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Geological Development of Eastern Indonesia and the Northern Australia Collision Zone: a Review
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Geological development of eastern Indonesia and the northern Australia collision zone: A review
Peter W. Baillie1,Tom H. Fraser2, Robert Hall3 and Keith Myers4
Keywords: Banda Arc, eastern Indonesia, Timor Trough, Tanimbar Trough, Aru Trough
• Relatively rigid lithosphere of the northern margin of the Australian craton.
• Deepwater troughs, including the Java Trench and its bathymetric extensions the Timor, Tanimbar, Aru and Seram troughs.
• The Banda Arc comprising an Outer Banda Arc (including the islands of Timor, Tanimbar and Seram) and the volcanic arc of the Inner Banda Arc and the Weber Deep.
• The island of New Guinea comprising (a) West Papua Central Range fold belt, (b) the West Papua foreland lying south of the fold belt, (c) the Bird’s Head microcontinent, and (d) Cenderawasih Bay and the
“Bird’s Neck”.
In general terms, the geological evolution of the region may be considered in terms of:
• building blocks formed during Proterozoic and Palaeozoic times;
• development of a Mesozoic passive margin; and
• Neogene collision of Australia and the Indonesian Sunda archipelago.
The first two phases occurred when the region was part of Gondwanaland.
While the geological literature on both the northern margin of Australia (essentially the North West Shelf) and South East Asia (including East Indonesia) is voluminous, it is essentially endemic and the overlap of political and geological boundaries has tended to mask the common geology between the two regions (Longley et al., 2002).
In this paper, we review the geology of the collision zone between the Australian craton and the Indonesian archipelago – one of the most geologically-complex areas on Earth.
Much useful information is found in the Journal of Asian Earth Sciences (formerly the Journal of Southeast Asian Earth Sciences), the AAPG Bulletin, specialist publications of The Geological Society together with conference proceedings published by the Petroleum Exploration of Australia (PESA), the Australian Petroleum Producing and Exploration Association (APPEA), the Indonesia Petroleum Association (IPA) and the South E a s t A s i a P e t r o l e u m E x p l o r a t i o n A s s o c i a t i o n (SEAPEX).
The seismic database available to us includes several non-exclusive surveys acquired by TGS-NOPEC Geophysical Company and/or WesternGeco, including East Indonesia regional surveys EIR98 and EIR99, together with the GP-ARI and Matahari surveys acquired near the Australian border (Fig. 1).
Abstract
For much of its history, the region encompassing the northern margin of the Australian Plate and eastern Indonesia has been part of, or relatively close to a continental margin.
Three significant periods of Palaeozoic–Mesozoic terrane dispersion occurred, where continental fragments separated from the margin of Gondwanaland, drifted northward and were subsequently accreted to the Eurasian craton. Separation of these blocks in the Early Devonian, late Early Permian and Late Triassic–Late Jurassic was accompanied by the opening of the Palaeo- Tethys, Meso-Tethys and Ceno-Tethys ocean basins.
By the end of the Jurassic, the northern margin of the Australian continent was part of a passive margin facing an open ocean containing several continental blocks or microcontinents.
The present geology results from two tectonic forces:
(1) northward movement of the Australian Plate driven by the Southern Ocean spreading centre at 9 cm/year, and (2) rotation of the Pacific Plate which sets up sinistral shear on the northern margin of New Guinea.
This paper summarises the geological evolution as:
building blocks formed during Proterozoic and Palaeozoic times; development of a Mesozoic mid-latitude passive margin on the eastern margin of Gondwanaland;
and Neogene collision of Australia and the Indonesian archipelago.
Introduction
Eastern Indonesia lies within a complex tectonic zone formed as a result of Neogene collision and interaction of the Australian and Eurasian (Sundaland) plates, and the Caroline and Philippine Sea oceanic plates, and the Pacific Plate. The region has been likened to a giant jigsaw puzzle, consisting of a complex of small ocean basins separated by slivers or fragments of sometimes-thickened continental crust, which seem ultimately destined to form part of a single complex terrane (Milsom, 1991; Metcalfe, 1998).
The active tectonics of the region have long been recognised and are clearly expressed in its morphology, volcanicity and seismicity (e.g. Hamilton, 1979; Veevers, 2000). The following morphotectonic features are recognised:
1 TGS-NOPEC Geophysical Company, Level 5, 1100 Hay Street West Perth, WA, 6005. Australia. Email: [email protected]
2 Resource System Diagnostics, Jakarta, Indonesia
3 SE Asia Research Group, Royal Holloway University of London, United Kingdom
4 WesternGeco, Kuala Lumpur, Malaysia
540 Geological development of eastern Indonesia and northern Australia collision zone
Figure 1. Locality map showing seismic database used in this study. Letters A-D indicate position of seismic lines shown on Figure 7.
Proterozoic and Palaeozoic building blocks
Proterozoic and Lower Palaeozoic rocks form the effective basement of the study area and are presently found near the margin of the Australian Craton (Arafura Platform) and the Bird’s Head of West Papua. Although the early tectonic history is poorly defined, the overall north–south structural grain of the basement rocks of the Arafura Platform is well illustrated by satellite gravity data (Fig. 2) where several kilometres of gently-deformed Lower Palaeozoic and Proterozoic section is present.
By the early Palaeozoic, the region was part of the eastern margin of Gondwanaland (Metcalfe, 1998).
The Goulburn Graben (Fig. 3) developed within the Proterozoic basement during the early Palaeozoic (Bradshaw et al., 1990; McLennan et al., 1990). This northwest- trending graben is asymmetric, with major faults developed along its northern boundary and a Palaeozoic to lower Mesozoic fill. Similar intracratonic rifts include the Barakan Basin (Barber et al., 2003), Petrel Sub-basin (Bonaparte Basin) and the Fitzroy Trough (Canning Basin;
McLennan et al., 1990).
Gondwanaland tectonic development
During Palaeozoic and Mesozoic times, three significant periods of terrane dispersion occurred, where continental fragments separated from the margin of Gondwanaland, drifted northward and were subsequently accreted to the Eurasian craton (Hutchison, 1989). Separation of these blocks in the Early Devonian, late Early Permian and Late Triassic–Late Jurassic was accompanied by the opening of the Palaeo-Tethys and Meso-Tethys ocean basins (Fig. 4; Metcalfe, 1998).
The first important tectonic event was the Early Devonian rifting of South China, North China, Indochina, Tarim and Qaidam from Gondwanaland and the subsequent formation of the Palaeo-Tethys Ocean (Fig. 4b; Metcalfe, 1998).
The second event was the Carboniferous to Permian rifting of Sibumasu and Qiangtiang as part of the Cimmerian continent, now preserved in parts of Central and Southeast Asia (Fig. 4c; Metcalfe, 1998). On the North West Shelf this important rifting event gave rise to the Westralian Super-basin (Bradshaw et al., 1988). It relates to the onset of the Sibamasu block separation and resulted in a thick (10 km) continuous fill of mainly Permian and Mesozoic sediments, covering the
Baillie et al. 541
Figure 2. Regional satellite gravity map (source – http://topex.ucsd.edu/marine_grav/mar_grav.html)
entire Westralian Super-basin (Longley et al., 2002). Late Carboniferous uplift, erosion and local folding in the Bonaparte Basin have been related to this event (Baillie et al., 1994).
The Late Triassic Carnian to Norian succession of the North West Shelf was deposited following a regionally extensive period of significant tectonism, erosion and uplift along the edges of the craton (Fitzroy Movement), related either to breakup events along the Gondwanan margin or to docking of continental blocks along the West Papua/PNG subduction margin (Longley et al., 2002).
The third significant period of terrane dispersal from the Gondwanaland margin occurred in the Late Triassic to Late Jurassic, associated with the Late Triassic drift of the Lhasa block and the subsequent drift of the West Burma and Woyla blocks in the Late Jurassic (Metcalfe, 1998). Longley et al.
(2002) recognized the separation of three West Burma sub- blocks: Block I began to rift in the latest Hettangian and extension continued until breakup in the Sinemurian; rifting of Block II subsequently began in the Callovian and was complete by the Oxfordian; and Block III separated from a position outboard of the Bonaparte Basin in the Tithonian.
It is possible that a fourth block occupied the position of the present Banda Sea.
Late Jurassic continental breakup along the Argo Abyssal Plain, followed by earliest Cretaceous breakup along the rest of the margin, brought the first deep-marine conditions into close proximity to the continental-edge basins (Baillie et al., 1994).
Mesozoic passive margin
Data from the Mesozoic exposures in eastern Indonesia range from mapping traverses led by BMR/AGSO
personnel during the 1980s to a limited amount of subsurface data from seismic and well-bore penetrations.
The Upper Palaeozoic and Mesozoic stratigraphy of southeast Indonesia is a continuation of the northern Australian rift-drift sequence, and was controlled by Gondwanaland breakup and the subsequent opening of the Indian Ocean (Fig. 5, modified after Pairault, 2002, originally sourced from Pigram and Panggabean 1981,1983 and Fraser et al., 1993). Three major tectonic stages are recognised, similar in broad character to the North West Shelf:
• A Permian to Late Triassic pre-rift stage, represented by shallow marine and fluvio-deltaic deposition on a stable platform.
• A Late Triassic to Early Jurassic break-up stage characterised by block faulting, uplift and thermal doming, and followed by erosion and accumulation of continental red beds.
• Post-rift from the Middle Jurassic when the region started to cool and subside, leading to the deposition of restricted marine sediments until rising sea level in the Cretaceous led to open marine conditions. A thick carbonate succession was deposited during the Tertiary (New Guinea Limestone).
Fraser et al. (1993) created a stratigraphy for western West Papua based on depositional cycles:
• The early Toarcian to late Bajocian Inanwatan Polysequence (present in the Agung-1, CS-1, TBF-1 and Gunung-1 wells) was deposited in a nearshore setting, progressively passing into fluvial and lacustrine environments.
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Figure 3. Regional tectonic setting (compiled from various sources with some new interpretation).
Baillie et al. 543
CENOZOIC
PLEIST.
MIOCENEOLIGO-EOCENE
PLIOCENE ERA EPOCH
LATELATEEARLYMIDDLE
TRIASSIC
MESOZOICPALAEOZOIC
MID.
EAR.
JURASSICCRETACEOUS EARLYLATELATE MID.
EAR.
EAR.
EAR.
MID.
L.
L.
PERMIAN CARBONIF.
DEVONIAN SILURIAN PALEOEAR.
L.
Pre- Breakup Stable platform
Breakup Uplift, block faulting, local volcanism; rapid
sedimentation followed by erosion Post- Breakup
? Restricted marine Open marine, low sedimentation rates
Establishment of widespread carbonate
shelf facies Mid Oligocene
sea-level fall
? ?
KETUM FORMATION AIFAM GROUP TIPUMA FORMATION
INANWATAN POLYSEQUENCE
ROABIBA POLYSEQUENCE
KEMBELANGAN GROUP FORMATION
LOWER NEW GUINEA LIMESTONE GROUP
= FAUMAI & WARIPI FORMATIONS UPPER NEW GUINEA
LIMESTONE GROUP
= KAIS & SIRGA FORMATIONS KLASAMAN & KLASAFET
FORMATIONS TECTONIC
STAGES STRATIGRAPHY OF WESTERN WEST PAPUA
Metamorphosed turbidites (mostly slate and shale).
Marine mudstone and limestone passing up into non marine clastic sequence.
Continental red beds (shale and sandstone).
Nearshore marine to progressively non marine:
shallow water marine limestone + lacustrine
& anoxic sands, clays and coal.
Transgressive sequence: claystone pass up into limestone, sandy in places. Little
terrestrial influence.
Open marine fine clastic deposits and deep water clays.
Deep marine argillaceous section. No terrestrial influence. Volcanic event at the base.
Faumai Fm: bank and shoal arenaceous limestone with abundant foraminifers.
Waripi Fm: oolitic and bioclastic foraminiferal limestone, calcareous sandstone and
mudstone in places.
Kais Fm: reefal limestone.
Sirga Fm: transgressive clastic sequence (mudstone, locally sandstone).
SEBYAR POLYSEQUENCE
JASS POLYSEQUENCE
Major regional transgression
Major erosion:
50 m.y.
Major regional transgression
Klasaman Fm: sandy mudstone and calcareous sandstone. Conglomerates
and lignite in upper part.
Klasafet Fm: marl, siltstone and a little limestone.
Clastic sedimentation in new basins
Figure 5. Stratigraphy of western West Papua (modified after Pairault, 2002).
• The early Callovian to early Kimmeridgian Roabiba Polysequence (CS-1 and TBF-1 wells) follows a 12 million year period of erosion or non-deposition and comprises significant sandstones, together with claystone and limestone deposited in a nearshore environment.
• The mid-Tithonian to early Valanginian Sebyar Polysequence represents a major regional transgression following a major stratigraphic break.
The Coniacian to Maastrichian Jass Polysequence (Agung-1, CS-1, TBF-1, Gunung-1 and Onin South-1 wells) followed early Cretaceous erosion. A major transgression took place from the mid-Campanian, leading to the deposition of deepwater argillaceous sediments.
The Triassic rocks of eastern Indonesia have a predominantly marine carbonate aspect. Rich oil-prone source rocks exist in both Buru (Fraser et al., 1993) and Seram islands. However, in the Bird’s Head, an NNGPM well, Puragi-1, penetrated a reservoir quality sandstone that is thought to be Triassic in age.
The Jurassic has been sampled by fieldwork within West Papua, and also on the islands of Banggai, Sula, Tanimbar and Kai. Numerous wells have penetrated the Jurassic, predominantly as marine clastics. The thickest succession is recorded within the well TBJ-1, just west of the Onin Peninsula (Fig. 1). The Jurassic appears to pinch out towards the Bird’s Head and an east–west depositional margin is
considered to exist within the northern Bird’s Head, parallel to the coastline of Berau Bay. Existing northwest–southeast structural lineaments controlled deposition within the Bird’s Head. It is thought that the Jurassic coastline transgressed northward through the Jurassic (subject to eustatic variations). Throughout the eastern Indonesia region and the Timor Sea, reservoir quality sandstones were deposited in nearshore marine settings during the early Late Jurassic (see also Barber et al., 2003), prior to the so-called “Break-up Event”. It is proposed here that the Plover Sandstone reservoir of the Timor Sea is synchronous with both the reservoir of the Abadi discovery (Indonesia Masela PSC; Nagura et al., 2003) and the Tangguh Field, suggesting a similar depositional setting over an enormous area of coastline and nearshore marine environment.
The Late Jurassic and Early Cretaceous are less widely recognised because their deposition was adversely affected by major structural events. The “Break-up Event” resulted in regional inundation of the low-lying coastline and shallow marine fringe, resulting in widespread deep marine claystone deposition. Depositional packages are complex and hard to correlate, due to subsequent partial erosion and varied structural attitudes as the rift basins stabilized and subsequently failed again. A major sediment-starved flooding (deepening) event in Aptian/Albian times resulted in the deposition of the Darwin Radiolarite in the Timor Sea region.
To the authors’ current knowledge, the radiolarite lithofacies has yet to be recognised within West Papua.
544 Geological development of eastern Indonesia and northern Australia collision zone The rest of the Late Cretaceous saw continuing deep
marine deposition with occasional shallowing, probably controlled by structural tilting linked to post-rift thermal sag.
Carbonate rocks are recognised within some wells drilled in the northern Australian margin.
Key petroleum system attributes of the Mesozoic are:
1. Oil-prone source rocks within (a) marine Late Triassic marine carbonates, and (b) paralic Middle–Late Jurassic clastics.
2. High quality sandstone reservoirs of Middle–Late Jurassic age widespread throughout the north Australian margin.
3. Thick Cretaceous claystone successions provide effective seal to reservoir and burial for source.
4. The “Break-up Event” of Late Jurassic age (climax) provides the initial structural setting for most of today’s hydrocarbon discoveries.
Neogene Collision
Between 45 and 25 Ma, there was subduction north of Australia, remnants of which are seen in the mantle (Hall and Spakman, 2002).
Initial collision between the Australian and the Philippine plates began around 25 Ma when transcurrent motion of the Philippine Plate resulted in the shaving of blocks from the northerly protrusion of the Australian continent (Pigram and Panggabean, 1984; Hutchison, 1989; Longley, 1997). The leading edge of the Australia Plate arrived at the Indonesian arc subduction zone and initiated the New Guinea fold belt (Hamilton, 1979; Hall, 1996).
Bird’s Head (Kepala Burung) and the Sorong Fault Zone The western extremity of the island of New Guinea lying to the north of 2°S is known as the “Bird’s Head” (Kepala Burung; Fig. 1). The core region comprises Palaeozoic and southward-younging continental rocks of the Kemum Block, thought to have formed on a passive continental margin (Pigram and Davies, 1987). The Kemum Block is separated from three principal fault-bounded blocks to the north by the Sorong Fault. The Tamrau, Netoni and Tosem blocks comprise disparate successions of Mesozoic and Cenozoic arc-derived sedimentary and igneous rocks (Pigram and Davies, 1987;
Hall and Wilson, 2000).
There is some uncertainty as to the origin of the Bird’s Head, in particular its location relative to Australia in Jurassic times and whether it has rotated significantly during the Neogene. There is no compelling evidence for either (a) significant rotation, or (b) an origin from the eastern side of Australia (e.g. Struckmeyer et al., 1993).
The origin of Cenderawasih Bay is also uncertain. The triangular shape and water depths of more than 2 km suggests that it could have opened to the north (cf. sphenochasm concept of Carey, 1958). However, the geometric arguments rely on shapes of poorly mapped features and depths in the order of 2 km are not typical of normal oceanic crust. If the underlying crust is oceanic, it must be overlain by a considerable thickness of sediment. Its age is unknown. In addition, satellite gravity data (Fig. 6) indicates a strong gravity high along the Wandamen Peninsula due to very
young metamorphic rocks. Very young ages (<5 Ma) suggest recent extension and a possible core complex formation;
these young metamorphic rocks require further investigation.
A gravity low is present only along the eastern margin of Cenderawasih Bay, therefore it appears unlikely that the entire bay has opened by rotation. Transpressional extension along the north-northeast lineament close to the eastern margin may be responsible for the rhomboidal shape of the deepest part of Cenderawasih Bay, possibly related to the development of the Lengguru Fold Belt to the west and south.
The once contiguous microcontinents of eastern Indonesia were dismembered and dispersed by Sorong fault splays around 20 Ma, resulting in the formation of the Bird’s Head, Seram- Buru, Obi-Bacan, Buton-Tukang Besi and Banggai-Sula complexes (Pigram and Panggabean, 1984; Hall, 2002). Since 12 Ma, the authors consider that the Bird’s Head has not moved any significant distance. A small counter-clockwise rotation took place between 4 and 8 Ma, and relatively small strike-slip movements between 4 and 8 Ma and also 0.5 and 2 Ma (Hall, 2002). These events may well have direct relevance to the development of Cenderawasih Bay.
The Sorong Fault System, the southern boundary of both the Molucca Sea and the Philippine Sea plates with the Australian Plate (Hall and Wilson, 2000), is one of the most important structural elements of South East Asia. The Sorong Fault has been responsible for translating continental fragments from the northern margin of the Australian Plate (i.e. the Bird’s Head) into the outer edge of Sundaland (Pigram and Panggabean, 1984; Hutchison, 1989). This fault system was initiated in the early Miocene, the result of oblique convergence of Australia and the Philippine Sea, Caroline and Pacific plates, and continues to the present day.
The Banda Arc
The Banda Arc, a west-facing horseshoe-shaped arc, lying between the Timor and Arafura seas and the Birds Head micro-continent is the locus of the convergence and critical to understanding the tectonic development of the region.
From the Banda Sea side outwards, the arc comprises an inner volcanic arc, which has been active since the Late Miocene, and an outer non-volcanic arc of islands formed principally of allochthonous sedimentary, metamorphic and some igneous rocks (Darman and Sidi, 2000; Hall, 2002).
The Inner Banda Arc includes the Solor and Alor island groups, Wetar and Banda, and the Outer Banda Arc includes Timor, Tanimbar, Kai Besar, Seram and Buru (Fig. 1).
The Outer Banda Arc comprises a series of belts of rocks (Bowin et al., 1980; Darman and Sidi, 2000) comprising:
• a discontinuous inner belt of “ophiolite”, which in general is blocky and narrow;
• a belt of low- to high-grade metamorphic rocks;
• a fold-and-thrust belt dominated by Permo-Triassic sedimentary rocks derived from the Australian continental margin;
• a fold-and-thrust belt dominated by Late Mesozoic and Cenozoic deepwater sediments; and
• an outer belt of uplifted late Neogene basins.
The rocks of the Outer Banda Arc comprise Late Triassic to Early Jurassic shallow water clastics and carbonates
Baillie et al. 545
-400 -400
-400 -800
400 0 400
800 -400 0
-800 0 0
400 800
800 800
1200
130°E 132°E 134°E 136°E
5°S 3°S 1°S
0 km 100
Figure 6. Satellite gravity, Bird’s Head and Cenderawasih Bay (source: www.deos.tudelft.nl/altim/ceo/pim.html), with onshore Bouguer anomalies and trends (after Dow et al., 1986) indicated.
unconformably overlain by a Late Jurassic to Palaeogene condensed sequence of deep-water carbonate and chert – stratigraphies vary only after the Oligocene. Indeed, the virtually identical (“Australian”) Mesozoic stratigraphies of Buton, Buru and Seram (and possibly East Sulawesi) suggest that they originally formed part of the Australian craton (Audley-Charles et al., 1979;
Milsom, 2000).
Major thrust faults have been mapped north of the Timor Trough to the west of, and on the island of Timor, north of Wetar (the currently seismically active Wetar Thrust), north of the island of Flores (Flores Thrust) and north of Tanimbar. A number of northeast-trending strike- slip faults and fracture zones have been mapped along the Banda Arc deformation zone, extending from Sumba to Tanimbar (McCaffrey, 1996).
Earthquakes suggest that the present stage of the collision of the Australian continental margin with the Banda Arc probably involves more rapid convergence at the backarc thrusts than at the Timor Trough. The Wetar and Flores backarc zones are considerably more energetic seismically than the subduction thrust fault north of the eastern Java Trench and Timor Trough; all known large thrust earthquakes recorded during the 1990s occurred at the Flores and Wetar thrusts (McCaffrey, 1996). A change from south- to north-directed thrusting after the emergence of Timor is noteworthy (Price and Audley-Charles, 1987;
Hall and Wilson, 2000); subduction reversal may be in progress, accompanied by the recent disappearance of the
Banda allochthon. Deep seismic studies have reinforced the suggestion of subduction polarity reversal (Snyder et al., 1996).
Trough systems
Many previous workers have described a single trough system resembling a giant horseshoe extending from the Java Trench through the Timor, Tanimbar and Aru troughs into the Seram Trough (Fig. 3). This may be fortuitous, but incorrect. The deep-water crustal lineaments and corresponding gravity lows (Figs 2, 6) observed throughout the study area owe their origin to a number of widely different mechanisms.
The Java Trench (Fig. 3) marks the boundary between the Indian Ocean-Australian plates and Sunda-Indonesian arc lithosphere, and has been the location of subduction through most of the Cenozoic. The Java Trench and all present-day subduction terminate near the island of Sumba (Fig. 3). Subduction has completed consumption of oceanic crust outboard of the Tertiary accretionary wedge (green colour on Fig. 3).
The Timor, Tanimbar, Aru and Seram troughs, generally less than 50 km across and only 1.0–2.5 km deep, mark the edge of relatively undeformed Australian continental crust – the deformed leading edge of the continental crust is indicated by vertical overprint on Figure 3. Modern high quality seismic data (Fig. 7a-d) permits evaluation of these contact zones.
546 Geological development of eastern Indonesia and northern Australia collision zone
Figure 7. Representative seismic lines across collision zone, (a) Seram Trough, seismic line EIR98–201, (b) Aru Trough, line EIR98–106, (c) Tanimbar Trough, line MH01–11, (d) Timor Trough, line MH01–18; location of lines indicated on Figure 1.
Figure 7a shows a representative line across the Seram Trough, broadly similar to the Timor Trough, albeit with reversed polarity. Gravity data (Fig. 2) indicates a gravity high centred on Kai Besar, showing that the Seram Trough is not contiguous with the Aru Trough.
The Aru Trough (Fig. 7b), immediately north of the Tanimbar Trough, is totally different in character and shows abundant evidence of strike-slip movements with rapid ponding of recent sediment in deep, linear depocentres (Fig. 8). Along the eastern flank of the trough, dredged samples include Carboniferous–Permian siliciclastics, Jurassic–Cretaceous black shale, late Albian to late Eocene open-marine pelagic sediment, early Miocene shallow-water platform carbonate and late Miocene to Holocene pelagic sediment (Cornee et al., 1997).
It is possible that the eastern contact continues though the “Bird’s Neck” and along the eastern (markedly linear) side of Cenderawasih Bay where it is probably terminated by the Yapen Fault, a splay of the Sorong Fault. West Papua gravity data (Fig. 6) indicates the presence of dislocation across the “Bird’s Neck”, supporting this conclusion.
Figures 7c and 7d traverse the Tanimbar and Timor troughs respectively. The downwarped Australian plate shows evidence of relatively minor, brittle extensional deformation. The contact zone is relatively sharp and displays relatively minor compressional deformation. Rocks of the Outer Banda Arc are highly deformed and include a series of stacked thrust sheets.
Most previous workers have recognized that the trough system is a relatively young and active tectonic feature.
New seismic data from the Tanimbar Trough gives independent supporting evidence for a very young age (Fig. 9). Modern and fossil submarine canyons indicative of deepwater conditions probably developed during Pleistocene lowstands, when sediment was made available from the Arafura Platform. These are confined to the Pliocene and younger section.
Discussion
Subduction and accretion
Almost as contentious as the origin of the Banda Arc, has been the location and timing of post-Jurassic subduction in the region. From at least early Eocene times until the present day, oceanic crust has been subducting west and south of Sundaland (Sumatra, Java, Borneo, west Sulawesi) and north of New Guinea (Hall 2002). The current (fast) northward movement of the Australian Plate of 9 cm/year was initiated around 48 Ma (e.g. Baillie et al., 1994) implying that in excess of 4,000 km of shortening must be accommodated at the northern margin.
While conventional subduction of Indian Ocean oceanic crust (oceanic part of the Australian plate) continues in the Java Trench (Masson et al., 1990), all other troughs are not as straightforward. Rocks of the Indonesian archipelago are juxtaposed with continental rocks of the Australian plate where there is no subduction currently taking place. The boundary between the two regimes is clearly at Sumba where a promontory of Australian continental lithosphere collided with Sumba between 8 Ma (Keep et al., 2003) and 16 Ma (Rutherford et al ., 2001), also resulting in the mid-Miocene unconformity across Timor and subduction of Jurassic oceanic crust in the Banda Sea (Hall, 2002, fig. 24).
GPS measurements indicate that Timor appears to moving northward at about the same rate as Australia (Genrich et al., 1994; McCaffrey 1996).
Banda Arc and Trough systems
The Banda Arc appears to have achieved the impossible by being in collision simultaneously, and with the same plate, to the north and to the south (Milsom et al., 1996).
The cause of the extreme curvature of the Banda Arc is one of the major unsolved problems of South East Asian geology (Milsom et al., 1996). As noted by Hall (2002), the curvature itself has suggested derivation as a result of rotation driven by collision – many other authors have had many other ideas. There are four disparate groups of theories on the origin of the arc:
• Folding of an earlier east-west arc.
• The arc achieved its present curvature in the Cretaceous.
• The arc was formed by accretion of Australian continental material leading to enclosure of the Banda Sea oceanic basin.
• The arc inherited its present shape from the irregular Jurassic Australian continental margin.
Baillie et al. 547
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Figure 8. Detail of Aru Trough seismic line.
Figure 9. Detail of seismic line MH01–18, showing modern and fossil submarine canyons; prominent seismic marker below canyons is Base Pliocene (green arrow). Note canyons young away from trough axis, suggesting dynamic system with sequential activation rather than a single event.
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548 Geological development of eastern Indonesia and northern Australia collision zone The age of the Banda Sea is critical for tectonic
reconstructions – assumed ages of formation range from Mesozoic to Neogene (Norvick, 1979; Milsom, 2000; Pigram and Panggabean, 1983). More recently, the age of the Banda Sea has been thought to be Late Triassic, related to the opening of Meso-Tethys (Metcalfe, 1998), or Jurassic related to separation of the West Burma block from northern Australia (Longley et al., 2002; Hall, 2002). It now appears that there have been two episodes of oceanic crust formation in the region (a) Jurassic oceanic crust generated during the breakup of the Gondwanaland margin (“Proto-Banda Sea” of Hall, 2002), which has now been subducted, and (b) Neogene oceanic crust of the present Banda Sea, as originally suggested by Hamilton (1979), and confirmed by dredging and interpretation of anomalies (Hall, 2002).
The geology of Timor has been used to constrain tectonic models for the region. Three main structural models have been proposed for Timor (Richardson and Blundell, 1996):
1. The Imbricate Model – Timor is interpreted as an accumulation of chaotic material imbricated against the hanging wall of a subduction trench, the Timor Trough, and essentially forms a large accretionary prism.
2. The Overthrust Model – Timor is interpreted in terms of Alpine-style thrust sheets.
3. The Rebound Model – the Australian continental margin entered a subduction zone in the vicinity of the Wetar Strait. Subsequently, the oceanic lithosphere detached from the continental part due to “slab snapping”, resulting in the uplift of Timor by isostatic rebound on steep faults.
Various combinations of these differing models have also been proposed.
The trough systems of the collision zone are less well studied. A recent study of the Seram Trough system (Pairault, 2002; Pairault et al., 2003), has shown that evolution is complex, with the following sequence of Neogene events recognised:
• A Pliocene deformation event, probably around 5 Ma, which produced the MOKA (Misool-Onin-Kumawa Anticlinorium of Fraser et al., 1993).
• A period of erosion in the early Pliocene – the resulting unconformity is now covered by about 1 km of sediments near the culmination of the anticlinorium.
• The unconformity was folded and now dips south towards the Seram Trough south of the anticlinal axis.
Pairault et al. (2003) suggested that thrust loading on Seram may have caused the trough to develop and that the Seram Trough is a foredeep and not a subduction trench.
Pairault et al. (2002) interpreted the evolution of the Seram Trough in the context of an intraplate shortening model, to indicate that oceanic lithosphere of the Australian Plate started to roll back towards the southeast at the beginning of the late Miocene.
Conclusions
For much of its history, the study area has been part of, or relatively close to a continental margin. The geological evolution may be considered in terms of Proterozoic and
Palaeozoic building blocks, development of a Mesozoic mid- latitude passive margin on the eastern margin of Gondwanaland, and the Neogene collision of Australia and the Indonesian archipelago. Present day regional tectonic elements are shown as Figure 3.
By the early Palaeozoic, the region was part of the eastern margin of Gondwanaland (Metcalfe, 1998).
During Palaeozoic and Mesozoic times, three significant periods of terrane dispersion occurred where continental fragments separated from the margin of Gondwanaland, drifted northward and were subsequently accreted to the Eurasian craton (Hutchison, 1989).
At around 160 Ma, the northern margin of Australia lay at approximately 40°S latitude – and remained in mid-latitudes through the Cretaceous (Baillie et al., 1994). By the end of the Jurassic, this somewhat irregular margin was part of a passive margin facing an open ocean containing several continental blocks or microcontinents – much of this oceanic crust has subsequently been subducted.
The present complex geology of the region results from two disparate tectonic forces: (1) northward movement of the Australian Plate, and (2) sinistral shear on the northern margin of New Guinea (Sorong fault system), resulting from relative motion of the Australian and Pacific plates. Initial collision between the Australian and the Philippine plates began around 25 Ma when sinistral transcurrent motion of the Philippine Plate resulted in the shaving of blocks from the northerly protrusion of the Australian continent.
Acknowledgements
Figure 2 was compiled by Dino Loschi. We thank Nick Hoffman and Ron Noble for useful reviews and TGS-NOPEC Geophysical Company and WesternGeco for permission to publish proprietary information.
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Tom Fraser was born in Scotland and educated at Queen Mary College, London University under the eagle eye of Prof. Jake Kirkaldy. Tom’s baptism into the oil and gas industry was by mudlogging in the North Sea. He progressively worked his way into interpretation and New Ventures tasks. He has worked in 17 different countries, including South America and West Africa. During the last 15 years he has worked principally in South East Asia, in both Mesozoic and Tertiary basins. His principal interests are in integration of seismic and geological information, and the quantification and display of Petroleum System attributes. He enjoys a significant after-hours relationship with his Fender bass.
Keith Myers graduated in geology at Curtin University, Western Australia and undertook post-graduate studies in petrology and mineralogy. After a period in gold exploration, he moved to petroleum consulting firm Dolan
& Associates in 1992, working on projects in various locations in Australia, South East Asia and Africa. In 1999, he joined Western Geophysical as Manager of Exploration &
Reservoir Services, with continued involvement in Asia, especially China and Indonesia. After the formation of the WesternGeco joint venture, he moved to Kuala Lumpur as Business Development Manager for the Asia Pacific region. Keith is a member of PESA, SEAPEX and the Geological Society of Malaysia.
Robert Hall is Professor of Geology and Director of the SE Asia Research Group at Royal Holloway University of London. He completed his PhD in 1974 on ophiolites and sutures in eastern Turkey and later worked in the Eastern Mediterranean and Middle East for several years. Since 1984, he has worked in South East Asia, mainly in Indonesia, carrying out field-based research supported by industrial consortia, with post-doctoral researchers and postgraduate research students.
These studies have been the basis for computer animations of the plate tectonic reconstructions of SE Asia and the SW Pacific for the Cenozoic. Research interests: Regional geology of SE Asia and the western Pacific. Island arc origin and evolution. Plate tectonic reconstructions of SE Asia and the west Pacific. Seismic tomography, mantle processes and tectonics of the region. Tropical sedimentation: links to provenance, climate and tectonics. Implications of plate tectonics for biogeography of SE Asia.
Peter Baillie is a graduate of the University of Tasmania (geology) and Macquarie University in Sydney (sedimentology, basin analysis). He was employed at the Tasmanian Department of Mines from 1970 until 1993 and held various positions in regional geological mapping and petroleum exploration administration. In 1993, he moved to the Department of Minerals and Energy in Western Australia as Manager of the Exploration and Production Branch in the Petroleum Division. He joined TGS-NOPEC Geophysical Company in 1997 and is Chief Geologist Asia Pacific. Formerly the Managing Editor of the PESA Journal, Peter was Deputy Chairman of WABS II (1998) and Chairman of WABS III (2002).
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