• Volume 109, Issue 3

      September 2000,   pages  315-391

    • Ocean circulation in the tropical Indo-Pacific during early Pliocene (5.6 - 4.2 Ma): Paleobiogeographic and isotopic evidence

      M S Srinivasan D K Sinha

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      A Comparison of late Neogene planktic foraminferal biogeography and stable isotopic records of shallow dwelling and deep dwelling planktic foraminifera from DSDP sites 214 (Ninetyeast Ridge, northeast Indian Ocean) and 586B (ontong-Java Plateau, western Equatorial Pacific) provides a clue to the nature of the ocean circulation in the tropical Indo-Pacific during early Pliocene.

      The Present study reveals that the late Neogene planktic foraminiferal data from the eastern and western sides of the Indonesian Seaway are very similar. The only distinct inter-ocean difference however is the absence of Pulleniatina spectablis from the Indian Ocean. This species makes its first evolutionary appearance in the Equatorial Pacific at about 5.6 Ma (Early Gilbert reversed) and ranges up to 4.2 Ma (Top Conhiti subchron). The complete absence of Pulleniatina spectablis from the Indian Ocean is attributed to blocking of westward flow of tropical waters of the Pacific to the Indian Ocean resulting in a major change in the tropical Pacific and Indian oceans during 5.6 to 4.2 ma.

      In order to understand the nature of this blockage, isotopic depth ranking of selected planktic foraminifera and thus may be interpreted that the shallow sills that mark the Seaway in modern times were present as early as 5.6 Ma.

      The distribution of Pulleniatina spectablis throughout the Equatorial Pacific reveals that Modern Equatorial Pacific Under Current (Cromwell Current) flowing towards east at a depth of 200-300 m (which is also the depth habitat of Pulleniatina spectablis) was present at the beginning of the Pliocene (5.6 Ma).

      As a dequel to the blocking of the Indonesian Seaway and the resultant interruption in the flow of central Equatorial Current System of the Pacific to the west there was an increase in the western Pacific Warm Pool Waters and strengthening of the gyral circulation in the Pacific and Indian Oceans. This eventually triggered the intensification of the Asian Monsoon System.

    • Response of an ocean general circulation model to wind and thermodynamic forcings

      A Chakraborty H C Upadhyaya O P Sharma

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      The stretched-coordinate ocean general circulation model has been designed to study the observed variability due to wind and thermodynamic forcings. The model domain extends from 60‡N to 60‡S and cyclically continuous in the longitudinal direction. The horizontal resolution is 5‡ x 5‡ and 9 discrete vertical levels. First a spin-up experiment has been done with ECMWF-AMIP 1979 January mean fields. The wind stress, ambient atmospheric temperature, evaporation and precipitation have been used in order to derive mechanical and thermodynamical surface forcings. Next, the experiment has been extended for another 30 years (3 cycles each of 10 year period) with varying surface boundary conditions (from January 1979 to December 1988 of ECMWF-AMIP monthly fields for each cycle) along with 120 years extended spin-up control run's results as initial conditions. The results presented here are for the last 10 years simulations. The preliminary results of this experiment show that the model is capable of simulating some of the general features and the pattern of interannual variability of the ocean.

    • Precipitation reconstruction using ring-width chronology of himalayan cedar from western himalaya: Preliminary results

      Ram R Yadav Won-Kyu Park

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      Himalayan cedar (Cedrus deodara (D. Don) G. Don) due to its long age and wide ecological amplitude in the Himalayan region has strong dendroclimatic potential. A well replicated ring-width chronology of it, derived from the ensemble of tree-ring samples of two adjacent homogeneous sites, has been used to reconstruct precipitation for the non-monsoon months (previous year October to concurrent May) back to AD 1171. This provides the first record of hydrological conditions for the western Himalayan region, India during the whole of the ‘Little Ice Age’ and latter part of the ‘Medieval Warm Period’. The reconstruction revealed the wettest and the driest non-monsoon months during the fourteenth and the thirteenth centuries, respectively. The seventeenth century consistently recorded dry non-monsoon months in the western Himalayan region. Surplus precipitation, especially more pronounced since the 1950s, is recorded in the current century.

    • Diverse signatures of deformation, pressure-temperature and anatexis in the Rayagada sector of the Eastern Ghats granulite terrane

      S K Sen S Bhattacharya

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      The granulites and granitoids around Rayagada in the north central part of the Eastern Ghats belt display structural and petrological differences when compared to similar rocks from Chilka and Jenapore in the northern Eastern Ghats. The impress of F1 deformation is almost erased while that ofF3 is muted. The metapelites have a restricted chemical range and are non-migmatitic. There are two varieties of leptynitic granitoids, one of which is interlayered with yet another S-type granite containing cordierite. The maximum recorded temperature from geothermometers is 780‡C, but the magnitude of pressure is comparatively low, the highest value being 6.3 kbar. Another distinctive feature of the pressuretemperature record is the absence of evidence of decompression in the lower realms of pressure and temperature. Metamorphic reactions that could be identified indicate cooling, a noteworthy reaction being the sillimanite to andalusite transformation. Integration of data from pressure-temperature sensors suggest cooling at two pressures, 6 and 5 kbar. The generation of two types of granitoids from metapelites is interpreted to be due to intersection with solidus curves for pelitic and graywacke-like compositions, constrained by recent experiments, at 6 and 5 kbar. The first melting occurred on a prograde path while the second one was due to increase in temperature during exhumation at tectonic rates.

      Thus inspite of a broad similarity in the geodynamic scenario across the northern part of the Eastern Ghat belt, differences in exhumation rates and in style of melting were responsible for producing different signatures in the Rayagada granulite terrane.

    • Plain strain problem of poroelasticity using eigenvalue approach

      Rajneesh Kumar Aseem Miglani N R Garg

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      A plain strain problem of an isotropic elastic liquid-saturated porous medium in poroelasticity has been studied. The eigenvalue approach using the Laplace and Fourier transforms has been employed and these transforms have been inverted by using a numerical technique. An application of infinite space with concentrated force at the origin has been presented to illustrate the utility of the approach. The displacement and stress components in the physical domain are obtained numerically. The results are shown graphically and can be used for a broad class of problems related to liquid-saturated porous media.

    • Aeromagnetic study of peninsular India

      P Harikumar Mita Rajaram T S Balakrishnan

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      The degree sheet Aeromagnetic maps up to 17‡N, acquired from the Geological Survey of India, have been manually redigitised at 6 minute intervals to study the long wavelength anomalies over peninsular India. These data have been collected at different survey altitude, epochs, flight line directions, etc. Great care has been taken to correct the total field map and remove the contribution due to the core field and prepare an accurate crustal anomaly map. For the first time, a regional map, depicting the NW-SE structural features north of the orthopyroxene isograd with the essentially E-W features to the south of it and revealing several well known structures, is presented. The analytical signal is calculated to delineate the source fields of these anomalies. It dramatically maps the charnockites and is able to delineate the orthopyroxene isograd. In the Dharwar region the magnetic signatures are associated with the intrusives/ iron ore bodies. Thus, we find that the source rocks of the aeromagnetic anomalies are the host province of charnockites in the SGT and the intrusives/iron ore bodies in the Dharwar belt. Gravity residuals are calculated and a tectonic map of the region is presented from the combined geopotential data.

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