, T., Yanai, S., 2010. New insight into a subduction- related orogen: reappraisal of geotectonic framework and evolution of the Japanese Islands. Gondwana Res. 18, 82–105. Iwamori, H., 2000. Thermal effects of ridge subduction and its implications for the origin of granitic batholith and paired metamorphic belts. Earth Planet. Sci. Lett. 181, 131–144. Iwano, H., Orihashi, Y., Hirata, T., Ogasawara, M., Danhara, T., Horie, K., Hasebe, H., Sueoka, S., Tamura, A., Hayasaka, Y., Katsube, A., Ito, H., Tani, K., Kimura, J., Chang, Q., Kouchi, Y., Haruta, Y., Yamamoto, K., 2013. An inter-laboratory evaluation of OD3 zircon for use as a secondary U-Pb dating standard. Isl. Arc 22, 382–394. Jackson, S.E., Pearson, N.J., Griffin, W.L., Belousova, E.A., 2004. The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U-Pb zircon geochronology. Chem. Geol. 211, 47–69. Joo, Y.J., Lee, Y.L., Hisada, K., 2007. Provenance of Jurassic accretionary complex: Mino terrane, inner zone of south-west Japan –implications for palaeogeography of eastern Asia. Sedimentology 54, 515–543. Kagami, H., Iizumi, S., Tainosho, Y., Owada, M., 1992. Spatial variations of Sr and Nd isotope ratios of Cretaceous-Paleogene granitoid rocks, Southwest Japan Arc. Contrib. Miner. Petrol. 112, 165–177. Kano, H., Yamamoto, K., Okamura, M., 2003. Lithostratigraphy of the Domeki Formation in the Nakamura and Sukumo Cities, southwestern Shikoku, and its depositional setting as a slope-basin deposit. Res. Rep. Kochi Univ. Nat. Sci. 52, 1–24 (in Japanese with English abstract). Katto, J., Tashiro, M., 1979. A study on the molluscan fauna of the Shimanto Terrain Southwest Japan, Part 2, Bivalve fossils from the Muroto-hanto Group in Kochi prefecture, Shikoku. Res. Rep. Kochi Univ. Nat. Sci. 28, 1–11. Kiminami, K., Miyashita, S., Kawabata, K., 1994. Ridge collision and in situ greenstones in accretionary complexes: an example from the Late Cretaceous Ryukyu Islands and southwest Japan margin. Isl. Arc 3, 103–111. Kiminami, K., Hamasaki, A., Matsuura, T., 1999. Geochemical contrast between the

43

Journal of Asian Earth Sciences 170 (2019) 29–44

H. Hara, K. Hara

Shikoku, Japan –evolution of Cretaceous to Miocene accretionary prism. Modern Geol. 12, 25–46. Taira, A., Ogawa, Y., 1991. Cretaceous to Holocene forearc evolution in Japan and its implication to crustal dynamics. Episodes 14, 205–212. Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell Scientific Publications, Oxford. Terabayashi, M., Okamoto, K., Yamamoto, H., Kaneko, Y., Ota, T., Maruyama, S., Katayama, I., Komiya, T., Ishikawa, A., Anma, R., Ozawa, H., Windley, B.F., Liou, J.G., 2005. Accretionary complex origin of the mafic-ultramafic bodies of the Sanbagawa belt, central Shikoku, Japan. Int. Geol. Rev. 47, 1058–1073. Terakado, Y., Nohda, S., 1993. Rb–Sr dating of acidic rocks from the middle part of the Inner Zone of southwest Japan: tectonic implications for the migration of the Cretaceous to Paleogene igneous activity. Chem. Geol. 109, 69–87. Thió-Henestrosa, S., Martín-Fernández, J.A., 2005. Dealing with compositional data: the freeware CoDaPack. Math. Geol. 37, 773–793. Toriumi, M., Teruya, J., 1988. Tectono-metamorphism of the Shimanto Belt. Modern Geol. 12, 303–324. Tsutsumi, Y., Miyashita, A., Terada, K., Hidaka, H., 2009. SHRIMP U-Pb dating of detrital zircons from the Sanbagawa Belt, Kanto Mountains, Japan: need to revise the framework of the belt. J. Min. Petrol. Sci. 104, 12–24. Ujiie, K., Kameyama, M., Yamaguchi, A., 2010. Geological record of thermal pressurization and earthquake instability of subduction thrusts. Tectonophysics 485, 260–268. von Huene, R., Scholl, D.W., 1991. Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust. Rev. Geophys. 29, 279–316. Wakita, K., 2013. Geology and tectonics of Japanese islands: a review – the key to understanding the geology of Asia. J. Asian Earth Sci. 72, 7587. Wakita, K., 2015. OPS mélange: a new term for mélanges of convergent margins of the world. Int. Geol. Rev. 57, 529–539. Wallis, R.S., Okudaira, T., 2016. Paired metamorphic belts of SW Japan: the geology of the Sanbagawa and Ryoke metamorphic belts and the Median Tectonic Line. In: Moreno, T., Wallis, R., Kojima, T., Gibbsons, E. (Eds.), The Geology of Japan. Geological Society of London, pp. 101–124. Whittaker, J.M., Müller, R.D., Leitchenkov, G., Stagg, H., Sdrolias, M., Gaina, C., Goncharov, A., 2007. Major Australian-Antarctic plate reorganization at HawaiianEmperor bend time. Science 318, 83–86. Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., von Quadt, A., Roddick, J.C., Spiegel, W., 1995. Three natural zircon standards for U-Th–Pb, Lu–Hf, trace element and REE analyses. Geostand. Geoanal. Res. 19, 1–23. Yamaguchi, A., Ujiie, K., Nakai, S., Kimura, G., 2012. Sources and physicochemical characteristics of fluids along a subduction-zone megathrust: a geochemical approach using syn-tectonic mineral veins in the Mugi mélange, Shimanto accretionary complex. Geochem. Geophys. Geosyst. 13, Q0AD24. https://doi.org/10.1029/ 2012GC004137. Yamamoto, T., 2003. Lithofacies and eruption ages of Late Cretaceous caldera volcanoes in the Himeji-Yamasaki district, southwest Japan: implications for ancient large-scale felsic arc volcanism. Isl. Arc 12, 294–309. Yamazaki, T., Yokota, Y., Okumura, K., 1993. Cretaceous radiolarians from the eastern part of Aki City, Kochi Prefecture–with reference to the boundary between the North and South Shimanto Subbelts. News Osaka Micropaleontol. Spec. Vol. 9, 215–223 (in Japanese with English abstract). Yoshida, K., Takahashi, G., Imaoka, T., 2009. The Cretaceous Shirakiyama cauldron in northwest Yamaguchi Prefecture, Japan: an example of asymmetric subsidence. J. Geol. Soc. Jpn. 115, 643–657 (in Japanese with English abstract).

magnetobiostratigraphic chronology of Cenozoic sequences from ODP sites 1218, 1219, and 1220, equatorial Pacific. In: Ailson, P.A., Lyle, M., Firth, J.V., (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results 199, pp.1–76. Nishida, K., Imaoka, T., Iizumi, S., 2005. Cretaceous-Paleogene magmatism in the central San-in district, Southwest Japan: an examination based on Rb–Sr isochron ages. J. Geol. Soc. Jpn 111, 123–140 (in Japanese with English abstract). Ochi, M., Mamiya, T., Kusuhashi, N., 2014. A reexamination of the stratigraphy of the Miocene Kuma Group, Shikoku, SW Japan: the “Shimosakabatoge Formation” and the “Tomishige Formation”. J. Geol. Soc. Jpn 120, 165–179 (in Japanese with English abstract). Ogawauchi, Y., Iwamatsu, A., Tanabe, A., 1984. Stratigraphy and geologic structures of the Shimanto Supergroup in the northeastern part of Nobeoka City, Miyazaki Prefecture, Japan. Reports of the Faculty of Science, Kagoshima University, no.17, 67–88 (in Japanese with English abstract). Okada, H., Okamura, M., 1980. Calcareous nannofossils obtained from the Shimanto Belt in Kochi Prefecture. In: Taira, A., Tashiro, M. (Eds.), Geology and Paleontology of the Shimanto Belt in Honor of Prof. Jiro Katto, Rinyakosaikai Press, Kochi, pp.147–152 (in Japanese with English abstract). Onishi, C.T., Kimura, G., 1995. Change in fabric of melange in the Shimanto Belt, Japan: change in relative convergence? Tectonics 14, 1273–1289. https://doi.org/10.1029/ 95TC01929. Oyaizu, A., Miura, K., Tanaka, T., Hayashi, H., Kiminami, K., 2002. Geology and radiolarian ages of the Shimanto Supergroup, western Shikoku, Southwest Japan. J. Geol. Soc. Jpn 108, 701–720 (in Japanese with English abstract). Sakaguchi, A., 1996. High geothermal gradient with ridge subduction beneath Cretaceous Shimanto accretionary prism, southwest Japan. Geology 24, 795–798. Sakaguchi, A., 1999. Thermal structure and paleo-heat flow in the Shimanto accretionary prism, Southwest Japan. Isl. Arc 8, 359–372. Sato, D., Matsuura, H., Yamamoto, T., 2016. Timing of the Late Cretaceous ignimbrite flare-up at the eastern margin of the Eurasian Plate: new zircon U-Pb ages from the Aioi–Arima–Koto region of SW Japan. J. Volcanol. Geoth. Res. 310, 89–97. Seton, M., Müller, R.D., Zahirovic, S., Gaina, C., Torsvik, T., Shephard, G., Talsma, A., Gurnis, M., Turner, M., Maus, S., Chandler, M., 2012. Global continental and ocean basin reconstructions since 200 Ma. Earth Sci. Rev. 113, 212–270. Seton, M., Flament, N., Whittaker, J., Müller, R.D., Gurnis, M., Bower, D.J., 2015. Ridge subduction sparked reorganization of the Pacific plate-mantle system 60–50 million years ago. Geophys. Res. Lett. 42. https://doi.org/10.1002/2015GL063057. Sláma, J., Košler, J., Condon, D.J., Crowley, J.L., Gerdes, A., Hanchar, J.M., Horstwood, M.S.A., Morris, G.A., Nasdala, L., Norberg, N., Schaltegger, U., Schoene, B., Tubrett, M.N., Whitehouse, M.J., 2008. Plešovice zircon – A new natural reference material for U-Pb and Hf isotope microanalysis. Chem. Geol. 249, 1–35. Suyari, K., Yamazaki, T., 1987. Boundary between the North and South Shimanto Subbelts in Tokushima Prefecture-The Aki Tectonic Line reconsidered. J. Sci. Univ. Tokushima 20, 37–46 (in Japanese with English abstract). Suyari, K., Yamazaki, T., 1988. Microfossil age of the northern margin of the Shimanto South Subbelt in Shikoku. J. Sci. Univ. Tokushima 21, 107–133 (in Japanese with English abstract). Suzuki, H., Fukuda, O., 2012. Paleogene radiolarian fossils found from the Shimanto accretionary prism in the Kii Peninsula, Southwest Japan. Assoc. Geol. Collab. Japan, Monogr. 59, 237–247 (in Japanese with English abstract). Taira, A., Tashiro, M., Okamoto, M., Katto, J., 1980. The geology of the Shimanto Belt in Kochi Prefecture, Shikoku, Japan. In: Taira, A., Tashiro, M. (Eds.), Geology and Paleontology of the Shimanto Belt in Honor of Prof. Jiro Katto, Rinyakosaikai Press, Kochi, pp. 319–389 (in Japanese with English abstract). Taira, A., Katto, J., Tashiro, M., Okamura, M., Kodama, K., 1988. The Shimanto Belt in

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