IAGR Memoir No. 10, pp. 233-242.
© 2007 International Association for Gondwana Research, Japan.
ISBN: 978-4-938925-16-1 C3344
MEMOIR 10
Aeromagnetic Signatures of the Cratons and Mobile Belts
Over India
S.P. Anand and Mita Rajaram*
Indian Institute of Geomagnetism, Kalamboli Highway, New Panvel (W), Navi Mumbai - 410 218, India
* Corresponding author: E-mail: [email protected]
Abstract
An aeromagnetic image map of India up to 25° North latitude and between 74° to 84° East longitude is prepared. The three cratons: Dharwar,
Bastar and Sighbhum are clearly demarcated and are separated by the NW-SE trending Godavari and Mahanadi grabens and are surrounded by
the mobile belts of the Central Indian Tectonic Zone, the Eastern Ghats and the Southern Granulite Terrain. The analytic signal map of the
aeromagnetic data depicts that the main magnetic sources within the cratons are related to iron ore belts, schist belts and dyke systems while
the sources within the mobile belts are due to the exhumed crust reflecting the high-grade granulite belts. The Euler solutions bring out the
block structure and fractured nature of the region. Spectral estimates are utilised to evaluate the Curie isotherm depth. Regions of exhumed
crust and mobile belts show a thinner magnetic crust than the cratons. Utilising the 1D heat conduction, steady state thermal model for the
continental crust, we calculate the geothermal heat flux from the Curie isotherm depths, incorporating available surface heat flow and
thermal conductivity for the various tectonic blocks of India. The calculated heat flux, for several areas match the surface heat flow
measurements.
Keywords: Aeromagnetic anomalies, crustal structure, thermal regime, charnockites, Euler solutions.
Introduction
The International Association of Geomagnetism and
Aeronomy (IAGA) has declared the current decade as a
Decade of Geopotential Research in which a major impetus
has been given to satellite magnetic data collection.
Beginning with the Oersted Satellite several other satellites
have either been put in orbit or are slated for launch within
this decade. It is only befitting that in addition to the global
satellite magnetic anomaly map a supporting World
Aeromagnetic Anomaly map be compiled, and IAGA had
set up a task force that has just released the world digital
magnetic anomaly map on 5 July 2007 at the IUGG 2007
Meeting held at Perugia, Italy. In the present paper, we
compile available aeromagnetic data over India up to
25 degrees North, collected at the reconnaissance scale
of 1:250,000 and analyse it to provide regional
interpretation in terms of the structural, tectonic and
thermal history of this region. Very little aeromagnetic
data is available above 25 degrees North latitude.
Peninsular India is a multi-cratonic assembly of
Precambrain crustal blocks, surrounded by mobile belts,
with varied lithologies, tectonic style and evolutionary
history, that have been brought into juxtaposition and
sutured together during different epochs. However the
surface cover often complicates the picture and it is in
this context that the aeromagnetic data can play a very
crucial role in delineating the sub-surface structures.
Tectonic models available so far are based largely on either
gravity and/or geology maps that have huge data gaps as
data collection are heavily dependent on road access. On
the contrary, aeromagnetic data has excellent data
coverage with the data points being closely spaced along
flight line with a uniform flight line spacing of 4 km. The
inherent nature of this data set leads to very reliable
information content almost independent of any
extrapolation in the maps generated and their subsequent
interpretation/analysis. The Indian region up to 25° N,
includes three Archaean Cratons (Dharwar, Bastar and
Singhbhum) separated by two Gondwana Grabens
(Godavari and Mahanadi) and fringed to the south by
the exhumed lower crust of the high-grade domain and
to the east by the Proterozoic mobile belt (Eastern Ghat
Mobile Belt) and the north by the Central Indian Tectonic
Zone. The west is covered by the Deccan trap flow and no
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S.P. ANAND AND MITA RAJARAM
aeromagnetic data has been collected in this region. The
study of the Indian region is important as it witnessed
tectonic activity from the Archaean to the present, and
the paper deals with the aeromagnetic signatures of this
area to understand the exposed and sub-surface structural
and tectonic features and to throw light on their
evolutionary history.
Generalised Geology and Tectonics
The geology and tectonics of the study region is
depicted in figure 1 (redrawn from GSI 1993, 1994, 2000).
The most characteristic structural feature of the Archaean
cover sequence of the Dharwar Craton is their arcuate
NNW-SSE trend with convexity towards the east. The NS
trending Closepet granite is a conspicuous feature in the
Dharwar block. The rocks of the Dharwar Craton are
mainly sedimentary in origin, and occur in narrow
elongated synclines resting on the gneisses (Radhakrishna
and Vaidyanadhan, 1997). These rocks are enriched in
manganese and iron ore and are also extensively
mineralised with gold. Granites and granitoids of the ca.
age 2600 to 2500 million years have extensively intruded
the Dharwar Craton. The northern part of Dharwar Craton
is made up of Kaladgi, Badami and Bhima group of
sediments, approximately of Proterozoic age. Further
north the terrain is covered by extensive volcanic flows.
Krishna River with some of its tributaries viz., the Bhima,
Fig. 1. Generalised Geology and Tectonic Map of Peninsular India, redrawn from GSI (1993, 1994, 2000). AKSZ–Achan Kovil Shear Zone,
VFl–Vaigai River fault, CFl–Cauvery River fault, SAFl–Salem-Attur fault, MFl–Moyar fault, BhlLn–Bhavali Lineament, HmLn–Hemavati
Lineament, BLn–Bhadra Lineamnet, CBFl–Chitradurga Boundary fault, ArFl–Arkavati fault, DTFl–Dharma-Tungabadhara fault, KrFl– Krishna
river fault, KKLn–Kolhapur Kurnool Lineament, DFl–Dindi fault, KFl–Kadam river fault, BgLn-Brahmagarh lineament, KLn–Kondagaon
lineament, CIS–Central Indian Shear, GvFl–Gavilgarh fault, TnSh–Tan Shear, NPS–North Purulia Shear.
IAGR Memoir No. 10
AEROMAGNETIC SIGNATURES OF CRATONS AND MOBILE BELTS
Tungabadhra, Dharma, Kumudavati and Hemavati along
with the Pennar and its tributaries drains the area (GSI,
1994). The known faults and shears of the peninsular
shield closely follow the pattern of major rivers.
The southern region, below orthopyroxene isograd
(extending from 8° to 13° N latitude), is one of the few
terrains in the world that has preserved Archaean crust
with extensive granulites, believed to be of lower-crustal
origin. There is a marked change in the structural
trend from the dominant NS in the Dharwar block to
EW in the SGT. The lithologies of SGT include
charnockites, both banded/gneissic and massive types, and
two pyroxene granulites, inter-layered with high-grade
quartzofeldspathic gneisses and khondalites, with
extensive alkaline granite-granite magmatism (Sarkar, 2001).
The terrain is traversed by fairly dense network of NW
and NE trending lineaments. The WNW-ESE trending
lineaments include Bhavali lineament (BhLn), Moyar fault
(MFl), Salem Atur fault (SAFl), Cauvery fault (CFl), Vaigai
River fault (VFl); NW-SE trending Achan Kovil Shear Zone
(AKSZ) and NNE-SSW trending Metur east fault (MeFl)
are some of the very prominent discontinuities which have
affected all the basement elements of the terrain.
Bastar Craton is bounded in the north by Narmada Rift,
in the northeast by Mahanadi Rift and in the south by
Godavri Rift. This craton is covered by rocks, belonging
to the Sakoli, Iron ore, Bengpal, Saussar, Chilpighat,
Dongargarh and Kotri Groups which contain acid and basic
volcanic rocks, sediments, iron formations, and granites
ranging in age from Archaean to recent. In places, these
rocks have been intruded by Deccan basalts.
The Eastern Ghat Mobile Belt (EGMB) towards the east
of the Dharwar block, is a highly deformed terrain with
lithological assemblages varying from Archaean to recent.
Recent alluvial sediments form long coastal strips at the
periphery of the peninsular shield. The EGMB is separated
from the Dharwar Craton by the volcano-sedimentary
dominated Nellore type schist belt forming a major thrust
along the eastern margin of the Proterozoic Cuddapah
Basin. The basement constitutes mainly of Proterozoic
high-grade gneiss-granulites consisting of charnockites,
khondalites and high-grade gneiss (GSI, 2000) whose
structural trend is NE-SW.
The Singhbhum Craton to the east, is separated from
the rest of the Indian Peninsular Shield, by the Mahanadi
Graben (MG) occurring to its west and the Sukinda thrust
(Mahadevan, 2002) to the south. Tectonically, this block
includes the Singhbhum granite (which includes banded
iron formation) to the south, and the Proterozoic
Singhbhum mobile belt in the middle separated from the
Chotanagpur Granite Gneiss (CGG) terrain in the north
by the North Purulia Shear (NPS).
The Central Indian Tectonic Zone (CITZ) (Yedekar
et al., 1990) lying to the north of the Central Indian Shear
IAGR Memoir No. 10
235
(CIS) is marked by several sub-parallel ENE trending
faults: Narmada North and South, Tapti fault, Gavilgarh
fault (GVFl), Tan shear (TnSh), Bamni-Chilpa fault and
Tatapani fault. The Tatapani area has several thermal
springs that are responsible for the high heat flow in
Central India (Ravi Shankar, 1991).
Aeromagnetic Anomaly Map
Aeromagnetic surveys over the Dharwar and Bastar
Cratons, Southern Granulite Terrain and Eastern Ghat
Mobile Belt was carried out by the National Remote
Sensing Agency (NRSA) during the period from 1980 to
1994 at altitudes of 5000 ft, 7000 ft with a narrow strip
in the central part towards the south covered with a flying
height of 9500 ft. Aeromagnetic data was collected over
the western part of Cuddapah Basin and the adjoining
crystalline at a flight altitude of 500 ft during 1980-81
with line spacing of 500–1000 m. Degree sheet
aeromagnetic contour maps produced at 1:250,000 scale
were purchased from GSI. These maps were digitised,
corrected for main field, gridded at 2km interval and
continued to a common altitude of 5000 ft to obtain the
final aeromagnetic image map of the region. The image
map is represented as figure 2; in this map, WGS84 datum
with UTM projection has been used. The identified major
faults, shears, lineaments, etc. have been demarcated on
the map.
The total field anomaly map shows very clearly the
tectonic elements of the region. In the Dharwar and Bastar
region the anomalies show NW-SE trend changing to
essentially EW trend north of Main Peninsular Shear
(MPS) and south of orthopyroxene isograd (Rajaram and
Anand, 2003a, b) bounded by typical NE-SW trend of the
EGMB towards the east. Based on the magnetic anomaly
pattern, the image map can be broadly classified into
distinct blocks viz., Block I between 8° and 13° N which
includes SGT; Block II covering area between 13° to 22° N
latitude up to MPS which includes Dharwar and Bastar
Craton; Block III between 22° and 25° N latitude including
CITZ and Singhbhum Craton and to the east of Block II is
the EGMB (Block IV) with typical NE-SW trends. Block I
and Block III are dominated by 2D-linear anomalies
trending mainly ENE-WSW to EW and contain localised
3D features. Block-II is heterogeneous and characterised
by sparsely distributed broad anomalies, besides isolated
2D-linears. In the description that follows, the identified
faults, lineaments and shear from figure 2 are noted in
brackets.
Block I: SGT
Within Block I, the trend of the anomalies from
12°–13° N, swerve from NW-SE in the west to NE-SW in
the east through E-W in the central part. These anomalies
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S.P. ANAND AND MITA RAJARAM
can be easily correlated to the strike of the charnockitic
rocks out cropping in this region. The NE-SW trending
anomaly stretching form Mysore to Bangalore can be
associated with the Hunsur lineament (HLn) representing
deep seated crustal fracture with which are associated
emplacement of tholeiitic and acidic-alkaline dyke systems
and granitoids (GSI, 1994). The major shear zones in the
region including the Moyar-Bhavani Shear Zone (MBSZ)
trending WNW-ESE to EW, the EW trending PalaghatCauvery Shear Zone (PCSZ) stretching from west to east
coast and separating the Nilgiri-Madras granulite block
from the Madurai-Kerala block, NW-SE trending steep
gradient anomaly zone separating Kerala Khondalite Block
(KKB) from the Madurai Block (MB) representing the
AKSZ, etc. (Ramakrishnan, 2003) are evident on the
anomaly map suggesting that these are associated with
major lithological changes. The Bhavani lineament, the
Moyar fault and the Salem-Attur fault appear as a single
system (MBSZ) as evidenced from the anomalies and may
be so in the subsurface. Strong NE-SW- trending linear
anomaly patterns mark the Proterozoic alkali complex and
mafic ultra-mafic dominated Salem-Dharmapuri Belt
(H3), besides the associated Salem-Attur Shear Zones. To
the east of the Attur fault is the large magnetic high (H4)
also associated with the Ariyalur gravity high
(Balakrishnan, 1997). Cauvery fault (CFl), Palar river fault
(PFl), Vaigai river fault (VFl) etc., are identified from the
anomaly map where there is sudden change in the
anomaly pattern or trend. The khondalites south of AKSZ
are showing very low gradient magnetic high extending
to offshore. Although SGT is dominated by EW trends,
towards the western margin the trends change essentially
to NW-SE. This throws light on the possibility that the
rifting processes, along the margin, have affected the
magnetic anomalies. The various basins such as Cauvery
(CB), Krishna-Godavari (KG), Palar (P) and Cuddapah
Fig. 2. Aeromagnetic anomaly map of
Peninsular India (the abbreviations
used in the figure are as mentioned
in the text).
IAGR Memoir No. 10
AEROMAGNETIC SIGNATURES OF CRATONS AND MOBILE BELTS
are characterised essentially by low magnetic gradients
reflecting thick non-magnetic sediments. These basins,
except the Cuddapah Basin, is well known for their
hydrocarbon potential.
Block II: Dharwar and Bastar Cratons
The major anomaly trends in Block II correlate well
with the typical NW-SE Dharwarian trend with a few EW
and NE-SW trends. The greenstone and allied
supracrustals, Kadam River fault (KFl), the exposed and
sub-surface granulite belt (Anand and Rajaram, 2003)
extending from Karimnagar to Khammam along the
shoulders of the Godavari Graben (GG) and the
topographically elevated region (H1) bounded by the
Bharmagarh lineament to the southwest and Kondagaon
lineament (KLn) to the northeast respectively, broadly
represents the NW-SE trends (Anand and Rajaram, 2002)
seen in Block II. The Dharwar and Bastar blocks have a
large number of 3D anomalies with high amplitude
elongated and circular closures, that represents iron ore
belts of Bailadilla (B), Sandur (Sn), Kudremukh (Kh),
Bababudan (Bb) and Goa (Pj). The NW-SE trending GG
is conspicuous by broad and flat anomaly suggesting thick
non-magnetic sediments. The anomalous zone (H2) reflects
the basaltic lava flows covering the Bhima and Kaladgi
basin. A major NW-SE lineament in the Dharwar Craton,
the Wajrakarur lineament (WkLn) (Harikumar et al.,
2000), cutting through the Cuddapah Basin to the south
of Madras, has a tectonomagmatic importance as major
kimberlite pipes are known to exist in this region. Within
the Dharwar Craton, the western part of the Proterozoic
Cuddapah basin is characterised by E-W trending high
amplitude broad anomaly reflecting the undulations in
the basement. The NW-SE trending high-low-high pair
(HL1) in the southwestern margin of the Cuddapah Basin
represents basic-ultrabasic magmatism as well as mafic
dyke swarms in the adjoining terrain.
A persistent ENE-WSW to EW trending anomaly zone
in the northwestern part of the Bastar Craton marks the
Central Indian Shear Zone (CISZ) delineating the contact
of Sakoli fold belt in the south with the Sausar Mobile
Belt (GSI, 2000). ENE-WSW trending linear anomaly zone
(Ln1) extending for several hundred kilometres, delineates
the major Proterozoic mafic dyke swarms in the northern
part of the Bastar Craton. This linear anomaly zone
appears to control the northward extension of the
structural elements within the Bastar Craton and may have
major implications. Major part of the region bounded by
this lineament and MPS is magnetically flat. From the
aeromagnetic picture it appears that Chattisgarh Basin
extends further west than that mapped geologically.
Several NW trending anomalies that are identified within
the Proterozoic Chattisgarh Basin may represent basement
IAGR Memoir No. 10
237
shears and subsurface mafic dyke swarms and can form
locales of deposition of radioactive minerals.
Block III: CITZ and Singhbhum Craton
There is a marked change in anomaly pattern and trend
to region north of MPS (block III) being characterised by
dense ENE-WSW to EW trending anomaly patterns parallel
to the trends of the mobile belts of CITZ. The contact of
the Sausar Mobile Belt with the Son-Narmada lineament
(GSI, 2000) is defined by linear anomaly zone trending
ENE-WSW to EW along the trend of the Betul-Chhindwara
belt (east of Jabalpur) and by strong linear anomalies
over the Deccan trap cover. The Singhbhum Craton shows
a fairly homogeneous aeromagnetic picture. The elongated
3D feature (HL2) represents the Iron Ore Group within
the Singhbhum Craton. All the ENE trends associated with
the Singhbhum Craton are terminated towards east by
NS linear anomalies of the Rajmahal trap and the alluvial
cover. North Purulia Shear (NPS) that separates the
Singhbhum Craton from the Chottanagpur Granitic
Gneissic (CGG) Complex towards the north appears as
ENE trending high anomaly bordering the northern fringe
of the craton. Part of the CGG (between 23° and 25° N) is
characterised by ENE-WSW to EW trending narrow
anomaly zone comprising gneiss-supracrustals, alkali
complexes and younger granulites.
Block IV: EGMB
The EGMB (Block IV) is represented by high to
moderate amplitude NE-SW trending linear and short
wavelength anomalies, extending from north of Chennai
to south of Bhubaneswar. The Sileru Shear Zone (SlSZ),
where the trend of anomalies changes from NW-SE in
cratonic part to NE-SW, marks the contact of the craton
and EGMB. The EGMB can be divided into two segments
based on magnetic signature (Anand and Rajaram, 2003).
The region north of 18.5° N being magnetically flat while
the south is represented by linear anomalies. Strong WNWESE trending anomalies along the Mahanadi graben in
the north reflect the Sukinda thrust that marks the contact
between EGMB and the Singhbhum Craton.
Magnetic Sources and their Relative Depths
The analytic signal of the total field (Nabighian, 1972;
Nabighian, 1974; Roest et al., 1992) and Euler solutions
(Thompson, 1982; Reid et al., 1990) have been computed
to get a picture of the aerial distribution, nature and depth
of magnetic sources in Peninsular India. The generated
analytic signal map is shown in figure 3; highs represent
magnetic sources. We find that within the cratons, the
magnetic sources are mainly related to iron ore, schist
belts, intrusives and dyke systems, while within the mobile
belts: EGMB, CITZ, and SGT, the magnetic sources
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S.P. ANAND AND MITA RAJARAM
identified from the analytic signal map are related to
exhumed rocks of high-grade granulites mainly
charnockites. A visual interpretation of the analytic signal
map (Fig. 3) shows heavy concentration of magnetic
sources in SGT that are confined to the north and south
by the Orthopyroxene isograd (OPx) and AKSZ
respectively. Laboratory measurements (Ramachandran,
1990; Ajaykumar, 2004) of the major rock types in Block
I revealed that unaltered charnockites are having the
highest susceptibility while the khondalites, hornblende,
biotite gneiss, etc. have very low susceptibilities. Hence
the magnetic sources in Block I calculated using the
analytic signal method is interpreted in terms of unaltered
charnockites that are exposed and in the subsurface. The
Kerala Khondalite Block (KKB), where there is large
exposure of khondalitic pellite appears as a broad high in
the anomaly map (Fig. 2), shows no signature in the
analytic signal map suggesting that magnetisation of
khondalite is less than that of charnockites. The west coast
fault is seen as a system of faults, intersected by various
cross faults. The NE-SW trending belt of the basic igneous
rocks associated with the intrusives of Hunsur lineament
(HLn), finds expression in the analytic signal map. The
zone between MBSZ and the AKSZ is characterised by
many maxima representing extensive sources signifying
that the host province is magnetic. On comparison with
figure 1 it can be seen that most of this region is covered
by gneiss that have very low susceptibilities. It is therefore
inferred that the retrogression of charnockites into
hornblende and biotite gneiss in the subsurface is very
less in this region. It is interesting to note that there are
no magnetic sources in the small region bounded by PCSZ,
Bhavani lineament (BhLn) and Cauvery fault, suggesting
that the process of retrogression of charnockites into
hornblende biotite gneiss is very high. Signature of the
Nilgiri block (S1) is evident on the analytic signal map.
The line of separation of the crystallines and sedimentary
basins can be clearly discerned from this map. In the MB,
the magnetic sources (charnockites) are concentrated
towards the east of the Kerala/Periyar lineament (KPLn),
where there are large exposures of hornblende-biotite
gneiss (GSI, 1995). Hence it is inferred that charnockites
underlie the hornblende-biotite gneiss at shallow depths
in this region. On comparison with geology map (Fig. 1)
it can be seen that the region west of KPLn is also occupied
by charnockites but their signatures in the analytic signal
map is not prominent. This maybe related to the low
susceptibility of these charnockitic rocks either due to
extensive weathering or due to processes associated with
retrograde metamorphism supported by rock susceptibility
measurements (Ramachandran, 1990; Ajaykumar, 2004).
Within Block II, the mapped and subsurface extension
of iron ore belts viz, Bailadilla (B), Sandur (Sn),
Bababudan (Bb) and Kudremukh (Kh) and the mafic flows
associated with schist belts are clearly brought out. There
is a marked difference in the magnetic anomaly pattern
and magnetic sources in the Western and Eastern Dharwar
Craton that has been explained in terms of the difference
in environment of deposition and/or the difference in
grades of metamorphism (Anand and Rajaram, 2002).
The magnetic sources along the shoulders of the GG and
EGMB are inferred as charnockites exposed and in the
sub-surface. Susceptibility measurements (Murthy and
Rao, 2001) in the EGMB show that khondalites have an
average susceptibility of 10 μcgs units, leptenites
300 μcgs units while that of charnockites is 2000 μcgs
units and supports our inference from the analytic signal.
The high-grade metamorphic rocks of the Karimnagar
Granulite Belt (KGB), forming the western shoulder of
the GG (Anand and Rajaram, 2003), turns south and
appears to join the EGMB. The EGMB is divided into two
segments based on the metamorphic history (Anand and
Rajaram, 2003). The GG and the Chattisgarh Basin (ChB)
are devoid of major magnetic sources. Two parallel lines
running E-W (Ln1, Ln3) across the Bastar Craton evident
on this map is possibly related to system of dykes,
suggesting that rift-related tectonics was active in the
geologic past of this region.
The magnetic sources in the CITZ are restricted to the
south by the MPS. Signature of the mapped Central Indian
Shear is very weak. Much of the sources in the northwest
part of CITZ are related to the Deccan traps that extend
up to the western shoulder of Mahanadi graben below
the alluvium. The northern and southern edge of the
Singhbhum block, viz., the NPS and the Sukhinda thrust
is found associated with magnetic minerals. The highs
related to the Singhbhum iron (HL2) ore are amply evident
on the map.
Euler solutions (Thompson, 1982; Reid et al., 1990)
were generated for different structural indices and using
different grid intervals. We found that with smaller grid
interval, the solutions were noisy. The Euler solution
presented here are for grid interval of 5 km. The best
solutions (tight clustering) were obtained for SI = 2.
Theoretically SI = 2 represents two-dimensional sources
(horizontal cylinder/pipe). The Euler solutions as seen in
figure 4 would give an idea of the depth estimate of the
magnetic sources. It may be noted that the exposed
charnockites, due to weathering may have their
susceptibility lower than the sub-surface charnockites that
would then represent the magnetic sources. Within Block
I, the Euler solutions along the AKSZ show shallower
sources on the exposed charnockites and dips southeast
to greater depths. The West Coast fault between 10° and
11° N is shallow, implying sources to the north and south
are deeper and that the West Coast fault is dissected. The
crystalline sedimentary contact fault does show deep
sources. Further, the sources along the Cauvery fault (CFl)
IAGR Memoir No. 10
AEROMAGNETIC SIGNATURES OF CRATONS AND MOBILE BELTS
Fig. 3. Analytic signal map of the aeromagnetic anomaly of Peninsular
India depicting the magnetic sources (the abbreviations used
in the figure are as mentioned in the text). The highs represent
magnetic sources.
are deep, with the fault being terminated by the Bhavani
lineament (BhLn). It is interesting to note that the source
depths are shallow around the region where the
charnockites are exposed and deep where it is in the subsurface. The PCSZ shows up as a continuous linear band
in the Euler solutions and the sources associated are at an
average depth of 5.5 km. Thus it appears that the Palghat
Cauvery Shear is a major tectonic element, possibly a
suture zone, which according to Gopalkrishnan (2003)
divides the shield into two distinct tectonic terrains.
Another EW fault/contact zone is evident south of PCSZ
at an average depth of 4 km. This was not previously
identified by geological or geophysical studies. Large
numbers of EW faults/shears identified within SGT
suggests that NS compressive forces might have been
active in the geological past that triggered the exhumation
process within SGT.
Within Block II, the sources along the Chitradurga
Boundary fault appear to be terminated by a NE-SW
lineament to the north of Dharma-Tungabadhra lineament.
The magnetic sources associated with Hungund-Kustagi
schist belt are relatively shallow compared with RaichurDeodurg belts. The ENE-WSW (Ln2) trending magnetic
sources, at an average depth of 3.5 km can possibly be
IAGR Memoir No. 10
239
related to subsurface intrusives and may have implications
in the formation of Shimoga Basin in the Western Dharwar
Craton.
Within Block III, from Euler solution the depth to top
of the magnetic sources associated with Main Peninsular
Shear is found to be at an average depth of 5 km. It is
interesting to note that in the region where no fault has
been mapped in the surface between the Tan shear and
the Sukinda thrust, the Euler solutions are very deep. They
have been down faulted by the Mahanadi Gondwana
Graben and covered with sediments thereby having no
surface expression and leading to the conjecture that the
Tan shear extends below the Gondwanas into the
Barabhum thrust/NPS (Yedekar et al., 1990). From the
spectral analysis of regional Bouguer gravity data, Agarwal
et al. (1995) proposed a fault zone in the crust below the
Deccan traps. This fault zone matches rather well with
the Main Peninsular Shear. There is a NW-SE fault within
the Proterozoic Chattisgarh Basin, possibly forming a
conducive zone for mineralisation. The Singhbhum block
stands out as a separate block with its edge being defined
by the Main Peninsular shear to the south and west and
the NE-SW trending shear that merges with the NPS to
the north. The contact of the Mahakhosals, within the
CITZ, is also brought out clearly in the Euler solution map.
Fig. 4. Euler solutions of the aeromagnetic anomaly map of Peninsular
India for SI = 2. The depths mentioned are from the observation
height of 1.5 km.
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S.P. ANAND AND MITA RAJARAM
Curie Isotherm Depth and Heat Flow
The depth of the Curie isotherm (Rajaram, 2007) is
the depth at which crustal rocks reach their Curie
temperature. As magnetite with a Curie temperature of
580 °C is believed to be the dominant magnetic mineral
in the deep crust within the continental region, it is
reasonable to assume that below the Curie isotherm depth
the lithosphere is virtually non-magnetic. Here we utilise
the available aeromagnetic data over the Indian
subcontinent to calculate the Curie isotherm depths. The
one dimensional heat conduction equation is then utilised
to convert these depths into heat flow values so that these
may be compared with heat flow measurements.
To estimate the Curie isotherm depth of the region
(Anand, 2005), depth estimates were made using spectral
analysis (Spector and Grant, 1970). It is very difficult to
estimate the depth to the bottom of the magnetic sources,
Zc as the spectra in the Fourier domain is dominated at all
wavelengths by the contribution from the shallower parts.
The limited depth extent of the body leads to a maximum
in the power spectrum and when a significant spectral
maximum does occur, indicating that the source bottoms
are detectable, the wavenumber of this maximum kmax is
related to the depth to the Curie isotherm Zc and the
depth to the top of the body, Zt, by the following relation
(Blakely, 1995):
2.5 degrees by 2.5 degrees (2°×2° for southern region) on
the edges of the studied area.
A very striking feature of the calculated Curie isotherm
depth is the fact that the three cratons depict high values,
being surrounded by thin mobile belts. The Curie depths
are shallow in the Southern Granulite terrain and the
region to the north of the Bastar and Singhbhum Cratons
(the Central Indian Tectonic Zone, CITZ). The highest
Curie depth within the Dharwar Craton is 39 km while
that in the Bastar Craton is 38.4 km and that in the
Singhbhum craton is 35.5 km. The Curie isotherm depth
in the Godavari graben and the Eastern Ghat Mobile Belt
is low at around 30 km while that in the CITZ ranges
from 25.4 km to 30.2 km. Also the depth in the Southern
Granulite Terrain can be as low as 23.2 km. These results
seem to compare well with those obtained by Rajaram
et al. (2003) using filtering techniques where they find
that the shallow depth obtained in the SGT region is
related to a velocity change observed in the seismic
reflection/refraction profiles at the same depth in the
Kuppam-Palani transect and could reflect a compositional
change (Reddy et al., 2003).
We utilise the 1-D heat conduction, steady state model
for the continents (Fox Maule et al., 2005) assuming that
the depth to the Curie isotherm, represents the 580 °C
Curie isotherm of magnetite. The heat flux Q at the surface
(1)
To check if a spectral peak exists, the whole region
was initially divided into 2°×2° overlapping blocks and
for each block a radially average power spectrum was
calculated. Well-defined spectral peaks were not obtained
for 2°×2° blocks indicating that the source bodies have
deep base. Hence 3°×3°, 4°×4° and 5°×5° blocks were
considered and the procedure repeated. Well-defined
spectral peaks were obtained for 4°×4° in the Southern
Peninsular India up to 18° N and 5°×5° for the rest of the
region. Before computing the spectrum, the aeromagnetic
data were upward continued to 5 km to eliminate the
interference from shallow near surface sources. After
computing the radially averaged power spectrum of the
aeromagnetic data, a best fit straight line was drawn and
its slope helped to determine the depth to the top of the
magnetic sources. The depths to the bottom of the
magnetic sources were calculated iteratively using
equation (1). The entire area was divided into overlapping
cells with each cell being increased by a degree in
longitude and latitude; thus the whole of Peninsular India
was divided into 72 blocks. Of these 12 blocks did not
yield a well-defined spectral peak. It may be noted that
due to the stringent requirement of 5°×5° grid size (4°×4°
for southern region), no information on the depth to the
Curie isotherm could be ascertained for a region of
Fig. 5. Heat flow map of Peninsular India derived from the Curie
isotherm depths calculated from the aeromagnetic anomalies.
IAGR Memoir No. 10
AEROMAGNETIC SIGNATURES OF CRATONS AND MOBILE BELTS
(z = 0), is calculated using the equation (Fox Maule et
al., 2005) given below, where d is the scale depth assumed
as 8 km, k is the thermal conductivity and H0 is the surface
heat production:
In this equation we utilise the parameters k and H0
from published literature (Rao et al., 2003). The k and H0
values used are: 2.8 W/mK and 1.2 μW/m 3 for
Western Dharwar (WD), 2.8 W/mK and 2.3 μW/m3 for
Eastern Dharwar (ED), 2.8 W/mK and 0.3 μW/m3 for the
Southern Granulite Terrain and 3.0 W/mK and
1.1 μW/m3 for the Central Indian region. Figure 5 is a plot
of the heat-flow map thus generated. We find high heat
flow values are associated with SGT (49 to 79 mW/m2)
and relatively low values are associated with the Dharwar
Craton with Western Dharwar ranging essentially from
50 to 58 mW/m2 with some isolated values at 74 mW/m2
and Eastern Dharwar ranging from 56 to 74 mW/m2.
Within the Dharwar Craton, we find from figure 5 that by
and large the heat flow values over Eastern Dharwar (ED)
are higher than those in the Western Dharwar (WD),
and this is borne out by measurements as reported
by Rao et al. (2003); the measured range of heat flow
values in the Western Dharwar is from 25 to 50 mW/m2
while that in the Eastern Dharwar is 40 to 75 mW/m2.
However, the values within the SGT (49–79 mW/m2)
do not match the measurements (28–42 mW/m2). This is
related to the fact that the bottom of the magnetic crust
does not reflect the Curie isotherm but a compositional
change as noted by Rajaram et al. (2003) corresponding
to the change in seismic velocity. The comparison of the
calculated heat flow values with the observations reveal
an excellent match in the Central Indian region which
comprises of the CITZ (64–75 mW/m2) and two cratons
(Singhbhum 56–60 mW/m2 and Bastar 52–64 mW/m2).
As expected we obtain high heat flow values in the CITZ
(observed: 69–79 mW/m2) and relatively low values in
the two cratons (observed: 51–63 mW/m2). The reasonably
good match of the calculated heat flow with the
measurements lends credence to the methodology adopted.
Conclusions
The available aeromagnetic data collected at the
reconnaissance scale (1:250,000) have been compiled to
generate an aeromagnetic map of India up to 25° N. The
varying structural trends within this map are controlled
by the tectonics of the region and one can clearly identify
the three cratons: Dharwar, Bastar and Singhbhum, and
the mobile belts: the EGMB, SGT, and CITZ. An analysis
of the data helps to identify the magnetic sources and
estimate the depth to the top of the magnetic sources.
IAGR Memoir No. 10
241
We find that the main magnetic sources within the cratons
(Dharwar, Bastar and Singhbhum) are related to iron ore
belts, schist belts and dyke systems while the sources
within the mobile belts (EGMB, SGT and the CITZ) are
due to the exhumed crust reflecting the high-grade
granulite belts like charnockites. The magnetic data helps
to unravel the subsurface, below the sediments and trap
cover. The aeromagnetic data have also been utilised to
calculate the Curie isotherm depth and the heat flow using
1D conductive, steady state heat flow model for the
continents. The heat-flow values match reasonably well
with the observations. The heat flow values in the cratons
are low and they are high within the EGMB and CITZ.
Rajaram et al. (2006) have published a composite
magnetic anomaly map by putting together available
ground and marine magnetic data to complement the
available aeromagnetic data over India and its contiguous
region, to bring out the utility of magnetic data. Even in
this map crucial data gaps remain, for example, over the
Deccan trap covered regions. If the aeromagnetic data
over the whole country were available this would provide
a uniformly distributed data set to unravel the subsurface
geology and the tectonic structure and would help to
generate a heat flow map of the country.
Acknowledgment
We wish to thank Sri T.M. Mahadevan for several
discussions and suggestions and for his continued interest
in our work.
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