CONTENTS OF APRIL 2013, Vol.XXXIV No.2
The Global Tectonics of the Indian Ocean and its Relevance to India’s Western Margin.
Imprints of Gadwal-Raichur Schist Belt Beneath Cuddapah Sediments, Evidences from Gravity and Magnetic Anomalies, Kurnool Area, Andhra Pradesh.
J. V. Rama Rao, N. V. S. Murty and B. Balakrishna
Thematic Integrated Interpretation of Geological, Remote Sensing, Aeromagnetic, AGRS and Gravity Data for Uranium Exploration, Parts of Alwar and Bayana-Lalsot Basins, Rajasthan.
Shailesh Tripathi, A. K. Maurya and V. Ramesh Babu
Application of Geophysics in Landslide Studies – A Case Study from Manthada, Nilgiri District, Tamil Nadu.
M. Pradeep Kumar, Dinesh Gupta and B. Hari Rao
Gravity and Magnetic Techniques to Decipher the Basement Structures for Uranium Mineralization – A Case study from Subbareddy Palem area, Palnad Sub-Basin, Guntur District, Andhra Pradesh, India.
S.Vijaya Kumar, Srinivasulu, B. V. L. Kumar, P. Allipeera and R. L. Narasimha Rao
The Global Tectonics of the Indian Ocean and its Relevance to India’s Western Margin
Earthworks bv, Delft, The Netherlands.
A pre-existing plate tectonic computer model of Indian Ocean development has been rebuilt (a) using new marine magnetic anomaly data to improve control of the relative movement of Africa-Antarctica in the Jurassic and (b) sea-floor topographic features (principally fracture zones) and magnetic anomalies to refine the entire Antarctica-India movement. The latter offers better constraints than the Africa-Madagascar-Mascarene-India route used previously and so defines more closely some of the constraints on the relationship between India and Madagascar that close the new plate circuit. Working backwards in time, India is found to ‘dock’ with Madagascar at about 88 Ma. This timing coincides with the ages of volcanic remnants found along the common coast of both fragments. Continuing further backwards in time, the common coastlines were constrained to remain approximately parallel since any separation in backward time corresponds to crustal consumption in forward time, for which there is no evidence. Such a constraint leads to a credible model for all the Gondwana fragments in which only strike-slip between India and Madagascar takes place between Gondwana break-up (167.2 Ma) and the initiation of the Mascarene ocean basin (88 Ma). In model CR13AAID (June 2013) this movement is first sinistral (~80 km, Jurassic), then dextral (~165 km, 145-114 Ma, Berriasian to Albian) and finally sinistral again (114-88 Ma, Albian to Coniacian) ending with a sinistral offset of about 50 km from the ‘fit’ position, prior to separation. The potential for transtension exists in all these phases, with the development of an 1800-km-long rift/shear zone that could have hosted sediment derived from both India and Madagascar. After separation , there is another 20 myr period of a passive-margin situation (Turonian to Maastrichtian), post-dating the 88 Ma Marion volcanicity and pre-dating the 68 Ma onset of the Deccan Trap activity. The resulting continental shelf of western India, up to 300 km in width, represents one of the most accessible and extensive areas of shelf geology of which only the post-Trap Cenozoic succession has so far been explored in any detail. An animation of the latest plate model may be viewed and downloaded at http://www.reeves.nl/gondwana
Imprints of Gadwal-Raichur Schist Belt Beneath Cuddapah Sediments, Evidences from Gravity and Magnetic Anomalies, Kurnool Area, Andhra Pradesh
J. V. Rama Rao*, N. V. S. Murty** and B. Balakrishna***
Geological Survey of India, *Eastern Region - Kolkata, **Central Region - Nagpur, ***Southern Region,
Hyderabad - E-mail: email@example.com , firstname.lastname@example.org
Gravity and magnetic anomalies obtained from systematic geophysical mapping have yielded interesting inferences other than depicting the schist belts in the granite-gneissic terrain of Eastern Dharwar Craton. Extension of Dharwarian trends, as mapped, into the Cuddapah basin reveals the sub-surface geology effectively and makes a case in the utility of geophysical mapping under covered areas. Correlation of gravity anomalies with the signal enhanced magnetic anomalies is striking and together these fields unfold the near surface geology and structures. We present here the results obtained over the western part of Cuddapah basin, covering part of Kurnool Sub-Basin. Lack of physical property contrast amongst various sediments of Kurnool group and the strong regional influence of basement explains the negative relation of the trend of anomalies with the general strike of geological units. The influence of subsurface geological features is so strong that the well-known Gani-Kalva fault is discernible only in the form of termination of anomalies and a shift in the continuation of gravity high zone.
Frequency analysis of gravity and magnetic anomalies indicate common depth interfaces coinciding with thickness of the Cuddapah sediments in this part of the area to be of the order of 500 m and the extent of granite-greenstone formations to be about 2 Km. A third interface observed around 6 km could be due to changes in the nature of basement. Modelling of gravity anomalies based on the frequency analysis and the physical properties have depicted the sub surface geological features. Profiles drawn across the general strike of anomalies depict the typical nature of schist belts as bifurcations and merger.
Key words: Gravity and Magnetic anomalies, Gadwal-Raichur schist belt, Cuddapah Basin.
Thematic Integrated Interpretation of Geological, Remote Sensing, Aeromagnetic, AGRS and Gravity Data for Uranium Exploration, Parts of Alwar and Bayana-Lalsot Basins, Rajasthan
Shailesh Tripathi , A. K. Maurya and V.Ramesh Babu
Airborne Survey and Remote Sensing Group, Atomic Minerals Directorate for Exploration and Research
Department of Atomic Energy, Begumpet, Hyderabad-500016
Middle Proterozoic Delhi Supergroup of rocks exposed in north Rajasthan in Khetri, Alwar, and Bayana-Lalsot Basins are known to host a number of uranium, base metals and other economic mineral deposits/prospects. Uranium mineralization in these basins is associated with structures such as faults, deep fractures, shears, and fold axes. Geological, remote sensing and geophysical datasets of parts of Alwar and Bayana-Lalsot basins have been processed and interpreted to decipher favourable structures for uranium mineralization. Digital image processing such as principal component analysis and other enhancement techniques of Landsat ETM+ satellite image helped in delineating the regional geological set up and in mapping the complex folded pattern and lineaments such as fold axes, faults/shears. Interpretation of gravity data indicated a gravity low over Lalsot basin, whereas Alwar Basin is characterised by gravity high. Bayana basin lies between two gravity highs. This suggests the presence of a horst and graben structure in the area. Alwar and Bayana sediments are relatively more magnetic compared to Lalsot sediments. Interpretation of magnetic data using total magnetic intensity images and its derivatives such as reduced to pole, tilt derivative and analytical signal delineated sub-surface structures. Majority of magnetic linears are trending in NE-SW and N-S directions and are well co-relatable with the lineaments mapped from the satellite data. Reduced to the pole, tilt derivative and analytical signal images reflected the folded pattern of the sediments in the Alwar Basin. Processed Airborne Gamma Ray Spectrometric (AGRS) data presented as radiometric ternary (U-Th-K) image helped in demarcating the lithological variations and understanding the radio elemental distributions in the area. The integrated study helped in identifying potential target areas for further uranium exploration.
Application of Geophysics in Landslide Studies – A Case Study from Manthada, Nilgiri District, Tamil Nadu
M. Pradeep Kumar , Dinesh Gupta and B. Hari Rao
Geological Survey of India, Bandlaguda, Southern Region, Hyderabad-500068
Electrical Resistivity surveys in the form of spot Vertical Electrical Soundings (VES) were conducted in Manthada landslide area of Nilgiri district, Tamil Nadu, to estimate the thickness of overburden / depth to bed rock. Twenty seven VES using Schlumberger array have been carried out over the study area. Interpretation of the field data suggests three and four-layer earth sections to exist with thickness varying from 1 metre to more than 30 m. The bed rock has resistivity higher than 500 ohm.m. Resistivity lows and highs of the order of a few hundred ohm.m to a few thousands of ohm.m correspond to highly jointed to massive Charnockite rocks respectively. Depth to the bed rock or overburden thickness map reveals that the depth to bed rock is shallow in the western and north western portions whereas it is deep in the southeastern part varying from 2.5 metre to more than 30 metres in the surveyed area Apparent Resistivity, resistivity pseudo-section and depth to basement rock maps were prepared. The apparent resistivity maps bring out resistivity lows of order 200 ohm.m to 400 ohm.m disposed in NNE-SSW direction indicative of a weak zone which may be prone to landslides. On the other hand, high resistivity trends may indicate a stable land mass of massive charnockite rocks.
Key words: Landslides, Resistivity, depth to bed rock, Manthada, Nilgiri Hills
Gravity and Magnetic Techniques to Decipher the Basement Structures for Uranium Mineralization – A Case study from Subbareddy Palem area, Palnad Sub-Basin, Guntur District, Andhra Pradesh, India
S.Vijaya Kumar* , Srinivasulu , B. V. L. Kumar , P. Allipeera and R. L. Narasimha Rao
Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Begumpet, Hyderabad - 500 016.
The Crescent-shaped Cuddapah basin is one of the major Proterozoic basins in India that host a number of mineral deposits of economic interest. In this basin, uranium mineralization is mostly controlled by basement structures – faults/fractures, shear and unconformity related stratiform/stratabound types. Subbareddy Palem area falls in the southwestern part of the Palnad Sub-basin of Cuddapah basin.
The primary uranium mineralization mostly occurs as veins, fractures/cavity fillings in basement granite and in sediments above the unconformity. The fertile basement granitoid is the main source of uranium in this area, carbonaceous matter acts as a reducing agent. Geophysical surveys comprising gravity and magnetic methods are carried out to decipher the favorable basement structural features for uranium mineralization.
An area of about 12 sq km was covered by gravity and magnetic methods. The magnetic and gravity anomalies trend in NW-SE direction representing the general strike of the formations in the area. A major basement fault (F1) has been identified in the northeastern part of the survey area from Bouguer gravity map. Part of the fault/ fracture zone is emplaced by basic dyke of limited strike extent as inferred from magnetic data. First vertical derivative image of gravity as well as magnetic data reveal another basement fault parallel to the above major fault.
This fault/ fracture (F2) trending in WNW-ESE direction is characterized by high order of gravity and magnetic linears by intrusion of basic material. The fault/fractures delineated in WNW-ESE direction in this area are sympathetic to the regional Kandlagunta basement fault to the south of Koppunuru area and corroborate with the established mineralized trends/basement features observed in that area. The inferred basement structures and undulations in the basement topography form favorable geological environment for the deposition of uranium mineralization.