INTRODUCTION

The continental promontory of the Eurasian plate in SEAsia called Sundaland (Figure 1) contains a large number ofCenozoic sedimentary basins which formed between the Pale-ogene and the Middle Miocene. Many are very deep and some

contain up to 14 km of Cenozoic section. The origin of manyof the basins is problematic in terms of the causes of thestresses which formed them, and the amount of sedimentaccumulated. The Sundaland continental region is surroundedby active and complex tectonic boundaries. To the south andsouthwest there is subduction of oceanic crust of the IndianPlate at the Sunda–Java Trench, to the southeast Australiancontinental crust is colliding in eastern Indonesia with theSundaland margin, associated with young extension in theBanda Sea. To the east there are a number of marginal basins

Book TitleBook SeriesCopyright 2004 by the American Geophysical Union10.1029/Series#LettersChapter#

1

Sundaland Basins

Robert Hall

SE Asia Research Group, Department of Geology, Royal Holloway University of London, Egham,Surrey, U.K.

Christopher K. Morley

Department of Petroleum Geoscience, Universiti of Brunei Darussalam, Tunku Link, Gadong,Brunei Darussalam

The continental core of Sundaland, comprising Sumatra, Java, Borneo, theThai–Malay Peninsula and Indochina, was assembled during the Triassic Indosin-ian orogeny, and formed an exposed landmass during Pleistocene lowstands. Becausethe region includes extensive shallow seas, and is not significantly elevated, it isoften assumed to have been stable for a long period. This stability is a myth. The regionis today surrounded by subduction and collision zones, and merges with theIndia–Asia collision zone. Cenozoic deformation of Sundaland is recorded in thenumerous deep sedimentary basins alongside elevated highlands. Some sediment mayhave been supplied from Asia following Indian collision but most was locallyderived. Modern and Late Cenozoic sediment yields are exceptionally high despitea relatively small land area. India–Asia collision, Australia–SE Asia collision,backarc extension, subduction rollback, strike-slip faulting, mantle plume activity,and differential crust-lithosphere stretching have been proposed as possible basin-forming mechanisms. In scale, crustal character, heat flow and mantle characterthe region resembles the Basin and Range province or the East African Rift, but isquite unlike them in tectonic setting. Conventional basin modeling fails to predictheat flow, elevation, basin depths and subsidence history of Sundaland and overes-timates stretching factors. These can be explained by interaction of a hot uppermantle, a weak lower crust, and lower crustal flow in response to changing forcesat the plate edges. Deformation produced by this dynamic model explains the main-tenance of relief and hence sediment supply over long time periods.

which opened during the Cenozoic, including the South ChinaSea whose origin remains controversial. To the west and north-west are the Himalayan orogenic belt, associated collision beltsin Myanmar, and large strike-slip faults which accommodateIndia–Asia collision. This setting is very different from largecontinental areas such as Africa, Australia and South Americathat are mostly surrounded by major mid-oceanic rift systemsor linear, relatively simple, subduction zones. Consequentlyit might be expected that sedimentary basins developed onSundaland crust would exhibit some different characteristicsfrom classic rifts, foredeep and strike-slip basins. However,most studies of the Sundaland basins, particularly rift basins,have tried to fit them into established models, such as theMcKenzie model [McKenzie, 1978; White and McKenzie,1988] for extension [e.g. Longley, 1997; Wheeler and White,2002], or interpreted them as strike-slip pull-apart basins [e.g.Huchon et al., 1994; Leloup et al., 2001; Polachan et al., 1991;Tapponnier et al., 1986]. A common theme concerning thedevelopment of Sundaland basins has been the attempt tostrongly link many of them tectonically, and in terms of sedi-

ment sources and supply, to escape tectonics and the upliftand erosion of Asia. In many cases, the whole region has beenseen as merely a far-field expression of India–Asia collision.It is argued in this paper that this collision, and its influence,have been over-emphasized, and that important local tectonicactivity driving subsidence and uplift within Sundaland needsto be considered. We discuss how these basins differ fromthose in other continental regions and suggest that Sundalandis an unusual region to which conventional models for basin for-mation cannot be applied without important changes. Thenature and timing of subsidence and inversion suggest a com-plex and shifting pattern of extension and contraction withtime. The long and active tectonic history of the Sundalandmargins has determined the character of the crust and mantleand strongly influenced basin development.

2. SUNDALAND

Sundaland is the region of SE Asia that formed an exposedlandmass during the Pleistocene sea level lowstand, com-

2 SUNDALAND BASINS

Figure 1. Location of Sundaland and the principal sedimentary basins within the region.

prising Indochina, the Thai–Malay Peninsula, Sumatra, Java,Borneo, and the shallow marine shelf (the ‘Sunda Shelf ’)between them. It is the promontory extending southeast fromEurasia although seismicity [Cardwell and Isacks, 1978; John-ston and Bowin, 1981] and GPS measurements [Rangin etal., 1999; Simons et al., 1999] indicate that a SE Asian Plateor Sunda Block is moving separately from the Eurasian plateat the present day.

Sundaland (Figure 1) is the continental core of SE Asia. Itis now bordered to the west, south and east by subduction andcollision zones. To the north it merges with the regiondeformed in the Cenozoic by India–Asia collision. Geologi-cally it is convenient to separate Sundaland from Asia (Figure2) to the northeast along the Red River Shear Zone, whichfollows the Carboniferous Song Ma suture, and to the north-west from a Burma block along the Hpakan–Tawmaw jadetract, a Cretaceous suture and ophiolite zone [Hutchison,1975]. The western and southern margins of Sundaland followthe Sunda and Java Trenches. The eastern margin is irregular;it is usual to draw the boundary of the Sundaland continentalcore through west Java, northeast into Borneo and then north-west towards the South China Sea, to exclude the mainly ophi-olitic rocks added to its eastern margin in the Cretaceous.However, we include east Java, west Sulawesi and northernBorneo within Sundaland since these areas formed its easternactive margins throughout the Cenozoic. These boundariesare similar to those suggested by seismicity and GPS meas-urements for the Sunda Block.

Sundaland was essentially in its present form, and in a sim-ilar position with respect to Asia, by the Early Mesozoic [Met-calfe, 1990; 1996] and since then has been largely emergentor submerged to very shallow depths (Figure 3). It was formedby amalgamation of continental blocks during the TriassicIndosinian orogeny and is surrounded by material mostlyadded later in the Mesozoic. During the Permo–Triassic stagesof suturing there was extensive granite magmatism from Thai-land to Malaya, associated first with subduction precedingcollision, and later with post-collisional thickening of the con-tinental crust [Hutchison, 1989; 1996b]. In west Borneo theoldest rocks known are Paleozoic metamorphic rocks intrudedby Mesozoic granites. Isotopic studies indicate the MalayPeninsula has a Proterozoic continental basement [Liew andMcCulloch, 1985; Liew and Page, 1985] and there is somegeochemical evidence to suggest that west Sulawesi mayinclude Proterozoic basement [Bergman et al., 1996]. TheMesozoic stratigraphic record is patchy but suggests that muchof Sundaland was emergent after the Late Triassic. Mesozoicterrestrial deposits are not extensive (except in northern Sun-daland from the Khorat to the Lanping–Siamo fold belt region)but are found throughout Sundaland; Mesozoic marine depositsare rare. From Sumatra to Borneo the older continental core

is surrounded by Mesozoic ophiolitic and arc igneous rocks,which may include fragments of older continental material,accreted to the Sundaland margins during the Cretaceous.There appears to have been an important tectonothermal eventin the mid to late Cretaceous which led to crustal thickeningand elevation [e.g. Barley et al., 2003; Charusiri et al., 1993;Cobbing et al., 1986; 1992; Mitchell, 1993; Mitchell et al.,2002; Morley, 2004; Upton et al., 1997]. There are granites ofCretaceous age which may represent Andean-type marginsfacing the Pacific and Indian oceans, or the products of a con-tinent collision [Barley et al., 2003]. However, little is knownof the Late Cretaceous and Paleocene history because of thepaucity of sedimentary rocks of this age on land or offshore.

3. BASIN SETTING

The areas surrounding Sundaland are currently tectonicallyactive, and include a variety of tectonic settings. The Cenozoicsedimentary basins formed near to regions of subduction-related deformation, arc–continent and continent–continentcollision, and intra-continental strike-slip deformation [Wickerand Stearn, 1999]. At present the contrast between intensetectonic activity, manifested by seismicity and volcanism, atthe margins of Sundaland and the apparent stability of itsinterior is remarkable. Most of the Sunda Shelf is flat,extremely shallow (Figure 3), with depths considerably lessthan 200 m, and was emergent at times during the Pleistocene.Data from this region have been used in construction of globaleustatic sea level curves [e.g. Fleming et al., 1998]. Perhapsfor these reasons, Cenozoic Sundaland has been viewed inthe same way, and its core has been described by some authorsas the Sunda shield or craton, widely regarded as a region ofstability [e.g. Ben-Avraham and Emery, 1973; Gobbett andHutchison, 1973; Tjia, 1996], which has remained undeformedand close to sea level since the Mesozoic. However, the char-acter of the crust and mantle in this region differ from nearbycontinental regions and there are a number of reasons to doubtthe supposed long-term stability and rigidity of Sundaland.

3.1. Tomography

P and S wave seismic tomography models (Plate 1) showSundaland is an area of low velocities in the lithosphere andunderlying mantle [e.g. Bijwaard et al., 1998; Lebedev andNolet, 2003; Ritsema and van Heijst, 2000; Widyantoro andvan der Hilst, 1997], in strong contrast to Indian and Aus-tralian continental lithosphere to the NW and SE which areprobably colder, thicker and stronger. Sundaland is an excep-tional area; the only other continental areas of comparablesize with similar low velocities at such depths are East Africaand the Basin and Range province, both regions of known

HALL AND MORLEY 3

4 SUNDALAND BASINS

Figure 2. Principal geographical and geological features of Sundaland and the surrounding region. The Sunda Shelf andcontiguous continental shelf of SE Asia is shaded in light gray up to the 200 m bathymetric contour. The other bathymet-ric contours are at 2000, 4000 and 6000 m.

active high extension. Low mantle velocities are commonlyinterpreted in terms of elevated temperature, and this is con-sistent with regional high heat flow, but they may also reflectelevated H2O or CO2 contents, partial melting or seismic

anisotropy [Lebedev and Nolet, 2003]. Many oil companywells report high CO2 levels, although it is not known if thishas a mantle source, or a deep crustal source, in which caseit may be the product of high heat flow and metamorphic

reactions in the lower or upper crust. The mantle heat flowcontribution estimated by basin modeling [e.g. Madon andWatts, 1998; Watcharanantakul and Morley, 2000] also sug-gests a high value, although because such modeling is basedon simple isostatic assumptions the mantle heat flow may beoverestimated. Worldwide heat flow from the mantle is typ-ically in the order of 20–30 mW/m2 and may exceptionallyapproach 40 mW/m2 [Artemieva and Mooney, 1999; Wanget al., 2000]. The high heat flows seen regionally acrossSundaland, and the generally low mantle velocities observedin tomographic models suggest mantle heat flow values ofthe order of 40 mW/m2.

3.2.Heat Flow

Global data clearly show that Sundaland is a region of highheat flow with surface heat flow greater than 80 mW/m2

[Artemieva and Mooney, 2001]. Plate 2 shows contoured heatflow for SE Asia based on published databases and oil com-

pany compilations [Kenyon and Beddoes, 1977; Pollack etal., 1990; 1993; Rutherford and Qureshi, 1981]. Heat flowthroughout the region is high and is not simply a result of arcmagmatism as sometimes suggested [e.g. Nagoa and Uyeda,1995]. The data used in the contoured map are mainly from oilcompany wells, which are generally far from present-day vol-canoes. A number of factors may have contributed to producethe high observed upper crustal heat flows.

Offshore Vietnam is an area of high heat flow probablyrelated to Oligo–Miocene oceanic spreading in the SouthChina Sea [Shi et al., 2003]. The area from the Malacca Straitnorth of Central Sumatra to NW Java also shows high heatflows, which are probably the consequence of nearby sub-duction-related arc magmatism in Sumatra and Java. Thereare few heat flow measurements close to volcanoes and themap probably underestimates the heat flow in the region of themodern volcanic arcs. A contribution from active volcanoeshas been estimated by assigning a heat flow value at the siteof each volcano of 140 mW/m2, based on observations in the

HALL AND MORLEY 5

Figure 3. DEM showing principal surface features of the Sundaland region based on Smith and Sandwell [1997] show-ing seafloor bathymetry from satellite altimetry combined with topography from GTOPO30.

6 SUNDALAND BASINS

Plate 1. Depth slices through S20RTS S wave tomographic model [Ritsema and van Heijst, 2000] for SE Asia. High shearvelocities are represented by blue and low shear velocities by red with an intensity which is proportional to the amplitudeof the shear velocity perturbations. The range in shear velocity variation is given below each map.

Plate 2. Contoured heat flow map for SE Asia based on the database of Pollack et al. [1990; 1993] and oil company com-pilations [Kenyon and Beddoes, 1977; Rutherford and Qureshi, 1981].

Japan Arc [Nagoa and Uyeda, 1995], but this is not incorpo-rated in Plate 2.

Heat flows measured in the interior Sundaland basins fromthe Gulf of Thailand to west Borneo are also high where arcmagmatism is not a feasible explanation. These basins aremore than 800 km from the active Sunda volcanic arc andsimilar distances from oceanic crust of the South China Sea.The Malay Basin, Pattani Basin and Gulf of Thailand are char-acterized by high to very high present-day geothermal gra-dients (30–75°C/km). In the Pattani and Malay Basinsgeothermal gradients and heat flows are very high in wellswith 4–8 km of post-rift Miocene–Recent sediments. Therange of heat flow values for 30 wells in the Pattani Basin isfrom 78 mW/m2 to 101 mW/m2 [Bustin and Chonchawalit,1995; Pigott and Sattayarak, 1993]. A similar range of heatflows is reported for the Malay Basin [Madon et al., 1999].These are values 15–25 million years after the onset of ther-mal subsidence yet they are similar to heat flows in the activeEast African Rift away from very high heat flow volcanic cen-ters. The highest heat flows and geothermal gradients in theMalay and Pattani Basins occur around the basin centers,whilst the lowest values occur around the basin flanks, sug-gesting the basins are contributing significantly to the ele-vated values. Basins can affect heat flow because of theinsulating effects of the sedimentary rocks (particularly shalesand coals) and by concentrating the radiogenic erosional prod-ucts from granitic terrains. Sundaland, particularly the zoneextending from Thailand to the Malay Peninsula, has a largenumber of granite intrusions of Triassic and Late Cretaceousage [Cobbing et al., 1992; Dunning et al., 1995; Hutchison,1973; Krähenbuhl, 1991; Schwartz et al., 1995] and thesemay also be present offshore. Consequently some of the highheat flow may be attributed to radiogenic heat production,either from the granites or from sediments derived from gran-ite sources, in the upper crust. Waples [2001; 2002] estimatedhigh radiogenic heat production from sediments could con-tribute up to 1.8 mW/m2 in the Malay and Pattani Basins.

A prolonged history of high heat flow within Sundaland isindicated by other lines of evidence. There was sporadicMiocene–Quaternary basaltic volcanism in SE Vietnam, Thai-land, the Malay Peninsula and north Borneo [Hutchison,1996b]. There are metamorphic core complexes east of theSagaing Fault in Myanmar, at Doi Inthanon and Doi Suthepin NW Thailand and around the Red River Fault Zone in Viet-nam [Bertrand et al., 1999; Dunning et al., 1995; Jolivet et al.,2001]. The metamorphic core complexes are direct indica-tors that flow of the lower crust has occurred in places. Thereare Oligocene granites at Doi Inthanon in NW Thailand. Inaddition, subduction may have contributed by arc volcanismaround the western to southern Sundaland margin betweenthe Myanmar Central Basin and the Indonesian archipelago,

and within Borneo [reviewed in Hutchison, 1996b; Macpher-son and Hall, 2002].

Sundaland, particularly the zone extending from Thailandto the Malay Peninsula, has a large number of granite intru-sions of Triassic and Late Cretaceous age [Cobbing et al.,1992; Dunning et al., 1995; Hutchison, 1973; Krähenbuhl,1991; Schwartz et al., 1995] and these may also be presentoffshore. Consequently some of the high heat flow may beattributed to radiogenic heat production, either from the gran-ites or from sediments derived from granite sources, in theupper crust. High radiogenic heat production and the insu-lating effects of argillaceous sediments appear to be importantfactors in explaining why the Pattani and Malay Basins havehigh heat flow [Waples, 2001; 2002]. However, the tomo-graphic models suggest a significant heat flow contributionfrom deeper in the lithosphere. That deep processes causehigh heat flow which affect much of the crust is supportedby gravity data and subsidence histories that suggest a thinelastic thickness for the lithosphere [Madon and Watts, 1998;Watcharanantakul and Morley, 2000]. In summary, at themargins of Sundaland high heat flows are related to subduc-tion-related processes and magma rise. In contrast, the hotinterior of Sundaland appears to be the result of high mantleheat flow, a significant upper crustal heat flow from radi-ogenic granites and their erosional products, as well as theinsulation effects of thick sediments.

3.3. Sediment Yields

Many estimates of global sediment yields ignore SE Asia.However, high present-day and long-term sediment yields forSundaland imply that relief in the region has been maintained,indicating important tectonic activity over a prolonged periodof the Cenozoic. At present, sediment discharge into the globaloceans is estimated at between 12 and 20 ? 109 t/yr [Hay,1998; Ludwig and Probst, 1998; McLennan, 1993; Millimanand Meade, 1983; Milliman and Syvitski, 1992]. Millimanand Meade [1983] suggested that more than 70% of this dis-charge comes from rivers of SE Asia and Oceania but their dataset included only 8 rivers from the region, in New Guineaand Taiwan. Milliman and Syvitski [1992] estimated a figureof 20 to 25% based on a data set including 31 rivers from SEAsia. Milliman et al. [1999] suggested a similar percentage,based on 56 rivers from Indochina, New Guinea, Java, andthe Philippines, and concluded that six islands in SE Asia(Sumatra, Java, Borneo, Sulawesi, Timor and New Guinea)provide about 20–25% of the sediment discharge to the globalocean, although they account for only about 2% of the globalland area. Although these rivers may not be representative ofthe entire region all estimates indicate the region is currentlyone of high sediment supply to the oceans [Suggate and Hall,

HALL AND MORLEY 7

2003]. This is supported by studies of short-term sedimentyields in the region by hydrologists [e.g. Douglas, 1967; 1999;Douglas et al., 1992]. However, additional data are neededfor SE Asia and the West Pacific islands, particularly fromthe large ever-wet tropical islands with small human popula-tions such as New Guinea and Borneo.

There have been few studies of long-term sediment yields butthose studies [Hall and Nichols, 2002; Morley et al., 2003]show that the late Cenozoic rates are consistent with estimatesof current rates. For example, the amount of sediment derivedfrom Borneo in the Neogene is about one third of that erodedfrom the Himalayas during the Neogene [Hall and Nichols,2002], indicating similar denudation rates per unit area.

Although there are complex relationships between rockuplift, weathering, erosion and downslope transport [Hovius,1998] it is clear that the high rates of sediment supply indicateelevation and relief, although factors such as rainfall andrunoff are also important [Hay, 1998; Ludwig and Probst,1998; Milliman and Syvitski, 1992; Summerfield and Hulton,1994]. Sundaland has been close to its present position at theequator throughout the Cenozoic. There was a change from anOligocene drier climate to a wetter Neogene climate [Mor-ley, 1998; 2000]. Parts of the region, such as Borneo, remainedever-wet throughout the Neogene, whereas areas north andsouth of the equator, such as Sumatra, Java and Indochina,developed a monsoonal, highly seasonal climate by thePliocene [Morley, 1998; 2000]. However, although precipi-tation and runoff may have changed during the Cenozoic, cli-mate alone cannot account for high sediment fluxes, and reliefmust have been maintained. Was the relief within Sundaland,or was sediment derived from elevated regions outside Sun-daland? The sediments in basins in and around Sundaland areoften interpreted to be the product of erosion much furthernorth in Asia following Indian collision [e.g. Métivier et al.,1999]. In contrast, we suggest that the sedimentary basins ofthe region were filled from local sources and there is no needto assume significant input from Asia. This in turn impliestectonic activity to produce the relief required to maintainsediment input as discussed below.

3.4. Sediment Sources Within Sundaland

The location of Sundaland adjacent to the Himalayas and thegreat rivers of Indochina (Mekong, Red River, Salween) flow-ing from the north (Figure 2) has suggested links betweensedimentation and the rise of the Himalayas and Tibet. Hutchi-son [1996a] and Hall [1996] suggested that rivers such as theMekong and Chao Phraya crossed Sundaland to feed the NWBorneo margin, largely to explain the amount of sedimentthere, although both later amended this view [Hall and Nichols,2002; Hutchison et al., 2000]. Métivier et al. [1999] identified

changes in sedimentation rates during the Cenozoic and con-cluded that extrusion tectonics (consistent with lack of an ele-vated mountain belt) was more important between 50 and 30Ma, whereas crustal thickening and uplift became more impor-tant after 30 Ma when sedimentation rates increased. Theyfocused on the effect of the India–Asia collision and ignoredsediment input from local sources. However, local sourcesare significant and recent work has cast doubt on a long his-tory of erosion in Asia supplying sediment to the south [Clark,2003]. We have highlighted above the importance of high sed-iment yields in the region. We suggest that despite being adja-cent to the India–Asia collision zone most of the sedimentwas derived from elevated regions within Sundaland.

Before 30 Ma there were barriers to sediment supply fromAsia. Thailand is at the lower end of the topographic gradientfrom the Himalayas, which has been explained as a result of awedge of ductile lower crust flowing out from Tibetan Plateau[Clark and Royden, 2000]. However, to view this slope as a con-sequence only of India–Asia collision is too simple. Radio-metric and apatite fission track studies discussed below suggestthat there was initial uplift in the Late Cretaceous–Paleogene,between the Shan Plateau of Myanmar and northern Thailand,through Laos, into the Lanping–Siamo fold belt (Figure 4).During the Oligocene–Early Miocene a narrow N–S elevatedzone was imposed on the earlier uplift in western Thailand.Thus, for much of the Cenozoic, northern Sundaland was rel-atively high ground and a broad topographic saddle may haveexisted between the Sundaland highlands and the Himalayas.In the Middle Miocene uplift spread out radially from easternTibet [e.g. Clark and Royden, 2000] and linked with existinghighlands of northern Sundaland (Plate 3).

Further barriers were produced by active faulting. In Thai-land the largest transverse structural barrier occurs where theMae Ping Fault cuts NW–SE across the Cenozoic rift basinsat the Chainat strike-slip duplex (Figures 4 and 5). This restrain-ing bend high separated the Phitsanulok Basin to the northfrom the Suphan Buri and Ayutthaya Basins to the south dur-ing the Oligo–Miocene [O’Leary and Hill, 1989] and liesdirectly across the path of the present-day Chao Phraya River.

There is also the question of whether the large fluvial sys-tems of Indochina were even established by the MiddleMiocene. It is unlikely that any through-going fluvial systemrecognizable as the Chao Phraya River (Figures 2 and 5) existedduring the Late Oligocene and Miocene when the rift basins ofThailand were active because active rift basins tend to be sitesfor interior drainage into long-lived lakes. In Thailand there isconsiderable evidence for prolonged periods of lacustrine con-ditions during the Late Oligocene–Middle Miocene in many ofthe rift basins both onshore (e.g. Phitsanulok, Mae Sot, Fang,Suphan Buri, Li, Lampang, Chiang Muan, and Mae Moh [Flintet al., 1988; Morley et al., 2001; O’Leary and Hill, 1989; Pra-

8 SUNDALAND BASINS

ditan, 1989]) and offshore (Chumphon and Pattani Basins[Jardine, 1997; Praditan, 1989]).

In the Late Miocene extension began to diminish in activ-ity, and by the Pliocene inversion marks the end of extensionin most basins [Morley et al., 2001; Uttamo et al., 2003]. If anearly Chao Phraya River had existed it would not have linkedwith Tibetan plateau drainage systems. The different branchesof the Chao Phraya fluvial system probably linked up only inthe Pliocene, when erosion and thermal subsidence began toeliminate the rift topography of central Thailand (Figure 5).

The absence of major through-going rivers is also indicatedby the very proximal character of sediment in several basins[O’Leary and Hill, 1989; Praditan, 1989; Uttamo et al., 1999]which was derived from adjacent highlands. Syn-rift sand-stones are generally sub-arkoses, litharenites and arkoses sug-

HALL AND MORLEY 9

Figure 4. Northern Sundaland (Myanmar, Laos, Thailand, Cambo-dia, China) showing the many extensional and strike-slip faults andassociated basins which indicate significant internal deformation ofthe Sundaland region during the Cenozoic (based on satellite images,surface mapping and seismic reflection data). The Mae Ping andThree Pagodas Fault Zones display numerous branching, splaying andduplex geometries, and affect a large area (unlike the Sagaing and RedRiver Faults), possibly indicating a transpressional setting duringthe Paleogene.

Figure 5. The present-day drainage of Thailand and adjacent regions,shown with the principal rift basins and associated faults. Some ofthe main barriers to through-going drainage such as long-lived lakes,and structural barriers (Chainat Ridge) are highlighted. The pres-ent-day drainage pattern reflects the older syn-rift pattern (drainagenormal to rift orientation) and late linkage of rift axial systems.

10 SUNDALAND BASINS

Plate 3. Reconstructions of the tectonic evolution of SE Asia based on Morley [2002a] modified to show the nature of basinfill (marine or continental) and approximate location and timing of rifting and deformation in Sundaland.

gesting a short transport distance [e.g. Flint et al., 1988;O’Leary and Hill, 1989; Praditan, 1989; Trevana and Clark,1986; Uttamo et al., 1999]. Provenance studies of Miocenesandstones and conglomerates in the Chiang Mai Basin, thelargest basin in northern Thailand, show a history of unroof-ing of the metamorphic core complexes west of the basin[Rhodes et al., 2003]. In northern Thailand the earliest basinfill was derived from metamorphic rocks, and in the LateMiocene–Pliocene granitic rocks were exposed [Praditan,1989; Uttamo et al., 1999]. Further south the Oligocene–Mid-dle Miocene fill of the Ayutthaya Basin is predominantly allu-vial fan and fluviatile sediments, with large volumes of arkosederived from granites immediately north of the basin, andvolcanic and clastic input from the Khorat Plateau to the east[O’Leary and Hill, 1989].

The Tibetan rivers were deflected away from Thailand byPaleogene and Oligo–Miocene uplift, and carried their sedi-ment to the west and east of Sundaland (Figure 2 and Plate 3).In the west the Bengal Fan and Myanmar Central Basin (Fig-ures 1 and 2) were the two major sites of Himalayan sedi-mentation until the Middle Miocene [e.g. Pivnik et al., 1998;Shamsuddin and Abdullah, 1997]. Until the Middle Miocenesome rivers from the Tibetan Plateau flowed towards the Cen-tral Basin, but uplift in Assam, SE Tibet and northern Myan-mar caused this route to be cut. One likely consequence ofthe uplift was to cut off the Irrawaddy River from the TibetanPlateau and restrict its headwaters to a region just north ofthe Central Basin. It has been proposed [Clark et al., 2004] thatcapture of river systems occurred when the Tibetan Plateauuplift propagated eastwards so that other drainage systems,notably the Salween and Mekong Rivers, started to emergefrom the Tibetan Plateau. There appears to have been a markedchange in the site of deposition of much of the sedimentderived from the Tibetan Plateau; before the Middle Mioceneit was in the Bengal Fan and Central Basin, after the MiddleMiocene it was the Gulf of Martaban (Salween River) andthe South China Sea (Mekong River).

Much less is known about drainage patterns and develop-ment in the southern part of Sundaland. However, the palaeo-geography of the region indicates a fluvial connection betweenIndochina and NW Borneo was unlikely. By the MiddleMiocene there was a marine shelf between Indochina andBorneo (Plate 3) and the many basins (Pattani, Malay, NamCon Son) within this shelf would have trapped sediments longbefore they reached the Borneo margin. Provenance studies[van Hattum et al., 2003] show that Paleogene deep watersediments of the Crocker Fan [Crevello, 2001; William et al.,2003] were probably derived from Borneo itself. 3D seismicdata reveal complex meandering rivers in offshore basins ofthe Sunda Shelf [e.g. Crevello, 2001; Fatt, 1999] but there istoo little published information to construct a coherent picture

of drainage patterns. Throughout the Neogene major riversflowed onto the Sunda Shelf from Borneo and smaller riversfrom the Malay Peninsula. The volume of sediment erodedfrom Borneo was huge and the Neogene depocentres offshoreSarawak and Sabah are extremely deep, as recorded above,and filled entirely with sediment derived from Borneo. Suma-tra and Java, at the edge of Sundaland, appear to have been iso-lated from rivers of the Sunda Shelf by elevated regionsrunning from the central Malay Peninsula via the SingaporeRidge into western Borneo. Most rivers probably flowed southand west from these highlands filling the Mergui, Sumatraand Java Basins until about the Late Miocene when Java andSumatra became elevated causing rivers to flow north andeast into the Malacca Straits and Java Sea.

4. BASIN HISTORIES

Recent reviews of Sundaland Cenozoic sedimentary basinsand their setting have been given by several authors [Hall,2002; Longley, 1997; Morley, 2001; 2002a; Pertamina BPPKA,1996a; 1996b; 1996c; Petronas, 1999; Sandal, 1996]. Thecharacteristics of the basins and timing of basin initiation andinversion (Plates 4 and 5) are topics too large to be discussedin detail but here we summarize some key features.

4.1. Early Cenozoic Setting of Basin Formation

As noted above, there are few Upper Cretaceous and Pale-ocene rocks on land or offshore. Their absence is usually inter-preted to indicate that most of Sundaland was above sea levelat the beginning of the Cenozoic. Emergence may have beenthe result of a tectonothermal event that affected the regionfrom Indochina to the Malay Peninsula [e.g. Ahrendt et al.,1993; Dunning et al., 1995; Krähenbuhl, 1991; Upton et al.,1997]. There is also evidence of Late Cretaceous deformationassociated with emplacement of ophiolites and metamorphismat the Sundaland margins in Borneo, Java and Sumatra.

At the beginning of the Cenozoic (Plate 3) northern Sun-daland was relatively high. Uplift in the Late Cretaceous–Pale-ogene extended from the Shan Plateau of Myanmar andnorthern Thailand through Laos into the Lanping–Siamo foldbelt (Figure 4) as the result of a diffuse, poorly defined oro-genic event [Charusiri et al., 1993; Morley, 2002b; 2004;Upton, 1999; Wang and Burchfiel, 1997]. In Thailand, out-side the region of metamorphic core complexes, apatite fissiontrack studies [Racey et al., 1997; Upton, 1999; Upton et al.,1997] indicate the onset of cooling between 70 and 50 Ma, andgeological observations [Mouret et al., 1993] indicate anepisode of inversion during the early Paleocene (about 65Ma). In the frontal monocline and Loei–Petchabun fold belt,the amount of post-Khorat Group (Jurassic–Cretaceous) over-

HALL AND MORLEY 11

Plate 4. Ages of basin initiation in Sundaland. The record typically begins in the Eocene or Oligocene although since theolder parts of most sequences are terrestrial, and deeper parts of many basins are not drilled, most are relatively poorly dated.

Plate 5. Ages of basin inversion or elevation due to tectonism in and around Sundaland.

burden thickness removed since inversion is estimated at about2.3 to 4.4 km, depending on the geothermal gradient used(25°C km-1 [Mouret et al., 1993]; 35°C km-1 [Racey et al.,1997]) whereas some 2–6 km of section may have beenremoved during the Paleogene in parts of western Thailand[Morley, 2004; Upton, 1999].

Further south the region is interpreted to have been conti-nental based largely on negative evidence. The Malay Penin-sula has probably been above sea level since the Mesozoic.Upper Jurassic–Lower Cretaceous sediments in the MalayPeninsula are continental and described as molasse, and over-lie older rocks with a marked unconformity [Gobbett andHutchison, 1973; Harbury et al., 1990] They indicate fluvial,lacustrine and deltaic environments. There are a few Cenozoicrocks in isolated lacustrine basins [Stauffer, 1973]. K–Ar datesand fission track ages indicate a local increase in cooling ratein the Late Cretaceous [Krähenbuhl, 1991; Kwan et al., 1992],at a similar time to the uplift interpreted for Thailand. Else-where, there are unconformable contacts of Paleogene strataon Cretaceous or older sedimentary, metamorphic and igneousrocks where the basement to sedimentary basins is seen (forexample, in Sumatra, Java and Borneo), or has been pene-trated by offshore drilling. Sediment provenance studies [vanHattum et al., 2003] indicate that an extensive area of SWBorneo was elevated by the Eocene, and hydrocarbon explo-ration has shown that the area between south Borneo and Javaremained elevated until the Neogene [Bishop, 1980; Matthewsand Bransden, 1995]. To the east of Sundaland was the Proto-South China Sea where there was already extension in theEarly Cenozoic along the Vietnam–South China margin [Chenet al., 1993; Hayes et al., 1995; Jinmin, 1994; Lee et al., 2001;Matthews et al., 1997; Rangin et al., 1995; Ru and Pigott,1986; Zhou et al., 1995]. To the south of the Proto-SouthChina Sea some authors have suggested subduction beneaththe NW Borneo margin in Sarawak [Hazebroek and Tan, 1993;Hutchison, 1996a; Taylor and Hayes, 1983] whereas othersinterpret a passive margin during the Paleocene and EarlyEocene [Hall, 1996; 2002].

4.2. Basin Formation

From the Eocene onwards numerous sedimentary basinsformed throughout Sundaland (Figures 1 and 2), many ofwhich are unusually deep. Cenozoic sediments were depositedon a variety of basement rocks ranging from granites to ophi-olites. The record typically begins in the Eocene or Oligocene(Plate 4) although since the older parts of most sequences areterrestrial all are relatively poorly dated.

The basins can be divided into three main areas and tec-tonic provinces: 1) active margin basins that lie inboard ofthe Andaman–Sumatran–Java Trench (Mergui, North, Cen-

tral and South Sumatra, West and East Java Basins); 2) SundaShelf basins that trend NW–SE to N–S within the interior ofSundaland (Thailand, Gulf of Thailand, and Malay, Penyu andWest Natuna Basins) which formed at a similar time to basinsfurther east which are offshore Vietnam (Nam Con Son Basin),and 3) Borneo margin basins (Baram Delta province, San-dakan, Tarakan and Kutai Basins) that are situated on the for-mer active margin of northern Borneo and passive margin ofeastern Borneo.

4.2.1. Sundaland active margins. At the southern edge ofSundaland are basins that formed close to Cenozoic subduc-tion margins. In gross geometry the Mergui, North, Central,South Sumatra, West and East Java Basins appear to lie in asimple backarc location, distributed along an arcuate trend,mimicking the trench, some 300–700 km into the upper plate.These basins developed across a broad area of southern Sun-daland during the Paleogene [Eubank and Makki, 1981;Matthews and Bransden, 1995; Pertamina BPPKA, 1996a;1996b; 1996c; Petronas, 1999; Williams and Eubank, 1995;Williams et al., 1995]. The earliest sediments in all of thebasins are continental, and therefore the exact age of basininitiation is uncertain. Many basins (e.g. Sumatra) remainedlargely continental until the Miocene whereas others (EastJava, Barito) became marine by the Late Eocene [Cole andCrittenden, 1997; Mason et al., 1993; Matthews and Brans-den, 1995]. Rifting had ceased by the Early Miocene, there wasa transition to thermal subsidence, and marine deposition waswidespread.

The orientation of basin-controlling structures varies; someare trench-parallel whereas others are highly oblique to thetrench. The basins have been described as failed backarc riftbasins [e.g. Eubank and Makki, 1981], the Sumatran basinshave been interpreted as formed in a strike-slip setting althoughthe Sumatran Fault system became active long after their for-mation, in the Miocene [McCarthy and Elders, 1997], and ithas also been suggested that they formed in response to sub-duction rollback [Morley, 2002a]. At the northern end of thesystem the Mergui–North Sumatra Basin has a broadly N–Strench-parallel orientation [Pertamina BPPKA, 1996a]. TheN–S orientation of the Sumatran rift basins may reflect a pre-existing N–S basement fabric controlling fault orientation.During Miocene–Pliocene inversion the N–S fabric was over-printed by NW–SE trends [Pertamina BPPKA, 1996a; 1996b;1996c]. In the Central and Southern Sumatra Basins the dom-inant structural grain of the Eocene–Oligocene rifts is N–S toNNE–SSW, 30–40° oblique to the trench, although secondaryNW–SE trends are also present [Madon and Ahmad, 1999;Pertamina BPPKA, 1996b; 1996c]. In the southern part ofthe system in the offshore West and East Java Basins (Plate 6)the rifts strike NE–SW and appear to reflect basement struc-

HALL AND MORLEY 13

14 SUNDALAND BASINS

Plate 6. Basin cross-sections in the Sundaland region based on published studies: a) East Java Sea [Matthews and Bransden,1995]; b) Triton Horst, offshore Vietnam [Dang and Sladen, 1997]; c) Cuu Long–Nam Con Son Basins, offshore Vietnam [Simonet al., 1997]; d) West Natuna Basin, Sunda Shelf [Phillips et al., 1997]; e) northern Malay Basin [Petronas, 1999].

tural trends [Hamilton, 1979] and the orientation of Paleo-gene extension in the Makassar Strait [e.g. Hall, 2002].Onshore in East Java the NE–SW structures interact with anarc-parallel trend and volcanic loading contributed to basin sub-sidence [Smyth et al., 2003].

4.2.2. Sunda Shelf. Perhaps the most remarkable and unusualbasins within Sundaland are those in the central region. Thisregion stretches from Sarawak to southern Laos, where thereis a discontinuous string of basins of highly variable strikecontaining up to 14 km [e.g. Petronas, 1999] of Cenozoicsediments. These basins include the onshore Thailand basins,Gulf of Thailand basins including the large Pattani Basin, andthe Malay, Penyu and West Natuna Basins of Malaysia andIndonesia (Figure 1 and Plate 6).

Some of these basins have been suggested to have a strike-slip, rather than an extensional, origin [Petronas, 1999;Polachan et al., 1991; Replumaz et al., 2004; Tapponnier etal., 1986]. However, seismic reflection data from the Gulf ofThailand indicate the basins are extensional, and while somerecord oblique extension there is no evidence of major through-going strike-slip faults [Morley, 2001; 2002a]. Basement fab-rics must have influenced basin orientation, but other factorssuch as regional tectonic controls on stress orientation andmagnitude probably played a significant role. Most of thebasins are characterised by several phases of extension, typ-ically with different fault orientations during the differentphases of extension.

The Malay and Pattani Basins are characterized by Ceno-zoic sedimentary rock thicknesses greater than 6 km, rapid sub-sidence rates (maximum time-averaged rates over 5 Ma fromthe main depocentres of 0.2–1 cm/yr), high to very high pres-ent-day geothermal gradients (30–75°C/km), and the absenceor minor importance of fault-controlled subsidence for themajority of the basin history. When compared with typicalrift basins such as the North Sea, subsidence rates for theseSunda Shelf basins are almost an order of magnitude higher(Figure 6). Many of the central Sundaland basins, in particu-lar the Malay, Pattani and Nam Con Son Basins, have exten-sive and thick post-rift sequences. Consequently the basincenters are too deep and thick to be completely penetrated bydrilling. Dating of rift sequences is either from shallow incom-plete sections on the basin flanks, or is inferred from the ageof overlying sequences. However, it appears that during theEocene (and possibly earlier) there was an extensive area ofrift basins filled predominantly by continental deposits in theheart of Sundaland.

Thermal subsidence began in the Late Oligocene, accom-panied by some late extensional faulting. Extension in theMalay Basin became progressively inactive from south tonorth. In some basins, such as the Malay Basin [Petronas,

1999] and Nam Con Son Basin [Lee et al., 2001; Matthews etal., 1997; Olson and Dorobek, 2000] there is evidence formore than one phase of extension. Thermal subsidence beganin offshore Sarawak in the Early Miocene, in the northwest-ern Gulf of Thailand in the Middle Miocene, and in centralThailand in the Late Miocene–Pliocene. There are threeremarkable features about this string of basins: the progressivenorthward-younging of the onset of thermal subsidence, the

HALL AND MORLEY 15

Figure 6. Comparison of subsidence rates of North Sea basins andthree super-deep basins of SE Asia. The North Sea curves are fromthe Viking Graben and show a rift to post-rift subsidence rate typi-cal of average continental crust. The SE Asian basins have subsidedan order of magnitude faster, possibly indicating unusual continen-tal crust and mantle. Subsidence curves are from the deepest parts ofthe basins.

very large component of thermal subsidence in the Malay andPattani Basins for the dimensions of the rifts, and the associ-ation with metamorphic core complexes on the western mar-gin of the rift system in northern Thailand.

The Malay and Pattani Basins are rift basins. However theyexhibit small amounts of extension (β < 1.5) for the thick-ness of post-rift section (4–10 km). The Malay Basin (Plate 6)has an Eocene–Oligocene syn-rift section greater than 4 kmthick and a Lower Miocene–Recent post-rift section thatexceeds 8 km thickness in places [e.g. Madon et al., 1999]. Theamount of upper crustal extension indicated by offsets on syn-rift faults visible on seismic reflection lines is less than β = 1.5[e.g. Madon et al., 1999] but back-stripping of the post-rift sec-tion suggests lithosphere thinning up to β = 4 [Madon andWatts, 1998]. The Pattani Basin has a maximum of about 7.5km of Upper Oligocene to Recent section. The MiddleMiocene-Recent thermal subsidence component reaches amaximum thickness of 6 km. Flexural and Airy backstrip-ping of the Pattani Basin post-rift section indicates β = 2–2.5.Like the Malay Basin this v value does not match estimates ofextension from the syn-rift section using direct measurementsfrom seismic sections and from STRETCH modeling (β = 1.3[Watcharanantakul and Morley, 2000]). Wheeler and White[2000] suggested that post-rift subsidence in the Pattani Basinis not anomalous. However, they interpreted the base of thepost-rift section to be located near the tops of small normalfaults (assuming they were syn-rift structures) and incorrectlyplaced the base of the post-rift section at very shallow depths(maximum of 2 km). In fact, the post-rift section is affectedby minor displacement conjugate normal faults [Kornsawanand Morley, 2002] and the thickness of the post-rift section ismuch greater (6 km).

In the northern and western parts of Sundaland there arefew Cenozoic rocks and most areas were probably emergentfor much of the Cenozoic. The Gulf of Thailand basins can betraced north onland into Thailand where there are many smallbasins, predominantly terrestrial. The onland basins were ini-tiated in the Late Oligocene–Early Miocene and containsequences which are predominantly fluviatile or lacustrinewith a few possible marine horizons. This contrasts with theGulf of Thailand where the basins became fully marine fromthe Middle to Late Miocene. Radiometric age dating indi-cates the major phases of left-lateral displacement on the MaePing and Three Pagodas Faults ended in the Late Oligocene[Lacassin et al., 1997] before the main phase of onland basinformation. Uttamo et al. [2003] suggested that fault tip, pull-apart, fault wedge and extensional basins were related to NW-trending dextral and NE-trending sinistral strike-slip faultswhich developed during Late Oligocene–Early Mioceneeast–west extension. Structural relationships within severalbasins demonstrate that normal faults have in places followed

underlying strike-slip faults (as pre-existing fabrics), but arenot kinematically related to them [Morley, 2001; Morley,2002a]. Strike-slip faults oblique to the N–S rift trend havebeen reactivated as oblique extensional faults.

4.2.3. Borneo margins. Around Borneo are several verydeep basins mainly filled by the erosional products of upliftedorogenic highlands but little is published on some of the basinsand for many their deep structure is unknown. These basinsinclude the Baram Delta province, NW Borneo; the Sandakanand Tarakan Basins of NE Borneo; and the Kutai Basin, eastBorneo (Plate 7). Some of the basins were initiated in thePaleogene but in all cases most of their sediment fill is Mioceneand younger. They are not well understood and there appearto be several different basin-forming mechanisms but like theSunda Shelf basins they contain large amounts of sedimentdeposited very quickly.

During the Early Cenozoic there were marine areas to thenorth and east of Borneo: the Crocker Fan and accretionaryprism lay to the northwest [Hutchison, 1989; Hutchison etal., 2000] and developed in response to southeastward sub-duction of the Proto-South China Sea along the northern mar-gin of Borneo (Plate 3). To the east the Kutai and TarakanBasins form part of a passive continental margin that passeseast into thinned continental or oceanic crust beneath thenorthern Makassar Straits [Calvert and Hall, 2003; Cloke etal., 1999].

The Kutai Basin formed by Eocene rifting of the MakassarStrait which separates east Borneo from west Sulawesi. Thereis up to 14 km of Eocene–Recent sediment [Moss and Cham-bers, 1999] and the majority of this was probably depositedsince the Early Miocene and was derived from erosion of theBorneo highlands and inversion of older parts of the basinmargins (Plate 7), to the north and west, which began in theEarly Miocene [Ferguson and McClay, 1997; van de Weerdand Armin, 1992]. The thick Miocene–Recent basin sequenceshave filled accommodation space created during the Eocenerifting. Offshore NE Borneo are other deep basins of differ-ent origins. In the Tarakan Basin the Plio-Pleistocene sequencealone is about 4 km thick. Wight et al. [1993] recorded that thedeepest Pleistocene depocentre contains a sediment thicknessof up to 4.5 km and that Pleistocene depocentres do not coin-cide with Pliocene depocentres. No modeling of basin his-tory has been published but strike-slip fault influence issuggested by published maps [Wight et al., 1993]. Swauger etal. [1995] reported 10 km of Neogene sediments in the San-dakan Basin, and Graves and Swauger [1997] suggested theremay be as much as 15 km of sediment. Like the Kutei Basinthe thick Miocene–Recent basin sequences offshore Sandakanhave filled pre-existing accommodation space, in this casecreated during Miocene rifting of the Sulu Sea, but there are

16 SUNDALAND BASINS

HALL AND MORLEY 17

Plate 7. Regional cross-sections across northern and eastern Borneo. Deep structure is schematic, but is based on grav-ity and seismic reflection data. The more detailed cross-sections focus on basin geometry and are based largely on indus-try seismic reflection data: Kutai Basin [Calvert, 2000a; Calvert, 2000b] NW Borneo [Morley et al., 2003; Sandal, 1996].

also thick sequences onshore (more than 6 km) in circularbasins which are remnants of a formerly more extensive basin[Balaguru et al., 2003]. These sediments were deposited aftera period of uplift and erosion in the Early Miocene resultingfrom collision between South China continental crust and thenorth Borneo active margin. Soon after the collision therewas rapid subsidence, in the Middle Miocene, depositing thickclastic sequences in delta and pro-deltaic environments whichwere supplied from erosion of the central Borneo highlandsto the south and west.

Uplift in the northern part of Borneo is asymmetric andgreatest towards the NW margin, due to partial subductionof buoyant thinned continental crust beneath the NW margin[Hutchison et al., 2000]. This Middle Miocene-Recent uplifthas resulted in considerable erosion, and deposition of theerosional products on the shelf and deepwater region of NWBorneo. Episodic folding, thrusting and strike-slip deforma-tion, reflected in local and regional unconformities [e.g. Lev-ell, 1987], affect the Sabah margin, where MiddleMiocene–Recent sequences are generally less than 4 km thick.To the SW the Sabah margin passes into the Baram Deltaprovince which contains up to 12 km vertical thickness ofMiddle Miocene–Recent section in places [Morley et al.,2003; Sandal, 1996]. The province is characterized by grav-ity-style deformation typical of large deltas (growth faults,mobile shales, toe thrusts) and by tectonic uplift events andinversion of growth faults [Morley et al., 2003].

Although all these basins may have formed by several dif-ferent processes we suggest they also should be viewed withinthe Sundaland context because they have unusually thick sed-imentary sequences that were deposited in short periods oftime and were derived from local sources.

4.3. Inversion

During the Cenozoic the sedimentary basins were exposedto erosion as a consequence of eustatic changes in sea level aswell as a variety of local and regional tectonic events. Erosionof elevated regions occurred during several episodes of upliftand inversion that affected Sundaland since the Late Cretaceous(Plate 5). Identifying and understanding the causes is an ongo-ing problem. Despite a long-term fall in global sea level fromthe Early Miocene there was a change to more widespreadmarine conditions beginning in the Early Miocene. A long-termgradual subsidence producing such flooding has been sug-gested to be a dynamic topographic effect resulting from long-term subduction [Lithgow-Bertelloni and Gurnis, 1997].However, the area of marine conditions increased from theeast from the region of South China Sea oceanic crust (Plate3) and accompanied post-rift thermal subsidence in the SundaShelf basins from the Late Oligocene to Early Miocene which

suggests cessation of rifting and onset of thermal subsidencewas one driver for the onset of marine conditions. In addi-tion, seismic reflection data suggest renewed subsidence inthe Miocene with a tectonic origin, at least in southern Sun-daland. Although many unconformities are of local extent, atleast three events are widespread, and are probably princi-pally of tectonic origin: at about 25 Ma, 15–17 Ma and 5–6 Ma.However the important widespread unconformity around 11Ma seen in many basins including the Pattani, Malay andNam Con Son Basins and NW Borneo [e.g. Ginger et al.,1993; Higgs, 1999; Jardine, 1997; Matthews et al., 1997]probably has a primarily eustatic cause [e.g. Haq et al., 1987].

4.3.1. Thailand to Malaysia. A significant period of upliftwithin Sundaland occurred during the Oligocene–EarlyMiocene and formed a N–S belt running from northwesternThailand through at least as far as peninsular Malaya (Plate 5).40Ar-39Ar dating of feldspar and micas from within the ThreePagodas and Mae Ping Shear Zones yield cooling ages ofaround 36–30 Ma [Lacassin et al., 1997]. These ages are inter-preted to represent the cessation of strike-slip faulting. Apatitefission track central ages around the fault zone range between29-20 Ma [Upton, 1999]. The ages are similar to those obtainedfor uplift of the metamorphic core complexes further north,west of Chiang Mai, with muscovite ages of 24–16 Ma, andapatite fission track ages of 23–18 Ma [Dunning et al., 1995;Rhodes, 2002]. However Oligocene–Miocene uplift is not justconfined to the strike-slip faults and metamorphic core com-plexes, In western Thailand between the Mae Ping and ThreePagodas Faults apatite fission track central ages are 18±4 and22±1 Ma [Upton, 1999]. Along the Thai Peninsula south of theThree Pagodas Fault Cretaceous granites yield apatite fissiontrack central ages of 25–20 Ma and the Kaeng Kratchen For-mation has ages of 35–20 Ma [Upton, 1999]. In the MalayPeninsula K–Ar dates and fission track ages indicate a localincrease in cooling rates to 5°C/m.y. in the Oligo–Miocene[Krähenbuhl, 1991; Kwan et al., 1992] suggesting uplift.These data suggest a N-S belt of uplift developed during theOligo–Miocene, which would have provided an importantlocal sediment source both for the Mergui Basin to the west,and the onshore Thailand, Gulf of Thailand and Malay Basinsto the east.

4.3.2. SW Borneo. The Oligo–Miocene uplift may haveextended even further southeast into SW Borneo (Plate 5).An extensive area between Malaya and western Borneoremained or became elevated during the Cenozoic although thetiming of elevation, the amount of crust eroded, and exactsites of sediment deposition are not known. Sediment prove-nance studies [van Hattum et al., 2003] show that SW Borneoprovided sediment to the Crocker Fan from the Eocene. The

18 SUNDALAND BASINS

Malay Peninsula was probably elevated from much earlier,judging from the character of Mesozoic rocks and the absenceof Cenozoic rocks summarized above, and is connected toBorneo by the Singapore Platform, which at its shallowestpoint now has a water depth of 29 m. There, a thin Quaternarycover on probable Cretaceous basement is indicated by seis-mic surveys and drilling [Ben-Avraham and Emery, 1973].This suggests very recent erosion, although nothing is knownof the timing of uplift and amount of erosion. However, fur-ther south, thick fluviatile sequences in the hydrocarbon-richNW Java Basins pass south into thick Mio–Pliocene turbiditesin West Java, which are now up to 2000 m above sea level. Thisimplies significant sediment supply, a source area to north(the Singapore Platform?), and uplift since the Late Miocene.

4.3.3. Sundaland interior. In the interior of Sundaland seis-mic surveys show there were inversion events from the EarlyMiocene, apparently at different times in different areas [e.g.Morley, 2001; Petronas, 1999]. In northern Thailand there wereat least four episodes of inversion from the Late Oligocene tothe Pliocene [Morley, 2001; Uttamo et al., 1999]. There wasonly minor inversion in the Gulf of Thailand, whereas furthersouth in the Malay and West Natuna Basins inversion is moreimportant (Plate 6), with several episodes of Miocene inver-sion and associated intra-Miocene unconformities [Ginger et al.,1993; Higgs, 1999; Jardine, 1997; Lockhart et al., 1997; Phillipset al., 1997; Tjia and Liew, 1996]. In the Malay, Penyu andNatuna Basins there is up to 4000 feet of missing section [Higgs,1999]. Discontinuities in backstripped subsidence curves [e.g.Madon and Watts, 1998; Wheeler, 2000] suggest renewed sub-sidence after Late Miocene inversion. This is also suggestedby stratigraphy because after inversion the Sunda Shelf becameentirely marine, although global sea level was falling, indicat-ing significant vertical movements during the Neogene. Apatitefission track data from onshore central Vietnam show a sig-nificant increase in uplift rate during the Late Miocene [Carteret al., 2000]. Inferred denudation rates changed from about 34m/m.y. to 390–500 m/m.y. coinciding with enhanced progra-dation of sediments in adjacent offshore basins. The increase indenudation rate coincided with eruption of basalts during theLate Miocene and could be linked with magma underplating andregional uplift related to a thermal anomaly below Indochina[Carter et al., 2000; Hoang and Flower, 1998].

4.3.4. Malay Peninsula, Sumatra and Java. There are indi-cations of uplift, inversion, or more significant orogenic con-traction, from many parts of southern Sundaland. In the LateNeogene there was significant deformation in Sumatra, Javaand Borneo resulting in widespread elevation [e.g. Pertam-ina BPPKA, 1996a; 1996b; 1996c]. For example, on the westside of the Malay Peninsula are the Mergui and Sumatra

Basins, which received sediment from the Malay Peninsula inthe Oligocene and Early Miocene. In both Sumatra and Javathere is evidence of significant folding and thrusting duringthe Neogene [Eubank and Makki, 1981; Matthews and Brans-den, 1995; Pertamina BPPKA, 1996a; 1996b; 1996c; Petronas,1999; Williams and Eubank, 1995; Williams et al., 1995]. Inmany areas these were more profound events than suggestedby the term inversion but their cause is not obvious. They aredifficult to explain in terms of regional plate motions, whichsuggest relatively continuous subduction at the Sunda–JavaTrenches. It suggests variation in the strength of couplingbetween the overriding and downgoing plates.

4.3.5. North Borneo. After collision of South China conti-nental fragments with the former active margin in NE Borneo(Plate 7) sediments were deposited in basins above the colli-sion-related unconformity. Later thrusting and folding beganin the Middle Miocene [Hazebroek and Tan, 1993; Hutchisonet al., 2000; Morley et al., 2003; Petronas, 1999; Sandal, 1996].In eastern Borneo inversion in the Kutai Basin (Plate 7) beganin the Early Miocene, not driven by deformation to the eastas often assumed [e.g. McClay et al., 2000; van de Weerd andArmin, 1992], but by events within Borneo [Calvert, 2000a;Calvert and Hall, 2003; Hall, 2002; Hall and Wilson, 2000].Inversion moved from west to east during the Early and Mid-dle Miocene [Ferguson and McClay, 1997; McClay et al.,2000; Moss et al., 1997]. In SE Borneo inversion of the Bar-ito Basin and uplift of the adjacent Meratus Mountains occurredepisodically from the Late Miocene to Pleistocene [Mason etal., 1993; Satyana et al., 1999]. NE Borneo is a region inwhich there has been significant Neogene uplift and denuda-tion. It includes the highest mountain in SE Asia, the 4100 mgranite peak of Mt Kinabalu. Intrusion ages of about 14 Ma[Jacobsen, 1970; Swauger et al., 1995] indicate rapid uplift, andfission track data indicate 4–8 km of Late Miocene denudationthroughout the Crocker Range [Hutchison et al., 2000; Swaugeret al., 1995]. A mantle-driven cause is suggested by strati-graphic, structural and geochemical studies.

5. ORIGIN OF CENOZOIC BASINS OF SUNDALAND

5.1. Strike-slip Origin

The escape tectonics model for Asia originated in obser-vations made in the 1970s which identified major strike-slipfaults such as the Sagaing, Three Pagodas, Mae Ping and RedRiver Faults, radiating from eastern Tibet [e.g. Molnar andTapponnier, 1975; Tapponnier et al., 1986]. The model sug-gests that large crustal blocks have been ejected laterally fromthe India–Asia collision zone, and deformation is concen-trated on a few major strike-slip faults. Subsequently, many of

HALL AND MORLEY 19

the Cenozoic basins in Sundaland (offshore Vietnam, Thailand,and the Gulf of Thailand) were interpreted to be pull-apartbasins controlled by strike-slip faults [e.g. Molnar and Tap-ponnier, 1975; Polachan et al., 1991; Tapponnier et al., 1986].The escape tectonics model is attractive in its simplicity andthe way in which it relates disparate major tectonic blocks,structures and basins in the region. By the 1990s it became pos-sible to test the model as a result of investigating the rela-tionships between major strike-slip faults and Cenozoicsedimentary basins, particularly those of offshore Vietnam,the Gulf of Thailand and onshore Thailand [Matthews et al.,1997; Morley, 2002a; Rangin et al., 1995].

The largest of the strike-slip faults is the Red River ShearZone, which was initially suggested to have about 1000 kmof left-lateral displacement, later reduced to about 500 km[Briais et al., 1993; Lee and Lawver, 1995]. The only way toterminate such large displacement fault systems is at majorplate boundaries; hence the South China Sea has been inter-preted as a giant pull-apart at the tip of the Red River ShearZone, the Mae Ping Fault has been interpreted to terminate ina subduction zone off NW Borneo, and the Sagaing Fault to ter-minate at the Andaman Sea spreading center [see reviews inLeloup et al., 2001; Morley, 2002a]. The South China Seaoceanic crust forms a wedge that tapers to the WSW in whichthere was a gradual decrease in the amount of sea floor spread-ing towards the inferred offshore prolongation of the Red RiverShear Zone. Consequently, the relationship between the SouthChina Sea and the Red River Shear Zone as a giant pull-apartdoes not work geometrically [Morley, 2002a]. Furthermore,details of the geometry and timing of extensional faults in thepassive margin continental crust indicate there is not a sim-ple pull-apart relationship with the Red River Shear Zone[Matthews et al., 1997; Morley, 2002a; Rangin et al., 1995].

The escape tectonics model relates South China Sea ocean-floor spreading to Red River Fault motion [Leloup et al., 2001;Replumaz and Tapponnier, 2003] but does not address themechanism of earlier continental extension. The model alsoeliminates the Proto-South China Sea by linking thrusting inNW Borneo to movements on the Mae Ping Fault [Leloup etal., 2001]. This is not only geometrically unsatisfactory [Mor-ley, 2002a] but also does not explain the diachronous cessa-tion of thrusting along the NW Borneo margin. Moreover,the Mae Ping Fault cannot be traced across the Vietnam mar-gin, South China Sea and into NW Borneo. An alternativesingle driving mechanism for South China Sea spreading isslab-pull [Hall, 1996; 2002]. In this interpretation, subductionof the Proto-South China Sea oceanic crust beneath Borneoduring the Paleogene initially drove continental extension tothe north, and later to sea-floor spreading in the South ChinaSea (Plate 8). Thus, continental extension and sea floor spread-ing were the result of the same driving mechanism. However,

although this alternative may account for basins around theSouth China Sea it does not explain other Sundaland basins.

The escape tectonics model emphasizes the rigid nature of thecrust and the concentration of deformation on the boundingslip faults. As understanding of internal deformation of crustalblocks has improved, particularly based on satellite images,seismic reflection data, and field observations, it has becomeapparent that the Yunnan–Laos–Thailand–offshore Malaysiapart of Sundaland contains a dense network of Cenozoic faultsand rift basins. Some have experienced multiple phases ofextension and inversion [Ginger et al., 1993; Madon and Ahmad,1999; Morley et al., 2001; Phillips et al., 1997]. Not only are thesupposedly rigid continental blocks highly fractured, in placesthere is evidence for hot ductile flowing lower crust in meta-morphic core complexes, such as the Mogok gneisses of Myan-mar [Bertrand et al., 1999], in NW Thailand [Dunning et al.,1995; Rhodes et al., 1997] and near the Red River Shear Zone,Vietnam [Jolivet et al., 2001]. Deformation style and stressevolution are more complex than predicted.

Active strike-slip faulting along the offshore extension of theRed River Shear Zone has clearly affected basin structure andorientation in a narrow zone close to the fault trace [e.g. Leeet al., 2001; Matthews et al., 1997; Rangin et al., 1995; Zhouet al., 1995; Zhu et al., 1999], but the effects are local notregional. Elsewhere, as discussed above, the evidence forstrike-slip control on basin formation is weak to absent. Argu-ment continues about the significance, age, sense of dis-placement and amounts of movement on all the majorstrike-slip faults [e.g. Briais et al., 1993; Leloup et al., 1995;Morley, 2002a; Rangin et al., 1995; Roques et al., 1997a;1997b; Taylor and Hayes, 1983; Wang and Burchfiel, 1997].Nonetheless, despite criticism of the escape tectonics model,major strike-slip faults were important in the evolution ofSundaland. However, they are not the explanation of all aspectsof its tectonic evolution.

5.2. Rift Origin

Most basins appear to be rift basins capped by thermal sagor passive margin sequences, depending upon proximity tothe South China Sea oceanic crust [e.g. Cole and Crittenden,1997; Lee et al., 2001; Longley, 1997; Matthews et al., 1997;Zhou et al., 1995; Zhu et al., 1999]. However, an explanationof basin origin simply by rifting has two particular problems:the great depths of many of the basins, and the orientation ofthe rifts.

A possible solution is that the basins are the dynamic resultof subduction. Lithgow-Bertelloni and Gurnis [1997] arguedthat a large component of vertical motion of Sundaland maybe a dynamic topographic consequence of subduction andestimated that more than 1000 m of subsidence may be dynam-

20 SUNDALAND BASINS

HALL AND MORLEY 21

Pla

te 8

.Rec

onst

ruct

ions

to il

lust

rate

pri

ncip

al f

orce

s ac

ting

on

Sun

dala

nd b

ased

on

mod

el o

f H

all

[200

2]. I

n th

e O

ligo

cene

(30

Ma)

the

re w

as e

xtru

sion

of

Indo

chin

abl

ocks

due

to

Indi

a–A

sia

coll

isio

n, s

lab-

pull

due

to

subd

ucti

on o

f th

e P

roto

-Sou

thC

hina

Sea

, and

min

or m

ovem

ents

of

the

Sund

a–Ja

va s

ubdu

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The

net

res

ult

was

obl

ique

ext

ensi

on w

ithin

Sun

dala

nd. I

n th

e E

arly

Mio

cene

(20

Ma)

Aus

tral

ia–S

EA

sia

colli

sion

beg

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Sul

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i and

led

to ro

tatio

n of

Bor

neo,

and

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f the

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In

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thru

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in d

iffer

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of S

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. In

the

Lat

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the

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Ban

da s

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in t

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SE to

for

m th

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anda

Sea

, and

sub

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of th

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Chi

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Pla

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the

east

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som

e pa

rts

of S

unda

land

, at t

he s

ame

tim

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exh

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and

upli

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her

part

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the

reg

ion.

Thr

ough

out

the

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ic th

ere

wou

ld a

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have

bee

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ically maintained by mantle flow. Although accepting thattheir model of mantle flow predicted dynamic topographymuch larger than observed they argued that it predicted “thecorrect sign of flooding and exposure: emergence in the Pale-ocene and inundation over the past ~30 m.y.”. This claim isunconvincing. Lithgow-Bertelloni and Gurnis [1997] infer anover-simplified and large-scale pattern of differential verti-cal motion over the entire region, for which they cite Ronovet al. [1989], although as described above, the pattern of upliftand subsidence is very much more complex. Lithgow-Bertel-loni and Gurnis [1997] accept that their failure to predict thecorrect vertical motions for India and South America mayarise from ambiguities in their tectonic reconstructions. Pre-Cenozoic reconstructions of SE Asia are far more uncertainthan those for India and South America, although it is clear thatSE Asia was surrounded by subduction zones throughout theMesozoic as well as the Cenozoic. Nonetheless, a dynamictopographic effect could explain why many basins becomefully marine relatively late in the Neogene when global sealevel was falling although it is difficult to see how dynamictopography can account for the complex distribution, size,scale, depth and timing of basin subsidence in Sundaland.

In contrast, Wheeler and White [2000; 2002] suggested thatdynamic topographic effects were insignificant in East andSE Asian basins and that there was little or no anomaloussubsidence in Sundaland. They argued that the wavelengthand amplitude of subsidence were inconsistent with a dynamictopographic explanation. For the continental region of Eastand SE Asia that they analyzed, which extended from theJapan Sea south to the Malay-Penyu Basins, they interpreteda pattern of north to south and east to west migration of rift ini-tiation and marine inundation. Wheeler and White [2002] con-cluded that lithospheric stretching and cooling predominantlycontrols subsidence of continental sedimentary basins in SEAsia. As noted above, we consider the subsidence of severalbasins to be much greater than expected for the extensionobserved. However, this anomalous subsidence seems to be rel-atively late (as also observed by Wheeler and White [2002] foroffshore Vietnam) and is not predicted by dynamic topo-graphic models. Furthermore, if the whole of Sundaland isconsidered there is no north to south migration of rift initia-tion, rather the opposite, or possibly a progression towardsthe Sundaland interior.

As with mantle-driven subsidence models, a simple riftingmodel cannot account for the complex distribution, size, scale,depth and timing of basin subsidence. In particular, a simplemodel of lithospheric stretching begs the question of whatwas driving extension. We suggest that there are three com-ponents required for a more comprehensive explanation ofbasin formation. A better model of an unusual crust and lith-osphere, a better understanding of the balance of forces on

the region, and a better understanding of how these forceschanged with time.

The anomalous orientations of the rifts, the evidence formultiple phases of rifting, and the regional onset and termi-nation of extensional events, suggest that to fully understandthe Sundaland rift basins it is necessary to look beyond indi-vidual driving mechanisms in different areas. Published finiteelement models for the region have focused on the indentoreffect of India on regional stresses [Huchon et al., 1994; Kongand Bird, 1997; Seyferth and Henk, 2004]. However, thesetend to produce an oversimplified stress pattern that only par-tially explains the known sedimentary basin history of multipleextension and inversion events (see reviews by Morley [2001;2002a]). We suggest that it is particularly important to considerthe forces acting at the plate boundaries (Plate 8) which var-ied significantly throughout the Cenozoic because of the com-plexity of events in the region [Hall, 2002]. The variable natureof the plate boundaries, temporally and spatially, makes Sun-daland a difficult region to model but could be achieved usingfinite element modeling based on regional tectonic models. Inthe adjacent Australian plate the modern stress pattern hasbeen well matched by finite element modeling of stresses atthe plate boundaries [Hillis and Reynolds, 2000; Reynolds etal., 2002] and this approach would help understand the originof the Sundaland basins.

Some of the important plate tectonic events that affectedthe boundary and interior of Sundaland during the Cenozoic[Hall, 2002] that would have affected stress patterns includethe following events. Early Cenozoic: convergence betweenBurma and Thailand as India passed the Burma block. Possi-ble thickening of the Shan–Thai block and local source ofbuoyancy forces. Eocene onwards: progressive shorteningand thickening of Himalayan–Tibetan plateau region.Eocene–Early Miocene: slab pull of Proto-South China Sea.Oligocene–Early Miocene: ridge push from South China seaspreading center. Miocene: counter-clockwise rotation of Bor-neo. Middle Miocene–Recent: collision of Australia with SESundaland. Late Miocene: rise of Tibetan Plateau and prop-agation of uplift into the northern part of Sundaland. MiddleMiocene–Recent: buoyant uplift of NE Borneo with possi-ble loss of lithospheric root. Cenozoic: continuous subduc-tion at Sunda–Java Trench, although with variation in couplingbetween upper and lower plates. Variation in subduction-related forces would also have been influenced by arrival at thesubduction zone of thickened crust, such as the Ninety-EastRidge, the Investigator Fracture Zone and the distal parts ofthe Bengal Fan. Late Miocene–Recent: rollback of subductionhinge east of SE Sundaland to form the Banda Sea. Plate 8illustrates in a qualitative way how changing forces acting onthe whole region could account for the different orientation ofrifts and their timing.

22 SUNDALAND BASINS

We have drawn attention above to the evidence for theunusual character of Sundaland crust and lithosphere. Someother unusual features, such as depths of sedimentary basinsmay be partly accounted for by varying some of the assump-tions in conventional stretching models, such as thicker crustand thinner lithosphere, at the time of rifting. However, high heatflow may mean that models also need to incorporate a morecomplex model of the lithosphere. It is now recognized that con-tinental crust is capable of flow when its temperature is over400-500°C [McKenzie and Jackson, 2002; McKenzie et al.,2000]. To have most of the lower crust capable of flow eithergeothermal gradients must be elevated, and/or the crust mustbe considerably thicker than 30 km. Both are potential factorsthat would increase the effect of sediment loading-driven sub-sidence, which could account for the unusually deep basinsand the relative importance of the thermal subsidence.

5.3. Deep Basins

Many rift-post rift basins around the world modeled usingvariants of the McKenzie model have problems matching theamount of extension with the amount of thermal subsidence.A number of explanations have been advanced for such incon-sistencies, such as poorly constrained mantle physical prop-erties [White, 1990], incorrect estimation of upper crustalextension due to neglect of extension on faults which arebelow the resolution of seismic reflection data [Marrett andAllmendinger, 1991; 1992], and non-uniform stretching ofthe lithosphere [Hellinger and Sclater, 1983]. Modeling ofthe Malay and Pattani Basins suggests non-uniform stretching.The solution satisfies the observations but provides no reasonwhy non-uniform stretching has occurred. Watcharanantakuland Morley [2000] suggested subduction roll-back could causenon-uniform stretching, but this would require the effects ofsubduction roll-back to occur in a region more than 1000 kmfrom the trench. Non-uniform thinning of the mantle litho-sphere could be caused by a mantle plume, but there is novolcanic or domal uplift evidence for a plume.

In Sundaland the problem of very deep basins is not just con-fined to post-rift thermal subsidence. The Myanmar CentralBasin, NW and NE Borneo Basins are not post-rift basins,and in most cases there is little or no evidence of control of sub-sidence by deep basement faults. Although there are tectonicreasons for the basin location and initiation, we suggest thatthe main reason for the great depth is enhancement of tec-tonic subsidence by sediment loading. The standard models forAiry and flexural isostasy do not result in such extreme sub-sidence in shallow marine and continental settings [e.g. McKen-zie, 1978; Steckler and Watts, 1978] but the initial assumptionsof such models may be varied if there is flow of the lowercrust (Figure 7). Rapid post-rift subsidence in deep basins

such as the Pattani and Malay Basins would require 1) initi-ation of post-rift subsidence by cooling [McKenzie, 1978], 2)a lower crust capable of flow but in pressure equilibrium (i.e.initially no tendency for flow from the rift flanks towards therift center after the syn-rift stage has finished) and 3) an influxof sediment into the basin which triggers flow of the lowercrust to accommodate the load (analogous to synformaldepocentres associated with mobile salt or shale basins). Amechanism may be required to initiate regional flow, for exam-ple creation of space by crustal thinning around the SouthChina Sea spreading center, or subduction rollback at theAndaman Trench.

If flow of the lower crust can occur then the standardassumption that the thickness and density of the crust can beeliminated from the isostatic equations, because they do notchange during loading, becomes invalid. The SE Asian crustunder the rift basins is hot, with conditions favorable for thelower crust to flow. Such flow would provide a mechanismfor accommodating large thicknesses of sediment, and avoidisostatic imbalance. However flow of the lower crust alonestill does not produce isostatic compensation. For a 10 kmthick sedimentary basin, compensation of the upper 3–5 kmof section (2.4 g/cm3 sediment replacing 2.7 g/cm3 uppercrust) is critical. In most circumstances isostatic compensationcannot occur within the crust, but requires buoyancy forces todrive mantle uplift of 3–5 km beneath the basin (displacing theflowing lower crust) to achieve isostatic compensation (3.3g/cm3 mantle replacing 2.9 g/cm3 lower crust; Figure 7). Wesuggest that the deep SE Asian basins pose real questionsabout the assumptions behind widely accepted basin and iso-static models and understanding SE Asian basins requiresmore than application of models developed for simpler, morestable, regions.

6. CONCLUSIONS

The geological contrast between Sundaland and theIndia–Asia collision zone has produced a similar contrast in

HALL AND MORLEY 23

Figure 7. Model for subsidence of super-deep basins in which tec-tonic subsidence is significantly enhanced by lower crustal flowaway from the site of sediment loading. To create isostatic balancewith a compensation depth within the mantle, some mantle uplift atthe base of the crust is required.

ideas about the importance of key geological features. Thecollision, with its major themes of crustal flow, escape tec-tonics, massive uplift, sediment supply, and climatic influ-ence is so well-known that there is a tendency to assume it wasthe major influence on all of the very large surrounding region.Sundaland by contrast is a much less obviously homogeneousregion with small spreading centers opening and closing nearits margins, one major and several minor subduction margins,small collisional events, numerous rifts, and a few very largestrike-slip faults. Tectonic events within Sundaland have notobviously influenced India–Asia collision, whereas India–Asiacollision events may have affected Sundaland.

The Himalayas and Tibetan Plateau dominate such a largearea, and have such high topography that in many tectonicmodels they are regarded as the only significant regions driv-ing the Cenozoic tectonics of Indochina and further south,regardless of whether the driving mechanism is escape tec-tonics of rigid crustal blocks or flow of lower crust. The topog-raphy of northern Indochina and China has been related toflow of ductile lower crust away from the Tibetan Plateausince the Late Miocene [Clark and Royden, 2000]. However,in Thailand much of the present-day topography appears tohave been established earlier, during the Paleogene to EarlyMiocene. Cretaceous–Paleogene deformation in the Lan-ping–Simao fold belt of NE Thailand, and Oligocene–EarlyMiocene metamorphic core complex uplift in Myanmar, Thai-land and Vietnam, indicate that significant topography wasgenerated by tectonic events within Indochina prior to theoutwards propagation of Tibetan Plateau topography. Thetopographic expression of Sundaland is a result of severaldifferent tectonic events, and is not just the result of India–Asiacollision or lower crustal flow from the Tibetan Plateau.

There is a widespread perception that Sundaland was a sta-ble area during the Cenozoic and rigid plate motion for Sun-daland is often inferred [e.g. Replumaz et al., 2004; Replumazand Tapponnier, 2003]. GPS measurements have even beenused to suggest that Sundaland rotated clockwise as a rigidblock during the last 8–10 million years [e.g. Rangin et al.,1999]. In contrast, paleomagnetic results indicate large counter-clockwise rotations of Borneo and other parts of Sundaland[Fuller et al., 1999; Richter et al., 1999; Schmidtke et al.,1990] but which, if modelled as a single block, result in over-lap with Indochina in reconstructions. As argued above, theregion has been very far from stable, and it cannot havebehaved as a single rigid block during most of the Cenozoic.The considerable evidence for a complex pattern of subsi-dence and elevation requires a much more complex tectonicmodel for the region with significant internal deformation ofSundaland [Hall, 1996; Hall, 2002]. Much of the character ofnorthern Sundaland may be due to small collisional eventsthat were precursors to the main India–Asia collision, includ-

ing left-lateral movements of the Mae Ping and Three Pago-das Faults.

The known differences in nature and timing of basin for-mation and inversion suggest a complex and shifting patternof uplift and subsidence with time. The region is one of rela-tively high heat flow, and gravity observations suggest a thinelastic thickness for the lithosphere. Tomography shows unusu-ally low velocities at shallow depths for a continental regionsuggesting a thin, warm and weak lithosphere. Weak andwarm continental lithosphere may be the result of long-termsubduction beneath Sundaland. Although subduction may bea driving force, we do not suggest that dynamic topographiceffects are the explanation. If there is a dynamic topographicsignal it is small and long wavelength in comparison to the pat-terns of vertical motions within the region. On the other hand,the unusual depths of sedimentary basins, their orientationand ages require explanations more complex than offered byconventional stretching models. We have suggested abovethat these require a better model of an unusual crust and lith-osphere, a better understanding of the balance of forces, anda better understanding of how these forces changed with time.Lower crustal flow offers an explanation of the depths ofbasins but basin initiation and inversion requires a changingstress field which caused the variations in timing, distribu-tion and amounts of vertical and horizontal motions withinSundaland. We suggest this was the result of changing intra-plate stresses which reflect the changing pattern of forces atthe plate edges, driven by long-term subduction, acting on avery weak and probably thin lithosphere.

The complexity of the background plate-driven tectonicevents suggest that a continuum model may offer a betterdescription of deformation than a rigid block model. Analy-sis of the entire region and the forces acting on it over time[Cloetingh and Wortel, 1986; Richardson, 1992; Richardsonet al., 1979] may provide a better understanding than shoe-horning the problems into an India–Asia ‘solution’. To fullytest whether the rift basin pattern can be understood by finiteelement modeling, stresses originating from all the plateboundaries surrounding Sundaland and from topography mustbe considered to see how the effects sum and vary across thewhole plate [e.g. Reynolds et al., 2002]. Local sources ofstress will exert a stronger influence closer to the source (e.g.the indentor influence in the NW, subduction in the west andSW, slab pull and ridge push in the east, collisions in the east),and the stress magnitude and orientation will be the sum of dif-ferent forces plus the effects of variable torques acting on theplate [e.g. Coblentz and Richardson, 1995; Richardson, 1992;Zoback et al., 1989]. Stresses and torques will vary spatiallyand temporally in response to such factors as collision, chang-ing obliquity of subduction, and transition from plate bound-aries involving oceanic crust to continental crust.

24 SUNDALAND BASINS

The sedimentary basins of the region, onshore and offshorealso contain a record of tectonics, in the changing volumes andcharacter of sediment. There are currently few good estimatesof volumes of sediment supplied at different times, and thereare few provenance studies which could identify the importanceof the Asian input into basins in the region. Much of thisrecord is potentially there, as a result of the exploration forhydrocarbons in the region, but relatively little of it is yet inthe public domain. There remains a final important problem,which is that of timing because, for example, it is not possi-ble to identify the causes of basin formation unless the age ofbasin initiation is adequately dated. Most of the Sundalandbasins have an early and extended terrestrial history, and oldersequences often contain few fossils which have limited bios-tratigraphic value [see, for example, Cole and Crittenden,1997], beside being beyond the reach of drilling. Similar, butgreater, problems surround dating of uplift. In order to under-stand the region it is vital to have better dating of events, in par-ticular by thermochronological methods.

Acknowledgments. Financial support to RH has been provided atdifferent times by NERC, the Royal Society, the London UniversityCentral Research Fund, and the London University and Royal Hol-loway SE Asia Research Groups, supported by a number of industrialcompanies. Work in Indonesia has been facilitated by GRDC, Lemi-gas and LIPI. We thank Wim Spakman for help in producing Plate1. Financial support to CKM has been provided at various times bythe Universiti of Brunei Darussalam, The Royal Society and BruneiShell Berhad (BSB). Subsurface data from Thailand and BruneiBasins have been provided by PTTEP, Unocal, Thai Shell, TFE,EGAT, Banpu Mines and Lanna Lignite.

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