Implications of gravity data from East Kalimantan and the Makassar Straits: a solution to the origin of the Makassar Straits?

PERGAMON

Journal of Asian Earth Sciences 17 (1999) 61±78

Implications of gravity data from East Kalimantan and the Makassar Straits: a solution to the origin of the Makassar Straits? I.R. Cloke a, *, J. Milsom b, D.J.B. Blundell a a

University of London Southeast Asia Research Group, Geology Department, Royal Holloway, Egham, Surrey TW20 OEX, UK b University of London Southeast Asia Research Group, Geology Department, University College, London, UK Received 10 July 1997; accepted 26 September 1998

Abstract Recent free-air gravity data covering the Makassar Straits is integrated with Bouguer gravity data from onshore East Kalimantan to provide new insights into the basement structure of the region. Onshore Kalimantan, gravity highs on the northern margin of the Kutai Basin trend NNE±SSW and N±S and correspond with the axes of inverted Eocene half-grabens. NW±SE trending lows correspond to deep seated basement weaknesses reactivated as normal faults during the Tertiary. An intra-basin gravity high trending NNE±SSW, the Kutai Lakes Gravity High, is modelled as folded high density Paleogene sediments ¯anked by syn-inversion synclines in®lled with low density sediments. O€shore Kalimantan, the Makassar Straits include two basins o€set by an en-echelon fault zone, suggestive of an extensional origin. The regional signature of the free-air anomaly data mirrors the bathymetry, but this e€ect can be reduced by the use of ®lters in order to examine the basin architecture. The free-air gravity minimum in the Makassar Strait is only ÿ20 mGal, much smaller than that appropriate for a foreland basin, and more indicative of an extensional basin. The steepness of the gradients on the ¯anks of the basins indicates fault control of their margins. A regional 2D pro®le across the North Makassar Basin suggests the presence of attenuated crust (<14 km) in the basin axis at the present day, whereas ¯exural backstripping implies the presence of oceanic crust of middle Eocene age. The presence of oceanic crust in the North Makassar Straits Basin has implications for regional plate tectonic models. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Kutai; Makassar; Gravity; Oceanic crust; SE Asia

1. Introduction This paper investigates the evolution of the Kutai Basin and Makassar Straits using a geophysical dataset to document the structural con®guration of the basin and ¯exural backstripping (Watts, 1978) to ascertain the crustal type beneath the Mahakam Delta (Fig. 1). A high resolution Bouguer gravity map covering East Kalimantan (Fig. 2) is integrated with a free-air gravity map (Fig. 3) recorded over the Makassar Straits (Fig. 1; location shown inset) and important structural

features identi®ed. A crustal pro®le compiled from deep seismic re¯ection and well data, orthogonal to the axis of the Mahakam Delta is ¯exurally backstripped and used to determine the crustal type (Fig. 6) beneath the delta, whereas static gravity modelling is used to create a density model across the basin (Fig. 5). The ®nal section of the paper compares the narrow Makassar Straits Basin with suitable analogues around the world, and discusses the implications of this study for the evolution of the eastern margin of Sundaland.

2. Makassar Straits * Corresponding author. Present address: MOBIL North Sea Ltd, 3 Clements Inn, London, UK. Fax: 00-44-171-412-4124; E-mail: [email protected].

The Makassar Straits, which separate the large islands of Kalimantan and Sulawesi (Fig. 1), are

1367-9120/99 $ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 7 4 3 - 9 5 4 7 ( 9 8 ) 0 0 0 5 6 - 7

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Fig. 1. Geological map of East Kalimantan and the surrounding region showing distribution of basement and Tertiary sediments. Areas covered by gravity data onshore Kalimantan (Fig. 2) and o€shore (Fig. 3, inset right) and the positions of seismic pro®les (Fig. 4a,b, 5 and 6) described in the text are shown, as is the location of gravity recorded onshore Kalimantan (map modi®ed from Moss et al., 1997).

bounded to the west by the Kutai Basin, currently one of Indonesia's most important oil producing provinces. Sulawesi, to the east, contains far fewer proven hydrocarbon reserves, but gas is produced in the Sengkang basin and active exploration continues in the Kalosi area, a little further to the north. The origin of the Makassar Straits, which has implications for the origin of the Tertiary basins located onshore east Kalimantan, has been and still is the subject of considerable scienti®c debate. Mechanisms proposed have included extension (Katili, 1971; Carey, 1976; Hamilton, 1979; Untung et al., 1983; Situmorang, 1982; Wissmann, 1984; Hutchison, 1989), transtension (Rangin et al., 1989) and load-induced ¯exure (Van Bemmelen, 1970; Bergman et al., 1996). Whilst the majority of authors have favoured extensional origins, the ages proposed for rifting have varied from the middle Eocene (Situmorang, 1982; Hutchison, 1989; Rangin et al., 1990; Hall, 1996; Moss et al., 1997), Miocene (Bendang, 1993), Pliocene (Katili, 1971) through to Quaternary (Van Bemmelen,

1970). Extension is inferred to have formed both the North and South Makassar Straits basins, which are separated by the Adang transform fault (Hamilton, 1979). The structure of the Makassar Straits has been studied extensively by a number of di€ering methods. Coastline matching was used by Katili (1971) to reconstruct the extensional basins, but may be invalid because the rapidly evolving Neogene coastline of western Sulawesi does not lend itself to this method (Bergman et al., 1996). Other workers have used geophysical data to study the basin origin. These have included aeromagnetic maps (Untung et al., 1983) and pro®les of magnetic data recorded along ship tracks (Weissel and Hayes, 1978; Wissmann, 1984; CGG - 104). Unfortunately, the interpretation of magnetic data has provided little detailed information on basin structure. Deep seismic pro®les have been recorded both across and down basins (BGR cruisesÐValdivia 16/1977 and Sonne 16/1981; CGG megaregional project, 1994) and interpretations of

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Fig. 2. Grey scale illuminated Bouguer gravity map of East Kalimantan compiled from data provided by LASMO Runtu. Location of data is shown on Fig. 1.

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Fig. 3. Grey scale illuminated free-air gravity map of the Makassar Straits compiled from Smith and Sandwell (1995). Location of data shown on Fig. 1.

Fig. 4. Line interpretations of seismic re¯ection data recorded across the Adang fault zone, bordering the southern margin of the North Makassar Straits, and the Sangkulirang fault zone, bordering the northern margin of the North Makassar Straits. Location of seismic lines shown Fig. 1.

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Fig. 5. Density model created to match the Bouguer (over land) and free-air (over sea) pro®le from East Kalimantan to West Sulawesi. Location of pro®le shown Fig. 1.

pro®les across the basins have been published (Katili, 1971; Burollet and Salle, 1981; Situmorang, 1982; Wissmann, 1984; Guntoro, 1995; Bergman et al., 1996). Faulted basement was resolved, although poorly, in the South Makassar Straits Basin (Situmorang, 1982; Wissmann, 1984), but the structure of the North Makassar Straits Basin is more dif®cult to interpret, due to the lack of a strong seismic re¯ector at basement level. Whether the crust beneath the North Makassar Straits Basin is continental (Burollet and Salle, 1981) or oceanic (Hamilton, 1979; Untung et al., 1983; Guntoro, 1995) is thus still open to question. Density models have been used by Guntoro (1995) to model the free-air pro®le over the North Makassar Straits Basin, but gravity models by themselves provide non-unique solutions and require validation by other means. A further method proposed for determining the type of crust beneath the basins has been the analysis of the distribution patterns of basement terranes, plate motion vectors, and structural deformation. Using this method Malacek et al. (1993) proposed that a fragment of a late Cretaceous ocean basin is trapped between the relict subduction zones of northwest Borneo and western Sulawesi. Bergman et al. (1996) have used ¯exural modelling to support their contention that the lithospheric loads of the western Sulawesi thrust belt and the Mahakam Delta are suf-

®cient to explain the origin of the North Makassar Basin.

3. Kutai Basin The geological evolution of the Kutai Basin, which is located to the west of the North Makassar Basin, has been described by Wain and Berod (1989), Van de Weerd and Armin (1992), Pieters et al. (1993) and Moss et al. (1997). It is bounded to the southwest by the Schwaner mountains and to the northwest by Jurassic±Cretaceous melange and the Cretaceous-aged Embaluh Group turbidites, which form mountain ranges (Moss, 1998). Most recent models favour an extensional origin for the basin, with initiation by the mid-middle Eocene (Moss et al., 1997), subsequent basin modi®cation during the late Oligocene±early Miocene (Moss et al., 1997; Cloke, 1997) and inversion of pre-existing middle Eocene depocentres from the middle Miocene to the present day (Chambers and Daley, 1995). Bouguer gravity data across the Kutai Basin have been used to constrain structural cross-sections and to determine the location of structures within the basin (Ott, 1987; Sunaryo et al., 1988; Wain and Berod, 1989; Chambers and Daley, 1995; Cloke, 1997). The regional gravity pattern has also been used to demonstrate that the Kutai Basin has an

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Fig. 6. Comparison of the observed free-air and Bouguer gravity anomaly with the calculated anomaly based on an elastic thickness of 20 km. The calculated anomaly can be considered as the sum of the rifting and sedimentation anomalies. Location of line is shown on Fig. 1.

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extensional rather than an origin as a foreland basin (Moss et al., 1997).

4. Gravity data Gravity measurements, whether at land or sea, contain contributions from sources of interest at shallow depths (residuals) as well as contributions from sources at greater depths (regionals). Deeper sources produce Bouguer anomalies that are of longer wavelength and are smooth over considerable distances, whereas shallow sources generate sharper anomalies of shorter wavelengths. The main boundaries at which density contrasts signi®cantly a€ect the gravity ®eld are: (a) the sediment-water interface; (b) the base of shallow low density sediments, in the range of 1±500 m thickness; (c) major basin structures in the range of 1±10 km; and (d) the Moho at a depth of 10±30 km. Various ®ltering techniques may be used to enhance particular aspects of these boundaries and have been applied to maps of East Kalimantan and the Makassar Straits by Cloke (1997).

5. East Kalimantan Bouguer map The Bouguer map for east Kalimantan was illuminated from the south-east to highlight subtle structural features (Fig. 2; location shown in Fig. 1). In the Kutai Basin a number of prominent Bouguer gravity highs are superimposed on a slightly, to moderately, positive background ®eld (Fig. 2). The highest values are over outcropping basement on the northern margin of the basin, where the Bouguer anomaly is as high as +56 mGal. In the basin axis values are as low as 0 mGal, but then rise gradually towards the coast where, across the onshore portion of the Mahakam Delta, values are in the order of +30 mGal (Fig. 2). Thus there is no substantial di€erence between the Bouguer values recorded over basement and those recorded over the basin ®ll (Moss et al., 1997). This is despite the presence of a thick Paleogene and Neogene sedimentary section, which should represent a considerable mass de®cit. A mass excess, which has to be regional in extent, is therefore required at depth to balance the mass de®ciency of the sediment ®ll, implying that the basin is underlain by relatively thin or extended crust (Moss et al., 1997). The observations are not compatible with the ¯exural basin origin suggested by Wain and Berod (1989), Van de Weerd and Armin (1992), Pieters et al. (1993) and, more recently, by Bergman et al. (1996). Depression of a foreland basin occurs because of loading of elastic lithosphere, and the load

is by de®nition displaced from the depocentre. Local isostatic compensation does not apply, and low to negative gravity is observed over the sedimentary accumulations. Three NNE±SSW oriented gravity highs, are apparent on the Bouguer gravity map of East Kalimantan (Fig. 2). The most prominent is the Kutai Lakes Gravity High (KLGH), with an amplitude of approximately +40 mGal. A density model created for a transect across the Kutai Basin suggests that the high is the result of the inversion of Paleogene sediments (Chambers and Daley, 1995; Cloke, 1997) rather than isostatic readjustment as a result of a massive gravitational landslide as suggested by Ott (1987). The Kedang Kepala High (Cloke et al., 1997) forms a broader anomaly of approximately +54 mGal to the north±northeast of the KLGH and is o€set by about 10 km in a right lateral sense by a NW±SE oriented gravity low, identi®ed as the Belayan Axis (Wain and Berod, 1989; Cloke et al., 1997; Moss et al., 1997) (Fig. 2). The Mawai well sited on this high (Fig. 1) has penetrated syn-rift sediments, with high vitrinite values (R0 > 1.2%). To the north the Kedang Kepala High is itself o€set from another positive anomaly by a further linear NW±SE trending gravity low identi®ed as the Kedang±Kepala Axis (KKA) (Fig. 2). Bouguer gravity values in the two lows range from minima close to zero, up to more than 20 mGal. The KKA coincides in part with an en-echelon fault zone interpreted on seismic sections (Cloke, 1997), and a lineament interpreted using Synthetic Aperture Radar (Moriya and Nishidai, 1996; the Bengalon lineament; Cloke, 1997) but gravity anomalies have been interpreted as indicating a continuation further to the east (Sunaryo et al., 1988). Magnetic anomalies across the region indicate an increase in depth to magnetic basement across the KKA (Sunaryo et al., 1988).

6. Gravity data covering the Makassar Straits The Makassar Straits have been divided into three major physiographic units using bathymetric data. These are the shallow Paternoster Platform, which is generally accepted as a subsea extension of eastern Kalimantan, and the deep North and South Makassar Straits basins. Both basins are roughly rectangular and ¯at ¯oored in their central parts (seismic line CGG104). The North Makassar Straits Basin, with maximum depths of more than 2300 m, is the larger and deeper of the two. The most obvious feature of the free-air gravity map of the Makassar Straits (Fig. 3) is the presence of two separate lows, each of between 0 and ÿ20 mGal, corresponding to the two distinct o€set basins. Also appar-

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ent are free-air gravity lows along the line of the Adang Fault and in the Doang Trough, and free-air gravity highs at the margins of the basins. The steepness of the gradients on the ¯anks of the basins indicates possible tectonic control of the margins and suggests a transition from normal thickness to thinned crust (Talwani and Eldholm, 1973; Whittaker et al., 1997). The South Makassar Straits Basin coincides with a rectangular free air low with minimum values of ÿ20 mGal. It is linked to the North Makassar Straits Basin by the narrow NNW±SSE Adang linear gravity low (Fig. 3). Steep free-air gravity gradients along the western and eastern sides of the basin coincide quite precisely with the regions of steepest bathymetric slope. Previous workers have identi®ed both extensional and reverse faulting at these margins (Situmorang, 1982; Letouzey et al., 1990; Van de Weerd and Armin, 1992; Bransden and Matthews, 1992). The Taku±Talu fault zone, which forms the western margin to the South Makassar Basin, correlates with the steep free air gradient trending NNW±SSW (see Fig. 14 in Wilson and Bosence, 1996), indicating a rapid step down of the margin accommodated by extensional faults. Middle Miocene inversion has resulted in reverse movement on some faults and the formation of asymmetric anticlines. The continuity of the free-air anomaly associated with this fault zone may be used to show the lateral extent of the fault zone. From the gravity data the Taku±Talu Fault Zone does not appear to be divided into a number of overlapping fault segments (cf. Adang fault zone). Positive (>30 mGal) free air values over the Paternoster Platform indicate a shelf area with shallow basement and/or dense carbonates. Ophiolitic rocks are present on Laut Island and in the adjacent Meratus Mountains (Sikumbang, 1986), where high gravity values are interpreted as recording the presence of ultrama®c rocks with a high density within the basement complex (Guntoro, 1995). The high free air anomaly recorded over the o€shore Mahakam Delta was considered by Hamilton (1979) to be due to the presence of a basement high, and has been suggested by Guntoro (1995) to mark a continuation of the Laut ridge, o€set along the Adang fault zone. An alternative explanation is presented here based on observations of free-air anomalies recorded over deltas prograding over continental-oceanic crust transition zones (cf. Watts and Marr, 1995). At the south-western end of the South Makassar Straits Basin an elongated nose of low gravity values follows the line of the Doang Trough. The anomaly is narrow (approximately 30 km in width), about 100 km in length, has an amplitude of ÿ20 mGal and can be regarded as de®ned by the 0 mGal contour. Wissmann (1984) has postulated that this represents a rift which separates two continental blocks. The steep free air

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gradient associated with the south-eastern margin of the Doang Trough infers a fault controlled margin. It is interesting to note the change in polarity of faulting from the Taku±Talu fault zone to Doang Trough, which suggests the presence of an accommodation zone similar to those identi®ed in other continental rifts (Bosworth, 1985). Other fault-bounded depocentres are also identi®ed on the Sunda Shelf as gravity minima (Fig. 3). A number of localised (+50 to 60 mGal) free air highs identi®ed at the southwest tip of Sulawesi may be due to late Miocene intermediate and basic volcanic rocks since these also show up as magnetic anomalies on the shelf. The North Makassar Straits Basin is associated with a trapezoidal shaped free-air low reaching minimum values of ÿ20 mGal. The free-air minima correlate with bathymetric minima (Fig. 3) (Wissmann, 1984). The southern margin of the basin is dominated by gradients de®ning the Adang Gravity Low along the north edge of the Paternoster Platform. The margin appears to be less regular than would be expected if, as is the case with the Taku±Talu fault in the South Makassar Basin, it were de®ned by a single fault, cf. (Fig. 3) (cf. Hamilton, 1979; Wissmann, 1984; Van de Weerd and Armin, 1992). At its eastern end, close to the coast of West Sulawesi, the Paternoster/Adang gravity gradient coincides with a steep bathymetric slope trending NNW±SSE, but further west the gradient is less steep, gravity trends are more NW±SE and the gravity pattern suggests segmentation of the fault into a number of en-echelon overlapping splays. Seismic line 116 is perpendicular to the Adang fault zone and may be used to correlate between the free-air gravity map and the basin structure. In the interpretation of seismic line 116 shown in Fig. 4a, the step down of the Paternoster Platform is accommodated by a number of closely spaced faults. Interpretation of a 3D seismic re¯ection dataset recorded across the Seppinngan ®eld, which sits astride the fault zone, shows that it consists of a number of overlapping segments (Unocal, personal communication) and this is clearly observed on seismic basemaps of the Balikpapan fault zone (van de Weerd and Armin, 1992). Similar curvilinear gravity traces observed over NW±SE fault zones onshore Kalimantan have also been suggested to be result of an overlapping segmented fault zone (Cloke, 1997). O€shore West Sulawesi, the eastern margin of the basin slopes less steeply, and this change is re¯ected in more gentle gravity gradients. Seismic sections from this area show that sediments have been deformed into a thin-skinned, west-vergent fold and thrust belt (Bergman et al., 1996; Cloke, 1997). Active thrusting is demonstrated by the deformation of the sea bed by a number of thrust propagation folds. Approximately 14±16% shortening is required to generate the

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observed structures (Bergman et al., 1996; Cloke, 1997). High positive anomalies adjacent to the Sulawesi coastline should be due to the pre-Tertiary basement identi®ed on o€shore seismic pro®les (Burollet and Salle, 1981). The Lariang Delta coincides with a NW±SE trending gravity high which has been interpreted by Wissmann (1984) to represent the o€shore trace of the southward downthrowing Bontang fault (Fig. 3). Seismic data across this fault cannot discriminate between oceanic or continental basement (Wissmann, 1984) but support the idea of a structural depression (Untung et al., 1983). The northern margin of the North Makassar Straits Basin is bounded by a prominent (+50 mGal) high trending NW±SE which coincides with the Sangkulirang/Palu-Koro fault zone identi®ed by previous authors (Hamilton, 1979; Wissmann, 1984; van de Weerd and Armin, 1992). Bathymetric contours show a rough and irregular sea ¯oor with two NW±SE trending zones separated by a 3000 m deep and 20 km wide basin. On seismic sections through this region the fault zone is characterised by a number of positive ¯ower or tulip structures, suggesting transpressional stress along a strike-slip fault (Fig. 4b). Low density upper Miocene±Pliocene successions onlap on to a strongly deformed Miocene±Paleogene succession. The fault zones correlate with the trend of a number of earthquakes with epicentres at depths of less than 30 km (Fig. 7) (Hamilton, 1974; Hayes and Taylor, 1978; McCa€rey and Sutardjo, 1982). This basement high appears to have been a relatively small emergent area in the Paleogene (Samuel and Muchsin, 1975; Rose and Hartono, 1978) as evidenced by onlap of re¯ectors onto underlying basement (Fig. 4b). The prominent series of overlapping NW±SE trending gradients associated with the northern margin of the Mangkalihat Peninsula mark the traces of a number of separate extensional and sinistral strike-slip faults, including the Maratua fault or Mangkalihat fault (Wissmann, 1984; IRC observations). The presence of these faults is supported by seismic data across the basin. The Celebes Sea to the north is underlain by Eocene age oceanic crust (Weissel, 1980; Rangin et al., 1990). The nature of the transition from attenuated continental crust to oceanic crust is unclear, although a transform margin linking to the North Sulawesi trench has been proposed by Hamilton (1979). The Mahakam Delta is associated with a prominent, oval shaped free air high of up to +90 mGal, surrounded by a subtle moat (Fig. 3). Similar highs have been identi®ed over other deltas, such as the Mississippi, Amazon and Ebro, which prograde across attenuated crust or over the continental-oceanic crust transition zone (Watts and Marr, 1995). In these cases e€ects have been explained as due to the initial margin

con®guration and sediment loading (Walcott, 1972). Applying this explanation to the Mahakam Delta removes the need to assume either a basement high beneath the delta (Hamilton, 1979) or the presence of high density ultrama®cs (Guntoro, 1995). Moreover, the presence of the free-air high allows the crustal type beneath the delta to be identi®ed using ¯exural backstripping (Watts, 1988). 7. Modelling a cross-section of the Makassar Straits The line chosen for detailed analysis is perpendicular to the axis of the delta and is shown in Fig. 1 with the contoured free-air gravity map for the North Makassar Straits Basin (Fig. 3). It extends across the onshore section of the Kutai Basin, starting just to the west of the Kutai Lakes. It trends on an azimuth of 1288 (Fig. 5), crosses the prominent Kutai Lakes Gravity High (KLGH) (+38 mGal) (Ott, 1987) and then transects the high associated with the Mahakam Delta (+96 mGal) before crossing the low associated with the Makassar Straits (ÿ20 mGal). To match the gravity pro®le, a density model had to be created to explain the features observed on the pro®le. Densities used in the modelling (shown in Fig. 5 as density contrasts with standard continental crust) were based on density models constructed for seismic sections on the northern margin of the basin (Cloke, 1997). Middle Eocene syn-rift sediments were modelled as only slightly less dense (r =ÿ 0.1 Mgm ÿ 3) than the basement. The KLGH was produced by invoking an inversion fold of these relatively high density sediments at depth. Given that well Kahala-B1 (Fig. 2) drilled directly into upper Oligocene sediments and that this well is astride the high this scenario is considered highly probable. The alternative model presented by Ott (1987) and more recently supported by Bates (1996) is that of a gravitational slide induced by uplift of the Upper Kutai Basin during the late Oligocene and Miocene. The slumping and removal of sediments from the area surrounding the KLGH is thought to have resulted in isostatic rebound and a gravity high, produced by elevated basement (Ott, 1987). The westerly vergence of the majority of fold structures identi®ed in the Lower Kutai Basin and the sheer size of landslide required, provides arguments against this model. Alternative models presented more recently for the structures observed in the Lower Kutai Basin (Chambers and Daley, 1995; Cloke et al., 1997; Ferguson and McClay, 1997) do not require gravitational mechanisms to generate the fold structures. The gravity lows observed on either side of the KLGH may be produced by in®lling of the synclines observed on seismic sections (DD-0-02) with low density Lower and Middle Miocene (r =ÿ 0.55 Mgm ÿ 3) sediments

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Fig. 7. Comparison of the observed free-air and Bouguer anomaly and the calculated anomaly expected for a value of Te = 20 km to more typical values expected at rift-type basins (Te = 0, 5 and 10 km) and from older oceanic crust (Te = 50 km). Bottom pro®le is the topography, bathymetry and sediment distribution (from Duval et al., 1992; Patterson personal communication). Thin dashed line is the tectonic subsidence/uplift based on a Te = 20 km. Location of line shown in Fig. 1.

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which enhance the narrow high. If there are in fact high density, folded syn-rift sediments beneath the Kutai Lakes, then an extension of the Meratus ophiolite beneath the Neogene axis of the basin is not required by the gravity data. Free-air gravity ``edge e€ects'' are amongst the most distinctive features of the oceanic gravity ®eld and are the gravity e€ects of thinning of crust from continental to oceanic values (Walcott, 1972; Talwani and Eldholm, 1973). The free-air anomaly covering the Mahakam Delta can only be produced by thinning the crust, which is directly contrary to the foreland basin model of Bergman et al. (1996) and Van Bemmelen (1970) for this area, which requires thickening of the crust, and also to the interpretation of Wissmann (1984) who suggested the Mahakam Delta was supported by a basement ridge. In the model shown in Fig. 5, the Mahakam high is produced by rapid thinning of the crust from 30 km to less than 14 km in the centre of the North Makassar Straits Basin, in agreement with models presented by Guntoro (1995).

8. Determination of crustal type beneath the North Makassar Straits Basin Analysis of the gravity data can be taken a stage further by introducing values of Te (elastic thickness of the crust). Flexural backstripping, following the methodology of Watts (1988), can then be used to determine the ¯exural rigidity and type of crust beneath the Mahakam Delta. The necessary details of delta thickness and structure were derived from seismic re¯ection data and published interpretations of the delta front (Duval et al., 1992). Prior to ¯exural backstripping, the regional gradient was calculated using linear regression, and then subtracted from the observed gravity values, leaving the residual gravity anomaly. The gravity anomaly for the crustal cross-section was computed using a procedure similar to that described by Watts (1988). It is useful to consider the edge e€ects at rifted margins as the result of two competing contributions, one derived from ``rifting'' and the other from ``sedimentation'' (Fig. 6b). The ``rift anomaly'' is associated with the crust and upper mantle at the time of rifting and, in the case of the stretching model is given by the combined gravity e€ect of the water-®lled subsided basin (negative) and the uplifted mantle (positive). The positive gravity e€ect extends across a broader region at the surface than the negative e€ect, because its source is deeper (Fig. 6a). The ``sedimentation anomaly'' consists of a positive anomaly, due to the displacement of water by denser sediment, and a negative anomaly due to their compensation. Since the sediment load is shallower than

the compensation, the positive anomaly dominates immediately over the sediment pile, i.e. the sedimentation anomaly is made up of a gravity high ¯anked by lows (Fig. 6b). Rifting and sedimentation e€ects were evaluated separately and then summed. The rifting anomaly was calculated using the position of the backstripped basement (TSU-Total Subsidence/Uplift: Watts, 1988) as the upper surface of the crust and the backstripped Moho as the base. The backstripped Moho is de®ned as the base of the restored crust in the absence of sediment loading and therefore represents the crustal structure at the time of rifting (Watts and Torne, 1992). The ``sedimentation anomaly'' was calculated by combining the positive gravity e€ect of the sediment load (which is calculated from the TSU and the present day water depth) and the negative e€ect of the ¯exure of the basement. The sedimentation anomaly consisted of four sequences [0±5.5 m.y., 5.5±8.2 m.y., 8.2±10.5 m.y. and 10.5±14.8 m.y.] based on Duval et al. (1992), Burrus et al. (1992) and Paterson, personal communication. Densities were derived from well logs. The analysis gives a good agreement between the observed and predicted gravity anomaly for a value of e€ective elastic thickness, Te = 20 km (Fig. 6). The amplitude of the high predicted for this thickness is approximately the same as that actually observed, and the steep gradients on either side are also similar (shown as asterisks on Fig. 6). In addition the model generates a Makassar Straits low of approximately the correct amplitude. The 20 km value is consistent with normal oceanic crust of middle Eocene (47 m.y.) age, based on the relationship: Te ˆ 3

r …age of oceanic crust-age of oceanic crust at time of loading† …Watts; 1978†

which was derived from studies of world-wide variations of Te with age of oceanic lithosphere at the time of loading (Watts, 1978). The equation yields a value of Te =020 km when an age of crust at time of loading is 33 m.y. [rift age (47 2 5 m.y.)Ðbase sediment ®ll at time of loading (14.8 m.y.)]. In order to examine the sensitivity of the gravity anomaly to changes in Te, and therefore to di€ering crustal models, the predicted gravity for the Te = 20 km model was compared with three other cases. The ®rst was where lithosphere had the strength of attenuated continental crust (Te = 5 km) established from ¯exure studies of passive continental margins (cf. Barton and Wood, 1984; Watts, 1988) (Fig. 7e). The other two cases involved oceanic crust (Te = 10 km; 24 m.y. oceanic crust, Te = 50 km; 260 m.y. oceanic crust). It has been shown by oceanic ¯exure studies that Te is given by the depth to the 4508C oceanic isotherm which suggests that as the oceanic plate cools

I.R. Cloke et al. / Journal of Asian Earth Sciences 17 (1999) 61±78

73

Fig. 8. Compilation of Te against age of the oceanic crust at time of loading for the Mahakam Delta, together with results of studies from other rift and oceanic basins around the world. The age is an average age of the crust since rifting/spreading. The heavy lines show the 3008, 4508C (isotherm for normal oceanic crust) and 6008C isotherms (modi®ed from Watts and Torne, 1992).

with age it increases in strength (Watts, 1978). Another important observation is that re-heating or thermal stress does not modify the value of Te. Fig. 7 shows that none of the alternative models adequately explains the amplitude and wavelength of the observed gravity anomaly. The gravity predicted using Te = 5 km fails to produce the high associated with the Neogene depocentre of the Mahakam delta, although the gradient onshore does resemble the observed gravity. The model also fails to match the low associated with the axis of the Makassar Straits. These observations imply that the North Makassar Straits Basin is not underlain by low strength attenuated continental crust, as suggested by Burollet and Salle (1981), but that there is likely to be a transition to low strength continental crust here near the present-day coastline of Kalimantan. Neither model using an alternative value of Te appropriate to oceanic crust produced the amplitude and wavelength of the gravity high associated with the

Mahakam Delta. For younger crust (Te = 10 km) the amplitude and wavelength of the high are too small, for older crust (Te = 50 km) they are too large. The models suggest that the correct values of age and e€ective elastic thickness lies somewhere between these two extremes. Fig. 7b and c shows the comparison between the predicted and observed free-air pro®les, and asterisks are used to show the similarities between the two. The modelling indicates that the Mahakam high is generated as a result of loading of high strength oceanic crust of approximately 47 2 5 m.y. age. The tectonic subsidence expected for oceanic crust of 47 m.y. age can be calculated using Stein and Stein (1992), and is shown on the crustal pro®le (Fig. 7) as a dashed black line. O€shore the expected subsidence pro®le agrees to within 20.5 km with the position of the backstripped basement (Fig. 7). The two curves diverge approximately 20 km shoreward of the present day shelf-break, where there is the transition to the onshore segment with Te = 5 km. This change from

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Fig. 9. Grey shaded free-air gravity map of the Makassar Straits and Celebes Sea with a structural interpretation superimposed. The map shows the interpreted oceanic-continental transition zone beneath the Mahakam Delta interpreted from ¯exural studies. This is extrapolated to the West Sulawesi margin.

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continental to oceanic crust has been indicated on the crustal pro®le (Fig. 6) by a change in shading.

9. Implications for the origin of the Makassar Straits and Kutai Basin The free-air gravity maps (Fig. 3), static gravity model (Fig. 5) and ¯exural model (Figs. 6 and 7) across the Mahakam Delta provide an interpretation for the structure of the Makassar Straits and indicate the nature of the crust beneath the centre of the basin. Static density models for the North Makassar Straits Basin indicate that the crustal thickness is no more than 14 km in the basin axis (Fig. 5) (Guntoro, 1995; Cloke 1997). The density models are dependent on the density and thickness of sediments assumed to be present in the basin (Milsom, 1996). Both of these are poorly constrained, since there are no well penetrations of the basin axis, and densities have been extrapolated from those of exposed onshore sediments. In addition, seismic re¯ection pro®les (cf. CGG-104) have no seismically resolvable basement re¯ector. Further data are therefore required to constrain the model, but it is possible that the crust is less than 10 km thick in places, which is within the range for oceanic crust (Wilson, 1988). The age vs depth relationships for oceanic crust formed in back-arc environments (Park et al., 1990) suggest that the depth to middle Eocene oceanic crust in the Makassar Straits may be greater than for normal oceanic crust (Stein and Stein, 1992) of the same age. There are striking resemblances between the North Makassar Straits and another small ocean basin of similar dimensions in Ban Bay, between Ban island and Greenland. Seismic refraction pro®les have shown the crust to be very thin, with the Moho at 11 km (Keen et al., 1972). Bouguer anomalies across the basin are between 150 and 200 mGal, similar to the values over the North Makassar Straits Basin but signi®cantly greater than those recorded in the southern basin (Cloke, 1997). Positive free-air gravity anomalies rimming the deep basin, which are similar in amplitude and wavelength to those observed adjacent to the Makassar Straits (Fig. 9), have been correlated with the transition from continental to oceanic crust (Whittaker et al., 1997). Magnetic anomalies are weak, and, as with the Makassar Straits basins (Weissel and Hayes, 1978; Untung et al., 1983), there has been no consensus on the identi®cation of linear sea ¯oor magnetic anomalies (Whittaker et al., 1997). Oblique spreading has often been invoked to explain the lack of distinct sea ¯oor spreading anomalies (Roots and Srivastava, 1984) but in very small basins it is probable that there would be a low degree of organization of

75

spreading segments and no linear magnetic anomalies (Roots and Srivastava, 1984; Dauteil and Brun, 1996). Flexural modelling of free-air gravity (Figs. 6 and 7) across the Mahakam Delta suggests the presence of high ¯exural rigidity crust (Te = 20 km) beneath the present day depocentre and indicates that the transition to low ¯exural rigidity crust (Te = 5 km) takes place 20 km landward of the present day shelf-break (Figs. 6 and 7). We suggest that the match of the models (Fig. 7), plus the approximate tectonic subsidence expected for oceanic crust of this age (Fig. 8) (Stein and Stein, 1992), is evidence for the presence of Middle Eocene age oceanic crust (47 25 m.y.) in the North Makassar Straits Basin. This is a maximum age of oceanic crust, since the spreading axis was probably located further to the east. It is not clear how wide the Makassar Straits originally were, since the eastern ¯ank is now being overridden by the west Sulawesi, Marjene thrust belt (Bergman et al., 1996; Cloke, 1997). A minimum estimate of 14% shortening has been obtained from section restorations (Bergman et al., 1996; Cloke, 1997). The width of the transition zone between oceanic and attenuated continental crust (Fig. 6) cannot be resolved, although the available evidence suggests it is sharp (<5 km) (South China Sea: Taylor and Hayes, 1980) rather than broad. There is no evidence of any dipping re¯ectors such as have been observed at other volcanic passive margins (cf. Norwegian continental margin, Hinz and Weber, 1976; Hatton Bank, Roberts et al., 1984; Vùring Plateau, Eldholm et al. 1989). The major change in basement elevation observed at the Mangkalihat Peninsula is similar to observations from the Norwegian continental margin (Talwani and Eldholm, 1973). The age of the oceanic crust predicted by the modelling results is similar to that obtained by ODP leg 124 (Silver and Rangin, 1991) and identi®cation of magnetic anomalies (Weissel, 1980; Wissmann, 1984) in the adjacent Celebes Sea.

10. Conclusions The interpretation of the free-air map of the Makassar Straits has identi®ed the presence of two o€set lows that correspond to the North and South Makassar Straits basins. These NNE±SSW trending basins are separated by the Adang fault zone. The gravity signature across this fault suggests an en-echelon segmented fault zone, rather than a linear fault as traditionally interpreted. Flexural studies indicate that the Mahakam Delta has loaded crust of a high ¯exural rigidity. Modelling of the gravity anomaly implies that the crust corresponds to oceanic crust of a Middle Eocene age. The transition zone to low strength atte-

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nuated continental crust occurs landward of the present day shelf break of the Mahakam Delta. Interpretation of gravity data from onshore Kalimantan has identi®ed prominent NW±SE trending gravity anomalies (Fig. 2). These are an inherited basement fabric and not the result of Tertiary deformation, although they were reactivated at various stages throughout the Tertiary. In addition N±S and NNE± SSW positive anomalies correspond to the axis of inverted Middle Eocene half-graben. Integration of the above observations with regional tectonic models implies that oceanic spreading was synchronous in both the Celebes Sea and the Makassar Straits. The basins developed onshore Kalimantan are therefore the remnants of a rifted passive margin which has subsequently been inverted. This study has shed new light on the formation of the Makassar Strait and provides a regional integrated model for the formation of the inverted extensional basins in Kalimantan.

Acknowledgements This work formed part of NERC studentship GT4/ 94/370/G to I.R.C. Onshore gravity data was provided by LASMO Runtu Ltd whereas o€shore gravity data was from the Smith and Sandwell (1995) grid. The work was carried out under the auspices of the SE Asia Research Group which is funded by ARCO Indonesia, LASMO Indonesia Ltd, MOBIL Oil Indonesia, EXXON Exploration, Canadian Petroleum, Union Texas Petroleum and UNOCAL Indonesia Ltd, who are all gratefully acknowledged. R. Hall, J. Goudain and S. Ronge are thanked for their comments which improved the quality of the ®nal manuscript.

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