Tectonophysics, 178 (1990) 337-356 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 337 Gravity field, deep-seismic sounding and crust-mantle structure over the Cuddapah basin and Dhawar Craton of India R.K. VERMA and Y. SATYANARAYANA Department of Applied Geophysics, Indian School of Mines, Dhanbad 826 004 (India) (Received December 17,1988; accepted August 24,1989) Abstract Verma, R.K. and Satyanarayana, Y., 1990. Gravity field, deep-seismic sounding and crust-mantle structure over the Cuddapah basin and Dhawar Craton of India. Tecronophysics, 178: 337-356. South of 21° N, the Indian Peninsula is character&d by very low Bpuguer gravity anomalies ranging from - 110 mGal to -60 mGa1. A low gravity field is present over the eastern margin of the Cuddapah basin, Dharwar craton. as well as areas covered by the Deccan traps. Several profiles drawn in the E-W direction across the southern peninsula show that the gravity anomalies have long wavelengths, of the order of several hundred kilometers. This feature appears to be related to the geoid low brought out by Gaposchkin and Lambeck (1970, 1971) on the basis of satellite gravity data. Deep-seismic sounding investigations carried out by Kaila et al. (1979) along Kavali-Udipi profile, running from the east coast of India to the west coast, provide a good deal of subsurface information concerning the configuration of the Cuddapah basin, Dharwar schist belts and the Moho along the entire profile. Various factor contribute to the gravity field. The gravity field over Cuddapah basin has been analysed in terms of sediments comprising the basin, the Moho configuration and intrusive bodies present underneath the basin. In order to explain satisfactorily the low gravity field over the Dharwar craton (to the west of the Cuddapah basin), a lower density upper mantle has been inferred. This feature appears to be related to processes that have taken place during the eruption of the Deccan traps over the Indian Peninsula and the formation of the Indian Ocean. Introduction One of the most prominent geoid anomalies This geoid anomaly appears to extend from observed over the Earth’s surface is the one over India towards Australia, covering a large part of the Indian Ocean and the surrounding Indian the Indian Ocean. The presence of this anomaly Peninsula (Gaposchkin and Lambeck, 1970,1971). has been confirmed by surface gravity measure- This anomaly has a wavelength of nearly 2500 km, ments in the Indian Ocean using 1 O x lo average a major part of which extends from the Carlsberg free-air anomaly values (figs. 2 and 3 of Kahle and ridge in the west to the Malaya Peninsula in the Talwani, 1973). An interesting feature of this east, and from 20” N to 20 o S in an N-S direc- anomaly is that it extends over a large part of the tion. The Gaposchkin and Lambeck (G and L) Indian Ocean, south of 21” N latitude. Un- geoid, obtained from spherical harmonic coeffi- doubtedly this feature suggests the presence of a cients up to degree and order 16, and terrestrial major mass anomaly which appears to be related gravity data, referred to an ellipsoid of flattening in some way to the processes that led to the l/297.0 is shown in Fig. 1 (after Kahle and Talwani, 1973). 0040-1951/90/$03.50 0 1990 - Elsevier Science Publishers B.V.
Transcript
Tectonophysics, 178 (1990) 337-356
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
337
Gravity field, deep-seismic sounding and crust-mantle structure over the Cuddapah basin and Dhawar Craton
of India
R.K. VERMA and Y. SATYANARAYANA
Department of Applied Geophysics, Indian School of Mines, Dhanbad 826 004 (India)
(Received December 17,1988; accepted August 24,1989)
Abstract
Verma, R.K. and Satyanarayana, Y., 1990. Gravity field, deep-seismic sounding and crust-mantle structure over the Cuddapah basin and Dhawar Craton of India. Tecronophysics, 178: 337-356.
South of 21° N, the Indian Peninsula is character&d by very low Bpuguer gravity anomalies ranging from - 110 mGal to -60 mGa1. A low gravity field is present over the eastern margin of the Cuddapah basin, Dharwar craton. as well as areas covered by the Deccan traps. Several profiles drawn in the E-W direction across the southern peninsula show that the gravity anomalies have long wavelengths, of the order of several hundred kilometers. This feature appears to be related to the geoid low brought out by Gaposchkin and Lambeck (1970, 1971) on the basis of satellite gravity data.
Deep-seismic sounding investigations carried out by Kaila et al. (1979) along Kavali-Udipi profile, running from the east coast of India to the west coast, provide a good deal of subsurface information concerning the configuration of the Cuddapah basin, Dharwar schist belts and the Moho along the entire profile. Various factor contribute to the gravity field.
The gravity field over Cuddapah basin has been analysed in terms of sediments comprising the basin, the Moho configuration and intrusive bodies present underneath the basin. In order to explain satisfactorily the low gravity field over the Dharwar craton (to the west of the Cuddapah basin), a lower density upper mantle has been inferred. This feature appears to be related to processes that have taken place during the eruption of the Deccan traps over the Indian Peninsula and the formation of the Indian Ocean.
Introduction
One of the most prominent geoid anomalies This geoid anomaly appears to extend from
observed over the Earth’s surface is the one over India towards Australia, covering a large part of
the Indian Ocean and the surrounding Indian the Indian Ocean. The presence of this anomaly
Peninsula (Gaposchkin and Lambeck, 1970,1971). has been confirmed by surface gravity measure-
This anomaly has a wavelength of nearly 2500 km, ments in the Indian Ocean using 1 O x lo average
a major part of which extends from the Carlsberg free-air anomaly values (figs. 2 and 3 of Kahle and
ridge in the west to the Malaya Peninsula in the Talwani, 1973). An interesting feature of this
east, and from 20” N to 20 o S in an N-S direc- anomaly is that it extends over a large part of the
tion. The Gaposchkin and Lambeck (G and L) Indian Ocean, south of 21” N latitude. Un-
geoid, obtained from spherical harmonic coeffi- doubtedly this feature suggests the presence of a
cients up to degree and order 16, and terrestrial major mass anomaly which appears to be related
gravity data, referred to an ellipsoid of flattening in some way to the processes that led to the
Fig. 1. Gaposchkin and Lambeck (1971) geoid obtained from spherical harmonic coefficients up to degree and order 16, referred to
an ellipsoid of flattering l/297.0. Contour interval 10 m.
evolution of the Indian Ocean, formed as a result
of the break-up of Gondwanaland (McKenzie and
Sclater, 1971).
This major anomaly in the gravity field is also
reflected on the Bouguer as well as the isostatic
anomaly maps of the Southern Peninsula of India.
Recently, Kaila et al. (1979) have carried out
deep-seismic sounding (DSS) along a profile (Fig.
2) extending from Kavali on the east coast to
Udipi on the west coast, traversing the major
geological/ tectonic units of the Indian Peninsula.
This profile has provided vital information con-
cerning the crust and the upper mantle in the area.
Analysis of the gravity field along this profile
gives us a better insight into the processes leading
to the evolution of the Dharwar Craton, the East-
ern Ghats and the nature of the upper mantle
underneath the southern peninsula of India. These
studies throw some light on the nature of the
geoid anomaly mentioned earlier. Results of this
analysis are discussed in this paper.
Gravity field over the southern Indian Peninsula
A study of the Bouguer anomaly map of India
(NGRI, 1975; Verma, 1985) shows that the Indian
Peninsula can be broadly divided into two parts.
North of 20” N, Bouguer anomalies over shield
areas vary from 0 to - 50 mGal values (barring
the Indo-Gangetic plains), while to the south of
this latitude, the values vary from - 50 to - 120
mGa1, the Eastern Ghats belt being the only ex-
ception. The large negative anomaly in the south-
ern Peninsula of India (Fig. 3) cannot be ex-
plained in terms of near-surface geological or
topographical features (as shown in this paper). At
least a part of this anomaly appears to be related
to the constitution of the upper mantle.
That the anomalous gravity field is of deep-
seated origin is confirmed partly by the Airy iso-
static anomaly (T = 30 km) map of the southern
peninsula of India (Fig. 4). South of 21” N, the
isostatic anomalies are largely negative (- 20 to
A R
AE3
1
SEA
BA
Y
OF
BEN
GA
L -
I 74O
sp8o
SH
OT
POIN
T 0
I I
I
76'
7B0
80°
L--
ALL
UVI
UM
L/
/ B
ASI
C
INTR
USI
VES
18
12O
I
- D
SS
PRO
FILE
ix
xl
DEC
CA
N
TRAP
S I-c
G
RA
NIT
ES
F[
CUDD
APAH
SE
DIM
ENTS
f-1
G
NEI
SSIC
C
OM
PLEX
DH
AR
WA
RS
AB
BR
. :
CC-C
LOSE
PET
GR
AN
ITE
; C
HT
- C
HIT
RA
DU
RG
A
i PN
P-
PAR
NA
PALL
E
CH
TB
- C
HIT
RA
DU
RG
A
SCH
IST
BEL
T
SHTB
-
SHIM
OG
A
SCH
IST
BEL
T Fi
g. 2
. nh
e lo
catio
n ,,f
the
Kav
ali-U
dipi
D
SS p
rofil
e,
in t
he s
outh
ern
pen
insu
la o
f In
dia
, af
ter
Kai
la e
t al
. (1
979)
. T
he
pro
file
tra
vers
es (
from
ea
st t
o w
-t),
the
hte
m
Gh
ats,
th
e
Cu
dd
apah
bas
in,
Pen
insu
lar
Gn
eiss
es,
Clo
sep
et g
ran
ite
and
th
e D
han
var
sch
ist
bel
t. :
R.K. VERMA AND Y. SATYANARAYAMA
5 s:
]I W
0 2 u m
=:
a
nQ t
> 0 >
GEOPHYSICAL STUDY OF CUDDAPAH BASIN AND DHARWAR CRATON. INDIA
UDlPl
ARABIAN
SEA I
BAY
OF
BENGAL
TRIVANDRUM~ \fl 0
72O I I I I
74O 76’ 70O 80”
MADRAS
ISOSTAT IC ANOMALY MA
(AIRY-HEISKANEN,T=30i
NGRI-1975
100 0 100 Km I 1 fi ’ 1
Contour Interval 10 mGal
I I
82’ i%’
Fig. 4. Airy isostatic anomaly (T = 30 km) map of southern India (after NGRI, 1975), showing large isostatic anomalies over parts of
the southern peninsula.
- 90 mGal), while over the northern shield areas, the anomalies vary from +50 to -20 mGal. Fig- ure 5 shows the nature of Bouguer and isostatic (Airy, T = 30 km) anomalies along several profiles trending in an E-W to ENE-WSW direction in the southern peninsula of India. The profiles show the nature of long wavelength anomalies. Over the southern peninsula the part of Eastern Ghats (E.G.), north of 16 o N, is again an exception, the isostatic anomalies being largely positive (0 to + 30 mGa1). Barring the E.G. the entire southern peninsula is characterised by negative isostatic anomaly values.
Problems concerning interpretation of the gravity field
The gravity field observed over the surface is influenced by several factors, such as the near- surface geological features, the Moho configura- tion and in some cases the anomalous structure of the asthenosphere or upper mantle (Fairhead, 1976; Brown and Girdler, 1980).
In order to interpret the regional gravity field, basic information is required regarding the Moho configuration and the velocity-depth function. The DSS profile shot by Kaila et al. (1979) from
n “V” DECCAN TRAPS __ p3??zJ GNEISSES WITH CHARNOCKITES
i!zzl CUDDAPAHS GNEISSES
SCALE 0 100 200 KM f % I 1 ,
Fig. 5. Profiles A-A’ to E-E’ traversing the southern peninsula of India in an E-W to ENE-WSW direction, showing the large
negative Bouguer as we11 as isostatic anomalies over the peninsuta. It may be noted that in genera1 the negative anomaties are
characterised by a long wavelength.
GEOPHYSICAL STUDY ( 3F CUDDAPAH BASIN AND DHARWAR CRATON. INDIA 343
lV3W ‘A-IVWONV AllAVM WY ‘Hld30
344 R.K. VERMA ANDY. SATYANARAYAMA
Kavali along the east coast to Udipi along the
west coast, provides us this basic information.
Kavali-Udipi DSS profile
The profile is located between latitude 13-
14’ N and longitude 74.5-80 o E and extends from
Kavali (14.8” N, 80 o E) on the east cost to Udipi
(13.5 o N, 74.7 ‘E) on the west coast (Fig. 2). The
profile traverses (from east to west), the Eastern
Chats, the Nellore Schist belt, the southern part of
the Cuddapah basin, the Peninsular Gneisses, the
Closepet granite, the Chitradurga and Schimoga
Schist belts and ends near Udipi on the west
coast. Figure 6 shows the results from the DSS
profile along with surface geology, elevation and
gravity field.
The basic information obtained from DSS in-
vestigation is the interval velocity vs depth func-
tion for the southern peninsula. This function
INTERVAL VELOCITY, KM/SEC
_5 6 7 8 9
obtained along the Kavali-Udipi profile, as well
as for the Koyna I and II profiles after Kaila et al.
(1979, 1981a, b) is shown in Fig. 7. The figure
shows that the interval velocity increases from 5.4
to 6.5 km s-’ between a depth of 0 and 15 km,
remaining constant at about 6.5 km s-’ upto 22
km, and then increasing gradually to 7.1 km s-’
down to about 40 km depth, where it jumps to 8.0
km s-‘. The interval velocity has been converted
into density values using the curve given by Ludwig
et al. (1970). It may be seen that the crustal
density at a depth of about 35-40 km is of the
order of 3.0 g/cmp3. The upper mantle density
has been taken to be 3.3 g/cme3, corresponding
to the velocities reported.
Analysis of the gravity field along Kavali-Udipi
profile has been carried out in two parts using all
available DSS information. Part I discusses the
analysis over the Eastern Ghats and the Cudda-
pah basin and Part II deals with the Dharwar
Craton (west of the Cuddapah basin).
DENSITY, 9/cm3
I I 1 , I ’ I ’ I I I 1
2.4 3.0 3.2 3.4 3.6
30 - I Y
60 -
70 -
- S.PENINSULAR SHIELD
--x-x- KOYNA REGION
Fig. 7. Interval velocity vs depth relationship obtained for the southern peninsular shield (Kavali-Udipi profile) and Koyna region
according to Kaila et al. (1979, 1981a, b). The interval velocity has been converted into density using the relationship given by
Ludwig et al. (1970).
GEOPHYSICAL STUDY OF CUDDAPAH BASIN AND DHARWAR CRATON, INDIA 345
I. Geology and tectonics of the Cuddapab basin
The Cuddapah basin, with its crescent shape, occupies a large area, about 44,000 km’, in the southern part of the Indian Peninsula, between latitudes 1395 and 17 o N, and longitudes 77 “45’ and 85”15’E. It is presumed to be a continental basin formed during Late Proterozoic times, a considerable lapse of time after the deposition, uplifting and metamorphism of the Dharwars.
The basin is filled with Precambrian sediments consisting of quartzites, limestones, dolomites, shales, slates (Cuddapah Group) and shales, limes- tones and quart&e (Kurnool Group). A large part of the basin is occupied by Cuddapah sediments, but the northwestern and the northern parts, which are generally low lying, are overlain by the Kumool formations. Major geological features of the Cud- dapah basin are shown in Fig. 8, together with isostatic anomaly contours (after Qureshy et al.,
1 15’ KAVALI
BAY
I N d.E X
I--1 ALLUVIUM pq < Ir ‘ DHARWAR
(ml LATERITE 11 UNCLAS Pl3”CTAI -t_n,a,nLLINES
vm UPPER GONDWANA F1 DOLERITES
m KURNOOLS I++1 GRANITE DOMES
f---l CUDDAPAH __AXIS OF
GRAVITY LOW
[ml KHONDALITES -a-a-DSS PROFILE 79” 80” 81’
,-10~ ISOSTATIC ANOMALY CONTOUR
Fig. 8. Tectonic map of the Cuddapah basin and adjoining areas showing Airy-Heiskanen anomaly contours (for T= 30 km)
superimposed on simplified geology (after Qureshy et al., 1968).
346
1968). The total thickness of sediments is geologi-
cally estimated to be about 6000 m (Murthy,
1982). Narayanaswamy (1970) had discussed the
geology and tectonics of the Cuddapah basin in
detail.
The contact of the eastern part of the basin
with the Dharwars and the Peninsular Gneisses is
marked by a thrust (Murthy, 1982) all along the
margin, except in the northern part, as shown in
Fig. 8. According to Sen and Narasimha Rao
(1967), igneous activity in the Cuddapah basin
was restricted to the western half of the basin. To
the east of the Cuddapah basin lies the Eastern
7O 78’ 7
rO_ ’ 79O 80' 81'
' 10 0 1020304050Km ’
SCAtE
R.K. VERMA AND Y. SATYANARAYAMA
Ghats belt which runs mostly along the east coast
of India from Bhubaneswar on the northwest to
Cape Comorin in the south. The Eastern Ghats
are associated with a charnockite and khondalite
suite of rocks, mafic intrusives, anorthosites as
well as extensive granitisation and migmatisation
(Narayanaswamy, 1970).
Gravity field over the Cuddapah basin and its sur-
roundings
The Bouguer anomaly map of the Cuddapah
basin after Qureshy et al. (1968) is shown in Fig.
16’
I
7B"
-/ ClJDiAPAH BASIN
79O BOO
Fig. 9. Bouguer anomaly map of the Cuddapah basin and the adjoining areas (after Qureshy et al., 1968).
GEOPHYSICAL STUDY OF CUDDAPAH BASIN AND DHARWAR CRATON, INDIA 347
9. The Cuddapah depression is well pronounced on the gravity anomaly map. The contours follow the shape of the basin to a large extent. Bouguer anomaly values are nearly - 100 mGal outside the western margin, rise to -60 mGal in the south- western part, fall again to -110 mGa1 near the eastern margin and rise to nearly -20 mGal over the Eastern Ghats.
A large gravity high situated in the southwest- em part of the basin is marked by a -60 mGal contour. This high has a general N-S trend, and has an amplitude of +40 mGal as compared to the surrounding regions, with a steep gradient over its flanks. This anomaly has been attributed to high-density trap rocks which are extensively found in the western part of the basin (Kaila and Bhatia, 1981; Mishra et al., 1985).
The gravity anomaly contours between the east- em margin of the basin and the east coast run more or less parallel to the eastern margin of the Cuddapah basin. There is a steep positive gradient near the contact of the basin to the Eastern Ghats. Here the Bouguer anomaly rises sharply from - 110 mGal over the eastern part of the Cudda- pah basin to -20 mGa1 over the Eastern Ghats (Fig. 9). It may be seen from Fig. 5 that the isostatic anomaly also rises sharply from - 90 mGal to -20 mGa1 in the same area. The steep gradient is probably due to a faulted contact be- tween the Eastern Ghats and the Dharwar Craton as suggested by Subrahmanyam and Verma (1986).
Gravity data alone do not give us adequate information concerning mass inhomogeneities lying in the crust. Recently, the DSS group of NGRI has shot two profiles across the Cuddapah basin, from Kavali to Pamapalle in the southern part and Alampur to Koniki and Ganapeswaram in the northern part of the basin (see Fig. 9). The results of these studies, reported by Kaila et al. (1979) and Tewari et al. (1986), have given great deal of information concerning the basin and the underlying crust, as well as the Moho along the two profiles. Interpretation of the gravity field along the Kavali-PamapalIe profile in the light of results obtained from DSS is discussed herewith.
Major features of the Kavali-Parnapalle DSS pro-
file
A crustal depth section along the Kavali- Parnapalle section of the Kavali-Udipi profile (after Kaila et al., 1979) is shown in Fig. 10.
The crustal depth section along the profile has been divided into eight blocks, numbered Z to VZZZ, which are separated by ten deep-seated faults and thrusts marked 2 to 10. These faults have been inferred on the basis of seismic reflection data, discontinuity of reflections or their absence. The Cuddapah basin lies between a large deep- seated fault numbered IO in the western margin and a low-angle thrust (fault 2) in the eastern margin of the basin (Fig. 10). Faulting in the Moho is a noticeable feature of the profile. In general, the Moho dips eastward, to the east of Pamapalle.
In the eastern part of the profile, block ZZ mostly constituting the Eastern Ghats, appears to have moved upwards with respect to the adjacent blocks Z and ZZZ, marked by a low-angle thrust. The Moho appears to have gone down underneath the Eastern Ghats along the Kavali-Pamapalle profile (Fig. 10). The basement underlying the sediments (having P-wave velocity of 5.8 km s-l), is well marked by the refraction segment with a velocity of 6.4 km s- ‘. It shows an eastward dip and is terminated along the eastern margin by a low-angle thrust. The maximum depth of the sedi- ments along the profile is found to be nearly 10 km near Maidukuru, where the basement has been downthrown along a deep-seated fault (7 in Fig. 10). The Moho underneath the basin is highly faulted, showing consistent downdip towards the east. The depth of the Moho varies from 35 km in the west to nearly 46 km underneath the Eastern Ghats.
Analysis of the gravity field along the Kavali- Parnapalle profile
The observed Bouguer anomaly along the Kavali-Pamapalle profile appears to be in- fluenced by several factors, including the sedi-
-40
BOUG
UER
ANOM
ALY
(OBS
ERVE
D)
-60
J -8
0 -1
00
I ‘.
I ,
,
I- 40
n
c .
,
8.59
: -.
_i
,”
‘_m
^ __-
i
I \,
l- -
- 6G
--
-
J 60
cl
$0
SHO
T PO
INT
FAUL
T WI
TH
I _._
REF
LEC
TOR
SE
GM
ENTS
yh
R;;$
ONOF
1-
_1
ALL
UVI
UM
1 70
WIT
H BO
UNDA
RY
l-2-J
G
RAN
ITIC
VE
LOC
ITY(
KM
/SEC
) BA
SEM
ENT
m
$;y;
&TS
m
&~;
~;SL
’:N1~
;NE,
SSE
S
I 1”
CR
USTA
L B
LOC
K
@
MO
HO
BO
UNDA
RY
m
~;;~
;f$~
123
BASI
C IN
TRU
SIVE
S
r Y
I + a w
a
Fig.
10
. Si
mpl
ifie
d cr
usta
l de
pth
sect
ion
alon
g th
e K
aval
i-Pa
rnap
alle
pr
ofile
fr
om
DSS
ac
ross
th
e so
uthe
rn
part
of
th
e C
udda
pah
basi
n (a
fter
K
aila
et
al.,
19
79).
DiST
ANCE
,KM
2.
40
220
200
o~~
,,,,,,
,l,,l,
,,,l,,
,,,,,,
reo
160
1LO
120
100
80
60
40
20
0 1
IO
EASTERN
6HAlS
AP
=-
0.3
s 6o
l-
70
I 60
70
80
90
* l .:*
.. I . -
s-
KU
RN
OO
LS
a
* -.
*.
.
R CU
DDAP
AHS
III ;:z
vT
DHAR
WAR
S BA
SIC
INTR
USI
VES
m
WIT
H AP
= 0.
15
g/m
3
HIG
H-D
ENSI
TY
LAYE
R W
ITH
BASI
C C
OM
PO
SIT
ION
I
UN
CLA
SSIF
IED
C
RYS
TALL
INE
GN
EISS
ES
Fig
. Il
. O
bser
ved
and
com
pute
d gr
avity
an
omal
ies
alon
g th
e K
aval
i-Pa
rnap
alle
pr
ofile
. C
urve
ar
e as
fa
llow
s:
I -o
bser
ved
Bou
guer
an
omal
y;
2 -A
iry-
&st
atic
an
omal
y (T
=
30
km);
j-
grav
ity
effe
ct
of
sedi
men
ts
com
pris
ing
the
Cud
dapa
h ba
sin
and
crus
tal
root
(e
xcee
ding
35
km
) ex
tend
ing
up
to
the
Moh
o,
us@
de
nsity
as
sh
own
in
the
figu
re
(the
conf
igur
atio
n of
th
e ba
sin
and
the
Moh
o ar
e as
pe
r D
SS
resu
lts),
4
-Bou
guer
an
omal
y co
rrec
ted f
or
the
sedi
men
ts,
Moh
o co
nfig
urat
ion
and
a hi
gh-d
ensi
ty
bloc
k un
dern
eath
th
e
Eas
tern
G
hats
; 5-
-ano
mal
y w
ith
corr
ectio
ns
sam
e as
fo
r cu
rve
4, w
ith
addi
tiona
l co
rrec
tion
for
a lo
w-d
ensi
ty
uppe
r m
antle
to
th
e w
est
of
Cud
dapa
h ba
sin
as s
how
n in
th
e fi
gure
; 6
-Bou
guer
an
omal
y w
ith
addi
tiona
l co
rrec
tion
for
a hi
gh-d
ensi
ty
laye
r/ba
sic
intr
usiv
e in
th
e lo
wer
cr
ust
in
the
wes
tern
pa
rt
of
the
Cud
dapa
h ba
sin
(thi
s cu
rve
satis
fies
th
e ob
serv
ed
grav
ity
fiel
d fa
irly
w
eil)
; 7-
isos
tatic
an
omal
y co
rrec
ted
for
the
effe
ct
of
sedi
men
ts
com
pris
ing
the
Cud
dapa
h ba
sin,
S
350 R.K. VERMA ANDY. SATYANARAYAMA
ments of the Cuddapah basin and the Moho con-
figuration, as well as density variations within the
crust. The upper mantle density may also be a
contributing factor. Various factors which in-
fluence the gravity field were studied one by one.
Figure 11 shows the general configuration of
the Cuddapah basin and the Moho along the
Kavali-Pamapalle profile, along with the eleva-
tion, the surface geology and the observed Bouguer
(curve I ), as well as Airy isostatic anomaly (curve
2). The basin extends from SP 225 in the west to
nearly SP 80 in the east and has a maximum
thickness of nearly 10 km. Although the reflec-
tions from the Moho are not continuous, the gen-
eral configuration of Moho as shown in Fig. 11 is
consistent with the DSS results. In the absence of
these results (the basin as well as the Moho con-
figuration), several models could be constructed to
explain the observed gravity field. However, the
basin configuration as well as the Moho depth put
appreciable constraints on the acceptable models.
By applying corrections for the sediments and the
crustal root (> 35 km) the bias due to these fac-
tors can be removed.
Considering the velocity information available
from the DSS profiles, the density values of sedi-
ments, the underlying basement, average crust and
Moho were determined using the velocity-density
relationship given by Ludwig et al. (1970).
The sediments comprising the Cuddapah basin
show a lesser seismic wave velocity as compared to
the underlying basement. Hence these must be of
lower density as compared to the underlying
gneisses. The density contrast between the two
was estimated to be of the order of -0.1 g cmW3.
Using this value, the gravity effect of the sedi-
ments was computer (assuming a two-dimensional
model; Talwani et al., 1959) for the basin config-
uration as revealed by DSS.
Moho depth at sea level was assumed to be 35
km, as shown in the western part of the basin (Fig.
lo), where the average elevation is about 200 m
and the sediments are absent. The velocity func-
tion for the Kavali-Udipi profile (Kaila et al.,
1979; Kaila and Bhatia, 1981) as shown in Fig. 7
suggests that the lower-crustal density should be
of the order of 3.0 g cmp3. Assuming an upper-
mantle density of 3.3 g cmp3, the effect of crustal
root formation (exceeding 35 km depth) under-
neath the Cuddapah basin was computed, using a
density contrast of - 0.3 g cm-3 between the crust
and the mantle.
The combined effect of the sediments and the
Moho configuration is shown by curve 3 in Fig.
11 for the Kavali-Parnapalle profile. It is ob-
served that the combined effect of the sediments
and the Moho largely explains the observed
anomaly in the central part of the basin. However,
there are large discrepancies between curves 1 and
3 in the western part of the basin and to the east
of the basin (areas underlain by the Eastern
Ghats). The observed gravity field should be nearly
- 20 to - 30 mGa1 over the metamorphics to the
west of the basin, whereas it is nearly -80 mGa1.
In the eastern part (over the Eastern Ghats), the
gravity field is 50-60 mGa1 more positive as com-
pared to the computed values. It is apparent there-
fore that sediments and the Moho configuration
do not explain the observed gravity field along the
entire DSS profile.
It may be observed from Fig. 11 that, upon
correcting the isostatic anomaly for the effect of
sediments, the magnitude of this anomaly is con-
siderably reduced in the western part of the basin
(curve 7) but the anomaly does not disappear
entirely. The isostatic anomaly over the Eastern
Ghats remains more positive as compared to val-
ues over the Cuddapah basin. The geologically
corrected isostatic anomalies are relatively free
from any surface mass inhomogeneities and there-
fore should be attributed only to deeper causes.
This suggests that the overall density of the crust
underlying the Eastern Ghats should be higher
than that underneath the Cuddapah basin. As the
Eastern Ghats are a granulite belt, it is logical to
expect that these are underlain by a crust of
relatively higher density.
Keeping this in mind, a high-density crustal
block with a density contrast of +0.07 g cme3
underneath the Eastern Chats (as compared to the
crust underneath the Cuddapah basin) was as-
sumed and the gravity effect was computed. The
combined gravity effect of sediments, the crustal
root (exceeding 35 km thickness) and the Eastern
Ghats block is shown by curve 4 in Fig. 11. The
assumed high-density crustal block shows a low-
GEOPHYSICAL STUDY OF CUDDAPAH BASIN AND DHARWAR CRATON. INDIA 351
angle thrust relationship with the Cuddapah sedi- ments as suggested by DSS results, and dips steeply towards the east. This block has a width of about 80 km at shallower depths and is relatively narrow (35-40 km) at a depth of about 35 km.
It may be mentioned here that the Eastern Ghats, a granulite terrain, is characterised by a gravity high all along its length (Subrahmanyam and Verma, 1986) and may represent a continental suture of Precambrian times. How this suture was evolved is difficult to envisage at present.
The decrease in the Bouguer anomaly further east, which may be due to alluvium along the east coast (Kaila and Bhatia, 1981), has not been con- sidered here. The assumed high-density crust in the eastern part can explain the observed gravity field in this area satisfactorily. However, a major discrepancy between the observed and the com- puted values in the western part remains.
As discussed earlier, the Bouguer anomaly map of the southern peninsula of India reveals the presence of appreciably large negative anomalies over the Peninsular Shield (see Fig. 3). At least a part of this anomaly appears to be of deep-seated origin, unrelated to near-surface features or the Moho configuration. In order to explain the large negative Bouguer anomaly, about -70 to - 80 mGal to the west of Cuddapah basin, the presence of a low-density upper mantle (Ap = - 0.06) ex- tending from a depth of about 35 km to 80-90 km has been invoked, as shown in Fig. 11. This low- density upper mantle, which may be a part of the lithosphere, appears to extend further to the west for a few hundred km (discussed in Part II). The combined gravity effect of sediments, the Moho configuration, high-density crust underneath the Eastern Ghats and a low-density upper mantle is shown by curve 5 (Fig. 11).
The gravity effects of sediments comprising the Cuddapah basin, the Moho configuration, the high-density crust underneath the Eastern Ghats and a low-density upper mantle accounts for most of the anomaly along the profile. However, a discrepancy of about + 35 mGal between the ob- served and computed values in the western part of the basin remains to be accounted for.
As mentioned earlier, Bouguer anomaly values rise from nearly - 90 to - 120 mGal over areas to
the west of the Cuddapah basin, to -60 mGa1 locally in the western part of the Cuddapah basin, where considerable evidence of igneous activity
has been seen geologically. In this area Kaila and Bhatia (1981) have inferred a thick basaltic layer underneath the Cuddapah sediments, on the basis of DSS results. Following this, the discrepancy between curve 5 and the observed anomaly (curve 1) has been explained in terms of a high-density (basic) crust with a density contrast of +0.05 g cmp3 with respect to the surroundings. This body lies at a depth of about lo-20 km below the sea level. This body might have been the source of basic intrusives found within the Cuddapah sedi- ments (see Figs. 8 and 11). The thickness of this body varies from 10 to 15 km, and it lies between Pamapalle in the west and Maidukuru in the east.
The gravity effect of basic intrusives within the ‘Cuddapah sediments has been computed using the assumed density contrast of +0.15 g cme3 with respect to the surrounding Cuddapah sediments. The intrusive bodies extend to a depth of about 3-5 km, dipping towards the east. The maximum gravity effect of intrusives was found to be about + 10 to + 14 mGa1, locally.
The combined gravity effect of sediments, crustal thickening (exceeding 35 km and extending down to Moho), a high-density crust underneath the Eastern Ghats, basic intrusives, a high-density crustal layer underneath the Cuddapah basin, and a low-density upper mantle to the west of Cud- dapah basin is shown by curve 6 in Fig. 11. The curve shows fairly good correlation with the ob- served gravity field. The analysis of the gravity field presented here is consistent with the major geological information available and the Moho configuration, as well as with the DSS results.
II. The gravity field over Dharwar Craton
Introductory remarks
The southern peninsula of India, south of the Deccan trap and to the west of the Cuddapah basin, is known as the Dharwar Craton. It is an important part of the Indian shield, being well known for greenstone-granite-gneiss association, and is a typical low-grade Archaean terrain
352 R.K. VERMA AND Y. SATYANARAYAMA
1000 -
800 - 2z
.
5 600 - -
2
z w’ 400 -
. UDIPI - KAVALI
o KOYNA
UK-10 a
UK;14
K-90 UK-13
~~-15~~ ,' ,K-2
K- 4" ' KT5UK-'2 0
a UK-2
UK-l.
H,MOHO DEPTH , KM
Fig. 12. Average elevation vs Moho Depth relationship obtained for different blocks located along the Kavali-Udipi and Koyna I and II profiles; the average Moho depth at sea level is inferred to be 35 km, as discussed in the text.
(Swaminath et al., 1976; Naqvi et al., 1974, 1978).
It has a wide distribution of volcano-sedimentary
schist belts, such as Chitradurga, Shimoga, Sargur,
Kolar and others, as well as intrusive granites
surrounded by a vast Peninsular Gneiss complex.
Some workers believe that the Dharwars con-
stitute a nucleus around which continental growth
in the southern peninsula of India has taken place
(Pichamuthu, 1970). The Major geological features
of the Dharwar Craton are shown in Fig. 2.
Gravity field: general remarks
Gravity field over the Dharwar greenstone-
gneiss-granite terrain has been studied by Subrah-
manyam and Verma (1982). The field is char-
acterised by several highs and lows of short wave-
length. The highs, ranging from +lO to + 30
mGa1 in amplitude, are found to be generally
associated with syncline-filled volcano-sedimen-
tary sequences or greenstone belts, while lows
ranging from - 5 to - 20 mGa1 amplitude are
found over granite outcrops or granites concealed
underneath the gneisses.
Apart from near-surface geological features, the
gravity field is influenced to a large degree by
variations in crustal thickness. It is a well known
observation that in general, over continental areas,
as the elevation increases, the Moho depth in-
creases (Woollard, 1962) contributing to large
negative Bouguer anomalies. In order to examine
whether this is the case in the southern peninsular
shield we have plotted average elevation (i o x : o
areas) vs. Moho depth for different parts of the
Kavali-Udipi profile, as well as for the Koyna I
and II profiles (Kaila et al., 1979, 1981a, b). The
latter have been shot over the Deccan trap region,
as shown in Fig. 3. The relationship obtained is
shown in Fig. 12. The figure shows that, in gen-
eral, the Moho depth increases with elevation.
However, there is significant departure for points
which lie along the Eastern Ghats (UK-Z, UK-2
and UK-3, Fig. 13) where the Moho lies at a
depth of about 40-46 km for very low elevations.
The average Moho depth at sea level is expected
to be 34-35 km for crust of normal composition.
The minimum depth along the Kavali-Udipi pro-
file is about 34 km.
Parnapalle- Udipi section of the DSS profile
Figure 13 shows the observed Bouguer anomaly
(curve Z) along this section of the profile, which
extends from the western margin of the Cuddapah
GEOPHYSICAL STUDY OF CUDDAPAH BASIN AND DHARWAR CRAl-ON, INDIA
( lQ3W) AlQWONV t13nmoa NOlltiI3i3 (WM) Hld3Q
+ . . . *I. eon + : x . . . + x : . .
354 R.K. VERMA ANDY. SATYANARAYAMA
basin to the west coast of the Indian Peninsula.
Our present interest is to interpret the regional
gravity field, which is shown by curve 2 in the
same figure. The regional Bouguer anomaly shows
values of the order of - 100 to - 110 mGa1 near
the west coast. The values rise to nearly - 90
mGal in the central part of the area (near the
Closepet granite) and further increase to - 70
mGa1 in the eastern part of the area, near Pama-
palle. The possible causes of this regional varia-
tion are as follows: (a) variation in the crustal
thickness/Moho depth; (b) variation in crustal
density; (c) variation in the upper mantle density.
In order to examine the various factors, correc-
tion for crustal thickness/Moho depth variation
was applied to the observed gravity field. The
Moho depth as reported by DSS was used for this
purpose. As in the case of the Kavali-Pamapalle
profile (Fig. 11) a density contrast of -0.3 g
cm - 3 between the lower crust and the mantle was
assumed for this purpose. The anomaly corrected
for this factor is shown by curve 4 in Fig. 13.
It is apparent from this curve that the corrected
regional anomaly varies from - 90 mGa1 to - 40
mGa1 and lies at a level of about -60 to - 70
mGa1 over a large part of the profile. It is difficult
to explain such large anomalies of regional nature
in terms of crustal inhomogeneities. A large part
of this anomaly appears to be of deep-seated
origin. Curve 4 of Fig. 13 shows a broad high of
about +35 mGa1 amplitude lying over the
Schimoga schist belt, the Chitradurga schist belt
and the adjoining gneissose terrane, extending up
to the Closepet granite. Outlining this high, a
regional anomaly ZZ has been defined, as shown
by curve 5 in Fig. 13. This broad regional anomaly
has been interpreted in terms of a low-density
upper mantle (with a density contrast of -0.06 g
cmp3 with respect to its surroundings) underlying
this profile, extending from the Moho up to a
depth of about 70-90 km. The residual (ZZ)
anomaly, defined as the difference between curves
4 and 5, has been interpreted in terms of a high-
density crustal layer (Ap = +0.08 g cm-3), lying
at a depth of about lo-23 km below the surface in
the above-mentioned area. This high-density crust
could be related to the processes leading to the
evolution of the Dharwar schist belts, which con-
sist predominantly of volcano-sedimentary rocks.
A combined effect of crustal thickness variation, a
low-density upper mantle and a high-density
crustal layer can explain the observed regional
gravity field in this part of the DSS profile ade-
quately, as shown by curve 6 (Fig. 13).
implication of the model
The observed Bouguer anomaly is a cumulative
effect of mass inhomogeneities lying below sea
level at any depth within the lithosphere, the up-
per mantle or the asthenosphere. In order to at-
tribute the anomalies to sources at different de-
pths, information obtained from DSS is very help-
ful.
The appreciably large negative Bouguer anoma-
ly observed over large parts of the southern
Peninsula of India cannot be explained in terms of
a crustal root effect or variations in Moho depth.
A low-density upper mantle underlying large parts
of the southern peninsula can account for the
observed Bouguer as well as isostatic anomalies.
This feature extends from the western part of the
Cuddapah basin in the east up to the Western
Ghats in the west, along the Kavali-Udipi DSS
profile, and attains a thickness of the order of
40-55 km underneath the Western Ghats.
Recently, Ramakrishna Rao et al. (1989) have
studied the upper mantle structure in the sub-
crustal lithosphere in the southern Indian shield
from phase velocities of body waves, using data
for shallow earthquakes originating from the east
as well as the west coasts of India. The phase
velocity was measured by using well located
seismic stations set up by the National Geophysi-
cal Research Institute (NGRI). These studies show
that the upper mantle in the western part of the
southern Indian shield has lower velocities as
compared to that along the eastern part, the veloc-
ity contrast (in Pn) being of the order of 0.3-0.5
km s-l. These results strongly support ani-
sotropy/ inhomogeneity in the lithosphere of the
southern Indian shield. Since velocity contrast
represents density contrast as well, these results
support the presence of a lower-density upper
mantle, as inferred from gravity data.
GEOPHYSICAL STUDY OF CUDDAPAH BASIN AND DHARWAR CRATON. INDIA 355
It may be recalled here that, as shown in Fig. 1, the geoid anomaly which is centered near Ceylon
covers a large part of the Indian Ocean and ex- tends over the southern part of the Indian Peninsula. The low-density upper mantle inferred from analysis of gravity data can explain the ob- served geoid anomaly to some extent. The origin of such a broad regional feature has to be ex- plained in terms of processes that have taken place over the Indian Peninsula and the surround- ing Indian Ocean since Tertiary times.
Over the Indian Peninsula, a major geological feature observed today is the Deccan volcanics, which presently cover an area of approximately 500,000 km2. The volcanics (also known as traps) have been dated as 59-62 m-y. in age (McElhinny, 1968). According to Krishnan (1960) the volcanics covered a much larger area, probably double the present size, during Eocene times. The Indian Ocean has been evolved mostly during the late Cretaceous to Present period (McKenzie and Sclater, 1971), as a result of the break-up of Gondwanaland. The Decqn trap eruptions have definitely taken place during the period of active drift of the Indian subcontinent, as revealed by palaeomagnetic studies (Klootwijk, 1979). It ap- pears that, as a result of these processes, convec- tion currents were generated in the deep mantle, resulting in mass inhomogeneity over large parts of the Indian Ocean and the surrounding region.
Bott (1987) and Ricard et al. (1989) have ex- plained the presence of a geoid low in the Indian Ocean and the surrounding region in terms of presence of deep convection currents in the man- tle. These currents result in mass inhomogeneities over large areas. It is possible that the convection current existing in the Indian Ocean is related to the processes leading to its evolution. The geoid anomaly is likely to exist for a few million years, since the convection currents are of deep-seated origin and may have very low velocities.
Snmmary of results
The gravity field over Southern Peninsular In- dia is characterised by large negative Bouguer as well as isostatic anomalies. The results of the DSS profile shot from Kavali along the east coast to
Udipi on the west coast of India (Kaila et al., 1979) enable us to interpret satisfactorily the large negative anomalies over Dharwar craton and the eastern and western parts of the Cuddapah basin, as well as the positive anomaly observed over the Eastern Ghats.
The large negative Bouguer and isostatic anomalies observed over the eastern part of the Cuddapah basin appear to be due to the effect of sediments, as well as the eastward dip of the Moho underneath the basin. In comparison, the appreciably positive Bouguer anomaly over the Eastern Ghats can be explained only in terms of a higher-density crustal block underlying the Ghats.
Analysis of the gravity field over the Dharwar Craton suggests that the large negative Bouguer anomaly is not caused by variations in the Moho or crustal density. A low-density upper mantle underlies a large part of the Dharwar Craton, contributing to Bouguer anomalies of the order of -60 to -70 mGal. It appears that similar condi- tions may prevail over large parts of the southern peninsular shield. To some extent, this can explain the large negative geoid anomaly observed over the Indian Ocean as well as the southern part of Indian Peninsula. It is inferred that the upper mantle underlying these areas is inhomogeneous, perhaps as a result of convection currents related to the evolution of the Indian Ocean.
Acknowledgement
We are grateful to S.K. Mitra for drafting the figures and to M.K. Roy for typing the manuscript.
References
Bott, M.H.P., 1987. Geoid anomalies, mantle convection, hot
spots and subduction zones. Paper presented at IUGG
