Research ArticleCenozoic geodynamic evolution of the Andaman–Sumatra subduction

margin: Current understanding

PARTHA P. CHAKRABORTY1†* AND PROSANTA K. KHAN2

1Department of Applied Geology (email: parthageology@gmail.com) and 2Department of Applied Geophysics,Indian School of Mines University, Dhanbad, 826 004, India

Abstract The Andaman–Sumatra margin displays a unique set-up of extensionalsubduction–accretion complexes, which are the Java Trench, a tectonic (outer arc) prism, asliver plate, a forearc, oceanic rises, inner-arc volcanoes, and an extensional back-arc withactive spreading. Existing knowledge is reviewed in this paper, and some new data on thesurface and subsurface signatures for operative geotectonics of this margin is analyzed.Subduction-related deformation along the trench has been operating either continuouslyor intermittently since the Cretaceous. The oblique subduction has initiated strike–slipmotion in the northern Sumatra–Andaman sector, and has formed a sliver plate betweenthe subduction zone and a complex, right-lateral fault system. The sliver fault, initiated inthe Eocene, extended through the outer-arc ridge offshore from Sumatra, and continuedthrough the Andaman Sea connecting the Sagaing Fault in the north. Dominance ofregional plate dynamics over simple subduction-related accretionary processes led to thedevelopment and evolution of sedimentary basins of widely varied tectonic character alongthis margin. A number of north–south-trending dismembered ophiolite slices of Creta-ceous age, occurring at different structural levels with Eocene trench-slope sediments,were uplifted and emplaced by a series of east-dipping thrusts to shape the outer-arcprism. North–south and east–west strike–slip faults controlled the subsidence, resulting inthe development of a forearc basins and record Oligocene to Miocene–Pliocene sedimen-tation within mixed siliciclastic–carbonate systems. The opening of the Andaman Seaback-arc occurred in two phases: an early (~11 Ma) stretching and rifting, followed byspreading since 4–5 Ma. The history of inner-arc volcanic activity in the Andaman regionextends to the early Miocene, and since the Miocene arc volcanism has been associated withan evolution from felsic to basaltic composition.

Key words: Andaman–Sumatra margin, forearc basin, oblique subduction, subduction–accretion complexes, trench-slope basin.

INTRODUCTION

The 1200 km long Andaman–Sumatra subductionmargin offers a rare insight for understanding tec-tonogeomorphic evolution at the leading edge ofthe Indian Plate in the course of its fragmentationfrom Gondwanaland in the late Cretaceous and

subsequent northward journey in the Tertiary. Thesubducting Indian Plate, Java Trench, Andaman–Sumatra accretionary prism, Andaman Seaback-arc basin, oceanic rises (Alcock and Sewellseamounts), and active volcanoes such as Barrenand Narcondam constitute major morphotectonicelements in the NNE part of this plate margin(Fig. 1). The sequential evolution of these elementsat different stages of plate motion framed theactive margin geology. Despite the importance ofthe region in terms of tectonics, palaeoceano-graphic significance, and petroleum potential,

*Correspondence.†Present address: Rajiv Gandhi Institute of Petroleum Technology, Rae

Bareli, Uttar Pradesh, India

Received 5 January 2008; accepted for publication 31 May 2008.

Island Arc (2008)

© 2008 The AuthorsJournal compilation © 2008 Blackwell Publishing Asia Pty Ltd

doi:10.1111/j.1440-1738.2008.00643.x

geological and geophysical studies undertaken sofar on this plate margin are limited. The unique andcomplex geology of this tectonic province reflectsthe mutual interplay of various spatio–temporalfactors, such as oblique subduction, spatiallyvariable subduction rate, rotation of platelets,offsetting through various fault systems of variedgeometry, arc volcanism, and an ill-defined spread-ing center (Fitch 1972; McCaffrey 1992; Raju et al.2004; Curray 2005). The variation in direction (fromnorthward to northeastward) and velocity of the

Indian Plate subduction vector through the Ceno-zoic is well known, and the dynamic evolution of theplate margin through the coupling and decoupl-ing of different platelets, crustal movement alongstrike–slip faults, rotation of continental blocks,and opening of the marginal basin to form theAndaman Sea, are also reasonably well established(Curray et al. 1979; Tapponnier et al. 1986; Maung1987; McCaffrey et al. 2000; Raju et al. 2004).

Following the seminal papers of Rodolfo (1969),Curray & Moore (1974), Ray (1982), and Curray

Fig. 1 (a) Regional tectonic framework for Burma–Andaman–Sumatra margin (after Curray & Munasinghe 1991; Curray 2005). Note double-arcgeometry of the Java Trench. A clockwise rotation (Ninkovich 1976) of the Java Trench (~20°) and a counterclockwise rotation for southern Malaya andnorthern Sumatra are shown. Relative motions of different plates or platelets are shown by open arrows. A solid arrow at lower left represents the directionof Indian Plate velocity vector, and (b) The study area demarcated by a dashed box. Different morphotectonic elements of accretionary prism, sliver plate,inner architectural element volcanoes, back-arc spreading, transcurrent faults (Sagaing, Sumatra, West Andaman fault systems) are shown. Note theincrease in plate obliquity (f) towards north. ALK, Alcock Seamount; ASR, Andaman Sea Ridge; B, Barren Volcano; DF, Dauki Fault; KBF, Kawab Fault; MBT,Main Boundary Thrust; MR, Mergui Ridge; N, Narcondam Volcano; SGF, Sagaing Fault; SMT, Sewell Seamount; WAF, West Andaman Fault.

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(1988), a number of studies published over thelast 30 years has resulted in a substantialincrease in the database on the geology of theAndaman–Sumatra margin, in terms of bothevent chronology and operative tectonics. Most ofthese data, however, are heavily biased towardsseismic, palaeomagnetic, and geophysical aspectsin the Andaman–Sumatra offshore (Roy 1992;Dasgupta & Mukhopadhyay 1993; Curray 2005).For example, multi-channel seismic reflectioninvestigations in the Andaman Sea in a sectionalong 11°N, carried out by hydrocarbon explora-tion companies (Roy 1992), imaged some ofthe tectonic elements in the subsurface. Bycontrast, there exist only limited results fromon-land investigations (Chakraborty et al. 1999;Chakraborty & Pal 2001, Pal et al. 2003; Bando-padhyay 2005; Allen et al. 2008). Furthermore, todate most work has focused on specific geotec-tonic elements, with only brief discussion of thesignificance of various elements within a regionalgeological framework. Thus, at this plate marginattempts have rarely been made to evaluate therelative importance of the roles of regional platedynamics and simple subduction-related accretionprocesses in the formation of basins of widely dif-ferent tectonic character: trench-slope, forearc,and back-arc.

This article presents a collation of early andmore recent data (both geological and geophysical)published on the Andaman–Sumatra subductionmargin, aimed at providing a refined understand-ing of the geotectonic history of this plate margin.Besides discussing the chronological evolutionof different geotectonic elements, the articleaddresses the subducting Indian Plate geometry,its variation in space and time, and controls ofsubduction plate geometry on the tectonic evolu-tion of the overriding plate. In particular, thedouble-arc geometry of the Java Trench, thedecoupling of the Indo–Burma Ranges andthe Andaman–Sumatra Ridge, and the control ofregional fault systems including the WestAndaman Fault, Sagaing Fault, and Sumatra Faultsystem, is discussed.

In addition, as a member of the GeologicalSurvey of India Thematic mapping party, the firstauthor has been involved in generating an exten-sive database covering key geological transects inthe north, middle, and south Andaman Islands.In the present study, selected data are reporteddealing with field observations of south Andamanand north Andaman islands. The aim is to providea refined understanding of the geotectonic history

of this plate margin taking into consideration bothsurface and subsurface signatures.

REGIONAL GEOLOGY

Internal deformation and crowding of a number ofoceanic and continental sub-plates at the leadingedge of the indenting Indian subcontinent framedthe complex Cenozoic tectonics of Southeast Asia(Fitch 1972; Weissel et al. 1980; Mitchell 1981;Tapponnier et al. 1986; Hall 2002). Subduction isconsidered to have started along the western SundaArc following the break-up of Gondwanaland inthe early Cretaceous (Scotese et al. 1988). Defor-mation of Eocene–Oligocene–Miocene–Pliocenesediments on the Andaman Islands, accretion ofCretaceous–Eocene sediments, arc volcanic activ-ity in the Miocene–Pliocene, and the occurrence ofyoung volcanoes in the Andaman and Nicobarislands suggest that subduction-related processesin the Java Trench were operative either con-tinuously or intermittently from the Cretaceousonwards. Convergence between Indian and Asianmegaplates is being taken up primarily by thecontinuous Burma–Andaman–Sumatra–Banda–Java subduction system. From south to north, thechanging orientation of the trench (east–westnear Banda–Java, southeast–northwest aroundSumatra, and north–south near Andaman–Burma)is responsible for varying subduction character:frontal in the southern sector, and tangential withsubstantial strike–slip motion in the northernSumatra–Andaman sector. It is considered that theobliquity of the convergence is accommodated bylarge strike–slip fault system running parallel tothe Sumatra trench: the Sumatra fault zone, or theSemangko Fault (a dextral strike–slip fault withseveral splays, Diament et al. (1992).

The Andaman–Sumatra margin, the focus ofthe present article, represents the central partof the 5000-km-long Burma–Java subductioncomplex, and has been active since the middle Ter-tiary (Hamilton 1988). The margin displays majortectonostratigraphic elements striking approxi-mately parallel to the trend of the Java Trench(Fig. 1). In the west of the Andaman–Nicobarisland chain the oceanic part of the Indian Plate issubducting eastwardly below the oceanic part ofthe Southeast Asian Plate (Seely et al. 1974).

The Andaman outer arc represents the Creta-ceous to Miocene period (Table 1) and has beenemplaced as tectonically imbricated fold–thrustpackages, up- and out-built from submarine to

Evolution of Andaman–Sumatra margin 3

© 2008 The AuthorsJournal compilation © 2008 Blackwell Publishing Asia Pty Ltd

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4 P. P. Chakraborty and P. K. Khan

© 2008 The AuthorsJournal compilation © 2008 Blackwell Publishing Asia Pty Ltd

subarial settings with continuing subduction(Chakraborty et al. 1999; Pal et al. 2003; Curray2005). Underthrusted fold packets within theAndaman Forearc Basin are identified fromgravity–magnetic geophysical data and seismicprofiles (Peter et al. 1966; Weeks et al. 1966;Mukhopadhyay & Krishna 1991; Roy 1992). Thestrongly varying dimension of the fold packetsfrom west to east across the forearc subsurfaceis commonly explained based on the difference inrate and direction of subduction with time andplate stresses involved. This idea of temporallyvariable subduction dynamics is supported bymore recent seismotectonically based interpreta-tion of the opening and evolution of the AndamanSea, east of the outer-arc prism (Raju et al. 2004;Khan & Chakraborty 2005).

Time bracketing for most of the tectonic eventswas done with the help of biostratigraphicalstudies. Microfossil assemblages, their ages, andpalaeobathymetric implications were carried outby the Oil and Natural Gas Commission onsamples collected from wells drilled in the forearcbasin on the west of the Andaman outer arc prism.Table 1 presents the foraminiferal assemblagesin rocks of different periods, and the inferredbathymetry. Reworking and mixing of fossils fromolder strata with assemblages of younger strata isobserved throughout the succession suggestingrecurrent uplifting of depositional substrateswith ongoing subduction. Despite this confoundingeffect of mixed index forms, stratigraphic subdivi-sions were conducted in the subsurface successionby Karunakaran et al. (1968), Ray (1982) and Roy(1983). Shelf to open marine fauna is abundant inrocks of all age groups, suggesting the evolution ofthe margin in an open marine setting since theCretaceous (Pal et al. 2003).

DATA AND METHODS

Besides collation of recent databases, the presentwork incorporates results of: (i) systematic the-matic mapping done in five consecutive fieldseasons (1995 to 1999); and (ii) analysis of earth-quake data on two key east–west transects nearNorth Andaman and Sumatra. Outlining the outerarc and trench-slope basin geometry includinglithological disposition and structural control isdone by detailed mapping over a 90-km2-area innorth Andaman Island. The configuration of thesubducting lithosphere at a plate margin can bereconstructed on the basis of the distribution of

earthquake hypocenters, which is otherwiseknown as the Benioff zone trajectory. Reconstruc-tions of the Benioff profile for the subductingIndian slab at the Andaman–Sumatra margin weredone from earthquake data (mb � 4.0, maximumrecorded depth 260 km) lying within a lateralwidth of about 200 km on the downgoing IndianPlate and recorded at 15 or more stations. Thesedatasets covered the period between 1964 and1999 (International Seismological Centre 2001). Aliterature review for slab geometry reconstructionfrom hypocentral distribution revealed that theprocedures followed for Benioff zone reconstruc-tion have inherent inconsistency. The trend of theBenioff zone trajectory can be estimated eitherassuming the best fit of the slab upper surface(Ruff & Kanamori 1983; Ponko & Peacock 1995;Christova 2004), or as a best fit down through thehypocentral distribution (Luyendyk 1970; Isacks& Molnar 1971; Khan 2003, 2005). The well definedseismicity trend obtained under the present studyallowed reconstructions of slab upper surface inthe depth–dip angle profile. Besides, gravity pro-files across different morphotectonic elements andshort seismic transects across some key tectonicboundaries (after Mukhopadhyay & Krishna 1991;Rao & Krishna 1997) are also used in the studyto provide a comprehensive surface–subsurfacepicture.

DISCUSSION

Understanding the geometry of arc–trenchsystems is heavily constrained by the limited scopeof onland studies, which often result in strongreliance on geophysical signatures and scant DeepSea Drilling Project and Ocean Drilling Pro-gram feedback. The Andaman–Sumatra subduc-tion margin is also not an exception. The presentstudy is the first of its kind on this arc–trenchsystem as it addresses the geometry of differentmorphotectonic elements and their mutual rela-tionships through collation of both field-basedobservations from the accretionary prism andappraisal of geophysical datasets. The descriptionsto follow will, therefore, focus on the salient fea-tures delineated for each of these elements.

TRENCH

Mukhopadhyay & Krishna (1991) provided grav-ity profiles extending between the Indian andMalayan continental margins, and offered probing

Evolution of Andaman–Sumatra margin 5

© 2008 The AuthorsJournal compilation © 2008 Blackwell Publishing Asia Pty Ltd

views on crustal and deeper lithospheric massanomalies. West of the Andaman Islands, theJava Trench is associated with a gravity low of-40 mgal, becomes relatively lower (-60 mgal)towards Burma in the north, while towards thesouth, near Nicobar Island, the gravity anomalyattains a minimum of -100 mgal. The bathymetryof the trench varies between 2.5 and 4.5 km alonga north–south transect from the north of northAndaman Island to off the Sumatra margin(Ramana et al. 1997; Curray 2005). Availablebathymetric data on the Indian Ocean around theJava Trench show closely spaced contour charac-ter off the western coast of the Sumatra islands,and widely spaced character towards the north inthe west of the Andaman Islands (Ghatak & Ban-erjee 2001; fig. 2 of Curray 2005). This suggests achange in trench morphometry from a narrow,steeply dipping, starved type in the south to awide, relatively gently dipping and sediment-filledtype towards the north.

The trench–forearc system received sedimentsboth longitudinally and transversely. Hamilton(1988) suggested major clastic inputs in the trenchfrom Ganges and Brahmaputra river deltas in theform of longitudinal fans. Recent works (Bando-padhyay & Ghosh 1998; Bandopadhyay 2005),however, provided undoubted field and petro-graphic evidence for lateral pyroclastic input fromsubarial or shallow subaqueous explosive activearc volcanism on the western margin of theBurma–Malaya continent. Considering high sedi-mentation rates, Chakraborty & Mukhopadhyay(2003) argued that the shedding of large amount ofdetritus in the forearc may be the result of activegrowth of two large delta systems, the Ganges–Brahmaputra delta and Irrawady delta. However,the supply is cut off in the south by the collisionbetween Ninetyeast Ridge and Java Trencharound the Andaman sector. Tectonically slicedtrench sediments are recorded in the sliced thrustpackets of the outer arc and are represented byblack shale with olistostromes (of limestone, ultra-mafics, basalt) and tectonic mélange containing achaotic mixture of diverse lithic fragments. Thearc–trench gap in the Andaman–Sumatra sectorvaries between 220 and 320 km (Dasgupta et al.2003).

SUBDUCTION OBLIQUITY AND BENIOFF ZONES

Plate obliquity (f) along the subduction margin isdefined as the angle between trench normal andthe converging plate velocity vector. This is illus-

trated in Figure 2 based on the work of Liu et al.(1995). Various workers (Fitch 1972; Diament et al.1992; McCaffrey 1992, McCaffrey et al. 2000) haveshown the importance of this parameter in assess-ing variations in strain partitioning, subductioncharacter, and back-arc dynamics among differentplate margins. Diament et al. (1992) observed anincrease in plate convergence obliquity from thesouthern tip of Sumatra to the north AndamanIslands. McCaffrey (1992) suggested that the arc-parallel shear force would not be large enough tocause motion on forearc shear faults for conver-gence obliquity lower than the critical angle. Hefurther evaluated a critical value of convergenceobliquity of about 20 � 5° for the Sumatra region,and interpreted this as a decoupled margin in asso-ciation with both the Sumatra Fault and thrust-related subduction movements perpendicular tothe arc. It was predicted by Hall (1997) that by15 Ma north Sumatra had rotated counterclock-wise with south Malaya and, as the rotation pro-ceeded, the orientation of the Sumatran marginbecame less oblique to the Indian Plate motionvector. This resulted in the partitioning of conver-gence into an orthogonal subduction componentand a trench-parallel strike–slip component,leading to the formation of the dextral Sumatranstrike–slip fault systems, and extension in theAndaman region.

Based on fault plane solutions of earthquakedata, Raju et al. (2004) suggested that a tensionalstress regime is dominant in the northernAndaman Sea, which gradually becomes com-pressive in the southern Andaman and northern

Fig. 2 Diagram (after Liu et al. 1995) illustrating formation ofsliver plate, b. Incoming oceanic plate and overriding plate are a and c,respectively. Angles f and y represent plate obliquity and slip obliquity,respectively.

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Sumatran sectors. Khan & Chakraborty (2005)estimated an increase in the obliquity value from18 to 102° along the Sumatra–Andaman sub-duction margin between latitudes 2 and 17°N.Whereas strike–slip rates in the forearcs of mostof the subduction zone segments are generally lessthan 10% of the overall plate convergence rate,exceptions are the Aleutian and Sumatra marginswhere the rates are markedly higher as about25 and 50% (Jarrard 1986), respectively. The mostillustrative effect of this oblique convergence hasbeen the formation of a sliver plate between thesubduction zone and a complex right-lateral faultsystem (Figs 1,2). The initial sliver fault, whichprobably started in the Eocene, extended throughthe outer-arc ridge offshore from Sumatra andfurther through the present region of theAndaman Sea into the Sagaing Fault (Fig. 1).

High-resolution reconstruction of depth–dipangle Benioff profiles across the Andaman–Sumatra margin (Khan & Chakraborty 2005)shows minimum dip and attainment of early(~25 km depth) dip angle saturation in the Benioffzone trajectory at the subduction segment between2.0 and 3.2°N, where the slip vector residual (i.e.f – y, where f represents the angle between trenchnormal and subducting plate velocity vector, and ythe angle between trench-normal and slip vector,Fig. 2) reaches a minimum value (nearly zero, Liuet al. 1995). In the north near the Andaman Islands(9.8 to 12.2°N), along with an increase in slip vectorresidual, the Benioff zone trajectories becomesteeper and attain dip saturation at much higherdepths (~50 km, Fig. 3).

OUTER-ARC PRISM

Paralleling the trench, the Andaman–Nicobarisland chain represents the outer-arc prism inthe northern segment of the Sunda subductionmargin. Dismembered bodies of Cretaceous ophio-lite slices and Tertiary sediments of trench-slope–forearc setting, scrapped off the underthrustedIndian Plate, constitute the outer-arc systemwhich has up- and out-built with continuing sub-duction since the Cretaceous. A strong negativefree-air gravity anomaly characterizes the outerarc, and suggests an excess of relatively low-density sediment within the ridge (Curray 2005).The crust underlying the outer-arc prism varieswidely in character, ranging from tectonicmélange, including ultramafic rocks, off Sumatra(Kieckhefer et al. 1981), to continental crust offMyanmar (Acharya 1994, 1998) and to oceanic

crust off Indo–Burma below the Andaman–Nicobar Ridge (Curray 2005).

The stratigraphic sequence established from thetectonogeomorphic units of this arc is given inTable 1 (after Ray 1982; Pal et al. 2003). Metamor-phic rocks, tectonic mélange, and an ophiolitesuite sensu stricto constitute the Ophiolite Groupof rocks, and occur as a number of north–south-trending dismembered bodies encased at differentstructural levels within the Eocene ophiolite-derived clastics of trench-slope origin, i.e. theMithakhari Group (Fig. 4). From earthquake first-motion solutions, a preponderance of southeast–ESE-striking normal and reverse faults arepredicted (Dasgupta et al. 2003) within the outerarc, with a few right-lateral strike–slip faults par-allel to the strike of the arc. Except for the sheeteddyke complex, all other members of a classic ophio-lite stratigraphy are exhibited, in the sequence of aplutonic complex, intrusives, extrusive lava series,and pelagic sediments. Based on the presence of apre-subduction tectonic fabric within the harzburg-ites of the ophiolite package, Pal et al. (2003) sur-mised a two-fold subdivision: a mantle sequence,and a crustal sequence. A complete sequence from

Fig. 3 Depth sections showing Benioff trajectories (reconstructed fromearthquake hypocentral distributions and marked by solid curved lines) ofsubducting Indian lithosphere at (a) Andaman, and (b) Sumatra sectors.Earthquake data (mb � 4.0 and recorded at more than 15 stations) aretaken from the International Seismological Centre 2001 catalog for 1964to 1999.

Evolution of Andaman–Sumatra margin 7

© 2008 The AuthorsJournal compilation © 2008 Blackwell Publishing Asia Pty Ltd

the Moho is exposed in the Bedonabad–Chidiyatapu Road section of south AndamanIsland, and the Nimbutala–Mayabandar Roadsection of middle Andaman Island. The ophioliteslices invariably show thrust contacts with under-lying sediments, and occasionally with overlyingsediments. Systematic thematic mapping in south

Andaman Island revealed a systematic decrease inthe dip of thrust planes from east to west (Fig. 4).In the west, the thrusts show low eastward dips(8–10°), whereas in the east they have much steeper(65–70°) eastward dips (Fig. 4). Metapelites andmetabasics of greenschist to amphibolite gradeoccur in a mélange zone of ophiolites (Fig. 5).

Fig. 4 Detailed geological map and repre-sentative geological sections for parts of northAndaman Island showing lithostratigraphicunits, tectonic elements, and thrust contactsof ophiolites. Note eastward increase in thethrust angle.

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The Eocene Mithakhari Group represents depo-sition within isolated trench-slope basins charac-terized by: (i) rapid facies changes both along andacross depositional strike; (ii) coarse-graineddeposits with synsedimentary basinal disturbance;(iii) isolated sedimentary successions with unre-lated depositional patterns and widely variablepaleocurrents; (iv) wide paleo-environmentalvariation ranging from non-marine deltaic to sub-marine starved set ups; (v) short truncated sedi-mentary succession; and (iv) mass-flow unitscontaining blocks of underlying accretionaryslices. The Mithakhari succession is constituted bya wide range of sedimentary settings, varyingfrom coal-bearing delta plains (Pal et al. 2003) todistal prodelta or basinal set-ups (Chakrabortyet al. 1999), dominated by channelized laminardebris flow (Ricci Lucci 1985), high concentrationflow dispersion (Deckie & Hein 1995), or low-concentration unconfined turbidity currents. Thediscontinuous nature of these basins in the entireSunda Arc has also been reported from seismicreflection studies (Karig et al. 1980; Moore et al.1982). From fission-track and (U/Th)–He thermo-chronometry of apatite and zircon grains, Allenet al. (2008) constrained the depositional age ofMithakhari sediments to between about 60 and40 Ma.

Subsequent to the development of the accretion-ary prism, the north–south and east–west strike–slip faults resulted in the development of a forearcbasin with deposition of Oligocene and Miocene–Pliocene sediments: the Andaman flysch Groupand Archipelago Group of rocks. Curray et al.(1979) and subsequent workers (Ray 1982; Roy1983) interpreted the Andaman flysch sedimentsas essentially part of the Bengal fan. By contrast,

using detailed process-based facies analysis,Chakraborty and Pal (2001) identified these sedi-ments as of forearc origin, barred by the outer arcridge from the turbiditic deposition of the Bengalfan. The forearc developed either because of: (i)oblique subduction of the Indian Plate (cf. Uyeda& Kanamori 1979); or (ii) failure of the wedge sta-bility along normal/strike–slip faults, transverseand parallel to the strike of the accretionary prism(east–west). The fault-controlled forearc was initi-ated with a longitudinal geometry to the east of theprism, and received sediments both transverselyfrom the rising accretionary orogen and longitudi-nally from the active Ganges–Brahmaputra andIrawaddy delta systems in the north (Chakraborty& Pal 2001; Mukhopadhyay et al. 2004). In additionto siliciclastic turbidites (Fig. 6), the forearc wasalso fed by recurrent pyroclastic influx in theMiocene–Pliocene (Pal et al. 2005). From theyoungest argon mica dating, Allen et al. (2008)suggested initiation of Andaman flysch depositionat or after 35 to 30 Ma. Application of a non-parametric ‘sign-of-difference’ Waldron test onturbidite successions reveals the presence of athinning-upward bed-thickness motif withinvarying palaeogeography of the forearc and areinterpreted as products of non-random deposi-tional processes (Mukhopadhyay et al. 2003,2004). From seismite layers within the fan system(Fig. 7), Chakraborty & Mukhopadhyay (2003)estimated a high sedimentation rate in the fansystem, varying between 5.8 and 21.7 m per1000 years. On its eastern flank, the forearc isunderlain by continental crust. However, the loca-tion and nature of the continental margin isobscured beneath the pronounced unconformityunderlying recent continental shelf strata. It hasbeen speculated that the basement beneath thedeeper part of the forearc basin is either trappedoceanic crust (Hamilton 1977) or pre-Miocene

Fig. 5 Tectonic mélange showing clasts in sheared matrix developed atthe base of ophiolite slices (length of ruler 0.12 m).

Fig. 6 Siliciclastic turbidites within the Andaman Flysch Group, southAndaman Island (exposure length 120 m).

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accretionary material (Karig et al. 1980). Thedominance of shallow-water carbonate clastics inthe top part of the Miocene–Pliocene section ofthe Andaman–Nicobar Islands (the ArchipelagoGroup) suggests that the proximal part of theforearc basin was uplifted above the storm wavebase, and resulted in shedding of carbonate clas-tics in the deeper basinal part. A thick sequence ofinterbedded tuff, described recently from theArchipelago succession (Pal et al. 2005), has beenidentified to be either vitric or crysto-vitric incharacter and deposited as subarial to subaqueoushigh- to low-concentration turbidity current.

A noteworthy feature in this arc system is itsdouble-arc geometry, represented by the Indo–Burma Range and the Andaman–Nicobar Ridgesystem, as shown in Figure 1. Maung (1987)suggested that the Indo–Burma Range andAndaman–Nicobar Ridge were originally parts ofa single arc and that the double-arc geometry wasframed by the northward translation of the single-arc system causing the bending of the tail end ofthe arc of the Andaman–Nicobar–Sumatra section,rotating into the Burma Plate and causing thebending of the arc. It is considered that the north-ward movement of Burma Plate has resulted in a460-km-wide opening of the Andaman Sea.

BACK-ARC BASIN

Opening onto the northward-moving Indian Plate,the Andaman Sea, with its subduction boundariestowards the north, west and south, is associatedwith poorly understood marginal basins of intrigu-ing tectonic setting (Raju et al. 2004; Khan &Chakraborty 2005). The Andaman Basin has beenclassified as an oblique convergence extensionalbasin, rather than a typical back-arc basin (Rajuet al. 2004). It includes the Alcock and Sewell sea-mounts, the central Andaman Basin between thetwo rises, the East Basin, and some other, smaller

topographic features of unknown character(Curray 2005). Swath bathymetry data revealedseveral morphotectonic features that divide thebasin into a complex western part comprising arc-parallel seamount chains and north–south trend-ing fault systems, and a relatively smooth easternpart. Gravity values are low, varying between -40and -60 mgal (Mukhopadhyay & Krishna 1991).

The existing models of the opening of theAndaman Sea are based on a long tectonic history,which includes collision-induced extrusive tecton-ics (resulting from the rigid indentation of theIndian Plate with Asia; Peltzer & Tapponnier,1988, Raju et al. 2004) in the latest Oligocene–earlyMiocene, post-collision northward movement ofIndia, and the clockwise rotation of Burma–Javasubduction zone. The study of Bertrand & Rangin(2003) in the central Myanmar–Andaman Searegion supported a long extensional history duringthe last 45 Ma on this plate margin. By contrast,assessments of slip vectors from thrust earth-quakes at the Java Trench and estimation of arc-parallel stretching for the Sumatran forearc arguein favor of spreading in the Andaman Sea for thepast 13 Ma only. The integrated study of swathbathymetry, magnetic, and seismological data byRaju et al. (2004) showed active seafloor spreadingin the Andaman Sea Basin to be a much youngerphenomenon, operative only for the last 4–5 Ma.This finding, concerning principally only the north-ern part of this geotectonic belt (at ~11.8° latitude),raised a major question on the pre-spreadingdeformation history of the southern part of theAndaman forearc. The more recent study of Khan& Chakraborty (2005) has resolved this debate byconstraining the period for major episodes ofchange in dip angle trajectories in the Andaman–Sumatra sector, based on isochron reconstruction(at 1-Ma resolution) on the subducting Indianlithosphere. A two-stage evolution for the Mergui–Andaman Backarc Basin is suggested by Khan &Chakraborty (2005): (i) extension and rifting ataround 11 Ma; and (ii) extension through seafloorspreading since 4–5 Ma. Nonconformity betweenplate shape and subduction margin geometry isinterpreted as the cause for mid-Miocene intra-plate extension and tearing. Enhanced stretchingin the overriding plate consequently led to activeforearc subsidence, recorded all along this platemargin. It is presumed that the initial phase ofAndaman Sea opening is concealed in Early–Middle Miocene forearc subsidence history. Thelate Miocene–Pliocene pull-apart opening andspreading was possibly initiated near the western

Fig. 7 Soft sediment deformation (seismite) within Andaman FlyschGroup (exposure length 1.2 m).

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part of the Mergui–Sumatra region, and propa-gated northwards in subsequent periods.

The full-scale opening of present-day deepAndaman Sea Basin possibly resulted from extru-sive tectonics which prompted extension by sea-floor spreading during the last 4 Ma, and riftingalong the plane joining the Sagaing and Semangko(Sumatra) fault systems. A temporary halt inrifting at this pull-apart stage, and northeastwardveering of the Andaman Sea Ridge (ASR), arerelated to the uplifting of oceanic crust in the post-Middle Miocene in the form of the Alcock andSewell seamounts, lying symmetrically north andsouth of this spreading ridge. Also perhaps sig-nificant is that the seamounts overlapped whenthe Andaman Sea rift system was partially closed.Concurrently, the present plate between theSoutheast Asia and the Burma sliver blocks is con-sidered to have formed (Maung 1987).

INNER-ARC

Inner-arc volcanic activity on the Andaman andNicobar island group has so far been reported forQuaternary volcanoes: the dormant volcano onNarcondam Island and the active volcano onBarren Island (Ray 1982; Pal et al. 2007). Activelava flow from Barren has been registered asrecently as in 1994, and restricted phreatic andfumarolic activities coinciding with the devastating2004 Sumatra earthquake have been recorded.The eruptions are highly explosive–pulsativestrombolian type (15–30 s interval, Pal et al. 2007),gradually merging into pilinian style (Haldaret al. 1996). Andesite, dacite, and basalt (includingolivine basalt) have been reported. A local gravityhigh of as much as 40 mgal may be related with avolcanic plug at shallow depths under the Barrenand Narcondam volcanics in the north Andamanarea. The lava chemistry closely matches that ofthe high-alumina island arc tholeiites enriched inlight rare earth elements (LREEs) and largeion lithophile elements (LILEs), and depleted inheavy REEs (HREEs) and high field strength ele-ments (HFSEs), compared to N-type mid-oceanicridge basalt (MORB) (Haldar et al. 1999; Pal et al.2007). Electron probe microanalysis of mineralsfrom the 2004 to 2005 volcanics revealed anorthite(An94.5) and bytownite (An81.6–89.0) as prime feldsparphenocrysts, with labradorite (An57.3) as the majormicrolite phase in the groundmass (Pal et al. 2007).Olivines are forsteritic (Fo72–79) and pyroxenes areof diopside and pigeonite affinity. Sr-isotope com-positions show varying crustal contamination, and

a possible E-type MORB source has been sug-gested for the lava suite.

From the recent study of interbedded pyroclas-tic sequences within the Archipelago Group ofsediments, Pal et al. (2005) extended the history ofactive inner arc volcanism up to Miocene–Pliocene.High SiO2 (69.44 to 75.37 wt%) and Al2O3 (12.90to 19.17 wt%) contents of tuff samples allowedthese workers to surmise felsic volcanism associ-ated with the pyroclastic emplacement. Consider-ing the basaltic–andesitic character of present-dayBarren or Narcondam island magmatism, a felsicto basaltic compositional evolution in arc volcanismhas been suggested.

In the north, the volcanic area of Mount Popa,Mayanmar and in the south, the Wey and Bruehislands off the northern tip of Sumatra, form aregional strike-wise extension of the volcanic belt,of possible inner-arc status. In the south of thesubduction zone, between Sumatra and Java,absence of volcanicity from the early Eocene to theearly Miocene has been correlated with a north-ward advancement of the subduction zone duringcounterclockwise rotation of northern Sumatra(Hall 2002); this may be valid for the Andamanregion also.

INTEGRATION

The relationship between the subduction zoneand convergent plate margin is better appreciatedwhen their typical representations are consideredtogether. Whereas geometry of convergent platemargin appears in map view, a subduction zoneis typically shown in cross-section. Along theAndaman–Sumatra subduction margin, details ofmargin evolution have proven difficult to interpretbecause of overprinting of sequential phases ofdeformation, lack of detailed geophysical imagingof deep structural features, uplift and erosion ofEocene and younger marine strata during the lateCenozoic, and lack or poor quality of surface expo-sure onshore. Figure 8 illustrates the gravitysignatures (Fig. 8a), Benioff trajectory of the sub-ducting Indian slab and overlying morphotectonicsetup at the Andaman Plate margin in an east–west transect (~12.5°N, Fig. 8b), a map view withdifferent tectonic stages of Andaman Sea opening(Fig. 8c) and sub-surface profiles from two seismictransects (Fig. 8d,e). It can be argued from thesteeply dipping Benioff zone trajectory (Fig. 3),estimated about 70 Ma (Müller et al. 1997) age forthe subducting slab and presence of extensionalback-arc system with active spreading on the

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overriding plate, that the Andaman sector nearlyresembles Mariana-type subduction margin be-longing to the class 1 strain system of Jarrard(1986). Towards the south in the Sumatran sector,the shallower dip of the Benioff zone trajectory,attainment of early dip angle saturation (~25 kmdepth) and younger age for the subducting slab(47 Ma), dominance of strike–slip motion with littlecompression or extension, the subduction charac-ter can be approximated as a class 4 strain cat-egory of Jarrard (1986). This strike-wise variablesubduction character is consistent with the varia-tion in convergent margin geometries between theAndaman and Sumatra sectors and is well corrobo-rated by the estimations of widely varying cou-pling coefficient estimated at different parts of thismargin: high to very high (c = 0.6 to 1.00, Scholz &Campos 1995; Prawirodirdjo et al. 1997), interme-diate (Christensen & Ruff 1988), and very low(c = 0.02 to 0.05, Peterson & Seno 1984; Pacheco

et al. 1993). Understandably, the subductioncharacter in the Sumatran sector corroborates ahigher coupling coefficient and had preferentiallyresulted higher magnitude earthquakes in com-parison to the Andaman sector in the north.Historical records for the last 200 years in theSumatra forearc clearly suggest incidences ofgreat earthquakes (1797, seismic moment magni-tude [Mw] ~ 8.4; 1833, Mw ~ 8.75; 1861, Mw ~ 8.25–8.5; 2004, Mw ~ 9.3; 2005, Mw ~ 8.5; 2007, Mw ~ 8.4)in the south and southeastern part of the SumatraPlate margin only; the Andaman–Nicobar andnorthern Sumatra segments preferentially record-ing the major shocks not exceeding Mw = 8.0 (1847,Mw = 7.5; 1881, Mw = 7.9; 1941, Mw = 7.7).

Separated from Antarctica and Australia ofeast Gondwana, India started its journey in theCretaceous with concomitant subduction along theeastern Asian margin. Identified in seismic pro-files, deformations in the Cretaceous sediments of

Fig. 8 Model showing Benioff trajectoryfor the subducting Indian slab and spatial dis-tribution of various morphotectonic elementsdeveloped on the overriding plate in the northAndaman sector, (a) gravity signatures (afterMukhopadhyay & Krishna 1991) of differentelements; horizontal axis represents distanceseastward from 12.5°N, 91.2°E, (b) Beniofftrajectory of the subducting Indian slab andspatial distribution of various morphotectonicelements; open triangles represent earthquakehypocenters and vertical axis represents depthin km, (c) map view illustrating two-phaseopening of Andaman Back-arc Basin; forma-tion of the rift-related Mergui Basin in thesouth (~11 Ma) preceded active spreading (4to 5 Ma onwards) towards north. Small boldarrows indicate the stretching and openingdirection of back arc basin, and (d,e) seismictransects showing the subsurface signaturesfor trench–accretionary complex (d, after Rao& Krishna 1997) and rifted back-arc system(e, after Raju et al. 2004).

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the Bengal fan in the west of the Java Trench,and the occurrence of Cretaceous folded thrustpackets in the east of middle Andaman Islandconfirm onset of subduction since the Cretaceous(Roy 1992; Pal et al. 2003). With continuous sub-duction, a number of ophiolite slices along withpelagic sediments were scrapped off to form anaccretionary prism. The sequential change in thedip of the thrusts from west to east within theouter-arc prism suggests accretion of ophioliteslices in their present position, rather theiremplacement as allochthonous nappe sheets, assuggested by Sengupta et al. (1990). During theEocene, the basins were isolated and separated bythrust-bounded structural ridges of the ophiolitebasement (Chakraborty et al. 1999). Individualbasins of ophiolite provenance and coarse detrituswere uplifted and became part of the accretionarycomplex. Frontal accretion eventually led to agradual increase in the slope of the accretionarywedge, and ultimately to extension during theLate Eocene–Early Oligocene. A fault-controlledforearc basin was initiated which accommodatedthe turbidite package (i.e. Andaman flysch sedi-ments) supplied from both the outer-arc prism andthe delta systems in the north.

Attempts have been made to constrain the drifthistory of the Indian Plate, based on detailed inter-pretation of marine magnetic anomalies in thecentral Indian Basin. The dramatic decrease in theIndian Plate velocity around 50 Ma is interpretedas a signal for the closure of the Neotethys in Tibet(Patriat & Achache 1984). The northward motion ofIndia continued until 20 Ma, when a new regimeof plate tectonics was established in which Indiaand Australia became integral parts of a singleplate (Patriat & Achache 1984). Concomitant withthis, a counterclockwise rotation of India relative toEurasia caused the plate convergence directionto become more oblique at this plate margin sincethe Middle to Late Eocene (Fitch 1972; Patriat &Achache 1984). Initiation of oblique convergencewould have been associated with a partitioning ofmotion: the lateral component is frequently takenup by an arc-parallel sliver fault, and the normalcomponent controls subduction. This partitionedforce component along the sliver fault is respon-sible for development of an extensional domain onthe overriding plate. In the case of the Andaman–Sumatra margin, the strike–slip rate associatedwith the oblique subduction is estimated to be sub-stantial (i.e. 50%), in contrast to very low (<10% ofoverall convergence rate) strike–slip rates in theforearcs of most of the plate segments with frontal

subduction. Such high strike–slip motion resultedin the development of a dextral strike–slip systemin Sumatra, and also caused extension in theAndaman region. Hall (1997) related this exten-sional event with basin history near the presentSumatran Fault, the Ombilin Basin, for which defi-nite forcing of strike–slip motion on sedimentationhas been found in the Neogene. Furthermore, anincrease in plate convergence obliquity is observedalong the Sumatra margin, from its southern tipto the Andaman Sea. Besides causing significantchange in the stress distribution at this platemargin, the new organization of plates with higherconvergence obliquity triggered deformation, andthe generation of new tectonic grains within boththe downgoing slab and the overriding plate. Onthe overriding plate sliver fault, the West Andamanand Sagaing fault systems were formed, alongwhich the relative movements caused extensionand opening of early Andaman back-arc system(Khan & Chakraborty 2005) in the formation of theMergui–Sumatra Basin during the Oligocene.

This long extensional history, covering the last45 Ma, for this plate margin is supported also by thestudies of Bertrand & Rangin (2003) and Khan(2005) in the central Mayanmar–Andaman Sea.However, the onset of active spreading at theAndaman back-arc has been suggested to be ataround 4 to 5 Ma, based on both the periods ofslab deformation episodes for the downgoingplate (Khan & Chakraborty 2005) and on magneticanomaly studies (Raju et al. 2004) of the back-arccrust. The southern segment of the Andaman SeaRidge possibly records the initiation history of thisspreading phase. The ocean–continent transitionrecorded in the Mergui terrace (Curray et al. 1979)hints towards a possible locale from where thespreading phase initiated. Subsequent veeringin the rotation of the Andaman Sea Ridge inthe northeast–southwest direction is possibly anoutcome of multiple tectonic forces including north-ward velocity inhomogeneity between the Indianand Burma plates (Le Dain et al. 1984; Maung1987), and north–south compression resulted therefrom uplifting of oceanic crust in the post-MiddleMiocene through the Alcock and Sewell Seamounts(in the north and south of the ridge respectively),and the post-Middle Miocene ‘stop and go’ spread-ing character in this ridge system.

CONCLUSIONS

1. The Andaman subduction margin represents atrench–outer-arc–back-arc system that evolved

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sequentially with spatially and temporallyvarying subduction character. Although asubduction–accretion process was initiatedat this plate margin in the Cretaceous, thecounter-clockwise rotation of India with respectto Eurasia in the Middle to Late Eocene causedonset of oblique convergence and reorganizedthe stress distribution on the overriding plateleading to the present-day morphotectonicset-up including the ongoing motion of thesliver plate in the south and active back-arcspreading in the north.

2. The reconstruction of subduction geometriesfor the Andaman and Sumatra sectors andtheir comparison with corresponding conver-gent margin architectures allowed visualizationof strike-wise variation in subduction character.The Andaman sector with active back-arcextension and spreading represents the class 1strain system of Jarrard (1986) whereas theSumatra sector approximates the class 4variety.

3. The study reveals northward sequential evolu-tion and outboarding of basins with widelyvarying geometry and filling history at differentmorphotectonic domains of the arc–trenchsystem.

4. Comparison of the Andaman–Sumatra marginevolution to general models of subduction-margin behavior and to specific accretionaryprisms along the Sunda margins suggest morecomplex mechanisms than simple accretion-dominated processes. Evolution of basins alongthis margin as trench-slope, forearc, or back-arc appears to be strongly overprinted byregional expression of plate dynamics ratherthan simple subduction-related accretion.

ACKNOWLEDGEMENTS

The authors thank the Indian School of MinesUniversity and also Professor M. Delafontaine foroffering probing comments on an early version ofthe manuscript.

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