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Rao, M. B. S. V.
- Godavari Basin - A Geophysical Study
Authors
1 National Geophysical Research Institute, Hyderabad 500007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 30, No 6 (1987), Pagination: 469-476Abstract
Detailed gravity and magnetic surveys in and around the Godavari basin have helped in delineating the subsurface structures which, in turn, facilitated reconstructing its evolution. The general NW-SE trend of the magnetic and the gravity anomalies towards north takes a swift turn along an E-W line south of Bhadrachalam. This may be attributed to a deep-seated fault dividing the basin into Godavari and Chintalapudi sub-basins. The same fault may be responsible for limiting the southern extent of Pakhal sediments. A similar deep-seated fault north of Bhadrachalam along the Mailaram 'high' is also indicated. Modelling of some critical gravity and magnetic anomalies indicates the maximum thicknesses for Gondwana and Sullavai sediments as 4.5-5 km and 1.5-2 km respectively. A large gradient in the Bouguer anomaly on the eastern side of the Gondwana basin suggests the existence of a long continuous normal fault which is the master fault of the Gor.dwana rift valley. The other side of the basin is characterised by variations in the gradient of the Bouguer anomaly suggesting enechelon faulting.
The Gondwana basin is flanked on either side by heavy rocks of density 2.95 g/cm3 between the depths of 6km to 13 km which might be due to the block uplift of the lower crust forming the 'shoulders' of the rift valley or large sub-basic intrusions in the region. The definite magnetic anomalies in this region providing a high susceptibility of 0.1-0.2 emu almost at the same depth as obtained from the Bouguer anomaly further substantiate them. The subsequent uplifts of the basin is apparent from 'highs' in Bouguer anomaly inside the basin such as around Chinnur which shows sharp gradients on either side suggesting faulted margins. The Regional Bouguer anomaly separated from the observed field along a few representative profiles suggests the presence of a high density (3.1 g/cm3) material along the Moho between the depths of 30km to 38km.
The magnetic data reveals several basement ridges and intrusions as far west as. Manthani and Sirpur-Kagaznagar which are transverse to the general trend of the Gondwana basin and almost parallel to the eastern ghat trends suggesting that they may be the subsurface reflections of eastern ghat orogeny. A well-defined magnetic anomaly in the Chintalapudi sub-basin suggests the depth to the basic intrusion in the basement as 3.1 kilometres. There are high magnetic anomalies along the Precambrian faults defining the Purana basin suggesting basic intrusions along them. The basin as a whole depicts a high heat flow specially its southern part which might be due to several intrusive bodies as inferred from the magnetic and the gravity studies. The Godavari basin is a typical rift structure formed during the Gondwana period. The high density material along the Moho and in the upper crust suggests the diapiric upwelling of the asthenosphere as the main cause for the development of a typical continental graben in this region and subsequent uplifts along the shoulders.
