フィッション・トラック ニュースレター

第 18 号

19-21 2005 年

野島断層の FT 熱年代学研究
田上高広・村上雅紀・永原健嗣

Fission-track thermochronology of the Nojima fault zone
Takahiro Tagami, Masaki Murakami and Kenji Nagahara

京都大学大学院理学研究科，Graduate School of Science, Kyoto Univ.

Quantitative assessment of heat generation

within 25 m from the fault in the GSJ-750.

and transfer associated with a fault motion isOn

critically important to constrain the thermal

conduction modeling as well as the generalbudget of orogeny as well as to estimate thepositive correlation between the FT annealing

shear stress that controls the energy budget

and deformation/alteration of borehole rocks,of earthquakes.

These thermal signatures

those cooling ages in both boreholes probably

also provide a tool for constraining the ages

represent ancient thermal overprints by heatof faults.

dispersion or transfer via fluids in the fault

In the present contribution, wethebasisof

one-dimensionalheatsummarize our recent results of the zircon

zone.

fission-track (FT) thermochronologic analysis

indicates the resulted temperature increase ofof the Nojima fault, which was activated1duringhomogeneously

the1995(Hyogoken-Nanbu

Kobeearthquake

earthquake;

Calculation of in-situ heat dispersion

degreeC,

iftheandfrictional

heatisinstantaneously

M7.2)

dispersed via fluids to a 3 m-wide zone.

(Tagami et al., 2001; Murakami et al., 2002;

Because such a small temperature increase

Murakami and Tagami, 2004).

Rock samples

does not advance significantly the zircon FT

were collected from the University Groupannealing, it is likely that the thermal

500 m (UG-500) borehole, Geological Survey

overprints were caused by migration of hotof Japan 750 m (GSJ-750) borehole, the fault

fluids along the fault zone from deep crustal

trench at Hirabayashi, and nearby outcrops.

interior.

In the two boreholes that penetrate the fault

FTs of the 2 - 10 mm thick pseudotachylyte

at depth, zircon FTs were partially annealed

layer were totally annealed and subsequently

in the samples neaby the fault (Figures 1 andcooled through ZPAZ at 56 Ma, which is2).

interpreted as the time of (final stage) of

The age of onset of cooling from theFor the fault trench samples, zircon

zircon partial annealing zone (ZPAZ) was

pseudotachylyte formation (Figure 3).

estimated by the inverse modeling of FT datasuggested, therefore, that the present Nojima

using the Monte Trax program; i.e., 2.5 Ma

fault was formed in the Middle Quaternary by

within 3 m (in the hanging wall only) fromreactivating an ancient fault initiated at 56 Ma

the fault plane in the UG-500, and 35 Ma

at mid-crustal depth.

19It isReferences
Tagami, T., Hasebe, N., Kamohara, H. andsystem: Zircon fission-track evidence of Nojima

Takemura, K., 2001. Thermal anomaly aroundGSJ 750m borehole samples. Geophys. Res. Lett.,

Nojima fault as detected by the fission-track

29, doi: 10.1029/2002GL015679.

analysis of Ogura 500M borehole samples. TheMurakami, M. and Tagami, T., 2004.

Island Arc, 10, 457-464.

Datingpseudotachylyte of the Nojima fault using theMurakami, M., Tagami, T. and Hasebe, N., 2002.

zircon fission-track method. Geophys. Res. Lett.,

Ancient thermal anomaly of an active fault

31, doi: 10.1029/2004GL020211

Figure 1. Zircon FT length and age versus distance to the fault in the GSJ 750 borehole. Zircons from theoutcrop are characterized by mean lengths of ˜10 - 11 µm and unimodal length distributions, showing no signsof appreciable reduction of FT length. In contrast, those from adjacent to the fault at depth show significantly
reduced means of ˜6 - 8 µm and distributions having two components of long and short tracks. Error bars
are ±2 SE. Note that the track-length distribution is shown in a histogram with different patterns for tracks
marking azimuth angles greater (solid column) and less (open column) than 60° to the crystallographic c-axis,
because track lengths in zircon depend on etching and annealing properties which show an angular variation.
Also shown are the distribution of fault rocks (Tanaka et al., 1999) and sample localities (Murakami et al., 2002).

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Figure 2. Zircon mean FT length versus distance to the fault in the UG 500 borehole, with track
length histograms of representative samples. Zircons from localities >30 m away from the fault plane
as well as those from outcrops are characterized by the mean lengths of ˜10 - 11 µm and unimodaldistributions with negative skewness, showing no signs of appreciable reduction of FT length. Incontrast, those from adjacent to the fault at depth show significantly reduced means of ˜6 - 8 µm anddistributions having a dominant peak around 6 - 7 µm with rather positive skewness. Error bars are±1 SE. Also shown is the lithology of the borehole section (Murata et al., 2001), along with the threesubzones within the fracture zone that represent dominant fault rock types (Tanaka et al., 2001);
subzone 1 = non- to weakly deformed and altered rocks, subzone 2 = weakly deformed and alteredrocks, subzone 3 = fault breccia and fault gouge.

Figure 3. The photograph (A) and sketch (B) of a sampled fault rock section in the Hirabayashi trench,with a plot of mean age, mean length and length distributions of zircon fission tracks measured. NT-PTb
is also plotted (open circle). The blue boxes represent the mean age and length of NF-HB2 (the Ryoke
host rock sample). The sample from the pseudotachylyte layer (NT-PTa) has an age significantly younger
than that of initial cooling of the Ryoke host rock sample (NF-HB1 and 2). The photograph (C) is“syringe-shaped” tracks found in NT-UG1, which shows the evidence of partial annealing. Error bars
are ±1 SE. After Murakami and Tagami (2004).

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