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Studies on a new ESR signal (R signal) of fault gouges for fault dating
QuaternaryScienceReviews(QuaternaryGeochronology),Vol. 16, pp. 477-48 l, 1997.
Pergamon
PII: $0277-3791(96)00091-1
© 1997Elsevier ScienceLtd. All rights reserved. Printed in Great Britain. 0277-3791/97 $32.00
STUDIES ON A NEW ESR SIGNAL (R SIGNAL) OF FAULT GOUGES FOR FAULT DATING RYUTA HATAYA,* KAZUHIRO TANAKA* and TOSHIKATSU MIKI?
*Abiko Research Laboratory, Central Research Institute of Electric Power Industry, Abiko, Chiba 270-11, Japan ?Division of Materials Science and Engineering, Yamaguchi University, Ube, Yamaguchi 755, Japan Abstract - - A new ESR signal, designated R, is used for the ESR fault dating. The R signal is masked by the E ~ signal at commonly used microwave powers, typically 1 mW, but is clearly distinguished from it at higher microwave powers. The R signal of quartz grains of the youngest fault gouge of the Atotsugawa fault, a major active fault in central Japan, was the most intense and increased remarkably by artificial gamma-ray irradiation, while source rocks have no R signal. It indicates that precursors of paramagnetic centres associated with the R signal are produced during or after faulting and that the R signal increases later. Our experiments reveal that heating prior to artificial irradiation is ineffective for the production of the precursors, and the precursors is made by another mechanism, probably mechanical fracturing at faulting. If so, the DE value by the R signal means the equivalent dose of natural radiation which a sample received after faulting. Therefore, the R signal might directly give the age of the most recent fault movement. © 1997 Elsevier Science Ltd
INTRODUCTION
QG
signals by microwave power response. Hataya et al. (in press) also showed that the g=2.001 signal at a moderate microwave power (typically 1 mW; e.g. Kosaka and Sawada, 1985; Toyokura et al., 1985) is mixture of E ~ and R signals and that the equivalent dose (DE value) for the R signal may be lower than ones for the g=2.001 signal at a moderate microwave power in previous works. We have no idea for the source of the R signal. However, we consider that the R signal is characteristic of intrafault materials and that this signal is more useful for fault dating than the E' signal, because complete zeroing of the E' centres by mechanical fracturing has not been shown yet. Both Ariyama (1985) and Tanaka (1990) have reported complete zeroing of E' centre by the ringshearing test, but their shearing experiments have a serious problem. In their experiment, iron powder which is derived from the apparatus disturbed ESR signals and made the results look complete zeroing (Hataya and Tanaka, 1992). Moreover, Fukuchi (1989) showed that the E' centres are hardly expected to be annealed out by the frictionals hearing of faulting near the ground surface. In this paper, we describe the R signal in quartz grains taken from intrafault materials and source rocks of the Atotsugawa fault, a major active fault in Japan, and investigate effects of pre-heating on growth of the centre associated with the R signal by irradiation. From these results, we show that the R signal grows after faulting. It gives a new possibility of ESR fault dating.
The electron spin resonance (ESR) dating method is one of the potential techniques for determining the most recent movement of faults. The basic principle of this method is complete zeroing of ESR signals in the quartz grains during faulting (Ikeya et al., 1982; Miki and Ikeya, 1982). In previous works on ESR fault dating, a paramagnetic centre called E' (an oxygen vacancy with one electron: Weeks, 1956; McMorris, 1969; McMorris, 1970; Griscom, 1979), which is detected around g=2.001 at room temperature, has been used and has given ESR ages. Moreover, many researchers have studied on mechanism of resetting at faulting and increase of the E' centre; for example, resetting by mechanical fracturing (e.g. Miki and Ikeya, 1982; Ariyama, 1985; Tanaka and Shidahara, 1985; Buhey et al., 1988; Tanaka, 1990; Grtin, 1992; Lee and Schwarcz, 1994), resetting by frictional heating (e.g. Fukuchi, 1989; Lee and Schwarcz, 1994), and formation of oxygen vacancy (precursor of the E ' centre) in quartz grains by mechanical fracturing (Fukuchi, 1992). On the other hand, recently, Hataya et al. (1997) have detected a new ESR signal in quartz grains taken from fault gouges of two active faults, and designated it R signal. This signal has nearly the same g-value as the E ~ signal, but has several different characteristics: e.g. broader and isotropic spectrum, and short spinlattice relaxation time. We can distinguish the two 477
478
Quaternary Science Reviews (Quaternao, Geochronology): Volume 16 137 E30N •
137E15N
~-
36N30E
Sample Locality i
t
ol
36N20E 10km
FIG. 1. Locality map of the Atotsugawa fault and the sampling site. The Atotsugawa fault is a maior active fault in Japan. SAMPLE W e took samples taken from the shear zone of the Atotsugawa fault, a major active fault in Japan. Figure 1 shows the location of this fault and the sampling site. At this site, cataclasite derived from both Cretaceous granodiolite and andesite dyke is in fault contact with the Quaternary gravel layer (Tanaka, 1990). Figure 2 shows the detailed sketch of the new shear zone of the Atotsugawa fault at the sampling site. The new shear zone was made after Quaternary gravel was deposited. The intrafault material in the new shear zone is divided into five parts, A - E zones. The A zone is made of wet soft olive-gray clay less than 3 mm in width; it was formed at the most recent faulting, because it cuts the others. The B zone is wet soft gray-olive clay and its width varies from 0 to 20 mm, and it is the second youngest, because it cuts the others except the A zone. The C zone consists of granitic cataclasite and is 0 to 20 mm wide; however, the C zone is not so much intrafault material as a block. The D zone is less than 5 mm wide and is made of tight gray clay. The E zone is fault breccia and varies from 50 to 70 mm in width. This structure indicates that the Atotsugawa fault has moved many times during the Quaternary Era.
METHOD Quartz grains were extracted from the samples by the following procedure. The samples were first washed in water and then cleaned in 36% hydrochloric acid for about 6 hrs, in 20% sodium hydroxide for about 2 hrs, and finally in 9.2% hydrofluoric acid for about 2 hrs. Magnetic minerals were r e m o v e d with a magnetic separator. Minerals other than quartz were separated by the density separation method. Finally, the sample was sieved to 0.075-0.25 mm. G a m m a ray irradiation to quartz samples was performed using a 6°Co gamma-ray source at a dose rate of 8.73 Gy/h at the Japan Atomic Energy Research Institute.
The ESR data were taken with an X-band spectrometer (JEOL TE200) at room temperature. The first derivative spectra were obtained with the following measurement conditions: 100 kHz magnetic field modulation; modulation amplitude =0.02 mT; field scan speed =1.5 mT/min; time constant =0.1 sec and spectrum accumulation =10 times. The magnetic field intensity was calibrated with an NMR gauss meter (JEOL ES-FC5), and the microwave frequency was obtained with a microwave counter. The gvalue is determined directly from the magnetic field and microwave frequency. The peak-to-peak height of the first derivative spectrum was taken as the signal intensity.
RESULTS AND DISCUSSION
R Signal in the Sheared Zone Figure 3 shows ESR spectra of quartz grains taken from the youngest gouge (A zone). A sharp anisotropic signal of E' centres is observed around g=2.001 at microwave powers of 0.01 m W (Fig. 3a). On the other hand, a broad isotropic signal, the R signal, is clearly detected at g=2.0007 at 25 m W of microwave powers (Fig. 3c). Artificial gamma ray irradiation increases the R signal intensity of the A zone sample as depicted (Fig. 3d), but causes no significant change in the line width of the R signal and in the part of the spectrum except for the R signal. So, we can evaluate the lineshape of R signal roughly by subtracting the non-irradiated spectrum from the irradiated one. The resultant trace exhibits that R signal is almost isotropic (Fig. 3e), whereas the E / signal is anisotropic. Figure 4 shows ESR spectra at 25 m W around g=2.001 of quartz grains taken from intrafault materials and source rocks of the Atotsugawa fault. The R signal is detected and strengthened by additive irradiation in the sample of the A zone. In case of the B zone sample, the R signal is weak but recognized, and slightly strengthened. However, in case of samples in other parts and source rocks, the R signal is not detected and is not introduced after artificial irradiation. Quartz grains of the gravels have a weak
R. Hataya ef al: A New ESR Signal of Fault Gouges
signal around g=2.001. This signal might be due to E’ centre, but its intensity doesn’t increase by artificial irradiation. Therefore, not all intrafault materials have centres associated with the R signal (R centre) and host rocks do not have them. These suggest that intrafault materials which have R centres may have been made under a limited condition. This will be an issue for future investigations. Furthermore, these indicate that detailed geological observation of a shear zone at sampling and comparison of ESR signals among intrafault materials and source rocks are indispensable to obtain appropriate ESR ages. Our result reveals that the R centres are produced after faulting. However, an R center is not produced after artificial gamma irradiation, because it is detected in a natural sample. The production of the precursors of the R centres may relate to faulting directly. If natural radiation produced the precursors of the R centres and accumulated the R signal, the older sample should have larger signal
479
intensity. However, the newer sample has larger signal intensity and source rocks have no R signal. Therefore, it shows that source rocks have no precursor of the R signal, and that precursors of the R centres were not introduced by only natural radiation. One explanation of this result may be that precursors were introduced by faulting and that natural radiation since then has accumulated these centres. In such case, newer intrafault materials derived from older ones should have more precursors just after faulting and have larger signal intensity. In fact, geological structure of the new shear zone suggests that the A zone is made chiefly of the B zone (Fig. 2).
Effects of Pre-heating Irradiation
on Production
of the R Centre by
If the precursors of R centers were introduced time of faulting, it is due to frictional heating
at the and/or
the youngest fault gouge I
1Ocm FIG. 2. Detailed sketch of the outcrop at the sampling site. Cataclasitic rock derived from both Cretaceous granodiolite and andesite dyke is in fault contact with Quaternary gravels. The intrafault material which composes the fracture zone is divided into five parts, A-E zones.
480
Quaternar?; Science Reviews (Quaterna O, Geochronology): Volume 16 g=2.0018 g=2.0005 (a) natural at 0.01 mW (b) natural at 1.00 mW
(c) natural at 25.0 mW
(d) irradiated (261.9 Gy) at 25.0 mW
(e) substract
((d)-(c))
experiment using samples of granitic cataclasite, and the R signal is not clearly detected in all cases, too. The results reveal that thermal heating under 500°C with in 15 min and irradiation do not introduce the R signal. From the computer simulation of Fukuchi (1989), temperature rising over 500°C at the point within 2 mm at a slip-velocity of 100 cm/s is estimated at the depth of a few hundred metres, and continues less than 30 sec. As he said, however, assuming a dry fault surface, the above temperature rise is expected at much deeper parts. Therefore, under a wet condition, the long-term and high temperature rising more than the our annealing condition is hardly expected by faulting near the ground surface. Thus, if the precursors of the R centres are introduced at faulting, frictional heating is not effective to production of the precursors of the R centres, and mechanical fracturing may contribute.
0.5 mT FIG. 3. The ESR signal spectra around g=2.001 of quartz grains taken from gouge of the Atotsugawa fault. Trace (a) was taken at microwave power of 10 laW, trace (b) were at 1 mW and trace (c) was at 25 roW. The E~ signal is clearly detected at 10 btW, but the R signal arises at 1 mW and is better detected at 25 mW. Trace (d) is at 25 mW after artificial gamma ray irradiation (261.9 Gy). Trace (e) is the difference between traces (d) and (c). mechanical fracturing at a fault sliding plane. Therefore, to investigate the effect of frictional heating at faulting on production of the precursors of the R centres, we carried out the thermal annealing experiment using quartz grains taken from the source rocks of the Atotsugawa fault. The quartz grains were pre-heated at various temperatures before gamma ray irradiation. The pre-heating temperature was varied from 25 (room temperature) to 500°C, but the heating duration was fixed to be 15 min. Figure 5 shows spectra of pre-heated samples of the Quaternary gravel layer after irradiation. The R signal is not clearly detected in all cases. W e have done the same
E S R Fault Dating Using the R Signal The E' signal at microwave power of 0.01 m W in the A zone sample decrease by g a m m a ray irradiation while the R signal at microwave power of 25 m W increases, and the signal at g=2.001 at moderate microwave power (typically l mW, Fig. 3b) is mixture of E' signal and R signal. In such case, when the increase of the R signal intensity exceeds the decrease of the E ~ signal one, the mixture signal gradually increases as the result of the R signal increasing and the E' signal decreasing with the additive dose, otherwise the mixture signal decreases gradually. In the former case, the DE value of the R signal is smaller than that of the mixture signal. Therefore, the E' signal would not be appropriate for dating ESR ages obtained by the E' signal have to be re-examined. On the other hand, our results give a new possibility of ESR fault dating using the R signal. As mentioned above, the precursors of the R centres are introduced after faulting, and the R signal starts to grow after faulting. Mechanical fracturing at faulting may contribute to
g=2.0~7 g=2.0007 A zone
\-'----,~,j/" ~ f ~ m p . =
B zone
~..~.,L..//' /'~
250
C zone
~
250
D zone
"~-'~'-.-J~l'-'"-""~ 250 ~,_~,, ,/Q/---"/'~* 250
E zone
~
Granitic . -M--~, cataclasite ~ , , ~ , ~ Quaternary ~ , gravel ~'~ '~/
~ t'
~
50
250 -
~
room temp.
200°C
.,,,'j
'/A] /
300°C
V'~~i~//
35°°c
\~f
400oc
2~0
0.5mT
FIG. 4. The ESR signal spectra around g=2.001 at 25 mW of non-irradiated quartz grains taken from intrafault materials and source rocks of the Atotsugawa fault. The R signal is clearly detected in the A and B zone samples.
500°C 0.5mT
FIG. 5. The ESR signal spectra around g=2.001 at 25 mW of pre-heated and consequently irradiated quartz grains taken from Quaternary gravel. Heating temperature is shown on the right side of each spectrum. Irradiation dose is 261.9 Gy for all samples. The R signal is not clearly detected in all cases.
R. Hataya et al: A New ESR Signal of Fault Gouges production of the R centres. If the precursors of the R signal are produced at faulting, the DE value means the total dose which a sample has received since faulting. Therefore, in the case that source rocks have no R centre, the R signal may give the age of the most recent fault m o v e m e n t directly, without taking the condition of complete zero-setting into account, which is essentially important for other ESR signals, e.g. Ti, AI and, of course, E' signals. However, the precursors may be produced and accumulated at faulting, and older intrafault materials have the R signal, so it is not easy to date the most recent fault movement. Moreover, Hataya et al. (1997) suggested that the thermal stability of the R signal is sufficient for late Quaternary dating. Therefore, it may be appropriate for ESR dating of fault movements to use the R signal. Several veiled properties of the R signal should be made clear for establishing the new ESR dating method.
CONCLUSION W e have investigated the R signal in quartz grains taken from intrafault materials and source rocks of the Atotsugawa fault, a major active fault in Japan. The R signal is the most intense and increases by artificial gamma-ray irradiation in the youngest gouge. The R signal gives the smaller DE value than the signal at g=2.001 at moderate microwave power (typically 1 mW), because the signal at g=2.001 at moderate microwave power is the mixture of E' and R signals. Heating and consequence artificial irradiation are not effective for f o r m a t i o n o f the precursors of the R centres, so mechanical fracturing at faulting may be effective. This result shows that the precursors of the R centres are produced after faulting and that the R signal starts to grow after faulting. If the R signal is produced at faulting, when no R signal is observed in source rocks, the DE value obtained using the R signal means the equivalent dose of natural radiation which a sample received after faulting, and the ESR method using the R signal might give the age of the fault movement, without taking account the condition of complete zeroing of ESR signals during faulting. Thus, the R signal gives a new possibility of ESR fault dating.
ACKNOWLEDGEMENTS We greatly thank Dr T. Fukuchi, Dr H. Yoshida, Dr M. Chigira, Mr Y. Nakato, Mr K. Sone and Mr S. Nakakuki for their helpful suggestions and discussions. This research was financially supported by ten electric power companies in Japan.
481
REFERENCES Ariyama, T. (1985) Conditions of resetting the ESR clock during faulting. In: Ikeya, M. and Miki, T. (eds), ESR Dating and Dosimetry, pp. 251-258. IONICS, Tokyo. Buhey, W.N., Schwarcz, H.P. and GriJn, R. (1988) ESR dating of fault gouge: the effect of grain size. Quaternao" Science Reviews 7, 515-522. Fukuchi, T. (1989) Theoretical study on frictional heat by faulting using electron spin resonance. Applied Radiation and Isotopes 40, 1181-1193. Fukuchi, T. (1992) ESR studies for absolute dating of fault movements. Journal of the Geological Society, London 149, 265-272. Griscom, D.L. (1979) Point defect and radiation damage processes in c~-quartz. Proc. 33rd Frequency Control Symp., 98-109. Grtin, R. (1992) Remarks on ESR dating of fault movements. Journal of the Geological Society, London 149, 261-264. Hataya, R. and Tanaka, K. (1992) Studies on applicability of fault dating by ESR method (D-Experiments on zero-setting of ESR signals in quartz. Central Research Institute of Electric Power Industry Report No. U93019. (In Japanese with English abstract). Hataya, R., Tanaka K. and Miki, T. (1997) A new ESR signal (R signal) in quartz grains taken from fault gouges: its properties and significance for ESR fault dating. Applied Radiation and Isotopes, 48, 423-429. Ikeya, M., Miki, T. and Tanaka, K. (1982) Dating of a fault by electron spin resonance on intrafault materials. Science 215, 1392-1393. Kosaka, K. and Sawada, S. (1985) Fault gouge analysis and ESR dating of the Tsurukawa fault. In: Ikeya, M. and Miki, T. (eds), ESR Dating and Dosimeto,, pp. 257-266. IONICS, Tokyo. Lee, H.K. and Schwarcz, H.P. (1994) Criteria for complete zeroing of ESR signals during faulting of the San Gabriel fault zone, southern California. Tectonophysics 235, 317337. McMorris, D.W. (1969) Trapped-electron dating: ESR studies. Nature 222, 870-871. McMorris, D.W. (1970) ESR detection of fossil alpha damage in quartz. Nature 226, 146-148. Miki, T. and Ikeya, M. (1982) Physical basis of fault dating with ESR. Naturwiss 69, 390-391. Tanaka, K. (1990) Dating of fault movement by the ESR method: basis and application. Ph.D. thesis, University of Kyushu, Japan. Tanaka, K. and Shidahara, T. (1985) Fracturing, crushing and grinding effect on ESR signal of quartz. In: Ikeya, M. and Miki, T. (eds), ESR Dating and Dosimeto,, pp. 239-247. IONICS, Tokyo. Toyokura, I., Sakuramoto, Y., Ohmura, K., Iwasaki, E. and Ishiguchi, M. (1985) Determination of the age of fault movement--the Rokko fault. In: Ikeya, M. and Miki, T. (eds), ESR Dating and Dosimetr3', pp. 219-228. IONICS, Tokyo. Weeks, R.A. (1956) Paramagnetic resonance of lattice defects in irradiated quartz. Journal of Applied Physics 27, 1376-1384.
Pergamon
PII: $0277-3791(96)00091-1
© 1997Elsevier ScienceLtd. All rights reserved. Printed in Great Britain. 0277-3791/97 $32.00
STUDIES ON A NEW ESR SIGNAL (R SIGNAL) OF FAULT GOUGES FOR FAULT DATING RYUTA HATAYA,* KAZUHIRO TANAKA* and TOSHIKATSU MIKI?
*Abiko Research Laboratory, Central Research Institute of Electric Power Industry, Abiko, Chiba 270-11, Japan ?Division of Materials Science and Engineering, Yamaguchi University, Ube, Yamaguchi 755, Japan Abstract - - A new ESR signal, designated R, is used for the ESR fault dating. The R signal is masked by the E ~ signal at commonly used microwave powers, typically 1 mW, but is clearly distinguished from it at higher microwave powers. The R signal of quartz grains of the youngest fault gouge of the Atotsugawa fault, a major active fault in central Japan, was the most intense and increased remarkably by artificial gamma-ray irradiation, while source rocks have no R signal. It indicates that precursors of paramagnetic centres associated with the R signal are produced during or after faulting and that the R signal increases later. Our experiments reveal that heating prior to artificial irradiation is ineffective for the production of the precursors, and the precursors is made by another mechanism, probably mechanical fracturing at faulting. If so, the DE value by the R signal means the equivalent dose of natural radiation which a sample received after faulting. Therefore, the R signal might directly give the age of the most recent fault movement. © 1997 Elsevier Science Ltd
INTRODUCTION
QG
signals by microwave power response. Hataya et al. (in press) also showed that the g=2.001 signal at a moderate microwave power (typically 1 mW; e.g. Kosaka and Sawada, 1985; Toyokura et al., 1985) is mixture of E ~ and R signals and that the equivalent dose (DE value) for the R signal may be lower than ones for the g=2.001 signal at a moderate microwave power in previous works. We have no idea for the source of the R signal. However, we consider that the R signal is characteristic of intrafault materials and that this signal is more useful for fault dating than the E' signal, because complete zeroing of the E' centres by mechanical fracturing has not been shown yet. Both Ariyama (1985) and Tanaka (1990) have reported complete zeroing of E' centre by the ringshearing test, but their shearing experiments have a serious problem. In their experiment, iron powder which is derived from the apparatus disturbed ESR signals and made the results look complete zeroing (Hataya and Tanaka, 1992). Moreover, Fukuchi (1989) showed that the E' centres are hardly expected to be annealed out by the frictionals hearing of faulting near the ground surface. In this paper, we describe the R signal in quartz grains taken from intrafault materials and source rocks of the Atotsugawa fault, a major active fault in Japan, and investigate effects of pre-heating on growth of the centre associated with the R signal by irradiation. From these results, we show that the R signal grows after faulting. It gives a new possibility of ESR fault dating.
The electron spin resonance (ESR) dating method is one of the potential techniques for determining the most recent movement of faults. The basic principle of this method is complete zeroing of ESR signals in the quartz grains during faulting (Ikeya et al., 1982; Miki and Ikeya, 1982). In previous works on ESR fault dating, a paramagnetic centre called E' (an oxygen vacancy with one electron: Weeks, 1956; McMorris, 1969; McMorris, 1970; Griscom, 1979), which is detected around g=2.001 at room temperature, has been used and has given ESR ages. Moreover, many researchers have studied on mechanism of resetting at faulting and increase of the E' centre; for example, resetting by mechanical fracturing (e.g. Miki and Ikeya, 1982; Ariyama, 1985; Tanaka and Shidahara, 1985; Buhey et al., 1988; Tanaka, 1990; Grtin, 1992; Lee and Schwarcz, 1994), resetting by frictional heating (e.g. Fukuchi, 1989; Lee and Schwarcz, 1994), and formation of oxygen vacancy (precursor of the E ' centre) in quartz grains by mechanical fracturing (Fukuchi, 1992). On the other hand, recently, Hataya et al. (1997) have detected a new ESR signal in quartz grains taken from fault gouges of two active faults, and designated it R signal. This signal has nearly the same g-value as the E ~ signal, but has several different characteristics: e.g. broader and isotropic spectrum, and short spinlattice relaxation time. We can distinguish the two 477
478
Quaternary Science Reviews (Quaternao, Geochronology): Volume 16 137 E30N •
137E15N
~-
36N30E
Sample Locality i
t
ol
36N20E 10km
FIG. 1. Locality map of the Atotsugawa fault and the sampling site. The Atotsugawa fault is a maior active fault in Japan. SAMPLE W e took samples taken from the shear zone of the Atotsugawa fault, a major active fault in Japan. Figure 1 shows the location of this fault and the sampling site. At this site, cataclasite derived from both Cretaceous granodiolite and andesite dyke is in fault contact with the Quaternary gravel layer (Tanaka, 1990). Figure 2 shows the detailed sketch of the new shear zone of the Atotsugawa fault at the sampling site. The new shear zone was made after Quaternary gravel was deposited. The intrafault material in the new shear zone is divided into five parts, A - E zones. The A zone is made of wet soft olive-gray clay less than 3 mm in width; it was formed at the most recent faulting, because it cuts the others. The B zone is wet soft gray-olive clay and its width varies from 0 to 20 mm, and it is the second youngest, because it cuts the others except the A zone. The C zone consists of granitic cataclasite and is 0 to 20 mm wide; however, the C zone is not so much intrafault material as a block. The D zone is less than 5 mm wide and is made of tight gray clay. The E zone is fault breccia and varies from 50 to 70 mm in width. This structure indicates that the Atotsugawa fault has moved many times during the Quaternary Era.
METHOD Quartz grains were extracted from the samples by the following procedure. The samples were first washed in water and then cleaned in 36% hydrochloric acid for about 6 hrs, in 20% sodium hydroxide for about 2 hrs, and finally in 9.2% hydrofluoric acid for about 2 hrs. Magnetic minerals were r e m o v e d with a magnetic separator. Minerals other than quartz were separated by the density separation method. Finally, the sample was sieved to 0.075-0.25 mm. G a m m a ray irradiation to quartz samples was performed using a 6°Co gamma-ray source at a dose rate of 8.73 Gy/h at the Japan Atomic Energy Research Institute.
The ESR data were taken with an X-band spectrometer (JEOL TE200) at room temperature. The first derivative spectra were obtained with the following measurement conditions: 100 kHz magnetic field modulation; modulation amplitude =0.02 mT; field scan speed =1.5 mT/min; time constant =0.1 sec and spectrum accumulation =10 times. The magnetic field intensity was calibrated with an NMR gauss meter (JEOL ES-FC5), and the microwave frequency was obtained with a microwave counter. The gvalue is determined directly from the magnetic field and microwave frequency. The peak-to-peak height of the first derivative spectrum was taken as the signal intensity.
RESULTS AND DISCUSSION
R Signal in the Sheared Zone Figure 3 shows ESR spectra of quartz grains taken from the youngest gouge (A zone). A sharp anisotropic signal of E' centres is observed around g=2.001 at microwave powers of 0.01 m W (Fig. 3a). On the other hand, a broad isotropic signal, the R signal, is clearly detected at g=2.0007 at 25 m W of microwave powers (Fig. 3c). Artificial gamma ray irradiation increases the R signal intensity of the A zone sample as depicted (Fig. 3d), but causes no significant change in the line width of the R signal and in the part of the spectrum except for the R signal. So, we can evaluate the lineshape of R signal roughly by subtracting the non-irradiated spectrum from the irradiated one. The resultant trace exhibits that R signal is almost isotropic (Fig. 3e), whereas the E / signal is anisotropic. Figure 4 shows ESR spectra at 25 m W around g=2.001 of quartz grains taken from intrafault materials and source rocks of the Atotsugawa fault. The R signal is detected and strengthened by additive irradiation in the sample of the A zone. In case of the B zone sample, the R signal is weak but recognized, and slightly strengthened. However, in case of samples in other parts and source rocks, the R signal is not detected and is not introduced after artificial irradiation. Quartz grains of the gravels have a weak
R. Hataya ef al: A New ESR Signal of Fault Gouges
signal around g=2.001. This signal might be due to E’ centre, but its intensity doesn’t increase by artificial irradiation. Therefore, not all intrafault materials have centres associated with the R signal (R centre) and host rocks do not have them. These suggest that intrafault materials which have R centres may have been made under a limited condition. This will be an issue for future investigations. Furthermore, these indicate that detailed geological observation of a shear zone at sampling and comparison of ESR signals among intrafault materials and source rocks are indispensable to obtain appropriate ESR ages. Our result reveals that the R centres are produced after faulting. However, an R center is not produced after artificial gamma irradiation, because it is detected in a natural sample. The production of the precursors of the R centres may relate to faulting directly. If natural radiation produced the precursors of the R centres and accumulated the R signal, the older sample should have larger signal
479
intensity. However, the newer sample has larger signal intensity and source rocks have no R signal. Therefore, it shows that source rocks have no precursor of the R signal, and that precursors of the R centres were not introduced by only natural radiation. One explanation of this result may be that precursors were introduced by faulting and that natural radiation since then has accumulated these centres. In such case, newer intrafault materials derived from older ones should have more precursors just after faulting and have larger signal intensity. In fact, geological structure of the new shear zone suggests that the A zone is made chiefly of the B zone (Fig. 2).
Effects of Pre-heating Irradiation
on Production
of the R Centre by
If the precursors of R centers were introduced time of faulting, it is due to frictional heating
at the and/or
the youngest fault gouge I
1Ocm FIG. 2. Detailed sketch of the outcrop at the sampling site. Cataclasitic rock derived from both Cretaceous granodiolite and andesite dyke is in fault contact with Quaternary gravels. The intrafault material which composes the fracture zone is divided into five parts, A-E zones.
480
Quaternar?; Science Reviews (Quaterna O, Geochronology): Volume 16 g=2.0018 g=2.0005 (a) natural at 0.01 mW (b) natural at 1.00 mW
(c) natural at 25.0 mW
(d) irradiated (261.9 Gy) at 25.0 mW
(e) substract
((d)-(c))
experiment using samples of granitic cataclasite, and the R signal is not clearly detected in all cases, too. The results reveal that thermal heating under 500°C with in 15 min and irradiation do not introduce the R signal. From the computer simulation of Fukuchi (1989), temperature rising over 500°C at the point within 2 mm at a slip-velocity of 100 cm/s is estimated at the depth of a few hundred metres, and continues less than 30 sec. As he said, however, assuming a dry fault surface, the above temperature rise is expected at much deeper parts. Therefore, under a wet condition, the long-term and high temperature rising more than the our annealing condition is hardly expected by faulting near the ground surface. Thus, if the precursors of the R centres are introduced at faulting, frictional heating is not effective to production of the precursors of the R centres, and mechanical fracturing may contribute.
0.5 mT FIG. 3. The ESR signal spectra around g=2.001 of quartz grains taken from gouge of the Atotsugawa fault. Trace (a) was taken at microwave power of 10 laW, trace (b) were at 1 mW and trace (c) was at 25 roW. The E~ signal is clearly detected at 10 btW, but the R signal arises at 1 mW and is better detected at 25 mW. Trace (d) is at 25 mW after artificial gamma ray irradiation (261.9 Gy). Trace (e) is the difference between traces (d) and (c). mechanical fracturing at a fault sliding plane. Therefore, to investigate the effect of frictional heating at faulting on production of the precursors of the R centres, we carried out the thermal annealing experiment using quartz grains taken from the source rocks of the Atotsugawa fault. The quartz grains were pre-heated at various temperatures before gamma ray irradiation. The pre-heating temperature was varied from 25 (room temperature) to 500°C, but the heating duration was fixed to be 15 min. Figure 5 shows spectra of pre-heated samples of the Quaternary gravel layer after irradiation. The R signal is not clearly detected in all cases. W e have done the same
E S R Fault Dating Using the R Signal The E' signal at microwave power of 0.01 m W in the A zone sample decrease by g a m m a ray irradiation while the R signal at microwave power of 25 m W increases, and the signal at g=2.001 at moderate microwave power (typically l mW, Fig. 3b) is mixture of E' signal and R signal. In such case, when the increase of the R signal intensity exceeds the decrease of the E ~ signal one, the mixture signal gradually increases as the result of the R signal increasing and the E' signal decreasing with the additive dose, otherwise the mixture signal decreases gradually. In the former case, the DE value of the R signal is smaller than that of the mixture signal. Therefore, the E' signal would not be appropriate for dating ESR ages obtained by the E' signal have to be re-examined. On the other hand, our results give a new possibility of ESR fault dating using the R signal. As mentioned above, the precursors of the R centres are introduced after faulting, and the R signal starts to grow after faulting. Mechanical fracturing at faulting may contribute to
g=2.0~7 g=2.0007 A zone
\-'----,~,j/" ~ f ~ m p . =
B zone
~..~.,L..//' /'~
250
C zone
~
250
D zone
"~-'~'-.-J~l'-'"-""~ 250 ~,_~,, ,/Q/---"/'~* 250
E zone
~
Granitic . -M--~, cataclasite ~ , , ~ , ~ Quaternary ~ , gravel ~'~ '~/
~ t'
~
50
250 -
~
room temp.
200°C
.,,,'j
'/A] /
300°C
V'~~i~//
35°°c
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400oc
2~0
0.5mT
FIG. 4. The ESR signal spectra around g=2.001 at 25 mW of non-irradiated quartz grains taken from intrafault materials and source rocks of the Atotsugawa fault. The R signal is clearly detected in the A and B zone samples.
500°C 0.5mT
FIG. 5. The ESR signal spectra around g=2.001 at 25 mW of pre-heated and consequently irradiated quartz grains taken from Quaternary gravel. Heating temperature is shown on the right side of each spectrum. Irradiation dose is 261.9 Gy for all samples. The R signal is not clearly detected in all cases.
R. Hataya et al: A New ESR Signal of Fault Gouges production of the R centres. If the precursors of the R signal are produced at faulting, the DE value means the total dose which a sample has received since faulting. Therefore, in the case that source rocks have no R centre, the R signal may give the age of the most recent fault m o v e m e n t directly, without taking the condition of complete zero-setting into account, which is essentially important for other ESR signals, e.g. Ti, AI and, of course, E' signals. However, the precursors may be produced and accumulated at faulting, and older intrafault materials have the R signal, so it is not easy to date the most recent fault movement. Moreover, Hataya et al. (1997) suggested that the thermal stability of the R signal is sufficient for late Quaternary dating. Therefore, it may be appropriate for ESR dating of fault movements to use the R signal. Several veiled properties of the R signal should be made clear for establishing the new ESR dating method.
CONCLUSION W e have investigated the R signal in quartz grains taken from intrafault materials and source rocks of the Atotsugawa fault, a major active fault in Japan. The R signal is the most intense and increases by artificial gamma-ray irradiation in the youngest gouge. The R signal gives the smaller DE value than the signal at g=2.001 at moderate microwave power (typically 1 mW), because the signal at g=2.001 at moderate microwave power is the mixture of E' and R signals. Heating and consequence artificial irradiation are not effective for f o r m a t i o n o f the precursors of the R centres, so mechanical fracturing at faulting may be effective. This result shows that the precursors of the R centres are produced after faulting and that the R signal starts to grow after faulting. If the R signal is produced at faulting, when no R signal is observed in source rocks, the DE value obtained using the R signal means the equivalent dose of natural radiation which a sample received after faulting, and the ESR method using the R signal might give the age of the fault movement, without taking account the condition of complete zeroing of ESR signals during faulting. Thus, the R signal gives a new possibility of ESR fault dating.
ACKNOWLEDGEMENTS We greatly thank Dr T. Fukuchi, Dr H. Yoshida, Dr M. Chigira, Mr Y. Nakato, Mr K. Sone and Mr S. Nakakuki for their helpful suggestions and discussions. This research was financially supported by ten electric power companies in Japan.
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