Gravity anomalies and deep structure of the Andaman arc

Gravity Anomalies and Deep Structure of the Andaman Arc MANOJ MUKHOPADHYAY Department of Geophysics, Indian School of Mines, Dhanbad-826004, India (Received January 1986; revised 24 November, 1987) Keywords: plate subduction, gravity field, deep structure, sediment distribution, mafic mass within sediments, low density zone below the volcanic axis, and crustal transition. Abstract. The Andaman arc is associated with a major Free-air anomaly pair of mean amplitude 180 mgal. Two-dimensionalgravity interpretation suggests significant mass anomalies below the arc that presumably have resulted due to subduction of the Indian plate below the Burma plate. It is inferred that the Andaman trench is of asymmetric V-shape containing about 7 km sediments. An outer bathymetric rise seaward of the trench possibly corresponds to a lithospheric flexure by 500 m. The Cretaceous-Tertiary sediments constituting the Andaman sedimentary arc attain their maximum thickness of about 13 km under the Nicobar Deep at the subduction zone. At this location a mafic mass is emplaced within the sedimentary section. The underlying oceanic crust apparently experiences phase transition at about 27 km depth in a Benioffzone environment. The Andaman volcanic arc underlies a low density zone that is at least 60 km wide. Along the east margin of the Andaman Sea, crustal transition presumably occurs below the Mergui Terrace at the Malayan coast. zone of m a x i m u m gravity generally outlines the volcanic arc located further east within the overriding Burma plate. A similar disposition of gravity anomalies is known for the Burmese arc (cf. Evans and Crompton, 1946). Peter et al. (ibid.) interpreted the A n d a m a n gravity low in terms of excess crustal thickness underlying the A n d a m a n - N i c o b a r sedimentary islands (called the A n d a m a n - N i c o b a r Ridge, ANR). Here we interpret the gravity anomaly pair in terms of subduction of the Indian plate on the basis of available geologic data, seismic control and seismologic data. Our main conclusions are that the descending lithospheric slab below the A n d a m a n arc is a zone of mass excess, two prominent areas of mass deficiency underlie the subduction zone and volcanic axis respectively, and ocean-continent crustal transition appears to occur under the Mergui terrace at the Malayan continental margin. Introduction The A n d a m a n arc in the N E Indian Ocean together with the Burmese arc further north define a nearly 2100-km-long margin of the underthrusting Indian plate with the Burma plate (cf. Fitch, 1970; Curray et al., 1979). The two arc systems provide an important tectonic link between the Himalayan collision zone and a major island arc-trench system of south Asia, the Indonesian arc (Figure 1). The A n d a m a n arc is of particular interest for its Neogene back-arc spreading; this activity is presumably related to leaky transform tectonics (Uyeda and Kanamori, 1979). A major Free-air anomaly pair of average amplitude 180 mgal coincides with the A n d a m a n arc over a distance of 1100 km in north-south direction (Peter et al., 1966). The zone of minimum gravity corresponds to the trench and sedimentary islands whereas the Marine Geophysical Researches 9 (1988) 197 210. 9 1988 by Kluwer Academic Publishers. Regional Geology and Tectonic Setting The A N R is believed to have formed in OligoMiocene times due to east-west compression of sediments derived from the Malayan shelf (Rodolfo, 1969a). Its chief constituent rocks are: Cretaceous serpentinites, ophiolites with radiolarian cherts, Cretaceous to Eocene cherty pelagic limestone, grit, conglomerate, and a thick section of Eo-Oligocene flysch overlain by Neogene shallow water sediments (Table I). Results of seismic surveys (Curray et al., 1979) suggest that the surface trace of subduction below the A n d a m a n arc lies at the western base of the A N R where the trench is filled with sediments of the Bengal Fan (Figures 2 and 3). The process of subduction and offscraping of ocean-floor sediments continues today MANOJ 198 91 ~ 95 ~ MUKHOPADHYAY 99 ~ O 25 20* 15" ~0. Thrust Fault s Eastern Boundary Thrust ~ , Active fault - - - Inactive fault AA Volcanic axis Spreading Ridge Edge Of _~--"~ Cant inenta| Crust Fig. 1. Generalized tectonic map for the Andaman arc and adjoining Burmese and Indonesian arcs (redrawn after Curray et al., 1979). The Indian plate underthrusts the Burma plate eastward below both the Andaman and Burmese arcs. GRAVITY ANOMALIES AND DEEP S T R U C T U R E OF THE A N D A M A N ARC 199 TABLE I Generalizedstratigraphic sequenceand sedimentcharacter for the Andaman Islands region (data source: Chatterjee, 1967; Eremenko and Sastri, 1977; and Roy, 1983) Age Formation RecentPleistocene PlioceneMiocene Long Guitar Round Strait OligoceneLate Eocene Port Blair PaleoceneLate Cret. Baratang Cretaceous Port Meadow Generalized lithology Beach and tidal deposits coral reef, raised beaches unconformity Foraminiferal clay, thin bands of silt Max. thickness (m) 60 Foraminiferal limestone, calcareous sandstone and siltstone unconformity Chalk, sandstone, siltstone 450 Sandstone, grit, conglomerate, marl and siltstone unconformity Thick to massive sandstone, shale, siltstone unconformity Shale, associated greywackes, limestone unconformity Radiolarian chert, jaspers, quartzite, limestone, marble. Oceanic basement/ophiolite suite 500 as evidenced by deformation of the Pleistocene sediments near the base of the landward wall of the filled trench, and also by current seismicity (see below). The structure in the A N R is dominated by east-dipping nappes having gentler folding in the north part of the arc as compared to tighter folding and more intense deformation within the nappes further south off Sumatra (Weeks et al., 1967; Moore and Curray, 1980). Also the structures in the CretaceousOligocene sequences are generally more deformed than those developed in younger sequences (Eremenko and Sastri, 1977). Several north-south faults and thrusts within the A N R and offshore areas are known from surface mapping and seismic surveys; the most extensive of them is the Jarwa thrust developed in the main Andaman Islands (Roy, 1983). Curray et al. (1979) describe a set of north-south faults slicing the sea floor along the eastern edge of the A N R in the Nicobar deep and under the western part of the Andaman Sea, most significant of them being the West Andaman fault (Figures 1 and 2). Some of these faults/thrusts are also seismically active (Mukhopadhyay, 1984). 520 750 1370 500 + The eastern edge of the A N R gently slopes down eastward to the floor of the Andaman basin whose deepest portions are located in the 100-200 km wide central Andaman trough. The depth of the trough varies from 2 km below sea level at its northern end to beyond 3 km midway along its 750 km length. The northwestern margin of the trough is marked by a mosaic of steep and elongate seavalleys and seamounts such as the Nicobar Deep, Barren-Narcondam volcanic islands, Invisible Bank, Alcock and Sewell seamounts (Rodolfo, 1969a, b) (Figure 2). The Narcondam is now an extinct volcano but the Barren last erupted in 1832. All the seamounts which together form the Andaman volcanic arc possibly share a common origin; their principal rock constituents are: basalt, augite basalt or andesite (Rodolfo, 1969a). The central trough is bisected by the Andaman back-arc spreading ridge that has produced nearly symmetric spreading for the last 11 m.y. with a half-spreading rate of 1.86 cm yr -1 (Curray et al., 1979). The Andaman basin contains an average sediment thickness of 4 km, but it is delimited eastward by the Mergui Terrace at the Malayan continental margin. t~ yjl~ ~'~ Andaman Trench 1 1 Fautt Spreding R)dge (partly active) Seamount ( B a s a l t / A ride s~te ) Edge of continental crust 5ravity station location I 120 Km Boy of Benga 91 ~ "-I 93* "I"7" . 95 ~ _ 99* , B ~= o . - - - . ~~' c ~ = - ~ ^ - '-~ ^~" i L ' o = ' A I . . A i --_-'- -" . . . . . . ~ " I ctm ctn ~i O . _- O - I C ' . -- " "--- . : "1 m z -----=-E<: . ~=:~qz. U II . = i- k,C e n trial ~ ".-, 2 - 2 - . 7 : o - .- . . . . . . ^^\^, . I I 97 ~ . . . . . . E:_- --'-_g ~ ~ .... ~ ~ ~ 9 E o 89 ~ o 91" 93 ~ 95* 97* Fig. 2. Tectonic details o f the A n d a m a n arc and the A n d a m a n marginal sea (source: Rodolfo, 1969a, b and Curray et al., 1979). A A ' is the interpreted gravity profile; small circles are gravity station locations. BB' through D D ' are seismic profiles (after Curray et al., 1979) whose underlying structures are depicted on Figure 3. EE' is a seismologic section (redrawn after Mukhopadhyay, 1984) also illustrated on Figure 3. 99 ~ 201 G R A V I T Y A N O M A L I E S A N D DEEP S T R U C T U R E OF T H E A N D A M A N ARC Plate Setting Indian plate corresponding to an east-west section, EE', in the central portion of the Andaman arc is shown on Figure 3 (redrawn after Mukhopadhyay, 1984). In preparing the section all hypocenters located within a distance of 200 km of the section line were The Andaman arc is an area of high seismicity where focal depths of earthquakes extend down to 150 km. The Benioff zone configuration for the descending Middle Andaman B VE =7.2 X Bengal Island ~ Narcondam Smf. WAF ~, ~ Atcock / Smt. B-0 Spreading I.I v1 Invisible Bank .I/WAF /~J Barren Smt . .~,~\ _ _ "Spreading Andaman-Nicobar C Ri_dge V E = 2 2 X ~ ~,._.~ ".'-~" ~ Benga I~'~v~.~. -" Mergui Terrace /~'~'~/~" r~ ~0 I--2 I.I QI Fan O VE-22X Bengal Andaman-Nicobar Ridge Sevet[ WAF Smf. ~,, A Spreading _ Mergui Ridge ~ North "6 Sumatra O/ rO Basin ~ I.I Sediment s t r a t a s Volcanics West Andaman Fault WAF Trench . . E AXiS West i, 9. .. . " SO~" Indian ~. 1001- " SCALE I Andaman Nicobar ~ Ridge 9 9 . .~ plate : Central Andaman Sea Earthquakes 140 L_ (period 1916-1975) I **" e* ~ ~ I 200 Km Andaman sea . - - - - - ~ c / Volcan!c arc East eex 9 9 ee "*"?;~""" -',~": B u r m a~ ~; tat ~ * " ~ ~ -'i "? ~ e~ I'~ I J Fig. 3. Structures present under the seismic profiles BB' through DD' crossing the Andaman arc, and the Benioff zone configuration below the section EE'. For bottom figure: A.B Aseismic Belt; EFZ Earthquake Free Zone. 202 MANOJ M U K H O P A D H Y A Y projected onto a vertical plane. The horizontal lower boundary of the Indian Ocean lithosphere is placed immediately below the lowest foci at a depth of 75 km; traced eastward this boundary dips at about 30~ under the arc and plunges to nearly 150 km depth below the Andaman Sea. The upper boundary for the inclined lithosphere is rather difficult to locate unless a basic assumption is made that it lies in the western vicinity of the ANR where a megathrust is already proposed by Fitch (1970). By definition, this zone of megashear therefore outlines the surface trace of the boundary between the descending Indian plate and the overriding Burma plate. Further east of the Benioff zone two shallower seismic zones are located within the Burma plate (Figure 3). Following Sacks et al. (1978) and Yamashina et al. (1978), we designate this as the 'seismic slab' of the overriding plate. The relatively shallow seismic zone closest to the Benioff zone occurs below the western part of the Andaman Sea near the frontal arc (ANR); whereas the crustal seismic zone east of the volcanic arc corresponds to back-arc activity. It also appears from section EE' that the fore-arc and back-arc seismic zones are distinguished by an earthquake free zone between them. A triangular 'asesimic belt' occurs at shallower depths defined by the upper surfaces of the Benioff zone and the 'seismic slab' of the overriding plate; the apex of the aseismic belt is deflected downward - concordant with the dip direction of the Benioff zone, particularly under the east flank of the ANR and the Nicobar Deep. This bending of the 'Burma plate seismic slab' is presumably a consequence of downward drag experienced by the overriding plate near the subduction zone. According to Curray et al. (1979) the Burma plate is of elongated shape in north-south direction; it encompasses the Andaman basin underlying the Andaman Sea and also a substantial part of inland Burma. The east margin of the Burma plate with the Asian plate is marked by the Sagaing fault - a major transform extending through Burma toward the Himalayan collision zone (Figure 1). It is suggested that extensional stress dominates the landward wall of the Andaman trench whereas compressive stress prevails within the overriding Burma plate (Mukhopadhyay, 1984). This, however, gives way to deviatoric tensional stress near the Andaman back-arc spreading ridge in the interior of the Burma plate. Note that the back-arc spreading ridge bisects the Andaman volcanic arc in the central Andaman Sea as it passes between the Barren and Sewell seamounts (Figure 2). Gravity Anomalies The gravity field for the Andaman arc, first described by Peter et al. (1966), is clearly bipolar in nature, and is quite similar to that for the Western Pacific arcs. A revised Free-air anomaly map for the region is given on Figure 4. The most significant feature present on the Free-air map is a gravity anomaly pair of average amplitude 180 mgal coinciding with the Andaman arc. The maximum anomaly, however, locally varies up to 300 mgal (peak to trough) near the Invisible Bank in the north Andaman Sea. The general character of the anomaly shows a gravity low over the island-arc and trench area and a gravity high over the volcanic arc. In the north part of the Andaman arc _the axis of the gravity minimum shifts its position to a more easterly location to follow the Nicobar deep along the eastern edge of the ANR (Figure 4). The gravity minimum is seen to shift again to its supposedly initial location only to the north of the Preparis Channel close to the Burmese coast. Peter et al. (ibid.) have interpreted the low gravity in terms of excess crustal thickness reaching up to 40 km underlying the ANR. Further east, the Andaman basin underlying the Andaman Sea is associated with Free-air anomalies varying up to _+ 50 mgal. Mild positive anomalies are also observed over a bathymetric swell to the immediate west of the Andaman trench as well as over the Ninety East Ridge. The Ninety East Ridge strikes nearly subparallel to the Andaman arc in this region. The gravity anomaly variation along a 980-kmlong profile, AA', taken across the central portion of the Andaman arc (Figure 2 for profile location) is illustrated on Figure 5. Major tectonic features traversed by the profile from west to east are: Bengal Fan, Ninety East Ridge, Andaman trench, ANR, Nir Deep, volcanic arc and back-arc spreading ridge in the region of the Sewell seamount, Andaman basin, and finally the Mergui terrace at the Malayan continental margin. Gravity and bathymetric point values plotted on Figure 5 follow a ship's track along the Ten Degree Channel (data source: Defense Mapping Agency, U.S.A.). Free-air values at both ends of the profile are within 0 to - 2 0 mgal, suggesting isostatic compensation for the Indian Ocean lithosphere below the Bengal Fan on the west as well as for 203 G R A V I T Y A N O M A L I E S A N D D E E P S T R U C T U R E OF T H E A N D A M A N A RC 91~ 900 92" 93 ~ [ 9/,0 9S ~ I Scale r"80 Km o 7" "v 0 P r e p a r i s South Channel coTI-I1[]/*" N2 Fan B e n g a l 1/, 0 rth . . ' G r a v i t y =daStation nan ^ Free a i r 13 If 12 contour interval varinble H/L f i r a v i t y high or -':" ~ ~176 low ...Trench axis A o IC 9~ ol 8 "~ = a~ o '~ ~ t Little Nicobar 0 g~a 88 ~ 890 90 ~ 91 ~ 9Z ~ 93 ~ 9/*~ 950 Fig. 4. Free-air gravity anomaly map for the Andaman arc-trench region, and generalized geology for the Andaman-Nicobar Islands. Data sources are cited in text. Note the north-south linearity of the positive anomaly zone over the 'outer high' and Ninety East Ridge, negative anomaly over the Andaman trench, and positive anomalies further east over the Andaman volcanic arc. The volcanic arc gravity contours are adopted from Peter et al. (1966). The axis of the gravity minimum, however, shifts its position eastward to follow the Nicobar Deep in the north part of the arc as compared to more southern areas. Legend: 1, Unmapped; 2, Laterite; 3, Cretaceous-Tertiary sediments; 4, Ophiolite. AA' is the gravity profile (also see Figure 2) interpreted in Figure 5. 204 MANOJ M U K H O P A D H Y A Y -~o /,Or" -- AA A A 0 9 AA A AA A A -z,,0 - -80 -80 -12( -120FObserved -160t- -" .~ -'- Computed -16( 4 ~. Km A -200 0 100 200 300 WRst' e e , O.B.H & B. F._-,~-90eost Ridge . - ~ 500 zOO | t T~A~- A.N.R 600 I NO. 700 800 t I A.S. ~- 900 980- -20q I I M.T, Enst 0 10 20 30 5O l 70 E 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 profile location). Inset shows computed gravity effect due to the descending Indian lithosphere below 75 km depth. See text for discussion on interpreted 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., Andaman Sea; M.T., Mergui Terrace. Digits refer to density values in g cm 3. G R A V I T Y A N O M A L I E S A N D D E E P S T R U C T U R E O F T H E A N D A M A N ARC the Malayan continental margin on the extreme east. The Bengal Fan gravity field changes over the Ninety East Ridge and its adjoining area to the immediate west of the Andaman trench to describe a relative gravity high of about 30 mgal. This change in gravity corresponds to a bathymetric swell of more than 350 m over a horizontal span of about 150 km (also see Figure 4). Obviously these bathymetric and gravity changes are related in part to both the Ninety East Ridge and lithospheric flexuring in the form of an 'Outer High' to the immediate west of the Andaman trench. Watts and Talwani (1974) have already noted that the outer high in the present region is less prominent as compared to that in the Indonesian arc further south. However, bathymetric relief for the Ninety East Ridge is also much subdued in the Andaman region compared to more southern areas; the ridge is practically buried at this latitude (cf. Curray et al., 1982). Observed Free-air values are close to - 50 mgal over the Andaman trench, but they fall off rapidly to - 1 9 2 regal over the Nicobar Deep across the ANR, thence rise to about 12 mgal over the volcanic arc toward the east. Further east the anomaly values are of the order of - 2 5 mgal over the Andaman basin before attaining mild positive values again over the Mergui Terrace at the Malayan coast. This anomaly variation along the Ten Degree Channel and across the Andaman Sea is interpreted here in terms of a subduction zone model. Deep Structure For purposes of gravity interpretation for profile AA', we use some guidelines on velocity and depth distribution for near surface geologic layers underlying the Andaman arc as known from previous studies. They include: an interpreted section for a sub-bottom profile along the Ten Degree Channel (after Weeks et al., 1967), results of wide-angle seismic reflection and refraction study on crustal layers under the Ganges Cone (after Naini and Leyden, 1973), information from DSDP leg 217 located on north part of the Ninety East Ridge, sediment velocities from sonobuoys from Bengal and Nicobar Fans and Andaman basin (Hamilton et al., 1977), and information from three seismic profiles, BB' through DD', across the Andaman arc (after Curray et al., 1979) (Figures 2 and 3). Structural interpretation for these seismic sections as given by Curray et al. clearly demonstrates 205 that major geologic structures underlying the Andaman arc are grossly similar in the north-south direction. The present gravity profile AA' and aforesaid seismologic section EE' are located in the close vicinity of these seismic profiles. The gravity anomaly variation for many island arctrench areas has been explained in the framework of sea-floor spreading and plate tectonics. Such gravity model studies commonly infer that the descending lithosphere under island-arc/trench areas produces a considerable gravity effect on the surface (cf. Grow, 1973; Grow and Bowin, 1975). This is possibly a consequence of thermal effects of cold oceanic lithosphere along Benioff zones (Minear and Toksoz, 1970) that predict long wavelength gravity anomalies of amplitudes of 50-100 mgal due to thermal perturbations in the upper mantle. The gravity effect produced by the descending Indian Ocean lithosphere at the location of the present gravity profile has been computed using the configuration shown by section EE' (Figure 3) where we assume the lithosphere to asthenosphere density contrast to be 0.05 g cm -3. A similar density value was assumed by Grow (ibid.) for the Aleutian arc. Grow and Bowin (ibid.) consider a more complex density distribution for the descending lithosphere under the Chilean trench. However, in the present case since no other geophysical data support is available we have used the lithospheric configuration shown in Figure 3, and a uniform density distribution for the lithosphere and the surrounding asthenosphere. For two-dimensional computation of gravity effects we use the polygonal method of Talwani et al. (1959). As can be seen on Figure 5, the computed gravity effect due to the subducting lithosphere under the Andaman arc has a maximum value of 68 mgal which tails off quite symmetrically from the peak value, and diminishes to less than 5 mgal within a distance of 500 km from the deepest part of the subduction zone. The positive gravity effect due to the descending lithosphere is 20 mgal at the trench axis and 35-40 mgal over the volcanic arc. The observed anomaly along profile AA' is supposedly a combination of this deep gravity effect plus the effect due to the overlying layers. Hence the positive effect due to the descending lithospheric slab (below 75 km) must be compensated by a low density zone overlying it at the location of the volcanic arc in the vicinity of the Sewell seamount. For achieving this a low density zone is envisaged in terms of a less dense (by an estimated value of 206 MANOJ M U K H O P A D H Y A Y -0.01 g cm -3 with respect to the surrounding lithosphere) vertical rock column, at least 60 km wide, penetrating the lithosphere under the volcanic arc. Construction of the geometry for the low density zone is purely arbitrary at this stage; the model is assumed to merely fit the observed gravity anomalies. Alternately, the positive gravity effect in question may be balanced by asthenospheric shallowing to the base of the crust beneath the volcanic arc at the location of the earthquake free zone (shown in Figure 3) or by some form of mass anomalies arising out of mantle flow at the Andaman back-arc region as the asthenosphere tends to pull the overriding Burma plate westward in the direction of the Andaman trench. Any such mantle flow, if operative, will have to be related to the dynamics of lithospheric subduction. Such a process of mantle flow is believed to be a dominant mechanism for several active back-arc regions of the world (cf. Hager et al., 1983). However, at present we have very little to choose from these various alternative explanations. It has been argued (cf. Minear and Toksoz, 1970) that heating due to friction between the underthrust and overriding plates may produce a high-temperature, low-density zone generally below the volcanic arc, where seismic waves are also known to attenuate. No such elaborate seismic velocity model is however available for the Andaman arc. It is likely that the presence of volcanoes themselves requires a higher temperature (low-density) source at depth below them as already noted by Grow (1973). The Andaman volcanic arc is intriguingly split by the backarc spreading ridge at the location of the present gravity profile. Published values for heat-flow measurements so far made in the region of the volcanic arc 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 distance away 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 Model Another distinct source of density anomaly lies in the upper part of the lithosphere at the subduction zone. This is produced by the Indian Ocean crust at the Andaman island arc-trench area where the crustal layer gets highly deformed as it is carried with the subducting plate. For the purpose of gravity modeling we assume a simplified two-layer ocean crust that consists of a sediment layer of mean density 2.5 gcm -3 overlying a basal part (corresponding to oceanic layers 2 and 3) of mean density 2.9 g cm -3. These density values are estimated on the basis of observed seismic velocities under the Ganges Cone and Andaman arctrench 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 the Ganges Cone in the north part of the Indian Ocean basin lies at a depth of 14 km below sea-level where the average water depth is 3.5 km. This ocean basin crust is supposedly in isostatic equilibrium according to the Airy scheme as suggested by near-zero Free-air anomalies under the western part of profile AA'. That isostatic equilibrium prevails on a regional scale in most parts of the northern Indian Ocean is also exemplified by averaged Free-air anomalies over 1~ • 1~ areas (cf. Kahle and Talwani, 1973); such anomalies are however, superposed on a much longer wavelength gravity low due to deeper sources and a geoidal anomaly centered south of Ceylon in the north Indian Ocean (cf. Watts and Daly, 1981). The sediment layer underlying this part of the Bengal Fan corresponding to the western flank of profile AA' is considered to be 4.5 km thick; the sediments range in age from Campanian to Holocene. Note that sediment thickness in the Bengal Fan is quite variable and may attain a high figure (cf. Curray and Moore, 1974). Curray et al. (1982) suggest that the sediment thickness possibly exceeds 5 km in the present area. Seismic reflection and refraction spot surveys reported by Naini and Leyden (1973) in the vicinity of the present gravity profile indicate that sediments are, in general, 4-5 km thick and they directly overlie the oceanic basement of distinct higher velocity of 6.22 km sec -~. Therefore, the 4.5 km sediment thickness considered here for gravity modeling purposes is perhaps a minimum estimate. A time section constructed by Curray et al. (ibid.) from seismic reflection records with averaged velocities from results of wideangle reflection and refraction measurements along the Ten Degree Channel basin clearly demonstrates that almost flat-lying sediments of the Bengal Fan thin out eastward over the Ninety East Ridge but thicken again with eastward dip upon approaching the Andaman trench located some 70 km further east. G R A V I T Y A N O M A L I E S A N D D E E P S T R U C T U R E O F T H E A N D A M A N ARC This simplified crustal model for northern Indian Ocean crust is quite comparable to an average oceanic crustal model (for layers 2 and 3) as reported by Christensen and Salisbury (1975) who propose layer 2 and 3 thicknesses of 1.39 __+0.50 and 4.97 + 1.25 km, having respective compressional wave velocities of 5.04 ___0.69 and 6.73 ___0.19 km s 1. They further conclude that there is little change in thickness for layers 2 and 3 if the ocean basin is any older than 40 m.y. As the crust underlying the northern Indian Ocean near the Ninety East Ridge is definitely much older than this (cf. Luyendyk and Rennick, 1977 for magnetic anomaly ages and DSDP results on the area), we believe that the simplified oceanic crustal model considered here for gravity modeling may not be far from the real picture. This Bengal Fan crust is carried down the Andaman trench with the descending Indian plate. While some of the foregoing assumptions about the normal crustal configuration under the NE Indian Ocean are bound to remain ambiguous until deep crustal seismic control and crustal drilling data become available, certain important inferences can be made on the approximate configuration of the trencharc geometry, sedimentary and crustal layers below the ANR, and about the subduction zone, which satisfy the gravity data. Trench and Outer Bathymetric Rise Geometry Along the western flank of the ANR, the Andaman trench is clearly demarcated by a continuous gravity low of amplitude - 48 mgal, although the bathymetric expression for the trench is much less clear (also see Figure 4). At the Ten Degree Channel the gravity gradients across the outer and inner walls of the trench are -0.63 mgal km 1 and 1.90 mgal km i respectively. The positive gravity effect due to the descending Indian lithosphere at the trench axis is 20 mgal. For gravity model calculations we have considered density contrasts between sediments and oceanic crust and the latter versus the lithosphere as uniform throughout ( = - 0 . 5 g e m - 3 ) . The model shows that the Andaman trench has an asymmetric V-shape; the apex of the trench axis is filled with sedinaents and the maximum sediment thickness is 7 km under the area (Figure 5). In other words, the trench contains an excess of 2.5 km of sediments as compared to the Bengal Fan crust to its immediate west. 207 There is a corresponding downbulge of oceanic layers 2 and 3 into the lithosphere. Seaward of the trench, in the region of a bathymetric swell of more than 350 m, the gravity model shows a lithospheric flexuring by 500 m to account for a gravity high of 30 mgal. For constructing the swell geometry, we use a similar density distribution as before, and assume the crustal thickness as constant following Watts and Talwani (1974). Such a lithospheric swell seaward of the Andaman trench occurs over a horizontal distance of about 150 km. Obviously the swell is related in part to the buried topography of the 90 East Ridge as well as that due to the 'outer high' typical of western Pacific arcs. Continuation of the outer gravity high parallelling the Andaman trench in the north-south direction clearly suggests that lithospheric flexuring associated with the Andaman subduction zone occurs on a regional scale, and that the outer gravity and bathymetric rises are not entirely due to the 90 East Ridge. The ridge is practically buried at this latitude; further north it may die out altogether. Crustal Configuration below ANR and Subduetion Zone The gravity model (Figure 5) shows that the sediment thickness for the Cretaceous-Tertiary section reaches an average value of 5~5 km under the ANR, but down the subduction zone eastward, sediment thickness may increase to 13 km. The stratigraphic estimate for the Andaman flysch of mid-Eocene to Oligocene age under the ANR is 3 km; and the gross sediment thickness for Upper Cretaceous to Recent sediments, excluding older sediments, is about 4.2 km (Table I). Grow (1973) and Grow and Bowin (1975) have discussed the effect on density as the oceanic crust experiences transition from lower to higher pressure assemblages (basalt to eclogite) at pressures between 10 and 20 kb (30 to 60 km depth) in a Benioff zone environment. This process is grossly generalized here by a simple density change of the oceanic crust from 2.9 to 3.4 g cm -3 at about 27-28 km depth. The gravity model shows that both sedimentary and deeper crustal layers are depressed into their respective substrata over a distance of 200 km below the ANR and at the subduction zone. It is envisaged that sediments and the underlying crust of the Burma plate are thrust over the layers of the descending Indian plate through 208 MANOJ M U K H O P A D H Y A Y an efficient decoupling marked by a great decollement, in particular under the Nicobar Deep along the eastern flank of the ANR. This location is marked by a prominent boundary thrust along the east edge of the ANR (Mukhopadhyay, 1984). It is noted above that the A N R is sliced by several north-striking subparallel faults and thrusts including the most extensive Jarwa thrust. Quite a few of these faults are seismically active at depth. The faults and thrusts under the ANR produce a pattern of east-dipping thrust sheets and nappes (Curray et al., 1979). fault activity affecting the A N R near the Ten Degree Channel. Rather, slicing off wedges of ocean crust during subduction of the Indian plate seems to be supported by the eastward thickening of the marie/ ultramafic mass and its concordant dip with eastward subduction of the Indian plate. Welland and Mitchell (1977) also note that tectonic slices of marie and ultramarie rocks occur within or on the landward margin of the Andaman flysch belt. Crustal Transition at the Malayan Continental Margin Mafic Mass in ANR Several mafic/ultramafic bodies including a few ophiolites are known from the Andaman-Nicobar Islands (see Figure 4), although very little is known about their nature, origin and their relationship with ANR sediments. Two such relatively large-sized bodies are known from the Car Nicobar and Teressa Islands near the Ten Degree Channel where gravity profile AA' is located. The profile shows a relative gravity high about 50 km wide having an amplitude of 44 mgal along the east edge of the A N R in the vicinity of the subduction zone (Figure 5). The gravity high is located offshore north of the Car Nicobar island. The shape and gradient of the anomaly suggest a shallow causative mass. To explain the anomaly we infer a mafic/ultramafic mass of assumed density 3.0 gcm -3 surrounded by sediments of the ANR. The gravity model for the causative mass requires it to be nearly 3 km thick having subparallel dip to the sense of subduction below the Andaman arc. Although the nature and origin of the causative mass are not known, certain inferences on them can be made on the basis of the model of Figure 5. For this, three widely believed hypotheses regarding the obduction of ophiolites and emplacement of other mafic/ultramafic rocks may be cited here: (a) obduction of oceanic crust/ophiolites along faults that dip opposite to the direction of subduction (cf. Coleman, 1971; Christensen and Salisbury, 1975); (b) slicing off wedges of ocean crust during subduction along faults which are subparallel to the sense of subduction (cf. Dewey and Bird, 1971); (c) emplacement of a mafic/ultramafic mass related to changing motion along tansform faults (see Silver et al., 1978). Of these, a process of ophiolite obduction is clearly not favoured by the present gravity model. Also, we are not aware of any transform The opposite, eastern boundary of the Andaman Sea is formed by the Malay peninsula, its continental shelf, and the Mergui Terrace (Figure 2). The bathymetric pattern present along the western flank of the Mergui Terrace (Figure 5) is clearly representative of continental shelf and slope geometry, where the observed Free-air anomaly varies from - 2 4 regal over the eastern Andaman Sea to 12 regal over the Mergui Terrace landward. Curray et al. (1979) suggest that the Mergui-North Sumatra basin is underlain by thinned continental crust; and the north-south block faulting pattern in the area is similar to tensional faulting - typical of rifting young continental margins. This interpretation implies that the crustal transition possibly occurs below the Mergui Terrace. The observed Free-air anomaly is similar in nature to that observed for typical transitional crust underlying Atlantic-type continental margins (cf. Dehlinger, 1978, pp. 230-232). Negative anomalies over the eastern Andaman Sea, that corresponds to the seaward side of the continental slope, are however somewhat subdued in the present case. This evidently results from the positive gravity effect of the descending Indian lithosphere whose effect is nearly 15 mgal here (inset in Figure 5). Assuming Airy compensation as valid for the Malayan continental margin, the observed anomalies are interpreted in terms of a thicker oceanic crust under the Andaman Sea, and by a transitional crust, of assumed average density 2.84 gcm -3, underlying the Mergui Terrace and Mergui-North Sumatra basin. The oceanic Moho in our model deepens from 17 km under the eastern Andaman Sea to 19 km below the latter areas; the Moho dip is 16~ eastward. This east-west crustal transitional zone under the Mergui Terrace evidently marks the eastern limit of GRAVITYANOMALIESAND DEEP STRUCTUREOF THE ANDAMANARC thick sediment accumulation and thicker oceanic crust underlying the Andaman Sea. Summary Active subduction of the Indian plate is currently occurring beneath the Andaman arc along an east dipping Benioff zone that extends to a depth of about 150 kin; the deepest part of the Benioff zone is attained below the Andaman volcanic arc. The overriding Burma plate is, in part, defined by an active seismic slab at least 50 km thick. This lithospheric slab is deflected downwards in the vicinity of the Benioff zone. There, a triangular aseismic wedge in the top part of the Burma plate is generally outlined beneath the east edge of the ANR as well as the Nicobar Deep. A revised Free-air anomaly map for t h e Andaman arc and its immediate environs shows that the arc is associated with a major gravity anomaly pair of amplitude 180 regal extending over a distance of about l l 0 0 k m in a north-south direction. The gravity anomaly variation along a profile in the central Andaman Sea is interpreted here in terms of plate subduction following the general pattern of plate configuration as given by the seismicity data. This suggests that sediments below the Andaman arc increase from about 7 km to as much as 13 km at the mouth of the subduction zone underlying the east flank of the ANR and the Nicobar Deep. At this location a mafic mass is emplaced within the sedimentary section. The underlying oceanic crust apparently experiences phase transition at about 27 km in a Benioff zone environment. The Andaman volcanic arc is intriguingly split by the Andaman back-arc spreading ridge which has remained active since at least 11 m.y.b.p. At the location of the present gravity profile, a low density zone of nearly 60 km width underlies the volcanic axis within the overriding plate. 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Phys. Earth 26, Suppl., $447-$458.