10th Asian Regional Conference of IAEG (2015)

Formative mechanism of inhomogeneous distribution of fractures, an example of theToki Granite, Central Japan
Eiji SASAO(1), Takashi YUGUCHI(2), Yasuto ITOH(3), Takashi INOUE(3)and Masayuki ISHIBASHI(4)(1) Tono Geoscience Center, Japan Atomic Energy Agency, Japan
E-mail:sasao.eiji@jaea.go.jp(2) Japan Atomic Energy Agency, presently Yamagata University, Japan,(3) Graduate School of Science, Osaka Prefecture University, Japan,(4) Tono Geoscience Center, Japan Atomic Energy Agency, Japan
Abstract
Understanding of the fracture network is important for safety assessment for disposalof high-level radioactive waste (HLW). The number of fractures in Japanese granites ismuch more than those in Sweden and Finland. Such differences are generally believed to becaused by geological setting of which Japan locates in tectonically active plate boundary
while Sweden and Finland locate in the stable East European craton.
Japan Atomic Energy Agency pursues the geoscientific study in the Toki Granite,
distributed in Central Japan, to establish scientific basis for HLW disposal in Japan. In thispaper, we present our hypothesis on the formative mechanism of inhomogeneous
distribution of fracture in the Toki Granite.
In the Toki Granite, fractures are abundant at the shallower part where both low- andhigh-angle fractures are dominant, while less at the deeper part where high-angle fracture isdominant. Low-angle fracture was formed by unloading during the exhumation of granite
prior to deposition of the overlying sedimentary sequence. Distribution of the high-angle
fracture is inhomogeneous.
Thermochronological study revealed that the incipient cooling rate was c.30-70oC/million years from monazite chemical Th–U–total Pb isochron method closure to zircon
fission-track (FT) closures in the range of c.70 to 60 Ma. This was quite rapid comparedwith that of c.3-13 oC/million years from zircon FT to apatite FT closures in the range ofc.60 to 20 Ma. Paleomagnetic study of the intact granite indicated that the paleomagnetic
directions were dispersed, even though any structural disturbance has not been observed.
This suggests granite was plastically deformed during rapid cooling period.
The rapid cooling might cause inhomogeneous distribution of cooling strain. Whenthe granite reached to brittle deformation field, inhomogeneous fracture distribution wasformed by the inhomogeneous strain. If so, recognition of the cooling history of the granitic
body is essential to understand the distribution of the fracture network.
Keywords: fracture, cooling rate, thermochronology, paleomagnetism, Toki Granite,
Mizunami Underground Research Laboratory
1.

Introduction

Such differences are generally believed to be caused
by geological setting of which Japan locates intectonically active plate boundary while Sweden and
Finland locate in the stable East European craton.
Japan Atomic Energy Agency pursues thegeoscientific research and development project
namely by the Mizunami Underground ResearchLaboratory (MIU) Project in the Toki Granite,
distributed in Central Japan, to establish scientific

Fracture in rock can act as groundwater pathways.
Therefore, understanding of the fracture network isimportant for construction of underground facility
and also for safety assessment for disposal ofhigh-level radioactive waste (HLW). The number offractures in Japanese granites is much more thanthose in northern Europe such as Sweden and Finland.

110th Asian Regional Conference of IAEG (2015)

basis for HLW disposal in Japan. In this paper, we
present our hypothesis on the formative mechanismof inhomogeneous distribution of fracture in the Toki
Granite, on the basis of observation of fractures in theshafts and galleries at the MIU site as well as drill
cores, and thermochronological and paleomagnetic
studies at and around the MIU site.
2.

(whole-rock Rb–Sr, Shibata and Ishihara, 1979). The
Toki granite is a stock, 14×12 km2 in areal extent(Ishihara and Suzuki, 1969), with a vertical thicknessof a least 1.5 km based on borehole investigations.
The Toki granite intruded Jurassic sedimentary rocksof the Mino Terrane and the Nohi Rhyolite (85±5 Ma,
allanite CHIME dating; Suzuki et al., 1998). The
granite is overlain unconformably by the Mizunami
and Tokai Groups (Itoigawa, 1974, 1980). The Toki
granite is characterized by systematic spatial changes
in rock facies (mode and mineral assemblage) and acorresponding change in bulk chemical composition,and thus is recognized as a zoned pluton (Yuguchi etal., 2010).

Geological setting

The MIU is under construction in the Mizunami
City, Gifu Prefecture, Central Japan (Fig. 1). Geology
around the MIU site consists of Jurassic–Cretaceous
accretionary
complex(Mino
Terrane),Cretaceous–Paleogene volcano–plutonic complex(the Nohi Rhyolite, the Toki Granite and granitic
rocks in the Ryoke Belt), Miocene Mizunami Group(c.20–15 Ma; Sasao et al., 2006, 2011) and
Mio–Pliocene Tokai Group (c.12–1.5 Ma; Todo
Collaborative Research Group, 1999), in ascendingorder (e.g. Sasao, 2013; Saegusa and Matsuoka,2011).

3.

Macroscopic fractures are well developed in theToki Granite (e.g. Fujii, 2000). The previous
surface-based investigation such as Saegusa andMatsuoka (2011) revealed that two structural domains,
an upper highly fractured domain (UHFD) and alower sparsely fractured domain (LSFD), have been
distinguished in Toki granite based on the fracture
frequency with low dip angle (<30 o). The low angle
fractures are considered to be formed by unloading
during exhumation of the granite prior to depositionof the overlying Mizunami Group.
We have investigated the distribution and modeof occurrence of fractures at the MIU site (e.g.
Ishibashi et al., 2014). The investigation has been
performed at the main and ventilation shafts whichwere excavated to the depth of c.500 meters below
ground surface, and horizontal galleries excavated at
the GL-300 and -500 meters levels (Fig. 1). Fracture
data was also acquired by the borehole investigation.
Data on strike and dip of fracture were mainly
obtained by numerical analysis of borehole television(BTV) image. At present, more than 50 boreholes has
been drilled in the MIU site both from surface andunderground galleries. In this study, data obtained by
two boreholes drilled at the GL-300 and -500 meters
galleries were used for discussion (Fig. 1).
In the ventilation shaft, the boundary between
UHFD and LSFD is located at the -460 meters below
ground level (GL-460m) (Fig. 2). In the UHFD, highdip angle fractures are densely developed with low
dip angle (>30 o) fractures in the upper part (above
GL-300m). Below the boundary, fracture density isapparently decreased.
Based on the horizontal borehole data drilledfrom the GL-300m and GL-500m galleries, dense andsparse fracture densities are repeatedly appeared (Fig.
3). This tendency is observed in both of UHFD at the
GL-300m and LSFD at the GL-500m.

MIZ-1 borehole ★

Ventilation

Investigation of distribution of fractures at the
MIU site

Main Shaft

Shaft-300m Access/Research
Gallery-500m Access/Research
Gallery

Fig. 1 Location and facility allocation of the
Mizunami Underground Research Laboratory(MIU). Red lines indicate the location of boreholes
examined, and yellow star indicates the location of
MIZ-1 borehole referred in Figure 4.
The Toki granite is one of the Late Cretaceous
plutonic bodies in the Sanyo Belt (Ishihara andChappell, 2007), and has been dated at 68.3±1.8 Ma(monazite chemical Th–U–total Pb isochron method;
CHIME, Suzuki and Adachi, 1998) and 72.3±3.9 Ma

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10th Asian Regional Conference of IAEG (2015)

UHFD
LSFD

Fig. 2 Sketch map of fracture distribution with in the ventilation shaft (development view of shaft wall), andSchmidt stereonet diagrams showing fracture orientations (lower hemisphere projections of poles tofracture).
The low dip angle (>30 o) fractures are mainly developed above GL-300m.
4.

Cooling rate of the Toki Granite inferredfrom geochronological data

Apatite fission-track dating was carried out at
Kyoto Fission-Track Co. Ltd. Apatite samples wereanalyzed by the external detector method (ED1,internal mineral surface) using the geometry factor of
0.5 (Gleadow, 1981).
We adopted chronological data obtained from thesingle borehole to avoid local differences of coolinghistory as shown in Fig. 4. The result indicates thatthe incipient cooling rate was c.30–70 oC/million
years from monazite Pb to zircon FT closures in the

In the Toki Granite, many chronological data
have been obtained such as monazite CHIME age(Suzuki and Adachi, 1998), whole rock Rb–Sr age(Shibata and Ishihara, 1979), and hornblende K–Ar,
biotite K–Ar and zircon fission-track (FT) ages
(Yuguchi et al., 2011) with some newly obtained
apatite FT ages (Table 1).

Table 1 Compilation of age dating at the Toki Granite.
MethodAge (Ma)

Closure temperature (oC)

Reference

monazite CHIME

68.3±1.8c.700 ?

Suzuki and Adachi (1998)

whole rock Rb–Sr

72.3±3.9c.700 ?

Shibata and Ishihara (1979)

hornblende K-Ar

74.3±2.6510±25

Yuguchi et al. (2011)

biotite K-Ar
zircon fission-track

78.5±3.9 to 59.7±1.5
75.6±3.3 to 52.8±2.6

300±50240±50

Yuguchi et al. (2011)
Yuguchi et al. (2011)

apatite fission-trackc.32-49120±20This study3

10th Asian Regional Conference of IAEG (2015)

range of c.70 to 60 Ma. This was quite rapid
compared with that of c.3–13 oC/million years fromzircon FT to apatite FT closures in the range of c.60
to 40 Ma.
5.

horizontal borehole at the GL-300m (Figs. 1 and 3).
Geographical coordinates of potentially rotated coresamples were carefully determined by comparison of
azimuths of more than one fractures between core andborehole surface observed by BTV image. Some
examples of magnetic measurement are shown in Fig.
5 on the equal-area projections of site-mean magnetic

Paleomagnetic Study
Six intact granite samples were collected from the

(a)

(b)

MI2205MI2202

Fig. 3 Density of fractures confirmed by borehole investigation at two different level. (a) at the GL-300m,
and (b) at the GL-500m.
Dark blue and pale blue indicate horizontal distribution of fault core and surrounding damaged zone,
respectively. Arrows indicate sampling points for paleomagnetic study.

Fig. 4 Relationship between age dating and closed temperature (cooling history) of the Toki
Granite. K-Ar and fission-track ages are obtained from the MIZ-1 borehole drilled in the MIU
site, as shown in Figure 1. CHIME and Rb-Sr ages are quoted from Suzuki and Adachi (1998)
and Shibata and Ishihara (1979), respectively.

410th Asian Regional Conference of IAEG (2015)

directions. The directions of natural remanent
magnetization (NRM) are dispersed (Fig. 5). The
radius of the 95 % confidence circle is in the range of
19 to 30.
Magnetite is estimated to be the dominant
contributor of the stable remanent magnetization on
the basis of the stepwise acquisition experiment ofisothermal remanent magnetization.
6.

Characterization of water conducting fracture andtheir long-term behavior in deep crystalline rock: acase study of the Toki Granite. Journal of the Japan
Society of Engineering Geology, Vol. 55, No. 4,
pp.156-165 (in Japanese with English abstract).
Ishihara, S., and Chappell, B. (2007): Chemicalcompositions of the late Cretaceous Ryoke
granitoids of the Chubu District, central Japan –Revisited, Bulletin of the Geological Survey ofJapan, Vol. 58, No. 9/10, pp. 323-350.
Ishihara, S., and Suzuki, Y. (1969): Basement
granites of the Toki uranium deposits in Tono
region, Reports of the Geological Survey of Japan,
No. 232, pp. 113-127.
Itoigawa, J. (1974): Geology of the Mizunami district,central Japan, Bulletin of the Mizunami Fossil
Museum, No. 1, pp. 9-42 (in Japanese).
Itoigawa, J. (1980): Geology of the Mizunami district,central Japan, Monograph of the Mizunami Fossil
Museum, No. 1, pp. 1-50 (in Japanese).
Maruyama, S. and Seno, T. (1986): Orogeny andrelative plate motions: example of the Japaneseislands, Tectonophysics, Vol. 127, Issues 3-4, pp.
305-325.
Saegusa, H. and Matsuoka, T., eds. (2011): Final
Report on the Surface-based investigation Phase(Phase I) at the Mizunami Underground ResearchLaboratory Project, Japan Atomic Energy Agency
Reports, JAEA-Research 2010-067, 377p.
Sasao, E. (2013): Petrographic study of the Miocene
Mizunami Group, Central Japan: Detection of
unrecognized volcanic activity in the Setouchi
Province, Island Arc, Vol. 22, Issue 2, pp. 170-184.
Sasao, E., Iwano, H. and Danhara, T. (2006): Fission
track ages of tuffaceous sandstone from the Toki
Lignite-bearing Formation of the Mizunami Groupin the Tono district, Gifu Prefecture, central Japan,
Journal of Geological Society of Japan, Vol. 112,
No. 7, pp. 459-468.
Sasao, E., Danhara, T., Iwano, H. and Hayashi, J.
(2011): Fission track ages of the Miocene Mizunami
and Iwamura Groups in southeastern Gifu
Prefecture, Central Japan, Journal of GeologicalSociety of Japan, Vol. 117, No. 8, pp. 476-481.
Shibata, K., and Ishihara, S. (1979): Rb-Sr
whole-rock and K-Ar mineral ages of granitic rocksin Japan, Geochemical Journal, Vol. 13, No. 3, pp.
113-119.
Suzuki, K., and Adachi, M. (1998): Denudation
history of the high T/P Ryoke metamorphic belt,
southwest Japan: constraints from CHIME monazite
ages of gneisses and granitoids, Journal of
Metamorphic Geology, Vol. 16, Issue 1, pp. 27-37.
Suzuki, K., Nakazaki, M., and Adachi, M. (1998) An
855 Ma CHIME age for the Agigawa welded tuff
sheet in the oldest volcanic sequence of the Nohi
Rhyolite, central Japan, Journal of Earth andPlanetary Sciences, Nagoya University, Vol. 45, pp.

Discussion

Key finding of this study is dispersion of the
NRM direction. The measured granite is intact
without apparent deformation in the core-scale andwas carefully sampled from the narrow range of thecore without fracture. Such careful treatment ofsample preparation avoids artificial error of thepaleomagnetic measurement.
Curie temperature of magnetite is 580 oC (e.g.
Butler, 1992). Fracture network could be formed at
the brittle regime, i.e. below 300–400 oC. Thissuggests granite was plastically deformed during thetemperature regime ranging from c. 570 to 300–400
o
C. At this time, cooling rate was quite high, assuggested by closure temperature and age dating(Table 1, Fig. 4).
The pacific plate began to subduct at 70 million
years ago (Maruyama and Seno, 1986). Thus theJapanese islands was placed under compressed field
up to now. Such compressed stress field could cause
rapid uplift of the granitic body, indicated by rapid
cooling rate, and might cause inhomogeneous plastic
deformation like “starch syrup”. This mode of
deformation
could
generate
inhomogeneous
distribution of cooling strain.
When the granite reached to brittle deformation
field, fractures were formed by the inhomogeneous
strain. At the part of strain concentration, fracture
could be densely developed.
If it is the case that the above-mentioned model istrue, recognition of the cooling history of the granitic
body is essential to understand the distribution of thefracture network.
References
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Domains to Geologic Terranes, Blackwell Science
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in the Tono district, central Japan, Journal of
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249-263.
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methods: what are the real alternatives? Nuclear
Tracks, Vol. 5, Nos. 1/2, pp. 3-14.
Ishibashi, M., Ando, T., Sasao, E., Yuguchi, T.,
Nishimoto, S. and Yoshida, H. (2014):

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10th Asian Regional Conference of IAEG (2015)17-27.
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pluton: an example from the Toki granite, Central
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Yuguchi T., Tsuruta T., Nishiyama T. (2010): Zoningof rock facies and chemical composition in the Toki
granitic body, Central Japan. , Japanese Magazineof Mineralogical and Petrological Sciences, Vol. 39,
No. 2, pp. 50-70, (in Japanese with English
abstract).

Fig. 5 Equal-area lower hemisphere projections of site-mean magnetic directions obtained from the some intact
granite samples from GL-300m.
Dm and Im, mean declination and mean inclination in degrees, respectively; α95, radius of 95% confidence
circle in degrees; N, number of specimens; κ, Fisherian precision parameter, respectively.

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