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31.
32.
A suite of garnetiferous amphibolites and mafic granulites occuras small boudins within layered felsic migmatite gneiss in thenorthern part of the Sausar Mobile Belt (SMB), the latter constitutingthe southern component of the Proterozoic Central Indian TectonicZone (CITZ). Although the two types of metabasites are in variousstages of retrogression, textural, compositional and phase equilibriastudies attest to four distinct metamorphic episodes. The earlyprograde stage (Mo) is represented by an inclusion assemblageof hornblende1 + ilmenite1 + plagioclase1 ± quartz andgrowth zoning preserved in garnet. The peak assemblage (M1)consists of porphyroblastic garnet + clinopyroxene ±quartz ± rutile ± hornblende in mafic granulitesand garnet + quartz + hornblende in amphibolites and stabilizedat pressuretemperature conditions of 910 kbarand 750800°C and 8 kbar and 675°C, respectively.This was followed by near-isothermal decompression (M2), andpost-decompression cooling (M3) events. In mafic granulites,the former resulted in the development of early clinopyroxene2Ahornblende2Aplagioclase2Asymplectites at 8 kbar and 775°C (M2A stage), synchronouswith D2 and later anhydrous clinopyroxene2Bplagioclase2Bilmenite2Bsymplectites and coronal assemblages at 7 kbar, 750°C (M2Bstage) and post-dating D2. In amphibolites, ilmenite + plagioclase+ quartz ± hornblende symplectites appeared during M2at 6·4 kbar and 700°C. During M3, coronal garnet+ clinopyroxene + quartz ± hornblende-bearing symplectitesin metabasic dykes and hornblende3plagioclase3 symplectitesembaying garnet in mafic granulites were formed. PT estimatesshow near-isobaric cooling from 7 kbar and 750°C to 6 kbarand 650°C during M3. It is argued that the decompressionin the mafic granulites is not continuous, being punctuatedby a distinct heating (prograde?) event. The latter is alsocoincident with a period of extension, marked by mafic dykeemplacement. The combined PT path of evolution has aclockwise sense and provides evidence for a major phase of earlycontinental subduction in parts of the CITZ. This was followedby a later continentcontinent collision event duringwhich granulites of the first phase became tectonically interleavedwith younger lithological units. This tectonothermal event,of possibly Grenvillian age, marks the final amalgamation ofthe North and the South Indian Blocks along the CITZ to producethe Indian subcontinent. KEY WORDS: Central Indian Tectonic Zone; clockwise PT path; continental collision; metabasite 相似文献
33.
NARAYANAPANICKER SURESH TRILOKI N. BAGATI ROHTASH KUMAR VIKRAM C. THAKUR 《Sedimentology》2007,54(4):809-833
Quaternary alluvial fans in the tectonically active Pinjaur Dun, an intramontane valley in the Sub‐Himalaya, were deposited in front of the Nalagarh Thrust and were influenced both by tectonics and glacial climate fluctuations. The surface morphology indicates that an earlier set of first‐order fans (Qf1) became entrenched and onlapped by a series of second‐order fans (Qf2). The younger fan segments were then cut by a pair of terraces (T1 and T2). Quartz optically stimulated luminescence dating establishes that the Qf1 aggradational phase was initiated before 96·5 ± 25·3 ka and terminated after 83·7 ± 16·3 ka. This was followed by a period of incision, before Qf2 fan deposition started at 72·4 ± 13·4 ka and continued until 24·5 ± 4·9 ka. Sediment was deposited on the T1 (upper) and T2 (lower) terraces at 16·3 ± 2·1 and 4·5 ka, respectively, recording a return to overall degradation punctuated by minor deposition on terraces. The period of incision separating the younger and older fan deposits coincided with enhanced SW monsoon precipitation. The subsequent development of the Qf2 fans and their progradation until 20 ka suggest erosional unloading of the thrust hangingwall during a tectonically quiescent phase. Toe cutting, deposition of axial river and lacustrine facies, and retreat of Qf2 around 45 ka, indicate fanward shift of the axial river due to tilting of the valley towards the NE in response to reactivation of the Nalagarh Thrust. The cessation of Qf2 deposition around 20 ka and the onset of through‐fan entrenchment suggest reduced sediment supply but relatively high stream power during the last glacial maxima (LGM). The prolonged stream incision since the cessation of Qf2 deposition, with only minor depositional phases at 16·3 ± 2·1 and 4·5 ka, resulted from high water discharge and low sediment input during intensification of the SW monsoon and vegetation changes in the hinterland. 相似文献
34.
Pan-African high-grade metamorphism in the Kerala KhondaliteBelt (South India) led to the in situ formation of garnet-bearingleucosomes (L1) in sodic quartzalkali feldsparbiotitegneisses. Microtextures, mineralogy and the geochemical characteristicsof in situ leucosomes (L1) and gneiss domains (GnD) indicatethat the development of leucosomes was mainly controlled bythe growth of garnet at the expense of biotite. This is documentedby the selective transfer of FeO, MgO, , Sm and the heavy rareearth elements into the L1 domains. P-T constraints (T>800C,P>6kbar, aH2O0.3) suggest that the leucosomes were formedthrough complete melting of biotite in fluid-absent conditions,following the model reaction Biotite+Alkali feldspar+QuartzlGarnet+Ilmenite+Melt.The fraction of melt generated during this process was low (<10vol.%). The identical size of the leucosomes as well as theirhomogeneous and isotropic distribution at outcrop scale, whichlacks any evidence for melt segregation, suggest that the migmatiteremained a closed system. Subsequent to migmatization, the leptyniticgneisses were intruded by garnet-bearing leucogranitic melts(L2), forming veins parallel and subperpendicular to the foliation.The leucogranites are rich in potassium (K2O5.5 wt%), (Ba400p.p.m.) and Sr (300 p.p.m.), and exhibit low concentrationsof Zr (40 p.p.m.), Th (<1 p.p.m.) and (<10 p.p.m.). Thechondrite-normalized REE spectra show low abundances (LaN20,LuN3) and are moderately fractionated (LaN/LuN7). An Eu anomalyis absent or weakly negative. The higher 87Sr/86Sr ratio at550 Ma (0.7345) compared with the migmatite (0.7164) precludesa direct genetic relationship between leptynitic gneisses andleucogranites at Manali.Nevertheless, the chemical and mineralogicalcompositions of the leuocogranites strongly favour a derivationthrough fluid-absent biotite melting of isotopically distinctbut chemically comparable Manali-type gneisses. The undersaturationof Zr, Th and REE, a typical feature of leucogranitic meltsgenerated during granulite facies anatexis of psammo-peliticlithologies and attributed to disequilibrium melting with incompletedissolution of accessory phases (zircon, monazite), is weaklydeveloped in the leucogranites of Manali.It is concluded thatthis is mainly due to the sluggish migration of the melts instatic conditions, which facilitated equilibration with therestitic gneisses.
*Fax: 0228-732763; e-mail: ingo.braun{at}uni-bonn.de 相似文献
35.
Linear relationship between dipole and Schlumberger sounding resistivities leads to the use of digital filters to transform the former to the latter. This transformation is of importance from the viewpoint that Schlumberger interpretational techniques and know-how could then be applied to the pseudo-Schlumberger field curve. Filters for this transformation are presented for the radial, perpendicular, and parallel (30°) dipole method. The characteristics of these filters are similar to the ones for transforming dipole data to the kernel and are favourable in that they do not amplify noise. A sampling interval of (In 10) /6 has been used in determining the filter yielding good accuracy. Like previous filters the present one is handy and fast in operation. 相似文献
36.
The combined observation of first and later arrivals in shallow seismic refraction surveys, particularly on hard rock terrains, is discussed. Details of experimental weathered-zone investigations by the correlation refraction method in a granite terrain (i.e. field procedure, seismograms obtained, plotting of the data, and identification of the waves are presented). Complete travel time data and interpreted subsurface sections of a few test refraction surveys are included. In one instance the interpreted results of normal and converted refracted wave data have been tested by drilling at three points along a 220 m long profile. 相似文献
37.
A VANEET KUMAR N C KOTHIYAL SARUCHI KUMARI R MEHRA A PARKASH R R SINHA S K TAYAGI R GABA 《Journal of Earth System Science》2014,123(3):479-489
The objective of the present study was to ascertain contamination levels, distribution behaviour and PAHs exposure during summer, winter and autumn during 2011–2012 in one of the developing cities of northern India. Average PAHs concentration was found to be 18.17, 4.04 and 16.38 μg g ?1, whereas, concentration of 16 individual PAHs was found to vary between 0.02 and 200.23, 0.008 and 28.4 μg g ?1, and 0.01 and 252.55 μg g ?1 during summer, winter and autumn seasons, respectively. The average concentration of low and high carcinogenic PAHs during summer, winter and autumn was found to be 5.1 and 31.29, 2.1 and 6.4, 4.74 and 35.08 μg g ?1 at most intercepts. The average ratio of low to high carcinogenic PAHs was found to be 1:6, 1:3, 1:7.6 during summer, winter and autumn, respectively. Five ringed PAHs were found in higher concentration in all seasons. Dib(ah)A and B(a)P were the two individual PAHs found in highest concentration during summer, winter and autumn seasons. Two tailed T-test was applied for authenticity of the results. Toxic equivalency factor of B(a)P and Dib(ah)A was maximum as compared to other PAHs. The study could be of great significance for the planners while considering environmental remedial measures. 相似文献
38.
Meso-scale atmospheric events promote phytoplankton blooms in the coastal Bay of Bengal 总被引:1,自引:0,他引:1
K MANEESHA V V S S SARMA N P C REDDY Y SADHURAM T V RAMANA MURTY V V SARMA M DILEEP KUMAR 《Journal of Earth System Science》2011,120(4):773-782
The Bay of Bengal is considered to be a low productive region compared to the Arabian Sea based on conventional seasonal observations.
Such seasonal observations are not representative of a calendar year since the conventional approach might miss episodic high
productive events associated with extreme atmospheric processes. We examined here the influence of extreme atmospheric events,
such as heavy rainfall and cyclone Sidr, on phytoplankton biomass in the western Bay of Bengal using both in situ time-series observations and satellite derived Chlorophyll a (Chl a) and sea surface temperature (SST). Supply of nutrients through the runoff driven by episodic heavy rainfall (234 mm) on
4–5 October 2007 caused an increase in Chl a concentration by four times than the previous in the coastal Bay was observed within two weeks. Similar increase in Chl a, by 3 to 10 times, was observed on the right side of the cyclone Sidr track in the central Bay of Bengal after the cyclone Sidr. These two episodic events caused phytoplankton blooms in the western Bay of Bengal which enhanced ~40% of fishery production
during October–December 2007 compared to that in the same period in 2006. 相似文献
39.
NAVEEN GANDHI SANJEEV KUMAR S PRAKASH R RAMESH M S SHESHSHAYEE 《Journal of Earth System Science》2011,120(1):99-111
Various experiments involving the measurement of new, regenerated and total productivity using 15N and 13C tracers were carried out in the Bay of Bengal (BOB) and in the Arabian Sea. Results from 15N tracer experiments indicate that nitrate uptake can be underestimated by experiments with incubation time <4 hours. Indirect
evidence suggests pico- and nano-phytoplankton, on their dominance over microphytoplankton, can also influence the f-ratios.
Difference in energy requirement for assimilation of different nitrogen compounds decides the preferred nitrogen source during
the early hours of incubation. Variation in light intensity during incubation also plays a significant role in the assimilation
of nitrogen. Results from time course experiments with both 15N and 13C tracers suggest that photoinhibition appears significant in BOB and the Arabian Sea during noon. A significant correlation
has been found in the productivity values obtained using 15N and 13C tracers. 相似文献
40.
FABIEN DURAND FABRICE PAPA ATIQUR RAHMAN SUJIT KUMAR BALA 《Journal of Earth System Science》2011,120(5):859-872
This study investigates the impact of monthly Ganges–Brahmaputra river discharge variations on Bay of Bengal salinity and
temperature during the period 1992–1999. The Ganges–Brahmaputra river discharge is characterized by a well-defined seasonal
cycle with strong interannual variations. The highest/lowest yearly peak discharge occurs in summer 1998/summer 1992, with
1998 value amounting to twice that of 1992. This river discharge is then used to force an ocean general circulation model.
Our main result is that the impact of these rivers on the variability of Bay of Bengal sea surface salinity is strong in the
northern part, with excess run-off forcing fresh anomalies, and vice versa. Most of the years, the influence of the interannual variability of river discharge on the Bay salinity does not extend south
of ~10°N. This stands in contrast with the available observations and is probably linked to the relatively coarse resolution
of our model. However, the extreme discharge anomaly of 1998 is exported through the southern boundary of the Bay and penetrates
the south-eastern Arabian Sea a few months after the discharge peak. In response to the discharge anomalies, the model simulates
significant mixed-layer temperature anomalies in the northern Bay of Bengal. This has the potential to influence the climate
of the area. From our conclusions, it appears necessary to use a numerical model with higher resolution (both on the horizontal
and vertical) to quantitatively investigate the upper Bay of Bengal salinity structure. 相似文献