Tertiary evolution of the Eastern Indonesia Collision Complex
T.R. Charlton
Ridge House, 1 St. Omer Ridge, Guildford, Surrey, GU1 2DD, UK
Received 14 January 1999; accepted 6 July 1999
Abstract
Eastern Indonesia is the zone of interaction between three converging megaplates: Eurasia, the Paci®c and Indo-Australia. The geological basis for interpretations of the Tertiary tectonic evolution of Eastern Indonesia is reviewed, and a series of plate tectonic reconstructions for this region at 5 million year intervals covering the last 35 million years is presented.
The oldest reconstruction predates the onset of regional collisional deformation. At this time a simple plate con®guration is interpreted, consisting of the northward-moving Australian continent approaching an approximately E±W oriented, southward-facing subduction zone extending from the southern margin of the Eurasian continent eastwards into the Paci®c oceanic domain. Beginning at about 30 Ma the Australian continental margin commenced collision with the subduction zone along its entire palinspastically-restored northern margin, from Sulawesi in the west to Papua New Guinea in the east. From this time
untilca24 Ma, the Australian continent indented the former arc trend, with the northward convergence of Australia absorbed
at the palaeo-northern boundary of the Philippine Sea Plate (the present-day Palau-Kyushu Ridge).
Atca24 Ma the present-day pattern of oblique convergence between the northern margin of Australia and the Philippine Sea
Plate began to develop. At about this time a large portion of the Palaeogene colliding volcanic arc (the future eastern
Philippines) began to detach from the northern continental margin by left-lateral strike slip. Fromca18 Ma oblique
southward-directed subduction commenced at the Maramuni Arc in northern New Guinea. Atca12 Ma the Sorong Fault Zone strike-slip
system developed, eectively separating the Philippines from the Indonesian tectonic domain. The Sorong Fault Zone became
inactive atca6 Ma, since which time the tectonics of eastern Indonesia has been dominated by the anticlockwise rotation of the
Bird's Head structural block by some 30±408.
Contemporaneously with post-18 Ma tectonism, the Banda Arc subduction±collision system developed o the northwestern margin of the Australian continent. Convergence between Indo-Australia and Eurasia was accommodated initially by northward
subduction of the Indian Ocean, and subsequently, since ca 8 Ma, by the development of a second phase of arc-continent
collision around the former passive continental margin of NW Australia.72000 Elsevier Science Ltd. All rights reserved.
1. Introduction
Eastern Indonesia, situated at the intersection of the Alpine-Himalayan and Circum-Paci®c orogenic belts, is the location of the Earth's fundamental convergent triple junction. Long-term interaction of these three plates (taking the de®nition of plates at the coarsest scale) has resulted in the present-day situation where a complex arrangement of platelets with poorly de®ned plate boundaries cover an area equivalent to most of
western Europe or the western United States. It is my contention, however, that this complexity is only a relatively recent development, and that back in time, through the Tertiary, the tectonic situation rapidly simpli®es, so that by the mid-Palaeogene the fragmen-ted platelets have resolved themselves into a relatively simple pattern of coherent plates. Starting from this simple pre-Neogene con®guration, it is the aim of this paper to generate the complexity of the present-day by a few fairly simple changes in regional dynamics, with these changes largely explicable in terms of plate boundary interactions taking place within the evolving eastern Indonesia region.
1367-9120/00/$ - see front matter72000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 ( 9 9 ) 0 0 0 4 9 - 8
The region under consideration comprises the east-ern half of Indonesia from Sulawesi island in the west to the Papua New Guinea border in the east (Fig. 1). Peripheral regions beyond these limits are, quite delib-erately, only treated in the most general terms. The reason for this exclusive focus on eastern Indonesia is that, as will be discussed in the following section, there is no clear consensus on how the larger region viewed at a sub-global scale has evolved through the Tertiary.
2. External constraints on the collision model
The Eastern Indonesia Collision Complex involves the interaction of three or four major plates:
Indo-Australia, Eurasia and the Paci®c (or Caroline and Philippine Sea) plates. The gross kinematics of these plates (Fig. 1) provides an important framework within which to interpret the evolution of this region. In particular, the relative movements of the two main continental fragments, Australia and Southeast Asia, should provide a good constraint on the evolution of the collision complex. However, this constraint is not as strong as could be desired because the two conti-nents form the end-points in a long global plate motion circuit (Southeast Asia±Eurasia±Africa±Ant-arctica±Australia), and uncertainties at each stage in the circuit are compounded in the ®nal result.
It is commonly stated in the literature (e.g. Audley-Charles et al., 1988; Packham, 1990; Rangin et al., Fig. 1. Regional tectonic setting of the eastern Indonesia collision zone. Inset: approximate convergence vectors for the three main plates Eurasia (EU), Paci®c (PA) and Indo-Australia (AU). M.S.C.Z.: Molucca Sea collision zone. N.B.B.: North Banda Basin. S.B.B.: South Banda Basin.
Fig. 2. Palaeolatitudinal change for a nominal point on the Australian continent (108S, 1248E, currently located in southern West Timor). The in-dividual curves are linear interpolations between measured points (the dots) from published palaeogeographic maps. All apart from the Rangin et al. (1990) curve are measured directly from the position of Timor on the maps; the Rangin et al. (1990) curve is extrapolated from the southern tip of the Aru Islands (ÿ38). In some of the maps for older time intervals, extrapolation was necessary from another point on the
1990; Hall, 1996) that the position of the Australian continent through time is reasonably well constrained. This, however, is not strongly supported by Fig. 2 which shows the palaeolatitude of a point on the Aus-tralian continental margin (108S, 1248E, currently located in the Timor collision zone) through the Ter-tiary, as interpreted by these authors, together with a number of other published reconstructions. This shows that interpreted mean rates of northward movement for Australia during the last 30 Ma vary by a factor of two from about 40±80 km/m.y. Only ®fteen million years ago there was as much as 880 km (88) dierence in interpreted palaeolatitude between the extremes, which is close to the present-day north±south spread of the entire eastern Indonesia collision complex. Even excluding the Dercourt et al. (1993) track, which is clearly out of line with the others, only reduces the uncertainty to ca700 km at 30 Ma (dashed line in the lower part of Fig. 2). This allows an uncertainty of up toca 9 Ma for the time at which a particular point on the northern Australian continental margin reached a particular latitude, given the current `absolute'
north-ward motion of Australia at ca 75 km/m.y. (see
below).
The faster rates in Fig. 2 are associated with models that utilise a hot spot reference frame, whilst the models with a slower northward motion use a palaeo-magnetically-derived reference framework. This is illus-trated in Fig. 2 by the reconstructions generated in the Soeding et al. (1997) program, which allows calcu-lation of continental palaeopositions in both reference frames. Thus the latitudinal spread in Fig. 2 is princi-pally a result of the methodology used in determining the palaeolatitudes (hotspot or palaeomagnetic frame-works). This explains the spread, but does not remove the problem for the palaeomagnetically-derived recon-structions that the rate of northward movement of Australia has apparently slowed considerably since 10 Ma relative to latitude. Reconstructions in the hotspot reference framework generally do not show this appar-ent slowing.
Present-day plate motion calculations suggest rela-tively high convergence rates between Australia and Eurasia. The plate motion calculations of Minster and Jordan (1978), which are determined in the hotspot reference frame, indicate that the reference point (108S, 1248E) is moving ca 198 east of north at a rate of about 79.5 km/m.y. (i.e., a northward velocity of ca 75 km/m.y.). In the NUVEL-1A model of de Mets et al. (1990, 1994) the relative convergence of Australia with Eurasia is 79.8 km/m.y. bearing 0168 Ð i.e. a northward velocity of 76.5 km/m.y. assuming Eurasia to be stationary. These ®gures are at the top of the range of long-term northward displacements for Aus-tralia shown in Fig. 2, in line with long-term north-ward displacements in the hotspot reference frame, but
considerably faster than those constructed in the palaeomagnetic framework.
The palaeoposition of Southeast Asia is also less than adequately constrained. Taking Java island as a representative element of this platelet, there is signi®-cant disagreement in the published literature as to its movement history during the Tertiary. Thus Rangin et al. (1990) and Daly et al. (1991) inferred clockwise and overall westward displacement of Java through the Tertiary, whilst Hall (1996) interpreted overall anti-clockwise and eastward displacement. Similar disagree-ment is shown in the palaeomagnetic literature with regard to the larger region, with many authors propos-ing large anticlockwise rotations of Southeast Asia during the Tertiary (e.g. Fuller et al., 1991), whilst others have concluded little or no overall rotation (Lumadyo et al., 1993). However, it should be added that most palaeomagnetic studies ®nd only small changes in latitude for Southeast Asia through the Tertiary.
In summary, whilst the globally determined palaeo-continental positions and instantaneous plate motion calculations provide important external constraints on the tectonic evolution of eastern Indonesia, they can-not at present provide more than a very generalised framework. Tying an evolutionary model too closely to any particular global or very-large-regional tectonic model is probably not sensible at this stage because there is such a wide variation between alternatives.
The model developed here assumes fairly high rela-tive velocities between Australia and Southeast Asia in line with the hot spot reference frame, principally because this seems to ®t best with inferences on the timing of collision arising from interpretation of east-ern Indonesia geology. A constant northward velocity is assumed here for Australia since the Eocene because most calculations of the motion of Australia based on hot spot traces (e.g. Duncan, 1981; Wellman, 1983) or relative to Antarctica (Royer and Sandwell, 1989; Veevers et al., 1991) do not recognise major changes in the rate of northward displacement through this period. In terms of available models, the chosen path of Australia most closely follows the Paleomap recon-structions of Scotese and co-workers (see Scotese, 1999).
3. Regional structural elements
Sec-tion 4 by the smaller crustal elements within the col-lision zone.
3.1. Southeast Asia
In the context of eastern Indonesia, the Southeast Asian Plate (a sub-element of the Eurasian Plate) con-sists of the Sundaland continental block and the Cel-ebes Sea oceanic fragment (Fig. 1). Eastern Sundaland comprises Java and Borneo islands and the intervening shelfal Java Sea. To the south, Sundaland is bounded
by the Sunda Arc subduction system. In the southeast the margin of Sundaland is transitional into the South-west Sulawesi structural province discussed later. This transitional region is characterised structurally by Palaeogene rift grabens inverted by Miocene com-pression, which has produced a distinctive cross-sec-tional expression of `Sunda folds' (e.g. Letouzey et al., 1990). In the eastern Sundaland margin these inversion structures predominantly trend E±W, but near to Sula-wesi the trend changes markedly to nearly N±S (Fig. 3).
Further north the Makassar Straits between Borneo and Sulawesi probably developed initially from the same Palaeogene phase of extension recognised on the eastern Sundaland margin, although the degree of extension in the Makassar Straits was considerably greater. The South Makassar Basin (Fig. 3) is thought to be underlain by continental crust strongly attenu-ated by Palaeogene extension (Situmorang, 1982), whilst the North Makassar Basin may be underlain by Palaeogene oceanic crust, as is found in the Celebes Sea to the north (Weissel, 1980; Smith et al., 1990). The eastern and southern margins of the Makassar Straits and Celebes Sea respectively comprise fold and thrust belts (described in more detail later) associated with the Neogene westward displacement of Sulawesi. Consequently the Makassar Straits additionally possess some facets of a foreland basin superimposed on the older rifted margin structure (Bergman et al., 1996).
3.2. Australia
The northern part of the Australian continental block comprises Australia, the Northwest and Arafura shelf seas, and the southern half of New Guinea island (Irian Jaya province of Indonesia and Papua New Gui-nea: Fig. 1). The boundary between autochthonous Australian crust essentially undeformed during the Tertiary and the collision complex to the north is marked by the Timor-Tanimbar Trough thrust front, the extensional front of the Aru Trough, the Tarera-Aiduna strike-slip fault, and the Central
Ranges-Papuan Foldbelt deformation front. Immediately
north of this orogenic front is deformed Australian crustal material (the parautochthonous zone) and then crustal elements allochthonous to the Australian mar-gin. Many parts of the parautochthonous zone are linked directly to the autochthon to the south, and can be restored by standard palinspastic techniques where sucient data is available. The precise point of origin of other `Australian' crustal elements (e.g. the Bird's Head and Banggai-Sula crustal fragments to be dis-cussed below) is more questionable, and some authors (Pigram and Panggabean, 1984; Pigram and Davies, 1987; Struckmeyer et al., 1993, etc.) have suggested that these terranes have had long and complex kin-ematic histories independent of the main Australian Plate. Detailed discussion of these so-called allochtho-nous models is beyond the scope of this paper, but in summary I would suggest that there is no convincing geological argument that requires an allochthonous origin for these terranes, and on the contrary their geological anities strongly support a relatively local origin. In this paper I suggest that the present struc-tural isolation of these terranes from autochthonous Australia is the result of processes acting after initial collision of a coherent Australian continent with an
island arc system, rather than a pre-collisional disag-gregation of the Australian margin as implied by the allochthonous terrane models.
Immediately to the west of the Australian continent is Mesozoic oceanic crust of the Indian Ocean. The western continental margin of Australia was formed by rifting phases in the Late Jurassic and Early Cretac-eous, with extension directions implied by ocean ¯oor magnetic lineation patterns oriented between about WNW±ESE and NW±SE. This oceanic crust is sub-ducting northward beneath the southern and eastern Sunda Arc (from southern Sumatra eastwards), whilst the Australian continental margin is in collision with the Banda Arc system, which is the direct eastward extension of the Sunda Arc. Further west still, the Indian Ocean is underlain by Cenozoic oceanic crust, which is being subducted at the SW continental margin of Sundaland beneath most of Sumatra.
3.3. The Paci®c
The northeastern quadrant of this region comprises the Paci®c ocean crustal domain. The Paci®c Plate proper is separated from eastern Indonesia by two smaller plates of essentially oceanic crustal type: the Philippine Sea and Caroline plates (Fig. 1). The Caro-line Sea comprises oceanic crust formed between ap-proximately 34±29 Ma (magnetic anomalies 12-10: Weissel and Anderson, 1978). Relative motion between the Caroline and Paci®c plates is currently small, and the distinction of a separate Caroline Plate at the pre-sent day is questionable. Whether the Caroline Sea formed a distinct plate at any time after its formation is not clear.
The southern part of the Philippine Sea also has only small motion relative to the Paci®c Plate, but overall has a very dierent trajectory, rotating about a pole close to the northern apex of the plate relative to Eurasia, and about a pole NE of present-day Halma-hera relative to the Paci®c (e.g. Ranken et al., 1984; Seno et al., 1993). The western part of the Philippine Sea Plate formed in two phases of sea¯oor spreading during the Palaeogene, whilst the eastern third of the plate formed in two phases of mid-late Tertiary back-arc or interback-arc spreading forward of the Palau-Kyushu Ridge (e.g. Hilde and Lee, 1984; Mrozowski and Hayes, 1979). Hall et al. (1995a,b) and Hall (1996) have suggested that the development of the Philippine Sea Plate is intimately linked with the evolution of eastern Indonesia because they interpret the Halma-hera±Bacan±Waigeo region as an integral part of the Philippine Sea Plate since the Palaeogene. This in-terpretation will be discussed in more detail sub-sequently.
According to Weissel and Anderson (1978), the most likely interpretation of the Ayu Trough is that it devel-oped in two phases of sea¯oor spreading: from 12±6 Ma at a relatively high spreading rate (40 km/m.y. full spreading rate at the southern end of the trough), and from 6±0 Ma at a slower rate (8 km/m.y.). The open-ing of this triangular trough accommodated approxi-mately 208 of relative rotation between the Philippine Sea and Caroline plates.
3.4. The Philippines
At the present day the Philippine archipelago forms an obliquely convergent buer zone between the Phi-lippine Sea and Eurasian plates. The main structural elements are N±S oriented subduction zones east and west of the archipelago, and a left-lateral wrench sys-tem (the Philippine Fault) also with a predominantly N±S orientation. The eastern trench system (the Philip-pine Trench) and the PhilipPhilip-pine Fault are relatively young features, apparently only having developed in the last few million years (e.g. Cardwell et al., 1980; Rangin et al., 1990; Aurelio et al., 1991). However, geological interpretations indicate that left-lateral shear may have been active regionally since the Oligo-cene (Karig et al., 1986), and westward-directed sub-duction pre-dating that at the Philippine Trench is recorded in the Wadati-Benio zone associated with the Sangihe Arc, which links Mindanao island in the southern Philippines with northern Sulawesi. This seis-mic zone is shown by Cardwell et al. (1980) to extend as far north as the central Philippines.
Palaeomagnetism indicates that much of the Philip-pines has been translated northward through the Ter-tiary. For instance McCabe et al. (1985) suggested that before the Neogene the entire Philippines region was located in equatorial latitudes, and was translated northward in the late Palaeogene or early Neogene. The regional tectonic model of Rangin et al. (1990) interpreted a similar northward translation of the Phi-lippine arc terranes. However, Fuller et al. (1991) have cautioned that this may be an over-simplication.
Karig et al. (1986) noted the paucity of accretionary complexes in the Philippines. Most of the archipelago is composed of non-continental volcanic arc terranes ranging in age from Cretaceous through to the present day. In the central Philippines islands of Panay and Bicol the main arc activity took place between 50±30 Ma, ceasing in early Oligocene times (Rangin et al., 1990). Ophiolite terranes are locally present (e.g. the Zambales and Angat Terranes of Luzon: Karig et al., 1986), but are of lesser importance regionally than they are in eastern Indonesia. Continental terranes are found in the west-central part of the Philippines (the North Palawan Terrane: e.g. McCabe et al., 1985), and in SW Mindanao in the south of the archipelago
(Pubellier et al., 1991). These are usually interpreted as fragments of the Eurasian continental margin, but it will be speculatively suggested in the evolutionary model presented later that the SW Mindanao continen-tal fragment may have originated from the pre-colli-sional Australian continental margin.
Most of the complexity of the Philippines island arc history is beyond the scope of this study. Apart from SW Mindanao, the outline of which is shown in the older reconstructions presented subsequently, the remainder of the arc terranes in the eastern Philip-pines, from Luzon in the north to eastern Mindanao in the south, are treated here as a loosely de®ned vol-canic arc terrane, the East Philippines Terrane.
4. The Eastern Indonesia Collision Complex
4.1. Southwest, west-central and north Sulawesi and the Sangihe Arc
Southwest Sulawesi has a transitional boundary with eastern Sundaland, and originated as part of that con-tinent (Fig. 3). However, palaeomagnetic declination plots by Panjaitan and Mubroto (1993) suggest that Southwest Sulawesi has at least locally undergone anti-clockwise rotations of up to 808 since the Miocene, with one Pliocene site rotated 708 anticlockwise. This probably represents local block rotation associated with the Walanae left-lateral fault system (Fig. 3) which was active during the Plio-Pleistocene (Berry and Grady 1987).
The ®rst signi®cant Tertiary structural event recog-nised in SW Sulawesi is the phase of Palaeogene exten-sion also widely recognised on the eastern Sunda shelf, marked by extensional tectonics and arc volcanism, which, at least locally, commenced as early as the Palaeocene (Polve et al., 1997). Subsequently, during the Middle Eocene±Middle Miocene, the western half of SW Sulawesi (present-day coordinates) formed part of an extensive eastern Sundaland carbonate platform (Fig. 3; Wilson and Bosence, 1996). In the eastern half of the peninsula arc volcanism may have continued contemporaneously through much of this period (van Leeuwen, 1981; Priadi et al., 1994). From the Middle Miocene volcanism was widespread across both halves of SW Sulawesi, although geochemically this younger volcanism re¯ects an extensional structural
environ-ment rather than typical island arc conditions
(Yuwono et al., 1988).
earlier through to the Pliocene or Quaternary (18±3 Ma according to Bergman et al., 1996; as young as 0.6 Ma according to Priadi et al., 1994). However R. Hall (personal communication, 1999) questions the re-liability and volumetric signi®cance of the earlier part of this age range, considering the high-K igneous ac-tivity to be younger than 11 Ma, whilst Polve et al. (1997) dated the peak of this igneous phase to between 13±10 Ma.
Structurally central Sulawesi consists of a west-ward-directed fold and thrust belt with a thrust front in the Makassar Straits (Fig. 3; Coeld et al., 1993). Radiometric dating (Bergman et al., 1996) suggests that this foldbelt developed between about 13±5 Ma. An earlier phase of ophiolite obduction on the eastern margin of this province (Lamasi Complex) is dated at about 21 Ma.
The north arm of Sulawesi forms a similar volcano-plutonic belt. The western end of the north arm has a basement of Sundaland continental crust, whilst the eastern and central segments of the arm are built upon oceanic crust (Fig. 3; Kavalieris et al., 1992). The old-est Tertiary volcanism is of island arc type, and is dated as Eocene±Oligocene in age. After a volcanic hiatus with associated tectonism, a second phase of volcanism is dated between 22±16 Ma (Early Mio-cene). A third phase of volcanism commenced at about
9 Ma, and continues through to the present-day at the eastern end of the north arm. The latter volcanism is associated with the active Sangihe Arc to the north, which is related to westward consumption of the Molucca Sea Plate. According to Hamilton (1979), the Sangihe Arc originated in the early Middle Miocene, and was particularly active through to the Late Mio-cene.
Palaeomagnetic studies suggest that the north arm of Sulawesi has undergone approximately 20±258 of clockwise rotation since the Miocene (Surmont et al., 1994). This rotation has been accommodated by underthrusting of Celebes Sea oceanic crust beneath the north arm of Sulawesi at the North Sulawesi Trench (Hamilton, 1979; Silver et al., 1983a). The hangingwall of the North Sulawesi Trench has a fold and thrust belt structural style (Neben et al., 1998, ®g. 5) comparable to the fold and thrust belt on the east-ern margin of the Makassar Straits. Oset between the oshore North Sulawesi foldbelt and the east Makas-sar Straits foldbelt is taken up on the Palu-Koro strike slip fault (Fig. 3).
4.2. Eastern Sunda-Banda Arc
The eastern Sunda Arc is the segment of the arc east of the main Sundaland continent up to its intersection
with the colliding Australian margin at the latitude of Sumba island (Fig. 4). East of this the feature con-tinues as the Banda Arc, with direct geological conti-nuity between the two, particularly in the volcanic arc. Although the eastern Sunda Arc separates oceanic crust of the Indian Ocean from oceanic crust of the southern Banda Sea, the arc itself is, at least in part, built on a basement of Sundaland continental crust. This basement is exposed in Sumba (Chamalaun et al., 1981) and in the allochthonous (overthrust non-Aus-tralian) sequence of Timor (Earle, 1983; Audley-Charles, 1985). The arc has a history of igneous ac-tivity through much of the Tertiary, with arc volcan-ism recognised in the Palaeocene (Masu Volcanics, Sumba), Eocene (Metan Volcanics, Timor), Early Oli-gocene (Kur island, western Kai group: Honthaas et al., 1997), Early Miocene (Jawila Volcanics, Sumba; volcanics associated with the Noil Toko Formation in Timor, e.g. de Waard, 1957; Rosidi et al., 1981), Middle Miocene (ca12 Ma in Wetar: Abbott and Cha-malaun, 1981), Late Miocene (Manamas Volcanics, Timor: e.g. Bellon, in Linthout et al., 1997) and younger. Arc volcanism has not been identi®ed during the Late Oligocene. East of Wetar Island the eastern Banda volcanic arc is apparently only as old as Late Miocene or Pliocene (van Bemmelen, 1949; Bowin et al., 1980), although Kur Island located in the inner-most part of the Kai forearc complex includes arc vol-canics of Early Oligocene age (Honthaas et al., 1997).
In the forearc ridge to the south of the present vol-canic arc, Sumba, located at the transition from ocea-nic subduction at the Sunda Arc to continental collision around the Banda Arc, is entirely composed of non-Australian crustal elements. In Timor and adja-cent islands, non-Australian crustal material originat-ing in the pre-collisional forearc complex is thrust onto the most distal parts of the Australian continental margin sequence, which is itself strongly imbricated by thrusting. Further west in the Tanimbar Islands an allochthonous sequence analogous to that on Timor and Sumba is volumetrically very reduced or absent; the imbricated Australian margin succession is by far predominant (Charlton et al., 1991b). Similarly the Kai Islands are composed predominantly of Austra-lian-anity crustal material (Charlton et al., 1991a), although radiometric dating of igneous and meta-morphic rocks on Kur Island in the extreme west of the Kai group suggests an allochthonous origin for these (Honthaas et al., 1997).
Fortuin et al. (1994, 1997) have identi®ed a phase of rifting along the axis of the Banda-eastern Sunda vol-canic arc in the Middle-Late Miocene which led to the isolation of Sumba and the Timor allochthon from the volcanic arc, and to the development of the Savu Basin. Rapid subsidence of the Savu Basin commenced at about the Early/Middle Miocene boundary (ca 16
Ma: Fortuin et al., 1997), approximately contempora-neously with the main volcanic arc to the north of the basin becoming inactive (Barberi et al., 1987). For at least part of the subsequent period (?Middle-Late Mio-cene), active volcanism was apparently transferred to the southern margin of the basin (Fortuin et al., 1994). This period coincides closely with the time of gener-ation (ca 16±9.5 Ma) and emplacement (ca 8 Ma) of ultama®c suites in Timor, Seram and possibly interven-ing parts of the eastern Banda Arc (Linthout et al., 1997). It will be suggested subsequently that these events are linked, and are related to Middle-Late Mio-cene southeastward expansion of the Sunda Arc prior to Late Miocene collision around the Banda Arc.
4.3. South Banda Basin
The South Banda Basin is a fragment of oceanic crust situated within the curve of the Banda Arc (Fig. 4). The age of the crust has been variously inter-preted as Mesozoic (Lapouille et al., 1985; Lee and McCabe, 1986), Palaeogene (Barber, 1979) and Neo-gene (Hamilton, 1979; Hall, 1996; Honthaas et al., 1997). One of the principal arguments that has been used in favour of a Mesozoic age is the parallelism and apparent lateral near-continuity of magnetic anomalies in the South Banda Basin with the known Mesozoic anomalies in the Indian Ocean. However, the apparent continuity is likely to be no more than coincidental because oceanic crust that formerly lay north of the Northwest Shelf in the vicinity of present-day Timor is now represented by the Wadati-Benio zone dipping north from the Sunda-Banda Arc. Restoring this to its pre-collisional position on the Australian passive margin indicates that the present-day South Banda Basin cannot have originated closer to Timor than present-day Sulawesi or Buru, which greatly reduces the apparent continuity between the two anomaly sets.
A recent study by Honthaas et al. (1997) strongly suggests a late Neogene age for the South Banda Basin, broadly contemporaneous with the North Banda Basin (see below). Dredging on the northern ¯ank of the basin has yielded fossil-bearing volcani-clastic sediments and radiometrically dated basalts giv-ing consistent ages of 7±3 Ma for the volcanism, interpreted as the age of sea¯oor spreading in the South Banda Basin.
4.4. Banda ridges
SE. Dredge samples from the Lucipara Ridge (Silver et al., 1985, Honthaas et al., 1997) comprise a mixture of
continental metamorphic and sedimentary rocks,
together with basic volcanics. Two metamorphic rocks yielded radiometric ages of about 11 and 22 Ma (Silver et al. 1985). Dredge samples from the Sinta Ridge (Vil-leneuve et al., 1994) consist of continental margin car-bonate and clastic sediments including Triassic reef limestones. Both ridges therefore appear to be com-posed predominantly of continental crust.
4.5. SE Sulawesi-Buton-Tukangbesi
This region, consisting of the southeast arm of Sula-wesi together with its topographic extension oshore to the southeast, is treated here as a single structural domain because, contrary to a number of alternative interpretations (e.g. Smith and Silver, 1991; Davidson, 1991), I will suggest that it has formed a structurally coherent body throughout the Neogene.
In broad outline, the region consists of a continental margin terrane of Australian anity overthrust by the East Sulawesi Ophiolite (to be discussed in more detail in the following section). In the SE of this region is the Tukang Besi Platform. Little geological and geophysi-cal data is available from this area, but gravity values suggest a continental terrane (Ali et al., 1996), and the absence of signi®cant sedimentary section imaged on available seismic data (Ali et al., 1996) suggests that the Buton foldbelt does not extend onto the platform, as previously interpreted by Davidson (1991). The platform is probably a relatively undeformed fragment of Australian-anity continental crust (Hamilton, 1979).
Westward the platform passes into a Neogene molassic basin in front of the Buton fold and thrust belt. The deeper parts of the foldbelt comprise sedi-mentary sequences with clear stratigraphic anities to the Banda forearc islands of Buru, Seram and Timor (e.g. Smit Sibinga, 1928; Davidson, 1991; Smith and Silver, 1991), which are are in turn widely interpreted to be composed for the most part of Australian conti-nental margin sequences. Minor fragments of a for-merly more extensive ophiolite complex are locally preserved in the structurally higher levels of the Buton foldbelt (Smith and Silver, 1991). In SE Sulawesi the ophiolite complex is more extensive, where it overlies Australian-anity continental metamorphic basement in the west, and overlies or is imbricated with Austra-lian-anity cover sequences towards the east (Surono, 1996).
In Buton the age of collision between the continent and the ophiolite complex can be dated stratigraphi-cally as Oligocene, based on the youngest age of the pre-collisional sequence (Upper Eocene-?Lower Oligo-cene: Smith and Silver, 1991) and the oldest syn- to
post-orogenic sediments (planktonic foraminiferal zone N3±N4 or latest Oligocene-earliest Miocene in the Bulu-1 well: P.T. Robertson Utama Indonesia, personal communication). Davidson (1991) also men-tions ages as old as planktonic foraminiferal zone N3/N4 for the syn- to post-orogenic Tondo For-mation in South Buton, whilst Surono (1996) dated molassic sediments in SE Sulawesi to the Early Mio-cene, and interpreted a latest Oligocene age of
col-lision between the continental terrane and the
ophiolite belt. This is signi®cantly earlier than the Middle Miocene initiation of collision interpreted by Smith and Silver (1991).
Southeast Sulawesi and Buton are cut by a number of large faults striking NW±SE (the Matano, Lasolo (or Lawanopo), Kolono, Kolaka, Kioko and Hamil-ton faults: Figs. 3 and 4). The Matano Fault is a left-lateral strike-slip fault based on geology (Ahmad,
1977) and seismology (McCarey and Sutardjo,
1982), and it has been generally assumed that the other faults have similar strike-slip displacements. However, most of the other faults do not show the geological characteristics of large strike-slip faults: there is for instance no clear braiding of fault strands, or linked oblique transpressional/transten-sional folding and faulting. Instead the faults have been mapped regionally as nearly straight lineaments with gentle folding subparallel to the fault trend (e.g. folding of the Tokala and Meluhu formations near the SE end of the main Lasolo Fault lineament on the Lasusua±Kendari map sheet: Rusmana et al., 1993b). These faults have more of the characteristics of large normal faults. Facies patterns in the Late Triassic suggest that these major block faults have existed since at least that time (Charlton, submitted), and have probably been reactivated several times since. The relatively young strike-slip oset on the Matano Fault may be such a reactivation with a dierent sense of displacement.
The SE arm of Sulawesi is separated from the SW arm by the Gulf of Bone. Seismic sections across the gulf (e.g. Guntoro, 1996) suggest an extensional ori-gin for the embayment, and Hamilton (1979) inter-preted a Middle Miocene age for the commencement of extension. The end of extension is marked on the seismic lines by an unconformity separating normal faulted and gently folded section below from essen-tially undeformed strata above. At the head of the gulf in the Malili geological map sheet (Simandjuntak
et al., 1991a) this unconformity can be traced
4.6. Central Sulawesi and East Sulawesi-Banggai-Sula
As with the SE Sulawesi-Buton-Tukangbesi struc-tural domain, East Sulawesi and the Banggai and Sula islands are here interpreted as having formed an essen-tially coherent structural block throughout the Neo-gene. This similarity extends to the general geological characteristics, with the East Sulawesi Ophiolite thrust eastwards onto a continental margin terrane of Austra-lian anity. The East Sulawesi Ophiolite is also thrust westward onto Sundaland-anity basement in the Poso region of central Sulawesi. As with SE Sulawesi and Buton, the age of collision between the continental terrane and the East Sulwesi Ophiolite has been widely dated to the Middle Miocene (e.g. Garrard et al., 1988), but again I will suggest that there is evidence for an earlier onset of collision in the Oligocene. In the East Arm of Sulawesi there is also a second phase of fold and thrust belt development in the Pliocene (Davies, 1990).
The geological evolution of central Sulawesi has been studied in detail by Parkinson (1991, 1996, etc.). The Pompangeo metamorphic complex was interpreted as Sundaland continental margin basement, perhaps formed by the accretion of a Gondwanan terrane in the mid Cretaceous (ca 110 Ma) when initial medium-high pressure metamorphism developed. During the Oligocene a second phase of metamorphism developed under high pressure conditions. Subsequently the East Sulawesi Ophiolite was obducted onto the Pompangeo basement complex by westward-directed thrusting. This obduction may have occurred contemporaneously with obduction of the Lamasi Complex in southwes-tern Sulawesi at about 21 Ma (cf. Bergman et al., 1996).
Separating the base of the East Sulawesi Ophiolite from the underlying Pompangeo Complex is a tectonic melange complex, the Peleru Melange. The upper con-tact of the Peleru Melange Complex with the base of the ophiolite is overprinted by a metamorphic sole with an inverted thermal gradient, formed by the obduction of a hot ultrabasic body onto cooler crustal material. Parkinson interpeted the Oligocene phase of deformation in central Sulawesi as the result of obduc-tion of young and hot backarc oceanic crust onto the Sundaland margin prior to collision with the Banggai Platform in the Middle Miocene. However, several el-ements of the sub-ophiolite tectonic complex upon which the inverted metamorphic gradient is over-printed (speci®cally, the Nanaka, Tetambahu and probably the Matano Broken Formations of the Peleru Melange Complex) show stratigraphic anities with the ``Australian'' sequence of the Banggai Platform. As the metamorphic sole separating the Peleru Melange Complex from the structurally overlying East Sulawesi Ophiolite has been dated radiometrically at about 31
Ma (Parkinson, 1996), the Banggai Platform must have arrived by (or at) that time. This Oligocene age of suturing between western (Sundaland-anity) and eastern (Australian-anity) Sulawesi is consistent with the stratigraphically-derived Oligocene age for the ophiolite-continent collision in Buton and SE Sulawesi. The origin of the East Sulawesi Ophiolite and indeed ophiolites in general is not well understood. Geochem-istry suggests an origin in a supra-subduction zone set-ting, possibly related to the backarc Celebes Sea (Monnier et al., 1995; also Bergman et al., 1996), which suggests linkage with the Asian/Paci®c plate margins. On the other hand, palaeomagnetic studies of Cretaceous or Palaeogene lavas from the east arm of Sulawesi indicate a relatively high southerly latitude (ca208S), possibly not far north of the Australian con-tinent at that time (Mubroto et al., 1994). Whatever the precise origin of the East Sulawesi Ophiolite, there is a common association regionally between ophiolite complexes and subduction forearcs. In the reconstruc-tions presented later the East Sulawesi Ophiolite is treated as part of an oceanic forearc complex paired with the Palaeogene volcanic arc terranes of western Sulawesi.
The East Sulawesi Ophiolite is separated from the Banggai-Sula continental fragment by the Tomori Basin (Davies, 1990; Handiwiria, 1990; Abimanyu, 1990). The basinal succession comprises a lower sequence of shelf carbonates and clastics ranging in age from Upper Eocene±Upper Miocene, succeeded by thick molassic sequences of Pliocene±Recent age. There is no direct evidence for major collision-related structural development before the Pliocene, and pre-sumably this region lay some distance in front of the ophiolite-continent collision front during the Oligo-Miocene period. The present fold and thrust belt struc-ture on the western ¯ank of the Tomori Basin only developed during the Pliocene (between 5.2±2.8 Ma according to Davies, 1990).
The Banggai-Sula continental fragment (Pigram et al., 1985; Garrard et al., 1988) has long been recog-nised as stratigraphically related to the continental part of New Guinea island, and hence to the Austra-lian continent (e.g. KlompeÂ, 1954; Visser and Hermes, 1962). These latter authors suggested stratigraphic similarity with the Bird's Head structural block of wes-tern New Guinea, whilst Pigram et al. (1985) argued for a more distant origin, adjacent to central Papua New Guinea. Elsewhere (Charlton, 1996) I have reviewed the evidence which leads me to favour a con-nection with the Bird's Head block.
modelling (Silver et al., 1983a). A possible indication of a Middle Miocene or earlier onset of extension is given by the Middle Miocene (and younger) Bongka Formation (Poso map sheet: Simandjuntak et al., 1991b), which is a deepwater turbiditic sequence unconformably overlying the central Sulawesi collision complex. Again latest Miocene±earliest Pliocene for-mations that unconformably overlie older strata (e.g. the Lonsio and Kintom formations of the Luwuk map sheet: Rusmana et al., 1993a) may mark the cessation of extension in the gulf.
4.7. Seram and Buru
Seram Island has been described as a mirror image across the Banda Sea of Timor in the south (Audley-Charles et al., 1979). It consists of a northward-di-rected fold and thrust belt forming the forearc complex of the northern Banda Arc. However, unlike Timor but like Tanimbar and Kai, there is no major `Asian' allochthon within the Seram collision complex. [N.B. this interpretation is in contrast to the original in-terpretation of Audley-Charles et al. (1979) who inferred an allochthonous origin for a major part of the Seram succession based on similarities with the Timor allochthon as then recognised, particularly with the Maubisse Formation of Timor. An Australian and therefore parautochthonous origin for the Maubisse Formation is now widely accepted (e.g. Audley-Charles and Harris, 1990), and the necessity for an extensive allochthon in Seram is negated]. As with Tanimbar and the eastern Banda Arc, the volcanic arc in the
hin-terland of Seram (Ambon and adjacent islands) is essentially Pliocene and younger in age.
Buru Island is also usually considered to be one of the islands in the Banda forearc chain. However, although there is close stratigraphic similarity between Seram and Buru, the two islands show very dierent structural styles. Whilst Seram consists of an imbricate stack of thrust sheets, Buru has a relatively simple anticlinorial structure with the principal fold axis fol-lowing the long axis of the island. It is likely that the Buru anticlinorium marks a westward dying out of Banda forearc deformation, and thus the island forms a pin-point termination for this convergent system.
Seram and Buru islands are separated by a small tri-angular marine embayment (Fig. 5) which I interpret as a triangular pull-apart structure (sphenochasm). This structure osets the Pliocene volcanic island of Ambelau south of Buru from the Ambon group south of Seram, which suggests that it opened during Late Pliocene±Recent times. This sphenochasm is inter-preted in the later reconstructions to have accommo-dated 458 of late-stage clockwise rotation between Buru and western Seram.
4.8. Bird's Head-Misool and the Sorong Fault Zone
the Banda Arc region to the southwest. There is thus no strong reason to suspect on stratigraphic grounds that the Bird's Head structural block is allochthonous, as has been proposed by Pigram and co-workers (see above). Neither is there any strong stratigraphic or structural evidence to support the contention (Pigram and Panggabean, 1984; Pigram and Davies, 1987) that Misool together with the Onin and Kumawa peninsulas of the Bird's Head formed a separate terrane independent of the main Bird's Head block prior to the Oligocene. On the contrary, seismic data (e.g. Perkins and Livsey, 1993, Fig. 2) strongly suggests simple tectonostrati-graphic continuity from the Misool-Onin-Kumawa Ridge into the main part of the Bird's Head block before the inversion that formed the ridge in the Pliocene, the inversion being related to the colli-sional development of the northern Banda Arc.
The Bird's Head block was aected by an important phase of deformation during the Oligocene, most nota-bly giving rise to the NW±SE trending Central Bird's Head (Vogelkop) Monocline (Visser and Hermes, 1962). Resultant uplift and erosion of the Kemum basement block to the north of the monocline led to the shedding of an extensive clastic sequence (the Sirga Formation of Late Oligocene±Early Miocene age) around the margins of this structural high. A series of en echelon NW±SE ridges, probably reverse faulted inversion structures cogenetic with the Central Bird's Head Monocline, also developed across the northern Bintuni Basin (the Inanwatan, Puragi and Ayot Ridges of Fraser et al., 1993); also the Wasian and Mogoi Ridges further east. A contemporaneous Oligocene structural event is also recognised in Misool Island where Late Oligocene±Early Miocene section uncon-formably overlies gently deformed Eocene±Oligocene and older strata (Pigram et al., 1982a).
Palaeomagnetic studies on the Bird's Head-Misool region are relatively sparse. In Misool, Thrupp et al. (1987) reported 338 and Wensink et al. (1989) ca 408 of anticlockwise rotation relative to Australia since the Late Cretaceous. In the northern Bird's Head Giddings et al. (1993) suggested a three-stage rotation history involving anticlockwise rotation of 558 about a local pole in the later Mesozoic or early Palaeogene, fol-lowed by a ca 208 anticlockwise rotation about a
dis-tant pole (equivalent to ca 2000 km of ESE±WSW
lateral translation) in the mid Tertiary, and a ®nal 108
anticlockwise rotation in the late Neogene. However, considering the uncertainties of the Australian polar wander curve (e.g. Klootwijk, 1996), this can only be considered one interpretation of the data.
The Bird's Head structural block is bounded to the north by the Sorong Fault, which is part of a major left-lateral fault system that extends eastwards through northern New Guinea and westwards as far as
Sula-wesi (the Sorong Fault Zone in the sense of Hall and others). The age of initial movement on the Sorong system has been variously estimated between Oligocene and mid Pliocene, with most estimates placing the main phase of movement in the mid Miocene±Pliocene (see Charlton, 1996). A minimum displacement of about 850 km can be inferred for the main fault strand south of the Banggai-Sula structural block based on relocation of the Tomori Basin in eastern Sulawesi north of the Salawati Basin in the northern Bird's Head (Charlton, 1996). Movement on any other fault strands such as those widely inferred through the Bacan-Halmahera region would be additional to this.
4.9. North Banda Basin
The North Banda oceanic basin separates the Bang-gai-Sula block to the north from Buru in the east, the Sinta Ridge in the south, and SE Sulawesi in the west. Dredge sampling in the basin (ReÂhault et al., 1994; Honthaas et al., 1997) has yielded basalts with a back-arc basin geochemical signature, dated radiometrically
between 11.4 2 1.15 Ma and 7.33 2 0.18 Ma (5
samples). As this small oceanic basin is bounded to the north by the main strand of the Sorong Fault Zone and the age of basin formation apparently coincides with the age of activity on the wrench fault system, it is likely that the two structures are genetically linked, with the North Banda Basin accommodating a trans-tensional component in the Sorong strike-slip system. A partial present-day analogue for the connection between the Sorong Fault Zone and the North Banda Basin might be the linkage between the Andaman Sea spreading centre and the Sumatra Fault strike-slip sys-tem in western Indonesia.
The western margin of the North Banda Basin is a young thrust zone (the Tolo Thrust: Silver et al., 1983b). The foreland loading of this thrust belt onto the oceanic crust of the North Banda Basin has been suggested as an explanation for the anomalous bathy-metric depth of the basin (ReÂhault et al., 1994).
4.10. Cenderawasih Bay-Aru Trough
Tarera-Aiduna Fault to the north, the western margin of the Arafura Shelf to the east, and the Kai Islands struc-tural block to the west.
Elsewhere (Charlton, 1999) I have interpreted Cen-derawasih Bay and the Aru Trough as triangular pull-apart structures (sphenochasms) that have
accommo-dated some 30±408 of anticlockwise rotation of the Bird's Head block relative to Australia (Fig. 6). This rotation postdates the formation of the Lengguru Foldbelt, which developed primarily during the Late Miocene (Pigram et al., 1982b; Dow and Robinson, 1985). The opening of Cenderawasih Bay is also marked in Yapen Island by a reversal in the sense of oset on the wrench fault system (from left-lateral to right-lateral), with the reversal dated as Early Pliocene.
4.11. Northern east Irian Jaya and Papua New Guinea
Northernmost east Irian Jaya comprises a number of relatively small Palaeogene volcanic arc terranes with, to the south of these, an extensive ophiolite com-plex, the Irian Jaya Ophiolite Belt. These together are presumed to represent a large part of the volcanic arc± forearc pair that collided with Australia in mid Ter-tiary times. The Palaeogene volcanic arc system of eastern Irian Jaya can be traced eastward through northern Papua New Guinea (the Bewani, Toricelli, Prince Alexander and Adelbert blocks, and the Finis-terre terrane of the Huon peninsula), and into the New Britain and Solomon arcs, whilst the ophiolitic forearc is represented in Papua New Guinea by the April, Marum and Papuan ophiolites. To the west,
comparable volcanic arc terranes can be traced
through Biak and Yapen islands north of Cenderawa-sih Bay, the Arfak Terrane on the eastern edge of the Bird's Head, the Tosem Terrane in the northernmost Bird's Head, and to Batanta and western Halmahera, whilst predominantly ophiolitic terranes are found in eastern Halmahera and Waigeo.
Initial arc±continent collision in northern Irian Jaya and Papua New Guinea has been widely interpreted as Oligocene in age (Dow, 1977; Pieters et al., 1983; Pigram et al., 1989; Pigram and Symonds, 1991; Davies et al., 1996, etc.), contemporaneous with the collision in Sulawesi and Buton already described, and with the structural event in the Bird's Head and Mis-ool. As in the Sulawesi belt, however, other workers have suggested younger ages for collision in Papua New Guinea: for instance late Early Miocene (Francis, 1990) and Middle Miocene (Carman, 1993), whilst Hill et al. (1993) have interpreted the Oligo-Miocene struc-tures as developing above a southward-directed sub-duction zone, with arc±arc collision delayed until the Late Miocene±Pliocene. Detailed discussion of these alternative interpretations of Papua New Guinea is beyond the scope of this paper, but with reference to the latter interpretation, it is perhaps signi®cant that the Late Miocene±Pliocene colliding arc (the Finis-terre±Adelbert Arc) largely comprises a Palaeogene volcanic arc terrane without a corresponding forearc complex, but it parallels the ophiolite belt which might be interpreted as the remnants of such a forearc com-Bird's Head 30-40°
anticlockwise rotation relative to Australia
Motion of Pacific relative to Australia c.128km/m.y. Northern Bird's Head effectively attached to Pacific plate
Bird's Head-Australia pole of net rotation approx. 4.9°S, 133.9°E Motion of Pacific
relative to Australia block into the Cenderawasih Bay sphenochasm
plex without a corresponding volcanic arc. Another possible interpretation of the Adelbert±Finisterre Arc, as shown in the reconstructions presented later, is that it originated as the volcanic arc counterpart of the ophiolite forearc complex which collided with the northern margin of Australia in the Oligocene; that the volcanic arc was subsequently split away from the forearc±collision complex to the south by opening of the (poorly dated) Solomon Sea by oceanic spreading in the mid Miocene; and that the volcanic arc was sub-sequently re-attached to the continent by excision of the Solomon Sea at the opposing New Britain Trench and Trobriand Trough subduction zones between the Pliocene and the present day.
In Papua New Guinea an igneous complex named the Maramuni Arc (Dow, 1977) has been dated to between about 18±12 Ma, and age equivalents can be found locally in Irian Jaya (e.g. the Moon Volcanics of the northern Bird's Head, and the Utawa Diorite of the Bird's Neck: Pieters et al., 1983). The Maramuni Arc has a calcalkaline geochemical signature, and is widely interpreted as marking a phase of southward-directed subduction at the New Guinea Trench. Sub-sequent volcanism in both Papua New Guinea and eastern Irian Jaya has a post-orogenic geochemical sig-nature, comparable to the contemporaneous shoshoni-tic and high-K post-collisional igneous suites of western Sulawesi.
A second phase of major tectonism commenced in Irian Jaya during the Middle Miocene (e.g. McDowell et al., 1996) with the development of the Central Ranges-Lengguru orogenic belt. The Central Ranges include large regions dominated structurally by base-ment-involved high-angle reverse faulting and by left-lateral wrench faulting (Visser and Hermes, 1962; Abers and McCarey, 1988; Kendrick et al., 1995), with thin-skinned fold and thrust belt styles of defor-mation of lesser signi®cance, e.g. in the relatively narrow frontal ranges. The Lengguru Foldbelt, in con-trast, is predominantly thin-skinned, with a northern pinpoint against the Kemum basement block, perhaps analogous to the interpreted termination of the Banda foldbelt in Seram and Buru. The present-day separ-ation of the Lengguru Foldbelt from the Central Ranges is likely to be a consequence of the Pliocene± Recent opening of Cenderawasih Bay.
It is a notable feature of the New Guinea collision complex that it is dominated by deformed continental margin sequences and the structurally overlying pre-collisional forearc complex, now represented by ophio-litic terranes. In comparison, terranes representing the pre-collisional volcanic arc are volumetrically relatively minor. This contrasts with the Philippines archipelago where volcanic arc terranes predominate, and forearc terranes are relatively scarce.
4.12. Halmahera-Bacan-Waigeo-Obi
The Palaeogene of Halmahera, Bacan and Waigeo consists of an intraoceanic volcanic arc complex in the west (Bacan and western Halmahera) and an ophiolite-sedimentary province in the east (eastern Halmahera and Waigeo). The volcanic arc and ophiolitic
com-plexes have, according to palaeomagnetic data,
together formed an essentially coherent structural domain throughout the Neogene (Hall et al., 1995a,b; see further discussion below).
In Bacan Island two Palaeogene arc terrains (North and South Bacan) are separated by a continental base-ment block (the Sibela Mountains) which is structu-rally overlain by ophiolitic material. The island arc and continental basement blocks are in steep (normal?) faulted contact (e.g. Malaihollo and Hall, 1996). A continental signature is recognised in young volcanic rocks from southern Bacan, suggesting that continental crust extends beneath this area (Morris et al., 1983). This continental signature is also recognised from Late Miocene volcanics in southern Bacan and Early Mio-cene intrusives in northern Bacan (Malaihollo and Hall, 1996), suggesting that continental crust has underlain Bacan since at least the Early Miocene. This juxtaposition of oceanic island arc with presumed Aus-tralian continental crust is therefore likely to have occurred during the Oligocene, as in New Guinea and Sulawesi.
Obi Island to the south of Bacan is broadly compar-able to Bacan in having a continental basement inlier surrounded by ophiolitic sequences and island arc vol-canics. In the south of Obi the non-continental units are probably thrust southward onto Australian-anity cover sequences which can be correlated with similar (Jurassic) rocks in the adjacent Sula islands.
On top of the Palaeogene arc complex in western Halmahera, Bacan and Obi is developed a second igneous sequence ranging in age from about 20 Ma through to the present (Baker and Malaihollo, 1996). These authors subdivided the sequence into a `pre-arc' suite dated between about 20±9 Ma which may be of post-collisional origin (cf. Sulawesi and New Guinea), and an island arc sequence dated from about 12 Ma through to the present. A possible additional break in the arc sequence is recognised at about 3±4 Ma, at which time the locus of arc activity shifted about 50 km to the west. The present-day arc is associated with the eastward subduction of the Molucca Sea beneath Halmahera.
Oligocene and Miocene (Hall et al., 1988b). A similar history is interpreted for Waigeo, where the late Palaeogene uplift event is associated with strong defor-mation (Charlton et al., 1991c).
A second phase of deformation is recognised during the Pliocene in both Halmahera and Waigeo. In Hal-mahera a westward-directed fold and thrust belt devel-oped between the western and eastern halves of the island (Hall et al., 1988b). This thrusting may be con-nected with the westward shift in the locus of igneous activity at about 3±4 Ma (Baker and Malaihollo, 1996). In Waigeo large-scale folding is associated with left-lateral wrench faulting (Charlton et al., 1991c).
As already mentioned, palaeomagnetism indicates that the Halmahera±Bacan±Waigeo region has acted as a coherent structural domain throughout the Neo-gene (Hall et al., 1995a,b). These authors have further suggested that since the Palaeogene the Halmahera domain has formed an integral part of the Philippine Sea Plate, and in the plate reconstructions of Hall (1996) this interpretation forms a primary constraint on the kinematics of the Philippine Sea Plate. How-ever, although this interpretation may be permissible on palaeomagnetic grounds, it is dicult to reconcile with the post-Oligocene regional geology, particularly with the geology of New Guinea during this time. Hall's reconstructions suggest that Halmahera collided with the northern margin of Australia in the region of present-day eastern Papua New Guinea during the Late Oligocene, and since that time has moved pro-gressively westward along the northern margin of New Guinea. The constraint of assuming that the Halma-hera block has also remained an integral part of the Philippine Sea Plate since the Palaeogene implies that the Halmahera±New Guinea plate boundary should have been purely transcurrent through the Neogene; it is not possible to invoke an obliquely convergent plate boundary because the Oligocene arc±continent col-lision precludes the existence of subductable oceanic crust to the south of Halmahera. Such a purely trans-current plate boundary is, however, inconsistent with the presence of the calcalkaline Maramuni Arc in New Guinea which developed during the Early-Middle Mio-cene, contemporaneously with the supposed east to west passage of Halmahera north of New Guinea. Even if the Maramuni Arc developed in the wake of Halmahera's westward passage, as suggested by Hall (1997), only very limited and highly oblique subduc-tion would be permitted which would not be likely to generate the volume of magma associated with the Maramuni Arc. Additionally, a clear east to west dia-chrony in the onset of the arc would be expected, evi-dence for which does not appear to have been reported in the literature. This suggests to me that it is unlikely that the Halmahera structural domain has formed a coherent fragment of the Philippine Sea Plate
since the Palaeogene as interpreted by Hall et al. (1995a,b).
4.13. Molucca Sea
The Molucca Sea marks the zone of collision between the young eastward-subducting Halmahera
Arc and the westward-subducting Sangihe Arc
(McCarey et al., 1980; McCarey, 1991). The
Wadati-Benio seismic zone associated with the San-gihe Arc extends to a depth of at least 600 km, whilst the eastward-dipping slab beneath Halmahera reaches only to about 230 km (Cardwell et al., 1980). The San-gihe forearc has overridden the northern end of the Halmahera arc, and is beginning to overthrust western Halmahera (Hall, 1997). The Molucca Sea collision complex is also being backthrust westwards onto the Sangihe volcanic arc (Hamilton 1979).
5. The development of the Eastern Indonesia collision complex
5.1. Palaeocene-Early Oligocene pre-collisional setting
In the following palaeotectonic reconstructions, the interpreted structure of the northern Australian conti-nental margin with the inferred pre-collisional disposi-tion of Australian-anity displaced terranes is the result of a detailed palaeogeographic evolutionary study of this region (Charlton, submitted; work in pro-gress; Robertson Research, 1999). It is broadly in line with a number of other reconstructions published pre-viously (e.g. Audley-Charles, 1976; Audley-Charles et al., 1988; Daly et al., 1991; Dercourt et al., 1993). The constructions are compatible with Tertiary palaeomag-netic latitudinal data from Sumba (Wensink, 1994; Wensink and van Bergen, 1995; Fortuin et al., 1997), Timor (Wensink and Hartosukohardjo, 1990), Buton
10 15 20 25 30 35 40 45 50
(Ali et al., 1996), Sulawesi (Panjaitan and Mubroto, 1993; Surmont et al., 1994), Kelang (Haile, 1978) and the Halmahera region (Hall et al., 1995b; Fig. 7). However, compared with the Halmahera regional declination data (Hall et al., 1995b, ®g. 8a) there is a consistent over-estimation for the rotation of Halma-hera by about 208. A possible explanation of this is that the present N±S elongation of Halmahera is re-lated to the Plio-Quaternary Halmahera arc, and the older Tertiary arc may have had a slightly dierent orientation, not elongated as shown in the (necessarily schematic) reconstructions.
The main geographic features of this reconstruction (Fig. 8) are a large continental promontory (for brevity here described as the Greater Sula Spur, following KlompeÂ, 1954) which includes present-day eastern/ southeastern Sulawesi, Banggai-Sula, Buton, Buru, Seram and the Bird's Head. Continental basement in
the Greater Sula Spur extends considerably further north than in equivalent areas to the east (cf. Pigram and Davies, 1987, etc.). This is interpreted to be a con-sequence of right-lateral displacement of the Greater Sula Spur relative to the main body of northern Aus-tralia during breakup of Gondwana in the Permian-Triassic (Charlton, submitted).
By the early Tertiary the Greater Sula Spur was sep-arated from present-day Timor and the southern Banda Arc by an oceanic embayment (the `Proto-Banda Sea' in Fig. 8) which developed as a result of Mesozoic sea¯oor spreading (Charlton, submitted). It should be noted, however, that this Proto-Banda Sea is only indirectly related to the present-day South Banda Basin in that both occupy locations between Timor and Seram. It is not implied that the present-day South Banda Basin is composed of Mesozoic ocea-nic crust.
The Sundaland block in the reconstructions is essen-tially that of the present-day; the details of possible Neogene rotation and/or internal deformation within Sundaland are not considered in this study. The pre-sent-day eastern Sunda/Banda Arc is initially shown restored against the SE Sundaland margin. In the older reconstructions the arc is shown with a schematic 50% lateral shortening to compensate for lateral exten-sion interpreted during the later stages of colliexten-sion, as will be described in more detail subsequently.
The third main element of the reconstructions is an intra-oceanic island arc system including the future western and northern Sulawesi and Halmahera arc ter-ranes of eastern Indonesia. Arc terter-ranes that end up in present-day Papua New Guinea and the Philippines are also shown, although the various crustal blocks of the present-day eastern Philippines are not shown indi-vidually.
An interpretation of the regional plate setting at 35 Ma (Early Oligocene) immediately before the onset of collision is shown in Fig. 8. During the Palaeogene the western half of northern Australia (the region that subsequently formed the eastern Indonesia collision complex) was evidently a structurally quiescent conti-nental margin, as indicated by the absence of anything other than passive margin sedimentary sequences (pre-dominantly carbonates). It is possible that arc-conti-nent interactions were already taking place during the Eocene±Early Oligocene in the eastern sector of the northern Australian margin (the future Papua New Guinea), as indicated by the interpreted age of ophio-lite obduction in the Papuan Peninsula (e.g. Davies et al., 1996). However, this is not strongly supported by the unpublished palaeogeographic reconstructions car-ried out as part of this study (Robertson Research, 1999).
During the Palaeocene±Eocene, the Sunda Shelf was the site of graben development, interpreted here as re-lated to the western Sulawesi island arc rifting away from the eastern margin of Sundaland. Other Palaeo-gene island arc terranes of northern east Indonesia
(Halmahera, Batanta, Arfak, Biak, Yapen, etc.)
together with comparable island arc terranes now found in Papua New Guinea and the Philippines, are interpreted to have formed part of a roughly east±west intraoceanic island arc system located at the plate boundary between the Philippine Sea Plate to the north and oceanic crust of the northern Indo-Austra-lian Plate to the south. This east±west plate boundary linked directly westwards with the forerunner of the
present-day Sunda Arc. Palaeogene island arc
sequences of the present-day Banda Arc at this time still formed part of the Sundaland continental margin, as indicated by the occurrence in the Timor allochthon of a Late Eocene land mammalAnthracothema verhoe-veni(von Koenigswald, 1967), which apparently
inhab-ited plains and marshes in the neighbourhood of great rivers.
5.2. Oligocene collision
The northern passive margin of Australia began to collide with the island arc system to the north during the Oligocene (Fig. 9). Indications of this collision are found from Buton and central Sulawesi in the west, through the Bird's Head and Bacan-Obi region, to northern Papua New Guinea and as far east as the Papuan Peninsula. Stratigraphic or radiometric resol-ution does not at this stage permit any interpretation of the synchrony or otherwise of this collision; at pre-sent available evidence points to collision in approxi-mately mid-late Oligocene times along the entire length of the reconstructed northern Australian margin.
5.3. Post-collisional indentation
The ®rst phase of post-collisional deformation in the
eastern Indonesia region from ca 30±24 Ma was
characterised by indentation of the former arc trend, and was accompanied only by relatively limited defor-mation (Fig. 11). In the present-day Halmahera sector of the colliding arc, which impinged upon Australian New Guinea immediately east of the relatively rigid Bird's Head structural block, arc±continent conver-gence continued slightly longer, as indicated by the somewhat younger end-collisional ages obtained from this region (ca 22 Ma) compared with other parts of the collision complex (Hall et al., 1995a). During this period of indentation the arc±continent collision zone probably no longer formed a well-de®ned plate bound-ary, but occupied a position perhaps analogous to pre-sent-day Italy which is not located on a clearly de®ned plate boundary but nevertheless remains tectonically
and volcanically active. Northern Australia eectively became welded to the Philippine Sea Plate during this interval, and continuing northward movement of Aus-tralia was absorbed at the northern (palaeo-coordi-nates) margin of the Philippine Sea, whilst the westward convergence of the Paci®c Plate was taken up at the eastern boundary of the plate.
Relative displacement between Australia and Sunda-land was accommodated during this interval and sub-sequently by the commencement of anticlockwise rotation in SW Sulawesi (Figs. 11 and 12), possibly as-sociated with greater or lesser rotation of Sundaland.
5.4. Post-collisional disaggregation by left-lateral shear
Fig. 10. Possible regional reconstruction at 30 Ma based on the positioning of the Philippine Sea Plate by Fuller et al. (1991).
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Fig. 11. Reconstruction at 25 Ma.
Fig. 13. Reconstruction at 15 Ma.
regional dynamics may have resulted from collision between the Ontong Java oceanic plateau and the Solomon Arc, which commenced at about 22 Ma (Kroenke et al., 1986).
In northern New Guinea a transcurrent plate boundary is interpreted to have developed between the pre-collisional volcanic arc and its counterpart forearc/ collision complex to the south, resulting in much of the Palaeogene volcanic arc that originally lay north of the present-day New Guinea collision complex being translated northwestwards towards the present-day Philippines. The translation of the East Philippines Terrane is not detailed in this study, but for the pur-poses of the reconstructions it is rotated at a rate of 1.758/m.y. clockwise about a pole located at latitude 138N, longitude 1808. This rotation rate lies within the spread of recent estimates for the present rotation rate of the Philippine Sea Plate relative to Eurasia (1.09± 1.958/m.y.: Hall et al., 1995c, table 5) and within the
spread of long-term palaeomagnetic declination
changes for the Philippine Sea Plate (Fuller et al., 1991, ®g. 7b). For comparison, Hall et al. (1995c) interpreted a rotation pole at 158N, 1608E and a ro-tation rate of 1.78/m.y. between 25 and 5 Ma.
During the interval ca 18±12 Ma the motion of the Paci®c relative to Australia was taken up by reversed (southward) subduction beneath eastern and central New Guinea (generating the Maramuni volcanic arc), and this arc±forearc system linked northwestwards
into the proto-Philippines Fault strike slip system. West of the proto-Philippine wrench fault, induced transtension in the future gulfs of Bone and Tomini resulted in limited westward translation of western Sulawesi relative to the Greater Sula Spur block (Fig. 13). Linkage between North Sulawesi and Mind-anao was maintained during this period by the devel-opment of the Sangihe Arc, which subsequently absorbed convergence between the Philippine Sea Plate and eastern Eurasia.
From about 15 Ma it is suggested that the poorly dated Solomon Sea began to open as a transtensional backarc basin in response to the oblique convergence between northern Australia and the Philippine Sea Plate (Figs. 13 and 14). This ocean basin developed along the line of weakness separating the Papuan Peninsula forearc collision complex from the Palaeo-gene arc terranes to the north (New Britain). A new volcanic arc may have developed north of New Britain (the shaded area shown in Figs. 13±15); this is the line of the future Bismarck Sea spreading centre (see below).
Commencing at about 12 Ma a new wrench system, the Sorong Fault Zone, began to develop, eectively separating the future Philippines from eastern Indone-sia (Fig. 14). This appears to be part of a major plate boundary reorganisation at this time, also seen for instance in the development of the Ayu Trough spreading centre. A new wrench fault probably
