Human Modification of the Geomorphically Unstable Salt River in Metropolitan Phoenix     

Martin Roberge 《The Professional geographer》2002,54(2):175-189

In‐stream gravel mining, massive bridge piers, and channelization have all contributed to the geomorphic instability of the Lower Salt River channel in Arizona. Dam closure, changing dam operating rules, and the frequent modification of the channel bed have decreased our ability to predict the Salt River hydrology. Engineering practice has adapted to this situation and to a public that is increasingly intolerant of service disruptions by constructing larger bridges and extending levees. Building these larger structures may be counterproductive; future construction should not constrict the channel and should re‐establish a braided river to decrease the energy available to the system.  相似文献   

SHRIMP Age and Geochemistry of the Bikou Volcanic Terrane: Implications for Neoproterozoic Tectonics on the Northern Margin of the Yangtze Craton   总被引：3，自引：0，他引：3  

A.D.HANSON  P.A.DRUSCHKE 《《地质学报》英文版》2003,77(4):479-490

The Bikou volcanic terrane is predominated by subalkaline tholeiitic lavas. Rock samples display lower initial ratios of Sr and Nd, 0.701248-0.704413 and 0.511080-0.512341 respectively. 207Pb and 208Pb are significantly enriched in the lavas. Most samples have positive εNd, which indicates that the magma was derived from EM-type mantle source, while a few samples with negative εNd indicate that there was contamination in the magma evolution. Magma differentiation is demonstrated by variations of LREE and LILE from depletion to enrichment. Additionally, normalized REE patterns and trace elements showed that lavas from the Bikou volcanic terrane have similar characteristics to those of basalts in arc settings caused by subduction and collision. Analyses showed that the Bikou volcanic terrane is a volcanic arc. New evidence proved that the Hengdan Group, north of the Bikou arc, is a turbidite terrane filling a forearc basin. Consequently, the Bikou volcanic terrane and the Hengdan turbidite terrane const  相似文献   

伊宁吐拉苏火山盆地金矿成矿构造系统与远景评估   总被引：6，自引：0，他引：6  

汪劲草  夏斌  漆树基 《新疆地质》2003,21(4):383-386

吐拉苏火山盆地金矿成矿构造系统由2种类型的成矿构造组成，一种是火山机构中的高角度塌陷式断裂；另一种是爆发．沉积相中的低角度水压式断裂．阿希金矿受控于弧形塌陷式断裂，其北东段坍塌强度大于南东段，最大张裂空间控制的矿体群向NE侧伏．阿希火山机构东侧的阿恰勒河组下可能隐伏着与西侧对应的高角度弧形塌陷式成矿断裂．爆发．沉积相中硅化岩型金矿受控于水压式断裂，不透水凝灰岩层圈闭富水粗火山碎屑岩层，组成压力仓构造并发生水力压裂作用．由水力压裂作用形成的层控水压式成矿断裂出现的几率，远大于后期由非成矿断裂切割抬升它们出现的几率．构造解析表明吐拉苏火山盆地2类成矿构造控制的金矿具有巨大的地质找矿潜力．  相似文献   

甘肃礼县新生代火山喷发碳酸岩的发现及意义   总被引：7，自引：8，他引：7  

喻学惠  莫宣学等 《岩石学报》2003,19(1):105-116

甘肃礼县新生代钾霞橄黄长岩中有大量火山喷发碳酸岩出露，本文对该区各种碳酸岩的地质产状，岩石特征，全岩化学及稀土，微量和C、O同位素地球化学进行了研究，结果表明本区碳酸岩属于一种高CaO低碱（Na2O K2O)的火成碳酸岩，其化学成分与东非裂谷乌干达FortPortal地区的碳酸岩相似，其中碳酸岩的火山砾岩和凝灰岩具有高的SiO2和Mg/Ca比值，化学成分显示了硅酸盐和碳酸盐混合的特征，可能代表直接由地幔部分熔融形成的原生碳酸盐岩浆。其它类型碳酸岩的成因与原生碳酸盐岩浆或原生钾霞橄黄长岩浆的液态不混溶作用和／或结晶分异作用有关。碳酸岩及共生的高钾火山岩不是典型的大陆裂谷岩浆作用的产物，其起源和成因与软流圈的上涌有关。碳酸岩的发现为查明本区地幔的组成和性质提供了新的岩石学证据。  相似文献   

黄河上游第四纪河流地貌演化--兼论青藏高原1∶25万新生代地质填图地貌演化调查   总被引：6，自引：0，他引：6  

张智勇  于庆文  张克信  顾延生  向树元 《地球科学》2003,28(6):621-626,633

在青藏高原1∶25万地质填图中,新生代地貌演化调查方法是查明地貌组成的形态、分布、形成年代等特征,分析地貌成因类型,研究地貌与构造、气候、沉积的关系,通过夷平面、河流阶地等反映隆升过程的标志性地貌面调查,分析地貌发展阶段,建立区域地貌演化史.由黄河上游羊曲段阶地地貌调查结果,推断黄河在0.03 Ma才切开共和南山.对比黄河上游不同发育地段阶地,表明黄河上游地貌演化过程是伴随高原阶段隆升而向上游阶段性溯源侵蚀发展的.1.6 Ma黄河稳定出现在民和-兰州-临夏,1.1 Ma切开积石峡到达化隆-贵德,0.15 Ma切开龙羊峡出现于共和盆地,约0.03 Ma经历最新抬升事件,切开贵南南山及西秦岭,并沟通若尔盖盆地抵达黄河源区.  相似文献   

Characterisation of the 1997 Vulcanian explosions of Soufrière Hills Volcano,Montserrat, by video analysis     

Formenti  Y.  Druitt  T. H.  Kelfoun  K. 《Bulletin of Volcanology》2003,65(8):587-605

The activity of Convention at Montserrat Soufrière Hills Volcano, Montserrat, during the period 1995–1999 included numerous violent explosions. Two major cycles of Vulcanian explosions occurred in 1997: a first of 13 explosions between 4 and 12 August and a second of 75 between 22 September and 21 October. The explosions were short-lived events lasting a few tens of seconds during which partial fountain collapse generated pyroclastic surges and pyroclastic flows, and buoyant plumes ascended 3–15 km into the atmosphere. Each explosion discharged on average 3×10⁵ m³ (dense-rock equivalent, DRE) of magma, draining the conduit to depths of 1–2 km. The paper focuses on the first few seconds of three explosions of the 75 that occurred in September/October 1997: 6 October 1997 at 17:50, 7 October 1997 at 16:02 and 9 October 1997 at 12:32. Physical parameters such as exit velocities, magmatic water contents and magma pressures at fragmentation are estimated by following and modelling the ascent of individual momentum-dominated finger jets visible on videos during the initial stages of each explosion. The model treats each finger jet as an incompressible flow sustained by a steady flux of gas and particles during the few seconds of ascent, and produces results that compare favourably with those using a multiphase compressible code run using similar eruptive parameters. Each explosion reveals a progressive increase in eruptive intensity with time, jet exit velocities increasing from 40 m s^–1 at the beginning of the explosion up to 140 m s^–1 after a few seconds. Modelling suggests that the first magma to exit was largely degassed, whereas that discharged after a few seconds contained up to 2 wt% water. Magma overpressures up to ~10 MPa are estimated to have existed in the conduit immediately prior to each explosion. Progressive increases in jet exit velocity with time over the first few seconds of each explosion provide direct evidence for strong pre-eruptive gradients in water content and magma pressure in the upper reaches (probably 100–500 m) of the conduit. Fountain collapse occurred during the first 10–20 s of each explosion because the discharging jets had bulk densities up to 100 times that of the atmosphere and were unable to entrain enough air to become buoyant. Such high eruptive densities were due to the presence of partially degassed magma in the conduit.Editorial responsibility: A. Woods  相似文献   

The design of post‐mining landscapes using geomorphic principles     

G. R. Hancock  R. J. Loch  G. R. Willgoose 《地球表面变化过程与地形》2003,28(10):1097-1110

Nature can provide analogues for post‐mining landscapes in terms of landscape stability and also in terms of the rehabilitated structure ‘blending in’ with the surrounding undisturbed landscape. In soil‐mantled landscapes, hillslopes typically have a characteristic pro?le that has a convex upper hillslope pro?le with a concave pro?le lower down the slope. In this paper hillslope characteristic form is derived using the area–slope relationship from pre‐mining topography at two sites in Western Australia. Using this relationship, concave hillslope pro?les are constructed and compared to linear hillslopes in terms of sediment loss using the SIBERIA erosion model. It is found that concave hillslopes can reduce sediment loss by up to ?ve times that of linear slopes. Concave slopes can therefore provide an alternative method for the construction of post‐mining landscapes. An understanding of landscape geomorphological properties and the use of erosion models can greatly assist in the design of post‐mining landscapes. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

滇西思茅大平掌矿区火山岩特征及其构造环境   总被引：3，自引：0，他引：3  

李峰  范柱国  李保珠 《大地构造与成矿学》2003,27(1):48-55

滇西大平掌铜多金属矿床的含矿建造(龙洞河组)主要由细碧岩-角斑岩-石英角斑岩和流纹岩-硅质岩组成,伴有部分火山角砾岩、凝灰岩,属双峰式火山岩组合。火山岩具明显的Na2O>K2O和Nb、Ta、Zr、Hf、Ti亏损。岩石化学和微量元素判别标志指示含矿建造形成于岛弧裂陷环境。结合区域构造古地理资料分析,它是滇西南古特提斯洋壳在晚石炭世-早二叠世由西向东俯冲过程中岩浆作用的结果。  相似文献   

Use of Geomorphological Information in Indirect Landslide Susceptibility Assessment   总被引：31，自引：6，他引：31  

van Westen  C. J.  Rengers  N.  Soeters  R. 《Natural Hazards》2003,30(3):399-419

The objective of this paper is to evaluate the importance of geomorphological expert knowledge in the generation of landslide susceptibility maps, using GIS supported indirect bivariate statistical analysis. For a test area in the Alpago region in Italy a dataset was generated at scale 1:5,000. Detailed geomorphological maps were generated, with legends at different levels of complexity. Other factor maps, that were considered relevant for the assessment of landslide susceptibility, were also collected, such as lithology, structural geology, surficial materials, slope classes, land use, distance from streams, roads and houses. The weights of evidence method was used to generate statistically derived weights for all classes of the factor maps. On the basis of these weights, the most relevant maps were selected for the combination into landslide susceptibility maps. Six different combinations of factor maps were evaluated, with varying geomorphological input. Success rates were used to classify the weight maps into three qualitative landslide susceptibility classes. The resulting six maps were compared with a direct susceptibility map, which was made by direct assignment of susceptibility classes in the field. The analysis indicated that the use of detailed geomorphological information in the bivariate statistical analysis raised the overall accuracy of the final susceptibility map considerably. However, even with the use of a detailed geomorphological factor map, the difference with the separately prepared direct susceptibility map is still significant, due to the generalisations that are inherent to the bivariate statistical analysis technique.  相似文献   

浅述五大连池火山区浅层气体钍、氡与断裂构造和火山活动的关系     

孙如波  高清武  潘玉林  杨森林 《东北地震研究》2003,19(4):9-17

在五大连池火山区流体化学研究原有工作的基础上，对火山群的重要区段开展了野外踏勘、气体普查，特别是用放射性气体测量方法对火烧山、老黑山、笔架山、药泉山等火山区进行了Tn、Rn测量。通过大量实测资料，分析和研究了Tn、Rn等气体和火山、火山活动、断裂构造、断裂活动性及其他地质条件的关系，初步查明了五大连池火山区的气体组成特征、分布和形成规律，对地下主要气体的成因、气体和火山构造活动及其它地质条件的关系进行了初步探讨，并提出了对五大连池火山区气体观测的初步设想和建议。  相似文献