Earth, Ocean and Atomospheric Sciences - Publications

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  • Item
    Sound velocity, density, and related properties along a transect across the Bay of Bengal
    ( 1989-06-01) Krishna, K. S. ; Gopala Rao, D. ; Prabhakar S Murty, G. ; Ramana, Y. V.
    Physical properties, sound velocity and density of 95 core samples from ten locations along a transect (parallel to 15°N latitude) across the Bay of Bengal were measured. Results indicate that sound velocities range from 1,482 to 1,679 m/s (av: 1,583 m/s) and densities from 1,338 to 1,757 kg/m3 (av: 1,527 kg/m3). It has been observed that dark colored clays and sandy clays show higher values of sound velocity and density than the average values. A comparative study of the same with that of the sediment cores collected from the Norwegian basin, Mediterranean Sea, and Hatteras, Nares Abyssal plains, western North Atlantic Ocean was done. © 1989 Springer-Verlag New York Inc.
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    Magnetic anomalies across the southern Central Indian Ridge: evidence for a new transform fault
    ( 1990-01-01) Chaubey, A. K. ; Krishna, K. S. ; Subba Raju, L. V. ; Gopala Rao, D.
    Bathymetric and magnetic studies along seven northeast-southwest profiles across the Central Indian Ridge segment between latitudes 18 and 22°S and longitudes 66 and 69°E reveal a northwest-southeast trending median valley, associated with a broad and high-amplitude magnetic anomaly (the central anomaly) along the ridge crest. A series of linear magnetic anomalies parallel to the ridge are identified as sea-floor spreading lineations 2,2A,3,3A and 4. A half spreading rate of 2.2 cm y-1 is estimated for the last 10 Ma. The ridge jump between the anomalies 2-2A (approx. 2.5 Ma) and a new left lateral transform fault offsetting the ridge axis about 50 km are indicated to explain the magnetic pattern. © 1990.
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    Buried late Pleistocene fluvial channels on the inner continental shelf off Vengurla, west coast of India
    ( 1991-01-01) Raju, L. V.S. ; Krishna, K. S. ; Chaubey, A. K.
    Analysis of echosounding, side-scan sonar and shallow seismic data collected west of Burnt islands off Vengurla, west coast of India, revealed a featureless seabed, thicknesses of subsurface layers, and presence of buried channels filled with sediments. Cross sectional dimensions between 15 to 100 m width and 2 to 6 m depth suggest a fluvial origin of the channels. These buried channels appear to mark former positions of rivers flowing from the nearby coast and debouching into the Arabian Sea. These features, which formed during the late Pleistocene, are at approximately the same depth below the seabed. The seismic data suggest that depositions of this sytem occurred during eustatic sea-level fluctuations and formed an important part of shelf stratigraphy. -Authors
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    Magnetic studies over the northern extension of the Prathap Ridge complex, eastern Arabian Sea
    ( 1992-03-01) Krishna, K. S. ; Prabhakar S Murty, G. ; Srinivas, K. ; Rao, D. Gopala
    The northern parts of the Prathap and Laccadive Ridge system, eastern Arabian Sea, consist of three parallel basement ridge peaks at varied depths. The topographic highs are associated with either well-developed or subdued magnetic signatures. Model studies, constrained by seismic results, determine the varied nature and depth to the top of the causative basement bodies. Similarities of the geophysical signatures of the ridges and their structural resemblance perhaps point to their common origin. Hence we propose that the Prathap Ridge complex may be a part of the Chagos-Laccadive Ridge system and formed because of the Reunion hotspot activity. © 1992 Springer-Verlag New York Inc.
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    Age of younger tonalitic magmatism and granulitic metamorphism in the South Indian transition zone (Krishnagiri area); comparison with older Peninsular gneisses from the Gorur–Hassan area
    ( 1993-01-01) PEUCAT, J. J. ; MAHABALESWAR, B. ; JAYANANDA, M.
    Abstract A major episode of continental crust formation, associated with granulite facies metamorphism, occurred at 2.55–2.51 Ga and was related to accretional processes of juvenile crust. Dating of tonalitic–trondhjemitic, granitic gneisses and charnockites from the Krishnagiri area of South India indicates that magmatic protoliths are 2550–2530 ± 5 Ma, as shown by both U–Pb and 207Pb/206Pb single zircon methods. Monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 ± 10 Ma. These data provide precise time constraints and Sr–Nd isotopes confirm the existence of late tonalitic–granodioritic juvenile gneisses at 2550 Ma. Pb single zircon ages from the older Peninsular gneisses (Gorur–Hassan area) are in agreement with some previous Sr ages and range between 3200 ± 20 and 3328 ± 10 Ma. These gneisses were derived from a 3.3–3.5‐Ga mantle source as indicated from Nd isotopes. They did not participate significantly in the genesis of the 2.55‐Ga juvenile magmas. All these data, together with previous work, suggest that the 2.51‐Ga granulite facies metamorphism occurred near the contact of the ancient Peninsular gneisses and the 2.55–2.52‐Ga ‘juvenile’tonalitic–trondhjemitic terranes during synaccretional processes (subduction, mantle plume?). Rb–Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably related to the dextral major east–west shearing which displaced the 2.5‐Ga juvenile terranes toward the west. Copyright © 1993, Wiley Blackwell. All rights reserved
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    Parabolic density function in sedimentary basin modelling
    ( 1993-09-01) Rao, C. Visweswara ; Chakravarthi, V. ; Raju, M. L.
    For modelling sedimentary basins of large thickness from their gravity anomalies, the concept of parabolic density function which explains the variation of true density contrast of the sediments with depth in such basins is introduced in Bott's (1960) procedure. The analytical expression the gravity anomaly of a two-dimensional vertical prism with parabolic density contrast needed to estimate the gravity effect of the basin in modelling procedure is derived in a closed form. Two profiles of gravity anomalies, one across San Jacinto Graben, California and the other across Tucson basin, Arizona where the density of sediments is found to vary with depth are interpreted. © 1993 Birkhäuser Verlag.
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    Tectonic and sedimentary history of the Argo Abyssal Plain, eastern Indian Ocean
    ( 1994-01-01) Rao, D. G. ; Krishna, K. S. ; Pillipenko, A. I. ; Subrahmanyan, V. ; Dracheva, V. I. ; Exon, N. F.
    The present geomorphological and seismic investigations were done to map structural elements, the nature and temporal distribution of sediments, and the tectonics of the Argo Abyssal Plain region. Study of the sediments helps decipher the ongoing tectonic processes at the time of their deposition. -from Authors
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    Mesozoic anomalies in the Bay of Bengal
    ( 1994-01-01) Ramana, M. V. ; Nair, R. R. ; Sarma, K. V.L.N.S. ; Ramprasad, T. ; Krishna, K. S. ; Subrahmanyam, V. ; D'Cruz, Maria ; Subrahmanyam, C. ; Paul, John ; Subrahmanyam, A. S. ; Chandra Sekhar, D. V.
    The analysis of 8200 line km of total magnetic intensity data in the Bay of Bengal, northeastern Indian Ocean, revealed the presence of approximately N30°E-trending seafloor spreading type magnetic anomalies. These anomalies resemble the Mesozoic anomaly series (M11-M0) reported elsewhere. The oldest anomaly (M11, 132.5 Ma) identified close to the east coast of India is followed by the younger series of Mesozoic anomalies towards the offshore. Some of the anomalies are offset by 60-80 km. The configuration of the offsets of the isochrons allowed us to propose approximately N120°E-trending oceanic fracture zones. The Mesozoic crust of 132.5-118 Ma is estimated to evolve with an average half-spreading rate of 3.5 cm/yr, except for the ocean floor between the M9 and M4 magnetic isochrons. This part of the crust appears to be affected by the buried subsurface 85°E Ridge. © 1994.
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    Evidence for seafloor spreading in the Laxmi Basin, northeastern Arabian Sea
    ( 1994-01-01) Bhattacharya, G. C. ; Chaubey, A. K. ; Murty, G. P.S. ; Srinivas, K. ; Sarma, K. V.L.N.S. ; Subrahmanyam, V. ; Krishna, K. S.
    Marine magnetic anomaly data from the Laxmi Basin for the first time reveal the presence of fairly correlatable NNW-trending magnetic lineations. These magnetic lineations are symmetric about a central negative magnetic anomaly and the axis of symmetry coincides with a characteristic short-wavelength free-air gravity low. The anomalies are interpreted as representing a two-limbed seafloor spreading sequence which can be equated to the A28-A33 interval of the geomagnetic polarity reversal timescale. These results suggest that the Laxmi Basin is underlain by an oceanic crust. Evidence of seafloor spreading in this basin possibly implies a pre-A27 spreading episode in the evolutionary history of the Arabian Sea. © 1994.
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    Forward modeling: Gravity anomalies of two-dimensional bodies of arbitrary shape with hyperbolic and parabolic density functions
    ( 1994-01-01) Visweswara Rao, C. ; Chakravarthi, V. ; Raju, M. L.
    Computer programs in FORTRAN 77 to compute the gravity anomaly of a two-dimensional (2-D) body of irregular cross section with hyperbolic and parabolic variations in density contrast are developed and presented. The gravity anomaly of San Jacinto Graben, California, using a hyperbolic function and that of Los Angeles Basin, California, using a parabolic function, are computed and compared with respective observed anomalies. © 1994.
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    Identification and origin of a subsurface ridge on the continental margin of western India
    ( 1994-01-01) Krishna, K. S. ; Murty, G. P.S. ; Rao, D. Gopala
    Gravity and magnetic studies in the eastern Arabian Sea reveal the presence of subsurface and exposed ridges on and beyond the continental shelf of western India. Distinct patterns of geophysical anomalies are associated with the ridges and also reveal the presence of magnetic polarity reversals. The portions of negative and positive magnetic anomalies of the ridges indicate that the northern part was formed in normal polarity time and the southern part in reversed polarity time. The boundary line of magnetic polarity reversals is marked along 15°10′N. Further it is conjectured that these ridges probably originated because of the movement of the Indian Plate over the Reunion hotspot, during the Late Cretaceous and Early Tertiary. © 1994.
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    Structural interpretation of the Konkan basin, southwestern continental margin of India, based on magnetic and bathymetric data
    ( 1994-03-01) Subrahmanyam, V. ; Krishna, K. S. ; Murthy, G. P.S. ; Rao, D. Gopala ; Ramana, M. V. ; Rao, M. Gangadhara
    Magnetic and bathymetric studies on the Konkan basin of the southwestern continental margin of India reveal prominent NNW-SSE, NW-SE, ENE-WSW, and WNW-ESE structural trends. The crystalline basement occurs at about 5-6 km below the mean sea level. A mid-shelf basement ridge, a shelf margin basin, and the northern extension of the Prathap Ridge complex are also inferred. The forces created by the sea-floor spreading at Carlsberg Ridge since late Cretaceous appears to shape the present-day southwestern continental margin of India and caused the offsets in the structural features along the preexisting faults. © 1994 Springer-Verlag.
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    Magnetic studies in the northern Bay of Bengal
    ( 1994-06-01) Ramana, M. V. ; Subrahmanyam, V. ; Krishna, K. S. ; Chaubey, A. K. ; Sarma, K. V.L.N.S. ; Murty, G. P.S. ; Mittal, G. S. ; Drolia, R. K.
    Total magnetic intensity and bathymetric surveys were carried out in the northern Bay of Bengal between 6° to 11° 45′ N latitudes and east of 84° to 93° 30′ E longitudes. The hitherto known 85° E Ridge is characterised as a subsurface feature by a large amplitude, positive magnetic anomaly surrounded by Mesozoic crust. A newly identified NE to NNESSW trending magnetic anomaly between 7° N, 87° 30′ E and 10° 30′ N, 89-90° E may be one of the unidentified Mesozoic lineations in the northern Bay of Bengal. The Ninetyeast Ridge is not associated with any recognizable magnetic anomaly. The Sunda Trough to the east of the Ninetyeast Ridge is characterised by a positive magnetic anomaly. A combined interpretation, using Werner deconvolution and analytical signal methods, yields basement depths ~ 10 km below sea level. These depths are in agreement with the seismic results of Curray (1991). © 1994 Kluwer Academic Publishers.
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    Analysis of multi-channel seismic reflection and magnetic data along 13° N latitude across the Bay of Bengal
    ( 1994-06-01) Rao, D. Gopala ; Bhattacharya, G. C. ; Ramana, M. V. ; Subrahmanyam, V. ; Ramprasad, T. ; Krishna, K. S. ; Chaubey, A. K. ; Murty, G. P.S. ; Srinivas, K. ; Desa, Maria ; Reddy, S. I. ; Ashalata, B. ; Subrahmanyam, C. ; Mital, G. S. ; Drolia, R. K. ; Rai, S. N. ; Ghosh, S. K. ; Singh, R. N. ; Majumdar, R.
    Analysis of the multi-channel seismic reflection, magnetic and bathymetric data collected along a transect, 1110 km long parallel to 13° N latitude across the Bay of Bengal was made. The transect is from the continental shelf off Madras to the continental slope off Andaman Island in water depths of 525 m to 3350 m and across the Western Basin (bounded by foot of the continental slope of Madras and 85° E Ridge), the 85° E Ridge, the Central Basin (between the 85° E Ridge and the Ninetyeast Ridge), the Ninetyeast Ridge and the Sunda Arc. The study revealed eight seismic sequences, H1 to H8 of parallel continuous to discontinuous reflectors. Considering especially depth to the horizons, nature of reflection and on comparison with the published seismic reflection results of Curray et al. (1982), the early Eocene (P) and Miocene (M) unconformities and the base of the Quaternary sediments (Q) are identified on the seismic section. Marked changes in velocities also occur at their boundaries. In the Western Basin the acoustic basement deepening landward is inferred as a crystalline basement overlain by about 6.7 km of sediment. In the Central Basin possibly thicker sediments than in the Western Basin are estimated. The sediments in the Sunda Arc area are relatively thick and appears to have no distinct horizons. But the entire sedimentary section appears to be consisting of folded and possibly faulted layers. The comparatively broader wavelength magnetic anomalies of the Central Basin also indicate deeper depth of their origin. Very prominent double humped feature of the 85° E Ridge and broad basement swell of the Ninetyeast Ridge are buried under about 2.8 km thick sediments except over the prominent basement high near 92° E longitude. The positive structural relief of the buried 85° E Ridge in the area is reflected in magnetic signature of about 450 nT amplitude. Flexural bulge of the 85° E Ridge and subsidence of the Ninetyeast Ridge about 24 cm my-1 rate since early Eocene period have been inferred from the seismic sequence analysis. © 1994 Kluwer Academic Publishers.
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    Late Archaean crust-mantle interactions: geochemistry of LREE-enriched mantle derived magmas. Example of the Closepet batholith, southern India
    ( 1995-01-01) Jayananda, M. ; Martin, H. ; Peucat, J. J. ; Mahabaleswar, B.
    The Closepet batholith in South India is generally considered as a typical crustal granite emplaced 2.5 Ga ago and derived through partial melting of the surrounding Peninsular Gneisses (3.3 to 3.0 Ga). In the field, it appears as a composite batholith made up of at least two groups of intrusions. (a) An early SiO2-poor group (clinopyroxene quartz-monzonite and porphyritic phyritic monzogranite) is located in the central part of the batholith. These rocks display a narrow range in both initial 87Sr/86Sr (0.7017-0.7035) and e{open}Nd(-0.9to -4.1). (b) A later SiO2-rich group (equigranular grey and pink granites) is located along the interface between the SiO2-poor group and the Peninsular Gneisses. They progressively grade into migmatised Peninsular Gneisses, thus indicating their anatectic derivation. Their isotopic characteristics vary over a wide range (87Sr/86Sr ratios=0.7028-0.7336 and e{open}Nd values from-2.7 to-8.3, at 2.52 Ga). Field and geochronological evidence shows that the two groups are broadly contemporaneous (2.518-2.513 Ga) and mechanically mixed. This observation is supported by the chemical data that display well defined mixing trends in the e{open}Sr vs e{open}Nd and elemental variation diagrams. The continuous chemical variation of the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end-members derived from different source regions (enriched peridotitic mantle and Peninsular Gneisses). It is proposed that the intrusion of mantle-derived magmas into mid-crustal levels occurred along a transcurrent shear zone; these magmas supplied additional heat and fluids that initiated anatexis of the surrounding crust. During this event, large-scale mixing occurred between mantle and crustal melts, thus generating the composite Closepet batholith. The mantle-derived magmatism is clearly associated with granulite facies metamorphism 2.51±0.01 Ga ago. Both are interpreted as resulting from a major crustal accretion event, possibly related to mantle plume activity. © 1995 Springer-Verlag.
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    Age of the Holenarsipur Greenstone Belt, relationships with the surrounding gneisses (Karnataka, south India)
    ( 1995-01-01) Peucat, J. J. ; Bouhallier, H. ; Fanning, C. M. ; Jayananda, M.
    A rhyolitic volcanic flow included in the Holenarsipur greenstone belt, has a simple igneous population of zircons which has a SHRIMP 207Pb/206Pb age of 3298 ± 7 Ma, interpreted as the crystallization age of this flow and considered synchroneous with deposition of the coeval sedimentary pile. The Holenarsipur greenstone belt is now documented as the oldest supracrustal accumulation in the Dharwar craton. From field and other published geochronological results it is concluded that a large part of the surrounding gneisses are very close in age to this rhyolite. -from Authors
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    Tectonic model for the evolution of oceanic crust in the northeastern Indian Ocean from the Late Cretaceous to the early Tertiary
    ( 1995-01-01) Krishna, K. S.
    Bathymetry and magnetic studies in the northeastern Indian Ocean revealed seafloor topographic features, magnetic lineations and abandoned spreading centers. -Authors
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    Modelling of density interface with binomial density variation
    ( 1995-01-01) Chakravarthi, V.
    The concept of variable density plays an important role in interpreting the gravity anomalies of sedimentary basins and in obtaining the real basement depth values. A second-order binomial density function has been introduced to model the rapid density variation of sedimentary rocks with depth. A recursive analytical expression for the gravity anomaly of an out-cropping two-dimensional vertical prism is then derived. Based on the expression, a computer-based algorithm is developed to solve the structure of the sedimentary basin from its gravity anomalies. The sedimentary strata above the density interface are approximated by a series of juxtaposed prisms and their thicknesses are adjusted accordingly until the theoretical anomalies fit the observed anomalies. The first term of the binomial density function is used in the Bouguer slab formula to obtain the initial depth estimates of the basin. The gravity anomaly profile of the Godavari basin, India, where the density of sedimentary rocks is found to vary with depth, is interpreted. © 1995.
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    Archaean high-grade gneiss complex from Satnur-Halagur-Sivasamudram areas, Karnataka, southern India: petrogenesis and crustal evolution
    ( 1995-01-01) Mahabaleswar, B. ; Jayananda, M. ; Peucat, J. J. ; Swamy, N. S.
    The high-grade gneiss complex of the Satnur-Halagur-Sivasamudram area in southern Karnataka forms a part of amphibolite-granulite facies transition zone of southern India. The major lithologies are metasediments, amphibolite facies gneisses, foliated charnockites, mafic granulites, granite sheets. All these lithologies show a prominent N-S fabric which appears to have been produced during late Archaean shear deformation. Rb-Sr whole rock isochron and U-Pb zircon and monazite ages suggest that much of the crust accreted during 2.96 Ga magmatic event followed by the 2.5 Ga granulite facies metamorphism; the occurrence of a previous high-grade metamorphism close to 2.9 Ga remains debatable. -from Authors
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    Gravity modelling of an interface above which the density contrast decreases hyperbolically with depth
    ( 1995-01-01) Visweswara Rao, C. ; Raju, M. L. ; Chakravarthi, V.
    An interpretational procedure for determining the depth of an interface underlying sediments whose density contrast decreases hyperbolically with depth is developed. The approximate depth of the interface at each gravity station is calculated by using the gravity formula of an infinite slab with a hyperbolic density contrast. Based on these depth values, the sediment-basement interface is replaced by an n-sided polygon. Its gravity anomaly is computed by using the formula of Visweswara Rao et al. (1994). These depth values are refined using the procedure of Bott (1960). © 1995.