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, the Thai–Malay Peninsula and Indochina, was assembled during the Triassic Indosin- ian orogeny, and formed an exposed landmass during Pleistocene lowstands. Because the region includes extensive shallow seas, and is not significantly elevated, it is often assumed to have been stable for a long period. This stability is a myth. The region is today surrounded by subduction and collision zones, and merges with the India–Asia collision zone. Cenozoic deformation of Sundaland is recorded in the numerous deep sedimentary basins alongside elevated highlands. Some sediment may have been supplied from Asia following Indian collision but most was locally derived. Modern and Late Cenozoic sediment yields are exceptionally high despite a 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 character the region resembles the Basin and Range province or the East African Rift, but is quite unlike them in tectonic setting. Conventional basin modeling fails to predict heat flow, elevation, basin depths and subsidence history of Sundaland and overes- timates stretching factors. These can be explained by interaction of a hot upper mantle, a weak lower crust, and lower crustal flow in response to changing forces at the plate edges. Deformation produced by this dynamic model explains the main- tenance of relief and hence sediment supply over long time periods.

INTRODUCTION contain up to 14 km of Cenozoic section. The origin of many of the basins is problematic in terms of the causes of the The continental promontory of the Eurasian plate in SE stresses which formed them, and the amount of sediment Asia called Sundaland (Figure 1) contains a large number of accumulated. The Sundaland continental region is surrounded Cenozoic sedimentary basins which formed between the Pale- by active and complex tectonic boundaries. To the south and ogene and the Middle Miocene. Many are very deep and some southwest there is subduction of oceanic crust of the Indian Continent–Ocean Interactions Within East Asian Marginal Seas Plate at the Sunda–Java Trench, to the southeast Australian Geophysical Monograph Series 149 continental crust is colliding in eastern Indonesia with the Copyright 2004 by the American Geophysical Union Sundaland margin, associated with young extension in the 10.1029/149GM04 Banda Sea. To the east there are a number of marginal basins

55 56 SUNDALAND BASINS

Figure 1. Location of Sundaland and the principal sedimentary basins within the region. which opened during the Cenozoic, including the South China ment sources and supply, to escape tectonics and the uplift Sea whose origin remains controversial. To the west and north- and erosion of Asia. In many cases, the whole region has been west are the Himalayan orogenic belt, associated collision belts seen as merely a far-field expression of India–Asia collision. in Myanmar, and large strike-slip faults which accommodate It is argued in this paper that this collision, and its influence, India–Asia collision. This setting is very different from large have been over-emphasized, and that important local tectonic continental areas such as Africa, Australia and South America activity driving subsidence and uplift within Sundaland needs that are mostly surrounded by major mid-oceanic rift systems to be considered. We discuss how these basins differ from or linear, relatively simple, subduction zones. Consequently those in other continental regions and suggest that Sundaland it might be expected that sedimentary basins developed on is an unusual region to which conventional models for basin for- Sundaland crust would exhibit some different characteristics mation cannot be applied without important changes. The from classic rifts, foredeep and strike-slip basins. However, nature and timing of subsidence and inversion suggest a com- most studies of the Sundaland basins, particularly rift basins, plex and shifting pattern of extension and contraction with have tried to fit them into established models, such as the time. The long and active tectonic history of the Sundaland McKenzie model [McKenzie, 1978; White and McKenzie, margins has determined the character of the crust and mantle 1988] for extension [e.g. Longley, 1997; Wheeler and White, and strongly influenced basin development. 2002], or interpreted them as strike-slip pull-apart basins [e.g. Huchon et al., 1994; Leloup et al., 2001; Polachan et al., 1991; 2. SUNDALAND Tapponnier et al., 1986]. A common theme concerning the development of Sundaland basins has been the attempt to Sundaland is the region of SE Asia that formed an exposed strongly link many of them tectonically, and in terms of sedi- landmass during the Pleistocene sea level lowstand, com- HALL AND MORLEY 57 prising Indochina, the Thai–Malay Peninsula, Sumatra, Java, is surrounded by Mesozoic ophiolitic and arc igneous rocks, Borneo, and the shallow marine shelf (the ‘Sunda Shelf’) which may include fragments of older continental material, between them. It is the promontory extending southeast from accreted to the Sundaland margins during the Cretaceous. Eurasia although seismicity [Cardwell and Isacks, 1978; John- There appears to have been an important tectonothermal event ston and Bowin, 1981] and GPS measurements [Rangin et in the mid to late Cretaceous which led to crustal thickening al., 1999; Simons et al., 1999] indicate that a SE Asian Plate and elevation [e.g. Barley et al., 2003; Charusiri et al., 1993; or Sunda Block is moving separately from the Eurasian plate Cobbing et al., 1986; 1992; Mitchell, 1993; Mitchell et al., at the present day. 2002; Morley, 2004; Upton et al., 1997]. There are granites of Sundaland (Figure 1) is the continental core of SE Asia. It Cretaceous age which may represent Andean-type margins is now bordered to the west, south and east by subduction and facing the Pacific and Indian oceans, or the products of a con- collision zones. To the north it merges with the region tinent collision [Barley et al., 2003]. However, little is known deformed in the Cenozoic by India–Asia collision. Geologi- of the Late Cretaceous and Paleocene history because of the cally it is convenient to separate Sundaland from Asia (Figure paucity of sedimentary rocks of this age on land or offshore. 2) to the northeast along the Red River Shear Zone, which follows the Carboniferous Song Ma suture, and to the north- 3. BASIN SETTING west from a Burma block along the Hpakan–Tawmaw jade tract, a Cretaceous suture and ophiolite zone [Hutchison, The areas surrounding Sundaland are currently tectonically 1975]. The western and southern margins of Sundaland follow active, and include a variety of tectonic settings. The Cenozoic the Sunda and Java Trenches. The eastern margin is irregular; sedimentary basins formed near to regions of subduction- it is usual to draw the boundary of the Sundaland continental related deformation, arc–continent and continent–continent core through west Java, northeast into Borneo and then north- collision, and intra-continental strike-slip deformation [Wicker west towards the South China Sea, to exclude the mainly ophi- and Stearn, 1999]. At present the contrast between intense olitic rocks added to its eastern margin in the Cretaceous. tectonic activity, manifested by seismicity and volcanism, at However, we include east Java, west Sulawesi and northern the margins of Sundaland and the apparent stability of its Borneo within Sundaland since these areas formed its eastern interior is remarkable. Most of the Sunda Shelf is flat, active margins throughout the Cenozoic. These boundaries extremely shallow (Figure 3), with depths considerably less are similar to those suggested by seismicity and GPS meas- than 200 m, and was emergent at times during the Pleistocene. urements for the Sunda Block. Data from this region have been used in construction of global Sundaland was essentially in its present form, and in a sim- eustatic sea level curves [e.g. Fleming et al., 1998]. Perhaps ilar position with respect to Asia, by the Early Mesozoic [Met- for these reasons, Cenozoic Sundaland has been viewed in calfe, 1990; 1996] and since then has been largely emergent the same way, and its core has been described by some authors or submerged to very shallow depths (Figure 3). It was formed as the Sunda shield or craton, widely regarded as a region of by amalgamation of continental blocks during the Triassic stability [e.g. Ben-Avraham and Emery, 1973; Gobbett and Indosinian orogeny and is surrounded by material mostly Hutchison, 1973; Tjia, 1996], which has remained undeformed added later in the Mesozoic. During the Permo–Triassic stages and close to sea level since the Mesozoic. However, the char- of suturing there was extensive granite magmatism from Thai- acter of the crust and mantle in this region differ from nearby land to Malaya, associated first with subduction preceding continental regions and there are a number of reasons to doubt collision, and later with post-collisional thickening of the con- the supposed long-term stability and rigidity of Sundaland. tinental crust [Hutchison, 1989; 1996b]. In west Borneo the oldest rocks known are Paleozoic metamorphic rocks intruded 3.1. Tomography by Mesozoic granites. Isotopic studies indicate the Malay Peninsula has a Proterozoic continental basement [Liew and P and S wave seismic tomography models (Plate 1) show McCulloch, 1985; Liew and Page, 1985] and there is some Sundaland is an area of low velocities in the lithosphere and geochemical evidence to suggest that west Sulawesi may underlying mantle [e.g. Bijwaard et al., 1998; Lebedev and include Proterozoic basement [Bergman et al., 1996]. The Nolet, 2003; Ritsema and van Heijst, 2000; Widyantoro and Mesozoic stratigraphic record is patchy but suggests that much van der Hilst, 1997], in strong contrast to Indian and Aus- of Sundaland was emergent after the Late Triassic. Mesozoic tralian continental lithosphere to the NW and SE which are terrestrial deposits are not extensive (except in northern Sun- probably colder, thicker and stronger. Sundaland is an excep- daland from the Khorat to the Lanping–Siamo fold belt region) tional area; the only other continental areas of comparable but are found throughout Sundaland; Mesozoic marine deposits size with similar low velocities at such depths are East Africa are rare. From Sumatra to Borneo the older continental core and the Basin and Range province, both regions of known 58 SUNDALAND BASINS

Figure 2. Principal geographical and geological features of Sundaland and the surrounding region. The Sunda Shelf and contiguous 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 commonly anisotropy [Lebedev and Nolet, 2003]. Many oil company interpreted in terms of elevated temperature, and this is con- wells report high CO2 levels, although it is not known if this sistent with regional high heat flow, but they may also reflect has a mantle source, or a deep crustal source, in which case elevated H2O or CO2 contents, partial melting or seismic it may be the product of high heat flow and metamorphic HALL AND MORLEY 59

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. reactions in the lower or upper crust. The mantle heat flow pany compilations [Kenyon and Beddoes, 1977; Pollack et contribution estimated by basin modeling [e.g. Madon and al., 1990; 1993; Rutherford and Qureshi, 1981]. Heat flow Watts, 1998; Watcharanantakul and Morley, 2000] also sug- throughout the region is high and is not simply a result of arc gests a high value, although because such modeling is based magmatism as sometimes suggested [e.g. Nagoa and Uyeda, on simple isostatic assumptions the mantle heat flow may be 1995]. The data used in the contoured map are mainly from oil overestimated. Worldwide heat flow from the mantle is typ- company wells, which are generally far from present-day vol- ically in the order of 20–30 mW/m2 and may exceptionally canoes. A number of factors may have contributed to produce approach 40 mW/m2 [Artemieva and Mooney, 1999; Wang the high observed upper crustal heat flows. et al., 2000]. The high heat flows seen regionally across Offshore Vietnam is an area of high heat flow probably Sundaland, and the generally low mantle velocities observed related to Oligo–Miocene oceanic spreading in the South in tomographic models suggest mantle heat flow values of China Sea [Shi et al., 2003]. The area from the Malacca Strait the order of 40 mW/m2. north of Central Sumatra to NW Java also shows high heat flows, which are probably the consequence of nearby sub- 3.2.Heat Flow duction-related arc magmatism in Sumatra and Java. There are few heat flow measurements close to volcanoes and the Global data clearly show that Sundaland is a region of high map probably underestimates the heat flow in the region of the heat flow with surface heat flow greater than 80 mW/m2 modern volcanic arcs. A contribution from active volcanoes [Artemieva and Mooney, 2001]. Plate 2 shows contoured heat has been estimated by assigning a heat flow value at the site flow for SE Asia based on published databases and oil com- of each volcano of 140 mW/m2, based on observations in the 60 SUNDALAND BASINS

Plate 1. Depth slices through S20RTS S wave tomographic model [Ritsema and van Heijst, 2000] for SE Asia. High shear velocities are represented by blue and low shear velocities by red with an intensity which is proportional to the amplitude of 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]. HALL AND MORLEY 61

Japan Arc [Nagoa and Uyeda, 1995], but this is not incorpo- and within Borneo [reviewed in Hutchison, 1996b; Macpher- rated in Plate 2. son and Hall, 2002]. Heat flows measured in the interior Sundaland basins from Sundaland, particularly the zone extending from Thailand the Gulf of Thailand to west Borneo are also high where arc to the Malay Peninsula, has a large number of granite intru- magmatism is not a feasible explanation. These basins are sions of Triassic and Late Cretaceous age [Cobbing et al., more than 800 km from the active Sunda volcanic arc and 1992; Dunning et al., 1995; Hutchison, 1973; Krähenbuhl, similar distances from oceanic crust of the South China Sea. 1991; Schwartz et al., 1995] and these may also be present The Malay Basin, Pattani Basin and Gulf of Thailand are char- offshore. Consequently some of the high heat flow may be acterized by high to very high present-day geothermal gra- attributed to radiogenic heat production, either from the gran- dients (30–75°C/km). In the Pattani and Malay Basins ites or from sediments derived from granite sources, in the geothermal gradients and heat flows are very high in wells upper crust. High radiogenic heat production and the insu- with 4–8 km of post-rift Miocene–Recent sediments. The lating effects of argillaceous sediments appear to be important range of heat flow values for 30 wells in the Pattani Basin is factors in explaining why the Pattani and Malay Basins have from 78 mW/m2 to 101 mW/m2 [Bustin and Chonchawalit, high heat flow [Waples, 2001; 2002]. However, the tomo- 1995; Pigott and Sattayarak, 1993]. A similar range of heat graphic models suggest a significant heat flow contribution flows is reported for the Malay Basin [Madon et al., 1999]. from deeper in the lithosphere. That deep processes cause These are values 15–25 million years after the onset of ther- high heat flow which affect much of the crust is supported mal subsidence yet they are similar to heat flows in the active by gravity data and subsidence histories that suggest a thin East African Rift away from very high heat flow volcanic cen- elastic thickness for the lithosphere [Madon and Watts, 1998; ters. The highest heat flows and geothermal gradients in the Watcharanantakul and Morley, 2000]. In summary, at the Malay and Pattani Basins occur around the basin centers, margins of Sundaland high heat flows are related to subduc- whilst the lowest values occur around the basin flanks, sug- tion-related processes and magma rise. In contrast, the hot gesting the basins are contributing significantly to the ele- interior of Sundaland appears to be the result of high mantle vated values. Basins can affect heat flow because of the heat flow, a significant upper crustal heat flow from radi- insulating effects of the sedimentary rocks (particularly shales ogenic granites and their erosional products, as well as the and coals) and by concentrating the radiogenic erosional prod- insulation effects of thick sediments. ucts from granitic terrains. Sundaland, particularly the zone extending from Thailand to the Malay Peninsula, has a large 3.3. Sediment Yields number of granite intrusions of Triassic and Late Cretaceous age [Cobbing et al., 1992; Dunning et al., 1995; Hutchison, Many estimates of global sediment yields ignore SE Asia. 1973; Krähenbuhl, 1991; Schwartz et al., 1995] and these However, high present-day and long-term sediment yields for may also be present offshore. Consequently some of the high Sundaland imply that relief in the region has been maintained, heat flow may be attributed to radiogenic heat production, indicating important tectonic activity over a prolonged period either from the granites or from sediments derived from gran- of the Cenozoic. At present, sediment discharge into the global ite sources, in the upper crust. Waples [2001; 2002] estimated oceans is estimated at between 12 and 20 x 109 t/yr [Hay, high radiogenic heat production from sediments could con- 1998; Ludwig and Probst, 1998; McLennan, 1993; Milliman tribute up to 1.8 mW/m2 in the Malay and Pattani Basins. and Meade, 1983; Milliman and Syvitski, 1992]. Milliman A prolonged history of high heat flow within Sundaland is and Meade [1983] suggested that more than 70% of this dis- indicated by other lines of evidence. There was sporadic charge comes from rivers of SE Asia and Oceania but their data Miocene–Quaternary basaltic volcanism in SE Vietnam, Thai- set included only 8 rivers from the region, in New Guinea land, the Malay Peninsula and north Borneo [Hutchison, and Taiwan. Milliman and Syvitski [1992] estimated a figure 1996b]. There are metamorphic core complexes east of the of 20 to 25% based on a data set including 31 rivers from SE Sagaing Fault in Myanmar, at Doi Inthanon and Doi Suthep Asia. Milliman et al. [1999] suggested a similar percentage, in NW Thailand and around the Red River Fault Zone in Viet- based on 56 rivers from Indochina, New Guinea, Java, and nam [Bertrand et al., 1999; Dunning et al., 1995; Jolivet et al., the Philippines, and concluded that six islands in SE Asia 2001]. The metamorphic core complexes are direct indica- (Sumatra, Java, Borneo, Sulawesi, Timor and New Guinea) tors that flow of the lower crust has occurred in places. There provide about 20–25% of the sediment discharge to the global are Oligocene granites at Doi Inthanon in NW Thailand. In ocean, although they account for only about 2% of the global addition, subduction may have contributed by arc volcanism land area. Although these rivers may not be representative of around the western to southern Sundaland margin between the entire region all estimates indicate the region is currently the Myanmar Central Basin and the Indonesian archipelago, one of high sediment supply to the oceans [Suggate and Hall, 62 SUNDALAND BASINS

2003]. This is supported by studies of short-term sediment changes in sedimentation rates during the Cenozoic and con- yields in the region by hydrologists [e.g. Douglas, 1967; 1999; cluded that extrusion tectonics (consistent with lack of an ele- Douglas et al., 1992]. However, additional data are needed vated mountain belt) was more important between 50 and 30 for SE Asia and the West Pacific islands, particularly from Ma, whereas crustal thickening and uplift became more impor- the large ever-wet tropical islands with small human popula- tant after 30 Ma when sedimentation rates increased. They tions such as New Guinea and Borneo. focused on the effect of the India–Asia collision and ignored There have been few studies of long-term sediment yields but sediment input from local sources. However, local sources those studies [Hall and Nichols, 2002; Morley et al., 2003] are significant and recent work has cast doubt on a long his- show that the late Cenozoic rates are consistent with estimates tory of erosion in Asia supplying sediment to the south [Clark, of current rates. For example, the amount of sediment derived 2003]. We have highlighted above the importance of high sed- from Borneo in the Neogene is about one third of that eroded iment yields in the region. We suggest that despite being adja- from the Himalayas during the Neogene [Hall and Nichols, cent to the India–Asia collision zone most of the sediment 2002], indicating similar denudation rates per unit area. was derived from elevated regions within Sundaland. Although there are complex relationships between rock Before 30 Ma there were barriers to sediment supply from uplift, weathering, erosion and downslope transport [Hovius, Asia. Thailand is at the lower end of the topographic gradient 1998] it is clear that the high rates of sediment supply indicate from the Himalayas, which has been explained as a result of a elevation and relief, although factors such as rainfall and wedge of ductile lower crust flowing out from Tibetan Plateau runoff are also important [Hay, 1998; Ludwig and Probst, [Clark and Royden, 2000]. However, to view this slope as a con- 1998; Milliman and Syvitski, 1992; Summerfield and Hulton, sequence only of India–Asia collision is too simple. Radio- 1994]. Sundaland has been close to its present position at the metric and apatite fission track studies discussed below suggest equator throughout the Cenozoic. There was a change from an that there was initial uplift in the Late Cretaceous–Paleogene, Oligocene drier climate to a wetter Neogene climate [Mor- between the Shan Plateau of Myanmar and northern Thailand, ley, 1998; 2000]. Parts of the region, such as Borneo, remained through Laos, into the Lanping–Siamo fold belt (Figure 4). ever-wet throughout the Neogene, whereas areas north and During the Oligocene–Early Miocene a narrow N–S elevated south of the equator, such as Sumatra, Java and Indochina, zone was imposed on the earlier uplift in western Thailand. developed a monsoonal, highly seasonal climate by the Thus, for much of the Cenozoic, northern Sundaland was rel- Pliocene [Morley, 1998; 2000]. However, although precipi- atively high ground and a broad topographic saddle may have tation and runoff may have changed during the Cenozoic, cli- existed between the Sundaland highlands and the Himalayas. mate alone cannot account for high sediment fluxes, and relief In the Middle Miocene uplift spread out radially from eastern must have been maintained. Was the relief within Sundaland, Tibet [e.g. Clark and Royden, 2000] and linked with existing or was sediment derived from elevated regions outside Sun- highlands of northern Sundaland (Plate 3). daland? The sediments in basins in and around Sundaland are Further barriers were produced by active faulting. In Thai- often interpreted to be the product of erosion much further land the largest transverse structural barrier occurs where the north in Asia following Indian collision [e.g. Métivier et al., Mae Ping Fault cuts NW–SE across the Cenozoic rift basins 1999]. In contrast, we suggest that the sedimentary basins of at the Chainat strike-slip duplex (Figures 4 and 5). This restrain- the region were filled from local sources and there is no need ing bend high separated the Phitsanulok Basin to the north to assume significant input from Asia. This in turn implies from the Suphan Buri and Ayutthaya Basins to the south dur- tectonic activity to produce the relief required to maintain ing the Oligo–Miocene [O’Leary and Hill, 1989] and lies sediment input as discussed below. directly across the path of the present-day Chao Phraya River. There is also the question of whether the large fluvial sys- 3.4. Sediment Sources Within Sundaland tems of Indochina were even established by the Middle Miocene. It is unlikely that any through-going fluvial system The location of Sundaland adjacent to the Himalayas and the recognizable as the Chao Phraya River (Figures 2 and 5) existed great rivers of Indochina (Mekong, Red River, Salween) flow- during the Late Oligocene and Miocene when the rift basins of ing from the north (Figure 2) has suggested links between Thailand were active because active rift basins tend to be sites sedimentation and the rise of the Himalayas and Tibet. Hutchi- of interior drainage into long-lived lakes. In Thailand there is son [1996a] and Hall [1996] suggested that rivers such as the considerable evidence for prolonged periods of lacustrine con- Mekong and Chao Phraya crossed Sundaland to feed the NW ditions during the Late Oligocene–Middle Miocene in many of Borneo margin, largely to explain the amount of sediment the rift basins both onshore (e.g. Phitsanulok, Mae Sot, Fang, there, although both later amended this view [Hall and Nichols, Suphan Buri, Li, Lampang, Chiang Muan, and Mae Moh [Flint 2002; Hutchison et al., 2000]. Métivier et al. [1999] identified et al., 1988; Morley et al., 2001; O’Leary and Hill, 1989; Pra- HALL AND MORLEY 63

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 extension in most basins [Morley et al., 2001; Uttamo et al., 2003]. If an early Chao Phraya River had existed it would not have linked with Tibetan plateau drainage systems. The different branches of the Chao Phraya fluvial system probably linked up only in the Pliocene, when erosion and thermal subsidence began to eliminate the rift topography of central Thailand (Figure 5). The absence of major through-going rivers is also indicated by 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-

Figure 4. Northern Sundaland (Myanmar, Laos, Thailand, Cambo- dia, China) showing the many extensional and strike-slip faults and associated basins which indicate significant internal deformation of the Sundaland region during the Cenozoic (based on satellite images, Figure 5. The present-day drainage of Thailand and adjacent regions, surface mapping and seismic reflection data). The Mae Ping and shown with the principal rift basins and associated faults. Some of Three Pagodas Fault Zones display numerous branching, splaying and the main barriers to through-going drainage such as long-lived lakes, duplex geometries, and affect a large area (unlike the Sagaing and Red and structural barriers (Chainat Ridge) are highlighted. The pres- River Faults), possibly indicating a transpressional setting during ent-day drainage pattern reflects the older syn-rift pattern (drainage the Paleogene. normal to rift orientation) and late linkage of rift axial systems. 64 SUNDALAND BASINS

Plate 3. Reconstructions of the tectonic evolution of SE Asia based on Morley [2002a] modified to show the nature of basin fill (marine or continental) and approximate location and timing of rifting and deformation in Sundaland. HALL AND MORLEY 65 gesting a short transport distance [e.g. Flint et al., 1988; of drainage patterns. Throughout the Neogene major rivers O’Leary and Hill, 1989; Praditan, 1989; Trevana and Clark, flowed onto the Sunda Shelf from Borneo and smaller rivers 1986; Uttamo et al., 1999]. Provenance studies of Miocene from the Malay Peninsula. The volume of sediment eroded sandstones and conglomerates in the Chiang Mai Basin, the from Borneo was huge and the Neogene depocentres offshore largest basin in northern Thailand, show a history of unroof- Sarawak and Sabah are extremely deep, as recorded above, ing of the metamorphic core complexes west of the basin and filled entirely with sediment derived from Borneo. Suma- [Rhodes et al., 2003]. In northern Thailand the earliest basin tra and Java, at the edge of Sundaland, appear to have been iso- fill was derived from metamorphic rocks, and in the Late lated from rivers of the Sunda Shelf by elevated regions Miocene–Pliocene granitic rocks were exposed [Praditan, running from the central Malay Peninsula via the Singapore 1989; Uttamo et al., 1999]. Further south the Oligocene–Mid- Ridge into western Borneo. Most rivers probably flowed south dle Miocene fill of the Ayutthaya Basin is predominantly allu- and west from these highlands filling the Mergui, Sumatra vial fan and fluviatile sediments, with large volumes of arkose and Java Basins until about the Late Miocene when Java and derived from granites immediately north of the basin, and Sumatra became elevated causing rivers to flow north and volcanic and clastic input from the Khorat Plateau to the east east into the Malacca Straits and Java Sea. [O’Leary and Hill, 1989]. The Tibetan rivers were deflected away from Thailand by 4. BASIN HISTORIES Paleogene and Oligo–Miocene uplift, and carried their sedi- ment to the west and east of Sundaland (Figure 2 and Plate 3). Recent reviews of Sundaland Cenozoic sedimentary basins In the west the Bengal Fan and Myanmar Central Basin (Fig- and their setting have been given by several authors [Hall, ures 1 and 2) were the two major sites of Himalayan sedi- 2002; Longley, 1997; Morley, 2001; 2002a; Pertamina BPPKA, mentation until the Middle Miocene [e.g. Pivnik et al., 1998; 1996a; 1996b; 1996c; Petronas, 1999; Sandal, 1996]. The Shamsuddin and Abdullah, 1997]. Until the Middle Miocene characteristics of the basins and timing of basin initiation and some rivers from the Tibetan Plateau flowed towards the Cen- inversion (Plates 4 and 5) are topics too large to be discussed tral Basin, but uplift in Assam, SE Tibet and northern Myan- in detail but here we summarize some key features. mar caused this route to be cut. One likely consequence of the uplift was to cut off the Irrawaddy River from the Tibetan 4.1. Early Cenozoic Setting of Basin Formation Plateau and restrict its headwaters to a region just north of the Central Basin. It has been proposed [Clark et al., 2004] that As noted above, there are few Upper Cretaceous and Pale- capture of river systems occurred when the Tibetan Plateau ocene rocks on land or offshore. Their absence is usually inter- uplift propagated eastwards so that other drainage systems, preted to indicate that most of Sundaland was above sea level notably the Salween and Mekong Rivers, started to emerge at the beginning of the Cenozoic. Emergence may have been from the Tibetan Plateau. There appears to have been a marked the result of a tectonothermal event that affected the region change in the site of deposition of much of the sediment from Indochina to the Malay Peninsula [e.g. Ahrendt et al., derived from the Tibetan Plateau; before the Middle Miocene 1993; Dunning et al., 1995; Krähenbuhl, 1991; Upton et al., it was in the Bengal Fan and Central Basin, after the Middle 1997]. There is also evidence of Late Cretaceous deformation Miocene it was the Gulf of Martaban (Salween River) and associated with emplacement of ophiolites and metamorphism the South China Sea (Mekong River). at the Sundaland margins in Borneo, Java and Sumatra. Much less is known about drainage patterns and develop- At the beginning of the Cenozoic (Plate 3) northern Sun- ment in the southern part of Sundaland. However, the palaeo- daland was relatively high. Uplift in the Late Cretaceous–Pale- geography of the region indicates a fluvial connection between ogene extended from the Shan Plateau of Myanmar and Indochina and NW Borneo was unlikely. By the Middle northern Thailand through Laos into the Lanping–Siamo fold Miocene there was a marine shelf between Indochina and belt (Figure 4) as the result of a diffuse, poorly defined oro- Borneo (Plate 3) and the many basins (Pattani, Malay, Nam genic event [Charusiri et al., 1993; Morley, 2002b; 2004; Con Son) within this shelf would have trapped sediments long Upton, 1999; Wang and Burchfiel, 1997]. In Thailand, out- before they reached the Borneo margin. Provenance studies side the region of metamorphic core complexes, apatite fission [van Hattum et al., 2003] show that Paleogene deep water track studies [Racey et al., 1997; Upton, 1999; Upton et al., sediments of the Crocker Fan [Crevello, 2001; William et al., 1997] indicate the onset of cooling between 70 and 50 Ma, and 2003] were probably derived from Borneo itself. 3D seismic geological observations [Mouret et al., 1993] indicate an data reveal complex meandering rivers in offshore basins of episode of inversion during the early Paleocene (about 65 the Sunda Shelf [e.g. Crevello, 2001; Fatt, 1999] but there is Ma). In the frontal monocline and Loei–Petchabun fold belt, too little published information to construct a coherent picture the amount of post-Khorat Group (Jurassic–Cretaceous) over- Plate 4. Ages of basin initiation in Sundaland. The record typically begins in the Eocene or Oligocene although since the older 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. HALL AND MORLEY 67 burden thickness removed since inversion is estimated at about tral and South Sumatra, West and East Java Basins); 2) Sunda 2.3 to 4.4 km, depending on the geothermal gradient used Shelf basins that trend NW–SE to N–S within the interior of (25°C km-1 [Mouret et al., 1993]; 35°C km-1 [Racey et al., Sundaland (Thailand, Gulf of Thailand, and Malay, Penyu and 1997]) whereas some 2–6 km of section may have been West Natuna Basins) which formed at a similar time to basins removed during the Paleogene in parts of western Thailand further east which are offshore Vietnam (Nam Con Son Basin), [Morley, 2004; Upton, 1999]. and 3) Borneo margin basins (Baram Delta province, San- Further south the region is interpreted to have been conti- dakan, Tarakan and Kutai Basins) that are situated on the for- nental based largely on negative evidence. The Malay Penin- mer active margin of northern Borneo and passive margin of sula has probably been above sea level since the Mesozoic. eastern Borneo. Upper Jurassic–Lower Cretaceous sediments in the Malay Peninsula are continental and described as molasse, and over- 4.2.1. Sundaland active margins. At the southern edge of lie older rocks with a marked unconformity [Gobbett and Sundaland are basins that formed close to Cenozoic subduc- Hutchison, 1973; Harbury et al., 1990] They indicate fluvial, tion margins. In gross geometry the Mergui, North, Central, lacustrine and deltaic environments. There are a few Cenozoic South Sumatra, West and East Java Basins appear to lie in a rocks in isolated lacustrine basins [Stauffer, 1973]. K–Ar dates simple backarc location, distributed along an arcuate trend, and fission track ages indicate a local increase in cooling rate mimicking the trench, some 300–700 km into the upper plate. in the Late Cretaceous [Krähenbuhl, 1991; Kwan et al., 1992], These basins developed across a broad area of southern Sun- at a similar time to the uplift interpreted for Thailand. Else- daland during the Paleogene [Eubank and Makki, 1981; where, there are unconformable contacts of Paleogene strata Matthews and Bransden, 1995; Pertamina BPPKA, 1996a; on Cretaceous or older sedimentary, metamorphic and igneous 1996b; 1996c; Petronas, 1999; Williams and Eubank, 1995; rocks where the basement to sedimentary basins is seen (for Williams et al., 1995]. The earliest sediments in all of the example, in Sumatra, Java and Borneo), or has been pene- basins are continental, and therefore the exact age of basin trated by offshore drilling. Sediment provenance studies [van initiation is uncertain. Many basins (e.g. Sumatra) remained Hattum et al., 2003] indicate that an extensive area of SW largely continental until the Miocene whereas others (East Borneo was elevated by the Eocene, and hydrocarbon explo- Java, Barito) became marine by the Late Eocene [Cole and ration has shown that the area between south Borneo and Java Crittenden, 1997; Mason et al., 1993; Matthews and Brans- remained elevated until the Neogene [Bishop, 1980; Matthews den, 1995]. Rifting had ceased by the Early Miocene, there was and Bransden, 1995]. To the east of Sundaland was the Proto- a transition to thermal subsidence, and marine deposition was South China Sea where there was already extension in the widespread. Early Cenozoic along the Vietnam–South China margin [Chen The orientation of basin-controlling structures varies; some et al., 1993; Hayes et al., 1995; Jinmin, 1994; Lee et al., 2001; are trench-parallel whereas others are highly oblique to the Matthews et al., 1997; Rangin et al., 1995; Ru and Pigott, trench. The basins have been described as failed backarc rift 1986; Zhou et al., 1995]. To the south of the Proto-South basins [e.g. Eubank and Makki, 1981], the Sumatran basins China Sea some authors have suggested subduction beneath have been interpreted as formed in a strike-slip setting although the NW Borneo margin in Sarawak [Hazebroek and Tan, 1993; the Sumatran Fault system became active long after their for- Hutchison, 1996a; Taylor and Hayes, 1983] whereas others mation, in the Miocene [McCarthy and Elders, 1997], and it interpret a passive margin during the Paleocene and Early has also been suggested that they formed in response to sub- Eocene [Hall, 1996; 2002]. duction rollback [Morley, 2002a]. At the northern end of the system the Mergui–North Sumatra Basin has a broadly N–S 4.2. Basin Formation trench-parallel orientation [Pertamina BPPKA, 1996a]. The N–S orientation of the Sumatran rift basins may reflect a pre- From the Eocene onwards numerous sedimentary basins existing N–S basement fabric controlling fault orientation. formed throughout Sundaland (Figures 1 and 2), many of During Miocene–Pliocene inversion the N–S fabric was over- which are unusually deep. Cenozoic sediments were deposited printed by NW–SE trends [Pertamina BPPKA, 1996a; 1996b; on a variety of basement rocks ranging from granites to ophi- 1996c]. In the Central and Southern Sumatra Basins the dom- olites. The record typically begins in the Eocene or Oligocene inant structural grain of the Eocene–Oligocene rifts is N–S to (Plate 4) although since the older parts of most sequences are NNE–SSW, 30–40° oblique to the trench, although secondary terrestrial all are relatively poorly dated. NW–SE trends are also present [Madon and Ahmad, 1999; The basins can be divided into three main areas and tec- Pertamina BPPKA, 1996b; 1996c]. In the southern part of tonic provinces: 1) active margin basins that lie inboard of the system in the offshore West and East Java Basins (Plate 6) the Andaman–Sumatran–Java Trench (Mergui, North, Cen- the rifts strike NE–SW and appear to reflect basement struc- 68 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 [Simon et al., 1997]; d) West Natuna Basin, Sunda Shelf [Phillips et al., 1997]; e) northern Malay Basin [Petronas, 1999]. HALL AND MORLEY 69

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]. HALL AND MORLEY 70 [2002]. In the Oligocene (30 Ma) there was extrusion of Indochina [2002]. In the Oligocene (30 Ma) there was Hall Reconst r uctions to illustrate principal forces acting on Sundaland based mation, also with di f ferent principal stress orientations. Plate 8. model of due to India–Asia collision, slab-pull subduction of the Proto-South blocks The net result subduction hinge. of the Sunda–Java China Sea, and minor movements Australia–SE Miocene (20 Ma) within Sundaland. In the Early extension oblique was of the and led to rotation of Borneo, and movements in Sulawesi Asia collision began subduction hinge. In northern underthrusting Borneo Sunda–Java there was of South and inversion The net result was China continental crust, and crustal thickening. parts shortening of Sundaland. In the Late Miocene (10 Ma) there in different orogenic Andaman region subduction hinge in the rollback of the Sunda–Java–Banda was and in the SE to form the Banda Sea, and subduction of South China Sea beneath Luzon. Rotation of the Philippine Sea Plate led to transpression at eastern mar- gin. The net result w as ren e ed xtension in some pa r ts of Sundalan d , at the same Throughout the Ceno- and uplift in other parts time as exhumation of the region. at the subduction in forces due to arrival been variations also have zoic there would crust,zone of thickened deformation to the north due to India–Asia collision, and col- the stresses acting on other continental regions, many lisions in the east. Unlike r e gion changed signi f icant l y o v er sho t periods, leading to multiple phases of rifting defor- and orogenic orientations and later multiple phases of inversion with different HALL AND MORLEY 71 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 an arc-parallel trend and volcanic loading contributed to basin sub- sidence [Smyth et al., 2003].

4.2.2. Sunda Shelf. Perhaps the most remarkable and unusual basins within Sundaland are those in the central region. This region stretches from Sarawak to southern Laos, where there is a discontinuous string of basins of highly variable strike containing up to 14 km [e.g. Petronas, 1999] of Cenozoic sediments. These basins include the onshore Thailand basins, Gulf of Thailand basins including the large Pattani Basin, and the Malay, Penyu and West Natuna Basins of Malaysia and Indonesia (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 et al., 1986]. However, seismic reflection data from the Gulf of Thailand indicate the basins are extensional, and while some record 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 factors such as regional tectonic controls on stress orientation and magnitude probably played a significant role. Most of the basins are characterised by several phases of extension, typ- ically with different fault orientations during the different phases 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 from the main depocentres of 0.2–1 cm/yr), high to very high pres- ent-day geothermal gradients (30–75°C/km), and the absence or minor importance of fault-controlled subsidence for the majority of the basin history. When compared with typical rift basins such as the North Sea, subsidence rates for these Figure 6. Comparison of subsidence rates of North Sea basins and Sunda Shelf basins are almost an order of magnitude higher three super-deep basins of SE Asia. The North Sea curves are from (Figure 6). Many of the central Sundaland basins, in particu- the Viking Graben and show a rift to post-rift subsidence rate typi- lar the Malay, Pattani and Nam Con Son Basins, have exten- cal of average continental crust. The SE Asian basins have subsided sive and thick post-rift sequences. Consequently the basin an order of magnitude faster, possibly indicating unusual continen- centers are too deep and thick to be completely penetrated by tal crust and mantle. Subsidence curves are from the deepest parts of drilling. Dating of rift sequences is either from shallow incom- the basins. plete sections on the basin flanks, or is inferred from the age of overlying sequences. However, it appears that during the 1999] and Nam Con Son Basin [Lee et al., 2001; Matthews et Eocene (and possibly earlier) there was an extensive area of al., 1997; Olson and Dorobek, 2000] there is evidence for rift basins filled predominantly by continental deposits in the more than one phase of extension. Thermal subsidence began heart of Sundaland. in offshore Sarawak in the Early Miocene, in the northwest- Thermal subsidence began in the Late Oligocene, accom- ern Gulf of Thailand in the Middle Miocene, and in central panied by some late extensional faulting. Extension in the Thailand in the Late Miocene–Pliocene. There are three Malay Basin became progressively inactive from south to remarkable features about this string of basins: the progressive north. In some basins, such as the Malay Basin [Petronas, northward-younging of the onset of thermal subsidence, the 72 SUNDALAND BASINS very large component of thermal subsidence in the Malay and underlying strike-slip faults (as pre-existing fabrics), but are Pattani Basins for the dimensions of the rifts, and the associ- not kinematically related to them [Morley, 2001; Morley, ation with metamorphic core complexes on the western mar- 2002a]. Strike-slip faults oblique to the N–S rift trend have gin of the rift system in northern Thailand. been reactivated as oblique extensional faults. The Malay and Pattani Basins are rift basins. However they exhibit small amounts of extension (β < 1.5) for the thick- 4.2.3. Borneo margins. Around Borneo are several very ness of post-rift section (4–10 km). The Malay Basin (Plate 6) deep basins mainly filled by the erosional products of uplifted has an Eocene–Oligocene syn-rift section greater than 4 km orogenic highlands but little is published on some of the basins thick and a Lower Miocene–Recent post-rift section that and for many their deep structure is unknown. These basins exceeds 8 km thickness in places [e.g. Madon et al., 1999]. The include the Baram Delta province, NW Borneo; the Sandakan amount of upper crustal extension indicated by offsets on syn- and Tarakan Basins of NE Borneo; and the Kutai Basin, east rift faults visible on seismic reflection lines is less than β = 1.5 Borneo (Plate 7). Some of the basins were initiated in the [e.g. Madon et al., 1999] but back-stripping of the post-rift sec- Paleogene but in all cases most of their sediment fill is Miocene tion suggests lithosphere thinning up to β = 4 [Madon and and younger. They are not well understood and there appear Watts, 1998]. The Pattani Basin has a maximum of about 7.5 to be several different basin-forming mechanisms but like the km of Upper Oligocene to Recent section. The Middle Sunda Shelf basins they contain large amounts of sediment Miocene-Recent thermal subsidence component reaches a deposited very quickly. maximum thickness of 6 km. Flexural and Airy backstrip- During the Early Cenozoic there were marine areas to the ping of the Pattani Basin post-rift section indicates β = 2–2.5. north and east of Borneo: the Crocker Fan and accretionary Like the Malay Basin this β value does not match estimates of prism lay to the northwest [Hutchison, 1989; Hutchison et extension from the syn-rift section using direct measurements al., 2000] and developed in response to southeastward sub- from seismic sections and from STRETCH modeling (β = 1.3 duction of the Proto-South China Sea along the northern mar- [Watcharanantakul and Morley, 2000]). Wheeler and White gin of Borneo (Plate 3). To the east the Kutai and Tarakan [2000] suggested that post-rift subsidence in the Pattani Basin Basins form part of a passive continental margin that passes is not anomalous. However, they interpreted the base of the east into thinned continental or oceanic crust beneath the post-rift section to be located near the tops of small normal northern Makassar Straits [Calvert and Hall, 2003; Cloke et faults (assuming they were syn-rift structures) and incorrectly al., 1999]. placed the base of the post-rift section at very shallow depths The Kutai Basin formed by Eocene rifting of the Makassar (maximum of 2 km). In fact, the post-rift section is affected Strait which separates east Borneo from west Sulawesi. There by minor displacement conjugate normal faults [Kornsawan is up to 14 km of Eocene–Recent sediment [Moss and Cham- and Morley, 2002] and the thickness of the post-rift section is bers, 1999] and the majority of this was probably deposited much greater (6 km). since the Early Miocene and was derived from erosion of the In the northern and western parts of Sundaland there are Borneo highlands and inversion of older parts of the basin few Cenozoic rocks and most areas were probably emergent margins (Plate 7), to the north and west, which began in the for much of the Cenozoic. The Gulf of Thailand basins can be Early Miocene [Ferguson and McClay, 1997; van de Weerd traced north onland into Thailand where there are many small and Armin, 1992]. The thick Miocene–Recent basin sequences basins, predominantly terrestrial. The onland basins were ini- have filled accommodation space created during the Eocene tiated in the Late Oligocene–Early Miocene and contain rifting. Offshore NE Borneo are other deep basins of differ- sequences which are predominantly fluviatile or lacustrine ent origins. In the Tarakan Basin the Plio-Pleistocene sequence with a few possible marine horizons. This contrasts with the alone is about 4 km thick. Wight et al. [1993] recorded that the Gulf of Thailand where the basins became fully marine from deepest Pleistocene depocentre contains a sediment thickness the Middle to Late Miocene. Radiometric age dating indi- of up to 4.5 km and that Pleistocene depocentres do not coin- cates the major phases of left-lateral displacement on the Mae cide with Pliocene depocentres. No modeling of basin his- Ping and Three Pagodas Faults ended in the Late Oligocene tory has been published but strike-slip fault influence is [Lacassin et al., 1997] before the main phase of onland basin suggested by published maps [Wight et al., 1993]. Swauger et formation. Uttamo et al. [2003] suggested that fault tip, pull- al. [1995] reported 10 km of Neogene sediments in the San- apart, fault wedge and extensional basins were related to NW- dakan Basin, and Graves and Swauger [1997] suggested there trending dextral and NE-trending sinistral strike-slip faults may be as much as 15 km of sediment. Like the Kutei Basin which developed during Late Oligocene–Early Miocene the thick Miocene–Recent basin sequences offshore Sandakan east–west extension. Structural relationships within several have filled pre-existing accommodation space, in this case basins demonstrate that normal faults have in places followed created during Miocene rifting of the Sulu Sea, but there are 73 SUNDALAND BASINS also thick sequences onshore (more than 6 km) in circular suggests cessation of rifting and onset of thermal subsidence basins which are remnants of a formerly more extensive basin was one driver for the onset of marine conditions. In addi- [Balaguru et al., 2003]. These sediments were deposited after tion, seismic reflection data suggest renewed subsidence in a period of uplift and erosion in the Early Miocene resulting the Miocene with a tectonic origin, at least in southern Sun- from collision between South China continental crust and the daland. Although many unconformities are of local extent, at north Borneo active margin. Soon after the collision there least three events are widespread, and are probably princi- was rapid subsidence, in the Middle Miocene, depositing thick pally of tectonic origin: at about 25 Ma, 15–17 Ma and 5–6 Ma. clastic sequences in delta and pro-deltaic environments which However the important widespread unconformity around 11 were supplied from erosion of the central Borneo highlands Ma seen in many basins including the Pattani, Malay and to the south and west. Nam Con Son Basins and NW Borneo [e.g. Ginger et al., Uplift in the northern part of Borneo is asymmetric and 1993; Higgs, 1999; Jardine, 1997; Matthews et al., 1997] greatest towards the NW margin, due to partial subduction probably has a primarily eustatic cause [e.g. Haq et al., 1987]. of buoyant thinned continental crust beneath the NW margin [Hutchison et al., 2000]. This Middle Miocene-Recent uplift 4.3.1. Thailand to Malaysia. A significant period of uplift has resulted in considerable erosion, and deposition of the within Sundaland occurred during the Oligocene–Early erosional products on the shelf and deepwater region of NW Miocene and formed a N–S belt running from northwestern Borneo. Episodic folding, thrusting and strike-slip deforma- Thailand through at least as far as peninsular Malaya (Plate 5). tion, reflected in local and regional unconformities [e.g. Lev- 40Ar-39Ar dating of feldspar and micas from within the Three ell, 1987], affect the Sabah margin, where Middle Pagodas and Mae Ping Shear Zones yield cooling ages of Miocene–Recent sequences are generally less than 4 km thick. around 36–30 Ma [Lacassin et al., 1997]. These ages are inter- To the SW the Sabah margin passes into the Baram Delta preted to represent the cessation of strike-slip faulting. Apatite province which contains up to 12 km vertical thickness of fission track central ages around the fault zone range between Middle Miocene–Recent section in places [Morley et al., 29-20 Ma [Upton, 1999]. The ages are similar to those obtained 2003; Sandal, 1996]. The province is characterized by grav- for uplift of the metamorphic core complexes further north, ity-style deformation typical of large deltas (growth faults, west of Chiang Mai, with muscovite ages of 24–16 Ma, and mobile shales, toe thrusts) and by tectonic uplift events and apatite fission track ages of 23–18 Ma [Dunning et al., 1995; inversion of growth faults [Morley et al., 2003]. Rhodes, 2002]. However Oligocene–Miocene uplift is not just Although all these basins may have formed by several dif- confined to the strike-slip faults and metamorphic core com- ferent processes we suggest they also should be viewed within plexes, In western Thailand between the Mae Ping and Three the Sundaland context because they have unusually thick sed- Pagodas Faults apatite fission track central ages are 18±4 and imentary sequences that were deposited in short periods of 22±1 Ma [Upton, 1999]. Along the Thai Peninsula south of the time and were derived from local sources. Three Pagodas Fault Cretaceous granites yield apatite fission track central ages of 25–20 Ma and the Kaeng Kratchen For- 4.3. Inversion mation has ages of 35–20 Ma [Upton, 1999]. In the Malay Peninsula K–Ar dates and fission track ages indicate a local During the Cenozoic the sedimentary basins were exposed increase in cooling rates to 5°C/m.y. in the Oligo–Miocene to erosion as a consequence of eustatic changes in sea level as [Krähenbuhl, 1991; Kwan et al., 1992] suggesting uplift. well as a variety of local and regional tectonic events. Erosion These data suggest a N-S belt of uplift developed during the of elevated regions occurred during several episodes of uplift Oligo–Miocene, which would have provided an important and inversion that affected Sundaland since the Late Cretaceous local sediment source both for the Mergui Basin to the west, (Plate 5). Identifying and understanding the causes is an ongo- and the onshore Thailand, Gulf of Thailand and Malay Basins ing problem. Despite a long-term fall in global sea level from to the east. the Early Miocene there was a change to more widespread marine conditions beginning in the Early Miocene. A long-term 4.3.2. SW Borneo. The Oligo–Miocene uplift may have gradual subsidence producing such flooding has been sug- extended even further southeast into SW Borneo (Plate 5). gested to be a dynamic topographic effect resulting from long- An extensive area between Malaya and western Borneo term subduction [Lithgow-Bertelloni and Gurnis, 1997]. remained or became elevated during the Cenozoic although the However, the area of marine conditions increased from the timing of elevation, the amount of crust eroded, and exact east from the region of South China Sea oceanic crust (Plate sites of sediment deposition are not known. Sediment prove- 3) and accompanied post-rift thermal subsidence in the Sunda nance studies [van Hattum et al., 2003] show that SW Borneo Shelf basins from the Late Oligocene to Early Miocene which provided sediment to the Crocker Fan from the Eocene. The HALL AND MORLEY 74

Malay Peninsula was probably elevated from much earlier, Basins, which received sediment from the Malay Peninsula in judging from the character of Mesozoic rocks and the absence the Oligocene and Early Miocene. In both Sumatra and Java of Cenozoic rocks summarized above, and is connected to there is evidence of significant folding and thrusting during Borneo by the Singapore Platform, which at its shallowest the Neogene [Eubank and Makki, 1981; Matthews and Brans- point now has a water depth of 29 m. There, a thin Quaternary den, 1995; Pertamina BPPKA, 1996a; 1996b; 1996c; Petronas, cover on probable Cretaceous basement is indicated by seis- 1999; Williams and Eubank, 1995; Williams et al., 1995]. In mic surveys and drilling [Ben-Avraham and Emery, 1973]. many areas these were more profound events than suggested This suggests very recent erosion, although nothing is known by the term inversion but their cause is not obvious. They are of the timing of uplift and amount of erosion. However, fur- difficult to explain in terms of regional plate motions, which ther south, thick fluviatile sequences in the hydrocarbon-rich suggest relatively continuous subduction at the Sunda–Java NW Java Basins pass south into thick Mio–Pliocene turbidites Trenches. It suggests variation in the strength of coupling in West Java, which are now up to 2000 m above sea level. This between the overriding and downgoing plates. implies significant sediment supply, a source area to north (the Singapore Platform?), and uplift since the Late Miocene. 4.3.5. North Borneo. After collision of South China conti- nental fragments with the former active margin in NE Borneo 4.3.3. Sundaland interior. In the interior of Sundaland seis- (Plate 7) sediments were deposited in basins above the colli- mic surveys show there were inversion events from the Early sion-related unconformity. Later thrusting and folding began Miocene, apparently at different times in different areas [e.g. in the Middle Miocene [Hazebroek and Tan, 1993; Hutchison Morley, 2001; Petronas, 1999]. In northern Thailand there were et al., 2000; Morley et al., 2003; Petronas, 1999; Sandal, 1996]. at least four episodes of inversion from the Late Oligocene to In eastern Borneo inversion in the Kutai Basin (Plate 7) began the Pliocene [Morley, 2001; Uttamo et al., 1999]. There was in the Early Miocene, not driven by deformation to the east only minor inversion in the Gulf of Thailand, whereas further as often assumed [e.g. McClay et al., 2000; van de Weerd and south in the Malay and West Natuna Basins inversion is more Armin, 1992], but by events within Borneo [Calvert, 2000a; important (Plate 6), with several episodes of Miocene inver- Calvert and Hall, 2003; Hall, 2002; Hall and Wilson, 2000]. sion and associated intra-Miocene unconformities [Ginger et al., Inversion moved from west to east during the Early and Mid- 1993; Higgs, 1999; Jardine, 1997; Lockhart et al., 1997; Phillips dle Miocene [Ferguson and McClay, 1997; McClay et al., et al., 1997; Tjia and Liew, 1996]. In the Malay, Penyu and 2000; Moss et al., 1997]. In SE Borneo inversion of the Bar- Natuna Basins there is up to 4000 feet of missing section [Higgs, ito Basin and uplift of the adjacent Meratus Mountains occurred 1999]. Discontinuities in backstripped subsidence curves [e.g. episodically from the Late Miocene to Pleistocene [Mason et Madon and Watts, 1998; Wheeler, 2000] suggest renewed sub- al., 1993; Satyana et al., 1999]. NE Borneo is a region in sidence after Late Miocene inversion. This is also suggested which there has been significant Neogene uplift and denuda- by stratigraphy because after inversion the Sunda Shelf became tion. It includes the highest mountain in SE Asia, the 4100 m entirely marine, although global sea level was falling, indicat- granite peak of Mt Kinabalu. Intrusion ages of about 14 Ma ing significant vertical movements during the Neogene. Apatite [Jacobsen, 1970; Swauger et al., 1995] indicate rapid uplift, and fission track data from onshore central Vietnam show a sig- fission track data indicate 4–8 km of Late Miocene denudation nificant increase in uplift rate during the Late Miocene [Carter throughout the Crocker Range [Hutchison et al., 2000; Swauger et al., 2000]. Inferred denudation rates changed from about 34 et al., 1995]. A mantle-driven cause is suggested by strati- m/m.y. to 390–500 m/m.y. coinciding with enhanced progra- graphic, structural and geochemical studies. dation of sediments in adjacent offshore basins. The increase in denudation rate coincided with eruption of basalts during the 5. ORIGIN OF CENOZOIC BASINS OF SUNDALAND Late Miocene and could be linked with magma underplating and regional uplift related to a thermal anomaly below Indochina 5.1. Strike-slip Origin [Carter et al., 2000; Hoang and Flower, 1998]. The escape tectonics model for Asia originated in obser- 4.3.4. Malay Peninsula, Sumatra and Java. There are indi- vations made in the 1970s which identified major strike-slip cations of uplift, inversion, or more significant orogenic con- faults such as the Sagaing, Three Pagodas, Mae Ping and Red traction, from many parts of southern Sundaland. In the Late River Faults, radiating from eastern Tibet [e.g. Molnar and Neogene there was significant deformation in Sumatra, Java Tapponnier, 1975; Tapponnier et al., 1986]. The model sug- and Borneo resulting in widespread elevation [e.g. Pertam- gests that large crustal blocks have been ejected laterally from ina BPPKA, 1996a; 1996b; 1996c]. For example, on the west the India–Asia collision zone, and deformation is concen- side of the Malay Peninsula are the Mergui and Sumatra trated on a few major strike-slip faults. Subsequently, many of 75 SUNDALAND BASINS the Cenozoic basins in Sundaland (offshore Vietnam, Thailand, although this alternative may account for basins around the and the Gulf of Thailand) were interpreted to be pull-apart South China Sea it does not explain other Sundaland basins. basins controlled by strike-slip faults [e.g. Molnar and Tap- The escape tectonics model emphasizes the rigid nature of the ponnier, 1975; Polachan et al., 1991; Tapponnier et al., 1986]. crust and the concentration of deformation on the bounding The escape tectonics model is attractive in its simplicity and slip faults. As understanding of internal deformation of crustal the way in which it relates disparate major tectonic blocks, blocks has improved, particularly based on satellite images, structures and basins in the region. By the 1990s it became pos- seismic reflection data, and field observations, it has become sible to test the model as a result of investigating the rela- apparent that the Yunnan–Laos–Thailand–offshore Malaysia tionships between major strike-slip faults and Cenozoic part of Sundaland contains a dense network of Cenozoic faults sedimentary basins, particularly those of offshore Vietnam, and rift basins. Some have experienced multiple phases of the Gulf of Thailand and onshore Thailand [Matthews et al., extension and inversion [Ginger et al., 1993; Madon and Ahmad, 1997; Morley, 2002a; Rangin et al., 1995]. 1999; Morley et al., 2001; Phillips et al., 1997]. Not only are the The largest of the strike-slip faults is the Red River Shear supposedly rigid continental blocks highly fractured, in places Zone, which was initially suggested to have about 1000 km there is evidence for hot ductile flowing lower crust in meta- of left-lateral displacement, later reduced to about 500 km morphic core complexes, such as the Mogok gneisses of Myan- [Briais et al., 1993; Lee and Lawver, 1995]. The only way to mar [Bertrand et al., 1999], in NW Thailand [Dunning et al., terminate such large displacement fault systems is at major 1995; Rhodes et al., 1997] and near the Red River Shear Zone, plate boundaries; hence the South China Sea has been inter- Vietnam [Jolivet et al., 2001]. Deformation style and stress preted as a giant pull-apart at the tip of the Red River Shear evolution are more complex than predicted. Zone, the Mae Ping Fault has been interpreted to terminate in Active strike-slip faulting along the offshore extension of the a subduction zone off NW Borneo, and the Sagaing Fault to ter- Red River Shear Zone has clearly affected basin structure and minate at the Andaman Sea spreading center [see reviews in orientation in a narrow zone close to the fault trace [e.g. Lee Leloup et al., 2001; Morley, 2002a]. The South China Sea et al., 2001; Matthews et al., 1997; Rangin et al., 1995; Zhou oceanic crust forms a wedge that tapers to the WSW in which et al., 1995; Zhu et al., 1999], but the effects are local not there was a gradual decrease in the amount of sea floor spread- regional. Elsewhere, as discussed above, the evidence for ing towards the inferred offshore prolongation of the Red River strike-slip control on basin formation is weak to absent. Argu- Shear Zone. Consequently, the relationship between the South ment continues about the significance, age, sense of dis- China Sea and the Red River Shear Zone as a giant pull-apart placement and amounts of movement on all the major does not work geometrically [Morley, 2002a]. Furthermore, strike-slip faults [e.g. Briais et al., 1993; Leloup et al., 1995; details of the geometry and timing of extensional faults in the Morley, 2002a; Rangin et al., 1995; Roques et al., 1997a; passive margin continental crust indicate there is not a sim- 1997b; Taylor and Hayes, 1983; Wang and Burchfiel, 1997]. ple pull-apart relationship with the Red River Shear Zone Nonetheless, despite criticism of the escape tectonics model, [Matthews et al., 1997; Morley, 2002a; Rangin et al., 1995]. major strike-slip faults were important in the evolution of The escape tectonics model relates South China Sea ocean- Sundaland. However, they are not the explanation of all aspects floor spreading to Red River Fault motion [Leloup et al., 2001; of its tectonic evolution. Replumaz and Tapponnier, 2003] but does not address the mechanism of earlier continental extension. The model also 5.2. Rift Origin eliminates the Proto-South China Sea by linking thrusting in NW Borneo to movements on the Mae Ping Fault [Leloup et Most basins appear to be rift basins capped by thermal sag al., 2001]. This is not only geometrically unsatisfactory [Mor- or passive margin sequences, depending upon proximity to ley, 2002a] but also does not explain the diachronous cessa- the South China Sea oceanic crust [e.g. Cole and Crittenden, tion of thrusting along the NW Borneo margin. Moreover, 1997; Lee et al., 2001; Longley, 1997; Matthews et al., 1997; the Mae Ping Fault cannot be traced across the Vietnam mar- Zhou et al., 1995; Zhu et al., 1999]. However, an explanation gin, South China Sea and into NW Borneo. An alternative of basin origin simply by rifting has two particular problems: single driving mechanism for South China Sea spreading is the great depths of many of the basins, and the orientation of slab-pull [Hall, 1996; 2002]. In this interpretation, subduction the rifts. of the Proto-South China Sea oceanic crust beneath Borneo A possible solution is that the basins are the dynamic result during the Paleogene initially drove continental extension to of subduction. Lithgow-Bertelloni and Gurnis [1997] argued the north, and later to sea-floor spreading in the South China that a large component of vertical motion of Sundaland may Sea (Plate 8). Thus, continental extension and sea floor spread- be a dynamic topographic consequence of subduction and ing were the result of the same driving mechanism. However, estimated that more than 1000 m of subsidence may be dynam- 76 SUNDALAND BASINS ically maintained by mantle flow. Although accepting that the region, and a better understanding of how these forces their model of mantle flow predicted dynamic topography changed with time. much larger than observed they argued that it predicted “the The anomalous orientations of the rifts, the evidence for correct sign of flooding and exposure: emergence in the Pale- multiple phases of rifting, and the regional onset and termi- ocene and inundation over the past ~30 m.y.”. This claim is nation of extensional events, suggest that to fully understand unconvincing. Lithgow-Bertelloni and Gurnis [1997] infer an the Sundaland rift basins it is necessary to look beyond indi- over-simplified and large-scale pattern of differential verti- vidual driving mechanisms in different areas. Published finite cal motion over the entire region, for which they cite Ronov element models for the region have focused on the indentor et al. [1989], although as described above, the pattern of uplift effect of India on regional stresses [Huchon et al., 1994; Kong and subsidence is very much more complex. Lithgow-Bertel- and Bird, 1997; Seyferth and Henk, 2004]. However, these loni and Gurnis [1997] accept that their failure to predict the tend to produce an oversimplified stress pattern that only par- correct vertical motions for India and South America may tially explains the known sedimentary basin history of multiple arise from ambiguities in their tectonic reconstructions. Pre- extension and inversion events (see reviews by Morley [2001; Cenozoic reconstructions of SE Asia are far more uncertain 2002a]). We suggest that it is particularly important to consider than those for India and South America, although it is clear that the forces acting at the plate boundaries (Plate 8) which var- SE Asia was surrounded by subduction zones throughout the ied significantly throughout the Cenozoic because of the com- Mesozoic as well as the Cenozoic. Nonetheless, a dynamic plexity of events in the region [Hall, 2002]. The variable nature topographic effect could explain why many basins become of the plate boundaries, temporally and spatially, makes Sun- fully marine relatively late in the Neogene when global sea daland a difficult region to model but could be achieved using level was falling although it is difficult to see how dynamic finite element modeling based on regional tectonic models. In topography can account for the complex distribution, size, the adjacent Australian plate the modern stress pattern has scale, depth and timing of basin subsidence in Sundaland. been well matched by finite element modeling of stresses at In contrast, Wheeler and White [2000; 2002] suggested that the plate boundaries [Hillis and Reynolds, 2000; Reynolds et dynamic topographic effects were insignificant in East and al., 2002] and this approach would help understand the origin SE Asian basins and that there was little or no anomalous of the Sundaland basins. subsidence in Sundaland. They argued that the wavelength Some of the important plate tectonic events that affected and amplitude of subsidence were inconsistent with a dynamic the boundary and interior of Sundaland during the Cenozoic topographic explanation. For the continental region of East [Hall, 2002] that would have affected stress patterns include and SE Asia that they analyzed, which extended from the the following events. Early Cenozoic: convergence between Japan Sea south to the Malay-Penyu Basins, they interpreted Burma and Thailand as India passed the Burma block. Possi- a pattern of north to south and east to west migration of rift ini- ble thickening of the Shan–Thai block and local source of tiation and marine inundation. Wheeler and White [2002] con- buoyancy forces. Eocene onwards: progressive shortening cluded that lithospheric stretching and cooling predominantly and thickening of Himalayan–Tibetan plateau region. controls subsidence of continental sedimentary basins in SE Eocene–Early Miocene: slab pull of Proto-South China Sea. Asia. As noted above, we consider the subsidence of several Oligocene–Early Miocene: ridge push from South China sea basins to be much greater than expected for the extension spreading center. Miocene: counter-clockwise rotation of Bor- observed. However, this anomalous subsidence seems to be rel- neo. Middle Miocene–Recent: collision of Australia with SE atively late (as also observed by Wheeler and White [2002] for Sundaland. Late Miocene: rise of Tibetan Plateau and prop- offshore Vietnam) and is not predicted by dynamic topo- agation of uplift into the northern part of Sundaland. Middle graphic models. Furthermore, if the whole of Sundaland is Miocene–Recent: buoyant uplift of NE Borneo with possi- considered there is no north to south migration of rift initia- ble loss of lithospheric root. Cenozoic: continuous subduc- tion, rather the opposite, or possibly a progression towards tion at Sunda–Java Trench, although with variation in coupling the Sundaland interior. between upper and lower plates. Variation in subduction- As with mantle-driven subsidence models, a simple rifting related forces would also have been influenced by arrival at the model cannot account for the complex distribution, size, scale, subduction zone of thickened crust, such as the Ninety-East depth and timing of basin subsidence. In particular, a simple Ridge, the Investigator Fracture Zone and the distal parts of model of lithospheric stretching begs the question of what the Bengal Fan. Late Miocene–Recent: rollback of subduction was driving extension. We suggest that there are three com- hinge east of SE Sundaland to form the Banda Sea. Plate 8 ponents required for a more comprehensive explanation of illustrates in a qualitative way how changing forces acting on basin formation. A better model of an unusual crust and lith- the whole region could account for the different orientation of osphere, a better understanding of the balance of forces on rifts and their timing. HALL AND MORLEY 77

We have drawn attention above to the evidence for the unusual character of Sundaland crust and lithosphere. Some other unusual features, such as depths of sedimentary basins may be partly accounted for by varying some of the assump- tions in conventional stretching models, such as thicker crust and thinner lithosphere, at the time of rifting. However, high heat flow may mean that models also need to incorporate a more complex model of the lithosphere. It is now recognized that con- Figure 7. Model for subsidence of super-deep basins in which tec- tinental crust is capable of flow when its temperature is over tonic subsidence is significantly enhanced by lower crustal flow 400-500°C [McKenzie and Jackson, 2002; McKenzie et al., away from the site of sediment loading. To create isostatic balance 2000]. To have most of the lower crust capable of flow either with a compensation depth within the mantle, some mantle uplift at geothermal gradients must be elevated, and/or the crust must the base of the crust is required. be considerably thicker than 30 km. Both are potential factors that would increase the effect of sediment loading-driven sub- such as the Pattani and Malay Basins would require 1) initi- sidence, which could account for the unusually deep basins ation of post-rift subsidence by cooling [McKenzie, 1978], 2) and the relative importance of the thermal subsidence. a lower crust capable of flow but in pressure equilibrium (i.e. initially no tendency for flow from the rift flanks towards the 5.3. Deep Basins rift center after the syn-rift stage has finished) and 3) an influx of sediment into the basin which triggers flow of the lower Many rift and post-rift basins around the world modeled crust to accommodate the load (analogous to synformal using variants of the McKenzie model have problems match depocentres associated with mobile salt or shale basins). A ing the amount of extension with the amount of thermal subs- mechanism may be required to initiate regional flow, for exam- sidence. A number of explanations have been advanced for ple creation of space by crustal thinning around the South such inconsistencies, such as poorly constrained mantle physi- China Sea spreading center, or subduction rollback at the ical properties [White, 1990], incorrect estimation of upper Andaman Trench. crustal extension due to neglect of extension on faults which If flow of the lower crust can occur then the standard are below the resolution of seismic reflection data [Marrett and assumption that the thickness and density of the crust can be Allmendinger, 1991; 1992], and non-uniform stretching of eliminated from the isostatic equations, because they do not the lithosphere [Hellinger and Sclater, 1983]. Modeling of change during loading, becomes invalid. The SE Asian crust the Malay and Pattani Basins suggests non-uniform stretching. under the rift basins is hot, with conditions favorable for the The solution satisfies the observations but provides no reason lower crust to flow. Such flow would provide a mechanism why non-uniform stretching has occurred. Watcharanantakul for accommodating large thicknesses of sediment, and avoid and Morley [2000] suggested subduction roll-back could cause isostatic imbalance. However flow of the lower crust alone non-uniform stretching, but this would require the effects of still does not produce isostatic compensation. For a 10 km subduction roll-back to occur in a region more than 1000 km thick sedimentary basin, compensation of the upper 3–5 km from the trench. Non-uniform thinning of the mantle litho- of section (2.4 g/cm3 sediment replacing 2.7 g/cm3 upper sphere could be caused by a mantle plume, but there is no crust) is critical. In most circumstances isostatic compensation volcanic or domal uplift evidence for a plume. cannot occur within the crust, but requires buoyancy forces to In Sundaland the problem of very deep basins is not just con- drive mantle uplift of 3–5 km beneath the basin (displacing the fined to post-rift thermal subsidence. The Myanmar Central flowing lower crust) to achieve isostatic compensation (3.3 Basin, NW and NE Borneo Basins are not post-rift basins, g/cm3 mantle replacing 2.9 g/cm3 lower crust; Figure 7). We and in most cases there is little or no evidence of control of sub- suggest that the deep SE Asian basins pose real questions sidence by deep basement faults. Although there are tectonic about the assumptions behind widely accepted basin and iso- reasons for the basin location and initiation, we suggest that static models and understanding SE Asian basins requires the main reason for the great depth is enhancement of tec- more than application of models developed for simpler, more tonic subsidence by sediment loading. The standard models for stable, regions. Airy and flexural isostasy do not result in such extreme sub- sidence in shallow marine and continental settings [e.g. McKen- 6. CONCLUSIONS zie, 1978; Steckler and Watts, 1978] but the initial assumptions of such models may be varied if there is flow of the lower The geological contrast between Sundaland and the crust (Figure 7). Rapid post-rift subsidence in deep basins India–Asia collision zone has produced a similar contrast in 78 SUNDALAND BASINS ideas about the importance of key geological features. The ing left-lateral movements of the Mae Ping and Three Pago- collision, with its major themes of crustal flow, escape tec- das Faults. tonics, massive uplift, sediment supply, and climatic influ- The known differences in nature and timing of basin for- ence is so well-known that there is a tendency to assume it was mation and inversion suggest a complex and shifting pattern the major influence on all of the very large surrounding region. of uplift and subsidence with time. The region is one of rela- Sundaland by contrast is a much less obviously homogeneous tively high heat flow, and gravity observations suggest a thin region with small spreading centers opening and closing near elastic thickness for the lithosphere. Tomography shows unusu- its margins, one major and several minor subduction margins, ally low velocities at shallow depths for a continental region small collisional events, numerous rifts, and a few very large suggesting a thin, warm and weak lithosphere. Weak and strike-slip faults. Tectonic events within Sundaland have not warm continental lithosphere may be the result of long-term obviously influenced India–Asia collision, whereas India–Asia subduction beneath Sundaland. Although subduction may be collision events may have affected Sundaland. a driving force, we do not suggest that dynamic topographic The Himalayas and Tibetan Plateau dominate such a large effects are the explanation. If there is a dynamic topographic area, and have such high topography that in many tectonic signal it is small and long wavelength in comparison to the pat- models they are regarded as the only significant regions driv- terns of vertical motions within the region. On the other hand, ing the Cenozoic tectonics of Indochina and further south, the unusual depths of sedimentary basins, their orientation regardless of whether the driving mechanism is escape tec- and ages require explanations more complex than offered by tonics of rigid crustal blocks or flow of lower crust. The topog- conventional stretching models. We have suggested above raphy of northern Indochina and China has been related to that these require a better model of an unusual crust and lith- flow of ductile lower crust away from the Tibetan Plateau osphere, a better understanding of the balance of forces, and since the Late Miocene [Clark and Royden, 2000]. However, a better understanding of how these forces changed with time. in Thailand much of the present-day topography appears to Lower crustal flow offers an explanation of the depths of have been established earlier, during the Paleogene to Early basins but basin initiation and inversion requires a changing Miocene. Cretaceous–Paleogene deformation in the Lan- stress field which caused the variations in timing, distribu- ping–Simao fold belt of NE Thailand, and Oligocene–Early tion and amounts of vertical and horizontal motions within Miocene metamorphic core complex uplift in Myanmar, Thai- Sundaland. We suggest this was the result of changing intra- land and Vietnam, indicate that significant topography was plate stresses which reflect the changing pattern of forces at generated by tectonic events within Indochina prior to the the plate edges, driven by long-term subduction, acting on a outwards propagation of Tibetan Plateau topography. The very weak and probably thin lithosphere. topographic expression of Sundaland is a result of several The complexity of the background plate-driven tectonic different tectonic events, and is not just the result of India–Asia events suggest that a continuum model may offer a better collision or lower crustal flow from the Tibetan Plateau. description of deformation than a rigid block model. Analy- There is a widespread perception that Sundaland was a sta- sis of the entire region and the forces acting on it over time ble area during the Cenozoic and rigid plate motion for Sun- [Cloetingh and Wortel, 1986; Richardson, 1992; Richardson daland is often inferred [e.g. Replumaz et al., 2004; Replumaz et al., 1979] may provide a better understanding than shoe- and Tapponnier, 2003]. GPS measurements have even been horning the problems into an India–Asia ‘solution’. To fully used to suggest that Sundaland rotated clockwise as a rigid test whether the rift basin pattern can be understood by finite block during the last 8–10 million years [e.g. Rangin et al., element modeling, stresses originating from all the plate 1999]. In contrast, paleomagnetic results indicate large counter- boundaries surrounding Sundaland and from topography must clockwise rotations of Borneo and other parts of Sundaland be considered to see how the effects sum and vary across the [Fuller et al., 1999; Richter et al., 1999; Schmidtke et al., whole plate [e.g. Reynolds et al., 2002]. Local sources of 1990] but which, if modelled as a single block, result in over- stress will exert a stronger influence closer to the source (e.g. lap with Indochina in reconstructions. As argued above, the the indentor influence in the NW, subduction in the west and region has been very far from stable, and it cannot have SW, slab pull and ridge push in the east, collisions in the east), behaved as a single rigid block during most of the Cenozoic. and the stress magnitude and orientation will be the sum of dif- The considerable evidence for a complex pattern of subsi- ferent forces plus the effects of variable torques acting on the dence and elevation requires a much more complex tectonic plate [e.g. Coblentz and Richardson, 1995; Richardson, 1992; model for the region with significant internal deformation of Zoback et al., 1989]. Stresses and torques will vary spatially Sundaland [Hall, 1996; Hall, 2002]. Much of the character of and temporally in response to such factors as collision, chang- northern Sundaland may be due to small collisional events ing obliquity of subduction, and transition from plate bound- that were precursors to the main India–Asia collision, includ- aries involving oceanic crust to continental crust. HALL AND MORLEY 79

The sedimentary basins of the region, onshore and offshore Makassar Strait, Indonesia: evidence for a Miocene continent- also contain a record of tectonics, in the changing volumes and continent collision, in Tectonic Evolution of SE Asia, edited by character of sediment. There are currently few good estimates R. Hall, and D.J. Blundell, pp. 391–430, Geological Society of of volumes of sediment supplied at different times, and there London Special Publication, 1996. are few provenance studies which could identify the importance Bertrand, G., C. Rangin, H. Maluski, T.A. Han, M. Thein, O. Myint, W. Maw, and S. Lwin, Cenozoic metamorphism along the Shan of the Asian input into basins in the region. Much of this scarp (Myanmar): evidences for ductile shear along the Sagaing record is potentially there, as a result of the exploration for fault or the northward migration of the eastern Himalayan syn- hydrocarbons in the region, but relatively little of it is yet in taxis?, Geophys. Res. Lett., 26, 915–918, 1999. the public domain. There remains a final important problem, Bijwaard, H., W. Spakman, and E.R. Engdahl, Closing the gap which is that of timing because, for example, it is not possi- between regional and global travel time tomography, J. Geophys. ble to identify the causes of basin formation unless the age of Res., 103, 30055–30078, 1998. basin initiation is adequately dated. Most of the Sundaland Bishop, W.P., Structure, stratigraphy and hydrocarbons offshore basins have an early and extended terrestrial history, and older southern Kalimantan, Indonesia, AAPG Bull., 64, 37–58, 1980. sequences often contain few fossils which have limited bios- Briais, A., P. Patriat, and P. Tapponnier, Updated interpretation of tratigraphic value [see, for example, Cole and Crittenden, magnetic anomalies and sea floor spreading stages in the South 1997], besides being beyond the reach of drilling. Similar, but China Sea: Implications for the Tertiary tectonics of Southeast Asia, J. Geophys. Res., 98, 6299–6328, 1993. greater, problems surround dating of uplift. In order to under- Bustin, R.M., and A. Chonchawalit, Formation and tectonic evolu- stand the region it is vital to have better dating of events, in par- tion of the Pattani Basin, Gulf of Thailand, Int. Geol. Rev., 37, ticular by thermochronological methods. 866–892, 1995. Calvert, S.J., The Cenozoic evolution of the Lariang and Karama Acknowledgments. Financial support to RH has been provided at basins, Sulawesi, in Indonesian Petroleum Association, Proceed- different times by NERC, the Royal Society, the London University ings 27th Annual Convention, Jakarta, pp. 505–511, 2000a. Central Research Fund, and the London University and Royal Hol- Calvert, S.J., The Cenozoic geology of the Lariang and Karama loway SE Asia Research Groups, supported by a number of industrial regions, western Sulawesi, Indonesia, Ph.D. thesis, University of companies. Work in Indonesia has been facilitated by GRDC, Lemi- London, 2000b. gas and LIPI. We thank Wim Spakman for help in producing Plate Calvert, S.J., and R. Hall, The Cenozoic geology of the Lariang and 1. Financial support to CKM has been provided at various times by Karama regions, Western Sulawesi: new insight into the evolu- the Universiti of Brunei Darussalam, The Royal Society and Brunei tion of the Makassar Straits region, in Indonesian Petroleum Asso- Shell Berhad (BSB). Subsurface data from Thailand and Brunei ciation, Proceedings 29th Annual Convention, Jakarta, pp. Basins have been provided by PTTEP, Unocal, Thai Shell, TFE, 501–517, 2003. EGAT, Banpu Mines and Lanna Lignite. Cardwell, R.K., and B.L. Isacks, Geometry of the subducted litho- sphere beneath the Banda Sea in eastern Indonesia from seismic- REFERENCES ity and fault plane solutions, J. Geophys. Res., 83, 2825–2838, 1978. Ahrendt, H., C. Chonglakmani, B.T. Hansen, and D. Helmcke, Carter, A., D. Roques, and C.S. Bristow, Denudation history of Geochronological cross-section through northern Thailand, J. SE onshore central Vietnam: constraints on the Cenozoic evolution of Asian Earth Sci., 8, 207–217, 1993. the western margin of the South China Sea, Tectonophysics, 322, Artemieva, I.M., and W.D. Mooney, Mantle heat flow in stable con- 265–277, 2000. tinental regions: A global study, Eos Trans. AGU, 80, F 967, 1999. Charusiri, P., A.H. Clark, E. Farrar, D. Archibald, and B. Charusiri, Artemieva, I.M., and W.D. Mooney, Thermal thickness and evolution Granite belts in Thailand: evidence from the 40Ar/39Ar geochrono- of Precambrian lithosphere: A global study, J. Geophys. Res., 106, logical and geological syntheses, J. SE Asian Earth Sci., 8, 127–136, 16,387–16,414, 2001. 1993. Balaguru, A., G.J. Nichols, and R. Hall, The origin of the circular Chen, P.P.H., Z.Y. Chen, and Q.M. Zhang, Sequence stratigraphy basins of Sabah, Malaysia, Bull. Geol. Soc. Malaysia, 46, 335–351, and continental margin development of the northwest shelf of the 2003. South China Sea, AAPG Bull., 77, 842–862, 1993. Barley, M.E., A.L. Pickard, K. Zaw, P. Rak, and M.G. Doyle, Juras- Clark, M.C., Late Cenozoic Uplift of Southeastern Tibet, Ph.D. the- sic to Miocene magmatism and metamorphism in the Mogok sis, Massachusetts Institute of Technology, USA, 2003. metamorphic belt and the India-Eurasia collision in Myanmar, Clark, M.K., and L.H. Royden, Topographic ooze: Building the east- Tectonics, 22, doi:10.1029/2002TC001398, 2003. ern margin of Tibet by lower crustal flow, Geology, 28, 703–706, Ben-Avraham, Z., and K.O. Emery, Structural framework of Sunda 2000. Shelf, AAPG Bull., 57, 2323–2366, 1973. Clark, M.K., L.M. Schoenbohm, L.H. Royden, K.X. Whipple, B.C. Bergman, S.C., D.Q. Coffield, J.P. Talbot, and R.J. Garrard, Tertiary Burchfiel, X. Zhang, W. Tang, E. Wang, and L. Chen, Surface tectonic and magmatic evolution of western Sulawesi and the uplift, tectonics, and erosion of eastern Tibet from large-scale 80 SUNDALAND BASINS

drainage patterns, Tectonics, 23, doi:10.1029/2002TC001402, Fleming, K., P. Johnston, D. Zwartz, Y. Yokoyama, K. Lambeck, and 2004. J. Chappell, Refining the eustatic sea-level curve since the last Cloetingh, S., and R. Wortel, Stress in the Indo-Australian plate, glacial maximum using far- and intermediate-field sites, Earth Tectonophysics, 132, 49–67, 1986. Planet. Sci. Lett., 163, 327–342, 1998. Cloke, I.R., J. Milsom, and D.J.B. Blundell, Implications of gravity Flint, S., D.J. Stewart, T. Hyde, E.C.A. Gevers, O.R.F. Dubrule, data from east Kalimantan and the Makassar Straits: a solution and E.D. van Riessen, Aspects of reservoir geology and pro- to the origin of the Makassar Straits?, J. Asian Earth Sci., 17, duction behaviour of Sirikit Oil Field, Thailand: An integrated 61–78, 1999. study using well and 3-D seismic data, AAPG Bull., 72, Cobbing, E.J., D.I.J. Mallick, P.E.J. Pitfield, and L.H. Teoh, The gran- 1254–1269, 1988. ites of the Southeast Asian tin belt, J. Geol. Soc. London, 143, Fuller, M., J.R. Ali, S.J. Moss, G.M. Frost, B. Richter, and A. Mahfi, 537–550, 1986. Paleomagnetism of Borneo, J. Asian Earth Sci., 17, 3–24, 1999. Cobbing, E.J., P.E.J. Pitfield, D.P.F. Darbyshire, and D.I.J. Mallick, The Ginger, D.C., Ardjakusumah, R.J. Hedley, and J. Pothecary, Inversion granites of the South-East Asian tin belt, Overseas Mem. British history of the West Natuna basin: examples from the Cumi-Cumi Geol. Surv., 10, 1–369, 1992. PSC, in Indonesian Petroleum Association, Proceedings 22nd Coblentz, D.D., and R.M. Richardson, Statistical trends in the Annual Convention, Jakarta, pp. 635–658, 1993. intraplate stress field, J. Geophys. Res., 100, 20,245–20,255, 1995. Gobbett, D.J., and C.S. Hutchison, Geology of the Malay Peninsula Cole, J.M., and S. Crittenden, Early Tertiary basin formation and (West Malaysia and Singapore), 438 pp., Wiley-Interscience, New the development of lacustrine and quasi-lacustrine/marine source York, 1973. rocks on the Sunda Shelf of SE Asia., in Petroleum Geology of Graves, J.E., and D.A. Swauger, Petroleum systems of the Sandakan Southeast Asia, edited by A.J. Fraser, S.J. Matthews, and R.W. Basin, Philippines, in Proceedings of the International Confer- Murphy, pp. 147–183, Geological Society of London, 1997. ence on Petroleum Systems of SE Asia and Australia, edited by Crevello, P.D., The great Crocker submarine fan: A world-class fore- J.V.C. Howes, and R.A. Noble, pp. 799–813, Indonesian Petro- deep turbidite system, in Indonesian Petroleum Association, Pro- leum Association, 1997. ceedings 28th Annual Convention, Jakarta, pp. 378–407, 2001. Hall, R., Reconstructing Cenozoic SE Asia, in Tectonic Evolution of Dang, H.N., and C. Sladen, Petroleum Geology of offshore Da Nang, SE Asia, edited by R. Hall, and D.J. Blundell, pp. 153–184, Geo- Central Vietnam, in Proceedings of the International Conference logical Society of London Special Publication, 1996. on Petroleum Systems of SE Asia and Australia, edited by J.V.C. Hall, R., Cenozoic geological and plate tectonic evolution of SE Howes, and R.A. Noble, pp. 449–468, Indonesian Petroleum Asso- Asia and the SW Pacific: computer-based reconstructions, model ciation, 1997. and animations, J. Asian Earth Sci., 20, 353–434, 2002. Douglas, I., Man, vegetation and the sediment yield of rivers, Nature, Hall, R., and G.J. Nichols, Cenozoic sedimentation and tectonics in 215, 925–928, 1967. Borneo: climatic influences on orogenesis, in Sediment Flux to Douglas, I., Hydrological investigations of forest disturbance and Basins: Causes, Controls and Consequences, edited by S.J. Jones, land cover impacts in South-East Asia: a review, Philos. Trans. and L.E. Frostick, pp. 5–22, Geological Society of London Spe- R. Soc. London Ser. B, 354, 1725–1738, 1999. cial Publication, 2002. Douglas, I., T. Spencer, T. Greer, K. Bidin, W. Sinun, and W.M. Wong, Hall, R., and M.E.J. Wilson, Neogene sutures in eastern Indonesia, The impact of selective commercial logging on stream hydrology, J. Asian Earth Sci., 18, 787–814, 2000. chemistry and sediment loads in the Ulu Segama Rain Forest, Sabah, Hamilton, W., Tectonics of the Indonesian region, USGS Prof. Paper, Philos. Trans. R. Soc. London Ser. B, 335, 397–406, 1992. 1078, 345 pp., 1979. Dunning, G.R., A.S. Macdonald, and S.M. Barr, Zircon and monazite Haq, B.U., J. Hardenbol, and P.R. Vail, Chronology of fluctuating U-Pb dating of the Doi Inthanon core complex, northern Thai- sea levels since the Triassic, Science, 235, 1156–1167, 1987. land: implications for extension within the Indosinian orogen, Harbury, N.A., M.E. Jones, M.G. Audley-Charles, I. Metcalfe, and Tectonophysics, 251, 197–215, 1995. K.R. Mohamed, Structural evolution of Mesozoic Peninsular Eubank, R.T., and A.C. Makki, Structural geology of the Central Malaysia, J. Geol. Soc. London, 147, 11–26, 1990. Sumatra back-arc basin, in Indonesian Petroleum Association, Hay, W.W., Detrital sediment fluxes from continents to oceans, Chem. Proceedings 10th Annual Convention, Jakarta, pp. 153–196, 1981. Geol., 145, 287–323, 1998. Fatt, R.W.H., Petroleum resources, peninsular Malaysia, in The Petro- Hayes, D.E., S.S. Nissen, P. Buhl, J. Diebold, B.C. Yao, W.J. Zeng, and leum Geology and Resources of Malaysia, pp. 253–272, Petronas, Y.Q. Chen, Throughgoing crustal faults along the northern mar- Kuala Lumpur, 1999. gin of the South China Sea and their role in crustal extension, J. Ferguson, A., and K. McClay, Structural modelling within the Sanga Geophys. Res., 100, 22,435–22,446, 1995. Sanga PSC, Kutei Basin, Kalimantan: its implication to pale- Hazebroek, H.P., and D.N.K. Tan, Tertiary tectonic evolution of the ochannel orientation studies and timing of hydrocarbon entrap- NW Sabah continental margin, Bull. Geol. Soc. Malaysia Special ment., in Proceedings of the International Conference on Petroleum Publication, 33, 195–210, 1993. Systems of SE Asia and Australia, edited by J.V.C. Howes, and Hellinger, S.J., and J.G. Sclater, Some comments on two-layer exten- R.A. Noble, pp. 727–743, Indonesian Petroleum Association, sion models for the evolution of sedimentary basins, J. Geophys. 1997. Res., 88, 8251–8269, 1983. HALL AND MORLEY 81

Higgs, R., Gravity anomalies, subsidence history and the tectonic by A. Yin, and T.M. Harrison, pp. 18–34, Cambridge University evolution of the Malay and Penyu Basins (offshore Peninsula Press, Cambridge, 1997. Malaysia)—Discussion, Basin Res., 11, 285–290, 1999. Kornsawan, A., and C.K. Morley, The origin and evolution of com- Hillis, R.R., and S.D. Reynolds, The Australian stress map, J. Geol. plex transfer zones (graben shifts) in conjugate fault systems Soc. London, 157, 915–921, 2000. around the Funan Field, Pattani basin, Gulf of Thailand, J. Struct. Hoang, N., and M. Flower, Petrogenesis of Cenozoic basalts from Geol., 24, 435–449, 2002. Vietnam: Implication for origins of a ‘Diffuse igneous province’, Krähenbuhl, R., Magmatism, tin mineralization and tectonics of the J. Petrol., 39, 369–395, 1998. Main Range, Malaysian Peninsula: Consequences for the plate Hovius, N., Controls on sediment supply by large rivers, in Relative tectonic model of Southeast Asia based on Rb-Sr, K-Ar and fis- Role of Eustasy, Climate, and Tectonism in Continental Rocks, sion track data, Bull. Geol. Soc. Malaysia, 29, 1–100, 1991. edited by K.W. Shanley, pp. 3–16, SEPM Society for Sedimentary Kwan, T.S., R. Krähenbuhl, and E. Jager, Rb-Sr, K-Ar and fission- Geology Special Publication, 1998. track ages for granites from Penang Island, West Malaysia—an Huchon, P., X. Le Pichon, and C. Rangin, Indochina peninsula and interpretation model for Rb-Sr whole-rock and for actual and the collision of India and Eurasia, Geology, 22, 27–30, 1994. experimental mica data, Contrib. Mineral. Petrol., 111, 527–542, Hutchison, C.S., Plutonic activity, in Geology of the Malay Penin- 1992. sula (West Malaysia and Singapore), edited by D.J. Gobbett, and Lacassin, R., H. Maluski, P.H. Leloup, P. Tapponnier, C. Hinthong, C.S. Hutchison, pp. 215–252, Wiley-Interscience, New York, K. Siribhakdi, S. Chuaviroj, and A. Charoenravat, Tertiary 1973. diachronic extrusion and deformation of western Indochina: Struc- Hutchison, C.S., Ophiolites in Southeast Asia, Geol. Soc. Am. Bull., tural and 40Ar/39Ar evidence from NW Thailand, J. Geophys. Res., 86, 797–806, 1975. 102, 10,013–10,037, 1997. Hutchison, C.S., Geological Evolution of South-East Asia, 376 pp., Lebedev, S., and G. Nolet, Upper mantle beneath Southeast Asia Oxford Monographs on Geology and Geophysics, Clarendon from S velocity tomography, J. Geophys. Res., 108, Press, 1989. doi:10.1029/2000JB000073, 2003. Hutchison, C.S., The ‘Rajang Accretionary Prism’ and ‘Lupar Line’ Lee, G.H., K. Lee, and J.S. Watkins, Geologic evolution of the Cuu problem of Borneo, in Tectonic Evolution of SE Asia, edited by R. Long and Nam Con Son basins, offshore southern Vietnam, South Hall, and D.J. Blundell, pp. 247–261, Geological Society of Lon- China Sea, AAPG Bull., 85, 1055–1082, 2001. don Special Publication, 1996a. Lee, T.Y., and L.A. Lawver, Cenozoic plate reconstruction of south- Hutchison, C.S., South-East Asian Oil, Gas, Coal and Mineral east Asia, Tectonophysics, 251, 85–139, 1995. Deposits, 265 pp., Oxford Monographs on Geology and Geo- Leloup, P.H., N. Arnaud, R. Lacassin, J.R. Kienast, T.M. Harrison, physics, Clarendon Press, 1996b. T.T.P. Trong, A. Replumaz, and P. Tapponnier, New constraints Hutchison, C.S., S.C. Bergman, D.A. Swauger, and J.E. Graves, A on the structure, thermochronology, and timing of the Ailao Shan- Miocene collisional belt in north Borneo: uplift mechanism and Red River shear zone, SE Asia, J. Geophys. Res., 106, 6683–6732, isostatic adjustment quantified by thermochronology, J. Geol. Soc. 2001. London, 157, 783–793, 2000. Leloup, P.H., R. Lacassin, P. Tapponnier, U. Schärer, D.L. Zhong, Jacobsen, G., Geology and mineral resources of the Gunong Kina- X.H. Liu, L.S. Zhang, S.C. Ji, and P.T. Trinh, The Ailao Shan- balu Area, Sabah, E. Malaysia, Geol. Surv. Malaysia Rep., 8, Red River shear zone (Yunnan, China), Tertiary transform bound- 1–111, 1970. ary of Indochina, Tectonophysics, 251, 3–84, 1995. Jardine, E., Dual petroleum systems governing the prolific Pattani Levell, B.K., The nature and significance of regional unconformities Basin, offshore Thailand, in Proceedings of the International Con- in the hydrocarbon-bearing Neogene sequences offshore west ference on Petroleum Systems of SE Asia and Australia, edited by Sabah, Bull. Geol. Soc. Malaysia, 21, 55–90, 1987. J.V.C. Howes, and R.A. Noble, pp. 351–363, Indonesian Petro- Liew, T.C., and M.T. McCulloch, Genesis of granitoid batholiths of leum Association, 1997. Peninsular Malaysia and implications for models of crustal evo- Jinmin, W., Evaluation and models of Cenozoic sedimentation in lution: evidence from a Nd-Sr isotopic zircon study, Geochim. the South China Sea, Tectonophysics, 235, 77–98, 1994. Cosmochim. Acta, 49, 587–600, 1985. Johnston, C.R., and C. Bowin, Crustal reaction resulting from the mid- Liew, T.C., and R.W. Page, U-Pb zircon dating of granitoid plutons Pliocene to recent continent-island collision in the Timor region, from the West Coast of Peninsular Malaysia, J. Geol. Soc. London, BMR J. Aust. Geol. Geophys., 6, 223–243, 1981. 142, 515–526, 1985. Jolivet, L., O. Beyssac, B. Goffe, D. Avigad, C. Lepvrier, H. Maluski, Lithgow-Bertelloni, C., and M. Gurnis, Cenozoic subsidence and and T.T. Thang, Oligo-Miocene midcrustal subhorizontal shear uplift of continents from time-varying dynamic topography, Geol- zone in Indochina, Tectonics, 20, 46–57, 2001. ogy, 25, 735–738, 1997. Kenyon, C.S., and L.R. Beddoes, Geothermal Gradient Map of Lockhart, B.E., O. Chinoroje, C.B. Enomoto, and G.A. Hollomon, Southeast Asia, 50 pp., South East Asia Petroleum Exploration Early Tertiary deposition in the southern Pattani Trough, Gulf of Society and Indonesian Petroleum Association, 1977. Thailand., in Proceedings of the International Conference on Kong, X., and P. Bird, Neotectonics of Asia: thin-shell finite-ele- Stratigraphy and Tectonic Evolution in Southeast Asia and the ment models with faults, in The Tectonic Evolution of Asia, edited South Pacific, Bangkok, Thailand, pp. 476–489, 1997. 82 SUNDALAND BASINS

Longley, I.M., The tectonostratigraphic evolution of SE Asia., in McLennan, S.M., Weathering and global denudation, J. Geol., 101, Petroleum Geology of Southeast Asia, edited by A.J. Fraser, S.J. 295–303, 1993. Matthews, and R.W. Murphy, pp. 311–339, Geological Society Metcalfe, I., Allochthonous terrane processes in Southeast Asia, Phi- of London Special Publication, 1997. los. Trans. R. Soc. London Ser. A, 331, 625–640, 1990. Ludwig, W., and J.L. Probst, River sediment discharge to the oceans: Metcalfe, I., Pre-Cretaceous evolution of SE Asian Terranes, in Tec- present-day controls and global budgets, Am. J. Sc., 298, 265–295, tonic Evolution of SE Asia, edited by R. Hall, and D.J. Blundell, 1998. pp. 97–122, Geological Society of London Special Publication, Macpherson, C.G., and R. Hall, Timing and tectonic controls on 1996. magmatism and ore generation in an evolving orogen: evidence Métivier, F., Y. Gaudemer, P. Tapponnier, and M. Klein, Mass accu- from Southeast Asia and the western Pacific, in The Timing and mulation rates in Asia during the Cenozoic, Geophys. J. Int., 137, Location of Major Ore Deposits in an Evolving Orogen, edited 280–318, 1999. by D.J. Blundell, F. Neubauer, and A. von Quadt, pp. 49–67, Geo- Milliman, J.D., K.L. Farnsworth, and C.S. Albertin, Flux and fate logical Society of London Special Publication, 2002. of fluvial sediments leaving large islands in the East Indies, Nether- Madon, M.B., P. Abolins, M.J.B. Hoesni, and M.B. Ahmad, Malay lands J. Sea Res., 41, 97–107, 1999. Basin, in The Petroleum Geology and Resources of Malaysia, pp. Milliman, J.D., and R.H. Meade, World-wide delivery of river sed- 173–217, Petronas, Kuala Lumpur, 1999. iment to the oceans, J. Geol., 91, 1–21, 1983. Madon, M.B., and M.B. Ahmad, Basins in the Straits of Melaka, in Milliman, J.D., and J.P.M. Syvitski, Geomorphic/tectonic control of The Petroleum Geology and Resources of Malaysia, pp. 237–249, sediment discharge to the ocean: the importance of small moun- Petronas, Kuala Lumpur, 1999. tainous rivers, J. Geol., 100, 525–544, 1992. Madon, M.B., and A.B. Watts, Gravity anomalies, subsidence history Mitchell, A.H.G., Cretaceous-Cenozoic tectonic events in the west- and the tectonic evolution of the Malay and Penyu Basins (off- ern Myanmar (Burma) Assam region, J. Geol. Soc. London, 150, shore Peninsular Malaysia), Basin Res., 10, 375–392, 1998. 1089–1102, 1993. Marrett, R., and R.W. Allmendinger, Estimates of strain due to brit- Mitchell, A.H.G., T. Hiang, and N. Htay, Mesozoic orogenies along tle faulting: Sampling of fault populations, J. Struct. Geol., 13, the Mandalay-Yangon margin of the Shan Plateau, in Symposium 735–738, 1991. on Geology of Thailand, 26–31 August 2002, Bangkok, pp. Marrett, R., and R.W. Allmendinger, Amount of extension on ‘small’ 136–149, Department of Mineral Resources Thailand, 2002. faults: An example from the Viking Graben, Geology, 20, 47–50, Molnar, P., and P. Tapponnier, Cenozoic tectonics of Asia: effects of 1992. a continental collision, Science, 189, 419–426, 1975. Mason, A.D.M., J.C. Haebig, and R.L. McAdoo, A fresh look at the Morley, C.K., Combined escape tectonics and subduction rollback- Barito Basin, Kalimantan, in Indonesian Petroleum Association, backarc extension: a model for the Tertiary rift basins in Thai- Proceedings 22nd Annual Convention, Jakarta, pp. 589–606, land, Malaysia and Laos, J. Geol. Soc. London, 158, 461–474, 1993. 2001. Matthews, S.J., and P.J.E. Bransden, Late Cretaceous and Cenozoic Morley, C.K., A tectonic model for the Tertiary evolution of strike- tectono-stratigraphic development of the East Java Sea Basin, slip faults and rift basins in SE Asia, Tectonophysics, 347, 189–215, Indonesia, Mar. Petroleum Geol., 12, 499–510, 1995. 2002a. Matthews, S.J., A.J. Fraser, S. Lowe, S.P. Todd, and F.J. Peel, Struc- Morley, C.K., Tectonic settings of continental extensional provinces ture, stratigraphy and petroleum geology of the SE Nam Con Son and their impact on sedimentation and hydrocarbon prospectivity, Basin, offshore Vietnam, in Petroleum Geology of Southeast Asia, in Sedimentation in Continental Rifts, edited by R.W. Renaut, and edited by A.J. Fraser, S.J. Matthews, and R.W. Murphy, pp. 89–106, G.M. Ashley, pp. 23–53, 2002b. Geological Society of London Special Publication, 1997. Morley, C.K., Nested strike-slip duplexes and other evidence for McCarthy, A.J., and C.F. Elders, Cenozoic deformation in Sumatra: Late Cretaceous-Paleogene transpressional tectonics before and Oblique subduction and the development of the Sumatran Fault during India-Eurasia collision, in Thailand, Myanmar and Malaysia, System., in Petroleum Geology of Southeast Asia, edited by A.J. J. Geol. Soc. London, in press, 2004. Fraser, S.J. Matthews, and R.W. Murphy, pp. 355–363, Geologi- Morley, C.K., S. Back, P. van Rensbergen, P. Crevello, and J.J. Lam- cal Society of London Special Publication, 1997. biase, Characteristics of repeated, detached, Miocene-Pliocene McClay, K., T. Dooley, A. Ferguson, and J. Poblet, Tectonic evolution tectonic inversion events, in a large delta province on an active of the Sanga Sanga Block, Mahakam Delta, Kalimantan, Indone- margin, Brunei Darussalam, Borneo, J. Struct. Geol., 25, sia, AAPG Bull., 84, 765–786, 2000. 1147–1169, 2003. McKenzie, D., and J.A. Jackson, Conditions for flow in the continental Morley, C.K., N. Woganan, N. Sankumarn, T.B. Hoon, A. Alief, and crust, Tectonics, 21, 5-1–5-7, 2002. M. Simmons, Late Oligocene-Recent stress evolution in rift basins McKenzie, D.P., Some remarks on the development of sedimentary of northern and central Thailand: implications for escape tecton- basins, Earth Planet. Sci. Lett., 40, 25–32, 1978. ics, Tectonophysics, 334, 115–150, 2001. McKenzie, D.P., P.B. Gans, and E.L. Miller, Characteristics and con- Morley, R.J., Palynological evidence for Tertiary plant dispersals in sequences of flow in the lower crust, J. Geophys. Res., 105, the SE Asian region in relation to plate tectonics and climate, in 11,029–11,046, 2000. Biogeography and Geological Evolution of SE Asia, edited by R. HALL AND MORLEY 83

Hall, and J.D. Holloway, pp. 211–234, Backhuys Publishers, Lei- Pollack, H.N., S. Hurter, and J.R. Johnson, The new global heat flow den, The Netherlands, 1998. data compilation, Eos Trans. AGU, 71, 1604, 1990. Morley, R.J., Origin and Evolution of Tropical Rain Forests, 362 Pollack, H.N., S.J. Hurter, and J.R. Johnson, Heat flow from the pp., John Wiley, 2000. Earth’s interior: analysis of the global data set, Rev. Geophys., 31, Moss, S.J., J. Chambers, I. Cloke, A. Carter, D. Satria, J.R. Ali, and 267–280, 1993. S. Baker, New observations on the sedimentary and tectonic evo- Praditan, S., Characteristics and controls of lacustrine deposits of lution of the Tertiary Kutai Basin, East Kalimantan, in Petroleum some Tertiary basins in Thailand, in International Symposium of Geology of Southeast Asia, edited by A.J. Fraser, S.J. Matthews, and Intermontane Basins, pp. 135–144, Geology and Resources, Chi- R.W. Murphy, pp. 395–416, Geological Society of London Spe- ang Mai, Thailand, 1989. cial Publication, 1997. Racey, A., I.R. Duddy, and M.A. Love, Apatite fission track analy- Moss, S.J., and J.L.C. Chambers, Tertiary facies architecture in the sis of Mesozoic red beds from northeastern Thailand and western Kutai Basin, Kalimantan, Indonesia, J. Asian Earth Sci., 17, 157– Laos, in Proceedings of the International Conference on Stratig- 181, 1999. raphy and Tectonic Evolution in Southeast Asia and the South Mouret, C., H. Heggemann, J. Gouadain, and S. Krisadasima, Geo- Pacific, pp. 200–209, Bangkok, Thailand, 1997. logical history of the siliciclastic Mesozoic strata of the Khorat Rangin, C., M. Klein, D. Roques, X. Le Pichon, and L.V. Trong, The Group in the Phu Phan Range area, Northeastern Thailand, in Pro- Red River fault system in the Tonkin Gulf, Vietnam, Tectono- ceedings International Symposium on Biostratigraphy of Main- physics, 243, 209–222, 1995. land Southeast Asia: Facies and Palaeontology, Chiang Mai, pp. Rangin, C., X. Le Pichon, S. Mazzotti, M. Pubellier, N. Chamot- 23–49, 1993. Rooke, M. Aurelio, A. Walpersdorf, and R. Quebral, Plate con- Nagoa, T., and S. Uyeda, Heat flow distribution in southeast Asia vergence measured by GPS across the Sundaland/Philippine Sea with consideration of volcanic heat, Tectonophysics, 251, 153–161, plate deformed boundary: the Philippines and eastern Indonesia, 1995. Geophys. J. Int., 139, 296–316, 1999. O’Leary, J., and G.S. Hill, Tertiary basin development in the South- Replumaz, A., H. Karason, R.D. van der Hilst, J. Besse, and P. Tap- ern Central Plains, Thailand, in International Symposium on Inter- ponnier, 4-D evolution of SE Asia’s mantle from geological recon- montane Basins, Geology and Resources, Chiang Mai, Thailand, structions and seismic tomography, Earth Planet. Sci. Lett., 221, pp. 254–264, 1989. 103–115, 2004. Olson, C.C., and S.L. Dorobek, Comparison of structural styles, Replumaz, A., and P. Tapponnier, Reconstruction of the deformed col- regional subsidence patterns, and depositional patterns across the lision zone between India and Asia by backward motion of lithos- Nam Con Son and Cuu Long Basins, offshore southeast Vietnam, pheric blocks, J. Geophys. Res., 108, doi:10.1029/2001JB000661, in AAPG Annual Meeting Abstracts, pp. 109, New Orleans, 2000. 2003. Pertamina BPPKA, Petroleum Geology of Indonesian Basins. Volume Reynolds, S.D., D.D. Coblentz, and R.R. Hillis, Tectonic forces con- I: North Sumatra Basin, 85 pp., 1996a. trolling the regional intraplate stress field in continental Australia: Pertamina BPPKA, Petroleum Geology of Indonesian Basins. Volume Results from new finite element modeling, J. Geophys. Res., 107, II: Central Sumatra Basins, 232 pp., 1996b. doi:10.1029/2001JB000408, 2002. Pertamina BPPKA, Petroleum Geology of Indonesian Basins. Volume Rhodes, B.P., New U-Pb and Ar/Ar ages from the Doi Inthanon and X: South Sumatra Basins, 81 pp., 1996c. Doi Suthep metamorphic core complexes, Northwestern Thai- Petronas, The Petroleum Geology and Resources of Malaysia, 665 pp., land, in The Symposium on Geology of Thailand, pp. 284–308, 1999. Department of Mineral Resources, Bangkok, Thailand, 2002. Phillips, S., L. Little, E. Michael, and V. Odell, Sequence stratigra- Rhodes, B.P., J. Blum, T. Devine, and K. Ruangvataasirikul, Geology phy of Tertiary petroleum systems in the West Natuna Basin, of the Doi Suthep metamorphic complex and adjacent Chiang Indonesia, in Proceedings of the International Conference on Mai Basin, in Proceedings of the International Conference on Petroleum Systems of SE Asia and Australia, edited by J.V.C. Stratigraphy and Tectonic Evolution in Southeast Asia and the Howes, and R.A. Noble, pp. 381–401, Indonesian Petroleum Asso- South Pacific, pp. 305–313, Bangkok, Thailand, 1997. ciation, 1997. Rhodes, B.P., R. Conejo, T. Benchawan, S. Titus, and R. Lawson, Pigott, J.D., and N. Sattayarak, Aspects of sedimentary basin evolu- Paleocurrents and provenance of the Mae Rim Formation, north- tion assessed through tectonic subsidence analysis. Example: ern Thailand: Implications for tectonic evolution of the Chiang northern Gulf of Thailand, J. SE Asian Earth Sci., 8, 407–420, Mai basin, in 8th International Congress on Pacific Neogene 1993. Stratigraphy, Chiang Mai University, Thailand, pp. 311–331, 2003. Pivnik, D.A., J. Nahm, R.S. Tucker, G.O. Smith, K. Nyein, M. Nyunt, Richardson, R.M., Ridge forces, absolute plate motions, and the and P.H. Maung, Polyphase deformation in a fore-arc/back-arc intraplate stress-field, J. Geophys. Res., 97, 11,739–11,748, 1992. basin, Salin subbasin, Myanmar (Burma), AAPG Bull., 82, Richardson, R.M., S.C. Solomon, and N.H. Sleep, Tectonic stress 1837–1856, 1998. in the plates, Rev. Geophys. Space Phys., 17, 981–1017, 1979. Polachan, S., S. Pradidtan, C. Tongtaow, S. Janmaha, K. Intarawijitr, Richter, B., E. Schmidtke, M. Fuller, N. Harbury, and A.R. Sam- and C. Sangsuwan, Development of Cenozoic basins in Thailand, sudin, Paleomagnetism of peninsular Malaysia, J. Asian Earth Mar. Petroleum Geol., 8, 84–97, 1991. Sci., 17, 477–519, 1999. 84 SUNDALAND BASINS

Ritsema, J., and H.-J. van Heijst, Seismic imaging of structural het- Smith, W.H.F., and D.T. Sandwell, Global seafloor topography from erogeneity in the Earth’s mantle: Evidence for large-scale mantle satellite altimetry and ship depth soundings, Science, 277, flow, Sci. Prog., 83, 243–259, 2000. 1956–1962, 1997. Ronov, A., V. Khain, and A. Balukhovsky, Atlas of Lithological-Pale- Smyth, H., R. Hall, J. Hamilton, and P. Kinny, Volcanic origin of ogeographical Maps of the World, Mesozoic and Cenozoic of Con- quartz-rich sediments in East Java, in Indonesian Petroleum Asso- tinents and Oceans, 79 pp., U.S.S.R. Academy of Sciences, ciation, Proceedings 29th Annual Convention, Jakarta, pp. Leningrad, 1989. 541–559, 2003. Roques, D., S.J. Matthews, and C. Rangin, Constraints on strike- Stauffer, P.H., Cenozoic, in Geology of the Malay Peninsula (West slip motion from seismic and gravity data along the Vietnam mar- Malaysia and Singapore), edited by D.J. Gobbett, and C.S. Hutchi- gin offshore Da Nang: implications for hydrocarbon prospectivity son, pp. 143–176, Wiley-Interscience, New York, 1973. and opening of the East Vietnam Sea, in Petroleum Geology of Steckler, M.S., and A.B. Watts, Subsidence of the Atlantic-type mar- Southeast Asia, edited by A.J. Fraser, S.J. Matthews, and R.W. gin off New York, Earth Planet. Sci. Lett., 41, 1–13, 1978. Murphy, pp. 341–353, Geological Society of London Special Pub- Suggate, S., and R. Hall, Predicting sediment yields from SE Asia: lication, 1997a. A GIS approach, in Indonesian Petroleum Association, Proceed- Roques, D., C. Rangin, and P. Huchon, Geometry and sense of motion ings 29th Annual Convention, Jakarta, pp. 289–304, 2003. along the Vietnam margin: onshore/offshore Da Nang area, Bull. Summerfield, M.A., and N.J. Hulton, Natural controls of fluvial Geol. Soc. France, 168, 413–422, 1997b. denudation rates in major world drainage basins, J. Geophys. Res., Ru, K., and J.D. Pigott, Episodic rifting and subsidence in the South 99, 13871–13883, 1994. China Sea, AAPG Bull., 70, 1136–1155, 1986. Swauger, D.A., S.C. Bergman, A.P. Marillo, E.S. Pagado, and T. Rutherford, K.J., and M.K. Qureshi, Geothermal gradient map of Surat, Tertiary stratigraphy and tectonic framework of Sabah, Southeast Asia 2nd edition – 1981, 51 pp., South East Asia Petro- Malaysia: a field and laboratory study, in GEOSEA 95: 8th leum Exploration Society and Indonesian Petroleum Association, Regional Conference on Geology, Minerals, and Energy Resources 1981. of SE Asia, Manila, pp. 35–36, 1995. Sandal, S.T., The Geology and Hydrocarbon Resources of Negara Tapponnier, P., G. Peltzer, and R. Armijo, On the mechanism of col- Brunei Darussalam, 243 pp., Syabas, Bandar Seri Begawan, Brunei lision between India and Asia, in Collision Tectonics, edited by Darussalam, 1996. M.P. Coward, and A.C. Ries, pp. 115–157, Geological Society Satyana, A.H., D. Nugroho, and I. Surantoko, Tectonic controls on London Special Publication, 1986. the hydrocarbon habitats of the Barito, Kutei, and Tarakan Basins, Taylor, B., and D.E. Hayes, Origin and history of the South China Sea eastern Kalimantan, Indonesia: major dissimilarities in adjoining Basin, in The Tectonic and Geologic Evolution of Southeast Asian basins, J. Asian Earth Sci., 17, 99–122, 1999. Seas and Islands Part 2, edited by D.E. Hayes, pp. 23–56, Amer- Schmidtke, E.A., M.D. Fuller, and R. Haston, Paleomagnetic data ican Geophysical Union, Geophysical Monograph Series, 1983. from Sarawak, Malaysian Borneo, and the late Mesozoic and Tjia, H.D., Sea-level changes in the tectonically stable Malay-Thai Cenozoic tectonics of Sundaland, Tectonics, 9, 123–140, 1990. peninsula, Quaternary Int., 31, 95–101, 1996. Schwartz, M.O., S.S. Rajah, A.K. Askury, P. Putthapiban, and S. Tjia, H.D., and K.K. Liew, Changes in tectonic stress field in north- Djaswadi, The Southeast Asian Tin Belt, Earth Sci. Rev., 38, ern Sunda Shelf Basins, in Tectonic Evolution of SE Asia, edited 95–290, 1995. by R. Hall, and D.J. Blundell, pp. 291–306, Geological Society of Seyferth, M., and A. Henk, Syn-convergent exhumation and lat- London Special Publication, 1996. eral extrusion in continental collision zones—insights from Trevana, A.S., and R.A. Clark, Diagenesis of sandstone reservoirs of three-dimensional numerical models, Tectonophysics, 382, 1–29, Pattani Basin, Gulf of Thailand, AAPG Bull., 70, 299–308, 1986. 2004. Upton, D., A Regional Fission Track Study of Thailand: Implica- Shamsuddin, A.H.M., and S.K.M. Abdullah, Geologic evolution of tions for Thermal History and Denudation, Ph.D. thesis, Univer- the Bengal Basin and its implications for hydrocarbon exploration sity of London, 1999. in Bangladesh, Indian J. Geol., 69, 93–121, 1997. Upton, D.R., C.S. Bristow, C.S. Hurford, and A. Carter, Tertiary tec- Shi, X., X. Qiu, K. Xia, and D. Zhou, Characteristics of surface heat tonic denudation in Northwestern Thailand: Provisional results flow in the South China Sea, J. Asian Earth Sci., 22, 265–277, from apatite fission-track analysis, in Proceedings of the Inter- 2003. national Conference on Stratigraphy and Tectonic Evolution in Simon, B., H.L. ten Haven, and C. Cramez, The petroleum systems Southeast Asia and the South Pacific, Bangkok, Thailand, pp. of the south Con Son Basin, offshore South Vietnam, in Pro- 421–431, 1997. ceedings of the International Conference on Petroleum Systems of Uttamo, W., G.J. Nichols, and C.F. Elders, The Tertiary sedimentary SE Asia and Australia, edited by J.V.C. Howes, and R.A. Noble, pp. basins of northern Thailand, in Symposium on Mineral, Energy 467–479, Indonesian Petroleum Association, 1997. and Water Resources of Thailand: Towards the Year 2000, edited Simons, W.J.F., B.A.C. Ambrosius, R. Noomen, D. Angermann, P. Wil- by C. Khantaprab, pp. 71–92, Department of Geology, Chula- son, M. Becker, E. Reinhart, A. Walpersdorf, and C. Vigny, Observ- longkorn University, Bangkok, Thailand, 1999. ing plate motions in SE Asia: Geodetic results of the GEODYSSEA Uttamo, W., G.J. Nichols, and C.F. Elders, Relationships between project, Geophys. Res. Lett., 26, 2081–2084, 1999. Cenozoic strike-slip faulting and basin opening in northern Thai- HALL AND MORLEY 85

land, in Intraplate Strike-Slip Deformation Belts, edited by F. Widyantoro, S., and R.V. van der Hilst, Mantle structure beneath Storti, R.E. Holdsworth, and F. Salvini, pp. 89–108, Geological Indonesia inferred from high-resolution tomographic imaging, Society of London Special Publication, 2003. Geophys. J. Int., 130, 167–182, 1997. van de Weerd, A., and R.A. Armin, Origin and evolution of the Ter- Wight, A.W.R., L.H. Hare, and J.R. Reynolds, Tarakan Basin, NE tiary hydrocarbon bearing basins in Kalimantan (Borneo), Indone- Kalimantan, Indonesia: a century of exploration and future hydro- sia., AAPG Bull., 76, 1778–1803, 1992. carbon potential, Bull. Geol. Soc. Malaysia Special Publication, van Hattum, M., R. Hall, and G.J. Nichols, Provenance of northern 33, 263–288, 1993. Borneo sediments, in Indonesian Petroleum Association, Pro- William, A.G., J. Lambiase, S. Back, and M.K. Jamiran, Sedi- ceedings 29th Annual Convention, Jakarta, pp. 305–319, 2003. mentology of the Jalan Salaiman and Bukit Melinsung outcrops, Wang, E., and B.C. Burchfiel, Interpretation of Cenozoic tectonics western Sabah: Is the West Crocker Formation an analogue for in the right-lateral accommodation zone between the Ailao Shan Neogene turbidites offshore?, Bull. Geol. Soc. Malaysia, 47, shear zone and the eastern Himalayan syntaxis, Int. Geol. Rev., 39, 63–75, 2003. 191–219, 1997. Williams, H.H., and R.T. Eubank, Hydrocarbon habitat in the rift Wang, X., J. Wang, Q. Li, and H. Yu, Deep heat flow and geothermal graben of the Central Sumatra Basin, Indonesia, in Hydrocarbon structure in Sichuan basin of China, in Proceedings World Geot- Habitat in Rift Basins, edited by J.J. Lambiase, pp. 331–371, Geo- hermal Congress, Kyushu-Tohoku, Japan, pp. 1937–1940, 2000. logical Society of London Special Publication, 1995. Waples, D.W., A new model for heat flow in extensional basins: radi- Williams, H.H., M. Flower, and R.T. Eubank, Characteristics of ogenic heat, asthenospheric heat, and the McKenzie model, Nat- selected Palaeogene and Cretaceous lacustrine source basins of ural Resources Res., 10, 227–238, 2001. Southeast Asia, in Hydrocarbon Habitat in Rift Basins, edited by Waples, D.W., A new model for heat flow in extensional basins: esti- J.J. Lambiase, pp. 241–282, Geological Society of London Special mating radiogenic heat production, Natural Resources Res., 11, Publication, 1995. 125–133, 2002. Zhou, D., K. Ru, and H.Z. Chen, Kinematics of Cenozoic extension Watcharanantakul, R., and C.K. Morley, Syn-rift and post-rift mod- on the South China Sea continental margin and its implications for elling of the Pattani Basin, Thailand: evidence for a ramp-flat the tectonic evolution of the region., Tectonophysics, 251, 161–177, detachment, Mar. Petroleum Geol., 17, 937–958, 2000. 1995. Wheeler, P., Cenozoic Basin Formation in SE Asia, Ph.D. thesis, Zhu, W.L., M.B. Li, and P.K. Wu, Petroleum systems of the Zhu III Cambridge University, 2000. subbasin, Pearl River Mouth Basin, South China Sea, AAPG Bull., Wheeler, P., and N. White, Quest for dynamic topography: Obser- 83, 990–1003, 1999. vations from Southeast Asia, Geology, 28, 963–966, 2000. Zoback, M.L., M.D. Zoback, J. Adams, M. Assumpcao, S. Bell, E.A. Wheeler, P., and N. White, Measuring dynamic topography: An Bergman, P. Blumling, N.R. Brereton, D. Denham, J. Ding, K. analysis of Southeast Asia, Tectonics, 21, 10.1029/2001TC900023, Fuchs, N. Gay, S. Gregersen, H.K. Gupta, A. Gvishiani, K. Jacob, 2002. R. Klein, P. Knoll, M. Magee, J.L. Mercier, B.C. Muller, C. Paquin, White, N., Does the uniform stretching model work in the North K. Rajendran, O. Stephansson, G. Suarez, M. Suter, A. Udias, Sea?, in Tectonic Evolution of the North Sea Rifts, edited by D.J. Z.H. Xu, and M. Zhizhin, Global Patterns of Tectonic Stress, Blundell, and A.D. Gibbs, pp. 217–239, Clarendon Press, Oxford, Nature, 341, 291–298, 1989. 1990. White, R., and D. McKenzie, Formation of the steer’s head geome- R. Hall, SE Asia Research Group, Department of Geology, Royal try of sedimentary basins by differential stretching of the crust Holloway University of London, Egham, Surrey, TW20 0EX, U.K. and mantle, Geology, 16, 250–253, 1988. (e-mail: [email protected]) Wicker, J.J., and J.E.F. Stearn, Baram field — The 3D marine re- C.K. Morley, Department of Petroleum Geoscience, Universiti of processing challenge, Bull. Geol. Soc. Malaysia, 43, 439–450, Brunei Darussalam, Tunku Link, Gadong, Brunei Darussalam. 1999. (e-mail: [email protected])