National Center for Earth Science Studies
National Center for Earth Science Studies
Soumya G.S.,National Center for Earth Science Studies |
Radhakrishna T.,National Center for Earth Science Studies
Journal of Geology | Year: 2017
Garnet is a commonly occurring accessory mineral in many anorthosite plutons, and it is variably interpreted as a metamorphic, magmatic, or xenocrystic mineral. The Neoproterozoic Oddanchatram anorthosite in the South Indian granulite terrain is one such example. Igneous textures are remarkably well preserved, with large grains (up to 5 cm) of garnet in the margins of the pluton. Typical prograde metamorphic textures, such as garnet formation by isobaric reaction between mafic silicate and plagioclase or by dehydration breakdown of amphibole, are lacking, and Fe-Ti oxides mostly occur as exsolutions in the associated plagioclase or scarce disseminated oxide grains. The garnets are almandine rich in composition, with higher CaO (>5 wt%) than the low-CaO (<2 wt%) almandine of garnets in the adjoining metamorphic assemblages. They cannot be described as xenocrysts because of their large size, compared to their smaller size in the country rocks, apart from the chemical distinctions. A reaction of hydrous fluid influx with plagioclase and mafic phases in the anorthosite, as described for similar large-garnet formation in the Adirondacks by McLelland and Selleck, appears to be a more reasonable explanation. The fluid source may be external, released from hydrous minerals of the hornblende-biotite-bearing gneiss country rock during the prograde metamorphism that the terrain experienced, or internal, from the late magmaticenrichedhydrousliquids, as evident from primary amphibole. Formationof garnetand exsolution of Fe-Tioxides appear to have developed when the anorthosite was still at a depth corresponding to upper-amphibolite-facies metamorphic conditions and experienced isothermal decompression and exhumation subsequently, as evidenced by the plagioclase-orthopyroxene symplectite development from garnet. © 2017 by The University of Chicago.
Radhakrishna T.,National Center for Earth Science Studies |
Soumya G.S.,National Center for Earth Science Studies |
Satyanarayana K.V.V.,Indian Institute of GeomagnetismNavi Mumbai
Journal of Geodynamics | Year: 2017
The paper presents new palaeomagnetic results and reassesses complete set of published palaeomagnetic results on the lamproite intrusions in the Gondwana formations of the Eastern India. Altogether eleven sites register reliable characteristic magnetisations corresponding to the c. 110 Ma emplacement age of the lamproites. A mean ChRM is estimated with D = 331.3°; I = −62.4° (α95 = 6.2°, k = 55; N = 11). The palaeomagnetic pole of λ = 14.9°: Φ = 287.6° (A95 = 8.4°) is established for the lamproites and it averaged the secular variation and confirms to the Geocentric Axial Dipole (GAD). The pole compares remarkably well with the grand mean pole reported for the Rajmahal traps that are attributed to represent location of the Kerguelen mantle plume head. The palaeolatitudes transferred to Rajmahal coordinates (25.05°: 87.84°) are situated ∼6° north of the present location of the Kerguelen hotspot location. The interpretations are consistent with earlier suggestions of southward migration of the plume based on palaeomagnetic results of Site 1138 of the ODP Leg 183 and with the predictions of numerical models of global mantle circulation. © 2017
Unnikrishnan C.K.,National Center for Earth Science Studies |
Rajeevan M.,Ministry of Earth science
Meteorology and Atmospheric Physics | Year: 2017
High resolution hybrid atmospheric water budget over the South Asian monsoon region is examined. The regional characteristics, variability, regional controlling factors and the interrelations of the atmospheric water budget components are investigated. The surface evapotranspiration was created using the High Resolution Land Data Assimilation System (HRLDAS) with the satellite-observed rainfall and vegetation fraction. HRLDAS evapotranspiration shows significant similarity with in situ observations and MODIS satellite-observed evapotranspiration. Result highlights the fundamental importance of evapotranspiration over northwest and southeast India on atmospheric water balance. The investigation shows that the surface net radiation controls the annual evapotranspiration over those regions, where the surface evapotranspiration is lower than 550 mm. The rainfall and evapotranspiration show a linear relation over the low-rainfall regions (<500 mm/year). Similar result is observed in in NASA GLDAS data (1980–2014). The atmospheric water budget shows annual, seasonal, and intra-seasonal variations. Evapotranspiration does not show a high intra-seasonal variability as compared to other water budget components. The coupling among the water budget anomalies is investigated. The results show that regional inter-annual evapotranspiration anomalies are not exactly in phase with rainfall anomalies; it is strongly influenced by the surface conditions and other atmospheric forcing (like surface net radiation). The lead and lag correlation of water budget components show that the water budget anomalies are interrelated in the monsoon season even up to 4 months lead. These results show the important regional interrelation of water budget anomalies on south Asian monsoon. © 2017 Springer-Verlag Wien
Maya K.,National Center for Earth Science Studies |
Mohan S.V.,National Center for Earth Science Studies |
Limaye R.B.,Agharkar Research Institute |
Padmalal D.,National Center for Earth Science Studies |
Kumaran N.K.P.,Agharkar Research Institute
PLoS ONE | Year: 2017
The coastal lands of southern Kerala, SW India in the vicinity of Achankovil and Thenmala Shear Zones reveal a unique set of geomorphic features like beach ridges, runnels, chain of wetlands, lakes, estuaries, etc. The chain of wetlands and water bodies that are seen in the eastern periphery of the coastal lands indicates the remnants of the upper drainage channels of the previously existed coastal plain rivers of Late Pleistocene age that are later broadened due to coastal erosion under the transgressive phase. The terrain evolutionary model developed from the results of the study shows that the Late Pleistocene transgressive events might have carved out a major portion of the land areas drained by the coastal plain rivers and as a result the coastal cliff has been retreated several kilometers landwards. The NNESSW trending beach ridges located close to the inland wetlands indicate the extent of shoreline shift towards eastwards during Late Pleistocene period. The present beach parallel ridges in the younger coastal plain indicate the limit of the Mid Holocene shoreline as the transgression was not so severe compared to Late Pleistocene event. The zone of convergence of the two sets of beach ridges coincides with the areas of economically viable heavy mineral placers that resulted from the size and density based sorting under the repeated transgressive events to which the coast had subjected to. The chain of wetlands in the eastern side of the study area has been evolved from a mega lagoon existed during Late Pleistocene. The Pallikkal River that links discrete eastern wetland bodies has been evolved into its present form during Early Holocene. © 2017 K. et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Srinivas R.,National Center for Earth Science Studies |
Shynu R.,National Center for Earth Science Studies |
Sreeraj M.K.,National Center for Earth Science Studies |
Ramachandran K.K.,National Center for Earth Science Studies
Marine Pollution Bulletin | Year: 2017
Metal concentrations (Al, Cr, Ni, Cu, Zn, and Pb), grain size, and total organic carbon content in 29 surface sediment samples from the nearshore area off Calicut were analyzed to determine their distribution and pollution status. Surface sediments were dominantly silts with low percentage of clay and sand at nearshore and offshore areas. The mean metal concentrations were in the following order: Cr. >. Ni. >. Zn. >. Pb. >. Cu. The enrichment factor and geo-accumulation index of metals suggest that the surface sediments were not polluted by Zn and moderately polluted by Cu and Ni. By contrast, Cr and Pb showed significant enrichment levels. Results from a multivariate statistical analysis suggested that the spatial enrichment of these heavy metals was related to sediment type. Thus, the sediment distribution and their metal enrichment were mainly controlled by local hydrodynamic conditions that caused the winnowing of fine-grained sediments. © 2017.
Padmalal D.,National Center for Earth Science Studies
Environmental Monitoring and Assessment | Year: 2015
The paper deals with the hydrochemical characterization and water quality assessment of springs emerging from the Archaean crystalline basements at the foothills of Western Ghat mountains in the highlands and Neogene sedimentary formations in the coastal lowlands of Kerala in south west India. A total of 19 springs from two important river basins of southern Kerala such as Ithikkara and Kallada river basins were studied for 18 physico-chemical (temperature, pH, electrical conductivity (EC), total dissolved solids (TDS), dissolved oxygen (DO), total hardness (TH), Na+, K+, Ca2+, Mg2+, CO3 2−, HCO3 −, Cl−, SO4 2− , NO3 −, SiO2, Fe2+, and F−) as well as bacteriological parameters. The discharge computations show that free-falling type of springs in the area discharge about 256.23 million liters of water a year. A comparative study between the spring water samples of highland and lowland regions reveal that the quality of spring water, except pH and bacteriological contents, satisfies the standards set by the Bureau of Indian Standards and World Health Organization for drinking water. Spring water samples collected from the lowlands register high value of Na+ and Cl− compared with the highlands. Bicarbonate, Ca2+, Mg2+, and K+ values are high in highland than lowland springs. The present study reveals that the spring water sources in the region can be developed as an alternate source for drinking water, provided pH correction and proper disinfection are done prior to its end use. © 2015, Springer International Publishing Switzerland.
Noujas V.,National Center for Earth Science Studies |
Thomas K.V.,National Center for Earth Science Studies |
Badarees K.O.,National Center for Sustainable Coastal Management
Ocean Engineering | Year: 2016
Coastal processes along the Munambam to Chettuwa coastline located at the south-west part of India have been studied to understand the role of a mudbank and artificial structures like breakwaters in causing shoreline changes. The mudbank occurring along this coast is different from those in South America, Lousiana and China. The morphology and coastal processes of the coast have been studied by collecting nearshore and beach data from 2009 to 2013. Location of the mudbank area has been shifting over the years from north to south. The beach at the mudbank and its north is accreting while the southern side is eroding. Harbor breakwaters and seawalls are other factors influencing beach stability. An erosion hotspot along this mudbank coast is identified and management interventions proposed based on numerical model studies using MIKE 21 SW model and LITPACK. The model results are calibrated and validated with field observations. Sediment transport along this sector was estimated using LITDRIFT module. Shoreline change was calculated using the LITLINE module. The production run was carried out with different management options and the most suitable option is recommended for the eroding sector. The other sectors do not require any intervention to sustain the beaches. © 2015 Published by Elsevier Ltd.
Ravindra Kumar G.R.,National Center for Earth Science Studies |
Sreejith C.,National Center for Earth Science Studies |
Sreejith C.,P.A. College
Lithos | Year: 2016
The Kerala Khondalite Belt (KKB) of the southern India encompasses volumetrically significant magmatic components. Among these, orthopyroxene-bearing, felsic ortho-granulites, popularly known as charnockites in Indian context, constitute an important lithology. In contrast to the well-known phenomena of arrested charnockitization, the geochemical characteristics and petrogenesis of these ortho-granulite suites remain poorly studied, leaving geodynamic models envisaged for the KKB highly conjectural. In this paper, we try to bridge this gap with detailed results on orthopyroxene-bearing, felsic ortho-granulites spread over the entire KKB and propose a new petrogenetic and crustal evolution model. Based on geochemical characteristics, the orthopyroxene-bearing, felsic ortho-granulites (charnockites sensu lato) of KKB are classified into (1) tonalitic (TC), (2) granitic (GC), and (3) augen (AC) suites. Members of the TC follow sodic (characterized by decreasing CaO/Na2O), whereas those of the GC and AC follow calc-alkaline trends of differentiation. Geochemical patterns of the TC resemble those of the Archaean tonalite–trondhjemite–granodiorite (TTG) suites, with slightly magnesian character (average Mg# = 33), moderate LREE (average LaN = 154), low HREE (average YbN = 6) and Y (1–53 ppm; average 11 ppm). The TC is also characterized by positive to slightly negative europium anomalies (Eu/Eu* = 0.7 to 1.67). The GC and AC suites, on the other hand, resemble post-Archaean arc-related granites. The GC displays ferroan nature (average Mg# = 22), low to moderate degrees of REE fractionation (average [La/Yb]N = 34.84), high contents of Y (5–128 ppm; average 68), and low Sr/Y (1–98) ratios. Significant negative Eu anomalies (Eu/Eu* = 0.18–0.91; average 0.50) and low Sr (65–690 ppm) are also noted in the GC. Similar chemical characteristics are shown by the AC, with ferroan nature (average Mg# = 21), low to moderate degrees of REE fractionation (average [La/Yb]N = 26), high contents of Y (71–99 ppm; average 87), and low Sr/Y (average 2) ratios with significant negative Eu anomalies (Eu/Eu* = 0.03–0.31; average 0.23) and low Sr (average 160 ppm) contents. The protoliths of the TC are interpreted as being derived from partial melting of thickened oceanic-arc crust composed of Archaean mafic composite source rocks (i.e., eclogite and/or garnet amphibolite) with a garnet amphibolite residue. Geochemical features of the GC, such as high Rb/Sr (average 1.80) and Ba/Sr ratios (average values > 6), are considered as evidence for crustal reworking in their genesis, suggesting remelting of a quartzo-feldspathic (TTG) source, within the plagioclase stability fields. The geochemical features of the felsic ortho-granulite suite, substantiated with published geochronological data on members of the TC, GC, and AC suites, suggest a four-stage crustal evolution of the KKB. The first stage is marked by the formation of an over-thickened oceanic-arc. Zircon Hfc model ages of the TC and GC suites constrain the time of this juvenile magmatic crust-forming event as Meso- to Neoarchaean (2.8 to 2.6 Ga). The second stage corresponds to the production of TTG magmas by melting of the over-thickened oceanic-arc crust, subsequent to basaltic underplating during Palaeoproterozoic (ca. 2.1 Ga). The third stage was initiated by a transition in subduction style from shallow to steep due to continent-arc accretion. This stage is marked by the formation of granitic magmas through partial melting of the TTG crust and their differentiation into GC and TC. The zircon crystallization ages (1.89 and 1.85 Ga) of the GC indicate arc accretion occurred during the Palaeoproterozoic. The fourth stage of crustal evolution is correlated with the Mesoproterozoic (~ 1.5 Ga) emplacement of megacrystic K-feldspar granites (protoliths to the AC and augen gneisses). The distinct petrography, geochemistry and crystallization ages of the AC suggests recurrence of megacrystic, high-K calc-alkaline granitoids as the product of final phases of crustal-remelting marking subduction cessation. All these magmatic events are fairly well correlated with the major episodes of crustal growth observed in the once contiguous continental fragments (Sri Lanka and Madagascar) and worldwide events (2.7, 1.9, and 1.2 Ga) implying similar episodic nature in the lower crustal evolution of the KKB. © 2016 Elsevier B.V.
Dubey C.P.,CSIR - Central Electrochemical Research Institute |
Tiwari V.M.,CSIR - Central Electrochemical Research Institute |
Tiwari V.M.,National Center for Earth Science Studies
Computers and Geosciences | Year: 2016
New measuring instruments of Earth's gravity gradient tensors (GGT) have offered a fresh impetus to gravimetry and its application in subsurface exploration. Several efforts have been made to provide a thorough understanding of the complex properties of the gravity gradient tensor and its mathematical formulations to compute GGT. However, there is not much open source software available. Understanding of the tensor properties leads to important guidelines in the development of real three dimensional geological models. We present a MATLAB computational algorithm to calculate the gravity field and full gravity gradient tensor for an undulated surface followed by regular geometries like an infinite horizontal slab, a vertical sheet, a solid sphere, a vertical cylinder, a normal fault model and a rectangular lamina or conglomerations of such bodies and the results are compared with responses using professional software based on different computational schemes. Real subsurface geometries of complex geological structures of interest are approximated through arrangements of vertical rectangular laminas. The geological application of this algorithm is demonstrated over a horst-type structure of Oklahoma Aulacogen, USA and Vredefort Dome, South Africa, where measured GGT data are available. © 2015 Elsevier Ltd.
Harley S.L.,University of Edinburgh |
Nandakumar V.,National Center for Earth Science Studies
Precambrian Research | Year: 2016
In-situ zircon U-Pb isotopic and monazite Th-U-Pb chemical ages from leucosome in migmatitic paragneiss at Kanjampara in the Trivandrum Block, India, demonstrate the occurrence of high-T metamorphism and anatexis at 1.92 Ga. This Palaeoproterozoic tectonothermal event was strongly overprinted by the main Neoproterozoic-Cambrian granulite facies tectonism which itself involved significant partial melting and formation of garnet-bearing assemblages at 0.65 GPa and >820-860 °C. Monazite grains, which enclose zircon grains and locally preserve Palaeoproterozoic chemical age domains, were extensively reset and recrystallized at 565 ± 6 Ma. These monazite grains were further modified to form high-Th cuspate rims at 517 ± 15 Ma, equivalent to the lower intercept age for extensive Pb-loss from the highly discordant 1.92 Ga zircon, 528 ± 18 Ma. These results confirm that at least some of the metasedimentary paragneisses in the Trivandrum Block are polymetamorphic, initially metamorphosed in the Palaeoproterozoic in an event older than the 1.89-1.85 Ga granitic orthogneisses recognised from the area. The nature of the relationship between the paragneisses and older (2.0-2.1 Ga) granitic and charnockitic orthogneisses of the Trivandrum and Nagercoil Blocks, in particular whether they share the same Palaeoproterozoic history or were interleaved after 1.92 Ga, requires further investigation focussed on the presence or absence of the isotopic imprint of this event in both rock suites throughout the area.The growth or recrystallisation of monazite at ca. 565 Ma and its further modification at ca. 520 Ma indicates that the high-T metamorphism in the Pan-African was long-lived, with a duration of at least 45 Myr. This adds further weight to recent proposals, based on zircon-monazite isotopic studies from other localities in the Trivandrum Block, that the region formed the mid- to deep-crust of a long-lived collisional hot orogenic belt that welded Gondwana from ca. 580-510 Ma. © 2016 .