ESR dating of the Wangsan fault,South Korea

The Wangsan fault is exposed at Kyeongju, Korea. The andesite is unconformably covered by Quaternary alternating conglomerate and sandstone deposits. The unconformity is cut by the thrust fault which displaces a hanging wall block of about 30 m. Exposed at the surface of this fault is a light gray and brown fault gouge, about 40–120 cm thick. In order to test the consistency of ESR ages from a single fault gouge zone, we collected six gouge samples systematically along and across the Wangsan fault. We found that six samples collected from the same gouge zone show consistent ESR date estimates (average 550 ka). Because fault rock is rare along faults in unconsolidated sediments due to low confining stress near the surface, we consider that this fault gouge has been moved up with the hanging wall block along the fault. The estimated average uplift rate of the hanging wall block is about 0.04 cm/year based on the age of the displaced Quaternary deposits (vertical separation; about 20 m) dated by OSL dating methods. The depth of the fault gouge at the time of reactivation, which was estimated from uplift rate and the ESR ages, is about 220 m. Therefore, we conclude that the results of ESR age estimates represent the time of reactivation of the fault gouge at a depth of 220 m in the past during fault movement, because later movements which occurred during uplift near the surface may not have zeroed ESR signals significantly for ESR dating of fault movements.     

Fractal geochronology — dating of the past, planning for the future

Dates for regional-scale geological events are typically determined by analysis of radioactive decay processes within micron-scale closed systems, demonstrating that geochronology information is “fractal” — that is, largely independent of the scale of magnification. This has important implications for the way geochronology data should be stored, retrieved and processed in order to maximize extraction of useful geological information.     

Permo-Triassic tectonic evolution and metallogenesis of the Rockhampton—Maryborough area, Queensland — A photogeological investigation

Structural analysis of remotely sensed data provides a method of assessing the tectonic significance of regional metallogenic lineaments in the New England Orogen of southeastern Queensland. Photogeological analysis of Landsat imagery and small-scale aerial photography reveals a pattern of WNW—NNW-oriented structures, which were apparently generated in response to Mesozoic crustal extension and reactivated during Early Tertiary block faulting. These structures tend to overprint arcuate late Palaeozoic to early Mesozoic trends and batholith belts, and exert a control over Middle to Late Triassic rifting and epizonal plutonism. The distribution of epigenetic base and precious metal deposits in the Rockhampton—Maryborough area is locally but not regionally related to identifiable structural lineaments.     

ESR detection of seismic frictional heating events in the Nojima fault drill core samples, Japan

We have analyzed the Nojima fault NIED 1800 m drill core samples by ESR (Electron Spin Resonance) to detect seismic frictional heating events, especially during the 1995 Kobe Earthquake. Dark gray fault gouge with foliation > 10 cm away from the fault plane at about 1140 m in depth, which was produced by ancient fault movements, has a FMR (ferrimagnetic resonance) signal. Heating experiments show that this FMR signal is derived from ferrimagnetic trivalent ion oxides (γ-Fe₂O₃: maghemite) with imperfect crystallinity, which is produced by thermal dehydration of γ-FeOOH (lepidocrocite) or Fe(OH)₃ (limonite). The existence of the FMR signal means that dry heating such as frictional heating once occurred, and that the frictional heat temperature along the dark gray fault gouge may have risen to over 350 °C during ancient seismic fault slip. In order to detect frictional heating events in fault zones, the increase of the FMR signal and the color change of fault gouge into dark gray or black are important indexes. On the other hand, no FMR signal is detected from the fault gouges just on two fault planes at about 1140 m and 1300 m in depth, which are considered to be possible main fault planes in the 1995 Kobe Earthquake. These two fault planes may not have played an important role of fault slip in the Earthquake.     

K–Ar dating of fault gouge in the northern Sydney Basin, NSW, Australia—implications for the breakup of Gondwana

The occurrence of synkinematic and authigenic clay minerals is a common feature in fault gouges. Few attempts have been made to date fault gouges. We present the first age data in Australia for synkinematic illite–smectite growth in two fault zones of the northern Sydney Basin, NSW. The faults occur at Burwood Beach, NSW in the northern part of the Sydney Basin and are hosted by Early Permian siltstones, tuffs and coals of the Lambton Formation, Newcastle Coal Measures. The faults are 1.5 m apart, show normal displacement and trend N–S with steep easterly dips. Foliated gouge zones, comminution and dilational breccias are developed along both fault surfaces. K–Ar ages extracted from samples in the gouge and tuffs in the damage zones are 172 (6–10 μm) to 119 Ma (<0.4 μm), respectively. Older ages of 272–281 Ma for the coarse fractions (>2 μm), 237–245 Ma for the <2 μm fraction, 218 Ma for the <0.4 μm fraction and 196 Ma for the <0.1 μm fraction have been obtained from siltstones within and outside the damage zone. We believe the younger ages of 196–237 Ma indicate the time at which diagenetic illite–smectite formed and the 122–150 Ma dates from the <2 μm fraction represent the maximum age of gouge formation. The younger ages are thought to reflect the last slip event occurring on the faults, which is related to the rifting and dispersal of the eastern margin of the Australian continent.     

Planning and climate — The case of Di-Zahav

Climate is one of the important factors which must be taken into consideration in all levels of planning. Despite its importance many planners are still unaware of it. An excellent example of such a situation is the case of Di-Zahav, a new settlement located about 130km south of Elat, on the alluvial fan of Wadi Dahab, which flows to the Gulf of Elat.In addition to its extreme hot and arid climate, the region suffers from very strong winds blowing throughout the year from the northerly direction. Heavy rainfall on the 19—20 February 1975 caused a considerable flood in the Wadi Dahab. Because of a mistake in the construction of the road leading to Di-Zahav, the settlement was inundated. After the water dried up, the strong north winds blew sand and dust into the village and dunes accumulated between the buildings. The strong winds are also a negative factor in human comfort and are an obstacle in the development of tourism which is the economic basis of Di-Zahav. Therefore, it seems that the location of the settlement on the southern part of the alluvial fan instead of on the north-eastern edge was a mistake. Also the detailed plan did not take into consideration the negative effects of the climate. It was therefore necessary to plan and carry out a peripheral and internal protection system consisting of planted and artificial windbreaks. It was also suggested to carry out some modifications in the buildings in order to prevent the strong winds blowing into them.     

The origin of fluoride-rich groundwater in Mizunami area, Japan — Mineralogy and geochemistry implications

The aim of this paper was to explore new factors that might be reasons for the occurrence of fluoride-rich groundwater in the area around a construction site. During the construction of two deep shafts of the Mizunami Underground Research Laboratory (MIU) in Mizunami city, central Japan, a large quantity of groundwater with high fluoride concentration was charged into the shafts. Chemical investigation carried out during the excavation revealed that fluoride concentrations in the area around the MIU site greatly exceeded those prescribed by Japanese standards. Therefore, the origin of fluoride ion was experimentally investigated. Samples were collected from the core of a deep borehole drilled in the study area. The weathering - and alteration levels of the collected granites varied greatly. Granitic powders were used to measure fluoride content in the granitic rock mass. The fluoride content ranged between 200 and 1300 mg/kg. The powders were reacted with purified water for 80 days. The results of water–rock interaction showed granitic rock to be one of the main sources of fluoride-rich groundwater in Mizunami area. Fluoride concentrations in these solutions that were shaken for 80 days varied between 2 and 7 mg/l. This change may have occurred as a result of the spatial distribution of fluoride ions in the granite mass as evidenced by mineralogical analysis of fluoride content in several specimens. X-ray powder diffraction analysis of the rock before- and after the water–rock interaction tests manifested that the presence of fluorite mineral was relatively small compared to other minerals. The degree of weathering and alteration might be an additional factor causing dissolution of fluoride-rich minerals. However, it was difficult to interpret the change in fluorite composition by X-ray diffraction analysis.     

Inherent heterogeneity of sediments in Dhahran, Saudi Arabia — a case study

The behavior and properties of sediments depend on their compositional characteristics and formation processes, as well as the environmental conditions during their geological history, i.e. post-formation processes. A vertical cut made in a hill in Dhahran, Saudi Arabia, reveals a vivid picture of the inherent heterogeneity of sediments that have been deposited at different geological ages. A review of the geology of the area, as well as laboratory tests, help to determine the possible causes of the variability of soil types and properties in the area. Laboratory tests include basic geotechnical tests, chemical tests, X-ray diffraction analysis, scanning electron microscopy, and thermal analysis. These tests are used to identify different rock types and soils from the face of the cut. The results of this study indicate that the material from this cut varies from clayey shale and limestone rock (Tertiary, lower Eocene) formed some 52 M.Y. to calcite-cemented sand and pure calcite rock formed in the Quaternary age.     

The Vrancea earthquake of March 4, 1977 — a multiple seismic event and its seismotectonic implications

A study concerning the multiplicity character of Vrancea earthquake of March 4, 1977 based on records of the Romanian seismic network is presented. Four separate shocks within a 9 s interval were recognized of which three with M = 6.0–6.9. Some explanations regarding the macroseismic intensity are made. Also analysed are the shaking duration, the influence of the Carpathian Mountains on the shape of the isoseismal curves and the focal mechanism for the first two main shocks.     

Crustal movements in the tohoku district, Japan, during the period 1900–1975, and their tectonic implications

Vertical crustal movements in the Tohoku district in the past 75 years are discussed with reference to their tectonic implications.

For this purpose, the author first compiled a map of accumulated vertical movement in the past 75 years, by making proper correction for closure. In addition, a new presentation technique utilizing a time-space domain representation of elevation changes is applied to the data. These contour maps provide an informative summary of the vertical crustal movement history in this district. From them, we see that the northeastern part of the district has subsided continuously and aseismically, with an area of significant subsidence inland at a distance about 300 km or more from the trench axis.

In order to explain the mechanism of the remarkable extension of the subsiding area, the finite element method is applied to model the elastic strain field in the district. It is shown that a simple model of uniform dragging at the interface of a sinking slab does not provide a good interpretation of the inland distribution of subsidence. A modification of the conventional model is proposed which hypothesizes vertical movement of the subducting lithospheric slab under the Tohoku district.     

西秦岭北缘断层的多期变形演化及其构造动力学意义

西秦岭北缘断层是青藏高原东北缘新生代盆地与西秦岭地块之间的边界断层,其构造变形的几何学-运动学特征和变形历史等研究对于重建青藏高原东北缘新生代以来的构造变形时空动力学过程,限定新生代盆地构造属性,揭示印度板块-欧亚板块碰撞汇聚的远程构造响应和青藏高原东北缘隆升等重大科学问题具有重要地质约束。本文通过对西秦岭北缘新生代盆地南边界F1断层的断层岩类型及分带、构造要素的几何学-运动学特征等较详细的构造解析,辨认出F1断层6期构造变形：第一期为北西西走向、向北倾斜的韧性-韧脆性伸展正断层作用;第二期为北西西走向、向北陡倾或近直立的高角度逆冲断层作用,指示近南北向挤压缩短作用;第三期为走向近南北、向东或向西陡倾的对冲断层作用,指示了近东西向的挤压缩短作用;第四期为北东向右旋和北西向左旋的走滑共轭断层系统,指示了近东西向的挤压作用;第五期为断层面近直立的北东向左旋、北西向右旋的共轭破裂系统,指示了近南北向挤压作用;第六期为断层面近直立的近东西向左旋和近南北向右旋走滑断层构成了几何学-运动学协调的共轭破裂系统,指示了北东向挤压作用。结合西秦岭北缘渐新世-中新世沉积盆地具有断陷盆地沉积序列特征和上新世具有类磨拉石的冲洪积扇粗砾岩特征以及F1断层多期变形对新生代盆地沉积地层的控制和改造作用分析,认为F1断层第一期韧性-脆韧性伸展正断作用始于渐新世,控制了渐新世-中新世伸展断陷盆地沉积;F1断层第二期高角度逆冲缩短变形使得渐新世-中新世断陷盆地封闭、靠近F1断层的底部砾岩层卷入了挤压逆冲断层作用,断层拖曳使地层产状翘起变陡,这期变形持续到上新世冲洪积扇粗砾岩出现;F1断层第三期与第四期虽然都为近东西向挤压,但第三期为东西向对冲挤出,而第四期为北东向和北西向斜向走滑挤出,其动力学机制是否与青藏高原东北缘西部地壳增厚隆升诱发的中-下地壳向东流动拖曳导致的上地壳东西向挤压缩短尚待证实,由于第三和第四期变形的构造形迹在上新统韩家沟砾岩不存在,因此,这两期变形的时代只能是发生在中新世末期或上新世早期;第五期北东和北西向共轭破裂系统和第六期南北向和东西向共轭破裂系统在渐新统-中新统沉积地层和上新统粗砾岩地层中都存在,其时代无疑是上新世末期或第四纪以来的构造变形,但第五期共轭断层指示的最大主压应力为近南北向,而第六期最大主压应力为北东-南西向,两者夹角约30°,指示两期变形最大主应力方向发生了30°的顺时针旋转,这可能与青藏高原东北缘变形重组过程中块体旋转有关。上述F1断层丰富且复杂的构造变形形迹揭示的断层变形方式和历史演变对于澄清青藏高原东北缘新生代红层盆地构造属性认识上的分歧和高原变形是均匀增厚变形和块体沿断层挤出滑移地壳变形机制的争论等提供了重要的构造依据。     

False drainage anomalies (?) in a granite area, Kvalöya in Troms, Norway — a case study

An anomalous area (100 km²) revealed by a previous regional overbank sediment sampling survey by the Geological Survey of Norway was followed-up for Ti, Nb, Y, Zr, La and Ce by drainage and lithogeochemical sampling and analyses. The drainage samples were anomalous in the same elements as in the overbank samples and contained similar concentrations. The lithogeochemical results did not indicate mineralization and showed only a small enhancement of Ti, Zr, La and Ce in mylonitic rocks.Mineralogical studies of the drainage samples revealed, apart from proportions of quartz and feldspars, that stream sediments and heavy mineral concentrates are similar in mineralogy, with epidote predomination and several vol% of fine-grained titanite and zircon. Drainage samples contained amounts of Nb, Y, and Zr up to ten times larger than average contents of rock samples. Enhanced light REE (La and Ce) and Nb reflect host minerals, titanite, and enhanced Y possibly zircon and/or allanite.Anomalies in both media do not relate to mineralization. The overbank samples and the drainage samples provide comparable geochemical data.     

K-Ar dating of ring complexes and fault systems in Northern Kordofan,Sudan: evidence for independent magmatic and tectonic activity

From an area of brittle shearing and ring complexes in Northern Kordofan, Sudan 28 K-Ar ages were calculated to determine the geochronology of tectonic and magmatic events. The age results for deformational episodes are 560, 340 and 255 Ma, whereas alkaline magma was intruded at 280, 220 and 165 Ma. The incompatibility of age marks leads to the conclusion that magmatic and tectonic activity in this area have to be regarded as independent events. This holds true even when all dated intrusions from the entire region are compared with post-Pan-African continental tectonic features.     

腾冲地块内早印支期构造事件的厘定：来自那邦闪长岩锆石U-Pb定年和Lu-Hf同位素证据

腾冲地块是青藏高原东南缘的重要组成部分,在大地构造位置上与拉萨地块相对应.近年来,松多榴辉岩带的发现揭示拉萨地块内存在晚二叠-早三叠世的俯冲-碰撞事件,与其相对应的腾冲地块是否也经历了相应的构造事件随之成为人们关注的热点.早印支期那邦闪长岩为偏铝质、富钾、钙碱性岩浆岩.锆石LA-ICP-MS U-Pb定年结果表明闪长岩侵位于(245.0±2.9)Ma.锆石εHf(t)值变化于+7.8～+14.9,在εHf(t)-U-Pb年龄图解上所有样品都落在球粒陨石线和亏损地幔线之间、较小的单阶段模式年龄和地壳模式年龄,在Th/Yb-Ba/La图解上分布于俯冲板片流体区和Yb/Hf(＜1.2)比值,以及较高的Mg#值和Th、U负异常,说明其来源于幔源物质部分熔融,并有部分壳源物质加入的混合岩浆.LREE和LILE强烈富集、Th、U、Nb、Ta、Ti明显负异常,以及La/Yb-Sc/Ni和La/Yb-Th/Yb构造环境判别图解揭示其岛弧型岩浆岩的性质.形成时代与松多榴辉岩带相近、形成于俯冲-碰撞构造环境及腾冲地块内存在类似于拉萨地块内缺失晚二叠-早三叠世地层的区域性角度不整合,说明腾冲地块内也存在与拉萨地块和澳大利亚陆块北部边缘发生俯冲-碰撞相对应的构造事件,早印支期那邦闪长岩是腾冲地块内对这一事件的响应.