长江中下游成矿带区域构造格局的新认识

长江中下游成矿带作为我国重要的成矿带之一,矿产资源开发利用及区域地质研究历史悠久,成果丰硕。本文重点论述了长江中下游成矿带的构造格局和复合构造系统,探讨了董岭/埤城式基底与江南式基底的拼合时代,提出了南北对峙带的概念,明确了江南过渡带的内涵。主要认识有:(1)长江中下游成矿带平面构造格局为"一弧一线夹两喇叭"的形态;(2)董岭/埤城式基底与江南式基底的拼合时代介于Z_1与Z_2之间;(3)南北对峙带是"纲",控制了长江中下游成矿带的构造格局;(4)剪切阶段是"眼",区域上由"压扭"向"张扭"的转换过程中成岩成矿大爆发;(5)江南过渡带的南半部早期属于江南隆起,在印支期与下扬子地区一并发生褶皱,褶皱特征类似下扬子坳陷南缘而与江南隆起腹地不同,故划到长江中下游成矿带构造单元中。     

Origin and evolution of Suru Valley ophiolite peridotite slice along Indus suture zone,Ladakh Himalaya,India: Implications on melt-rock interaction in a subduction-zone environment

In this paper, we present whole-rock and mineral geochemistry of serpentinized peridotites from the Suru Valley ophiolite slice Ladakh Himalaya, in an attempt to put constraints on their petrogenesis and tectonic evolution in the context of Mesozoic Neo-Tethys Ocean. On the basis of petrographic study, Suru Valley serpentinized peridotites can be identified as serpentinized harzburgites. Relative to primitive mantle these rocks have depleted major and rare earth element (REE) geochemical characteristics comparable to ocean floor mantle rocks reflecting their mantle residual nature. However, higher abundance of highly incompatible large ion lithophile elements (e.g., Rb, Ba, Th, U, Pb and Sr), reflect metasomatism in a subduction zone environment. The presence of silicate assemblage includes Mg-rich olivine (Fo_90-92) and orthopyroxene (En_91-93 Fs_6.4-8.7) of supra-subduction zone affinity. Evaluation of mineral and whole-rock geochemistry suggests that the Suru Valley ophiolitic peridotites represent residues left after moderate degrees of partial melting thereby underwent metasomatism in a supra-subduction zone environment related to north dipping intra-oceanic island arc during Cretaceous in the context of Mesozoic Neo-Tethys ocean.     

Evolution of contourite systems in the late Cretaceous Chalk Sea along the Tornquist Zone

Based on integration of seismic reflection and well data analysis this study examines two major contourite systems that developed during the late Cretaceous in the southern Baltic Sea. The evolution of these Chalk Sea contourite systems between the Kattegat and the southern Baltic Sea started when Turonian to Campanian inversion tectonics overprinted the rather flat sea floor of the epeiric Chalk Sea. The Tornquist Zone and adjacent smaller blocks were uplifted and formed elongated obstacles that influenced the bottom currents. As a consequence of the inversion, the sea floor west of the Tornquist Zone tilted towards the north‐east, creating an asymmetrical sub‐basin with a steep marginal slope in the north‐east and a gentle dipping slope in the south‐west. A south‐east directed contour current emerged in the Coniacian or Santonian along the south‐western basin margin, creating contourite channels and drifts. The previously studied contourite system offshore Stevns Klint is part of this system. A second, deeper and north‐west directed counter‐flow emerged along and parallel to the Tornquist Zone in the later Campanian, but was strongest in the Maastrichtian. This bottom current moderated the evolution of a drift‐moat system adjacent to the elevated Tornquist Zone. The near surface Alnarp Valley in Scania represents the Danian palaeo‐moat that linked the Pomeranian Bay with the Kattegat. The previously studied contourite system in the Kattegat represents the north‐western prolongation of this system. This study links previous observations from the Kattegat and offshore Stevns Klint to the here inferred two currents, a more shallow, south‐east directed and a deeper, north‐west directed flow.     

Valley confinement as a factor of braided river pattern for the Platte River

The Platte River in Nebraska has evolved in the twentieth century from a predominantly braided river pattern to a mélange of meandering, wandering, anastomosed, island braided, and fully braided reaches. Identifying the factors that determine the occurrence of a fully braided main channel was the objective of this study. Aerial photography, gage flow data, ground-surveyed cross sections, bed material samples, and the results of sediment transport modeling were used to examine factors that control spatial change in main river pattern of the central Platte River. Valley confinement is identified as the determining factor of braided river in nine of eleven divisions of the central Platte River. Flow reduction and the interruption of sediment supply are identified as determining factors preventing fully braided river in the remaining two of eleven reaches.Valley confinement, the topography which limits the width of the floodplain, was initially measured as width between historical banks (predevelopment river banks). This metric was later refined to width between confining features (historical banks, remnant bars, bridge abutments, protected banks and levees). Under existing conditions, the main channel of the central Platte River is fully braided when valley confinement (width between confining features) is 600 m or less and begins to divide into the multiple channels of an anastomosed pattern when valley confinement (width between confining features) exceeds 600 m When Platte River flow is divided between two to four major anabranches, a fully braided pattern in the main channel of the main anabranch requires a more confined valley of 400 m or less.Valley confinement is demonstrated to be the dominant factor in determining river pattern in the central Platte River, although this factor is not normally considered in the continuum of channel pattern model. Conclusions from this study can be used to increase the occurrence of fully braided main channel in the central Platte River, to aid habitat recovery for endangered or threatened bird species that favor this river pattern. Consideration of valley confinement with river continuum factors can aid river managers by improving predictions of river pattern in response to management actions.     

哀牢山——元江河谷对区域地理分异的影响

基于对哀牢山是云南东西部自然界线的认识,对比分析哀牢山-元江河谷东西两侧5个纬度带、近几十年的重要自然要素、农业种植结构、农民收入来源及区域经济发展水平特征,结果表明:西部较东部地形高差较大、≤15°宜农土地比重较少、水汽和热量资源更丰富,形成东西部间显著自然环境差异;东部较西部的经济作物、旱作种植比重更高,表明东西部在农业生产结构方面存在明显分异,而各部的种植结构(以旱作种植比例为例)在各纬度间呈近纬向的相似性变化;东部较西部的人均农业产值、农民纯收入及人均GDP更高,表明东西部经济发展水平存在较大时空差异,而东西各部内则存在较好的相似性。从区域社会经济发展方面佐证了哀牢山作为云南东西部界线的存在。     

萨拉乌苏河流域MGS2层段主要氧化物变化及其指示的气候波动

萨拉乌苏河流域与MIS2相同时代的MGS2地层段记录了5.5个由风成砂与河流相或湖沼相构成的沉积旋回,通过对各沉积旋回的主要氧化物分析表明：河湖相中Al2O3、TOFE、K2O、MgO、CaO、Na2O、TiO含量通常明显高于风成砂,而SiO2含量则呈现相反的变化,构成了与沉积旋回相对应的5.5个主要氧化物变化旋回。结合各主要氧化物的化学迁移特性和剖面所在区域的地貌特征进行分析,认为该剖面的主要氧化物变化旋回是气候波动的反映,一个主要氧化物含量变动旋回指示了一个干冷-暖湿旋回,也即一个冬夏季风先后演替的气候旋回。MGS2持续的时间仅为12 ka,一个冬夏季风环境的变化大致平均仅需要2 ka,一个冬季风或者夏季风环境的平均持续时间大约1 ka。显然,这是一个千年尺度的环境变化。研究还表明,MGS2气候波动与晚冰期气候变化以及相关的Heinrich事件具有良好的对应关系。可以推断,MGS2记录的千年尺度环境变化是全球环境变化在本区域的反映。     

基于ALOS遥感数据的新丰江流域生态安全GIS评价

基于ALOS遥感影像,采用10m×10m的栅格精度,以乡镇行政区划作为综合评价单元,在G IS的支持下,获取了18种评价因子信息。通过建立生态安全指数模型,计算出每个栅格和每个乡镇区域的生态安全指数并将其划分为6个等级,即Ⅰ级到Ⅵ级(生态安全状况由非常安全到极不安全)。结果表明,新丰江流域内的生态安全状况整体上较好,但安全等级分布不平衡,具有明显的区域性特征,生态堪忧地区为居民及工矿用地密集区,主要分布在流域的北部、东南部和中部的采矿区,如上坪镇、元善镇和上莞镇等;生态较为安全的区域主要分布在流域中部,如新丰江库区附近的锡场镇、半江镇等。该方法为流域的生态安全评价、预警、保护及管理提供了有效手段,具有积极的实用价值。     

Measuring alluvial fan sensitivity to past climate changes using a self‐similarity approach to grain‐size fining,Death Valley,California

The effects of climate change on eroding landscapes and the terrestrial sedimentary record are poorly understood. Using mountain catchment–alluvial fan systems as simple analogues for larger landscapes, a wide range of theoretical studies, numerical models and physical experiments have hypothesized that a change in precipitation rate could leave a characteristic signal in alluvial fan sediment flux, grain size and down‐system fining rate. However, this hypothesis remains largely untested in real landscapes. This study measures grain‐size fining rates from apex to toe on two alluvial fan systems in northern Death Valley, California, USA, which each have well‐exposed modern and ca 70 ka surfaces, and where the long‐term tectonic boundary conditions can be constrained. Between them, these surfaces capture a well‐constrained temporal gradient in climate. A grain‐size fining model is adapted, based on self‐similarity and selective deposition, for application to these alluvial fans. This model is then integrated with cosmogenic nuclide constraints on catchment erosion rates, and observed grain‐size fining data from two catchment‐fan systems, to estimate the change in sediment flux from canyon to alluvial fan that occurred between mid‐glacial and modern interglacial conditions. In a fan system with negligible sediment recycling, a ca 30% decrease in precipitation rate led to a 20% decrease in sediment flux and a clear increase in the down‐fan rate of fining, supporting existing landscape evolution models. Consequently, this study shows that small mountain catchments and their alluvial fan stratigraphy can be highly sensitive to orbital climate changes over <10⁵ year timescales. However, in the second fan system it is observed that this sensitivity is completely lost when sediment is remobilized and recycled over a time period longer than the duration of the climatic perturbation. These analyses offer a new approach to quantitatively reconstructing the effects of past climate changes on sedimentation, using simple grain‐size data measured in the field.     

深切河谷斜坡地带岩溶地下水的赋存特征及开发利用——以石阡县安家寨地下水系统为例

本文以石阡县安家寨片区开放型岩溶地下水系统为研究对象,以石阡县安家寨地区的实际勘查成果为基础,指出了在该类型地区受地质构造控制,可能存在局部封闭型的阻水型蓄水构造,成为该类型地区寻找和开发利用地下水的有利地带。并在此提出在深切河谷斜坡地带寻找有开发价值的地下水源地的方向为:在"开放型"水文地质系统中寻找局部的"封闭型的地下水蓄水构造",可作为该类型地区勘查和开发岩溶地下水的借鉴。     

Isotopic evolution and climate paleorecords: modeling boundary effects in groundwater-dominated lakes

We used an isotopic mass-balance model to examine how the hydrogeologic setting of lakes influences isotopic response of evaporating lake water to idealized hydroclimatic changes. The model uses a monthly water and isotope balance approach with simplified water-column structure and groundwater exchanges. The framework for comparative simulations is provided by lakes in a region of the Northern Rocky Mountains that display high interlake geochemical variability, thought to be controlled by groundwater hydraulics. Our analysis highlights several isotopic effects of flow between aquifers and lakes, leading to possible divergence of isotopic paleorecords formed under a common climate. Amplitude of isotopic variation resulting from simulated climate forcing was greatly damped when high groundwater fluxes and/or low lake volume resulted in low lake fluid residence time. Differing precipitation and evaporation scenarios that are equivalent in annual fluid balance (P−E) resulted in different isotopic signatures, interpreted as a result of evaporation kinetics. Concentrating low-δ groundwater inflow during spring months raised springtime lake δ values, a counterintuitive result of coincidence between times of high groundwater inflow and the evaporation season. Transient effects of reduced fluid balance caused excursions opposite in sign from eventual steady-state isotopic shifts resulting from enhanced groundwater inflow dominance. Lags in response between climate forcing and isotopic signals were shortened by high groundwater fluxes and resulting short lake residence times. Groundwater-lake exchange exerts control over patterns of lake isotopic response to evaporation through effects on lake residence time, inflow composition, and seasonal timing of inflow and outflow. Sediments from groundwater-linked lakes, often used for paleoenvironmental analysis, should be expected to reflect isotopic complexities of the type shown here. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.