Gravity and Magnetic Signatures of Proterozoic Rifted Margins: Bundelkhand Craton and Bijawar and Mahakoshal Group of Rocks and Vindhyan Basin and their Extension under Ganga Basin

Gravity highs surrounding Bundelkhand Craton and three sets of sub parallel paired aeromagnetic anomalies along its SE margin are attributed to High-Density, high susceptibility volcano sedimentary sequences and mafic intrusives of Bijawar Group of rocks of Paleo-Proterozoic period. Modelling of airborne magnetic anomahes provide almost vertical intrusives of high susceptibility (1 x 10^-3emu) mafic rocks at a depth of about 850m below northern margin of Vindhyan sediments in the basement which is supported from high velocity rocks at the same depth in an adjoining seismic section. Both these anomalies are typical of those observed along rifted continental shelves world over as in case of western margin off India, Norway, Red Sea Rift, etc.

Gravity highs and linear magnetic anomalies along southern margin of the Vindhyan basin are attributed to mafic and ultramafic intrusives of older Mahakoshal group of rocks with volcano sedimentary sequences of sectoral nature at the rifted margin of Bundelkhand craton similar to canyon deposits along present day continental rise and slope. The undisturbed Vindhyan sediments of Meso-Neo-Proterozoic period were deposited on the platform provided by continental shelf of Bundelkhand craton during convergence as fore land basin. This is in conformity with relative ages of Mahakoshals, Bijawars and Vindhyan sediments as they formed during rifting and convergence phases, respectively. Volcanic plugs (~1.11 Ga) associated with northern margin of Vindhyan basin (Panna diamond belt) and Mahakoshal Group of rocks (Jungel pipes) towards the south lying with in zones of high magnetic anomahes (mafic/ultramafic rocks referred to above) might be related to the same rifting process, but extruded to surface at later dates. Gravity high east of Aravalli Delhi Mobile Belt forming the Agra-Shahjahnpur ridge bounded by northward extensions of Great Boundary Fault and Chambal fault, is attributed to rocks equivalent to Mahakoshal Group of rocks along the western margin of the Vindhyan basin extending under the Ganga basin up to the Himalayan front in Western Nepal where seismically active zone of Western Himalaya starts.