January, 2013

January, 2013   ||  Volume 17 No.1


Review Article

Evolution of the Indian Continental Lithosphere: Insights from episodes of crustal evolution and geophysical models


Sreebagh, Ammankoil Road, Kochi- 682035
Email: tmmahadevan@rediffmail.com

The Indian lithosphere is an ensemble of terranes that had distinctive patterns of thermal evolution through time. Recent researches of a global scale, through thermal modeling and study of mantle xenoliths and xenocrysts, have helped to graduate the properties of the continental lithospheres of the world with characteristic properties of essentially lithospheric thickness and density and intimately related aspects of chemistry ( composition) through the three major time bands, broadly the Archaean, the Proterozoic and the Phanerozoic. Viewed from the scales of distinction, the Indian shield lithosphere may be expected to have a dominantly Proterozoic stamp with Archaean lithospheric properties preserved below the Western Dharwar Craton, the Singhbhum Cratonic block and the Bundelkhand cratonic blocks. In the rest of the Shield more dispersed Archaean –Proterozoic lithosphere heterogeneity is envisaged with a poorly preserved Archaean lithosphere. The Phanerozoic impact has been relatively feeble in the south Indian shield and restrictive in the northern parts of the shield but has been sufficient to impart a denser and hotter lithosphere below the latter. An analysis of the geophysical data reveals that the two geophysical parameters, magnetic and gravity more or less conform to expectations of the character of the lithosphere while the velocity structure determined using different rather sophisticated methods provide a disparate picture. A spectrum of seismic thicknesses has been arrived at with a 80-100km thick lithosphere on one end and >200km on the other. Estimations of shallower thickness are a majority. A thin lithosphere is attributed to lithospheric erosion through a basal drag during the post-130Ma drift of the Indian shield and, if true, would erase the expectations of lithospheric properties imbibed through over 3.5 Ga in the span of some 130 million years of drift. Geological contradictions to this conclusion arise from a total absence of any dynamic or magmatic manifestations or a significantly high continental scale heat flow consistent with of such a large scale of erosion except some selected belts of uplift and elevated thermal fields that are related to local extensional structures much older to 130Ma. It seems possible that the low thickness estimates are due to the influence of compositional variations, fluids, partial melting and scattering. Differences in the scales of lithospheric heterogeneity and the wavelengths of geophysical surveys may also have contributed to an uniformly thin lithosphere. This discrepancy underlines the need for integrating more closely spaced seismic velocity studies with the expected compositional heterogeneity across the shield with geological history, geochemistry and potential field and Electro-magnetic signatures down to the 410 discontinuity that may have been the Early Archaean lithospheric thickness.


Grain Scale physical signatures of early Seismio-tectonic occurrences in fault rock: A study in Gajalia fold belt, Tripura (India)

Sudip Dey and Sushmita Paul

Department of Geography and Disaster Management,Faculty of Natural Sciences,
Tripura University,Suryamaninagar-799130
E-mail: sudip_geo@rediffmail.com

This paper attempts to assess the microstructural signatures of early seismio-tectonic occurrences which are preserved in fault rocks. For that purpose some samples were collected from a faulted zone of Gajalia, south Tripura. Digital imaging of the thin sections of those samples was performed for assessing grain scale deformation pattern. It has been observed that the tested samples are characterised by moderate foliations and various shapes of grain particles like round, sub-round, angular and sub-angular. The grain scale deformation patterns in the tested samples clearly show that very complex stress pattern at micro level developed during past seismic events. Fluid flows along the margins of foliations are found which is important for weakening mechanism to initiate seismic slip. From the microstructure of the samples it is assessed that various physical processes like stress development, friction and fluid flow play the key role for brittle deformation of grains during seismio-tectonic events. Finally a conceptual model has been developed to explain the physical mechanism of grain scale deformation during early seismo-tectonic occurrence.


Geomagnetic field variations in India – an analysis by Fuzzy c-means clustering
M.Sridharan1 and A.M.S.Ramasamy2
1Magnetic Observatory, Indian Institute of Geo-magnetism, Pondicherry University Campus, Puducherry- 605 014, India. e-mail:sri26357@gmail.com
2Ramanujan School of Mathematics, Pondicherry University, Puducherry- 605 014, India, e-mail: amsramasamy@gmail.com

The aim of this paper is to identify the pattern of geomagnetic field variations between the internationally declared quiet and disturbed days in the Indian region. The significance of this study is due to the fact that India has a unique set of geomagnetic observatories spanning the magnetic equator and the Sq focus in the region of the world where geomagnetic and geographic planes are least separated.
The technique of fuzzy c-means clustering which is powerful for pattern recognition is employed in this study. The distance between the cluster centers can be taken as a measure of geomagnetic activity. The abnormal variations at Ujjain due to conductivity anomalies, at Trivandrum due to equatorial electro jet current system have been identified. The pattern of variation differs between the geomagnetically quiet and the disturbed periods. The difference in pattern shows anomalous variations at Ujjain. A major geological feature i.e. Narmada-Son lineament, inspite of being tectonically active is not affecting the variations in the vertical Z-component in magnetic records of Ujjain. However, it is found to be significant in the horizontal H component. Analytical technique and the results of the analysis are presented here

Correlative study on Solar activity and all India rainfall: Cycle to Cycle Analysis

R. Samuel Selvaraj1, P.R. Umarani1 and N. Mahalakshmi
1 Dept of physics, Presidency College, Chennai.600 005
Dept of Physics Anna Adarsh College, Chennai.600 040
E- mail: maglev_htc@yahoo.co.in


The solar energy gets absorbed in the atmospheric system and acts as a basic driving force for the climate dynamics. The energy output of the sun varies on all timescales. The best known variation is the regular fluctuations in the number of sunspots, which shows up as small dark regions on the solar disk, and affect the energy output of the sun. This variation has the potential to induce fluctuations in the climate, especially the rainfall. In the present paper we try to establish a relationship between the all India rainfall and the Sunspot numbers. We use a non-parametric statistical rank correlation to find the significance between the sunspot numbers and rainfall. The results indicate an anti correlation between the sunspot and rainfall. The all India rainfall is maximum when the sunspot numbers are minimum and vice versa. This suggests that the climate indeed is influenced by the solar activity. As such its effect cannot be ignored in analysing rainfall cycles  

Satellite derived precipitation estimates over Indian region during southwest monsoons
Harvir Singh1,* and O.P. Singh2
1National Centre for Medium Range Weather Forecasting, Noida-201307, INDIA
2Regional Meteorological Centre, IMD, New Delhi-110003, INDIA
Email: harviriitkgp@gmail.com

Quantitative Precipitation Estimates (QPE) derived from Kalpana-1 Satellite and Tropical Rainfall Measuring Mission (TRMM) rainfall have been compared with observed gridded rainfall over six representative regions of India during southwest monsoon 2009 & 2010. The results have shown that no single satellite precipitation estimates are close to the gridded actual rainfall over all the regions in the same year and over the same region in two different types of monsoon, though good correlation coefficients (CC) exist between the satellite precipitation estimates and actual rainfall. While cumulative seasonal TRMM rainfall for June to September is an under-estimation during deficient monsoon year 2009 by about 6 cm, cumulative seasonal QPE is very close to the actual seasonal rainfall. In a normal monsoon year 2010 cumulative seasonal TRMM rainfall was very close to observed seasonal rainfall whereas the difference between the cumulative seasonal QPE and actual rainfall was large.


South Indian Convergence Zone Model: A new approach to seasonal forecasting of summer monsoon rainfall in India Part III: Forecast for meteorological subdivisions
Onkari Prasad and O.P. Singh¹
43, Ritu Apartments, A-4 Paschim Vihar, New Delhi-110 063
¹India Meteorological Department, Lodi Road, New Delhi-110003
Email:prasadonkar123@yahoo.in; singh.op@imd.gov.in

Long Range Forecast (LRF) of southwest monsoon rainfall for India as a whole is being issued by India Meteorological Department (IMD) since 1886. But issuing LRF of rainfall for smaller spatial and temporal resolutions has remained a challenging task till date. LRF of rainfall, if available at sub-division and district level, could get integrated in agricultural planning, water management strategies and a number of other activities. Issuing subdivision-wise forecasts of rainfall for monthly, bi-monthly periods of June+ July, July+ August, August+ September and for the season as a whole (June-September), and seasonal forecast in the cluster of districts/districts in the subdivision of Tamilnadu and Pondicherry are discussed in the present study. The study has shown that forecasts of southwest monsoon rainfall could be issued for a number of subdivisions with an accuracy of 80% to 100% for monthly forecasts, 70% to 100% for bi-monthly forecasts and 85% to 100% for seasonal forecasts respectively. The accuracy of seasonal forecast for cluster of districts/districts in Tamilnadu and Pondicherry ranges from 70% to 100%.

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