阿尔金断裂带中段现代沉积物样品钾长石红外激发后红外释光的残留信号——对年轻古地震事件测年的指示意义

RESIDUAL POST-IR IRSL SIGNALS OF POTASSIUM FELDSPAR FROM MODERN SAG POND DEPOSITS OF CENTRAL ALTYN TAGH FAULT: IMPLICATION FOR DATING YOUNG PALEOSEISMIC EVENTS

阿尔金断裂中段车尔臣河—清水河沿线发育新鲜的地震地表破裂带,据推测可能由千年以来的强震事件所致。然而,由于目前缺乏可有效应用于ka时间尺度的第四纪测年方法,建立该地区历史时期古地震事件的精确时间序列十分困难。文中在该断裂带采集分选好、中、差3种不同类型的具有代表性的现代断塞塘沉积样品,探讨了基于钾长石红外激发后红外释光(post-IR IRSL)方法测定年轻古地震事件的可能性。通过"积分区间-激发温度坪"实验,确认了在首步IR激发温度为110℃时,post-IR IRSL170信号在ka时间尺度上不受异常衰减影响。采用粗颗粒2mm小测片技术,利用最小年龄模型和有限混合模型,可有效识别出断塞塘沉积物中post-IR IRSL170信号晒退彻底的钾长石颗粒。实验表明,这一方法得到的post-IR IRSL170信号的非光敏残留剂量为0. 6~0. 8Gy,如通过合理的方法扣除其影响,post-IR IRSL170技术将有望用于数百a至ka时间尺度的古地震事件测年。

The Altyn Tagh Fault(ATF)is one of the most prominent active strike-slip faults in the India-Eurasia collision.Fresh features of surface ruptures,which are attributed to seismic events taking place in the last millennium,are identified at several sites along the Che'erchen River to Qingshui River section on the central part of ATF.Accurate chronology of these earthquake events would help understand the spatial-temporal relationship of the recent earthquakes.However,great difficulties are encountered.The central ATF is located in the arid area,and the vegetation cover is so limited that rare organic materials appropriate for radiocarbon dating can be found in the sediments.Luminescence dating technique may serve as an alternative to directly determine the burial ages of the earthquake related sediments.The optically stimulated luminescence(OSL)signal of quartz,which has been widely employed for luminescence dating,displays unwanted charateristics for accurate dating.Firstly,the quartz OSL signal is not sensitive to irradiation,which leads to low signal-to-noise ratio or even no measurable quartz OSL signal.Secondly,the targeted samples of the last millennium are very young,and the radiation dose received during the burial is expected to be less than 3~4Gy,which futher deteriorates the signal-to-noise ratio of the quartz OSL signal.Therefore,quartz OSL signal is not appropriate for dating the sediments relevant to the recent earthquakes on ATF.The infrared stimulated luminescence(IRSL)signal of potassium feldspar is an alternative,and it is in usual an order of maginitude more sensitive to raidation than the quartz OSL signal.The enhanced signal-to-noise ratio makes it applicable to young samples.The post-IR IRSL signal has been successfully applied to date the sediments beyond the Holocene,however,the relatively slow bleaching of the post-IR IRSL signal poses challenges on applying it to young sediments,especially for the sediments deposited during the last millennium.In this study,we investigated the feasibility of using post-IR IRSL signal from potassium feldspar to date the earthquake events of the last millennium by employing modern sag pond deposits with different sorting and expected equivalent dose(De)of 0Gy.Choosing an appropriate measurement procedure and identifying the well bleached pottassium feldspar grains are essential for post-IR IRSL dating of young sediments.The non-fading characteristic of the post-IR IRSL170 signal measured at 170℃following a prior IR stimulation at 110℃was verified by employing the De plateau test with respect to the signal integration interval and IR stimulation temperature together.Reducing the amount of potassium feldspar grains mounted on an aliquot would help reveal the among grains variation of bleaching level of post-IR IRSL170 signal before depostion and identify the most sufficiently bleached grains.Therefore,the post-IR IRSL170 De values of 2mm aliquots were measured for three samples with different sedimentary textures.The median of De distribution of well sorted and stratified sag pond deposits is consistent with the minimum De value inferred from the minimum age model(MAM-3)and finite mixture model(FMM),while for the poorly sorted deposits,the median is significantly overestimated compared with the minimum De values from the MAM-3 and the FMM.The minimum De values of 0.6~0.8Gy of all three samples are consistent with the unbleachable residual dose previously reported for post-IR IRSL signals measured at similar temperature for well bleached samples.It implies that by combined use of small aliquot and statistical age models,the well-bleached potassium feldspar grains could be identified.Such an intrinsic unbleachable component needs to be properly corrected when earthquake events of last millennium are to be dated in this area.Otherwise,the post-IR IRSL170 age would be overestimated by 200~300a.The post-IR IRSL170 procedure investigated in this study is not only applicable for dating the paleoearthquake events along the Altyn Tagh Fault,but also with great potential to be applied to other tectonically active area.With consideration of the potential variability in post-IR IRSL signal characteristics of potassium feldspar grains from different origins,the signal stability needs to be routinely inspected.The modern analog sample would also be informative for justifying the measurement procedure and analytical method employed.