Cenozoic plate tectonics of SE Asia 99 The plate tectonics of Cenozoic SE Asia and the distribution of land and sea

Robert Hall SE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK Email: robert*hall@gl*rhbnc*ac*uk

Key words: SE Asia, SW Pacific, plate tectonics, Cenozoic

Abstract Introduction

A plate tectonic model for the development of SE Asia and For the geologist, SE Asia is one of the most the SW Pacific during the Cenozoic is based on palaeomag- intriguing areas of the Earth$ The mountains of netic data, spreading histories of marginal basins deduced the Alpine-Himalayan belt turn southwards into from ocean floor magnetic anomalies, and interpretation of geological data from the region There are three important Indochina and terminate in a region of continen- periods in regional development: at about 45 Ma, 25 Ma and tal archipelagos, island arcs and small ocean ba- 5 Ma At these times plate boundaries and motions changed, sins$ To the south, west and east the region is probably as a result of major collision events surrounded by island arcs where lithosphere of In the Eocene the collision of India with Asia caused an the Indian and Pacific oceans is being influx of Gondwana plants and animals into Asia Mountain building resulting from the collision led to major changes in subducted at high rates, accompanied by in- habitats, climate, and drainage systems, and promoted dis- tense seismicity and spectacular volcanic activ- persal from Gondwana via India into SE Asia as well as cre- ity$ Within this region we can observe collision ating barriers between SE Asia and the rest of Asia Contin- between island arcs, between island arcs and ued indentation of Asia by India further modified Sundaland continents, and between continental fragments$ and created internal barriers affecting biogeographic pat- At the same time ocean basins are opening terns From a biogeographic and tectonic viewpoint, the major Cenozoic tectonic event in SE Asia occurred about 25 within this convergent region$ SE Asia includes million years ago, resulting in major changes in the configu- areas with the highest global rates of plate con- ration and character of plate boundaries, and caused effects vergence and separation$ which propagated westwards through the region This event It is clear from the geology of the region that led to the progressive arrival of Australian microcontinental the snapshot we see today is no less compli- fragments in Sulawesi, providing possible pathways for mi- gration of faunas and floras between Asia and Australia, but cated than in the past$ The region has devel- also creating new barriers to dispersal oped by the interaction of major lithospheric Tectonic reconstruction maps of lithospheric fragments plates, principally those of the Pacific, India- cannot be translated simply into maps of land and sea which Australia and Eurasia (Fig$1), but at the present are of greater value to biogeographers Determining the pal- day a description only in terms of these three aeogeography of the region is not yet possible, but an at- tempt is made to outline the main likely features of the ge- plates is a very great oversimplification$ Many ography of the region since the late Oligocene minor plates need to be considered, and in Evidence from all fields of biogeography is required to some parts of the region the boundaries be- test different tectonic models and identify the origin of tween these smaller plates are very uncertain$ It present biogeographic patterns but there must be a focus on is also clear that some of the deformation cannot plants and animals which have difficulty in dispersing, and be described in simple plate tectonic terms$ for which non-geological controls are unimportant The present distribution of plants and animals in SE Asia may Lithosphere has deformed internally, and mate- owe much more to the last one million years than the pre- rial has been added by arc volcanic processes, ceding 30 million years which means that at least one of the axioms of

Biogeography and Geological Evolution of SE Asia, pp 99-131 Edited by Robert Hall and Jeremy D Holloway © 1998 Backhuys Publishers, Leiden, The Netherlands 100 R* Hall

20oN PHILIPPINE South SEA China EURASIA Sea PACIFIC 10oN Sulu Sea

▲

▲ Celebes Sea CAROLINE

0o

Bismarck Sea Banda Sea Solomon Sea 10oS Woodlark Basin

INDIA INDIA-AUSTRALIA

90oE 100oE 110oE 120oE 130oE 140oE 150oE 20oS

Fig1 The larger tectonic plates of SE Asia and the SW Pacific The many small ocean basins and the major strike-slip fault systems at the margins of SE Asia and Australia are manifestations of the complexity of plate tectonics in the region which requires a description in terms of many more plates than those shown

plate tectonics, of rigid fragments moving on a of the region, into which large volumes of sphere, cannot be assumed$ sediments have been shed, removed from rising The complexity of the present-day tectonics mountains$ Thus the distribution of land and sea of the region and the observable rates of plate has changed during the Cenozoic, and many motions (e*g$, Hamilton, 1979; McCaffrey, 1996) parts of the region have seen dramatic vertical indicate that major oceans, or multiple small movements of several kilometres, with moun- oceans, have closed during the Cenozoic$ Sev- tains where once there were oceans, and deep eral major island arcs have certainly formed dur- marine regions where mountains had existed$ ing this time and some may have completely The abrupt division between Asian and Aus- disappeared$ At some plate boundaries strike- tralian floras and faunas in Indonesia, first recog- slip faulting has dismembered previously coher- nised by Wallace in the nineteenth century, has ent regions, and along these boundaries there its origin in the rapid plate movements and reor- can be both major crustal subsidence and uplift ganisation of land-masses in SE Asia$ Wallace due to deformation$ During the past 50 million realised that the region had changed dramati- years the configuration of the region has there- cally in the past without knowing the cause, and fore changed significantly in plate tectonic since his work there has been a general aware- terms$ Accompanying these large scale move- ness that the present distribution of land and sea ments have been equally significant vertical is not the same as that of the past, and that the movements, recorded in the sedimentary basins changes are in some way implicated in biogeo- Cenozoic plate tectonics of SE Asia 101 graphic patterns$ We are now confident that the culty, make reconstructions; the known motions geological changes are the result of plate move- of major plates do impose limits on possibilities; ments and are not the consequence of an ex- and the resulting interpretations do offer a panding Earth$ However, although the very means of identifying important tectonic events large-scale motions of major plates have been and highlighting key problems$ This paper ex- reasonably well known for the last 20 years or plains the background to a plate tectonic model so, the detail necessary to reconstruct SE Asia of SE Asia and the SW Pacific and discusses its has been lacking$ Furthermore, because of the implications for biogeography$ complexity of the geology of the region and be- cause so much of it has been remote and diffi- cult of access, geologists have not been able to Mesozoic to Cenozoic background clearly describe its long term development and it is only in the last few years that models ex- In very general terms, the region owes its origin plaining the development of the region have to the pre-Cenozoic break-up of the Gondwana been produced$ super-continent (Fig$2), the subsequent move- Making tectonic reconstructions of SE Asia ment of Gondwana fragments northwards, and becomes more difficult as the age of the recon- their eventual collision with Eurasia$ Metcalfe struction becomes greater, and examination of (1998 this volume) provides an account of the present tectonics of the region shows why present knowledge of the Palaeozoic and Meso- this is so$ Projecting motions that are known to- zoic development of SE Asia$ It is clear that day into the past is very problematical; our ob- many fragments separated from Gondwana and servations of the present tectonics indicate that amalgamated in SE Asia over a considerable pe- plates, plate boundaries and motions can be riod of time$ The process of rifting led to forma- geologically ephemeral features$ In some parts tion of new oceans, and the northward motion of the region, for example the Philippines and of Gondwana fragments required subduction of east Indonesia, it is not even certain that plate older oceanic crust at the edges of the growing tectonics provides a suitable model for a de- Eurasian continent$ By the Mesozoic, a region tailed understanding of the development of the composed of fragments derived from Gondwa- region$ Despite these difficulties, a plate tectonic na formed a Sundaland core surrounded by model does have value and by working back subduction zones$ from the present-day we can, albeit with diffi- Subduction meant that the Sundaland margins were complex$ Island arcs at the margins may have been underlain by continental and oceanic crust, and there were probably many small ocean basins behind the arcs and above the subduction zones$ The widespread ophiolites are fragments of oceanic lithosphere now found Avalonian- on land, and much of this lithosphere was Cadomian terranes formed in subduction-related settings, such as backarc basins and forearcs$ Ophiolites are Cimmerian commonly emplaced at some stage during the AFRICA terranes convergence of two plates and convergence is ultimately completed by collision between arc Pre- and continent, or continent and continent$ Palaeozoic INDIA Gondwana Throughout the Mesozoic there appear to have SOUTH core AMERICA AUSTRALIA been collisions of fragments with the Sundaland margins, and by the beginning of the Cenozoic ANTARCTICA SE Asia was a composite mosaic of continental crust, island arc material and oceanic crust$ Pacific margin Two major fragments separated from Gond- wana in the Cretaceous and moved northwards Fig2 Reconstruction of Gondwana (after Unrug, 1997) in as parts of different plates: India and Australia$ the early Palaeozoic showing outlines of the major conti- India completed its passage in the early Ceno- nental fragments which separated during the Palaeozoic and Mesozoic Many of the Cimmerian terranes had accreted to zoic and collided with the Asian continent about Laurasia to form the core of SE Asia by the end of the 50 million years ago (Fig$3)$ However, collision Mesozoic did not cause India to become fixed to Asia as 102 R* Hall

0 Ma

30Ma

45Ma

0Ma

30Ma 90Ma 45Ma

90Ma 120Ma 120Ma

120Ma

Fig3 India and Australia separated from Gondwana in the Cretaceous The map shows the late Cretaceous and Cenozoic movement of these two major continental fragments north with respect to Asia and SE Asia, both of which are shown in their present day positions for reference

predicted by early plate tectonic models$ In- faults which cut across Asia are considered to be stead, India continued to move northwards, al- zones of major displacements which link to mar- beit at a slower rate than during the Cretaceous$ ginal basins, such as the South China Sea, off- There is considerable disagreement amongst ge- shore$ If correct, this hypothesis implies major ologists about how the continued northward changes in SE Asia linked to India’s continued movement was accommodated during the northward movement$ In contrast, other work- Cenozoic, and its consequences$ According to ers (e*g* England and Houseman, 1986; Dewey Tapponnier and colleagues (e*g* Tapponnier et et al$ 1989; Houseman and England, 1993) dis- al$, 1982, 1986, 1990; Peltzer and Tapponnier, miss the extrusion hypothesis, arguing that the 1988; Briais et al$, 1993) the impact of a rigid displacement on the strike-slip faults has been Indian ‘indentor’ on an Asian margin weakened small and that the continued convergence of In- by subduction-related heating and magmatism dia and Asia has been accommodated by crustal caused eastward ‘extrusion’ and rotation of con- thickening with very little eastward movement tinental fragments, and opening of some of the of crust$ small oceanic marginal basins of SE Asia$ The Australia separated from Gondwana, leaving progressive extrusion of continental fragments Antarctica as its final remnant, at about the same to the east and consequent rotation of crustal time as India, but moved less quickly north- blocks has been simulated in laboratory experi- wards$ Instead of a direct collision with another ments using plasticine (Fig$4), and the strike-slip continent Australia is now making a glancing Cenozoic plate tectonics of SE Asia 103

AB

C D ASIA

SOUTH CHINA

SOUTH CHINA SEA

INDOCHINA INDIA

Fig4 The impact of a rigid Indian indentor with Asia has been postulated to have caused the development of major strike-slip faults as India progressively penetrated Asia (A to D) Analogue models produced during experiments (redrawn from Peltzer and Tapponnier, 1988) show striking similarities to the major features of SE Asia

collision with a composite SE Asia, which in- and marginal basins as it is at the present day$ At cludes some of the earlier Gondwana fragments present the Philippine Sea plate and Philippine to arrive, and also includes the island arcs island arcs separate the east Asian margin from formed due to the subduction of oceanic crust the Pacific plate south of Japan$ Unlike the In- north of Australia$ In east Indonesia the north- dian ocean and Pacific oceans, the Philippine ward movement of Australia during the Sea plate lacks well defined sea-floor magnetic Cenozoic has been marked by arc-continent col- anomalies which normally provide the basis for lision and major strike-slip motion within the reconstructing past plate motions$ The Philip- north Australian margin$ Further east, arc-conti- pine Sea plate is also difficult to link to the nent collisions have been the result of elimina- movements of the other major plates because it tion of marginal basins formed above subduc- is surrounded by subduction zones$ For these tion zones as Australia has moved north, and reasons the history of movement of plate move- this system of arcs and marginal basins can be ments in the west Pacific north of New Guinea traced east along the margin of the Pacific plate has been very uncertain, and consequently the in Melanesia$ eastern edge of SE Asia has been difficult to re- During the late Mesozoic and Cenozoic there construct$ Palaeomagnetic data from east Indo- was subduction of the Pacific ocean to the east nesia (Hall et al*, 1995) have provided the basis of Asia, although the eastern margin of Asia and for reconstructing the Philippine Sea plate and SE Asia was probably a region of small plates its motion since the early Cenozoic, and conse- 104 R* Hall

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

Fig5 Present-day tectonic features of SE Asia and the SW Pacific Light straight lines are selected marine magnetic anomalies and active spreading centres White lines are subduction zones and strike-slip faults The present extent of the Pacific plate is shown in mid grey Labelled filled areas are mainly arc, ophiolitic, and accreted material formed at plate margins during the Cenozoic, and submarine arc regions, hot spot volcanic products, and oceanic plateaus Pale grey areas represent submarine parts of the Eurasian continental margins Dark grey areas represent submarine parts of the Australian continental margins See pages 126-131 for colour plates of Figs5 to 10 Letters represent marginal basins and tectonic features as follows:

Marginal Basins Tectonic features A Japan Sea N Ayu Trough Ba Banda Arc Mk Makassar Strait Ry Ryukyu Arc B Okinawa Trough P Caroline Sea BH Bird’s Head Mn Manus Island Sa Sangihe Arc C South China Sea Q Bismarck Sea Ca Cagayan Arc NB New Britain Arc Se Sepik Arc D Sulu Sea R Solomon Sea Fj Fiji NC New Caledonia So Solomons Arc E Celebes Sea S Woodlark Basin Ha Halmahera Arc NH New Hebrides Arc Sp Sula Platform F Molucca Sea T Coral Sea IB Izu-Bonin Arc NI New Ireland Su Sulu Arc G Banda Sea U Tasman Sea Ja Japan Arc Nng North New Guinea TK Three Kings H Andaman Sea V Loyalty Basin Lo Loyalty Islands Terranes Rise J West Philippine Basin W Norfolk Basin Lu Luzon Arc Pa Papuan Ophiolite To Tonga Arc K Shikoku Basin X North Fiji Basin Pk Palau-Kyushu Ridge Tu Tukang Besi L Parece Vela Basin Y South Fiji Basin Platform M Mariana Trough Z Lau Basin Cenozoic plate tectonics of SE Asia 105 quently, for making reconstructions of the re- may not have existed, both for arc and continen- gions adjacent to the Philippine Sea plate$ tal terranes$ Thus, the plate model can only be Combining the Philippine Sea plate history an approximation$ Some of the elements of the with the known movements of the major plates, model are deliberately represented in a stylistic India, Australia, Pacific and Eurasia, provides manner to convey the processes inferred rather some limits within which reconstructions of SE than display exactly what has happened, for ex- Asia, and parts of the west Pacific, can be at- ample, the motion of the terranes of north New tempted$ Within this region there are recent in- Guinea$ terpretations of the South China Sea opening Previous reconstructions which cover all or (Briais et al$, 1993) and other marginal basins parts of the region discussed here include those which limit options still further and provide con- of Katili (1975), Crook and Belbin (1978), Hamil- straints of variable quality on modelling the tec- ton (1979), Briais et al$ (1993), Burrett et al$ tonic history of the region$ The importance of (1991), Daly et al* (1991), Lee and Lawver (1994), the marginal basins is that only they are likely to Rangin et al* (1990), and Yan and Kroenke (1993) contain a clear record, based on ocean floor who also produced an animated reconstruction magnetic anomalies, of the motion history of of the SW Pacific$ The reader is referred to the some of the minor plates$ However, many of the original papers for accounts of the earlier mod- marginal basins of SE Asia completely lack mag- els$ Some differences between the model here netic anomalies, many have not been drilled and other models result from the choice of refer- during the ocean drilling campaigns, and their ence frames; some use the hotspot reference ages and character are still poorly known$ frame, and others use a fixed Eurasia, whereas Therefore much of the evidence which must be these reconstructions use a palaeomagnetic ref- used in a regional tectonic model of SE Asia is erence frame$ These choices result in different based on interpretation of geological data from palaeolatitudes and can cause other differences$ the small ocean basins, their margins, and from There have also been improvements in our the geologically more complicated land areas knowledge of global plate motions since the around them$ The reader should be aware that, earlier regional reconstructions$ However, in as in other areas of science, geologists differ in many cases the principal differences between their interpretations of these data, and much of the different models result from different inter- the information does not lend itself to unam- pretations of geological data$ biguous reconstruction$ Nonetheless, a com- This paper gives an account of a plate tectonic plete tectonic history can only be deduced from model for the Cenozoic development of the re- the geology on land combined with data from gion based on my interpretations of a large the oceans$ The account here is therefore my range of geological data$ It summarises the re- view of the development of Cenozoic SE Asia gional tectonic development of SE Asia using a using plate tectonic reconstructions based on plate model which has been animated using 1 such deductions$ Ma time-slices$ Below is a brief account of the model and its major features, which is followed by a discussion of its principal implications for The model biogeographers relating to the distribution of land and sea during the last 30 million years$ The reconstructions were made using the AT- LAS computer program (Cambridge Paleomap Services, 1993) and plate motion model for the Reconstructions major plates$ The motion of Africa is defined relative to magnetic north and motions of the The model discussed here includes that devel- major plates are all relative to Africa$ Eurasia has oped earlier for SE Asia (Hall, 1996) which has been close to its present-day position through- been extended to include the SW Pacific (Hall, out the Cenozoic$ The reconstructions in this 1997)$ Reconstructions of SE Asia and the SW paper add to this model for the major plates by Pacific (Fig$5) shown on a global projection are including a large number of smaller fragments in presented at 10 Ma intervals for the period 50-10 SE Asia and the SW Pacific$ More than 100 frag- Ma (Figs$6-10)$ The reader is referred to Hall ments are currently used, and most retain their (1995, 1996) for a more complete account of the current size in order that they remain recognis- assumptions and data used in reconstructions of able$ During the 50 Ma period fragments repre- SE Asia and for maps showing only SE Asia but sented may have changed size and shape or with more detail$ 106 R* Hall

50 Ma End Early EURASIA Eocene

40oN

PACIFIC PLATE 20oN

INDIA

INDIAN PLATE

AUSTRALIAN ? PLATE 20oS

AUSTRALIA 40oS

90oE 180oE

ANTARCTICA 60oS

Fig6 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 continental margin and an island arc 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

Configuration at 50 Ma India collided with Asia in the early Tertiary but there remains considerable controversy about At 50 Ma (Fig$6) India and Australia were sepa- the exact age of collision, and its consequences rate plates although their motions were not (Packham, 1996; Rowley, 1996)$ The position of greatly different$ Transform faults linked the the Eurasian margin and the extent of Greater slow-spreading Australia-Antarctic and the fast India are major problems$ The reconstruction spreading India-Australia spreading centres$ shown in Fig$6 shows a conservative estimate Some of the ophiolites of Sulawesi probably and, since India-Asia collision began at about 50 formed at the India-Australia mid-ocean ridge$ Ma, this implies that the Asian margin extended Cenozoic plate tectonics of SE Asia 107 south to at least 30oN$ Many of the tectonic spreading centre$ Further east in the Pacific, In- events in SE Asia are commonly attributed to the dian and Australian oceanic lithosphere had effects of Indian indentation into Asia and the been subducting northwards beneath the Sepik- subsequent extrusion of continental fragments Papuan arc in the early Tertiary$ During the eastwards along major strike-slip faults$ Despite Paleocene and early Eocene the New Guinea the great attraction of this hypothesis and the Mesozoic passive margin collided with this intra- spectacular evidence of displacements on the oceanic arc causing emplacement of the Sepik Red River fault (Tapponnier et al$, 1990) the and Papuan ophiolites (Davies, 1971)$ Subse- predictions of major rotations, southeastward quently, most of the New Guinea margin was a extrusion of fragments, and the timing of events passive margin during the Paleogene but the (Tapponnier et al$, 1982), remain poorly sup- oceanic crust to the north is inferred to have ported by geological evidence in SE Asia$ formed during the Mesozoic in an intra-oceanic The east Eurasian continental margin was ori- marginal basin behind the Sepik-Papuan arc$ ented broadly NE-SW$ From Japan northwards The position and character of the east Australia- Asia was bounded by an active margin$ Taiwan, Pacific margin is also uncertain$ Tasman and Palawan and the now extended crust of the Coral Sea opening had probably been driven by South China Sea margins formed a passive mar- subduction but the site of subduction must have gin, established during Cretaceous times$ Sunda- been considerably east of the Australian conti- land was separated from Eurasia by a wide nent, beyond the Loyalty Rise and New Caledo- proto-South China Sea probably floored by nia Rise$ Spreading had ceased in both basins by Mesozoic ocean crust$ The southern edge of this about 60 Ma (Paleocene)$ By the Paleocene it ocean was a passive continental margin north of appears that subduction east of New Caledonia a continental promontory extending from Bor- was to the east not to the west (Aitchison et al$, neo to Zamboanga$ The Malay peninsula was 1995)$ The history of this region remains poorly closer to Indochina and the Malay-Sumatra mar- known since it is almost entirely submarine, and gin was closer to NNW-SSE$ Because rotation of magnetic anomalies in this area are poorly de- Borneo is part of this model the reconstruction fined$ differs from those of Rangin et al$ (1990) and Java and West Sulawesi were situated above a Daly et al$ (1991) who infer a margin oriented trench where Indian plate lithosphere was closer to E-W$ I see no evidence to support the subducting towards the north$ The character of almost E-W orientation of the Sundaland margin this boundary is shown as a simple arc but may in the region of Sumatra as shown on these and have included marginal basins and both strike- many other reconstructions (e*g* Briais et al$, slip and convergent segments depending on its 1993; Hutchison, 1996)$ Furthermore, such local orientation$ Extending plate boundaries models have major difficulties in explaining the into the Pacific is very difficult$ A very large area amount, timing and mechanism of rotation re- of the West Pacific has been eliminated by sub- quired to move Sumatra from an E-W to NW-SE duction since 50 Ma which will continue to orientation$ West Sumatra includes arc and cause major problems for reconstructions$ How- ophiolitic material accreted in the Cretaceous$ ever, there is clear evidence that this area resem- East Borneo and West Sulawesi appear to be bled the present-day West Pacific in containing underlain by accreted arc and ophiolitic material marginal basins, intra-oceanic arcs and subduc- as well as continental crust which may be early- tion zones$ The Java subduction system linked rifted Gondwana fragments$ This material had east into Pacific intra-oceanic subduction zones been accreted during the Cretaceous and may required by the intra-oceanic arc rocks within have resulted in a highly thickened crust in this the Philippine Sea plate; parts of the east Philip- part of Sundaland, possibly sustained by sub- pines, the West Philippine basin and Halmahera duction$ include arc rocks dating back at least to the Cre- Australia was essentially surrounded by pas- taceous$ North of the Philippine Sea plate there sive margins on all sides$ To the west the pas- was a south-dipping subduction zone at the sive margin was formed in the Late Jurassic, and southern edge of a Northern New Guinea plate$ Fig$6 postulates a failed rift, possibly floored by oceanic crust on the site of the present-day Banda Sea, partially separating the Bird’s Head 50-40 Ma microcontinent from Australia$ Mesozoic oce- anic lithosphere was present north of the Bird’s Whatever the timing of India-Asia collision, a Head, south of the active Indian-Australian consequence was the slowing of the rate of 108 R* Hall plate convergence after anomaly C21 and a ma- anic volcanic rocks (Stern and Bloomer, 1992) jor change in spreading systems between which formed the Izu-Bonin-Mariana arc sys- anomaly C20 and C19 at about 42 Ma$ India and tem, and the Philippine Sea plate was a recog- Australia became one plate during this period nisable entity by the end of this period$ There (Figs$6 and 7) and the ridge between them be- was significant rotation of the Philippine Sea came inactive$ Northward subduction of Indian- plate between 50 and 40 Ma and the motion his- Australian lithosphere continued beneath the tory of this plate (Hall et al$, 1995) provides an Sunda-Java-Sulawesi arcs although the direction important constraint on development of the of convergence may have changed$ Rift basins eastern part of SE Asia$ The West Philippine ba- formed throughout Sundaland, but the timing of sin, Celebes Sea, and Makassar Strait opened as their initial extension is uncertain because they single oceanic basin within the Philippine Sea contain continental clastics which are poorly plate although the reconstructions probably un- dated, and their cause is therefore also uncer- derestimate the width of the Makassar Strait and tain$ They may represent the consequences of Celebes Sea, which may have been partially oblique convergence or extension due to re- subducted in the Miocene beneath west laxation in the over-riding plate in response to Sulawesi$ India-Asia collision, enhanced by slowing of The opening of the West Philippine-Celebes subduction, further influenced by older struc- Sea basin required the initiation of southward tural fabrics$ subduction of the proto-South China Sea be- The Java-Sulawesi subduction system contin- neath Luzon and the Sulu arc$ It is this subduc- ued into the West Pacific beneath the east Phil- tion which caused renewed extension along the ippines and Halmahera arcs$ Further east, the South China margin, driven by slab-pull forces direction of subduction was southward towards due to subduction between eastern Borneo and Australia and this led to the formation of a Luzon, and later led to sea-floor spreading in the Melanesian arc system$ During the Eocene the South China Sea, rather than indentor-driven extended eastern Australasian passive margin tectonics$ had collided with the intra-oceanic arc already emplaced in New Guinea resulting in emplace- ment of the New Caledonia ophiolite (Aitchison 40-30 Ma et al$, 1995; Meffre, 1995) followed by subduc- tion polarity reversal$ Subduction began be- In this interval (Figs$7 and 8) the spreading of neath Papua New Guinea with major arc growth the marginal basins of the West and SW Pacific producing the older parts of the New Britain, continued$ Indian ocean subduction continued Solomons and Tonga-Kermadec systems, lead- at the Sunda-Java trenches, and also beneath the ing to development of major marginal basins in arc extending from Sulawesi through the east the SW Pacific whose remnants probably sur- Philippines to Halmahera$ Sea floor spreading vive only in the Solomon Sea$ This model postu- continued in the West Philippine-Celebes Sea lates the initial formation of these arcs at the basin until about 34 Ma$ This spreading centre Papuan-east Australian margin as previously may been linked to backarc spreading of the suggested by Crook and Belbin (1978) following Caroline Sea which formed from about 40 Ma subduction flip, rather than by initiation of intra- due to subduction of the Pacific plate$ The oceanic subduction within the Pacific plate out- Caroline Ridge is interpreted in part as a rem- board of Australia as suggested by Yan and nant arc resulting from Caroline Sea backarc Kroenke (1993)$ The evidence for either pro- spreading, and the South Caroline arc ultimately posal is limited but this model has the simplicity became the north New Guinea arc terranes$ By of a single continuous Melanesian arc$ 30 Ma the Caroline Sea was widening above a During this interval there were major changes subduction zone at which the newly-formed in the Pacific$ The Pacific plate is widely said to Solomon Sea was being destroyed as the have changed its motion direction at 43 Ma, Melanesian arc system migrated north$ The based on the age of the bend in the Hawaiian- backarc basins in the SW Pacific were probably Emperor seamount chain, although this view very complex, as indicated by the anomalies in has recently been challenged by Norton (1995) the South Fiji basin, and will never be com- who attributes the bend to a moving hotspot pletely reconstructed because most of these ba- which became fixed only at 43 Ma$ Subduction sins have been subducted$ of the Pacific-Northern New Guinea ridge The Philippines-Halmahera arc was station- (Fig$7) led to massive outpouring of intra-oce- ary, so spreading in the West Philippine-Celebes Cenozoic plate tectonics of SE Asia 109

40 Ma Middle Eocene EURASIA

40oN

PACIFIC PLATE 20oN INDIA

INDIAN PLATE

20oS

AUSTRALIA

40oS

90oE 180oE

ANTARCTICA 60oS

Fig7 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 Australasia producing the Solomon Sea and the island arcs of Melanesia

Sea basin maintained subduction between NE mal faulting east of Vietnam often shown as Borneo and north of Luzon$ The pull forces of kinematically linked to the Red River fault sys- the subducting slab therefore account for tem$ There was approximately 500-600 km left- stretching of the Eurasian margin north of lateral movement on the Red River fault (Briais Palawan, and later development of oceanic crust et al$, 1993) during the extrusion of Indochina in the South China Sea which began by 32 Ma$ (32-15 Ma)$ In contrast, the indentor model does not ac- The dextral Three Pagodas and Wang Chao count for stretching at the leading edge of the faults are simplified as a single fault at the north extruded blocks, such as Indochina, or the nor- end of the Malay peninsula$ There are a host of 110 R* Hall

30 Ma Mid Oligocene EURASIA

40oN

PACIFIC INDIA PLATE 20oN

INDIAN PLATE

20oS AUSTRALIA

40oS

90oE 180oE

ANTARCTICA 60oS

Fig8 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

faults through this region, and a plate tectonic may have been initiated in a different tectonic model can only oversimplify the tectonics of the setting, in which a pre-existing structural fabric continental regions by considering large and influenced their development (Hutchison, 1996)$ simple block movements and broadly predicting regional stress fields$ The implication of this sim- plified model is that basins such as the Malay 30-20 Ma and Gulf of Thailand basins have a significant component of strike-slip movement on faults This period of time (Figs$8 and 9) saw the most controlling their development$ However, they important Cenozoic plate boundary reorganisa- Cenozoic plate tectonics of SE Asia 111 tion within SE Asia$ At about 25 Ma, the New 20-10 Ma Guinea passive margin collided with the leading edge of the east Philippines-Halmahera-New The clockwise rotation of the Philippine Sea Guinea arc system$ The Australian margin, in the plate necessitated changes in plate boundaries Bird’s Head region, was also close to collision throughout SE Asia which resulted in the tec- with the Eurasian margin in West Sulawesi and tonic pattern recognisable today (Figs$9 and 10)$ during this interval ophiolite was emplaced in These changes include the re-orientation of Sulawesi$ spreading in the South China Sea, and the devel- By 30 Ma the Sulawesi margin may have been opment of new subduction zones at the eastern complex and included ocean crust of different edge of Eurasia and in the SW Pacific$ Contin- types (mid-ocean ridge, backarc basin)$ Thus ued northward motion of Australia caused the the Sulawesi ophiolite probably includes mate- counter-clockwise rotation of Borneo$ Northern rial formed within the Indian Ocean (Mubroto et Borneo is much more complex than shown$ al$, 1994) as well as ocean basins marginal to There was volcanic activity and build-out of Eurasia (Monnier et al$, 1995)$ The arrival of the delta and turbidite systems into the proto-South Australian margin at the subduction zone China Sea basin$ Major problems include the caused northward subduction to cease$ The source of sediment in the basins surrounding ocean crust trapped between Sulawesi and central Borneo and the location and timing of Halmahera first became part of the Philippine volcanic activity in Borneo$ The remaining oce- Sea plate and later the Molucca Sea plate$ The anic crust of the western proto-South China Sea, Philippine Sea plate began to rotate clockwise and thinned continental crust of the passive and the trapped ocean crust began to subduct margin to the north, was thrust beneath Borneo beneath Sulawesi in the Sangihe arc$ thickening the crust, resulting in rapid erosion Soon afterwards the Ontong Java plateau col- of sediments into the Neogene circum-Borneo lided with the Melanesian arc$ These two major deltas, and ultimately leading to crustal melting$ collisions caused a significant change in the The rotation of Borneo was accompanied by character of plate boundaries in the region be- counter-clockwise motion of west Sulawesi, and tween about 25 and 20 Ma (Early Miocene)$ smaller counter-clockwise rotations of adjacent They also linked the island arcs of Melanesia to Sundaland blocks$ In contrast, the north Malay the New Guinea terranes at the southern margin peninsula rotated clockwise, but remained of the Caroline plate, and to the Halmahera- linked to both Indochina and the south Malay Philippines arcs$ This linkage seems to have peninsula$ This allowed widening of basins in coupled the Pacific to the marginal basins of the the Gulf of Thailand, but the simple rigid plate West Pacific, and the Caroline and Philippine model overestimates the extension in this re- Sea plates were subsequently driven by the Pa- gion$ This extension was probably more widely cific$ Both began to rotate, almost as a single distributed throughout Sundaland and plate, and the Izu-Bonin-Mariana trench system Indochina on many different faults$ The Burma rolled back into the Pacific$ Rifting of the Palau- plate became partly coupled to the northward- Kyushu ridge began, leading first to opening of moving Indian plate and began to move north the Parece Vela basin and later to spreading in on the Sagaing fault leading to stretching of the the Shikoku basin$ The change in plate bounda- Sunda continental margin north of Sumatra, and ries led to subduction beneath the Asian margin$ ultimately to ocean crust formation in the Subduction beneath the Halmahera-Philip- Andaman Sea$ pines arc ceased and the New Guinea sector of North Sumatra rotated counter-clockwise with the Australian margin became a strike-slip zone, south Malaya, and as the rotation proceeded the the Sorong fault system, which subsequently orientation of the Sumatran margin changed moved terranes of the South Caroline arc along with respect to the Indian plate motion vector$ the New Guinea margin$ The consequent increase in the convergent Advance of the Melanesian arc system led to component of motion, taken up by subduction, widening of the South Fiji basin and Solomon may have increased magmatic activity in the arc Sea basin (now mainly subducted)$ At the Three and weakened the upper plate, leading to for- Kings Rise subduction seems to have been initi- mation of the dextral Sumatran strike-slip fault ated soon after ocean crust was formed to the system taking up the arc-parallel component of east, allowing the rise to advance east and India-Eurasia plate motion$ spreading to propagate behind the rise into the East of Borneo, the increased rate of subduc- Norfolk basin from a triple junction to the north$ tion caused arc splitting in the Sulu arc and the 112 R* Hall

20 Ma Early Miocene EURASIA

40oN

INDIA PACIFIC PLATE 20oN

INDIAN PLATE

AUSTRALIA 20oS

40oS

90oE 180oE

ANTARCTICA 60oS

Fig9 Reconstruction of the region at 20 Ma Collision of the north Australian margin in the region 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 events caused major reorganisation of plate boundaries Subduction of the Solomon Sea began at the eastern New Guinea margin Spreading began in the Parece Vela and Shikoku marginal basins The north Australian margin became a major left-lateral strike-slip system as the Philippine Sea-Caroline plate 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

Sulu Sea opened as a backarc basin (Hinz et al$, edge of the Philippine Sea plate below the north 1991; Silver and Rangin, 1991) south of the Sulawesi-Sangihe arc which extended north to Cagayan ridge$ The Cagayan ridge then moved south Luzon$ This was a complex zone of op- northwards, eliminating the eastern proto-South posed subduction zones linked by strike-slip China Sea, to collide with the Palawan margin$ faults$ The Philippine islands and Halmahera New subduction had also begun at the west were carried with the Philippine Sea plate to- Cenozoic plate tectonics of SE Asia 113

10 Ma Late Miocene EURASIA

40oN

INDIA PACIFIC PLATE 20oN

AUSTRALIA 20oS

40oS

90oE 180oE

ANTARCTICA 60oS

Fig10 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 development of opposed subduction zones Rotation of Borneo was complete but motion of the Sumatran forearc slivers was associated with new spreading in the Andaman Sea

wards this subduction zone$ North of Luzon, sin- north Palawan resulted in a jump of subduction istral strike-slip movement linked the to the south side of the Sulu Sea$ Southward subducting west margin of the Philippine Sea subduction beneath the Sulu arc continued until plate to subduction at the Ryukyu trench$ Colli- 10 Ma$ The remainder of the Philippines contin- sion of Luzon and the Cagayan ridge with the ued to move with the Philippine Sea plate, pos- Eurasian continental margin in Mindoro and sibly with intra-plate strike-slip motion and sub- 114 R* Hall duction resulting in local volcanic activity$ At the new subduction began at the Negros and Philip- east edge of the Philippine Sea plate spreading pine trenches$ These subduction zones were terminated in the Shikoku basin$ linked by strike-slip systems active within the As a result of the change in plate boundaries, Philippines, and this intra-plate deformation cre- fragments of continental crust were emplaced in ated many very small fragments which are diffi- Sulawesi on splays at the western end of the cult to describe using rigid plate tectonics$ Sorong fault system$ The earliest fragment to The Molucca Sea continued to close by sub- collide is inferred to have been completely duction on both sides$ At present the Sangihe underthrust beneath West Sulawesi and contrib- forearc has overridden the northern end of the uted to later crustal melting (Polvé et al$, 1997)$ Halmahera arc, and is beginning to over-thrust Later, the Tukang Besi platform separated from west Halmahera$ In the Sorong fault zone, accre- the Bird’s Head and was carried west on the tion of Tukang Besi to Sulawesi locked a strand Philippine Sea plate to collide with Sulawesi$ of the fault and initiated a new splay south of Locking of splays of the Sorong fault caused the Sula platform$ The Sula platform then col- subduction to initiate at the eastern margin of lided with the east arm of Sulawesi, causing ro- the Molucca Sea, thus producing the Neogene tation of the east and north arms to their present Halmahera arc$ In this way the Molucca Sea be- position, leading to southward subduction of came a separate plate as the double subduction the Celebes Sea at the north Sulawesi trench$ system developed$ The Eurasia-Philippine Sea plate-Australia tri- After the collision of the Ontong Java plateau ple junction was and remains a zone of micro- with the Melanesian arc the Solomons became plates but within this contractional setting new attached to the Pacific plate$ Westward subduc- extension began in the Banda Sea$ The Bird’s tion began on the SW side of Solomon Sea, be- Head moved north relative to Australia along a neath eastern New Guinea, eliminating most of strike-slip fault at the Aru basin edge$ Mesozoic Solomon Sea and resulting in the formation of ocean crust north of Timor was eliminated at the Maramuni arc system$ As the Solomon Sea was eastern end of the Java trench by continued eliminated, the South Caroline arc began to con- northern motion of Australia which brought the verge on the north New Guinea margin and the Australian margin into this trench as the volcanic arc terranes were translated west in the major inner Banda arc propagated east$ Seram began left-lateral shear zone, probably accompanied to move east requiring subduction and strike- by some rotation$ In the southern part of the slip motion at the edges of this microplate$ Since Solomons Sea subduction was in the opposite 5 Ma the southern Banda Sea has extended to its direction (eastward) and created the New Hebri- present dimensions, and continental fragments des arc system$ Spreading ceased in the South are now found in the Banda Sea ridges within Fiji basin$ young volcanic crust$ The Banda Sea is here in- terpreted to be very young as suggested by Hamilton (1979) and others$ 10-0 Ma In west Sundaland, partitioning of conver- gence in Sumatra into orthogonal subduction At the beginning of this period SE Asia was and strike-slip motion effectively established largely recognisable in its present form (Fig$10)$ one or more Sumatran forearc sliver plates$ Ex- Rotation of Borneo was complete$ This, with tension on the strike-slip system linked to the collision in the central Philippines and Mindoro, spreading centre in the Andaman Sea (Curray et and continued northward movement of Aus- al$, 1979)$ Within Eurasia reversal of motion on tralia, resulted in reorganisation of plate the Red River system may have been one conse- boundaries and intra-plate deformation in the quence of the regional change in plate motions$ Philippines$ The Luzon arc came into collision Opening of the Ayu trough separated the with the Eurasian margin in Taiwan$ This may Caroline plate and Philippine Sea plate, al- be the cause of the most recent regional change though the rate of separation at this spreading in plate motions at about 5 Ma$ The Philippine centre was very low$ North of the Bird’s Head, Sea plate rotation pole moved north from a po- and further east in New Guinea, transpressional sition east of the plate; clockwise rotation con- movements were marked by deformation of arc tinued but the change in motion caused re-ori- and ophiolite slivers separated by sedimentary entation of existing, and development of new, basins$ Progressive westward motion of the plate boundaries$ Subduction continued at the South Caroline arc within the left-lateral trans- Manila, Sangihe and Halmahera trenches, and pressional zone led to docking of the north New Cenozoic plate tectonics of SE Asia 115

30 Ma Mid Oligocene

aa 20o N aa

10o N

aa aa aa

aa aa aa aa 0o

a aaaa a aaaa aaaa aaaa aaaaaa aa aa VOLCANOES aa aa

HIGHLAND 10o S a aa LAND aa CARBONATE aa PLATFORMS aaa aaaaaaaa aaa aaaaaaaaaaa SHALLOW SEA aaaaa aaaaaaaaaaa aaaaa aaaaaaaaaaa DEEP SEA aaaaaaaaaaa aaaaaaaaaaa 90o E 100o E 110o E 120o E 130o E aaaaaaaaaaa140o E 150o E 20o S

Fig11 Postulated distribution of land and sea in SE Asia at 30 Ma No attempt has been made to represent topography with Asia and Indochina Much of the area north and east of the Indian collision zone must have been highlands

Guinea terranes$ This caused the cessation of know where there was land and sea, where the southward subduction of the Solomon Sea plate sea was shallow and deep, and how wide were but resulted in its northward subduction be- the seas$ For the land, there needs to be some neath New Britain$ The New Britain subduction knowledge of topography, particularly where led to rapid spreading in Woodlark basin as a there were mountainous regions$ The distribu- consequence of slab-pull forces and rapid rip- tion and character of land and sea will have pro- ping open of continental crust beneath the vided physical pathways and barriers to disper- Papuan peninsula$ Elimination of most of the sal, and may well have influenced plant and ani- remaining Solomons marginal basin by east- mal distribution by effects on other controlling ward subduction led to formation of the New factors such as local and global climate, oceanic Hebrides arc and ocean crust formation in the circulation patterns, and sea-level$ North Fiji basins$ However, moving from tectonic reconstruc- tion maps to detailed palaeogeographical maps involves further complexities$ In many ways the Determining the extents of land and sea geological record is a marine record$ Most of Earth history is recorded in rocks deposited at For the biogeographer, the tectonic develop- the surface, and the areas where most sediments ment of the region is only a starting point for are deposited are close to or below sea-level, understanding$ In order to understand the distri- and mainly at continental margins$ Dating of bution of most organisms it is also necessary to rocks is largely based on fossils, and marine or- 116 R* Hall

25 Ma End Oligocene

aaaa aaaa aaaa aa aaaa aa aaaa aaaa

aa aa aa

aa aa aaaa aa aa aa aa aaaa aaaa aaaa aaaaaa aa aa aa VOLCANOES aa HIGHLAND LAND

aa CARBONATE aa aa aa PLATFORMS aa aa SHALLOW SEA aa aa DEEP SEA aa

Fig12 Postulated distribution of land and sea in SE Asia at 25 Ma

ganisms generally provide the fossils of greatest restricted types of environments, and usually biostratigraphic value which usually provide contain few fossils which have limited biostrati- some insight into the environment of deposi- graphic value$ Unlike marine fossils, fossil as- tion$ Geologists are therefore usually able to re- semblages from land rarely yield information construct the history of marine areas$ In the about the history of their enclosing sediments deep oceans sedimentary rocks may lack fossils relative to sea-level$ but the history of sediments deposited on ocean However, there are ways to solve some of crust is known because ocean crust subsides these problems, and mapping palaeogeography with age due to lithospheric cooling and age- onto the reconstructions is not, in principle, im- depth relationships are well established$ Thus, possible although much of the information re- many postulated land-bridges in oceanic re- quired is not yet available$ It is possible to iden- gions can be dismissed with some confidence$ tify the positions of former coastlines, interpret In contrast, mapping environments and phys- the location of former river systems, and indi- iography of former land areas is a great deal rectly infer areas of mountains$ In SE Asia some more difficult$ Uplift, erosion and periods of of the information can be compiled from the lit- emergence are mainly recorded by negative ev- erature; an attempt to do this for the region of idence, such as unconformities and stratigraphic Wallace’s Line is discussed by Moss and Wilson incompleteness$ Even when there is a rock (1998, this volume)$ Some data, for example lo- record it will often be difficult to date because cation of former coastlines, could be determined sediments deposited on land typically represent from records of oil companies acquired during Cenozoic plate tectonics of SE Asia 117

20 Ma aaa aaa Early Miocene aa aa aa aa

aaa aaa aaaa aaaa aaa aaaa aaa aaaa aaaa

aa aa aa aa aa aa

aa aa

aaa aaa aaa aaa aaaa aaa aaaa aaa aaaaaaaa aaaa aaaaaaaa aaaa aaaa aaaa aaaaaa aa aa aaaa aaaaaaa VOLCANOES aa aa aaaaaaa aa aaaaaaa HIGHLAND aaaaaaa aaaaaaa aaaaaaaaaaaaaaaaaaaa LAND aaaaaaaaaaaaaaaaaaaa aaaa aaaaaaaaaaaaaaaaaaaa aa CARBONATE aaaa aaaaaaaaaaaaaaaaaaaa aa aaaaaaaaaaaaaaaaaaaa PLATFORMS aaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaa SHALLOW SEA aaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaa DEEP SEA aaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaa

Fig13 Postulated distribution of land and sea in SE Asia at 20 Ma

extensive seismic surveys of SE Asia for hydro- those few sources, some proprietary information carbons$ New research could provide further from oil companies, and a wide range of litera- detail and biogeographers themselves could ture and maps$ The sources are too numerous to also contribute by, for example, mapping distri- cite and the quality of coverage is very variable$ butions of fossil plants and interpreting their The task is a very large one, given the size of the environments$ area, and the results should therefore be regard- ed as a first order approximation only$ I have not attempted to draw palaeogeographical maps Land and sea for 30-0 Ma for periods before 30 Ma$ The period 30-0 Ma is of most interest to biogeographers; before then Figs$11 to 16 are an attempt to compile the gen- the separation between Asia and Australia was eral features of land and sea onto maps of the greater and the tectonic reconstructions are also tectonic reconstructions showing 5 million year more uncertain$ intervals between 30 and 5 Ma for the region of The limited ranges of environments and dis- SE Asia$ The maps may be useful in indicating tributions shown are best estimates$ Broadly the likely geographical connections and barriers speaking, each area shown should be regarded and the periods when these were in existence$ as a probability$ For example, for an area shown There are few studies that compile this type of as deep marine, the probability of that area be- information and all cover limited parts of area ing shallow marine is low, and of it being land is for limited times$ Thus these maps are based on very low$ Some of the assignments are educated 118 R* Hall

15 Ma Middle Miocene

aa

aa aa aa

a a aaaaa aaaaa aaaaa aaaaa

aaa aaaa aaa aaaa aaa aaaa aaaa aa aa aaaaaaa aa aaaaaaa aa aaaaaaa aa aa aa aaaaaaa aa aa aaaaaaa aaaaaaa VOLCANOES aaaaaaa aaaaaaa aaaaaaaaaaaaa HIGHLAND aaaaaaa aaaaaaaaaaaaa aaaaaaaaaaa aaaa aaaaaaaaaaaaa LAND aaaaaaa aaaaaaaaaaaaa aaaaaaaaaaaaa aaaaaaaaaaaaa aa CARBONATE aaaaaaaaaaaaa aa PLATFORMS aaaaaaaaaaaaa aaaaaaaaaaaaa aaaaaaaaaaaaa SHALLOW SEA aaaaaaaaaaaaa DEEP SEA aaaaaaaaaaaaa

Fig14 Postulated distribution of land and sea in SE Asia at 15 Ma

guesses$ For example, areas of long-lived island Asia down major river systems$ Much of south- arcs develop thickened crust, implying relative ern Sundaland was the site of deposition of allu- shallow water areas and local emergence$ When vial, fluvial and deltaic sediments$ There were volcanoes are active, magma production, ther- major embayments in the eastern Asian margin mal expansion and crustal buoyancy can lead to formed by the South China Sea, the proto-South emergence but individual volcanoes can be very China Sea and the Celebes Sea-Makassar Strait$ short-lived on a geological time scale (typically Separating these were elongate bathymetric fea- less than one million years) even though an arc tures which were probably mainly shallow wa- may have been a long-lived feature$ It is usually ter with intermittent emergent areas, notably not possible to identify precisely which areas where arc volcanoes were active$ The southern- were emergent, simply that there are likely to most promontory was the Sulawesi-Philippines- have been such areas$ Halmahera arc which could have provided a The mid Oligocene (Fig$11) was the time of a pathway into the Pacific, via volcanic island major fall in global sea-level (Haq et al$, 1987)$ stepping stones, for organisms that could cross Very large areas of Sundaland and Sunda shelf seawater$ The other promontories terminated in were exposed and there were probably more the deep ocean area of the Pacific$ emergent areas than at any subsequent time un- At about 25 Ma (Fig$12) the north Australian til the end of the Cenozoic$ North of Sundaland, margin came into contact with Sulawesi and the Asia was a persistent highland area, and large Halmahera arc, and this could have created a amounts of sediment moved south from central discontinuous land connection via the island Cenozoic plate tectonics of SE Asia 119

10 Ma Late Miocene

aaa aaa aaa aaa

aa aa

a a aaa aaa aaa

aa aa

aa aa aa aa aa aa aa aa aa aa aaa a aaa aa aaaa aaaaa aaaa aaa aaaaa a aaaaa aaaaaa aaaaaaaaaaaaa aaaaaa aaaaaaaaaaaaa aaaa aaaaaa VOLCANOES aaaa aaaaaaaaaaaaa aaaaaa aaaaaaaaaaaaa HIGHLAND aaaaaaaaaaaaa aaaaaaaaaaaaa LAND aa aa CARBONATE aa aa PLATFORMS SHALLOW SEA DEEP SEA

Fig15 Postulated distribution of land and sea in SE Asia at 10 Ma

arcs of Halmahera and the Philippines into zone of shallow water within which there could Sulawesi$ The arc-continent collision closed the have been numerous islands$ Furthermore, deep water passage between the Pacific and In- strike-slip fault movements led to the arrival of dian oceans (Kennett et al$ 1985) by about 20 numerous fragments of continental crust in Ma (Fig$13) and there must have been major Sulawesi, sliced from the Bird’s Head changes of oceanic currents (Fig$17 and 18) with microcontinent$ The northern Makassar Strait re- implications for the distribution of many marine mained a deep water area, and presumably organisms, particularly those of shallow marine formed a barrier to migration for many plant and environments$ North-central Borneo was up- animals (Moss and Wilson, 1998 this volume)$ lifted and shed huge volumes of sediments into From 15 Ma to 5 Ma (Figs$14, 15, 16) was a the deltas which formed in north and east Bor- period in which emergent Sundaland reduced in neo$ area, while the deep marginal basins in the east From about this time there was probably al- were eliminated (proto-South China Sea) or re- ways some land in the area of Sulawesi, and the duced in size (Sulu, Celebes and Molucca Sea)$ extensive but poorly dated Celebes molasse Local collision and volcanic arc activity led to (Kündig, 1956) represents the products of sub- intermittent emergence in many of the arc re- aerial erosion, although there were no perma- gions but these probably always resembled the nent land links to Sundaland nor to Australia$ present Philippine and North Molucca arcs, with However, there were intermittently emergent ar- land separated by sea, which could locally have eas between Australia and Sulawesi, and a broad been quite deep$ More of Borneo became emer- 120 R* Hall

5Ma Early Pliocene

aa aa

a a aa aa aa aa aaaa aa aaaa aa aa aa aa aa aa aa aaa aaaaa a a aa aaa aaaaa a aaaaa VOLCANOES aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa HIGHLAND aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa LAND aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aa CARBONATE aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa PLATFORMS aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa SHALLOW SEA aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa DEEP SEA aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

Fig16 Postulated distribution of land and sea in SE Asia at 5 Ma

gent and the central mountains on the Sarawak- panied by changes in land area and drainage Kalimantan border extending into Sabah be- systems$ Huge volumes of sediment began to came wider and higher with time$ It is important move south from central Asia into the sedimen- to be aware that within this convergent setting tary basins of the Sunda shelf$ Ultimately all this deep basins also formed (e*g*, Sulu Sea, Banda would have driven dispersal from Gondwana Sea) which must have represented new barriers via India into SE Asia (e*g* Harley and Morley, to dispersal which formed at the same time as 1995), and later speciation centred in Sundaland new land pathways were established$ which for many organisms became separated from Asia by climate and topography, and which remained separated from Australia by Conclusions marine barriers$ Continued indentation of Asia by India modified the Eurasian continent but There are three important periods in regional much more knowledge is required of the timing development$ At about 45 Ma plate boundaries of fault movements and the amounts of changed, probably as a result of India-Asia colli- displacements before Sundaland can be ad- sion$ From a biogeographical viewpoint the ar- equately understood$ The deformation within rival of India would have led to a movement of Asia and Sundaland is likely to have led to the Gondwana plants and animals into Asia$ Moun- formation of geographical barriers, principally tain building resulting from the collision led to mountains, some of which were associated with major changes in habitats, and climate, accom- strike-slip faults and geologically short-lived$ Cenozoic plate tectonics of SE Asia 121

8 Ma 10 Ma Equatorial Undercurrent

16 Ma 20 Ma

South Equatorial Current

22 Ma 30 Ma

Indonesian Seaway

South Equatorial Current

Fig17 Circulation patterns of surface and near-surface wa- Fig18 Possible circulation patterns of surface and near-sur- ters in the Pacific ocean inferred by Kennett et al (1985) at face waters in eastern Indonesia shown on the tectonic re- three stages during the Neogene as the Indonesian sea-way constructions of this paper The currents postulated are closed Black arrows indicate cold currents and unfilled ar- based on Kennett et al (1985) and present-day circulation rows indicate warm currents patterns (Fine et al, 1994) 122 R* Hall

The second major period is around 25 Ma and tectonics could be a controlling factor in when plate boundaries and motions changed some cases$ Cicada distributions in New Guinea again, partly due to collision between the north suggest a geological control (Boer and Duffels, Australian margin and arcs to the north$ This, 1996), and slicing of crustal fragments from the together with collision of the Melanesian arcs Bird’s Head could have caused influxes of fau- and the Ontong Java plateau, changed the tec- nas and floras into Sulawesi from Australia at tonics of the oceanic-arc region east of Asia intervals in the last 20 Ma$ However, geology (Philippines, Celebes Sea, Sulu Sea, Philippine and tectonics also influence other variables Sea, Caroline Sea, north New Guinea, New Brit- which are more subtle controls on biogeo- ain, Solomons, Tonga)$ The 25 Ma event was graphic patterns$ Sea-level, elevation of land ar- probably the most important tectonic event eas, soil, wind and water movements, and cli- from the biogeographical point of view as it led mate are all examples of factors upon which to new, albeit discontinuous, links between there is some geological influence$ Climatic con- Australia via Sulawesi into SE Asia across areas trols are too difficult to model at present, but at which were mainly shallow marine and locally some time in the future it will be possible to use included land$ It also resulted in a very long dis- the tectonic models as the basis for simulation of continuous island arc link between Asia and ancient climates in SE Asia$ It is notable that at Melanesia$ However, as the pathways between present there are more highland areas, and a Australia and Sundaland came into existence, greater area of land than at any time during the new barriers also formed$ The central Borneo last 30 million years$ This is consistent with mountains began to rise in the early Miocene rather restricted areas of modern carbonate plat- and became a regional drainage divide sending forms which are limited in part by clastic sedi- sediment north into the Sarawak basins and ment influx$ The present distribution and size of Baram delta, and southeast into the Tarakan and shallow water carbonate areas may in part re- Mahakam deltas$ North of Borneo, as the proto- flect a period of relatively low sea-level, but also South China Sea closed, the Oligo-Miocene record the recent rise of mountains due to tec- South China Sea widened and the Sulu Sea tonic forces as the region is compressed be- opened$ As the distance between Australia and tween Asia and Australia$ Sulawesi closed, the deep Banda Sea opened$ Some of the biogeographic patterns in SE Asia Thus, movement of plants and animals between at present are difficult to relate simply to geol- Australia and Sundaland would have remained ogy, for example, the distance between Borneo difficult$ Perhaps it was this zone of barriers, and Sulawesi (Wallace’s line and equivalents) close to a region of deep and former deep should have been as easy to cross as the barriers ocean barriers separating Borneo and Australia, between Australia and Sulawesi$ This raises the which is the origin of Wallace’s line$ The narrow question of the longevity of biogeographic pat- Makassar Strait, which at its south end termi- terns, about which we currently lack adequate nates in a long-lived discontinuous carbonate information$ During the last million years there platform, could not alone have been a major have been periods of low sea-level associated barrier to dispersal$ with glacial intervals when far greater areas of Plate motions and boundaries changed again land were emergent than at present, and the at about 5 Ma, possibly as a consequence of arc- present areas are significantly greater than those continent collision in Taiwan, and in the last 5 during the Neogene$ Much of the Sunda shelf Ma there has been renewed tectonic activity and would have been emergent although in eastern a significant increase in land and highlands all Indonesia there are many narrow deep water round the margins of SE Asia$ A number of new areas (such as the Makassar Strait) which would dispersal pathways developed across the re- have remained physical barriers$ However, else- gion, for example those linking Taiwan and where large sea-level falls would have separated New Guinea through the Philippines and North some formerly connected ocean basins as shal- Moluccas, and connecting New Guinea to Thai- low water areas became emergent, changing land via the Banda and Sunda arcs$ It is also oceanic circulation patterns and modifying probable that there was an increase in the range weather and climate (e*g$ Huang et al$, 1997)$ of habitats along these routes, due to elevation Fluctuations in temperatures and rainfall are of mountains, and likely associated variations in likely to have been more extreme at intervals in rainfall$ the last million years than in the preceding 30 Disentangling the contribution of geology to million years$ Therefore, the last period of geo- biogeographic patterns is not simple$ Geology logical history, perhaps one million years or Cenozoic plate tectonics of SE Asia 123 even much less, may have had a far greater in- References fluence on biogeographic patterns than the much longer period before$ Aitchison, J C, Clarke, D L, Meffre, S and Cluzel, D 1995 To go further, detailed maps of land and sea, Eocene arc-continent collision in New Caledonia and im- plications for regional southwest Pacific tectonic evolu- and palaeo-topography must be compiled from tion Geology 23: 161-164 published maps and papers, and unpublished Briais, A, Patriat, P and Tapponnier, P 1993 Updated inter- coastline, shelf edge, age and lithofacies infor- pretation of magnetic anomalies and seafloor spreading mation, much in oil company files$ In particular stages in the South China Sea: implications for the Terti- the display of uplift and subsidence, and timing ary tectonics of Southeast Asia Journal of Geophysical Research 98: 6299-6328 of magmatic events, on tectonic reconstructions Boer, A J de and Duffels, J P, 1996 Historical biogeogra- would help in identifying underlying processes phy of the cicadas of Wallacea, New Guinea and the and give more confidence in mapping land and West Pacific: a geotectonic explanation Palaeogeogra- sea into areas where there is little direct evi- phy, Palaeoclimatology, Palaeoecology 124: 153-177 dence$ Biogeographers must contribute, for ex- Burrett, C, Duhig, N, Berry, R and Varne, R 1991 Asian and South-western Pacific continental terranes derived ample, distributions of fossil plants can provide from Gondwana, and their biogeographic significance In information on palaeo-temperatures and envi- Austral Biogeography Edited by P Y Ladiges, C J Hum- ronments$ There is a need to focus on biogeo- phries and L W Martinelli, Australian Systematic Botany graphic patterns which are most likely to reveal 4: 13-24 the links to geology using plants and animals Cambridge Paleomap Services 1993 ATLAS version 33 Cambridge Paleomap Services, PO Box 246, Cambridge, which have difficulty in dispersing, and for UK which non-geological controls are unimportant$ Crook, K A W and Belbin, L 1978 The Southwest Pacific It is for the biogeographers to identify such criti- area during the last 90 million years Journal of the Geo- cal floral and faunal indicators$ We still know logical Society of Australia 25: 23-40 Curray, J R, Moore, D G, Lawver, L A, Emmel, F J, Raitt, little about rates of speciation and dispersal, and R W, Henry, M and Kieckheffer, R 1979 In Tectonics of for most animals and plants the fossil record is the Andaman Sea and Burma American Association of poor or non-existent$ DNA studies offer one Petroleum Geologists Memoir 29: 189-198 way of determining a time-scale for biological Daly, M C, Cooper, M A, Wilson, I, Smith, D G and development which could contribute to an ex- Hooper, B G D 1991 Cenozoic plate tectonics and ba- sin evolution in Indonesia Marine and Petroleum Geolo- planation of biogeographic patterns$ Another gy 8: 2-21 way forward is mathematical simulation of the Davies, H L 1971 Peridotite-gabbro-basalt complex in east- biological variables, testing biogeographic pat- ern Papua: an overthrust plate of oceanic mantle and terns against predictions$ It is certain that no sin- crust BMR Journal of Australian Geology and Geophysics gle factor will account for the distribution of 128: 48 pp Dewey, J F, Cande, S and Pitman W C III 1989 Tectonic plants and animals in SE Asia; tectonic move- evolution of the India/Eurasia collision Zone Eclogae ments may be a control but their importance is Geologicae Helvetiae 82: 717-734 still far from clear$ England, P and Houseman, G 1986 Finite strain calcula- tions of continental deformation 2 Comparison with the India-Asia collision zone Journal of Geophysical Re- search 91: 3664-3676 Acknowledgements Fine, R A, Lukas, R, Bingham, F M, Warner, M J and Gammon, R H 1994 The western equatorial Pacific: a Financial support has been provided by NERC, water mass crossroads Journal of Geophysical Research the Royal Society, the London University Central 99 (C12): 25,063-25080 Research Fund, and the London University SE Hall, R 1995 Plate tectonic reconstructions of the Indone- sian region Proceedings of the Indonesian Petroleum Asia Research Group currently supported by Association 24th Annual Convention: 71-84 Arco, Canadian Petroleum, Exxon, Lasmo, Hall, R 1996 Reconstructing Cenozoic SE Asia In Tectonic Mobil, Union Texas and Unocal$ Work in Indo- Evolution of SE Asia Edited by R Hall and D J Blundell nesia has been facilitated by GRDC, Bandung Geological Society of London Special Publication 106: and Directors including H$ M$ S$ Hartono, M$ 153-184 Hall, R 1997 Cenozoic tectonics of SE Asia and Australasia Untung, R$ Sukamto and I$ Bahar$ I am grateful In Petroleum Systems of SE Asia and Australasia pp 47- to Moyra Wilson, Steve Moss and Alistair Fraser 62 Edited by J V C Howes and R A Noble Indonesian for information$ Clive Burrett, Bob Musgrave, Petroleum Association, Jakarta Gordon Packham and Rupert Sutherland made Hall, R, Fuller, M, Ali, J R and Anderson, C D 1995 The helpful comments on the reconstructions and Philippine Sea Plate: magnetism and reconstructions American Geophysical Union Monograph 88: 371-404 the manuscript$ I thank Kevin Hill for consider- Hamilton, W 1979 Tectonics of the Indonesian region able help and discussion in reconstructing the USGS Professional Paper 1078: 345 pp New Guinea and the SW Pacific$ Haq, B U, Hardenbol, J and Vail, P R 1987 Chronology of 124 R* Hall

fluctuating sea levels since the Triassic Science 235: Palaeomagnetism of the Balantak ophiolite, Sulawesi 1156-1167 Earth and Planetary Science Letters 125: 193-209 Harley, M M and Morley, R J 1995 Ultrastructural studies Norton, I O 1995 Tertiary relative plate motions in the of some fossil and extant palm pollen, and the recon- North Pacific: the 43 Ma non-event Tectonics 14: 1080- struction of the biogeographical history of subtribes Igua- 1094 nurinae and Calaminae Review of Palaeobotany and Pal- Packham, G 1996 Cenozoic SE Asia: Reconstructing its ag- ynology 85: 153-182 gregation and reorganisation In Tectonic Evolution of SE Hinz, K, Block, M, Kudrass, H R and Meyer, H 1991 Asia Edited by R Hall and D J Blundell Geological So- Structural elements of the Sulu Sea Geologische Jahr- ciety of London Special Publication 106: 123-152 buch A127: 883-506 Peltzer, G and Tapponnier, P 1988 Formation of strike-slip Houseman, G and England, P 1993 Crustal thickening ver- faults, rifts, and basins during the India-Asia collision: an sus lateral expulsion in the Indian-Asian continental colli- experimental approach Journal of Geophysical Research sion Journal of Geophysical Research 98: 12,233-12,249 93: 15085-15117 Huang, C-Y, Liew, P-M, Zhao, M, Chang, T-C, Kuo, C- Polvé, M Maury, R C, Bellon, H, Rangin, C, Priadi, B, M, Chen, M-T, Wang, C-H and Zheng, L-F, 1997 Yuwono, S, Joron, J L and Soeria Atmadja, R 1997 Deep sea and lake records of the Southeast Asian pale- Magmatic evolution of Sulawesi (Indonesia): constraints omonsoons for the last 25 thousand years Earth and on the Cenozoic geodynamic history of the Sundaland Planetary Science Letters 146: 59-72 active margin Tectonophysics 272: 69-92 Hutchison, C S 1996 South-East Asian Oil, Gas, Coal and Rangin, C, Jolivet, L, Pubellier, M 1990 A simple model for Mineral Deposits Clarendon Press, Oxford 265 pp the tectonic evolution of southeast Asia and Indonesia Katili, J A 1975 Volcanism and plate tectonics in the Indo- region for the past 43 m y Bulletin de la Société nesian island arcs Tectonophysics 26: 165-188 géologique de France 8 VI: 889-905 Kennett, J P, Keller, G, and Srinivasan, M S 1985 Miocene Rowley, D B 1996 Age of initiation of collision between planktonic foraminiferal biogeography and paleogeo- India and Asia: a review of stratigraphic data Earth and graphic development of the Indo-Pacific region In The Planetary Science Letters 145: 1-13 Miocene ocean: paleogeography and biogeography Ge- Silver, E A and Rangin, C 1991 Leg 124 tectonic synthesis ological Society of America Memoir 163: 197-236 In Proceedings of the Ocean Drilling Program, Scientific Kündig, E 1956 Geology and ophiolite problems of east Results 124: 3-9 Edited by E A Silver, C Rangin, M T Celebes Verhandelingen van het Koninklijk Nederland- von Breymann et al sch Geologisch-Mijnbouwkundig Genootschap, Geolo- Stern, R J and Bloomer, S H 1992 Subduction zone infan- gische Serie 16: 210-235 cy: examples from the Eocene Izu-Bonin-Mariana and Lee, T-Y and Lawver, L A 1994 Cenozoic plate tectonic Jurassic California arcs Bulletin of the Geological Society reconstruction of the South China Sea region Tectono- of America 104: 1621-1636 physics 235: 149-180 Tapponnier, P, Lacassin, R, Leloup, P, Scharer, U, Dalai, McCaffrey, R 1996 Slip partitioning at convergent plate Z, Haiwei, W, Xiaohan, L, Shaocheng, J, Lianshang, Z boundaries of SE Asia In Tectonic Evolution of SE Asia and Jiayou, Z 1990 The Ailao Shan/Red River metamor- Edited by R Hall and D J Blundell Geological Society of phic belt: Tertiary left lateral shear between Indochina London Special Publication 106: 3-18 and South China Nature 343: 431-437 Meffre, S 1995 The development of island arc-related ophi- Tapponnier, P, Peltzer, G, LeDain, A, Armijo, R and Cob- olites and sedimentary sequences in New Caledonia bold, P 1982 Propagating extrusion tectonics in Asia: PhD thesis, University of Sydney, Australia, 239 pp new insights from simple experiments with plasticine Metcalfe, I 1998 Palaeozoic and Mesozoic geological evolu- Geology 10: 611-616 tion of the SE Asian region: multi-disciplinary constraints Tapponnier, P, Peltzer, G and Armijo, R 1986 On the me- and implications for biogeography In Biogeography and chanics of the collision between India and Asia In Colli- Geological Evolution of SE Asia Edited by R Hall and J sion Tectonics Edited by M P Coward and A C Ries D Holloway (this volume) Special Publication of the Geological Society of London Monnier, C, Girardeau, J, Maury, R C and Cotten, J 1995 19: 115-157 Back arc basin origin for the East Sulawesi ophiolite Unrug, R 1997 Rodinia to Gondwana: the geodynamic map (eastern Indonesia) Geology 23: 851-854 of Gondwana supercontinent assembly GSA Today 7 Moss, S J and Wilson, M J 1998 Tertiary evolution of Su- (1): 1-6 lawesi-Borneo: implications for biogeography In Bioge- Yan, C Y and Kroenke, L W 1993 A plate tectonic recon- ography and Geological Evolution of SE Asia Edited by struction of the SW Pacific 0-100 Ma In Proceedings of R Hall and J D Holloway (this volume) the Ocean Drilling Program, Scientific Results 130: 697- Mubroto, B, Briden, J C, McClelland, E and Hall, R 1994 709 Edited by T Berger, L W Kroenke, L Mayer et al Cenozoic plate tectonics of SE Asia 125

Colour plates for:

The plate tectonics of Cenozoic SE Asia and the distribution of land and sea

Robert Hall SE Asia Research Group, Department of Geology, Royal Holloway University of London

Captions

Fig5 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 plateaus 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 follows:

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

Fig6 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 continental margin and an island arc 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 Fig7 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 Australasia producing the Solomon Sea and the island arcs of Melanesia Fig8 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 Fig9 Reconstruction of the region at 20 Ma Collision of the north Australian margin in the region 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 events caused major reorganisation of plate boundaries Subduction of the Solomon Sea began at the eastern New Guinea margin Spreading began in the Parece Vela and Shikoku marginal basins The north Australian margin became a major left-lateral strike-slip system as the Philippine Sea-Caroline plate 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 Fig10 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 development of opposed subduction zones Rotation of Borneo was complete but motion of the Sumatran forearc slivers was associated with new spreading in the Andaman Sea 126 R* Hall

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 Cenozoic plate tectonics of SE Asia 127

50 Ma End Early EURASIA Eocene

40oN

PACIFIC PLATE 20oN

INDIA

INDIAN PLATE

AUSTRALIAN ? PLATE 20oS

AUSTRALIA 40oS

90oE 180oE

ANTARCTICA 60oS 128 R* Hall

40 Ma Middle Eocene EURASIA

40oN

PACIFIC PLATE 20oN INDIA

INDIAN PLATE

20oS

AUSTRALIA

40oS

90oE 180oE

ANTARCTICA 60oS Cenozoic plate tectonics of SE Asia 129

30 Ma Mid Oligocene EURASIA

40oN

PACIFIC INDIA PLATE 20oN

INDIAN PLATE

20oS AUSTRALIA

40oS

90oE 180oE

ANTARCTICA 60oS 130 R* Hall

20 Ma Early Miocene EURASIA

40oN

INDIA PACIFIC PLATE 20oN

INDIAN PLATE

AUSTRALIA 20oS

40oS

90oE 180oE

ANTARCTICA 60oS Cenozoic plate tectonics of SE Asia 131

10 Ma Late Miocene EURASIA

40oN

INDIA PACIFIC PLATE 20oN

AUSTRALIA 20oS

40oS

90oE 180oE

ANTARCTICA 60oS