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41.
By measuring the ratio of wavelength to arclength of folds in closely associated disharmonically folded competent layers,
it is possible to determine which layer has undergone a greater layer-parallel strain and has a smaller competence. This method
may lead to a paradoxical situation. For example when foliated quartzite and mica schist layers are folded together, the mica
schist laminae show a much larger buckle shortening than the quartzite layers. On the other hand, the geometry of folds in
quartzite indicates that quartzite was more competent than the mica schist. The structure can be explained by different modes
of buckling, general buckling in quartzite layers and internal buckling in strongly anisotropic mica schists. 相似文献
42.
Maitrayee Chakraborty Sayan Biswas Pulak Sengupta 《International Geology Review》2015,57(11-12):1619-1632
The Arcuate Singhbhum Shear Zone (SSZ) forms an integral part and occurs at the southern fringe of the Palaeoproterozoic North Singhbhum Fold Belt (NSFB) of the East Indian Shield. Repeated folding, ductile shearing, and accompanying hydrothermal activities in the SSZ during the late Palaeoproterozoic (ca. 1.66–1.60 Ga) orogeny resulted in a highly tectonized ensemble of rocks including a suite of peraluminous kyanite-rich quartzite (KQR). Near Kanyaluka village, the KQR shows millimetre- to decimetre-thick alternation of kyanite- and quartz-rich bands. The banded rock is intensely sheared and is cross-cut by weakly deformed to undeformed kyanite-quartz veins. In many places, kyanite-rich bands show sea-green coloured pods rich in lazulite. Textural studies reveal that deformed kyanite and quartz grains are sequentially replaced by augelite and lazulite (XMg > 0.97) at the terminal phase of shearing in the SSZ. Modelling of observed textures and mineral compositions with the C-Space program shows the following augelite- and lazulite-forming reactions:
1.829Kyanite + 0.998P + 1.5H2O + 0.001Ca = 1Augelite + 1.666Al + 0.001 Mg + 1.822SiO2 + 0.002Fe + 0.0002Na
1.778Kyanite + 0.667Augelite + 1.294P + 1.011 Mg + 0.011Fe + 0.0001Na = 1Lazulite + 2.833Al + 1.78SiO2 + 0.001 Ca
43.
Prerona Das Abhijit Mukherjee Syed Aaquib Hussain Md. Shahid Jamal Kousik Das Ashok Shaw Mrinal K. Layek Probal Sengupta 《水文研究》2021,35(1):e14002
Groundwater depletion has been an emerging crisis in recent years, especially in highly urbanized areas as a result of unregulated exploitation, thus leaving behind an insufficient volume of usable freshwater. Presently Ganges river basin, the sixth largest prolific fluvial system and sustaining a huge population in South Asia, is witnessed to face (i) aquifer vulnerability through surface waterborne pollutant and (ii) groundwater stress due to summer drying of river as a result of indiscriminate groundwater abstraction. The present study focuses on a detailed sub-hourly to seasonally varying interaction study and flux quantification between river Ganges and groundwater in the Indian subcontinent which is one of the first documentations done on a drying perennial river system that feeds an enormous population. Contributing parameters to the total discharge of a river at its middle course on both temporal and spatial scale is estimated through three-component hydrograph separation and end-member mixing analysis using high-resolution water isotope (δ18O and δ2H) and electrical conductivity data. Results from this model report groundwater discharge in river to be the highest in pre-monsoon, that is, 30%, whereas, during post-monsoon the contribution lowers to 25%; on the contrary, during peak monsoon, the flow direction reverses thus recharging the groundwater which is also justified using annual piezometric hydrographs of both river water and groundwater. River water-groundwater interaction also shows quantitative variability depending on river morphometry. The current study also provides insight on aquifer vulnerability as a result of pollutant mixing through interaction and plausible attempts towards groundwater management. The present study is one of the first in South Asian countries that provides temporally and spatially variable detailed quantification of baseflow and estimates contributing parameters to the river for a drying mega fluvial system. 相似文献
44.
Dipak Ghosh Argha Deb Swarup Ranjan Sahoo Subrata Haldar Rosalima Sengupta 《Natural Hazards》2011,58(3):877-889
This paper reports the measurement of radon concentration in well water at the site of Jalpaiguri (26°32′N, 88°46′E) near
the active fault zone of West Bengal, India. Radon concentration has been measured in well water with the help of solid-state
nuclear track detectors (SSNTD). The study indicates a positive correlation between radon anomaly and earthquake. The data
of radon content in well water have been compared with that in soil gas at the same site. 相似文献
45.
The Samchampi-Samteran alkaline igneous complex (SAC) is a near circular, plug-like body approximately 12 km2 area and is emplaced into the Precambrian gneissic terrain of the Karbi Anglong district of Assam. The host rocks, which
are exposed in immediate vicinity of the intrusion, comprise granite gneiss, migmatite, granodiorite, amphibolite, pegmatite
and quartz veins.
The SAC is composed of a wide variety of lithologies identified as syenitic fenite, magnetite ± perovskite ± apatite rock,
alkali pyroxenite, ijolite-melteigite, carbonatite, nepheline syenite with leucocratic and mesocratic variants, phonolite,
volcanic tuff, phosphatic rock and chert breccia.
The magnetite ± perovskite ± apatite rock was generated as a cumulus phase owing to the partitioning of Ti, Fe at a shallow
level magma chamber (not evolved DI = O1). The highly alkaline hydrous fluid activity indicated by the presence of strongly
alkalic minerals in carbonatites and associated alkaline rocks suggests that the composition of original melt was more alkalic
than those now found and represent a silica undersaturated ultramafic rock of carbonated olivine-poor nephelinite which splits
with falling temperature into two immiscible fractions—one ultimately crystallises as alkali pyroxenite/ijolite and the other
as carbonatite. The spatial distribution of varied lithotypes of SAC and their genetic relationships suggests that the silicate
and carbonate melts, produced through liquid immiscibility, during ascent generated into an array of lithotypes and also reaction
with the country rocks by alkali emanations produced fenitic aureoles (nephelinisation process). Isotopic studies (δ18O and δ13C) on carbonatites of Samchampi have indicated that the δ13C of the source magma is related to contamination from recycled carbon. 相似文献
46.
Ph. Négrel B. Lemière H. Machard de Grammont P. Billaud B. Sengupta 《Hydrogeology Journal》2007,15(8):1535-1552
Geochemical observations, including major ion and trace element analysis, and isotopic tracing have been carried out in the Subarnarekha River system (northeastern India) during a surface-water- and groundwater-monitoring program aimed at evaluating impacts of mining. The aquifer is of fracture type. Groundwater flow conditions and pollutant transfer were observed through a network of 69 wells. δ18O and δ2H results suggest that transfer from rainfall towards groundwater storage through soils and the unsaturated zone is fast, without any major transformation like evaporation. The scatter of 87Sr/86Sr signatures in surface water and groundwater are explained by three end-members. One is compatible with rainwater inputs. The most mineralised end-member represents anthropogenic inputs (agricultural practices and ore processing). The third end-member, characterised by a high 87Sr/86Sr signature, is believed to be controlled by natural geochemical processes, although affected by human activities (e.g. drainage of mine waste). Potential flow paths, investigated north of the area, reveal that all groundwater types seem to evolve more in pockets than along a flow path. The limited extent of transfer and the predominance of natural phenomena help to explain the moderate level of groundwater contamination and the characteristics of surface water contamination by mining and the metallurgy industry. 相似文献
47.
In the Schirmacher Hills, most of the ductile shearing took place under high to medium grade amphibolite facies metamorphism. The microstructure of the mylonites shows characteristic features of high temperature deformation and thus gives us an idea of deformation mechanisms of the constituent minerals at great crustal depth. The variation in microstructure of the sheared rock is partly due to heterogeneity of the intensity of strain from domain to domain, producing protomylonites, orthomylonites and ultramylonites. However, a large part of the microstructural variation has resulted from syn- to post-tectonic recrystallization and grain growth of constituent minerals. Both quartz and feldspar have deformed by crystal plastic processes with dominant grain boundary migration. The present aspect ratio of the feldspar grains is a result of various degrees of dynamic recrystallization along the grain boundary. The ratio varies between 1.5 and 2. Presence of exsolution lamellae in perthites and formation of myrmekite at the strained grains of K-feldspar suggest diffusion assisted dislocation creep. These mylonites are characterized by the presence of weakly strained or unstrained long quartz ribbons. The development of quartz ribbons with the absence of significant strain suggests grain recovery and grain growth during high temperature mylonitization. The growth of quartz ribbons took place by coalescing neighbouring grains both along and across the ribbon length. At the ultramylonite stage the fine-grained matrix of quartz and feldspar mostly accommodates the bulk strain. 相似文献
48.
S. Sengupta 《Journal of the Geological Society of India》2009,74(5):597-600
Megalospheric form of a striate Nummulites, provisionally identified here as Nummulites sp. aff. chavannesi de la Harpe, is documented from the Early Oligocene rocks of SW Kutch, Gujarat. This striate Nummulites occurs in association with N. fichteli-clipeus Group, N. cf. fichteli, Heterostegina, Operculina and Gypsina. High abundance of microspheric tests of reticulate Nummulites and virtual absence of microspheric tests of striate Nummulites reflect contrasting success of growth and sexual reproduction of the two groups of sympatric Nummulites. 相似文献
49.
Dr. Sudipta Sengupta 《International Journal of Earth Sciences》1977,66(1):175-192
The Singhbhum Shear Zone separates the rocks of a highly metamorphosed northern group from an unmetamorphosed southern group. It had been recognised by earlier workers as a thrust zone in which the thrust movements were achieved by slip along a pre-existing schistosity. Deformed conglomerates crop out in discontinuous bands along the Shear Zone. Strain determinations from these conglomerates have been made to ascertain the strain pattern within the Shear Zone. None of the existing methods of determining strain from stretched pebbles can be applied to these conglomerates. An approximate value for shortening perpendicular to the schistosity can however be obtained from cross-cutting buckled veins. Detailed study of various small-scale structures clearly indicates that the principal structures of this region cannot be explained simply by a slipping on the schistosity.
Zusammenfassung Die Singhbhum-Scherzone trennt die Gesteine der stark metamorphen nördlichen Gruppe von denjenigen einer nicht metamorphen südlichen Gruppe. Nach früheren Autoren ist es eine Überschiebungszone, worin die Überschiebungsbewegungen durch das Gleiten entlang einer älteren Schieferung zustande kamen. Deformierte Trümmergesteine treten in ununterbrochenen Bändern entlang der Scherzone auf. Man hat Deformationsbestimmungen von diesen Trümmergesteinen gemacht, um das Deformationsbild in der Scherzone zu erklären. Eine ausführliche Untersuchung der verschiedenen kleinmaßstäblichen Strukturen zeigt deutlich, daß die Hauptstrukturen dieser Gegend nicht einfach durch das Gleiten auf der Schieferung erklärt werden können.
Résumé La zone de cisaillement de Singhbhum sépare les roches très métamorphiques au nord, de celles d'un groupe non-métamorphique au sud. Elle avait été reconnue lors de recherches précédentes comme étant la base de charriages suivant laquelle la poussée se serait faite par glissement le long d'une schistosité pré-existante. Des conglomérats déformés affleurent par bandes discontinues le long de cette zone. Grâce à ces conglomérats, des estimations sur la déformation ont été faites pour définir le style de déformation dans la zone de glissement. Une étude détaillée de différentes structures à petite échelle indique clairement qu'on ne peut pas expliquer les principales structures de cette région uniquement par un glissement sur la schistosité.
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50.
Abani R Samal Raja R Sengupta Richard H Fifarek 《Journal of Earth System Science》2011,120(4):583-593
Linear trends of anomalously high gold values in the Florida Canyon gold deposit, Nevada have been identified using a combination
of contour maps of gold (Au) concentration developed with a geographic information system (GIS) and variogram maps created
using a geostatistical analysis package. These linear trends are interpreted to represent major fault zones that exerted a
prinicipal control on gold mineralization and therefore imparted a spatial anisotropy to gold concentrations. 相似文献