Gcophys. J . R . astr. SUC.(1975) 40, 441-452.
Gravity and Crustal Thickness in the Indo-Gangetic
Plains and Himalayan Region, India
S. K. Choudhury
(Received 1974 August 2)
Summary
Bouguer gravity data of the Indo-Gangetic plains and the Himalayan
region have been interpreted in terms of crustal configuration. Since the
available empirical relations for converting gravity anomalies to crustal
thickness do not take into account the effect due to the presence of low
density sediments and also the lateral variation in the density contrast
between the crust and the mantle, a new approach to the interpretation,
well suited for a digital computer, was evolved.
The results indicate that the thickness of the crust below the Central
Himalayas is of the order of 70-72 km, thereby implying a crustal
thickening of about 35-37 km. A crustal high observed south of Delhi
and Lucknow may have important geological significance.
Introduction
Unveiling the secrets of the Earth's crust has been one of the principal objectives
of the geophysicists from the very early days. Seismology has contributed the most
towards the knowledge of the structure and composition of the crust. Reflection and
refraction seismology and dispersion of surface waves are the main methods of studying
the crust. There are other geophysical methods, namely, gravity, heat flow determination and palaeomagnetism which can significantly contribute towards understanding
of the intricate crustal structure and its composition. Since seismic information on
the crustal structure of the Himalayan region is extremely scanty, an attempt was
made to study the crustal thickness of this region from the available gravity data.
The area studied and its location are shown on the Fig. 1 . Towards north the
area is limited by the lofty Central Himalayas and towards south by the metasediments
of the Peninsular India. Towards east and west the area is bounded by longitudes
88" E and 75" E, respectively. The gravity data in the Indo-Gangetic plains and
foothill areas have been collected by the Oil & Natural Gas Commission in their
country-wide exploration programme for petroleum. The Bouguer anomalies were
derived after applying the Bouguer correction with density 2 g cm-3 and latitude
correction according to the 1930 International gravity formula. The data were
reduced to mean sea level. The density of the station is approximately 1 station per
2 . 5 sq km. Towards north in the Himalayas and in the south in the Peninsular shield
area the gravity data obtained by Survey of India (Gulatee 1956) have been incorporated for the preparation of the map. The data collected by National Geophysical
Research Institute (Qureshy 1971) near Badrinath are also included. The composite
Bouguer anomaly map which is a smoothed version of the detailed, large scale maps,
is shown in Fig. 2.
441
I
.
I
750
---l-701
750
B
3,
*
1
8 5O
Main Boundary.Fault
. __
9'0°
I
8 50
lndan shield
7
- ,
~~
11.1
---.-..-...-~..
----,
,,,,,,,,,,
.,--..I
T
950
-"",,#I
Oragenetic sediments and Oohiolites
,
.
. f i .n
.-d ~ sFlvsch)
.
Subhirnolayas
m
h Main Central Thrust
ILY
!-.-IArea studied
N I .
FIG.1. Index map (after Gansser 1964).
80"
I
60°
___
1
-
I
950
25O-
30°-
350-
m
FIG.2. Bouguer anomaly map of the Indo-Gangetic Plains. Contour values are in milligal. The eleven profiles along which crustal sections
have been interpreted are shown by 1,2, etc.
444
S . K. Choudhury
Method of interpretation
Using empirical re!ations:
Initially, the crustal thickness was calculated by using the empirical relations
deduced by different workers. These relations are as foIlows:
(Worzel & Shurbet 1955)
2 = 33-0.055 Ag
(1)
(Demenitskaya 1958)
2 = 35 (I-tanh 0.0037 Ag)
(2)
(Andreev 1958)
2 = 30-0.1 Ag
(3)
(Woollard 1959)
2 = 32-0.08 Ag
(4)
(Woollard & Strange 1962)
2 = 40.5-
32.5 tanh
~
275
Where Z is the crustal thickness in kilometres and the gravity anomaly Ag in milligals.
The crustal thickness along a meridional profile (through 88" E) up to Darjeeling
(Fig. 3) was calculated by the relations (1)-(5) and the results are given in Table 1.
Furthermore, as suggested by Woollard (1959) to avoid the local anomalies due to
shallow causatives Bouguer anomalies averaged over 100 km were utilized for
80
-
0-
-80 -120L
4-r
MSL
km
n
FIG.3. Gravity profile I , from Contai to Darjeeling and the interpreted crustal
section along it. The dashed line represents the depth obtained with uniform density
contrast and the solid line with variable density contrast.
39,3
44.8
32.6
34.9
29.8
32.3
39.1
46-0
38.1
41-3
34-6
37.8
46.2
34.4
35.7
32.9
34-3
38.0
41.8
- 26
-49
-2
-23
-91
- 160
53.5
34.3
32.9
38.8
34.6
- 34
- 29
33.8
31.8
35.9
34.1
34.7
33.4
39-5
34.2
32.7
38-5
33-8
32-2
37.8
34.2
- 22
34- 5
34.2
32.7
38.5
34.5
-27
34.9
33.1
31.4
36.7
33.8
- 14
-27
Woollard
(1959)
5.
Andreev
(1958)
4.
Demenickajya
(1958)
3.
Worzel &
Shurbet
(1955)
2.
Method
used
Bouguer
gravity
(mg4
1.
44.0
39.7
34.5
33.5
34.0
34.0
34.0
34.0
34.2
34.0
34.0
33.0
Woollard
100 km
av (1959)
6.
Woo11ard
50-9
42.5
34.3
31.1
37.4
34.6
35.0
35-6
34.7
34.1
34.7
33.2
(1962)
7.
& Strange
42-8
36,6
33-1
33.0
35-0
32.0
31.6
30.8
30-9
29.6
30.0
28.5
8.
Tsuboi
(1956)
43.8
39.2
35.6
35.6
36.6
36.2
34.8
34.0
33.2
33.6
34.0
52.0
45.2
39.8
36.2
36.6
36.2
34.8
34.0
33-2
33.6
34.0
Present method of Iteration
With
With
uniform
variable
density
density
contrai t
contrasts
9.
10.
34.2
34.2
Comparison of crustal thicknesses obtained for direrent methods along the profile of fig No. 3 , thickness in kilonietres
Table 1
5
P
2.
3
k
$
g
E:
446
S. K. Choudhury
calculating the crustal thickness by Woollard’s relation (2). The values thus calculated
ai e also given in Table 1.
Tsuboi’smethod
It was considered that the method described by Tsuboi (1956) which takes into
account the effect of the sediments, might yield more objective interpretation. In
this method if the values of gravity anomalies observed at a series of equidistant
grid points along the surface are given, the total mass beneath the grid points can be
calculated. The distance between the consecutive grid points and the depth to the mass
plane are taken to be 35 km in this case. The density contrast was assumed to be
0.43 g cm-3 between the crustal and sub-crustal material (Worzel & Shurbet, 1955).
The thickness of the crust was calculated by assuming the maximum effect of the low
density sediments to be -60 mgal. This method is obviously expected to give better
results than those obtained by the empirical relationships. However, the method
does not take into account the lateral change in the density contrast between the
crustal and sub-crustal material. The results obtained by this method, when applied
on the profile (Fig. 3) are indicated in Table I .
Modifed approach to interpretation
The relations and formulae used above are based on the assumption of homogeneous composition of the crust; moreover, these do not take into account the
effect of the presence of any appreciable thickness of sediments. In the present work,
attempts to rectify these two deficiencies have been made as follows:
(a) Towards north the gravity values decrease rapidly indicating that the thickness
of the crust is increasing northwards. Woollard (1959) and Woollard, Ostenso &
Thiel(l960) had indicated that as the crust becomes thicker its mean density increases;
so the density differences between the crust and the mantle decrease as the thickness
of the crust increases. Woollatd (1959) had suggested the density contrast below
mountains and high plateau to be 0.24 g
between the crust and the mantle,
whereas normal density contrast below the shield areas may be 0.43 g cm-3. Cook
(1962) has pointed out that below the mountains, the crust and mantle may become
mixed during the mountain building process. The crustal section may not only
shorten and thicken due to the action of the horizontal forces, but the crust may
become mixed with the underlying mantle. Cook has further observed that the mantlecrust mix probably exists beneath continents in tectonic belts which lie along the
landward extension of oceanic ridges. In the Arabian Sea the north-eastern branch
of the Carlsberg Ridge points towards the sub-Himalayan zone and this might be an
indication that considerable crust-mantle mixing took place in the Himalayan belt.
Qureshy (1969) on the basis of gravity data, suggested that the main attribute to the
crustal thickening below the Himalayas to be the thickening of the basaltic layer.
Therefore, the assumption of a decreasing density contrast between the crust and the
mantle with increasing crustal thickness seems to be quite well supported.
Since the present area adjoins and also includes the highest and one of the youngest
mountains of the world, it was decided to use varying density contrast to compute
the crustal thickness. In the exposed shield area the density contrast between the
crust and the mantle was taken to be 0.43 gcmW3,after Worzel & Shurbet (1955)
who assigned the density values for crust and mantle as 2.84 and 3 * 2 7 g ~ m - ~ ,
respectively. The density contrast was linearly decreased to 0.24 g cm-3 near the
Central Himalayas.
The reference crustal thickness was assumed to be 35 km below the southern
fringe of area. This assumptim is based cn the following consideration:
Indo-Gangetic Plains and Himalayan Region
447
(1) The average Bouguer anomaly value over the exposures in the southern part
of the area is about -40 mgal. Worzel and Shurbet proposed a standard continental
crust of 33 km for the land at sea level with zero Bouguer anomaly. From the
Bouguer slab formula, taking Ag = 40 mgal, A p = 0.43 ~ m - the
~ , thickness comes
out to lx.2.2 km. This means that the normal crustal thickness in the southern
periphery of the area, where basement rocks are exposed, is about 35 km. Woollard’s
value corresponding -40 mgal anomaly is also 35.2 km.
(2) The average elevation of the southern part of the area is about 300 ms. The
relations between the crustal thickness and surface elevation are as follows:
(Woollard 1959)
H = 32+7*8h,
(Worzel & Shurbet 1955) H = 33+7.21 h.
(a)
(b)
Where H = crustal thickness, It = surface elevation. Taking h to be 300 m, %hevalues
of H, with the formulas (a) and (b) come out to be 34.3 and 35.1 km, respectively.
(b) The aeromagnetic survey of the Indo-Gangetic Plains indicates the presence
of a considerable thickness of sedimentary section (Agocs 1956), a substantial part
of which are of Neogene age. This should greatly influence the Bouguer anomalies.
The average density of the sedimentary column in the foothill and plain area is known
at many places from the deep exploratory wells and taken to be 2 . 4 g ~ m - ~For
.
example, a sedimentary thickness of 5.0-6.0 km. with a density contrast of0.25 gcm-3
between the sediments and the basement will give rise to an anomaly of about
50-60mgal. The maximum effect due to these lighter sediments is caused somewhat south of the foothills belt, after which the effect will be almost negligible because
the rocks whether sedimentary, metamorphic or igneous, being highly lithified and
compact, do attain densities comparable to the normal crustal density.
To overcome the limitation in the assumption of uniform density contrast and to
accommodate the effect of low density sediments, it was thought necessary to develop
a comparatively easier and faster method well suited to a digital computer. In the
present method of interpretation the sediments, the thickness of which was known
from the aeromagnetic survey, were replaced with the crustal material having density
of 2-84 g ~ m - The
~ . gravity effect due to this was calculated by the method outlined
by Talwani, Worzel & Landisman (1959). This effect when added to the Bouguer
values provided a new gravity anomaly (henceforth called the geologically corrected
Bouguer anomaly) which can be considered to be due to the effect of a single density
differential at the base of a crust, i.e. at the level of Mohorivicic discontinuity.
To interpret these geologically corrected Bouguer anomalies an iterative procedure,
very well suited to a digital computer, was used. This is based on the method described
by Bott (1960), but modified to the extent that the cross-section of the model is not
represented by vertical rectangular prisms, but by a single n-sided polygon as described
by Talwani et al. (1959). The assumption of two dimensionality required in this
method, is satisfied since the crustal structure is linear and the profiles have been
taken at right angle to the strike of the isogals. The first trial model is calculated on
the basis of the Bouguer slab formula and with each iteration the model is modified
in such a way that the calculated anomaly agrees better with the geologically corrected
Bouguer anomaly values at every point along the profile. Usually eight iterations
were sufficient to minimize the residual errors. The configuration of the base of the
crust is thus obtained along the profile. The different steps of interpretation by this
method of iteration are shown in Fig. 3. The reference depth of the crust near the
exposed shield area at the southern portion is taken to be 35 km as mentioned earlier.
The two different configurations, one with uniform density contrast of 0.43 g cm-3
and the other with the variable density contrast from 0.43 to 0.24 g cm-3 are shown.
The results obtained by the iteration method along the profile of Fig. 3 are also given
in Table 1 for ease of comparison.
448
S. K. Choudhury
The calculations were carried out along eleven profiles using variable density
contrasts. The depth values thus obtained were contoured at an interval of 2 km and
the results are presented in Fig. 4.
Results
The isopach map of the crust depicts only the first order undulations of the crust.
Locally, the crust might have smaller undulations, but those second order effects
cannot be evaluated by this interpretation.
The salient features can be summarized as follows: The contours of the isopach
map strike parallel to the axis of the Himalayas. In the plains, there is a crustal hump
between the longitudes 78" and 84" E, enclosed by the 34 km. contour. This hump
can probably be correlated with the Shillong Plateau and may have interesting
significance in Indian geology in limiting the Vindhyan Sea towards further north.
The thickness of crust in the Central Himalayas is about 70-72 km. This implies
that the total crustal thickening from the plains to the Central Himalayas is about
35-37 km. In the sub-Himalayan zone, where crust is dipping steadily, the order of
average dip is quite low being of the order of 7"-8". The amount of dip increases
to about 15" as the axis of the Himalayas is approached.
Comparison of the present results with the previous work
The information on the crustal structure in India is far from adequate. However
different workers have estimated the crustal thickness in the Himalayan and subHimalayan region from the study of P waves and dispersion of Love and Rayleigh
waves. The results obtained by them are given in Table 2.
Datta (1961) considered the data of two sets of earthquakes; the first set of 18
earthquakes having epicentres in the northern side of the Himalayas, i.e. in Tibet
and South China and the second set of 16 earthquakes having epicentres on the
southern side of the Himalayas. By finding the difference in intercept times for the
time-distance plot of two sets of earthquakes, the crustal thickening below the Himalayas could be found out. Datta obtained the value of 70-75 km as the depth to
Mohorovicic discontinuity below the Himalayas as compared to that of about 50 km
in north-east India. Roy & Jain (1968) have reconsidered the same sets of data used
by Datta in a slightly modified manner and arrived at almost the same value of crustal
thickening. They further calculated the additional crustal thickness below the
Himalayas as 20 km on the basis of the gravity data. Evans & Crompton (1946)
also calculated the crustal configuration from the gravity data along almost the same
profile shown in the Fig. 3. They corrected the Bouguer values for the low density
sediments and fitted a crustal model to the corrected values. They obtained a value
of 50 km of crustal thickness below Darjeeling. Gulatee (1958) from the consideration
of isostasy estimated a crustal thickness of about 56-60 km below the Himalayan
region.
The normal crustal thickness in the southern part of the area has been assumed to
be 35 km. In north America with so much of close seismic controls, the estimation
of the normal sea level crustal thickness at zero Bouguer anomaly varies from 32 km
(Woollard 1959) to 38.4 km (Steinhart & Meyer 1961). So if the normal crustal
thickness in the southern part of the area is found to be different from 35 km by seismic
studies, then the contour values of the map in Fig. 4 will have to undergo modification
accordingly. But what is important is that still the slope of the Mohorovicic discontinuity and total crustal thickening as has been deduced from the present investigation
will remain unchanged.
The limitations of interpreting the gravitydataare well knownand need no repetition.
North-east Assam
Himalayan region
40
52
70-75
65-70
2Yk8
Chaudhury (1966)
Chouhan & Singh (1965)
Datta (1961)
Gupta & Hari Narain (1967)
Kaila, Reddy & Narain (1965)
3.
4.
5.
6.
40.2
45*
45.3
55-60*
Saha (1965)
Tandon (1954)
Tandon & Chaudhury (1964)
8.
9.
10.
11.
* denotes measurement from artifical explosion.
50
Mukherjee (1942)
Roy (1939)
7.
Basis of investigation
Body waves.
Surface wave dispersion.
Under the Himalayas
Surface wave dispersion
North-east Assam
Himalayan region
Body waves study of shallow earthquakes.
Body waves study of shallow eqrthquakes.
Central India
Gangetic basin
Body waves from shaliow earthquakes.
0
P
E.
m
a
P
Surface wave dispersion
Body wave study of shallow earthquakes.
E.
5
Body waves from shallow earthquakw
Surface wave dispersion
Body waves from shallow earthquakes.
Punjab Plains
Tibetan Plateau
Under Himalayan &
Below the Himalayas
Himalayan region
2.
38.1
Chakravorty & Ghosh (1960)
I.
Region for which
calculated
Name of the author
S1. No.
Thickness of
the crust
in km
Comparison of Crustal structure for Himalayan and sub-Himalayan region by different authors
Table 2
-.
,*r;F-
FIG.4. Isopach map of the crust in the Indo-Gangetic plains. Contour interval 2 km.
_ _ .L
Darpelvx)
L
+
\
407
,b’
Cwta!
,
Calcutta
I
Indo-Gangetic Plains and Himalayan Region
45 1
This gives only a plausible crustal model which has to be proved or disproved by the
seismic results. But one must notice the wide divergence of the values obtained for the
thickness of the crust (Table 2) by the seismic methods. One reason may be that most
of the workers have neglected the effect of large thickness of low velocity (2-7 km s-')
sediments in their calculations. The earthquake data have also some limitation
regarding the location, depth of origin and time of initiation. Moreover, in India
there is lack of suitably situated adequate number of seismological observatories.
It has also been observed that refraction results do not agree everywhere with the
results obtained from surface waves. The exact answer can be expected only from the
explosion seismology, but the first-order approximation model can be conveniently
derived from the gravity data.
Acknowledgment
The author is thankful to Shri S. N. Sengupta, Chief of Geophysical Services
and to Shri A. N. Datta, Chief Geophysicist for their guidance and keen interest
in the project. Thanks are also due to Shri D. K. Trehan and Shri R. Amaravadi
for many stimulating discussions and to Shri N. K. Sharma for the drafting of the
figures.
Oil and Natural Gas Commission,
Geophysics Directorate,
TeI Bhavan,
Dehradun (UP),
India.
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