• Mita Rajaram

      Articles written in Journal of Earth System Science

    • Induction by long period geomagnetic variations in the Indian sub-continent

      Mita Rajaram B P Singh S Y Waghmare

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      In the present paper storm time variations and 27-day geomagnetic periodicity have been analysed to estimate the depth of the substitute conductor, assuming an infinitely (super) conducting core model of the earth. The advantage of using data from a restricted longitude range is that the uncertainties arising from lateral contrasts in the upper mantle and contributions from Sq current systems are considerably reduced. The result of the present analysis, which has been done in the time domain, gives a value of 522 km for the depth of the substitute conductor in case of storm time variations which rises to 870 km for 27-day recurrent storms. A higher value of the depth for 27-day variations indicate that the rise in conductivity inside the earth is not like a step function rather is a gradual one. The value of 522 km for storm time variations for the Indian region is smaller than the global average. This is natural to expect because the Indian sub-continent is known to be a tectonically active region.

    • Magsat studies over the Indian region

      B P Singh Mita Rajaram

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      Data collected by Magsat have been extensively used by Indian scientists in studies of the crust beneath India. Results obtained by various workers have been summarized and the reasons for differences in findings have been discussed. It is concluded that methods that work well for higher latitudes do not give the best estimates of crustal field and magnetization in equatorial regions. A better estimate of the crustal component is obtained when the external current contribution is estimated using the symmetry properties of associatedX and Z-fields. Inversion technique that provides stable crustal magnetization in midlatitudes, becomes unstable near the equator. Why such an instability arises and how it can be circumvented are discussed. That the Peninsular shield, the Ganga basin and the Himalayas are three different geotectonic blocks is clearly reflected in the magnetization distribution. A thick magnetic crust under Aravalli, Singhbum and Dharwar suggest these areas to be comparatively stable. In general, seismic, gravity and heat flow data agree characteristically well with the magnetization estimates.

    • 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.

    • A relook into the crustal architecture of Laxmi Ridge, northeastern Arabian Sea from geopotential data

      Nisha Nair S P Anand Mita Rajaram P Rama Rao

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      In this study, we undertake analysis of ship-borne gravity-magnetic and satellite-derived free-air gravity (FAG) data to derive the crustal structure of Laxmi Ridge and adjacent areas. 2D and 3D crustal modelling suggests that the high resolution FAG low associated with the ridge is due to underplating and that it is of continental nature. From Energy Spectral Analysis, five-depth horizons representing interface between different layers are demarcated that match those derived from 2D models. Magnetic sources from EMAG2 data, various filtered maps and absence of underplating in the EW section suggest that the EW and NW–SE segment of the Laxmi Ridge is divided by the Girnar fracture zone and probably associated with different stages of evolution. From the derived inclination parameters, we infer that the region to the north of Laxmi Ridge, between Laxmi and Gop Basins, is composed of volcanic/basaltic flows having Deccan affinity, which might have been emplaced in an already existing crust. The calculated inclination parameters derived from the best fit 2D model suggests that the rifting in the Gop Basin preceded the emplacement of the volcanics in the region between Laxmi and Gop Basins. The emplacement of volcanic/basaltic flows may be associated with the passage of India over the Reunion hotspot.

    • Structural framework of the Wagad uplift and adjoining regions, Kutch rift basin, India, from aeromagnetic data


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      The Kutch sedimentary basin formed during the Late Triassic breakup of Gondwanaland is characterised by horst and graben structures consisting of several east–west trending uplifts surrounded by low-lying plains. The eastern part of the basin has a diverse landscape comprising the Wagad uplift, Banni plain, Island Belt uplift and the Rann of Kutch. This area is bounded by major faults like the South Wagad Fault (SWF), Gedi fault and the Island Belt Fault. The lineaments/faults present in the region at different depth levels and the propagation of these features through the different sedimentary layers are studied using the semi-detailed aeromagnetic data collected over the basin. The aeromagnetic anomaly map depicts several major E–W, NE–SW and NW–SE oriented lineaments/faults, which probably represent structural trends associated with different stages of evolution of this rift basin. Power spectral analysis of the differential reduced to pole magnetic data indicates the presence of four magnetic interfaces. The slopes identified from the 1D power spectra were used for designing matched bandpass filters for isolating and enhancing the magnetic signatures present within those interfaces. Different edge detection techniques were used to delineate the magnetic contacts/faults/lineaments present in those interfaces. In addition, we have computed the radially averaged power spectrum of 121 subset grids each with a dimension of $\rm{20 km \times 20 km}$ from which three magnetic interfaces were delineated and compared with the stratigraphic sequence of the Wagad uplift and adjoining regions. A major NE–SW fault is delineated from this analysis and suggests that this fault has depth persistence as it dislocates the different magnetic interfaces. Integration with stratigraphic data suggests that this fault was formed prior to the deposition of Miocene Kharinadi formation. We have interpreted that this fault, forming the eastern limit of the Banni basin, might have formed during the passage of the Indian plate over the Reunion hotspot. Based on the results of the aeromagnetic data analysis and other published data, we propose a generalised evolutionary model for the study region.

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