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A. Banerjee Abhik Kumar Sanyal Subenoy Chakraborty 《Astrophysics and Space Science》1990,166(2):259-268
In this paper we study the exact solutions for a viscous fluid distribution in Bianchi II, VIII, and IX models. The metric is simplified by assuming a relationship between the coefficients of the metric tensor. Solutions are obtained in two special cases: in one an additional assumption is made where the matter density and the expansion scalar have a definite relation and in the other a barotropic equation of state of the formp= is assumed. While the Bianchi II solutions are already found in the literature the other two classes of solutions are apparently new. 相似文献
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Noether gauge symmetry for F(R) theory of gravity has been explored recently. The fallacy is that, even after setting gauge to vanish, the form of F(R)∝R n (where n≠1 is arbitrary) obtained in the process, has been claimed to be an outcome of gauge Noether symmetry. On the contrary, earlier works proved that any nonlinear form other than $F(R) \propto R^{\frac{3}{2}}$ is obscure. Here, we show that, setting gauge term zero, Noether equations are satisfied only for n=2, which again does not satisfy the field equations. Thus, as noticed earlier, the only form that Noether symmetry admits is $F(R) \propto R^{\frac{3}{2}}$ . Noether symmetry with non-zero gauge has also been studied explicitly here, to show that it does not produce anything new. 相似文献
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GEOFFREY STANO T. N. KRISHNAMURTI T. S. V. VIJAYA KUMAR ARUN CHAKRABORTY 《地球,A辑:动力气象学与海洋学》2002,54(4):370-381
This paper addresses the use of a satellite-based radar for obtaining the composite structure (from several monsoon depressions) of the distribution of precipitation elements in the horizontal and the vertical. This composting is based on the use of a simple elliptical layout of coordinates along the major and minor axes of each storm as it passed over north central India. This satellite, called the Tropical Rainfall Measuring Mission (TRMM), carries onboard a microwave instrument known as the Precipitation Radar (PR). The vertical structure of hydrometeors provided by the radar is somewhat of the same quality as the ground-based Doppler radar units. The PR could identify many features such as the melting layers, height of convection, extent of anvils and types of precipitation over different sections of the composited monsoon depression. Furthermore, the asymmetric nature of surface rainfall that intensifies around the composited monsoon depression has also been mapped, which could provide several more details than was possible from other satellite-based estimates. It is found that the most intense precipitation occurs in the south-southwest region of the monsoon depression. The preponderance of stratiform rain and the coverage of fewer deep convective elements, especially over the orographic upslope region, are some other noticeable features obtained using the TRMM PR. The stratiform rain was noted to arise where the melting layers (in the radar reflectivity signatures) were located near 5 km. In those few occasions where tall rain clouds were discernible, the cloud tops were seen to extend all the way from 12 to 15 km. Rainfall amounts across the composite monsoon depression range from 10 to 100 mm d−1 . The 3–4 d passage time of one of those disturbances resulted in local rainfall totals of the order of 200–300 min d−1 . 相似文献
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The metamorphic history of the Himalayas has been constrained mostly through studies of the ubiquitous metapelitic rocks. Non‐eclogitic metabasite rock lenses that occur intercalated with the metapelites have received little attention and it is not clear whether they share a common metamorphic history. This study reports the results of a petrological study of the metabasite lenses (dm3–m3) from the Lesser Himalayan (LH) and the Higher Himalayan (HH) domains in Sikkim. These have similar bulk chemical compositions and chemical affinities (sub‐alkaline tholeiitic basalts), with plagioclase and amphibole as the dominant mineralogical constituents. Garnet and clinopyroxene occur in some samples depending on small variations in bulk chemistry; and orthopyroxene is developed as a retrograde phase in some rocks. Minor phases are ilmenite, chlorite, titanite and rutile. The rocks were metamorphosed at similar conditions (~9–12 kbar, 800 °C). Minor differences in bulk chemical composition lead to different phase assemblages and mineral chemistry in adjacent metabasite lenses, a feature that is used to demonstrate that metamorphic conditions (peak P–T as well as retrograde P–T path) can be reliably retrieved through a combination of pseudosection analysis and kinetically constrained individual thermobarometry. The peak P–T conditions of the metabasites from this region are independent of the present geographic or tectonic (i.e. within the LH or HH) location of the samples and they differ from the conditions at which the regional metapelites (i.e. metapelites not immediately adjacent to the metabasite lenses) were metamorphosed. Metapelites that are immediately adjacent to the metabasite lenses differ in their appearance, phase assemblage and recorded P–T history from those of the regional metapelites, either because they were emplaced as slivers along with the metabasites, or because they were modified when they came in contact with the metabasites. The retrograde P–T paths of the LH and HH metabasites are different: the HH samples underwent steep decompression whereas the LH followed a more gentle exhumation path. The P–T conditions of peak metamorphism (9–12 kbar, 800 °C) are commensurate with a thermal perturbation at the base of a crust of average thickness and may be the signature of a widespread (samples found across different regions in the Himalaya) and long‐lasting (e.g. homogeneous garnet compositions) crustal underplating event that occurred during the early stages (?subduction) of the Himalayan orogeny, or earlier if the metamorphism was pre‐Himalayan. 相似文献
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Abhik Kundu Abdul Matin Malay Mukul Patrick G. Eriksson 《Journal of the Geological Society of India》2011,78(4):321-336
The Himalayan fold-and-thrust belt has propagated from its Tibetan hinterland to the southern foreland since ∼55 Ma. The Siwalik
sediments (∼20 - 2 Ma) were deposited in the frontal Himalayan foreland basin and subsequently became part of the thrust belt
since ∼ 12 Ma. Restoration of the deformed section of the Middle Siwalik sequence reveals that the sequence is ∼325 m thick.
Sedimentary facies analysis of the Middle Siwalik rocks points to the deposition of the Middle Siwalik sediments in an alluvial
fan setup that was affected by uplift and foreland-ward propagation of Greater and Lesser Himalayan thrusts. Soft-sediment
deformation structures preserved in the Middle Siwalik sequence in the Darjiling Himalaya are interpreted to have formed by
sediment liquefaction resulting from increased pore-water pressure probably due to strong seismic shaking. Soft-sediment structures
such as convolute lamination, flame structures, and various kinds of deformed cross-stratification are thus recognized as
palaeoseismic in origin. This is the first report of seismites from the Siwalik succession of Darjiling Himalaya which indicates
just like other sectors of Siwalik foreland basin and the present-day Gangetic foreland basin that the Siwalik sediments of
this sector responded to seismicity. 相似文献
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KAUSHIK DAS SANKAR BOSE SUBRATA KARMAKAR SUPRIYA CHAKRABORTY 《Journal of Earth System Science》2012,121(1):1-17
Granulite-facies rocks occurring north-east of the Chilka Lake anothosite (Balugan Massif) show a complex metamorphic and
deformation history. The M1–D1 stage is identified only through microscopic study by the presence of S1 internal foliation shown by the M1 assemblage sillimanite–quartz–plagioclase–biotite within garnet porphyroblasts of the aluminous granulites and this fabric
is obliterated in outcrop to map-scale by subsequent deformations. S2 fabric was developed at peak metamorphic condition (M2–D2) and is shown by gneissic banding present in all lithological units. S3 fabric was developed due to D3 deformation and it is tectonically transposed parallel to S2 regionally except at the hinge zone of the F3 folds. The transposed S2/S3 fabric is the regional characteristic structure of the area. The D4 event produced open upright F4 folds, but was weak enough to develop any penetrative foliation in the rocks except few spaced cleavages that developed in
the quartzite/garnet–sillimanite gneiss. Petrological data suggest that the M4–D4 stage actually witnessed reactivation of the lower crust by late distinct tectonothermal event. Presence of transposed S2/S3 fabric within the anorthosite arguably suggests that the pluton was emplaced before or during the M3–D3 event. Field-based large-scale structural analyses and microfabric analyses of the granulites reveal that this terrain has
been evolved through superposed folding events with two broadly perpendicular compression directions without any conclusive
evidence for transpressional tectonics as argued by earlier workers. Tectonothermal history of these granulites spanning in
Neoproterozoic time period is dominated by compressional tectonics with associated metamorphism at deep crust. 相似文献
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Shamik CHAKRABORTY YASUDA Hiroshi Abhik CHAKRABORTY NABETA Hajime KAWAI Takayuki ISHIYAMA Shun 《资源与生态学报(英文版)》2015,6(5):345-352
The Nile is one of the longest rivers on the planet and an important freshwater source for the arid regions of Africa. It is also a river that is extensively affected by anthropogenic impact. This paper aims to provide an account of the social drivers that combine to cause extensive changes in the Nilotic environments. This paper is based on extensive review of literature backed up by field research. The main focus is on the lower Nile, where the effects of anthropogenic disturbances are most prominent. We argue that the Nile Basin is characterized by interrelated and compound problems of resource management, and managing this river system effectively requires shifting the focus from water related problems to a basin wide management agenda. We contend that knowledge of environmental history is important for this agenda shift, and the idea of benefit sharing can alleviate the growing stress on this extremely sensitive arid river basin. 相似文献
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Flume experiments were carried out to study the turbulence and its impact on suspension and segregation of grain-sizes under unidirectional flow conditions over the sand-gravel mixture bed. The components of fluid velocity with fluctuations were measured vertically using 3-D Micro-acoustic Doppler velocimeter (ADV). The theoretical models for velocity and sediment suspension have been developed based on the concept of mixing length that includes the damping effect of turbulence due to sediment suspension in the flow over the sand-gravel mixture bed. Statistical analysis of segregation of grain-sizes along downstream of the bed has been performed using the principle of unsupervised learning or clustering problem. Exploratory data analysis suggests that there is a progressive downstream fining of sediment sizes with selective depositions of gravels, sand-gravels and sand materials along the stream, which may be segmented into three regions such as, the upstream, the transitional and the downstream respectively. This contribution is relevant to understand the direction of ancient rivers, the bed material character in the river form, sorting process and its role in controlling the sediment flux through landscape. 相似文献