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ESR dating of the subsidiary faults in the Yangsan fault system, Korea
Quaternary Science Reviews 20 (2001) 999}1003
ESR dating of the subsidiary faults in the Yangsan fault system, Korea夽 Hee-Kwon Lee *, Henry, P. Schwarcz Division of Earth Science, Kangwon National University, Chunchon, Kangwon-do, 200-701, South Korea School of Geography and Geology, McMaster University, Hamilton, Ontario, L8S 4M1, Canada
Abstract The Yangsan fault system is located near heavy industrial complexes and nuclear power plants in the Korean peninsula. We carried out ESR dating of fault rocks from the Wonwonsa and Ihwa faults, two subsidiary faults of the Yangsan fault system. ESR ages are divided into three groups in order of decreasing con"dence: multiple plateau ESR ages; single plateau ESR ages; and maximum ESR ages. The Wonwonsa fault was reactivated at least 4 times and the Ihwa fault at least 2 times within the Quaternary period. Long-term cyclic fault activity of some subsidiary faults of the Yangsan fault system occurred in the Pleistocene, and suggests the possibility of continued activity on this fault system. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Heavy industrial complexes and nuclear power plants have been constructed along the southeastern coastal region of the Korean peninsula. Geoscientists have tried to evaluate potential seismic hazards associated with these industrial complexes. Because the Yangsan fault system is located near the southeastern part of the Korean peninsula (Fig. 1), most of the research has been focused on the geometry of this fault system and the temporal and spatial pattern of fault activity. However, there is still controversy about the movement on the Yangsan fault system continued into the Quaternary period. Some researchers suggest that the Yangsan fault system was reactivated during the late Quaternary period, based on the results of trench studies along the Yangsan fault system (Okada et al., 1994; KIGAM, 1998, Chapter 5). On the other hand, others insist that no major fault movement has occurred along the Yangsan fault system during the Quaternary period (KIGAM, 1998, Chapter 4). At present, there are not enough data to limit the timing of fault activity on the Yangsan fault system. In order to study this problem, we collected fault rock samples from the Wonwonsa fault and the Ihwa fault which are subsidiary faults of the Yangsan fault system. The time of last movement of fault rock bands were 夽
Paper published in December 2000. * Corresponding author. E-mail address: heekwon@cc kangwon.ac.kr (H.-K. Lee).
determined by the ESR plateau dating method. The relationship between ESR ages and the structural features within the fault rock zones were used to evaluate the space}time pattern of fault activities of these two faults.
2. ESR plateau dating of fault rocks Since Ikeya et al. (1982) "rst proposed the ESR dating method for dating the last movement of a fault, several researchers have dated fault movements using this method (Fukuchi et al., 1986; Lee and Schwarcz, 1994, 1996). The methods of dating fault rocks have been described in various papers (Fukuchi et al., 1986; Buhay et al., 1988; Lee and Schwarcz, 1994; Schwarcz and Lee, 2000) and was recently reviewed by Rink (1997). The method is based on the observation that several di!erent ESR signals in quartz (the E', OHC, Al and Ti amongst others) are reduced in intensity to near zero as a result of both local frictional heating and lattice deformation during fault movements. Subsequently, electrical charges produced by environmental ionizing radiation are trapped at charge-trapping defects in the quartz resulting in the growth of intensities of ESR signals. Therefore, the intensities of ESR signals are proportional to the time elapsed since the faulting event. The ESR age can be determined from the ratio of the equivalent dose (D ) to # the dose rate (d). In applications of ESR dating of fault movement, the ESR signal is supposed to have been reset to zero during
0277-3791/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 5 5 - X
1000
H.-K. Lee, H.P. Schwarcz / Quaternary Science Reviews 20 (2001) 999}1003
3. Methods Fault rock samples were collected from the Wonwonsa and the Ihwa fault as shown in Fig. 2. The samples were lightly disaggregated and sieved. Size fractions of 25}45, 45}75, 75}100, 100}150, 150}250 lm were prepared, and treated to isolate quartz, using magnetic separation, and attack with #uoboric and #uosilicic acid. Aliquots of the quartz were irradiated with Co gamma rays, and these together with an unirradiated aliquot were analyzed on a Bruker ESR spectrometer (EMX 300). The E signals were obtained at room temperature. Instrumental settings for E signals were: microwave frequency"9.44 GHz; microwave power"0.1 mW; scan width"5 mT; scan time"164 ms; modulation frequency"100 kHz; modulation amplitude"0.1 mT; and time constant"0.05 s. The A1 spectra can only be obtained at lower temperatures (here 77 K) due to short spin relaxation times at room temperature. For the Al spectra, the microwave power"2 mW, scan width"20 mT, and the other settings were the same as the room temperature measurements. The data were processed using the DATA program of R. GruK n.
4. Structural geology and results of ESR dating of fault rocks Sketches of outcrops with sampling localities and ESR ages are shown in Fig. 2. Typical examples of the plot of ESR ages vs. grain sizes are shown in Fig. 3; ESR data and calculated dates are described in Table 1. 4.1. The Wonwonsa fault Fig. 1. (a) Map of East Asia showing Korean Peninsula. (b) The Yangsan fault system and the locality of ESR dating of fault rocks.
fault movements. It is essential to demonstrate that the ESR signal was fully zeroed at the time of faulting events. Several criteria are now available to show completeness of zeroing (Buhay et al., 1988; Fukuchi, 1988; Lee and Schwarcz, 1994). In particular, Lee and Schwarcz (1994) proposed that grain boundary frictional sliding during fault movement should result in zones at the grain periphery which are zeroed more by both localized lattice deformation and local heating. Toyoda and Schwarcz (1996) used abrasion to study the spatial distribution of ESR centres in quartz isolated from the fault rock. They found that as the surface layers were removed by abrasion, ESR signals became larger, in agreement with the predictions by Lee and Schwarcz (1994). Rink et al. (1999) also found that heat-induced sensitization of OSL signals in quartz was the greatest in the "nest grains in the gouge.
A subhorizontal fault is exposed in an outcrop near the Wonwonsa temple. A complex of Quaternary conglomerate unconformably deposited on early Tertiary granite (KIGAM, 1998) is thrust over a footwall of Quaternary sedimentary rock and granite. A fault gouge zone 1}30 cm thick branches and merges on a scale of a few meters within the granite. The contact surface of the hanging wall side is very sharp and distinct but the foot wall contact is irregular. Cataclastic foliation is developed in parts of the gouge zone. Subrounded to rounded fragments of granite occur in the gouge. ESR ages were obtained on two samples collected about 1 m apart within a single fault gouge band in the granite (Won 3, 4). A plateau ESR age of 130$10 ka was obtained from E signals in fractions of Won 3. ESR ages of Won 4 decrease with decreasing grain size but do not show any plateau in the plot of ESR age vs. grain size. We can only estimate a maximum ESR age of 200$30 ka which is consistent with the plateau age for the other sample. Sample Won 5 was taken from a 1 cm thick gouge zone between granite and Quaternary deposits.
H.-K. Lee, H.P. Schwarcz / Quaternary Science Reviews 20 (2001) 999}1003
1001
Fig. 2. Sketches of the outcrops and ESR ages of (a) the Wonwonsa fault and (b) the Ihwa fault.
Fig. 3. Examples of ESR ages vs. grain sizes. (a) multiple plateau age, (b) multiple age and/or plateau age, (c) plateau age, (d) maximum age.
ESR ages obtained by Al and E signals of Won 5 gave a multiple plateau ESR age of 400$20 ka. Won 7 was collected from the hanging wall side of a gouge zone 30 cm thick between granite and Quaternary deposits. Al signals of this sample gives a plateau age of 460$50 ka which is consistent with the plateau age of Won 5 within the error range. Three samples were collected from three
branched fault gouge zones in the granite (Won 10, 11, 12). ESR ages for the Al signal of Won 10 decreased with decreasing grain size. The 25}45 lm fraction gave an ESR age of 230$60 ka which would therefore allow us to obtain an assumed maximum age of last movement on this branch. E signals of all grain sizes of Won 10 appeared to be saturated. Both E and Al signals of Won
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H.-K. Lee, H.P. Schwarcz / Quaternary Science Reviews 20 (2001) 999}1003
Table 1 Age estimates for time of last movement on faults Fault
Locality
Sample
Measured signals
ESR date (ka)
Con"dence
Plateau signals
Wonwonsa Wonwonsa Wonwonsa Wonwonsa Wonwonsa Wonwonsa Wonwonsa Wonwonsa Ihwa Ihwa Ihwa Ihwa
Main band Main band Boundary Hanging Branch Branch Branch Subsidiary Main band Branch Bounding Subsidiary
Won3 Won4 Won5 Won7 Won10 Won11 Won12 Won15 Ihwa1 Ihwa2 Ihwa3 Ihwa4
E E E , E , E , E , E , E , E , E , E , E ,
130$10 200$30 400$20 460$50 230$60 '3000 710$50 480$30 660$50 1300$100 '3000 '3000
Plateau Maximum Multiple Plateau Maximum Saturated Plateau Plateau Multiple Multiple Saturated Saturated
E
A1 A1 A1 A1 A1 A1 A1 A1 A1 A1
11 appeared to be either in steady state or saturated. Therefore, the last movement within this branch occurred more than 3 Ma ago (based on the estimated time needed to achieve steady state for these samples). Only the two "nest fractions of Won 12 gave "nite ages for Al signals, averaging 710$50 ka. The larger grains of this sample appeared to be saturated. Won 15 was collected from the gouge zone of one of the subsidiary faults developed in the footwall of the Wonwonsa fault. ESR ages for Al signals of Won 15 generate a plateau age of 480$20 ka. E signals of all grain sizes of this sample are in saturation.
4.2. The Ihwa fault The Ihwa fault, forming the boundary between Tertiary granite and Quaternary deposits, is exposed on the wall of the Ihwa creek at Hohye-dong in Ulsan. The gouge zone 1}2 cm thick branches and merges at the bottom of the creek. The central block about 20 cm thick, bounded by fault gouge, is composed of undeformed granite and basic dyke. Fault breccia (about 40 cm thick) is developed in a fractured granite and in contact with gouge zone but there is no evidence of deformation in the Quaternary alluvial sedimentary rocks, even near the fault gouge. Ihwa 1 was collected from the gouge zone between Quaternary deposits and granite. ESR ages for E and Al signals of Ihwa 1 gave a multiple plateau ESR age of 660$50 ka. Ihwa 2, a sample of gouge between fault breccia of granite and a basic dyke, gaves a multiple plateau of 1,300$100 ka. Ihwa 3 was taken from a fault breccia in granite developed adjacent to gouge of the Ihwa fault. Ihwa 4 was taken from a gouge zone about 20 m away from the Ihwa fault. ESR signals of both Ihwa 3 and 4 are in saturation indicating that faulting events occurred more than 3 Ma ago.
E , A1 A1
A1 A1 E , A1 E , A1
5. Discussion and conclusions Toyoda and Schwarcz (1997) have found the formation of a counterfeit E signal in the quartz by c-ray irradiation. The intensity of E' signal of some samples (Won 10, 11, 15 and Ihwa 3 and 4) collected from the gouge in homogeneous granite does not grow systematically by c-ray irradiation, which is characteristic for the saturated samples. Some of the E ages for the "nest fractions gave the same ESR ages as were obtained using the Al signal, while the others gave ages which were older than those using Al signals, or were in saturation. This suggests that the E signals generated by c-ray irradiation of these samples were not counterfeit. The sequence of resetting sensitivity of the ESR signals is not systematic (cf. Fig. 3a and b). The zeroing of ESR signals during fault movement was controlled by numerous factors such as temperature, stress, displacement, thickness of gouge zone, #uid, rock composition, as well as strain rate and grain size. Therefore, we cannot generalize the sequence of resetting sensitivity of the ESR signals during fault movements. We have performed ESR dating of fault rocks using both the plateau method developed at McMaster University and the multiple centre method by Fukuchi et al. (1986). We can divide estimated ESR ages into three groups in order of decreasing con"dence (Fig. 3): Multiple plateau ESR age: the ESR ages decreased systematically with decreasing grain size, reaching a minimum value (plateau) for grains below a critical grain size for more than two ESR signals (e.g., E and Al) and give the same plateau ages (Fig. 3a); Multiple ESR age and/or Plateau ESR age: (1) at least one signal gave a plateau ESR age and the ESR ages from the other signal decreased with decreasing grain size, approching the plateau ESR ages (Fig. 3b) or (2) at least one signal gave a plateau ESR age below some critical grain size (Fig. 3c); Maximum ESR age: (1) the ESR ages did not decrease systematically with decreasing grain size or (2) they did not show any
H.-K. Lee, H.P. Schwarcz / Quaternary Science Reviews 20 (2001) 999}1003
concordant ages or plateau ages (Fig. 3d). In this case, the age given by the "nest fraction is assumed to be a maximum estimate of time since fault movement. The time pattern of fault activity on the Wonwonsa fault deduced from ESR dating of fault rocks can be summarised as follows: this zone was formed or reactivated at least 3 Ma ago. A branch fault in the footwall was developed about 700 ka ago. After that movement, the Wonwonsa fault was reactivated about 440}500 ka and 200}250 ka ago. The last movement which we have recorded occurred about 130 ka ago. These data suggest that the Wonwonsa fault zone was reactivated at least 4 times during the Quaternary period. The evolution of the Ihwa fault is summarized as follows: a fault breccia zone in granite cut by the Ihwa fault zone was formed over 3 Ma ago. After the formation of this breccia zone in the granite, a basic dyke intruded along the breccia zone. A gouge zone was later formed between the basic dyke and fault breccia about 1.4 Ma ago. A later gouge zone was formed between Quaternary deposits and the granite about 650 ka ago. In summary, we see evidence from these ESR dates of continued and prolonged faulting activity on these branches of the Yangsan fault system throughout the Quaternary. This suggests that this fault system may continue to be active today and to present some seismic hazard to structures in its vicinity.
Acknowledgements This research was performed for the National Hazards Prevention Research Project, one of the Critical Technology-21 Programs, funded by the Ministry of Science and Technology of Korea. H.K. Lee wishes to acknowledge the "nancial support of KIGAM in contract with RIDE (the research institute for development of earth resources of Kangwon National University). We thank Dr. Ueechan Chwae and Dr. Deng-Yong Cho at KIGAM
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for guiding to the site and their help during "eld work. References Buhay, W.M., Schwarcz, H.P., GruK n, R., 1988. ESR dating of fault gouge: the e!ect of grain size. Quaternary Science Reviews 7, 515}522. Fukuchi, T., 1988. Applicability of ESR dating using multiple centres to fault movement * The case of the Itoigawa-Shizuoka tectonic line, a major fault in Japan. Quaternary Science Reviews 7, 509}514. Fukuchi, T., Imai, N., Shimokawa, K., 1986. ESR dating of fault movement using various defect centres in quartz; the case in the western South Fossa Magna, Japan. Earth and Planetary Science Letters 78, 121}128. Ikeya, M., Miki, T., Tanaka, K., 1982. Dating of a fault by Electron Spin Resonance on intrafault materials. Science 215, 1392}1393. KIGAM, 1998. Final report of the re-evaluation to the design base earthquake considering the Yangsan Fault. KIGAM (Korea Institute of Geology, Mining and Materials), Korea, 435 pp. Lee, H.K., Schwarcz, H.P., 1994. Criteria for complete zeroing of ESR signals during faulting of the San Gabriel fault zone, southern California. Tectonophysics 235, 317}337. Lee, H.K., Schwarcz, H.P., 1996. ESR plateau dating of periodicity of activity on the San Gabriel fault zone, southern California. Geological Society of America Bulletin 108, 735}746. Okada, A., Watanabe, M., Sato, H., Jun, M.S., Jo, W.R., Kim, S.K., Jeon, J.S., Chi, H.C., Oike, K., 1994. Active fault topography and trench survey in the central part of the Yangsan fault, Southeast Korea. Journal of Geography of Japan 103, 111}126. Rink, W.J., 1997. Electron spin resonance (ESR) dating and ESR applications in Quaternary science and archaeology. Radiation Measurements 27, 975}1025. Rink, W.J., Toyoda, S., Rees-Jones, J., Schwarcz, H.P., 1999. Thermal activation of OSL as a geothermometer for quartz grain heating during fault movements. Radiation Measurements 30, 97}105. Schwarcz, H.P., Lee, H.K., 2000. Electron spin resonance dating of fault rocks. In: Sowers, J., Noller, J. Lettis, W.J. (Eds.) Quaternary Geochronology: Methods and Applications, AGU Reference Shelf Series, 4, 177}186. Toyoda, S., Schwarcz, H.P., 1996. The spatial distribution of ESR in fault gouge revealed by abrading technique. Applied Radiation and Isotopes 47, 1409}1413. Toyoda, S., Schwarcz, H.P., 1997. The hazard of the counterfeit E signal in quartz to the ESR dating of fault movements. Quaternary Science Reviews (Quaternary Geochronology) 16, 483}486.
ESR dating of the subsidiary faults in the Yangsan fault system, Korea夽 Hee-Kwon Lee *, Henry, P. Schwarcz Division of Earth Science, Kangwon National University, Chunchon, Kangwon-do, 200-701, South Korea School of Geography and Geology, McMaster University, Hamilton, Ontario, L8S 4M1, Canada
Abstract The Yangsan fault system is located near heavy industrial complexes and nuclear power plants in the Korean peninsula. We carried out ESR dating of fault rocks from the Wonwonsa and Ihwa faults, two subsidiary faults of the Yangsan fault system. ESR ages are divided into three groups in order of decreasing con"dence: multiple plateau ESR ages; single plateau ESR ages; and maximum ESR ages. The Wonwonsa fault was reactivated at least 4 times and the Ihwa fault at least 2 times within the Quaternary period. Long-term cyclic fault activity of some subsidiary faults of the Yangsan fault system occurred in the Pleistocene, and suggests the possibility of continued activity on this fault system. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Heavy industrial complexes and nuclear power plants have been constructed along the southeastern coastal region of the Korean peninsula. Geoscientists have tried to evaluate potential seismic hazards associated with these industrial complexes. Because the Yangsan fault system is located near the southeastern part of the Korean peninsula (Fig. 1), most of the research has been focused on the geometry of this fault system and the temporal and spatial pattern of fault activity. However, there is still controversy about the movement on the Yangsan fault system continued into the Quaternary period. Some researchers suggest that the Yangsan fault system was reactivated during the late Quaternary period, based on the results of trench studies along the Yangsan fault system (Okada et al., 1994; KIGAM, 1998, Chapter 5). On the other hand, others insist that no major fault movement has occurred along the Yangsan fault system during the Quaternary period (KIGAM, 1998, Chapter 4). At present, there are not enough data to limit the timing of fault activity on the Yangsan fault system. In order to study this problem, we collected fault rock samples from the Wonwonsa fault and the Ihwa fault which are subsidiary faults of the Yangsan fault system. The time of last movement of fault rock bands were 夽
Paper published in December 2000. * Corresponding author. E-mail address: heekwon@cc kangwon.ac.kr (H.-K. Lee).
determined by the ESR plateau dating method. The relationship between ESR ages and the structural features within the fault rock zones were used to evaluate the space}time pattern of fault activities of these two faults.
2. ESR plateau dating of fault rocks Since Ikeya et al. (1982) "rst proposed the ESR dating method for dating the last movement of a fault, several researchers have dated fault movements using this method (Fukuchi et al., 1986; Lee and Schwarcz, 1994, 1996). The methods of dating fault rocks have been described in various papers (Fukuchi et al., 1986; Buhay et al., 1988; Lee and Schwarcz, 1994; Schwarcz and Lee, 2000) and was recently reviewed by Rink (1997). The method is based on the observation that several di!erent ESR signals in quartz (the E', OHC, Al and Ti amongst others) are reduced in intensity to near zero as a result of both local frictional heating and lattice deformation during fault movements. Subsequently, electrical charges produced by environmental ionizing radiation are trapped at charge-trapping defects in the quartz resulting in the growth of intensities of ESR signals. Therefore, the intensities of ESR signals are proportional to the time elapsed since the faulting event. The ESR age can be determined from the ratio of the equivalent dose (D ) to # the dose rate (d). In applications of ESR dating of fault movement, the ESR signal is supposed to have been reset to zero during
0277-3791/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 0 0 ) 0 0 0 5 5 - X
1000
H.-K. Lee, H.P. Schwarcz / Quaternary Science Reviews 20 (2001) 999}1003
3. Methods Fault rock samples were collected from the Wonwonsa and the Ihwa fault as shown in Fig. 2. The samples were lightly disaggregated and sieved. Size fractions of 25}45, 45}75, 75}100, 100}150, 150}250 lm were prepared, and treated to isolate quartz, using magnetic separation, and attack with #uoboric and #uosilicic acid. Aliquots of the quartz were irradiated with Co gamma rays, and these together with an unirradiated aliquot were analyzed on a Bruker ESR spectrometer (EMX 300). The E signals were obtained at room temperature. Instrumental settings for E signals were: microwave frequency"9.44 GHz; microwave power"0.1 mW; scan width"5 mT; scan time"164 ms; modulation frequency"100 kHz; modulation amplitude"0.1 mT; and time constant"0.05 s. The A1 spectra can only be obtained at lower temperatures (here 77 K) due to short spin relaxation times at room temperature. For the Al spectra, the microwave power"2 mW, scan width"20 mT, and the other settings were the same as the room temperature measurements. The data were processed using the DATA program of R. GruK n.
4. Structural geology and results of ESR dating of fault rocks Sketches of outcrops with sampling localities and ESR ages are shown in Fig. 2. Typical examples of the plot of ESR ages vs. grain sizes are shown in Fig. 3; ESR data and calculated dates are described in Table 1. 4.1. The Wonwonsa fault Fig. 1. (a) Map of East Asia showing Korean Peninsula. (b) The Yangsan fault system and the locality of ESR dating of fault rocks.
fault movements. It is essential to demonstrate that the ESR signal was fully zeroed at the time of faulting events. Several criteria are now available to show completeness of zeroing (Buhay et al., 1988; Fukuchi, 1988; Lee and Schwarcz, 1994). In particular, Lee and Schwarcz (1994) proposed that grain boundary frictional sliding during fault movement should result in zones at the grain periphery which are zeroed more by both localized lattice deformation and local heating. Toyoda and Schwarcz (1996) used abrasion to study the spatial distribution of ESR centres in quartz isolated from the fault rock. They found that as the surface layers were removed by abrasion, ESR signals became larger, in agreement with the predictions by Lee and Schwarcz (1994). Rink et al. (1999) also found that heat-induced sensitization of OSL signals in quartz was the greatest in the "nest grains in the gouge.
A subhorizontal fault is exposed in an outcrop near the Wonwonsa temple. A complex of Quaternary conglomerate unconformably deposited on early Tertiary granite (KIGAM, 1998) is thrust over a footwall of Quaternary sedimentary rock and granite. A fault gouge zone 1}30 cm thick branches and merges on a scale of a few meters within the granite. The contact surface of the hanging wall side is very sharp and distinct but the foot wall contact is irregular. Cataclastic foliation is developed in parts of the gouge zone. Subrounded to rounded fragments of granite occur in the gouge. ESR ages were obtained on two samples collected about 1 m apart within a single fault gouge band in the granite (Won 3, 4). A plateau ESR age of 130$10 ka was obtained from E signals in fractions of Won 3. ESR ages of Won 4 decrease with decreasing grain size but do not show any plateau in the plot of ESR age vs. grain size. We can only estimate a maximum ESR age of 200$30 ka which is consistent with the plateau age for the other sample. Sample Won 5 was taken from a 1 cm thick gouge zone between granite and Quaternary deposits.
H.-K. Lee, H.P. Schwarcz / Quaternary Science Reviews 20 (2001) 999}1003
1001
Fig. 2. Sketches of the outcrops and ESR ages of (a) the Wonwonsa fault and (b) the Ihwa fault.
Fig. 3. Examples of ESR ages vs. grain sizes. (a) multiple plateau age, (b) multiple age and/or plateau age, (c) plateau age, (d) maximum age.
ESR ages obtained by Al and E signals of Won 5 gave a multiple plateau ESR age of 400$20 ka. Won 7 was collected from the hanging wall side of a gouge zone 30 cm thick between granite and Quaternary deposits. Al signals of this sample gives a plateau age of 460$50 ka which is consistent with the plateau age of Won 5 within the error range. Three samples were collected from three
branched fault gouge zones in the granite (Won 10, 11, 12). ESR ages for the Al signal of Won 10 decreased with decreasing grain size. The 25}45 lm fraction gave an ESR age of 230$60 ka which would therefore allow us to obtain an assumed maximum age of last movement on this branch. E signals of all grain sizes of Won 10 appeared to be saturated. Both E and Al signals of Won
1002
H.-K. Lee, H.P. Schwarcz / Quaternary Science Reviews 20 (2001) 999}1003
Table 1 Age estimates for time of last movement on faults Fault
Locality
Sample
Measured signals
ESR date (ka)
Con"dence
Plateau signals
Wonwonsa Wonwonsa Wonwonsa Wonwonsa Wonwonsa Wonwonsa Wonwonsa Wonwonsa Ihwa Ihwa Ihwa Ihwa
Main band Main band Boundary Hanging Branch Branch Branch Subsidiary Main band Branch Bounding Subsidiary
Won3 Won4 Won5 Won7 Won10 Won11 Won12 Won15 Ihwa1 Ihwa2 Ihwa3 Ihwa4
E E E , E , E , E , E , E , E , E , E , E ,
130$10 200$30 400$20 460$50 230$60 '3000 710$50 480$30 660$50 1300$100 '3000 '3000
Plateau Maximum Multiple Plateau Maximum Saturated Plateau Plateau Multiple Multiple Saturated Saturated
E
A1 A1 A1 A1 A1 A1 A1 A1 A1 A1
11 appeared to be either in steady state or saturated. Therefore, the last movement within this branch occurred more than 3 Ma ago (based on the estimated time needed to achieve steady state for these samples). Only the two "nest fractions of Won 12 gave "nite ages for Al signals, averaging 710$50 ka. The larger grains of this sample appeared to be saturated. Won 15 was collected from the gouge zone of one of the subsidiary faults developed in the footwall of the Wonwonsa fault. ESR ages for Al signals of Won 15 generate a plateau age of 480$20 ka. E signals of all grain sizes of this sample are in saturation.
4.2. The Ihwa fault The Ihwa fault, forming the boundary between Tertiary granite and Quaternary deposits, is exposed on the wall of the Ihwa creek at Hohye-dong in Ulsan. The gouge zone 1}2 cm thick branches and merges at the bottom of the creek. The central block about 20 cm thick, bounded by fault gouge, is composed of undeformed granite and basic dyke. Fault breccia (about 40 cm thick) is developed in a fractured granite and in contact with gouge zone but there is no evidence of deformation in the Quaternary alluvial sedimentary rocks, even near the fault gouge. Ihwa 1 was collected from the gouge zone between Quaternary deposits and granite. ESR ages for E and Al signals of Ihwa 1 gave a multiple plateau ESR age of 660$50 ka. Ihwa 2, a sample of gouge between fault breccia of granite and a basic dyke, gaves a multiple plateau of 1,300$100 ka. Ihwa 3 was taken from a fault breccia in granite developed adjacent to gouge of the Ihwa fault. Ihwa 4 was taken from a gouge zone about 20 m away from the Ihwa fault. ESR signals of both Ihwa 3 and 4 are in saturation indicating that faulting events occurred more than 3 Ma ago.
E , A1 A1
A1 A1 E , A1 E , A1
5. Discussion and conclusions Toyoda and Schwarcz (1997) have found the formation of a counterfeit E signal in the quartz by c-ray irradiation. The intensity of E' signal of some samples (Won 10, 11, 15 and Ihwa 3 and 4) collected from the gouge in homogeneous granite does not grow systematically by c-ray irradiation, which is characteristic for the saturated samples. Some of the E ages for the "nest fractions gave the same ESR ages as were obtained using the Al signal, while the others gave ages which were older than those using Al signals, or were in saturation. This suggests that the E signals generated by c-ray irradiation of these samples were not counterfeit. The sequence of resetting sensitivity of the ESR signals is not systematic (cf. Fig. 3a and b). The zeroing of ESR signals during fault movement was controlled by numerous factors such as temperature, stress, displacement, thickness of gouge zone, #uid, rock composition, as well as strain rate and grain size. Therefore, we cannot generalize the sequence of resetting sensitivity of the ESR signals during fault movements. We have performed ESR dating of fault rocks using both the plateau method developed at McMaster University and the multiple centre method by Fukuchi et al. (1986). We can divide estimated ESR ages into three groups in order of decreasing con"dence (Fig. 3): Multiple plateau ESR age: the ESR ages decreased systematically with decreasing grain size, reaching a minimum value (plateau) for grains below a critical grain size for more than two ESR signals (e.g., E and Al) and give the same plateau ages (Fig. 3a); Multiple ESR age and/or Plateau ESR age: (1) at least one signal gave a plateau ESR age and the ESR ages from the other signal decreased with decreasing grain size, approching the plateau ESR ages (Fig. 3b) or (2) at least one signal gave a plateau ESR age below some critical grain size (Fig. 3c); Maximum ESR age: (1) the ESR ages did not decrease systematically with decreasing grain size or (2) they did not show any
H.-K. Lee, H.P. Schwarcz / Quaternary Science Reviews 20 (2001) 999}1003
concordant ages or plateau ages (Fig. 3d). In this case, the age given by the "nest fraction is assumed to be a maximum estimate of time since fault movement. The time pattern of fault activity on the Wonwonsa fault deduced from ESR dating of fault rocks can be summarised as follows: this zone was formed or reactivated at least 3 Ma ago. A branch fault in the footwall was developed about 700 ka ago. After that movement, the Wonwonsa fault was reactivated about 440}500 ka and 200}250 ka ago. The last movement which we have recorded occurred about 130 ka ago. These data suggest that the Wonwonsa fault zone was reactivated at least 4 times during the Quaternary period. The evolution of the Ihwa fault is summarized as follows: a fault breccia zone in granite cut by the Ihwa fault zone was formed over 3 Ma ago. After the formation of this breccia zone in the granite, a basic dyke intruded along the breccia zone. A gouge zone was later formed between the basic dyke and fault breccia about 1.4 Ma ago. A later gouge zone was formed between Quaternary deposits and the granite about 650 ka ago. In summary, we see evidence from these ESR dates of continued and prolonged faulting activity on these branches of the Yangsan fault system throughout the Quaternary. This suggests that this fault system may continue to be active today and to present some seismic hazard to structures in its vicinity.
Acknowledgements This research was performed for the National Hazards Prevention Research Project, one of the Critical Technology-21 Programs, funded by the Ministry of Science and Technology of Korea. H.K. Lee wishes to acknowledge the "nancial support of KIGAM in contract with RIDE (the research institute for development of earth resources of Kangwon National University). We thank Dr. Ueechan Chwae and Dr. Deng-Yong Cho at KIGAM
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