Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

P. K. Agrawal , J. G. Negi

Abstract

An aeromagnetic map of the area west of Koyna (lr23' N, 73°45' E), bounded between latitude 17°5' Nand 17°35' N and longitude 73°15' E and 73°40' E, is spectrally analysed to derive a basement relief model. The model reflects structural heterogeneity conforming well with the existing estimates of trap thickness from other geophysical methods. Two prominent structural features-one near Chatnali and the other to the west of Durga Dongar-are observed which indicate fault-like structures. Between south of Alore and northeast of Mutagaon, a body with high reverse magnetization with its top lying at a depth of about 1200 m is also indicated. These results seem to indicate a limited triggering role of reservoir-induced seismicity in the region and point to the existence of pre-reservoir instability of the Koyna zone.

Keywords

Geophysics, Aeromagnetic Data, Deccan Traps, Koyna, Maharashtra, Spectral Analysis.

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P. K. Agrawal ¹, O. P. Pandey ¹

Affiliations
1 Theoretical Geophysics Group, National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, IN

Abstract

The nature of tectonic configuration between SE coast of India and NW coast of Sri Lanka has been a subject of considerable debate for over past two decades. In the present work, we analyse long wavelength gravity-magnetic, seismological, satellite imagery and related tectonic, geologic and geomorphic information over India, Madagascar, Sri Lanka and Antarctica to suggest that no intracratonic rift could ever develop between India and Sri Lanka. It appears that in the erstwhile Gondwanaland, Sri Lanka rested in the Lutzo-Holm Bay of Antarctica and had the same relative paleoposition with respect to India as it has today, although a small scale translational and rotational motion as suggested by Yoshida et al. (1992) may not be ruled out. The study throws significant light on the paleoassembly of these continental fragments and suggests the possibility of weak mantle upwelling between India and Sri Lanka which may have resisted the formation of oceanic crust between them.

Keywords

Tectonics, Intracontinental Rift, Gondwanaland, Antarctica, Madagascar, Sri Lanka, India.

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O. P. Panday ¹, P. K. Agrawal ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, IN

Abstract

The nature of lithospheric mantle deformation beneath the continental regions is a problem that has attracted global attention. It is being questioned whether the continental segments have remained the same at deeper levels since their evolution or whether recent tectonic and geodynamic events have deformed them. In the present work, we study the lithospheric thermal structure beneath the elongated, anomalous, seismically active and rifted western continental margin of India and its surroundings. Our study, based on the hitherto known evolutionary history of the Indian subcontinent, suggests that the lithospheric mantle beneath this part of the Indian peninsula has been considerably sheared, deformed and weakened due to mainly late Cretaceous - early Tertiary catastrophic and geodynamic events apart from Proterozoic thermal reactivations. Beneath this region, the asthenosphere has upwarped to a depth as shallow as 38 to 80 km due to rise of isotherms caused by subcrustal melting. Our study further reveals that the segments situated north of latitude 16° N are much warmer with shallow Moho depths than those situated south of it.

Keywords

Lithosphere, Mantle Deformation, Continental Margin, Western India.

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