Availability of dating of fault movement by ESR method : the case of mylonite
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Longgang volcano group is located in Huinan and Jingyu counties of Jilin Province,which occupies the middle segment of the Longgang mountain ranges.The volcanic activity in Quaternary,especially in the early Pleistocene,is very intensive,and as a result,over 2/3 of the total area is covered by Q1p(Xiaoyizishan Group)basalt sheet.The study area of this paper lies in the northeastern part of the Longgang volcano group,where some small-scaled faults are developed and their activity in the Quaternary Period is critical to the potential earthquake risk evaluation for the local engineering.In order to study the activities of these faults,a comprehensive project has been carried out by means of field investigation and application of ESR dating technique to determine the last active time of each fault.However,an inconsistent phenomenon was found in the study area,in which the of fault gouge from ESR dating method is much younger that the of Q1p basalt sheet that covers both fault plane and fault deformed materials.We propose two facts that would be responsible for this contravention.Firstly,the extensive volcanic activity in the study area in Quaternary may produce a significant amount of heat regionally.For example,the temperature of the molten basalt lava is between 1050℃ and 1200℃,which is high enough to cause the effect of annealing and resetting of ESR signals.Consequently,such effect may result in the of fault gouge with ESR dating method younger than the time when this heat event occurred.Secondly,after a fault is formed,chemical weathering will occur at the fault zone,which may result in dissolution on the grain surface of fault gouge.Such effect may significantly increase the amount of annual ESR dose and make the sample of weathered material younger than the un-weathered ones.Therefore,in this study area with extensively volcanic activity,the from fault gouge dated with ESR method should be considered as the upper limit age of the last fault activity rather than the last time when the fault was active.Even though the upper limit age does not represent the of the last activity of a fault,it is very useful in the practice of determining the capable fault for earthquake safety evaluation of large engineering sites.
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In order to outline the kinematics and movement history of a new Quaternary fault, Jingwan Fault in Dangjin, West Korea, we analyzed the geometry of the fault zone composed of a few gouge zones, and made ESR dating for fault gouge materials. The striking Jingwan Fault is a normal fault and exhibits a gradual change in dip (gentle in the lower part, steep in the upper part), indicating a listric fault. As for the fault gouge zone, its thickness varies and reaches 2~3 cm in the lower part or between basement rocks, and 20~30 cm in the middle-upper part or between the basement and Quaternary deposit. It is observed in the latter case that more than three gouge zones develop with different colors, and branch out and re-merge, or they are partly superimposed, indicating different movement episodes. The cumulative displacement is estimated to be about 10 m using the geological cross-sections, from which it is inferred that the total length of fault may be about 2.5 km on the basis of the empirical relation between cumulative displacement and fault length. Therefore, a more study would be needed to verify the entire fault length. The results of ESR dating for three gouge samples at different spots along the fault yields ages of , , and , indicating at least two movement episodes. Slickenlines observed on the fault planes indicate a pure dip slip (normal faulting), which suggests that the ENE-WSW trending Jingwan Fault was presumably moved under a NNW-SSE extensional environment.
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The Piqiang fault is a NW-trending strike-slip fault in the Keping uplift on the north margin of Tarim Basin.Satellite images of the fault show a left-lateral strike-slip faulting structure with local right-lateral characteristics in the south.The Piqiang fault and Selibu fault on the Bachu uplift in the basin shows well spatial matching relationship.Therefore,chronology evidence of ESR dating of the Piqiang fault bears great significance to the study of late deformation of Tarim Basin.Field surveys shows that the fault offset the Paleozoic in the Keping uplift and dragged it to the surface.The Ordovician in the area is dominated by carbonates and contains well-developed large calcite veins due to faulting and late fluid filling and crystallization.The dating of calcite veins in the fault belt indicates that the age recorded by the Piqiang fault at late stage is 62-66 ka,and that recorded by the middle part of the fault is 3 583±350 ka.The similar chronological age data of the southern and northern parts shows that the Piqiang fault has the characteristics of synchronous activity during the later period.Based on the genetic analysis of the big age differences of the calcite veins in the Piqiang fault,it is suggested that the Keping uplift still possibly experienced vertical uplifting even after overthrusting of Keping Uplift in SW direction.The vertical uplifting in turn is the result of differential modification at the south and north ends of the Piqiang fault.
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The variations of microstructures associated with the motion transformation in the central southern part of the Red River fault were studied in this paper.The researches included careful observation of microstructures in eighteen mylonite,twelve breccia,ten cataclasite,and eight sandy conglomerate samples from the fault,of which some were dated by the fission track in apatite (FT) method.In addition,the fault slip data were used to infer tectonic stress tensors.The results show that the microstructure of mylonite is indicative of left-slip during the early activity of the Red River fault.The FT ages of the mylonite and undeformed sandy conglomerate exceed 20 Ma,implying that a thermal event occurred in the Red River fault at that time.The microstructures of cataclastite and breccia imply the existence of right-slip during the late activity period of the fault.The above features,the FT ages (9.9～12.7 Ma,6.8～8.4 Ma,2.0～4.6 Ma),and the different stress fields suggest that the Red River fault has undergone three dislocations since the Miocene,of which the first was strike-slip,and the second and third were of normal faulting.
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We have studied the variations of microstructures associated with the motion transformation in the central-southern section of the Red River Fault.Our work includes careful observation of microstructures of the 18 mylonite,12 breccia,10 cataclasite,and 8 sandy conglomerate samples from the fault,some of which are dated by the fission track in apatite(FT)method.Meanwhile,fault slip data are used to infer tectonic stress tensors.The results show that the microstructure of mylonite is indicative of left-slip during the early stage of movement of the Red River Fault.The FT ages of the mylonite and undeformed sandy conglomerate exceed 20Ma,implying a thermal event occurred on the Red River Fault at that time.The microstructure of catalsite and breccia shows the right-slip during the late period of the fault movement.These features and FT ages(9.9～12.7,6.8～8.4,2～4.6Ma BP)suggest that the Red River Fault has experienced three dislocation events since the Miocene.The first is indicated by broken mylonite with compressive microstructure,and fault occurrence and slickenlines on the fault plane plunging both less than 30°,indicative of dominant strike-slip,and an activity age of 9.9～12.7Ma.The second has slickenlines plunging between 35°～45°(42° on average),implying dominant right-lateral slip with normal faulting component,which occurred during 6.8～8.4Ma.And the third event is featured by slickenlines plunging 35°～70°(65° on average),meaning normal faulting with a strike-slip component,which happened probably during 2.0～4.6Ma BP.
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Gouge is the carrier for information of the fault activity.In this study,fault gouges from the eastern Liupanshan piedmont fault zone were analyzed by ESR method.However,the measured ESR age is much older than that estimated in the field.On the other hand,SEM images of the quartz grains separated from the fault gouge showed a variety of morphologies,which indicate several periods of fault movements.The ESR age estimates failing to represent the latest fault movement suggest that the latest movement may not have zeroed ESR signals significantly for ESR dating and the quartz may be from different periods of the movements.Although there are still many uncertain factors in the ESR dating of fault gouge,it is essential to study systematically in this area.
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