ORIGINAL RESEARCH PAPER

Geoid height versus topography of the Northern Ninetyeast Ridge:implications on crustal compensation

S. Rajesh • T. J. Majumdar

Received: 31 August 2009 / Accepted: 17 May 2010

� Springer Science+Business Media B.V. 2010

Abstract In this paper we focused on understanding the

isostatic compensation of the Ninetyeast Ridge in the

overall context of the Bay of Bengal oceanic lithosphere

and the interaction of the ridge system with the north

Andaman subduction zone from north of 7–18�N. This

region is characterized by the initial interaction of the

Kerguelen hotspot with the Bay of Bengal oceanic litho-

sphere. We used satellite altimeter-derived marine geoid,

as it should comprehensively reflect the compensations

caused by large spatial wavelength dominated deeper

anomaly sources in a hotspot affected lithospheric load like

the Ninetyeast Ridge. Our analyses of the geoid-to-topog-

raphy ratio (GTR), residual geoid, gravity-to-topographic

kernel and upward continuation of anomalies show the

existence of two different types of source compensation

bodies beneath the northern (12–18�N) and southern (7–

12�N) Ninetyeast Ridge. In the northern region, the geoid

to topography ratio varies from 0.63 ± 0.05 to

0.44 ± 0.03, while in the southern region it ranges from

1.34 ± 0.09 to 1.31 ± 0.07 which resulted in a north to

south increase in the apparent compensation depth from

*9 to 28 km. The presence of a shallow Moho, low GTR,

broader gravity to topography kernel and the absence of a

ridge anomaly from the mantle density dominated upward

continued anomaly at z = 300 km indicates that at the

northern segment the underplated low density crustal melt

is the dominant isostatic compensating body. However, at

the southern ridge segment the high GTR, strong gravity-

to-topography kernel and the subsistence of the anomaly at

long wavelengths, even at z = 300 km represents the

existence of large volumes of hotspot related underplated

dense material as the source of compensation. The prox-

imity of the dense source compensating body of the

southern Ninetyeast Ridge to the Andaman subduction

zone affected the regional mantle driven density gradient

flow, as observed from the z = 300 km continued gravity

anomaly. The existence of a southern Ninetyeast Ridge in

such a transpressional regime has caused the formation of a

forearc sliver at its eastern flank, which is a major crustal

deformational structure developed as a result of ridge-

trench collision.

Keywords Ocean lithosphere � Bay of Bengal �Crustal compensation � Residual geoid � Sediment

thickness � Isostatic gravity � Geoid to Topography ratio �Upward continuation

Introduction

The Ninetyeast Ridge is a major aseismic ridge in the

Indian Ocean lithosphere as identified by Stokes (1960).

The Ninetyeast Ridge extends almost 5,000 km in a north–

south direction between 18�N and 31�S. Even though the

focus of the current study is limited to north of 5�N, it is

worthwhile getting an overall idea about the origin and the

morphological structure of the ridge in both the Northern

and Southern latitudes. Morphologically, as observed from

the bathymetry, it is possible to say that the Ninetyeast

Ridge does not have any bathymetric expression further

north than 10�N. Beyond 10�N the ridge is buried under

S. Rajesh

Wadia Institute of Himalayan Geology, Dehradun 248 001, India

T. J. Majumdar (&)

Institute of Seismological Research, Gandhinagar 382 009, India

e-mail: tjmajumdar@rediffmail.com

123

Mar Geophys Res

DOI 10.1007/s11001-010-9088-7

Bengal fan sediments (Curray et al. 1982). Around half a

kilometer of sediment is present over the ridge and hence

its bathymetry is subdued. The Ninetyeast Ridge shows

different widths along its north–south strike and has an

average width of 200 km. The morphological features of

the Ninetyeast Ridge could be revealed from the earlier

works of Sclater and Fisher (1974) and later Krishna et al.

(1995) observed that the ridge is wider and shallower at its

southern latitudes, based upon bathymetric and marine

magnetic anomaly studies. In northern latitudes the western

flank of the buried ridge is gentler in comparison with its

eastern flank (Subrahmanyam et al. 2008). The origin of

Ninetyeast Ridge was attributed to Kerguelen hotspot

activity through geochemical (Mahoney et al. 1983; Weis

et al. 1991), geo-chronological (Duncan 1991), subsidence

history (Bowin 1973) and marine geophysical studies

(Neprochnov et al. 1979). Clift (2005) reported slower

subsidence history of the Ninetyeast Ridge than predicted

for normal oceanic crust of corresponding age. The

Ninetyeast Ridge was formed as a hotspot trail during the

northward movement of the Indian plate over the Ker-

guelen hotspot during the period 82–38 Ma, following the

formation of the Rajmahal Traps on the Indian subconti-

nent (Mahoney et al. 1983; Kent et al. 2002). Later,

Krishna et al. (1999) have proposed a three phase forma-

tion of the Ninetyeast Ridge system in its Southern and

Northern latitudes. Thus, the formation of northern (above

equator) Ninetyeast Ridge occurred purely because of the

intraplate Kerguelen hotspot magmatism; while at the

southern latitudes, the combined interaction of the hotspot

with the Wharton Ridge spreading axis during the period

60–54 Ma has been proposed. The occurrence of intraplate

seismic event of M 7.7 (1928) and large scale surface

deformational structures identified at the southern Indian

Ocean close to the ridge has helped to re-define the

southern tectonic boundary of the Indian plate as diffused

plate boundary (Wiens et al. 1985). Thus, towards the south

of the equator, the geodynamic traits of this ridge, its for-

mation, and mode of isostasy are quite well studied.

In this paper we focused on understanding the isostatic

compensation of the Ninetyeast Ridge, its regional gravi-

tational stability in the overall context of the Bay of Bengal

oceanic lithosphere and the interaction of the ridge system

with the north Andaman subduction zone from north of 7–

18�N. Little information is available about the geody-

namics of this region in the context of the Ninetyeast

Ridge. The characteristics of this region are: first it signi-

fies the area of the initial eruption of Kerguelen hotspot

magma and its first interaction with the Bay of Bengal

oceanic lithosphere, especially between 12�S and 18�N.

Thus, factors such as the nature of compensation of the

ridge and the role of hotspot magma in determining those

compensation processes are yet to be investigated.

Secondly, the interaction of the ridge with the north And-

aman convergence zone and other major lithospheric

structures in the Bay of Bengal basin, like the Central basin

depression, where average sediment thickness is quite high

(*9 km) (Rajesh and Majumdar 2004). In this work, these

aspects have been addressed through the analysis of

residual geoid, geoid-to-topography ratio, gravity-to-

topography ratio and the upward continuation of marine

gravity anomalies obtained from satellite altimeter-derived

instantaneous sea surface height. Analysis of results along

with the continuation of the anomaly of the ridge system

shows that the source of compensation at its northern (12–

18�N) and southern (7–12�N) segments, respectively are

different. Less dense underplated crustal melt fraction

dominates in the shallow root of the northern segment,

while in south, strong positive anomalies and greater

crustal root indicate the role of accumulated dense hotspot

material as the dominant source of compensation.

Objectives

Major objectives of this study are as follows:

(i) To study the geoid-to-topography and the Pratt

apparent isostatic compensation depths along the

northern Ninetyeast Ridge,

(ii) To study the upward continuation of gravity anom-

alies of the Bay of Bengal basin at different heights,

and ascertain the role of lithospheric density anom-

alies in the isostatic compensation processes, and

(iii) Generation of an integrated crustal structure of the

northern Ninetyeast Ridge in the Bay of Bengal

basin.

Study area and data sources

The area selected for lithospheric investigations is a part of

the Bay of Bengal and is also known as the Northeastern

Indian Ocean. The coordinate locations of the area are from

7 to 20�N and 82 to 95�E in the Bay of Bengal and shown

as box in Fig. 1.

Satellite altimeter and ship-borne data

In this study we used (i) Geosat Exact Repeat Mission

(ERM) instantaneous sea surface height data with a repeat

period of 17.6 days and (ii) Two data sets of ERS-1/2

satellites in the ocean mode measurement. One set of data

is from the geodetic phase of the ERS-1 with 168 day

repeat orbit period and with approximately 16 km track

spacing and the other is ERM-2 with a 35 day repeat period

orbit and a 80 km track spacing at the equator. The ERS-1

Mar Geophys Res

123

geodetic data consists of a total of 165 tracks, which fall

over the whole Northern Indian Ocean, ranging from 0 to

25�N latitude and 55 to 105�E. (iii) The third set of data

used is the merged 1 year TOPEX/POSEIDON data

obtained from the Physical Oceanography Distributed

Active Archive Center (PODAAC), USA. Satellite-derived

geoid and gravity data have been generated from the

observed instantaneous sea surface height using these three

sets of altimeter data. Details of data processing have been

discussed by Majumdar et al. (1998) and validation of the

satellite data with ship-borne gravity by Rajesh and

Majumdar (2004).

Regional bathymetry

The gridded bathymetry data of the Bay of Bengal obtained

from ETOPO2 has a spatial resolution of roughly

3.6 km 9 3.6 km as shown in Fig. 1. In general the sub-

marine topography shows a smooth variation from north to

south, with a general southward dip, and water depths of

greater than around 4.6 km. Sharp gradients in the depth

anomaly can also be observed along the eastern continental

margin of India and in the Andaman island arc region. The

known basement or crustal structural features such as 85�E

Ridge, the Ninetyeast Ridge and mega-fracture zones were

not evident from the regional bathymetry. Thus, even

though the Bay of Bengal basin is impregnated with many

N–S lithospheric loads, none of these were observed as

submerged topographic relief in the smoothly varying

ocean floor. The thick pile of sediments supplied by major

east and northeast flowing rivers, especially the Ganges and

the Brahmaputra, had covered the basement topography of

the Bay of Bengal basin.

Methodology

Geoid height to topography and the ridge apparent

compensation depth

The geoid height or the geoid anomaly over shorter

wavelengths of less than 4,000 km could effectively be

used to understand the density distribution within the

lithosphere and the mode of isostatic compensation of

lithospheric loads. The correlation of geoid height to

topography is a good indicator of isostatic compensation

and also can be used to calculate the depth of compensation

of ocean ridge topography, continental rises and other

anomalous sea floor topographies. The methodology for the

computation of geoid height to topography ratios (GTR)

over the Ninetyeast Ridge are discussed below.

It has been observed that gravity anomalies alone are not

sufficient to explain the isostatic compensation because

these do not provide a lucid picture of how the distributions

of long spatial wavelength density anomalies that may have

been produced by plume flow are affecting the compen-

sation processes. In particular, the role of viscosity in

determining the magnitude and polarity of geoid anomalies

in the vicinity of subduction zones has been mentioned by

Hager (1984) and Ravine and Morgan (1993). Thus, in

simple isostatic terms, once the isostatic crustal compen-

sation is achieved in some part of the lithosphere, the effect

of mass imbalance due to excess mass will be zero, which

subsequently produces zero free-air gravity anomaly.

However, the gravitational potential, and hence the geoid is

not zero in such regions of isostatic compensation. Haxby

and Turcotte (1978) explained geoid anomalies as a mea-

sure of the dipole moment of density distribution. Thus,

geoid anomalies in isostatically compensated regions are

good indicators of the density distributions. For a wave-

length less than the flexural wavelength (Watts 2001) of

400 km, a linear relationship between geoid and the

topography has been observed (Ockendon and Turcotte

1977). The classification based upon the geoid/topography

explains three modes of compensation. Features with

geoid/topography ratios greater than 6 m/km are compen-

sated dynamically beneath the lithosphere by convective

stresses (McKenzie et al. 1980). Intermediate geoid/

topography ratios of 2–6 m/km reflect average compensa-

tion depths of 50–80 km (Sandwell and McKenzie 1989).

Finally, low geoid/topography ratios of 0–2 m/km are

associated with compensation depths less than 50 km, i.e.,

Airy or Pratt compensated plateaus (Angevine and Turcotte

Fig. 1 The ETOPO2 bathymetry and topography of the Bay of

Bengal basin with adjacent continental areas. The study area is shown

inside the box. Bathymetry variations are in meters

Mar Geophys Res

123

1983). One of the basic assumptions, as far as the com-

pensation of features is concerned, is that the average depth

of compensation should be smaller than the characteristic

wavelength of the topography.

In this study, the residual geoid anomalies are obtained

after removing the deeper Earth anomalies from the grid-

ded classical geoid anomaly data. We computed deeper

anomaly effects through spherical harmonic analysis using

EGM96 coefficients (Lemoine et al. 1998) up to degree and

order of harmonics 50. The resultant residual geoid

anomaly data has been utilized for correlation with the

residual basement depth. The residual basement depth was

calculated by removing the sediment effect from the

observed gridded bathymetry over the study region of the

northern and the southern Ninetyeast Ridge systems. This

was followed by removing the regional long wavelength

basement undulation polynomial of the order of two from

the sediment-corrected equal length bathymetry profiles

selected from the gridded data taken across the Ninetyeast

Ridge in the northern (above 12�N) and the southern

regions (above 5�N). The sediment correction to bathym-

etry is justified because of the lack of expression of the

Ninetyeast Ridge basement topography against the ocean

floor. The geoid/topography ratio (GTR) is obtained from

the slope of the best fitting regression line through the

correlation of the residual geoid and the residual basement

(topography) along respective profiles taken across the

strike of the Ninetyeast Ridge in the northern and southern

latitudes, all north of 5�N. The estimated GTR has been

used to compute the Pratt’s isostatic apparent depths of

compensation of the Ninetyeast Ridge at various northern

and southern latitudinal profiles. The expression used for

the computation of apparent compensation depth (dc) using

GTR is given by Monnereau and Cazenave (1988):

dc ¼gðGTRÞ

pG qm � qwð Þ ð1Þ

where g = acceleration due to gravity, 9.8 ms-2,

G = Gravitational Constant, 6.672 9 10-11 m3 kg-1 s-2,

qm = density of mantle, 3,300 kg m-3 and qw = sea water

density, 1,030 kg m-3, respectively. The expression in

Eq. 1 shows that the geoid-to-topography ratio is propor-

tional to the apparent depth of compensation for a positive

topography like the Ninetyeast Ridge in the study area.

The gravity to topography ratio over the Ninetyeast

Ridge

The relation between gravity and the topography, as well as

the effect on gravity by the deflection or compensation

produced in the crust have been explained by Sandwell and

McKenzie (1989). Their combined effects will generate the

resultant gravity due to Airy crustal compensation (Watts

2001) of the lithospheric load, which, when expressed in

the wave number domain, is

DgðkÞtotal ¼ 2pGðqc � qwÞHðkÞe�kdð1� e�ktÞ ð2Þ

where G is the Universal Gravitational Constant, qc and qw

are the densities of crust and the water layer, respectively, d

and t are the mean water depth and the crustal thickness.

H(k) is the Fourier transform of topography of the

lithospheric load. Substitution of Z(k) for parameters

other than H(k) will give a linear relationship between

gravity and topography in wave number domain as

DgðkÞ ¼ ZðkÞHðkÞ ð3Þ

Here Z(k) is a linear filter function, which reflects the

system property or the Airy isostatic filter function in the

wave number domain. It can be defined that the input

topographic load of the ridge as H(k) causes the gravity

output Dg(k) through the kernel Z(k). Thus, the kernel Z(k)

is the admittance (Dorman and Lewis 1972; McKenzie and

Bowin 1976; McNutt 1979; Watts 2001; Tiwari et al.

2003), which reflects the mode of compensation of the

lithospheric load.

In this study, the cross-spectral analysis of the free-air

gravity and the topography as given by McKenzie and

Bowin (1976) has been utilized to find the gravity-to-

topography ratio of the Ninetyeast Ridge along various

profiles in its northern and southern latitudes, as shown in

Fig. 2a–c. Equal length profiles were selected across the

Ninetyeast Ridge along 8�, 13� and 17�N latitudes and 1D

Fourier transformation applied. Before Fourier transfor-

mation, no. of data points were rounded to 28 through

020406080

100120

0.0018 0.0218 0.0418 0.0618 0.0818 0.1018

020406080

100120140

(a)

0

2

4

6

8

10

(b)

(c)

8°N

13°N

17°N

k (km-1)

Z(k

) (m

Gal

/km

)

0.0018 0.0218 0.0418 0.0618 0.0818 0.1018

0.0018 0.0218 0.0418 0.0618 0.0818 0.1018

Fig. 2 a–c Variation of gravity to topography kernel across the

Ninetyeast Ridge along 8�N (a), 13�N (b) and 17�N (c) profiles

Mar Geophys Res

123

padding with zeroes at the end and profile edge filters have

been applied in order to avoid Gibb’s oscillations.

The cross spectra between the topography and gravity,

as well as their respective powers were calculated before

the computation of Z(k) of the Ninetyeast Ridge. The total

length of each profile was 555.55 km. The topography data

were corrected for the sediment effect before the compu-

tation of their Fourier transform. As per Eq. 3, it is clear

that unless the topography has a good spectral correlation

with the corresponding gravity anomaly, the Z(k) compu-

tation will not give satisfactory results. In the Bay of

Bengal basin, the ocean floor topography and the oceanic

basement undulation have little resemblance. As a result,

the sediment-corrected topography of the respective grav-

ity profiles has been taken for the computation of Z(k). The

geoid-to-topography ratio and the gravity-to-topography

kernel would give the compensation depth and the nature

of compensation of the ridge system, respectively. How-

ever, relative strength of the deeper anomaly signals that

may contribute by the hotspot material in large spatial

wavelength windows are also to be investigated through

the upward continuation of anomalies.

Upward continuation of free-air gravity anomaly

over the Bay of Bengal basin

The continuation of free-air gravity anomalies over the Bay

of Bengal has been studied at various heights from the

geoid datum in order to understand the role of mantle level

compensation of the basin, and particularly over the

Ninetyeast Ridge. The continuation of anomalies at various

heights gives a clearer picture of the distribution of various

density anomaly structures by removing the unwanted,

very shallow or near surface anomaly signals.

Gravity data measured on a given plane can be trans-

formed mathematically to data measured at a higher or

lower elevation, thus attenuating or emphasizing shorter

wavelength anomalies. These analytic continuations lead to

convolution integrals that can be solved either in the space

or in the frequency domain (Bhattacharya 1965). The

theory behind this filtering technique is that, if the sub-

surface body is at great depth compared to the datum then

its gravity signals will be weak, but of long wavelength in

nature compared with the strong and short wavelength

nature of the shallow subsurface bodies. Thus, if one is

interested in deep anomaly signals then the shallow noise

anomaly can be removed from the observed data by just

shifting the observed datum plane upwards by a specific

amount and then computing the anomaly at a new plane.

By doing this one can retain the long wavelength waveform

and hence the distribution of the anomaly sources. Thus,

the upward continuation effectively acts as a low-pass

spatial frequency filter.

Since the objective of this study is to isolate the signals

with a deep origin associated with mantle level compen-

sation of the Ninetyeast Ridge system, in particular, along

with the general overall background of the basement, we

assumed a flat Earth approximation of the coordinates with

linear combination of the harmonic functions with an

exponential decay in the anomaly signals with height for

the computation. Based upon Bhattacharya (1965) the two-

dimensional gridded free-air gravity anomaly was expan-

ded in terms of the linear combination of harmonic terms in

the flat Earth approximation or in a Cartesian coordinate

system. The respective equations can be written as,

Dgðx; y; zÞ ¼X1

m¼0

X1

n¼0

Dgðx; yÞe2pz m2=L2xþn2=L2

y½ �1=2 ð4Þ

Here Dg(x, y) is the 2D gridded free-air gravity data, Dg(x,

y, z) is the respective continued data either upward or

downward by a factor of z. If z is positive then it will be a

downward continuation, while negative value of z

corresponds to an upward continuation. Lx and Ly are the

fundamental wavelengths in two directions x and y,

respectively. Thus, with negative z, the exponential term

decays the effect of gravity at the datum to the specified

height z. Lx and Ly represent the fundamental wavelengths

such that Lx = Nxdx and Ly = Nydy, respectively. Here dx

and dy represent the cell width of the 2D gridded data in

the x and y directions and Nx, Ny represents the number of

data points along the profiles in the x and y directions,

respectively. The upper limit for converging the series is

m0� Nx � 1ð Þ=2 and n0� Ny � 1� �

=2 respectively. The

Dg(x, y) can be expanded as the linear combination of

harmonic terms as:

Dgðx; yÞ ¼Amn cos 2pmx=Lx

� �cos 2pny

�Ly

� �

þ Bmn cos 2pmx=Lx

� �sin 2pny

�Ly

� �

þ Cmn sin 2pmx=Lxð Þ cos 2pny�

Ly

� �

þ Dmn sin 2pmx=Lxð Þ sin 2pny�

Ly

� �ð5Þ

The coefficients Amn, Bmn, Cmn, and Dmn are computed

from the respective normal equations expressed in terms of

Dg(x, y) and the combination of cosine and sin harmonic

terms (Bhattacharya 1965).

Results and discussion

The results of gravity-to-topographic kernel of the ridge

taken along a few crossing profiles are shown in Fig. 2a–c.

In general, Airy isostatic compensation takes place at

longer wavelength (*555 km) lithospheric loads and has a

lower cut off wavelength at around 45 km. Accordingly,

lithospheric loads with wavelength \ 45 km, have not

Mar Geophys Res

123

been effectively compensated. The compensation wave-

length range is compatible with the average 200 km width

of the Ninetyeast Ridge. The amplitude of the Airy com-

pensation filter along 13�N is less than 10 mGal/km, which

is less than the 140 and 120 mGal/km of the other two

profiles. This indicates a relatively better state of isostatic

balance of the Ninetyeast Ridge at 13�N. The gravity

anomaly caused by the topography could be effectively

countered by the near approximate thickening of the crustal

root into the mantle. But in the southern 8�N profile, the

amplitude shows a clear, single, narrow, straight peak with

high amplitude; this suggests that the gravity anomaly due

to topography is subdued compared to the gravity effect of

the thickened crustal root. The gravity effect of the crustal

root is more because of the increase in density of the

compensating material. The single lobe of the Z(k) ampli-

tude, rather than being observed along with the side lobes,

represents the non-fragmentation of the ridge compensat-

ing load along these latitudes, such that only a single

prominent component is taking part in the process of

compensation. However, in the 13�N profile, even though

the ridge is in a near balanced state of compensation, the

presence of side lobes indicates its broader compensation,

which takes place not only at its center, but also on its

widened flanks. The same is true for the case of the

northernmost 17�N profile, where the number of side peaks

is more in addition to the main lobe.

Thus north of 12�N, the compensating source body of

the ridge anomaly is obscured, possibly because of the

presence of short wavelength shallow density anomaly

structures, while at southern (between 5 and 12�N) lati-

tudes the compensating source body is stronger, clear and

predominantly dense. Hence, the amplitude of gravity-to-

topography ratio can give constraints on the state of iso-

static compensation. Furthermore, its distribution in the

wave number domain also shows whether the ridge is

compensated by a single unit or by fractured form at a

single location. However, this analysis cannot give the

depth of compensation of the crustal roots at the southern

and the northern regions of the Ninetyeast Ridge.

Figure 3 shows the residual geoid anomaly of the Bay of

Bengal oceanic lithosphere, which ranges from -11.5 m in

the north Andaman subduction zone region to more than

4 m at the southern portion of the Ninetyeast Ridge system.

As explained above, geoid anomalies are sensitive to

compensations by large wavelength lithospheric density

Fig. 3 The residual geoid anomaly contours in meters over the Bay of Bengal basin. NN0 represents variation in trend of the Ninetyeast Ridge.

PP0 at the eastern flank of the Ninetyeast Ridge shows the trend of the detached forearc sliver as a result of ridge-trench collision

Mar Geophys Res

123

anomalies and their consequent lithospheric flow. The

observed north Andaman trench anomaly lows are rela-

tively wide owing to the gentle bending of the down-going

subducted slab. But at south Andaman, the trench geoid

anomalies juxtaposing the southern Ninetyeast Ridge are

narrower which suggests steep bending of the subducted

slab. In addition, other basin features, such as the Central

basin depression, the Mahanadi Basin and the 85�E Ridge

are also observed as geoid anomaly lows.

The trend of the Ninetyeast Ridge is shown in Fig. 3 as

NN0. The ridge geoid anomaly as well as its area increases

from the ridge head at the north to its tail at the south. This

variation shows how the density of lithospheric mantle

influences the compensation process of the ridge. Rela-

tively high geoid observed at the southern segment (south

of 12�N) of the ridge represents occurrences of thicker

crust that may have been generated by major phases of

magmatism linked to the Kerguelen hotspot, although the

northern segment oceanic lithosphere experienced the ini-

tiation phase. The estimated GTR and the Pratt apparent

depths of compensation of the profiles ranging from 7 to

17�N across the strike of Ninetyeast Ridge north of the

equator, as obtained in this study, are shown in Table 1. In

addition, the GTR estimated by Sandwell and McKenzie

Table 1 Geoid to topography

and the Pratt apparent isostatic

compensation depths computed

along the Ninetyeast Ridge at

above and below the equator

a After Sandwell and McKenzie

(1989)

Serial no. Latitude (�N) Geoid/

topography

(m/km)

Compensation

depth (km)

Below equator

1 North Ninetyeast Ridge 1.1 ± 0.4a 23.0 ± 8.2

Mid-Ninetyeast Ridge 1.0 ± 0.3a 20.6 ± 6.2

South Ninetyeast Ridge 0.7 ± 0.1a 14.4 ± 2.0

Above equator in this study

2 7.0 1.34 ± 0.09 28.0 ± 2.0

3 8.0 1.23 ± 0.03 25.0 ± 0.7

4 10.0 1.31 ± 0.07 27.0 ± 1.4

5 12.0 0.63 ± 0.05 13.0 ± 1.0

6 13.0 0.42 ± 0.03 8.6 ± 0.7

7 15.0 0.43 ± 0.04 8.9 ± 0.8

8 16.0 0.41 ± 0.04 8.4 ± 0.7

9 17.0 0.44 ± 0.03 9.0 ± 0.6

-4

-3

-2

-1

0

1

2

3

4

-3

-2

-1

0

1

2

Res

idua

l Geo

id(m

)

-1

0

1

2

3

4

-4 -3 -2 -1 0 1

-2.5 -2.0 -1.5 -1.0 -0.5 0.0

-4 -2 0 2 4 6

-6 -4 -2 0 2 4-2

-1

0

1

2

3

(a) 7°N

(b) 8°N

(c) 16°N

(d) 17°N

R2 = 0.95GTR = 1.34 ± 0.09 m/km

R2 = 0.79GTR = 1.23 ± 0.03 m/km

R2 = 0.90GTR = 0.41 ± 0.04 m/km

R2 = 0.93GTR = 0.44 ± 0.03 m/km

Residual basement (km)

Fig. 4 a–d The Geoid to

Topography Ratio (GTR) of the

Ninetyeast Ridge at southern

and northern latitudes. (a) and

(b) are along 7�N and 8�N

profiles, while (c) and (d) are

along 16�N and 17�N profiles,

respectively

Mar Geophys Res

123

(1989) in the southern region of the Ninetyeast Ridge south

of the equator is also given. The Pratt apparent depths of

compensations of the Ninetyeast Ridge south of the equa-

tor, corresponding to Sandwell’s estimation of GTR, have

been computed using Eq. 1. Figure 4a–d show the best fit

between the residual geoid and residual topography across

the Ninetyeast Ridge at its southern and the northern lati-

tudes, respectively, for the estimation of GTR.

It is evident from Table 1 that the GTR computed across

the Ninetyeast Ridge in the southern latitudes (above 5�N)

is in general agreement with the GTR values obtained

south of the equator by Sandwell and McKenzie (1989),

except in the case of the southernmost (south Ninetyeast

Ridge value) region of the Ninetyeast Ridge. Thus, from

the south of the equator to the 10�N region, the estimated

GTR shows values of less than 2.0 m/km thus suggesting a

shallow Pratt level of apparent compensation depth at less

than 50 km. The estimated apparent depth of compensation

in the region between the central Ninetyeast Ridge (San-

dwell and McKenzie 1989) to 10�N is 20.6 ± 6.2 and

27.0 ± 1.4 km (in this study), which indicates an increase

in the Ninetyeast Ridge compensation depth. The increase

in sediment load over the ridge towards the north causes

downflexing of the lithosphere on which the Ninetyeast

Ridge is built. This could be a reason for the observed

increase in the compensation depth. The average com-

pensation depth in this region is 22.3 ± 0.5 km. The var-

iation of the compensation depth along the profile of the

subsurface ridge also indicates non-uniform levels of

crustal compensation of the Ninetyeast Ridge, obtained

through the multiple wavelength isostatic gravity anomaly

studies.

However, in the northern parts of the Ninetyeast Ridge

at 11–17�N the GTR and the apparent compensation depths

interestingly show a reducing trend, which is contrary to

that been expected from the theories of lithospheric flexure

due to increased sediment load. In this region, the GTR and

the compensation depth are less than 1.0 m/km and 15 km,

respectively. The variation of GTR ranges from

0.41 ± 0.04 to 0.44 ± 0.03 m/km at 17�N, with no

appreciable variation compared to the southern region.

Similarly, the average depth of isostatic compensation in

this region is 8.7 ± 0.4 km. Thus, the apparent depth of

compensation of the Ninetyeast Ridge shows an increase

(from the north to the south) from 8.7 ± 0.4 to

22.3 ± 0.5 km (Rajesh and Majumdar 2005). However,

the question of why the depth of compensation in the

southern region is more despite the thinner sediment

thickness in this region compared to the northern region,

still remains. The answer to this can be explained in the

light of Eq. 1, which was used to compute the apparent

depth of compensation. As per the expression, the apparent

depth of compensation is proportional to the geoid-to-

topographic ratio (GTR). However, the geoid height, or the

geoid anomaly, has already been explained as an indicator

of the dipole moment of vertical density distribution.

Unlike gravity, the geoid gives information on the density

structure of the ridge and its supporting base, the upper

mantle lithosphere. Thus, if a high density mantle exists

beneath the ridge in its southern latitudes, the geoid

anomaly or the geoid height can be high irrespective of the

topography of the ridge, which, in turn, could give rise to

high GTR and greater compensation depths. Thus, the

increased GTR and the increased apparent depth of com-

pensation in the southern parts of the Ninetyeast Ridge

indicate the existence of a high-density, supporting basal

mantle. However, this hypothesis has to be verified through

other mathematical means, and the effects of the topogra-

phy and the near surface crustal density have to be removed

to study the effect of compensation within the mantle under

the Ninetyeast Ridge system at its northern and the

southern latitudes.

Three upward continued gravity anomalies over the Bay

of Bengal oceanic lithosphere at heights z = 50, 200, and

300 km above the geoid datum were obtained to study the

Fig. 5 Free-air gravity anomaly of deep seated crustal density

structures computed at a height of z = 50 km from the geoid datum

over the BOB basin with prominent gravity highs at northern and

southern Ninetyeast Ridge. AA0 represents a major oblique strike slip

component which separates the northern and the southern ridge

segments. PP0 represents the detached forearc sliver. L1–L7 represent

the gravity anomaly lows along the Andaman subduction zone.

Anomalies L5–L7 in the vicinity of the ridge are severely affected by

the crustal deformation owing to ridge-trench collision. The consec-

utive trench anomaly lows are separated by parallel strike slip faults

shown as white dashed lines

Mar Geophys Res

123

features of interest in the study area. These are shown in

Figs. 5, 6, and 7, respectively.

The free-air gravity anomalies derived from the satellite

altimeter measured instantaneous sea surface height (SSH)

and have an anomaly range from -86.0 to 64.0 mGal over

the Bay of Bengal basin (Majumdar and Bhattacharyya

2004). After applying the upward continuation at a height

of z = 50 km from the geoid datum, the free-air anomaly

shows a range between -26.0 and 16.0 mGal. Around

50 mGal reduction has occurred in the amplitudes of the

free-air gravity anomalies and most of the short wavelength

features have been removed, so that only the broad crustal

features like 85�E Ridge, the Central Basin, the Ninetyeast

Ridge, the Marginal High and the Andaman subduction

zone, along with the continental highs are retained (Fig. 5).

The sources of these anomalous gravity features lie below

the upper crust. The northern and the southern segments of

Ninetyeast Ridge system are very clear, and are separated

by a NE–SW oriented fault, as shown as AA‘ in Fig. 5. In

addition, the eastern flank of the Ninetyeast Ridge that has

subsided into the Andaman subduction zone which is evi-

dent with the presence of a faulted region represented by

PP0 (Fig. 5). The effect of the topography and the presence

of sediment-load-induced flexure of the lithosphere, whose

spatial wavelength is more than 100 km, are clearly

distinguished in Fig. 5. The effect of the topographic

depression on the anomaly magnitudes is evident from the

very low value of less than -19.0 mGal seen in the area of

the Andaman Trench, as well as in the depression produced

in the Central Basin region. The over-compensation of the

85�E Ridge causes the flexural uplift of its western region,

which could be observed as a Marginal High or Margin

flexural uplift.

Considering the Ninetyeast Ridge system, it is evident

that in its northern portion, the anomalies are almost

14 mGal less compared to the southern region. The high

anomaly of the Ninetyeast Ridge in the southern region

could be explained by the enhanced topography of the

ridge. As a result, the free-air anomaly at z = 50 km level

is mainly dominated by the topographic effect and the

sediment load thicknesses. Thus, in order to study the

mantle level compensation of the ridge, still larger heights

are required to filter out the effect of topography and the

crustal surface loads from the gravity anomaly.

Figure 6 shows the computed free-air gravity anomaly

at a height of z = 200 km from the geoid datum over the

Bay of Bengal. The anomaly ranges from -7.4 to

1.5 mGal, with a reduction of around 18.0 mGal in the

minimum value with respect to the case of z = 50 km

height. The anomalies at this height are mildly influenced

by the density effects of the crust. Thus, in comparison

with Fig. 5, the fault structures and the 85�E Ridge are

absent from Fig. 6 because they are short wavelength

features compared to the broad, basin-scale features of the

Ninetyeast Ridge and the Central Basin depression. The

cause for the Central Basin depression is the presence of a

sediment load of more than 9.0 km thickness, but its effect

on basin subsidence reflects the weak support of the basin

by the low density lithospheric mantle. Basin depression

could also occur as a result of the down flexure of the

lithosphere. Thus, in the Central Basin depression region

the plate is plausibly less thick; as a result, its flexure

would be large and have less rigidity with respect to the

adjacent hotspot-affected lithosphere. The reduced anom-

aly amplitude of -2.9 mGal of the northern Ninetyeast

Ridge compared to 1.5 mGal in its southern region indi-

cates the possible existence of low density mantle.

This can be confirmed by considering the gravity

anomalies at the next height of z = 300 km, as shown in

Fig. 7. The northern Ninetyeast Ridge system has totally

vanished from the z = 300 km height free-air gravity

anomaly map, thus indicating the apparent low density

observation at this portion of the ridge from previous

height—continued anomalies that are dominated by the

crustal contribution, rather than that driven by the mantle

lithosphere. However, in the southern region the gravity

anomaly is still retained, which indicates the existence of

higher density lithospheric mantle material with a thicker

Fig. 6 Free-air gravity anomaly of lower crust and upper mantle

density structures computed at a height of z = 200 km from the

geoid datum over the BOB basin. The prominent density anomaly of

the southern Ninetyeast Ridge is evident along with the subdued

anomaly of northern ridge segment. The Central basin depression,

Andaman Trench anomaly low are other major lithospheric density

anomalies

Mar Geophys Res

123

crustal root. The high density mantle material beneath the

southern region of the ridge supports it isostatically. In

contrast, in the northern region, the ridge exists physically;

its less compensation depth coupled with the presence of

near normal lithospheric mantle causes a subdued appear-

ance to the ridge’s gravity anomaly at 300 km height.

Another important aspect of the z = 300 km anomaly is

that of the Andaman subduction region, where the total

absence of the trench’s low anomaly on the eastern flank of

the southern Ninetyeast Ridge could be seen. Using the

geoid anomalies at short wavelengths, as shown in Fig. 3,

we argue that the subducting slab in that portion of the

Andaman subduction region which juxtaposes to the

southern Ninetyeast Ridge is deflected at a greater angle

(Rajesh 2006). Long wavelength gravity anomalies mimic

geoid anomalies at larger wavelength e.g., at z = 300 km

shown in Fig. 7. Hager (1984) inferred that an increase in

viscosity with depth along the subduction zone could affect

the downward movement of the lithospheric slab. Thus,

coupling the greater deflection of the subducted slab with

the presence of high density and viscous mantle material on

the eastern flank of the southern Ninetyeast Ridge, it is

plausible that the downward movement of the subducted

limb of the lithosphere in this region is very much restricted.

Mukhopadhyay and Krishna (1995) observed that the

abutment of the Ninetyeast Ridge into the Andaman arc

trench zone caused complexity in the subduction process

and modified the arc-trench gravity field. The crustal

Fig. 7 Free-air gravity computed at a height of z = 300 km above

the geoid datum over the BOB basin. This shows the vanished

northern Ninetyeast Ridge anomaly with respect to the existence of a

strong southern ridge anomaly, where the low density underplated

crustal melt and hotspot related underplated basal material acting as

compensating loads, respectively. Relatively high gravity anomaly

observed along the trench in between the eastern flank of the southern

Ninetyeast Ridge and the backarc high represents the interaction of

ridge material with the Andaman subduction zone

Mar Geophys Res

123

signature of hindered down going of slab is evident from

the south Andaman trench gravity anomaly, as shown in

Fig. 5 (blue color) juxtaposed against the southern Ninet-

yeast Ridge. The trench anomalies (L6 and L7) are very

close and narrow in comparison with the trench anomalies

(L2–L4) close to the northern Ninetyeast Ridge region.

This suggests that the crust of the southern portion of the

ridge in the vicinity of the trench experiences greater

deformation. In general subduction zone anomalies are also

affected by crustal shortening where complex stress

regimes are developed: (1) due to the combined movement

of thrust systems either oblique or perpendicular to the

convergent boundary and (2) formation of strike-slip

components which cause a series of parallel faults to form

along the axis of the convergent boundary and transverse in

the EW thrust direction. Such transverse structures are

evident from Fig. 5 and are shown as white dashed lines.

The anomaly lows marked from L1 to L7 are formed in

between such transverse structures, along the axis of the

Andaman convergent zone. Thus, the undercompensated

southern Ninetyeast Ridge modulates the regional scenario

of the crustal deformation and hence tightening of the

trench anomaly contours L6 and L7. Deformational

structures associated with the ridge in such a transpres-

sional regime are evident on the eastern flank of the

southern Ninetyeast Ridge as a forearc sliver, which is

marked as dashed violet lines in Fig. 5 and as PP0 in Fig. 3.

Interestingly, such structures are not present in Figs. 6 and

7, where the effect of mantle lithosphere dominates the low

gravity anomaly of the north Andaman subduction zone

and the southern Ninetyeast Ridge. The absence of the

continuation of the low density subducted slab anomaly

between the eastern flank of the southern Ninetyeast Ridge

and the backarc high is conspicuous. This shows that at

lithospheric depth, the compensating load of the ridge

affects the internal driving density contrast that leads to

subduction and appears at surface as a case of oblique

ridge-trench collision.

An integrated crustal structure of northern and southern

portion of the Ninetyeast Ridge in the Bay of Bengal

basin

An integrated crustal structural model of the Ninetyeast

Ridge system north of 5�N has been developed by putting

together all the results gathered through geoid-to-

Water Column (1.03 gm/cm3)

Sediment (2.5 gm/cm3)

Oceanic Basalt (2.9 gm/cm3)

Mantle (3.4 gm/cm3)

Mantle (3.0 gm/cm3)8.7 ±±±± 0.4 km

Ridge crest

Ridge flank

Water Column (1.03 gm/cm3)

Sediment (2.5 gm/cm3)

Oceanic Basalt (2.9 gm/cm3)

Mantle (3.4 gm/cm3)

Mantle (3.0 gm/cm3) 22.3 ±±±± 0.5 km

Ridge crest

Ridge flank

(a)

(b)

Support of low density crustal melt

Support of viscous, dense under plated hotspot material

Fig. 8 a and b An integrated

crustal structure beneath the

Northern (above 12�N) and the

southern Ninetyeast Ridge

(between 5 and 12�N) in the

northern BOB basin. The

northern Ninetyeast Ridge

shown as (a) has less crustal

root and is compensated by low

density crustal melt product.

However, the southern

Ninetyeast Ridge shown as (b)

is having a deeper crustal root

which is compensated by

relatively viscous high density

hotspot material underplated

to crust

Mar Geophys Res

123

topography, residual geoid, gravity-to-topography and the

harmonic analyses of the Bay of Bengal gravity anomaly.

Figure 8a and b show the integrated crustal structure of the

Ninetyeast Ridge, representing its northern (north of 12�N)

and the southern portions (between 12 and 5�N) in the Bay

of Bengal. The isostatic compensation processes prevailing

in the Bay of Bengal basin, especially along the aseismic

Ninetyeast Ridge, reflect not only a simple crustal root

compensation process, but also depends upon the gravita-

tional support provided by the upper mantle. Grevemeyer

and Flueh (2000) have reported the compensation phe-

nomena as subsurface loading at a depth of 25 km in the

southern portion of the ridge, south of the equator.

Mahoney et al. (1983) reported upper mantle accretion

beneath the ridge at its southern latitudes south of the

equator.

In the present study, based upon geoid to topography

ratio, we inferred that the average crustal compensation of

the southern Ninetyeast Ridge system from 5 to 12�N is

around 22.3 km (Fig. 8b), which is much higher in com-

parison with the observed average 8 km compensation

depth for the northern ridge system between 12 and 17�N.

This indicates a shallow Moho in the northern portion of

the Ninetyeast Ridge compared to the southern region.

Recent studies based upon geoid to age offsets of the

Ninetyeast Ridge, Majumdar et al.(2009) estimated an

average lithospheric thickness of around 125 km. Krishna

et al.(2001) estimated effective elastic thickness (Te) as

35 km by considering an average lithospheric age of

80 Ma. The continuation of gravity anomalies at various

heights above the geoid datum have been used to under-

stand the strength of long wavelength mantle density

anomalies. This shows that from 5 to 12�N there exist

strong lithospheric mantle density anomalies below the

ridge, which dominate in all the upward continuation

heights. Furthermore, the gravity-to-topography kernel of

the southern Ninetyeast Ridge also shows the presence of a

single strong compensating source body. Thus, in general,

a substantial portion of mantle lithosphere is supporting the

ridge system towards its gravitational stability, especially

between 5 and 12�N. The dominance of mantle lithosphere

in supporting the ridge as a subsurface load can be

explained on the basis of the hotspot related material un-

derplated to the crust beneath the ridge during the north-

ward movement of the Bay of Bengal oceanic lithosphere

over the Kerguelen hotspot (Phipps Morgan et al. 1995).

From Fig. 5, it is evident that the area occupied by the

emplacement of Kerguelen hotspot magma over the Bay of

Bengal lithosphere grows from 17 to 5�N with an apparent

break at around 12–13�N (shown as black oblique arrows).

The region from 17 to 12�N represents the northernmost

and the initial phase of interaction of Kerguelen hotspot

magma with the Bay of Bengal oceanic lithosphere. While

from south of 13–5�N and even further south up to the

equator, the aerial occupation of hotspot magma was not

only large but also strong, representing the later phase of

Kerguelen hotspot magmatism (Phipps Morgan et al.

1995). Thus, there exists a subsurface load below the ridge

from the northernmost 17–5�N at varying compensation

depths due to variation in the hotspot related underplated

magmatic material during the northward movement of the

Bay of Bengal oceanic lithosphere over the Kerguelen

hotspot.

The presence of underplated magmatic mantle material

beneath the ridge between 5 and 12�N can also be

explained on the basis of residual geoid anomalies and

thermal diffusion. Geoid anomalies are sensitive to vis-

cosity of the mantle lithosphere and hence its flow due to

thermal diffusion. Figure 3 shows high residual geoid

anomaly of more than 3.5 m over the ridge system between

5 and 12�N. High geoid anomalies are expected if the

underplated hotspot related material beneath the ridge is

highly viscous and would not diffuse away from the ridge,

as shown in Fig. 8b.

However, beneath the northern ridge segment (from 12

to 17�N), the presence of low density crustal melt product,

as shown in Fig. 8a, has a role in determining the relatively

subdued gravity anomaly. The shallow compensation depth

of around 8 km also suggests the possibility of the existence

of low density melt products at the crustal level. Mukho-

padhyay and Krishna (1995) also reported the presence of

relatively low density crustal material in this portion of the

ridge. The gravity-to-topography kernel also shows multi-

ple side lobes in this region, which indicates that the com-

pensating source body of the ridge anomaly is possibly

obscured by the presence of short wavelength shallow

density anomaly structures. This is also evident in the

continuum field of the northern segment, shown in Fig. 7,

where at z = 300 km the density effect of the northern ridge

segment had completely vanished. If there was mantle

plume material present or dominant beneath the northern

segment of the ridge as a subsurface load, then its density

effect would have prevailed at z = 300 km. Apart from

this, the northern segment of the ridge system represents the

initial phase of interaction of the Kerguelen magmatism

with the Bay of Bengal oceanic lithosphere and occupies a

smaller area of eruption not much influenced by the plume

mantle density. Thus, the subdued gravity effect of the

northern segment is plausibly due to the dominance in the

low density effect of the underplated crustal melt product.

Conclusions

In this paper, the isostatic compensation mechanism of the

Ninetyeast Ridge system north of 5�N was studied using

Mar Geophys Res

123

satellite altimeter-derived geoid and gravity data. This

region is characterized by the first interaction phase of the

Kerguelen hotspot magma with the Bay of Bengal oceanic

lithosphere. Results were analysed through geoid-to-

topography ratio, residual geoid, gravity-to-topography

kernel and the upward continuation of free-air gravity

anomalies. In this area, the ridge has been observed as

having different northern (between 12 and 17�N) and

southern segments (between 5 and 12�N) with two different

sources of isostatic compensating load.

The analysis of Geoid/Topography ratios (GTR) in the

northern Ninetyeast Ridge system and the subsequent

computation of the apparent compensation depth show that

the Ninetyeast Ridge is in Pratt’s mode of isostatic com-

pensation with an average compensation depth of

8.7 ± 0.4 km. The subdued gravity signal of the ridge in

this region and its total absence from the z = 300 km

height continued anomaly indicates the source compensa-

tion body is mainly dominated by the low density under-

plated crustal melt. Gravity-to-topography kernel also

corroborates the existence of shallow multiple sources as

compensation loads.

The southern Ninetyeast Ridge system shows greater

depth of compensation of 22.3 ± 0.5 km obtained through

GTR and hence relatively deeper Moho. Gravity-to-

topography kernel and residual geoid show that a single,

prominent and long wavelength source body, which dom-

inated in all the continuation heights, provides the much

gravitational support of the ridge in this region. This

indicates the presence of dense, relatively cool and viscous

hotspot related underplated basal magmatic material that

accumulated beneath the ridge during the fast (*9 cm/a)

movement of the Indian plate over the Kerguelen hotspot.

Such relatively dense and viscous underplated magmatic

body might not diffuse away and could remain as a rela-

tively buoyant, under-compensated feature to gravitation-

ally stabilize the ridge system.

Results from residual geoid and free-air anomaly con-

tinued at a height of z = 50 km revealed the interaction of

the Ninetyeast Ridge systems with the Andaman Trench

zone, especially with the southern Ninetyeast Ridge. The

southern Ninetyeast Ridge is situated in a transpressional

regime, where the undercompensated ridge is subjected to

subduction-related thrust movement and shear along the

convergent plate boundary, which leads to the formation of

a forearc sliver at its eastern flank. Thus, the low trench

anomalies (L6 and L7 in Fig. 5) in the vicinity of southern

Ninetyeast Ridge bear the signatures of severe crustal

shortening compared to other trench anomalies in the

vicinity of the northern Ninetyeast Ridge. It has also

revealed, from large wavelength gravity anomalies, that the

cause of the observed trench crustal deformational signa-

tures in z = 50 km juxtaposing the southern Ninetyeast

Ridge is because of the interaction of the ridge compen-

sating source body with the depth-dependent internal

mantle driving density contrast of the plate subduction.

Acknowledgments The authors are thankful to Dr. Peter Clift,

Editor-in-Chief, MGR and another anonymous Referee for their

valuable suggestions for improvement of the manuscript. They also

wish to thank Director, WIHG, Dehradun, Dr. B. K. Rastogi,

Director General, ISR, Gandhinagar, Dr. R. R. Navalgund, Director,

SAC, Dr. J. S. Parihar, DD, RESA/SAC and Dr. Ajai, GD, MESG/

RESA/SAC, Ahmedabad for their keen interest in this study. They

also thank Mr. K. M. Sreejith, MESG for his help. Figures are

plotted using GMT software.

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