Extension of structural and tectonic trends from the Indian subcontinent into the Eastern Arabian Sea

Short Communication Extension of structural and tectonic trends from the Indian subcontinent into the Eastern Arabian Sea V. Kolla Elf A q u i t a i n e Petroleum, 1000 Louisiana, Houston, TX 77002, USA

and F. Coumes Soci~t6 Nationale Elf A q u i t a i n e (Production), Pau, France

Received 23 October 1989; revised 2 January 1990; accepted 10January 1990 Trends of basement highs, faults and structural patterns on the India-Pakistan (Indus) continental shelf and slope, and in the deep waters extending to the Lakshmi (Laxmi)-Chagos-Laccadive Ridge complex in the eastern Arabian Sea, are parallel to the ancient Precambrian structural and tectonic grain of the Indian subcontinent. These trends, the occurrence of horst-graben style features of the basement of the Lakshmi Ridge and of the region north and east of it, both in deep waters and shelf areas, and the oceanic nature of the basement west of the Ridge, suggest that the rifted transitional crust extends as far as the Lakshmi-Chagos-Laccadive Ridge complex. The extension of rifted crust with horst-graben style tectonic features into deep offshore areas of the Arabian Sea, has implications for future hydrocarbon exploration in the region. Keywords: Structural trends; Indian subcontinent; Eastern Arabian Sea; Laxmi-Chagos-Laccadrive Ridge Complex

Introduction Several investigators (Rao, 1984; Bhattacharya and Subrahmanyam, 1986; Ramana, 1986; Biswas, 1982, 1987; Biswas and Singh, 1988: Eremenko, 1968) have commented on the extension of structural and tectonic trends from the Indian subcontinent into shelf areas of the western Indian margin. Narain et al. (1968), based on the seismic refraction work of Francis and Shor (1966), and Babenko et al. (1981), based on CDP data and refraction shooting, suggested that the Chagos-Laccadive Ridge (Figures 1 and 2) forms a transition between the continental crust on the east and oceanic crust on the west. Naini and Talwani (1982) concluded from seismic refraction, gravity and magnetic studies that much of the eastern Arabian Sea between the Chagos-Laccadive-Lakshmi (Laxmi) Ridges and the Indian continental slope (Figures 1 find 2) is undcrlain by a transitional crust (transitional between continental and oceanic crust), formed due to rifting and related processes. South and west of the Lakshmi-Chagos-Laccadive Ridge, the crust is oceanic. The Lakshmi Ridge and the northern part of the Chagos-Laccadive Ridge were thought by Naini and Talwani (ltJ82) to be continental slivers. Biswas and Singh (1988). on the other hand, inferred that the rifted, transitional crust is much closer to the continental slope areas of the western Indian margin, and much of the area east of the Lakshmi-Chagos-Laccadive Ridge has oceanic crust. In this paper, we attempt to show that the structural and tectonic trends of the Indian subcontinent extend as far as the Lakshmi-Chagos-Laccadive Ridge in the eastern Arabian Sea, much farther offshore than hitherto 0264-8172/90/020188-09 $03.00 <1990 Butterworth & Co. (Publishers) Ltd 188

Marine and Petroleum Geology, 1990, Vol 7, May

reported and discussed. We further corroborate the inferences of Naini and Talwani (1982) that the region between the Lakshmi-Chagos-Laccadive Ridge and the Indian shelf margin has a transitional crust, and then point out possible implications for hydrocarbon exploration in deep waters in the future. We hope our conclusions will provide an impetus for further detailed studies of the vast unexplored eastern Arabian Sea in the future. Structural and tectonic trends on the Indian subcontinent The major portion of the Indian peninsula is a shield area, consisting of Archaen gneisses and schists and metamorphosed sedimentary rocks. The rest of the peninsula is covered by Deccan and Rajamahal traps (Krishnan, 1968). The most persistent and dominant Precambrian structural and tectonic trend is the N W - S E to N N W - S S E Dharwar trend (Figure 2," Krishnan, 1968; Biswas, 1987). This trend is parallel to the faulted west coast of India and extends from southern India all the way to the north into Kutch area, although buried for hirgc distances by the Deccan Traps (Das and Ray, 1976). The Dharwar trend is intersected in many places by NE SW trending lineaments (Ramana, 1986). In the northern part of western India, the N E - S W Aravalli trend, and in southern India, the Eastern Ghat trend ( N E - S W ) become the dominant structural elements. The E N E - W S W Satpura trend is the fourth dominant trend of western India. The main Aravalli trend ( N E - S W ) splays, towards the south-west, into the Delhi E - W trend, and another trend towards the

Structural and tectonic trends in the Eastern Arabian Sea." V• Kolla and F. Coumes ~°

7'~ °

7~

°

i

PAKISTAN •4•1

,

INDIA

&;¢;L

,

,,

/

o

0 ,e

}!

i Bomba',/

,/ U

55', '

%

'0

"t

'i

i

.-"

$

-

,!:7

0.

<;L@o

0

4,UO

O o ,< , o .

4e~ ¸¸

~-

•,°

(Jq~k°'JJ~" .t,i

, ~,;

~.,:¢

~'<

+

"

,, L)

27 °

~';'

°

,

> -

,

~

f

°

Figure 1 B a t h y m e t r y (in m e t e r s ) o f the A r a b i a n Sea. P r o m i n e n t b a t h y m e t r i c f e a t u r e s such as the C h a g o s - L a c c a d i v e Ridge, the L a k s h m i Ridge, the Indus Fan, the O w e n R i d g e - M u r r a y Ridge and the C a r l s b e r g Ridge are indicated. Scale bar = 600 km

south-east which merges into the Satpura trend. These major tectonic trends controlled the structural styles of several intracratonic and pericratonic sedimentary basins of India (e.g. the Kutch, Cambay and Narmada Basins; Raju, 1979; Biswas, 1987). Movements along the ancient structural trends greatly influenced the evolution of these basins. For example, the Cambay rift-basin (aborted rift basin, Raju, 1979) was formed by movements primarily along the Dharwar (NW-SE) and Aravalli (NE-SW) trends (Biswas, 1987). Movements along the major structural trends continue

to occur even at present, as indicated by earthquake zones on the Indian subcontinent (Chandra. 1977). Structural and basement trends in the Eastern Arabian Sea The shelf area of the western Indian margin consists of horst-graben complexes (Biswas and Singh, 1988). Similarly, off the Indus Delta (Pakistan). proprietary seismic reflection and published seismic refraction data suggest horst-graben complexes. The fault trends and basement arches on the shelf off Western India and

Marine and Petroleum Geology, 1990, Vol 7, May

189

Structural and tectonic trends in the Eastern Arabian Sea: V. Ko//a and F, Coumes also trend parallel or subparallel to the Indian margin. Pakistan are mainly parallel to the D h a r w a r ( N W SE)

and Aravalli (NE SW) trends (Figure 2). In the southern part, in addition to the NW SE trend, N E - S W trending faults that are parallel to the Eastern Ghat trend, extend from the southern Indian subcontinent to the offshore areas (Anonymous, 1968). The horst-graben complexes, basement arches and fault patterns of the shelf areas also formed due to rifting and movements along the ancient Precambrian structural trends (Biswas, 1987). Seaward, beyond the shelf break, several ridges and basement highs trend parallel or subparallel to the Indian margin. The water depths on these basement highs can be as much as 3000 m. Prominent among these ridges, farthest from land, are the Lakshmi Ridge and its northerly buried extensions, and the Chagos-Laccadive Ridge (Figures 1 and 2). The trend of the Lakshmi Ridge is NW-SE, similar to the Dharwar trend. North of 9°N, off India, the Chagos-Laccadive Ridge also trends NW SE or NNW SSE. However, south of 9°N, the trend of this Ridge is primarily N S (Figure 2; Heezen and Tharp, 1964). Landward of these ridges, several other ridges (e.g. Kori High and other unnamed ridges in Figure 2) o ~1

~"

1

I

I

I

60" I

I

I

[

Although the regional gravity field over the entire Arabian Sea is negative, free-air (Figure 3) as well as isostatic gravity anomalies (Naini and Talwani, 1982) define several N W - S E trends of relative, positive and negative anomalies, consistent with the basement trends discussed above. The Lakshmi Ridge has relative, negative anomalies, whereas the Chagos-Laccadive Ridge has positive anomalies along portions of its crest and negative anomalies along its flanks. Magnetic anomalies, corresponding to some of these basement highs, are also parallel to the Indian margin, but they are not of the seafloor-spreading type (Naini and Talwani, 1982). All these ridge trends are mainly parallel to the Dharwar trend. In addition, high quality seismic reflection data, where available in deep waters, show N E - S W and NW-SE trending faults (Figure 2 and 4(A); Bellaiehe and Droz, 1987, Droz and Bellaiche, in press) on the northerly buried extension of the Lakshmi Ridge. NE SW trending deep faults extend from the southern Indian shield to as far as the Chagos-Laccadive Ridge (Figure 2; Anonymous, 1968). Also, in the southern part, the basement and topographic highs form NE SW trends, 70.

•

I

r

I

KUTC'd R~FT . F..~N b

•

~

80" 25"'

i

~ x ~ -~t

~" /

7~"

.

~.

*

~,~- ~ v v ~

.

/

/ \ ~ /';~ ,

,

,

"]V v '\

~

V

\ ,

-V

\

x

.

V

V

9~GC"I~'t'RP V

~

\X

,.o. N

~!~o~ ~,, ,p.\ge~ \

,v

zo"

V V

k/

V'~/~V

v

"

v v

.~ ~ v 'E'

15"

Arabian

sea

~

N

"

%.

,

LEGEND VVV VVV VVV

DECCANVOLCANCS

PRINCIPAL DEEP-SEATED FAULTS

/ dl

MAJOR AND MINOR FAULTS

.~ ~

---

~]/~\S O~GC~)\"J~" O%-\~ P~

I.. ~ 1 ~ I

BOUNDARIESOF CHAGOS-LACCADIVE R,OGE

)

A×ES OE RIDGES RARALLEL 0.

/

SUBPARALLEL TO THE COASTLINE (CHAGOS-LACCADIVE, LAXMI, ETC]

BASEMENT ARCHES IN THE SHELF .co.sT.L

Io"

f

PRECAMBRIAh TECTONIC TRENDS BASEMENT RIDGE OR TOPOGRAPHC

\ a\., l

,

/ / j I 1

.EG,ON

5"

5' 55"

60°

65°

Figure 2 Basement and structural trends in the eastern Arabian Sea lines: Closs et a/. 1969; Naini and Kolla, 1982; Naini and Talwani, Shuaib, 1982; Rao, 1984; Ramana, 1986; Biswas, 1982, 1987; and Western Indian subcontinent are compiled from Anonymous, 1986; sectiQns of Figures 4A, 4B and 4C are also shown

190

7O"

75"

8O"

compiled from bathymetry (unpublished and proprietary seismic 1982; Harbison and Bassinger, 1973; Bellaiche and Droz, 1987; Biswas and Singh, 1988). Structural and tectonic trends on the Biswas, 1982, 1987; and Krishnan, 1968. Locations of the seismic

M a r i n e and P e t r o l e u m G e o l o g y , 1990, Vol 7, M a y

Structural and tectonic trends in the Eastern Arabian Sea: V. Kolla and F. Coumes

~"

70°

7~o

ARACHI

"~l/

/:;~5°

GULFOFCOl ~ : KATHIAWAR PENINSULA

. . . . ,

LEGEND

.

.

.

0

:.:.:.:.

lIwlm

~

o.,-,.

~L-~ ~

~. . . . .' . . . ' ....... .

~

r

:

....2o ~

:

~

:"~ :.:.~,~:...

, .li::!! :iiiii!iii!iiii! !iiiiii!i ::i!iiiiiii i .'." ".'.'.'.:.:.:-'." ~X-'.'~::.:.:......." ~.~:::::.~ • .-.....-...-:.:. ,~2.:.? i:!:i:i:i:i:i:i:i:i:!:i.:.:.:.

:.:..

::::::::::::::::::::::::::::::::::

::::::::::::::::::::::::::::::::::::::::

\--. ~0

65

.

.

.

.

.

.

.

I0~

Figure 3 Free-air gravity a n o m a l y m a p of the Arabian Sea, simplified after Naini and Talwani (1982). Contour interval is 25 mgals. Bathymetric contours at 200, 2000 and 3000 m are also indicated

superimposed on N W - S W or N N W - S S E trends. The N E - S W trends in the southern part are parallel to the Eastern Ghat trends, whereas the N W - S E and N N W - S S E trends are parallel to the Dharwar trend. Seismic reflection profiles clearly show the block-faulted nature of the buried extension of the Lakshmi Ridge [Figure 4(A)]. South of this Ridge, hyperbolic echoes typical of oceanic crust are indicated on seismic lines [Figure 4(B)]. North-east of the Ridge, between it and the India-Pakistan shelf edge, horst-graben complexes occur [Figure 4(C)]. The crust in this region does not exhibit hyperbolic echoes. The basement is smooth except when faulted and appears to be stratified as shown by reflections within the basement. Block faulting is also apparent on seismic lines across the Chagos-Laccadive Ridge (Babenko et al., 1981). Horst-graben style features, basement and structural trends discussed above do not continue west of the Lakshmi-Chagos-Laccadive Ridge. West of this ridge complex, E - W trending, seafloor-spreading type magnetic lineations and N - S trending fracture zones characterize the crust (e.g. Naini and Talwani, 1982). We note that these fracture zones are generally parallel

to the trend of the Chagos-Laccadive Ridge south of 9°N but not to that of its northern portion off India. It is as if the Chagos-Laccadive-Lakshmi Ridge complex forms a boundary between two regions of contrasting tectonic fabric. Discussion

We believe that the ancient structural grain is likely to be inherited and preserved in the region of rifting. Structural and basement trends in the eastern Arabian Sea parallel the structural and tectonic trends of the Indian subcontinent. In addition, typical oceanic crust occurs south and west of the Lakshmi Ridge, and horst-graben style features occur north and east of it. These observations corrobate the inference of Naini and Talwani (1982) that the Eastern Arabian Sea north and east of the Lakshmi Ridge has rifted transitional crust with thickness in excess of 17 km and with no seafloor-spreading magnetic lineations, whereas the region south and west of the Ridge has oceanic crust with seafloor-spreading magnetic lineations and crustal thickness of 11.5 kin. The northerly extension of the

Marine and Petroleum Geology, 1990, Vol 7, May

191

^Ulhl

Ilfl~qdl

VT/I.T

II'l,qV

I''IL

L IuA

UI56L

^UUlUU~

lIMP

iwu

u.-Irlal(J-IJ,~d

w.-~t

le^V,'l,

pue

liM

au].~elAI

e."

~EC. "o

0-(/)

~ o o ~ ~.~ (1) ~

--,

~.0"0

e8

~ ~ _,=.= ~ 5'

~ ~ ~._~

":,: ;~":/:'~"~:", ~",~1 :~ • I:,.,,'~, ~F ~'-"tll,, I~i., (

•~'p'~,~:}i,,;,H

:'l('l"

~-~ g 21" ~ ,

~

~

0"<

•.~.,~ ;-.~ .~,- .# ~2

~,),~

~J~ .~,,

~)lr* I T: ' ~;,~l.t a ,! "1

,t ~,,~

~" c ::r , .,~ ,:, ~ .~:~,. ,. , ~'~"~

t.%."~i"~

ij~.,~:r

(,;.i,~ . :

Jk,5

........

, ,!;!~.'-~i~t~¢,~,!p~ I

~_.~

_

:Mll'li~,~!b'|

'."!~' ...:I:;~'~'~:"iP

0

o

Z6L

k;61,

^UlAI L l e a O661, ^ O O l O e E ) kui~e4u.~J,e d p u e gu!.sUlh I

gO

lilV

u~" o

o

6

o

0

ll~l

|n~vr-..L.

",4

<3)

b

b

tl

~,

N

xi

~*~

~r)

~.,

i'X~' -

,~. ~! ,~;i~

.,',

I'P,,,

o~u.,,~,-,, d

j

Hu~

~llUTi

i~ .¢cT,b u~.~4~.4 V u . l c , ~ j

~ 4 ~ ~ ! ~IJ u o ' 4 ~ u ! u u ~ u o 4

H~

I~AH.~U~'.'Z.~b

ii1¥11~,

~o[.w~,~,~ T

o

'M

o

(D

o o

3

"o

"-'i

-.-~

.....

___

=

.~~ ~_--~

;

-

-

~--~-'-~:

"

-"

".

.

~.

--

.

_ ~=

~=

~

!

::'k'±"-;#~-~T~'~--~

~

~

-%

Figure

4(C)

.

" :-

-~__

"- : -

-~

-

~- ~ : : - ' : : : : : ::--;. -- ~

.

='~--- - - - - ~

~:~-~-~"-~

.

.

~

,.,

.

:~__ -

~

~

-

-::-:-

~ ~;~,-~

:---~:~_:_~--_~-=--- = = _

~ -

~---'~'~

.

:~"-.,.,,-.;-~--~-~k---~=:= ~ ' - . ~ "

~

~.:'~--~:~.'~'~--.!::~--'~:'-:!~'~,.~.-:.:::-":----."-"~:~

_ ~ . ~ - . ~ . U : - : - - - : - = . - , - . ; ~ ~ : ~ ~ - -"- : ; - . : ~ L : - ~

:~=-~

-

'

-,.-

-

~

~

.

.

.

BURIED EXTENSION OF LAKSHMIRIDGE

.__~ -- . ~ . ~ _ ~ . . ~ . . - . ~ _ ~ . L . ~

~-

. . . . .

.

--:~ ~ ~ " ~ : ~

- ~- ~--"-~:

---~_

.

~--~-"-S'=~': ~ ' ~ " -

-

.

.

_

....

~,-~.,-~,.-,~-~.~.-~..~..___--..;-.--..-~

.,~_=_~->~=.~_~_:~-_-~-~-=~-=

~

,

~

:_~

~. r

.

~

--

_.

.-'~

~

-

~

.

:

-

~

-. -~:-..--:-~-:

~. =-:.'~'~::~'~ ~ - ' : ~ - . - . . ~ . ~ , ~ . : ~ ' ~ : ~ " ~ : ~

".

" ~::d:

-~:-~-: -

_4-

-:

::~

" ::.'.

. :_

.

-:~::

.

.

:= ~;~..-"

:~

.

::-.~

~.-. ~ - ; : ' - : - ~

;

.-

~.

~

~---~

km

24.

-.~

" --

_

Mile

15

_=--~= ~ - _ ~ _ - = - - ~ _ : ~ _ _ ~ 2 . - . - _ : ~ _

~ - - _

I

;

-'=-'--'~-

--

: L ~

I

~

~_

RIDGE CLOSE TO THE FOOT OF CONTINENTAL SLOPE

BURIED

,~

7.0

4.0

3.0

B.O

B.O

5 , 0

- , ~-~ 4.0 __ _~--,~=~--,

-~:----.

~ - -

.

-, :

- . - ~ - ~ : ~ - : ~:'--~- : ~ : - > " :-- ~;--~'-

" :~:-:: "

.

~" -

,~-----%.:-"~~:~.~.:":--::-:~-:=-'-:~-i~~~:--:~-.:~.~:":;~:~ _ : ~ : : - - ' - : " ~ % ~

::~: ::-

._~- %.~_~.~:--k-.~.

~--q~_=__ - : ~ - _ - - - _ - - - _ ~

-:_:- ~

~ :.-:---~,; ~ L ~ : ~ . . , . . ~ - ~ . ~ - ' ~ 7 ~ ' ~

'

:~-~------:-_L_::L~-~:

-r~_--~--.-

~ - - ~

- - - ~ - ~ - ~:- _

- --3.£)

~-.-~..;~ . . - ~ . ~ . ~ - - ~ - - : : : " ~ - ~ : ~ 4 ~ . ~ ' ~ : ~ - . ~ - ' 7 " ~ - - ~ 7 _ - ~ _ . " ~_ ~--:-'~-'-L~--F'.~ : ~ - % ~ ' ~ : ~ - : _ : : ~-~'~~-"~-q~--'-!~.~.~L~

-~-=~= ~

,

-

----~

=_:~_~.~..~.~.:p~.::-.-.:__~

-:--.%~_=_~_~:--~-__~-~_.-~--.~---~-e~_

:~~"~::-'-~C:-_ _ ~ _ ~ - - ~ ; ~ - - ~ - : : : ~ ~ = - - : - -

-

=,

~ - ~ - ~ ~ - ~ , - _ - T ~ . . C : ~ Z ~ _ - : ~ - : ~ "~.-~...----~.-.." -~.~-,-~---:---:~.-'-~ ~ L ~

:-~C-~-~=~=

" m:~:- ~ . ~ : . - : ~ ~ ~ ~ ~ ~ . _ ~ : ~ - ~ ~ ; ~ ' : ~ T ~ : ~ : ~ : : : :

~ ~:-:~.~ ~ - - - ~ - ~ _ - ~ - - . . - : =- ..... - - ~ _ ~ :

~:-~>:~-

~-*~'--k :~: :S~:-~?-~---k'~~

--~

~":~-~L--:- ~---:--.. ~.:~":::,~ ~ r

"~'~%

~. ' ~ - ~ - ~ '

~.

--- - ~ - - - ~

"5".-':~:k:-'~,.

.

~

~

.-~'-.~.~,~.:':~':~:~:kZ~---~i

_

_ ~

-=~--~.--_~_~_~---~C

: - ~ - ~ < : - : ~ . :~:.'::'-";~"~-2-:~,,~,~;-~'----"~d:'::.~;~-'-'~-.kc.~--~.

-~-:~__ - - - - :

_

.

~4.>~:::"~,.::~::-~::~?.:~.~-~'a.~-

-

. 7 ~

: - - ~

(Cl

P v

r

<

0

,o

c~

e.s.

ck

c~

S t r u c t u r a l a n d t e c t o n i c t r e n d s in the Eastern A r a b i a n Sea: V. Kolla a n d F. C o u m e s

Lakshmi Ridge that exhibits a block-faulted nature on seismic lines [Figure 4(A)] may be a continental sliver similar to the Lakshmi Ridge (Naini and Talwani, 1982). The origin of the Chagos-Laccadive Ridge has been explained as a transform fault (e.g. Sclater and Fisher, 1974), a hot-spot trace (e.g. Whitmarsh, 1974; Duncan, in press) or a micro-continent (Heezen and Tharp, 1964). The portion of the Chagos-Laccadive Ridge that is off India trends N W - S E or NNW-SSE, parallel or subparallel to the Indian margin. In addition, there are similiarities in the crustal thicknesses of this Ridge and of the region east of it to those of the Lakshmi Ridge and the region east of it, respectively. From these considerations, Naini and Talwani (1982) concluded that the northern portion of the Chagos-Laccadive Ridge is a continental sliver. We believe that the N W - S E trend of the Chagos-Laccadive Ridge, with superposed N E - S W basement and fault trends that extend from land, suggests that the ancient structural grain of the Indian shield might have significantly influenced the mode of origin of the Chagos-Laccadive Ridge. This conclusion does not necessarily rule out any of the origins mentioned above. Biswas and Singh (1988) extended the rift-originated horst-graben complexes and basins from the shelf to slope areas, based on reflection seismic lines. Farther west, but to the east of the Chagos-Laccadive Ridge, these authors interpreted oceanic crust from seismic reflection profiles. However, their published seismic sections are ambiguous at best in characterizing the crustal nature. Our seismic reflection profiles of the Lakshmi Ridge to the north, however, show that the oceanic crust begins much farther west. Babenko et al. (1981), implied, based on CDP and refraction data, that thick crust extends to as far as the Chagos-Laccadive Ridge. The extension of continental structural trends into far offshore areas does not necessarily point unequivocally to a transitional crust or a crust with continental affinities in the region. However, the extension of the continental structural trends combined with other types of information presented here and discussed by Naini and Talwani (1982) and Babenko et al. (1981), suggest that the rifted crust extends offshore to as far as the Chagos-Laccadive Ridge. Deep penetration, high quality seismic reflection profiles are needed to validate this suggestion. The extension of rifted, transitional crust to as far as the Lakshmi-Chagos-Laccadive Ridge has implications for future hydrocarbon exploration. We expect suitable reservoir lithologies, source beds and thermal history for maturation, seals, and traps in the basins that resulted from rift-graben style tectonics. The petroleum prospects of the shelf horst-graben complexes onshore India, and in the shelf areas of the Indian margin are being actively explored. Similarly, the rift-style basins of the Indus (Pakistan) shelf deserve exploration in the future. Depending upon the prevailing technology and economics, exploration can extend first into slope areas both off India (Biswas and Singh, 1988; Babenko et al., 1981) and off Pakistan, and then farther into deep waters.

Conclusion The trends of basement highs, faults and structural patterns in the eastern Arabian Sea (from the

India-Pakistan shelf to the Lakshmi-Chagos-Laccadive Ridge) are parallel to the tectonic and structural trends of the Indian subcontinent. Also, typical oceanic crust occurs west and south of the LakshmiChagos-Laccadive Ridge complex, and horst-graben style tectonic features occur north and east of the Ridge. This suggests that the eastern Arabian Sea between the Lakshmi-Chagos-Laccadive Ridge and the Indian margin has a rifted, transitional crust.

Acknowledgements We thank R. G. Martin, B. R. Naini, M. Talwani and N. E. Barnes for reading the manuscript and offering helpful comments.

References Anonymous (1968) Tectonic map of India Oil and Natural Gas Commission of India, Dehra Dun, India Babenko, K. M., Panayev, V. A., Svistunov, Yu. I. and Shelzinger, A.ye. (1981 ) Tectonics of the eastern part of the Arabian Sea, according to seismic data Geotectonics 16, 139-147 Bellaiche, G. and Droz, L. (1987) Accidents de Socle dans le Nord-Ouest de'locean Indien: Premiers resultats de la Campagne 'MD51/Profindus' C.R. Acad. ScL Paris 305, 37-41 Bhattacharya, G. C. and Subrahmanyam, V. (1986) Extension of the Narmada-Son lineament on the continental margin off Saurashtra, Western India as obtained from magnetic measurements Mar. Geophys. Res. 8, 329-344 Biswas, S. K. (1982) Rift basins in the western margin of India and their hydrocarbon prospects AAPG Bull. 66, 1497-1513 Biswas, S. K. (1987) Regional tectonic framework structure and evolution of the western marginal basins of India Tectonophysics 135, 307-327 Biswas, S. K. and Singh,. N. K. (1988) Western continental margin of India and hydrocarbon potential of deep-sea basins 7th Offshore South Asia Conference, Singapore, 170-181 Chandra, U. (1977) Earthquakes of peninsular India-A seismotectonic study Bull. SeismoL Soc. Am. 67, 1387-1413 Closs, H., Bungenstock, H. and Hinz, K. (1969) Results of seismic refraction measurement in the Northern Arabian Sea: A contribution to the international Indian Ocean Expedition Meteor Research Results, C, 3, p. 1-28 Das, S. R. and Ray, A. K. (1976) Fracture pattern, hot springs and lineaments of the west coast Geol. Surv. India, 125 Year News Lett. 1-6 Droz, L. and Bellaiche, G. (in press) Seismic facies and geological evolution of the central portion of the Indus Fan, in: Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems (Eds P. Weimer and M. H. Links) Springer-Verlag, New York, USA Duncan, R. and ODP Leg 115 Scientific Party (1989) R~union hot-spot activity through Tertiary time: Initial results from the Deep Sea Drilling program, Leg 115 J. VolcanoL Geotherm. Res. 36, 193-198 Eremenko, N. A. (1968) The tectonics and petroleum possibilities of the southern part of India Mem. GeoL Soc. India 2, 371-376 Francis, T. J. G. and Shor, G. C. (1966) Seismic refraction measurements in the northwest Indian Ocean J. Geophys. Res. 71,427-449 Harbison, R. N. and Bassinger, B. G. (1973) Marine geophysical study off Western India J. Geophys. Res. 78, 432-440 Heezen, B. C. and Tharp, M. (1964) Physiographic diagram of the Indian Ocean, the Red Sea, the South China Sea, the Sulu Sea and the Celebes Sea (with the descriptive sheet) GSA Inc., New York, USA Krishnan, M. S. (1968) Geology of India and Burma (5th edition) Higinbothams, Madras, India, 536 pp. Naini, B. R. and Kolla, V. (1982) Acoustic character and thickness of sediments of the Indus Fan and the continental margin of Western India Mar. GeoL 47, 181-195

M a r i n e a n d P e t r o l e u m G e o l o g y , 1990, V o l 7, M a y

195

S t r u c t u r a l a n d t e c t o n i c t r e n d s in the Eastern A r a b i a n Sea: V, Kolla a n d F. C o u r n e s Naini, B. R. and Talwani, M. (1982) Structural framework and the Rao, D. G. (1984) Marine magnetic anomalies off Ratnagri evolutionary history of the continental margin of Western western continental shelf of India Mar. Geol. 61, 103-110 India, in: Studies in Continental Margin Geology (Eds J. S. Sclater, J. G. and Fisher, R. L. (1974) Evolution of the east central Watkins and C. L. Drake AAPG Mem. 34, 167-191 indian Ocean, with emphasis on the tectonic setting of the Narain, H., Kaila, K. L. and Verma, R. K. (1968) Continental Ninetyeast Ridge GSA Buff. 85, 683-702 margins of India Can. J. Earth Sci. 5, 1051-1065 Shuaib, S. M. (1982) Geology and hydrocarbon potential of Raju, A. T. R. (1979) Basin analysis and petroleum exploration offshore Indus Basin, Pakistan AAPG Bull. 66, 940-946 with some examples from Indian sedimentary basins J. Geol. Soc. India 20, 49-60 Whitmarsh, R. B. (1974) Some aspects of plate tectonics in the Ramana, M. V. (1986) Regional tectonic trends on the inner Arabian Sea Initial Reports of the Deep Sea Drilling Project continental shelf off Konkan and central west coast of India 23, US Government Printing Office, Washington, DC, USA Geo-Mar. Lett. 6, 1-5 527-535

196

M a r i n e a n d P e t r o l e u m G e o l o g y , 1990, V o l 7, M a y