Gravity Anomalies and Deep Structure of the Andaman Arc

8/8/2019 Gravity Anomalies and Deep Structure of the Andaman Arc

1/14

Gravi ty Anomal ies and Deep S tructure o f the Andaman ArcM ANOJ M UKHOP ADHYAYDepartment o f Geophysics , Indian School o f Mines , Dhanbad-826004, India

(Received January 1986; revised 24 Novem ber, 1987)

Keywords: plate subduction, gravity field, deep structure, sedimentdistribution, ma fic mass within sediments, low density zone belowthe volcanic axis, and crustal transition.

Abstract. The Andaman arc is associated with a major Free-airanomaly pair of m ean am plitude 180 mgal. Two-dime nsionalgrav-ity interpretation suggests significant mass anomalies below the arcthat presum ably have resulted due to subduction of the Indian platebelow the Burm a plate. It is inferred that the A ndaman trench is ofasymm etric V-shape containing about 7 km sediments. An outerbathymetric rise seaward of the trench possibly corresponds to alithospheric flexure by 500 m. The Cretaceous-Tertiary sedimentsconstituting the Andam an sedim entary arc attain their ma ximumthickness of about 13 km under the N icobar De ep at the subduc-tion zone. At this location a m afic m ass is emplaced within thesedimentary section. The underlying oceanic crust apparently ex-periences phas e transition a t about 27 km dep th in a Benioffzoneenvironment. The Andam an volcanic arc underlies a low densityzone that is at least 60 km wide. Along the east m argin of theAndam an S ea, crustal transition presum ably occurs below the M er-gui Terrace at the M alayan coast.

IntroductionT h e A n d a m a n a r c i n t h e N E I n d i a n O c e a n t o g e t h e rw i t h t h e B u r m e s e a r c f u r t h e r n o r t h d e f i n e a n e a r l y2 1 0 0 - k m - l o n g m a r g i n o f t h e u n d e r t h r u s t i n g I n d i a np l a t e w i t h t h e B u r m a p l a t e ( c f . F i t c h , 1 97 0; C u r r a y e ta l . , 1 97 9) . T h e t w o a r c s y s t e m s p r o v i d e a n i m p o r t a n tt e c t o n i c l i n k b e t w e e n t h e H i m a l a y a n c o l l i s i o n z o n ea n d a m a j o r i s l a n d a r c - t r e n c h s y s t e m o f s o u t h A s i a ,t h e I n d o n e s i a n a r c ( F i g u r e 1 ). T h e A n d a m a n a r c is o fp a r t i c u l a r i n t e r e s t f o r it s N e o g e n e b a c k - a r c s p r e a d i n g ;t h is a c t i v i ty is p r e s u m a b l y r e l a t e d t o l e a k y t r a n s f o r mt e c t o ni c s ( U y e d a a n d K a n a m o r i , 1 97 9).

A m a j o r F r e e - a i r a n o m a l y p a i r o f a v e r a g e a m p l i-t u d e 1 80 m g a l c o i n c id e s w i t h th e A n d a m a n a r c o v e r ad i s t a n c e o f 1 10 0 k m i n n o r t h - s o u t h d i r e c t io n ( P e t e r e ta l . , 1 96 6) . T h e z o n e o f m i n i m u m g r a v i t y c o r r e s p o n d st o t h e t r e n c h a n d s e d i m e n t a r y i s l a n d s w h e r e a s t h eMarine Geophysical Researches 9 (1988) 197 210.9 1988 by Kluwer Academic P ubl ishers .

z o n e o f m a x i m u m g r a v i t y g e n e ra l l y o u tl i n e s t h e v o l-c a n i c a r c l o c a t e d f u r t h e r e a s t w i t h i n t h e o v e r r i d i n gB u r m a p l a te . A s i m i l a r d is p o s i t io n o f g r a v i t y a n o m a -l ie s i s k n o w n f o r t h e B u r m e s e a r c ( c f . E v a n s a n dC r o m p t o n , 1 9 4 6 ) . P e t e r e t a l . ( i b i d . ) i n t e r p r e t e d t h eA n d a m a n g r a v i t y l o w i n t e r m s o f ex c e ss c r u s ta l t h i c k -n e ss u n de r l y in g t he A n d a m a n - N i c o b a r s e d i m e n t a r yi s l a n d s ( c a l l e d t h e A n d a m a n - N i c o b a r R i d g e , A N R ) .H e r e w e in t e r p r e t t h e g r a v i t y a n o m a l y p a i r i n te r m s o fs u b d u c t i o n o f th e I n d i a n p l a t e o n t h e b a s is o f a v a il -a b l e g e o l og i c d a t a , s e i sm i c c o n t r o l a n d s e i s m o l o g icd a t a . O u r m a i n c o n c l u s i o n s a r e t h a t t h e d e sc e n d i n gl i t h o s p h e r i c s l a b b e l o w t h e A n d a m a n a r c i s a z o n e o fm a s s e x c es s , t w o p r o m i n e n t a r e a s o f m a s s d e f i c i en c yu n d e r li e t h e s u b d u c t i o n z o n e a n d v o l c a n i c a xi s re s p e c -t iv e ly , a n d o c e a n - c o n t i n e n t c r u s t a l t r a n s i t io n a p p e a r st o o c c u r u n d e r t h e M e r g u i t e r r a c e a t t h e M a l a y a nc o n t i n e n t a l m a r g i n .

Regi ona l Geo l ogy and Tectonic S e t t i n gT h e A N R i s b e l i e ve d to h a v e f o r m e d i n O l ig o -M i o c e n e t i m e s d u e t o e a s t - w e s t c o m p r e s s i o n o f se d i-m e n t s d e r i v e d f r o m t h e M a l a y a n s h e lf ( R o d o l f o ,1 9 6 9a ) . I t s c h i e f c o n s t i t u e n t r o c k s a r e : C r e t a c e o u s s e r -p e n t i n i t e s , o p h i o l i t e s w i t h r a d i o l a r i a n c h e r t s , C r e t a -c e o u s t o E o c e n e c h e r t y p e l a g i c l i m e s t o n e , g r i t ,c o n g l o m e r a t e , a n d a t h i c k s e c t i on o f E o - O l i g o c e n ef l ys c h o v e r l a i n b y N e o g e n e s h a l l o w w a t e r s e d i m e n t s( T a b l e I ) .

R e s u l t s o f s e i sm i c s u r v e y s ( C u r r a y e t a l . , 1979) sug-g e s t t h a t t h e s u r f a c e t r a c e o f s u b d u c t i o n b e l o w t h eA n d a m a n a r c li es a t th e w e s t e rn b a s e o f th e A N Rw h e r e t h e t r e n c h i s f il le d w i t h s e d i m e n t s o f t h e B e n g a lF a n ( F i g u r e s 2 a n d 3 ). T h e p r o c e s s o f s u b d u c t i o n a n do f f s c r a p i n g o f o c e a n - f l o o r s e d i m e n t s c o n t i n u e s t o d a y


2/14


3/14

GRAVITY ANOMALIES AND DEEP STRUCTUR E OF THE ANDAM AN ARC 199

TABLE IGeneralized stratigraphic sequence and sediment character for the And aman Islands region (data source: Chatterjee,1967; Eremenko and Sastri, 1977; and Ro y, 1983)A ge F o r m a t i o n G e n e r a l i z e dithology Max. thickness

(m )Recent- Beach and tidal depositsPleistocene coral reef, raised beachesunconformityPliocene- Long Foraminiferal clay, thinMiocene bands of silt 60

Guitar Foraminiferal limestone,calcareous sandstone andsiltstoneunconformityRound Chalk, sandstone, siltstone

450

520Strait

Oligocene- Por t BlairLate EocenePaleocene- BaratangLate Cret.Cretaceous Port Meadow

Sandstone, grit, conglome-rate, ma rl and siltstoneunconformityThick to massive sandstone,shale, siltstoneunconformityShale, associated grey-wackes, limestoneunconformityRadiolarian ch ert, jaspers,quartzite, limestone, marble.Oceanic basement/ophiolitesuite

500

750

1370

500 +

a s e v i d e n c e d b y d e f o r m a t i o n o f t h e P l e i s to c e n e s ed i -m e n t s n e a r t h e b a s e o f t h e l a n d w a r d w a l l o f t h e fi ll edt r e n c h , a n d a l s o b y c u r r e n t s e i s m i c i t y (s e e b e l o w ) . T h es t r u c t u r e i n t h e A N R i s d o m i n a t e d b y e a s t - d i p p i n gn a p p e s h a v i n g g e n t l e r fo l d i n g in t h e n o r t h p a r t o f t h ea r c a s c o m p a r e d t o t i g h t e r f o l d i n g a n d m o r e i n t e n s ed e f o r m a t i o n w i t h in t h e n a p p e s f u r t h e r s o u t h o f fS u m a t r a ( W e e k s e t a l . , 1 96 7; M o o r e a n d C u r r a y ,1 9 8 0 ) . A l s o t h e s t r u c t u r e s i n t h e C r e t a c e o u s -O l i g o c e n e s e q u e n c e s a r e g e n e r a l l y m o r e d e f o r m e dt h a n t h o s e d e v e l o p e d i n y o u n g e r s e q u e n c e s ( E r e -m e n k o a n d S a s tr i , 1 9 7 7) . S e v e r a l n o r t h - s o u t h f a u l tsa n d t h r u s t s w i t h i n th e A N R a n d o f f s h o r e a r e a s a r ek n o w n f r o m s u r f a c e m a p p i n g a n d s e is m i c s u r v ey s ; t h em o s t e x t e n si v e o f t h e m i s t h e J a r w a t h r u s t d e v e l o p e di n t h e m a i n A n d a m a n I s l a n d s ( R o y , 1 9 8 3 ) . C u r r a y e ta l . ( 1 9 7 9 ) d e s c r i b e a s e t o f n o r t h - s o u t h f a u l t s s li c in gt h e s e a f lo o r a l o n g t h e e a s t e r n e d g e o f t h e A N R i n t h eN i c o b a r d e e p a n d u n d e r t h e w e s t e rn p a r t o f t h e A n -d a m a n S e a , m o s t s i g n if i c an t o f t h e m b e i n g t h e W e s tA n d a m a n f a u l t ( F i g u r e s 1 a n d 2 ) . S o m e o f t h e sef a u l t s / th r u s t s a r e a l s o s e i s m ic a ll y a c ti v e ( M u k h o p a d -h y a y , 1 9 8 4 ) .

T h e e a s t e r n e d g e o f t h e A N R g e n t l y s l o pe s d o w ne a s tw a r d t o t h e f l o o r o f t he A n d a m a n b a s in w h o s ed e e p e s t p o r t i o n s a r e l o c a t e d i n t h e 1 0 0 - 2 0 0 k m w i d ec e n t ra l A n d a m a n t r o u g h . T h e d e p t h o f th e t r o u g hv a r ie s f r o m 2 k m b e l o w s e a l ev e l a t i ts n o r t h e r n e n d t ob e y o n d 3 k m m i d w a y a l o n g i t s 7 5 0 k m l e n g th . T h en o r t h w e s t e r n m a r g i n o f t h e t r o u g h i s m a r k e d b y am o s a i c o f st e e p a n d e l o n g a t e s e a v a l le y s a n d s e a m o u n t ss u c h a s t h e N i c o b a r D e e p , B a r r e n - N a r c o n d a m v o l -c a n i c i s l a n d s , I n v i s i b l e B a n k , A l c o c k a n d S e w e l ls e a m o u n t s ( R o d o l f o , 1 9 6 9 a , b ) ( F i g u r e 2 ) . T h e N a r -c o n d a m i s n o w a n e x t i n c t v o l c a n o b u t t h e B a r r e n l a s te r u p t e d i n 1 8 3 2 . A l l t h e s e a m o u n t s w h i c h t o g e t h e rf o r m t h e A n d a m a n v o l c a n i c a r c p o ss i b l y sh a r e a c o m -m o n o r i g i n ; t h e i r p r i n c i p a l r o c k c o n s t i t u e n t s a r e :b a s a l t , a u g i t e b a s a l t o r a n d e s i t e ( R o d o l f o , 1 9 6 9 a ) . T h ec e n t r a l t r o u g h i s b i s e c t e d b y t h e A n d a m a n b a c k - a r cs p r e a d i n g r i d g e t h a t h a s p r o d u c e d n e a r l y s y m m e t r i cs p r e a d i n g f o r t h e l a s t 1 1 m . y . w i t h a h a l f - s p r e a d i n gr a t e o f 1 .8 6 c m y r - 1 ( C u r r a y e t a l . , 1 9 7 9 ) . T h e A n -d a m a n b a s i n c o n t a i n s a n a v e r a g e s e d i m e n t t h i c k n e s so f 4 k m , b u t i t is d e li m i t e d e a s t w a r d b y t h e M e r g u iT e r r a c e a t t h e M a l a y a n c o n t i n e n ta l m a r g i n .


4/14

91~

93*

95~

97~

99*

"-

"I7"

._

yj~Anma

Tre

11Faut

SpenR)d

(paryacv)

~'~Semo

(BaslAdse

Ed

ocntnental

cus

5raviysaton

locton

I

1KmI

Boy

of

BengaI

8~

, ~=

~'

~

"2-2-.7:o-

B

o.

~c

.

.

.

.

.

.

.

.

=

~

-

^^\^,

-

=

-

=-E


5/14

GRAVITY ANOMALIES AND DEEP STRUCTURE OF THE ANDAMAN ARC 201Plate SettingThe Andaman arc is an area of high seismicity wherefocal depths of earthquakes extend down to 150 km.The Benioff zone configuration for the descending

Indian plate corresponding to an east-west section,EE', in the central portion of the Andaman arc isshown on Figure 3 (redrawn after Mukhopadhyay,1984). In preparing the section all hypocenters locatedwithin a distance of 200 km of the section line were

M idd le A ndam anB I s l a n dVE =7 .2 X

B e n g a l ~ ~

NarcondamSmf. A tcock /W A F S m t . B~ , S p r e a d i n g - 0

A n d a m a n - N i c o b a rC R i _ d g eV E = 2 2 X ~Ben ga I ~ ' ~ v ~ . ~ . - "F a n

O A n d a m a n - N i c o b a r S e v e t [ M e rg u i S u m a t r a OV E - 2 2 X R i d g e

I n v i s i b l eB a n k M e r g u i r ~. I/ W A F T e r r a c e ~ 0~ , . _ . ~ /~ J B a r r e n S mt ." . ' -~ " ~ .~ ,~ \ _ _ " S p r e a d i n g / ~ ' ~ ' ~ / ~ " I--2

N o r t h " 6B e n g a l W A F S m f . R i d g e B a s i n r O~,, A S p r e a d in g _ ~ ~

S CA LEI I2 0 0 Km

S e d i m e n t s t r a t a sV o l c a n i c s

W A F W e st A n d a m a n F a u l t

I . I

v 1

I .IQ I

I . I

A n d a m a nT r e n c h ~ A n d a m a n. . N i c o b a r s e a . - - - - - ~ c /E A X iS ~ R i d g e ~ V o l c a n ! c a r ci , . . . . . Easte s t 9 . 9 9

~ " : " * "? ; ~ " - ',~ " : B u r m a ~ ; e ~e * e e x 9 9 eeS O I n d i a n p l a t e * * " " " ~ t a t~ . 1 0 01 - C e n t r a l A n d a m a n ~ * ~ -'i" I S ea E a r t h q u a k e s " ~ "? I140 L_ ( p e r io d 1916- 1975) ~ I ' ~ J

F i g . 3 . S t r u c tu r e s p r e s e n t u n d e r t h e s e is m i c p r o f il e s B B ' t h r o u g h D D ' c r o s s in g t h e A n d a m a n a r c , a n d t h e B e n i o f f z o n e c o n f i g u r a t i o n b e lo wt h e s ec t io n E E ' . F o r b o t t o m f ig u re : A . B A s e i sm i c B e l t; E F Z E a r t h q u a k e F r e e Z o n e .


6/14

202 MANOJ MUKHOPADHYAY

projected onto a vertical plane. The horizontal lowerboundary of the Indian Ocean lithosphere is placedimmediately below the lowest foci at a depth of75 km; traced eastward this boundary dips at about30 ~ under the arc and plunges to nearly 150 km depthbelow the Andaman Sea. The upper boundary for theinclined lithosphere is rather difficult to locate unlessa basic assumption is made that it lies in the westernvicini ty of the ANR where a megathrust is alreadyproposed by Fitch (1970). By definition, this zone ofmegashear therefore outlines the surface trace of theboundary between the descending Indian plate andthe overriding Burma plate. Further east of the Be-nioff zone two shallower seismic zones are locatedwithin the Burma plate (Figure 3). Following Sacks etal. (1978) and Yamashina et al . (1978), we designatethis as the 'seismic slab' of the overriding plate. Therelatively shallow seismic zone closest to the Benioffzone occurs below the western part of the Anda manSea near the frontal arc (ANR); whereas the crustalseismic zone east of the volcanic arc corresponds toback-arc activity. It also appears from section EE'that the fore-arc and back-arc seismic zones are dis-tinguished by an earthquake free zone between them.A triangular 'asesimic belt' occurs at shallower depthsdefined by the upper surfaces of the Benioff zone andthe 'seismic slab' of the overriding plate; the apex ofthe aseismic belt is deflected downward - concordan twith the dip direction of the Benioff zone, particularlyunder the east flank of the ANR and the NicobarDeep. This bending of the 'Burma plate seismic slab'is presumably a consequence of downward drag expe-rienced by the overriding plate near the subductionzone.

According to Cur ray et al . (1979) the Burma plateis of elongated shape in north-south direction; it en-compasses the Andaman basin underlying the An-daman Sea and also a substantial part of inlandBurma. The east margin of the Burma plate with theAsian plate is marked by the Sagaing fault - a majortransform extending through Burma toward theHimalayan collision zone (Figure 1).

It is suggested that extensional stress dominates thelandward wall of the Andaman trench whereas com-pressive stress prevails within the overriding Burmaplate (Mukhopadhyay, 1984). This, however, givesway to deviatoric tensional stress near the Andamanback-arc spreading ridge in the interior of the Burmaplate. Note that the back-arc spreading ridge bisectsthe Andaman volcanic arc in the central Andaman

Sea as it passes between the Barren and Sewellseamounts (Figure 2).

Gravity AnomaliesThe gravity field for the And aman arc, first describedby Peter et al . (1966), is clearly bipolar in nature, andis quite similar to that for the Western Pacific arcs. Arevised Free-air anomaly map for the region is givenon Figure 4. The most significant feature present onthe Free-air map is a gravity anomaly pair of averageamplitude 180 mgal coinciding with the Andamanarc. The maximum anomaly, however, locally variesup to 300 mgal (peak to trough) near the InvisibleBank in the north Andaman Sea. The general charac-ter of the anomaly shows a gravity low over theisland-arc and trench area and a gravity high over thevolcanic arc. In the north part of the Andaman arc_the axis of the gravity minimum shifts its position toa more easterly location to follow the Nicobar deepalong the eastern edge of the ANR (Figure 4). Thegravity minimum is seen to shift again to its suppos-edly initial location only to the north of the PreparisChannel close to the Burmese coast. Peter et al. (ibid.)have interpreted the low gravity in terms of excesscrustal thickness reaching up to 40 km underlying theANR.

Further east, the Andaman basin underlying theAndaman Sea is associated with Free-air anomaliesvarying up to _+ 50 mgal. Mi ld positive anomalies arealso observed over a bathymetric swell to the immedi-ate west of the Andaman trench as well as over theNinety East Ridge. The Ninety East Ridge strikesnearly subparallel to the Andaman arc in this region.

The gravity anomaly variation along a 980-km-long profile, AA', taken across the central portion ofthe Andaman arc (Figure 2 for profile location) isillustrated on Figure 5. Major tectonic features tra-versed by the profile from west to east are: BengalFan, Ninety East Ridge, Andaman trench, ANR,Nir Deep, volcanic arc and back-arc spreadingridge in the region of the Sewell seamount, Andamanbasin, and finally the Mergui terrace at the Malayancontinental margin. Gravity and bathymetric pointvalues plotted on Figure 5 follow a ship's track alongthe Ten Degree Channel (data source: Defense Map-ping Agency, U.S.A.). Free-air values at both endsof the profile are within 0 to -2 0 mgal, suggestingisostatic compensation for the Indian Ocean litho-sphere below the Bengal Fan on the west as well as for


7/14

GRAVITY ANOMALIES AND DEEP STRUCTURE OF THE ANDAMAN ARC 203

1 /,

1 3

12

9 0 0 91 ~ 92 " 9 3 ~[ I

S c a l er " 80 K m o 7 "

B e n g a l F a n

9 /, 0 9 S ~

" v 0P r e p a r i s S o u thC h a n n e lc o T I -I 1 N 2[ ] / * "r t h . . ' G r a v i t y= d a - S t a t i o nn a n ^ F r e e a i r

I f -':"~ 1 7 6

c o n t o u ri n t e r v a lv a r i n b l eH / L f i r a v i t yh i g h o r

l o w. . . T r e n c ha x i s

Ao

I C

9~

o l8

"~ = a ~ o ' ~ ~ L i t t l e N i c o b a r

0t

g~a

8 8 ~ 8 9 0 9 0 ~ 9 1 ~ 9 Z ~ 9 3 ~ 9 / *~ 9 5 0

F i g . 4. F r e e -a i r g r av i t y a n o m a l y m a p f o r t h e A n d a m a n a r c - tr e n c h r e g io n , a n d g e n e r a l iz e d g e o l og y f o r t h e A n d a m a n - N i c o b a r I s l a n d s . D a t as o u r c e s a re c i t ed i n t ex t . N o t e t h e n o r t h - s o u t h l i n e a r it y o f t h e p o s i t i v e a n o m a l y z o n e o v e r t h e ' o u t e r h i g h ' a n d N i n e t y E a s t R i d g e , n e g a t i v ea n o m a l y o v e r t h e A n d a m a n t r e n ch , a n d p o s i ti v e a n o m a li e s f u r t h e r e a st o v e r t h e A n d a m a n v o l c a ni c a rc . T h e v o l c a n i c a rc g r a v i ty c o n t o u r sa r e a d o p t e d f r o m P e t e r e t a l . ( 1 9 6 6 ) . T h e a x i s o f t h e g r a v i t y m i n i m u m , h o w e v e r , s h i ft s i t s p o s i t i o n e a s t w a r d t o f o l l o w t h e N i c o b a r D e e p i nt h e n o r t h p a r t o f t h e a r c a s c o m p a r e d t o m o r e s o u t h e r n a r e a s. L e g e n d : 1 , U n m a p p e d ; 2 , L a t e r it e ; 3 , C r e t a c e o u s - T e r t i a r y s e d i m e n t s ; 4 ,

O p h i o l i t e . A A ' i s th e g r a v i t y p r o f i l e (a l s o s e e F i g u r e 2 ) i n t e r p r e t e d i n F i g u r e 5 .


8/14


/,Or"- - AA A A

0 9AA A A

-8 0

-120F-Observed

-" .~ -'- Com pu ted-160t-~.-20 0 0 100 200 300 zOO

WRst' e e , tO.B.H &B. F._-,~-90eost Ridge .-~ T~A~-500|

A.N.R

4

600I

- ~ o

AA A

-z,,0

- -80

700 800t I

-12(

-16(

N O . A.S.

Km ~-900 980- -20qI IEnstM.T,

0102030

l5O

70

90

11C

13t

151

Fig. 5. Two-dimensional interpretation of the gravity field along profile AA' in the central Andaman arc (see Figures 2 and 4 for profilelocation). Inset shows com puted gravity effect due to the descending Indian lithosphere below 75 km d epth. See text for discussion oninterpreted model. B.F., Bengal Fan; O.B.H., Outer Bathymetric High; T.A., Trench Axis; N.D., Nicobar Deep; V.A., Volcanic Arc; A.S.,And ama n Sea; M.T., Mergu i Terrace. Digits refer to density values in g cm 3.


9/14

GRAVITY ANOMALIES AND DEEP STRUCTU RE OF THE ANDAMAN ARC 205

t h e M a l a y a n c o n t i n e n t a l m a r g i n o n t h e e x t r e m e e a s t .T h e B e n g a l F a n g r a v i t y fi el d c h a n g e s o v e r t h e N i n e t yE a s t R i d g e a n d i t s a d j o i n i n g a r e a t o t h e i m m e d i a t ew e s t o f th e A n d a m a n t r e n c h t o d e s c r i b e a re l a ti v eg r a v i t y h ig h o f a b o u t 3 0 m g a l. T h i s c h a n g e i n g r a v i tyc o r r e s p o n d s t o a b a t h y m e t r i c s w e ll o f m o r e t h a n3 5 0 m o v e r a h o r i z o n t a l s p a n o f a b o u t 1 5 0 k m ( a l sos e e F i g u r e 4 ) . O b v i o u s l y t h e s e b a t h y m e t r i c a n d g r a v -i t y c h a n g e s a r e r e l a te d i n p a r t t o b o t h t h e N i n e t y E a s tR i d g e a n d l i th o s p h e r i c f l e x u ri n g i n t h e f o r m o f a n' O u t e r H i g h ' t o t h e i m m e d i a te w e s t o f th e A n d a m a nt r e n ch . W a t t s a n d T a l w a n i ( 1 9 7 4 ) h a v e a lr e a d y n o t e dtha t the ou te r h igh in the presen t reg ion i s le s s prom i -n e n t a s c o m p a r e d t o t h a t i n t h e I n d o n e s i a n a r c f u r t h e rs o u t h . H o w e v e r , b a t h y m e t r i c r e li e f f o r t h e N i n e t yE a s t R i d g e i s a l s o m u c h s u b d u e d i n t h e A n d a m a nr e g i o n c o m p a r e d t o m o r e s o u t h e r n a r e a s ; t h e r i d g e i sprac t i ca l ly bur i ed a t th i s l a t i tude (c f . Cur ray e t a l . ,1982) . Observed F ree -a i r va lues a re c lose to- 5 0 m g a l o v e r t h e A n d a m a n t r e n c h , b u t t h e y fa l l o f fr a p i d l y t o - 1 9 2 r e g a l o v e r t h e N i c o b a r D e e p a c r o s st h e A N R , t h e n c e r is e to a b o u t 1 2 m g a l o v e r th e v o l -c a n i c a r c t o w a r d t h e e a s t . F u r t h e r e a s t t h e a n o m a l yv a l ue s a re o f t h e o r d e r o f - 2 5 m g a l o v e r th e A n -d a m a n b a s i n b e f o r e a t t a i n i n g m i l d p o s i t i v e v a l u e sa g a i n o v e r t h e M e r g u i T e r r a c e a t t h e M a l a y a n c o a s t .T h i s a n o m a l y v a r i a t i o n a l o n g t h e T e n D e g r e e C h a n -n e l a n d a c r o s s th e A n d a m a n S e a is i n t e r p r e t e d h e r e i nt e r m s o f a s u b d u c t i o n z o n e m o d e l .

Deep StructureF o r p u r p o s e s o f g r a v i ty i n t e r p r e t a ti o n f o r p r o f il eA A ' , w e u s e s o m e g u i d e l i n e s o n v e l o c i t y a n d d e p t hd i s t r ib u t i o n f o r n e a r s u r f a c e g e o l o g i c l a y e rs u n d e r l y -i n g t h e A n d a m a n a r c a s k n o w n f r o m p r e v i o u s s tu d i e s.T h e y i n c lu d e : a n i n t e r p r e t e d s e c t i o n f o r a s u b - b o t t o mp r o f il e a lo n g t h e T e n D e g r e e C h a n n e l ( a f t e r W e e k s e ta l . , 1967), resu l t s o f wide-angle s e i smic re f l ec t ion andr e f r a c t i o n s t u d y o n c r u s t a l l a y e r s u n d e r t h e G a n g e sC o n e ( a f t e r N a i n i a n d L e y d e n , 1 9 7 3 ) , i n f o r m a t i o nf r o m D S D P l eg 2 1 7 l o c a te d o n n o r t h p a r t o f t h eN i n e t y E a s t R i d g e , s e d i m e n t v e l o c i t i e s f r o ms o n o b u o y s f r o m B e n g a l a n d N i c o b a r F a n s a n d A n -d a m a n b a s in ( H a m i l t o n e t a l . , 1977) , and informat ionf r o m t h r e e s e i s m i c p r o f i l e s , B B ' t h r o u g h D D ' , a c r o s st h e A n d a m a n a r c ( a f te r C u r r a y e t a l . , 1979) (F igures2 and 3) . S t ruc tura l in t e rpre ta t ion for these s e i smics e c ti o n s a s g i ve n b y C u r r a y e t a l . c l e ar l y d e m o n s t r a t e s

t h a t m a j o r g e o l o g i c s t r u c t u r e s u n d e r l y i n g t h e A n -d a m a n a r c a r e g r o s s l y s i m i l a r i n t h e n o r t h - s o u t h d i -r e c ti o n . T h e p r e s e n t g r a v i t y p r o f il e A A ' a n d a f o r e s a i dse i smologic s ec t ion EE ' a re loca ted in the c lose v ic in-i ty o f these s e i smic prof i l e s.T h e g r a v i t y a n o m a l y v a r i a t i o n f o r m a n y i s la n d a r c -t r e n c h a r e a s h a s b e e n e x p l a i n e d i n t h e f r a m e w o r k o fsea - f loor spreading and p la te t ec tonics . Such gravi tym o d e l s t u d i e s c o m m o n l y i n f e r t h a t t h e d e s c e n d i n g

l i t h o s p h e r e u n d e r i s l a n d - a r c / t r e n c h a r e a s p r o d u c e s ac o n s i d e r a b l e g r a v i t y e f fe c t o n t h e s u r f a c e ( c f . G r o w ,1 9 73 ; G r o w a n d B o w i n , 1 9 7 5 ) . T h i s i s p o s s i b ly a c o n -s e q u e n c e o f t h e r m a l e f fe c ts o f c o l d o c e a n i c l i t h o s p h e r ea l o n g B e n i o f f z o n e s ( M i n e a r a n d T o k s o z , 1 9 70 ) t h a tp r e d i c t l o n g w a v e l e n g t h g r a v i t y a n o m a l i e s o f a m p l i -t u d e s o f 5 0 - 1 0 0 m g a l d u e t o t h e r m a l p e r t u r b a t i o n s i nt h e u p p e r m a n t l e. T h e g r a v i t y e f fe c t p r o d u c e d b y t h ed e s c e n d i n g I n d i a n O c e a n l i t h o s p h e r e a t t h e l o c a t i o no f t h e p r e s e n t g r a v i t y p r o fi le h a s b e e n c o m p u t e d u s i n gt h e c o n f i g u r a t i o n s h o w n b y s e c t i o n E E ' ( F i g u r e 3 )w h e r e w e a s s u m e t h e l i t h o s p h e r e t o a s t h e n o s p h e r edens i ty con t ras t to b e 0 .05 g cm -3 . A s imi la r dens i tyv a l u e w a s a s s u m e d b y G r o w ( i b i d . ) f o r t h e A l e u t i a na rc . G r o w a n d B o w i n ( i b i d . ) c o n s i d e r a m o r e c o m p l e xd e n s i t y d i s tr i b u t i o n f o r t h e d e s c e n d i n g l i th o s p h e r e u n -d e r t h e C h i l e a n t r e n c h . H o w e v e r , i n t h e p r e s e n t c a s es i n c e n o o t h e r g e o p h y s i c a l d a t a s u p p o r t i s a v a i l a b l ew e h a v e u s e d t h e l i t h o s p h e ri c c o n f i g u r a t i o n s h o w n i nF i g u r e 3 , a n d a u n i f o r m d e n s i t y d i s t r i b u t i o n f o r t h el i t h o s p h e r e a n d t h e s u r r o u n d i n g a s t h e n o s p h e r e . F o rt w o - d i m e n s i o n a l c o m p u t a t i o n o f g r a v i ty e f fe c ts w eu s e th e p o l y g o n a l m e t h o d o f T a l w a n i e t a l . (1959) .

A s c a n b e s e e n o n F i g u r e 5 , t h e c o m p u t e d g r a v i t ye f fe c t d u e t o t h e s u b d u c t i n g l i t h o s p h e r e u n d e r t h e A n -d a m a n a r c h a s a m a x i m u m v a l u e o f 6 8 m g a l w h i c ht a il s o f f q u i t e s y m m e t r i c a ll y f r o m t h e p e a k v a l u e , a n ddimini shes to l e s s than 5 mg a l wi th in a d i s t anc e o f5 0 0 k m f r o m t h e d e e p e s t p a r t o f th e s u b d u c t i o n z o n e .The p os i t ive gravi ty e f fec t due to the descen ding l i tho-s p h e r e i s 2 0 m g a l a t t h e t r e n c h a x i s a n d 3 5 - 4 0 m g a lo v e r t h e v o l c a n i c a r c . T h e o b s e r v e d a n o m a l y a l o n gp r o f il e A A ' i s s u p p o s e d l y a c o m b i n a t i o n o f th i s d e e pgravi ty e f fec t p lus the e f fec t due to the over ly ing l ay-e rs . He nce the p os i t ive e ffec t due to the d escendingl i th o s p h e r i c s l a b ( b e l o w 7 5 k m ) m u s t b e c o m p e n s a t e db y a l o w d e n s i t y z o n e o v e r l y i n g i t a t t h e l o c a t i o n o fthe volcanic a rc in the v ic in i ty of the Sewel l s eam oun t .For achieving th i s a low dens i ty zone i s envi sagedi n t e rm s o f a l es s d e n s e ( b y a n e s t i m a t e d v a l u e o f


10/14


-0 .01 g cm -3 with respect to the surrounding litho-sphere) vertical rock column, at least 60 km wide,penetrating the lithosphere under the volcanic arc.Construction of the geometry for the low density zoneis purely arbit rary a t this stage; the model is assumedto merely fit the observed gravity anomalies. Alter-nately, the positive gravity effect in question may bebalanced by asthenospheric shallowing to the base ofthe crust beneath the volcanic arc at the location ofthe earthquake free zone (shown in Figure 3) or bysome form of mass anomalies arising out of mantleflow at the Andaman back-arc region as the astheno-sphere tends to pull the overriding Burma plate west-ward in the direction of the Andaman trench. Anysuch mantle flow, if operative, will have to be relatedto the dynamics of lithospheric subduction. Such aprocess of mantle flow is believed to be a dominantmechanism for several active back-arc regions of theworld (cf. Hager et al . , 1983). However, at present wehave very little to choose from these various alterna-tive explanations.

It has been argued (cf. Minear and Toksoz, 1970)that heating due to friction between the underthrustand overriding plates may produce a high-tempera-ture, low-density zone generally below the volcanicarc, where seismic waves are also known to attenuate.No such elaborate seismic velocity model is howeveravailable for the Andaman arc. It is likely that thepresence of volcanoes themselves requires a highertemperature (low-density) source at depth belowthem as already noted by Grow (1973). The An-daman volcanic arc is intriguingly split by the back-arc spreading ridge at the location of the presentgravity profile. Published values for heat-flow mea-surements so far made in the region of the volcanicarc indicate appreciably high values such as 5.27 hfu(Burns, 1964), 5.9 and 3.3 hfu (Curray et al . , 1979).Also the heat-flow values fall off linearly with distanceaway from the axis of the volcanic arc. This, in a way,tends to support the presence of a high-temperature,low-density zone under the volcanic arc.

Crustal Configuration for the Bengal Fan ModelAnother distinct source of density anomaly lies in theupper part of the lithosphere at the subduction zone.This is produced by the Indian Ocean crust at theAndaman island arc-trench area where the crustallayer gets highly deformed as it is carried with the

subducting plate. For the purpose o f gravity modelingwe assume a simplified two-layer ocean crust that con-sists o f a sediment layer of mean density 2.5 gc m -3overlying a basal part (corresponding to oceanic layers2 and 3) of mean density 2.9 g cm -3. These densityvalues are estimated on the basis of observed seismicvelocities under the Ganges Cone and Andaman arc-trench areas as given by Naini and Leyden (1973),Hamilton et al. (1977) and Curray et al . (1979, 1982)using the Nafe-Drake velocity-density relationship.We further assume that the oceanic Moho under theGanges Cone in the north part of the Indian Oceanbasin lies at a depth of 14 km below sea-level where theaverage water depth is 3.5 km. This ocean basin crustis supposedly in isostatic equilibrium according to theAiry scheme as suggested by near-zero Free-airanomalies under the western part of profile AA'. Thatisostatic equilibrium prevails on a regional scale inmost parts of the northe rn Indian Ocean is also exem-plified by averaged Free-air anomalies over 1~ 1~areas (cf. Kahle and Talwani, 1973); such anomaliesare however, superposed on a much longer wavelengthgravity low due to deeper sources and a geoidalanomaly centered south of Ceylon in the north IndianOcean (cf. Watts and Daly, 1981).

The sediment layer underlying this part of the Ben-gal Fan corresponding to the western flank of profileAA' is considered to be 4.5 km thick; the sedimentsrange in age from Campanian to Holocene. Note thatsediment thickness in the Bengal Fan is quite variableand may attain a high figure (cf. Curray and Moore,1974). Curray et al. (1982) suggest that the sedimentthickness possibly exceeds 5 km in the present area.Seismic reflection and ref raction spot surveys reportedby Naini and Leyden (1973) in the vicinity of thepresent gravity profile indicate that sediments are, ingeneral, 4-5 km thick and they directly overlie theoceanic basement of distinct higher velocity of6.22 km sec -~. Therefore, the 4.5 km sediment thick-ness considered here for gravity modeling purposes isperhaps a minimum estimate. A time section con-structed by Curray et al. (ibid.) from seismic reflectionrecords wi th averaged velocities from results of wide-angle reflection and refraction measurements alongthe Ten Degree Channel basin clearly demonstratesthat almost flat-lying sediments of the Bengal Fanthin out eastward over the Ninety East Ridge butthicken again with eastward dip upon approachingthe Andaman trench located some 70 km further east.


11/14

GRAVITY ANOMALIES AND DEEP STRUCTU RE OF THE ANDAMAN ARC 207

This simplified crustal model for northern IndianOcean crust is quite comparable to an average oceaniccrustal model (for layers 2 and 3) as reported byChristensen and Salisbury (1975) who propose layer 2and 3 thicknesses o f 1.39 __+0.50 and 4.97 + 1.25 km,having respective compressional wave velocities of5.04 ___0.69 and 6.73 ___0.19 km s 1. They further con-clude that there is little change in thickness for layers2 and 3 if the ocean basin is any older than 40 m.y. Asthe crust underlying the northern Indian Ocean nearthe Ninety East Ridge is definitely much older thanthis (cf. Luyendyk and Rennick, 1977 for magneticanomaly ages and DSDP results on the area), we be-lieve that the simplified oceanic crustal model consid-ered here for gravity modeling may not be far fromthe real picture. This Bengal Fan crust is carried downthe Andaman trench with the descending Indianplate.

While some of the foregoing assumptions about thenormal crustal configuration under the NE IndianOcean are bound to remain ambiguous until deepcrustal seismic control and crustal drilling data be-come available, certain important inferences can bemade on the approximate configuration of the trench-arc geometry, sedimentary and crustal layers belowthe ANR, and about the subduction zone, which sat-isfy the gravity data.

Trench and Outer Bathymetric Rise GeometryAlong the western flank of the ANR, the Andamantrench is clearly demarcated by a continuous gravitylow of amplitude - 48 mgal, although the bathymet-ric expression for the trench is much less clear (alsosee Figure 4). At the Ten Degree Channel the gravitygradients across the outer and inner walls of thetrench are -0. 63 mgal km 1 and 1.90 mgal km i re-spectively. The positive gravity effect due to the de-scending Indian lithosphere at the trench axis is20 mgal. Fo r gravity model calculations we have con-sidered density contrasts between sediments andoceanic crust and the latter versus the lithosphere asuniform throughout (= -0 .5 ge m- 3) . The modelshows that the Andaman trench has an asymmetricV-shape; the apex of the trench axis is filled with sed-inaents and the maximum sediment thickness is 7 kmunder the area (Figure 5). In other words, the trenchcontains an excess of 2.5 km of sediments as com-pared to the Bengal Fan crust to its immediate west.

There is a corresponding downbulge of oceanic layers2 and 3 into the lithosphere.

Seaward of the trench, in the region of a bathymet-ric swell of more than 350 m, the gravity model showsa lithospheric flexuring by 500 m to account for agravity high of 30 mgal. For construct ing the swellgeometry, we use a similar density distribution as be-fore, and assume the crustal thickness as constant fol-lowing Watts and Talwani (1974). Such a lithosphericswell seaward of the Andaman trench occurs over ahorizontal distance of about 150 km. Obviously theswell is related in part to the buried t opography of the90 East Ridge as well as that due to the 'outer high'typical of western Pacific arcs. Continuation of theouter gravity high parallelling the An daman trench inthe north-south direction clearly suggests that litho-spheric flexuring associated with the Andaman sub-duction zone occurs on a regional scale, and that theouter gravity and bathymetric rises are not entirelydue to the 90 East Ridge. The ridge is practicallyburied at this latitude; further north it may die outaltogether.Crustal Configurat ion below ANR andSubduet ion ZoneThe gravity model (Figure 5) shows that the sedimentthickness for the Cretaceous-Tertiary section reachesan average value of 5~5 km under the ANR, butdown the subduction zone eastward, sediment thick-ness may increase to 13 km. The stratigraphic esti-mate for the Andaman flysch of mid-Eocene toOligocene age under the ANR is 3 km; and the grosssediment thickness for Upper Cretaceous to Recentsediments, excluding older sediments, is abou t 4.2 km(Table I).

Grow (1973) and Grow and Bowin (1975) havediscussed the effect on density as the oceanic crustexperiences transit ion from lower to higher pressureassemblages (basalt to eclogite) at pressures between10 and 20 kb (30 to 60 km depth) in a Benioff zoneenvironment. This process is grossly generalized hereby a simple density change of the oceanic crust from2.9 to 3.4 g cm -3 at about 27-28 km depth. The grav-ity model shows that both sedimentary and deepercrustal layers are depressed into their respective sub-strata over a distance of 200 km below the ANR andat the subduction zone. It is envisaged that sedimentsand the underlying crust of the Burma plate are thrustover the layers of the descending Indian plate through


12/14


an e f f i c i en t decoupl ing marked by a grea t decol l e -m e n t , i n p a r t i c u l a r u n d e r t h e N i c o b a r D e e p a l o n g t h ee a s t e r n fl a n k o f th e A N R . T h i s l o c a t i o n is m a r k e d b ya p r o m i n e n t b o u n d a r y t h r u s t a l o n g t h e e a s t e d g e o ft h e A N R ( M u k h o p a d h y a y , 1 9 8 4 ) . I t i s n o t e d a b o v et h a t t h e A N R i s s li ce d b y s e v e r al n o r t h - st r i k in g s u b -para l l e l fau l t s and thrus t s inc luding the mos t ex ten-s ive J a rw a thru s t . Q ui t e a few of these fau l t s a rese i smica lly ac t ive a t depth . The fau l t s and th rus t s un -d e r th e A N R p r o d u c e a p a t t e r n o f e a s t -d i p p i n g t h r u s ts h e e t s a n d n a p p e s ( C u r r a y e t a l . , 1979).

M a f i c M a s s i n A N RSevera l maf ic /u l t ramaf ic bodies inc luding a few ophi -o li te s ar e k n o w n f r o m t h e A n d a m a n - N i c o b a r I s la n d s(see F igure 4) , a l though very l i t t l e i s known aboutthe i r na ture , o r ig in and the i r re l a t ionship wi th ANRsediment s . T wo such re la t ive ly l a rge-s ized bo dies a rek n o w n f r o m t h e C a r N i c o b a r a n d T e r e s s a I s l a n d sn e a r t h e T e n D e g r e e C h a n n e l w h e r e g r a v i t y p r o f i l eAA' i s loca ted . The prof i l e shows a re l a t ive gravi tyh i g h a b o u t 5 0 k m w i d e h a v in g a n a m p l i t u d e o f4 4 m g a l a l o n g t h e e a s t e d g e o f t h e A N R in the vicini tyo f t h e s u b d u c t i o n z o n e ( F i g u r e 5 ). T h e g r a v i t y h ig h i sl o c a t e d o f f s h o r e n o r t h o f th e C a r N i c o b a r i s l an d . T h es h a p e a n d g r a d i e n t o f t h e a n o m a l y s u g g e s t a s h a ll o wc a u s a t i v e m a s s . T o e x p l a i n t h e a n o m a l y w e i n f e r am a f i c / u l tr a m a f i c m a s s o f a s s u m e d d e n s i ty 3 . 0 g c m - 3s u r r o u n d e d b y s e d im e n t s o f t h e A N R . T h e g r a v i tym o d e l f o r t h e c a u s a t i v e m a s s r e q u i r e s i t t o b e n e a r l y3 k m t h i c k h a v i n g s u b p a r a l l e l d i p t o t h e s e n s e o f su b -d u c t i o n b e l o w th e A n d a m a n a rc . A l t h o u g h t h e n a t u r ea n d o r i g in o f th e c a u s a t iv e m a s s a r e n o t k n o w n , c e r -t a i n i n f e r e n c e s o n t h e m c a n b e m a d e o n t h e b a s i s o ft h e m o d e l o f F i g u r e 5 . F o r t h is , t h r e e w i d e l y b e li e v e dh y p o t h e s e s r e g a r d i n g t h e o b d u c t i o n o f o p h io l i te s a n de m p l a c e m e n t o f o t h e r m a f i c / u lt r a m a f i c r o c k s m a y b ec i te d h e r e: ( a ) o b d u c t i o n o f o c e a n i c c r u s t /o p h i o l i t e sa l o n g f a u l t s t h a t d i p o p p o s i t e t o t h e d i r e c ti o n o f s u b -duc t ion (c f . Colema n, 1971 ; Chr i s t ensen and Sa li s-bury , 1975); (b) s l ic ing of f wed ges of ocean c ru s td u r i n g s u b d u c t i o n a l o n g f a u l t s w h i c h a r e s u b p a r a l l e lt o t h e s e n se o f s u b d u c t i o n ( c f . D e w e y a n d B i r d ,1 9 71 ); ( c ) e m p l a c e m e n t o f a m a f i c /u l t r a m a f ic m a s sr e l a te d t o c h a n g i n g m o t i o n a l o n g t a n s f o r m f a u l ts ( se eSi lver e t a l . , 1978) . Of these , a proces s of ophio l i t eo b d u c t i o n i s c le a r ly n o t f a v o u r e d b y t h e p r e s e n t g r a v -i t y m o d e l . A l s o , w e a r e n o t a w a r e o f a n y t r a n s f o r m

f a u l t a c ti v i ty a f fe c ti n g th e A N R n e a r t h e T e n D e g r e eChan ne l . Ra th er , s li cing of f wedg es of ocean c rus td u r i n g s u b d u c t i o n o f th e I n d i a n p l a t e s ee m s to b es u p p o r t e d b y t h e e a s t w a r d t h i c k e n i n g o f th e m a r i e/u l t r am a f i c m a s s a n d i ts c o n c o r d a n t d i p w i t h e a s t w a r ds u b d u c t i o n o f t h e In d i a n p l a t e . W e l l a n d a n d M i t c h e l l(1977) a l so note tha t t ec tonic s l ices of mar ie and u l t ra -m a r i e r o c k s o c c u r w i t h i n o r o n t h e l a n d w a r d m a r g i no f t h e A n d a m a n f ly s c h b e l t.

Crustal Transition at the Malayan ContinentalMarg i nT h e o p p o s i t e, e a st e rn b o u n d a r y o f t h e A n d a m a n S e ai s f o r m e d b y t h e M a l a y p e n i n s u l a , i t s c o n t i n e n t a lshe l f , and the Mergui Ter race (F igure 2) . The ba thy-m e t r i c p a t t e r n p r e s e n t a l o n g t h e w e s t e r n f l a n k o f th eMergui Ter race (F igure 5) i s c l ea r ly representa t ive ofc o n t i n e n t a l s h e l f a n d s l o p e g e o m e t r y , w h e r e t h e o b -s e r v e d F r e e - a i r a n o m a l y v a r i e s f r o m - 2 4 r e g a l o v e rt h e e a s te r n A n d a m a n S e a t o 1 2 r e g a l o v e r t h e M e r g u iT e r r a c e l a n d w a r d . C u r r a y e t a l . (1979) sugges t tha tt h e M e r g u i - N o r t h S u m a t r a b a s i n i s u n d e r l a i n b yt h i n n e d c o n t i n e n t a l c r u s t ; a n d t h e n o r t h - s o u t h b l o c kfaul t ing pa t t e rn in the a rea i s s imi la r to t ens iona lfau l t ing - typ ica l of r i f ting youn g cont inenta l m ar-g ins . Thi s in te rpre ta t ion impl ies tha t the c rus ta l t ran-s i ti o n p o s s i b l y o c c u r s b e l o w t h e M e r g u i T e r r a c e . T h eo b s e r v e d F r e e - a i r a n o m a l y i s si m i la r i n n a t u r e t o t h a to b s e r v e d f o r t y p i c a l t r a n s it i o n a l c r u s t u n d e r l y i n g A t -l an t i c - type cont inenta l margins (c f . Dehl inger , 1978,p p . 2 3 0 - 2 3 2 ) . N e g a t i v e a n o m a l i e s o v e r t h e e a s t e r nA n d a m a n S e a , t h a t c o r r e s p o n d s t o t h e s e a w a r d s i d eo f t h e c o n t in e n t a l s l o p e , a re h o w e v e r s o m e w h a t s u b -dued in the present case . Thi s ev ident ly resu l t s f romthe pos i t ive gravi ty e f fec t of the descending Indianl i thosphere wh ose e f fec t i s nea r ly 15 mg al he re ( inse tin F igure 5) .

A s s u m i n g A i r y c o m p e n s a t i o n a s v a l i d f o r t h eM a l a y a n c o n t i n e n t a l m a r g in , t h e o b s e r v e d a n o m a l i e sa re in te rpre ted in t e rm s o f a th icker ocean ic c rus tu n d e r t h e A n d a m a n S e a , a n d b y a t r a n s i t i o n a l c r u s t ,o f a s s u m e d a v e r a g e d e n s i t y 2 . 8 4 g c m - 3 , u n d e r l y i n gt h e M e r g u i T e r r a c e a n d M e r g u i - N o r t h S u m a t r ab a s i n . T h e o c e a n i c M o h o i n o u r m o d e l d e e p e n s f r o m1 7 k m u n d e r t h e e a st e r n A n d a m a n S e a t o 1 9 k m b e -low the l a t t e r a reas ; the Moho d ip i s 16~ e a s t w a r d .Thi s eas t -wes t c rus ta l t rans i t iona l zone under theM e r g u i T e r r a c e e v i d e n t l y m a r k s t h e e a s t er n l i m i t o f


13/14

GRAVITY ANOMALIESAND D EEP STRUCTUREOF THE ANDAMANAR C 209

thick sediment accumulation and thicker oceaniccrust underlying the Andaman Sea.

SummaryActive subduct ion of the Indian plate i s current ly oc-curr i ng beneath the Andam an arc a l ong an eas t d i p-p i ng Ben i o f f zone tha t ex tends to a depth o f about150 kin; the deepest part of the Benioff zone is at-tained below the Andaman volcanic arc. The overrid-ing Burma plate is, in part, defined by an activeseismic slab a t least 50 km thick. This lithosphericslab is deflected downwards in the vicinity of theBenioff zone. There, a triangular aseismic wedge inthe top part of the Burma plate i s general ly out l inedbeneath the eas t edge of the ANR as well as the Nico-bar Deep. A revised Free-air anomaly map for theAndaman arc and its immediate environs shows thatthe arc i s associated with a major gravi ty anomalypair of ampl i tude 180 regal extending over a distanceo f a b o u t l l 0 0 k m i n a n o r th - s o u th d i r e ct io n . T h egravity anomaly variation along a profile in the cen-tral Andaman Sea is interpreted here in terms of platesubduction following the general pattern of plateconfiguration as given by the seismicity data. Thissuggests that sediments below the Andaman arc in-crease from about 7 km to as much as 13 km at themouth of the subduction zone underlying the eastflank of the ANR and the Nicobar Deep. At thislocation a mafic mass is emplaced within the sedimen-tary section. The underlying oceanic crust apparentlyexperiences phase transit ion at about 27 km in aBenioff zone environment. The Anda man volcanic arcis intriguingly split by the And ama n back-arc spread-ing ridge which has remained active since at least11 m.y.b.p. At the location of the present gravityprofile, a low density zone of near ly 60 km width un-derlies the volcanic axis within the overriding plate.Farther east, crustal transition presumably occurs be-low the Mergui Terrace at the Malayan coast wherethe crust is about 19 km thick with a gentle Moho-dipeastward.

AcknowledgmentGravi ty data used in this art icle were suppl ied by theDefense Mappi ng Agency , S t . Loui s , U .S .A . Thi sstudy was partly supp orted by a project grant No. 154of the Indi an Schoo l o f Mi nes .

ReferencesB ur ns , R . E . , 1964 , Sea B o t tom Hea t - F low M eas ur ement s i n t he

A n d a m a n S e a , J. Geophys . Res . 69, 4918-4919C hat t e r j ee , P . K . , 1967 , Geo logy o f t he M ain I s l ands o f t he An-

d a m a n A r e a , P r oe . Syr up. U pper Man t le P r o jec t , H y d e r a b a d ,India, Jan. 1967, 348-362.

Chr is tensen, N. I . and Sal isbury, M . H. , 1975, Structure an d Con -s t i t u t ion o f t he L owe r Ocean ic C r us t , R ev . G eophys. Space P hys .I3, 57-86.

C olema n , R . G. , 19 71 , P l a t e T ec ton ic E mplace ment o f Upp erM ant l e Per iodo t i t es Along C on t inen ta l E dges , J. Geophys . Res .76, 1212-1222.

C ur r ay , J . R . and M oor e , D . G . , 1971 , Gr ow th o f t he B enga lD e e p - S e a F a n a n d D e n u d a t i o n i n t h e H i m a l a y a s, G eol . Soc . A m .Bull. 82, 563-572.

C ur r ay , J . R . and M oor e , D . G. , 1974 , Sed imen ta r y and T ec ton icPr ocess es i n t he B enga l Deep- Sea Fa n and Geos ync l ine , in B ur k ,C. A. and Drake, C. L. (eds . ) , T h e G e o l o g y o f C o n t i n e n t a l M a r -gins , Spr inger -Ver t ag , New Yor k , pp . 617-627 .

C ur r ay , J . R . , M oor e , D . G. , L aw ver , L . A . , E mm el , F . J ., R a i t t , R .W. , Henr y , M . , and Kieckhef e r , R . , 1979 , T ec ton ics o f t he An-d a m a n S e a a n d B u r m a , A m . A s s oc . P e t r o l . G eo l . Mem oir 29 ,189 198.

C ur r ay , J . R . , E mmel , F . J . , M o or e , D . G . , and R a i t t , R . W. , 1982 ,S t r uc tu r e , T ec ton ics and Geo log ica l Hi s to r y o f t he Nor th -eas tern Indian Ocean, in Nairn, A.E.M. and Stehl i , F .G. (eds . ) ,T he O cean B as ins and Mar g ins , Plenum Pub l . C or p . 6 , pp .399-450.Deh l inger , P . , 1978, Mar ine G r av i ty , Elsevier , Amsterdam, pp.322.Dewey , J . F . and B i rd , J . M . , 1971, Or ig in and E mplacem ent o f t heOphio l i t e Su i t e : Appa lach ian Ophio l i t es i n Newf ound land , J .Geophys . Res . 76, 3179-3206.

E r menk o , N . A. and Sas t ri , V . V. , 1977 , On the Pe t r o l eum Geo lo gyo f A n d a m a n I sl a n ds , B ul l. O i l & Na tu r a l G as Com m n . Ind ia 14 ,1-13.E vans , P . and C r om pton , W . , 1946 , Geo log ica l Fac to r s i n Gr av i tyI n t e r p r e t a t ion by E v idence f r om I nd ia and B ur ma, Q. J. Geol.Soc . L ondon 102, 211-249.

F i t ch , T . J . , 1970 , E ar thqua ke M echan i s ms in the Himalaya n ,B ur mes e , and An dam an R eg ions and C on t inen ta l T ec ton ics i nC en t r a l As ia , J. Geophys . Res . 75, 2699 2709.

Gr ow , J . A . , 1973 , C r us t a l and Up per M ant l e S t r uc tu r e o f t heC en t r a l Aleu t i an Ar c , Geol. Soe. Am. Bull . 84, 2169-2192.

Gr ow , J . A . and B owin , C . O. , 1975 , E v idence f o r High- Dens i tyC r us t and M a nt l e B enea th the C h i le T r ench D ue to t he Des cend-ing L i thos pher e , J. Geophys . Res . 80, 1449-1458.

Hager , B . H . , O 'C onne l l , R . J ., and R aef s ky , A. , 1983 , Subduc t ion ,B ack- Ar c Spr ead ing and Globa l M ant l e F low, T ee tonophys . 99 ,165-189.Ham i l ton , E . L ., B achman , R . T . , C ur r ay , J . R . , and M o or e , D . G . ,1977, Sediment Veloci t ies f rom Sonobouys : Bengal Fan, SundaT r ench , Andaman B as in , and Nicobar Fan , J. Geophys . Res . 82 ,3003-3012.

Kah le , H . - G. an d T a lwan i , M . , 1973 , Gr av im et r i c I nd ian Oce anG e o i d , Z. Geophys . 39 , 162187 .Kar un akar an , C . , R ay , K. K . , and Saha , S . S ., 1968 , T er t i a r y Sed-i m e n t a t io n i n t h e A n d a m a n - N i c o b a r G e o s y n c li n e , J. Geol. Soe.Ind ia 9, 32-39.L uyendyk , B . P . and R enn ick , W . , 1977 , T ec ton ic Hi s to r y o f As e is -mic R idges in t he E as t e r n I nd ian Ocean , Geol. Soc. Am. Bull . 88,1347-1356.


14/14

210 MANOJ MUKHOPAI)HYAYMinear, J. H. and Toksoz, M. N., 1970, Thermal Regime of aDowngoing Slab and New Global Tectonics, J. Geophys. Res. 75,1397-1419.Moore, G. F. and Curray, J. R., 1980,Structure of he Sunda TrenchLower Slope off Sumatra from Multichannel Seismic ReflectionData, Marine Geophys . Res . 4, 319 340.Mukhopadhyay, M., 1984, Seismotectonicsof Subduction and Back-Arc Rifting under the Andaman Sea, Tectonophys. 108, 229-239.Naini, B. and Leyden, R., 1973, Ganges Cone: A Wide AngleSeismic Reflection and Refraction Study, J. Geophys . Res . 78,8711-8720.Peter, G., Weeks, L. A., and Burns, R. E., 1966, A ReconnaissanceGeophysical Survey in the Andaman Sea and Across the An-daman-Nicobar Island Arc, J. Geophys . Res . 71, 495-509.Rodolfo, K. S., 1969a, Bathymetry and Marine Geology of theAndaman Basin and Tectonic Implications for Southeast Asia,

Am. Assoc. Petrol. Geol. Bull. 52, 2422-2437.Rodolfo, K. S., 1969b, Sediments of the Andaman Basin, North-eastern Indian Ocean, Mar ine Geo l . 7, 371402.Roy, T. K., 1983, Geology and Hydrocarbon Prospects of An-daman-Nicobar Basin, Petrol. Asia J. Nov. 1983, 37-50.Sacks, I. S., Linde, A. T., Rodriguez, A. B., and Snoke, J. A., 1978,Shallow Seismicity in Subduction Zones, Geophys. Res. Lett. 5,901-903.

Silver, E. A., Joyodiwiryo, Y., and McCaffrey, R., 1978, GravityResults and Emplacement Geometry of the Sulawesi UltramaficBelt, Indonesia, Geology 6, 527-531.Talwani, M., Worzel, J. L., and Landisman, M., 1959, RapidGravity Computations for Two Dimensional Bodies with Appli-cation to the Mendocino Fracture Zone, J. Geophys. Res. 64,49-59.Uyeda, S. and Kanamori, H., 1979, Back-Arc Opening and theMode of Subduction, J. Geophys. Res. 84, 1049-1061.Watts, A. B. and Talwani, M., 1974, Gravity Anomalies Seawardof Deep-Sea T,'enches and Their Tectonic Implications, Geophys .Roya l Astron. Soc. J . 36, 57-90.Watts, A. B. and Daly, S. F., 1981, Long Wavelength Gravityand Topography Anomalies,Ann. Rev. Earth Planet. Sci. 9, 415-448.Weeks. L. A., Harbison, R. A., and Peter, G., 1967, Island ArcSystem in Andaman Sea, Am. Assoc. Petrol. Geol. Bull. 51, 1803-1815.Welland, M. J. P. and Mitchell, A. H. G., 1977, Emplacement ofOman Ophiolite: A Mechanism Related to Subduction and Col-lision, Geol . Soc. Am . Bul l . 88, 1081-1088.Yamashina, K., Shimazaki, K., and Kato, T., 1978, Aseismic Beltalong the Frontal Arc and Plate Subduction in Japan, J. Phys .E ar th 26, Suppl., $447-$458.

Date post:	09-Apr-2018
Category:	Documents
Upload:	sujit-dasgupta
View:	219 times
Download:	0 times

Gravity Anomalies and Deep Structure of the Andaman Arc

Documents