Gravity field and structures of the Rajmahal Hills: Example of the Paleo-Mesozoic continental margin in eastern India

Tectonophysics,

131 (1986) 353-367

Elsevier Science Publishers

353

B.V., Amsterdam

- Printed

in The Netherlands

GRAVITY FIELD AND STRUCTURES OF THE RAJMAHAL HILLS: EXAMPLE OF THE PALEO-MESOZOIC CONTINENTAL MARGIN IN EASTERN

MANOJ

INDIA

MUKHOPADHYAY,

Department

R.K. VERMA

and M.H. ASHRAF

of Geophysics, Indian School of Mines, Dhanbad

(Received

July 5, 1984; revised version

accepted

*

- 826004 (India)

May 6, 1986)

ABSTRACT Mukhopadhyay, Hills:

M., Verma,

example

R.K. and Ashraf,

of the Paleo-Mesozoic

M.H., 1986. Gravity

continental

margin

field and structures

in eastern

India.

of the Rajmahal

Tectonophysics,

131:

353-367. A narrow eastern

strip

India.

of Gondwana

basins

wavelength

and 48 mGal amplitude

Gondwana

sediments

constrained Rajmahal

Gondwanas Rajmahal

and granulite) against

suggests

the wider

flank

nearly

from

the peninsular gravity

traps and younger

of the Rajmahal over a rifted

100 m.y.

ago.

rifting,

perhaps

preceded

the continental

orogeny,

along the east coast of India.

sediments

High-grade occurring

layer also extends have covered basin

to the Bengal

faulted

shield

metamorphism

along

in the Precambrian

in

anomalies

due to partly

metamorphic

layer

below the eastern

it. The Gondwanas

are

floor whose deepest

part

basin.

margin

shield

high of 100 km

interpretation,

by a denser

over an irregular

hills adjacent and highly

high. Gravity

high is caused

up to 3.5 km thick. The metamorphic

were deposited traps

that

on this wider gravity

the shield edge and are preserved

the eastern

traps

with a conspicuous

over an area of 25,000 km2. Second order residual

hills where the Gondwanas,

downfaulted underlies

data,

the Rajmahal

is associated

and traps are superposed

by seismic

(amphibolite

separates

This part of the shield margin

It is inferred

that was intruded

the shield

edge

that the by the

presumably

as a part of the Eastern

Chats

INTRODUCTION

The Rajmahal hills are located in the northeast corner of the Indian shield. Their eastern part is separated from the shield by a narrow and discontinuous stretch of Gondwana (Carboniferous-Lower Cretaceous) coal-bearing basins (Fig. 1). The eastern hills are composed of bedded basaltic traps, called Rajmahal volcanics, that form flat-top hills over an area of 4100 km*. The Rajmahal volcanics extend southward below the surface for at least 100 km, as far as the Ghatal well, below the Tertiary sediments of the Bengal basin (Sengupta, 1966). Subsurface volcanics, in

* Present

address:

0040-1951/86/$03.50

Petro Bangla,

Dacca,

Bangladesh.

0 1986 Elsevier Science Publishers

B.V.

PUINEA -----

JAMALOANJ

KANSU

BURDWAN

OHAlAL 08”

GANGETIC

26’

GAL

N ‘BASIN _

INDEX

-:I t

10 ltza

Alluvium

cover

Neogene

sediments

INDIAN

Fig. 1. Regional eastern

geologic

setting of the Rajmahal

part of the Indian

map after the Geology Rajmahal

shield, and the Ranjit

Map of India (Geological

hills are shown in Fig. 5. Subsurface

hills and the Damodar

valley Gondwana

valley tectonic

in the eastern

Survey of India,

extension

are partly

overlain

by the Rajmahal

traps, and are covered

shield margin below the western part of the Bengal basin. -Damodar k-Kansat,

valley Gondwana B -Burdwan,

basin,

R V-Rajmahal

J -Jamalganj,

G -Ghatal.

SP --Shillong

Volcanics.

sediments

foredeep

plateau,

Inset shows

Base for the

by hachures. sediments

Ra V-Ranjit

P-Pumea,

general

details

from the Rajmahal

is indicated

by the Tertiary

Drillholes:

basins in the Himalaya.

1962). More geologic

of the Gondwana

hills below the western part of the Bengal basin and the Gangetic Gondwanas

window

lithology

The at the

valley,

D

Ku -Kuchma, encountered

in

these.

turn, partly cover a deep Gondwana trough at least 275 km long striking north-south. Sediment thickness in the concealed Gondwana trough reaches up to 3 km (Choudhury and Datta, 1973). Seismic and drilling data to the north and east of the Rajmahal hills indicate that the Gondwanas continue below the surface in both directions (cf. Farah, 1973; Rao, 1973; Agrawal, 1977), before they are again exposed in the Ranjit valley tectonic window in the Darjeeling Himalaya 100 km due north of the Rajmahal hills. The Rajmahal volcanics and Gondwana sediments together with their subsurface continuations under the Bengal basin he over the depressed Indian shield where several north-south basement faults have been mapped by seismic and other

355

Fig. 2. Basement Choudhury

configuration

and Datta,

oriented north-south margin

Gondwana

basement Gondwana

faults. basins,

map for the shield margin in eastern India (drawn from Evans, lW,

1973; and Rao, 1973) showing

the fault-controlled

deep Gondwana

trough

below the western part of the Bengal basin and the Gangetic foredeep. The shieid sediments

Depth

underlying

contours

RS-Rangpur

the Rajmabal

are in meters.

Volcanics

Rk’--Rajmahal

are downfaulted Vokanics,

to the east by

D-Damodar

valley

saddle.

geophysical surveys carried out by the Indo-Stanvac Petroleum Project and the Oil and Natural Gas Commission. The basement faults underlying the Bengal basin are subparallel to the shield margin, describing a set of buried ridges and depressions (Fig. 2). The overall structural pattern, history of sedimentation, and Rajmahal volcanicity at the shield margin apparently represent features of the Mesozoic continental margin in eastern India. Here we examine structures and tectonics of this rifted margin on the basis of recent gravity data for the Rajmahal hills, and other geological and geophysical evidence from adjoining parts of the Bengal basin. REGIONAL

GEOLOGY

The Indian shield rocks exposed to the west of the Rajmahal hills presumably constitute the basement complex for the region. The basement mostly consists of

356 TABLE

1

Stratigraphic

succession

in the Rajmahal

hills (modified

Formation

Age

Lithology

Recent

Alluvium

L. Cretaceous

Rajmahal

Series

and laterite

Fine-grained dolerites,

Upper Gondwana U. Triassic

from Ball, 1877)

hard and tough micro-

basaltic

carbonaceous quartzitic

1 Dubrajpur

Series

traps, shales,

shales, sandstones,

Coarse grits, conglomerates, and fine grained

L. Permian

Barakar

Series

Lower

i

Fine-grained

shaly

sandstones.

sandstones,

blue argillaceous

Gondwana L. Carboniferous

hard

grits.

conglomerates,

shales, carbonaceous

shales and coals. Talchir

Series

Boulder bed, fine-grained

sandstones

and yellow shales Precambrian Archean

to

Basement

complex

Gneiss, alternating and hornblende

with micaceous

schists. The gneiss is,

in general, granitoid. granulitic

Amphibohtes,

rocks.

granitoid gneiss (locally known as Bengal gneiss) and micaceous and hornblende schists (Ball, 1877) (Table 1). However, a detailed geologic description of shield rocks for this region is not available; hence our current knowledge of the shield rocks is sketchy. Our study shows that the metamorphic basement under the Rajmahal hills and their surroundings is quite heterogeneous it contains an unknown proportion of quartzo-feldspathic gneiss, garnetiferous schist, amphibolite and some granulites. Sampling localities for high-grade metamorphics generally correspond to the Rajmahal hills where a gravity high is observed (see below). The Gondwana sediments exposed along the shield edge directly overlie the basement metamorphics, but they are incompletely developed here Gondwanas of the Damodar Valley (Fig. 1). The Lower Gondwanas, where exposed along the shield

as compared margin,

to the

are partially

overlain by the Upper Gondwana Dubrajpur Series; elsewhere they are concealed by the Rajmahal Series which is most extensive in the eastern hills (Table 1). Stratigraphic thickness for the Rajmahal Series is about 0.6 km (Pascoe, 1975) in which the non-volcanic part never exceeds 35 m in aggregate, the individual beds varying between 20 and 70 m (Klootwijk, 1971). The Rajmahal bedded basalts presumably erupted from local fissures in central parts of the eastern hills (Hobson, 1929), where no less than 28 lava flows were recently identified by the Geological Survey of India (pers. commun.). The Rajmahal basalts are nearly 100 m.y. old (McDougall and McElhinny, 1970). Other surficial rocks in the area include laterite and alluvium, forming irregular patches.

357

GRAVITY

SURVEY

The Bouguer anomaly map for the Rajmahal hills and neighbouring areas (Fig. 3) is based on 850 gravity stations, of which nearly 500 were established as part of the

INDEX .

GRAVITYSTATION

Fig. 3. Bouguer

anomaly

For the sake of clarity,

ROUCUER

map for the Rajmahal not all station

S CUE

CRAVrrY ANOMALY CONTOURS

locations

1. Rr

>

hills and adjoining

areas based on 850 gravity

are shown.

anomaly

edge of the map are adopted

from Verma and Mukhopadhyay,

CGH is the Central

High referred

Gravity

,

to in text.

Gravity

1977. Gravity

contours contour

stations.

along the eastern interval

is 2 mGal.

3%

present

study.

The other

values

were taken

(1969) and Ghosh (1974). Bouguer anomaly the map (east of 88OE) have been adopted Mukhopadhyay, stations

(1956). Woollard

for the extreme eastern

from our previous

study

et al. part of

(Verma

and

1977).

The measurements gravity

from Gulatee contours

were made with a Worden

was guided

gravimeter.

Position

control

by 1” to 1 mile or 1” to 4 mile topographic

for

sheets

prepared by the Survey of India. Adequate gravity base station control was available from Gulatee (1956). Woollard et al. (1%9), Qureshy and Warsi (1973) and Verma et al. (1979). Station elevation values were obtained from spot heights, bench marks etc. Where such elevation values were not available, two American Paulin altimeters (having least count = 2 ft.) were employed for elevation control using topographic maps as guides. Necessary corrections were applied to the altimeter data. Gravity data collection and reductions were carried out according to normal procedures. Density for topographic mass above sea level was assumed to be 2.67 g/cm3. Terrain correction was computed for about 50 critical gravity stations spread out over the Rajmahal hills, its magnitude varying from 0.06 to 1.18 mGal. Terrain corrected gravity values were used to prepare the Bouguer anomaly map. For other stations however, this correction was not applied. ROUGUER

ANOMALY

.MAP

Bouguer anomalies for the Rajmahal hills and adjacent areas range from 26 to - 80 mGal (Fig. 3). The most prominent anomaly appearing in Fig. 3 is a central gravity high (CGH) which is open southward but terminates towards the north in east-trending gravity anomalies of the Gangetic foredeep. The CGH attains its peak amplitude over the shield edge along the western margin of the Rajmahal volcanics, with an average wavelength of 100 km in an east-west direction. Note that the gravity

high extends

beyond

the metamorphic-Gondwana

contact

encompassing

an

even greater part of the Rajmahal volcanics (see below for further analysis of CGH). A steep gravity gradient outlines the eastern limit of the CGH against the Gangetic foredeep gravity low that veers southeastward around the east flank of the Rajmahal volcanics.

The axis of this pronounced

gravity

low passes

close to the towns

of

Manihari, Rajmahal and areas further southeast. Another steep gravity zone separates the Gangetic foredeep gravity low from a gravity high over the Malda basement high, located further east in the Bengal basin (Figs. 2 and 3). Locally, the metamorphic-Gondwana contact along the west flank of the Rajmahal volcanics is clearly outlined by a discontinuous zone of low gravity, of about 10 mGal amplitude, distributed over the Gondwana basins. Such “lows” are superposed over the more extensive CGH. Similarly, local gravity highs or lows observed in the area of the Rajmahal volcanics are superposed over the longer-wavelength CGH. These second order anomalies are due to density differences between the

359

formations underlying the Rajmahal volcanics. For the purposes of geologic interpretation, the Bouguer anomalies will now be separated into regional and residual components.

I

87’ I

1

I

I

I

8Z I III

-Bougurr

anomaly

’

20Km

contour,

contour

interval

5 m6al

’

Fig. 4. Regional Bouguer anomaly map, obtained by graphical method, for the Rajmahal hills and adjoining areas. The contours are generally east-striking over the shield and the Gangetic foredeep but change to north-south over the Bengal plains across the Rajmahal Volcanics at the shield margin.

m

RAJYANAL

TRAP

SONDWANA

S

Fig. 5. Residual

Rouguer

anomaly

contour

interval

2 mGal.

A north-striking

margin

Gondwana

of the Rajmahal

basins are delineated Volcanics

edge. Steep gravity against

map for the Rajmahal

hills and adjoining gravity

by a discontinuous

are superposed

in the northeast

which the Gondwanas

pronounced

high outlines

zone of gravity

on the more extensive

part of the map is associated

are downfaulted.

residual

areas obtained

graphically;

the shield edge. The shield lows. Local gravity high developed

with the Kishanganj

anomalies

at the shield basement

fault

361 REGIONAL-RESIDUAL

Regional-residual structing

23 mutually

zones appearing profiles

MAPS

separation

of the observed

orthogonal

on the Bouguer

was selected

as controls.

ANOMALY

In doing

gravity

anomalies

profiles

across

map. The regional

the significant

gravity

by trial and error adjustment so, due consideration

was achieved

by conanomaly

curve along individual

using profile

intersection

was given to the overall

nature

points of the

observed gravity as well as to the symmetrical pattern of the CGH. The resultant regional and residual anomalies, in contoured form, are shown in Figs. 4 and 5. Figure 4 shows that regional gravity contours are of east-west strike in this part of the Indian shield; the same trend also continues into the Gangetic foredeep, illustrating a smooth fall by 80 mGa1 from south to north. This pronounced fall in gravity is a consequence of the distant root effect of the Himalaya as the north Indian shield crust underthrusts the Himalayan collision front (cf. Choudhury, 1975;

Verma

east-west

and

Mukhopadhyay,

strike of the regional

1977;

Bouguer

and

Warsi

anomalies

and

however,

Molnar,

changes

1977).

The

to north-south

around the Rajmahal volcanics and in areas further east, as originally noted by Evans and Crompton (1946). The most significant anomaly appearing on the residual map is a north-south gravity high, of 48 mGal amplitude, at the location of the CGH (Fig. 5). The

INDEX

L

LOKm

Laterite

cover

I-_

Alluvium

m

Rajmahal

m

Gondwana

Ix

Bengal gneiss (basement High-grade metamorphics

m

Density

values

and

1

trap sediment

are

in

I

g/cm3

Fig. 6. Three representative residual gravity profiles, BB’B”, DD’D” and FF’F”, in an east-west direction across the shield edge (for profile locations see Fig. 5), to illustrate the nature of the gravity high due to the metamorphics. Onto this are superposed anomalies of the Rajmahal volcanics. The most extensive gravity high has an average wavelength of 100 km, and strikes north-south. for the three profiles are also shown.

Interpreted models

362

residual high centers on the shield edge but encroaches on the Rajmahal volcanics eastward across the intervening Gondwana basins. The nature of the residual anomalies

along three representative

on Fig. 6 to illustrate direction

and

anomalies

that

symmetrical

about

due to the Gondwana

Tertiary

sediments

profiles

the residual the

shield

sediments,

are superposed

crossing

the Rajmahal

high is generally edge.

Shorter

Rajmahal

on this more

hills is shown

similar traps

extensive

in north-south

wavelength and

their

residual

residual overlying

high

of the

Rajmahal hills. Following the symmetric nature of the broader high, it is therefore possible to delineate the second order residuals due to shallower geologic masses underlying the eastern Rajmahal hills. Geologic interpretations for both first and second order residuals will be discussed in terms of two-dimensional structure sections.

ROCK

DENSITY

Rock density values for all the major lithologic groups of the Rajmahal hills area were determined (Table 2). Two distinct density groups can be observed in the shield rocks: the more prevalent Bengal gneiss that constitutes the basement complex for the region and has an average rock density of 2.68 g/cm3; and high grade metamorphics (mainly amphibolites and granulites) which are locally present in the western part of the Rajmahal hills and have an average density of 3.08 g/cm3. It should be pointed out here that sampling localities for these high grade metamorphics were mostly restricted to areas of gravity observations over the shield area, so our knowledge about the extent of the high-grade metamorphics is quite poor at present. In fact, a recent geological map prepared by the Geological Survey of India (Mazumdar, 1986) for a small portion of this part of the eastern Indian shield

TABLE

2

Rock density

values for major rock sequences

Rock type

of the Rajmahal

No. oi

Density

samples

(g/cm3

area range

)

Weighted density (g/cm’

Rajmahal

trap

(basalts) Gondwana

sediments

(shale and sandstone) High-grade metamorphics

29

2.79-2.92

2.85

8

2.23-2.60

2.47

15

2.92-3.35

3.08

27

2.55-2.R5

2.68

(amphibolites, granulites) Basement rocks (granite-gneiss)

)

average

363

(located

between

87 and 87OlO’E, at about 24O45’N) shows a rather narrow

amphibolite

and hornblende

is therefore

essential

metamorphics. are present location

north-south.

in order to know the extent

Nonetheless, usually

of CGH)

gneiss striking

to the longer

over the western

assume

that the high-grade

Bengal

gneisses.

Mean

geologic mapping

and true nature

of the high-grade

we find that localities

correspond

part

metamorphics

density

values

traps are taken as 2.47 and 2.85 g/cm3

band of

Detailed

where high-grade wavelength

of the Rajmahal

high

(at

hills. This leads

occur as a near-surface

for the Gondwana

metamorphics

residual

layer within

sediments

the

us to the

and Rajmahal

respectively.

STRUCTURE MODELS

Two-dimensional east-west profiles,

structure interpretation for residual anomalies along six AA’A” through FF’F”, crossing the Rajmahal hills, is shown in

Fig. 7. Density data given in Table 2 were used for this purpose. Two-dimensional gravity computation, carried out using the polygonal method (after Talwani et al., 1959), is assumed to be valid for the present case as the residual anomalies are very elongated in the north-south direction. Gravity interpretation assumes that the longer-wavelength residual high over the shield edge and the Rajmahal hills is produced by a layer of high density metamorphics occurring within the Bengal gneisses. This metamorphic layer, in turn, is covered by the Gondwana sediments and Rajmahal traps below the eastern hills. That the Gondwanas continue below the traps is supported by geologic (Ball, 1877, and Hobson, 1929) and geophysical data from the western part of the Bengal basin. Corresponding to the easternmost portions of the profiles, a thin layer of Tertiary sediments and lateritic cover is incorporated

in the models,

as required

by surface

geology.

Density

for Tertiary

sediments and laterites is assumed to be 2.04 g/cm3. To constrain the models, we use basement-depths and Gondwana thickness as known from previous geophysical studies of the western parts of the Bengal basin (after Choudhury and Datta, 1973, and Rao, 1973). The available basement depth-values (Fig. 2) correspond only to the eastern parts of the profiles, and presumably represent depths to the top surface of the high-grade metamorphics. It is inferred above that the broader residual high at the location of the Rajmahal hills is caused by this layer of metamorphics; consequently the shield margin Gondwanas, traps and Tertiary-laterite cover produce only second order gravity residuals. In the absence of other geophysical data, we assume, that the Rajmahal trap is a thin layer 0.6 km thick, as per the stratigraphic estimate (Pascoc, 1975). The traps dip both southward and eastward from the type area. They are ultimately covered by the Tertiary sediments below the Bengal basin along the southern and eastern margins of the Rajmahal volcanics (for details of the subsurface relationship between the Rajmahal traps and Tertiary sediments, see: Khan and Azad, 1963; Sengupta, 1966; and Farah, 1973).

364 87’ I

I

I

/

* 1

@Ma&had

Fig. 7. Two-dimensional structural models along six profiles (see Fig. 5 for profile locations} as interpreted from the residual gravity field of the Rajmahal area. Models show the interpreted mass distribution for the Gondwana sediments and tbe traps underlying the Rajmahal Volcauics, (see text for discussion). I = laterite and alluvium; 2 = Rajmahal trap; 3 = Gondwana sediments; 4 = high-grade metamorphics; 5 = basement rocks (Bengal gneiss).

The results of the model interpretation, features in common: (a) A high-grade rnet~o~~c 100 km in an east-west

synthesized

in Fig. 7, have several

layer overlying the basement is present for about

direction from the peninsular shield across the Rajmahal

hills. This anomalous layer attains its maximum thickness of 3.5 km at the shield edge; its topography below the Rajmahal volcanics is quite rugged, producing a pattern of ridges and depressions covered by the Gondwana sediments. The metamorphic layer cannot possibly extend beyond the Bhagalpur-Mahihari line north of profile AA’, but is clearly open to the south, beyond the map area of Fig. 5. The Malda basement high, below the Bengal basin, which is partly traversed by profile AA’%“, is an area of residual gravity high of 38 mGa1 amplitude. The interpreted model for the profile suggests that the Malda high may also consist of high-grade met~o~~cs.

365

(b) The Gondwana the

shield

edge

are

volcanics

and Bengal

thickness

varies

from

depressions

underlying

basin,

thickness

their

sediments

exposed

preserved

over

basin.

in narrow

the

The Gondwanas

0.4 km at the shield the Rajmahal increases

and discontinuous

depressed

shield

are downfaulted edge to more

volcanics.

Further

to over 3 km within

north-south basement faults (cf. Choudhury continue northward with appreciable thickness

below

basins the

eastward than

east,

along

Rajmahal and their

1.5 km in local below

the depressions

the Bengal marked

by

and Datta, 1973). The Gondwanas beyond the map area of Fig. 5. The

Pumea well, located in the Gangetic foredeep 80 km north of profile AA’A” (Figs. 1 and 7), penetrates the Gondwana sediments at a depth of 1223 m, passes through thick Gondwana coal seams at between 1700 and 2794 m, and reaches the basement at 2845 m. The western limit of Gondwana sedimentation below the Gangetic foredeep is possibly marked by a basement fault running north; in the area of the Rajmahal hills this fault zone is mapped on surface at the locations A’ to F’ shown in Fig. 5. (c) The Rajmahal traps overlying the Gondwanas in the Rajmahal hills as well as below the Bengal basin are believed to thin out northward and eastward. Traps are absent

below

the

Pumea

well located

in the

Gangetic

foredeep

north

of the

Rajmahal hills; eastward, below the Ku&ma well in the Bengal basin, they are only 15 m thick (cf. Khan and Azad, 1966; Rao, 1973). Below the Bengal basin, however, the traps are encountered in the Burdwan and Ghatal wells at depths of 2645 and 3049 m respectively, although they are believed to be thinner there (cf. Sengupta, 1966). Available seismic data demonstrate that the traps dip monoclinally southward. Thus in the Rajmahal area the traps are thickest. These factors indicate that the fissures through which the Rajmahal basalts erupted were probably located in the central Rajmahal hills, as originally suggested by Hobson (1929). DISCUSSION

The gravity interpreted structure sections discussed in the foregoing seem to describe a set of basement ridges and depressions below the Rajmahal volcanics. This pattern, however, is better pronounced under adjacent areas of the Bengal basin.

We believe

that the structural

pattern

under

the Indian

shield margin

in this

region has been mostly inherited from the rifting of Gondwanaland, as in other east coast basins of India. Geophysical data collected by the Oil and Natural Gas Commission (cf. Talukdar, 1982; Roybarman, 1983) for various east coast basins and their offshore extensions, demonstrate that their underlying basement topography is typically dominated by a series of linear ridges and depressions generally outlined by tensional faults. This situation is common to all the east coast basins; from the Cauvery basin near the southern tip of India through the Palar, Godavari, Mahanadi and Bengal basins. Such subsurface ridges and depressions also run subparallel to the shield margin. Furthermore, the Cretaceous basalts/traps, of

varying

proportions,

together

with intertrappean

beds, overlie the Gondwana

ments in most of these coastal basins and their offshore of structural

style,

corroborated

from the Rajmahal

clearly points India

Gondwana

to pull-apart

following

rifting

sedimentation,

and

hills and adjoining

tectonics

affecting

parts. This common

Cretaceous

volcanism

sedipattern is also

areas of the Bengal basin.

the east coast continental

This

margin

of

of Gondwanaland.

The spreading history of the Indian Ocean and reconstruction of Gondwanaland proposed by Johnson et al. (1976) show that the oldest identifiable magnetic anomaly near the east coast of India is about 130 m.y. old, which suggests that the structures present under the east coast Gondwana basins must have already been in existence. Lower Cretaceous volcanism under the eastern continental margin of India occurred during the more advanced stages of rifting and northward drift of India. Curray et al. (1982) propose that Rajmahal volcanism had a hot spot origin, and that this hotspot activity later shifted to Burma, finally to form the Ninety East Ridge; presently it is below the Heard-Kerguelen plateau in the South Indian Ocean. However, we are inclined to believe that the Rajmahal volcanism was not that isolated an event since the same basaltic traps continue southward below the Bengal basin, and varying degrees of volcanism had also affected other east coast basins in India in intimate association with rifting in Gondwana times. Certainly in the Rajmahal area the Cretaceous volcanism was very intense. The high-grade metamorphic layer which we infer to overlie the basement gneisses below the Rajmahal hills is conspicuously developed at the shield margin. Its associated gravity high has an average wavelength of about 100 km in an east-west direction and extends north-south for nearly 400 km. Our recent surveys in the Singhbhum region (Verma et al., 1984) have shown that this gravity high extends much further south with nearly the same characteristics all along the shield margin in eastern India. It further appears that this high may ultimately connect to the Eastern Ghats gravity high that runs all along the east coast of India. Field evidence,

though

granulites producing Rajmahal

that form the bulk of the high grade metamorphics in the Rajmahal area the gravity high. We do not know at present if these metamorphics of the area were produced as a part of the Eastern Ghats orogeny in Pre-

scanty

at this stage, indicates

that it is mainly

amphibolites

and

cambrian times, or whether they are due to a much younger thermal event related to the Gondwanaland break-up. To answer this we need more geologic data and age determinations. REFERENCES AgrawaI, R.K., 1977. Structure and tectonics of Indo-Gangetic plains. In: V.L.S. Bhimsankaram and V.K. Gaur (Editors), Geophysical Case Histories in India. A.E.G. Seminar, Hyderabad, 1: 29-46. Ball, V., 1877. Geology of the Rajmahal hills. Mem. Geol. Surv. India, 13: 155-248. Curray, J.R., Emmel, F.J., Moore, D.G. and Rain, R.W., 1982. Structure, tectonics and geological history of the northeastern Indian Ocean. In: A.E.M. Naim and F.G. Stehli (Editors), The Ocean Basins and Margins, Vol. 6. Plenum, New York, N.Y., pp., 399-450.

367 Choudhury, SK., 1975. Gravity and crustal thickness in the Indo-Gangetic

plains and Himalayan region,

India. Geophys. J.R. Astron. Sot., 40: 441-452. Choudhury, SK. and Datta, A.N., 1973. Bouguer gravity and its geological evaluation in the western part of the Bengal basin and adjoining areas, India. Geophysics, 38: 691-700. Evans, P., 1964. The tectonic framework of Assam. J. Geol. Sot. India, 5: 80-96. Evans, P. and Crompton, W., 1946. Geological

factors in gravity interpretation

illustrated by evidence

from India and Burma. Q.J. Geol. Sot. London, 102: 211-249. Farah, A., 1973. Example of application of geophysical techniques in determining geological environment of a coal deposit. Geol. Sot. Am. Bull., 84: 2433-2444. Ghosh, D., 1974. Regional gravity and magnetic studies over Damodar valley Gondwana basins and the surrounding areas. Unpubl. Ph.D. Thesis, Indian School of Mines, Dhanbad. Gulatee, B.L., 1956. Gravity data in India. Surv. India. Tech. Pap. 10: 195. Hobson, G.V., 1929. General Rep. Rec. Geol. Surv. India, 62: 145-146. Johnson, B.D., Powell, C.M. and Veevers, J.J., 1976. Spreading history of the eastern Indian Ocean and greater India’s northward flight from Antarctica and Australia. Geol. Sot. Am. Bull, 87: 1560-1566. Khan, A.H. and Azad, J., 1963. The geology of Pakistan gas fields. Proc. 2nd Symp. Dev. Pet. Resour., ECAFE,

Miner. Resour. Develop. Ser., 18(l):

Klootwijk,

CT.,

1971. Paleomagnetism

Tectonophysics, McDougall,

275-282.

of the Upper Gondwana

Rajmahal

traps, northeast

India.

12: 449-467.

I. and McElhinny,

M.W., 1970. The Rajmahal

traps of India, K-Ar ages and paleomagne-

tism. Earth Planet. Sci. Lett., 9: 371-378. Mazumdar, S., 1986. Crustal evolution of the Chhotanagpur gneissic complex and the mica belt of Bihar. In: Precambrian

of the eastern Indian shield. Seminar Geol. Sot. India, Indian School of Mines,

Dhanbad, 1986. Pascoe, E.H., 1975. A Manual of Geology of India and Burma. Government of India Publ., New Delhi, 1345-2130. Qureshy, M.N. and Warsi, W.E.K., 1973. Gravity bases established in India by NGRI, Part III. Geophys. Res. Bull., Hyderabad, II: 9-16. Rao, M.B.R., Roybarman,

1973. The subsurface geology of Indo-Gangetic

plains. J. Geol. Sot. India, 14: 217-242.

A., 1983. Geology and hydrocarbon prospects of West Bengal. Petrol. Asia J., 6: 51-56.

Sengupta, S.. 1966. Geological and geophysical studies in western part of Bengal basin, India. Am. Assoc. Pet. Geol. Bull., 50: 1001-1017. Talukdar,

S.N., 1982. Geology and hydrocarbon

relationship

prospects

of east coast basins of India and their

to evolution of the Bay of Bengal. Offshore South-East

Asia. Conf., 1982, Singapore,

Explor. I, Gen. Sess., pp. l-8. Talwani, M., Worzel, J.L. and Landisman, M., 1959. Rapid gravity computations

for two dimensional

bodies with application to the Mendocino submarine fracture zone. J. Geophys. Res., 64: 49-59. Verma, R.K.

and Mukhopadhyay,

Tectonophysics, Verma, R.K.,

M., 1977. An analysis of the gravity field in northeastern

India.

42: 283-317.

Mukhopadhyay,

M., Ashraf, M.H., Nag, A.K. and Sarma, A.U.S.,

1979. Gravity bases

established in eastern India by ISM. Geophys. Res. Bull., Hyderabad, 17: 45-56. Verma, R.K., Sarma, A.U.S. and Mukhopadhyay, M., 1984. Gravity field over Singhbhum, its relationship to geology and tectonic history. Tectonophysics,

106: 87-107.

Warsi, W.E.K. and Molnar, P., 1977. Gravity anomalies and plate tectonics in the Himalaya. In: Ecologic et Geologic de 1’Himalaya. Colloq. Int. C.N.R.S.,

Paris, 268: 463-478.

Woollard, G.P., Manghnani, M. and Mathur, S.P., 1969. Gravity measurements in India. Hawaii Inst. Geophys., Univ. Hawaii. Final Rep., pt. 2, p. 52.