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J. geol. Soc. London, Vol. 136, 1979, pp. 519-527,2 figs. Printed inNorthern Ireland. The tectonic history of the Banda Arcs, eastern : a review

M. S. Norvick

SUMMARY: The Banda is underlain by a small marginal oceanic plate, which is believed to have formed during the early Tertiary. Unlike other marginal to the N, the spreading of the Banda platelet was restricted by surrounding continental blocks. Its present complexity is a result of late Miocene-earlyPliocene collision and obduction of theBanda Sea arc system over the leading edge of the Australian-lrian continental plate. Transcurrent faultingon the northern limb of the collision zone may have accentuated curvature of the arc. and volcanicitv ceased after collision in the and Seram sectors, but are still active at the eastern extremity of the arc.

Adouble spiral line of geologicallyvery complex, based on bathymetry (Mammerickx et al. 1976), reg- small, mountainous make up the Banda Arcs ional gravity (Vening Meinesz 1954; Chamalaun et al. of eastern Indonesia (Fig. 1). Although onshore out- 1976; Milsom 1977)and earthquake distribution crop information is scattered, and geophysical data on (Hamilton1974a; Cardwell & Isacks1978). Seismic the deep seas that cover 90% of the area sparse, the lineshave been published across the Timor tectonic history of the region has stimulated discussion (Branson1974; Beck & Lehner1974; Crostella & formany years, and produced different (sometimes Powell 1975) and the Seram Trough (Audley-Charles conflicting) interpretations. The present review attempts& Carter 1978). Two 1976 oceanographic surveys run to reinterpretto critically thedata and some of by Scripps(‘Indopac’ programme) and CCOP (‘Val- theplate tectonic theories on theevolution of the divia’ cruise) collected valuable seismic refraction and Banda Arcs in the light of recent geological mapping reflection data over the deep water areas (Purdy et al. and oceanographic surveys. 1977; Bowin et al. 1977; Shor et al. 1977). Datafrom most of thepre-1939 reconnaissance surveys have been compiled by van Bemmelen (1949). Tectonic framework These early studies suffer from the serious shortcom- ings that recognition of rock units in difficult terrain was often based on float material, and that palaeon- 4 major crustal plates occur around the margins of the tological dating was not as refined as it is now. Banda Arcs (Fig. 1). In the W, the Sunda , its The Geological Survey of Indonesia has a continu- forelandbasins and the extinct, accreted arcs of ing mapping programme on Timor, Seram, , Hal- Sulawesiforms southwesterna extension of the mahera and the Sula Islands (Sukamto, pers. comm.). Laurasiancontinental plate. In the S, theIndian Oil companystudies have led toseveral published OceanPlate and the attached Australian reports on Seram (Zillman & Paten 1975), the Timor extends northwards into Irian Jaya and westwards into et region(Crostella & Powell1975; Crostella 1977), the Sula Spur (Audley-Charles al. 1972). A seriesof accretedisland arcs are preserved in the northern Misool(Froidevaux 1975), Irian Jaya (Visser & Hermes1962; Vincelette 1973; Redmond & part of Irian as the Central New Guinea Orogen (Vis- Koesoemadinata 1976; Froidevaux 1978) and the Au- ser & Hermes1962). The Pacific OceanPlate is stralianNW Shelf (Balke et al. 1973;Warris 1973; moving westwards across the N side of Irian. The plate Laws & Kraus 1974; Powell 1976). boundary is a complex of active sinistral transcurrent Thegeological map of Audley-Charles (1968), to- faults, extending from northern Papua New Guinea to gether with recent mapping by the Geological Survey -the Sorong Fault System. of Indonesia,provides for the first timea complete TheBanda Arcs are thus enclosed by continental blocks all4 sides, and these are involved ina geological map of Timor on a scale of 1 : 250,000. In on spite of criticism by Grady(1975), Grady & Berry complex series of stresses. (1977),Chamalaun & Grady(1978)and 5 elements can bedistinguished within the Banda Brunnschweiler (1978), the complex geology appears arcs: to havebeen unravelled by Audley-Charles(1968), Carter et al. (1976)and Barber et al. (1977).Their 1. Theinternal oceanographic basins include the interpretation appears to be applicable to other parts BandaSea, Deep and Gulf of Bone,all of of the arc, and Audley-Charles et al. (1979) showed which areprobably underlain by oceaniccrust. A that Seram is geologically related to Timor. poorlyunderstood ridge, the Buton-Tukang Besi Little information is available on the sub-sea parts block,protrudes SE fromSulawesi into the Banda of theregion, and interpretations have mainly been Sea. This may be a micro-continent (Hamilton 1978).

0016-7649/79/090045 19$02.00 @ 1979 The Geological Society

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LEGEND Active Quaternary volcanoes 0 Areas of crustoceanlc m] Dormant Quaternary WlCanOeS 0 Active rubduction platr boundary Areas of Australian A crystalline El Active tramcurrent plate boundary -- Acfm plate boundary, over-ridden -e--- Areaswith probable Asian .-. A AA allochmanthrust OVW Actiw pbta boundary,uncertain Aurtralianautochlhon . .. . FIG.1. Simplified tectonic map of eastern Indonesia.

2. The InnerBanda Arc isa chain of activeand Java, but, as will be shown later, are thought to be a extinct Cenozoic volcanoes. Superficially, it is an ex- modern subduction trench. tension of the Java-Sumatravolcanic arc. However 4. The outer Banda non- extends from thesector E of Sumbawaprobably had adifferent Buruto Savuas an enormous U-shaped chain. A geological history to the islands in the W and appears complex of thrust sheets has been mapped on some of to have been initiated much more recently. the islands, and these hold the key to the elucidation 3. A series of deeps, culminating in the 7000+m of theentire region. Sumba, in the SW, appearsto Weber Deep, lie external and parallel to the volcanic lack thrust sheets, although it is now on-trend with the chain in an inter-arc position. These are probably not rest of thearc. This island may also be amicro- a continuation of the inter-arc troughsoff Sumatra and continent(Hamilton 1978). Audley-Charles (1975)

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believed Sumba to be a detached portion of pre-late continuation of thePliocene foreland. Nappes of Jurassic , which moved to its present allochthonous material have been mapped in Timor and positionalong hypothetical transform faults (the Seram.They overlie a para-autochthon of Permian Sumba Fracture) during the opening of the Wharton and Mesozoic clastic sediments, which can be closely Basin. comparedwith rocks of thesame age on the Au- 5. Theboundary between the arcs and the Aus- stralianNW Shelf and in Irian.The outcrops on tralian foreland is marked by a discontinuous series of Timor,which Hamilton identified as a tectonicsub- depressions,namely the Timor and Seram Troughs, ductionmtlange, are infact part of asuperficial and the Aru Deep. These were once thought to be a sedimentary slump breccia or olistostrome (Bobonaro continuation of the Java subduction trench, but deep Scaly Clay). A deep water subduction melangemay be sea drilling has shown that they formed in relatively present in theouter arc, but it occurs onlyas the recenttimes, and seismic refraction studies suggest lowestKolbano and Nief thrustsheets, whichlie that they are underlain by continental crust. structurally above the para-autochthon and consist of an imbricate wedge of folded and faulted but coherent sediments. Nature and origin of the Banda Arcs Pre-late Miocene history 2 theories have been used to explain the structure of Pliocenetectonics in the Banda Arc region have theBanda Arcs. Hamilton (1974b, 1976, 1978) be- been so disruptive that the pre-Neogene history of the lieved that the double arc is an active and long-lived area is difficult to decipher. subductionsystem. He thoughtthat the Timor, Aru Audley-Charlesand his co-workershave argued and Seram Troughs represent the outcrop of the sub- that and were separated by many duction zone that generated the volcanoes in the inner thousands of kilometres of during the Permian arc, and that the outer non-volcanic arc is a subduc- andMesozoic. They based this theory on supposed tion mtlange. Audley-Charles and co-workers (Carter differences in latitude, indicated by tropical faunas in et al. 1976; Barber et al. 1977) believed that the outer reefal limestones onthe Asian side and their absence in arc isessentially azone of Pliocene collision and presumedtemperate or circum-polar clastics onthe obduction between Asian-derived overthrusted mater- Australian side. They further supported their theory ialand Australian basement, that the present active withsparse palaeomagnetic data. However, shallow subduction trench lies between the inner and the outer water coral limestones are now known in the Permian arcs, and that both the external troughs and the outer and Triassic of the northern NW Shelf, Irian, Misool arc are underlain by Australian continental crust. and Papua New Guinea. Also, palaeomagnetic poles Earthquake and gravity data can be used to support for autochthon and allochthon on Timor are very close either theory. A Benioff zone dipping N and E under- (Chamalaun 1977), although this is based on a small lies the southern and eastern limbs of the arcs (Fitch amount of preliminary data, which may become mod- 1972; Cardwell & Isacks 1978). Active vulcanicity in ifiedby futurework (Audley-Charles, pers. comm.). the inner arc can be explained by partial melting of Unless climatic belts were much broader than at the down-going slabs of crust within this subduction sys- presentday, the wide latitudinal late Palaeozoic to tem. However, it is not possible to determine where Mesozoicseparation of Asiaand Australia postu- thesubduction zone reaches the surface. The outer latedby Audley-Charles (1977) isstill farfrom non-volcanic arc is marked by a well defined series of proven. negativegravity anomalies. These are generally ac- Onthe Australian-Irian foreland, the Permian- cepted to represent thick sections of low density ma- Middle Jurassic was a time of deltaic to fluvial clastic terial, which wouldbe consistent with either Hamil- sedimentation. Intermittent shelf carbonate deposition ton’s theory of the outer arc being a steeply dipping occurred in areasprotected from clastic influx. tectonic mtlange, orwith Audley-Charles’s contention Lithofaciesbelts onthe Australian NW Shelf show that a thick pile of low density thrusts is present in this progressively greater marine influence away from the region. continent, and this trend can be extrapolated to Timor However,recent seismic refraction work in the (Audley-Charles 1977). There is evidence on the Au- and the Aru Deep has shown that crust stralianNW Shelf forperiods of vulcanicity,rifting of continentalthickness underlies these depressions and block faulting, primarily in the late Middle Juras- (Shor et al. 1977), which tendsto support Audley- sic (Callovian) but also in the late Triassic, which were Charles’sinterpretation. DSDP borehole 262, in the probablyassociated with the break-up of Indiaand axis of the Timor Trough, penetrated Plio-Pleistocene Australia (Warris 1973). deep sea sediments, overlying upper Pliocene shallow TheUpper Jurassic-UpperMiocene Kolbano and marine sediments. Thus, the external troughs are rela- Nief imbricateunits on Timor and Seram are inter- tively recent, perhaps fault controlled depressions in a preted as accretionary wedges, which were originally

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deformed in a deep sea subduction zone. Pre-Upper marineclastic sedimentation occurred in thelate Jurassic elements have yet to be found in these units. Jurassic on theAustralian-Irian foreland, and this This apparent absence suggests that deep sea sedimen- lasted, in places, until the late Cretaceous. This time tation began in the late Jurassic, which in turn implies interval is poorly represented in Sulawesi. thatthe between Sunda and Australia The late Cretaceous (Fig. 2a) was the time that the began forming during the late Jurassic. It is possible first geologically well represented orogeny began in E thatan ocean existed in thisposition prior to late Sulawesi and northern Irian. It also saw the spread of Jurassictimes, but it is notknown how the crust pelagic limestone deposition onto shelf areas in NW beneath it was formed or removed. Australia,western Irian and the Sula Spur. This Northwestern Sulawesi (Palu area) and the 'Asian' changedto shallow water carbonate sedimentation allochthon of Timor(Lolotoi complex) contain high during the Tertiary, which extended over most of the grademetamorphics, which are presumed to have shelves of NW Australia and Irian by the Miocene. beendeformed in avery old, probably pre-Permian In Sulawesi, W-verging subduction caused orogeny.The allochthon of Timoralso includes a developmentand intermittent deformation, which sequence of limestones and volcanics (Maubisse For- lastedfrom the late Cretaceous until the middle mation),which Audley-Charles et al. (1972)used as Miocene.Carter et al. (1976)speculated that the evidencefor Permian island arc development. They Timor Aileu-Maubisse was thrust over the Lolotoi at laterreinterpreted these rocks as belonging to the theAsian margin during the late Cretaceous or Asian (Carter et al. 1976). Clastic Palaeocene.During the middle Miocene, the Sula andcarbonate shelves were present inSulawesi Spur, which had been steadily approaching Sundaland (Buton) and the Banda allochthon (Asinepe limestone since the late Cretaceous, collided with Sulawesi, re- of Seram)during the Triassic. A change to more sultingin intense folding and metamorphism. The

<*..>

h MIDDLE MIOCENE

1;; 1;; -F a LATE CRETACEOUS 0 1000 2000 kms

LEGEND

Old oceanic crust M Subductlon trench # Spreadmg D New oceanic crust Tmnscurnnt fault =O= Rlftlng Metamorphic belt Thrust fault 4 Tedoasm *e* Wcanic arc 6 Plate movement

c PLIOCENE

FIG. 2. Schematic palinspastic palaeogeography of eastern Indonesia. In these reconstructions, the SE coast of Kalimantan is treated as the reference line. The relationship between western and central Irian has not been reconstructed; its attitude is subject to speculation.

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deepsea floor, that had previously lain between W provenance and a northward increase in metamorphic Sulawesi and Sula, may have become obducted onto grade. The provenance of their ‘Australian’ age equi- the leading edge of the Sula Spur. It is now preserved valentshas the opposite polarity. Finally, metamor- in eastern Sulawesi as one of the largest ophiolite belts phics within the Lolotoi have suffered very high grade, in the world (Kundig 1956). granulitefacies metamorphism. Such rocks occur in the central parts of the Australian shield, but at pres- Opening of the ent are covered by up to 15 000 m of sediment on the NW Shelf. The southern Banda Seais thought to be a marginal This evidence suggests that the Aileu, Maubisse and ocean basin, which opened sometime during the early Lolotoi are unlikely to have come from Australia. It is Tertiary.Together with the northward advance of not possible to prove their Asian origin or to match Australia,the opening of theBanda marginal sea the allochthon with specific parts of Sundaland. How- probably provided the driving mechanism for subduc- ever, the SE arm of Sulawesi contains metamorphics tionin the Banda Arcs (Fig. 2b). Marginalseas are and ultramafics, while Triassic limestones and clastics commonphenomena along the western seaboard of areknown on Buton (van Bemmelen 1949). Thus, the Pacific; the Sea of , , Philip- they may well have come from this area, in the ab- pinesSea and are comparable features. sence of betteralternatives. Buton shows divergent Some form of crustal instability or local high tempera- structural and stratigraphic trends compared with the ture beneath the crust may have been responsible for rest of Sulawesi, and appears to have suffered much theirinitiation (Karig 1971). The Gulf of Boneand lesstectonism. This can possibly be explained by its possiblyalso the Flores Deep may represent early partial detachment and rotation, perhaps along a con- riftedstages in the opening of marginalseas. In the tinuation of theButon-Buru bathymetric lineament, latter case, this was accompanied or perhaps causedby during the opening of the Banda Sea. alkalinevulcanism. Based on present water depths (about 5 km), Pacific age/depth graphs, and the very low heat flow values, Bowin et al. (in press) thought Late Miocene-Pliocene sobduction thatthe southern Banda Sea could be as old as and collision Palaeocene (perhaps 69 Ma). A spreading centre has yet to be identified in the Banda Sea, although magne- tic anomaly lineations were reported by Bowin et al. As the Banda marginal sea increased in size, a W- (1977). It is likely that the original spreading centre and N-dipping subduction zone developed on the Au- becamedeformed during constriction of thearcs in stralian side of the fragments of ‘Asian’ material, the the Pliocene and Quaternary. ‘Lolotoi arc’ of Carter et al. (1976). During the early Tertiary-Miocene, deep sea sediments of late Jurassic- lateMiocene age, which laterbecame the imbricate Origin of the ‘Asian’ AUochthon Kolbanoand Nief thrustsheets, were torn up and accreted to the hanging wall of the advancing subduc- tion trench. Presumably, these rocks represent Asian During the early development of the Banda margi- and Australian continental slope and rise sediments, as nalsea, fragments of ‘Asian’material, which were well asdeposits from the deep ocean floor. A calc- eventuallyemplaced as thrust sheets on Timor and alkalinevolcanic island arc, which later became the Seram,were detached from the edge of Sundaland. inner of Flores to Romang, developed The originalposition of these elements is unknown. behind the subduction trench. The volcanic arc corres- Carter et al. (1976) thought that they were once part pondingto the present northern part of thesystem of theCretaceous-Eocene orogen of Java,and that cannotbe identified with certainty. Amblau and the they were moved to the eastern corner of Sundaland Uliasser Islands may be part of such a volcanic arc; the by strike-slipfaulting along what is nowthe Java remainder is possiblysubmerged as the submarine Trench.A less complicated and perhaps preferable ridges and valleys SE of Seram. explanation is that they originated near SE Sulawesi. During the latest Miocene and Pliocene, the advanc- Thethrust sheets of Timor(Barber et al. 1977) ing subductionsystem collided with theAustralian- include a heavily deformed suite of Permian-Triassic Irian continental block(Fig. 2c). The deep sea subduc- limestones,basic submarine volcanics and clastics tion milange, the fragments of ‘Asian’ material, and (Aileu-Maubisse),and a complex of ultrabasicand probablyalso a portion of oceaniccrust from the metamorphicrocks (Lolotoi). They are thus quite arc-trench gap were chiselled off and obducted onto different to the less deformed but coeval clastic rocks the leading edge of the Australian Craton. There, they in the underiying outer arc para-autochthon andin the wouldhave formed the thrust sheets of Timor and Australian-Irian foreland. In addition, distribution of Seram.On Timor, the collision can be dated accu- lithofacies in the ‘Asian’ clastics (Aileu) suggestsNW a rately as middle Pliocene (zone N.20), by reference to

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/5/519/4885922/gsjgs.136.5.0519.pdf by guest on 27 September 2021 5 24 M. S. Noroick the youngest tectonized rocks in the deep-water thrust review in thelight of itsapparent occurrence in the sheet and the oldest rocks in the untectonized sequ- possibly thrust-free areas of the eastern islands. ence. Obduction and thrusting took place slightly ear- lier in Seram;during the latest Miocene or earliest Pliocene-Quaternary post-collision history Pliocene (zone N.18-N.19). In the final stages of colli- sion,volcanics andintrusives, contaminated by The emplacement of the thrust sheets over a thick metamorphicminerals perhaps from the sub-thrust ‘Australian’ sedimentary section must have resulted in Australian continental crust, were emplaced in , excessiveisostatic inequilibrium. This is believedto SE Seram and the Uliasser Islands as ‘ambonites’. havecaused rapid uplift and the formation of the Overthrust‘Asian’ allochthon has only been iden- outer chain of islands(Tjokrosapoetro 1978). The tified in the western islands of the outer arc. Thus, it sedimentaryslide breccias probably became reactiv- occurs in W Seram, central Seram and possibly Buru atedduring these movements, and this could have in the N limb of the arc, and in the islands from causedsome of theapparent thrusts interpreted on throughTimor to Leti (possibly also Babar) in the seismiclines N of Seram(Audley-Charles 8c Carter southernpart of thearc. Eastern Seram, the Kai 1978)and S of Timor(Crostella & Powell1975; Islands and the Tanimbar group contain ‘Australian’ Crostella1977). Mesozoicpara-autochthon. Possible crystalline base- Rapiderosion of theemerging islands provided mentoutcrops in centralSeram (Kobipoto complex) large quantities of detritus, which led to the formation and the western (Kur, Fadol). Evidence forof thepost-tectonic molasse basins. The history of ‘Asian’ elements is either fragmentary or absent, al- uplift is preservedin their stratigraphy. Thus, basal though this may be due to poor data. sequences consist of Pliocene deep water chalks and This can be explained by the hypothesis that, in the planktonic foraminiferal siltstones. These are replaced eastern sector of the outer arc, collision of the subduc- in the uppermost Pliocene by turbidites and conglom- tion system with the foreland has not yet taken place, erates, and finally, in the Pleistocene, by shallow water and that subduction may still be active. The eastern limestonesand beach sands. Uplift has continued to volcanic arc is apparently in a young and active state thepresent day, and the area has still not reached between Banda and Damar, and a major concentra- isostaticequilibrium. Thus, raised Quaternary reefs tion of earthquakes lies adjacent to the southern part and terrace gravels occur to 2000 m above sea level, of this sector. Although seismic reflection records are and the outer arc is marked by thenegative gravity inconclusive,the Weber Deep, which is one of the anomalies mentioned above. deepest sub-sea areas in Indonesia, may thus possibly Collision and obduction in the outer arc appears to representa recently active subduction trench. The have resulted in cessation of active subduction along WeberDeep may also contain a portion of original, itsoriginal surface. Thereafter, the northern part of unconsumed, Upper Jurassic-Recent ocean floor. Seis- the arc probably became inactive. However, vulcanic- mic refractionstudies suggest that it is underlain by ity and earthquake activity has continued to the pres- thinoceanic crust (Purdy et al. 1977;Bowin et al. ent inthe Flores- sector of theinner arc, so 1977). presumably compression and subduction must still be Objectionsto this theory are that the deep-sea in progress.Subduction is believedto have ‘stepped Kolbano-Nief facies occurs in eastern Seram, and that out’ to the S, and the subduction trenchis now thought there is evidence for sedimentary olistostromes in all to be located in the (Carter et al. 1976). theeastern islands. The presence of imbricatedeep Further freezing of subduction appears to have oc- water Nief beds in E Seram cannot be refuted. How- curred N of Timor. The Alor-Wetar-Romang sector ever,these rock types are not restricted to oceanic of the volcanic arc is inactive, and may have been so environments. For example, similar Upper Cretaceous since the Pliocene (Carter et al. 1976). It is thus likely limestones are widespread in an autochthonous setting that the inner arc has become accreted to Timorin this on the Sula Spur and NW Shelf, although, unlike the sector. A line of immature and active, subaerial and Nief-Kolbano, they are associated with shallow water submarine volcanoes runs through Gunung Api in the Jurassic and Lower Tertiary formations. Carter et al. southern Banda Sea. This is tentatively thought to be (1976)used palaeontological evidence to show that the result of a local, second ‘step-out’ of the subduc- the Bobonaro olistostrome of Timor was formed ear- tion zone, which developed as a pressure release when lier than the date of overthrusting (late Miocene, zone subduction froze to the S. N.17;as against Pliocene, zone N.20). They thought that it initally formed, perhaps from montmorillonitic Curvature of the Banda Arcs muds in thearc-trench gap, during migration of the subduction‘system towards Australia, and was thus The curvature of the Banda Arcs remains enigmatic. carriedinto the outer arc on theback of thethrust In particular, the divergent attitude of thrust sheets on sheets. It may have acquired its exotic clast component the opposing limbs is difficult to explain in terms of during overthrusting. This evidence will need critical root zones, as the intervening area is occupied by the

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deep Banda Sea. Convergent subduction beneath the Timor and the other islands. Some of this dislocation N, S and E limbs of the arcs atthe same time is on the northern limb of the arc is probably related to geologically and topologically improbable. Cardwell & continuedmovement on theSorong fault system, Isacks (1978) explainthe present day earthquake which extends the length of northern Irian (Visser & distribution by postulatingseparate Java-Timor-Aru Hermes 1972; Tjia 1973). The bathymetric separation Troughand Seram Trough subduction systems, con- and en echelon arrangement of many of the islands, nectedby an extension of the Tarera-Aiduna trans- and the suspected cross faults in the volcanic arc (van current fault. Audley-Charles et al. (1972) interpreted Bemmelen 1949) suggest that there has been consider- the curvature as an acquired feature, caused by west- able ‘stretching’ of both island chains since their for- ward movement of the Sula Spur after the collision of mation.It is thus likely that a certainamount of the subduction system with the Australian-Irian fore- post-collision movementhas occurred, whichmay land. However, age relationshipsin the eastern arm of have accentuated the curvature of the arcs. Sulawesisuggest thatthe Sula Spur collided with E Sulawesiin themiddle Miocene, i.e. prior tothe Conclusions emplacement of the allochthon on Timor and Seram (P. R. Ashton, pers. comm.). Post-Pliocene curvature Onshoregeology and offshore geophysics appear to of thearc could thus only have occurred if either support the hypothesis that the Banda Arcs represent Sunda or Australia-Irian had suffered major rotation anisland arc/continent collision zone.Rifting and in the last few million years, an unlikely event. sea-floorspreading, perhaps during the Palaeocene, Itnow seems possible that an eastern embayment broke off a sliver of Asian continental crust from the (between the Australian NW Shelf and the Sula Spur) Sulawesiarea and carried it eastwardstowards Au- wasin existence prior tothe arrival of theBanda stralia. Upper Jurassic-Miocene ocean floor sediments subduction system. The migrating arc would have en- became accreted to the hanging wall of the subduction tered this embayment from the W, and allochthonous zoneat the leading edge of the newoceanic plate. elementswould have collided and perhaps been During the latest Miocene and Pliocene, the subduc- smeared out by strike-slip faulting along the N and S tionzone collidedwith the Australian foreland. The limbs of the foreland. There has probably also been a fragments of ‘Asian’ material and the prism of oceanic great deal of internal deformation in the Banda margi- sedimentwere obducted over Australian continental nalsea. The submarine ridges and troughs between crust, and these elements are now preserved on Timor Buton and Seram may represent a series of transform andSeram as aseries of thrustsheets. Although faults which developed to accommodate this deforma- collision appears to have caused the cessation of sub- tion. There is also evidencefor brittle transcurrent duction S of Seram and N of Timor, continued N-S faulting in the western peninsulas of Irian (Froidevaux compression is thoughtto have resulted in thede- 1978;M. W. Howe, pers. comm.), whichmight be velopment of new Benioff zones in the Savu Sea and consistent with overall sinistral strike-slip movement. southernBanda Sea. The original Benioff zonemay Tensionrelease is thoughtto have occurred at the still be active in the eastern Banda Sea, and the 7000+ northern edge of the arc, along a postulated splay of m WeberDeep may represent Tertiary-Recenta the Sorong Fault running across the northern Banda subduction trench. Seaas the Buton-Buru bathymetric lineament. The small part of theBanda Sea whichlies N of the ACKNOWLEDGMENTS.The writer wishes to thank the British lineament does not appear to have had any association Company Limited for permission to publish this paper. He is also particularly indebted to Dr P. R. Ashton, with the thrust zone. Like the Weber Deep, a portion Dr T. D. Adams, Dr G. V. Wood, Dr M. G. Audley-Charles, of undeformed pre-Neogene ocean floor may be pre- Dr A. J. Barber and Mr D. J. Carter for their encouragement served in this area. and critical comments. Drafting and typing by the staff of BP Pleistocene, post-thrusting, transcurrent faults have PetroleumDevelopment Ltd., Aberdeen, isgratefully ack- probably been active on Seram, and possibly also on nowledged.

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Received and read6 December 1978; revisedtypescript received 25 January 1979.

MARTIN S. NORVICK,BP PetroleumDevelopment Limited, Farburn Industrial Estate, Dyce, Aberdeen AB2 OPB.

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