The plate tectonics of Cenozoic SE Asia and the distribution...

Cenozoic plate tectonics of SE Asia 99

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

Robert HallSE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham, SurreyTW20 0EX, UK Email: [email protected]

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

Abstract

A plate tectonic model for the development of SE Asia andthe SW Pacific during the Cenozoic is based on palaeomag-netic data, spreading histories of marginal basins deducedfrom ocean floor magnetic anomalies, and interpretation ofgeological data from the region. There are three importantperiods in regional development: at about 45 Ma, 25 Ma and5 Ma. At these times plate boundaries and motions changed,probably as a result of major collision events.

In the Eocene the collision of India with Asia caused aninflux of Gondwana plants and animals into Asia. Mountainbuilding resulting from the collision led to major changes inhabitats, climate, and drainage systems, and promoted dis-persal from Gondwana via India into SE Asia as well as cre-ating barriers between SE Asia and the rest of Asia. Contin-ued indentation of Asia by India further modified Sundalandand created internal barriers affecting biogeographic pat-terns. From a biogeographic and tectonic viewpoint, themajor Cenozoic tectonic event in SE Asia occurred about 25million years ago, resulting in major changes in the configu-ration and character of plate boundaries, and caused effectswhich propagated westwards through the region. This eventled to the progressive arrival of Australian microcontinentalfragments in Sulawesi, providing possible pathways for mi-gration of faunas and floras between Asia and Australia, butalso creating new barriers to dispersal.

Tectonic reconstruction maps of lithospheric fragmentscannot be translated simply into maps of land and sea whichare of greater value to biogeographers. Determining the pal-aeogeography of the region is not yet possible, but an at-tempt is made to outline the main likely features of the ge-ography of the region since the late Oligocene.

Evidence from all fields of biogeography is required totest different tectonic models and identify the origin ofpresent biogeographic patterns but there must be a focus onplants and animals which have difficulty in dispersing, andfor which non-geological controls are unimportant. Thepresent distribution of plants and animals in SE Asia mayowe much more to the last one million years than the pre-ceding 30 million years.

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

Introduction

For the geologist, SE Asia is one of the mostintriguing areas of the Earth. The mountains ofthe Alpine-Himalayan belt turn southwards intoIndochina and terminate in a region of continen-tal archipelagos, island arcs and small ocean ba-sins. To the south, west and east the region issurrounded by island arcs where lithosphere ofthe Indian and Pacific oceans is beingsubducted at high rates, accompanied by in-tense seismicity and spectacular volcanic activ-ity. Within this region we can observe collisionbetween island arcs, between island arcs andcontinents, and between continental fragments.At the same time ocean basins are openingwithin this convergent region. SE Asia includesareas with the highest global rates of plate con-vergence and separation.

It is clear from the geology of the region thatthe snapshot we see today is no less compli-cated than in the past. The region has devel-oped by the interaction of major lithosphericplates, principally those of the Pacific, India-Australia and Eurasia (Fig.1), but at the presentday a description only in terms of these threeplates is a very great oversimplification. Manyminor plates need to be considered, and insome parts of the region the boundaries be-tween these smaller plates are very uncertain. Itis also clear that some of the deformation cannotbe described in simple plate tectonic terms.Lithosphere has deformed internally, and mate-rial has been added by arc volcanic processes,which means that at least one of the axioms of

100 R. Hall

Fig.1. The larger tectonic plates of SE Asia and the SW Pacific. The many small ocean basins and the major strike-slip faultsystems at the margins of SE Asia and Australia are manifestations of the complexity of plate tectonics in the region whichrequires a description in terms of many more plates than those shown.

plate tectonics, of rigid fragments moving on asphere, cannot be assumed.

The complexity of the present-day tectonicsof the region and the observable rates of platemotions (e.g., Hamilton, 1979; McCaffrey, 1996)indicate that major oceans, or multiple smalloceans, have closed during the Cenozoic. Sev-eral major island arcs have certainly formed dur-ing this time and some may have completelydisappeared. At some plate boundaries strike-slip faulting has dismembered previously coher-ent regions, and along these boundaries therecan be both major crustal subsidence and upliftdue to deformation. During the past 50 millionyears the configuration of the region has there-fore changed significantly in plate tectonicterms. Accompanying these large scale move-ments have been equally significant verticalmovements, recorded in the sedimentary basins

of the region, into which large volumes ofsediments have been shed, removed from risingmountains. Thus the distribution of land and seahas changed during the Cenozoic, and manyparts of the region have seen dramatic verticalmovements of several kilometres, with moun-tains where once there were oceans, and deepmarine regions where mountains had existed.

The abrupt division between Asian and Aus-tralian floras and faunas in Indonesia, first recog-nised by Wallace in the nineteenth century, hasits origin in the rapid plate movements and reor-ganisation of land-masses in SE Asia. Wallacerealised that the region had changed dramati-cally in the past without knowing the cause, andsince his work there has been a general aware-ness that the present distribution of land and seais not the same as that of the past, and that thechanges are in some way implicated in biogeo-

INDIA

PACIFIC

CAROLINE

PHILIPPINESEA

▲

▲

EURASIA

Woodlark Basin

SolomonSea

BismarckSea

SouthChinaSea

SuluSea

CelebesSea

BandaSea

INDIA-AUSTRALIA

20oS

10oS

0o

10oN

20oN

110oE 120oE 130oE 140oE 150oE90oE 100oE


graphic patterns. We are now confident that thegeological changes are the result of plate move-ments and are not the consequence of an ex-panding Earth. However, although the verylarge-scale motions of major plates have beenreasonably well known for the last 20 years orso, the detail necessary to reconstruct SE Asiahas been lacking. Furthermore, because of thecomplexity 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 toclearly describe its long term development andit is only in the last few years that models ex-plaining the development of the region havebeen produced.

Making tectonic reconstructions of SE Asiabecomes more difficult as the age of the recon-struction becomes greater, and examination ofthe present tectonics of the region shows whythis is so. Projecting motions that are known to-day into the past is very problematical; our ob-servations of the present tectonics indicate thatplates, plate boundaries and motions can begeologically ephemeral features. In some partsof the region, for example the Philippines andeast Indonesia, it is not even certain that platetectonics provides a suitable model for a de-tailed understanding of the development of theregion. Despite these difficulties, a plate tectonicmodel does have value and by working backfrom the present-day we can, albeit with diffi-

culty, make reconstructions; the known motionsof major plates do impose limits on possibilities;and the resulting interpretations do offer ameans of identifying important tectonic eventsand highlighting key problems. This paper ex-plains the background to a plate tectonic modelof SE Asia and the SW Pacific and discusses itsimplications for biogeography.

Mesozoic to Cenozoic background

In very general terms, the region owes its originto the pre-Cenozoic break-up of the Gondwanasuper-continent (Fig.2), the subsequent move-ment of Gondwana fragments northwards, andtheir eventual collision with Eurasia. Metcalfe(1998 this volume) provides an account ofpresent knowledge of the Palaeozoic and Meso-zoic development of SE Asia. It is clear thatmany fragments separated from Gondwana andamalgamated in SE Asia over a considerable pe-riod of time. The process of rifting led to forma-tion of new oceans, and the northward motionof Gondwana fragments required subduction ofolder oceanic crust at the edges of the growingEurasian continent. By the Mesozoic, a regioncomposed of fragments derived from Gondwa-na formed a Sundaland core surrounded bysubduction zones.

Subduction meant that the Sundaland marginswere complex. Island arcs at the margins mayhave been underlain by continental and oceaniccrust, and there were probably many smallocean basins behind the arcs and above thesubduction zones. The widespread ophiolitesare fragments of oceanic lithosphere now foundon land, and much of this lithosphere wasformed in subduction-related settings, such asbackarc basins and forearcs. Ophiolites arecommonly emplaced at some stage during theconvergence of two plates and convergence isultimately completed by collision between arcand continent, or continent and continent.Throughout the Mesozoic there appear to havebeen collisions of fragments with the Sundalandmargins, and by the beginning of the CenozoicSE Asia was a composite mosaic of continentalcrust, island arc material and oceanic crust.

Two major fragments separated from Gond-wana in the Cretaceous and moved northwardsas parts of different plates: India and Australia.India completed its passage in the early Ceno-zoic and collided with the Asian continent about50 million years ago (Fig.3). However, collisiondid not cause India to become fixed to Asia as

Fig.2. Reconstruction of Gondwana (after Unrug, 1997) inthe early Palaeozoic showing outlines of the major conti-nental fragments which separated during the Palaeozoic andMesozoic. Many of the Cimmerian terranes had accreted toLaurasia to form the core of SE Asia by the end of theMesozoic.

AFRICA

INDIA

ANTARCTICA

AUSTRALIASOUTH

AMERICA

Pre-PalaeozoicGondwana

core

Cimmerianterranes

Avalonian-Cadomianterranes

Pacific margin

102 R. Hall

predicted by early plate tectonic models. In-stead, India continued to move northwards, al-beit at a slower rate than during the Cretaceous.There is considerable disagreement amongst ge-ologists about how the continued northwardmovement was accommodated during theCenozoic, and its consequences. According toTapponnier and colleagues (e.g. Tapponnier etal., 1982, 1986, 1990; Peltzer and Tapponnier,1988; Briais et al., 1993) the impact of a rigidIndian indentor on an Asian margin weakenedby subduction-related heating and magmatismcaused eastward extrusion and rotation of con-tinental fragments, and opening of some of thesmall oceanic marginal basins of SE Asia. Theprogressive extrusion of continental fragmentsto the east and consequent rotation of crustalblocks has been simulated in laboratory experi-ments using plasticine (Fig.4), and the strike-slip

faults which cut across Asia are considered to bezones of major displacements which link to mar-ginal basins, such as the South China Sea, off-shore. If correct, this hypothesis implies majorchanges in SE Asia linked to Indias continuednorthward movement. In contrast, other work-ers (e.g. England and Houseman, 1986; Deweyet al. 1989; Houseman and England, 1993) dis-miss the extrusion hypothesis, arguing that thedisplacement on the strike-slip faults has beensmall and that the continued convergence of In-dia and Asia has been accommodated by crustalthickening with very little eastward movementof crust.

Australia separated from Gondwana, leavingAntarctica as its final remnant, at about the sametime as India, but moved less quickly north-wards. Instead of a direct collision with anothercontinent Australia is now making a glancing

45Ma

0 Ma

90Ma

120Ma

120Ma

0Ma

30Ma

45Ma

90Ma120Ma

30Ma

Fig.3. India and Australia separated from Gondwana in the Cretaceous. The map shows the late Cretaceous and Cenozoicmovement of these two major continental fragments north with respect to Asia and SE Asia, both of which are shown in theirpresent day positions for reference.


collision with a composite SE Asia, which in-cludes some of the earlier Gondwana fragmentsto arrive, and also includes the island arcsformed due to the subduction of oceanic crustnorth of Australia. In east Indonesia the north-ward movement of Australia during theCenozoic has been marked by arc-continent col-lision and major strike-slip motion within thenorth Australian margin. Further east, arc-conti-nent collisions have been the result of elimina-tion of marginal basins formed above subduc-tion zones as Australia has moved north, andthis system of arcs and marginal basins can betraced east along the margin of the Pacific platein Melanesia.

During the late Mesozoic and Cenozoic therewas subduction of the Pacific ocean to the eastof Asia, although the eastern margin of Asia andSE Asia was probably a region of small plates

and marginal basins as it is at the present day. Atpresent the Philippine Sea plate and Philippineisland arcs separate the east Asian margin fromthe Pacific plate south of Japan. Unlike the In-dian ocean and Pacific oceans, the PhilippineSea plate lacks well defined sea-floor magneticanomalies which normally provide the basis forreconstructing past plate motions. The Philip-pine Sea plate is also difficult to link to themovements of the other major plates because itis surrounded by subduction zones. For thesereasons the history of movement of plate move-ments in the west Pacific north of New Guineahas been very uncertain, and consequently theeastern edge of SE Asia has been difficult to re-construct. Palaeomagnetic data from east Indo-nesia (Hall et al., 1995) have provided the basisfor reconstructing the Philippine Sea plate andits motion since the early Cenozoic, and conse-

ASIA

SOUTHCHINA

INDOCHINA

SOUTHCHINASEA

A B

C D

INDIA

Fig.4. The impact of a rigid Indian indentor with Asia has been postulated to have caused the development of major strike-slipfaults as India progressively penetrated Asia (A to D). Analogue models produced during experiments (redrawn from Peltzerand Tapponnier, 1988) show striking similarities to the major features of SE Asia.

104 R. Hall

0 MaPresent Day

INDIA

AUSTRALIA

PACIFICPLATE

EURASIA

40oN

20oN

20oS

40oS

60oS

90oE120oE 150oE

180oE

ANTARCTICA

INDIANPLATE

H

NEWZEALAND

PHILIPPINESEA

PLATE

Fig.5. Present-day tectonic features of SE Asia and the SW Pacific. Light straight lines are selected marine magnetic anomalies. andactive spreading centres. White lines are subduction zones and strike-slip faults. The present extent of the Pacific plate is shownin mid grey. Labelled filled areas are mainly arc, ophiolitic, and accreted material formed at plate margins during the Cenozoic, andsubmarine arc regions, hot spot volcanic products, and oceanic plateaus. Pale grey areas represent submarine parts of the Eurasiancontinental margins. Dark grey areas represent submarine parts of the Australian continental margins. See pages 126-131 for colourplates of Figs.5 to 10. Letters represent marginal basins and tectonic features as follows:

A Japan SeaB Okinawa TroughC South China SeaD Sulu SeaE Celebes SeaF Molucca SeaG Banda SeaH Andaman SeaJ West Philippine BasinK Shikoku BasinL Parece Vela BasinM Mariana Trough

N Ayu TroughP Caroline SeaQ Bismarck SeaR Solomon SeaS Woodlark BasinT Coral SeaU Tasman SeaV Loyalty BasinW Norfolk BasinX North Fiji BasinY South Fiji BasinZ Lau Basin

Ba Banda ArcBH Birds HeadCa Cagayan ArcFj FijiHa Halmahera ArcIB Izu-Bonin ArcJa Japan ArcLo Loyalty IslandsLu Luzon Arc

Mk Makassar StraitMn Manus IslandNB New Britain ArcNC New CaledoniaNH New Hebrides ArcNI New IrelandNng North New Guinea

TerranesPa Papuan OphiolitePk Palau-Kyushu Ridge

Ry Ryukyu ArcSa Sangihe ArcSe Sepik ArcSo Solomons ArcSp Sula PlatformSu Sulu ArcTK Three Kings

RiseTo Tonga ArcTu Tukang Besi

Platform

Marginal Basins Tectonic features


quently, for making reconstructions of the re-gions adjacent to the Philippine Sea plate.

Combining the Philippine Sea plate historywith the known movements of the major plates,India, Australia, Pacific and Eurasia, providessome limits within which reconstructions of SEAsia, and parts of the west Pacific, can be at-tempted. Within this region there are recent in-terpretations of the South China Sea opening(Briais et al., 1993) and other marginal basinswhich limit options still further and provide con-straints of variable quality on modelling the tec-tonic history of the region. The importance ofthe marginal basins is that only they are likely tocontain a clear record, based on ocean floormagnetic anomalies, of the motion history ofsome of the minor plates. However, many of themarginal basins of SE Asia completely lack mag-netic anomalies, many have not been drilledduring the ocean drilling campaigns, and theirages and character are still poorly known.Therefore much of the evidence which must beused in a regional tectonic model of SE Asia isbased on interpretation of geological data fromthe small ocean basins, their margins, and fromthe geologically more complicated land areasaround them. The reader should be aware that,as in other areas of science, geologists differ intheir interpretations of these data, and much ofthe information does not lend itself to unam-biguous reconstruction. Nonetheless, a com-plete tectonic history can only be deduced fromthe geology on land combined with data fromthe oceans. The account here is therefore myview of the development of Cenozoic SE Asiausing plate tectonic reconstructions based onsuch deductions.

The model

The reconstructions were made using the AT-LAS computer program (Cambridge PaleomapServices, 1993) and plate motion model for themajor plates. The motion of Africa is definedrelative to magnetic north and motions of themajor plates are all relative to Africa. Eurasia hasbeen close to its present-day position through-out the Cenozoic. The reconstructions in thispaper add to this model for the major plates byincluding a large number of smaller fragments inSE Asia and the SW Pacific. More than 100 frag-ments are currently used, and most retain theircurrent size in order that they remain recognis-able. During the 50 Ma period fragments repre-sented may have changed size and shape or

may not have existed, both for arc and continen-tal terranes. Thus, the plate model can only bean approximation. Some of the elements of themodel are deliberately represented in a stylisticmanner to convey the processes inferred ratherthan display exactly what has happened, for ex-ample, the motion of the terranes of north NewGuinea.

Previous reconstructions which cover all orparts of the region discussed here include thoseof Katili (1975), Crook and Belbin (1978), Hamil-ton (1979), Briais et al. (1993), Burrett et al.(1991), Daly et al. (1991), Lee and Lawver (1994),Rangin et al. (1990), and Yan and Kroenke (1993)who also produced an animated reconstructionof the SW Pacific. The reader is referred to theoriginal papers for accounts of the earlier mod-els. Some differences between the model hereand other models result from the choice of refer-ence frames; some use the hotspot referenceframe, and others use a fixed Eurasia, whereasthese reconstructions use a palaeomagnetic ref-erence frame. These choices result in differentpalaeolatitudes and can cause other differences.There have also been improvements in ourknowledge of global plate motions since theearlier regional reconstructions. However, inmany cases the principal differences betweenthe different models result from different inter-pretations of geological data.

This paper gives an account of a plate tectonicmodel for the Cenozoic development of the re-gion based on my interpretations of a largerange of geological data. It summarises the re-gional tectonic development of SE Asia using aplate model which has been animated using 1Ma time-slices. Below is a brief account of themodel and its major features, which is followedby a discussion of its principal implications forbiogeographers relating to the distribution ofland and sea during the last 30 million years.

Reconstructions

The model discussed here includes that devel-oped earlier for SE Asia (Hall, 1996) which hasbeen extended to include the SW Pacific (Hall,1997). Reconstructions of SE Asia and the SWPacific (Fig.5) shown on a global projection arepresented at 10 Ma intervals for the period 50-10Ma (Figs.6-10). The reader is referred to Hall(1995, 1996) for a more complete account of theassumptions and data used in reconstructions ofSE Asia and for maps showing only SE Asia butwith more detail.

106 R. Hall

50 MaEnd EarlyEocene

INDIA

AUSTRALIA

PACIFICPLATE

INDIANPLATE

AUSTRALIANPLATE

EURASIA

40oN

20oN

20oS

40oS

60oS

90oE 180oE

?

ANTARCTICA

Fig.6. Reconstruction of the region at 50 Ma. The possible extent of Greater India and the Eurasian margin north of India areshown schematically. Shortly before 50 Ma collision between the north Australian continental margin and an island arc hademplaced ophiolites on the north New Guinea margin, and in New Caledonia, eliminating ocean crust formed at the formerAustralian-Indian ocean spreading centre. Double black arrows indicate extension in Sundaland.

Configuration at 50 Ma

At 50 Ma (Fig.6) India and Australia were sepa-rate plates although their motions were notgreatly different. Transform faults linked theslow-spreading Australia-Antarctic and the fastspreading India-Australia spreading centres.Some of the ophiolites of Sulawesi probablyformed at the India-Australia mid-ocean ridge.

India collided with Asia in the early Tertiary butthere remains considerable controversy aboutthe exact age of collision, and its consequences(Packham, 1996; Rowley, 1996). The position ofthe Eurasian margin and the extent of GreaterIndia are major problems. The reconstructionshown in Fig.6 shows a conservative estimateand, since India-Asia collision began at about 50Ma, this implies that the Asian margin extended


south to at least 30oN. Many of the tectonicevents in SE Asia are commonly attributed to theeffects of Indian indentation into Asia and thesubsequent extrusion of continental fragmentseastwards along major strike-slip faults. Despitethe great attraction of this hypothesis and thespectacular evidence of displacements on theRed River fault (Tapponnier et al., 1990) thepredictions of major rotations, southeastwardextrusion of fragments, and the timing of events(Tapponnier et al., 1982), remain poorly sup-ported by geological evidence in SE Asia.

The east Eurasian continental margin was ori-ented broadly NE-SW. From Japan northwardsAsia was bounded by an active margin. Taiwan,Palawan and the now extended crust of theSouth China Sea margins formed a passive mar-gin, established during Cretaceous times. Sunda-land was separated from Eurasia by a wideproto-South China Sea probably floored byMesozoic ocean crust. The southern edge of thisocean was a passive continental margin north ofa continental promontory extending from Bor-neo to Zamboanga. The Malay peninsula wascloser to Indochina and the Malay-Sumatra mar-gin was closer to NNW-SSE. Because rotation ofBorneo is part of this model the reconstructiondiffers from those of Rangin et al. (1990) andDaly et al. (1991) who infer a margin orientedcloser to E-W. I see no evidence to support thealmost E-W orientation of the Sundaland marginin the region of Sumatra as shown on these andmany other reconstructions (e.g. Briais et al.,1993; Hutchison, 1996). Furthermore, suchmodels have major difficulties in explaining theamount, timing and mechanism of rotation re-quired to move Sumatra from an E-W to NW-SEorientation. West Sumatra includes arc andophiolitic material accreted in the Cretaceous.East Borneo and West Sulawesi appear to beunderlain by accreted arc and ophiolitic materialas well as continental crust which may be early-rifted Gondwana fragments. This material hadbeen accreted during the Cretaceous and mayhave resulted in a highly thickened crust in thispart of Sundaland, possibly sustained by sub-duction.

Australia was essentially surrounded by pas-sive margins on all sides. To the west the pas-sive margin was formed in the Late Jurassic, andFig.6 postulates a failed rift, possibly floored byoceanic crust on the site of the present-dayBanda Sea, partially separating the Birds Headmicrocontinent from Australia. Mesozoic oce-anic lithosphere was present north of the BirdsHead, south of the active Indian-Australian

spreading centre. Further east in the Pacific, In-dian and Australian oceanic lithosphere hadbeen subducting northwards beneath the Sepik-Papuan arc in the early Tertiary. During thePaleocene and early Eocene the New GuineaMesozoic passive margin collided with this intra-oceanic arc causing emplacement of the Sepikand Papuan ophiolites (Davies, 1971). Subse-quently, most of the New Guinea margin was apassive margin during the Paleogene but theoceanic crust to the north is inferred to haveformed during the Mesozoic in an intra-oceanicmarginal basin behind the Sepik-Papuan arc.The position and character of the east Australia-Pacific margin is also uncertain. Tasman andCoral Sea opening had probably been driven bysubduction but the site of subduction must havebeen considerably east of the Australian conti-nent, beyond the Loyalty Rise and New Caledo-nia Rise. Spreading had ceased in both basins byabout 60 Ma (Paleocene). By the Paleocene itappears that subduction east of New Caledoniawas to the east not to the west (Aitchison et al.,1995). The history of this region remains poorlyknown since it is almost entirely submarine, andmagnetic anomalies in this area are poorly de-fined.

Java and West Sulawesi were situated above atrench where Indian plate lithosphere wassubducting towards the north. The character ofthis boundary is shown as a simple arc but mayhave included marginal basins and both strike-slip and convergent segments depending on itslocal orientation. Extending plate boundariesinto the Pacific is very difficult. A very large areaof the West Pacific has been eliminated by sub-duction since 50 Ma which will continue tocause major problems for reconstructions. How-ever, there is clear evidence that this area resem-bled the present-day West Pacific in containingmarginal basins, intra-oceanic arcs and subduc-tion zones. The Java subduction system linkedeast into Pacific intra-oceanic subduction zonesrequired by the intra-oceanic arc rocks withinthe Philippine Sea plate; parts of the east Philip-pines, the West Philippine basin and Halmaherainclude arc rocks dating back at least to the Cre-taceous. North of the Philippine Sea plate therewas a south-dipping subduction zone at thesouthern edge of a Northern New Guinea plate.

50-40 Ma

Whatever the timing of India-Asia collision, aconsequence was the slowing of the rate of

108 R. Hall

plate convergence after anomaly C21 and a ma-jor change in spreading systems betweenanomaly C20 and C19 at about 42 Ma. India andAustralia became one plate during this period(Figs.6 and 7) and the ridge between them be-came inactive. Northward subduction of Indian-Australian lithosphere continued beneath theSunda-Java-Sulawesi arcs although the directionof convergence may have changed. Rift basinsformed throughout Sundaland, but the timing oftheir initial extension is uncertain because theycontain continental clastics which are poorlydated, and their cause is therefore also uncer-tain. They may represent the consequences ofoblique convergence or extension due to re-laxation in the over-riding plate in response toIndia-Asia collision, enhanced by slowing ofsubduction, further influenced by older struc-tural fabrics.

The Java-Sulawesi subduction system contin-ued into the West Pacific beneath the east Phil-ippines and Halmahera arcs. Further east, thedirection of subduction was southward towardsAustralia and this led to the formation of aMelanesian arc system. During the Eocene theextended eastern Australasian passive marginhad collided with the intra-oceanic arc alreadyemplaced in New Guinea resulting in emplace-ment of the New Caledonia ophiolite (Aitchisonet al., 1995; Meffre, 1995) followed by subduc-tion polarity reversal. Subduction began be-neath Papua New Guinea with major arc growthproducing the older parts of the New Britain,Solomons and Tonga-Kermadec systems, lead-ing to development of major marginal basins inthe SW Pacific whose remnants probably sur-vive only in the Solomon Sea. This model postu-lates the initial formation of these arcs at thePapuan-east Australian margin as previouslysuggested by Crook and Belbin (1978) followingsubduction flip, rather than by initiation of intra-oceanic subduction within the Pacific plate out-board of Australia as suggested by Yan andKroenke (1993). The evidence for either pro-posal is limited but this model has the simplicityof a single continuous Melanesian arc.

During this interval there were major changesin the Pacific. The Pacific plate is widely said tohave changed its motion direction at 43 Ma,based on the age of the bend in the Hawaiian-Emperor seamount chain, although this viewhas recently been challenged by Norton (1995)who attributes the bend to a moving hotspotwhich became fixed only at 43 Ma. Subductionof the Pacific-Northern New Guinea ridge(Fig.7) led to massive outpouring of intra-oce-

anic volcanic rocks (Stern and Bloomer, 1992)which formed the Izu-Bonin-Mariana arc sys-tem, and the Philippine Sea plate was a recog-nisable entity by the end of this period. Therewas significant rotation of the Philippine Seaplate between 50 and 40 Ma and the motion his-tory of this plate (Hall et al., 1995) provides animportant constraint on development of theeastern part of SE Asia. The West Philippine ba-sin, Celebes Sea, and Makassar Strait opened assingle oceanic basin within the Philippine Seaplate although the reconstructions probably un-derestimate the width of the Makassar Strait andCelebes Sea, which may have been partiallysubducted in the Miocene beneath westSulawesi.

The opening of the West Philippine-CelebesSea basin required the initiation of southwardsubduction of the proto-South China Sea be-neath Luzon and the Sulu arc. It is this subduc-tion which caused renewed extension along theSouth China margin, driven by slab-pull forcesdue to subduction between eastern Borneo andLuzon, and later led to sea-floor spreading in theSouth China Sea, rather than indentor-driventectonics.

40-30 Ma

In this interval (Figs.7 and 8) the spreading ofthe marginal basins of the West and SW Pacificcontinued. Indian ocean subduction continuedat the Sunda-Java trenches, and also beneath thearc extending from Sulawesi through the eastPhilippines to Halmahera. Sea floor spreadingcontinued in the West Philippine-Celebes Seabasin until about 34 Ma. This spreading centremay been linked to backarc spreading of theCaroline Sea which formed from about 40 Madue to subduction of the Pacific plate. TheCaroline Ridge is interpreted in part as a rem-nant arc resulting from Caroline Sea backarcspreading, and the South Caroline arc ultimatelybecame the north New Guinea arc terranes. By30 Ma the Caroline Sea was widening above asubduction zone at which the newly-formedSolomon Sea was being destroyed as theMelanesian arc system migrated north. Thebackarc basins in the SW Pacific were probablyvery complex, as indicated by the anomalies inthe South Fiji basin, and will never be com-pletely reconstructed because most of these ba-sins have been subducted.

The Philippines-Halmahera arc was station-ary, so spreading in the West Philippine-Celebes


40 MaMiddle Eocene

INDIA

AUSTRALIA

PACIFICPLATE

INDIANPLATE

EURASIA

40oN

20oN

20oS

40oS

60oS

90oE 180oE

ANTARCTICA

Fig.7. Reconstruction of the region at 40 Ma. India and Australia were now parts of the same plate. An oceanic spreading centrelinked the north Makassar Strait, the Celebes Sea and the West Philippine basin. Spreading began at about this time in theCaroline Sea, separating the Caroline Ridge remnant arc from the South Caroline arc. Spreading also began after subduction flipin marginal basins around eastern Australasia producing the Solomon Sea and the island arcs of Melanesia.

Sea basin maintained subduction between NEBorneo and north of Luzon. The pull forces ofthe subducting slab therefore account forstretching of the Eurasian margin north ofPalawan, and later development of oceanic crustin the South China Sea which began by 32 Ma.In contrast, the indentor model does not ac-count for stretching at the leading edge of theextruded blocks, such as Indochina, or the nor-

mal faulting east of Vietnam often shown askinematically linked to the Red River fault sys-tem. There was approximately 500-600 km left-lateral movement on the Red River fault (Briaiset al., 1993) during the extrusion of Indochina(32-15 Ma).

The dextral Three Pagodas and Wang Chaofaults are simplified as a single fault at the northend of the Malay peninsula. There are a host of

110 R. Hall

30 MaMid Oligocene

INDIA

AUSTRALIA

PACIFICPLATE

INDIANPLATE

EURASIA

40oN

20oN

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180oE

Fig.8. Reconstruction of the region at 30 Ma. Indentation of Eurasia by India led to extrusion of the Indochina block bymovement on the Red River fault and Wang Chao-Three Pagodas (WC-TP) faults. Slab pull due to southward subduction of theproto-South China Sea caused extension of the South China and Indochina continental margin and the present South China Seabegan to open. A wide area of marginal basins separated the Melanesian arc from passive margins of eastern Australasia, shownschematically between the Solomon Sea and the South Fiji basin.

faults through this region, and a plate tectonicmodel can only oversimplify the tectonics of thecontinental regions by considering large andsimple block movements and broadly predictingregional stress fields. The implication of this sim-plified model is that basins such as the Malayand Gulf of Thailand basins have a significantcomponent of strike-slip movement on faultscontrolling their development. However, they

may have been initiated in a different tectonicsetting, in which a pre-existing structural fabricinfluenced their development (Hutchison, 1996).

30-20 Ma

This period of time (Figs.8 and 9) saw the mostimportant Cenozoic plate boundary reorganisa-


tion within SE Asia. At about 25 Ma, the NewGuinea passive margin collided with the leadingedge of the east Philippines-Halmahera-NewGuinea arc system. The Australian margin, in theBirds Head region, was also close to collisionwith the Eurasian margin in West Sulawesi andduring this interval ophiolite was emplaced inSulawesi.

By 30 Ma the Sulawesi margin may have beencomplex and included ocean crust of differenttypes (mid-ocean ridge, backarc basin). Thusthe Sulawesi ophiolite probably includes mate-rial formed within the Indian Ocean (Mubroto etal., 1994) as well as ocean basins marginal toEurasia (Monnier et al., 1995). The arrival of theAustralian margin at the subduction zonecaused northward subduction to cease. Theocean crust trapped between Sulawesi andHalmahera first became part of the PhilippineSea plate and later the Molucca Sea plate. ThePhilippine Sea plate began to rotate clockwiseand the trapped ocean crust began to subductbeneath Sulawesi in the Sangihe arc.

Soon afterwards the Ontong Java plateau col-lided with the Melanesian arc. These two majorcollisions caused a significant change in thecharacter of plate boundaries in the region be-tween about 25 and 20 Ma (Early Miocene).They also linked the island arcs of Melanesia tothe New Guinea terranes at the southern marginof the Caroline plate, and to the Halmahera-Philippines arcs. This linkage seems to havecoupled the Pacific to the marginal basins of theWest Pacific, and the Caroline and PhilippineSea plates were subsequently driven by the Pa-cific. Both began to rotate, almost as a singleplate, and the Izu-Bonin-Mariana trench systemrolled back into the Pacific. Rifting of the Palau-Kyushu ridge began, leading first to opening ofthe Parece Vela basin and later to spreading inthe Shikoku basin. The change in plate bounda-ries led to subduction beneath the Asian margin.

Subduction beneath the Halmahera-Philip-pines arc ceased and the New Guinea sector ofthe Australian margin became a strike-slip zone,the Sorong fault system, which subsequentlymoved terranes of the South Caroline arc alongthe New Guinea margin.

Advance of the Melanesian arc system led towidening of the South Fiji basin and SolomonSea basin (now mainly subducted). At the ThreeKings Rise subduction seems to have been initi-ated soon after ocean crust was formed to theeast, allowing the rise to advance east andspreading to propagate behind the rise into theNorfolk basin from a triple junction to the north.

20-10 Ma

The clockwise rotation of the Philippine Seaplate necessitated changes in plate boundariesthroughout SE Asia which resulted in the tec-tonic pattern recognisable today (Figs.9 and 10).These changes include the re-orientation ofspreading in the South China Sea, and the devel-opment of new subduction zones at the easternedge of Eurasia and in the SW Pacific. Contin-ued northward motion of Australia caused thecounter-clockwise rotation of Borneo. NorthernBorneo is much more complex than shown.There was volcanic activity and build-out ofdelta and turbidite systems into the proto-SouthChina Sea basin. Major problems include thesource of sediment in the basins surroundingcentral Borneo and the location and timing ofvolcanic activity in Borneo. The remaining oce-anic crust of the western proto-South China Sea,and thinned continental crust of the passivemargin to the north, was thrust beneath Borneothickening the crust, resulting in rapid erosionof sediments into the Neogene circum-Borneodeltas, and ultimately leading to crustal melting.

The rotation of Borneo was accompanied bycounter-clockwise motion of west Sulawesi, andsmaller counter-clockwise rotations of adjacentSundaland blocks. In contrast, the north Malaypeninsula rotated clockwise, but remainedlinked to both Indochina and the south Malaypeninsula. This allowed widening of basins inthe Gulf of Thailand, but the simple rigid platemodel overestimates the extension in this re-gion. This extension was probably more widelydistributed throughout Sundaland andIndochina on many different faults. The Burmaplate became partly coupled to the northward-moving Indian plate and began to move northon the Sagaing fault leading to stretching of theSunda continental margin north of Sumatra, andultimately to ocean crust formation in theAndaman Sea.

North Sumatra rotated counter-clockwise withsouth Malaya, and as the rotation proceeded theorientation of the Sumatran margin changedwith respect to the Indian plate motion vector.The consequent increase in the convergentcomponent of motion, taken up by subduction,may have increased magmatic activity in the arcand weakened the upper plate, leading to for-mation of the dextral Sumatran strike-slip faultsystem taking up the arc-parallel component ofIndia-Eurasia plate motion.

East of Borneo, the increased rate of subduc-tion caused arc splitting in the Sulu arc and the

112 R. Hall

20 MaEarly Miocene

AUSTRALIA

PACIFICPLATE

INDIANPLATE

EURASIA

40oN

20oN

20oS

40oS

60oS

90oE 180oE

ANTARCTICA

INDIA

Fig.9. Reconstruction of the region at 20 Ma. Collision of the north Australian margin in the region between the Birds Headmicrocontinent and eastern New Guinea occurred at about 25 Ma. The Ontong Java plateau arrived at the Melanesian trench atabout 20 Ma. These two events caused major reorganisation of plate boundaries. Subduction of the Solomon Sea began at theeastern New Guinea margin. Spreading began in the Parece Vela and Shikoku marginal basins. The north Australian marginbecame a major left-lateral strike-slip system as the Philippine Sea-Caroline plate began to rotate clockwise. Movement on splaysof 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 faultsystem was initiated.

Sulu Sea opened as a backarc basin (Hinz et al.,1991; Silver and Rangin, 1991) south of theCagayan ridge. The Cagayan ridge then movednorthwards, eliminating the eastern proto-SouthChina Sea, to collide with the Palawan margin.New subduction had also begun at the west

edge of the Philippine Sea plate below the northSulawesi-Sangihe arc which extended north tosouth Luzon. This was a complex zone of op-posed subduction zones linked by strike-slipfaults. The Philippine islands and Halmaherawere carried with the Philippine Sea plate to-


10 MaLate Miocene

AUSTRALIA

PACIFICPLATE

EURASIA

40oN

20oN

20oS

40oS

60oS

90oE 180oE

ANTARCTICA

INDIA

Fig.10. Reconstruction of the region at 10 Ma. The Solomon Sea was being eliminated by subduction beneath eastern new Guineaand beneath the New Hebrides arc. However, continued subduction led to development of new marginal basins within the period10-0 Ma, including the Bismarck Sea, Woodlark basin, North Fiji basins, and Lau basin. The New Guinea terranes, formed in theSouth Caroline arc, docked in New Guinea but continued to move in a wide left-lateral strike-slip zone. Further west, motion onstrands of the Sorong fault system caused the arrival of the Tukang Besi and Sula fragments in Sulawesi. Collision events at theEurasian 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 wascomplete but motion of the Sumatran forearc slivers was associated with new spreading in the Andaman Sea.

wards this subduction zone. North of Luzon, sin-istral strike-slip movement linked thesubducting west margin of the Philippine Seaplate to subduction at the Ryukyu trench. Colli-sion of Luzon and the Cagayan ridge with theEurasian continental margin in Mindoro and

north Palawan resulted in a jump of subductionto the south side of the Sulu Sea. Southwardsubduction beneath the Sulu arc continued until10 Ma. The remainder of the Philippines contin-ued to move with the Philippine Sea plate, pos-sibly with intra-plate strike-slip motion and sub-

114 R. Hall

duction resulting in local volcanic activity. At theeast edge of the Philippine Sea plate spreadingterminated in the Shikoku basin.

As a result of the change in plate boundaries,fragments of continental crust were emplaced inSulawesi on splays at the western end of theSorong fault system. The earliest fragment tocollide is inferred to have been completelyunderthrust beneath West Sulawesi and contrib-uted to later crustal melting (Polvé et al., 1997).Later, the Tukang Besi platform separated fromthe Birds Head and was carried west on thePhilippine Sea plate to collide with Sulawesi.Locking of splays of the Sorong fault causedsubduction to initiate at the eastern margin ofthe Molucca Sea, thus producing the NeogeneHalmahera arc. In this way the Molucca Sea be-came a separate plate as the double subductionsystem developed.

After the collision of the Ontong Java plateauwith the Melanesian arc the Solomons becameattached to the Pacific plate. Westward subduc-tion began on the SW side of Solomon Sea, be-neath eastern New Guinea, eliminating most ofSolomon Sea and resulting in the formation ofMaramuni arc system. As the Solomon Sea waseliminated, the South Caroline arc began to con-verge on the north New Guinea margin and thearc terranes were translated west in the majorleft-lateral shear zone, probably accompaniedby some rotation. In the southern part of theSolomons Sea subduction was in the oppositedirection (eastward) and created the New Hebri-des arc system. Spreading ceased in the SouthFiji basin.

10-0 Ma

At the beginning of this period SE Asia waslargely recognisable in its present form (Fig.10).Rotation of Borneo was complete. This, withcollision in the central Philippines and Mindoro,and continued northward movement of Aus-tralia, resulted in reorganisation of plateboundaries and intra-plate deformation in thePhilippines. The Luzon arc came into collisionwith the Eurasian margin in Taiwan. This maybe the cause of the most recent regional changein plate motions at about 5 Ma. The PhilippineSea plate rotation pole moved north from a po-sition east of the plate; clockwise rotation con-tinued but the change in motion caused re-ori-entation of existing, and development of new,plate boundaries. Subduction continued at theManila, Sangihe and Halmahera trenches, and

new subduction began at the Negros and Philip-pine trenches. These subduction zones werelinked by strike-slip systems active within thePhilippines, and this intra-plate deformation cre-ated many very small fragments which are diffi-cult to describe using rigid plate tectonics.

The Molucca Sea continued to close by sub-duction on both sides. At present the Sangiheforearc has overridden the northern end of theHalmahera arc, and is beginning to over-thrustwest Halmahera. In the Sorong fault zone, accre-tion of Tukang Besi to Sulawesi locked a strandof the fault and initiated a new splay south ofthe Sula platform. The Sula platform then col-lided with the east arm of Sulawesi, causing ro-tation of the east and north arms to their presentposition, leading to southward subduction ofthe Celebes Sea at the north Sulawesi trench.

The Eurasia-Philippine Sea plate-Australia tri-ple junction was and remains a zone of micro-plates but within this contractional setting newextension began in the Banda Sea. The BirdsHead moved north relative to Australia along astrike-slip fault at the Aru basin edge. Mesozoicocean crust north of Timor was eliminated at theeastern end of the Java trench by continuednorthern motion of Australia which brought theAustralian margin into this trench as the volcanicinner Banda arc propagated east. Seram beganto move east requiring subduction and strike-slip motion at the edges of this microplate. Since5 Ma the southern Banda Sea has extended to itspresent dimensions, and continental fragmentsare now found in the Banda Sea ridges withinyoung volcanic crust. The Banda Sea is here in-terpreted to be very young as suggested byHamilton (1979) and others.

In west Sundaland, partitioning of conver-gence in Sumatra into orthogonal subductionand strike-slip motion effectively establishedone or more Sumatran forearc sliver plates. Ex-tension on the strike-slip system linked to thespreading centre in the Andaman Sea (Curray etal., 1979). Within Eurasia reversal of motion onthe Red River system may have been one conse-quence of the regional change in plate motions.

Opening of the Ayu trough separated theCaroline plate and Philippine Sea plate, al-though the rate of separation at this spreadingcentre was very low. North of the Birds Head,and further east in New Guinea, transpressionalmovements were marked by deformation of arcand ophiolite slivers separated by sedimentarybasins. Progressive westward motion of theSouth Caroline arc within the left-lateral trans-pressional zone led to docking of the north New


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Fig.11. Postulated distribution of land and sea in SE Asia at 30 Ma. No attempt has been made to represent topography with Asiaand Indochina. Much of the area north and east of the Indian collision zone must have been highlands.

Guinea terranes. This caused the cessation ofsouthward subduction of the Solomon Sea platebut resulted in its northward subduction be-neath New Britain. The New Britain subductionled to rapid spreading in Woodlark basin as aconsequence of slab-pull forces and rapid rip-ping open of continental crust beneath thePapuan peninsula. Elimination of most of theremaining Solomons marginal basin by east-ward subduction led to formation of the NewHebrides arc and ocean crust formation in theNorth Fiji basins.

Determining the extents of land and sea

For the biogeographer, the tectonic develop-ment of the region is only a starting point forunderstanding. In order to understand the distri-bution of most organisms it is also necessary to

know where there was land and sea, where thesea was shallow and deep, and how wide werethe seas. For the land, there needs to be someknowledge of topography, particularly wherethere were mountainous regions. The distribu-tion and character of land and sea will have pro-vided physical pathways and barriers to disper-sal, and may well have influenced plant and ani-mal distribution by effects on other controllingfactors such as local and global climate, oceaniccirculation patterns, and sea-level.

However, moving from tectonic reconstruc-tion maps to detailed palaeogeographical mapsinvolves further complexities. In many ways thegeological record is a marine record. Most ofEarth history is recorded in rocks deposited atthe surface, and the areas where most sedimentsare deposited are close to or below sea-level,and mainly at continental margins. Dating ofrocks is largely based on fossils, and marine or-

116 R. Hall

25 MaEnd Oligocene

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SHALLOW SEA

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Fig.12. Postulated distribution of land and sea in SE Asia at 25 Ma.

ganisms generally provide the fossils of greatestbiostratigraphic value which usually providesome insight into the environment of deposi-tion. Geologists are therefore usually able to re-construct the history of marine areas. In thedeep oceans sedimentary rocks may lack fossilsbut the history of sediments deposited on oceancrust is known because ocean crust subsideswith age due to lithospheric cooling and age-depth relationships are well established. Thus,many postulated land-bridges in oceanic re-gions can be dismissed with some confidence.

In contrast, mapping environments and phys-iography of former land areas is a great dealmore difficult. Uplift, erosion and periods ofemergence are mainly recorded by negative ev-idence, such as unconformities and stratigraphicincompleteness. Even when there is a rockrecord it will often be difficult to date becausesediments deposited on land typically represent

restricted types of environments, and usuallycontain few fossils which have limited biostrati-graphic value. Unlike marine fossils, fossil as-semblages from land rarely yield informationabout the history of their enclosing sedimentsrelative to sea-level.

However, there are ways to solve some ofthese problems, and mapping palaeogeographyonto the reconstructions is not, in principle, im-possible although much of the information re-quired is not yet available. It is possible to iden-tify the positions of former coastlines, interpretthe location of former river systems, and indi-rectly infer areas of mountains. In SE Asia someof the information can be compiled from the lit-erature; an attempt to do this for the region ofWallaces Line is discussed by Moss and Wilson(1998, this volume). Some data, for example lo-cation of former coastlines, could be determinedfrom records of oil companies acquired during


extensive seismic surveys of SE Asia for hydro-carbons. New research could provide furtherdetail and biogeographers themselves couldalso contribute by, for example, mapping distri-butions of fossil plants and interpreting theirenvironments.

Land and sea for 30-0 Ma

Figs.11 to 16 are an attempt to compile the gen-eral features of land and sea onto maps of thetectonic reconstructions showing 5 million yearintervals between 30 and 5 Ma for the region ofSE Asia. The maps may be useful in indicatingthe likely geographical connections and barriersand the periods when these were in existence.There are few studies that compile this type ofinformation and all cover limited parts of areafor limited times. Thus these maps are based on

those few sources, some proprietary informationfrom oil companies, and a wide range of litera-ture and maps. The sources are too numerous tocite and the quality of coverage is very variable.The task is a very large one, given the size of thearea, and the results should therefore be regard-ed as a first order approximation only. I havenot attempted to draw palaeogeographical mapsfor periods before 30 Ma. The period 30-0 Ma isof most interest to biogeographers; before thenthe separation between Asia and Australia wasgreater and the tectonic reconstructions are alsomore uncertain.

The limited ranges of environments and dis-tributions shown are best estimates. Broadlyspeaking, each area shown should be regardedas a probability. For example, for an area shownas deep marine, the probability of that area be-ing shallow marine is low, and of it being land isvery low. Some of the assignments are educated

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20 MaEarly Miocene

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118 R. Hall

guesses. For example, areas of long-lived islandarcs develop thickened crust, implying relativeshallow water areas and local emergence. Whenvolcanoes are active, magma production, ther-mal expansion and crustal buoyancy can lead toemergence but individual volcanoes can be veryshort-lived on a geological time scale (typicallyless than one million years) even though an arcmay have been a long-lived feature. It is usuallynot possible to identify precisely which areaswere emergent, simply that there are likely tohave been such areas.

The mid Oligocene (Fig.11) was the time of amajor fall in global sea-level (Haq et al., 1987).Very large areas of Sundaland and Sunda shelfwere exposed and there were probably moreemergent areas than at any subsequent time un-til the end of the Cenozoic. North of Sundaland,Asia was a persistent highland area, and largeamounts of sediment moved south from central

Asia down major river systems. Much of south-ern Sundaland was the site of deposition of allu-vial, fluvial and deltaic sediments. There weremajor embayments in the eastern Asian marginformed by the South China Sea, the proto-SouthChina Sea and the Celebes Sea-Makassar Strait.Separating these were elongate bathymetric fea-tures which were probably mainly shallow wa-ter with intermittent emergent areas, notablywhere arc volcanoes were active. The southern-most promontory was the Sulawesi-Philippines-Halmahera arc which could have provided apathway into the Pacific, via volcanic islandstepping stones, for organisms that could crossseawater. The other promontories terminated inthe deep ocean area of the Pacific.

At about 25 Ma (Fig.12) the north Australianmargin came into contact with Sulawesi and theHalmahera arc, and this could have created adiscontinuous land connection via the island

15 MaMiddle Miocene

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arcs of Halmahera and the Philippines intoSulawesi. The arc-continent collision closed thedeep water passage between the Pacific and In-dian oceans (Kennett et al. 1985) by about 20Ma (Fig.13) and there must have been majorchanges of oceanic currents (Fig.17 and 18) withimplications for the distribution of many marineorganisms, particularly those of shallow marineenvironments. North-central Borneo was up-lifted and shed huge volumes of sediments intothe deltas which formed in north and east Bor-neo.

From about this time there was probably al-ways some land in the area of Sulawesi, and theextensive but poorly dated Celebes molasse(Kündig, 1956) represents the products of sub-aerial erosion, although there were no perma-nent land links to Sundaland nor to Australia.However, there were intermittently emergent ar-eas between Australia and Sulawesi, and a broad

zone of shallow water within which there couldhave been numerous islands. Furthermore,strike-slip fault movements led to the arrival ofnumerous fragments of continental crust inSulawesi, sliced from the Birds Headmicrocontinent. The northern Makassar Strait re-mained a deep water area, and presumablyformed a barrier to migration for many plant andanimals (Moss and Wilson, 1998 this volume).

From 15 Ma to 5 Ma (Figs.14, 15, 16) was aperiod in which emergent Sundaland reduced inarea, while the deep marginal basins in the eastwere eliminated (proto-South China Sea) or re-duced in size (Sulu, Celebes and Molucca Sea).Local collision and volcanic arc activity led tointermittent emergence in many of the arc re-gions but these probably always resembled thepresent Philippine and North Molucca arcs, withland separated by sea, which could locally havebeen quite deep. More of Borneo became emer-

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10 MaLate Miocene

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120 R. Hall

gent and the central mountains on the Sarawak-Kalimantan border extending into Sabah be-came wider and higher with time. It is importantto be aware that within this convergent settingdeep basins also formed (e.g., Sulu Sea, BandaSea) which must have represented new barriersto dispersal which formed at the same time asnew land pathways were established.

Conclusions

There are three important periods in regionaldevelopment. At about 45 Ma plate boundarieschanged, probably as a result of India-Asia colli-sion. From a biogeographical viewpoint the ar-rival of India would have led to a movement ofGondwana plants and animals into Asia. Moun-tain building resulting from the collision led tomajor changes in habitats, and climate, accom-

panied by changes in land area and drainagesystems. Huge volumes of sediment began tomove south from central Asia into the sedimen-tary basins of the Sunda shelf. Ultimately all thiswould have driven dispersal from Gondwanavia India into SE Asia (e.g. Harley and Morley,1995), and later speciation centred in Sundalandwhich for many organisms became separatedfrom Asia by climate and topography, andwhich remained separated from Australia bymarine barriers. Continued indentation of Asiaby India modified the Eurasian continent butmuch more knowledge is required of the timingof fault movements and the amounts ofdisplacements before Sundaland can be ad-equately understood. The deformation withinAsia and Sundaland is likely to have led to theformation of geographical barriers, principallymountains, some of which were associated withstrike-slip faults and geologically short-lived.

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Fig.17. Circulation patterns of surface and near-surface wa-ters in the Pacific ocean inferred by Kennett et al. (1985) atthree stages during the Neogene as the Indonesian sea-wayclosed. Black arrows indicate cold currents and unfilled ar-rows indicate warm currents

Fig.18. Possible circulation patterns of surface and near-sur-face waters in eastern Indonesia shown on the tectonic re-constructions of this paper. The currents postulated arebased on Kennett et al. (1985) and present-day circulationpatterns (Fine et al., 1994).

20 Ma

South Equatorial Current

30 Ma

Indonesian Seaway

South Equatorial Current

10 MaEquatorial

Undercurrent

8 Ma

16 Ma

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122 R. Hall

The second major period is around 25 Mawhen plate boundaries and motions changedagain, partly due to collision between the northAustralian margin and arcs to the north. This,together with collision of the Melanesian arcsand the Ontong Java plateau, changed the tec-tonics of the oceanic-arc region east of Asia(Philippines, Celebes Sea, Sulu Sea, PhilippineSea, Caroline Sea, north New Guinea, New Brit-ain, Solomons, Tonga). The 25 Ma event wasprobably the most important tectonic eventfrom the biogeographical point of view as it ledto new, albeit discontinuous, links betweenAustralia via Sulawesi into SE Asia across areaswhich were mainly shallow marine and locallyincluded land. It also resulted in a very long dis-continuous island arc link between Asia andMelanesia. However, as the pathways betweenAustralia and Sundaland came into existence,new barriers also formed. The central Borneomountains began to rise in the early Mioceneand became a regional drainage divide sendingsediment north into the Sarawak basins andBaram delta, and southeast into the Tarakan andMahakam deltas. North of Borneo, as the proto-South China Sea closed, the Oligo-MioceneSouth China Sea widened and the Sulu Seaopened. As the distance between Australia andSulawesi closed, the deep Banda Sea opened.Thus, movement of plants and animals betweenAustralia and Sundaland would have remaineddifficult. Perhaps it was this zone of barriers,close to a region of deep and former deepocean barriers separating Borneo and Australia,which is the origin of Wallaces line. The narrowMakassar Strait, which at its south end termi-nates in a long-lived discontinuous carbonateplatform, could not alone have been a majorbarrier to dispersal.

Plate motions and boundaries changed againat about 5 Ma, possibly as a consequence of arc-continent collision in Taiwan, and in the last 5Ma there has been renewed tectonic activity anda significant increase in land and highlands allround the margins of SE Asia. A number of newdispersal pathways developed across the re-gion, for example those linking Taiwan andNew Guinea through the Philippines and NorthMoluccas, and connecting New Guinea to Thai-land via the Banda and Sunda arcs. It is alsoprobable that there was an increase in the rangeof habitats along these routes, due to elevationof mountains, and likely associated variations inrainfall.

Disentangling the contribution of geology tobiogeographic patterns is not simple. Geology

and tectonics could be a controlling factor insome cases. Cicada distributions in New Guineasuggest a geological control (Boer and Duffels,1996), and slicing of crustal fragments from theBirds Head could have caused influxes of fau-nas and floras into Sulawesi from Australia atintervals in the last 20 Ma. However, geologyand tectonics also influence other variableswhich are more subtle controls on biogeo-graphic patterns. Sea-level, elevation of land ar-eas, soil, wind and water movements, and cli-mate are all examples of factors upon whichthere is some geological influence. Climatic con-trols are too difficult to model at present, but atsome time in the future it will be possible to usethe tectonic models as the basis for simulation ofancient climates in SE Asia. It is notable that atpresent there are more highland areas, and agreater area of land than at any time during thelast 30 million years. This is consistent withrather restricted areas of modern carbonate plat-forms which are limited in part by clastic sedi-ment influx. The present distribution and size ofshallow water carbonate areas may in part re-flect a period of relatively low sea-level, but alsorecord the recent rise of mountains due to tec-tonic forces as the region is compressed be-tween Asia and Australia.

Some of the biogeographic patterns in SE Asiaat present are difficult to relate simply to geol-ogy, for example, the distance between Borneoand Sulawesi (Wallaces line and equivalents)should have been as easy to cross as the barriersbetween Australia and Sulawesi. This raises thequestion of the longevity of biogeographic pat-terns, about which we currently lack adequateinformation. During the last million years therehave been periods of low sea-level associatedwith glacial intervals when far greater areas ofland were emergent than at present, and thepresent areas are significantly greater than thoseduring the Neogene. Much of the Sunda shelfwould have been emergent although in easternIndonesia there are many narrow deep waterareas (such as the Makassar Strait) which wouldhave remained physical barriers. However, else-where large sea-level falls would have separatedsome formerly connected ocean basins as shal-low water areas became emergent, changingoceanic circulation patterns and modifyingweather and climate (e.g. Huang et al., 1997).Fluctuations in temperatures and rainfall arelikely to have been more extreme at intervals inthe last million years than in the preceding 30million years. Therefore, the last period of geo-logical history, perhaps one million years or


even much less, may have had a far greater in-fluence on biogeographic patterns than themuch longer period before.

To go further, detailed maps of land and sea,and palaeo-topography must be compiled frompublished maps and papers, and unpublishedcoastline, shelf edge, age and lithofacies infor-mation, much in oil company files. In particularthe display of uplift and subsidence, and timingof magmatic events, on tectonic reconstructionswould help in identifying underlying processesand give more confidence in mapping land andsea into areas where there is little direct evi-dence. Biogeographers must contribute, for ex-ample, distributions of fossil plants can provideinformation on palaeo-temperatures and envi-ronments. There is a need to focus on biogeo-graphic patterns which are most likely to revealthe links to geology using plants and animalswhich have difficulty in dispersing, and forwhich non-geological controls are unimportant.It is for the biogeographers to identify such criti-cal floral and faunal indicators. We still knowlittle about rates of speciation and dispersal, andfor most animals and plants the fossil record ispoor or non-existent. DNA studies offer oneway of determining a time-scale for biologicaldevelopment which could contribute to an ex-planation of biogeographic patterns. Anotherway forward is mathematical simulation of thebiological variables, testing biogeographic pat-terns against predictions. It is certain that no sin-gle factor will account for the distribution ofplants and animals in SE Asia; tectonic move-ments may be a control but their importance isstill far from clear.

Acknowledgements

Financial support has been provided by NERC,the Royal Society, the London University CentralResearch Fund, and the London University SEAsia Research Group currently supported byArco, Canadian Petroleum, Exxon, Lasmo,Mobil, Union Texas and Unocal. Work in Indo-nesia has been facilitated by GRDC, Bandungand Directors including H. M. S. Hartono, M.Untung, R. Sukamto and I. Bahar. I am gratefulto Moyra Wilson, Steve Moss and Alistair Fraserfor information. Clive Burrett, Bob Musgrave,Gordon Packham and Rupert Sutherland madehelpful comments on the reconstructions andthe manuscript. I thank Kevin Hill for consider-able help and discussion in reconstructing theNew Guinea and the SW Pacific.

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Colour plates for:

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

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

Captions

Fig.6. Reconstruction of the region at 50 Ma. The possible extent of Greater India and the Eurasian margin north of India areshown schematically. Shortly before 50 Ma collision between the north Australian continental margin and an island arc hademplaced ophiolites on the north New Guinea margin, and in New Caledonia, eliminating ocean crust formed at the formerAustralian-Indian ocean spreading centre. Double black arrows indicate extension in Sundaland.Fig.7. Reconstruction of the region at 40 Ma. India and Australia were now parts of the same plate. An oceanic spreading centrelinked the north Makassar Strait, the Celebes Sea and the West Philippine basin. Spreading began at about this time in theCaroline Sea, separating the Caroline Ridge remnant arc from the South Caroline arc. Spreading also began after subduction flipin marginal basins around eastern Australasia producing the Solomon Sea and the island arcs of Melanesia.Fig.8. Reconstruction of the region at 30 Ma. Indentation of Eurasia by India led to extrusion of the Indochina block bymovement on the Red River Fault and Wang Chao-Three Pagodas (WC-TP) Faults. Slab pull due to southward subduction ofthe proto-South China Sea caused extension of the South China and Indochina continental margin and the present South ChinaSea 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.Fig.9. Reconstruction of the region at 20 Ma. Collision of the north Australian margin in the region between the Birds Headmicrocontinent and eastern New Guinea occurred at about 25 Ma. The Ontong Java plateau arrived at the Melanesian trenchat about 20 Ma. These two events caused major reorganisation of plate boundaries. Subduction of the Solomon Sea began atthe eastern New Guinea margin. Spreading began in the Parece Vela and Shikoku marginal basins. The north Australian marginbecame a major left-lateral strike-slip system as the Philippine Sea-Caroline plate began to rotate clockwise. Movement onsplays 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 Faultsystem was initiated.Fig.10. Reconstruction of the region at 10 Ma. The Solomon Sea was being eliminated by subduction beneath eastern new Guineaand beneath the New Hebrides arc. However, continued subduction led to development of new marginal basins within the period10-0 Ma, including the Bismarck Sea, Woodlark basin, North Fiji basins, and Lau basin. The New Guinea terranes, formed in theSouth Caroline arc, docked in New Guinea but continued to move in a wide left-lateral strike-slip zone. Further west, motion onstrands of the Sorong Fault system caused the arrival of the Tukang Besi and Sula fragments in Sulawesi. Collision events at theEurasian 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 wascomplete but motion of the Sumatran forearc slivers was associated with new spreading in the Andaman Sea.

Fig.5. Present-day tectonic features of SE Asia and the SW Pacific. Yellow lines are selected marine magnetic anomalies. Cyan linesoutline bathymetric features. Red lines are active spreading centres. White lines are subduction zones and strike-slip faults. Thepresent extent of the Pacific plate is shown in pale blue. Areas filled with green are mainly arc, ophiolitic, and accreted materialformed at plate margins during the Cenozoic. Areas filled in cyan are submarine arc regions, hot spot volcanic products, andoceanic plateaus. Pale yellow areas represent submarine parts of the Eurasian continental margins. Pale and deep pink areasrepresent submarine parts of the Australian continental margins. Letters represent marginal basins and tectonic features as follows:

A Japan SeaB Okinawa TroughC South China SeaD Sulu SeaE Celebes SeaF Molucca SeaG Banda SeaH Andaman SeaJ West Philippine BasinK Shikoku BasinL Parece Vela BasinM Mariana Trough

N Ayu TroughP Caroline SeaQ Bismarck SeaR Solomon SeaS Woodlark BasinT Coral SeaU Tasman SeaV Loyalty BasinW Norfolk BasinX North Fiji BasinY South Fiji BasinZ Lau Basin

Ba Banda ArcBH Birds HeadCa Cagayan ArcFj FijiHa Halmahera ArcIB Izu-Bonin ArcJa Japan ArcLo Loyalty IslandsLu Luzon Arc

Mk Makassar StraitMn Manus IslandNB New Britain ArcNC New CaledoniaNH New Hebrides ArcNI New IrelandNng North New Guinea

TerranesPa Papuan OphiolitePk Palau-Kyushu Ridge

Ry Ryukyu ArcSa Sangihe ArcSe Sepik ArcSo Solomons ArcSp Sula PlatformSu Sulu ArcTK Three Kings

RiseTo Tonga ArcTu Tukang Besi

Platform

Marginal Basins Tectonic features

126 R. Hall

0 MaPresent Day

INDIA

AUSTRALIA

PACIFICPLATE

EURASIA

40oN

20oN

20oS

40oS

60oS

90oE120oE 150oE

180oE

ANTARCTICA

INDIANPLATE

H

NEWZEALAND

PHILIPPINESEA

PLATE


50 MaEnd EarlyEocene

INDIA

AUSTRALIA

PACIFICPLATE

INDIANPLATE

AUSTRALIANPLATE

EURASIA

40oN

20oN

20oS

40oS

60oS

90oE 180oE

?

ANTARCTICA

128 R. Hall

40 MaMiddle Eocene

INDIA

AUSTRALIA

PACIFICPLATE

INDIANPLATE

EURASIA

40oN

20oN

20oS

40oS

60oS

90oE 180oE

ANTARCTICA


30 MaMid Oligocene

INDIA

AUSTRALIA

PACIFICPLATE

INDIANPLATE

EURASIA

40oN

20oN

20oS

40oS

60oS

90oE

ANTARCTICA

180oE

130 R. Hall

20 MaEarly Miocene

AUSTRALIA

PACIFICPLATE

INDIANPLATE

EURASIA

40oN

20oN

20oS

40oS

60oS

90oE 180oE

ANTARCTICA

INDIA


10 MaLate Miocene

AUSTRALIA

PACIFICPLATE

EURASIA

40oN

20oN

20oS

40oS

60oS

90oE 180oE

ANTARCTICA

INDIA

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