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

D. N. Kanungo ¹, D. S. N. Murthy ¹

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

Abstract

Structural studies of Tethys sediments between Malari and Lapthal show development of many minor structures e.g. - 'cleavages, tension and shear fractures' - which have been utilised to deduce the stress fields responsible for the deformation of these rocks. The primary sedimentary structures such as bedding, current bedding suggest shallow deposition conditions. The shape of folds, the fracture cleavages, quartz and calcite filled tension gashes and other fractures, and the thrusts are all taken as evidence for 'shallow-level' deformation of these rocks in which 'brittle deformation' was dominant.

The study suggests that major compressive stress responsible for the deformation of these rocks was always directed northeast-southwest and was lying sub-parallel to the earth's surface, the P-intermediate was directed NW-SE and was also horizontal, while the P-minimum was always invariably perpendic:ular to the earth's surface. Hence, it is concluded that these rocks have been deformed by horizontal compression and it appears that the major push was from the SW in this part of the ranges.

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D. Guptasarma ¹, T. R. K. Chetty ¹, D. S. N. Murthy ¹, A. V. Ramana Rao ¹, B. Venkatanarayana ¹, N. R. Baker ¹

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

Abstract

No Abstract.

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T. R. K. Chetty ¹, D. S. N. Murthy ¹

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

Abstract

LANDSAT TM data have been used as an aid to studying the regional geology of the granulite terrane in the southern part of Eastern Ghats. The technique in conjunction with regional traverses proved to be useful in identifying regional structural elements, such as shear zones, fold patterns of different scales and other lineament patterns, in poorly exposed and inaccessible areas. A major crustal scale ductile shear zone, designated as Sileru shear zone, with a dextral sense is highlighted and lies at the western margin of the high grade terrane. The other shear zones within the granulite terrane, characterised by migmatitic gneisses and typical L-S mylonitic fabrics, are associated with transpressional tectonics. Fold patterns varying from tight upright isoclinal folds to refolded large scale recumbent folds, along with complex fold interference structures on mesoscopic to regional scale are described. The interpretation of LANDSAT data, followed by limited ground based traverses from Sileru to Visakhapatnam in Visakhapatnam district and from Chinturu to Eleswaram in East Godavari district are the constraints in precisely defining the regional structures and understanding the tectonic processes that fashioned shear zones and fold patterns in granulite terranes. A fold thrust thin-skinned tectonic model involving two tectonothermal events for the study area is envisaged.

Keywords

Granulite Terrane, Tectonics, Eastern Ghats, Andhra Pradesh.

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D. S. N. Murthy ¹, A. M. Dayal ¹, R. Natarajan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-7, IN

Abstract

Petrography and chemistry of newly discovered Chigicherla Kimberlite pipe of Andhra Pradesh, South India are presented. The textural characteristics of the rock suggest it to be more close to the hypabyssal type of kimberlite. Presence of melilite classifies these rocks as extreme variety of kimberlite. Mineralogy of the spinel harzburgite xenoliths that occur within these IGmberlite reveals two types of xenoliths:

Harzburgite xenoliths having spinel which is Cr rich (61.6%) and Al (10.29%) poor. Harzburgite xenolith having spinel which is A1 rich (4358%) and Cr (25.90%) poor.

Temperature estimates reveal that the Cr-rich type xenolith has equilibrated at 972°C while the Cr-poor ones were equilibrated at 839°C ± 31°C. The maximum pressure estimated for these xenoliths is 25 kb. Presence of chrome spinel and absence of magnesium aluminium spinel in the xenoliths, indicate that the uppermost 45 or 50 km of the mantle section has highly "depleted" peridotite composition. The study supports the contention of the steady state of thermal condition in the mantle lithosphere beneath the continental shield and has not changed appreciably through the last one billion years.

Keywords

Kimberlite, Geothermometty, Anantapur District, Andhra Pradesh.

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D. S. N. Murthy ¹, A. M. Dayal ¹, R. Natarajan ¹, V. Balram ¹, P. K. Govil ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500007, IN

Abstract

Newly discovered kimberlite pipe 11 of Chigicherla area of Andhra Pradesh consists of two generations of olivine with phlogopite, spinel. perovskite, melilite etc., A comparative study of chemical characterstics of pipe 11 with those of pipe 1,2,3,4 and 5 of Wajrakarur- Lattavaram area and pipe 10 of Chighicherla indicate that Zr-Hf, Nb-Ta, Th-U have a high degree of correlation. The ratio of Ba/Nb( <10), Z/Hf( -50) and Nb/Zr (>0.5) are similar to the Group-I kimberlites. The chondri te normalised REE pattern of pipe 11 is similar to the other kimberlites and shows enrichment in LREE. Spider diagram of incompatible elements indicate similarity with those of many oceanic island alkalic basalts or other types of alkalic intraplate volcanics. They are highly enriched in incompatible elements. Major elements, Ce/Yb and La/Yb ratios suggest that the formation of kimberlite is due to low degree of partial melting of ultrabasic source along with an earlier subducted oceanic lithosphere.

Keywords

Petrology, Kimberlite, Phlogopite, Andhra Pradesh.

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A. M. Dayal ¹, D. S. N. Murthy ¹

Affiliations
1 National Geophysical Research institute, Hyderabad 500 007, IN

Abstract

Geochemistry of Lower Pakhal shales shows higher content of Si0₂ and A1₂0₃ and negligible amount of CaO and Na₂O. The Cr/Ni, Ti/Zr and Zr/Y ratios and chondrite normalized REE fractionation pattern in these shales are comparable to the Post-Archaean Australian shale (PAAS) Distribution of La-Th-Sc, Th-Sc-Zr/lO and Ti/Zr vs La/Sc ratios and association of these shales with quartzites and dolomites indicate that they were deposited in an intracratonic basin. The study reveals that the source rocks for these sediments were granitic in composition and the adjoining Archaean gneissic terrain is the predictable provenance. The fractionated LREE and negative Eu anomaly indicates either cratonic basin or a shallow water passive continental environment during deposition.

Keywords

Geochemistry, Provenance, Pakhal Shales, Pakhal Supergroup, Andhra Pradesh.

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M. V. M. S. Rao ¹, D. S. N. Murthy ¹, G. M. Nagaraja Rao ², S. K. Mohanty ², S. Udayakumar ²

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN
2 National Institute of Rock Mechanics, Kolar Gold Fields - 563 117, IN

Abstract

We carried out uniaxial compression tests and monitored the deformation and failure behaviour of a set of Godhra granites from Panchmahals district, Gujarat The testing was carried out by incrementing the stress at a constant rate on the test sample, and by recording the strain and acoustic emissions (AE) concurrently The static moduli have been computed from the stress-strain data, and the AE parameters, viz , hits, energy counts and peak amplitudes have been subjected to detaied analysis The results inferred from AE signatures of both medium grained and fine grained granites show that the stress-induced micro-cracking is stochastic during the early stages of rock deformation and it is followed successively by the on-set of dilatancy (inelastic volumetris strain) due to formation of new microcracks at = 65% failure stress, stable cracking, unstable cracking and crack coalescence leading finally,to the dynamic failure of the rock Owing to the preponderance of relatively high amplitude AE at stresses close to failure, the AE b value decreases quite sharply in accordance with the underlying physical processes of crack growth in both the medium grained and fine grained granite rock These observations agree well wlth those of accelerated increase in occurrence rate and energy counts of AE as the impending failure approaches in the rock.

Keywords

Uniaxial Compression Tests, Dilatancy, Crack propagation, AE Stattstics, Godhra granite, Gujarat.

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