Early Silurian tholeiitic-boninitic Mailisu ophiolite,South Tianshan,Kyrgyzstan: a geochemical record of subduction initiation

ABSTRACT

Ophiolite assemblages of the South Tianshan fold-and-thrust-belt (STS) track the sea floor dynamics of the late Cambrian to Carboniferous Turkestan Ocean in the western Central Asian Orogenic Belt. We interpret new geochronological and geochemical data for the Mailisu ophiolite from western Kyrgyzstan as evidence for an early Silurian incipient subduction zone outboard the South Chatkal Arc of the Middle Tianshan. Igneous zircons from a leucogabbro yielded a U-Pb age of 440 ± 6 Ma indicating one of the oldest sea floor fragments in the STS. Metabasalt samples have N- to E-MORB trace element patterns and initial ε_Nd values of ~5 to 9 support melting of depleted asthenosphere comprising a minor OIB component. Cross-cutting diabase dykes show geochemical characteristics of high-Ca basaltic boninites. Their low REE abundances and highly fractionated middle-heavy REE patterns indicate sources that were more melt-depleted than those of the metabasalts. Concave upwards LREE patterns are compatible with a residual peridotite source, fertilised by small-degree OIB-like melts with ε_Nd of ~5 to 6. Positive Ba, Sr, and Pb anomalies support the involvement of slab-derived hydrous fluids that probably facilitated the melting of the residual mantle. The association of MORB-like metabasalts with younger dykes of boninite affinity suggests subduction initiation for the origin of the Mailisu ophiolite. Our data elucidate one of the major yet little understood episodes in the Paleozoic history of the Turkestan Ocean. Future identification of similar rock assemblages will be helpful to understand the insufficiently constrained history of ocean floor subduction in the Paleozoic Oceans of the western CAOB.     

冰湖溃决泥石流的形成、演化与减灾对策

本文分析了主要由冰滑坡和冰崩入湖导致的冰湖溃决的机理和条件.进而,从气候条件、水文条件、终碛堤、冰湖规模、冰滑坡、沟床特征和固体物质补给等方面分析了冰湖溃决泥石流的形成条件和特点,归纳出冰湖溃决泥石流沿程演化的6种模式:溃决洪水-稀性泥石流、溃决洪水-黏性泥石流、溃决洪水-稀性泥石流-黏性泥石流、溃决洪水-黏性泥石流-稀性泥石流、溃决洪水-稀性泥石流-黏性泥石流-稀性泥石流和溃决洪水-黏性泥石流-稀性泥石流-洪水.针对冰湖溃决泥石流突发性强、频度低、洪峰高、流量大、流量过程暴涨暴落、破坏力强和灾害波及范围广等特点,提出了7点减灾对策.     

栗子坪水电站南桠村沟泥石流运动特征研究

南桠村沟位于南桠河栗子坪水电站首部枢纽区左岸，是水电站工程区内规模较大的一条泥石流冲沟，历史上曾经发生过较大规模的泥石流。水电站调节池位于南桠村沟的出口台地上，沟内第四纪松散堆积物及人工堆积物丰富，在暴雨，地震等自然因素的触发下，时有较大规模的泥石流发生的可能，对调节池的运行安全带来不利影响。笔者根据国内外有关研究成果，结合南桠村沟泥石流的具体特征，分析研究了南桠村沟泥石流的运动特征，包括泥石流的流速、流量、和冲击力，这一研究对评价南桠村沟泥石流对栗子坪水电站调节池的影响具有现实意义。     

黄土丘陵区土壤侵蚀链垂直带水沙流空间分布

采用多坡段组合模型、人工模拟降雨实验研究了土壤侵蚀链垂直带水沙流空间分布变化特征。结果表明:愈向下坡方向的坡段产流量愈大,单位面积、单位时间的产流量按坡段排列为:谷坡>梁峁坡下部>梁峁坡中部>梁峁坡上部。雨强为29.7mm/h时,坡面上没有沟蚀发育;雨强为60.5mm/h时,细沟主要分布在梁峁坡下部和谷坡处;雨强为90 2mm/h时,各种侵蚀形态在坡面均有较好发育,细沟的出现部位一直伸展到梁峁坡中部和上部之间。由于上坡来水来沙的作用,梁峁坡的产沙量增大了20.2%～63.5%,谷坡的产沙量增大了42.9%～74.5%。     

Concentrated flow erosion in cultivated catchments: Influence of soil surface state

Concentrated flow erosion is the dominant form of winter erosion in northern France. This study correlates the ephemeral rill and gully volumes measured in 20 cultivated catchments (4–95 ha) for three consecutive winters with the size of the potential runoff-contributing areas. These areas were identified by characterizing soil surface state through crust development stage, importance of surface wheel tracks and roughness grade. A single and significant relationship was found between the size of runoff-contributing areas, estimated by this criterion, and the rill and gully volumes. This identified the proportion of the catchment area occupied by fields with a degraded surface structure as the main factor controlling the variability of erosion in a context of concentrated flow erosion on cultivated land. The extension of degraded areas was shown to be controlled by dynamic interactions between weather, land occupation and soil physical properties. This criterion accounts for the uneven distribution of rainfall in space and time. Morphological factors, such as talweg length and slope, are believed to determine part of the residual variability.     

Hazardous earth processes in parts of Bauchi State,Nigeria: Their causes and environmental implications

The August 1985 crack has focussed attention on passive earth processes in parts of Bauchi State, Nigeria. These processes, encompassing groundwater solution and roof collapse, soil shrink-swell, and gullying, are widespread in the Akko, Gombe, and Bauchi Local Government Areas (LGAs) of the State. Soils in these areas are rich in montmorillonite clay minerals and possess appreciable shrink-swell capabilities. Surface heave ranges from 8.1 to 11 cm. Preliminary data on plasticity measurement show a plasticity index (PI) of 13.4 to 15%. Consequently, in response to seasonal changes, the soil usually splits, leaving gaping cracks. These cracks, once formed, constitute channels throug which meteoric water easily penetrates the bedrock. The water is chemically acid and the underlying bedrock contains layers of limestone and shale. Systematic solution of these chemically reactive rocks results in the formation of caverns, the roof of which may ultimately collapse leaving randomly distributed sink holes. These cracks and sink holes run through villages, destroying mud buildings, which often collapse overnight, and transforming most modern block buildings into death traps. They affect roads, bridges, and other foundations, and render farmlands waste, swallowing up livestock.The expansive soil processes, solution and roof collapse phenomena currently identified in these places, are passive phenomena and have been overlooked as geologic hazards. Preliminary results of investigations on the physical and chemical properties of the soils and underlying rocks indicate that these areas of Bauchi State of Nigeria need to be monitored closely to avert disaster.     

Surface runoff in a torrent catchment area in Middle Europe and its prevention

The Schesa, a sinister contributory torrent to the Ill river near Bludenz (federal province of Vorarlberg) is the largest basin-shaped gully of Middle Europe and endangers the underlying villages by torrential debris flow and gigantic mass movements. The catchment is characterized by a complex geological situation, high annual precipitation and torrential rains from spring to early autumn, which cause enormous amounts of surface runoff. Based on field investigations comprising rain simulation experiments on representative plots, investigations on land-use, vegetation cover, soil physical characteristics, geology, hydrogeology and other features of the catchment area, surface runoff coefficient maps were developed. They formed the basis for assessment of runoff potential for different scenarios in vegetation cover and land-use intensity. Calculation of runoff for the recurrent design event by use of an improved run-time method showed the urgent necessity of runoff reduction measures in large parts of the catchment area above the gully. Based on the modelling results a concept for reduction of both, surface runoff and amount of deep percolating water has been elaborated.     

Distribution and concentrations of anthropogenic platinum group elements in the surface environment of Hong Kong

Ecotourism environmental impact assessment is a traveling activity relevant influence prediction and appraisal. And the setting-up of the index system is very important to implement the appraisal of environmental impact of ecological travel. According to the index system, natural ecological environment, humane social environment, the environment of tourist resources and the atmosphere of capacity of tourist environment, four major systems form the first index. Each system formed the second index of several key elements. In addition, it is a key step to appraise the sureness of the factor weight, which influences the rationality of the result directly. Daily method of weighting has the Delphi＇s and the AHP. According to ＂Grade form of characteristic value of Standard of rating for quality of tourist attractions＂ which the National Tourism Administration issues, the paper takes the Jingshan mountains ecological tourist zone as an example and adopts AHP to weight the factors mainly, assisted with the Delphi＇s by the help of computer. The Jingshan ecological tourist zone is good. The result of calculation, comprehensive value of 8.35 after establishment, shows that it doesn＇t change dramatically, attributing to the index of natural ecological environment like atmosphere, quality of surface water, acoustics environment and vegetation coverage. Improvement of traffic states and sanitation caused by the project in the region increases the comprehensive value, while the water and soil loss in partial area caused by construction decrease.     

Comparison of organic pollutants in urban deposits of Guangzhou and Hong Kong metropolitan areas

With the rapid development of industrialization and urbanization in the Pearl River Delta region, organic as well as inorganic pollution is becoming a major environmental concern. Urban deposits including road dusts, corresponding gully sediments and topsoils proved to be useful tools for assessing the level and distribution of contaminants in metropolitan areas. Most of the previous studies utilizing urban deposits, however, focused on inorganic pollutants, while data on organic pollutants are rare. In this study, therefore, the organic pollutants in urban deposits of Hong Kong and Guangzhou were systematically investigated in terms of their abundance, composition and distribution. The road dusts, gully sediments and vehicular tunnel dusts as well as soils from different urban locations with variable traffic volumes （less than 1000 to more than 75000 AADT） were examined. The results indicated that vehicle-emitted particles were the major source of organic pollutants, while the contributions from background soils were very limited. Surprisingly, the soil in a remote island of Hong Kong with no vehicular traffic has also shown signs of pollution from vehicular particles likely through dry-wet atmospheric deposits. In general, the characteristics of organic pollutants in urban dusts from both cities are comparable although there were also some significant differences. For example, the amount of extractable organic matter in urban deposits of Hong Kong was significantly higher than that of Guangzhou. Differences in their extractable asphaltene contents not only reflected the differences in quality and types and fuels used in these cities, but also sample residence times, and thus different extents of weathering processes.     

黑水县芦花沟泥石流灾害治理现状与建议

芦花沟是岷江水系黑水河中游右岸一级支流,历史上曾多次暴发泥石流,对黑水县城芦花镇造成了严重的危害和威胁.自上世纪80年代对其进行综合治理后,泥石流得到了有效控制.但由于工程年久失修,加之近几年降水量偏高,该沟泥石流有进一步活跃的趋势.仅2004年就发生了3次泥石流,其中09-30泥石流冲出物近1×104 m3,规模较大,并在排导槽中产生堵塞,幸得当地政府及时启动紧急预案进行了疏通,县城才得以平安.为了消除隐患,减轻灾害,我们对该沟泥石流进行了调查分析,提出了相应的应急措施和进一步防治对策.