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

N. Srinivasulu ¹, A. Shivakeshavalu ¹, G. Ramadass ¹, I. B. Ramaprasada Rao ¹

Affiliations
1 Centre of Exploration Geophysics, Osmania University Hyderabad - 500 007, IN

Abstract

No Abstract.

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G. Ramadass ¹, I. B. Ramaprasada Rao ¹, D. Himabindu ²

Affiliations
1 Center of Exploration Geophysics, Osmania University, Hyderabad - 500 007, IN
2 CSIR Research Associate, Center of Exploration Geophysics, Osmania University Hyderabad - 500 007, IN

Abstract

Regional gravity investigations were carried out in the northern part of Dharwar craton along a 600 km long Jadcharla-Raichur-Gadag-Hubli-Dharwar-Goa transect to obtain a clearer perception of the deep geological structure and to throw light on some of the debatable aspects such as the contact between the eastern and western Dharwar cratons From qualitative analysis of the horizontal and vertical gradients and analytical signal amplitude computed from the Bouguer gravity, the boundaries of the supracrustals were delineated

The polynomial fitting technique was used to obtain the regional (long wavelength component of Bouguer gravity) to infer deep-seated features The 5^th order polynomial fit of the anomaly curve was modeled wlth GM-SYS software to obtain the crustal structure along the transect From the crustal upwelling/downwarps as indicated by modeling and the first horizontal gradient of regional Bouguer gravity the locations of four deep seated faults were determined Running west to east, the first of these, the west coast fault,is located near Ponda at station 34 The second, third and fourth faults, situated at stations 198, 307 and 530, correspond to the Bababudan-Nallur shear, the western margin of the Closepet batholith and the Makthal schist belt, respectively The structural configuration so obtained, contrary to established hypotheses of the geological classification of the craton into the eastern and western blocks, prompted to a modified tectonic classification of the craton into the western, an upthrust intervening, and eastern Dharwar blocks, whlch provided a new insight in to the structural configuration of the area While the average crustal thickness obtained for the western and eastern Dharwar blocks are 39 Km and 37 km respectively, the crustal thickness beneath the Chitradurga thrust fault, which lies in the middle of the intervening block IS 34 5 km.

Keywords

Gravity studies, Jadcharla-Goa subtransect, Dharwar Craton.

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G. Ramadass ¹, I. B. Ramaprasada Rao ¹, N. Srinivasulu ¹, D. Himabindu ¹

Affiliations
1 Centre of Exploration Geophysics, Osmania University, Hyderabad - 500 007, IN

Abstract

The Dharwar craton constitutes part of one of the most significant Precambrian shields in the world. Different episodes of deformation undergone by the craton make it difficult to decipher its complex geology. Density studies were conducted along the Jadcharla-Goa subtransect (820 samples), which cuts across the northern part of the craton to understand the geological nature of the region as well as provide a sound base for gravity surveys. Modal densities of 2.65 gm/cc, 2.92 gm/cc and 2.57 gm/cc were obtained for the three major constituents in the region - peninsular gneisses, schists and Closepet granites, respectively. Application of the Kolmogorov-Smimov statistical test on samples from the western and eastern Dharwars along the transect as a whole, as well as from Gadag (western Dharwars) and Makthal (eastern Dharwars) areas indicated that they differed in density values, suggesting they belonged to different population distributions. This, in the light of recent contrary studies, lends support to the concept of division of the Dharwar craton into two groups.

Keywords

Density studies, Jadcharla-Goa subtransect, Dharwar craton, South India.

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I. B. Ramaprasada Rao ¹, M. Venkata Chary ¹, Ram Raj Mathur ¹, S. Srinivas ²

Affiliations
1 Centre of Exploration Geophysics, Osmania University, Hyderabad - 500 007, IN
2 BHP Pvt LTd, IN

Abstract

Insitu electrical conductivity measurements were made over Deccan Traps using EM38 ground conductivity meter The equipment is a light weight dipole - dipole system wIth a coil separation of 1 m utilizing a frequency of 146KHz The measurements were made over chosen lava flow Units viz, compact basalt, amygdaloidal basalt and redbole The statistical analysis shows that (1) the mean resistivity of amygdaloldal basalt is 38 Ωm (range 25-42 Ωm), (2) the mean resistivity of the compact basalt under whIch a thm redbole is present is 113 Ωm, (range 105 - 131 Ωm), (3) the mean resistivity of red bole which is present under the compact basalt is 11 Ωm (range 10-13 Ωm) and (4) all the lnsitu resistivities show lognormal dIstnbution.

The EM approach conductivity values (converted to resistivity values) compare well with the earher reported mwu resistivity values usmg DC resistivity measurements (Sathyamurthy, 1985) However, the values of the resistivity obtamed on bulk samples In the laboratory show markedly higher values Synthetic MT/CSAMT data generated using the insitu resistivity values obtamed via EM approach is utilized to estimate the errors In the Inverted geoelectnc parameters of resistivity and thickness.

Keywords

EM Conductivity, Insitu Rocks, Deccan Traps.

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