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Annual Report 2004-2005
Deep Continental Studies
Deep Continental Studies (DCS) Programme aims
at delineating and understanding deep crustal configuration
and related processes of the Indian Lithosphere. During the
period, 2 meetings of the PAMC were organized and 11 new projects
were considered and 20 ongoing projects were reviewed. The
achievements made during last one year are given below:
New Projects Sanctioned
- Lower crustal Playground in Orogenesis: Integrated structural,
Metamorphic and fluid Inclusion studies on the Formation
and exhumation of the Angul Granulites, Eastern Ghats Belt,
Orissa
- Petrology and radioactive isotope systematics of Precambrian
mafic igneous rocks from the southern Bastar: Implication
for evolution of sub-continental lithosphere of central
Indian craton
- A pilot project on preparation of maps on rock properties
in some parts of Eastern Ghats.
- Paleomagnetic studies on basic dykes occurring in the
North-eastern part of the southern granulite terrain of
India.
- Geophysical investigations of the Andaman Island Arc –
Trench System and Back Arc Basin.
- Mafic crustal xenoliths in the Eastern Ghats belt: a window
to mantle processes.
- Use of Optimal Basis functions for Modeling and Inversion
of full Waveform Seismic Data.
- Magnetotelluric data processing, analysis and modeling.
- Geochemical and paleomagnetic studies of mafic dykes from
Bundelkhand and Bastar cratons, Central India: Implication
for lithospheric evolution and mesopalaeo Proterozoic palaeocontinental
reconstruction.
- Petrogenetic evaluation of Lamproite Rocks from Andhra
Pradesh
Research Highlights
- Petromineralogical, geochemical and economic mineralization
studies in the Naga Ophiolite complex and their bearing
on the nature of deep continental/ oceanic crust
- Studies on the granites occurring in the Phukungri-Avangkhu
area of the Naga Hills Ophiolite Belt have indicated the
metaluminous nature of these granites (Figure 2.14(a)).
- These granites have been emplaced as “volcanic-arc
granites (Figure 2.14 (b)).
- The overall mineralogical and geochemical characters of
these granites suggest derivation of the melt parental to
these granites might have been derived from a mantle or
juvenile crustal source with little crustal residence time.
- Great Boundary Fault: A Possible Crustal Scale Ductile
Shear Zone in the Northwestern Indian Shield
Ductile shear zones occur as m-scale lensoid
or tabular zones that consist of well developed mylonite foliations
and stretching lineations and, cut through the Nimbahera shale
exposed within the Great Boundary Fault zone in the Berach
river section. These shear zones are made up of two compositionally
contrasting bands: (i) well foliated quartz bands developed
due to mylonitisation of quartz veins and, (ii) massive, brown
coloured, fine grained, opaque bands containing relict fragments
of the Nimbahera shale. The quartz bands are predominantly
made up of quartz ribbons showing undulose extinction, subgrain
development, deformation lamellae, serrated grain boundaries,
ó-porphyroclasts, S-C fabric and mortar texture. These
microstructures imply that crystal plasticity and dynamic
recrystallisation were the dominant mechanisms of deformation
during the mylonitisation process. The brown bands are formed
due to the concentration of opaque minerals and dark residue
remaining after dissolution of light by coloured and readily
soluble minerals in the Nimbahera shale during the development
of quartz veins and ductile shearing.
- Comparison of heat flow and heat production regimes in
the eastern and western parts of the Archaean Dharwar craton,
southern India
- The sites for drilling were selected.
They are: Pavagada and Magadi in Closepet Granite, Hassan
and Piriyapatna in Western Dharwar Craton, Ernakulam, Malappuram
and Nagercoil in western parts of Gneiss-Granulite province
of south India (Figure 2.15 a)
- Drilling of two boreholes in Closepet Granite (Pavagada:
435 m; Magadi: 445 m) and two in western Dharwar Craton (Piriyapatna:
473 m; Hassan: 291 m), sampling of borehole chips at 5-6 m
depth intervals for laboratory measurements, temperature measurements
in these holes (Figure 2.15 b) and estimation of geothermal
gradient.
Figure 2.15 (a)
Simplified geological map of south India showing the distribution
of heat flow data. The seven sites selected for determination
of heat flow in specially drilled deep (up to 500 m) boreholes
in the present study are shown (solid red circles).
Figure 2.15 (b)
Temperature-depth profiles for the four boreholes drilled
so far (PP: Piriyapatna; HS: Hassan; MD: Magadi; PG: Pavagada).
Temperatures in the case of boreholes PP and HS are shifted
for clarity; the amounts of shifts in oC are also shown.
- Characterization of thermal conductivity for
the rock types met with in the boreholes in Closepet Granite
and Western Dharwar Craton.
- Field, model deformation, petrofabric and magnetic strain
studies along frontal and oblique ramps of the western Himalaya
Experimental results of the model deformation
studies reveal that the true magnitude and direction of particle
movement paths can be obtained only by choosing a fix point
outside of the deforming body undergoing translation. Location
of the fix point inside the body can lead to serious errors
of measurement. Since the velocity vectors obtained from the
GPS studies are based on location of the fix point inside
the deforming body, the results are open to questions, especially
in an active orogenic belt like Himalaya.
- Geological, Geochemical and Palaeomagnetic studies of
West Garo Hills dykes, Meghalaya and their Petrogenetic
and Tectonic implications.
The Garo Hills dykes and basalts are contaminated
by crustal material to a lesser extent. The low- mg number
in these dykes suggests that the source rocks did not equilibrate
with mantle mineralogies and the primary magma may have been
high- Mg picrite which underwent lowpressure fractionation.
Palaeomagnetic results show that ChRM vectors
from all the dykes reveal very good grouping with a mean value
of Dm=321, Im=-65 and their ChRM resulted in a mean VGP for
all dykes with Lp=64.4oW, ëP=9.4o
with a palaeolatitude ëm of 38.7oS to the
Indian subcontinent.
Detailed work on Sonapahar granulite belt which
is very close to the presently studied area, have shown the
peak metamorphic pressure- temperature conditions established
on the basis of reaction equilibria to be 750±50oC
and 5±0.5 Kb followed by later retrograde decompression.
In the Nongstoin and Nynsi areas symplectitic textures and
marginal granulations suggest a decompressional regime implying
a tectonic uplift of the Meghalaya plateau
- Age, Sr-Nd isotopic and geochemical studies of kimberlites
from the Jharia, Raniganj and Bokaro, Eastern India: characterization
of their source
- Petrological and geochemical studies on the
Jharia kimberlites show their similarity with Group II kimberlites
of the Kaapvaal craton.
- Trace and Rare Earth Element comparisons of the Jharia kimberlites
with the Kaapvaal craton rocks suggest a genetic relationship
between the two. Further study could establish the extent
of a Group II kimberlite province in Eastern India.
- Though the Rb-Sr ages of the Jharia kimberlites and their
chemical compositions are similar, their Sr-Nd isotopic compositions
are dissimilar suggesting different types of sources for the
two (Figure 2.16).
Figure 2.16: Sr vs. Nd isotopic composition of Group I and
II kimberlites.
Jharia kimberlite isotopic composition is shown in red.
- Delineation of subsurface thermal structure of the Narmada-Son
lineament
- Crustal thermal structure of central India
along the (I) Hirapur - Mandla, (II) Khajuriakalan - Pulgaon,
(III) Ujjain - Mahan DSS profile have been obtained. Numerical
results reveal that the surface heat flow value in the Central
India varies between 45 to 49 mW/m2. Out of which
21-23 mW/m2 is the contribution from the mantle
heat flow and remaining from within the crust. The calculated
heat flow values are found in close agreement with the heat
flow values measured at Lonar (47 mW/m2), Satephal
(47 mW/m2) and Singdad (49 mW/m2) in
Yawatmal area, Mandwa (48 mW/m2) in Wardha area
towards southern tips of Profile II and III, and Mohapani
(49 mW/m2) located in the northern tips of Satpura
basin south of NSL.
- The Curie depth is found to vary between 36 to 43 km and
is in close agreement with Curie depth estimated from MAGSAT
data.
- The Moho temperature varies between 500 to 580oC.
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Introduction
Introduction
