Proceedings of the Indian Academy of
Sciences
Earth and Planetary Sciences
Vol. 109, No. 3, September 2000
Abstracts
Ocean
circulation in the tropical Indo-Pacific during early Pliocene (5.6 -
4.2 Ma): Paleobiogeographic and isotopic evidence
M S Srinivasan and D K
Sinha
Department
og Geology Banaras Hindu University, Varanasi 221 005, India
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 and O P SHARMA
Centre for Atmospheric Sciences, Indian Institute of
Technology, New Delhi 110 016, India
*Present address: Mody College of Engineering and Technology,
Lakshmangarh 332 311,
Sikar (Rajasthan), India.
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_ _ 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* and WON-KYU PARK**
*Birbal Sahni Institute of Palaeobotany, Lucknow 226 007,
India.
**Department of Forest Products, College of Agriculture, Chungbuk
National University,
Cheongju 361 763, Republic of Korea.
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 1 and S BHATTACHARYA 2
1 Department of Geological Sciences, Jadavpur University, Calcutta, India
2 Geological Studies Unit, Indian Statistical Institute, Calcutta, India
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 of F3 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 pressure- temperature 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 1 , ASEEM MIGLANI 2 and N R GARG 3
1 Department of Mathematics, Kurukshetra University, Kurukshetra 136 119, Haryana, India.
2 Department of Mathematics, M. M. Engineering College, Mullana 133 203, Ambala, Haryana, India.
3 Department of Mathematics, Maharshi Dayanand University, Rohtak 124 001, Haryana, India.
Aeromagnetic study of peninsular India
P HARIKUMAR, MITA RAJARAM* and T S BALAKRISHNAN
Indian Institute of Geomagnetism, Colaba, Mumbai 400 005, India.
e-mail: mita@iig.iigm.res.in
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.