Technical Note of the National Research Institute for Earth Science and Disaster Prevention, No. 273; June, 2005
��

Apatite Fission-Track Dating of NIED Nojima Fault Drilling Cores
at Hirabayashi Borehole�����
Ryuji YAMADA , Tatsuo MATSUDA , and Kentaro OMURA�Solid Earth Science Research Group,
National Research Institute for Earth Science and Disaster Prevention, Japan ��ryamada@bosai.go.jp, mtatsuo@bosai.go.jp, omura@bosai.go.jp ���Abstract�Apatite fission-track (FT) dating is performed on 16 samples from two fracture zones at the depths of 1,140 and 1,310 m
observed along the 1,838 m borehole core penetrating the Nojima Fault, drilled at Nojima-Hirabayashi, Awaji Island,
Japan, by National Research Institute for Earth Science and Disaster Prevention just after the 1995 Hyogo-ken Nanbu
earthquake. Distribution of apatite FT age is between ca. 47 to 67 Ma, broader than that of zircon age. Apatite FT ages of
two samples just below the centre of fracture zones are significantly younger than zircon ages, indicating the heterogeneityof the effect of secondary heating related to fault activity. High temperature crustal fluid is a strong candidate as the sourceof the secondary heat considering the distance of the heterogeneity to the centres of fracture zones.
Key words: Thermal anomaly, Fracture zone, Apatite fission-track dating, Nojima fault���

1. Introduction
Thermochronological analysis with radiometric dating
methods around active faults may reveal the history ofheat generation and transport associated with fault
activities in and around fracture zones (FZs), and thus isimportant in understanding the dynamics of active faults.
There have been several attempts to use thermochronology
to demonstrate thermal anomalies around natural faults(e.g., Scholz, 1979; Tagami et al., 1988; Xu and Kamp,
2000; Comacho et al., 2001) focusing mainly on the broad
anomalies produced by the accumulation of heat overgeologic time scale. Of various radiometric dating
methods, the fission track (FT) method has several merits
for thermochronological analysis around active faults (e.g.,
Gallagher et al. 1998). (1) Environmental factors tominerals such as pressure and fluid acidity other thantemperature do not affect FT annealing behavior
significantly. (2) Minerals commonly used for analyses(i.e., apatite, zircon) are durable to weathering so that they
are likely to survive under the hydrothermal conditions
around FZ. (3) Closure temperatures for FT methods are
relatively low (approximately 110 and 240°C for apatite
and zircon, respectively; e.g., Gallagher et al., 1998.) sothat they can serve as sensitive indicators for thermal�

Tennodai 3-1, Tsukuba, Ibaraki, 305-0006, Japan−1−

events in the upper crust. (4) FT measurement allows
quantitative analysis of the heating or cooling behavior ofrocks by means of inverse modeling of thermal history
using laboratory based annealing kinetics. (5) Recent
laboratory annealing experiments have confirmed zircon
FT system is reset under the impulsive heating at ~1000°C for a couple of seconds at which pseudotachylyte
is supposed to be formed (Yamada et al., 2003, 2005a;
Murakami, 2005).
We performed FT dating using apatite on the granitic
samples around the FZs to assess the earthquake-related
thermal anomalies based on the closure temperature forapatite in addition to zircon analysis (Yamada et al.,2005b). Samples were collected from the 1,838 mborehole core drilled by National Research Institute forearth Science and Disaster Prevention (NIED) at
Nojima-Hirabayashi, Awaji Island, Japan, to penetrate theNojima fault just after the shock (Ikeda, 2001). Three
major FZs were found along the core at about 1,140, 1,310
and 1,800 m. The distribution of apatite and zircon FTages at 1,140 and 1,310 m fracture zone is presented andthe constraints on upper limit of the effect of earthquake
related heat at the fracture zone along the Nojima core isdiscussed in terms of the closure temperature of the dating

Technical Note of the National Research Institute for Earth Science and Disaster Prevention, No. 273; June, 2005

system applied.
2. Samples and experiments
The Nojima-Hirabayashi NIED borehole was drilled to adepth of 1,838m, approximately 320 m southeast of thesurface trace of the Nojima Fault on Awaji Island (Fig. 1).
The basement of the island is comprised of Cretaceous
Ryoke Granitic Rocks which consist mainly of
granodiorite (Mizuno et al., 1990) with hornblende andbiotite K-Ar ages ranging from 88 ± 4 to 90 ± 5 Ma and 70± 4 to 88 ± 4 Ma, respectively (Takahashi, 1992). Fig. 2
shows simplified lithological description and depths ofcollected samples along the core. Detailed description is
given elsewhere (e.g., Tanaka et al., 2001; Kobayashi etal., 2001a; Kobayashi et al., 2001b). Each of three FZs,
which is recognized as the distribution of cataclastic rocks,
has a width of approximately 30 to 100 m along the core
length with fault gouges in the central part. In the 1,140 m
FZ, pseudotachylyte was identified at 1,140.6 m (Tanaka
et al., 2001). In the 1300 m FZ, a fault gouge 10-20 cm
thick at 1313.9 m and that several cm at 1289.5 m wereidentified respectively (Kobayashi et al., 2001b). A
central part of each FZ (CFZ) is defined here by theexistence of pseudotachylyte and/or fault gouge of 10’s
cm thickness where the largest slip is expected from theview point of material distribution in each FZ that isindicated with bold lines on core columns in Fig. 2. Eightsamples for each FZ at 1,140 m and 1300 m were collectedfrom fresh or cataclastic granitic rocks in FZs, the depths

Fig. 1. Geological map showing the Nojima Fault and thedrilling site location (34 34’ 42.9’’ N, 134 58’
23.6’’ E, 65 m altitude). Modified from Mizuno etal. (1990).

�−2−Fig. 2. Depth profiles of geological column and samples
analyzed. Sample codes are given on the rightside. Widths of three gouges at center of fracturezones from the top are approximately 10, 10 and
20 cm, respectively.

and distances from the closest CFZ being shown in Table
1. The correlated distance of samples to suppositional
spread of each CFZ is approximated at first order bymultiplying the coefficient of 0.5, based on the inclinationof fault gouge observed as approximately 30° to the coreaxis (Kobayashi et al., 2001a).
Sufficient apatites were concentrated using crushing,sieving, panning, and standard magnetic and heavy liquid
separation techniques. Apatites are euhedral for all thesamples. FT dating was carried out using the external
detector method, which is applied to internal-polishedsurfaces of apatite grains (ED1 method; Gleadow, 1981).
Ages were calculated following the ζ-approach (Hurford,1990). The detailed description of experimental procedureand system calibration is documented elsewhere (e.g.,
Danhara et al., 1991; Danhara et al., 2003). The results arelisted in Table 1. 30 grains with good shape andhomogeneous spontaneous track distribution were selected
randomly and employed for age determination for eachsample. Some analyses failed the χ2-test at the 5%
significance level (Galbraith, 1981) although thedispersion of single grain ages was not significant. This isprobably because of the failing tendency of the χ2-test in
ED1 data that are affected by additional variation otherthan the Poisson variation in track counts. Danhara et al.
(1991) pointed out that the non-Poisson variation for ED1
method is caused by the difference in uranium contents

Apatite Fission-Track Dating of NIED Nojima Fault Drilling Cores at Hirabayashi Borehole – R. Yamada et al.
Table 1. Results of apatite FT dating.

Sample depth is measured along the borehole from the ground surface. Distance indicates the interval to the closest
center of fracture zones at 1,140.6, 1289.5 and 1313.9 m. Negative sign means a sample located shallower than therelevant gouges. 30 grains were employed for each measurement. All track densities ( ρ) are given in 10 5 tracks/cm2 , with
the numbers of counted tracks (N) in parentheses. All samples were analyzed with the external detector method, usingζ -calibration with dosimeter glass SRM 612. The ζ value for apatite analysis is 335 ± 5 (1 σ). The 2π/4π geometry
correction factor is 0.5. P(χ2 ) is the probability of obtaining χ2 value for ν degrees of freedom where ν = (number ofcrystals - 1) (Galbraith, 1981). Etching time of apatite was 4 minutes. Uranium content is assessed by the correlation
between spontaneous and induced tracks, and thus not necessarily concordant with results of chemical analysis.

above and below the observed internal surfaces due to thezoned distribution of uranium, which was observed forsome grains. In this study, 30 grains were measured foreach because it is desirable to measure greater number ofgrains to overcome this non-Poison factor (> 25; Green,1981). Thus, the low score of χ2-test does not necessarilymean grains have been derived different sources for eachsample.

m below the CFZ. In the 1,310 m FZ, apatite FT age
descends toward the CFZ and the youngest age is 47.1 ±3.8 Ma for NHFT16 located 3.6 m below the CFZ. The
difference in age between apatite and zircon for these two
samples is significant at 2σ error level (Table 1), and thatfor other 14 samples is almost insignificant at 1σ error
level. The discordance between apatite and zircon FT ages

3. Results
Fig. 3 shows depth profiles of apatite FT ages anduranium content measured by densities of spontaneous andinduced FTs. Apatite FT age varies between ~ 47 to ~ 67
Ma (Fig. 3). It is suggested that some samples near CFZ
have undergone secondary heating up to apatite FT closure
temperature (~ 110°C) although the timing of thesecondary heating is not sure because whether apatite FT
system in these samples was reset or not is uncertain yet.
Similar asymmetric distribution of apatite ages relative to
CFZ is recognized in the lower parts of 1,140 m and 1,310
m FZs, whilst the difference is almost insignificant at 1 σerror level for NHFT05 and NHFT15 that are located at
0.8 and 0.3 m above the each CFZ, respectively. In the
1,140 m FZ, apatite FT age descends toward the CFZ and
the youngest age is 48.6 ± 6.0 Ma for NHFT06 located 1.9

�Fig. 3. Apatite FT ages and uranium content versus
sample depths along the borehole core. CFZ are
indicated with dotted lines. Uranium content ismeasured by track counting methods. Error bars
are 1σ standard error.

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project by NIED in the vicinity of the 1995 Hyogo-ken
Nanbu earthquake, Japan. The Island Arc, 10, 199-205.
10) Kobayashi, K., Hirano, S., Arai, T., Ikeda, R., Omura,K., Sano, H., Sawaguchi, T., Tanaka, H., Tomita, T.,
Tomida, N., Matsuda, T., Yamazaki, A. (2001a):
Distribution of fault rocks in the fracture zone of theNojima Fault at a depth of 1,140 m: Observations fromthe Hirabayashi NIED drill core. The Island Arc, 10,411-421.
11) Kobayashi, K., Arai, T., Ikeda, R., Omura, K.,
Shimada, K., Tanaka, H., Tomida, T., Hirano, S.,
Matsuda, T. (2001b): Textures of fault rocks in thefracture zone of the Nojima Fault at a depth of 1,300 m:
Observations from the Hirabayashi NIED drilling core.
Rep. NIED, 61, 223-229.
12) Mizuno, K., Hattori, H., Sangawa, A., Takahashi, T.
(1990):
GeologyoftheAkashi
district,
quadrangle-series (in Japanese with English abstract),scale 1:50,000, 90pp. Geol. Surv. Japan.
13) Murakami, M., Yamada, R., Tagami, T. (2005):
Short-term annealing characteristics of spontaneous
fission tracks in zircon: A qualitative description, Chem.
Geol. Submitted.
14) Scholz, C.H. (1979): Frictional metamorphism, argon
depletion, and tectonic stress on the Alpine Fault, New
Zealand. J. Geophys. Res. 84, 6770-6782.
15) Tagami, T., Lal, N., Sorkhabi, R.B., Nishimura, S.
(1988): Fission track thermochronologic analysis of the
Ryoke Belt and the Median Tectonic Line, Southwest
Japan. J. Geophys. Res., 93, 13705-13715.
16) Takahashi, Y. (1992): K-Ar ages of the granitic rocksin Awaji Island with an emphasis on timing of
mylonization (in Japanese with English abstract),
Gankou, 87, 291-299.
17) Tanaka, H., Matsuda, T., Omura, K., Ikeda, R.,
Kobayashi, K., Shimada, K., Arai, T., Tomita, T.,
Hirano, S. (2001): Complete fault rock distribution
analysis along the Hirabayashi NIED core penetrating
the Nojima Fault at 1,140 m depth, Awaji Island,Southwest Japan. Rep. NIED, 61, 195-221.
18) Xu, G., Kamp, P.J.J. (2000): Tectonics and denudationadjacent to the Xianshuihe fault, eastern Tibetan
Plateau:
Constraints
fromfissiontrack
thermochronology. J. Geophys. Res., 105, 19231-19251.
19) Yamada, R., Murakami, M., Tagami, T. (2003): Zircon
fission track annealing: short-term heating experimenttoward the detection of frictional heat along active
faults. Geochim. Cosmochim. Acta, 67, A548.
20) Yamada, R., Galbraith, R.F., Murakami, M., Tagami, T.
(2005a): Statistical modelling of annealing kinetics offission-tracks in zircon; reassessment of laboratory
experiments. in prep.

at very local depths reflects the secondary heating there.
Because the conductive heat from the centre of fracturezones generated by frictional heat at the fault activity
seems insufficient to heat samples located at the distanceof some tens of centimeters, high temperature crustal fluid
is a strong candidate as the source of the secondary heat.
Simulating the conductive heat under such condition isrequired in order to confirm this supposition. It should be
noted that the definition of CFZ decides whether samples
are located above CFZ or not. CFZ is defined here by theexistence of pseudotachylyte and fault gouge where thelargest slip is expected in each FZ. Uranium content
measured with the correlation between spontaneous andinduced track densities in apatite is lower near CFZ except
samples in the hanging wall side of 1,140 m CFZ. As
inferred by the variation in content of uranium in zircons(Yamada et al., 2005b), the preferential breakage of grains
with high uranium content due to the immense pressure at
the CFZ is also suggested.
Acknowledgements
This research was supported by basic research fund of
NIED, 2003, in relation with the project of Research on
Earthquake Occurrence.
References�
1) Comacho, A., McDougall, I., Armstrong, R., Braun, J.
(2001): Evidence for shear heating, Musgrave block,central Australia. J. Struct. Geol., 23, 1007-1013.
2) Danhara, T., Kasuya, M., Iwano, H., Yamashita, T.
(1991): Fission-track age calibration using internal andexternal surfaces of zircon. J. Geol. Soc. Japan, 97,
977-985.
3) Danhara, T, Iwano, H., Yoshioka, T., Tsuruta, T. (2003):
Zeta calibration values for fission track dating with adiallyl phthalate detector. J. Geol. Soc. Japan, 109,665-668.
4) Galbraith, R.F. (1981): On statistical models for fission
track counts. Math. Geol., 13, 471-488.
5) Gallagher, K., Brown, R., Johnson, C. (1998): Fission
track analysis and its applications to geologicalproblems. Annu. Rev. Earth Planet. Sci., 26, 519-572.
6) Gleadow, A.J.W. (1981): Fission-track dating method:what are the real alternatives? Nucl. Track Detection, 2,
105-117.
7) Green, P.F. (1981): A new look at statistics infission-track dating. Nucl. Tracks, 5, 77-86.
8) Hurford, A.J. (1990): Standardization of fission track
dating calibration: Recommendation by the Fission
Track Working Group of the I.U.G.S. Subcommission ofGeochronology. Chem. Geol., 80, 171-178.
9) Ikeda, R. (2001): Outline of the fault zone drilling�

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Apatite Fission-Track Dating of NIED Nojima Fault Drilling Cores at Hirabayashi Borehole – R. Yamada et al.

21) Yamada, R., Matsuda, T., Omura, K. (2005b): Zircon
Fission-Track Dating of NIED Nojima fault Drilling
Cores at Hirabayashi Borehole. Tech. Note NIED, 272,

�

1-38.
(Accepted: April 11, 2005)−5−

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Apatite Fission-Track Dating of NIED Nojima Fault Drilling Cores at Hirabayashi Borehole – R. Yamada et al.

Appendix
Four sets of tables and figures in two pages comprise supplemental information on analytical results for each sample.
(A): top of first page
Single grain age distribution list. Statistical aspects at the bottom of the list are obtained to describe the distribution ofeach parameter of individual grains. Therefore the arithmetical mean of the number of tracks, track density, and single
grain age are irrelevant to parameters in Table 1 that are obtained by ζ-approach.
(B): bottom of first page
Analyst’s comprehensive evaluation on results of individual samples(C): top of second page
Photographs of each grain analyzed after etching(D): bottom of second page
Statistical aspects of analytical resultsa. Age histogramb. Correlation between the number of spontaneous tracks and the number of induced tracksc. Correlation between the density of spontaneous tracks and the density of induced tracks
d. Age spectra
e. Radial plotf. U content histogram

Conversion of sample codes in this appendix to those in main text is as follows.

�

Text

Appendix

Text

Appendix

NHFT01
NHFT02
NHFT03
NHFT04
NHFT05
NHFT06
NHFT07
NHFT08

FT020531(1)FT020531(2)FT020531(3)FT020531(4)FT020531(5)FT020531(6)FT020531(7)FT020531(8)NHFT11NHFT12
NHFT13
NHFT14
NHFT15NHFT16NHFT17NHFT18

FT020531(9)FT020531(10)FT020531(11)FT020531(12)FT020531(13)FT020531(14)FT020531(15)FT020531(16)−7−Technical Note of the National Research Institute for Earth Science and Disaster Prevention, No. 273; June, 2005

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