Insights into the Crustal Structure and Geodynamic Evolution of the Southern Granulite Terrain, India, from Isostatic Considerations

Insights into the Crustal Structure and Geodynamic Evolution of the Southern Granulite

Terrain, India, from Isostatic Considerations

NIRAJ KUMAR,1 A. P. SINGH,1 and B. SINGH1

Abstract—The Southern Granulite Terrain of India, formed

through an ancient continental collision and uplift of the earth’s

surface, was accompanied by thickening of the crust. Once the

active tectonism ceased, the buoyancy of these deep crustal roots

must have supported the Nilgiri and Palani-Cardamom hills. Here,

the gravity field has been utilized to provide new constraints on

how the force of buoyancy maintains the state of isostasy in the

Southern Granulite Terrain. Isostatic calculations show that the

seismically derived crustal thickness of 43–44 km in the Southern

Granulite Terrain is on average 7–8 km more than that required to

isostatically balance the present-day topography. This difference

cannot be solely explained applying a constant shift in the mean sea

level crustal thickness of 32 km. The isostatic analysis thus indi-

cates that the current topography of the Southern Granulite Terrain

is overcompensated, and about 1.0 km of the topographic load

must have been eroded from this region without any isostatic

readjustment. The observed gravity anomaly, an order of magni-

tude lower than that expected (-125 mGal), however, shows that

there is no such overcompensation. Thermal perturbations up to

Pan-African, present-day high mantle heat flow and low Te toge-

ther negate the possible resistance of the lithosphere to rebound in

response to erosional unloading. To isostatically compensate the

crustal root, compatible to seismic Moho, a band of high density

(2,930 kg m-3) in the lower crust and low density (3,210 kg m-3)

in the lithospheric mantle below the Southern Granulite Terrain is

needed. A relatively denser crust due to two distinct episodes of

metamorphic phase transitions at 2.5 Ga and 550 Ma and highly

mobilized upper mantle during Pan-African thermal perturbation

reduced significantly the root buoyancy that kept the crust pulled

downward in response to the eroded topography.

Key words: Southern granulite terrain, gravity anomalies,

Isostasy, depth of compensation, Moho.

1. Introduction

After the Himalayas, the Southern Granulite Ter-

rain (SGT) comprises the second highest peak of the

Indian subcontinent occupied by high-grade (granulite

facies) metamorphic rocks that generally originate at

20–30 km depth (Fig. 1). It has traditionally been

described as a vestigial facet of the Dharwar craton,

however, owing its present height to intermittent

movements taking place since the Late Mesozoic

through Quaternary (RADHAKRISHNA, 1993; VALDIA,

1998). Several orogenic models having different burial

and subsequent uplift to deep crustal rocks have been

attempted for uplifted high-grade Archaean rocks of

the SGT (RADHAKRISHNA, 1969; DRURY et al., 1984;

THAKUR and NAGARAJAN, 1992; SRINAGESH and RAI,

1996; CHETTY and BHASKAR RAO, 2006; SANTOSH et al.,

2009). On the one hand, these models of the SGT

evoke long-lived crustal roots that have remained

undisturbed, being imprints of Precambrian tectonism

(SRINAGESH and RAI, 1996; GUPTA et al., 2003) and, on

the other hand, repeated remobilization in response to

seven episodes of thermo-tectonic perturbations at

*2.5 Ga, *2.0 Ga, *1.6 Ga, *1.0 Ga, *800 Ma,

600 Ma and *550 Ma, and two distinct episodes of

metamorphism at *2.5 Ga and *550 Ma (e.g.,

RADHAKRISHNA, 1993; VALDIA, 1998; GHOSH et al.,

2004). To understand these processes, extensive geo-

morphological, geological and geophysical studies

have been carried out in the recent past (RAMAKRISH-

NAN, 2003; CHETTY et al., 2006; SANTOSH et al., 2009).

In spite of all these studies, the precise nature of the

lower crust beneath the SGT and complex nature of

crust-mantle connections are still a matter of specu-

lation and debate.

The orogenic mountains with charnockite and

khondalite along the ancient sutures represent expo-

sures of the lower continental crust through continental

collision and deep erosion (FOUNTAIN and SALISBURY,

1981). According to DEWEY and BURKE (1973), contin-

ued convergence leading to uplift of the earth’s surface

is accompanied by thickening of the passive continental

1 National Geophysical Research Institute (Council of Sci-

entific and Industrial Research), Uppal Road, Hyderabad 500007,

India. E-mail: [email protected]

Pure Appl. Geophys.

� 2010 Springer Basel AG

DOI 10.1007/s00024-010-0210-1 Pure and Applied Geophysics

crust, and buoyancy of these deep crustal roots supports

their topography (THOMAS, 1992; FISCHER, 2002). Fol-

lowing a period of erosion, successive lower levels of

the crust become exposed in the reactivated terrain.

Unless the lithosphere is mechanically very rigid, post-

tectonic erosion of the topographic mass is accompa-

nied by uplift of a buoyant crustal root and/or inflow of

mantle. On the other hand, if the lithosphere is very

weak, the net change in mass over time would be zero,

maintaining local isostatic equilibrium (FISCHER, 2002).

Subsequent tectonic processes, such as the lithospheric

delamination or convective removal of tectonically

thickened lithospheric mantle, extensional collapse and

re-equilibrium of the Moho, may modify the crustal

roots (LEECH, 2001) vis-a-vis the surface topography in

a manner compatible with the state of isostatic

equilibrium.

The surface elevation, density contrast between

the lower crust and upper mantle, and the crustal root

structure are intimately related through the phenom-

enon of isostasy and best manifested in the gravity

data. The main objective of this paper is therefore to

provide a 3D crustal structure of the SGT that

maintains the state of isostatic equilibrium.

2. Topography, gravity anomalies and state

of isostasy

2.1. The data

The topographic map (Fig. 2) is prepared with data

from the original version of the SRTM 90M eleva-

tion data (ftp://edcsgs9.cr.usgs.gov/pub/data/srtm)

over the continent and GEBCO bathymetry data

Figure 1Generalized geology of the Southern Granulite Terrain, south of 13.5�N (after GSI, 1998). Thin dashed line outlines the three geological

provinces, namely EDC Eastern Dharwar Craton, SGT Southern Granulite Terrain and WDC Western Dharwar Craton. Major geological

formations of the region are 1 Quaternary sediment, 2 Gondwana Supergroup, 3 K-Granite, 4 Archaean Schist belt, 5 Alkaline complex, 6

Gneiss (undifferentiated), 7 Charnockite and 8 Khondalite

N. Kumar et al. Pure Appl. Geophys.

ftp://edcsgs9.cr.usgs.gov/pub/data/srtm

(http://www.ngdc.noaa.gov/mgg/gebco/gebco.htm)

for the oceanic region. Based on gravity data collected

at 7,862 stations and merged with an almost equal

amount of the available data, a gravity anomaly map

(Figs. 3a, 4a) of the region was prepared (GMSI, 2006;

NIRAJ KUMAR et al., 2009). The geodetic survey team

provided the position location and elevation of about

1,100 gravity stations along the Kuppam-Kanyaku-

mari seismic line. A large number of other gravity

stations were located near the benchmarks and the spot

elevations appearing in the Survey of India toposheets.

About 300 differential global positioning stations were

also established to fill the gaps and give better control

to the elevation. Tied to these places of known eleva-

tion, the rest of the gravity stations were established

using altimeters with a maximum possible error of

5 m. The entire data set, consisting of over 14,000

gravity station points, corrected for the earth’s tide and

linear drift of the instrument, was tied to the IGSN

1971 gravity base (MORELLI et al., 1974) and pro-

cessed using the GRS 80 formula (MORITZ, 1980).

Density used for the complete Bouguer reduction was

2,670 kg m-3, as appropriate global crustal average

(HINZE et al., 2005). Following LEAMAN (1998), terrain

correction was computed by approximating the topo-

graphic masses with polyhedrons within a radius of

167 km using high-resolution digital elevation data

obtained by merging the station elevation data pro-

jected on same datum with the SRTM 90M elevation

data over the continent and GEBCO bathymetry data

for the oceanic region. Computations show that the

value of terrain correction has a maximum value of

about 19 mGal over the Palani-Cardamom hills. The

data were further subjected to ‘‘indirect effect’’

removal to account for the Central Indian Ocean Geoid

low (CHAPMAN and BORDINE, 1979), which would

Figure 2Topography (in m) of the Southern Granulite Terrain, south of 13.5�N and adjoining Oceans. Thin dashed line outlines the Dharwar craton and

the SGT. ACSZ Achankovil shear zone, AH Agastyamalai hill, BH Biligirirangan hill, CH Cardamom hill, MP Mysore plateau, NH Nilgiri

hill, PG Palghat Gap, SH Shevaroy hill, TNP Tamil Nadu plain, VH Varushanad hill

Insights into the Crustal Structure and Geodynamic Evolution

http://www.ngdc.noaa.gov/mgg/gebco/gebco.htm

otherwise cause a negative bias varying from about

18.4 mGal towards the north to 19.3 mGal towards the

south in the study region (NIRAJ KUMAR et al., 2009).

The source of this long-wavelength gravity anomaly is

assumed to be situated deep in the mantle and signif-

icant for our isostatic studies being within its

characteristic wavelength. The gravity anomalies,

corrected for indirect effect, are referred to hereafter as

free air and Bouguer anomaly only. It may be men-

tioned here that the gravity anomaly values are subject

to errors mostly caused by uncertainty in the station

elevations and probably yield a maximum uncertainty

of ±1.5 mGal, which may most likely occur in the

areas of extreme topographic relief.

Isostatic anomaly maps have intuitively been used

to interpret the state of isostasy, a practice that is

generally unproductive. It is more particularly so

because the effects of deep crustal and upper mantle

density distributions that support the topography in a

manner compatible with the principle to isostasy

were removed as isostatic correction and what is left

as isostatic anomaly is mainly caused by intra-crustal

masses (SIMPSON et al., 1986). Recently, the words

‘‘regional’’ and ‘‘residual’’ have been inserted to be

more explicit in the interpretation of such isostatic

gravity maps. A gravity field is caused by the masses

that significantly compensate topography as the

isostatic regional field, and the gravity anomalies

remain after subtracting the isostatic regional field

from the Bouguer gravity field as an isostatic residual

field (SIMPSON et al., 1986). In the present case, two-

dimensional isostatic residual anomalies were com-

puted taking into account the Airy-Heiskanen model

for isostatic compensation of topography using the

Parker algorithm (PARKER, 1972). It was computed by

subtracting the gravity effect of a crustal root

(isostatic regional anomaly) that compensates for

the present-day topographic load from the Bouguer

anomaly. The parameters used for the calculation

were topographic density (2,670 kg m-3), mean sea

level crustal thickness (32 km) and density contrast

across the base of the model crust (400 kg m-3).

2.2. Topography

The most prominent topographic feature of the

region is the Western Ghats, with a maximum

elevation of about 2,500 m (Fig. 2). The Palghat

Gap is the major breach within the Western Ghats that

connects the west coast with the peninsular region of

the east. With an average width of 13 km and an

elevation of about 70 m, the Palghat Gap is sur-

rounded by the Nilgiri (2,670 m) and Biligirirangan-

Shevaroy (1,630 m) hill ranges towards the north and

the Palani-Cardamom (2,500 m) hills in the south.

Further south, the relatively low-lying Achankovil

Shear Zone is surrounded by the approximately

1,700-m-high Varushanad hill range to the north and

the more than 1,800-m-high Agastyamalai hill to the

south. The Western Ghats merge with the Mysore

plateau to the north of Palghat Gap and the low-level

landform of the Tamil Nadu plains toward the east.

Many consider the Western Ghats a relict mountain

range, while others suggest that Mesozoic tectonic

uplift is responsible for the lofty heights and youthful

stage of the ranges (RADHAKRISHNA, 1993; VALDIA,

1998). Apparently, the present-day landscape of the

SGT is a coupled response to the kinematic of

the plate convergence, material properties of the

lithosphere, surface erosion and isostatic adjustment

of the thickened crust.

2.3. Free air anomaly

The free air gravity anomalies over isostatically

compensated orogenic belts will be independent

of long-wavelength topographic variation but are

representative of short-wavelength topography or

subsurface density distribution (TURCOTTE and

SCHUBERT, 1982). In such cases, long-wavelength

topographic masses match with negative mass distri-

bution at depth; their gravity effect becomes equal

with an opposite sign, and the free air gravity closes to

zero values (WOOLLARD, 1959; SUBBA RAO, 1996). One

way to judge whether complete isostatic compensa-

tion has been attained is therefore to examine the

pattern of free-air anomalies over the region of

interest. The general level of the free air anomalies

ranging from 25 to -50 mGal, however, shows large

fluctuations ranging from less than -50 mGal along

the west coast to more than ?100 mGal over the

Nilgiri and Palani-Cardamom hills (Fig. 3a). Despite

a ±50 mGal scatter in free-air values caused mostly

by variations in surface and near-surface geology, the


mean free-air anomaly value approaches -25 mGal

up to a height of about 500 m and then follows a

distinct positive correlation with the elevation

(Fig. 3b). Following WOOLLARD (1959), negative mass

distribution at depth overcompensates for topographic

masses of up to 500 m, whereas topographic masses

of 500–600 m apparently match with negative mass

distribution at depth. Since the topography above

600 m is undercompensated (the negative mass dis-

tribution at depth is too small), the free air anomaly

does not average to zero, and a positive free air effect

to that of the topographic mass is observed. This

positive nature of free air anomalies apparently

corresponds to the region of excess of masses (Nilgiri

and Palani-Cardamom hills), and the crust has suffi-

cient strength to sustain this local feature. In the

absence of topography, isolated patches of positive

free air anomaly over the Tamil Nadu plains are

nevertheless representative of subsurface density

distribution.

2.4. Bouguer anomaly

The short-wavelength Bouguer anomalies (Fig. 4a)

are the manifestation of the near-surface density

distribution associated with exposed geological for-

mations. Particularly in Dharwar craton relatively

high gravity values correlate well with the schistose

rocks and gravity lows mostly with the granitic

intrusions (NAQVI, 1973; KRISHNA BRAHMAM, 1993).

Similarly, a pronounced relative gravity high of -25

to ?30 mGal characterizes the Tamil Nadu coastal

plain towards the east. With little or no topography,

this anomaly was attributed to the basic difference in

the crustal composition (SUBRAHMANYAM and VERMA,

1986). However, this correlation with exposed geo-

logy is less obvious over Hassan towards northwest

where the most prominent long-wavelength negative

gravity anomaly of the southern India shield is not

limited to any specific geological formation but is

well spread over granites, gneisses and schists as

well. Bouguer anomalies over the SGT also exhibit

considerable variation over the charnockites them-

selves (SUBRAHMANYAM and VERMA, 1986). The most

pronounced gravity anomaly of the SGT is the broad

gravity low of -85 mGal centered over the Palani-

Cardamom hills. The long-wavelength nature of these

lows and the lack of correlation with the surface

geology suggest that the sources of the anomalies lie

at greater depth. Another prominent gravity low over

the elevated Western Ghats is also assumed to be by

crustal thickening due to isostatic compensation. A

relative gravity high over elevated Nilgiris is, how-

ever, an anomalous feature since the high region is

expected to have a Bouguer anomaly low due to the

compensation of topography.

Over SGT the Bouguer anomaly (plotted against

surface elevation) decreases with elevation up to

600 m, whereas elevations more than 600 m show no

apparent trend with the Bouguer anomaly, which

remains at about -60 mGal (Fig. 4b). This non-

linearity of the relationship of the Bouguer anomaly

to elevations above 600 m probably indicates that the

crust has sufficient strength to sustain this local

feature of denser crust. Alternatively, it indicates the

decrease in the crust-mantle density contrast with

further increases in elevation vis-a-vis crustal thick-

ness (WOOLLARD, 1959). It may be due to a

‘‘significant increase in the mean density of the crust

with increasing thickness,’’ and/or ‘‘there must be an

anomalous negative mass within the upper part of the

mantle whose proximity, where the crust is thickest,

is sufficient to lower the apparent mean density

contrast between the crust and the mantle’’ (FISCHER,

2002). Both of these factors tend to reduce the

effective density contrast between the crust and sub-

crust in elevated regions, causing Bouguer anomalies

that may not be as low as required by the average

elevation (WOOLLARD, 1959).

2.5. Isostatic residual anomaly

The isostatic residual map (Fig. 5) is no different

from a Bouguer residual map constructed by incor-

porating a correction for isostatic compensating

masses of the long-wavelength topography (short

wave-length topography is maintained by the strength

of the upper lithosphere) and is mainly caused by

intra-crustal masses (SIMPSON et al., 1986). In topo-

graphic highs like Nilgiri positive free air, Bouguer

and isostatic residual anomalies clearly indicate an

excess of mass beneath the region. QURESHY (1971,

1981) opined that the uplift of the Nilgiri hill

was caused by thickening of the crust through


incorporation of heavy material moving from the

upper mantle into the crust. Recent waveform

inversion examination delineated a deep Moho lying

at a depth of about 60 km and high-velocity/density

material at a depth of about 10 km beneath the region

(GUPTA et al., 2003; GUPTA and RAI, 2005). Another

significant feature is the existence of a belt of

negative isostatic residual anomaly on the west coast.

This feature extends from Mangalore to the southern

tip of India, a portion of which veers eastward along

the Palghat Gap and further northeast. Intuitively, a

low free air, Bouguer and isostatic residual anomaly

indicates a thicker than normal crust due to isostatic

compensation. SUBRAHMANYAM and VERMA (1980),

however, hold that the isostatic residual anomaly

does not seem to be controlled by topography as it

spreads over both the low-lying Palghat Gap/coastal

plains and the elevated Western Ghats. The magni-

tude of negative isostatic residual anomaly could be

explained by a number of factors, singly or collec-

tively, such as lighter density material like granite, an

undissipated root of the Western Ghats, a local

decrease in upper mantle density, probably caused by

incorporation of the root into the mantle, or devel-

opment of a low-density zone in the upper mantle

resulting from some deep-seated phenomena that

might have caused the epeirogenic uplift of the SGT

(QURESHY, 1971, 1981). How the condition of isostasy

is maintained is not very clear, but it does seem that

some sort of material exchange between the crust and

upper mantle is involved (QURESHY, 1981; GUPTA and

RAI, 2005).

3. Implications of Isostatic regional anomaly

The isostatic regional anomaly (Fig. 6) is the

manifestation of (1) the mechanical strength of the

lithosphere, (2) the Moho configuration, and/or (3) the

deep crustal and upper mantle density distributions

that support the topography in a manner compatible

with the principles of isostasy. In the presence of a

crustal load, a flat Moho geometry corresponds either

to a very high flexural rigidity or to compensation in

the mantle lithosphere (EBBING et al., 2007). In such

cases short-wavelength topography (equivalent to

zero loads) is maintained by the strength of the

lithosphere; the Moho interface has no undulation and

is located at the normal crustal depth. The observed

Moho depths, largely derived from the seismic

sounding profiles, 3D tomography and receiver func-

tion analysis, show a larger variation ranging from a

minimum of 33 km beneath the Kolar to a maximum

of 60 km beneath the Nilgiri (Fig. 8; KAILA et al.,

1979; RAVI KUMAR et al., 2001; GUPTA et al., 2003;

REDDY et al., 2003; GUPTA and RAI, 2005; PRASAD

et al., 2007; ABHISHEK RAI et al., 2009). These large

thickness variations occurring over a horizontal dis-

tance much shorter than the *150-km scale lead us to

assume that the crust is in local isostatic equilibrium

with the mantle.

In Airy-Heiskanen model for local isostatic

compensation surface topography was compensated

at the Moho, which can be calculated using the

formula:

t xð Þ ¼ h xð Þ q=DqþT

where t(x) is the depth to the crust/mantle boundary

at location x, h(x) is the elevation at location x, q is

the crustal density at the atmosphere-crust interface,

Dq is the density contrast between the lower crust

and upper mantle, and T is the mean sea level

crustal thickness (CHAPIN, 1996). In this case the

Moho relief is expected to be an amplified mirror

image of the topography. The amplification factor is

the ratio of the density of topographic masses

(*2,670 kg m-3) to the density contrast across the

Moho (*400 kg m-3). Thus, the expected amplifi-

cation factor is about 6.7; given an elevation change

of 2,500 m from the coast to the major hill ranges,

the Moho relief should be at least 17 km over and

above the mean sea level crustal thickness.

An indirect approach to the state of isostasy is

thus made by first calculating T using the crustal

thickness values obtained by seismic investigations

(KAILA et al., 1979, 1981; RAVI KUMAR et al., 2001;

GUPTA et al., 2003; GUPTA and RAI, 2005;

REDDY et al., 2003; PRASAD et al., 2007) and Bouguer

gravity data of the region. The Moho depth-Bouguer

gravity curve crosses the zero Bouguer axes at a

continental sea level crustal column of approximately

32 km (Fig. 7). Our value is close to the average sea

level crustal thickness value of 33–35 km calculated

for India (QURESHY, 1981; RAMBABU, 1997) and


Figure 3a Free air anomaly map (in mGal) of the Southern Granulite Terrain. Station distribution of gravity data are marked by stars. Thin dashed line

and abbreviations are as in Fig. 2. b Elevation (in m) plotted against the free air anomaly (in mGal) of the region


Figure 4a Bouguer anomaly map (in mGal) of the Southern Granulite Terrain (NIRAJ KUMAR et al., 2009). Thin dashed line and abbreviations are as in

Fig. 2. b Surface elevation (in m) plotted against the Bouguer anomaly (in mGal) of the region


32 km for the world (WOOLLARD, 1959). It also falls

near the seismic depths of 30 km beneath the Kelsi-

Guhagar region on the west coast in the Deccan

volcanic province (KAILA et al., 1981), and 30 and

35 km beneath the Kavali and Udipi coasts, respec-

tively, in Dharwar craton (KAILA et al., 1979), about

36 km beneath Trivendrum (RAVI KUMAR et al.,

2001; GUPTA et al., 2003; ABHISHEK RAI et al., 2009)

and 38–40 km beneath Chennai (RAVI KUMAR et al.,

2001; GUPTA et al., 2003). Adopting this value for the

mean sea level crustal thickness and taking crustal

and sub-crustal densities as 2,900 and 3,300 kg m-3

(WOOLLARD, 1970), we subjected the isostatic regio-

nal anomaly (Fig. 6) to single density-interface

inversion using the Parker algorithm (PARKER, 1972).

With a full 3D interface depth so derived in the fre-

quency domain, we arrived at the Airy-Heiskanen

depth of compensation in SGT varying between 32

and 45 km (Fig. 8). The one-to-one correlation

between the seismic Moho and the computed depth of

compensation still shows a maximum variation of

about 15 km beneath the Nilgiri hill, on average an

approximately 7–8 km deeper seismic crustal

boundary than expected. These variations mean that

the offset between the seismic Moho and the Airy-

Heiskanen depth of compensation cannot be adjusted

merely by applying a constant shift in the average sea

level crustal thickness value. The observation either

suggests a state of overcompensation for the given

topography or the masses needed to remove the offset

lie within the lower crust and/or the upper mantle.

The 7–8 km of extra deflection in seismic Moho

would result from topographic loads that are about

1.0 km higher than those at present. This implies that a

thickness of about 1.0 km of material must have been

eroded from this region without any isostatic

Figure 5Isostatic residual gravity anomaly (in mGal) map of the Southern Granulite Terrain. It was computed by subtracting from the Bouguer

anomaly the gravity effect of a crustal root that compensates for the present-day topographic load.


adjustment (MISHRA et al., 2004). This non-equilib-

rium in isostatic compensation may be explained if we

assume that the rebound in response to about 1.0 km of

erosion-induced crustal unloading was resisted by the

mechanical strength of the lithosphere (WATTS, 2001).

The seven thermo-tectonic perturbations between Late

Archaean to Neo-Proterozoic (GHOSH et al., 2004),

however, indicate a significantly lower lithospheric

rigidity until the Neo-Proterozoic. During subsequent

erosion the lithosphere was cooled, and the increased

rigidity must have resisted the lithosphere from iso-

static uplift maintaining a thick crustal root. A low Te

(11–16 km; STEPHEN et al., 2003; RAJESH and MISHRA,

2004) together with the Late Cretaceous thermal per-

turbation induced a thinned seismic lithosphere

(100–120 km; PRAKASH KUMAR et al., 2007; JAGADEESH

and RAI, 2008); the present-day high mantle heat

flow (23–32 mW m-2; RAY et al., 2003), however,

indicates a significantly weak lithosphere that could

resist the rebound in response to about 1.0 km of ero-

sion-induced crustal unloading. The 7–8 km of the

extra thickened seismic crust means a mass deficit

associated with the compensating crustal root that

overcompensates the present-day topography. In this

situation we would observe a long wavelength gravity

anomaly of the order of -125 mGals, which is not the

case. The gravity data as such show neither any sig-

nature of overcompensation at Moho depth nor any

requisite mechanical strength present that might have

resisted the rebound to about 1.0 km of erosion-

induced crustal unloading. The isostatic compensation

of the present-day topography may therefore be

explained by the waned buoyancy in the old crustal

root, i.e., (1) the crustal root is anomalously dense, and/

or (2) the upper mantle is anomalously light (FISCHER,

2002).

Figure 6Isostatic regional anomaly map of the Southern Granulite Terrain. It was computed taking into account the Airy-Heiskanen model for isostatic

compensation of topography using the Parker algorithm (PARKER, 1972). Abbreviations are as in Fig. 2


4. Estimated density contrast across isostatically

compensated Moho

The density contrast across the Moho can be

estimated by balancing the mass excess of the

topography against the mass deficit associated with

the crustal root provided the interface geometry is

constrained by other information (MENKE, 1999;

FISCHER, 2002). Density within the crustal root and

density as a uniform upper mantle layer below the

root were treated as two independent parameters and

were allowed to vary until best fitting values, which

maximized the depth variance reduction between the

seismic Moho and the depth of compensation. Our

modeling already showed a difference of the order of

7–8 km between the seismic Moho and the local

depth of compensation for a normal density contrast

of 400 kg m-3 between crust (2,900 kg m-3) and

upper mantle (3,300 kg m-3). The average P-wave

seismic velocities that have been directly measured in

the SGT are 7.1 km s-1 immediately above the Moho

and 8.0 km s-1 in the upper mantle (Fig. 9;

REDDY et al., 2003). A density of 2,910 kg m-3 for

the lower crust and 3,300 kg m-3 for the upper

mantle constrained by the above seismic velocities

(CHRISTENSEN and MOONEY, 1995) was found to be

insufficient in the depth variance reduction. Trusting

the seismic velocity vis-a-vis density, we increased

the lower crustal density to a maximum of

2,930 kg m-3, and the density of the upper mantle

was reduced to 3,210 kg m-3. A further decrease in

the density contrast (280 kg m-3) compatible with

the seismic observations has a minor impact on the

depth variance reduction. By adopting the Airy-He-

iskanen compensation model, MARTINEC (1994) also

observed the same density jump at the Moho under

the continental areas.

The derived 3D crustal root geometry, with a

maximum of over 50 km beneath the Nilgiri and

Palani-Cardamom hills, presents a crustal thickness

of 40–48 km under a large part of the SGT, and

finally reduces to 33–35 km along the coastal regions

(Fig. 10). Despite a reasonably good fit between the

seismic Moho and the isostatically compensated

crustal root, local differences are evident in the error

anomaly obtained by subtracting the gravity response

of the compensated crustal root from the isostatic

regional anomaly (Fig. 11). These can be partly

explained by the resolution of the 3D seismic Moho,

which was intended to explain the isostatic regional

anomalies. Assumption of uniform density contrast

between the lower crust and the upper mantle must

have also added to the lack of resolution. For exam-

ple, a shallower Moho beneath the Nilgiri hill and

deeper Moho beneath the Palani-Cardamom hills as

compared to seismic Moho indicate that the mecha-

nism of isostatic compensation for SGT must involve

lateral variations in density in both the crust and

upper mantle. A linear up-warp beneath the Palghat

Gap, which includes part of the Cauvery basin along

the east coast, also shows departure from the seismic

observations. According to MISHRA et al. (2004),

Palghat Gap is the manifestation of a passive rift-like

structure, probably due to local isostatic adjustments

along the pre-existing shear zones/faults. Alterna-

tively, the up-warp up to a depth of 38 km beneath

the Palghat Gap is suggested as an imprint of conti-

nental collision and crustal decoupling (SINGH et al.,

2003, 2006; RAO and PRASAD, 2006). Continuity of

surface topography and deep crustal roots beneath the

Nilgiri and Palani-Cardamom hills may indicate a

preserved collisional structure in the SGT. Finer

adjustments are thus required for detailed modeling

Figure 7Relation of the depth of seismic Moho discontinuity to Bouguer

gravity anomaly indicates *32 km as average sea level crustal

thickness in the Southern Granulite Terrain


of the complex crustal structure constrained by seis-

mic observations.

5. Possible evidence of density contrast reduction

A close consideration of the gravity data, topog-

raphy and compiled crustal structure suggests the

deep influence of the lithospheric density distribution

on the isostasy of the SGT. Nearly 7–8 km of extra

depth of the seismic Moho over and above the normal

depth of local compensation requires corresponding

density variations in the lower crust and upper mantle

to maintain the isostatic compensation. The relatively

abundant seismic data helped us to reasonably fix the

crustal structure and the density contrast across the

Moho. Further changes in the densities were slight

and restricted to the permissible range to fit the

measured gravity. A lower density contrast than the

normal one (280 kg m-3) between the lower crust

(2,930 kg m-3) and the upper mantle (3,210 kg m-3)

can be explained by an increase of the overall crustal

density by *1% through metamorphic phase trans-

formations in an over-thickened crust.

A higher average Vp velocity, between 6.9 and

7.1 km s-1 (REDDY et al., 2003), provides indirect

evidence of the petrophysical properties of the lower

crust. One plausible composition for such a lower

crust would be a mix of mafic granulite (RAMACHAN-

DRAN, 1992; RUDNICK and FOUNTAIN, 1995). Rocks

close to a mix of mafic granulite and localized zones

of garnet-omphasite-quartz bearing typical eclogite

are observed in the SGT (SHIMPO et al., 2006;

SAJEEV et al., 2009). Mafic granulite to eclogite phase

transition during two episodes of metamorphism at

*2.5 Ga and *550 Ma (GHOSH et al., 2004)

Figure 8Seismic Moho depths (in km; after KAILA et al., 1979, RAVI KUMAR et al., 2001; GUPTA et al., 2003; GUPTA and RAI, 2005; REDDY et al., 2003;

PRASAD et al., 2007) superimposed on the relief of the depth of local isostatic compensation (in km) calculated by taking into account a density

contrast of 400 kg m-3 between the lower crust and upper mantle and the average sea level crustal thickness of 32 km. Correlation of the two

depths shows a mean crustal depth variance of about 8 km


apparently increased the crustal root density of the

SGT largely because of an increase in the volume

fraction of garnet (FISCHER, 2002). Moreover, the

petrological and seismological evidence indicates

that the lower crustal densities probably do not

exceed about 3,060 kg m-3 (MENKE, 1999).

The Pn velocity, a major source of information on

the petrologic nature of the upper mantle, varies from

8.59 km s-1 beneath the western Dharwar craton to

7.8 km s-1 beneath the eastern Dharwar craton with

8.0 km s-1 beneath the SGT (KAILA et al., 1979,

REDDY et al., 2000, 2003; SARKAR et al., 2001). The

three-dimensional P-wave velocity tomography using

teleseismic rays from a variety of azimuths also

indicates 2–3% lower velocities at the depth range of

40–177 km beneath the SGT compared to the Dhar-

war craton (SRINAGESH and RAI, 1996). Based on the

modeling of receiver functions (computed from the

IRIS broadband station in Sri Lanka), PATHAK et al.

(2006) suggest a hotter upper mantle beneath the

region. The present day mantle heat flow in the SGT

(23-32 mW m-2) is indeed distinctly higher than

that of the adjacent Dharwar craton (11–16 mW m-2;

RAY et al., 2003). Under isostatic conditions, heating

of the lithosphere results in a further decrease in

velocity/density through thermal expansion and

apparently indicates a lighter upper mantle beneath

the region.

That the gravitational potential decreases with

distance from the surface of the earth at a slower rate

than gravity [i.e., potential a (1/R) and gravity a(1/R2), where R is radial distance from the centre of the

earth], the geoid height also provides complimentary

information about subcrustal mass distribution. The

gravimetric geoid map of South India, south of 14�N,

evidently contains high-resolution sub-crustal infor-

mation, which is closely correlated with the known

structural elements of the region (Fig. 12). It clearly

shows that the first order long-wavelength circular

global geoid low over the Indian Ocean is dominated

by a second order concentric high and low in the

region. The relative geoid high over the Dharwar

craton (north of 12�N) corresponds to an excess of

mass usually associated with the continental massifs

and shields due to old, well-compacted and dense

rocks. In contrast the SGT (south of 12�N) is co-

located by a relative geoid low as compared to the

Dharwar craton. This relative geoid low thereby

validates the presence of mass deficiency under the

SGT, as shown by our density distribution across the

Moho. This inferred density distribution in the lower

crust and upper mantle that maintains the present-day

isostatic compensation may be the imprint of Pre-

cambrian tectono-thermal perturbations.

6. Precambrian tectonic imprints

The average crustal thickness of 43–44 km with

high-grade lower crustal rocks exposed in this area

suggests a palaeo-crustal thickness of 65–75 km,

which is almost equal to the present-day crustal

thickness (75–80 km) under the Himalayas and Tibet

(MUNT et al., 2008 and references therein). Such a

large palaeo-crustal thickness of SGT has also been

attributed to convergence and collision with Dharwar

craton during the Late Archaean-Early Proterozoic

period (BHASKAR RAO et al., 1996; RADHAKRISHNA

et al., 1999). In practice, significant non-zero iso-

static anomalies exist in the active continental

collision zones, whereas shield regions are invariably

Figure 9Vp-depth plot of the Dharwar craton (after REDDY et al., 2000) and

the Southern Granulite Terrain (SGT). The dashed line and the

solid line in SGT indicate the velocity-depth relation in Palghat

Gap and in the region to its north, respectively (after REDDY et al.,

2003). Mantle heat flow (MHF) is also given for the ready

reference (after RAY et al., 2003). Abbreviations are as in Fig. 1


characterized by overcompensation of the present-

day topography (HACKNEY, 2004). Positive bias in

isostatic anomalies associated with the active orog-

eny is dynamically supported by stresses associated

with the plate motion. When subduction ceases, the

undercompensated topography will subside as iso-

static equilibrium is re-established (GOTZE et al.,

1991) in a short characteristic time compared to the

time scale of orogeny (GRATTON, 1989). It is quite

likely that the isostatic equilibrium in the SGT must

have been reached within a few million years of the

Archaean-Proterozoic boundary collision. However,

this might have been disturbed subsequently when

mountain belts underwent exhumation, plutonism and

tectonic collapse (CHETTY and BHASKAR RAO, 2006;

RAO and PRASAD, 2006), arguably caused by eclogi-

tization and the subsequent delamination or

convective removal of the tectonically over-thick-

ened crustal root (SINGH et al., 2003, 2006), with the

result that buoyancy forces operated again to bring in

the isostatic equilibrium.

A lower Poisson’s ratio (0.25–0.28) than that

expected for a high-grade metamorphic terrain indi-

cates a felsic to intermediate crust beneath the SGT

(GUPTA et al., 2003). The observation is hypothesized

as a consequence of post-collision modification of the

initial mafic lower crust through the process of litho-

spheric delamination leaving a felsic to intermediate

component (JAGADEESH and RAI, 2008). An almost

flat Moho along the Kuppam-Palani geotransect

(REDDY et al., 2003; RAO et al., 2006) together with an

upper crustal extensional regime convincingly indi-

cates that the upper crust must have decoupled from

the lower crust (RAO and PRASAD, 2006). A reduced Te

value of 11–16 km (STEPHEN et al., 2003; RAJESH and

MISHRA, 2004) for a 43–44-km-thick Proterozoic crust

(REDDY et al., 2003; PRASAD et al., 2007) and suffi-

cient heat at the Moho depth (current mantle heat flow

Figure 10Locally compensated 3D crustal root (Moho depth) calculated by taking into account the loading of topography and an average sea level

crustal thickness of 32 km. The crustal root comparable to the seismically derived Moho depths requires a reduced density contrast of

*280 kg m-3


is 23–32 mW m-2; RAY et al., 2003) further indicate

a mechanically decoupled upper crust and the upper-

most mantle (BUROV and DIAMENT, 1996). An enriched

mantle (EM-I type) beneath the SGT further provides

direct evidence for assimilation of the crustal material

into the mantle (ANIL KUMAR et al., 1998). The high

mantle heat flow, Moho temperature of about 550�C,

thinned underlying thermal lithosphere (*104 km)

and an enriched mantle thus provide unequivocal

evidences for a highly mobilized subcrustal mantle

(PANDEY and AGRAWAL, 1999). This post-collisional

crustal decoupling is expected to bring the astheno-

sphere into close proximity with the lower crust

(HARLEY, 1998; KRONER and BROWN, 2005). In that

case, the imprint of the mantle remobilization, which

might have been part of the pervasive 550-Ma Pan-

African and Late Cretaceous signatures (BART-

LETT et al., 1995), is apparently seen today as lighter

subjacent mantle beneath the SGT, and together with

the denser lower crust must have reduced the root

buoyancy that kept the crust pulled downward in a

manner compatible with the state of isostatic

equilibrium.

It can be mentioned here that the presented

interpretation is too simplistic and depends on the

assumption that the lithosphere of the SGT behaves

as an elastic plate loaded only by the topography. The

final model also relies on the assumption that the

SGT was once in isostatic equilibrium. The Late

Mesozoic to Quaternary perturbations and dominant

upper-crustal mass anomalies that characterize this

region complicate elastic models for isostatic com-

pensation. Part of the topographic compensation at

the lithospheric level needs further investigation.

7. Conclusions

The isostatically compensated 3D Moho config-

uration for the SGT varies from about 33–48 km, the

Figure 11The gravity error anomaly (in mGal) for the crustal root model that also includes small lower crustal density variations


maximum of 53 km being beneath the Nilgiri and

Palani-Cardamom hills. The isostatic calculations

give an impression that the moderate topographic

mass in the SGT is overcompensated, and the mass of

about 1.0 km of eroded surface material is yet to be

accounted for. The low Te/low rigidity negates the

possible presence of requisite mechanical strength

that could resist the rebound to erosion-induced

crustal unloading. To isostatically compensate the

crustal root, compatible to seismic Moho, an anom-

alously low-density contrast of about 280 kg m-3

between the lower crust (*2,930 kg m-3) and the

sub-crustal lithospheric mantle (*3,210 kg m-3)

below the SGT is needed. A relatively denser crust

due to two episodes of metamorphic phase transitions

at *2.5 Ga and *550 Ma and highly mobilized

upper mantle due to Pan-African thermo-tectonic

perturbations reduced significantly the root buoyancy

that kept the crust pulled downward in response to the

isostatic adjustment.

Acknowledgments

The authors thank the Director, NGRI, Hyderabad,

for his encouragement and permission to publish this

work. Thanks are also due to Prof. W. Jacoby, Dr

M.R.K. Prabhakar Rao and two anonymous reviewers

for their critical comments and suggestions, which

improved the manuscript considerably. A part of the

data collected under Grant-in-aid Projects, supported

by the Department of Science and Technology, New

Delhi, is gratefully acknowledged.

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(Received January 22, 2010, revised July 27, 2010, accepted September 14, 2010)


http://dx.doi.org/10.1029/2001JB000688

http://dx.doi.org/10.1016/j.gr.2008.12.012

http://dx.doi.org/10.1029/2003GL01686

Insights into the Crustal Structure and Geodynamic Evolution of the Southern Granulite Terrain, India, from Isostatic Considerations

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