Tectonophysics 485 (2010) 52–61

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

Hidetoshi Hara a,⁎, Toshiyuki Kurihara b

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

⁎ Corresponding author. Tel.: +81 298 61 3981; fax:E-mail address: hara-hide@aist.go.jp (H. Hara).

0040-1951/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.tecto.2009.11.017

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 September 2008Received in revised form 23 September 2009Accepted 22 November 2009Available online 3 December 2009

Keywords:Low-grade metamorphismIllite crystallinityIllite K–Ar datingKula–Pacific ridgeShimanto Belt

We reconstructed the tectono-metamorphic evolution of the low-grademetamorphosed Cretaceous Shimantoaccretionary complex in the Kanto Mountains, Central Japan, based on radiolarian fossils, metamorphictemperatures derived from illite crystallinity analysis, and timing of metamorphism based on illite K–Ardating. The accretionary age of the Kobotoke Group is Turonian to Maastrichtian (66–94 Ma), based onradiolarian fossils. Illite crystallinity data indicate metamorphic temperatures of approximately 300 °C. Theillite K–Ar ages constrain the timing of metamorphism to the Middle Eocene around 40 Ma. Combining ourresults and previous study, we defined two types of low-grade metamorphism within Cretaceous Shimantoaccretionary complex of the KantoMountains. The earlymetamorphism, in excess of 300 °C, was related to theuplift of the Sambagawa metamorphic rocks, in turn associated with the subduction of the Kula–Pacific ridgeduring the Late Cretaceous (65–75 Ma). This metamorphism is recorded in the Otaki Group within thenorthernmost part of the complex in the Kanto Mountains. Subsequent to the subduction of the Kula–Pacificridge, a later period of metamorphism, recorded in the Kobotoke Group, is characterized by the thermal effectsof the subduction of the young, hot Pacific Plate during the Middle Eocene. The effect of the early meta-morphism occurred synchronously 500 km along the trench from Southwest to Central Japan. The latermetamorphism occurred at 50 Ma in Kyushu and Shikoku of Southwest Japan, and at 40 Ma in the KantoMountains of Central Japan. This difference in the timing of metamorphism between Southwest and CentralJapan is explained by the northward migration of the young, hot Pacific Plate.

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1. Introduction

The Cretaceous Shimanto accretionary complex (e.g., Taira et al.,1988) iswidely exposed fromSouthwest to Central Japan (Fig. 1a),whereit consists of low-grade metamorphic rocks subjected to prehnite–actinolite and greenschist facies metamorphism. These rocks includethe 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, theOtaki and Kobotoke groups contain low-grade metamorphosed rocksof the Cretaceous Shimanto accretionary complex (Fig. 1). Metamor-phism of the Otaki Group is estimated to be in excess of 300 °C andgreater than 270 MPa during 65–75 Ma, based onmicro-thermometryof fluid inclusions, illite crystallinity analyses and illite K–Ar dating(Hara and Hisada, 2005, 2007), however, that of the Kobotoke Grouphas yet to be analyzed in detail. The low-grade metamorphosed rockswithin the accretionary complex are important in terms of under-standing the tectonic linkages between the accretionary complex

and metamorphism associated with subduction of the oceanic platebeneath the continental plate. In particular, the Cretaceous Shimantoaccretionary complex in the Kanto Mountains is characterized by twolow-grade metamorphosed rocks within the Otaki and Kobotokegroups. Analyses of two low-grade metamorphisms recorded in theOtaki and Kobotoke groups possibly lead to oceanic plate subductionhistory.

The aim of this paper is to reconstruct the tectonic evolution of thelow-grade metamorphosed rocks of the Kobotoke Group for under-standing of two low-grade metamorphisms within the CretaceousShimanto accretionary complex. We estimate the maximum meta-morphic temperatures based on illite crystallinity, and the timing ofmetamorphism based on illite K–Ar dating. We also report the firstoccurrence of radiolarian fossils from argillaceous rock in a mélangeunit within the group. Based on the combined results of illite crys-tallinity analysis, illite K–Ar dating, and new fossil data, we proposethe accretion and metamorphic history of the low-grade metamor-phosed Cretaceous Shimanto accretionary complex, as recorded bythe Kobotoke Group of the Kanto Mountains. We also discuss the sig-nificance of the low-grade metamorphism of the Otaki and Kobotokegroups in relation to Kula–Pacific ridge subduction and migration ofthe Pacific Plate during the Late Cretaceous to Paleogene.

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.

53H. Hara, T. Kurihara / Tectonophysics 485 (2010) 52–61

2. Geological outline

The Kanto Mountains, Central Japan, contain a Mesozoic island arcsystem that consists of (from northeast to southwest) Sambagawametamorphic rocks, the Jurassic Chichibu accretionary complex withCretaceous strike–slip basin sediments, and the Cretaceous to Paleo-gene Shimanto accretionary complex (Fig. 1b). The Cretaceous Shi-manto accretionary complex is subdivided into the Otaki, Ogochi, andKobotoke groups (Fig. 1b). The Kobotoke Group is in fault contactwith the Ogochi Group to the north along the Itsukaichi–KawakamiLine, and with the Eocene to Oligocene Shimanto accretionary com-plex (Sagamiko Group) to the south along the Matsuhime Fault (Yagi,2000).

The Otaki Group is mainly composed of phyllite, sandstone, andargillaceous mélange-type rocks, with blocks of chert, limestone, tuff,and basalt, and also is characterized by intensive deformation andmetamorphism (Hara andHisada, 2007). TheOtakiGroupwas affectedby greenschist facies metamorphism (Fujimoto et al., 1950; Ogawaet al., 1988). Hara and Hisada (2007) estimated peak metamorphictemperatures for the Otaki Group in excess of 300 °C and fluid pres-sures greater than 270 MPa, based on the micro-thermometry of fluidinclusions within quartz veins and illite crystallinity analyses of argil-laceous 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 accretionarycomplex in the Kanto Mountains, and is in fault contact with theOgochi Group to the south (Fig. 1).

The Ogochi Group is composed of six units from northeast tosouthwest, all of which are characterized by either coherent turbi-dite unit (the coherent unit) or argillaceous mélange-type rocks (themélange unit) (Iyota et al., 1994). All six units are bounded by high-angle reverse faults and are imbricated. The depositional ages of theOgochi Group range from late Albian to Campanian, and ages youngingsouthwestward.

The Kobotoke Group is also divided into the coherent unit and themélange unit (see Fig. 1). According to Yagi (2000) and Sakai (2007),the coherent unit is composed of sandstone, shale, and interbeddedsandstone and shale, named the Bonborigawa and Kosuge units. Themélange unit is subdivided into the Miyama, Uzuhiki, and Kobuseunits, comprising shale and broken beds of sandstone, with minormélange that consists of basalt and chert blocks in an argillaceousmatrix; the matrix is generally phyllitic and foliated. These three unitsare stratigraphically repeated due to thrusting. The Kobotoke Groupwas subjected to prehnite–pumpellyite facies to greenschist faciesmetamorphism (Toriumi and Teruya, 1988) and ductile deformation(Ogawa et al., 1988; Fabbri et al., 1990); however, the low-grademeta-morphism of this group has yet to be analyzed in detail because of ascarcity of suitable basaltic rocks.

54 H. Hara, T. Kurihara / Tectonophysics 485 (2010) 52–61

3. Texture of low-grade metamorphosed rocks

Feldspathic sandstone is the predominant lithology of the Kobo-toke Group (Sakai, 1987). Deep-burial diagenetic textures (see Liu, 2002;Egawa and Lee, 2008) are commonly observed in sandstones of thegroup. Detrital grains show significant mechanical compaction, and con-tain fractures and textures indicative of pressure-solution seams, includ-ing sutured grain boundaries (arrowhead in Fig. 2a). Illite (mica) rims arecommonly observed around detrital grains (arrowhead in Fig. 2b). Thefoliation is largely defined by the preferred orientation of clay mineralsand dark-colored pressure-solution seams (arrowheads in Fig. 2c). Theboundary between lithic fragments and matrix is generally unclear.

Argillaceous rocks are usually phyllitic with a foliation. The folia-tion is defined by the preferred orientation of clay minerals, segrega-tion of layers of recrystallized quartz and mica-rich layers, and dark-colored pressure-solution seams (Fig. 2d). Silt-size detrital grainscomposed mostly of quarts are observed in phyllitic shale, occasion-ally showing asymmetric fabrics around quartz grains due to sheardeformation.

Chert and basaltic rocks occur as small blocks within argillaceousrocks of the mélange units. The chert, consisting of recrystallizedmicrocrystalline quartz, is usually gray, green, or red in color, and con-tains radiolarian fossils deformed into ellipsoidal shapes. The basalticrocks consist of massive lava, volcaniclastics, and basalt tuff, and con-tain veins of epidote, chlorite, and prehnite, as well as metamorphicminerals such as actinolite and pumpellyite (Fig. 3). This metamor-phic mineral assemblage indicates sub-greenschist to greenschistfacies metamorphism (Fettes and Desmons, 2007).

Fig. 2. Photomicrographs of clastic rocks, showing the low-grade metamorphosed textures oM: mica. (a–b) Sandstone under plane polarized light (a) and crossed polarized light (b).providing evidence of pressure-solution. (c) Foliated sandstone (crossed polarized light). Tminerals and pressure-solution seams. (d) Sample of phyllite (KB-01) collected for K–Ar da

4. Radiolarian fauna and age

Previous studies have reported radiolarian fossils from shale with-in the coherent units of the Kobotoke Group. For example, radiolarianfossils within shale from the Bonborigawa Unit indicate a Campanianage (Sakai, 1987). Takahashi and Ishii (1995) andYagi (2000) reportedTuronian to Maastrichtian radiolarian ages for shale from the KosugeUnit; however, radiolarian fossils have yet to be obtained from themélange unit.

The present study describes the first occurrence of radiolarianfossils from silty shale within a mélange unit (Kobuse Unit) in theKobotoke Group (Fig. 4; see Fig. 5a. for sample locations). The radio-larian fossil-bearing shale also contains chert blocks.

The siliceous residue remainingafter thehydrofluoric acid (HF)etch-ing of samples of silty shale contains rare radiolarian shells, stronglyrecrystallized by low-grade metamorphism. Most multi-segmentednassellarians do not retain their external shell shape and surface struc-tures, thereby hampering detailed taxonomic analysis. After carefulsample preparation for scanning electron microscope (SEM) observa-tions, involving 15 repetitions of HF etching, we identified the follow-ing species (Fig. 4): Dictyomitra multicostata Zittel, Dictyomitra sp.aff. Dictyomitra koslovae Foreman, Amphipyndax stocki (Campbell andClark), and Pseudoaulophacus floresensis Pessagno.

Dictyomitra multicostata, first described from the Campanian inGermany (Zittel, 1876), has been reported fromUpper Cretaceous stratain many parts of the world (e.g., Foreman, 1968; Pessagno, 1976; Yama-zaki, 1987; Popova-Goll et al., 2005).Dictyomitra sp. aff.D. koslovae cor-responds to the morphotype treated by Hollis and Kimura (2001) as

f sandstone and phyllite. All scale bars are 1 mm. F: feldspar, R: rock fragment, Q: quartz,Arrowhead indicates sutured grain boundary and illite (mica) seams between grains,he foliation (indicated by arrowheads) is defined by the preferred orientation of clayting.

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 basaltviewed under plane polarized light (c) and crossed polarized light (d).

Fig. 4. Selected age-diagnostic radiolarians recovered from silty shale of the mélangeunit 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:Dictyomitramulticostata Zittel. 4:Dictyomitrasp. aff. D. koslovae Foreman. 5–6: Amphipyndax stocki (Campbell and Clark). 7: Pseudo-aulophacus floresensis Pessagno.

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“Dictyomitra cf. koslovae,” which is distinguished from D. koslovae by arelatively smooth outline from the cephalis to the prominent segment,and less developed structures for subsequent segments. According toHollis and Kimura's (2001) compilation of Upper Cretaceous biostrati-graphic data for Japan, this morphotype ranges from the D. koslovaeInterval Zone (Dk1: Santonian) to the Pseudotheocampe abschnitta Inter-val Zone (Pa: lower Maastrichtian), and is most common over the San-tonian to Campanian.

Pseudoaulophacus floresensis, the most representative species ofpseudoaulophacids, ranges from the Amphipyndax pseudoconulus Zoneto the Amphipyndax tylotus Zone, indicating a Campanian to Maas-trichtian age (Sanfilippo and Riedel, 1985). Hollis and Kimura (2001)reported that this species occurs within the Santonian (Dk1) to lowerMaastrichtian (Pa) zones mentioned above. For Amphipyndax species,we obtained only A. stocki: we did not recover other age-diagnosticand evolved forms such as A. pseudoconulus (Pessagno) and A. tylotusForeman.

The above data indicate amaximumage range of Santonian to earlyMaastrichtian for the analyzed radiolarian fauna. Fossil ages obtainedfrom argillaceous rocks are usually interpreted to indicate the age ofaccretion, based on the ocean plate stratigraphy (Matsuda and Isozaki,1991; Wakita and Metcalfe, 2005). The recovered radiolarian fossilsindicate that the age of accretion of the mélange unit is Santonian toCampanian, coincident with the age of accretion of the coherent units.The Kobotoke Group yields an accretion age of Turonian to Maas-trichtian without ages younging southward. We consider that inter-nal structure of the Kobotoke Group presents repeat of mélange andcoherent units controlled by decollment through underplating processduring Late Cretaceous (Fig. 5b).

Fig. 5. (a) Geological map of the Kobotoke and Sagamiko groups in the Kanto Mountains, showing illite crystallinity values. Symbols indicate sample localities; the accompanyingnumbers 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.

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

56 H. Hara, T. Kurihara / Tectonophysics 485 (2010) 52–61

5. Illite crystallinity

Illite crystallinity (IC) is determined from the Kübler index, whichis 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 thepreferred orientation of illite generally increases with decreasing ICvalues. The Kübler index is used to divide meta-sedimentary rocksinto 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 samplesof black pelitic rock collected from the Kobotoke Group, and from 14samples of comparable lithologies collected from the Sagamiko Group.The samples were washed, crushed in a swingmill for 10 seconds, andpassed through a 72-mesh sieve. Ten grams of the resulting powderwere then suspended in a test tube. The ≤2 µm clay fraction wasseparated by gravity settling and concentrated by centrifuging beforebeing pipetted onto two glass slides to make sedimented slides. Theaverage thickness of clay on the slides was maintained between 5 and10 mg/cm2 because clay thickness is known to affect the intensity ofthe crystallinity (Kisch, 1991).

We employed a JEOL 8030 X-ray diffractometer housed at theGeological Survey of Japan (Tsukuba, Ibaraki, Japan) using the fol-lowing measurement conditions: CuKα radiation at 40 kV and 40 mA,step scan speed of 0.01°2θ/s, divergence and scatter slits of 1°, re-ceiving slit of 0.2 mm, and scan range of 6.5–10.5°2θ. Two slides ofeach sample were scanned to check for errors. Samples with IC val-

ues of N0.30 Δ°2θ were treated with ethylene glycol to remove thesmectite peak that overlaps with the illite peak. For inter-laboratorycalibration, Warr and Rice (1994) proposed the CIS (Crystallinity-Index Standard), which comprises standard samples used for cali-bration in studies of illite crystallinity. Based onmeasured values of CISat the Geological Survey of Japan (GSJ), Hara and Kimura (2003) re-ported 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 mapof the study area. We focus on variations in IC along three transectsfrom 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

57H. Hara, T. Kurihara / Tectonophysics 485 (2010) 52–61

Kobotoke Group fall within the range 0.22 to 0.31 Δ°2θ (epizone toanchizone, mean=0.26 Δ°2θ, 1σ=0.024), while the western part ofthe coherent units yields values between 0.42 and 0.60 (diageneticzone, mean=0.52 Δ°2θ, 1σ=0.074). There are no meaningful hori-zontal variations in IC values within the mélange unit. IC values forthe 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 betweenthe Kobotoke Group and Sagamiko Group is estimated to be approx-imately 0.5–1.0 Δ°2θ.

6. Illite K–Ar dating

For K–Ar dating of illite, we collected three samples of black phyl-litic shale from the southernmost part of the mélange unit (KobuseUnit; see Fig. 5 for sample locations). Fig. 3d shows a representativephotomicrograph of the phyllitic shale used for dating.

Samples of illite intended for K–Ar datingwere prepared as follows.The rock sample was crushed using a jaw crusher and disk grinderbefore being passed through a 120–200 mesh sieve. The sieved frac-tion was then subjected to ultrasonic washing. Illite grains were con-centrated using isodynamic magnetic separation and heavy liquidseparation techniques, with hydrochloric acid treatment employed todissolve chlorite. K–Ar dating was carried out at the Hiruzen Institutefor Geology and Chronology (Okayama, Japan). The decay constant andisotopic abundance ratios used in the age calculation are after Steigerand Jäger (1977): λβ=4.962×10−10 y−1, λε=0.581×10−10 y−1, and40K/K=1.167×10−4 at.%, respectively.

The obtained K–Ar ages are listed in Table 1. The K–Ar ages are40.2±0.89, 48.3±1.1, and 38.4±0.86 Ma, with a mean age of 42 Ma.

7. Maximum metamorphic temperatures of the Kobotoke Group

A quantitative estimation of the temperature conditions indicatedby illite crystallinity data can be made based on the relationship be-tween IC values and vitrinite reflectance data (Guthrie et al., 1986;Underwood et al., 1993; Kosakowski et al., 1999). The IC data obtainedin the present study were converted into temperature values as fol-lows. Mukoyoshi et al. (2007) described a relationship between illitecrystallinity and mean random vitrinite reflectance (Rm%) based ondata fromnine localities in the Shimanto accretionary complex, coastalarea of eastern Kyushu. Their analyses of illite crystallinity were per-formed at the Geological Survey of Japan using the same sample pre-paration andmeasurement conditions as those employed in the presentstudy. The correlation between Rm and IC values indicates a linear re-gression equation of Rm (%)=6.9–8.2 IC (Δ°2θ), with a correlation co-efficient of 0.91.

Using the equation of Sweeney and Burnham (1990) to convertvitrinite reflectance into temperature over a heating duration of 10 Ma,the temperature conditions represented by the illite crystallinity dataare calculated using the following equation: T (°C)=353−206 IC(Δ°2θ), with a correlation coefficient of 0.92 (Mukoyoshi et al., 2007).

Table 1K–Ar ages of illite-rich fractions separated from argillaceous rocks of the Kobotoke Group, Csamples as those used for K–Ar dating.

Sample number IC value(Δ°2θ)

Radiogenic 40Ar(10−8cm3 STP/g)

Non-rad(%)

KB-01 0.26 731.1±7.2 4.7728.4±7.2 4.5

KB-02 0.27 764.7±7.8 7.3761.6±7.7 6.7

KB-03 0.25 597.1±6.1 8.1599.8±6.2 10

Applied to the KobotokeGroup in the present study, this equation indi-cates a temperature for the anchizone of 266–302 °C. The high-tem-perature boundary of the anchizone in the Shimanto accretionarycomplex, as determined in the present study, is about 25 °C lower thanthe estimate reported byUnderwood et al. (1993); however, the lowerboundaries are similar between the two studies.

Most of the IC values determined for the Kobotoke Group yieldtemperatures between approximately 290 and 310 °C (mean=300 °C).The calculated temperature for the western part of the coherent unit(KosugeUnit) is 245 °C (mean ICvalue=0.52 Δ°2θ). Similarly, the tem-perature calculated for the SagamikoGroup is 285 °C (mean IC value=0.33 Δ°2θ). Most of the Kobotoke Group has been subjected to peakmetamorphic temperatures of around 300 °C. In contrast, temperatureconditions estimated for the western coherent unit are disturbedaround the Tsurukawa Fault, yielding relatively low temperaturesof less than 250 °C. The Tsurukawa Fault has been active from theMiocene to recent (Murata et al., 1986). Yanai and Yamakita (1987)proposed that the Kobotoke Group was subjected to wrench tecto-nics associated with the development of faults such as the TsurukawaFault and Itsukaichi–Kawakami Line. It is also possible that IC valuesindicating low temperature around the western part of the coher-ent unit has been disturbed by wrench tectonic events. The KofuGranodiorite intruded into the western part of the Shimanto accre-tionary complex during Middle Miocene (Fig. 1). According to Haraet al. (1998), the thermal influence by intrusion for IC values occurredwithin several km from this granodiorite body. IC values from theKobotoke and Sagamiko groups in the study area indicate the low-grade metamorphic condition without thermal effect of granodioriteintrusion.

8. Accretion and metamorphic history of the Kobotoke Group

The K–Ar ages obtained for the Kobotoke Group is clearly youngerthan the Turonian to Maastrichtian depositional age (65.5–93.5 Maaccording to the time scale of Gradstein et al., 2004). The estimatedmetamorphic temperature of the Kobotoke Group is lower than theclosure temperature of the K–Ar system inwhitemica, which is 350 °C(Jäger, 1979). For low-grade metamorphism, K–Ar ages are assumedto represent the timing of peak metamorphism (Takami and Itaya,1996; Nishimura et al., 2000; Hara and Kimura, 2008). K–Ar agesare 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 thatthe influence of detrital mica is apparent in metapelite samples upto the metamorphic conditions of the anchizone–epizone boundary.Almost all of the illite crystallinity data obtained for the KobotokeGroup correspond to the epizone and anchizone (Figs. 5 and 6). Var-iations in K–Ar ages of the Kobotoke Group are considered to reflectthe influence of detrital mica. The oldest K–Ar age of 48 Ma is possiblyunsuitable in terms of interpreting the peak of low-grade metamor-phism. We adopt two K–Ar ages of 38 and 40 Ma (mean=39 Ma) asthe peak metamorphic age in the study.

retaceous Shimanto accretionary complex. The IC values were estimated from the same

. 40Ar Potassium(wt.%)

Isotopic age(Ma)

Average age(Ma)

4.628±0.093 40.26±0.89 40.2±0.8940.11±0.88

4.020±0.080 48.4±1.1 48.3±1.148.2±1.1

3.971±0.079 38.34±0.85 38.4±0.8638.51±0.86

Fig. 7. Timing of accretion and metamorphism of low-grade metamorphic rocks withinthe Kobotoke and Otaki groups. Bars labeled KB indicate the age of accretion of the Kobo-toke Group during the Turonian to Maastrichtian. The age of accretion and metamor-phism 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 onillite crystallinity analysis and K–Ar metamorphic ages. The black dot indicates that theOtaki Group cooled below 260±50 °C at 54–59 Ma, based on fission-track zircon ages(Hara et al., 2007).

58 H. Hara, T. Kurihara / Tectonophysics 485 (2010) 52–61

Based on radiolarian fossils and K–Ar age data, we propose the fol-lowing history of accretion andmetamorphismby low-grademetamor-phic rocks within the Kobotoke Group (Fig. 7). Radiolarian assemblagewithin shale indicates Turonian to Maastrichtian deposition andaccretion ages (66–94 Ma). Following accretion, the Kobotoke Groupwas subjected to low-grade metamorphism, with peak metamorphictemperatures of around 300 °C during Middle Eocene around 40 Ma.

9. Significance of low-grade metamorphism recorded in theCretaceous Shimanto accretionary complex

Metamorphic age of the Kobotoke Group is estimated to be during40 Ma, suggesting younger than that of the Otaki Group during 65–75 Ma. We discuss significance of two types of low-grade metamor-phism in the Cretaceous Shimanto accretionary complex, related to theSambagawa metamorphism and history of oceanic plate subduction.

The Sambagawa metamorphic rocks are typical high-P/T meta-morphic rocks, considered to represent deep-level tectonics in theaccretionary wedge (Wallis and Banno, 1990; Takasu et al., 1994). Inparticular, the chlorite zone of the Sambagawa metamorphic rocksis interpreted as the lowest grade in the Sambagawa metamorphism(Banno and Sakai, 1989). The origin of the chlorite zone rocks hasrecently been ascribed to the deeper part of the Cretaceous Shimantoaccretionary complex, based on the bulk chemistry of clastic rocks(Kiminami et al., 1999; Kiminami and Ishihama, 2003; Kiminami andToda, 2007), a reconstruction of oceanic plate stratigraphy (Okamotoet al., 2000; Terabayashi et al., 2005), and the timing of accretion andmetamorphism supported by geochronological studies of U–Pb zirconages and phengite K–Ar ages (Aoki et al., 2007, 2008). Low-grademetamorphism of the Cretaceous Shimanto accretionary complex isespecially important in terms of understanding the relationship be-tween the complex and the Sambagawa metamorphic rocks.

K–Ar ages of mica from the Sambagawa metamorphic rocks inthe Kanto Mountains are estimated to be during 58–84 Ma (Hirajimaet al., 1992 Miyashita and Itaya, 2002). In particular, K–Ar ages forthe chlorite zone rocks are estimated to be 72–84 Ma, indicating thetiming of peak metamorphism due to temperature conditions belowclosure temperature of 350 °C in K–Ar mica system (Hirajima et al.,1992). The timing of metamorphism for the chlorite zone rocks isslightly older than the ages of 65–75 Ma estimated from the OtakiGroup (Fig. 7; Hara and Hisada, 2005).

The Sambagawa metamorphic rocks were also uplifted between60 and 90 Ma, associated with subduction of the buoyant Kula–Pacificridge beneath the Asian continent (Maruyama, 1997). According toMasago et al. (2005) and Aoki et al. (2007), structural analyses andchronological data indicate that the Sambagawa metamorphic rocks

were thrust over the Cretaceous Shimanto accretionary complexduring uplift. Subsequence to metamorphism during 72–84 Ma, theSambagawa metamorphic rocks of chlorite zone in the Kanto Moun-tains were transported close to the low-grade metamorphosed Creta-ceuos Shimanto accretionary complex (Otaki Group) by thrustingduring the period 65–75 Ma, with temperature conditions around300 °C (Fig. 8a). In addition, Kiminami et al. (1994) reported thatMORB-type basaltic rock extruded upon and intruded into uncon-solidated clastic sediment (in situ basaltic rocks) within CretaceousShimanto accretionary complex, associatedwith collision between theKula–Pacific ridge and the Asian continent. Metamorphic event of theOtaki Group during 65–75 Ma was caused by thrusting of the Sam-bagawa metamorphic rocks and high geothermal gradient by theKula–Pacific ridge subduction.

Subsequent tometamorphic period around 65–75 Ma, the youngerlow-grade metamorphic event at around 50 Ma is recorded in theCretaceous Shimanto accretionary complex in the Kyushu and Shikokuareas (Agar et al., 1989; Hara and Kimura, 2008). According to Enge-bretson et al. (1985) and Maruyama (1997), the Kula–Pacific ridgehad already passed Northeast Japan by the Paleocene. Subsequent tosubduction of the Kula–Pacific ridge, a very young, hot section of thePacific Plate was subducted beneath the Asian continent during thePaleocene (Maruyama, 1997). Thermal modeling of a shallow sub-duction zone indicates that the regional extent of prehnite-bearingmetamorphic rocks in the Cretaceous Shimanto accretionary com-plex is related to the subduction of a hot slab (Miyazaki and Okumura,2002). The subduction of young plate also causes increase of geo-thermal gradientwithin accretionary complex (James et al., 1989). Thelater metamorphism around 50 Ma is related to a thermal event asso-ciated with subduction of the young and hot Pacific Plate. Low-grademetamorphism of the Kobotoke Group at around 40 Ma is consideredto be associated with subduction of the young Pacific Plate (Fig. 8b).

As noted above, the Otaki Group in the Kanto Mountains were notsubjected to younger thermal overprinting at around 50 Ma. FT datingof zircons suggests that the Otaki Group cooled below 260±50 °Cat around 54–59 Ma (Hara et al., 2007). In brief, the Otaki Group wasuplifted to shallower levels of the accretionary wedge at 50 Ma, with-out overprinting by younger thermal event (Figs. 7 and 8b).

10. Plate tectonics during the Late Cretaceous to Paleogene aroundthe northwestern Pacific region

We now reconstruct the configuration of Kula and Pacific Platesduring the Late Cretaceous to Paleogene, based on the timing andsignificance of low-grade metamorphism of the Cretaceous Shimantoaccretionary complex. The Kula–Pacific ridge was subducted beneaththe Asian continental margin during the Late Cretaceous, as indicatedby geological evidence such as uplift of the Sambagawa metamor-phic rocks (Maruyama, 1997) and large-scale magmatism recorded inSouthwest Japan (Kinoshita, 2002). The K–Ar ages of mica from thepart of Cretaceous Shimanto accretionary complex (Hanazono Forma-tion, Otaki Group) are around 70 Ma, indicating almost the same be-tween the Kii Peninsula and Kanto Mountains areas (Kurimoto, 1993;Hara and Hisada, 2005). The similarity of metamorphic ages betweenthe two areas suggests that the thermal effect associated with sub-duction of the Kula–Pacific ridge extended at least 500 km along thetrench (Fig. 8a).

Subsequent to subduction of the Kula–Pacific ridge, the young,hot Pacific Plate was orthogonally subducted beneath the Asian conti-nent during the Paleocene at a rate of 10.9 cm y−1 (Maruyama, 1997).Metamorphism associated with subduction of the young Pacific Platecontinued until 50 Ma in Kyushu and Shikoku of Southwest Japan, anduntil 40 Ma in the Kanto Mountains of Central Japan. The difference inage of the later low-grade metamorphic event recorded in the Creta-ceous Shimanto accretionary complex at Kyushu to Shikoku and thatat the Kanto Mountains is estimated to be 10 Ma. Assuming that later

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) and40 Ma (b). The left-hand figures show reconstructions of plate tectonics, modified fromMaruyama (1997) with our own interpretations. The right-hand figures show cross-sectionsthrough the accretionary prism, thermal conditions (not to scale), and the locations of relevant geological units. Thermal isograds are assumed to be parallel within the accretionaryprism, 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.

59H. Hara, T. Kurihara / Tectonophysics 485 (2010) 52–61

metamorphism reflecting the movement of the Pacific Plate, we con-sider that the subduction style of the Pacific Plate was not only orthog-onally, but also oblique with migration of the Kula–Pacific ridge tothe north. The distance between Kyushu and the Kanto Mountains isapproximately 800 km. To the north vector, the young Pacific Platemoved in tandem with the Kula–Pacific ridge at 8.0 cm y−1.

11. Summary and conclusions

Based on radiolarian fossil data, metamorphic temperatures de-rived from illite crystallinity analysis, and the timing of metamor-phism derived from illite K–Ar dating, we reconstructed the tectono-metamorphic evolution of the low-grade Cretaceous Shimantoaccretionary complex (Kobotoke Group) in the Kanto Mountains,Central Japan. The main findings of the study are summarized asfollows.

1) We report the first occurrence of radiolarian fossils from silty shalewithin a mélange unit (Kobuse Unit) in the Kobotoke Group. Theradiolarian assemblage is characterized by D. multicostata, Dic-tyomitra sp. aff. D. koslovae, A. stocki, and P. floresensis, which havebeen reported from the Santonian to Campanian. This age rangeoverlaps with the age obtained for coherent units within theKobotoke Group (Turonian to Maastrichtian).

2) Most of the illite crystallinity data indicate that the KobotokeGroup was metamorphosed at approximately 300 °C, based on ICvalues of 0.22–0.31 Δ°2θ.

3) The K–Ar ages of three samples collected from the Kobotoke Groupare 40.2±0.89, 48.3±1.1, and 38.4±0.86 Ma. The oldest K–Ar ageof 48 Ma is unsuitable in terms of interpreting the peak of low-grade metamorphism for influence of detrital mica. The ages of 38and 40 Ma are clearly younger than the Turonian to Maastrichtiandepositional age (66–94 Ma), and are assumed to represent thetiming of metamorphism.

4) Within the Cretaceous Shimanto accretionary complex distributedthroughout Southwest to Central Japan, we defined two typesof low-grade metamorphism. The first low-grade metamorphismoccurred synchronously in Southwest and Central Japan, related tothrusting of the Sambagawametamorphic rocks over the Shimantoaccretionary complex, which in turn occurred in association withsubduction of the Kula–Pacific ridge during the Late Cretaceous(65–75 Ma). The second metamorphic event is related to the ther-mal effect of the subduction of the young, hot Pacific Plate duringthe Middle Eocene (40–50 Ma). This later metamorphism contin-ued until 50 Ma in Kyushu and Shikoku, Southwest Japan, and until40 Ma in the Kanto Mountains of Central Japan. This difference intiming between Southwest and Central Japan reflects the north-ward migration of the Pacific Plate.

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Acknowledgements

We would like to thank Dr. K. Miyazaki and Dr. M. Aoya for theirvaluable comments concerning low-grade metamorphism, Dr. K.Kashiwagi and Dr. T. Tokiwa for suggestions regarding the descrip-tion of Cetaceous radiolarian fossils, and Dr. K. Hisada, Dr. N. Yagiand Mr. A. Kanematsu for their support during field surveys. Thanksare also due to the editor Professor M. Liu, and anonymousreviewers for their constructive and valuable comments of themanuscript.

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