Investigation of permeable fracture zones by tube-wave analysis     

Y. D. Li  W. Rabbel  R. Wang 《Geophysical Journal International》1994,116(3):739-753

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Investigation of the lithosphere beneath the Vogelsberg volcanic complex with P-wave travel time residuals     

T. Braun  H. Berckhemer 《International Journal of Earth Sciences》1993,82(1):20-29

With the aim of investigating the P-wave velocity structure below the Tertiary volcano Vogelsberg, a network of 10 mobile short period seismograph stations was installed in May 1987 for a period of 20 months. P-Wave travel time residuals relative to the station Kleiner Feldberg/Taunus (TNS) were determined for 168 seismic events using the Jeffreys - Bullen travel time tables. At all stations the relative residuals showed a positive sign, indicating a low velocity zone beneath the Vogelsberg. Maxima were found in the northern part of the Vogelsberg (station VAD +0.5 s) and in the region of the Amöneburger Basin (station RAU +0.28 s).The travel time residuals were inverted using the tomographic inversion method of Aki et al. (1977). The slowness perturbations of the single blocks were calculated relative to a crustal and upper mantle model of the Rhenish Massif. The results show an intracrustal low velocity body (about –9%) striking in a Variscan direction and underlying the north-eastern part of the Vogelsberg, and another velocity minimum (about – 6%) in the region of the Am6neburger Basin. In the lower crust and the upper mantle the velocities are reduced by about 4% relative to the starting model.The Variscan alignment of the low velocity zone under the Vogelsberg correlates with results of other geological studies. It can be assumed that during the rifting phase of the Upper Rhinegraben Variscan lineations have been reactivated, favouring uprising of magma along these old structures. The position and extension of the low velocity zone correlate with the assumed sediment distributions in the area of investigation. This may account for about one-half of the observed anomaly. The reason for the velocity reduction of about 4% in the entire underground region of the Vogelsberg down to a depth of about 70 km can be explained by the intensive fracturing of the lithosphere, caused by thermal and pressure gradients during the magma eruption process.  相似文献   

北秦岭基性火山岩地球化学特征与上地幔岩浆源区性质     

欧阳建平  张本仁  韩吟文  郑海飞  谢千里 《地球科学》1993,(1)

元古代和早古生代是北秦岭两个主要的火山岩浆作用时期,地球化学研究表明,元古代基性火山岩具有相对低SiO_2、Al_2O_3和高TiO_2特征,基性火山岩TiO_2-P_2O_5、MgO/FeO-Y、的变化显示了正常的、稳定的岩浆作用过程,相容元素分配模型表明这一时期基性火山岩与华北区上地幔部分熔融岩浆作用相吻合,早古生代基性火山岩除普遍具有高、低特征外,突出地表现出地球化学成分的不均匀性,多源、混合岩浆作用的干扰使单源岩浆演变特征遭受破坏,相容元素分配模型表明:这一时期基性火山岩浆与引起秦岭区、华北区上地幔岩浆共同作用有关系,北秦岭基性火山岩浆作用的特征与这一地区从元古代到早古生代的构造发展是相适应的,反映了该区上地幔性质及岩石圈构成的重大改变。  相似文献   

红碱淖—沟岔地区地下水流系统研究   总被引：3，自引：0，他引：3  

武选民  王树棕 《水文地质工程地质》1993,20(6):30-34

本文根据对红碱淖－沟岔地区地下水形成的背景条件、水化学特征、环境同位素组成的分析和剖面地下水势的计算，把研究区划分为２个区域地下水流系统和３个局部地下水流系统，对不同等级地下水流系统的边界、系统的循环作了详细探讨，深刻揭示了这种复合地形区、均质含水介质中地下水流的循环模式。  相似文献   

论边缘定位：玢岩型铁硫多金属矿床定位的动力学机制     

张少斌 《矿产与地质》1993,7(6):385-389

通过庐枞中生代火山岩地区成矿时空结构的研究,提出了玢岩型铁硫多金属矿床定位的动力学——边缘定位机制,总结出该区的4种边缘定位形式和引起边缘定位的两方面动力学因素,最后建立了该区构造、火山—岩浆、岩性多因素控制的边缘定位模式.  相似文献   

Chronological and Geochemical Characterisitics of Volcanic Rocks of the Yingcheng Formation in the Southern Songliao Basin: Constraints on the Early Cretaceous Evolution of the Songliao Basin     

CHEN Jingsheng  WANG Liwu  ZHANG Yanlong  GE Wenchun  YANG Hao  JI Zheng  ZHANG Lidong  TIAN Dexin 《《地质学报》英文版》2020,94(4):1276-1300

The discoveries of oil and gas reservoirs in the volcanic rocks of the Songliao Basin(SB) have attracted the attention of many researchers. However, the lack of studies on the genesis of the volcanic rocks has led to different opinions being presented for the genesis of the SB. In order to solve this problem, this study selected the volcanic rocks of the Yingcheng Formation in the Southern Songliao Basin(SSB) as the research object, and determined the genesis and tectonic setting of the volcanic rocks by using LA-ICP-MS zircon U-Pb dating and a geochemical analysis method(major elements, trace elements, and Hf isotopes). The volcanic rocks of the Yingcheng Formation are mainly composed of rhyolites with minor dacites and pyroclastic rocks. Our new zircon U-Pb dating results show that these volcanic rocks were erupted in the Early Cretaceous(113–118 Ma). The primary zircons from the rhyolites have εHf(t) values of +4.70 to +12.46 and twostage model age(TDM2) of 876–374 Ma. The geochemical data presented in this study allow these rhyolites to be divided into I-type rhyolites and A-type rhyolites, both of which were formed by the partial melting of the crust. They have SiO2 contents of 71.62 wt.%–75.76 wt.% and Al2 O3 contentsof 10.88 wt.% to 12.92 wt.%. The rhyolites have distinctively higher REE contents than those of ordinary granites, with obvious negative Eu anomalies. The light to heavy REE fractionation is not obvious, and the LaN/YbN(average value = 9.78) is less than 10. The A-type rhyolites depleted in Ba, Sr, P, and Ti, with relatively low Nb/Ta, indicating that the rocks belong A2 subtype granites formed in an extensional environment. The adakitic dacites are characterized by high Sr contents(624 to 1,082 ppm), low Y contents(10.6 to 12.6 ppm), high Sr/Y and Sr/Yb ratios, and low Mg# values(14.77 to 36.46), indicating that they belong to "C" type adakites. The adakitic dacite with high Sr and low Yb were likely generated by partial melting of the lower crust under high pressure conditions at least 40 km depth. The I-type rhyolites with low Sr and high Yb, and the A-type rhyolites with very low Sr and high Yb, were formed in the middle and upper crust under low pressure conditions, respectively. In addition, the formation depths of the former were approximately 30 km, whereas those of the latter were less than 30 km. The geochemical characteristics reveal that the volcanic rocks of Yingcheng Formation were formed in an extensional environment which was related to the retreat of subducted Paleo-Pacific Plate. At the late Early Cretaceous Period, the upwelling of the asthenosphere mantle and the lithosphere delamination caused by the retreat of the subducted Paleo-Pacific Plate, had resulted in lithosheric extension in the eastern part of China. Subsequently, a large area of volcanic rocks had formed. The SB has also been confirmed to be a product of the tectonic stress field in that region.  相似文献   

柴北缘阿木尼克山滩间山群火山岩地球化学特征及地质意义          下载免费PDF全文

江小强  肖渊甫  李建兵  史少飞  吴守智  李润根  彭建华 《地质找矿论丛》2020,35(1):73-84

柴达木盆地北缘阿木尼克山北坡滩间山群火山岩呈带状出露,广泛分布。本文通过对该套火山岩岩石组合、地球化学、锆石测年等研究,查明其岩性以流纹岩、英安质晶屑凝灰岩为主,间有碳酸盐岩、碎屑岩夹层。火山岩相包括溢流相、爆发沉积相。火山岩w(SiO2)=61.22%~83.88%,属中酸性岩范畴;里特曼指数(δ)均小于3.3,属于钙碱性岩,且A/CNK均小于或等于1.1。火山岩大离子亲石元素K、Rb、Ba及强不相容元素Th富集,高场强元素Ta、Nb亏损明显。中酸性火山岩稀土总量w(ΣREE)值介于75.58×10^-6~174.10×10^-6,w(LREE)/w(HREE)值介于7.09~13.31间,轻稀土明显富集,重稀土亏损,轻重稀土分馏程度中等到强烈;稀土分配曲线右倾,重稀土部分相对平坦。该区火山岩地球化学特征与赛什腾山地区吻合度很高,说明具有相似的大地构造属性,判断为岛弧环境。英安质晶屑凝灰岩和流纹岩两件样品206Pb/238U年龄分别是464.5Ma±8.5Ma、510Ma±15Ma,分别对应早-中奥陶世、早-中寒武世;通过综合研判认为柴达木盆地北缘地区早古生代秦祁昆多岛洋主域裂解离散阶段可能在早-中寒武世已经开始,该区滩间山群火山岩为洋壳俯冲的产物,发生的大规模的洋-陆俯冲发生在奥陶纪。  相似文献   

The importance of grain size and shape in controlling the dispersion of the Vedde cryptotephra     

Jennifer Saxby  Alison Rust  Katharine Cashman  Frances Beckett 《第四纪科学杂志》2020,35(1-2):175-185

Volcanic ash is dispersed in the atmosphere according to meteorology and particle properties, including size and shape. However, the multiple definitions of size and shape for non-spherical particles affect our ability to use physical particle properties to understand tephra transport. Moreover, although particles are often excluded from operational ash dispersion model setups, ash in tephra deposits 1000 km from source can exceed . Here we measure the shape and size of samples of Vedde ash from Iceland, an exceptionally widespread tephra layer in Europe, collected in Iceland and Norway. Using X-ray computed tomography and optical microscopy, we show that distal ash is more anisotropic than proximate ash, suggesting that shape exerts an important control on tephra dispersion. Shape also impacts particle size measurements. Particle long axis, a parameter often reported by tephrochronologists, is on average greater than geometric size, used by dispersion modellers. By using geometric size and quantifying shape, we can explain the transport of Vedde ash particles more than 1200 km from source. We define a set of best practices for measuring the size and shape of cryptotephra shards and discuss the benefits and limitations of using physical particle properties to understand cryptotephra transport.  相似文献   

Lahar-Triggering Mechanisms and Hazard at Ruapehu Volcano,New Zealand   总被引：1，自引：0，他引：1  

Lecointre  Jerome  Hodgson  Katy  Neall  Vincent  Cronin  Shane 《Natural Hazards》2004,31(1):85-109

Late Holocene volcanic activity at Ruapehu has been characterizedby the generation of small (<10⁵ m³) to very large (>10⁷ m³) lahars and repeated,small to medium (VEI 1-3) tephra-producing eruptions. The Onetapu Formation groupsall lahar deposits that accumulated during the last 2,000 years on the southeastern Ruapehu ring plain. The andesitic tephras are grouped within the Tufa Trig Formation and are intercalated within the laharic sequence. By correlating these two formations with new radiocarbon ages obtained on interbedded paleosols, we reconstruct a detailed volcanic history of Ruapehu for this period.Clast assemblages identified in the laharic sequences record thelithologies of synchronous tephras and rocks within the source region. These assemblages suggest a strong genetic link between the development of Crater Lake, the variation in eruptivestyles, and the production of lahars.Lahar-triggering mechanisms include: (1) flank collapse ofhydrothermally altered and unstable portions of the cone; (2) phreatic and phreatomagmatic eruptions favoring the generation of snow-rich slurries and hyperconcentrated stream flows; (3) suddenCrater Lake rim collapse, releasing large amounts of water inducing debris flows; and (4) eruptions that generate large volumes of tephra on snow-covered slopes, later remobilized by heavy rain.Two major lahars in the Onetapu sequence had a volume 4 × 10⁷ m³, roughly 1 to 2 orders of magnitude larger than the 1953event leading to the Tangiwai disaster (151 casualties). One of these lahars crossed over a lowinterfluve currently separating the Whangaehu River from a stream feeding the Tongariro River,sometime since peat accumulated between AD 1400 and AD 1660. A repetition of such a large-scaleevent would have devastating consequences on the infrastructure, economy and environment withinthe distal areas of the two catchments. The 1995–1996 eruptions were a timely reminder ofthe hazards posed by the volcano.  相似文献   

The impacts of pasture- and manure-spreading on microbial groundwater quality in carbonate aquifers   总被引：2，自引：0，他引：2  

Celico  F.  Musilli  I.  Naclerio  G. 《Environmental Geology》2004,46(2):233-236

Hydrogeological and microbiological research is in progress to analyze the interaction between groundwater and microbial pollutants, produced by pasture and/or manure spreading, in the areas of different carbonate aquifers of southern Italy. Several springs and wells were studied, and the precipitation, the discharge, the groundwater level and the classic microbial indicators of pollution were monitored weekly or daily. The experimental results show that the pasture and the manure spreading produced microbial contamination of the groundwater, even if runoff infiltration in swallow holes does not exist. The time dependence of microbial contamination shows a series of peaks irregularly distributed, related to the precipitation that produce effective infiltration.  相似文献