gravity field and deep structure of bengal fan

8/8/2019 Gravity Field and Deep Structure of Bengal Fan

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Tecronophysics, 186 (1991) 365-386Elsevier Science Publishers B.V., Amsterdam

365

Gravity field and deep structure of the Bengal Fan and itssurrounding continental margins, northeast Indian Ocean

Manoj Mukhopadhyay and M.R. KrishnaDepartm ent of Geophysics, Indian School of Mines, Dhanbad 826004, India

(Received October 23. 1989; revision accepted May 1, 1990)

ABSTRACT

Mukhopadhyay, M. and Krishna, M.R., 1991. Gravity field and deep structure of the Bengal Fan and its surroundingcontinental margins, northeast Indian Ocean. Tectonophysics, 186: 365-386.

A revised gravity anomaly map for the northeast Indian Ocean shows that the shelf edge underlying the eastern continentalmargin of India is a rather narrow but extensively linear gravity low (minimum free-air = -149 mGa1). The Bengal Fanseaward of the shelf has a depressed gravity field (average free-air = - 20 to - 30 mGa1) in spite of the enormous thickness ofsediments of as much as lo-15 km. The two buried ridges below the Bengal Fan-the 85 East and 90 o East Ridges-have alarge negative (- 75 mgal) and a substantial positive (40 mGa1) free-air anomaly, respectively. The Andaman and Burmesearcs lying along the east margin of the Bengal Fan are active subduction areas which have typical bipolar gravity signatureswith a maximum amplitude of 300 mGa1. Gravity interpretation for three regional traverses across the central and northernparts of the Bengal Fan and their surrounding continental margins suggests that a thickened oceanic crustal wedge juxtaposesthe transitional crust under the eastern continental slope of India; the 85 East Ridge, that was created when the IndianOcean lithosphere was very juvenile, appears to underlie a nearly 10 km thick and 120 km wide oceanic crustal blockconsisting of the ridge material embedded in the upper lithosphere; while the 90 o East Ridge submarine topography/buriedload below the Bengal Fan is probably isostatically compensated by a low-density mass acting as a cushion at the base of thecrust. The Bengal Fan crust, with its thick sediment layer, is carried down the Andaman subduction zone to a depth of about27 km where, possibly, phase transition takes place under higher pressure. The maximum sediment thickness at theAndaman-Burmese subduction zone is of the order of lo-12 km. The gravity model predicts a low density zone about 60 kmwide below the Andaman-Burmese volcanic arc, penetrating from crustal to subcrustal depths in the overriding Burma plate.A more complex density distribution is however, envisaged for the Andaman volcanic arc that is split by the Neogene back arcspreading ridge. The ocean-continent crustal transition possibly occurs farther east of the volcanic arc; below the Shanplateau margin in Burma or below the Mergui terrace at the Malayan continental margin east of the Andaman Sea.

Introduction

The Bengal basin in northeast India, togetherwith its offshore continuation, the Bengal Fan, inthe northeast Indian Ocean constitute the Bengalgeosyncline that evolved in the Mesozoic-Ceno-zoic. Geologically this is a fascinating region be-cause it contains up to 15 km of sediment. Thearea of the geosyncline shrunk with time in conse-quence of the eastward subduction of the Indianplate below the Andaman-Burmese arcs (Fig. 1).Along the eastern margin of the geosyncline, asmaller plate, the Burma plate, exists between the

underthrusting Indian plate and the overridingAsian plate at the location of the Andamar-Burmese arcs (Curray et al., 1979). The origin ofthe Burma plate relates to Neogene back-arcspreading in the Andaman Sea; the back-arcspreading ridge connects northward to the Sagaingtransform in Burma. The Andaman-Burmese arcsystem serves as an important transitional tectoniclink between the Himalayan collision zone to thenorth and a major island-arc trench system ofsouth Asia, the Sunda arc. Subduction of theIndian plate below the Andaman-Burmese arcsgenerally extends to a depth of 200-220 km

0040-1951/91/$03.50 Q 1991 - Elsevier Science Publishers B.V.


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366 M. MUKHOPADHYAY AND M.R. KRISHNA

I ~. Hinge zane t Thrust!& Margin of lndion Shield I&.& Back-arc spreading ridgec-_=l Edge of continental crust1

-

Fig. 1. Tectonic map for the Bengal Fan and the Andaman arc in the northeastern Indian Ocean and the adjacent continental marginin Burma and Malaya to the east and the Indian continental margin to the west. The most significant tectonic features below theBengal Fan are the 85 East and 90 East Ridges which have infhrenced the s~imentation history of the Fan since the Cretaceous.The location of the Eocene hinge zone, subparallel to the east coast of India, is inferred on the basis of seismic surveys in severalareas of the Bengal basin as well as offshore. AA, BB and CC are the interpreted gravity profiles. Two seismic sections (indicatedby bars) across the Andaman and Burmese arcs are located close to the gravity profiles. S.M. = Shillong massif; E.S = Bogra slope;E. F = Bengal foredeep; B. H. = Barishal high; S.T= Sylhet trough; H.T = Hatya trough; E.B.T. = Eastern Boundary thrust in

Burma; D. F. = Dauki fault; Mdr. = Madras: Wul. = Waltair; Bbr. = Bhubaneswar; Cnl. = Calcutta.

(Mukhopadhyay, 1984); however, under the Sundaarc farther south the subduction penetrates deeper(Hamilton, 1979).

Several stages are considered significant in thegeological history of the Bengal geosyncline sinceits formation through the rifting of easternGondwanaland. These are:

(a) The formation and evolution of the rifted,young, passive continental margins and the oceanbasin between them.

(b) Sea-floor spreading in the east to northeastIndian Ocean and closing of the Tethys.

(c) Collision of India with Asia, uplift anderosion of the Himalayas in the Tertiary-

Quaternary, transport of the eroded material bythe confluent Ganges and Bra~aputra rivers,and deposition of the sediment in the newly formedBengal geosyncline.(d) Deformation and subduction of this sedi-ment wedge below the Andaman-Burmese arcs,and formation of the successor basins (Curray andMoore, 1971; Johnson et al., 1976; Curray et al.,1982).

It is interesting to note that due to the con-tinued spreading of the Indian Ocean floor, stages(c) as well as (d) are still in operation. This dy-namic process has led to the formation of theBengal and Nicobar Fans in the northern part of


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GRAVITY FIELD AND DEEP STRUCTURE OF THE BENGAL FAN 367

the northeast Indian Ocean. Together these formthe largest deep-sea fan complex in the world,extending for some 3000 km from the Indiancontinental margin (Fig. 1).

Since the days of the Indian Ocean Expedition(Laughton et al., 1970) several structural andtectonic studies have been made of the BengalFan, and also of the 85 East Ridge and NinetyEast Ridge (NER) which underlie the Fan. It isbelieved that both these ridges are probably ofhot-spot origin; the former was created when theIndian Ocean lithosphere was young (10 iz 5 m.y.)and hot and flexed readily under the load of theridge. Most of the ridge was buried when theIndian Ocean hthosphere is 40-80 m.y. old (Liu etal., 1982). In contrast, the hot-spot that createdthe latter ridge was located under the Kerguelen-Heard Plateau in the southern Indian Ocean atthis time (Curray et al., 1982). The 85 o E Ridge iscompletely buried under the Bengal Fan, and couldonly be partly mapped by seismic surveys. How-ever 4800 km of the NER is exposed on the IndianOcean floor-only the 700 km long northernmostsegment is buried below the Bengal Fan, where

the ridge plunges northwards below the Fan sedi-ments. Due partly to the oblique subduction ofthe Indian plate at the Andaman-Burmese sub-duction zone, the surface area of the Bengal geo-syncline diminishes from south to north; the in-tensity of tectonic deformation also varies signifi-cantly along the Andaman-Burmese arcs. Mitchelland McKerrow (1975) suggested that the collisionprocess and mountain building started in thenorthern part of the Burmese arc (the Naga Hills),while active subduction of the Indian plate stilldominates the rest of the arc. Seismic activityresulting from subduction is however, highly sub-dued in coastal Burma and the north Andamanregion; it is possible that subduction of the Indianplate occurs here aseismically (Le Dain et al.,1984), or that this is a fossil plate boundary. Thesubduction zone geometry in the central Andamanarc area is apparently greatly affected by partialsubduction of the NER as observed on seismicdata (Curray et al., 1982). It is therefore expectedthat the subduction of the Indian plate below theAndaman-Burmese arcs, and the modifications tothe subduction zone geometry have produced con-

PrecambrianFold trend

Fig. 2. Continuity of Precambrian shield edge and fold trend from east to west Gondwanaland as shown by a plate reconstruction(redrawn after Powell et al., 1980). Also shown are the out l ines of the Deccan trap and the Eastern Continental Margin of India in

respect to the Antarctic peninsula.


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I h I.3I I 1


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GRAVITYFIELDAND DEEPSTRUCTUREOFTHEBENGALFAN

siderable mass anomalies at depth that are re-flected in the gravity field of the region (see be-low).

In this paper we give an inte~retation of thegravity along three regional traverses extendingfrom the rifted Indian continental margin to theAndaman-Burmese arcs and from the AndamanSea across the Bengal Fan. We use the availablegeological information and published seismic datato constrain the gravity interpretation. Our mainobjectives are to supplement the present in-ferences on the processes of crustal rifting andlithospheric stretching under the east coast con-tinental margin of India (ECMI), to provide infor-mation on the significant mass anomalies presentbelow the Bengal Fan, its underlying ridges, andthe Andaman-Burmese arcs, and to discuss theregional tectonics.Regional geologic setting and its evolution

The major geological features underlying theECMI and the Bay of Bengal are inherited fromthe break-up of eastern Gondwanaland and subse-quent spreading of the Indian Ocean floor. Vari-ous authors have used the magnetic anomalies,DSDP data, and the palaeomagnetic results tostudy its spreading history (for example, Heezenand Tharp, 1964; Le Pichon and Heirtzler, 1968;McKenzie and Sclater, 1971; Bowin, 1973; VonDer Borch et al., 1974; Johnson et al., 1976;Luyendyk and Rem&k, 1977; Curray et al., 1979,1982; Powell et al., 1980). Most authors agree thatspreading in the eastern Indian Ocean was ini-tiated during the Early Cretaceous (about 130 m-y.

369

B.P.) as India separated from combined Australiaand Antarctica (Fig. 2). The east coast bight ofIndia was possibly facing the protruberance ofEnderby Land of Antarctica. The eventual sep-aration occurred approximately parallel to theECMI that now trends NE-SW, which is in linewith the hinge zone or the edge of the continentalcrust extending from the Bengal delta into theoffshore region (Curray et al., 1982) (Fig. 1). Fourmajor spreading stages are envisaged for the crea-tion of the eastern Indian Ocean as a result ofmovement of the Indian, Australian and Antarcti-ca plates (130-80, 80-53, 53-32 and 32-O m.y.B.P.) (Johnson et al., 1976). Curray et al. (1982)however, invoke a five stage scenario: 130-105,105-90, 90-53, 53-32 and 32-O m.y. B.P. Thespreading history for the northern part of theeastern Indian Ocean is largely dependent on theidentification of magnetic anomalies off westernAustralia. So far there is very little control fromthe Bay of Bengal because no definite identifica-tion of spreading-type anomalies has yet beenpossible. Nevertheless, Johnson et al. (1976) pos-tulate that the oldest magnetic anomalies in thispart of the Indian Ocean are aligned subparallel tothe east coast of India, which also approximatelycorrespond to the 2 km isobath along the westernmargin of the Bengal Fan. Geophysical surveys bythe Oil and Natural Gas Commission (India) overthe five east coast basins and their offshore exten-sions suggest that their underlying basement istypically do~nated by an alternating set of NE-SW trending ridges and depressions (Sastri et al.,1973; Talukdar, 1982) (Fig. 3). This pattern iscommon for all the five east coast basins-the

Fig. 3. Basic geophysical evidence for the basement configuration below the east coast basins of India: (a) the basin locations:A-Cauvery; B-Palar; C-Krishna-Godavari; D-Mahanadi; E-Bengal basin. Their underlying basement configuration isillustrated in the subsequent figures; sediment thickness contours in km (source: Tectonic Map of India, O.N.G.C., 1968). Notice thatthe basin geometry is typically dominated by an alternating pattern of subsurface ridges and depressions bounded by basement faultsoriented NE-SW; all of the east coast basins open seaward. Rock stratigraphy for all five basins is synthesized in Table 1.Geographic names for the individual structural elements of the basins are: (b) 1 = Ramnad-Palk strait depression; 2 = Devakkottai-Mannargudi ridge; 3 = Thanjavur depression; 4 = Thirupundi-Vedaraniyam high; 5 = Tirutturaipundi-Nagapattinam depression;6 = Tranquebar depression; 7 = Kumbakonam-Shiyali ridge; 8 = Ariyalur-Pondicherry depression. (c) I= Varadayyapalaiyam-Durgarajupatnam ridge; 2 = Pulicat depression. (d) I = East Godavari depression; 2 = Bhimavaram ridge; 3 = West Godavar+depression; 4 = Bapatla Ridge; 5 = Krishna depression. (e) 1 = Puri depression; 2 = Bhubaneswar ridge; 3 = Paradeep depression;4 = Rajnagar ridge; 5 = Cuttack-Chandbali depression; 6 = shallow basement area. (f) Sediment isopach map for the Bengal basin

(after Evans, 1964) showing a monoclinal increase in sediment thickness towards deeper parts of the basin,


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370 M. MUKHOP ADHYAY AND M.R. KRISH NA

TABLE 1Stratigraphy for the five East Coast Basins of India (Source: Evans, 1964, Sastri et al., 1973, Jagannathan et al., 1983)Age

Holocene

Cauvery Basin

Alluvial sandsand clays

Palar Basin

Brown sands andriver alluvium (40)sandstone (15)

Krishna-GodavariBasinAlluvial sands,

Mahanadi Basin Bengal Basin

Reddish brown sandclays andkankar (25)

and clay (460)

Pleisto-cene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

LateCretaceous

Ferruginousgrits (95)

UnconformitySandstone, cong-lomerates lime-stone and coal(Cuddalore sand-stone, 300)

UnconformitySandstone, claystone, limestone(1190)UnconjormityClaystone, shale,sandstone andlimestone (340)

Shale, claystoneand limestone(1150)

Unconformity UnconformityClaystone, shale, Coarse grits,limestone calcareous(Pondicherry and sandstoneNiniyur, 700) (Intratrappeanbeds, 70)Unconformity Unconformity Unconformity ihconformitySandstone, clay-stone and lime-stone (UpperAriyalur, 730)Clayey sandstone,conglomerates,limestone(Lower Ariyalur)

Laterites andlateriticconglomerates (15)

Conglomeratic andclayey sandstone(Cuddalore sand-stone, 15)

Black claystone(400)

UnconformityCalcareous, gypsicclays (200)

Unconsolida-ted sandsand grits

Ferrugenous sand-stone grits andconglomerates(Rajahmundry sand-stone. 720)

Claystone andsandstone (110)

Argillaceous shalesand claystones(Debagram, 1445)

Claystone,siltstone andsandstoneUnconformity

Shale, siltstoneand claystone(Pandua, 340)

Siltstone(Memari, 80)

UnconformityLimestone. Deccantrap volcanics(Inte~rappeanbeds, 130)

Limestoneandclaystone

Claystone

Shale and compactlimestone(Sylhet, 342)

Sandstone withclay and shale(Jalangi, 721)

Calcareous shales,limestone(Bolpur, 125)


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GRAVITY FIELD AND DEEP STRUCTURE OF THE BENGAL FAN

TABLE 1 (continued)

371

Age Cauvery Basin Palar Basin Krishna-Godavari Mahanadi Basin Bengal BasinBasin

Early Limestone, black Conglomerates Clayey and Lateri- Basalt with fewCretaceous shale (Dalmiapuram) (Satyavedu tized sandstones Intertrappean

beds, 2000) (Tirupati sandstone, layersLimestone 830) (Rajmahal Traps,(Sriperumbudur Shales and clays 780)beds, 600) (Raghavapuram and

Vemavaram shales, 160)

Sandstone andlimestone(Trichnopally)UnconformifyClaystone, limestoneand shale (Uttatur)Unconformity

Jurassic Sandstone(Sivaganga beds,1090)

Unconformity UnconformityGrits, sandstone withclaystone(Gollapalli and Budavadasandstone, 200)

Permian

Carboni-ferousArchean

Unconformity

Granite Gneiss andother metamorphicrocks

UnconformityBoulder beds and Sands, shales andshales coal (Lower Gondwa-

na, 833)-Nonconformity-Igneous and Khondalites, schists Metamorphic andmetamorphic chamockites and local granitic andbasement pegmatites doleritic intru-

sions (Precambrian)Note: Names and numbers in parenthesis indicate the name of the local formation and its thickness in meters.

Cauvery, Palar, Krishna-Godavari, Mahanadi andBengal basins. The average sediment thickness inthe first four basins is about 3 km, while for theBengal basin it increases to as much as 13 km indeeper parts of the basin toward the Burmese arc(Evans, 1964; Talukdar, 1982; Baishya et al., 1986).A deep sedimentary trough of Gondwana age(Permo-Carboniferous) is commonly developed atthe Indian shield margin; the Gondwana sedi-ments are covered by the Tertiary sediments andthe coastal basalt traps. The shield marginGondwana sediments are nearly 3 km thick in

western parts of the Bengal basin, where they arepreserved in deep troughs covered by the EarlyCretaceous Rajmahal volcanics underlying theTertiary rocks (Sengupta, 1966; Choudhury andDatta, 1973; Mukhopadhyay et al., 1986). Stratig-raphy for the five east coast basins (Table 1)demonstrates that an average of 2 km of pre-Ter-tiary sediments, intertrap beds, basaltic flows andother volcanics are generally present in thesebasins. These characteristic basinal features underthe ECMI (both on land and offshore) apparentlyreflect the thermal and mechanical processes asso-


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ciated with crustal rifting following the easternGondwanaland break-up.Bathymetric pattern in the north par t of the BengalFan

Prominent morphological features under theECMI as well as the northeast Indian Ocean in-clude the ECMI, Bengal Fan, Andaman arc trench,and the Andaman Sea. They are briefly describedbelow.

The ECMI is a rather wide continental shelfwhose width gradually lessens from north to south-off the Madras coast it is possibly at its narrow-est (Fig. 4)-while the base of the slope deepensfrom north to south (also see, Rao and Rao, 1985).The Bengal Fan underlying the northeast IndianOcean has a smooth southward slope, where waterdepth varies from 200 m in the north part of theFan to more than 3500 m near the Ten DegreeChannel at about lOoN latitude (Curray andMoore, 1971). Sediment thickness in the Fan isenormous; it ranges from 5 to 15 km (Curray etal., 1982). Sediment generally thickens towards thenortheast of the Fan, in the vicinity of the GangesCone. The oldest dated sediments in the Fan areCampanian cherts underlain by unfossiliferouschert and chalk (Von der Borch and Sclater et al.,1974).

The most outstanding morphological feature inthe eastern Indian Ocean is the NER; the ridgeforms the eastern limit of the Bengal Fan againstthe Andaman arc where it separates the Bengaland Nicobar Fans (Figs. 1 and 5). To the north,the ridge is exposed to about 10 o N; beyond thatthe ridge plunges beneath the Bengal Fan sedi-ments. Seismic profiles have traced the ridge toabout 17 N where it is buried below the Fansediments, and it also impinges eastward on theAndaman arc (Curray et al., 1982).

The broad morphological features of the Anda-man arc include the Andaman-Nicobar sedimen-tary islands forming the fore-arc (popularly called,the Andaman-Nicobar Ridge, ANR) stretchingfor nearly 1100 km between the Burmese coastand Sumatra; a sharp bathymetric depression ofabout 2 km relief forming the Nicobar Deep eastof the ANR; and the Andaman volcanic arc farther

east forming a wider topographic high consistingof volcanoes and seamounts (Rodolfo, 1969).

The Andaman Sea, lying between the Andamanarc and the Malayan margin, is mostly delineatedby a 2 km N-S trending isobath which runs alongits entire 750 km length. The Mergui Terraceforms its eastern limit against the Malayan con-tinental margin. The Andaman back-arc spreadingcentre is a rather narrow bathymetric depressionin the middle of the Andaman Sea where, locally,the water depth increases to about 3 km.

Figure 4 illustrates the two contrasting bathy-metric trends which dominate the floor of thenortheast Indian Ocean-a north-south trend de-marcates the NER and the Andaman Sea, whilean east-west trend is common for the Bengal Fan.The latter has a gentle bathymetric slope south-ward, of the order of 1.6 m/km. This is in markedcontrast with an east-west bathymetric gradientof as much as 22 m/km across the NER in theequatorial region (Stein and Okal, 1978). The re-lief of the ridge in relation to the adjacent undis-turbed Indian Ocean floor at about 3 N is inexcess of 2 km; but farther north the ridge topog-raphy becomes gradually less distinct because ofits northward plunge beneath the Bengal Fan.Gravity field over the Bengal Fan and its adjacentcontinental margins

The gravity field in the Indian Ocean has beenmeasured using both submarine pendulum dataand surface ship measurements (see Le Pichonand Talwani, 1969 for a summary of the earlysurveys). Over the past two decades several authorshave prepared gravity anomaly maps for thenortheastern Indian Ocean (Peter et al., 1966;Bowin, 1973; Kahle and Talwani, 1973; Kahle etal., 1981; Kieckhefer et al., 1981; Watts and Daly,1981; Liu et al., 1982; Jaggannathan et al., 1983;Rao and Rao, 1985, 1986; Mukhopadhyay, 1988).Here we give a revised free-air anomaly map fornortheastern Indian Ocean and the Andaman Seabetween India and Malaya (data source: US De-fence Mapping Agency) (Fig. 5). The Bougueranomalies for the immediate onshore regions ofIndia and Burma surrounding the Bengal Fan arealso included on the figure so that the gravity


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374 M. MUKHOP ADHYAY AND M.R KRISH NA


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anomaly pattern can be followed for some dis-tance within the continental interior (source: Evansand Crompton, 1946; Gulatee, 1956; N.G.R.I.,1977). Gravity anomaly contours for the NER andthe Andaman arc, shown on the figure, are adoptedfrom Peter et al. (1966) and Mukhopadhyay (1988).

Generally, north-striking gravity contours pre-vail over the NER, Andaman arc and ECMI. TheBengal Fan has a depressed gravity field where thefree-air anomalies range from - 20 to - 30 mGa1.The most prominent gravity signature for theECMI is a gravity high over the inner shelf thatchanges to a more conspicuous gravity low overthe shelf edge across a zone of sharp gravitygradients. The shelf edge gravity minimum (aver-age F.A. = - 100 mGa1, lowest F.A. is - 149mGa1) is traceable along the east coast of Indiafrom offshore of Hambantota in south Sri Lankato the latitude of Puri (about 20 N in theMahanadi offshore basin, over a strike lengthwhich exceeds 1700 km. Lack of gravity coveragein areas farther north, in particular in the GangesCone, however, inhibits tracing the continuity ofthe ECMI gravity minimum in the offshore Bengalbasin. Available data indicate that the inner shelfgravity high that is traceable from offshore Co-lombo, through east Sri Lanka, to offshore Contaialong the ECMI (the gravity high shows a maxi-mum amplitude of 60 mGa1) is also present in theBurmese shelf between Akyab and Ramree Island.To follow the continuity of gravity anomalies be-tween the Indian and Burmese continentalmargins, however, detailed gravity coverage is es-sential in the 500 X 200 km2 data gap region in theoffshore Bengal basin. Note that these two funda-mentally different margins are closest here (alsosee Fig. 1).

Significant departures from the otherwise sub-dued gravity field for the Bengal Fan are seen forthe 85 East Ridge, which shows negative free-air

3 1 5

anomalies up to - 75 mGa1, and also in the NERwhere a clear gravity high of about 40 mGa1extends in north-south direction. Liu et al. (1982)consider that the 85 East Ridge gravity lowresults from excess crustal thickness, but the posi-tive free-air anomalies associated with NER areknown to be small relative to the ridge topogra-phy. Hence for isostatic compensation, the NERrequires a low density mass to support its uplift.The compensating mass may be gabbro orserpentinized peridotite lower in density than thenormal mantle rocks at equivalent depths (Bowin,1973) (see below). A zone of steep gravity gradi-ents outlines the east margin of the NER againstthe Andaman arc; the arc is associated with abipolar gravity signature with an average ampli-tude of 180 mGa1 (maximum amplitude increasesto 300 mGa1 near the Invisible Bank) in the northAndaman Sea (Fig. 5). A strong gravity minimumcoincides with the Andaman trench, whereas acomparable gravity high outlines the volcanic arcin the overriding Burma plate. The axis of thenegative anomaly changes its position from thetrench axis in south Andaman to the inner wall ofthe trench or even farther east to follow theNicobar Deep in the central area of the Andamanarc. The axis of the gravity minimum, however, isseen to follow the trench again in the northernpart of the arc (north of 15 N). A characteristicbipolar gravity field is also observed for theBurmese arc where the Bouguer anomaly ampli-tude is about 150 mGa1 (Mukhopadhyay and Das-gupta, 1988). The Burmese arc gravity minimumoutlines the Central Belt molasse basin lying tothe east of the Burmese Fold Mountain Belt(F.M.B. in Fig. 5), and the zone of high gravitycoincides with the volcanic arc. The positive grav-ity field for the volcanic arc descends eastward toa large and broader negative Bouguer anomalywith an amplitude of up to -100 mGa1 over the

Fig. 5. Revised gravity anomaly map for the northeast Indian Ocean and its immediate land areas in India and Burma (data sourcesare cited in text). Notice the characteristic gravity anomaly for the eastern continental margin of India (traceable for more than 1700km) and the bipolar gravity field for the Burmese and Andaman arcs (traceable for at least 1600 km). Three gravity traverses, AAthrough CC, whose locations are shown in Fig. 1, follow the ships tracks in offshore areas at about 13, 17O, and 20 N latitudes.All gravity station locations for offshore areas are not shown for better clarity of the map. F.M.B. = Fold Mountain Belt of Burma;

EB. T. = Eastern Boundary Thrust of the F.M.B. against the Burmese plains.


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7.5 Observed P- Velocl ty In Bengal fan7. 0 o - ProfIle AAll?N IA - Profile BB(17N 1

6.0

Observed P-Velocity I Bengol fan7.0 % - Southern part

x - Central portCl - Northern part IE

6. 0c:i

SoftSediments

1.0 1.5 2.0 25 30Density , g /cm3

Fig. 6. Plots showing the seismic velocity-density relationships for the Bengal Fan crust as inferred from the mean Nafe-Drakecurve. (a) Data reported by Curray et al. (1982) for two east-west seismic profiles across the Bengal Fan at about 13 and 17O Nlatitudes. (b) Data reported by Naini and Leyden (1973) for several spot seismic surveys in the Bengal Fan. For the Bengal Fan thesoft sediments are mostly defined by seismic velocities of less than about 2.6 km/sac; this is indicated by deviation of the meanNafe-Drake curve from its linear character for higher seismic velocities (shown by a dashed line on both curves). Inferred mean rock

density from the observed seismic velocity values for various crustal layers are discussed in the text.

Shan plateau across the Sagaing transform and theShan scarp (also see Fig. 1). According to Currayet al. (1979), the Sagaing transform defines theeastern margin of the Burma plate over a distancefrom north to south of 1000 km, and its serves asan important tectonic link between the Himalayancollision front and the Andaman back-arc spread-ing ridge. The Sagaing transform is believed tojoin the back-arc ridge through a short, but signifi-cant, seismically active offset fault in coastalBurma (Mukhopadhyay, 1984).Density values

In order to estimate densities for various litho-logical units underlying the Bengal Fan, we usethe seismic velocities mapped for different geo-logical horizons along two seismic reflection pro-files at about 13 and 17 o N over the Bengal Fan

(after Curray et al., 1982) where our gravitytraverses AA and BB are located (Fig. 1). Ad-ditional data on seismic velocity distribution inthe Bengal Fan crust are taken from several spotseismic surveys carried out by Naini and Leyden(1973) in the northern to southern parts of theBengal Fan. In Fig. 6 we have plotted the ob-served seismic velocity values on the mean Nafe-Drake curve defining the velocity-density rela-tionship (Nafe and Drake, 1963); the details ofseismic velocity distribution for the individualhorizons are illustrated in Figs. 7-9. These figuresshow that the compressional wave velocity in theBengal Fan sediments increases from 1.6 to 5.7km/s with depth and the velocities are as high as6.X-7.0 km/s in the deeper sediments or in thetop part of the oceanic igneous crust. The ob-served P wave velocities for the two long seismicprofiles referred to above are plotted in the mean


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Asthenosphere,3.35


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- Observedooooo Computed

Fig. 8. Gravity profile BB (location Figs. 1 and 5) and its interpretation in terms of crustal mass anomalies. All notations are thesame as in Fig. 7. At its eastern end the profile traverses south Burma where the Indian plate has a fossil boundary against the Burmaplate; An alternative interpretation is that subduction of the Indian plate is occurring aseismically, but this is not supported by thegravity model. Note that the gravity model predicts: (1) maximum sediment thickness off the Burmese coast where a relatively denseaccretionary wedge (shown by oblique dashes) is inferred; (2) a compensatory mass at the crustal base to support the excess mass ofthe 90 East Ridge (that acts as a positive subsurface load on the lithosphere); and (3) a thicker oceanic crustal wedge underlying theE.C.M.Z. and its adjacent abyssal plains. G.V. = Godavari Valley; E.G. = Eastern Ghats; F.M.B = Fold Mountain Belt in Burma;

W.T. and E. T. = Western and Eastern Troughs in Burma, respectively.

Nafe-Drake curve in Fig. 6a, while Fig. 6b is acorresponding plot using the spot refraction datafrom the Bengal Fan.

We infer a three-tier density distribution for theunderlying crust based on the correlation of theobserved seismic velocities with the mean Nafe-Drake curve (in particular for the linear segmentof the curve in the velocity range 2.7 to 7.4 km/s)and on the stratigraphic interpretation of theseismic horizons for the profiles across the BengalFan given by Curray et al. (1982). This three-tierstructure consists of a layer of semi-consolidatedsediment at shallow levels, with velocities of 2.7-4.8 km/s (with a mean estimated density of 2.4g/cm3), a deep sediment layer with velocities of4.8-5.7 km/s (with a mean estimated density of2.6 g/cm3), and oceanic igneous crust, havingvelocities exceeding 5.8 km/s (the highest ob-served velocity for the igneous crust is 7.5 km/s)with an inferred density of 2.9 g/cm3. This three-layer density structure is considered to be repre-sentative of the Bengal Fan crust, and is compara-

ble to an average oceanic crustal model (for layers2 and 3) (Christensen and Salisbury, 1975). Theypropose that layers 2 and 3 have thickness of1.39 f 0.50 and 4.97 f 1.25 km, and compres-sional wave velocities of 5.04 f 0.69 and 6.73 f0.19 km/s, respectively. The sediment thicknessunderlying the Bengal Fan is, however, highlyvariable; it varies from 5 to 15 km (Curray et al.,1982). Moho underlying the Fan has not yet beenmapped by the seismic method. These factors im-ply that the simplified density distribution in-ferred above for the Bengal Fan crust is a grossgeneralization of the actual situation. We notethat Christensen and Salisbury (1975) concludedthat there is little change in thickness for oceaniccrustal layers 2 and 3 if the ocean basin is olderthan 40 m.y. As the crust underlying the north-eastern Indian Ocean near the NER is much olderthan this (Luyendyk and Rennick, 1977; for mag-netic anomaly ages and DSDP results in thenorthern Indian Ocean), we believe that the sim-plified density model for the Bengal Fan crust


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0 2 0 0 f t o o 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 Km

1Qoo~:;.,,,,~,~;;;:,;~,.~:j:, , ,As t h e n o s p h e r e , 3 . 3 5

8 0 r LFig. 9 . Gravity profile CC (location, Figs. 1 and 5) and its interpretation in terms of crustal and deeper lithospheric mass anomaliesin a subduction zone model. The undert~sting Indian plate acts as a zone of mass excess below the Burmese arc, whose maximumgravity effect is about 12 mGaf at the location of the profile (see inset). All notations are the same as in Fig. 7. Notice that the Indiancontinental margin is widest under this profile and it is also closer to the subduction zone than the other two profiles. A denseraccretionary wedge (shown by oblique dashes) on the overriding Burma plate is inferred for the SW continental margin of Burma.

S.S. = Shan-Sagaing fault at the west margin of the Shan plateau in Burma.

considered here for gravity interpretation pur-poses may not be grossly different from the truesituation.

Hamilton et al. (1977) have reported the resultsof sonobuoy surveys for different geoacoustic pro-vinces of the Bengal Fan, in particular for theuppermost sediment layer (soft sediments of thesea bed). Different regression coefficients relatingthe mean compressional wave velocity as a func-tion of one-way travel time within the sedimentsfor different parts of the Fan were obtained. Thesevelocity values are in the range of 1.5-2.3 km/sfor the central part of the Fan through which oneof the present gravity lines traverses. The seismicvelocities corresponding to the first sediment layerof the Bengal Fan as observed for two profileslocated approximately along 13 N and 17 o Nlatitudes (after Curray et al., 1982) and thosegiven by Naini and Leyden (1973) are also in this

velocity range (values less than 2.7 km/s). Notethat this velocity range corresponds to the lowerpart of the mean Nafe-Drake curve (nonlinearsegment of the curve in Fig. 6). We attribute thevelocities to less compact and soft sediments ofthe sea bed whose thickness is presumably smallin comparison to the total thickness for the sedi-ments underlying the Fan. Hence we have notattempted to identify a mean density for the softsediments whose lithofacies also varies across theFan; rather we ascribe a mean density of 2.4g/cm3 for all of the shallower sediments below theFan. Densities were measured for a few dozenrock samples cored from the drill holes in theAndaman sedimentary arc (both in its eastern andwestern shelves) (samples obtained through thecourtesy of the Oil and Natural Gas Commission,India). These density data (Table 2) also suggestthat the average density for the sediments is of the


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TABLE 2Rock stratigraphy and lithology encountered in the Andaman wells (courtesy O.N.G.C., India) and the measured rock densities. Theinferred rock density from seismic velocities using the Nafe-Drake curve is also included here for correlationAge Gross lithology Number of Measured density

samples range (g/cm3)Inferred densityrange from seismicdata (g/cm3)

Miocene-Plioceneandyounger

Mid- UpperMiocene

Calcareous siltstoneand coarse fragmentsof clayPredominantly calcareouselasticPredominantly claystone,ash beds are present

3 1.90-1.99 1.9-2.20

6 2.20-2.30

3 1.90-1.93

7?Lower MioceneUpper Cretace-0s

Calcareous siltstoneSandy siltstoneClaystone and shaleClaystone, igneous andvolcanic materialmixture

3 2.35-2.546 2.42-2.559 2.51-2.645 2.52-2.67

Cretaceous

?

Altered basic intrusives,and extrusive igneous rocksBasic to ultrabasic volcanictuff

2 2.68-2.70

3 2.65-2.67

2.40

2.60

order of 2.40 g/cm3 to a depth of 2-3 km fromthe sea bed beneath the Andaman shelf.Gravity models

Figures 7-9 illustrate the broad gravity changesalong three regional traverses, AA through CC,acquired from the Indian and Malayan/Burmesecontinental margins across the Bengal Fan and theAndaman/Burmese arcs (Figs. 1 and 5 for traverselocations). The traverses use bathymetric andgravity point values along the ships tracks inoffshore areas (data source: US Defence MappingAgency), but they were extended onshore for some100 km or more in India and Burma in order todelineate broad gravity changes within the con-tinental interior away from the respective con-tinental margins. The gravity anomaly variationalong the three traverses is interpreted here interms of upper and lower crustal mass anomalies;seismic data from the Bengal Fan constrain theseinterpretations (see below). For interpretation pur-poses we first removed the regional gravity from

the observed anomalies using the GEM 10 gravityfield (Lerch et al., 1979); the amplitude of theregional gravity varies from 8 to 10 mGa1 for thetraverses. Two-dimensional computer-based grav-ity models given here for the three profiles werecomputed using the polygonal method of Talwaniet al. (1959). The computed values were matchedto the observed gravity anomalies within 2 mGa1.

Figures 7-9 show that the gravity anomaliesare largely negative, ranging from - 70 to - 80mGa1 over the cratonic areas of the Indian shieldand over the east coast basins underlying theprofiles. On approaching the ECMI, a relativegravity high over the inner shelf rapidly changesto a shelf edge gravity minimum (lowest free-airunder the profiles varies from - 50 to - 110mGa1). Farther oceanward, the free-air anomaliesrise to about - 30 mGa1 over the abyssal plains ofthe Bengal Fan. This gravity change from theIndian shield margin across the ECMI to theBengal Fan is typical of the crustal edge effectknown for passive continental margins (Dehlinger,1978)-a gravity high over the inner shelf and a


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narrow but conspicuous gravity low at the shelfedge, with a steep gravity gradient between them.The shape and magnitude of the shelf edge gravityminimum are, however, largely dependent on thelocal morphology of the continental slope and thesediment fill of the continental margin (see Schleeet al., 1979). The ECMI shelf edge gravity lowultimately changes to mild negative anomalies(F.A. = -20 to - 30 mGa1) oceanward over theBengal Fan. Figures 7-9 illustrate how this gravitychange occurs in different parts of the east coastunder the Palar basin (profile AA), the GodavariValley adjoining the Eastern Ghats (profile BB),and the Mahanadi Valley (profile CC) from southto north. For profile AA, the noted gravity changefor transitional crust occurs over a 60 km widepaleo- and present-day continental slope off theMadras coast where sediment thickness is nearly 4km (Sastri et al., 1973). Seismic data demonstratethat the sediment thickness increases to about 8km below the adjacent abyssal plains of the Ben-gal Fan (Curray et al., 1982). Seismic velocityvalues underlying both profiles AA and BB rangefrom 2.4-4.9 km/s in the sediment layer, while adistinct higher velocity of 6.-6.5 km/s is observedfor top part of the metamorphic crust under theECMI (Figs. 7 and 8). Detailed seismic control,however, is lacking for this critical region where acrustal transition is postulated. Velocity values inoceanic igneous crust below most parts of theBengal Fan are in the range 6.0-7.5 km/s andshow a gradual increase from shallow to deepercrust (a lone velocity value of 8.0 km/s at themargin of the 85 o E Ridge (Fig. 7) is taken here torepresent the mantle top), although the BengalFan Moho has not yet been mapped seismically.For gravity modelling we assumed a two-layermetamorphic crust at the Indian shield margin.This is supported by the available Deep SeismicSounding profile data across the Mahanadi andCuddapah basins (Kaila and Tewari, 1986). Usingthe Nafe-Drake velocity-density relationship, weinfer (Mukhopadhyay and Krishna, 1989) that theupper and lower crust at the margin of the Indianshield have average density values of 2.80 and 2.90g/cm3 repectively; this crust extends beneath theECMI across the east coast basins. We furtherassume that the continental crust is nearly 28 km

thick at the coastal region, and it thickens gradu-ally toward the shield interior. This crustal config-uration for the east coast agrees with that alreadyproposed by Subrahmanyam and Verma (1986) onthe basis of the regional gravity field and itscorrelation to elevation and surface rock densitiesfor the Eastern Ghats. The two-layer metamorphiccrust assumed for the east coast granulite terraincan only be a gross generalization of the crustalstructure in the absence of crustal seismic data.Smithson and Brown (1977) suggest that the lowercrustal rocks in normal continental areas aremostly metamorphics of approximate intermediatecomposition, rather than basaltic, gabbroic orcharnockitic rocks (also see Percival, 1989). Fromthis view point, the density value assigned to thelower crust seems generally appropriate. Gravityvariation across the ECMI is interpreted in Figs.7-9 in terms of a crustal transition between theIndian shield on the west and the Bengal Fan tothe east. The crustal mass anomalies depicted inthe models imply a wider transitional crust belowthe ECMI at the latitude of the Mahanadi basin(profile CC) as compared to that for more south-ern areas; the transition presumably occurs over adistance of about 300 km in the former area.Oceanic crust also clearly thickens in the samedirection; the gravity model infers a nearly 20 kmthick oceanic crust along the eastern edge of theMahanadi offshore basin, following the Eocenehinge zone proposed by Talukdar (1982) (Fig. 1).The thickened oceanic crustal wedge almost abutsthe transitional crust below the ECMI. This seemsto occur on a regional scale, exceeding 700 km innorth-south direction subparallel to the east coast.Some support for the thickned oceanic crustalwedge model comes from Raleigh- and Love wavedispersion studies (Brune and Singh, 1986). Thesestudies suggest a thicker than normal oceanic crustfor western parts of the Bengal Fan (Brune, 1989,pers. commun.). Deep refraction data are neces-sary however, for further refinement of the gravitymodel for this critical region. It is possible that thethick oceanic crustal wedge may have originatedthrough an initial stage of sea-floor spreadingvolcanism, analogous to what is proposed for otherpassive margins, such as the Norwegian andGreenland continental margins (e.g., Hinz, 1981;


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382 M. MUKHOPADHYAY AND M.R. KRI SHNA

Hinz et al., 1987) (also see, Talwani and Eldholm,1973; Foucher et al., 1982). By definition, it is alsothe oldest oceanic crust that now lies closest to thecontinental margin (see, among others, Schuep-bath and Vail, 1980) provided, that no westwardjumping of a ridge had occurred below the BengalFan at this location. On its landward side, thecontinental crust bears evidence of lithosphericstretching and cooling, producing subsiding basinson the surface. Future studies of magnetic anoma-lies in areas of the thickened oceanic crust underthe ECMI as proposed in our gravity modelscould prove interesting.

Buried beneath the sediments of the BengalFan is the 85 East Ridge, which was createdwhen the Indian Ocean lithosphere was veryyoung. The lithosphere flexed readily due to theload of the ridge (Liu et al., 1982) producingsignificant mass anomalies underlying the ridge.Evidence for this is a conspicuous gravity low(minimum F.A. = - 75 mGa1) oriented north-south coinciding with the buried ridge. The gravitylow is traceable from 6614N latitude (Fig. 5)but its continuity farther north is uncertain. Pro-file AA crosses the gravity low at about 14N,where the available seismic data (after Curray etal., 1982; Liu et al., 1982) define the top surface ofthe ridge and its flanks (Fig. 7). The gravity low isexplained by oceanic crustal materials forming theridge and its underlying root in the lithosphere;the density contrast between crust and sub-crust isassumed to be -0.5 g/cm3. The model shows thatthe anomalous crustal mass is about 10 km thickand 120 km wide, and is embedded in the upperlithosphere buried below about 6 km of BengalFan sediments.

The northernmost segment of the NER, whichplunges beneath the Bengal Fan sediments (seeabove), is traversed by both profiles AA and BB(Fig. 1); the ridge has a substantial positive grav-ity anomaly (around 40 mGa1) compared to theBengal Fan gravity field (Fig. 5). In order tointerpret the gravity high, we first remove the deepgravity effect due to the Indian Ocean lithospheresubducting below the Andaman arc. The Benioffzone geometry underlying the profile AA is de-fined by the teleseismic data for the period 1954-80 (source: I.S.S. and I.S.C. Bulletin). Along pro-

file BB crossing coastal Burma, no active subduc-tion of the Indian plate occurs. The Benioff zoneunder profile AA extends to a depth of about 180km. The descending lithospheric slab below 75 kmdepth along profile AA is considered to be a zoneof mass excess; following Grow (1973) we assumethat the lithosphere is denser than the surroundingasthenosphere by about 0.05 g/cm3. The com-puted gravity effect due to the descending litho-sphere (below 75 km depth) is greatest (about 32mGa1) over the deepest part of the Benioff zone atthe Andaman volcanic arc (inset in Fig. 7), anddecreases to about 7 mGa1 at the Andaman trench.The gravity effect is even less farther west over theNER. The observed free-air values are correctedfor the regional gravity as well as for the descend-ing lithospheric effect, and the remaining anoma-lies are interpreted (in Figs. 7 and 9) in terms ofsubduction of the Indian plate beneath the Anda-man-Burmese subduction zone. Our models forprofiles AA and BB show that the NER gravityhigh is probably caused by a compensatory massof inferred density 2.95 g/cm in the uppermostmantle. Although the proposed anomalous mass atthe base of the crust is conceptually similar to thatpreviously proposed by Bowin (1973) for portionsof the NER south of the equator, the presentmodel suggests that the anomalous mass has out-wardly dipping sides on a semi-regional scale,which act, possibly, as a cushion for isostaticcompensation at the crustal base. The gravity highof the NER is proportionately much less thanwould be otherwise required by its topography,which suggests isostatic compensation of the NERtopography. The proposed gravity models cannothowever, be used to determine whether the com-pensatory mass under the NER is a consequenceof isostatic root formation in an oceanic environ-ment, if it is a product of underplating of theupper mantle rocks at the base of the crust. Thepresent models are only partly constrained byseismic data from the northern parts of the NER(Figs. 7 and 8) where the anomalous mass notonly plunges below the Bengal Fan sediments butalso partially subducts eastwards beneath theAndaman subduction zone because of its obliqueorientation. Hence the NER gravity high observedon profiles AA and BB may also consist of


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GRAVYN FIELD AND DEEP STRUCTURE OF THE BENGAL FAN

contributions from crustal deformation in the formof an outer gravity high seaward of the Anda-man trench (also see Watts and Talwani, 1974).

The bipolar gravity signature for the Anda-man-Burmese subduction zone is observed in eachprofile, although the location of the gravityminimum with respect to the trench axis as well asthe amplitude and shape of the anomaly differalong the strike of the Andaman-Burmese arc.Such local gravity variations probably reflectchanges in the underlying mass distribution at thejuncture between the underthrusting Indian plateand the overriding Burma plate (Mukhopadhyay,1988; Mukhopadhyay and Dasgupta, 1988). Grav-ity models imply that the total sediment thicknessat the subduction zone is lo-12 km; underlyingigneous crust extends to a depth of about 27 kmaccording to the Benioff zone configurations (seeabove), where phase transition possibly takes placeunder higher pressure (also see Grow and Bowin,1975; Riddihough, 1979). For the southwest con-tinental margin of Burma, Paul and Lian (1975)reported comparable estimates of sediment thick-ness (Neogene-Upper Cretaceous) on the basis ofseismic reflection data. Between the trench andthe Burma coast, the presence of a local gravityhigh as traversed by the profile CC (Fig. 9) isexplained in terms of a denser, compared to trenchsediments at equivalent depths, (by at least 0.02-0.03 g/cm3) accretionary wedge forming the sub-duction complex. The same gravity high istraversed by profile BB in coastal Burma (Figs. 5and 8). At present no detailed seismic data areavailable to support this interpretation. However,Hamilton (1979) reported a seismic section for thesubduction complex across the Java trench (southof the Andaman arc) that showed distinctly higherseismic velocities for the rocks of the outer arcsubduction complex (e.g. melange, imbricatedsediments, and tectonically intercalated slices fromoceanic plate) (fig. 10 in Hamilton, 1979). Mostrecently, Clowes et al. (1987) have also reporteddistinctly higher velocity zones in the subductioncomplex under Vancouver Island in westernCanada. Such high velocity regions are believed torepresent the imbricated mafic or accretedmafic/ultramafic rock slices within the sedimentcolumn scraped off onto the overriding plate. The

gravity models corresponding ot the Andaman-Burmese subduction complex traversed by profilesBB and CC suggest that the sediments formingthe subduction complex are, in part, denser (byabout 0.02-0.03 g/cm3) than their neighbouringtrench sediments (of density 2.40 g/cm3) at equiv-alent depths. However for profile AA, we suggestthat the subduction complex underlying theNicobar Deep in the central Andaman arc alsopossibly contains slices of mafic rocks (of inferreddensity, 2.90 g/cm3) within about 2 km depth ofthe sea bed (Fig. 7). The mafic rock slice in thesediment column shown in the model possiblyoriginated through accretion along the east marginof the Andaman forearc, but confirmation mustcome from deep seismic reflection data (see Silveret al., 1978, for gravity models for the Indonesianarc). It is of interest to note that mafic rocks indifferent proportions have been encountered atdepths of 2-3 km in the wells drilled under theNicobar Deep, at locations just north of profileAA, by the Oil and Natural Gas Commission,India (DGM (Geol.), O.N.G.C., Madras, pers.commun., 1989).

The Andaman-Burmese volcanic arc traversedby all three profiles is delineated by a prominentnorth-south gravity high, both offshore and onland (see above). However, taking into account thegravity effect of the descending lithosphere, theapparent gravity high is really a moderate gravitylow. Hence, a low-density zone is inferred to un-derlie the volcanic arc in all three profiles. Thedensities under the volcanic arc shown in themodels are purely arbitrary; they are inferredmerely to fit the observed anomalies. Generally,we assume that the volcanic arc density is about0.03-0.1 g/cm3 less than adjacent densities, andthat they extend through the whole crust/litho-sphere. Grow (1973) has argued that the presenceof volcanoes requires lower densities and a ther-mally weak lithosphere. Closs et al. (1974) haveshown that heat flow values in the Andamanvolcanic arc region fall off linearly with theirdistance from the volcanic arc. Very high heatflow values have been observed in the Andamanregion; the highest measured value is 5.9 hfu.These factors possibly indicate that hotter (andpossibly lighter) crust/lithosphere is present un-


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der the Andaman volcanic arc; however a detailedseismic velocity model is necessary to supplementthe gravity models. A high density (or 2.90 g/cm3)volcanic plug is required at shallow depths toexplain a local gravity high of 40 mGa1 amplitudeover the Barren-Narcondam volcanic islands inthe north Andaman Sea (Fig. 7). The AndamanSea east of the volcanic arc is underlain by typicaloceanic crust, which ultimately joins continentalcrust of the Malayan continental margin at theMergui Terrace (Curray et al., 1982; Mukho-padhyay, 1988). The proposed gravity model forthe crustal transition under the Mergui Terraceinfers a two-layer, semi-continental crust nearly 24km thick, with densities of 2.80 and 2.90 g/cm3for the upper and lower crust, respectively. Forprofiles BB and CC, we use similar crustal den-sity configurations to infer semi-continental crustbelow the Eastern Trough (east of the Burmesevolcanic arc) in Burma (Mukhopadhyay andDasgupta, 1988) extending towards the Shan scarp.Further east lies the Shan Plateau forming theeastern Burma highlands that are covered byPaleozoic rocks (Mitchell and McKerrow, 1975)(Fig. 1). The Shan-Sagaing transform is believedto mark the boundary between the Burma andChina plates; the latter includes the Shan Plateau.Conclusion

On the basis of a revised gravity anomaly mapfor the northeast Indian Ocean and other seismic,geological, and density data, we present crustalstructure interpretations for three regional geo-physical traverses across the Bengal Fan and itsadjacent continental margins in India and Anda-man-Burmese arcs. The nature of the gravity fieldfor the east coast continental margin of India andfor the Andaman-Burmese subduction zones isquite diagonistic because these features representtwo fundamentally different continental margins.The former was inherited from the Gondwanalandbreak-up, whereas the latter developed at a laterstage by subduction of the Indian plate at theAsian plate margin. Subduction presently occursto a depth of about 200 km below these arcs.Gravity models suggest very significant massanomalies at the subduction zones, both at crustaland deeper lithospheric levels. Some lateral varia-

tion in rock density within the trench-arc sedi-ment column below the arcs is also inferred fromthe gravity models, in particular the accreted sedi-ments at the plate juncture. Verification of suchinferences can come only from further seismiccontrol. Thick sediments (8-10 km) may be ex-pected at the subduction zones, as shown by thegravity models. The Andaman-Burmese volcanicarc appears to underlie a regionally extensive, lowdensity rock column, penetrating the crust anddeeper lithosphere. This inference must also betested through seismic wave propagation studies.The Ninety East Ridge and the 85E Ridge pre-sent contrasting gravity signatures; the gravitymodels infer an isostatic compensatory mass atthe crustal base of the former, whereas the latterhas formed by thickening of the crust which isburied by the thick sediments of the Bengal Fan.

The geology of the northeast Indian Ocean andits surrounding continental margins present somevery fundamental and fascinating problems. How-ever, more geophysical data control in severalcritical areas are necessary to improve our presentunderstanding of the region.Acknowledgements

Gravity data used in this article were suppliedby the U.S. Defence Mapping Agency. The studywas partly supported by grant 24 (158)/EMR-IIof the C.S.I.R., Government of India. We be-nefitted from discussions with Professor J.R. Cur-ray of the Scripps Institution of Oceanography.ReferencesBaishya, N.C., Srivastava, S.K. and Singh, S.N., 1986. Identifi-

cation of lower Cretaceous sediment below thick volcanicssequence in Mahanadi offshore basin from vertical seismicprofiling: A case history. J. Assoc. Expl. Geophys. Hyde-rabad, 7: 195-203.

Bowin, C., 1973. Origin of the Ninety East Ridge from studiesnear the equator. J. Geophys. Res., 78: 6029-6043.

Brune, J.N. and Singh, D.D., 1986. Continent-like crustalthickness beneath the Bay of Bengal sediments. Seismol.Sot. Am. Bull., 76: 191-203.

Choudhury, S.K. and Datta, A.N., 1973. Bouguer gravity andits geological evaluation in the western part of the Bengalbasin and adjoining areas, India. Geophysics, 38: 691-700.

Christensen, HI. and Salisbury, M.H., 1975. Structure andconstitution of the lower oceanic crust. Rev. Geophys.Space Phys., 13: 57-86.


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gravity field and deep structure of bengal fan

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