Tectonic evolution of low-grade metamorphosed rocks of the Cretaceous Shimanto accretionary complex, Central Japan

Tectonophysics 485 (2010) 52–61

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Tectonic evolution of low-grade metamorphosed rocks of the Cretaceous Shimanto accretionary complex, Central Japan Hidetoshi Hara a,⁎, Toshiyuki Kurihara b a b

Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan Graduate School of Science and Technology, Niigata University, 8050 Nino-cho, Ikarashi, Niigata 950-2181, Japan

a r t i c l e

i n f o

Article history: Received 3 September 2008 Received in revised form 23 September 2009 Accepted 22 November 2009 Available online 3 December 2009 Keywords: Low-grade metamorphism Illite crystallinity Illite K–Ar dating Kula–Pacific ridge Shimanto Belt

a b s t r a c t We reconstructed the tectono-metamorphic evolution of the low-grade metamorphosed Cretaceous Shimanto accretionary complex in the Kanto Mountains, Central Japan, based on radiolarian fossils, metamorphic temperatures derived from illite crystallinity analysis, and timing of metamorphism based on illite K–Ar dating. The accretionary age of the Kobotoke Group is Turonian to Maastrichtian (66–94 Ma), based on radiolarian fossils. Illite crystallinity data indicate metamorphic temperatures of approximately 300 °C. The illite K–Ar ages constrain the timing of metamorphism to the Middle Eocene around 40 Ma. Combining our results and previous study, we defined two types of low-grade metamorphism within Cretaceous Shimanto accretionary complex of the Kanto Mountains. The early metamorphism, in excess of 300 °C, was related to the uplift of the Sambagawa metamorphic rocks, in turn associated with the subduction of the Kula–Pacific ridge during the Late Cretaceous (65–75 Ma). This metamorphism is recorded in the Otaki Group within the northernmost part of the complex in the Kanto Mountains. Subsequent to the subduction of the Kula–Pacific ridge, a later period of metamorphism, recorded in the Kobotoke Group, is characterized by the thermal effects of the subduction of the young, hot Pacific Plate during the Middle Eocene. The effect of the early metamorphism occurred synchronously 500 km along the trench from Southwest to Central Japan. The later metamorphism occurred at 50 Ma in Kyushu and Shikoku of Southwest Japan, and at 40 Ma in the Kanto Mountains of Central Japan. This difference in the timing of metamorphism between Southwest and Central Japan is explained by the northward migration of the young, hot Pacific Plate. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The Cretaceous Shimanto accretionary complex (e.g., Taira et al., 1988) is widely exposed from Southwest to Central Japan (Fig. 1a), where it consists of low-grade metamorphic rocks subjected to prehnite– actinolite and greenschist facies metamorphism. These rocks include the Makimine Formation in Kyushu (Miyazaki and Okumura, 2002) and the Hanazono Formation upon Kii Peninsula (Kurimoto, 1993; Awan and Kimura, 1996). In the Kanto Mountains, Central Japan, the Otaki and Kobotoke groups contain low-grade metamorphosed rocks of the Cretaceous Shimanto accretionary complex (Fig. 1). Metamorphism of the Otaki Group is estimated to be in excess of 300 °C and greater than 270 MPa during 65–75 Ma, based on micro-thermometry of fluid inclusions, illite crystallinity analyses and illite K–Ar dating (Hara and Hisada, 2005, 2007), however, that of the Kobotoke Group has yet to be analyzed in detail. The low-grade metamorphosed rocks within the accretionary complex are important in terms of understanding the tectonic linkages between the accretionary complex

⁎ Corresponding author. Tel.: + 81 298 61 3981; fax: +81 298 61 3653. E-mail address: [email protected] (H. Hara). 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.11.017

and metamorphism associated with subduction of the oceanic plate beneath the continental plate. In particular, the Cretaceous Shimanto accretionary complex in the Kanto Mountains is characterized by two low-grade metamorphosed rocks within the Otaki and Kobotoke groups. Analyses of two low-grade metamorphisms recorded in the Otaki and Kobotoke groups possibly lead to oceanic plate subduction history. The aim of this paper is to reconstruct the tectonic evolution of the low-grade metamorphosed rocks of the Kobotoke Group for understanding of two low-grade metamorphisms within the Cretaceous Shimanto accretionary complex. We estimate the maximum metamorphic temperatures based on illite crystallinity, and the timing of metamorphism based on illite K–Ar dating. We also report the first occurrence of radiolarian fossils from argillaceous rock in a mélange unit within the group. Based on the combined results of illite crystallinity analysis, illite K–Ar dating, and new fossil data, we propose the accretion and metamorphic history of the low-grade metamorphosed Cretaceous Shimanto accretionary complex, as recorded by the Kobotoke Group of the Kanto Mountains. We also discuss the significance of the low-grade metamorphism of the Otaki and Kobotoke groups in relation to Kula–Pacific ridge subduction and migration of the Pacific Plate during the Late Cretaceous to Paleogene.

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Fig. 1. (a) Location map of the Shimanto accretionary complex in Southwest and Central Japan. (b) Simplified geological map of the Kanto Mountains, Central Japan after Hara et al. (1998) and Yagi (2000). IKL: Itsukaichi–Kawakami Line, KF: Kitousan Fault, TF: Tsurukawa Fault, MF: Matsuhime Fault, TAL: Tonoki–Aikawa Line, C: Coherent unit, M: Mélange unit. The black rectangle indicates the area shown in Fig. 5.

2. Geological outline The Kanto Mountains, Central Japan, contain a Mesozoic island arc system that consists of (from northeast to southwest) Sambagawa metamorphic rocks, the Jurassic Chichibu accretionary complex with Cretaceous strike–slip basin sediments, and the Cretaceous to Paleogene Shimanto accretionary complex (Fig. 1b). The Cretaceous Shimanto accretionary complex is subdivided into the Otaki, Ogochi, and Kobotoke groups (Fig. 1b). The Kobotoke Group is in fault contact with the Ogochi Group to the north along the Itsukaichi–Kawakami Line, and with the Eocene to Oligocene Shimanto accretionary complex (Sagamiko Group) to the south along the Matsuhime Fault (Yagi, 2000). The Otaki Group is mainly composed of phyllite, sandstone, and argillaceous mélange-type rocks, with blocks of chert, limestone, tuff, and basalt, and also is characterized by intensive deformation and metamorphism (Hara and Hisada, 2007). The Otaki Group was affected by greenschist facies metamorphism (Fujimoto et al., 1950; Ogawa et al., 1988). Hara and Hisada (2007) estimated peak metamorphic temperatures for the Otaki Group in excess of 300 °C and fluid pressures greater than 270 MPa, based on the micro-thermometry of fluid inclusions within quartz veins and illite crystallinity analyses of argillaceous rocks. Illite K–Ar dating indicates an age of approximately 65– 75 Ma for this metamorphism (Hara and Hisada, 2005). The Otaki

Group occupies the northernmost part of the Shimanto accretionary complex in the Kanto Mountains, and is in fault contact with the Ogochi Group to the south (Fig. 1). The Ogochi Group is composed of six units from northeast to southwest, all of which are characterized by either coherent turbidite unit (the coherent unit) or argillaceous mélange-type rocks (the mélange unit) (Iyota et al., 1994). All six units are bounded by highangle reverse faults and are imbricated. The depositional ages of the Ogochi Group range from late Albian to Campanian, and ages younging southwestward. The Kobotoke Group is also divided into the coherent unit and the mélange unit (see Fig. 1). According to Yagi (2000) and Sakai (2007), the coherent unit is composed of sandstone, shale, and interbedded sandstone and shale, named the Bonborigawa and Kosuge units. The mélange unit is subdivided into the Miyama, Uzuhiki, and Kobuse units, comprising shale and broken beds of sandstone, with minor mélange that consists of basalt and chert blocks in an argillaceous matrix; the matrix is generally phyllitic and foliated. These three units are stratigraphically repeated due to thrusting. The Kobotoke Group was subjected to prehnite–pumpellyite facies to greenschist facies metamorphism (Toriumi and Teruya, 1988) and ductile deformation (Ogawa et al., 1988; Fabbri et al., 1990); however, the low-grade metamorphism of this group has yet to be analyzed in detail because of a scarcity of suitable basaltic rocks.

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3. Texture of low-grade metamorphosed rocks

4. Radiolarian fauna and age

Feldspathic sandstone is the predominant lithology of the Kobotoke Group (Sakai, 1987). Deep-burial diagenetic textures (see Liu, 2002; Egawa and Lee, 2008) are commonly observed in sandstones of the group. Detrital grains show significant mechanical compaction, and contain fractures and textures indicative of pressure-solution seams, including sutured grain boundaries (arrowhead in Fig. 2a). Illite (mica) rims are commonly observed around detrital grains (arrowhead in Fig. 2b). The foliation is largely defined by the preferred orientation of clay minerals and dark-colored pressure-solution seams (arrowheads in Fig. 2c). The boundary between lithic fragments and matrix is generally unclear. Argillaceous rocks are usually phyllitic with a foliation. The foliation is defined by the preferred orientation of clay minerals, segregation of layers of recrystallized quartz and mica-rich layers, and darkcolored pressure-solution seams (Fig. 2d). Silt-size detrital grains composed mostly of quarts are observed in phyllitic shale, occasionally showing asymmetric fabrics around quartz grains due to shear deformation. Chert and basaltic rocks occur as small blocks within argillaceous rocks of the mélange units. The chert, consisting of recrystallized microcrystalline quartz, is usually gray, green, or red in color, and contains radiolarian fossils deformed into ellipsoidal shapes. The basaltic rocks consist of massive lava, volcaniclastics, and basalt tuff, and contain veins of epidote, chlorite, and prehnite, as well as metamorphic minerals such as actinolite and pumpellyite (Fig. 3). This metamorphic mineral assemblage indicates sub-greenschist to greenschist facies metamorphism (Fettes and Desmons, 2007).

Previous studies have reported radiolarian fossils from shale within the coherent units of the Kobotoke Group. For example, radiolarian fossils within shale from the Bonborigawa Unit indicate a Campanian age (Sakai, 1987). Takahashi and Ishii (1995) and Yagi (2000) reported Turonian to Maastrichtian radiolarian ages for shale from the Kosuge Unit; however, radiolarian fossils have yet to be obtained from the mélange unit. The present study describes the first occurrence of radiolarian fossils from silty shale within a mélange unit (Kobuse Unit) in the Kobotoke Group (Fig. 4; see Fig. 5a. for sample locations). The radiolarian fossil-bearing shale also contains chert blocks. The siliceous residue remaining after the hydrofluoric acid (HF) etching of samples of silty shale contains rare radiolarian shells, strongly recrystallized by low-grade metamorphism. Most multi-segmented nassellarians do not retain their external shell shape and surface structures, thereby hampering detailed taxonomic analysis. After careful sample preparation for scanning electron microscope (SEM) observations, involving 15 repetitions of HF etching, we identified the following species (Fig. 4): Dictyomitra multicostata Zittel, Dictyomitra sp. aff. Dictyomitra koslovae Foreman, Amphipyndax stocki (Campbell and Clark), and Pseudoaulophacus floresensis Pessagno. Dictyomitra multicostata, first described from the Campanian in Germany (Zittel, 1876), has been reported from Upper Cretaceous strata in many parts of the world (e.g., Foreman, 1968; Pessagno, 1976; Yamazaki, 1987; Popova-Goll et al., 2005). Dictyomitra sp. aff. D. koslovae corresponds to the morphotype treated by Hollis and Kimura (2001) as

Fig. 2. Photomicrographs of clastic rocks, showing the low-grade metamorphosed textures of sandstone and phyllite. All scale bars are 1 mm. F: feldspar, R: rock fragment, Q: quartz, M: mica. (a–b) Sandstone under plane polarized light (a) and crossed polarized light (b). Arrowhead indicates sutured grain boundary and illite (mica) seams between grains, providing evidence of pressure-solution. (c) Foliated sandstone (crossed polarized light). The foliation (indicated by arrowheads) is defined by the preferred orientation of clay minerals and pressure-solution seams. (d) Sample of phyllite (KB-01) collected for K–Ar dating.

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Fig. 3. Photomicrographs of metamorphic minerals within veins within basaltic rocks. Scale bars are 0.2 mm. Act: actinolite, Chl: chlorite, Epi: epidote, Pmp: pumpellyite, Prh: prehnite. (a–b) Prehnite–pumpellyite vein in basalt viewed under plane polarized light (a) and crossed polarized light (b). (c–d) Epidote vein and chlorite–actinolite vein in basalt viewed under plane polarized light (c) and crossed polarized light (d).

Fig. 4. Selected age-diagnostic radiolarians recovered from silty shale of the mélange unit in the Kobotoke Group (see Fig. 5 for fossil locality). Scale bars A and B are 100 μm (bar A applies to 1–3; B applies to 4–7). 1–3: Dictyomitra multicostata Zittel. 4: Dictyomitra sp. aff. D. koslovae Foreman. 5–6: Amphipyndax stocki (Campbell and Clark). 7: Pseudoaulophacus floresensis Pessagno.

“Dictyomitra cf. koslovae,” which is distinguished from D. koslovae by a relatively smooth outline from the cephalis to the prominent segment, and less developed structures for subsequent segments. According to Hollis and Kimura's (2001) compilation of Upper Cretaceous biostratigraphic data for Japan, this morphotype ranges from the D. koslovae Interval Zone (Dk1: Santonian) to the Pseudotheocampe abschnitta Interval Zone (Pa: lower Maastrichtian), and is most common over the Santonian to Campanian. Pseudoaulophacus floresensis, the most representative species of pseudoaulophacids, ranges from the Amphipyndax pseudoconulus Zone to the Amphipyndax tylotus Zone, indicating a Campanian to Maastrichtian age (Sanfilippo and Riedel, 1985). Hollis and Kimura (2001) reported that this species occurs within the Santonian (Dk1) to lower Maastrichtian (Pa) zones mentioned above. For Amphipyndax species, we obtained only A. stocki: we did not recover other age-diagnostic and evolved forms such as A. pseudoconulus (Pessagno) and A. tylotus Foreman. The above data indicate a maximum age range of Santonian to early Maastrichtian for the analyzed radiolarian fauna. Fossil ages obtained from argillaceous rocks are usually interpreted to indicate the age of accretion, based on the ocean plate stratigraphy (Matsuda and Isozaki, 1991; Wakita and Metcalfe, 2005). The recovered radiolarian fossils indicate that the age of accretion of the mélange unit is Santonian to Campanian, coincident with the age of accretion of the coherent units. The Kobotoke Group yields an accretion age of Turonian to Maastrichtian without ages younging southward. We consider that internal structure of the Kobotoke Group presents repeat of mélange and coherent units controlled by decollment through underplating process during Late Cretaceous (Fig. 5b).

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Fig. 5. (a) Geological map of the Kobotoke and Sagamiko groups in the Kanto Mountains, showing illite crystallinity values. Symbols indicate sample localities; the accompanying numbers are IC values (Δ°2θ). Sample numbers (KB-01, KB-02, and KB-03) indicate those samples analyzed for illite K–Ar dating. (b) Cross-section along X–Y.

5. Illite crystallinity Illite crystallinity (IC) is determined from the Kübler index, which is the peak width at half maximum height of the 10 Å illite peak, as expressed in Δ°2θ (Kübler, 1968; Frey, 1987). The intensity of the preferred orientation of illite generally increases with decreasing IC values. The Kübler index is used to divide meta-sedimentary rocks into three zones: the diagenetic zone (N0.42 Δ°2θ), anchizone (0.25– 0.42 Δ°2θ), and epizone (b0.25 Δ°2θ). In the present study, we measured IC values from 34 samples of black pelitic rock collected from the Kobotoke Group, and from 14 samples of comparable lithologies collected from the Sagamiko Group. The samples were washed, crushed in a swing mill for 10 seconds, and passed through a 72-mesh sieve. Ten grams of the resulting powder were then suspended in a test tube. The ≤2 µm clay fraction was separated by gravity settling and concentrated by centrifuging before being pipetted onto two glass slides to make sedimented slides. The average thickness of clay on the slides was maintained between 5 and 10 mg/cm2 because clay thickness is known to affect the intensity of the crystallinity (Kisch, 1991). We employed a JEOL 8030 X-ray diffractometer housed at the Geological Survey of Japan (Tsukuba, Ibaraki, Japan) using the following measurement conditions: CuKα radiation at 40 kV and 40 mA, step scan speed of 0.01°2θ/s, divergence and scatter slits of 1°, receiving slit of 0.2 mm, and scan range of 6.5–10.5°2θ. Two slides of each sample were scanned to check for errors. Samples with IC val-

ues of N0.30 Δ°2θ were treated with ethylene glycol to remove the smectite peak that overlaps with the illite peak. For inter-laboratory calibration, Warr and Rice (1994) proposed the CIS (CrystallinityIndex Standard), which comprises standard samples used for calibration in studies of illite crystallinity. Based on measured values of CIS at the Geological Survey of Japan (GSJ), Hara and Kimura (2003) reported the following correlation equation: IC (CIS) = 1.55 IC (GSJ) − 0.07 (r = 0.99). Fig. 5 shows the obtained IC values plotted on a geological map of the study area. We focus on variations in IC along three transects from the Kobotoke Group to the Sagamiko Group (lines A–A′, B–B′, and C–C′ in Figs. 5 and 6). Most of the IC values determined for the

Fig. 6. North–south variations in illite crystallinity values along the transects A–A′, B–B′, and C–C′ shown in Fig. 5.

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Kobotoke Group fall within the range 0.22 to 0.31 Δ°2θ (epizone to anchizone, mean = 0.26 Δ°2θ, 1σ = 0.024), while the western part of the coherent units yields values between 0.42 and 0.60 (diagenetic zone, mean = 0.52 Δ°2θ, 1σ = 0.074). There are no meaningful horizontal variations in IC values within the mélange unit. IC values for the Sagamiko Group are higher than those for the Kobotoke Group. IC values for the Sagamiko Group range from 0.29 to 0.40 Δ°2θ (mean = 0.32 Δ°2θ, 1σ = 0.032). The difference in IC values between the Kobotoke Group and Sagamiko Group is estimated to be approximately 0.5–1.0 Δ°2θ. 6. Illite K–Ar dating For K–Ar dating of illite, we collected three samples of black phyllitic shale from the southernmost part of the mélange unit (Kobuse Unit; see Fig. 5 for sample locations). Fig. 3d shows a representative photomicrograph of the phyllitic shale used for dating. Samples of illite intended for K–Ar dating were prepared as follows. The rock sample was crushed using a jaw crusher and disk grinder before being passed through a 120–200 mesh sieve. The sieved fraction was then subjected to ultrasonic washing. Illite grains were concentrated using isodynamic magnetic separation and heavy liquid separation techniques, with hydrochloric acid treatment employed to dissolve chlorite. K–Ar dating was carried out at the Hiruzen Institute for Geology and Chronology (Okayama, Japan). The decay constant and isotopic abundance ratios used in the age calculation are after Steiger and Jäger (1977): λβ = 4.962 × 10− 10 y− 1, λε = 0.581 × 10− 10 y− 1, and 40 K/K = 1.167 × 10− 4 at.%, respectively. The obtained K–Ar ages are listed in Table 1. The K–Ar ages are 40.2 ± 0.89, 48.3 ± 1.1, and 38.4 ± 0.86 Ma, with a mean age of 42 Ma.

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Applied to the Kobotoke Group in the present study, this equation indicates a temperature for the anchizone of 266–302 °C. The high-temperature boundary of the anchizone in the Shimanto accretionary complex, as determined in the present study, is about 25 °C lower than the estimate reported by Underwood et al. (1993); however, the lower boundaries are similar between the two studies. Most of the IC values determined for the Kobotoke Group yield temperatures between approximately 290 and 310 °C (mean = 300 °C). The calculated temperature for the western part of the coherent unit (Kosuge Unit) is 245 °C (mean IC value = 0.52 Δ°2θ). Similarly, the temperature calculated for the Sagamiko Group is 285 °C (mean IC value = 0.33 Δ°2θ). Most of the Kobotoke Group has been subjected to peak metamorphic temperatures of around 300 °C. In contrast, temperature conditions estimated for the western coherent unit are disturbed around the Tsurukawa Fault, yielding relatively low temperatures of less than 250 °C. The Tsurukawa Fault has been active from the Miocene to recent (Murata et al., 1986). Yanai and Yamakita (1987) proposed that the Kobotoke Group was subjected to wrench tectonics associated with the development of faults such as the Tsurukawa Fault and Itsukaichi–Kawakami Line. It is also possible that IC values indicating low temperature around the western part of the coherent unit has been disturbed by wrench tectonic events. The Kofu Granodiorite intruded into the western part of the Shimanto accretionary complex during Middle Miocene (Fig. 1). According to Hara et al. (1998), the thermal influence by intrusion for IC values occurred within several km from this granodiorite body. IC values from the Kobotoke and Sagamiko groups in the study area indicate the lowgrade metamorphic condition without thermal effect of granodiorite intrusion. 8. Accretion and metamorphic history of the Kobotoke Group

7. Maximum metamorphic temperatures of the Kobotoke Group A quantitative estimation of the temperature conditions indicated by illite crystallinity data can be made based on the relationship between IC values and vitrinite reflectance data (Guthrie et al., 1986; Underwood et al., 1993; Kosakowski et al., 1999). The IC data obtained in the present study were converted into temperature values as follows. Mukoyoshi et al. (2007) described a relationship between illite crystallinity and mean random vitrinite reflectance (Rm%) based on data from nine localities in the Shimanto accretionary complex, coastal area of eastern Kyushu. Their analyses of illite crystallinity were performed at the Geological Survey of Japan using the same sample preparation and measurement conditions as those employed in the present study. The correlation between Rm and IC values indicates a linear regression equation of Rm (%) = 6.9–8.2 IC (Δ°2θ), with a correlation coefficient of 0.91. Using the equation of Sweeney and Burnham (1990) to convert vitrinite reflectance into temperature over a heating duration of 10 Ma, the temperature conditions represented by the illite crystallinity data are calculated using the following equation: T (°C) = 353 − 206 IC (Δ°2θ), with a correlation coefficient of 0.92 (Mukoyoshi et al., 2007).

The K–Ar ages obtained for the Kobotoke Group is clearly younger than the Turonian to Maastrichtian depositional age (65.5–93.5 Ma according to the time scale of Gradstein et al., 2004). The estimated metamorphic temperature of the Kobotoke Group is lower than the closure temperature of the K–Ar system in white mica, which is 350 °C (Jäger, 1979). For low-grade metamorphism, K–Ar ages are assumed to represent the timing of peak metamorphism (Takami and Itaya, 1996; Nishimura et al., 2000; Hara and Kimura, 2008). K–Ar ages are commonly affected by detrital mica that possess older K–Ar ages (Hunziker et al., 1986; Reuter and Dallmeyer, 1989; Itaya and Fukui, 1994; Nishimura et al., 2004). Hunziker et al. (1986) suggested that the influence of detrital mica is apparent in metapelite samples up to the metamorphic conditions of the anchizone–epizone boundary. Almost all of the illite crystallinity data obtained for the Kobotoke Group correspond to the epizone and anchizone (Figs. 5 and 6). Variations in K–Ar ages of the Kobotoke Group are considered to reflect the influence of detrital mica. The oldest K–Ar age of 48 Ma is possibly unsuitable in terms of interpreting the peak of low-grade metamorphism. We adopt two K–Ar ages of 38 and 40 Ma (mean = 39 Ma) as the peak metamorphic age in the study.

Table 1 K–Ar ages of illite-rich fractions separated from argillaceous rocks of the Kobotoke Group, Cretaceous Shimanto accretionary complex. The IC values were estimated from the same samples as those used for K–Ar dating. Sample number

IC value (Δ°2θ)

Radiogenic 40Ar (10− 8 cm3 STP/g)

Non-rad. (%)

KB-01

0.26

KB-02

0.27

KB-03

0.25

731.1 ± 7.2 728.4 ± 7.2 764.7 ± 7.8 761.6 ± 7.7 597.1 ± 6.1 599.8 ± 6.2

4.7 4.5 7.3 6.7 8.1 10

40

Ar

Potassium (wt.%)

Isotopic age (Ma)

Average age (Ma)

4.628 ± 0.093

40.26 ± 0.89 40.11 ± 0.88 48.4 ± 1.1 48.2 ± 1.1 38.34 ± 0.85 38.51 ± 0.86

40.2 ± 0.89

4.020 ± 0.080 3.971 ± 0.079

48.3 ± 1.1 38.4 ± 0.86

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Fig. 7. Timing of accretion and metamorphism of low-grade metamorphic rocks within the Kobotoke and Otaki groups. Bars labeled KB indicate the age of accretion of the Kobotoke Group during the Turonian to Maastrichtian. The age of accretion and metamorphism of the Otaki Group (OT), indicated by the dashed line, is based on Hara and Hisada (2007) and Hara et al. (2007). Open rectangles indicate temperature conditions based on illite crystallinity analysis and K–Ar metamorphic ages. The black dot indicates that the Otaki Group cooled below 260± 50 °C at 54–59 Ma, based on fission-track zircon ages (Hara et al., 2007).

Based on radiolarian fossils and K–Ar age data, we propose the following history of accretion and metamorphism by low-grade metamorphic rocks within the Kobotoke Group (Fig. 7). Radiolarian assemblage within shale indicates Turonian to Maastrichtian deposition and accretion ages (66–94 Ma). Following accretion, the Kobotoke Group was subjected to low-grade metamorphism, with peak metamorphic temperatures of around 300 °C during Middle Eocene around 40 Ma. 9. Significance of low-grade metamorphism recorded in the Cretaceous Shimanto accretionary complex Metamorphic age of the Kobotoke Group is estimated to be during 40 Ma, suggesting younger than that of the Otaki Group during 65– 75 Ma. We discuss significance of two types of low-grade metamorphism in the Cretaceous Shimanto accretionary complex, related to the Sambagawa metamorphism and history of oceanic plate subduction. The Sambagawa metamorphic rocks are typical high-P/T metamorphic rocks, considered to represent deep-level tectonics in the accretionary wedge (Wallis and Banno, 1990; Takasu et al., 1994). In particular, the chlorite zone of the Sambagawa metamorphic rocks is interpreted as the lowest grade in the Sambagawa metamorphism (Banno and Sakai, 1989). The origin of the chlorite zone rocks has recently been ascribed to the deeper part of the Cretaceous Shimanto accretionary complex, based on the bulk chemistry of clastic rocks (Kiminami et al., 1999; Kiminami and Ishihama, 2003; Kiminami and Toda, 2007), a reconstruction of oceanic plate stratigraphy (Okamoto et al., 2000; Terabayashi et al., 2005), and the timing of accretion and metamorphism supported by geochronological studies of U–Pb zircon ages and phengite K–Ar ages (Aoki et al., 2007, 2008). Low-grade metamorphism of the Cretaceous Shimanto accretionary complex is especially important in terms of understanding the relationship between the complex and the Sambagawa metamorphic rocks. K–Ar ages of mica from the Sambagawa metamorphic rocks in the Kanto Mountains are estimated to be during 58–84 Ma (Hirajima et al., 1992 Miyashita and Itaya, 2002). In particular, K–Ar ages for the chlorite zone rocks are estimated to be 72–84 Ma, indicating the timing of peak metamorphism due to temperature conditions below closure temperature of 350 °C in K–Ar mica system (Hirajima et al., 1992). The timing of metamorphism for the chlorite zone rocks is slightly older than the ages of 65–75 Ma estimated from the Otaki Group (Fig. 7; Hara and Hisada, 2005). The Sambagawa metamorphic rocks were also uplifted between 60 and 90 Ma, associated with subduction of the buoyant Kula–Pacific ridge beneath the Asian continent (Maruyama, 1997). According to Masago et al. (2005) and Aoki et al. (2007), structural analyses and chronological data indicate that the Sambagawa metamorphic rocks

were thrust over the Cretaceous Shimanto accretionary complex during uplift. Subsequence to metamorphism during 72–84 Ma, the Sambagawa metamorphic rocks of chlorite zone in the Kanto Mountains were transported close to the low-grade metamorphosed Cretaceuos Shimanto accretionary complex (Otaki Group) by thrusting during the period 65–75 Ma, with temperature conditions around 300 °C (Fig. 8a). In addition, Kiminami et al. (1994) reported that MORB-type basaltic rock extruded upon and intruded into unconsolidated clastic sediment (in situ basaltic rocks) within Cretaceous Shimanto accretionary complex, associated with collision between the Kula–Pacific ridge and the Asian continent. Metamorphic event of the Otaki Group during 65–75 Ma was caused by thrusting of the Sambagawa metamorphic rocks and high geothermal gradient by the Kula–Pacific ridge subduction. Subsequent to metamorphic period around 65–75 Ma, the younger low-grade metamorphic event at around 50 Ma is recorded in the Cretaceous Shimanto accretionary complex in the Kyushu and Shikoku areas (Agar et al., 1989; Hara and Kimura, 2008). According to Engebretson et al. (1985) and Maruyama (1997), the Kula–Pacific ridge had already passed Northeast Japan by the Paleocene. Subsequent to subduction of the Kula–Pacific ridge, a very young, hot section of the Pacific Plate was subducted beneath the Asian continent during the Paleocene (Maruyama, 1997). Thermal modeling of a shallow subduction zone indicates that the regional extent of prehnite-bearing metamorphic rocks in the Cretaceous Shimanto accretionary complex is related to the subduction of a hot slab (Miyazaki and Okumura, 2002). The subduction of young plate also causes increase of geothermal gradient within accretionary complex (James et al., 1989). The later metamorphism around 50 Ma is related to a thermal event associated with subduction of the young and hot Pacific Plate. Low-grade metamorphism of the Kobotoke Group at around 40 Ma is considered to be associated with subduction of the young Pacific Plate (Fig. 8b). As noted above, the Otaki Group in the Kanto Mountains were not subjected to younger thermal overprinting at around 50 Ma. FT dating of zircons suggests that the Otaki Group cooled below 260 ± 50 °C at around 54–59 Ma (Hara et al., 2007). In brief, the Otaki Group was uplifted to shallower levels of the accretionary wedge at 50 Ma, without overprinting by younger thermal event (Figs. 7 and 8b). 10. Plate tectonics during the Late Cretaceous to Paleogene around the northwestern Pacific region We now reconstruct the configuration of Kula and Pacific Plates during the Late Cretaceous to Paleogene, based on the timing and significance of low-grade metamorphism of the Cretaceous Shimanto accretionary complex. The Kula–Pacific ridge was subducted beneath the Asian continental margin during the Late Cretaceous, as indicated by geological evidence such as uplift of the Sambagawa metamorphic rocks (Maruyama, 1997) and large-scale magmatism recorded in Southwest Japan (Kinoshita, 2002). The K–Ar ages of mica from the part of Cretaceous Shimanto accretionary complex (Hanazono Formation, Otaki Group) are around 70 Ma, indicating almost the same between the Kii Peninsula and Kanto Mountains areas (Kurimoto, 1993; Hara and Hisada, 2005). The similarity of metamorphic ages between the two areas suggests that the thermal effect associated with subduction of the Kula–Pacific ridge extended at least 500 km along the trench (Fig. 8a). Subsequent to subduction of the Kula–Pacific ridge, the young, hot Pacific Plate was orthogonally subducted beneath the Asian continent during the Paleocene at a rate of 10.9 cm y− 1 (Maruyama, 1997). Metamorphism associated with subduction of the young Pacific Plate continued until 50 Ma in Kyushu and Shikoku of Southwest Japan, and until 40 Ma in the Kanto Mountains of Central Japan. The difference in age of the later low-grade metamorphic event recorded in the Cretaceous Shimanto accretionary complex at Kyushu to Shikoku and that at the Kanto Mountains is estimated to be 10 Ma. Assuming that later

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Fig. 8. Reconstruction of the plate tectonics and metamorphic history of the low-grade metamorphic rocks within the Kobotoke and Otaki groups for the periods 65–75 Ma (a) and 40 Ma (b). The left-hand figures show reconstructions of plate tectonics, modified from Maruyama (1997) with our own interpretations. The right-hand figures show cross-sections through the accretionary prism, thermal conditions (not to scale), and the locations of relevant geological units. Thermal isograds are assumed to be parallel within the accretionary prism, associated with subduction of the ridge and young, hot oceanic crust (e.g., James et al., 1989). KB: Kobotoke Group, OT: Otaki Group, SB: Sambagawa metamorphic rocks.

metamorphism reflecting the movement of the Pacific Plate, we consider that the subduction style of the Pacific Plate was not only orthogonally, but also oblique with migration of the Kula–Pacific ridge to the north. The distance between Kyushu and the Kanto Mountains is approximately 800 km. To the north vector, the young Pacific Plate moved in tandem with the Kula–Pacific ridge at 8.0 cm y− 1. 11. Summary and conclusions Based on radiolarian fossil data, metamorphic temperatures derived from illite crystallinity analysis, and the timing of metamorphism derived from illite K–Ar dating, we reconstructed the tectonometamorphic evolution of the low-grade Cretaceous Shimanto accretionary complex (Kobotoke Group) in the Kanto Mountains, Central Japan. The main findings of the study are summarized as follows. 1) We report the first occurrence of radiolarian fossils from silty shale within a mélange unit (Kobuse Unit) in the Kobotoke Group. The radiolarian assemblage is characterized by D. multicostata, Dictyomitra sp. aff. D. koslovae, A. stocki, and P. floresensis, which have been reported from the Santonian to Campanian. This age range overlaps with the age obtained for coherent units within the Kobotoke Group (Turonian to Maastrichtian).

2) Most of the illite crystallinity data indicate that the Kobotoke Group was metamorphosed at approximately 300 °C, based on IC values of 0.22–0.31 Δ°2θ. 3) The K–Ar ages of three samples collected from the Kobotoke Group are 40.2 ± 0.89, 48.3 ± 1.1, and 38.4 ± 0.86 Ma. The oldest K–Ar age of 48 Ma is unsuitable in terms of interpreting the peak of lowgrade metamorphism for influence of detrital mica. The ages of 38 and 40 Ma are clearly younger than the Turonian to Maastrichtian depositional age (66–94 Ma), and are assumed to represent the timing of metamorphism. 4) Within the Cretaceous Shimanto accretionary complex distributed throughout Southwest to Central Japan, we defined two types of low-grade metamorphism. The first low-grade metamorphism occurred synchronously in Southwest and Central Japan, related to thrusting of the Sambagawa metamorphic rocks over the Shimanto accretionary complex, which in turn occurred in association with subduction of the Kula–Pacific ridge during the Late Cretaceous (65–75 Ma). The second metamorphic event is related to the thermal effect of the subduction of the young, hot Pacific Plate during the Middle Eocene (40–50 Ma). This later metamorphism continued until 50 Ma in Kyushu and Shikoku, Southwest Japan, and until 40 Ma in the Kanto Mountains of Central Japan. This difference in timing between Southwest and Central Japan reflects the northward migration of the Pacific Plate.

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Acknowledgements We would like to thank Dr. K. Miyazaki and Dr. M. Aoya for their valuable comments concerning low-grade metamorphism, Dr. K. Kashiwagi and Dr. T. Tokiwa for suggestions regarding the description of Cetaceous radiolarian fossils, and Dr. K. Hisada, Dr. N. Yagi and Mr. A. Kanematsu for their support during field surveys. Thanks are also due to the editor Professor M. Liu, and anonymous reviewers for their constructive and valuable comments of the manuscript. References Agar, S.M., Cliff, R.A., Duddy, I.R., Rex, D.C., 1989. Accretion and uplift in the Shimanto Belt, SW Japan. J. Geol. Soc. London 146, 893–896. Aoki, K., Iizuka, T., Hirata, T., Maruyama, S., Terabayashi, M., 2007. Tectonic boundary between the Sanbagawa belt and the Shimanto belt in central Shikoku, Japan. J. Geol. Soc. Jpn. 113, 171–183. 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. 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. Banno, S., Sakai, C., 1989. Geology and metamorphic evolution of the Sanbagawa netanorphic belt, Japan. In: Daly, J.S., Cliff, R.A., Yardley, B.W.D. (Eds.), Evolution of Metamorphic Belts: Geol. Soc. Spec. Pub. No.43, pp. 519–532. Egawa, K., Lee, Y.I., 2008. Thermal maturity assessment of the Upper Triassic to Lower Jurassic Nampo Group, mid-west Korea: reconstruction of thermal history. Isl. Arc 17, 109–128. Engebretson, D., Cox, A., Gordon, R.G., 1985. Relative plate motions between ocean and continental plates in the Pacific basin. Geol. Soc. Am. Spec. Pap. 206, 1–59. Fabbri, O., Faure, M., Charvet, J., 1990. Back-thrusting in accretionary prism: microtectonic evidence from the Cretaceous–Lower Tertiary Shimanto belt of southwest Japan. J. Southeast Asian Earth Sci. 4, 195–201. Fettes, D., Desmons, J., 2007. Metamorphic Rocks: A Classification and Glossary of Terms. Cambridge University Press, Cambridge. 244 pp. Foreman, H.P., 1968. Upper maastrichtian radiolaria of California. Palaeontol. Assoc. London, Spec. Pap. Palaeontol. 3 82 pp. Frey, M., 1987. Very low-grade metamorphism of clastic sedimentary rocks. In: Frey, M. (Ed.), Low Temperature Metamorphism. Blackie Press, Glasgow, pp. 9–58. Fujimoto, H., Kawata, K., Miyazawa, S., Morikawa, R., Arai, F., Takano, T., Yoshida, S., Hara, K., Tazukem, H., Madoo, H., 1950. Geological studies of the Oku-chichibu. Bull. Chichibu Mus. Nat. Hist. 1, 1–28 (in Japanese with English Abstract). Gradstein, F.M., Ogg, J.G., Smith, A.G., 2004. A Geologic Time Scale 2004. Cambridge University Press, Cambridge. 589 pp. Guthrie, J.M., Houseknecht, D.W., Johns, W., 1986. Relationships among vitrinite reflectance, illite crystallinity, and organic geochemistry in the Carboniferous strata, Ouachita Mountains, Oklahoma and Arkansas. Am. Assoc. Petrol. Geol. Bull. 70, 26–33. Hara, H., Kimura, K., 2003. New proposal of standard specimens for illite crystallinity measurement: its usefulness as paleo-geothermal indicator. Bull. Geol. Sur. Jpn. 54, 239–250 (in Japanese with English Abstract). Hara, H., Hisada, K., 2005. Metamorphic age of the Southern Chichibu and Shimanto accretionary complexes in the Mitsumine district of the Kanto Mountains, central Japan: K–Ar ages of illite from phyllite. J. Geol. Soc. Jpn. 111, 217–223 (in Japanese with English Abstract). Hara, H., Hisada, K., 2007. Tectono-metamorphic evolution of the Cretaceous Shimanto accretionary complex, central Japan: constraints from a fluid inclusion analysis of syn-tectonic veins. Isl. Arc 16, 57–68. Hara, H., Kimura, K., 2008. Metamorphic and cooling history of the Shimanto accretionary complex, Kyushu, Southwest Japan: implications for the timing of out-of-sequence thrusting. Isl. Arc 17, 546–559. Hara, H., Hisada, K., Kimura, K., 1998. Paleo-geothermal structure based on illite crystallinity of the Chichibu and Shimanto Belts in the Kanto Mountains, central Japan. J. Geol. Soc. Jpn. 104, 705–717 (in Japanese with English Abstract). Hara, H., Danhara, T., Iwano, H., 2007. Cooling history estimated from fission-track dating of the Shimanto accretionary complex in the Mitsumine district of the Kanto Mountains, central Japan. J. Geol. Soc. Jpn 113, 73–76 (in Japanese with English Abstract). Hirajima, T., Isono, T., Itaya, T., 1992. K–Ar age and chemistry of white mica in the Sanbagawa metamorphic rocks in the Kanto Mountains, central Japan. J. Geol. Soc. Jpn. 98, 445–455 (in Japanese with English Abstract). Hollis, C.J., Kimura, K., 2001. A unified radiolarian zonation for the Late Cretaceous and Paleocene of Japan. Micropaleontology 47, 235–255. Hunziker, J.C., Frey, M., Clauer, N., Dallmeyer, R.D., Friedrichsen, H., Flehmig, W., Hochstrasser, K., Roggwiler, P., Schwander, H., 1986. The evolution of illite to muscovite: mineralogical and isotopic data from the Glarus Alps, Switzerland. Contrib. Mineral. Petrol. 92, 157–180. Itaya, T., Fukui, S., 1994. Phengite K–Ar ages of schists from the Sanbagawa southern marginal belt, central Shikoku, southwest Japan: influence of detrital mica and deformation on age. Isl. Arc 3, 48–58.

Iyota, N., Hisada, K., Sashida, K., Igo, H., 1994. The Ogochi Group of the Shimanto Terrane in the Kanto Mountains, central Japan. Sci. Rep., Inst. Geosci., Univ. Tsukuba, Sec. B 15, 47–69. Jäger, E., 1979. Introduction to geochronology. In: Jägaer, E., Hunziker, J.C. (Eds.), Lectures in Isotope Geology. Springer-Verlag, Berlin, pp. 1–12. James, T.S., Hollister, L.S., Morgan, W.J., 1989. Thermal modeling of the Chugach Metamorphic Complex. J. Geophy. Res. 94, 4411–4423. Kiminami, K., Ishihama, S., 2003. The parentage of low-grade metasediments in the Sanbagawa Metamorphic Belt, Shikoku, southwest Japan, based on whole-rock geochemistry. Sediment. Geol. 159, 257–274. Kiminami, K., Toda, Y., 2007. The parentage of low-grade metasediments in the Sanbagawa Metamorphic Belt, southern area of Mima City, Tokushima Prefecture, Japan. J. Geol. Soc. Jpn. 113, 158–167 (in Japanese with English Abstract). 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 Sanbagawa psammitic schists (Oboke unit) and the Cretaceous Shimanto sandstones in Shikoku, Southwest Japan and its geologic significance. Isl. Arc 8, 373–382. Kinoshita, O., 2002. Possible manifestations of slab window magmatisms in Cretaceous southwest Japan. Tectonophysics 344, 1–13. Kisch, H.J., 1991. Illite crystallinity: recommendations on sample preparation, X-ray diffraction settings, and interlaboratory samples. J. Metamorph. Geol. 9, 665–670. Kosakowski, G., Kunert, V., Clauser, C., Franke, W., Neugebauer, H.J., 1999. Hydrothermal transients in Variscan crust: paleo-temperature mapping and hydrothermal models. Tectonophysics 306, 325–344. Kübler, B., 1968. Evaluation quantitative du metamorphism par la cristallinite de l'Illite. Bull. Cent. Rech. Pau. SNPA 2, 385–397 in French with English abstract. Kurimoto, C., 1993. K–Ar ages of the rocks of the Sambagawa, Kurosegawa and Shimanto Terranes in the northeastern part of Wakayama Prefecture, Southwest Japan. Bull. Geol. Sur. Jpn. 44, 367–375 (in Japanese with English Abstract). Liu, K.W., 2002. Deep-burial diagenesis of the siliciclastic Ordovician Natal Group, South Africa. Sediment. Geol. 154, 177–189. Maruyama, S., 1997. Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present. Isl. Arc 6, 121–142. Masago, H., Okamoto, K., Terabayashi, M., 2005. Exhumation tectonics of the Sanbagawa high-pressure Metamorphic Belt, Southwest Japan — constraints from the Upper and Lower Boundary Faults. Int. Geol. Rev. 47, 1194–1206. 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. Miyashita, A., Itaya, T., 2002. K–Ar age and chemistry of phengite from the Sanbagawa schists in the Kanto Mountains, central Japan, and their implication for exhumation tectonics. Gond. Res. 5, 837–848. Miyazaki, K., Okumura, K., 2002. Thermal modelling in shallow subduction: an application to low P/T metamorphism of the Cretaceous Shimanto accretionary complex, Japan. J. Metamorph. Geol. 20, 441–452. 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. Sur. Jpn. 58, 23–31 (in Japanese with English Abstract). Murata, A., Kosaka, K., Kano, K., 1986. Age of activity of the Tsurukawa Fault in the Southern Kanto Mountains, as viewed from the relationship with the Kofu Plutonic Body. J. Geol. Soc. Jpn. 92, 905–908 (in Japanese with English Abstract). Nishimura, Y., Coombsm, D.S., Landis, C.A., Itaya, T., 2000. Continuous metamorphic gradient documented by graphitization and K–Ar age, southeast Otago, New Zealand. Am. Mineral. 85, 1625–1636. Nishimura, Y., Philippa, M.B., Itaya, T., 2004. Metamorphism and metamorphic K–Ar ages of the Mesozoic accretionary complex in Northland, New Zealand. Isl. Arc 13, 416–431. Ogawa, Y., Hisada, K., Sashida, K., 1988. Shimanto supergroup in the Kanto Mountains — a review. Modern Geol. 12, 127–146. Okamoto, K., Maruyama, S., Isozaki, Y., 2000. Accretionary complex origin of the Sanbagawa, high P/T metamorphic rocks, central Shikoku, Japan — Layer-parallel shortening structure and greenstone geochemistry. J. Geol. Soc. Jpn. 106, 70–86. Pessagno, E.A., 1976. Radiolarian zonation and stratigraphy of the Upper Cretaceous portion of the Great Valley Sequence, California Coast Ranges. Micropaleontol. Spec. Pub. 2 95 pp. Popova-Goll, I., Vishnevskaya, V., Baumgartner, P.O., 2005. Upper Cretaceous (Santonian– Campanian) radiolarians from Voronesh Anticline, southwestern Russia. Micropaleontology 51, 1–37. Reuter, A., Dallmeyer, R.D., 1989. K–Ar and 40Ar/39Ar dating of cleavage formed during very low-grade metamorphism: a review. In: Daly, J.S., Cliff, R.A., Yardley, B.W.D. (Eds.), Evolution of Metamorphic Belts: Geol. Soc. London Spec. Pub, vol. 43, pp. 161–171. Sakai, A., 1987. Geology of the Itsukaichi District. With Geological Sheet Map at 1:50,000, Geological Survey of Japan, Tsukuba, 75pp. (in Japanese with English abstract). Sakai, A., 2007. Geology of the Ome District. 4. Shimanto Sedimentary Complex, Quadrangle Series, 1:50,000, Geological Survey of Japan, AIST, Tsukuba, pp. 31–35 (in Japanese). Sanfilippo, A., Riedel, W.R., 1985. Cretaceous radiolaria. In: Bolli, H.M., Saunders, J.B., PerchNielsen, K. (Eds.), Plankton Stratigraphy. Cambridge University Press, Cambridge, pp. 573–630. Steiger, R., Jäger, E., 1977. Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359–362. Sweeney, J.J., Burnham, A.K., 1990. Evaluation of a sample model of vitrinite reflectance based on chemical kinetics. Am. Assoc. Petrol. Geol. Bull. 74, 1559–1570.

H. Hara, T. Kurihara / Tectonophysics 485 (2010) 52–61 Taira, A., Katto, J., Tashiro, M., Okamura, M., Kodama, K., 1988. The Shimanto Belt in Shikoku, Japan — evolution of Cretaceous to Miocene accretionary prism. Modern Geol. 12, 25–46. Takahashi, O., Ishii, A., 1995. Radiolarian assemblage-zones in the Jurassic and Cretaceous sequence in the Kanto Mountains, central Japan. Mem. Fac. Sci., Kyushu Univ., Ser. D, Earth Planet. Sci. 24, 49–85. Takami, M., Itaya, T., 1996. Episodic accretion and metamorphism of Jurassic accretionary complex based on biostratigraphy and K–Ar geochronology in the western part of the Mino–Tanba Belt, southwest Japan. Isl. Arc 5, 321–336. Takasu, A., Wallis, S.R., Banno, S., Dallmeyer, R.D., 1994. Evolution of the Sambagawa metamorphic belt, Japan. Lithos 33, 119–133. 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. Toriumi, M., Teruya, J., 1988. Tectono-metamorphism of the Shimanto Belt. Modern Geol. 12, 303–324. Underwood, M.B., Laughland, M.M., Kang, S.M., 1993. A comparison among organic and inorganic indicators of diagenesis and low-temperature metamorphism, Tertiary Shimanto Belt, Shikoku, Japan. In: Underwood, M.B. (Ed.), Thermal Evolution of the

61

Tertiary Shimanto Belt, Southwest Japan: An Example of Ridge–Trench Interaction: Geol. Soc. Am. Spec. Pap., vol. 273, pp. 45–61. Wakita, K., Metcalfe, I., 2005. Ocean plate Stratigraphy in East and Southeast Asia. J. Asian Earth Sci. 24, 679–702. Wallis, S.R., Banno, S., 1990. The Sambagawa Belt — trends in research. J. Metamorph. Geol. 8, 393–399. Warr, L.N., Rice, A.H.N., 1994. Interlaboratory standardization and calibration of clay mineral crystallinity and crystallite size data. J. Metamorph. Geol. 12, 141–152. Yagi, N., 2000. Stratigraphy of the Cretaceous and Paleogene sedimentary complexes of the Kobotoke Belt, Kanto Mountains, central Japan. Sci. Rep., Inst. Geosci., Univ. Tsukuba, Sec. B 21, 13–40. Yamazaki, T., 1987. Radiolarian assemblages of the Izumi Group in Shikoku and western Awaji Island, Southwest Japan. J. Geol. Soc. Jpn. 93, 403–417 (in Japanese with English Abstract). Yanai, S., Yamakita, S., 1987. Ductile deformation of the Kobotoke Group, South Kanto Mountains. Sci. Pap. College Arts Sci., the Univ. Tokyo 37, 59–71 (in Japanese with English Abstract). Zittel, K.A., 1876. Uber einige fossile Radiolarien aus der norddeutschen Kreide. Zeitsc. Deutsche. Geol. Gesell. 28, 75–86 (in German).