7-ecionop&sics. 135 (1987) 307-327

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

307

Regional tectonic framework, structure and evolution of the western marginal basins of India

SK. BISWAS

Oil and Natural Gas Commission, Born&v Regionul Business Centre, 6-C, Pri.vadarshini.

Eastern Express Highway, Bombyv-400 022 (India)

(Received June 261985: revised version accepted August 12.1986)

Abstract

Biswas, S.K., 1987. Regional tectonic framework, structure and evolution of the western marginal basins of India.

Tectonoph_ysics, 135: 307-327.

The Kutch-Saurashtra, Cambay and Narmada basins are pericontinental rift basins in the western margin of the

Indian craton. These basins were formed by rifting along Precambrian tectonic trends. Interplay of three major

Precambrian tectonic trends of western India, Dharwar (NNW-SSE), Aravalli-Delhi (NE-SW) and Satpura

(ENE-WSW), controlled the tectonic style of the basins. The geological history of the basins indicates that these basins

were formed by sequential reactivation of primordial faults. The Kutch basin opened up first in the Early Jurassic

(rifting was initiated in Late Triassic) along the Delhi trend followed by the Cambay basin in the Early Cretaceous

along the Dharwar trend and the Narmada basin in Late Cretaceous time along the Satpura trend. The evolution of the

basins took place in four stages. These stages are synchronous with the important events in the evolution of the Indian

sub-continent-its breakup from Gondwanaland in the Late Triassic-Early Jurassic, its northward drifting during the

Jurassic-Cretaceous and collision with the Asian continent in the Early Tertiary. The most important tectonic events

occurred in Late Cretaceous time. The present style of the continental margins of India evolved during Early Tertiary

time.

The Saurashtra arch, the extension of the Aravalli Range across the western continental shelf, subsided along the

eastern margin fault of the Cambay basin during the Early Cretaceous. It formed an extensive depositional platform

continuous with the Kutch shelf, for the accumulation of thick deltaic sediments. A part of the Saurashtra arch was

uplifted as a horst during the main tectonic phase in the Late Cretaceous.

The present high thermal regime of the Cambay-Bombay High region is suggestive of a renewed rifting phase.

Introduction

The Kutch, Cambay and Narmada basins are three major marginal rift basins in the western margin platform of the Indian craton (Fig. 1). The basins occur close to each other in the mid-west- em part of the Indian sub-continent. These basins are bounded by intersecting sets of faults whose trends follow three important Precambrian tectonic trends (Biswas, 1982). The Saurashtra peninsula occurs as a horst block between the rifts. These basins formed at different times dur-

~~1951/87/~03.50 0 1987 Elsevier Science Publishers B.V.

ing the Mesozoic, and, are thus, considered as important Mesozoic marginal marine basins (Bis- was, 1982).

The evolution of the three basins during differ- ent stages of India’s northward drift after breakup from Gondwanaland in the Late Triassic-Early Jurassic has been discussed by Biswas (1982). He brought out the sequential development of these basins from north to south. The Kutch rifting took place in the Late Triassic-Early Jurassic, Cambay rifting in Early Cretaceous and Narmada rifting in Late Cretaceous time. These rifting events

LEGEND

“L!srERhAh\’

TERTIARY

3ECCAN ‘RAP

UPPER CRETACEOUS

LOWER CRETACEOdS

JURASSIC

UPPER PROTEROZOIC

COWER PROTEROLO~C

/ DHARWAR SYSTEM 1

:RAwTES

CHARNOCKITES

LINEAMENTS, STRUCTURAL TRENDS.

FOLIATIONS, STRIKES, FOLD AXES e’c

MAJOR FAULT

309

are synchronous with the major events in India’s framework, stratigraphic and structural features,

drift history. Raju (1968) has described the tectonic and the depositional histories of these basins.

framework and history of the Cambay basin. The In this paper, the regional tectonic framework,

tectonic framework of the Narmada basin has not the structure of the three basins which controlled

been studied in detail. Rao and Talukdar (I980) the sedimentation and their geotectonic evolution

have described the regional tectonic set up of are discussed. The Kutch basin which has the

Narmada and Bombay offshore area. Biswas and longest record of the Mesozoic in western India, is

Deshpande (1983) have discussed the tectonic discussed in detail.

AREi

‘X - NAGAR PARKAR RiDGE 6

A - ISLANO BELT RIDGE

LR - MAIN LAND RIDGE

OR- WAGAO AlOGE

jGF- NARMAOA-SON GEOFRACTURE

CF - WEST COAST FAULT

%-- MAJOR FAULTS

- TECTONIC TRENDS

Fig. 2. Pericratonic rift basins with major structural lineaments.

Indian shield

In order to appreciate the role of the Prc-

cambrian structural trends in the tectonic evolu-

tion of the marginal basins, a brief review of the

Precambrian geology of India may be relevant.

The major portion of the Indian Peninsula is

the shield area with intracratonic and pericratonic

fault-bounded basins. More than half the area of

the peninsula exposes Archaean gneisses and

schists and Precambrian sediments and igneous

rocks which have been metamorphosed to various

degrees. The rest of the Peninsula is covered by

volcanic flows (Deccan and Rajmahal Trap flows),

Late Precambrian-Early Palaeozoic sediments.

and Gondwana rocks (Fig. 1).

Krishnan (1953) described structural and tec-

tonic features of India in detail. He brought out

that there are certain persistent regional trends

noticeable in the Precambrian rocks of different

parts of India. These trends are regional foliation

strikes, fold axes of ancient mobile belts and strike

faults which are manifest as major tectonic linea-

ments. In western India three such major trends

are seen (Figs. 1 and 2): Dharwar, Ara~~alli/Del~~

and Satpura. These trends are seen as metamor-

phic belts representing three major erogenic cycles.

Of these, Aravalli and Dharwar represent the

oldest erogenic cycles followed by Satpura. In the

Aravalli region (Fig. 1) the diastrophism was re-

peated in the Upper Precambrian and the rocks of

Delhi systems were intensly folded into a syn-

clinorium and intruded by granite and other ig-

neous rocks. This synclinorium, parallel to the

strike of the Aravalli fold belt, extends southwest

towards the Kutch region (Krishnan, 1953; Bis-

was. 1982).

Basement controlled tectonics have been dis-

cussed by many authors (Prucha et al.. 1965:

Beioussov, 1962; Eremenko, 1968; Milanovsky,

1972; Katz, 1978; Biswas, 1980, 1982). Naini and

Kolla (1982) also brought out that horsts and

grabens in the western continental margin of India

were formed by reactivation of Precambrian base-

ment trends, NNW-SSE and ENE-WSW. Katz

(1973, 1979) demonstrated in the context of the

India--Madagascar and India-Sri Lanka sep-

aration, that Precambrian lineaments were re-

juvenated in the Mesozoic-Tertiary to form oce-

anic transform faults and rifts which determined

phases of Indian Ocean floor spreading. In the

present case also, it is observed that the primor-

dial fault pattern and basement grain controlled

the structural styles of the sedimentary basins.

Reactivation of major basement faults and block

faulting along them gave rise to intracratonic and

marginal basins of the Indian Shield. Repeated

movetnents along them at different times con-

trolled the evolution of these basins. The parallel-

ism of fold axes and fault patterns with prevailing

basement lineaments are readily apparent in all

the basins. Predominance of a particular trend

and subsidiary cross trends in a basin depend on

the relative time of reactivation of different sets

of lineaments in the adjacent craton.

General geology

,411 three basins originated in different periods

during the Mesozoic and have varying thickness of

Mesozoic sediments. Whereas Kutch and Narmada

basins are Mesozoic basins, the Cambay basin is

mainly a Tertiary basin. It originated during the

Mesozoic but subsided at a greater rate during the

Tertiary.

Generalized stratigraphy, correlation and histo-

ries of marginal rift basins are summarized in

Table 1. The geology of the basins is depicted in

the regional geological map (Fig. 1).

The Kutch basin has the most complete Meso-

zoic record (Biswas, 1981) with thick accumula-

tions of Late Triassic to Lower Cretaceous sedi-

ments (3000 m). Sediments were deposited in a

sheltered gulf in sublittoral to deltaic environ-

ments in two major cycles: a Middle Jurassic

transgressive cycle and a Late Jurassic-Early

Cretaceous regressive cycle (Biswas, 1981). During

the transgressive cycle mainly carbonates and

shales were deposited, whilst deltaic elastics con-

stitute the regressive deposit. Sediments were laid

down on a Precambrian granitic basement exposed

only in the Nagar Parkar Hills (NP uplift, Fig. 5)

in Pakistan, bordering the northern flank of the

graben. The Mesozoic sediments were uplifted,

folded, intruded and covered by Deccan Trap

basaltic flows in Late Cretaceous-Early Palaeo-

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restrial volcano-elastic sediments. Early Eocene

transgression and subsequent Tertiary deposition

took place in peripheral lows bordering the

Mesozoic highs, and in lows between them.

Tectonic movements in the Tertiary were milder

and cyclic, represented in the stratigraphic column

as extensive unconformities.

Holes drilled in the Cambay basin have

penetrated over 400 m of the Lower Cretaceous

fluvio-deltaic sedimentary sequence (Roy Chow-

dhury et al., 1972). From seismic data, Biswas

(1982) concluded that at least 1200 m of Mesozoic

sedimentary rocks could be expected beneath the

Deccan Trap flows which form the floor of a

Tertiary (Palaeocene to Pliocene) sequence about

5000 m thick. In this basin also, the Tertiary

sequence starts with thick volcano-elastic pied-

mont fanglomerates deposited in yolked basin

condition during the initial stages of rifting.

Marine sedimentation started with an Early

Eocene transgression during the subsidence stage

when mainly dark shales were deposited. This was

followed by an oscillatory stage when lagoonal

and deltaic sedimentation took place in intra-basi-

nal lows. In the Early Miocene, as in the other two

basins, an extensive marine transgression took

place (Raju, 1968). The rocks of this basin are not

exposed. They are covered by thick alluvial de-

posits. Most of this geological information has

been obtained from wells drilled for oil and gas.

In the Narmada basin, shallow marine. Late

Cretaceous rocks (100 m) are exposed in scattered

outcrops underlying the Deccan Trap. These rocks

were deposited in a narrow embayment in the

western part of the basin as a result of transgres-

sion during the Turonian (Jafar, 1982). Among the

three marginal basins. Late Cretaceous marine

sediments occur only in this basin, indicating a

subsidence while in the other two basins this was a

period of non-deposition and volcanic activity.

However, the Turonian transgression was very

short-lived, as the Late Cretaceous tectonic activ-

ity also affected the sediments towards the end of

the Cretaceous or during the Early Palaeocene.

Uplifts, subsequent erosion and Deccan Trap

volcanic flows restricted the occurrence of marine

sediments in a few inliers. The Late Cretaceous

inundation is restricted to the western part of the

basin. These sediments lie on the Upper Protero-

zoic metasediments but towards the western part

they are underlain by Lower Cretaceous fluvio-

deltaic sediments and overlain by Eocene and

Miocene rocks near the coastal region where the

embayment opens out (see Fig. 13). Towards the

east, in the middle of this rift zone, remnants of

marine Permo-carboniferous sediments occur

(Krishnan, 1968).

During the Early Cretaceous the Kutch-..-

Saurashtra-Narmada region was a wide shelf

which received a thick pile of deltaic sediments

(over 1800 m) as the Mesozoic sea finally re-

gressed (Biswas, 1983).

The Deccan Trap basic lavas represent

widespread volcanic eruption towards the end of

the Cretaceous, which covered the Mesozoic sedi-

mentary rocks of all these basins. This volcanic

activity was a late-tectonic event at the close of

the Late Cretaceous diastrophic episode in west-

ern India (Biswas and Deshpande, 1983).

Regional tectonic framework

The western continental margin of India can be

classed as an Atlantic-type passive margin (Bis-

was, 1982). In Fig. 2, the major tectonic linea-

ments constituting the tectonic framework of

western India are depicted, while in Fig. 3 a part

of the tectonic map of India showing the major

tectonic elements of Western India is shown. Three

Precambrian erogenic trends - the NNW-SSE

Dharwar trend, the NE-SW Aravalli trend and

the ENE-WSW Satpura trend - dominate the

structural fabric of Western India.

The Narmada -Son line~ent along the Satpura

trend is a major tectonic boundary {West, 1962;

Choubey, 1971) dividing the Indian Shield into a

southern peninsular block and a northern foreland

block.

The Dharwar trend is parallel to the faulted

west coast of India and a series of extension faults

responsible for widening the western continental

shelf (Mitra et al., 1983). Its northward extension

into the western part of the Indian Shield across

the Narmada rift gave rise to the Cambay graben.

The third important tectonic trend is the

313

FIRST ORDER POSITIVE

STRUCTURE

+<MPORTANT FAULTS +’

&hK\-SLIP

0 D

I

DECCAN

YNE CLISE

TECTONIC FRAMEWORK OF WESTERN INOiA. ( MODIFIEG FROM ERMENKO 01. et., 1969 t

Fig. 3. Tectonic framework of western India (modified from Eremenko et al.. 1969)

NE-SW Aravalh trend which splays out into three shelf and a southern Bombay-Kerala shelf. The components at its south-western extremity. The northern component of the Aravalli orogen which main NE-SW trend continues across the Cambay is the trend of the Delhi fold belt, swings to E-W graben into Saurashtra as a southwesterly plung- and continues into the Kutch region across the ing arch. The arch extends across the continental Cambay graben. The Kutch basin rifted along this shelf dividing it into a northern Kutch-Saurashtra trend. The Aravalli and Delhi trends which cross

the Cambay graben, are seen as important cross

trends within the graben in the form of numerous

transverse and oblique faults, uplifts and geomor-

phic lineaments. Courses of some of the important

rivers like the Banas, Saraswati and Sabarmati

across this basin also illustrate such cross-trends

very well. Gravity highs clearly show these cross-

trends across the Cambay graben continuing into

Kutch where they form the major longitudinal

trend (Fig. 4). The third component veers anti-

clockwise and tends to merge with the Satpura

trend.

The Saurashtra horst remains as a foundered

block between the three intersecting rifts along the

major Precambrian trends. It is a more or less

square shaped block tilted to the southwest. Its

straight western margin is also a faulted margin

which follows the Dharwar trend. The western

margin fault cuts across the Saurashtra arch uplift-

ing this block.

The western part of the Indian sub-continent is

characterized by the Baluchistan orogen, Indus

basin (foredeep zone) and the Indus shelf cir-

cumscribing the northwestern part of the Indian

Shield (Fig. 3). The hinge zone of the Indus basin

appears to be passing along the Jaisalmer-Kutch

meridian as indicated by the thickening of the

sediments considerably to the west of this

meridian. The Indus shelf is characterized by “ir-

regular ridges or promontories of the Indian Shield

which extend northwestward into the basin for

varying distances” (Rahman. 1963) subdividing

the shelf into subsidiary basins. The southern part

of this shelf, south of the Jaisalmer arch (Figs. 2

and 3), has been named the Saurashtra--Kutch

shelf (Eremenko et al., 1969). This shelf is again

divided by an east-west trending Nagar Parkar--

Tharad ridge into two subsidiary basins----a north-

ern Shahgarh basin of South Rajasthan and a

southern Kutch-Saurashtra basin (Fig. 3). This

southern basin is thus situated at the sctuthern-

most end of the Indus shelf where it meets the

continental shelf of the Arabian Sea along the

Narmada “deep-fault”.

The Cambay basin occupies a narrow NNW-

SSE trending graben in the eastern part of the

Saurashtra-Kutch shelf (Fig. 3). A linear positive

feature of low magnitude, indicated by the gravity

map (Fig. 4) separates the graben and the

Kutch-Saurashtra basin. This high, called here the

Radhanpur-Barmer arch, extends along the west-

ern margin of the graben. Evidently. the high is

the result of uplift of the western shoulder of the

Cambay graben. This arch crosses the Nagar

/ --- l CAMLL?

0 50 100 Km , f

Fig. 4. Trends of basement highs in the Kutch-Cambay region showing important cross-trends in the Cambay basin.

Parkar-Tharad ridge almost at a right angle near Tharad and the Kutch-Saurashtra shelf occupies the angle between them. The Kutch rift basin occupies the northern part of this shelf between the Tharad ridge and the Saurashtra uplift.

Basin structure

The structural style of the basin is unique. Nowhere in India is a similar style to be seen. The numerous Late Cretaceous uplifts with associated plutonic bodies and inte~ening Tertiary basins, are comparable with the Laramide structures of the Colorado Plateau and Central Montana Rockies of North America (Eardly, 1962; Prucha et al., 196.5) and the structures of the Russian Platform (Beloussov, 1962).

The basin is distinguished by highlands which are the areas of uplifts and the plains which are the basins between the uplifts (Fig. 5). The margi- nal hill ranges of the highlands with escarpments facing the plains of the basins, are marginal flexures or monoclines along the master faults of the uplifts (Biswas, 1980).

The uplifts are oriented east-west along five principal faults: (1) the Nagar Parkar fault, (2) the Island Belt fault, (3) the South Wagad fault. (4)

315

the Kutch Mainland fault and (5) the North Kathiawar fault. Linear uplifts along the first four faults have given rise to four subp~allel positive elements or ridges: Nagar Parkar-Tharad, the Is- land Belt, Wagad and the Kutch Mainland (Figs. 2, 4 and 5). The Kathiawar or Saurashtra uplift along the North Kathiawar fault is a quadrangular horst bounded by faults on all sides (Fig. 2). The lineaments formed by these ridges are also indi- cated by subsurface gravity trends which can be traced through the Cambay graben into the areas of Delhi folding (Fig. 4). The trends swing east- ward from E-W to NE-SW following the Delhi tectonic trends. The uplifts, therefore, are the re- sult of the differential movements of discrete base- ment blocks along ancient faults parallel to the Precambrian tectonic trends. The comparatively thin sediment cover was drape-folded into linear flexures or “~~~c~~u~~e~~” (Prucha et al., 1965) along the faulted margins of the uplifts.

The ridges are separated by “sub-basins” be- tween the ridges which are in the form of half- grabens (Fig. 6).

The most striking feature of the basin is the occurrence of a meridional high in the middle of the basin. This Median High controlled the facies and thickness of the sediments (Fig. 7). It passes transversely across both positive and negative ele- ments of the basin so that the uplifts plunge

Fig. 5. Architecture of the Kutch basin.

316

Y)

4

m

317

EAST ‘i” x

STABLE PART

RSIIIINI: PART MEDIAN HIGH

Fig. 7. Stratigraphic section (along depositional strike) of Kutch M~nl~d across Median High. Note effect of the high on the

thickness and facies of the Upper Jurassic formations. The high has no effect on the thickness of the Middle Jurassic formations

indicating its synsedimentary growth in the Late Jurassic.

bilaterally and the sub-basins have a central high or shallow region. The Median High trends NNE-SSW. To the west of the high the basin is deeper with thicker accumulation of sediments showing change of facies from shallow to deeper shelf. To the east of the high, the basement is shallow with less thickness of sediments and the facies varies from shallow marine to littoral and fluvial. Most of the uplifts occur on the higher eastern part of the basin. Thus, this Median High occurs along the hinge line of the basin and forms a tectonized zone featured by intense faulting, folding and intrusions.

The Middle Jurassic formations, however, show uniform thickness across this but the Upper Jurassic-Lower Cretaceous elastic (deltaic) sedi- ments thicken considerably to the west of the high with the consequent change of facies (Fig. 7). This indicates that this high started to develop during the Late Jurassic sedimentation with a greater degree of subsidence on its western flank. It ap- pears that the high was formed by the regional bending of the basin along its hinge zone which is parallel to the regional hinge of the main Indus

basin (Fig. 3). Thus, the structure of the Kutch-Saurashtra

shelf consists of a Kutch basin bounded by the Nagar Parkar ridge, the Radhanpur-Barmer arch

and the Katbiawar uplift (Fig. 5). The basin plat- form slopes towards the southwest in which direc- tion the embayment opens up. The platform fea- tures parallel fault ridges and a Median High across them demarcating the hinge of the basin in Late Jurassic-Early Cretaceous time. The margins of this basin are defined by the Precambrian ter- rain now exposed in Rajasthan, Gujarat and Pakistan. The northern margin is faulted along the Nagar Parkar fault and the southern along the North Kathiawar fault. The structural axis of the basin as indicated by sediment thickness, plunges southwest, trending parallel and close to the Kathiawar uplift (Fig. 5).

The Saurashtra horst (Kathiawar uplift) which is the uplifted southern part of the Kutch- Saurashtra shelf, is bounded by two major inter- secting sets of faults parallel to the Dharwar and the Satpura tectonic trends (Fig. 2). This block is the uplifted portion of a WSW-plunging basement arch which continues across the continental shelf where it is known as the Kori High (Fig. 3). This part of the shelf thus formed an arched deposi- tional platform sloping towards the southwest.

Cambay

The Cambay Tertiary basin is a graben (Fig. 8) between the Kathiawar (Saur~htra) Uplift and

NE

KATHI AWAR UPLlf T CAMBAY GRABEN ARAVALII RANGE

Fig. 8. Geological Section across the Cambay basin along line RB’ of Fig. 3.

the Aravalli Range, extending along the west coast fluvial sands in the northern part of the basin

through the Gulf of Cambay in a roughly which are correlatable with the fluvio-deltaic

NNW-SSE alignment parallel to the Dharwar Lower Cretaceous formations of Kutch and

trend (Fig. 2). In the continental shelf. it is flanked Saurashtra (Hardas, 1980). Palaeocurrent data in-

by the Bombay platform and the Ratnagiri ridges dicate sediment transport uniformly towards

to the west and by peninsular India to the east west-southwest in the Saurashtra block during the

(Raju and Srinivasan, 1983; Mitra et al., 1983; see Early Cretaceous (Bhandari and Kumar, 1970). It

Fig. 3). A series of normal faults parallel to the appears that during that time the sediments were

same trend in the wide continental shelf defines transported mainly by the rivers flowing westward

the Atlantic-type margin of Western India (Figs. 9 from the Aravalli Range and adjacent uplands

and 10). The Cambay graben crosses the Narmada across the Cambay graben area. In the Early

graben near the mouth of the Gulf of Cambay. Cretaceous, therefore. the Cambay graben had not

The extension of the Narmada graben in the con- yet been formed. The whole area was a part of the

tinental shelf south of Saurashtra is called the developing western Indian shelf. The basin was a

Surat basin (Fig. 3). Very little information is part of an extensive west-sloping platform with a

available on the tectonic history of the Cambay hinge along the eastern fault (of the Cambay

basin during the Mesozoic. Drilling data indicate graben). This fault was activated across the

Fig. 9. Geological Section across the Surat basin along line DD’ of Fig. 3

BOMBAY PLATFORM P&NNh DEPRESS ION

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m OECCAN TRAP l..__2-

201tm

Fig. 10. Geological Section across Bombay Platform along line EE’ of Fig. 3 (based on seismic stratigraph~).

Aravalli Range initiating the rifting of the Saurashtra block. During the Late Cretaceous, the Sa~ashtra block was uplifted and the Cambay graben formed. It remained as a rift valley with a number of volcanic centres for Deccan Trap flows, now seen as intrabasinal highs (Fig. 11). The main subsidence of this graben with a floor of Deccan Trap lavas started in Early Tertiary time. The intrabasinal differential block movements along cross faults formed four discrete segments (Raju, 1968) during the Tertiary tectonic evidents (Fig. 11). Kaila et al. (1980) mapped seven blocks bounded by deep-seated faults in the southern part of the Cambay graben by deep seismic sound- ing. The palaeo-highs formed by subsidiary blocks and volcanic centres controlled the distribution of sedimentary facies.

The ENE-WSW graben bounded by a system of subparallel, dextral wrench faults (Das and Patel, 1984), slightly diverging towards the west, is now occupied by the Narmada river valley. This system of faults delineates the Narmada-Son mega-hneament. This mega-lineament (Fig. 2) is a geofracture or “deep-fault” as evident from its extension into the Moho at a depth of 35-40 km (Kaila et al., 1980, 1985). From deep seismic sounding profiles, Kaila et al. have shown that the rift zone is featured by block faulting along a set of deep faults which displace the Moho (see Fig.

12). The central block between SP 160 and SP 200 j has been trough faulted, accumulating about 1.7

km thick pre-Trap sediments. This zone of maxi- mum subsidence occurs at a place where the Godavari graben {trending parallel to the Dhar- war strike) meets the Narmada graben, forming a depocentre. Since the Godavari Graben is filled with Gondwana sediments, it is presumed that a

greater part of the thickness of pre-Trap sediments at the depocentre might belong to the Gondwana System. From the velocity data, Kaila et al. as- sumed that Late Cretaceous sediments are re- stricted in the shallower, north side of the trough (Fig. 12). As seen from the extent of the outcrop and the thickness of the sediments, the Cretaceous basin is confined to the western part of the rift zone within an embayment widening and deepen- ing towards the west (Fig. 13).

Kaila et al. (1985) further brought out the complexity of movements along this system of faults. They found a correlation between the dis- placements of the basement and Moho along the deep faults (Kaila et al., 1980). The fault at SP 135 forms a tectonic boundary between the Late Pro- terozoic-Early Palaeozoic epicontinental marine rocks (Vindhyan System) in the north and the paralic Gondwana rocks in the south (Fig. 12). Kaila et al. (1985) attributed this to the reversal of movements along deep faults bounding the central trough. Permian rifting along this fault zone is responsible for marine sedimentation in central India during that time (Eremenko et al., 1969).

320

I .

,..I“ ’ .:

,I ‘..., :‘: :

,’ : .: .’ . . . . . .

‘. :

:.:.. :.: ._,_; ,, t.: 1_...

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8 LOCK

OF CAMBAY

Fig. 11. Architecture of the Cambay Basin. (Modified from Raju and Srinivasan. 1983.)

Fig.

12

. G

eolo

gica

l Se

ctio

n ac

ross

th

e N

arm

ada

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ong

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. 3.

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322

n ALLUVIUM

m TERTIARY

I^ DECCAN TRAP

MATE BASIN L,M,T m CRETACEOUS

EASINA; AXIS WITH PLUNGE DIRECTION m PRECAMBRIAN

-

/ APPROX

Fig. 13. Architecture of the Narmada Basin.

Reactivation of the faults in the Cretaceous re-

sulted in the opening of the Mesozoic rift basin. A

Tertiary depocentre formed in the outer part of

the embayment (Broach sub-depression) where the

Narmada rift intersects the Cambay rift (Fig. 11).

This sub-depression is the extension of the central

trough mentioned above (Kaila et al., 1981). The

uplift of the Saurashtra arch terminated the Creta-

ceous sedimentation in the northern part of the

continental shelf and restricted the shallow marine

Late Cretaceous sedimentation within the embay-

ment through the Narmada graben and the em-

bryonic Cambay basin.

Pal and Bhimasankaran (1976) related post-

Jurassic drift of the Indian plate with the wrench-

type Narmada-Son mega-lineament. The dextral

motion along this (Das and Patel, 1984) in the

context of the anticlockwise rotation of India ap-

pears anomalous. A detailed discussion on this is

beyond the scope of the present paper. It may be

mentioned here that the right lateral movement

appears to be the result of differential transform

motion of the foreland and peninsular blocks

caused by the combined effect of the northward

motion of India as it continues to underthrust

below the Asian continent, eastward convergance

of the Bay of Bengal due to subduction below the

Andaman-Sumatra arc, and dextral transform

motion along the northeastern plate boundary of

India with the Indosinian plate (Parkar and Gea-

ley, 1983) (Fig. 14). Roy and Kacker (1982) sug-

gested a genetic association of the right lateral

Dauki tear fault (Fig. 14) on the south side of the

northeastern cratonic wedge of India, with the

EURASIA

CHlNA PLATE

Fig. 14. Sketch diagram showing differential movements along

the Narmada mega-shear.

323

Narmada-Son mega-lineament. They described the Narmada-Son-Da&i lineament as a mega- shear across the Indian sub-continent, generated

through a global stress system related to spreading of the Indian Ocean floor. The Narmada-Son- Da&i mega-shear appears to continue into the Naga Hill Schuppan belt near the northeastern collision margin with the Indosinian plate. The right lateral transform motion along this plate boundary (fig. 34 in Thompson, 1976) is compati- ble with the movement along the Narmada rift zone.

Evolution of the basin

The evolution of the western margin basins is related to the breakup of eastern Gondw~~~d from western Gondwanaland in the Late Triassic/ Early Jurassic (Norton and &later, 1979) and the subsequent spreading history of the Eastern In- dian Ocean. While India together with Antarctica and Australia rifted away from Africa, the graben faulting in the western margin of India took place by reactivation of ancient faults sequentially from north to south (Biswas, 1982). With the initiation of Indo-Antarctic separation in the Late Neoco- mian (Lawver et al., 1983, and the subsequent separation of Sri Lanka from India, the eastern margin basins of India began to develop. The present continental margin evolved when India’s drift motion along an anticlockwise path slowed down considerably after its collision with Eurasia in Late Eocene-Oligocene time (Norton and Sclater, 1979). The main tectonic events took place during the Late Cretaceous when the drift motion was at its maximum, with an average rate exceed- ing 15 cm/yr. (Powell, 1979).

The three marginal basins of western India, their structural style and architecture as described above, evolved in four stages as depicted in Fig. 15A-D.

Stage I

Kutch rifting along the Delhi trend was ini- tiated in the Late Triassic as evidenced by con- tinental Rhaetic sediments in the northern part of the basin (Kosal, 1984).

During Jurassic time, in the early stages of India’s northward drift away from Gondwana- land, the Kutch rift basin was formed by subsi- dence of a block between the Nagar Parkar Hills and the southwest extension of the Aravalli Range (Fig. 15A). The first occurrence of marine sedi- ments in the Middle Jurassic (Table 1) indicates that this graben became a fully marine basin dur- ing that time. It appears that the “Great Boundary Fault” of the Aravalli Range extended beyond the continental margin along the northern coastline of Saurashtra and acted as a principal weak zone (Roday and Singh, 1982; Das and Patel, 1984). The Kutch basin was the earliest of the three basins which received marine sediments. The Narmada geofracture and the West Coast fault remained passive as important structural linea- ments. The southeastern extremity of the Aravalli Range was subjected to erosion and peneplanation during this stage.

Stage II

In Early Cretaceous time, the Kutch basin was filled up and the sea began to recede (Table 1). The southwestern part of the Aravalli Range was peneplaned. The East Cambay fault which bounds the eastern margin of the Cambay basin (along the same NNW-SSE lineament as the West Coast fault) became active across the Aravalli Range. The entire region lying west of the East Cambay fault and north of the Narmada fault (Fig. 15B) subsided to form an extensive platform. The rivers flowing to the southwest from the Aravalli Hills deposited a large volume of deltaic sediments on this platform as it subsided with the East Cambay fault acting as the hinge (Biswas, 1983). The rift- ing along the Narmada geofracture was initiated during this time with the opening of a basin at its western end where deltaic sediments were also deposited by the rivers flowing along the geofrac- ture.

During this period, India-Antarctica rifting was initiated, and India’s east coast started to evolve forming the eastern pericontinental shelf basins. The occurrence of Albian paralic sediments indi- cates that at the end of this period, rifting of Sri

324

A - Jurorsic

C .- Late Cretoceous -Paleocene

K C s SBL

;P SB G

KUTCH BASIN CAMBAY BASIN SAURABHTRA BASIN SAURASHTRA GLOCK NARMADA BASIN BOMBAY PLATFORM SURAT BASiN GODAVARI GRABEN MAHANAOI GRABE N ARAVALL! RANGE

R - Early Cretocaous

SL << >> NG GBF WCF

:;.::.:.; .*

..---

2 7z

SRI LANKA BLOCK NAGAR PARKAR HILLS NARMADA GEOFRACTURE GREAT 8OUNOARY FAULT WEST COAST FAULT AREAS OF MARINE SEOIMLNTATION AREAS OF DELTALC SEDIMENTATION INCIPIENT FAULT FAULTS GRABENS VOLCANIC CENTRES

NOT TO SCdiE

Fig. 15. Stages of development of western margin basins. Set text for explanation.

Lanka along the NE-SW Precambrian trend was initiated (Katz, 1978).

Stage 111

The Late Cretaceous saw extensive regional uplift in the western part of India (Fig. 150. The Saurashtra block separated out at this time as a result of renewed movements along the western extension of the “Great Boundary Fault” (North Kathiawar fault), the western margin fauh of the

Cambay Basin. the western extension of the Narmada fault and the West Kathiawar fault (Fig. 3). The Cambay graben came into existence as a rift valley by reactivation of its boundary faults. The Narmada rift opened up and received marine sediments. Intensive block movement gave rise to the uplifts of Kutch. The foreland block north of the Narmada fault and the peninsular block south of it moved up as the Narmada graben subsided. The peninsular block tilted eastward, drowning the eastern intracratonic grabens (Godavari and

325

Mahanadi) and initiating marginal deltaic sedi- mentation along the eastern continental margin (1X). The Palk Strait rift started to develop as the Sri Lanka block separated away from India as evidenced by the marine Late Cretaceous sedi- ments resting over the basement in the intervening Palk Strait graben. Towards the end of the Creta- ceous (60-65 Ma) extensive subaerial eruption took place through a number of volcanic centres in the Cambay graben, Saurashtra and Kutch (Biswas and Deshpande, 1973) when one of the spreading ridge axis was close to the western margin of India (Norton and Sclater, 1979).

Stage IV

The present shape of the continental margins of India evolved during Early Tertiary time (Fig. 15D) when India collided with Asia and stabilized its present position. Eastern margin basins evolved and Sri Lanka moved to its present position. The West Coast fault was reactivated as the present western continental shelf subsided along it. The Cambay graben extended southward along the West Coast fault into the subsiding continental shelf. The Cambay and Narmada grabens crossed and mutually displaced each other. Conjugate shearing along the bounding faults of the two grabens resulted in a right-lateral movement along the Narmada faults and a left-lateral movement along the Cambay faults (Rao and Talukdar, 1980, Fig. 3). The Cambay graben subsided, accumulat- ing thick Tertiary sediments over a relatively thin floor of Deccan Trap flow. The Saurashtra block remained as a horst while the Kutch, Cambay and Surat basins subsided around it. The eastern part of the Narmada graben was uplifted, becoming a rift valley again.

The Cambay and Bombay platform areas are characterized by high geothermal gradient and heat flow today. The fact that the Tertiary sedi- ments are prolifically oil-bearing and that the oil generated in the Early Tertiary is not over-ma- tured or destroyed by the heat flow, suggests that an older thermal regime is responsible for oil generation in these basins. From this it follows that there had been repetitive thermal cycles, probably associated with different rifting phases

and mantle updoming. The present thermal re- gime appears to be quite young and associated with a recent rifting phase. This contention is supported by seismic evidence (Kaila et al., 1980) indicating a shallow depth of the mantle, 20-25 km, near the junction of the Cambay and Narmada rifts. Several earthquakes in this region in recent years (Guha and Power; 1982) have lent further support to such inte~retation.

Conclusions

(1) Kutch, Cambay and Narmada are three pe~continental rift basins on the western margin of India which evolved during different phases of India’s geotectonic history during its break up from Gondwanaland, its northward drift and final collision with Eurasia.

(2) Kutch and Narmada are Mesozoic basins whereas Cambay is mainly a Tertiary basin which originated during the Early Cretaceous as a fluvial valley. The Kutch and Narmada basins expe- rienced only peripheral subsidence during the Ter- tiary. The Kutch basin has the most complete stratigraphic record from the Middle Jurassic to the Holocene.

(3) The Kutch basin is fault-bounded with a southwesterly palaeoslope featured by sub-parallel residual fault ridges and crossed by a Median High along its hinge zone.

(4) The Saurashtra horst is the uplifted part of a WSW-plunging basement arch which divides the western continental margin into a northern Kutch-Saurashtra shelf and a southern Bombay- Kerala shelf. The arch forms the southern limit of the Jurassic sedimentation of the Indus shelf basin.

(5) Three Precambrian erogenic trends- NNW-SSE Dharwar trend, NE-SW Aravalli- Delhi trend and ENE-WSW Satpura trend-con- trolled the tectonic framework of the western margin of India.

(6) The Saurashtra arch is the extension of the Aravalli Range. It subsided along the eastern margin fault of the Cambay basin during the Early Cretaceous forming an extensive deposi- tional platform continuous with the Kutch shelf, for thick accumulation of deltaic sediments. Dur- ing the major tectonic phase in the Late Creta-

326

ceous, a part of the arch was uplifted as a horst

separating the three graben basins. The west coast

fault along the Dharwarian trend extended north-

ward to cause the rifting of the Cambay basin

between the Saurashtra horst and the Indian

Shield.

(7) Tectonic evolution of these basins took

place in four stages during the northward drift of

India.

(8) The subsidence of the Cambay basin and

western continental shelf basin in Tertiary time

was the result of thermal adjustment following the

Mesozoic rifting events.

(9) The present high thermal regime of the

Cambay-Bombay High region is suggestive of a

renewed rifting phase.

Acknowledgements

I am thankful to Mr. L.L. Bhandari, Director,

KDM Institute of Petroleum Exploration, India,

for his encouragement and for permitting me to

publish this paper. I am grateful to Dr. M.G.

Hardas and Mr. D.C. Srivastava for their help and

assistance in preparing the illustrations. I am espe-

cially indebted to Dr. M.G. Hardas for critically

reading the manuscript and giving many valuable

suggestions.

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