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Cenozoic of SE and Australasia 47

INDONESIAN PETROLEUM ASSOCIATION Proceedings of the Petroleum Systems of SE Asia and Australasia Conference, May 1997

CENOZOIC TECTONICS OF SE ASIA AND AUSTRALASIA

Robert Hall SE Asia Research Group, University of London

ABSTRACT quency and rapidity of changes in regional tecton- ics. A plate tectonic model for the development of the of SE Asia and Australasia is presented and INTRODUCTION its implications are summarised. The complexity of the present-day tectonics of the region and the The region of SE Asia and Australasia includes observable rates of plate motions indicate that ma- examples of almost every plate tectonic configura- jor , or multiple small oceans, have closed tion at different stages in the Wilson cycle between during the Cenozoic, and that the configuration of rifting and continental collision. It is the only place the region has changed significantly during this on where we can observe arcs in collision, time. Despite the long-term convergence there has one of the few places where an oceanic spreading been frequent opening of marginal basins, and ex- centre is actively propagating into continental tension related to strike-slip faults resulting from crust, and includes areas with the highest global partitioning of oblique convergence at plate rates of plate convergence and separation. But how boundaries. Present-day plate motions, based for useful is in describing the evolution example on GPS measurements and seismicity, il- of the region? It is good at describing interaction lustrate the complexity of processes but appear to between slowly moving, large plates with rela- have little relevance in understanding the long- tively simple geometries but its application to the term kinematic development of the region. There SE Asia-Australasian region is more difficult be- are three important periods in regional develop- cause of the number of small plates required to ac- ment: at about 45 Ma, 25 Ma and 5 Ma. At these count for the region’s development. Furthermore, times plate boundaries and motions changed, prob- many ideas about orogenic evolution through the ably as a result of major collision events. Indenta- Wilson cycle are based on the early, simple and tion of Asia by India may have modified the Eura- powerful plate tectonic concepts reproduced in sian but there is little indication that In- text-books but some of the axioms have changed or dia has been the driving force of tectonics in SE advances in knowledge have made them invalid. Asia. The movement of northwards has For example, ophiolites are rarely formed in nor- caused rotations of SE Asia blocks and mal basins, continental crust is now known of microcontinental fragments to SE Asia. Since to be subductable, convergence may continue long 25 Ma the oceanic region east of has been after continent-continent collision, and much de- driven by motion of the . To improve formation of may be distributed rather our tectonic models, detail is needed which can be than concentrated at plate margins. compiled from proprietary data, such as coastline, shelf edge, age and lithofacies information, held In discussing regional tectonics it is necessary to by companies. Improved dating of events is re- be aware of the many problems. How reliably and quired in order to identify regional events and their for how long can present plate motions, for exam- consequences and identify the processes that cause ple from GPS measurements, be projected into the the effects. Few sedimentary basins in the region past? How much of the record is missing? Subduc- will fit into simple basin models because of the fre- tion leads to destruction and formation of marginal

From: Petroleum Systems of SE Asia and Australasia. pp. 47-62. Edited by J. V. C. Howes & R. A. Noble 1997, Indonesian Petroleum Association, Jakarta. 48 R. Hall basins at the major plate boundaries during short structions which keep Eurasia fixed in its present time intervals and these basins may be impossible position (Rangin et al., 1990; Lee and Lawver, to reconstruct. There are considerable difficulties 1994). in identifying the importance of strike-slip mo- tions. What is the relevance of plate tectonics to Reconstructions of SE Asia and Australasia (Fig. smaller (e.g. basin) scale reconstructions? Distrib- 1) shown on a global projection are presented at 10 uted deformation, for example of Sundaland, may Ma intervals for the period 50-10 Ma (Figs. 2 to 6). not be amenable to simple plate tectonic analysis. More than 100 fragments are currently used, and There are important problems in identifying verti- most retain their current size in order that they re- cal axis rotations on a regional scale. main recognisable. During the 50 Ma period frag- ments represented may have changed size and A logical parsimony is necessary in reconstruc- shape or may not have existed, both for arc and tion, limiting the number of plates and their detail, continental terranes. Thus, the plate model can and observations of the present-day tectonics of only be an approximation. Some of the elements of the region show us that any plate model must be an the model are deliberately represented in a stylistic over-simplification. Nonetheless, accepting its manner to convey the processes inferred rather limitations, analysis of the region as rigid plates than display exactly what has happened, for exam- can still give important insights. It is important to ple, the motion of the terranes of north New work progressively back in time, recognising the Guinea. The reconstructions presented here extend increasing uncertainty in older reconstructions; re- the model developed previously for SE Asia and alistic Mesozoic reconstructions are not currently the reader is referred to Hall (1995, 1996) for a possible. 2-D plate tectonic cartoons are no longer more complete account of the assumptions and adequate descriptions or tools for understanding. It data used in reconstructing that region. Yan and is essential to test plate tectonic models by using Kroenke (1993) have produced an animated recon- maps which can be examined at short time inter- struction of the SW Pacific. Some important dif- vals. Animations (e.g. Yan and Kroenke, 1993; ferences between this model and theirs results Hall, 1996) can expose flaws in models, and major from the choice of reference frames; they used the gaps in our knowledge, but also help identify truly frame whereas this reconstruction uses a regional events. Compilations, for example of palaeomagnetic reference frame. Other differences lithofacies, do not make sense unless regional plate result from different interpretations of geological movements are considered. This paper attempts to data. The model presented here shows the move- summarise the regional tectonic development of ment of India and new interpretations of the SW SE Asia and Australasia based on such a plate tec- Pacific; a few important references are cited in the tonic model which has been animated using 1 Ma text but in the space available here the reasons for time-slices. For the petroleum industry these re- the differences from previous models and the justi- constructions may help in understanding the devel- fication of the reconstruction must await a more opment of sedimentary basins, and the distribution complete account which will be published else- of petroleum resources, by identifying important where. What follows below is a brief account of controls on their tectonic setting and the timing of the model and its major features, with discussion regionally important events. of its principal implications.

Basis for the Model RECONSTRUCTIONS

The reconstructions were made using the ATLAS Configuration at 50 Ma computer program (Cambridge Paleomap Serv- ices, 1993). In the ATLAS model the motions of At 50 Ma (Fig. 2) India and Australia were sepa- the major plates are defined relative to and rate plates although their motions were not greatly its movement is defined relative to magnetic north. different. Transform faults linked the slow-spread- There has been little Cenozoic motion of Eurasia ing Australia-Antarctic and the fast spreading In- and it remains in a similar position in all the recon- dia-Australia spreading centres. Older parts of the structions, although there are small movements of East ophiolite probably formed at the In- Eurasia due to the plate circuit used in the ATLAS dia-Australia ridge. India collided with Asia in the model, particularly for the last 5 Ma. Therefore early Tertiary but there remains considerable con- there are minor differences compared to recon- troversy about the exact age of collision, and its Cenozoic tectonics of SE Asia and Australasia 49 consequences (Packham, 1996; Rowley, 1996). margins on all sides. To the west the passive mar- The position of the Eurasian margin and the extent gin was formed in the Late Jurassic and Fig. 2 pos- of Greater India are major problems. The recon- tulates a failed rift, possibly floored by oceanic struction shown in Fig. 2 shows a conservative es- crust on the site of the present-day Banda , par- timate, and since India-Asia collision began at tially separating a Bird’s Head microcontinent about 50 Ma this implies that the Asian margin ex- from Australia. Mesozoic lithosphere was present tended south to at least 30°N. Many of the tectonic north of the Bird’s Head south of the active Indian- events in SE Asia are commonly attributed to the Australian spreading centre. Further east in the Pa- effects of Indian indentation into Asia and the sub- cific, Indian and Australian oceanic lithosphere sequent extrusion of continental fragments east- had been subducting northwards beneath the wards along major strike-slip faults. Despite the Sepik-Papuan arc before about 55 Ma. The New great attraction of this hypothesis and the spec- Guinea Mesozoic passive margin had collided tacular evidence of displacements on the Red with this intra-oceanic arc in the early Eocene River (Tapponnier et al., 1990) the predic- causing emplacement of the Sepik, Papuan and tions of major rotations, southeastward extrusion New Caledonia ophiolites. Subsequently, most of of fragments, and the timing of events (Tapponnier the margin remained a passive margin et al., 1982), remain poorly supported by geologi- during the Paleogene but the to the cal evidence in SE Asia. north is inferred to have formed during the Mesozoic in an intra-oceanic marginal basin be- The east Eurasian continental margin was oriented hind the Sepik-Papuan arc. The position of the east broadly NE-SW. From Japan northwards Asia was Australia-Pacific margin is also uncertain. Tasman bounded by an active margin. , Palawan and opening had probably been driven and the now extended crust of the by but if so the site of subduction was margins formed a passive margin, established dur- considerably east of the Australian continent, be- ing Cretaceous times. Sundaland was separated yond the Loyalty Rise and New Caledonia Rise. from Eurasia by a wide proto-South China Sea Spreading had ceased in both basins by about 60 probably floored by Mesozoic ocean crust. The Ma (Paleocene). The history of this region remains southern edge of this ocean was a passive conti- poorly known since it is almost entirely submarine nental margin north of a continental promontory and magnetic anomalies in this area are poorly de- extending from Borneo to Zamboanga. The Malay fined. peninsula was closer to Indochina and the Malay- Sumatra margin was closer to NNW-SSE. Because Java and West Sulawesi were situated above a rotation of Borneo is accepted here the reconstruc- trench where lithosphere was tion differs from those of Rangin et al. (1990) and subducting towards the north. The character of this Daly et al. (1991) who infer a margin oriented boundary is shown as a simple arc but may have closer to E-W. I see no evidence to support the al- included marginal basins and both strike-slip and most E-W orientation of the Sundaland margin in convergent segments depending on its local orien- the region of Sumatra as shown on these and many tation. Extending plate boundaries into the Pacific other reconstructions (e.g. Briais et al., 1993; is very difficult. A very large area of the West Pa- Hutchison, 1996). Furthermore, such models have cific has been eliminated by subduction since 50 major difficulties in explaining the amount, timing Ma which will continue to cause major problems and mechanism of rotation required to move for reconstructions. However, there is clear evi- Sumatra from an E-W to NW-SE orientation. West dence that this area resembled the present-day Sumatra includes arc and ophiolitic material West Pacific in containing marginal basins, intra- accreted in the Cretaceous. East Borneo and West oceanic arcs and subduction zones. The Java sub- Sulawesi appear to be underlain by accreted arc duction system is linked east into Pacific intra-oce- and ophiolitic material as well as continental crust anic subduction zones required by the intra-oce- which may be early-rifted fragments. anic arc rocks within the plate; This material had been accreted during the Creta- parts of the east , the West Philippine ceous and may have resulted in a highly thickened basin and Halmahera include arc rocks dating back crust in this part of Sundaland, possibly sustained at least to the Cretaceous. North of the Philippine by subduction. Sea plate there was a south-dipping subduction zone at the southern edge of a Northern New Australia was essentially surrounded by passive Guinea plate. 50 R. Hall

50-40 Ma tra-oceanic magmatic products, much greater in volume than typical arc magma production rates Whatever the timing of India-Asia collision, a (Stern and Bloomer, 1992). Fig. 2 implies that consequence was the slowing of the rate of plate boninitic magmatism was linked to subduction of convergence after anomaly C21 and a major the Pacific-Northern New Guinea ridge. This change in spreading systems between anomaly formed the Izu-Bonin and Mariana arc systems and C20 and C19 at about 42 Ma. India and Australia the was a recognisable entity became one plate during this period (Figs. 2 and 3) by the end of this period. There were major rota- and the ridge between them became inactive. tions of the Philippine Sea plate between 50 and 40 Northward subduction of Indian-Australian Ma and the motion history of this plate (Hall et al., lithosphere continued beneath the Sunda-Java- 1995) provides an important constraint on devel- Sulawesi arcs although the direction of conver- opment of the eastern part of SE Asia. The West gence may have changed. Rift basins formed Philippine Basin, , and Makassar throughout Sundaland, but the timing of their ini- Strait opened as single basin within the Philippine tial extension is uncertain because they contain Sea plate although the reconstructions probably continental clastics which are poorly dated, and underestimate the width of the and their cause is therefore also uncertain. They may Celebes Sea, which may have been partially represent the consequences of oblique conver- subducted in the Miocene beneath west Sulawesi. gence or extension due to relaxation in the over- riding plate in response to India-Asia collision, en- The opening of the West Philippine-Celebes Sea hanced by slowing of subduction, further influ- basin required the initiation of southward subduc- enced by older structural fabrics. tion of the proto-South China Sea beneath and the Sulu arc. It is this subduction which caused The Java-Sulawesi subduction system continued renewed extension along the South China margin, into the West Pacific beneath the east Philippines driven by slab-pull forces due to subduction be- and Halmahera arcs. Further east, the direction of tween eastern Borneo and Luzon, and later led to subduction was southward towards Australia and sea-floor spreading in the South China Sea, rather this led to the formation of a Melanesian arc sys- than indentor-driven tectonics. tem. Subduction began beneath with major arc growth producing the older parts of the New Britain, Solomons and Tonga- 40-30 Ma Kermadec systems, leading to development of ma- jor marginal basins in the SW Pacific whose rem- In this interval (Figs. 3 and 4) the spreading of the nants probably survive only in the . marginal basins of the West and SW Pacific con- This model postulates the initial formation of these tinued. subduction continued at the arcs at the Papuan-east Australian margin as previ- Sunda-Java trenches, and also beneath the arc ex- ously suggested by Crook and Belbin (1978) fol- tending from Sulawesi through the east Philippines lowing subduction flip, rather than by initiation of to Halmahera. Sea floor spreading continued in the intra-oceanic subduction within the Pacific plate West Philippine-Celebes Sea basin until about 34 outboard of Australia as suggested by Yan and Ma. This spreading centre may been linked to Kroenke (1993). The evidence for either proposal backarc spreading of the Caroline Sea which is limited but this model has the simplicity of a sin- formed from about 40 Ma due to subduction of the gle continuous Melanesian arc. Pacific plate. The Caroline Ridge is interpreted in part as a remnant arc resulting from Caroline Sea During this interval there were major changes in backarc spreading, and the South Caroline arc ulti- the Pacific. The Pacific plate is widely said to have mately became the north New Guinea arc terranes. changed its motion direction at 43 Ma, based on By 30 Ma the Caroline Sea was widening above a the age of the bend in the Hawaiian-Emperor subduction zone at which the newly-formed Solo- seamount chain, although this view has recently mon Sea was being destroyed as the Melanesian been challenged by Norton (1995) who attributes arc system migrated north. The backarc basins in the bend to a moving hotspot which became fixed the SW Pacific were probably very complex, as in- only at 43 Ma. There is regional boninitic dicated by the anomalies in the South Fiji Basin, magmatism, the cause of which is still not under- and will never be completely reconstructed be- stood, which resulted in massive outpouring of in- cause most of these basins have been subducted. Cenozoic tectonics of SE Asia and Australasia 51

The Philippines- remained station- well as ocean basins marginal to Eurasia (Monnier ary, so spreading in the West Philippine-Celebes et al., 1995). Sea basin maintained subduction between NE Bor- neo and north of Luzon. The pull forces of the The arrival of the Australian margin at the subduc- subducting slab therefore account for stretching of tion zone caused northward subduction to cease. the Eurasian margin north of Palawan, and later This trapped ocean crust between Sulawesi and development of oceanic crust in the South China Halmahera which first became part of the Philip- Sea which began by 32 Ma. This model incorpo- pine Sea plate and later the plate. The rates the 500-600 km movement on the Red River Philippine Sea plate began to rotate clockwise and fault postulated by Briais et al. (1993). In contrast, the trapped ocean crust began to subduct beneath the indentor model does not account for stretching Sulawesi in the Sangihe arc. at the leading edge of the extruded blocks, such as Indochina, or the normal faulting east of Vietnam Soon afterwards the Ontong Java plateau collided often shown as kinematically linked to the Red with the Melanesian arc. These two major colli- River Fault system. If these faults were linked, the sions caused a major change in the character of faults offshore Vietnam should be a restraining plate boundaries in the region between about 25 bend during the phase of left-lateral movement and 20 Ma (Early Miocene). They also linked the proposed (32-15 Ma) and consequently show island arcs of , the New Guinea terranes thrust faulting rather than the normal faulting - at the southern Caroline margin, and the served. Halmahera-Philippines arcs. This linkage seems to have coupled the Pacific to the marginal basins of The dextral Three Pagodas and Wang Chao faults the West Pacific, and the Caroline and Philippine are simplified as a single fault at the north end of Sea plates were subsequently driven by the Pacific. the Malay peninsula. There are a host of faults They both began to rotate, almost as a single plate, through this region and a plate tectonic model can and the Izu-Bonin- system rolled only oversimplify the tectonics of the continental back into the Pacific. Rifting of the Palau-Kyushu by considering large and simple block ridge began, leading first to opening of the Parece movements and broadly predicting regional stress Vela basin and later to spreading in the Shikoku fields. The implication of this simple model is that basin. The change in plate boundaries led to sub- basins such as the Malay and ba- duction beneath the Asian margin. sins have a significant component of strike-slip movement on faults controlling their development. Subduction beneath the Halmahera-Philippines However, they were initiated in a different tectonic arc ceased and the New Guinea sector of the Aus- setting, and in a region with an older structural fab- tralian margin became a strike-slip zone, the ric (Hutchison, 1996) which influenced their de- system, which subsequently moved velopment. terranes of the South Caroline arc along the New Guinea margin.

30-20 Ma Advance of the Melanesian arc system led to wid- ening of the South Fiji basin and Solomon Sea ba- This period of time (Figs. 4 and 5) saw the most sin (now mainly subducted). At the Three Kings important Cenozoic plate boundary reorganisation Rise subduction seems to have been initiated soon within SE Asia. At about 25 Ma, the New Guinea after ocean crust was formed to the east, allowing passive margin collided with the leading edge of the rise to advance east and spreading to propagate the east Philippines-Halmahera-New Guinea arc behind the rise into the Norfolk basin from a triple system. The Australian margin, in the Bird’s Head junction to the north. region, was also close to collision with the Eura- sian margin in West Sulawesi and during this inter- val ophiolite was emplaced in Sulawesi. By 30 Ma 20-10 Ma the Sulawesi margin may have been complex and included ocean crust components of different The clockwise rotation of the Philippine Sea plate types (MORB, backarc basins). Thus the Sulawesi necessitated changes in plate boundaries through- ophiolite probably includes material formed out SE Asia which resulted in the tectonic pattern within the Indian Ocean (Mubroto et al., 1994) as recognisable today (Figs. 5 and 6). These changes 52 R. Hall include the re-orientation of spreading in the South Palawan margin. New subduction had also begun China Sea, and the development of new subduc- at the west edge of the Philippine Sea plate below tion zones at the eastern edge of Eurasia and in the the north Sulawesi-Sangihe arc which extended SW Pacific. Continued northward motion of Aus- north to south Luzon. This was a complex zone of tralia caused the counter-clockwise rotation of opposed subduction zones linked by strike-slip Borneo. Northern Borneo is much more complex faults. The Philippine islands and Halmahera were than shown. There was volcanic activity and build- carried with the Philippine Sea plate towards this out of delta and turbidite systems into the proto- subduction zone. North of Luzon, sinistral strike- South China Sea basin. Major problems include slip movement linked the subducting west margin the source of sediment in the basins surrounding of the Philippine Sea plate to subduction at the central Borneo and the location and timing of vol- . Collision of Luzon and the canic activity in Borneo. Some of this sediment Cagayan ridge with the Eurasian continental mar- may have been derived from the north across the gin in Mindoro and north Palawan resulted in a Sunda shelf. Important igneous activity promoted jump of subduction to the south side of the Sulu economic mineralisation but its tectonic setting is Sea. Southward subduction beneath the Sulu arc not clear. The reconstruction exaggerates the continued until 10 Ma. The remainder of the Phil- width of oceanic crust remaining in the western ippines continued to move with the Philippine Sea proto-South China Sea, and much of this area may plate, possibly with intra-plate strike-slip motion have been underlain by thinned continental crust and subduction resulting in local volcanic activity. which was thrust beneath Borneo, thus thickening At the east edge of the Philippine Sea plate spread- the crust and ultimately leading to crustal melting. ing terminated in the Shikoku basin.

The rotation of Borneo was accompanied by coun- As a result of changing plate boundaries fragments ter-clockwise motion of west Sulawesi, and of continental crust were emplaced in Sulawesi on smaller counter-clockwise rotations of adjacent splays at the western end of the Sorong Fault sys- Sundaland blocks. In contrast, the north Malay pe- tem. The earliest fragment to collide is inferred to ninsula rotated clockwise, but remained linked to have been completely underthrust beneath West both Indochina and the south Malay peninsula. Sulawesi and contributed to later crustal melting. This allowed widening of basins in the Gulf of The potassic magmatism of Sulawesi (Polvé et al., Thailand, but the rigid simple plate model overes- 1997) is not typical of an arc setting and may be timates the extension in this region. This extension due to extension and rifting of over-thickened was probably more widely distributed throughout crust. Later, the Tukang Besi platform separated Sundaland and Indochina on many different faults. from the Bird’s Head and was carried west on the Here therefore, clockwise rotations are not attrib- Philippine Sea plate to collide with Sulawesi. uted to indentation as the limited evidence dating Locking of splays of the Sorong fault caused sub- the rotations also implies. duction to initiate at the eastern margin of the Molucca Sea, producing the Neogene Halmahera North Sumatra rotated counter-clockwise with arc. Thus the Molucca Sea became a separate plate south Malaya, and as the rotation proceeded the as the double subduction system developed. orientation of the Sumatran margin changed with respect to the Indian plate motion vector. The con- After the collision of the Ontong Java plateau with sequent increase in the convergent component of the Melanesian arc the Solomons became attached motion, taken up by subduction, may have in- to the Pacific plate. Westward subduction began creased magmatic activity in the arc and weakened on the SW side of Solomon Sea, beneath eastern the upper plate, leading to formation of the dextral New Guinea, eliminating most of Solomon Sea Sumatran strike-slip fault system taking up the arc- and resulting in the formation of Maramuni arc parallel component of India-Eurasia plate motion. system. As the Solomon Sea was eliminated the South Caroline arc began to converge on the north East of Borneo, the increased rate of subduction New Guinea margin and the arc terranes were caused arc splitting in the Sulu arc and the Sulu translated west in the major left-lateral shear zone, Sea opened as a back-arc basin (Silver and Rangin, probably accompanied by rotation. In the southern 1991) south of the Cagayan ridge. The Cagayan part of the Solomons Sea subduction was in the ridge then moved northwards, eliminating the east- opposite direction (eastward) and created the New ern proto-South China Sea, to collide with the Hebrides arc system. Spreading ceased in the Cenozoic tectonics of SE Asia and Australasia 53

South Fiji basin. Sea has extended to its present dimensions and continental fragments are now found in the ridges within young volcanic crust. The Banda 10-0 Ma Sea is interpreted to be very young as suggested by Hamilton (1979) and others. At the beginning of this period SE Asia was largely recognisable in its present form (Figs. 6 and 1). In west Sundaland, partitioning of convergence in Rotation of Borneo was complete. This, with colli- Sumatra into orthogonal subduction and strike-slip sion in the central Philippines and Mindoro, and motion effectively established one or more continued northward movement of Australia, re- Sumatran forearc sliver plates. Extension on the sulted in reorganisation of plate boundaries and in- strike-slip system linked to the spreading centre in tra-plate deformation in the Philippines. The the (Curray et al., 1979). Within Luzon arc came into collision with the Eurasian Eurasia reversal of motion on the Red River sys- margin in Taiwan. This may be the cause of the tem may have been one consequence of the re- most recent regional change in plate motions at gional change in plate motions. about 5 Ma. The Philippine Sea plate rotation pole moved north from a position east of the plate; Opening of the Ayu trough separated the Caroline clockwise rotation continued but the change in plate and Philippine Sea plate, although the rate of motion caused re-orientation of existing, and de- separation at this spreading centre was very low. velopment of new, plate boundaries. Subduction North of the Bird’s Head, and further east in New continued at the Manila, Sangihe and Halmahera Guinea, transpressional movements were marked trenches, and new subduction began at the Negros by deformation of arc and ophiolite slivers sepa- and Philippine trenches. These subduction zones rated by sedimentary basins. Progressive westward were linked by strike-slip systems active within the motion of the South Caroline arc within the left- Philippines and this intra-plate deformation cre- lateral transpressional zone led to docking of the ated many very small fragments which are difficult north New Guinea terranes. This caused cessation to describe using rigid plate tectonics. of southward subduction of the but resulted in its northward subduction beneath The Molucca Sea continued to close by subduction New Britain. The New Britain subduction led to on both sides. At present the Sangihe forearc has rapid spreading in Woodlark basin as a conse- overridden the northern end of the Halmahera arc, quence of slab-pull forces and rapid ripping open and is beginning to over-thrust west Halmahera. In of continental crust beneath the Papuan peninsula the Sorong fault zone, accretion of Tukang Besi to accompanied by major exhumation of the middle Sulawesi locked a strand of the fault and initiated a and lower crust to form core complexes. new splay south of the Sula platform. The Sula platform then collided with the east arm of Elimination of most of the remaining Solomons Sulawesi, causing rotation of the east and north marginal basin by eastward subduction led to for- arms to their present position, leading to south- mation of the New Hebrides arc and opening of the ward subduction of the Celebes Sea at the north North Fiji basins. There was significant local rota- Sulawesi trench. tion of small plates illustrated by Fiji. This region also illustrates the complexity of backarc basins The Eurasia-Philippine Sea plate-Australia triple processes: ridge jumps, propagating rifts, re-orien- junction was and remains a zone of microplates but tation of spreading and major rotations (90°) in within this contractional setting extension contin- very short periods (<10 Ma). ued in the Banda Sea. The Bird’s Head moved north relative to Australia along a strike-slip fault at the Aru basin edge. Mesozoic ocean crust north CONCLUSIONS of Timor was eliminated at the eastern end of the Java trench by continued northern motion of Aus- Studies of present-day motions, based for example tralia which brought the Australian margin into on GPS measurements and seismicity, demon- this trench as the volcanic inner Banda arc propa- strate the complexity of regional tectonics and the gated east. Seram began to move east requiring extremely high rates of plate tectonic processes. It subduction and strike-slip motion at the edges of is a salutary lesson to discover that in many parts this microplate. Since 5 Ma the southern Banda of the region the position of modern plate bounda- 54 R. Hall ries is not known, or they do not connect, and there In the long-term a greater systematic collection of remains considerable uncertainty about how to de- regional data sets is required to understand tec- scribe the modern region in plate tectonic terms tonic development. Palaeomagnetic studies are (e.g. McCaffrey, 1996). Our understanding of one example of the type of work which only be- modern tectonics should therefore warn us that the comes valuable when there are large data-sets for region must have changed considerably, at times large areas, for example to distinguish regional very rapidly, during the last 50 Ma. We should be from local vertical axis rotations. The requirement aware that much evidence has been lost during col- for conclusions from local studies in many cases lision and subduction, which makes any recon- prevents these regional data-sets from being ac- struction an over-simplification. Present-day plate quired, and often leads to over-interpretation of re- motions appear to have little relevance in under- sults. Dating of rocks and events, both isotopic and standing the long-term kinematic development of biostratigraphic, is in many areas still inadequate. the region. They cannot be projected back into the Most tectonic models are too restricted in area to past very far, at most 5 Ma and probably much less. contribute to a regional understanding. Such lim- Major plate boundaries may have remained in the ited models are often not useful since they solve same position but have changed character. New local problems by ignoring or moving larger prob- boundaries have been created at different times. lems outside the area of interest. To go further, de- To illustrate these points, the evolution of the New tail is needed which can be compiled from propri- Guinea orogenic belt cannot be understood from etary data, such as coastline, shelf edge, age and determination of present-day Australia-Pacific lithofacies information, held by companies. In par- motion and plate boundaries, and indeed an analy- ticular the display of uplift and subsidence, and sis in terms of the motions of these two plates tells timing of magmatic events, on reconstructions us practically nothing about its development. would permit examination of the relationships be- tween vertical motions and plate movements and There are three important periods in regional de- help in identifying underlying processes. The iden- velopment. At about 45 Ma plate boundaries tification of cause and effect is difficult or impos- changed, probably as a result of India-Asia colli- sible at present because there is still a tendency to sion. Indentation of Asia by India may have modi- identify an event in the area of interest as a ‘cause’. fied the Eurasian continent but much more detail is Global reconstructions put these ‘causes’ into con- required of the timing of fault movements and the text. As far as sedimentary basins are concerned it amounts of displacements before Sundaland can is necessary to be alert to the probability that few be adequately understood. However, even after in- basins have a simple history. They are unlikely to corporating the best documented evidence for ex- have the characteristics of a simple classical basin trusion (Briais et al., 1993) the reconstructions type because of the changes in regional tectonics show little indication that India has been the driv- and this will apply to any basin with a life greater ing force of tectonics in SE Asia. The second ma- than 10 Ma. jor period is around 25 Ma when plate boundaries and motions changed again, perhaps due to colli- sion between the north Australian margin and arcs to the north. This, together with collision of the Melanesian arcs and the Ontong Java plateau changed the tectonics of the oceanic-arc region ACKNOWLEDGEMENTS east of Asia (Philippines, Celebes Sea, , Philippine Sea, Caroline Sea, north New Guinea, Financial support has been provided by NERC, the New Britain, Solomons, Tonga). Since 25 Ma the Royal Society, the London University Central Re- region east of Eurasia has been driven by motion of search Fund, and the London University SE Asia the Pacific plate. The rotation of regions within the Research Group currently supported by Arco, Ca- marginal basins seems to be an expression of intra- nadian Occidental, Exxon, Lasmo, Mobil, Union Pacific plate deformation due to roll-back of the Texas and Unocal. Work in has been fa- Izu-Bonin-Mariana trench. It appears that the ca- cilitated by GRDC, Bandung and Directors includ- pacity for this may have reached its limit at about 5 ing H. M. S. Hartono, M. Untung, R. Sukamto and Ma, again possibly as a consequence of arc-conti- I. Bahar. I thank Kevin Hill for considerable help nent collision in Taiwan, and plate motions and and discussion during the reconstruction of the SW boundaries changed again. Pacific. Cenozoic tectonics of SE Asia and Australasia 55

REFERENCES Katili, J. A. 1975. Volcanism and plate tectonics in Briais, A., Patriat, P. and Tapponnier, P. 1993. the Indonesian island arcs. Tectonophysics, 26, Updated interpretation of magnetic anomalies and 165-188. seafloor spreading stages in the South China Sea: Lee, T-Y. and Lawver, L. A. 1994. Cenozoic plate implications for the Tertiary tectonics of Southeast tectonic reconstruction of the South China Sea re- Asia. Journal of Geophysical Research, 98, 6299- gion. Tectonophysics, 235, 149-180. 6328. McCaffrey, R. 1996. Slip Partitioning at Conver- Cambridge Paleomap Services. 1993. ATLAS ver- gent Plate Boundaries of SE Asia. In: Hall, R. & sion 3.3. Cambridge Paleomap Services, P.O. Box Blundell, D. J. (eds.), Tectonic Evolution of SE 246, Cambridge, U.K. Asia, Geological Society of London Special Publi- cation, 106, 3-18. Crook, K.A.W. and Belbin, L. 1978. The South- west Pacific area during the last 90 million years. Monnier, C., Girardeau, J., Maury, R. C. and Journal of the Geological Society of Australia, 25, Cotten, J. 1995. Back arc basin origin for the East 23-40. Sulawesi ophiolite (eastern Indonesia). Geology, 23, 851-854. Curray, J. R., Moore, D. G., Lawver, L. A., Emmel, F. J., Raitt, R. W., Henry, M. and Norton, I. O. 1995. Tertiary relative plate motions Kieckheffer, R. 1979. Tectonics of the Andaman in the North Pacific: the 43 Ma non-event. Tecton- Sea and Burma. American Association of Petro- ics, 14, 1080-1094. leum Geologists Memoir, 29, 189-198. Packham, G. 1996. Cenozoic SE Asia: Recon- Daly, M. C., Cooper, M. A., Wilson, I., Smith, D. structing its aggregation and reorganisation. In: G. and Hooper, B. G. D. 1991. Cenozoic plate tec- Hall, R. & Blundell, D. J. (eds.), Tectonic Evolu- tonics and basin evolution in Indonesia. Marine tion of SE Asia, Geological Society of London and Petroleum Geology, 8, 2-21. Special Publication, 106, 123-152.

Hall, R. 1995. Plate tectonic reconstructions of the Polvé, M. Maury, R. C., Bellon, H., Rangin, C., Indonesian region. Proceedings of the Indonesian Priadi, B., Yuwono, S., Joron, J. L. & Soeria Petroleum Association 24th Annual Convention, Atmadja, R. 1997. Magmatic evolution of 71-84. Sulawesi,(Indonesia): constraints on the Cenozoic geodynamic history of the Sundaland active mar- Hall, R. 1996. Reconstructing Cenozoic SE Asia. gin. Tectonophysics, in press. In: Hall, R. and Blundell, D. J. (eds.) Tectonic Evo- lution of SE Asia. Geological Society of London Rangin, C., Jolivet, L., Pubellier, M. 1990. A sim- Special Publication, 106, 153-184. ple model for the tectonic evolution of southeast Asia and Indonesia region for the past 43 m. y. Bul- Hall, R., Fuller, M., Ali, J. R. and Anderson, C. D. letin de la Société géologique de France, 8 VI, 1995. The Philippine Sea Plate: magnetism and re- 889-905. constructions. American Geophysical Union Monograph, 88, 371-404. Rowley, D. B. 1996. Age of initiation of collision between India and Asia: a review of stratigraphic Hamilton, W. 1979. Tectonics of the Indonesian data. Earth and Planetary Science Letters, 145, 1- region. USGS Professional Paper, 1078, 345 pp. 13.

Hinz, K., Block, M., Kudrass, H. R. and Meyer, H. Silver, E. A. and Rangin, C. 1991. Leg 124 tec- 1991. Structural elements of the Sulu Sea. tonic synthesis. In: Silver, E. A., Rangin, C., von Geologische Jahrbuch, A127, 883-506. Breymann, M. T. et al., Proceedings of the ODP, Scientific Results, 124, 3-9. Hutchison, C. S. 1996. South-East Asian Oil, Gas, Coal and Mineral Deposits. Clarendon Press, Ox- Stern, R. J. and Bloomer, S. H. 1992. Subduction ford. 265 pp. zone infancy: examples from the Eocene Izu- 56 R. Hall

Bonin-Mariana and Jurassic California arcs. Bulle- Tapponnier, P., Peltzer, G., LeDain, A., Armijo, R. tin of the Geological Society of America, 104, and Cobbold, P. 1982. Propagating extrusion tec- 1621-1636. tonics in Asia: new insights from simple experi- ments with plasticine. Geology, 10, 611-616. Tapponnier, P., Lacassin, R., Leloup, P., Scharer, U., Dalai, Z., Haiwei, W., Xiaohan, L., Shaocheng, Yan, C. Y. and Kroenke, L. W. 1993. A plate tec- J., Lianshang, Z. and Jiayou, Z. 1990. The Ailao tonic reconstruction of the SW Pacific 0-100 Ma. Shan/Red River metamorphic belt:Tertiary left lat- In: Berger, T., Kroenke, L.W., Mayer, L. et al., eral shear between Indochina and South China. Proceedings of the Ocean Drilling Program, Sci- Nature, 343, 431-437. entific Results, 130, 697-709.

KEY TO FIGURE 1

Marginal Basins Tectonic features

A Japan Sea Ba Banda Arc B BH Bird’s Head C South China Sea Ca Cagayan Arc D Sulu Sea Fj Fiji E Celebes Sea Ha Halmahera Arc F Molucca Sea IB Izu-Bonin Arc G Banda Sea Ja Japan Arc H Andaman Sea Lo Loyalty Islands J West Philippine Basin Lu Luzon Arc K Shikoku Basin Mk Makassar Strait L Parece Vela Basin Mn Manus Island M Mariana Trough NB New Britain Arc N Ayu Trough NC New Caledonia P Caroline Sea NH New Hebrides Arc Q NI New Ireland R Solomon Sea Nng North New Guinea Terranes S Woodlark Basin Pa Papuan Ophiolite T Coral Sea Pk Palau-Kyushu Ridge U Ry Ryukyu Arc V Loyalty Basin Sa Sangihe Arc W Norfolk Basin Se Sepik Arc X North Fiji Basin So Solomons Arc Y South Fiji Basin Sp Sula Platform Z Lau Basin Su Sulu Arc TK Three Kings Rise To Tonga Arc Tu Tukang Besi Platform Cenozoic tectonics of SE Asia and Australasia 57

0 Ma Present Day EURASIA

40oN

PHILIPPINE INDIA SEA PLATE

20oN H PACIFIC PLATE

INDIAN PLATE

AUSTRALIA 20oS

NEW ZEALAND 40oS

120oE 150oE 90oE 180oE

ANTARCTICA 60oS

Figure 1. Present-day tectonic features of SE Asia and the SW Pacific. Yellow lines are selected marine magnetic anomalies. Cyan lines outline bathymetric features. Red lines are active spreading centres. White lines are subduction zones and strike-slip faults. The present extent of the Pacific plate is shown in pale blue. Areas filled with green are mainly arc, ophiolitic, and accreted material formed at plate margins during the Cenozoic. Areas filled in cyan are submarine arc regions, hot spot volcanic products, and oceanic pla- teaus. Pale yellow areas represent submarine parts of the Eurasian continental margins. Pale and deep pink areas represent submarine parts of the Australian continental margins. Letters represent marginal basins and tectonic features as shown on the opposite page. 58 R. Hall

50 Ma End Early EURASIA Eocene

40oN

PACIFIC PLATE 20oN

INDIA

INDIAN PLATE

AUSTRALIAN ? PLATE 20oS

AUSTRALIA 40oS

90oE 180oE

ANTARCTICA 60oS

Figure 2. Reconstruction of the region at 50 Ma. The possible extent of Greater India and the Eurasian margin north of India are shown schematically. Shortly before 50 Ma collision between the north Australian passive continental margin and an had emplaced ophiolites on the north New Guinea margin, and in New Caledonia, eliminating ocean crust formed at the former Australian-Indian ocean spreading centre. Double black arrows indicate extension in Sundaland. Cenozoic tectonics of SE Asia and Australasia 59

40 Ma Middle Eocene EURASIA

40oN

PACIFIC PLATE 20oN INDIA

INDIAN PLATE

20oS

AUSTRALIA

40oS

90oE 180oE

ANTARCTICA 60oS

Figure 3. Reconstruction of the region at 40 Ma. India and Australia were now parts of the same plate. An oceanic spreading centre linked the north Makassar Strait, the Celebes Sea and the West Philippine basin. Spreading began at about this time in the Caroline Sea, separating the Caroline Ridge remnant arc from the South Caroline arc. Spreading also began after subduction flip in marginal basins around eastern Australa- sia producing the Solomon Sea and the island arcs of Melanesia. 60 R. Hall

30 Ma Mid Oligocene EURASIA

40oN

PACIFIC INDIA PLATE 20oN

INDIAN PLATE

20oS AUSTRALIA

40oS

90oE 180oE

ANTARCTICA 60oS

Figure 4. Reconstruction of the region at 30 Ma. Indentation of Eurasia by India led to extrusion of the Indochina block by movement on the Red River Fault and Wang Chao-Three Pagodas (WC-TP) Faults. Slab pull due to southward subduction of the proto-South China Sea caused extension of the South China and Indochina continental margin and the present South China Sea began to open. A wide area of marginal basins separated the Melanesian arc from passive margins of eastern Australasia, shown schematically between the Solomon Sea and the South Fiji basin. Cenozoic tectonics of SE Asia and Australasia 61

20 Ma Early Miocene EURASIA

40oN

INDIA PACIFIC PLATE 20oN

INDIAN PLATE

AUSTRALIA 20oS

40oS

90oE 180oE

ANTARCTICA 60oS

Figure 5. Reconstruction of the region at 20 Ma. Collision of the north Australian margin between the Bird’s Head microcontinent and eastern New Guinea occurred at about 25 Ma. The Ontong Java plateau arrived at the Melanesian trench at about 20 Ma. These two major events caused major reorganisation of plate boundaries. Subduction of the Solomon Sea began at the eastern New Guinea margin to produce the Maramuni arc. Spreading began in the Parece Vela and Shikoku marginal basins accompanied by roll-back of the Izu-Bonin-Mariana trench. The north Australian margin became a major left-lateral strike-slip system as the Philippine Sea- began to rotate clockwise. Movement on splays of the Sorong Fault system led to the collision of Australian continental fragments in Sulawesi. This in turn led to counter- clockwise rotation of Borneo and related Sundaland fragments, eliminating the proto-South China Sea. The Sumatra Fault system was initiated. 62 R. Hall

10 Ma Late Miocene EURASIA

40oN

INDIA PACIFIC PLATE 20oN

AUSTRALIA 20oS

40oS

90oE 180oE

ANTARCTICA 60oS

Figure 6. Reconstruction of the region at 10 Ma. The Solomon Sea was being eliminated by subduction beneath eastern new Guinea and beneath the New Hebrides arc. However, continued subduction led to development of new marginal basins within the period 10-0 Ma, including the Bismarck Sea, Woodlark basin, North Fiji basins, and Lau basin. The New Guinea terranes, formed in the South Caroline arc, docked in New Guinea but continued to move in a wide left-lateral strike-slip zone. Further west, motion on strands of the Sorong Fault system caused the arrival of the Tukang Besi and Sula fragments in Sulawesi. Collision events at the Eurasian continental margin in the Philippines, and subsequently between the Luzon arc and Taiwan, were accompanied by intra-plate deformation, important strike-slip faulting and complex develop- ment of opposed subduction zones. Rotation of Borneo was complete but motion of the Sumatran forearc slivers linked to new spreading in the Andaman Sea.