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101.
102.
Highly heterogeneous Precambrian basement under the central Deccan Traps, India: Direct evidence from xenoliths in dykes 总被引:1,自引:0,他引:1
Ranjini Ray Anil D. Shukla Hetu C. Sheth Jyotiranjan S. Ray Raymond A. Duraiswami Loyc Vanderkluysen Chandramohan S. Rautela Jyotirmoy Mallik 《Gondwana Research》2008,13(3):375-385
Crustal or mantle xenoliths are not common in evolved, tholeiitic flood basalts that cover huge areas of the Precambrian shields. Yet, the occasional occurrences provide the most direct and unequivocal evidence on basement composition. Few xenolith occurrences are known from the Deccan Traps, India, and inferences about the Deccan basement have necessarily depended on geophysical studies and geochemistry of Deccan lavas and intrusions. Here, we report two basalt dykes (Rajmane and Talwade dykes) from the central Deccan Traps that are extremely rich in crustal xenoliths of great lithological variety (gneisses, quartzites, granite mylonite, felsic granulite, carbonate rock, tuff). Because the dykes are parallel and only 4 km apart, and only a few kilometres long, the xenoliths provide clear evidence for high small-scale lithological heterogeneity and strong tectonic deformation in the Precambrian Indian crust beneath. Measured 87Sr/86Sr ratios in the xenoliths range from 0.70935 (carbonate) to 0.78479 (granite mylonite). The Rajmane dyke sampled away from any of the xenoliths shows a present-day 87Sr/86Sr ratio of 0.70465 and initial (at 66 Ma) ratio of 0.70445. The dyke is subalkalic and fairly evolved (Mg No. = 44.1) and broadly similar in its Sr-isotopic and elemental composition to some of the lavas of the Mahabaleshwar Formation. The xenoliths are comparable lithologically and geochemically to basement rocks from the Archaean Dharwar craton forming much of southern India. As several lines of evidence suggest, the Dharwar craton may extend at least 350–400 km north under the Deccan lava cover. This is significant for Precambrian crustal evolution of India besides continental reconstructions. 相似文献
103.
This paper assesses the key contributions made by Piers Blaikie to the field of political ecology. Those contributions relate to the way in which this scholar has sought to dismantle barriers to thought by: (1) integrating the insights of political economy with those from environmental science, (2) opening up theoretical space in political ecology by engaging fruitfully with post-structural critiques, (3) moving beyond a narrow area-studies and development studies focus, and (4) helping to internationalise the research field beyond its core American base. The paper also highlights ambiguity in the role played by Blaikie in political ecology, as stances that he has taken in his work have provoked wider debate over the field’s purpose and coherence. Ensuing tensions over theory and practice as well as single versus multiple truths persist yet, pace Blaikie and others (e.g. Robbins), a robust political ecology is nonetheless able to consider what people ‘do’ from a healthy diversity of theoretical positions. The multiple contributions of Piers Blaikie underpin a reputation for having produced a pioneering body of work that has inspired scholars across theoretical, empirical and disciplinary boundaries, thereby ensuring that his will be a reputation that is interpreted and re-interpreted for many years to come. 相似文献
104.
Kevin J. Smart William M. Dunne Raymond D. Krieg 《Journal of Structural Geology》1997,19(12):1443-1459
This paper focuses on the behavior of a roof sequence in the Appalachian Plateau of West Virginia, U.S.A., and emplacement of the Wills Mountain duplex with 17.5 km of displacement. Unlike the Plateau along strike in Pennsylvania and New York where forethrusting was previously documented, this roof sequence lacks an underlying salt-dominated roof décollement. Kinematic analyses reveal that the roof sequence in the West Virginian Plateau accommodated about two-thirds of the 17.5 km of shortening by the adjacent Wills Mountain duplex, as a forethrusting kinematic response. The remaining shortening imbalance of about 5 km between the duplexes and younger roof sequence rocks is accommodated by additional forethrusting further into the foreland and local compensation. This kinematic response matches that along strike in the central Appalachians despite the loss of the salt décollement. We interpret that an Ordovician shale-dominated formation was sufficiently weak to substitute for the salt horizon. Thus, a weak mechanical unit rather than specifically a salt décollement is a necessary prerequisite for forethrusting. A contributing factor to forethrusting may be the subvertical front of the Wills Mountain duplex, which inhibited other responses by the roof sequence. Mesoscale and smaller processes, including grain-to-grain pressure solution, twinning and cleavage formation account for over 75% of the shortening in the roof sequence, and, if ignored, would result in an erroneous interpretation of backthrusting or local compensation. This result suggests that failure to consider all deformation scales could lead to incorrect kinematic conclusions in other tectonic systems. 相似文献
105.
The issues associated with coupled transient heat and mass transfer in unsaturated clay barriers surrounding a buried heat source are considered, and the effects of driving forces in both the liquid and vapour phases are discussed. The reasons for the inability of the model of Philip and de Vries (1957) to accurately predict the redistribution of moisture in unsaturated clay barriers are explained. The model of Yong and Xu (1988) is used to predict the moisture redistribution of non-swelling unsaturated clay barriers. The large variations of the moisture values predicted by the two models are also discussed. 相似文献
106.
Raymond R. Jannas Richard E. Beane Bruce A. Ahler David R. Brosnahan 《Journal of Geochemical Exploration》1990,36(1-3)
A Middle Tertiary volcanic belt in the High Andes of north-central Chile hosts numerous precious- and base-metal epithermal deposits over its 150 km north-south trend. The El Indio district, believed to be associated with a hydrothermal system in the late stages of development of a volcanic caldera, consists of a series of separate vein systems located in an area of 30 km2 which has undergone intense argillic-sericitic-solfataric alteration. The majority of the known gold-copper-silver mineralization occurs within a structural block only 150 by 500 m in surface area, with a recognized vertical extent exceeding 300 m. This block is bounded by two high-angle northeast-trending faults oriented subparallel to the mineralized veins.Hypogene mineralization at El Indio is grouped into two main ore-forming stages: Copper and Gold. The Copper stage is composed chiefly of enargite and pyrite forming massive veins up to 20 m wide, and is accompanied by alteration of the wall rocks to alunite, kaolinite, sericite, pyrite and quartz. The Gold stage consists of vein-filling quartz, pyrite, native gold, tennantite and subordinate amounts of a wide variety of telluride minerals. Associated with this stage is pervasive alteration of the wall rocks to sericite, kaolinite, quartz and minor pyrophyllite. The transition from copper to gold mineralization is marked by the alteration of enargite to tennantite and by minor deposition of sphalerite, galena, huebnerite, chalcopyrite and gold. Mineral stability relations indicate that there was a general decrease in the activity of S2 accompanied by variations in the activity of Te2 during the Gold stage.Fluid-inclusion data show homogenization temperatures ranging from about 220 to 280°C, with salinities on the order of 3–4 eq. wt. % NaCl for the Copper stage. The Gold-stage inclusions indicate a similar range in homogenization temperatures, but significantly lower salinities (0.1–1.4 eq. wt. % NaCl). Fluid inclusions of transition minerals show a weak inverse relationship between homogenization temperatures (190–250°C) and salinities (3.4–1.4 eq. wt. % NaCl), which may represent mixing of hotter Gold-stage fluids with cooler late-Copper-stage fluids. No evidence of boiling was found in fluid inclusions, but CO2 vapor-rich inclusions were identified in wall-rock quartz phenocrysts which pre-date copper and gold mineralization.Mineral stability calculations indicate that given a fairly restricted range of solution compositions, the Copper-, Transition- and Gold-stage minerals at El Indio could have been deposited from a single solution, with constant total dissolved sulfur which underwent reduction through time. Limited sulfur-isotope data indicates that pyrite from the Copper stage was not in isotopic equilibrium with Copper-stage alunite or Transition-stage sphalerite. The sulfur-isotope and fluid-inclusion data indicate that two fluids with comparable temperatures but different compositions flowed through the El Indio system. The earlier fluid deposited copper attended by sericite-alunite-kaolinite alteration, and later epithermal fluids deposited gold with quartz-sericite-kaolinite-pyrite alteration. 相似文献
107.
108.
Christopher Uche Ezeh Michael Chukwuma Obeta Raymond N. C. Anyadike 《Arabian Journal of Geosciences》2016,9(16):681
The study compared the sequences of daily rainfall over coastal southern and semi-arid northern Nigeria. Daily rainfall occurrences for 41 years (1971–2011) over four meteorological stations in Lagos, Rivers, Borno, and Katsina were analyzed using frequency analysis and Markov chain model. Findings indicate that the coastal area had a predominance of 2–4-day wet spells while the semi-arid area showed a wet spell distribution that is geometric in nature with 1-day spell predominance. The dry spell behavior was nearly the opposite of the wet spell occurrence. The coastal region showed a dry spell of 1–4-day spell predominance while the semi-arid region showed a predominance of higher dry spells of 2–6 days. Accumulation of the amount of rainfall in each spell also showed that much rainfall from the coastal area was obtained from rains of spells of 3 days and above while the semi-arid had more of its rain from spells of 1–3 days. The mean annual rainfall was 1423.75 mm (Lagos), 2173.56 mm (Rivers), 517.50 mm (Katsina), and 578.34 mm (Borno). The wettest month was June (274.08 mm) in Lagos, September (378.18 mm) in Rivers, and August in Katsina and Borno (172.98 and 184.81 mm, respectively). The driest months were January for Lagos and Rivers (15.77 and 18.96 mm, respectively) and November–February for Katsina and Borno (0–0.06 mm). This showed that the coastal areas had nearly three times the volume of rain in the semi-arid area. The study further showed that onset of rain for the coastal area was March/April while the cessation of rainy season was October/November. On the other hand, the onset of rainy season in the semi-arid area was May/June and cessation of rainy season was September. Findings portend drier days for the semi-arid area due to dry spell persistence and hence, the consequent challenges of providing artificial water supply for agriculture and other purposes especially from October to May. 相似文献
109.
Concerns about police militarization have become an important public policy issue since the aggressive police response to the 2014 protests in Ferguson, Missouri, where police officers used military-style equipment to confront protestors. This event was a stark visual reminder that many U.S. police departments have used federal programs to acquire surplus military equipment, including weapons, armored vehicles, and body armor. We explore the geographies and histories of one the most important programs, called 1033, which supplies police with military equipment under the rationale of prosecuting the War on Drugs. We show that the legal blurring of the police and the military has been ongoing for decades at the national scale but this has resulted in an uneven landscape of police militarization at the county scale. We also investigate one of the most common global arguments for why police become militarized, which is the presence of Special Weapons and Tactics-style paramilitary teams, finding little support for that claim. More geographic inquiry is needed to understand the trajectories, causes, and consequences of police militarization. 相似文献
110.
Dr. Raoul Vajk 《Pure and Applied Geophysics》1951,19(3-4):129-143
Summary Methods for the elimination of the regional effect from gravity data are discussed in this paper. A historical review of the average gradient method used in torsion balance data is given. The connection between the averaging, or grid method, used on the gravity meter data and the average gradient method is pointed out and a criticism of these methods is given. Due to the ambiguity of the interpretation of gravity data no unique determination of the regional effect is possible and no mechanical method for its removal can be found. The determination of a regional gravity effect is an interpretation problem and for this reason it is unavoidably the subject of geological consideration and of the personal judgement of the interpreter. The method of smoothing the isogams and gravity curves is also discussed. A few suggestions are made for breaking the gravity data into two components (regional and residual) by the analysis of the gravity data. Finally the application of the second derivative methods for the elimination of the regional effect is discussed. 相似文献