Hydrocarbon basins in SE Asia: understanding why they are there

Robert HallSE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham,

Surrey TW20 0EX, UK(e-mail: robert.hall@gl.rhul.ac.uk)

ABSTRACT: There are numerous hydrocarbon-rich sedimentary basins in Indone-sia, Malaysia and southern Thailand. Almost all these basins began to form in theEarly Cenozoic, they are filled with Cenozoic sediments, most are rifted basins thatare the product of regional extension, and they formed mainly on continental crust.Many different models have been proposed to account for their formation and age.Understanding basin development requires a better knowledge of the Mesozoic andEarly Cenozoic history of Sundaland, which mainly lacks rocks of this age.Sundaland is a heterogeneous region, assembled from different continental blocksseparated by oceanic sutures, in which there has been significant Mesozoic andCenozoic deformation. It is not a ‘shield’ or ‘craton’. Beneath Sundaland there is amarked difference between the deep mantle structure west and east of about 100�Ereflecting different Mesozoic and Cenozoic subduction histories. To the west areseveral linear high velocity seismic anomalies interpreted as subducted Tethyanoceans, whereas to the east is a broad elliptical anomaly beneath SE Asia indicatinga completely different history of subduction. Throughout most of the Cretaceousthere was subduction north of India, preceding collision with Asia. However, northof Australia the situation was different. Cretaceous collisions contributed toelevation of much of Sundaland. Subduction beneath Sundaland ceased in theCretaceous after collision of Gondwana continental fragments and, during the LateMesozoic and Paleocene, there was a passive margin surrounding most of Sunda-land. When Australia began to move northwards from about 45 Ma, subductionresumed at the Sunda Trench. Basins began to form as the region went intocompression at the time of subduction initiation. There was widespread extension,broadly orthogonal to the maximum compressive stress but modified by pre-existingbasement structure. After subduction resumed, the weakness of the Sundalandlithosphere, unusually responsive to changing forces at the plate edges, meant thatthe basins record a complex tectonic history.

KEYWORDS: sedimentary basins, Sundaland, subduction

INTRODUCTION

There are numerous hydrocarbon-rich sedimentary basins inand around the Sunda Shelf in Indonesia, Malaysia and south-ern Thailand, in the continental area described as Sundaland(Fig. 1). Almost all of these basins began to form in the EarlyCenozoic, many are very deep and they are typically filled withterrestrial and shallow-marine sediments. The basins formed oncontinental crust, although this crust has a complex character,in terms of fabric and lithologies, and probably differs consid-erably from older major continents. From a tectonic point ofview the basins present two particular problems: exactly whendid they begin to form and why? The two questions cannot beseparated and it is not possible to identify a mechanism withoutknowing the timing. The abundance of basins suggests a majordriving force should be identifiable. There are several largetectonic causes that have been proposed or are feasible,including effects of India–Asia collision, collisions at the SEAsian margins, regional extension due to subduction-related

processes, such as hinge rollback, or plate boundary changesfollowing events elsewhere in the global plate system.

In this paper it is proposed that most of the basins began toform broadly synchronously in the Middle Eocene. It is arguedthat they formed in response to breaking of the plate of whichSundaland was part when new subduction systems were initi-ated around Sundaland. The considerable variation in thegeometry, character and distribution of the basins, and theirsubsequent histories, reflect two important features of Sunda-land: the composite character of its basement and the weaknessof its lithosphere.

BASIN FORMATION MODELS

A great deal has been written on the basins of the Sundalandregion and a large number of models have been proposed toaccount for the formation of some or all of the sedimentarybasins. It is not the intention of this paper to discuss the

Petroleum Geoscience, Vol. 15 2009, pp. 131–146 1354-0793/09/$15.00 � 2009 EAGE/Geological Society of LondonDOI 10.1144/1354-079309-830

distribution and details of different basins, or basin models, andthe reader is referred to reviews or special volumes (e.g.Williams et al. 1995; Howes & Noble 1997; Longley 1997;Petronas 1999; Hall & Morley 2004; Doust & Sumner 2007)and papers cited below for this information.

The basins have been classified in different ways andassigned to different categories. Many have been suggested tohave originated as pull-apart basins due to strike-slip faulting(e.g. Davies 1984; Tapponnier et al. 1986; Polachan et al. 1991;Huchon et al. 1994; Cole & Crittenden 1997; Shaw 1997;Leloup et al. 2001). A few have been suggested to have aflexural origin (e.g. DeCelles & Giles 1996; Wheeler & White2002). Most authors have suggested that all or most basins arerifts (e.g. Burri 1989; Williams & Eubank 1995; Longley 1997;Wheeler & White 2002; Hall & Morley 2004; Doust & Sumner2007).

A variety of mechanisms have been suggested to account forbasin formation. Many authors link basin formation to India–Asia collision, either related to extrusion-related strike-slipfaulting (e.g. Huchon et al. 1994; Madon & Watts 1998; Leloupet al. 2001; Tapponnier et al. 1986) or by driving extension (e.g.Cole & Crittenden 1997; Longley 1997). Many other authorshave explicitly or implicitly linked basin formation to subduc-tion. Some of the basins are described as forearc basins (e.g.Madon 1999), and many have been described as back-arc basins(e.g. Eubank & Makki 1981; Busby & Ingersoll 1995; Cole &Crittenden 1997; Barber et al. 2005), although it is not alwaysclear if this term is being applied in a purely descriptive sense,referring to their present setting, or a mechanistic sense, toindicate they formed behind an active arc. Morley (2001)suggested extension driven by subduction rollback to accountfor the formation of some Sundaland basins and Moulds (1989)proposed subduction-driven compression could account forextension of Sumatran and possibly other basins. Many authorshave noted the importance of pre-existing basement structuresin influencing basin development (e.g. Hamilton 1979; Moulds1989; Hutchison 1996a; Madon 1999; Hall & Morley 2004;Sapiie & Hadiana 2007).

One of the problems in interpreting the causes of basinformation is identifying the ages of basin initiation. Dating theinitiation of basin formation is problematical because the oldestparts of the sequences are terrestrial, typically lack fossils and, inmany cases, are observed only on seismic data. The older partsof many basins have not been sampled because they are toodeep. It is possible that there is a progressive change in the ageof basin formation from the external parts of Sundaland, withbasin initiation becoming younger towards the interior, but itappears more likely that subsidence began at approximately thesame time across most of Sundaland (Longley 1997; Hall &Morley 2004; Doust & Sumner 2007). More work is certainlyneeded to try to date basin initiation more accurately, but theproblems in doing this will not be overcome easily and ages willremain uncertain, because Upper Cretaceous and Paleocenesedimentary rocks are missing from most of Sundaland, prob-ably reflecting widespread uplift and subsequent erosion afterthe mid-Cretaceous. It is reasonably certain that rifting beganalmost everywhere except in Thailand in the Eocene, and theoldest proven rift sequences are Middle Eocene. Therefore, thebest estimate for basin initiation is suggested here to be about45 Ma. The ideas proposed below are not critically dependenton this proposed age or the synchronicity of basin formation.

SUNDALAND LITHOSPHERE

The interior of SE Asia, particularly the Sunda Shelf (Fig. 1)and surrounding emergent, but topographically relatively lowareas of Sumatra, the Thai-Malay peninsula and Borneo arelargely free of seismicity and volcanism, in marked contrast toits margins. This tectonically quiet region forms the continentalcore of Sundaland (Hall & Morley 2004). Sundaland extendsnorth into Indochina, much of it was terrestrial for most of theCenozoic and it formed an exposed landmass during thePleistocene. Most of the shelf is flat, extremely shallow, withwater depths considerably less than 200 m. Seismicity (Cardwell& Isacks 1978; Engdahl et al. 1998) and GPS measurements(Rangin et al. 1999; Michel et al. 2001; Bock et al. 2003; Simons

Fig. 1. Geography of SE Asia andsurrounding regions. Small black filledtriangles are volcanoes from theSmithsonian Institution, GlobalVolcanism Program (Siebert & Simkin2002), and bathymetry is simplifiedfrom the GEBCO (2003) digital atlas.Bathymetric contours are at 200 m,3000 m and 5000 m.

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et al. 2007) indicate that a SE Asian or Sunda microplate iscurrently moving slowly relative to the Eurasian Plate. TheSunda Shelf is widely regarded as a stable area (e.g. Geyh et al.1979; Tjia 1992; Hanebuth et al. 2000) and sea-level data fromthe region have been used in construction of global eustaticsea-level curves (e.g. Fleming et al. 1998; Bird et al. 2007),despite evidence of very young faulting and vertical movements(e.g. Bird et al. 2006).

The present stability and the extensive shallow area of theSunda Shelf has led to a widespread misconception thatSundaland was a stable area during the Cenozoic. It is oftendescribed as a shield or craton (e.g. Ben-Avraham & Emery1973; Gobbett & Hutchison 1973; Tjia 1996; Parkinson et al.1998; Barber et al. 2005), or plate (e.g. Davies 1984; Cole &Crittenden 1997; Replumaz & Tapponnier 2003; Replumaz et al.2004), implying a rigid block with deformation concentratedat the edges during the Cenozoic. Some authors suggestSundaland rotated clockwise as a block during the last8–10 million years (e.g. Rangin et al. 1999) or over a longerperiod (Replumaz et al. 2004).

However, the character of the Sundaland deep crust andmantle is quite different from nearby continental regions andfrom cratons (Hall & Morley 2004; Hyndman et al. 2005; Currie& Hyndman 2006). Unlike the well-known shields or cratons(e.g. Baltic, Canadian, African, Australian), Sundaland is notunderlain by a thick cold lithosphere that was stabilized in thePrecambrian. There has been significant deformation duringthe Cenozoic with formation of the sedimentary basins, as wellas localized inversion and widespread elevation of mountains.There are numerous faults that have been active during theCenozoic (e.g. Pubellier et al. 2005; Doust & Sumner 2007).The Sundaland interior has high surface heat flow values,typically greater than 80 mW m�2. Doust & Sumner (2007)noted exceptionally elevated heat flow (>150 mW m�2) andtemperature gradients in the area that extends from the CentralSumatra Basin northward into the southern Gulf of Thailandwith an unknown cause. At the Indonesian margins high heatflows reflect subduction-related magmatism but the hot interiorof Sundaland mainly reflects upper crustal heat flow fromradiogenic granites and their erosional products, the insulationeffects of sediments and a large mantle contribution (Hall &Morley 2004). Locally very high heat flows may reflect fluidmovements in the upper crust. P- and S-wave seismic tomogra-phy (Bijwaard et al. 1998; Ritsema & van Heijst 2000) also showthe region is characterized by low velocities in the lithosphereand underlying asthenosphere, in marked contrast to Indianand Australian continental lithosphere to the NW and SE. Suchlow mantle velocities are commonly interpreted in terms ofelevated temperature, and this is consistent with regionalhigh heat flow, but they may also partly reflect the mantlecomposition or elevated volatile contents.

Sundaland has been very far from stable (Hall & Morley2004) and it did not behave as a single rigid block during mostof the Cenozoic. The considerable evidence for a hetero-geneous pattern of subsidence and elevation indicates signifi-cant internal deformation (Hall 1996, 2002; Hall et al. 2008).The upper mantle velocities and heat flow observations suggestthe region is underlain by a thin and weak lithosphere (Hall &Morley 2004; Hyndman et al. 2005) that extends many hundredsof kilometres from the volcanic margins. Of critical importanceis the observation that such extensive regions of thin and hotlithosphere are weak (Hyndman et al. 2005; Currie & Hyndman2006) and have a total strength of similar magnitude to forcesacting at plate boundaries. When dry, plate boundary forcesmay exceed the total lithosphere strength during compressionand, when wet, during both compression and extension. As

suggested elsewhere (Hall 2002; Hall & Morley 2004), thismeans the entire region is likely to be unusually responsive tochanging plate boundary forces. This weakness has been ofmajor importance in the initiation of the Sundaland sedimen-tary basins, and in their later histories.

SUNDALAND HISTORY BEFORE THE EOCENE

The continental core of Sundaland was assembled from frag-ments (Fig. 2) that rifted from Gondwana during formation ofdifferent Tethyan oceans and this history is well described byMetcalfe (e.g. 1996, 1998). An Indochina–East Malaya blockseparated from Gondwana in the Devonian and, by theCarboniferous, was at tropical low latitudes where a distinctiveCathaysian flora developed. In contrast, Sibumasu separatedfrom Gondwana in the Permian, and collided with Indochina–East Malaya, already amalgamated with the South and NorthChina blocks, in the Triassic. Permian and Triassic granites ofthe Thai–Malay Tin Belt are the products of subduction andpost-collisional magmatism (Hutchison 1989).

The Mesozoic sedimentary record is very limited but sug-gests that much of Sundaland was emergent. Barber et al. (2005)suggested a complex shuffling of Cathaysian and Gondwanablocks during the Late Palaeozoic and Early Mesozoic, possiblyassociated with important strike-slip faulting. During the Meso-zoic there were several episodes of granite magmatism, inter-preted to have occurred at an Andean-type margin related tonorthward subduction, during the Jurassic and Cretaceous(McCourt et al. 1996). More continental fragments were addedto Sundaland in the Cretaceous (Fig. 3). The Kuching Zone(Hutchison 2005) includes rocks with Cathaysian affinitiessuggesting an Asian origin and a subduction margin continuingsouth from East Asia at which ophiolitic, island arc andmicrocontinental crustal fragments were added during theMesozoic.

Several important blocks were added in the Cretaceous. SWBorneo is interpreted here to be a continental block rifted from

Fig. 2. Sundaland at the end of the Cretaceous, modified afterMetcalfe (1996), Barber et al. (2005) and Barber & Crow (2009).West Sumatra, West Burma and Indochina–East Malaya formed partof a Cathaysia block added to Eurasia during the Palaeozoic.Sibumasu was accreted along the Raub–Bentong suture in theTriassic. West Burma and West Sumatra were subsequently movedalong the Sundaland margin. The Woyla Arc was accreted in theCretaceous. The Luconia block is interpreted here to be a Cathaysianfragment rifted from Asia and added to Sundaland during theMesozoic. SW Borneo (Metcalfe 1990) and East Java–West Sulawesi(Smyth et al. 2007) are interpreted to have been rifted from WestAustralia and added in the Late Cretaceous.

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the West Australian margin, and added to Sundaland in theEarly Cretaceous. The suture is suggested to run south from theNatuna area along the structural lineament named the BillitonDepression (Ben-Avraham 1973; Ben-Avraham & Emery1973) and originally interpreted by Ben-Avraham & Uyeda(1973) as a transform fault associated with Cretaceous openingof the South China Sea. After collision of the SW Borneo blockthe Cretaceous active margin ran from Sumatra into West Javaand continued northeast through SE Borneo into WestSulawesi. The intra-oceanic Woyla Arc collided with the Sumat-ran margin in the mid-Cretaceous, adding arc and ophioliticrocks to the southern margin of Sumatra (Barber et al. 2005).Further east the Early Cretaceous active margin is marked byCretaceous high pressure–low temperature subduction-relatedmetamorphic rocks in Central Java, the Meratus Mountains ofSE Borneo and West Sulawesi (Parkinson et al. 1998). Furtherfragments were added to Sundaland during the Late Cretaceous(Fig. 3) in East Java and West Sulawesi (Smyth et al. 2007;van Leeuwen et al. 2007).

Until recently, most authors (e.g. Metcalfe 1996) acceptedthat fragments rifted from the west Australian margin in theLate Jurassic and Early Cretaceous had collided in West Burmain the Cretaceous, or were uncertain about their presentposition. However, recent work suggests West Burma is not ablock added in the Cretaceous, but has been part of SE Asiasince the Early Mesozoic, possibly linked to West Sumatra(Barber & Crow 2009). The west Australian fragments are nowin Borneo, East Java and West Sulawesi. Outboard of theMeratus suture, East Java and West Sulawesi are underlain inpart by Archaean continental crust, and geochemistry (Elburget al. 2003) and zircon dating (Smyth et al. 2007; van Leeuwenet al. 2007) indicate a west Australian origin.

The Cretaceous arrival of continental fragments contributedto crustal thickening, magmatism (see below), emergence andwidespread erosion of Sundaland during the Late Cretaceousand Early Cenozoic, thus further diminishing the completenessof the stratigraphic record. Of greater importance for basindevelopment was the considerable variation in basement

lithologies and structure, which gave Sundaland at the begin-ning of the Cenozoic a highly complex basement fabric thatvaries from area to area, and which includes profound and deepstructural features that have been reactivated at different timesin different ways. This complex basement structure is thesecond important influence on the formation and character ofthe sedimentary basins of Sundaland.

MANTLE STRUCTURE BENEATH SUNDALAND

P-wave and S-wave seismic tomography not only indicate thelikely strength of the lithosphere but also display the mantlestructure beneath Sundaland (Widiyantoro & van der Hilst1997; Bijwaard et al. 1998; Ritsema & van Heijst 2000) fromwhich important insights into the subduction history of theregion can be obtained. Tomographic depth slices of the uppermantle (Fig. 4) reveal very clear, broadly curvilinear highvelocity anomalies, mainly parallel to present trenches, inter-preted as the principal lithospheric slabs subducted during theLate Cenozoic. However, of greater importance for interpretingthe older history are velocity contrasts in the lower mantle. Atdepths below 700 km there is a marked difference between themantle structure west and east of about 100�E (Fig. 5). To thewest there are a series of linear high velocity anomalies trendingroughly NW–SE, interpreted as subducted remnants ofTethyan oceans by van der Voo et al. (1999). East of 100�Ethese anomalies no longer exist, and only the southernmostanomaly can be traced eastwards. To the west of 100�E is aclear linear NW–SE-orientated high velocity anomaly, but tothe east the apparent continuation of the anomaly beneath SEAsia has a quite different appearance. It has a broad ellipticalshape with a long axis orientated approximately NE–SWthat terminates at the edge of the west Pacific beneath thePhilippines.

North of India the linear anomalies have been interpreted asa series of subduction zones active during India’s northwardmovement before collision with Asia (van der Voo et al. 1999;Aitchison et al. 2007). The high velocity anomaly in the lowermantle beneath Indonesia represents the accumulation ofsubducted lithosphere, but indicates a completely differentsubduction history. Replumaz et al. (2004) have suggested thatthe lower mantle tomography reflects a large clockwise rotation

Fig. 3. Inferred position of the Late Jurassic and the EarlyCretaceous Sundaland margin, adapted in part from Hamilton(1979), Parkinson et al. (1998) and Smyth et al. (2007). A suturebetween pre-Cretaceous Sundaland and the SW Borneo block isassumed to be present beneath the Sunda Shelf. Its position isuncertain. The extent and boundaries of the Cathaysian Luconiablock are also uncertain. The youngest Cretaceous suture runs fromJava and through the Meratus Mountains of SE Borneo and to thesouth and east of this is a continental fragment derived from westernAustralia (Smyth et al. 2007).

Fig. 4. Depth slice at 300 km from the P-wave tomographic modelof Bijwaard & Spakman (2000). High velocities are represented byblue and low velocities by red. The slice shows high velocityanomalies parallel to present trenches, interpreted as subducted slabsaround Indonesia, and relatively low velocities beneath Sundaland.

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of a single SE Asian block since 40 Ma, as proposed in areconstruction of the India–Asia collision zone by Replumaz &Tapponnier (2003). However, there is no evidence frompalaeomagnetism (Fuller et al. 1999) for the rotation or latitudechange suggested by Replumaz & Tapponnier (2003) and thearea of the anomaly extends significantly south of their recon-structed position of the Java Trench. The geological history ofthe region is also much more complex than that expected byrotation of a single rigid SE Asian plate (Hall et al. 2008).

The reconstruction of Hall (2002) offers an alternativeinterpretation of the deep anomaly beneath SE Asia. Both theupper and lower mantle reveal the importance of long-termsubduction at the Indonesian margins. However, most of whatis observed in the mantle records the northward movement ofAustralia during the Cenozoic, which began to move at asignificant rate only from about 45 Ma (Royer & Sandwell1989). There is no evidence for a similar series of Tethyanoceans to those subducted north of India, consistent withthe absence of subduction during the Late Cretaceous andPaleocene. The position of the deep lower mantle anomaly fitswell with that expected from Indian–Australian lithospheresubducted northward at the Java margin since about 45 Ma, andproto-South China Sea lithosphere subducted southward at thenorth Borneo trench since 45 Ma, with contributions fromseveral other subduction zones within east Indonesia, such asthose associated with the Sulu Arc, and the Sangihe Arc. Thedifference in subduction history north of India compared tothat north of Australia, with the change occurring at about100�E, is the third major feature relevant to the formation ofthe sedimentary basins of Sundaland.

IGNEOUS ACTIVITY FROM THE LATECRETACEOUS TO EOCENE

Typically, plate tectonic reconstructions have assumed subduc-tion was underway at the Sunda Trench at least by the EarlyEocene (e.g. Hall 1996, 2002) or from the Late Cretaceous (e.g.Metcalfe 1996; Sribudiyani et al. 2003; Barber et al. 2005;Whittaker et al. 2007). There is abundant evidence forsubduction-related magmatism since about 45 Ma fromSumatra eastwards but, in contrast, there is little volcanic recordin the expected position of the volcanic arc during the Late

Cretaceous and Paleocene, except in West Sulawesi and Sumba.However, there was widespread granite magmatism across mostof southern Sundaland in the Cretaceous, which unlike prod-ucts of older episodes (e.g. Hutchison 1989; Krähenbuhl 1991;Cobbing et al. 1992; McCourt et al. 1996) is not arranged inlinear belts, and suggests crustal melting across a large regionduring a long period, rather than subduction-related plutonism.

Volcanic activity

Throughout most of SE Asia, Upper Cretaceous and Paleocenerocks of all types are almost entirely absent. The evidence forLate Cretaceous and Early Cenozoic volcanic activity (Fig. 6) isalmost entirely based upon K–Ar ages, mainly from whole-rockdating, and there are many reasons to be cautious about suchages (McDougall & Harrison 1988; Harland et al. 1990;Macpherson & Hall 2002; Villeneuve 2004), particularly in SEAsia where the effects of tropical weathering are superimposedon tectonically and thermally altered rocks, commonly with lowpotassium contents. If these ages are accepted, they suggest aninterval of significantly diminished volcanic activity, possiblypost-collisional and often interpreted as not directly related tosubduction, succeeded in many parts of the Sundaland marginby renewed abundant volcanism from the Middle Eocene,which can be confidently linked to subduction.

Crow (2005) reviewed the record of Cenozoic volcanicactivity in Sumatra. No Upper Cretaceous rocks are known(Page et al. 1979), but there are a small number of Paleoceneages reported from boreholes, dykes and a few volcanic rocks,mainly basalts, which are mostly in south Sumatra (Bellon et al.2004). The Paleocene Kikim Volcanics include volcanic brec-cias, tuffs, and basaltic to andesitic lavas. De Smet & Barber(2005) interpreted a period of erosion in Sumatra between theLate Cretaceous and Early Eocene, with some volcanic activityin the area of the Barisan Mountains, although they noted thatthe age of the volcanic rocks is poorly constrained. Middle toUpper Eocene volcanic rocks are known from a few parts ofWest Sumatra (Crow 2005). They include subaerial pyroclasticsand lavas interpreted as arc volcanics in Aceh, and volcaniclas-tic sediments. K–Ar and zircon fission track ages are less than47 Ma. In Sumatra, prolonged erosion was followed in aboutthe Middle Eocene by a resumption of volcanic activity whichbecame widespread from the Late Eocene. Late Eocene toEarly Miocene volcanic activity is indicated by widespread andabundant volcanic rocks throughout Sumatra.

In Java there is an important Eocene unconformity and fewexposures of older rocks. Central Java includes the mostextensive area of basement rocks, the Lok Ulo Complex(van Bemmelen 1949; Asikin et al. 1992). This is regarded as theimbricated product of Cretaceous deformation (e.g. Parkinsonet al. 1998; Wakita 2000) at a subduction margin that extendedfrom West Java, through the Lok Ulo Complex, to the MeratusMountains. The youngest rocks from the Lok Ulo Complexcontain Campanian–Maastrichtian radiolaria with ages of70–80 Ma (Wakita et al. 1994). The Lok Ulo Complex isoverlain by the Eocene–Oligocene Karangsambung andTotogan Formations, which include scaly clays with nummuliticlimestone blocks, polymict conglomerates and basalt blocks,interpreted as deep-water olistostromes (A.H. Harsolumakso,pers. comm. 2005) or mud diapirs (A.J. Barber, pers. comm.2008). They lack abundant volcanic material but do containlarge amounts of quartzose debris. In West Java the oldestrocks are in the Ciletuh area where there are exposures of‘pre–Tertiary’ (van Bemmelen 1949) peridotites, serpentinitesand gabbros with chloritic schists interpreted as an ophiolitictectonic melange (e.g. Parkinson et al. 1998; Martodjojo et al.

Fig. 5. Depth slice at 1100 km from the P-wave tomographic modelof Bijwaard & Spakman (2000). The slice shows the linear highvelocity anomalies beneath India, in contrast to the deep wide highvelocity anomaly beneath Sundaland. The line at about 100�E isinterpreted to be the eastern end of the subduction zones north ofIndia.

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1978). K–Ar dating provided few results due to alteration(Schiller et al. 1991). A basalt pebble yielded a Late Cretaceousage, and a gabbro gave Paleocene to Middle Eocene ages. Theserocks are probably overlain, although subsequently juxtaposedtectonically (Clements & Hall 2007), by the Middle EoceneCiletuh Formation, which includes angular volcaniclasticmaterial, ophiolitic and chert debris, rare amphibolites, blocksof nummulitic limestones, and some volcaniclastic sandstones,incorporated when incompletely lithified. They are interpretedas deposits of a deep-marine forearc setting (Schiller et al. 1991;Clements & Hall 2007). In East Java there are a few, very small,exposures of basement greenschists and marbles (Smyth et al.2007). The contact with overlying sedimentary rocks, includingnummulitic limestones, is not exposed but is interpreted tobe an unconformity. The oldest sedimentary rocks, in theNanggulan area, are fluviatile quartzose conglomerates andsandstones lacking volcanic debris. These terrestrial depositspass rapidly up into Eocene marine deposits, which containincreasing amounts of volcaniclastic debris up-section. Thestratigraphy indicates an important change in the MiddleEocene, with initiation of volcanic activity after a prolongedperiod of emergence and erosion (Smyth et al. 2007).

In SE Borneo there is also a profound unconformity in theMeratus Mountains. The exposed basement includes ophiolites,Cretaceous volcaniclastic rocks (Sikumbang 1986, 1990;Yuwono et al. 1988) and subduction-related high pressure–lowtemperature metamorphic rocks (Parkinson et al. 1998), whichwere thrust together during a Late Cenomanian–Early Turonianarc–continent collision (before c. 91 Ma). The ManunggulGroup (Sikumbang 1986) or Formation (Yuwono 1987;Yuwono et al. 1988) includes Upper Cretaceous volcanogenic

tuffs and greywackes (91 to 65 Ma) which rest unconformablyon the older rocks and have a molasse character (Yuwono et al.1988). These rocks are intruded by basaltic to dacitic dykes andgabbroic to granitic stocks with K–Ar ages from 87�4 Ma to72�4 Ma, interpreted by Yuwono et al. (1988) to besubduction-related, based on their chemistry. The ManunggulFormation passes transitionally into the Tanjung Formation(Yuwono et al. 1988) which consists predominantly ofquartzose clastic sediments and coals. This was originallyinterpreted to be Middle and Upper Eocene, but Bon et al.(1996) interpreted the deepest part of the Tanjung Formation,dated from three wells, to be Upper Maastrichtian, based onnannoflora previously regarded as reworked. They interpretedthe Maastrichtian–Paleocene sediments to be deposits of apassive margin, including minor Paleocene volcanic rocks,formed after the Cretaceous collision. North of the MeratusMountains, in the Kutai Basin, rifting that led to formation ofthe Makassar Straits began in the Middle Eocene (Chambers &Moss 1999; Moss & Chambers 1999) and the oldest Cenozoicrocks resemble Upper Eocene quartzose clastic sequence in theMeratus Mountains. Upper Cretaceous volcanic rocks areknown from Kelian (Davies et al. 2008) in the Kutai Basin,where an inlier of felsic volcaniclastics is surrounded by Eoceneterrestrial and shallow-marine sedimentary rocks. Zircons froma pumice breccia have a U–Pb age of 67.8�0.3 Ma, consideredto be the age of eruption (Davies, 2002). Detrital zirconpopulations from the Kelian and Mahakam Rivers includeCretaceous ages from 126.3 to 67.6 Ma (Setiabudi et al. 2001).

Throughout West Sulawesi, Upper Cretaceous formationsinclude material with an igneous provenance and record minorvolcanic activity (T.M. van Leeuwen, pers. comm., 2007). In

Fig. 6. Distribution of (A) volcanicrocks and (B) minor intrusions insouthern Sundaland with ages between80 and 45 Ma. Sources are cited in thetext. The shaded area is the position ofthe Early Cretaceous arc. The totalnumber of dated rocks is small (63) butprobably does reflect the abundance ofLate Cretaceous to mid-Eocenevolcanic activity.

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the Lariang and Karama area of West Sulawesi, the oldestCenozoic sedimentary rocks rest in places on volcanic rocksand volcaniclastic sedimentary rocks that are mainly not welldated and may be Cretaceous or Paleocene (Calvert 2000;Calvert & Hall 2007). In the northern part of West Sulawesi theprobable Campanian–Maastrichtian Latimojong Formation iscomposed dominantly of marine sedimentary rocks with basal-tic to dacitic flows (van Leeuwen & Muhardjo 2005) and thereare volcaniclastic sandstones, tuffs and metabasites in theLatimojong Mountains further south (A.J. Barber, pers. comm.,2007). Volcaniclastic rocks contain Precambrian to Cretaceouszircons and the youngest age population has U–Pb ages of80–120 Ma (van Leeuwen & Muhardjo 2005). In the southernpart of West Sulawesi, similar rocks are dated in severalplaces and assigned to a number of different formations. TheBalangbaru Formation (Hasan 1990, 1991) includes olisto-stromes and turbidite fan deposits with a maximum age rangeof Turonian to Maastrichtian age (91–65 Ma). The MaradaFormation (van Leeuwen 1981) is Campanian–Maastrichtianand includes distal turbidites which contain quartz, freshfeldspar and andesite fragments as well as volcanic flows orsills.

Unconformably above the Balangbaru Formation is the Allaor Bua Formation (Sukamto 1982, 1986; Yuwono 1987),consisting of volcanic and intrusive rocks with K–Ar agesranging from 65–58 Ma, interpreted as subduction-related(Elburg et al. 2002). This passes upwards into the marginalmarine coal-bearing siliciclastic rocks of the ?Lower–MiddleEocene Malawa Formation. Further east are the Middle toUpper Eocene Langi Volcanics (van Leeuwen 1981), which arepredominantly andesitic volcanics with a typical arc geochem-istry (Elburg et al. 2002); some volcaniclastic rocks in the lowerpart contain zircons with a fission track age of 62�2 Ma. TheLangi Volcanics are intruded by a tonalite/granodiorite with aK–Ar age of 52–50 Ma and pass up into Upper Eocenelimestones.

The interpretation of these sequences is not entirely clear. Apost-collision passive margin setting has been suggested (Hasan1990, 1991; Wakita et al. 1996) for the lower part of theBalangbaru Formation but igneous debris in the upper part issuggested to be derived from a continental or magmatic arc andmay indicate a forearc setting (van Leeuwen 1981). Thegeochemistry of the Paleocene and Lower Eocene igneousrocks is generally interpreted to indicate a resumption ofsubduction after Cretaceous collision, and Hasan (1990) inter-preted a Late Paleocene–Early Eocene arc, but, if this was thecase, the episode of subduction seems to have been short lived.All these rocks are overlain by sedimentary sequences typical ofmarginal and shallow marine conditions, including platformcarbonates (Wilson et al. 2000), suggesting widespread riftingaround the present Makassar Straits, which began in the MiddleEocene (Moss & Chambers 1999; Calvert & Hall 2007).

On Sumba there are Upper Cretaceous–Paleocene rocks ofthe Lasipu Formation resembling the deep-marine turbidites ofWest Sulawesi in which there are some volcanic rocks (Burollet& Salle 1981). Abdullah et al. (2000) identified two calc-alkaline magmatic episodes by K–Ar dating, one Santonian–Campanian (86–77 Ma) and the other Maastrichtian–Paleocene(71–56 Ma), which they interpreted as the result of subductionat the Sundaland margin producing an Andean magmatic arc.However, the area of volcanic activity is small.

Widespread Cretaceous granite magmatism

In contrast to the paucity of volcanic rocks of Late Cretaceousand Early Cenozoic age, there are abundant Cretaceous and

Paleocene granites distributed across a very large area ofSundaland (Fig. 7). The majority of dated granites are EarlyCretaceous, and 215 of the 299 ages are older than 80 Ma. Forobvious reasons there is little information from offshore, andthere are some parts of Indochina for which there is littleinformation, such as Cambodia. None the less, it is difficult tosee linear belts that might be expected if they are subductionproducts, and many granites are far from probable subductionzones.

Some granites could be part of an east-facing Andeanmargin that continued south from South China. Cretaceousgranites are known in North China but it is still debated if theywere formed at a subduction margin (e.g. Lin & Wang 2006; Li& Li 2007; Yang et al. 2007). Jahn et al. (1976) argued that therewas a Cretaceous (120–90 Ma) thermal episode in the SE Chinamargin which may be related to west-directed Pacific subduc-tion. In South China, around Hong Kong, acid magmatismceased in the Early Cretaceous (Sewell et al. 2000) but mid-Cretaceous granites are reported from Vietnam (Nguyen et al.2004; Thuy et al. 2004) with youngest ages of 88 Ma. If this waspart of an Andean margin it is unclear if and where it continuedto the south.

The north–south belt of granites that follows the Thailand–Burma border is plausibly related to NE- or east-directedsubduction of the Indian plate (e.g. Mitchell 1993; Barley et al.2003). Early Cretaceous granites continue south into the Malaypeninsula and Sumatra (Fig. 7A) and could be related to thissubduction.

Granite magmatism in Borneo might be explained as theproduct of flat-slab subduction from the west or south, but thepositions of the Cretaceous arcs of southern Sundaland are wellknown in Sumatra, Java and SE Borneo (Hamilton 1979;Parkinson et al. 1998; Barber et al. 2005) and are far outboard ofmost of the granites. The Cretaceous granites in the SchwanerMountains (Williams et al. 1988) and western Sarawak (Tate2001) have been interpreted as the product of Andean-typemagmatism associated with subduction dipping beneathBorneo. If SE Sundaland was rotated by 90� from its presentposition, as suggested by palaeomagnetic studies of Borneo(Fuller et al. 1999), it is possible that the Schwaner granitescould be the product of subduction at the East Asian margin.Alternatively, if SW Borneo is an accreted continental fragmentthat originated in the south, it is possible that granite magma-tism was associated with south-directed subduction during itsnorthward movement in the Early Cretaceous.

The granites older than about 80 Ma may thus be explainedby subduction of the Indian Plate or Pacific plates. However,the younger granites in the area of Sumatra, Java and Borneoare less abundant and scattered across a wide area. A possibleexplanation is that they are products of collisional thickening.Further north in Sundaland, younger granites have been inter-preted as the product of thickening following continentalcollision (Barley et al. 2003). It is suggested here that LateCretaceous and Paleocene, and possibly some of the wide-spread Early Cretaceous, granites of Borneo and the SundaShelf represent post-collisional magmatism following Creta-ceous accretion of the SW Borneo continental fragment or theEast Java–West Sulawesi continental fragment (see below).This would account for their widespread distribution.

LATE CRETACEOUS TO MIDDLE EOCENETECTONIC SETTING

Many authors have assumed or implied that subduction at theSundaland active margin was continuous through the LateMesozoic into the Cenozoic (e.g. Yuwono et al. 1988; Metcalfe

Hydrocarbon basins in SE Asia 137

1996; Soeria-Atmadja et al. 1994, 1998; Abdullah et al. 2000;Sribudiyani et al. 2003) with an Andean magmatic arc stretchingfrom Sumatra to West Sulawesi. Most granite magmatism inIndonesia that could be associated with an Andean-margin isolder than 80 Ma. Furthermore, as observed above, there islittle evidence of volcanic activity in Sumatra, none in Java, verylittle in SE Borneo, and only in West Sulawesi and Sumba cana case be made for subduction-related volcanic activity. Vol-canic rocks are mainly flows or sills associated with deep-marine turbidites, and some volcaniclastic debris could bereworked from older arc rocks. The arc itself is not seen.Volcanic activity had ceased by the end of the Eocene in SWSulawesi, although probably continued into the Oligocenefurther north; its distribution in time and space suggests that itwas localized and intermittent, and radiometric age dates andstratigraphic information suggest little or no volcanism between30 and 19 Ma (T.M. van Leeuwen, pers. comm. 2007). Thearguments for subduction-related volcanic activity are based onthe subduction-character of the geochemistry, but the subduc-tion signature could simply reflect post-collisional magmatism(Macpherson & Hall 2002). However, even if there wassubduction beneath Sumba and West Sulawesi this does notrequire subduction in other parts of the Indonesian margin.

In contrast, some authors have either explicitly suggestedthat parts of the Sundaland margin were passive from the LateCretaceous to the Eocene, or implied a passive margin byidentifying Cenozoic resumption of subduction (e.g. Hamilton1979; Hasan 1990, 1991; Daly et al. 1991; Bon et al. 1996; Carlile& Mitchell 1994; Wakita et al. 1996; Parkinson et al. 1998).Parkinson et al. (1998) suggested that subduction ceased in theCretaceous after collision of a Gondwana continental fragmentthat is now beneath most of West Sulawesi. Carlile & Mitchell(1994) surmised that a collision of the Woyla Arc in Sumatrawas connected to collision and ophiolite emplacement in theMeratus Mountains. The age and character of volcanic andsedimentary rocks from SE Borneo and Sulawesi are consistentwith a major change in regime at about 90 Ma followingcollision. Smyth et al. (2005, 2007) have shown that there is acontinental crust beneath the Southern Mountains of East Java,and Smyth et al. (2007) proposed that a continental fragmentcollided in the Cretaceous and terminated subduction. The agesof radiolaria in accreted deep-marine sediments in Central Javashow the youngest subduction-related rocks are about 80 Ma. Itis therefore suggested here that subduction ceased all aroundthe south Sundaland margin in the Late Cretaceous, at about 90to 80 Ma (Fig. 8), and was not resumed until the Middle

Fig. 7. Distribution of Cretaceous granites in southern Sundaland.There may be more Cretaceous granites offshore beneath the SundaShelf, based on personal communications from oil company geolo-gists. (A) Granites older than 110 Ma, suggested to be the age ofdocking of the SW Borneo block. (B) Granites with ages between110 and 80 Ma, suggested to be the age of docking of the EastJava–West Sulawesi block. (C) Cretaceous to Paleocene granites.The shaded area is the position of the Early Cretaceous arc. Unfilledsquares on (A) are Cretaceous but with no precise age (ESCAP1994; Koning 2003). All other samples are radiometrically dated(Kirk 1968; Gobbett & Hutchison 1973; Pupilli 1973; Patmosukismo& Yahya 1974; Garson et al. 1975; Bignell & Snelling 1977; Haileet al. 1977; Beckinsale et al. 1979; Ishihara et al. 1980; Putthapiban &Gray 1983; Putthapiban 1984, 1992; van de Weerd et al. 1987;Darbyshire & Swainbank 1988; Williams et al. 1988, 1989; Yuwonoet al. 1988; Seong 1990; Krähenbuhl 1991; Cobbing et al. 1992;Nakapadungrat & Puttapiban 1992; Amiruddin & Trail 1993;Charusiri et al. 1993; de Keyser & Rustandi 1993; Nakapadungrat &Maneenai 1993; Pieters & Sanyoto 1993; Pieters et al. 1993; Supriatnaet al. 1993; Wikarno et al. 1993; Dunning et al. 1995; Dirk 1997;Hartono 1997; Utoyo 1997; Nguyen et al. 2004; Cobbing, 2005).

R. Hall138

Eocene, at about 45 Ma. The very limited amount of volcanicactivity between 90 and 45 Ma is interpreted as localizedpost-collisional magmatism, with the exception of WestSulawesi and Sumba where there was some west-directedsubduction.

A similar scenario can be inferred for the north side of theSundaland promontory. Deep-marine sediments of theCretaceous–Eocene Rajang Group form much of the CentralBorneo Mountains and Crocker Ranges in north Borneo andhave been widely interpreted as deposits of an active margin(Haile 1969, 1974; Hamilton 1979; Holloway 1982; Hutchison1973, 1989, 1996a, b; Williams et al. 1988, 1989). According toHutchison (1996b), deposition of Rajang Group sediments wasterminated by a Sarawak orogeny suggested to represent anintra-Eocene collision between a Balingian–Luconia and theSchwaner Mountains continents. In contrast, Moss (1998)presented evidence that the Rajang Group sediments weredeposited at a passive margin. Thus, an alternative interpreta-tion of the Late Cretaceous to Eocene setting is that theSarawak ‘orogeny’ marks the Middle Eocene initiation ofsubduction of the proto-South China Sea beneath northernBorneo, at the former passive margin, rather than a collision.The new subduction zone dipped towards the southeast andwas active from the Eocene until the Early Miocene (e.g. Tan &Lamy 1990; Hazebroek & Tan 1993; Hall 1996; Hutchison et al.2000).

Furthermore, considering tectonics at a much larger scale,although there is good evidence from magnetic anomalies southof India for its rapid northward movement in the LateCretaceous and Early Cenozoic, and hence subduction to thenorth of India, the magnetic anomalies south of Australiaindicate very slow separation of Australia and Antarctica until

about 45 Ma (Royer & Sandwell 1989). Hence, there is norequirement for subduction beneath Indonesia and, indeed, theonly way in which subduction could have been maintainedduring the Late Cretaceous and Early Cenozoic is to propose ahypothetical spreading centre between Australia and Sundalandwhich moved northward until it was subducted (e.g. Whittakeret al. 2007). Ridge subduction is often suggested to produce slabwindows associated with volumetrically or compositionallyunusual magmatism (e.g. Thorkelson & Taylor 1989; Hole et al.1995; Thorkelson 1996; Gorring & Kay 2001). A slab windowshould have swept westward beneath Java and Sumatra duringthe Late Cretaceous and Paleocene according to the Whittakeret al. (2007) model, but there is no evidence for it; indeed, asobserved above, there is no magmatism of this age in Java, andalmost none in Sumatra.

Thus, the new view proposed here of the large-scale tectonicsetting during the Late Cretaceous and Early Cenozoic betweenIndia and SE Asia is that it was markedly different in the westcompared to the east. West of about 100�E, India was movingrapidly northward and was yet to collide with Asia. East ofabout 100�E, there was no subduction. Australia was still far tothe south and, although Australia–Antarctica separation hadbegun in the Cretaceous, the rate and amount was small untilthe Eocene. The difference between east and west implies abroadly transform boundary between the Indian and Australianplates at about 100�E. Such a feature is supported by anomalyages on the unsubducted Indian Plate in the Wharton Basinsouth of Sumatra where there are large distances betweenanomalies across fracture zones indicating northward transformoffset of the Late Cretaceous to Eocene spreading centre ofc. 1700 km (anomaly 28) at 90�E and a further c. 1100 km(anomaly 28) between 90�E and 100�E (Sclater & Fisher 1974)or a total of c. 2000 km (anomaly 28) between the endCretaceous spreading centre west of 90�E and that east of100�E (Royer & Sandwell 1989). Similar large offsets aresuggested to have continued north on the now subductedboundary between the Indian and Australian Plates (Fig. 8).

At the beginning of the Cenozoic there was no northwardsubduction, and Sundaland is interpreted to have been anelevated continent due to the Cretaceous collisions of conti-nental blocks. South of Sumatra and Java, east of about 100�E,it is suggested that there was a passive margin. This passivemargin continued all round eastern Sundaland, which wasseparated from South China by the proto-South China Sea. Themargins changed from passive to active in the Eocene asAustralia began to move rapidly north. South-directed subduc-tion began beneath north Borneo while north-directed subduc-tion began south of Sumatra and Java.

EOCENE RESUMPTION OF SUBDUCTION ANDITS CONSEQUENCES

Only from about 45 Ma did Australia begin to separate signifi-cantly from Antarctica, and to move rapidly northwards, andthis caused subduction to resume at the Sundaland margins.The resumption of subduction initiated an active margin on thesouth side of Sundaland forming the Sunda Arc, and anotheractive margin on the north side of Sundaland in north Borneo.In most of Sumatra, volcanic activity became widespread fromthe Middle Eocene (Crow 2005). In Java, volcanic activitybegan in the Middle Eocene to form an arc that ran the lengthof the island (Hall & Smyth 2008; Smyth et al. 2008; Clementset al. 2009). In Sulawesi the Langi andesitic volcanics (vanLeeuwen 1981; Elburg et al. 2002) may represent a Middle toLate Eocene episode of subduction-related volcanism. OnSumba there was a significant episode of Eocene to Oligocene

Fig. 8. Reconstruction for the Late Cretaceous after collision of theSW Borneo block (SWB), and the later addition of the East Java (EJ)and West Sulawesi (WS) blocks (shaded dark) which terminatedsubduction beneath Sundaland. The inferred positions of theseblocks on the West Australian margin in the Late Jurassic at about155 Ma are shown by the numbered codes (SWB155, WS155 andEJ155). A transform or very leaky transform spreading centre isinferred to have separated the Indian and Australian Plates between80 and 45 Ma as India moved rapidly northwards.

Hydrocarbon basins in SE Asia 139

volcanism (Abdullah et al. 2000). The Sunda Arc continued eastinto the Pacific (Hall 2002).

Subduction also resumed on the north side of the Sundalandpromontory at about 45 Ma, and an active margin ran fromSarawak east towards the Philippines (Hall 1996, 2002). Innorth Borneo, in the Eocene to Early Miocene, there wassoutheast-directed subduction (Hall & Wilson 2000; Hutchisonet al. 2000) and the Crocker Fan (van Hattum et al. 2006)formed above the subduction zone at the active margin. Therewas relatively little Eocene to Early Miocene subduction-relatedmagmatism (Kirk 1968; Hutchison et al. 2000; Prouteau et al.2001) in Borneo, probably because the proto-South China Seanarrowed westwards (Hall 1996, 2002).

However, the most important effect was the initiation ofwidespread rifting throughout Sundaland leading to subsidenceand accumulation of vast amounts of sediment in sedimentarybasins. It is suggested here that this rifting was a response to theregional stresses imposed on Sundaland during the breaking ofthe plate which was required in order to begin subduction.

SUBDUCTION INITIATION

Starting subduction is widely considered to be a very difficultprocess (McKenzie 1977; Mueller & Phillips 1991) for whichthe dynamics remain poorly understood (Cloetingh et al. 1982;Toth & Gurnis 1998), and has been described as a fundamental,yet poorly understood, process (Hall et al. 2003) or as one ofthe unresolved problems of plate tectonics (Regenaur-Lieb et al.2001). None the less, the process must occur regularly andcontinuously, since long-lived subduction zones disappear andnew ones form in other places (Toth & Gurnis 1998). SinceMcKenzie’s (1977) demonstration that creating trenches isdifficult, the problem has been investigated theoretically usingnumerical and analogue modelling, and most studies haveconcentrated on breaking a plate in two types of tectonicsetting: within an oceanic plate and at a passive continentalmargin.

There is still considerable uncertainty. It is generally agreedthat the stresses required to initiate subduction are very large(McKenzie 1977; Cloetingh et al. 1982; Mueller & Phillips 1991)and, therefore, there have been many suggestions that oceaniclithosphere may break at pre-existing zones of weakness, suchas transform faults and fracture zones (Toth & Gurnis 1998;Hall et al. 2003), especially those that are serpentinized (Hilairetet al. 2007). Lateral buoyancy contrasts may also be important,such as might be found at the edges of large oceanic plateaux(Niu et al. 2003). In numerical modelling the assumption ofrheology is important; a non-Newtonian viscosity may makesubduction initiation easier (Billen & Hirth 2005) and conver-gence rate is also a significant factor (Hall et al. 2003; Billen &Hirth 2005).

All modelling confirms that old passive margins are difficultto turn into subduction zones. Increasing the age of passivemargin does not make it easier to initiate subduction (Cloetinghet al. 1982) and the negative buoyancy of old oceanic litho-sphere is insufficient in itself to lead to trench formation. Again,pre-existing weaknesses may be important (Cohen 1982) andfaults may be reactivated if convergence is oblique (Erickson1993). Sediment loading of young lithosphere may play a part(Cloetingh et al. 1982), and sediment loading over long periods(c. 100 Ma) may trigger subduction (Regenaur-Lieb et al. 2001).Water may play an important role in weakening young litho-sphere beneath a sediment load (Regenaur-Lieb et al. 2001).

Faccenna et al. (1999) described three scenarios based onanalogue modelling. The first produces only folding of oceaniclithosphere and no subduction is initiated at the passive margin;

this resembles the situation south of India at the present day(Martinod & Molnar 1995). In this case, it is easier to deformcontinental Asia than break the plate (Stern 2004). In the twoother cases, either an Atlantic-type passive margin evolves overa long period to form a trench or, in the other, the continentcollapses towards the ocean, producing back-arc extension andsubduction, resembling the Mediterranean. The result dependson the brittle and ductile strength of the passive margin, thenegative buoyancy of the oceanic lithosphere, and the horizon-tal body forces between continent and ocean. Mart et al. (2005)also experimented with analogue models, but in a centrifuge,and suggested that lateral density differences at continentalmargins may influence the initiation of subduction.

McKenzie (1977), Mueller & Phillips (1991) and Toth &Gurnis (1998) all included inclined pre-existing faults in theirmodels, but Mueller & Phillips (1991) differed in their estimateof resisting stresses on the faults and consequently foundsubduction initiation to be very unlikely. McKenzie (1977) andToth & Gurnis (1998) agreed on the magnitude of the stressesrequired, but McKenzie (1977) concluded that ridge-pushforces were insufficient to cause subduction initiation, whereasToth & Gurnis (1998) argued that they were sufficient, and thatcombined with additional forces from adjacent subductionzones, subduction initiation is not difficult and expected to becommon in the Western Pacific. Observations around SE Asiawhere there are numerous young subduction zones that haveinitiated in the last 15 Ma, including the Manila, Negros,Cotobato, North Sulawesi and Philippine Trenches (e.g.Cardwell et al. 1980; Karig 1982; Hall 1996, 2002), support thisconclusion. All have developed at the boundaries betweenocean crust and thickened arc crust, typically at the edges ofsmall basins, where there is both a topographic and buoyancycontrast.

SETTING OF BASIN FORMATION

Features that have been identified in different models asimportant or critical were present during the Eocene at theSundaland margins, including pre-existing dipping faults, bathy-metric and buoyancy contrasts, weak serpentinized zones,and probable considerable sediment loads. Early CretaceousSundaland had been surrounded by active margins and Karig(1982) identified former subduction zone margins which hecalled ‘deactivated continental or arc margins’ to be likely sitesof subduction initiation. At about 45 Ma, Australia began toseparate rapidly from Antarctica, hence increasing ridge-pushstresses on the Sundaland margin. SE Asia today, and duringthe Cenozoic, has been the site of exceptionally high rates ofsediment supply (e.g. Milliman et al. 1999; Hall & Nichols 2002;Suggate & Hall 2003; Hall & Morley 2004), and it is likely thatmuch of Sundaland was elevated following Cretaceous colli-sions. Thus, a considerable sediment load at the Sundalandmargins is probable.

It is not easy to identify the consequences of subductioninitiation on the region from the present limited understandingof the process and, therefore, the following scenario is some-what speculative. I suggest that in the Middle Eocene, theSundaland region went into regional compression as Australiaattempted to move north (Fig. 9). This would have meantapproximate N–S compression and, in consequence, approxi-mately E–W extension, causing basins to form. The localextension direction varied considerably, initially because ofvariations in basement fabric and, later, because once subduc-tion began, these were augmented by other forces aroundSundaland due to the different types and orientations of plate

R. Hall140

boundaries. There has been little work on topographic effectsas subduction begins, although Toth & Gurnis (1998) showedthat in their models a long wavelength depression developed onthe overriding plate with a wavelength of c. 400 km. All themodels, numerical and analogues, have used simple and typicalstrengths for different types of lithosphere but, as summarizedabove, Sundaland is not typical (Hall & Morley 2004) and thereis a very large region of weak lithosphere extending more than900 km to the north of the Sunda Trench (Hyndman et al. 2005;Currie & Hyndman 2006) where plate boundary forces aregreater than the strength of the plate in compression.

Thus, once subduction had started it is likely that stresses indifferent parts of Sundaland varied considerably, leading to

reactivation of different structures in the various basins atdifferent times, and resulting in changing orientations of faults,multiple episodes of extension, and inversion. For example, theextension direction observed in the Nam Con Son and CuuLong basins is likely to reflect an interaction of the regionalstresses imposed by subduction initiation, basement structuresin the Vietnam margin, slab-pull forces due to southeast-directed subduction of the proto-South China Sea, and far-fieldeffects of India–Asia convergence. The tectonic developmentof SE Asia in the last 45 Ma (Hall 2002), involving subduction,hinge movements, arc and microcontinent collisions, and newocean basin formation, led to a complex and frequentlychanging distribution of forces at the Sundaland margin; Hall &

Fig. 9. The situation just before andjust after subduction began beneathSundaland at about 45 Ma. Theinitiation of subduction required theplate to be broken at the Sundalandcontinental margin and it is suggestedthat compression induced broadly E–Wextensional stresses, which were locallyre-orientated by basement structures.After subduction was initiated, plateedge forces became increasinglyimportant.

Hydrocarbon basins in SE Asia 141

Morley (2004) illustrated these changing forces in a schematicway for different intervals during the Cenozoic and Figure 9shows the possible forces that influenced basin formation andorientation of local extensional stresses just before and justafter subduction began. The subsequent development of thebasins, including such features as relative age and sedimentaryfill, importance of continental rift inheritance for source andreservoir distribution, and heat flow histories, is described anddiscussed elsewhere (e.g. Williams et al. 1995; Howes & Noble1997; Longley 1997; Petronas 1999; Hall & Morley 2004; Doust& Sumner 2007).

CONCLUSIONS

Although it is now widely accepted that the Sundaland basinsare dominantly extensional, the origin of the extensional forcesremains uncertain. McKenzie-type models are widely applied,but the setting of SE Asian basins is very different fromintra-continental rifts like the North Sea, or passive margins likethose of the Atlantic, where such models were developed andwork well. India–Asia collision was not the cause of basinformation. Even if the indentor model is accepted, the timingof basin initiation is earlier than major movements on thestrike-slip faults and there are many other counter argumentsdiscussed elsewhere (e.g. Morley 2002; Hall & Morley 2004;Hall et al. 2008). The collision is often identified as a cause inrather a mystical manner with no clear indication of what washappening, or where India was in relation to SE Asia. Further-more, there is now doubt about the timing of India–Asiacollision which may be significantly younger than previouslyaccepted. Basin formation was well underway before the endEocene age for continent–continent collision proposed byAitchison et al. (2007).

Most of the models proposed for basin formation could beright in the sense that all could apply somewhere in theSundaland region. The regional compression proposed herewould cause extension by pure or simple shear, but a variety ofdifferent local stress systems may result from other influences,such as local basement structure, reactivated faults, shape of thecontinental margin, type of crust on each side of the new plateboundary, and locally weak or strong layers within the litho-sphere. The composite character of the Sundaland lithospheremeans that all of these could apply and local principal stressescould vary considerably around the region.

It is most likely that the plate broke by a tear propagatingthrough the region from an existing plate boundary. Thissuggests a west to east propagation of the tear from NorthSumatra eastwards. However, this is speculative, and thecharacter and orientation of plate boundaries at the Pacific endof the system are not known. The weakness of the lithospherein the region means that many basins did not subsequently havesimple histories. Many are characterized by changing stresses,reflected, for example, in multiple episodes of extension anddifferent directions of extension. This is due to changingbalance of forces at plate edges and their interaction withbasement fabric, all driven by multiple subduction-relatedprocesses. Subduction has been the most important influenceon SE Asia for all of its history, and still is.

The author is grateful to the University of London Central ResearchFund, the Royal Society and the Natural Environment ResearchCouncil for support, but especially to the consortium of oilcompanies who have supported projects in SE Asia for many years.Thanks to colleagues in Indonesia at the Pusat Survei GeologiBandung, Lemigas, Indonesian Institute of Sciences, and InstitutTeknologi Bandung, and many colleagues, friends and students inthe UK, Europe and SE Asia. Theo van Leeuwen and Tony Barber

are thanked for suggestions and comments on some of the ideas inthis paper.

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Received 12 November 2008; revised typescript accepted 5 February 2009.

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