TechnicalNoteof the National
for Earth
Science
Disaster Prevention,
272; etJune,
ZirconFission-Track
DatingResearchof NIED InstituteNojima Fault
Drilling
Cores atand
Hirabayashi
Borehole – R.No.
Yamadaal. 2005
��

Zircon 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�
Zircon 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. The age distribution ranging from ca. 57 to 69 Ma is concordant with previous reports on FT ages of the
Ryoke Granites that comprise the basement at Nojima-Hirabayashi area. This indicates that samples at fracture zones havenot undergone secondary heating up to zircon FT closure temperature (~ 250°C) by Quaternary tectonics including 1995
activity, and thus gives constraints on upper limit of the effect of earthquake related heat at the fracture zone along theNojima core.
�Key words: Thermal anomaly, Fracture zone, Zircon 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., zircon, apatite) 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 240 and 110°C for zircon
and apatite, respectively; e.g., Gallagher et al., 1998.) so�that they can serve as sensitive indicators for thermal
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, 2005;
Murakami, 2005).
We performed FT dating using zircon on the granitic
samples around the FZs to assess the earthquake-related
thermal anomalies based on the closure temperature forzircon. Samples were collected from the 1,838 m boreholecore drilled by National Research Institute for earth
ScienceandDisasterPrevention(NIED)
atNojima-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 zircon FT ages at 1,140
and 1,310 m fracture zone is presented and the constraints
on upper limit of the effect of earthquake related heat at
the fracture zone along the Nojima core is discussed in

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

Technical Note of the National Research Institute for Earth Science and Disaster Prevention, No. 272; June, 2005

terms of the closure temperature of the dating systemapplied.
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 that consist mainly of granodiorite(Mizuno et al., 1990) with hornblende and biotite 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 of collected
samples along the core. Detailed description is given
elsewhere (e.g., Tanaka et al., 2001; Kobayashi et al.,2001a; Kobayashi et al., 2001b). Each of three FZs, whichis recognized as the distribution of cataclastic rocks, has awidth of approximately 30 to 100 m along the core lengthwith 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 1,300 m FZ, a fault gouge 10-20 cm thick at
1313.9 m and that several cm at 1289.5 m were identified
respectively (Kobayashi et al., 2001b). A central part ofeach FZ (CFZ) is defined here by the existence ofpseudotachylyte and/or fault gouge of 10’s cm thickness
where the largest slip is expected from the view point ofmaterial distribution in each FZ that is indicated with boldlines on core columns in Fig. 2. Eight samples for each FZ
at 1,140 m and 1,300 m were collected from fresh or
cataclastic granitic rocks in FZs, the depths and distancesFig. 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).

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.

from the closest CFZ being shown in Table 1. The
correlated distance of samples to suppositional spread ofeach CFZ is approximated at first order by multiplying thecoefficient of 0.5, based on the inclination of fault gougeobserved as approximately 30° to the core axis (Kobayashi
et al., 2001a).
Sufficient zircons were concentrated using crushing,sieving, panning, and standard magnetic and heavy liquid
separation techniques. Zircons are euhedral for all thesamples. FT dating was carried out using the external
detector method, which is applied to internal-polishedsurfaces of zircon 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
above and below the observed internal surfaces due to the−2−Zircon Fission-Track Dating of NIED Nojima Fault Drilling Cores at Hirabayashi Borehole – R. Yamada et al.

Table 1. Results of zircon 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 1140.6, 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 zircon analysis is 380 ± 3 (1 σ). The 2π/4 π 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 zircon was 30 hours for NHFT06 and 18 hours for others. Uranium
content is assessed by the correlation between spontaneous and induced tracks, and thus not necessarily concordant withresults of chemical analysis.

zoned 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.

scale seems unlikely. Another is the preferential breakageof grains with high uranium content due to the immensepressure at the CFZ. Strength of crystals with highuranium or thorium content tends to weaken because ofmetamiction caused by internal exposure of alphaemission from radioactive elements. Highly metamicted
crystals might selectively be broken under high pressure to

3. Results
Fig. 3 shows depth profiles of zircon FT ages anduranium content measured by densities of spontaneous andinduced FTs. Zircon FT ages range between ~ 57-69 Ma
and show no significant variation within 2σ error level(Fig. 3). This range of zircon FT age agrees with theprevious works for Ryoke Granitic Rocks (Tagami et al.,
1988, 1995). This indicates that samples at fracture zones
have not undergone secondary heating up to zircon FT
closure temperature (~ 250°C) by Quaternary tectonics
including 1995 activity. NHFT06 located just beneath the
CFZ shows anomalously low uranium content comparedwith other samples. Two hypotheses are possible for thisdifference in crystal property. One is migration of thematerial in the size of crystals (~ 100 µm) along the CFZ.
According to the observation of cataclastic texture andsize of fractures around the CFZ, the migration in this�
−3−Fig. 3.

Zircon 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.

Technical Note of the National Research Institute for Earth Science and Disaster Prevention, No. 272; June, 2005

be disqualified for age dating, and accordingly the average
uranium content of crystals employed for dating mightlower.
Acknowledgements
This research was supported by basic research fund of
NIED, 2002, in relation with the project of Research on
Earthquake Occurrence.
References�
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Tomida, N., Matsuda, T., Yamazaki, A. (2001a):
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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.

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Kobayashi, K., Shimada, K., Arai, T., Tomita, T.,
Hirano, S. (2001): Complete fault rock distribution
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experiments. in prep.
(Accepted: April 11, 2005)−4−

Zircon Fission-Track Dating of NIED Nojima Fault Drilling Cores at Hirabayashi Borehole – R. Yamada et al.
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Zircon 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. 272; June, 2005

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