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The Tectonic History of the Banda Arcs, Eastern Indonesia: a Review J. geol. Soc. London, Vol. 136, 1979, pp. 519-527,2 figs. Printed inNorthern Ireland. The tectonic history of the Banda Arcs, eastern Indonesia: a review M. S. Norvick SUMMARY: The Banda Sea is underlain by a small marginal oceanic plate, which is believed to have formed during the early Tertiary. Unlike other marginal seas 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 island 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. Subduction and volcanicitv ceased after collision in the Timor 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 islands 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 (Hamilton 1974a; Cardwell & Isacks1978). Seismic the deep seas that cover 90% of the area sparse, the lineshave been published across the Timor region 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 Craton, its The Geological Survey of Indonesia has a continu- forelandbasins and the extinct, accreted arcs of ing mapping programme on Timor, Seram, Buru, 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 continent 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 The geological map of Audley-Charles (1968), to- faults, extending from northern Papua New Guinea to gether with recent mapping by the Geological Survey Sulawesi-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, Flores 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/5/519/4885922/gsjgs.136.5.0519.pdf by guest on 30 September 2021 5 20 M. S. Norvick 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-volcanic arc 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) Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/5/519/4885922/gsjgs.136.5.0519.pdf by guest on 30 September 2021 Tectonic history of BandaArcs, eastern Indonesia 521 believed Sumba to be a detached portion of pre-late continuation of thePliocene foreland. Nappes of Jurassic continental crust, 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 tectonic sub- 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 Sundaland and Australia were separated by many duction zone that generated the volcanoes in the inner thousands of kilometres of ocean 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.
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