Effect of Shear Deformation on Microfabric of Clay Using XRD Technique     

Ajanta Sachan  Dayakar Penumadu 《Geotechnical and Geological Engineering》2009,27(2):249-264

The present paper deals with the impact of shear deformation on the geometric arrangement of particles within the soil specimen, which is termed as the microfabric of soil. A series of compression and extension lubricated end triaxial tests are performed on cylindrical specimens of Kaolinite clay with two extreme microfabrics; dispersed and flocculated, which are obtained using slurry consolidation technique. Flocculated microfabric has random orientation of particles within the soil mass having face-to-edge particle contacts; however, dispersed microfabric has parallel orientation of particles containing face-to-face particle contacts. When the specimen is subjected to large stress levels during its shear deformation, the particle orientation and the geometric arrangement within the soil specimen gets affected due to the force acting on the clay platelets. The variation in microfabric of soil before and after shear deformation process is evaluated by obtaining X-ray diffraction patterns of the clay specimen at three different locations using standard X-ray diffractometer. The discussion includes an analysis of the orientation of soil particles located at shear banding zones of the clay specimens, which may be useful for understanding the strain localization development in clays.  相似文献   

Petrography,classification, oxygen isotopes,noble gases,and cosmogenic records of Kamargaon (L6) meteorite: The latest fall in India          下载免费PDF全文

D. Ray  R. R. Mahajan  A. D. Shukla  T. K. Goswami  S. Chakraborty 《Meteoritics & planetary science》2017,52(8):1744-1753

A single piece of meteorite fell on Kamargaon village in the state of Assam in India on November 13, 2015. Based on mineralogical, chemical, and oxygen isotope data, Kamargaon is classified as an L‐chondrite. Homogeneous olivine (Fa: 25 ± 0.7) and low‐Ca pyroxene (Fs: 21 ± 0.4) compositions with percent mean deviation of <2, further suggest that Kamargaon is a coarsely equilibrated, petrologic type 6 chondrite. Kamargaon is thermally metamorphosed with an estimated peak metamorphic temperature of ~800 °C as determined by two‐pyroxene thermometry. Shock metamorphism studies suggest that this meteorite include portions of different shock stages, e.g., S3 and S4 (Stöffler et al. 1991 ); however, local presence of quenched metal‐sulfide melt within shock veins/pockets suggest disequilibrium melting and relatively higher shock stage of up to S5 (Bennett and McSween 1996 ). Based on noble gas isotopes, the cosmic‐ray exposure age is estimated as 7.03 ± 1.60 Ma and nitrogen isotope composition (δ¹⁵N = 18‰) also correspond well with the L‐chondrite group. The He‐U, Th, and K‐Ar yield younger ages (170 ± 25 Ma 684 ± 93, respectively) and are discordant. A loss of He during the resetting event is implied by the lower He‐U and Th age. Elemental ratios of trapped Ar, Kr, and Xe can be explained through the presence of a normal Q noble gas component. Relatively low activity of ²⁶Al (39 dpm/kg) and the absence of ⁶⁰Co activity suggest a likely low shielding depth and envisage a small preatmospheric size of the meteoroid (<10 cm in radius). The Kr isotopic ratios (⁸²Kr/⁸⁴Kr) further argue that the meteorite was derived from a shallow depth.  相似文献   

Urban air pollution: process identification, impact analysis and evaluation of forecast potential     

P. Goswami  J. Baruah 《Meteorology and Atmospheric Physics》2011,110(3-4):103-122

The daily variations in the atmospheric pollutants like suspended particulate matter (SPM), respirable suspended particulate matter (RSPM), sulphur dioxide (SO₂) and nitrogen dioxide (NO₂) depend on both local meteorological processes and various natural as well as anthropogenic sources and sinks. It was shown in an earlier work (Goswami and Baruah in Mon Wea Rev 136(9):3597?C3607, 2008) that the daily variations in SPM over a location could be simulated quite well by considering daily meteorological fields from NCEP Reanalysis in combination with a model for natural and anthropogenic sources over Delhi. In the present work, we extend the scope of the model to include three other pollutants: RSPM, SO₂ and NO₂. While the basic conservation equations and the meteorological fields are common to all the three (and SPM) pollutants, the sources and sinks for each is modeled in a species-specific manner to obtain maximum skill. As we do not consider active chemistry, the present model provides the minimal dynamics of pollution over an urban location in terms of annual load; the model error is about 10% on the average, with no significant bias for any of the species.  相似文献   

http://www.sciencedirect.com/science/article/pii/S1674987113001345     

B. Goswami  C. Bhattacharyya 《地学前缘(英文版)》2014,5(6):821-843

Many elongated, lenticular plutons of porphyritic granitoids are distributed mainly near the southern and northern margin of the Chhotanagpur Gneissic Complex （CGC） which belongs to the EW to ENE-WSW tending 1500 km long Proterozoic orogenic belt amalgamat ng the North and South Indian cratonic blocks. The late Grenvillian （1071 ±64 Ma） Raghunathpur porphyritic granitoid gneiss （PGG） batholith comprising alkali feldspar granite, granite, granodiorite, tonalite, quartz syenite and quartz monzonite intruded into the granitoid gneisses of southeastern part of CGC in the Purulia district, West Bengal and is aligned with ENE-WSW trending North Purulia sr~ear zone, Mineral chemistry, geochemistry, physical condition of crystallization and petrogenetic model of Raghunathpur PGG have been discussed for the first time. The petrographic and geochemical features （including major and trace- elements, mineral chemistry and S7Sr/S6Sr ratio） suggest these granitoids to be classified as the shosh- onitic type. Raghunathpur batholith was emplaced at around 800 ~C and at 6 kbar pressure tectonic discrimination diagrams reveal a post-collision tectonic setting while structural studies reveal its emplacement in the extensional fissure of North Purulia shear zone. l＇he Raghunathpur granitoid is compared with some similar granitoids of Europe and China to draw its petrogenetic model. Hybridi- zation of mantle-generated enriched mafic magma and crustal magma at lower crust and later fractional crystallization is proposed for the petrogenesis of this PGG. Mafic magma generated in a post-collisional extension possibly because of delamination of subducting slab. Raghunathpur batholith had emplaced in the CGC during the final amalgamation （~ 1.0 Ga） of the North and South Indian cratonic blocks. Granitoid magma, after its generation at depth, was transported to its present level along megadyke channel, ways within shear zones.  相似文献   

CH stars at high Galactic latitudes     

Aruna Goswami 《Monthly notices of the Royal Astronomical Society》2005,359(2):531-544

High-velocity outflows from supermassive black holes have been invoked to explain the recent identification of strong absorption features in the hard X-ray spectra of several quasars. Here, Monte Carlo radiative transfer calculations are performed to synthesize X-ray spectra from models of such flows. It is found that simple, parametric biconical outflow models with plausible choices for the wind parameters predict spectra that are in good qualitative agreement with observations in the 2–10 keV band. The influence on the spectrum of both the mass-loss rate and opening angle of the flow are considered: the latter is important since photon leakage plays a significant role in establishing an ionization gradient within the flow, a useful discriminant between spherical and conical outflow for this and other applications. Particular attention is given to the bright quasar PG 1211+143 for which constraints on the outflow geometry and mass-loss rate are discussed subject to the limitations of the currently available observational data.  相似文献   

Application of Persistent Scatterer Interferometry (PSI) in monitoring slope movements in Nainital,Uttarakhand Lesser Himalaya,India     

Akano Yhokha  Pradeep K Goswami  Chung-Pai Chang  Jiun-Yee Yen  Kuo-En Ching  K Manini Aruche 《Journal of Earth System Science》2018,127(1):6

Orogenic movements and sub-tropical climate have rendered the slopes of the Himalayan region intensely deformed and weathered. As a result, the incidences of slope failure are quite common all along the Himalayan region. The Lesser Himalayan terrane is particularly vulnerable to mass-movements owing to geological fragility, and many parts of it are bearing a high-risk of associated disaster owing to the high population density. An important step towards mitigation of such disasters is the monitoring of slope movement. Towards this, the Persistent Scatterer Interferometry (PSI) technique can be applied. In the present study, the PSI technique is employed in Lesser Himalayan town of Nainital in Uttarakhand state of India to decipher and monitor slope movements. A total of 15 multi-date ENVISAT ASAR satellite images, acquired during August 2008 to August 2010 period, were subjected to PSI, which revealed a continuous creep movement along the hillslopes located towards the eastern side of the Nainital lake. The higher reaches of the hill seem to be experiencing accelerated creep of \({\sim }21\) mm/year, which decreases downslope to \({\sim }5\) mm/year. Based on spatial pattern of varying PSI Mean LOS Velocity (MLV) values, high (H), moderate (M), low (L) and very low (S) creeping zones have been delineated in the hillslopes. Given the long history of mass movements and continuously increasing anthropogenic activities in Nainital, these results call for immediate measures to avert any future disaster in the town.  相似文献   

An approach of understanding acid volcanics and tuffaceous volcaniclastics from field studies: A case from Tadpatri Formation,Proterozoic Cuddapah basin,Andhra Pradesh,India     

Sukanta Goswami  P K Upadhyay  Sangeeta Bhagat  Syed Zakaulla  A K Bhatt  V Natarajan  Sukanta Dey 《Journal of Earth System Science》2018,127(2):20

The lower stratigraphic part of the Cuddapah basin is marked by mafic and felsic volcanism. Tadpatri Formation consists of a greater variety of rock types due to bimodal volcanism in the upper part. Presence of bimodal volcanism is an indication of continental rift setting. Various genetic processes involved in the formation of such volcanic sequence result in original textures which are classified into volcaniclastic and coherent categories. Detailed and systematic field works in Tadpatri–Tonduru transect of SW Cuddapah basin have provided information on the physical processes producing this diversity of rock types. Felsic volcanism is manifested here with features as finger print of past rhyolite-dacite eruptions. Acid volcanics, tuffs and associated shale of Tadpatri Formation are studied and mapped in the field. With supporting subordinate studies on geochemistry, mineralogy and petrogenesis of the volcanics to validate field features accurately, it is understood that volcanism was associated with rifting and shallow marine environmental condition. Four facies (i.e., surge, flow, fall and resedimented volcaniclastic) are demarcated to describe stratigraphic units and volcanic history of the mapped area. The present contribution focuses on the fundamental characterization and categorization of field-based features diagnostic of silica-rich volcanic activities in the Tadpatri Formation.  相似文献   

The longest voyage: Tectonic,magmatic, and paleoclimatic evolution of the Indian plate during its northward flight from Gondwana to Asia     

Sankar Chatterjee  Arghya Goswami  Christopher R. Scotese 《Gondwana Research》2013,23(1):238-267

The tectonic evolution of the Indian plate, which started in Late Jurassic about 167 million years ago (~ 167 Ma) with the breakup of Gondwana, presents an exceptional and intricate case history against which a variety of plate tectonic events such as: continental breakup, sea-floor spreading, birth of new oceans, flood basalt volcanism, hotspot tracks, transform faults, subduction, obduction, continental collision, accretion, and mountain building can be investigated. Plate tectonic maps are presented here illustrating the repeated rifting of the Indian plate from surrounding Gondwana continents, its northward migration, and its collision first with the Kohistan–Ladakh Arc at the Indus Suture Zone, and then with Tibet at the Shyok–Tsangpo Suture. The associations between flood basalts and the recurrent separation of the Indian plate from Gondwana are assessed. The breakup of India from Gondwana and the opening of the Indian Ocean is thought to have been caused by plate tectonic forces (i.e., slab pull emanating from the subduction of the Tethyan ocean floor beneath Eurasia) which were localized along zones of weakness caused by mantle plumes (Bouvet, Marion, Kerguelen, and Reunion plumes). The sequential spreading of the Southwest Indian Ridge/Davie Ridge, Southeast Indian Ridge, Central Indian Ridge, Palitana Ridge, and Carlsberg Ridge in the Indian Ocean were responsible for the fragmentation of the Indian plate during the Late Jurassic and Cretaceous times. The Réunion and the Kerguelen plumes left two spectacular hotspot tracks on either side of the Indian plate. With the breakup of Gondwana, India remained isolated as an island continent, but reestablished its biotic links with Africa during the Late Cretaceous during its collision with the Kohistan–Ladakh Arc (~ 85 Ma) along the Indus Suture. Soon after the Deccan eruption, India drifted northward as an island continent by rapid motion carrying Gondwana biota, about 20 cm/year, between 67 Ma to 50 Ma; it slowed down dramatically to 5 cm/year during its collision with Asia in Early Eocene (~ 50 Ma). A northern corridor was established between India and Asia soon after the collision allowing faunal interchange. This is reflected by mixed Gondwana and Eurasian elements in the fossil record preserved in several continental Eocene formations of India. A revised India–Asia collision model suggests that the Indus Suture represents the obduction zone between India and the Kohistan–Ladakh Arc, whereas the Shyok-Suture represents the collision between the Kohistan–Ladakh arc and Tibet. Eventually, the Indus–Tsangpo Zone became the locus of the final India–Asia collision, which probably began in Early Eocene (~ 50 Ma) with the closure of Neotethys Ocean. The post-collisional tectonics for the last 50 million years is best expressed in the evolution of the Himalaya–Tibetan orogen. The great thickness of crust beneath Tibet and Himalaya and a series of north vergent thrust zones in the Himalaya and the south-vergent subduction zones in Tibetan Plateau suggest the progressive convergence between India and Asia of about 2500 km since the time of collision. In the early Eohimalayan phase (~ 50 to 25 Ma) of Himalayan orogeny (Middle Eocene–Late Oligocene), thick sediments on the leading edge of the Indian plate were squeezed, folded, and faulted to form the Tethyan Himalaya. With continuing convergence of India, the architecture of the Himalayan–Tibetan orogen is dominated by deformational structures developed in the Neogene Period during the Neohimalayan phase (~ 21 Ma to present), creating a series of north-vergent thrust belt systems such as the Main Central Thrust, the Main Boundary Thrust, and the Main Frontal Thrust to accommodate crustal shortening. Neogene molassic sediment shed from the rise of the Himalaya was deposited in a nearly continuous foreland trough in the Siwalik Group containing rich vertebrate assemblages. Tomographic imaging of the India–Asia orogen reveals that Indian lithospheric slab has been subducted subhorizontally beneath the entire Tibetan Plateau that has played a key role in the uplift of the Tibetan Plateau. The low-viscosity channel flow in response to topographic loading of Tibet provides a mechanism to explain the Himalayan–Tibetan orogen. From the start of its voyage in Southern Hemisphere, to its final impact with the Asia, the Indian plate has experienced changes in climatic conditions both short-term and long-term. We present a series of paleoclimatic maps illustrating the temperature and precipitation conditions based on estimates of Fast Ocean Atmospheric Model (FOAM), a coupled global climate model. The uplift of the Himalaya–Tibetan Plateau above the snow line created two most important global climate phenomena—the birth of the Asian monsoon and the onset of Pleistocene glaciation. As the mountains rose, and the monsoon rains intensified, increasing erosional sediments from the Himalaya were carried down by the Ganga River in the east and the Indus River in the west, and were deposited in two great deep-sea fans, the Bengal and the Indus. Vertebrate fossils provide additional resolution for the timing of three crucial tectonic events: India–KL Arc collision during the Late Cretaceous, India–Asia collision during the Early Eocene, and the rise of the Himalaya during the Early Miocene.  相似文献   

Chandrayaan-1 observation of distant secondary craters of Copernicus exhibiting central mound morphology: Evidence for low velocity clustered impacts on the Moon     

P. Senthil Kumar  A. Senthil Kumar  J.N. Goswami  A.S. Kiran Kumar 《Planetary and Space Science》2011,59(9):870-879

Analysis of the Chandrayaan-1 Terrain Mapping Camera image of a 20 km×27 km area in the Mare Imbrium region revealed a cluster of thousands of fresh and buried impact craters in the size range of 20-1300 m. A majority of the large fresh craters with diameter ranging from 160 to 1270 m exhibit near-circular mounds (30-335 m diameter and 10-40 m height) in the crater floor, and their size depends on the host crater size. The origin of this cluster of secondary craters may be traced to Copernicus crater, based on global lunar image and the analysis of Chandrayaan-1 Hyper Spectral Imager data. Our findings provide further evidence for secondary crater formation by low-velocity impact of a cloud of clustered fragments. The presence of central mounds can also distinguish the secondary craters from the primary craters and refine the chronology of lunar surface based on counting of small craters.  相似文献   

Geomorphology of the western Ionian Sea between Sicily and Calabria,Italy     

Rajasmita Goswami  Neil C. Mitchell  Andrea Argnani  Simon H. Brocklehurst 《Geo-Marine Letters》2014,34(5):419-433

In the westernmost Ionian Sea lies a steep, tectonically active marine basin influenced by turbidity currents generated by terrigenous river input from the adjacent mountains and strong tidal currents propagating through the Strait of Messina. Like many young marine rifts, the basin is lined by steep streams draining the uplifting coasts and supplying sediment across narrow shelves. However, unlike many rifts, this basin is semi-enclosed. The present study explores the seabed morphology and sediment structures in this complex environmental setting, based on multibeam sonar, chirp profiler and seismic reflection data collected in 2006. Offshore channels include many that can be directly linked to onshore streams, suggesting that hyperpycnal flows are important for their formation. Near the Strait of Messina in depths shallower than 400 m, the channels are subdued, plausibly explained as an effect of strong tidal currents. The Messina Channel is characterised by abundant mass-wasting features along its outer bends, particularly on the Calabrian side. Coincidence of the channel course with faults suggests that the channel is structurally controlled in places. The chirp profiles generally show only shallow penetration, the evidence for coarse texture being consistent with the steep gradient of the basin that inhibits deposition from turbidity currents. By contrast, some locally discontinuous mounds exhibiting layered sub-bottom reflectors in the chirp profiles are interpreted as modern levee deposits formed from channelised turbidity current overspill. Overall, this semi-enclosed basin shows little evidence of substantial accumulations associated with modern turbidity current activity, any contemporaneous sediment supply evidently bypassing the area to be deposited in the Ionian Trench; as a consequence, this trench should be an archive of local slope failure and flood events.  相似文献