Current Issue

July, 2016   ||  Volume 20 No.4

Editorial


1
Integrated geophysical study off Goa, Central West Coast of India

M. V. Ramana1, Maria Ana Desa*,2 and T. Ramprasad2
1 Sagarika Apartments, Nagali Hills, Dona Paula, Goa 403 004
2 CSIR-National Institute of Oceanography, Dona Paula, Goa, India-403 004
*Corresponding Author: mdesa@nio.org

 ABSTRACT
The western Indian offshore is characterized by several tectonic elements along its entire length. The prominent ones include the Laxmi Ridge (LR) and the Chagos-Laccadive Ridge (CLR) in the north and south, respectively. These ridges divide the western offshore into the western and eastern Arabian Basins, and have been attributed different evolution mechanisms and crustal types.
The Goa offshore (14.7 - 15.7°N and 70.3 -71.2°E), between the Laxmi Basin in the north and the CLR complex in the south, off the Central West Coast of India, may hold some clues of the connection/boundary between these diverse crustal domains. About 1550 km of gravity, magnetic, single and multibeam bathymetry data has been analyzed in the present study. The results indicate that the water depth varies between 2450 and 4000 m, and the region is characterized by ~NW-SE to NNW-SSE trending broad wavelength gravity and magnetic anomalies. Forward modeling of the geophysical data under seismic constraints highlight the presence of ~6-9 km thick oceanic crust, while towards the east, the crust is thicker and contaminated with Deccan volcanic intrusions. Magnetic data analysis suggests that the age of the underlying oceanic crust varies between 63.9 and 68.7 Ma (Chron 29n to 31r).
The present study indicates that the Goa offshore is an integral part of the Laxmi Basin. As such it is interpreted that a tectonic boundary between the oceanic crust in the north and the CLR complex in the south lies south of the study area. Further, this boundary may be associated with the Vengurla Arch and may have restricted the southward propagation of the Laxmi Basin spreading ridge.
Key words: West Coast of India, Goa offshore, Laxmi Basin, geophysical data, Chagos-Laccadive Ridge.



Multi-Parameter Geophysical Exploration Strategy Delineates PGE Host in Tasampalaiyam, Sittampundi Anorthosite Complex, Tamil Nadu

R. Ananda Reddy*, M. Mohan Babu, B. Ashish Giridhar, M. Pradeep Kumar and C.B.K. Sastry
Geophysics Division, Geological Survey of India, Southern Region, Hyderabad
*Corresponding Author: anand.rr55@gmail.com

ABSTRACT

A geophysical exploration strategy comprising closely spaced Gravity, Magnetic, IP, multi-electrode Resistivity Imaging and multi-parametric Geophysical Logging techniques was employed to bring out the controls of mineralization viz., structural features associated with regional folding in parts of T3 Sector of Tasampalaiyam block, Sittampundi Anorthosite Complex (SAC), Tamilnadu. It also helped in delineating the strike and depth extent of thin / narrow chromitite and chromiferous meta-pyroxenite bands hosting Platinum Group of Elements (PGE). Mineralization within this anorthosite Complex (SAC) is characterized by group of rocks consisting of anorthosite, amphibolite, pyroxenite, garnetiferous pyroxene granulite and meta-pyroxenite with or without chromitite.

Highly dominant magnetic anomalies were observed over the anorthosite in the north, compared to low magnetic signatures over pyroxene granulite in the south. In addition, subtle magnetic variations of the order of 10 to 20nT were recorded over chromiferous meta-pyroxenite bands. These bands are trending in ENE-WSW direction. Gravity image brought out feeble residual highs of around 0.15mGal trending ENE-WSW, within the broad gravity low zone over chromiferous meta-pyroxenites in the anorthosite. Resistivity low and chargeability high values trending in east-west direction were recorded over chromiferous meta-pyroxenite bands within the anorthosite, which are associated with fault / fracture / shears and kaolinised zones extending on either side. Thus, the multi-electrode resistivity imaging and multi-parametric borehole geophysical logging data, supported by subtle magnetic variations, feeble residual gravity highs, low resistivity and high chargeability signatures, have helped in demarcating the favourable zones for the possible occurrence of chromiferous meta-pyroxenite bands hosting the PGE mineralization within the Sittampundi Anorthosite Complex.
Key words: PGE Mineralization; Chromiferous meta-pyroxenite; Sittampundi Anortosite Complex; Gravity; Magnetic; I.P; Resistivity Imaging; Geophysical logging.



Moho temperatures along with their error bounds in Closepet Granite batholith region of Southern Indian Shield

Harini Guruhappa* and Kirti Srivastava
CSIR- National Geophysical Research Institute, Hyderabad, 500007, India.
*Corresponding Author: g.harini07@gmail.com

ABSTRACT
The subsurface temperatures along with their error structure in continental lithosphere are obtained using the available heat flow and heat production information. The stochastic analytical closed form solution using the Adomian method of decomposition is used to quantify the subsurface temperatures along with its error bounds. Heat flow data and controlling thermal parameters at two granite exposed drill holes, one Pavagada (PG), which is in greenschist facies and the other Magadi (MD) an amphibolite facies in Late Archaean Closepet Granite Batholith (CGB) located in the Dharwar Craton are used to model the subsurface temperatures. Results reveal that at the base of the crust the temperatures in Pavagada are 315 ± 55 o C and at Magadi it ranges from 300 ± 60 o C. The error bounds represented by the standard deviation are highly dependent on the variability in thermal conductivity hence the stochastic approach would help in quantifying the thermal state of the crust with its error details.
Key words: Stochastic, Adomian method, Heat conduction, Dharwar craton, Closepet granite.


A Search for Ionospheric Perturbations in VLF Electromagnetic Signals Associated with Large Earthquake in Greece Region 

Rahul Shrivastava*, D.K. Sondhiya, Shailesh Raghuwanshi and A.K. Gwal
Space Science Laboratory, Department of Physics and Electronics, Barkatullah University Bhopal – 462026, India
*Corresponding Author: raj106rahul@gmail.com 

ABSTRACT
Anomalous variations of electromagnetic signals in VLF range associated with large (M≥6) earthquakes are considered as short term earthquake precursors. For the present study, VLF signals observed by Sudden Ionospheric Disturbances (SID) monitoring station have been used. These signals are critically examined to delineate their correlation with Lixourion, Greece Earthquake occurred on January 26, 2014 (M=6.1). It was found that the amplitude and number of spikes of VLF signals have increased anomalously on January 9, 2014, before the preceding earthquake. It has been found that the variations in VLF electromagnetic signals, prior to seismic activity, might have been produced by an electromagnetic instability process in the Earth`s atmosphere. The ionosphere triggered by charged aerosols or electromagnetic waves found to have been induced in the preparatory stage of shallow (Depth≤55 km) earthquakes.
Key words: Earthquake Precursors, Seismology, Electromagnetic Signals, VLF, Ionospheric Perturbations.



Petrology and Geochemistry of Late Proterozoic Norite dykes from the Shillong plateau, NE India and their evolutionary history

J. Mallikharjuna Rao and M.Venkateshwarlu*
CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad-500007
*Corresponding Author: mamila_v@rediffmail.com

ABSTRACT
The Shillong plateau with an Archaean gneissic basement contains xenoliths and enclaves of diorite, amphibolite and various meta-sedimentary rocks. Dykes of norite, dolerite, alkaline lamprophyre together with pyroxenites and granites are intruded into these basement gneisses. The norites are relatively fresh, dark metallic brown in colour and show ophitic to sub-ophitic textures. The essential minerals in norites are plagioclase (An51-62), orthopyroxene (Mg81-84), clinopyroxene (Ca42-48, Fe14-16 and Mg38-43) and opeques, which show minor secondary alterations. The Late Proterozoic norites, which show quartz or hypersthene normative compositions, differ in their geochemical nature from the younger dolerites and basalts that exhibit olivine or quartz normative compositions. To establish geochemical characteristic of these norites, different binary and ternary plots were used. These plots along with total alkali-silica diagram (TAS) indicate that norites are basalt to basaltic andesite in composition and tholeiitic in nature and exhibit iron enrichment during differentiation from the source magma. Total rare earth element (REE) content varies from
116-291 ppm and exhibits fractionated trends (La/Yb=5.4 to 9.2) and a strong negative Europium anomaly (Eu/Eu* = 0.42-0.67). Primitive mantle normalised spidergram shows enrichment of large ion lithophile elements, slightly depleted rare earth element patterns and strong negative Nb, P, Hf and Ti peaks, indicating the contaminated nature. The present study is focussed on the Late Proterozoic mafic magmatic emplacement history and geochemical characteristics of these less frequently occurring norite dykes in this region in addition to the wide spread dolerite dykes in the region.

Key words: Petrology, Geochemistry, Dykes, Shillong plateau, NE India.



Forecasting rainfall trend over India during summer monsoon 

Vinod Kumar1, Ramesh Chand*2, M. Satya kumar3 and R. B. S. Narayan4
1Shyam Bhawan, Ashok Nagar, Road No. 11, Kankarbagh Colony, Patna-800020
2Ministry of Earth Sciences, Prithvi Bhavan, Lodi Road, New Delhi-110003
3H.No. 6-3-565, Flat No. 301, Akshaya Apartment, Somajiguda, Hyderabad- 500082
4Meteorological Office, C.S.I. Airport, NTC, Mumbai-400099
*Corresponding Author: vinodmanjusingh@gmail.com

ABSTRACT
Cold fronts with associated westerly waves (mid-latitude low pressure systems: 30°S-60°S) move north of 30°S from west coast of South Africa (10°E or even from 40°W) to west coast of Australia (120°E) during southwest monsoon season (winter season: southern hemisphere). They influence Indian summer monsoon rainfall significantly. Geo-potential height field of 850 hPa levels undergoes large changes in association with the movement of cold fronts/ upper air troughs from 040°W to 120°E / 140°E and north of 30°S/40° S during April-May. Moisture generated over Atlantic and Indian Oceans (from east of 40°W to 120°E) by cold fronts/upper air troughs augment south easterly trades through low level subtropical anticyclones during summer monsoon season as they move eastwards and equator-wards. Positive (high) and negative (low) anomalies of 850 hPa level geo-potential heights during April-May, north of 40°S from 40°W to 120°E/140°E have been used to forecast rainfall trend over India during the first week of June.
Key words: Cold fronts/upper air troughs, Atlantic and Indian Oceans, anomaly of 850 hPa geo- potential heights and quantitative forecast.


7

District level long range forecast of rainfall during southwest monsoon in Telangana 

Onkari Prasad*1, O.P. Singh2 and K. Prasad3
143, Ritu Apartments, A-4 Paschim Vihar, New Delhi-110063
2 B44, First Floor, Parshvnath Paradise, Mohan Nagar, Distt: Ghaziabad-,U.P
3 D-104, Seema Apartments, Sector 11, Dwarka, New Delhi-110085
*Corresponding Author: prasadonkari123@yahoo.in

ABSTRACT
Long range forecast of rainfall during southwest monsoon season, at spatial resolution of a district, could serve as an important input in planning of agricultural operations, water management and several other activities. However, such forecasts are not yet available. It has been shown that making use of South Indian Ocean Convergence Zone (SIOCZ) model, long range forecast of seasonal rainfall, rainfall for bi-monthly periods of July+ August and August+ September and during the month of September could be prepared for all the districts of Telangana. Long range forecast could also be prepared for monthly rainfall during July and August for the majority of the districts of the state.
Key words: Southwest monsoon, South Indian Ocean Convergence Zone Activity Index, District level long range forecast, Outgoing Longwave Radiation.


8

Research Note
Chemical Constituents in PM10 Aerosols during Diwali and Holi Fesitivals in Delhi 

Ruchi Singh1, Subhash Chandra1, Monika J Kulshrestha*1 and Rashmi2
1CSIR-National Physical Laboratory, New Delhi, India
2 Ex- chief scientist
*Corresponding Author: monikajk@yahoo.com

ABSTRACT
PM10 samples were collected during two major festivals of India, i.e. Diwali 2011 and Holi 2012 at a site in central Delhi. A very high contribution and sharp rise of various chemical constituents (K, S, Al, Cl, Mg, Ba, Sr, Cu) was observed during Diwali period. Correlation matrix indicated a common source of these chemical constituents due to the burning of firecrackers. During Holi period, most of the emitted species (Mg, Al, Ca) were crustal in nature. This study was aimed to assess the relative contribution of chemical constituents in PM10 aerosols, on day and night time basis during Diwali 2011 and Holi 2012 festivals in Delhi.
Key words: Chemical constituents, Aerosols, Diwali, Holi, Delhi.


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