Late Pleistocene–Holocene hydrologic changes in the interfluve areas of the central Ganga Plain, India     

Pradeep Srivastava  Indra Bir Singh  Shikha Sharma  Uma Kant Shukla  Ashok Kumar Singhvi   《Geomorphology》2003,54(3-4):279-292

Abandoned channel belts, ponds and point bar deposits of palaeochannels in the interfluve regions of the central Ganga Plain suggest changes in the morphohydrologic conditions during the Latest Pleistocene–Holocene. Stratigraphy of these ponds comprises channel sand at the base overlain by shell-bearing clayey silt. The contact of the two facies marks the phase when channels converted into standing water bodies. Point bar deposits of some palaeochannels are overlain by oxidised aeolian sand, indicating that the channel abandonment possibly occurred due to the desiccation and aridity in the region.Optically stimulated luminescence (OSL) chronometry of the pond sediments suggests that the deposition of the basal channel sand started before 13 ka and continued up to 8 ka. The ponds formed around 8–6 ka when the channel activity ceased. Evidence from the point bar deposits also indicates that the fluvial activity in the region ended sometime during 7–5 ka. This was followed by aeolian aggradation. The present study thus suggests that the hydrologic conditions in the Gangetic plains, i.e. initiation of channels and their abandonment, formation of microgeomorphologic features such as ponds and their eventual siltation, were controlled largely by climatic changes (i.e. monsoon changes) supported by tectonic activity. For the past 2 ka, increasing human and related agricultural activity has substantially accentuated the natural siltation rate of ponds.  相似文献   

Aftershock activity of Bhuj earthquake of january 26th, 2001     

Ashwani Kumar  S. C. Gupta  A. K. Jindal  Sanjay Jain  Vandana 《Journal of Earth System Science》2003,112(3):391-395

Following a large-sized Bhuj earthquake (M _s = 7.6) of January 26th, 2001, a small aperture 4-station temporary local network was deployed, in the epicentral area, for a period of about three weeks and resulted in the recording of more than 1800 aftershocks (-0.07 ≤M _L <5.0). Preliminary locations of epicenters of 297 aftershocks (2.0 ≤M _L <5.0) have brought out a dense cluster of aftershock activity, the center of which falls 20 km NW of Bhachau. Epicentral locations of after-shocks encompass a surface area of about 50 × 40 km² that seems to indicate the surface projection of the rupture area associated with the earthquake. The distribution of aftershock activity above magnitude 3, shows that aftershocks are nonuniformly distributed and are aligned in the north, northwest and northeast directions. The epicenter of the mainshock falls on the southern edge of the delineated zone of aftershock activity and the maximum clustering of activity occurs in close proximity of the mainshock. Well-constrained focal depths of 122 aftershocks show that 89% of the aftershocks occurred at depths ranging between 6 and 25 km and only 7% and 4% aftershocks occur at depths less than 5 and more than 25 km respectively. The Gutenberg-Richter (GR) relationship, logN = 4.52 - 0.89M_L, is fitted to the aftershock data (1.0<-M _L<5.0) and theb-value of 0.89 has been estimated for the aftershock activity.  相似文献   

Kinematics of the Gondwana basins of peninsular India     

Chandan Chakraborty  Nibir Mandal  Sanjoy Kumar Ghosh 《Tectonophysics》2003,377(3-4):299-324

The Gondwana successions (1–4 km thick) of peninsular India accumulated in a number of discrete basins during Permo-Triassic period. The basins are typically bounded by faults that developed along Precambrian lineaments during deposition, as well as affected by intrabasinal faults indicating fault-controlled synsedimentary subsidence. The patterns of the intrabasinal faults and their relationships with the respective basin-bounding faults represent both extensional and strike-slip regimes. Field evidence suggests that preferential subsidence in locales of differently oriented discontinuities in the Precambrian basement led to development of Gondwana basins with varying, but mutually compatible, kinematics during a bulk motion, grossly along the present-day E–W direction. The kinematic disparity of the individual basins resulted due to different relative orientations of the basement discontinuities and is illustrated with the help of a simple sandbox model. The regional E–W motion was accommodated by strike-slip motion on the transcontinental fault in the north.  相似文献   

A　Comparative　Study　of　the　Atmospheric　Layers　below　First　Lifting　Condensation　Level　for　Instantaneous　Pre-Monsoon　Thunderstorm　Ocurence　at　Agartala(23°30′N,91°15′E)　and　Ranchi(23°14′N,85°14′E)　of　India     

Sarbari Ghosh  Utpal Kumar De Atmospheric Science Research  Department of Physics  Jadavpur University  Calcutta  India Received April    revised July   《大气科学进展》1997,(1)

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On the heat budget of the Arabian Sea     

P. V. Hareesh Kumar  B. Mathew 《Meteorology and Atmospheric Physics》1997,62(3-4):215-224

Summary The rate of oceanic heat storage of the upper 200m of the Arabian Sea is explained in terms of net air-sea heat flux (Q _F), heat change due to horizontal divergence and vertical motion (Q _V) and heat change due to lateral advection (Q _A). The analysis revealed that the heat storage of the Arabian Sea is mainly controlled byQ _V while the effect ofQ _A is much larger than expected. Parameterisation of summer cooling revealed that the depletion of energy from the mixed layer is mainly due to upwelling and horizontal advection though large amount of heat is accumulated due to net air-sea heat flux. The annual heat balance of the upper 200m of the Arabian Sea suggested large heat gain by air-sea exchange processes. About two third of this heat gain is compensated by horizontal advection and one third by vertical advection.With 4 Figures  相似文献   

Multiple episodes of tectonothermal processes in the Eastern Ghats granulite belt     

Santanu Kumar Bhowmik 《Journal of Earth System Science》1997,106(3):131-146

A suite of rocks from Borra Carbonate Granulite Complex (BCGC) in the Eastern Ghats granulite belt displays superposed structures and overprinted mineral assemblages that reveal multiple episodes of tectonothermal reworking of the complex under granulite facies condition. Five distinct episodes of deformation (D₁, D₂, D₃, D₄ and D₅) and four phases of metamorphism (M₁, M₂, M₃ and M₄) are recorded. The signature of the earliest tectonothermal event, D₁ is a gneissic foliation (S₁) denned by segregation of peak granulite facies mineral assemblages corresponding to prograde M₁ metamorphism. M₂ metamorphic overprint represents an episode of near-isobaric cooling of the complex under a static condition. D₂ represents an episode of ductile deformation manifested by isoclinal folding (F₂) and associated extensional structures, within a broad framework of coaxial bulk deformation. The present study reveals that D₂ took place subsequent to M₂ - Subsequent deformation, D₃, produced F₃ folds and also deformations of boudins formed during D₂. M₃, which is synchronous with F₃, represents a near isothermal decompression of the BCGC. This was followed by a weak structural readjustment (D₄), producing E-W cross folds. The latter was not, however, associated with any recognizable petrological reworking. In the terminal events, deformation (D₅) and mineral reactions (M₄) were localized along narrow intersecting shear zones. The latter acted as channelways for carbonic and still later hydrous fluid infiltration. The available thermobarometric data from BCGC and other areas of the Eastern Ghats belt reveal that reworking during M₂ and M₃ ensued in a thermally perturbed regime. The high thermal regime might also have persisted during carbonic fluid infiltration related to terminal reworking (M₄).  相似文献   

Seismicity around Dhamni Dam, Maharashtra, India     

B. K. Rastogi  P. Mandal  N. Kumar 《Pure and Applied Geophysics》1997,150(3-4):493-509

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Electron cyclotron waves in the presence of parallel electric fields in the Earth’s auroral plasma     

S. Kumar  S. K. Dixit  A. K. Gwal 《Annales Geophysicae》1997,15(1):24-28

The electron cyclotron waves that originate at low altitudes (<0.5 R_E) and observed by ground facilities have been studied in the presence of a weak parallel electric field in auroral magnetoplasma consisting of trapped energetic auroral electrons and cold background electrons of ionospheric origin. The model distribution for auroral trapped electrons is taken as Maxwellian ring distribution. An expression for the growth rate has been obtained in the presence of parallel electric field assuming that the real frequency in the whistler mode is not affected by the presence of the electric field. The results show that waves grow (or damp) in amplitude for a parallel (or antiparallel) electric field. The influence of the electric field is more pronounced at a shorter wavelength spectrum. An increase in population of energetic electrons increases the growth rate and thus, plays a significant role in the wave excitation process in the auroral regions.  相似文献   

Seismic horizontal pullout capacity of shallow vertical anchors in sand     

Jyant Kumar  V. B. K. Mohan Rao 《Geotechnical and Geological Engineering》2004,22(3):331-349

The concept of the equivalent free surface has been extended to determine the seismic horizontal pullout capacity of shallow vertical strip plate anchors buried in sand. The analysis has been done rigorously by using the method of stress characteristics. The results have been expressed in the form of non-dimensional charts. The pullout resistance has been found to reduce quite extensively with increase in the magnitude of horizontal earthquake acceleration. The results were compared with the previously published data, and it was seen that the computed pullout resistance with the proposed method was found to be lowest. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Mechanism of arrested charnockite formation at Nemmara, Palghat region, southern India   总被引：3，自引：0，他引：3  

G. R. Ravindra Kumar   《Lithos》2004,75(3-4):331-358

The formation of arrested charnockite is an excellent example of structurally controlled channellised fluid flow along specific sites accompanied by selective elemental mobility and mineralogical changes. The present paper recognises and focuses study on three types of arrested charnockite formation from Palghat region, namely, shear-controlled, foliation parallel and boudin-neck types, and address their spatio-temporal relations to regional-scale charnoenderbite. The shear-controlled and foliation parallel types post-date deformation and migmatisation. The boudin-neck type, on the contrary, is coeval with partial melting and followed the path of cooling and decreasing water activity in the gneiss. K-feldspar veining around plagioclase and quartz, symplectitic intergrowth of biotite+quartz after orthopyroxene and K-feldspar, and fluid inclusion data suggests the presence of alkalic supercritical brine and low-density CO₂-rich fluid during charnockite formation. Charnockite domains developed following the breakdown of hornblende, biotite and quartz are characterised by a more or less pronounced depletion of Fe, Ca, Mg and Ti and trace elements Y and Zr, compared to their counterpart gneiss. REE spectra indicate a subtle depletion in the HREE near the centre of the charnockite domain. Although close-pair samples of gneiss–charnockite are isochemical, on a scale of a few millimetres, bi-directional element movement, related to the formation of new mineral was noted. It is postulated that arrested charnockite formation developed in situ on local scale within the granitic domains of the hornblende-biotite gneiss, in the presence of CO₂-rich fluids and alkalic supercritical saline brine. This process post-dated the time of regional granulite (charnoenderbite) and large regional scale retrogression and migmatisation.  相似文献