Proc. Indian Acad. Sci. (Earth Planet. Sci.), Vol. 99, No. 2, June 1990, pp. 187-199. Printed in .

Tectonics of the ophiolite belt from Naga Hills and Andaman Islands, India

S K ACHARYYA, K K RAY and S SENGUPTA Geological Survey of India, Calcutta 700016, India Abstract. The ophiolitic rocks of Naga Hills-Andaman belt occur as rootless slices, gently dipping over the Paleogene flyschoid sediments, the presence of blue-schists in ophiolite melange indicates an involvement of the subduetion process. Subduction was initiated prior to mid-Eocene as proved by the contemporaneous lower age limit of ophiolite-dedved cover sediment as against the accreted ophiolites and olistostromal trench sediment. During the late Oligocene terminal collision between the Indian and Sino-Burmese blocks, basement slivers from the Sino-Burmese block, accreted ophiolites and trench sediments from the subduction zone were thrust westward as nappe and emplaeed over the down-going Indian plate. The geometry of the ophiolites and the presence of a narrow negative gravity anomaly flanking their map extent, run counter to the conventional view that the Naga-Andaman belt marks the location of the suture. The root-zone of the ophiolite nappe representing the suture is marked by a partially-exposed eastern ophiolite belt of the same age and gravity-high zone, passing through central Burma-Sumatra-Java. The ophiolites of the Andaman and Naga Hills are also conventionally linked with the subduction activity, west of Andaman islands. This activity began only in late Miocene, much later than onland emplacement of the ophiolites; it further developed west of the suture in its southern part. Post-collisional northward movement of the Indian plate subparallel to the suture, also developed leaky dextral transcurrent faults close to the suture and caused Neogene-Quaternary volcanism in central Burma and elsewhere. Keywonls. Naga Hills; Andaman islands; ophiolites; India-Eurasia (Sino-Burma) collision; suture; subduction; transeurrent movement.

1. Introduction

The Naga Hills, in northeastern India, represent the northern segment of the N-S trend within the Indo-Burmese Range (IBR), which is generally recognized to be the on-land prolongation of the Andaman-Indonesian island-arc. Further north, the Naga hills is linked with the eastern end of the . A narrow trail of late Mesozoic-early Paleogene ophiolitic rocks occurs along the northern margin of the Himalayas and the eastern margin of the Indo-Burmese Range and then continue southward to the Andaman-Nicobar-Mentawi group of islands representing subaerial parts of the fore arc of the indonesian island arc system (figure 1). Thus the Naga- Andaman segment and its ophiolites are crucially located at the junction of the Tethyan-ophiolite belt of the Himalayas and the cordilleran-type ophiolite belt of the Indonesian island arc. Although the geological setting of this rather inaccessible belt was imperfectly known till the recent past, several attempts have been made to fit this region into global plate tectonic schemes and models. The ophiolite belt of the Indo-Burmese Range that continues into Andaman islands is conventionally regarded as representing the suture along which the eastern edge of the Indian plate collided with the Eurasia (Gansser 1980). The ophiolites of the Naga- 187 188 S K Acharyya, K K Ray and S Sengupta

3o-1 - .... \ I iro HIM AL&'~ AI

BENGAL [C~b r," V I~

~\ /= v~lHlO.~NOSi J t \~. a~l,.-OF i

: RAY OF '~ 1

i

Iz-o ~t .~AUOAiAU ~*

i 0 ,q\

PLAT e ~'~

Figure I. Tectonic setting of the lndo-Burmese Ranges and Andaman island arc. Location of Naga Hills and figure 2, South Andaman island and figure 3, and North Andaman island and figure 5 are shown. Legend: Solid black--Late Mesozoic-early Eocene ophiolites; V--Neogene-Quaternary volcano/volcanics; Double barded line--Trench, trace of active subduction zone: Saw toothed line--major thrust; parallel double line--Back-arc spreading axis. Transcurrent fault shown by sense of movement. Dash-dot line--Trace of negative gravity anomaly, Dashed line--Trace of gravity high (Bouguer on-land, Free Air on sea). Abbreviations: MAMO--Mandalay and Myitkyina Ophiolite, ITO--lndus-Tsangpo Ophiolite, SH--Shelf, WAF--West Andaman Fault.

Andaman belt are imputed to the ongoing Andaman-Java subduction activity which began since Cretaceous in time and extends northward in space (Karig et al 1979; Curray et al 1982; Mukhopadhyay and Dasgupta 1988), and these ophiolites are located within the accretionary prism (Moores et al 1984). The geometry, nature of contact, and the tectonic setting of the ophiolite bodies and associated rocks, from Naga Hills and Andaman islands, have been found to be at variance from that visualized by popular models. These ophiolites occur as rootless, thin sheet-like bodies at the highest tectonic level, being transported as nappe over the Paleogene flyschoid sediments. They do not represent the location of the eastern Indian suture zone as generally believed and are therefore analysed in this paper. Ophiolites from N a#a-Andaman belt 189

2. Geological framework

The Indo-Burmese Range (IBR) in its northern part is flanked on the west by the -Mikir Precambrian massif, representing NE prolongation of the Indian shield, and the shelf. Towards the south, IBR is flanked on the west by the Bengal basin and then the Bay of Bengal. Towards the east, IBR is flanked by the Central Burma Basin. The Andaman-Nicobar island arc occurs further south in structural continuity of the IBR (figure 1) as revealed by the continuity of morphotectonic units and regional gravity anomaly. In the Naga Hills and Andaman islands, a highly dismembered thin zone of serpentinite and deformed ultramafic rocks, ultramafic-mafic cumulates, basaltic and other volcanic rocks with closely associated oceanic pelagic sediments (Karunakaran et al 1964; Sen and Chattopadhyay 1978) represents an ophiolite assemblage (Anon 1972: Moores 1982). The principal rock types include dunite, harzburgite, lherzolite, wherlite, pyroxenite, olivine-gabbro, gabbro-norite and mafic volcanic rocks (Haldar 1985; Venkataramana et al 1986). There is an overall similarity of petrological characters of magmatic members of the ophiolite suite occurring in northern and southern Andaman sectors. The oceanic pelagic sediments are often interbanded with or overlie the basaltic volcanies as normal sedimentary cover and indicate that they belong to the Cretaceous-to-early-Eocene age for the oceanic domain (Acharyya et al 1989). The ophiolitic rocks in the Naga Hills and Andaman islands occur as a stack of thrust slices. Even in close traverses it is often difficult to correlate the different slices of the dismembered suite. These slices are open-folded and occur along a narrow linear belt occupying an elevated tectonic position and usually at cores of open synformal klippen. In the Naga Hills, the ophiolite belt is about 200 km long and relatively wider in its northern Nagaland sector. Towards south in the sector, the ophiolite belt is narrow and split into isolated bodies. In the Andaman belt it occurs as a trail of detached bodies and is better represented in the main Andaman islands (figures 1, 2 and 5). A partially exposed but parallel and contemporaneous ophiolite belt is located in central Burma, occurring below the cover of the Mio-Pliocene sediments and juxtaposed with the Neogene-Quaternary volcanic line. One other ophiolite belt is located further east and occurs along Myitkyina-Bhamo. Geological details of these ophiolite belts in Burma are not well known (Bender 1983). Further south the central Burma ophiolite belt follows the chain of volcanoes in Sumatra-Java and is concealed by the Andaman Sea that is located between them (figure 1). The ophiolite suite from Naga Hills and Andaman islands is also closely associated with other sedimentary and metamorphic rocks which give clues regarding their tectonic setting. The former includes ophiolite-derived clastics, olistostromal argillites and rhythmites. The associated sediments representing a diverse environment of deposition and disordered age relation, are usually tectonically jumbled up, and structurally form a melange which underlies the main ophiolite complex or occur at places within them (figures 2 and 3). In major parts of the Naga Hills, the ophiolite- derived clastics are imbricated with the ophiolitic rocks but have been often mapped separately and christened the Phokphur Formation (Acharyya 1986). In the Andaman ophiolite belt, and southern parts of Naga Hills, around Ukhrul (Manipur), these are unmappable in 1:50,000 scale and occur as a melange mainly with olistostromal t90 S K Acharyya, K K Ray and S Sengupta

INDEX 5Kin J N c- 2 .:..'.:-~ p~ AF o KYOI. I

_ r r r V 4 O C)" 5 PHOKPHUR D

D

D AF SARAMATI D D MOYA

COVE

"/-o I I--" --4~6

CH IR IYATAPU

0 icY v** **e c o* * Fig- 2 , . . ,~ Fig-3

Figures 2 and 3. Geological map of Naga Hills ophiolite belt, northern Nagaland sector 3. Geological map of Andaman ophiolite belt, South Andaman and Rutland islands. 1--Undifferentiated ultramafic and mafic rocks; 2--Cumulates; 3--Mafic volcanics; 4--Serpentinites; 5--Phokphur Formation (Ophiolite-derived clastic sediments); 6--Melange (Ophiolite-derived clastic sediments, ophiolitic rocks and ohstostromal sediments); 7--Disang Formation; 8--Andaman Flysch; 9--Nimi Formation; lO--Gneisses and other metamorphic rocks (Naga Metamorphic Complex and others); l 1--Archipelago Group; 12--Thrust contact; 13--Strike and dip of bedding, contact; 14--Axial trace of antiform and synform; 15--Faults; 16--Lines of cross sections shown in figure 6. OphioIRes from Naga-Andaman belt 191

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l"ox..,,/.v -' " ' -'<--:--" -f e ,I, o so?K.

x y Figure 4. Geological map of Phokphur area, northern Naga]and. Location of Phokphur shown in figure 2. Legends same as in figures 2 and 3.

argillites. In the Andaman belt, these have been informally designated the Namunagarh formation (Acharyya et al 1989). At Phokphur and other locations in Nagaland, and around the east-coast of the Saddle Hill, North Andaman, the incompatible relation between these ophiolite-derived clastics and the dismembered ophiolitic rocks is well preserved. The imbricated nature of occurrence of the ophiolitic rocks and the Phokphur Formation is also evident in the Phokphur area (figure 4). Lithologically the Phokphur and Namunagarh formations are closely similar and consist of gritty sandstone, lithic-wacke, conglomerate, siltstone, clay and minor limestone The clasts of these sediments are essentially of ophiolitic rocks such as basic rocks, radiolarian cherts, plagloclase, serpentines etc. Their grain size varies abruptly, both along and across the bedding, and often sediments of contrasting grain size are juxtaposed. Bedding character includes parallel or lenticular bedding or cross- bedding. Units with uneven scour base as trough and channel are present. All these indicate a shallow water high energy sedimentation. The Phokphur Formation also consists of volcaniclastics, tuffaceous wacke and minor volcanics. The Phokphur and Namunagarh formations contain similar shallow marine fauna characterized by larger foraminifera, invertebrates indicating middle and middle-to-late Eocene age respectively. The former also contains plant remains indicating paralic sedimentation (Acharyya et al 1989). The lower age limit of these cover sediments unconformably overlying the dismembered ophiolites, fixes the lower age limit of ophiolitr accretion when they were exhumed to shallow marine and on-land conditions (Moores 1982). In the southern parts of Naga Hills (Manipur) and the Andaman sector, is found an argillite facies with olistoliths of diverse types and sizes. Limestone blocks of late Cretaceous-to-mid Eocene age, ophiolitic rocks and gneissic and other continental metamorphic rocks occur as olistoliths. These represent trench sediments and have been dated mid-Eocene or younger based on the youngest age of olistoliths (Acharyya et al 1986, 1989). These sediments alone or in tectonic association with other sediments occur as a melange and structurally underlie the ophiolitic rocks consistently. 192 S K Acharyya, K K Ray and S Sengupta

Towards the east, the Naga Hills ophiolites and the sedimentary cover are tectonically bordered and overriden by an assemblage of quartz-mica-schist, garnet- mica-schist, gneiss, granite-gneiss etc., comprising the Naga Metamorphic Complex and their cover, the Nimi Formation made up of Orbitolina bearing limestone and microcline bearing arkosic quartzite (Acharyya et al 1986). Open-folded klippe of Naga Metamorphics occur over the ophiolites or even above the autochthonous Disang Formation (figure 2) (Acharyya 1986). The Naga Metamorphic Complex is also exposed as basement rocks in the northern parts of the Central Burma Basin (Brunneschweiler 1966). In the Andaman belt smaller bodies of similar metamorphic rocks occur within the ophiolite melange. In contrast to these continental metamorphic rocks, the blue-schist mineral assemblage with glaucophane, jadeite etc., occur sporadically within the ophiolite melange from Naga Hills (Ghose and Singh 1980; Chattyopadhyay et al 1983). Similar mineral assemblages have been recorded from the ophiolite rocks from Burma (Bender 1983), and noted by the present authors at Andaman. In the Naga Hills, the ophiolite pile structurally overlies a thick succesion of shale and rhythmite comprising the Disang Formation (Eocene) and representing autochthonous distal shelf to flyschoid sediments. In the adjacent area to the west, the Disang is conformably succeeded by arenaceous and turbiditic Barail sediments (Oligocene). In southern Naga Hills, often a wedge of olistostromal argillite, conventionally also grouped with the Disang, intervene between the overriding ophiolitic rocks and the rhythmite and turbidite bearing floor sediments of the Disang Formation (sensu stricto). In the Andaman belt, the Andaman Flysch of Eocene- Oligocene age underlies the ophiolites, with a tectonized melange of olistostromal sediments, rhythmites and ophiolite-derived elastics separating the two units. A pronounced late Oligocene angular unconformity is recorded from the entire region. Neogene sediments comprising the shallow marine Surma Group and younger fluvial sediments unconformably overlie the Barails in the westerly-located molasse basin flanking the Naga Hills. The Neogene cover is generally absent in the central Naga Hills. However, in the core of the syncline, a thin cover of the Surma sediments unconformably overlies tightly folded and eroded Barail sediments (Acharyya 1986). In both the belts the unconformity surface is affected by open folding (D2). Along the eastern margin of the Naga Hills, the Mio-Pliocene molasse sediments of the Central Burma Basin unconformably overlie the ophiolites and the Naga Metamorphic Complex (Brunnschweiler 1966). In the Andaman belt, the Mio- Pliocene cover includes pelagic limestone, clay etc., which unconformably overlies the Andaman Flysch and also ophiolite nappe rocks (Ray 1982). As in Nagaland, the entire pile together with the unconformity surface is affected by D2 folding. The palaeotectonic setting of the Naga Hills and Andaman ophiolites has been also inferred from chemical signatures in their basaltic rocks. Geochemical data from basaltic rocks from Naga Hills Ophiolite belt indicate the presence of basalts erupted in two different tectonic settings. The more dominant LREE-enriched basalts are characterstic of sea-mounts (Sengupta et al 1989). This type is not yet recorded from the Andaman ophiolite belt, but the presence of abundant limestones and their fauna in directly point towards the presence of seamounts also in this sector. The other type of ophiolitic basalts from Naga Hills and Andaman island show MORB like or back- arc-basin like character (Ray et al 1988; Sengupta et al 1989). Ophio/ites from Naga-Andaman belt 193

3. Geometry and contact relation

Members of the ophiolite suite and other rocks thus have been juxtaposed and stacked up because of thrusting, and simplified tectonic successions from Naga Hills and Andaman island are shown in table I. The tectonic pile is open folded in mesoscopic and macroscopic scale. Same style and scale of folds (D 2 ) have also affected the Mio-Pliocene cover sediments and have controlled the map pattern. The generalized geological map of the Nagaland (figure 2) and Andaman ophiolite belts (figure 3) even in a small scale, clearly depicts the open- folded nature of the ophiolites. In Naga Hills the synformal cores of ophiolites and their cover, at highest tectonic levels, are occupied by klippe of the Naga Metamorphics. These large scale folds are non-plunging or with a very low plunge. The NNE-SSW to N-S trending lithological units in the ophiolite belt are repetitions due to open, upright folds. As a result of these open folds, the floor and cover rocks of the ophiolites are often exposed along the antiformal cores (windows) and synformal klippe respectively. A synformal klippe of Naga Metamorphics and ophiolite voicanics occur over the Disang Formation, west of the main ophiolite belt in the Chipur area (_figure 2). The open folded nature of ophiolite slices and its tectonic floor, represented by a melange mainly constituted of sedimentary rocks like olistostromal sediments, ophiolite-derived elastics and also ophiolite volcanics and metamorphic rocks, from the North Andaman area is shown in figure 5. Therefore the geometry of these open folds has controlled the gentle dip of contacts of these thrust-bounded ophiolite bodies (figure 6). This common style is reflected uniformly by their map pattern from Naga Hills and Andaman islands. Contrary to these open folds (D2) a set of asymmetrical, tight to isoelinal folds of earlier generation (Dr)have been recognized mainly in mesoscopic scale from the

Tablet. Tectonicsuccession of ophiolite and associated rocks Naga Hillsand Andaman islands. Naga Hills Andaman Islands

Nimi Formation (Mid Cretaceous) unconformity Naga MetamorphicComplex Thrust Phokphur Formation (Mid Eocene) Dismembered ophiolite with Unconformity slices of continental Dismembered ophiolite metamorphic rocks Thrust Thrust Archipelago Group (Neogene) Unconformity Melange of ophiolite, Phokphur Melange of Namunagarh Formation, Olistostromal sediment formation. Olistostromal sediments (Lipa fm},Andaman Flysch Thrust Thrust Surma Group (Early Miocene) Archipelago Group (Neogene} ...... Unconformity Unconformity Barail Group (Late Eocene-Oligocene) Andaman Flysch(? Eocene-Oligocene) Disang Formation (Eocenel 194 S K Acharyya, K K Ray and S Sengupta

. :!: :.!!.i:i!:i:.. :..:,

Figure 5. Geological map of southern parts of North Andaman island. Legend: l--voicanies; 2--Undifferentiated ultramarie and marie rocks; 3--Melange of olistostromal and ophiolite-derived sediments; 4--Schist, phyllite; 5--Axial trace of antiform synform; 6--Thrust contact; 7--Strike and dip of contact.

D~ ~H///////////////~MSL

MT, TUK&R -""" ~ IADDLE HILL

C 0 F Figure 6. Geological cross-sections. Location of sections shown in figures 2, 3 and 5. Nature of occurrence of Namungagarh formation (ophiolite-derived sediments) shown diagrammatically by open circle symbol. entire length of the ophiolite belt. These are defined by compositional banding in the oceanic pelagic sediments of the ophiolite, in the ophiolite-derived cover sediments and also the floor sediments of the tectonic pile. Usually no planar structures have developed parallel to the axial-plane of these folds except as fanned fracture sets at places in competent layers. However, in eastern Nagaland close to the ophiolite belt such folds with well-developed axial-planar slaty-cleavage have been identified. The asymmetry and vergence of these folds indicate movement from east to west and these folds were possibly generated during the westward thrusting of the ophiolite and associated nappes (Sengupta et al 1987). Ophiolites from N aga- Andaman belt 195

The superposition of these two fold sets with axial planes at high angle has produced the type llI interference pattern (Sengupta et al 1987). The axial trend of the upright folds is generally coaxial with the earlier folds. The earlier folds have not developed in macroscopic scale to control the map pattern.

4. Tectonic setting and emplacement history of ophiolites

The ophiolites of Naga Hills (states of Nagaland-Manipur) and Andaman islands thus occur as a linear belt of shallow, open-folded, thin sheet-like bodies with a gentle-to- moderate dip of their contact. Occurrence of continental metamorphic rocks overriding the Naga Hills ophiolite stack and as klippen over the ophiolite or even over the Disang, the sense and vergence of earlier generation asymmetric folds indicating westward tectonic movement, presence of melange at the sole of the ophiolite stack and occasionally within the ophiolites, and occurrence of younger Eocene-Oligocene flyschoid sediments flooring the ophiolites indicate that these occur as low-dipping ophiolite nappe, overthrust from a root zone located further east (Acharyya et al 1989). Thus the geometry, structural style and tectonic setting of these ophiolites contradict prevailing views, according to which these are shown as steep-dipping bodies marking the suture zone along the eastern margin of the Indian plate (Rodolfo 1969; Gansser 1980; Curray et al 1982; Mukhopadhyay and Dasgupta 1988). Based on the tectonic setting of these ophiolites and associated rocks, the mechanism of their on-land emplacement can be reconstructed. Both in the Naga Hills and Andaman islands, oceanic domain having volcanism and pelagic sedimentation existed during Cretaceous to early Eocene. Based on discontinuous occurrences of ophiolitic rocks along this belt and a marginal basin-type chemical affinity of their volcanic rocks (Ray et al 1988; Sengupta et al 1989), it is presumed that several small ocean basins might have existed in this belt, a situation similar to the present-day Indonesian island arc system, where the Andaman Sea, Banda Sea etc., are like features. However, the tectonized nature of the ophiolites, indicated by their disrupted occurrence in both sectors, is also an added reason for their discontinuous occurrence. The geochemical signature of basaltic rocks from Naga Hills ophiolites, and the nature and age of limestones from Naga Hills and Andaman, indicate the presence of seamounts. Smooth continuous subduction seems to have been hindered by the presence of seamounts on the down-going ocean floor. Presence of blue-schists indicates involvement of the subduction process. Remnants from clipped-off seamounts and slivers from the ocean floor were accreted at the subduction zone. The olistostromal sediments were deposited in the subduction-related trench bordering the positive area of accreted ophiolites and slivers of basement rocks from the overriding continent. The accreted ophiolites were also unconformably overlain by the ophiolite-derived cover sediments in the hinterland basin to the east, where they were deposited in shallow marine and on-land condition. The lower age limit of both the olistoliths from the trench sediments and the ophiolite-derived cover sediments, is Mid-Eocene. This age therefore, fixes the lower age limit of subduction-related ophiolite accretion. The early structural grain, in the Naga Hills and Andaman ophiolites, indicates east-to-west accretionary process. Initially it imbricated the ocean floor contributing 196 S K Acharyya, K K Ray and S Sengupta

to ophiolite-derived clastics. The middle Eocene cover sediments were subsequently imbricated and also thrust over by the metamorphic basement rocks of the overriding continent. Accretionary process also affected the trench deposits. Towards west, however, the distal shelf and turbiditic floor sediments remained structurally unaffected by the early accretionary process, and thus preserve a continuous sedimentary record from Eocene to Oligocene. The accretionary folds (D~) thus developed over a wide span of time, and an east-to-west polarity of their development can be recognized. It appears that the entry of seamounts to the subduction zone temporarily jammed subduction, but the continued eastward oblique convergence between the Indian and Sino-Burmese blocks, as they broadly moved northward, enforced removal of these barriers and initiated on-land accretion of ophiolites which were accreted to the leading edge of the overriding Sino-Burmese continent. Ultimately, with consumption of the oceanic crust, there was terminal continent-continent collision during late Oligocene. This event is marked by widespread tectonism and a regional angular unconformity. Crustal shearing during terminal collision reworked the accreted slices. Gently dipping sheets of ophiolitic rocks, ophiolite-cover sediments, trench sediments, metamorphic rocks from basement of the overriding continent etc., were thrust westward from the suture zone as nappes and were emplaced over the Eocene- Oligocene distal shelf and flyschoid sediments of the western block. Geological evidence cited earlier clearly demonstrates that the root-zone of the Naga Hills and Andaman ophiolites, representing the India-Burma suture is located further east. Gravity data from the region also indicate that the Naga-Andaman ophiolite belt is flanked immediately on the east by a negative anomaly zone of the order of - 100 to - t50 mgal over 50-70 km (Evans and Crompton 1946; Peter et al 1966). This is consistent with the structural geometry of this zone indicating the shallow nature of the dense ophiolite bodies overlying the thick pile of lighter sediments. Further east, there is a parallel ophiolite belt passing through central Burma, Sumatra and Java, which is of late Mesozoic-Paleocene (Bender 1983; Page et al 1979) i.e. similar age. This eastern belt ophiolite, coinciding with a zone of regional gravity-high (figure 1), possibly represents the site of steeply dipping high density ophiolitic material. Several Eocene dioritic intrusives and related voicanics occurring in the Sino-Burmese ranges and Sumatra (Page et al 1979; Mitchell 1981; Bender 1983}, east or north of the eastern arcuate ophiolite belt, appear to be related to subduction. Thus the eastern ophiolite belt may represent the root-zone or the subduction zone where the ophiolites were accreted initially during middle Eocene, and subsequently during late Oligocene terminal collision, the ophiolite nappes were tectonically transported over the western block. The western ophiolite belt of the Naga Hills and Andaman islands thus represent the brow of the ophiolite nappe. In the Andaman islands and further Southeast, the brow of the ophiolite nappe lies adjacent to the currently active Andaman-Java subduction zone located towards the west and south of the island arc. Subduction along this zone seems to be active continuously'since Cretaceous (Curray et a! 1982) and the feature is also extrapolated northward to Naga Hills (Mukhopadhyay and Dasgupta 1988). The Andaman ophiolites are also proposed to have been generated in the Andaman Sea spreading centre iMoores et al 1984). The difference in age between the Andaman ophiolites and Ophiolites from Naga-Andaman belt 197

the Andaman Sea crust has been correctly pointed out as the main objection to such a hypothesis (Rod 1985). Several small ocean basins like Andaman Sea, Java Sea, Banda Sea opened up only since late Miocene (Curray et al 1982) and the Java trench was also formed around the same time (Umgrove 1948). The existence of this active trench, and the occurrence of Pliocene to recent volcanism in the inner arc suggest that the Andaman-Java subduction was active only since late Miocene. But the ophiolites of Andaman islands and its extension further north were accreted on-land, during mid- Eocene and thrust westward as nappe during late Oligocene. Thus they already existed as tectonized on-land features when the late Miocene Andaman-Java subduction and creation of Andaman Sea and such other small basins began. Thus in the southern parts of the Naga-Andaman ophiolite belt, a new subduction regime started during late Miocene, which was located west of the Late Oligocene suture. The emplacement of the Naga-Andaman ophiolite nappe over the Paleogene distal shelf and turbidite sediments of the western block, and the widespread late Oligocene unconformity marking a prominent break in sedimentation and tectonism recognized from the entire region, are inferred to have been caused by terminal continent- continent collision between the Indian and Sino-Burmese blocks (Acharyya et al 1989). In the Naga Hills sectors to the north, the Paleogene floor sediments are established to be laid on the Indian continent. Based on a close similarity of setting between the Naga Hills and the Andaman sector and geological continuity, it is postulated that the Andaman sector is also floored by continental substrate. In that case, the Indo-Burmese range, extending southward from Naga Hills and the Andaman island arc made up of non-volcanic rocks, would represent a part of the Indian continental block. This continental block in its southern part would be separated from the main block of the Indian continent by a portion of the Indian Ocean. Thus, whereas in the northern parts of the suture collision occurred between Sino-Burma and the main Indian block, in the south it was between Sino-Burma and the Indian continental fragment. To accommodate continued post-collisionai convergence, a new subduction regime, beneath the western margin of the continental fragment, was established later in the southern sector of the collision zone. This represents the currently active Andaman-Java subduction zone. Contrary to general belief, this structure does not continue northward into the internal parts of the lndo- Burmese Range and the Bengal Basin representing parts of the Indian continental block. The Indian and Eurasian plates were broadly moving northward and the Indian plate and Sino-Burmese micro-plate converged obliquely and ultimately collided during late Oligocene. Postcollisional drag, caused by the northward moving Indian plate subparallel to the suture, possibly produced dextral transcurrent faults located close to the suture zone (Acharyya 1989). 'Leaky' faults among some of these produced Neogene-Quaternary volcanism in Central Burma and elsewhere. In the southern part of the collision zone, these volcanics are likely to be mixed up with contemporaneous subduction-related volcanism. The Barren, Narcondam islands and other submarine volcanoes of the Andaman Sea are located along a N-S trending dextral transcurrent fault referred to as the West Andaman Fault (Curray et al 1982, Acharyya 1989). Northward this fault is joined with the Central Burma Volcanic line. 198 S K Acharyya, K K Ra)~ and S Senoupta

5. Conclusion

Ophiolitic rocks from the Naga Hills-Andaman island occur as nappes over the Paleogene distal shelf to flyschoid sediments. The ophiolites containing remnants from seamounts were accreted at the subduction zone. The ophiolite-derived elastic sediments of middle Eocene age were deposited in shallow marine to on-land condition over the initially accreted ophiolites, Contemporaneously a subduction- related trench bordered the accreted ophiolites and rocks from the overriding continent. The Indian and Eurasian plates were moving broadly northward and converged obliquely and ultimately collided. During late Oligocene terminal continent-continent collision of the Indian and Sino-Burmese blocks, basement fragments from the overriding continent, accreted ophiolites and olistostromal trench sediments from the subduction zone were thrust westward from the suture as nappes over the down-going Indian plate. The geometry of the Naga-Andaman ophiolites and their geological setting indicate that the root-zone of these ophiolite nappes, representing the suture, is located to the east. It is possibly marked by the partially exposed eastern ophiolite belt of the same age which passes through central Burma- Sumatra-Java. This belt has a gravity-high indicating steeply dipping high density ophiolite material. It is postulated that, whereas in the northern parts of the suture terminal collision occurred between the Sino-Burmese and the main Indian block, in the south it occurred between Sino-Burma and a continental fragment which was separated from the main Indian block by a part of the Indian Ocean. Thus, during post-collisional convergence, a new subduction regime, beneath the western margin of the continental fragment, was established since late Miocene in the southern sector. This represents the presently active Andaman-Java subduction zone. Post-collisional drag of northward moving Indian plate subparallel to the late Oligocene suture also produced dextral transcurrent faults located close the the suture zone. Some of these 'leaky' faults possibly caused Neogene-Quaternary volcanism in Central Burma area and elsewhere.

Acknowledgement

The authors are grateful to the Director General, Geological Survey of India, for providing facilities for the work and granting permission for this publication.

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