火山岩地区断层活动时代鉴定中的问题及其讨论

通过野外地质、地貌调查,对新生代玄武岩普遍发育的吉林省靖宇县龙岗火山群地区的断层活动性进行了研究,结合断层物质ESR测年分析结果以及本地区第四纪火山活动的特点,发现ESR测年数据与野外实际观察到的地质现象比较相对偏新。文中讨论了产生这一现象的两个可能原因:1)研究地区大规模的第四纪岩浆喷发活动作为一种重要的热事件,对于先存的断层物质中的ESR测年信号具有"退火"和零化作用;2)化学风化作用、地下水的淋滤作用、次生物质的混入和富硅热液作用而导致的次生石英颗粒的增长,都可能会导致断层带物质ESR测年剂量的增加,从而引起测年结果偏新。因此,研究区ESR测年结果反映出的并非是断层最后一次的活动年代,而可能是断层活动的上限年龄。尽管与实际的断层活动年龄有较大的差距,但对于大型工程场地的地震安全性评价,尤其是对于能动断层的鉴定,这种"上限年龄"仍然具有十分重要的应用价值     

岩石暴露与埋藏面释光测年进展及其应用

岩石暴露与埋藏面光释光测年是最近提出的一种新测年方法,其测年基本原理是当岩石暴露于太阳光下时,岩石表层的释光信号会被晒退,被晒退的深度随着岩石暴露时间的延长而增加,当岩石暴露面被沉积埋藏后其光释光信号又开始积累。利用岩石暴露(或埋藏)面残留释光信号与暴露(或埋藏)时间和深度的定量关系,就可以估算该岩石的暴露和埋藏年龄及历史。文中简要介绍了不同释光信号测定岩石暴露与埋藏面年龄的方法、进展、应用实例及存在的问题。这些研究实例表明,岩石暴露与埋藏面光释光测年方法在第四纪地质、地貌演化、考古和活动构造研究上可能具有广泛的应用前景。     

西南天山明尧勒背斜河流阶地沉积物的光释光测年

晚第四纪细颗粒河流阶地沉积物的准确定年是当前光释光测年研究中的一个难点。利用简单多片再生法(SMAR)、单片再生法(SAR)等对新疆西南天山喀什前陆盆地明尧勒背斜喀浪勾律克河阶地沉积物样品的细颗粒石英光释光(BLSL)信号、混合矿物红外释光信号(IRSL)和红外后蓝光释光信号([Post-IR]OSL)进行测量。通过对比同一样品的细颗粒石英BLSL、混合矿物IRSL和[Post-IR]OSL的等效剂量值,认为细颗粒石英简单多片再生法是河流阶地沉积物释光测年最为适用的方法之一。对于细颗粒混合矿物而言,由于长石矿物的异常衰退,对于等效剂量De>70Gy的样品,其IRSL等效剂量值均偏小;其[Post-IR]OSL等效剂量值仍需慎重使用。通过对同一样品细颗粒石英多个测片的单片再生法等效剂量值分布以及简单多片再生法多个测片校正后天然释光信号强度的一致性判断,认为样品LEDL03-87q,LEDL03-86q和LEDL04-54q在沉积时的光晒退是不均匀的,采用最小等效剂量组分来计算沉积年龄     

不同沉积环境相样品的释光古剂量比对

从天山北麓采集了晚第四纪风成、冰水相和河流相的粉砂样品,对它们进行了热释光、绿光释光和红外释光的古剂量测定。结果表明,对于经充分光晒退的风积物样品,三者的测量结果没有差别,而对于曝光不足的冰水相样品,热释光获得的古剂量值明显大于绿光释光和红外释光的古剂量值     

岷江上游堰塞湖沉积中软沉积物变形构造成因讨论

对在岷江上游及其支流河谷中的晚第四纪古堰塞湖地层中发现的大量软沉积物变形构造的类型、特征和成因进行了分析,发现这些软沉积物变形构造分为两类:一类是小型的层内变形构造,如液化卷曲变形、泄水构造、负荷构造、阶梯状微断裂等,其形成与软沉积物的塑性变形、液化和流动作用相关;另一类是崩(滑)塌体对软沉积物的扰动形成的大型褶曲构造.通过与其它成因的软沉积物变形构造的对比以及对区域构造环境的分析,得出地震、崩塌和滑坡等灾害性地质事件是造成河谷区古堰塞湖地层中软沉积物变形构造发育的最可能的驱动机制.     

第四纪松散沉积物中活断层滑动面的显微构造研究方法

第四纪松散深积物中的活断层滑动面是重要的古地震遗迹，也是鉴定古地震事件和确定古地震时代的重要标志。以往，以第四纪松散沉积物中活断层滑动面主要借助宏观观察进行研究，但遇到了一些难以解决的问题，如隐性断层问题。作者利用显微构造分析，找到了研究这些问题的途径，其方法是对第四纪松散沉积物进行定向原状样品采集，通过室内样品固结，显微薄片制作和显微构造现象观察等，从显微域里研究各种变形现象。用此方法，可以在没有宏观资料支持的情况下确定断面的运动性质；发现并描述第四纪松散沉积物中的隐性断层，研究断层的尖来机制，还可以对工程建设场地发现的不同松散沉积物相接触的现象进行活断层鉴别。     

火山熔岩烘烤沉积物的热释光年龄

火山熔岩溢流到下伏地层之上,使地层表层经受高温过程形成烘烤层。当该烘烤作用是本岩层的最后一次热事件时,方符合热释光技术测年龄的采样要求。本样品采自山西省大同县阁老山公社黑山南狼窝沟和阳高县下深井公社东水头村水库西岸两处,烘烤沉积物为第四纪黄土。测龄前首先对烘烤和非烘烤沉积物的物质成分、微结构和表面形态进行了分析鉴定     

活动断裂带内第四纪松散沉积物的变形类型

选择了一些典型活断层地质剖面,对经受了构造变动的第四纪松散沉积物进行了定向原状样品采集.通过室内样品固结、显微薄片制作和显微构造现象观察,从显微域里找到了第四纪松散沉积物变形的一些特点,并进行了分类.第四纪松散沉积物的变形类型有两种:一种是破裂变形,主要表现为平行滑动条带、平直滑动界面、碎屑颗粒的破碎等;另一种是流动变形,主要表现为:流动构造、碎砾旋转、粘土矿物的拖曳滑动等.     

西藏东南部更早冰川作用新发现

西藏东南部是青藏高原现代冰川及第四纪冰川作用中心之一.通过对冰川沉积物的定量测年研究,业已建立起本区第四纪冰川作用序列.其中,本区最早冰川作用即古乡冰期发生于深海氧同位素6阶段(MIS-6).近年我们发现念青唐古拉山东段波堆藏布谷地中高出古乡冰期冰碛500～600m的谷坡上有断续分布的高谷肩,其上残存更老的冰川沉积.应用ESR测年技术,获得冰碛物的年代为(506.3±60.4)ka.若考虑年代误差及气候环境,此次冰川作用发生于深海氧同位素12阶段的可能性最大,但也不排除发生于更早阶段的可能性.根据冰碛物出现的典型地点,本研究将其命名为"尼通冰期".     

龙门山断裂带北段晚第四纪活动性讨论

在野外考察的基础上 ,结合所采集的各条断裂之上的覆盖物或断层带物质的热释光 (TL)或电子自旋共振 (ESR)样品年龄 ,对龙门山断裂带北段的晚第四纪活动性进行了分析 ,认为 :后山断裂在第四纪早 -中期曾有过活动 ,晚更新世以来已不再活动 ;中央断裂早更新世或前第四纪是活动的 ;前山断裂在白龙江以北变成一些小的、零星分布的断裂 ,它们在第四纪早期以前有过活动。而已有研究表明龙门山断裂带中段和西南段晚第四纪以来仍在活动。造成龙门山断裂带不同段落新活动时代不同的主要原因 ,可能是区域应力场的变化所导致的活动地块边界的变化。龙门山断裂带的北段现在已不构成活动块体的边界 ,加之岷山隆起对龙门山断裂带东北段的屏障作用 ,使得龙门山断裂带北段活动减弱。而龙门山推覆构造带中南段和岷山隆起构造带共同成为块体持续挤压作用的东界。这为研究青藏高原的运动学及动力学等问题提供了重要信息     

ESR dating of glacial moraine deposits: Some insights about the resetting of the germanium (Ge) signal measured in quartz

The electron spin resonance (ESR) dating of tills using germanium-doped (Ge) paramagnetic centers in quartz has advantages over other dating techniques, as quartz is common, processing is easy, and the technique has the potential for dating features several hundreds of thousands years old. ESR dating of moraines is based on the supposition that either subglacial comminution or exposure to sunlight resets the signal. However, actual dating suggests that a signal that is initially present cannot be bleached to zero by grinding alone. We found that grinding coarse samples (0.5–1 mm in diameter) to the mean grain size of fine sand (0.125–0.193 mm) reduced the signal intensity to 53–69% of its original value. From the value of the signal difference, one can devise a correction factor for ESR ages of subglacial sediment. Polymineralic grains are commonly present in till. Exposure of them to sunlight for several days can reduce the signal intensity to 7–8% of its original value within 1–2 mm thick of the sediment surface. However, within 5–8 mm of the sediment surface, exposure to sunlight for over one week only reduced the signal intensity to mean plateau values of 42–50% of the initial value. Mixing upper and lower layers of the samples during exposure to sunlight changed the signal intensity. This suggests that the amount of bleaching varies spatially. Sediments initially deposited at the margins of ice caps or ice sheets and subsequently overridden may have been sufficiently exposed to sunlight to allow ESR dating of moraines. The purity of the quartz and the grain size have significant impacts on signal intensity; intensive purification and the use of a uniform fine sand fraction are thus recommended.     

Evaluation of ESR residual dose in quartz modern samples,an investigation on environmental dependence

Luminescence and ESR dating methods of quartz sediment are based on the natural resetting of the signal by light exposure (optical bleaching). When the bleaching is incomplete, a residual dose (D_eR) is added to the post-depositional dose accumulated since the deposit and hence the age is overestimated.Insufficient bleaching is usually linked to the environment and conditions of transport/deposition of the quartz grains affecting the light exposure duration. Indeed, each transportation mode – fluvial, marine or aeolian – is associated to specific conditions of light exposure, depending mainly to the location of grains in the transport agent during the transport phase, the opacity of the transport environment and the velocity of the transport.The present study attempts to discriminate the modes of transport/deposition providing a satisfying reset of the ESR signals of quartz grains. For this purpose, we investigated bleaching rates and ESR residual doses of aluminum centers from “present-day” aeolian, fluvial and marine sediments sampled in various sedimentary environments. The bleaching efficiency evaluation in these different environments may help for a better understanding of the resetting phenomenon for quartz signals which represents presently the main difficulty for ESR dating.The results show that the residual doses are small enough to allow an ESR dating of the main part of the sediment transported in almost all the context examined in this study. The smallest residual doses are obtained from quartz grains within the range of 100–200 μm and transported in clear water. Some limits for the application of optically bleached quartz ESR dating appears nevertheless, mainly when the residual dose and the dose accumulated after the deposit are quite similar, i.e. for Upper Pleistocene samples.     

ESR dating of fluvial quartz: Estimate of the minimal distance transport required for getting a maximum optical bleaching

An ESR experimental study of artificial optical bleaching of sedimentary quartz has shown that the aluminum center was maximally bleached after a 6-month illumination equivalent to natural light. This duration seems too long to apply in natural conditions. Nevertheless, the measurement of the ESR intensity of aluminum centers in quartz extracted from modern sediments and deposited in sandy bars shows that the maximum bleaching has effectively been reached.

In order to determine the relationship between the bleaching and the distance covered by a quartz grain in a river, samples were collected along the Creuse River (France) from its spring to about 170 km downstream, where maximum bleaching levels were observed in previous studies. The ESR intensities of the aluminum and titanium centers in quartz were measured, using X-band spectroscopy, before and after artificial bleaching. The difference measured between these sub-samples shows that the maximum bleaching level is obtained in the course of the first kilometer. Hence, the assumption that ESR dating of fluvial sediment is based on the optical bleaching was validated.     

Towards dating Quaternary sediments using the quartz Violet Stimulated Luminescence (VSL) signal

Quartz optically stimulated luminescence (OSL) dating is widely used to determine the time of deposition and burial of Late Quaternary sediments. Application of the method is usually limited to the past 150,000 years due to early saturation of the OSL signal. Here we explore the potential to date Quaternary sediments using the violet (402 nm) stimulated luminescence (VSL) signal of quartz. We develop and test a new post-blue VSL single aliquot regenerative dose dating protocol, and demonstrate that the VSL signal originates from a deep trap at about 1.9 eV with a thermal lifetime of 10¹¹ years at 10 °C, and that this trap is bleachable by sunlight. The VSL signal grows with dose to ∼6400 Gy, a factor ∼20 higher than the conventional quartz OSL signal, and with the proposed protocol we recover a known dose of 1000 Gy in three out of four samples. The potential of the VSL protocol for dating Quaternary sediments is highlighted by its successful application to a suite of geological samples ranging in age between 13 and 330 ka. Based on our investigations, we propose that the VSL protocol has the potential to extend the quartz dating range to cover the full Quaternary.     

Luminescence dating of mass-transport sediment using rock-surface burial methods: a test case from the Baksan valley in the Caucasus Mountains

Mass-transport sediments are generally difficult to date directly by existing methods. Conventional luminescence dating of sand is unsuitable because the short transport distances provide little opportunity for bleaching. However, larger clasts are often exposed to sunlight for prolonged periods before becoming entrained in mass-movements, and these clasts have the potential to be used for rock-surface luminescence dating. Mass-transport is a major component of landscape change in mountain regions; in the Baksan valley, Caucasus Mountains, high uplift rates and high precipitation create a rapidly eroding landscape where preservation of sediments is poor. The area is particularly prone to destructive debris flows, because of the large quantity of detrital material in the catchments. The debris-flow and hillslope sediment the Baksan valley are used here to test the applicability of rock-surface burial dating to mass-transport sediment. We find that colluvial clasts show a high degree of bleaching and give reproducible ages, with a large colluvial deposit dated to the early–mid Holocene. The bleaching of debris flow clasts is more variable – we suppose due to the more complex transport history of the clasts, with opportunities for lengthy storage in moraines or colluvium. Overall, the results are encouraging, and suggest that rock-surface methods can provide a useful approach to mass-transport dating in mountainous regions. However, improved targeting of samples and measurement efficiency is desirable for widespread application.     

Preliminary ESR dating results on loess samples from the loess–paleosol sequence at Luochuan, Central Loess Plateau, China

The quartz Al centre has been used in pioneering studies for ESR dating of sediments. Acceptable age estimates could be obtained for a range of deposits using this centre after estimating the residual level. To inspect the feasibility and reliability of ESR dating of windblown sediments, six loess samples from the loess–paleosol sequences of Luochuan profile on central Loess Plateau, China, were dated by ESR with regeneration method using the quartz Al centre measured at 115 K. The samples were exposed to sunlight for 430 h. Only about 25% signal intensity was bleached. According to our preliminary results, ESR age estimates increased with the depth along the loess profile, however, the ESR age of each sample is only about a half of the reference age of the corresponding strata. For example, the ESR age of a sample from the top of L₈ loess near B/M boundary (a known age of 780 ka) yields an age of only 385 ka. It seems that ESR dating using quartz Al centre with dose regeneration protocol may have the potential for dating of loess and other aeolian deposits, but the studies on the nature of the quartz Al centre and experiment protocols have to be studied further.     

ESR dating of sedimentary quartz: Possibilities and limitations of the single-grain approach

Usually, multiple-grain aliquots are used for electron spin resonance (ESR) dating of sediments. However, this approach excludes the ability of detecting insufficient bleaching, as would be the case when measuring several single aliquots or single grains. In this paper, we present preliminary results of single-grain ESR dating experiments on three different sedimentary deposits (fluvial, fluvio-aeolian and desert aeolian), ranging in age between 100 ka and 2 Ma. Titanium-related impurity centres (Ti–Li and Ti–H) were measured in a Q-band ESR spectrometer for estimating equivalent doses (D_e) of individual quartz grains (0.5–1 mm). Both additive and regenerative dose methods were used. The resulting D_e plots show a large scatter in D_e—from 100 Gy to more than 600 Gy—for the fluvial sample, probably reflecting different bleaching histories of the grains. On the contrary, a clear plateau can be observed in D_e for sands with a strong aeolian component. Preliminary single-grain ESR ages are in relatively good agreement with independent estimates, the best results being obtained for the desert aeolian sample (200 ka). It is concluded that ESR has a promising potential for estimating large naturally accumulated doses of well-bleached sediments. Nevertheless, the method is challenged by several experimental difficulties, such as long measurement times, excessive errors, and presumably a complex interrelationship between Ti–Li and Ti–H components.     

Effects of grain size on quartz ESR dating of Ti–Li center in fluvial and lacustrine sediments

The electron spin resonance (ESR) dating method has been introduced into the Quaternary chronology for nearly 40 years and has been successfully used for dating of fluvial and lacustrine sediments. In order to evaluate the possible effect of the sediment grain size on the dating estimate, the parallel ESR dating testing was carried out on quartz grains of five different size fractions ranging from 50 to 450 μm extracted from the same fluvial and lacustrine sediment collected close to the M/B boundary at the Donggutuo section, Nihewan Basin, China. The results show that equivalent doses and associated ages vary significantly. However the beta irradiation dose rate of the grains with different sizes accounts for only about 6% of the total deviation of the dating results. At the same time, the sensitivity of quartz Ti–Li center was calculated based on the additional irradiation. It shows that the larger grains are more sensitive than the smaller ones, which can leads to higher saturate ESR intensity and less equivalent dose. The variations of the sensitivity of quartz Ti–Li center of the grains with different sizes are responsible for the primary deviation of the dating results. The results also suggest that 100–150 μm grain size fraction would be priority size for the ESR dating of quartz Ti–Li center on fluvial and lacustrine sediments.     

RESEARCH PROGRESS ON LUMINESCENCE DATING OF ROCK SURFACES AND ITS APPLICATION

There are many examples of exposed or buried rock surfaces whose age is of interest to geologists and archaeologists. Luminescence dating is a well-established method of absolute chronology which has been successfully applied to a wide range of fine-grained sediments from hundreds of years to several hundred thousand years. Optical stimulated luminescence (OSL) has been recently proposed as a new method to date these rock surfaces (Laskariset al., 2011; Sohbatiet al., 2012a; Chapotet al., 2012; Pedersonet al., 2014; Sohbatiet al., 2015; Freieslebenet al., 2015). The basic principle is that luminescence signal of rock surface will soon decay when the rock is exposed to sunlight. When the rock surface is turned to be buried side, the OSL signal begins to accumulate again. With the variation of residual luminescence with depth, it is possible to estimate exposure and burial history of the rock. This article describes briefly the different luminescence dating methods for rock surfaces, its progress, application examples and present problems. For instance, Sohbatiet al. (2011) studied the depth dependence of the bleaching of the IRSL signal from granitic rocks, Laskaris and Liritzis (2011) proposed a mathematical function to describe the attenuation of daylight into rock surfaces, Sohbatiet al. (2012a) developed their model to include the environmental dose rate, Sohbatiet al. (2012b) overcame the problem of parameter estimation by using a known-age road-cut sample for calibration, Sohbatiet al. (2012c) further developed the OSL surface exposure dating model by including the simultaneous effect of daylight bleaching and environmental dose rate, and so on. These studies indicate that OSL dating method for rock surface can be applied widely to studies of geological and geomorphological evolution, archeology and Quaternary tectonic activity.     

Application of OSL dating to middle to late Holocene arroyo sediments in Kanab Creek,southern Utah,USA

Middle to late Holocene alluvium, identified as Quaternary alluvial unit 4 (Qa4), along Kanab Creek in southern Utah, USA was dated using optically stimulated luminescence (OSL) on quartz sand, and by radiocarbon dating of detrital charcoal. Entrenchment beginning in 1882 AD created arroyo walls that expose up to 35 m of the Qa4 alluvium. The stratigraphy and sedimentology suggest that fluvial aggradation along the study reach occurred rapidly. Due to the high sediment supply, short transport distances and semi-arid climate with flashy discharge, partial bleaching (zeroing) of the luminescence signal was expected to be a problem for OSL dating. We approached this problem by first using small-aliquot (∼20 grains) and single-grain dating of quartz sand to reduce the number of grains contributing to the OSL signal. Second, we used statistical parameters based on single-grain and small-aliquot equivalent dose (De) distributions of bleached sediment to help identify partial bleaching and to inform if a minimum age model (MAM) should be used for age calculation. Comparison of results with radiocarbon ages demonstrates the success of OSL dating on Kanab Creek arroyo-fill deposits, although careful attention should be paid to the sedimentary facies and stratigraphy of the targeted sample horizon to minimize the effects of partial bleaching. Thin, decimeter-scale plane-bedded and ripple cross-bedded sandy lithofacies were found to be the best target for OSL dating, as these sediments showed minimal evidence for incomplete solar resetting. Additionally, results generally indicate that better-bleached sediments are found in downstream reaches. Age control from these arroyo-fill deposits was acquired in order to fulfill larger research goals of understanding regional arroyo incision and aggradation cycles.