An imbrication model for the Rajang Accretionary Complex in Sarawak, Borneo

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1 Journal of Asian Earth Sciences 18 (2000) An imbrication model for the Rajang Accretionary Complex in Sarawak, Borneo E. Honza a, *, J. John b, R.M. Banda c a Department of Earth Sciences, Kumamoto University, Kurokami, Kumamoto , Japan b Bintulu Office, Minerals and Geosciences Department of Malaysia, Wisma Persekutuan, Jalan Bintulu-Miri, Bintulu, Sarawak, Malaysia c Sarawak Office, Minerals and Geosciences Department of Malaysia, P.O. Box 560, Kuching, Malaysia Received 30 April 1999; accepted 22 June 2000 Abstract The structural framework of the Rajang Accretionary Complex is interpreted on the basis of field surveys conducted around the Lupar Fault Zone in southwestern Sarawak. Bedding dips in the Rajang Accretionary Complex are generally southward, while the complex as a whole becomes younger northward. The complex is interpreted as a series of thrust slices formed by accretion at a subduction trench. Each slice forms a unit 10 to 15 km in width. Thrust slices of this order of this order of magnitude are consistent with those formed by accretion processes at modern subduction trenches. It is suggested that accretion of Late Jurassic to Cretaceous oceanic crust occurred initially from the Pacific (eastern) side of Borneo in the Late Cretaceous, forming part of an arc system extending all along the eastern margin of Asia from Japan to Kalimantan. Subsequently, in the Early Tertiary, due to bending of the southern end of the arc system in Borneo, the direction of subduction changed, so that accretion occurred from the north Elsevier Science Ltd. All rights reserved. Keywords: Imbrication model; Subduction; Accretion Borneo 1. Introduction The Lupar Fault Zone in southwestern Sarawak separates the Silantek Formation to the south from the Rajang Group to the north (Fig. 1). The Rajang Group comprises the Lubok Antu, Lupar and Belaga Formations (Haile, 1957; Liechti et al., 1960; Tan, 1979). The Silantek Formation is conformably overlain by the Plateau Formation, composed of massive sandstone, which forms a ridge of the Klingkang Range along the border between Sarawak and Kalimantan. The Lubok Antu Mélange to the north of the Lupar Fault Zone consists of tectonic fragments and blocks of sedimentary, volcanic and intrusive rocks, and their metamorphosed equivalents, ranging in size from a few cm to a few km, enclosed in a fine grained matrix. The Lupar Formation, north of the mélange, consists of turbidites, shale, slate, basalt and gabbro. The Belaga Formation, outcropping to the north of the Lupar Formation, consists of pelagic and terrigenous sediments and has been divided into the Layar, Kapit, Pelagus, Metah and Bawang members (Liechti et al., 1960; Tan, 1982). The Lupar Fault Zone, which is not directly observed in * Corresponding author. address: honza@sci.kumamoto-u.ac.jp (E. Honza). the field, is parallel to the trend of structural units in the Rajang Group in central and northeast Sarawak. The fault zone is considered to have formed as a result of subduction of oceanic crust from the north during the latest Cretaceous and Early Tertiary (Tan, 1979). The Rajang Accretionary Complex is considered to have been formed during the Early Tertiary (Hamilton, 1979; Tan, 1982) with the cessation of subduction in the Late Eocene, as the Upper Eocene Tatau Formation rests unconformably on the (?) Early Eocene Belaga Formation, forming part of the accretionary complex in northern Sarawak (Fig. 1 inset) (Kirk, 1957; Wolfenden, 1960; Hutchison, 1996). The Lupar Fault Zone and the mélange belt can be traced westwards to Natuna Island and, together with the associated units, can be traced eastwards into West Kalimantan. In Central Kalimantan the fault zone trends E W and swings round to the NE in northeastern Kalimantan (Heryanto et al., 1993; Pieters et al., 1993; Bénard et al., 1990). The Lubok Antu Mélange can be traced eastwards into Kalimantan as the Kapuas Mélange. Here also there is no obvious fault zone between the mélange and shallow marine to lagoonal, lacustrine and fluvial sediments to the south. The outcrop of the mélange can be traced further to the ENE by the occurrence of blocks in the sediments. The Silantek Formation occurs in the northern part of the /00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S (00)

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3 E. Honza et al. / Journal of Asian Earth Sciences 18 (2000) Fig. 2. Generalised stratigraphic columns in the area surveyed in western Sarawak. The thickness shown for the Lubok Antu Mélange is only approximate. Ketungau Basin in Kalimantan. The Ketangau Basin can be traced into the eastern part of the Mandai Basin to the south of the Kapuas Mélange. A further mélange belt, the Boyan Mélange occurs further to the south along the southern margin of the Ketungau and Mandai basins. Williams et al. (1988) have suggested that the Boyan Mélange was backthrust from the northern mélange zone. The present authors have carried out a geological survey in southwestern Sarawak to investigate the stratigraphic relationships among the various formations and to determine the structure of the accretionary complex. The survey area extended from the outcrop of the Plateau Formation in the south to Sarikei on the outcrop of the Kapit Member of the Belaga Formation of the Rajang Group to the north (Fig. 1). 2. Sediments and structure of the Silantek and Plateau Formations and the Rajang Group To the south of the Lupar Fault zone, the Silantek and Plateau formations consist of shale and alternations of shale, siltstone and sandstone with igneous intrusions. The Silantek Formation is composed of three members: the Basal Fig. 1. Geological boundaries in the area surveyed in western Sarawak. P1 P8 location of profiles illustrated in Fig. 5. The strike of the bedding and angles of dip are shown along the profiles.

4 754 E. Honza et al. / Journal of Asian Earth Sciences 18 (2000) Fig. 3. A measured section in the lowest part of the Silantek Formation to the south of the Lupar Fault Zone near Enkilili. PC: paleocurrent indicator; F: fault. Sandstone; the Temudok; and the Upper Silantek Redbed members. The age of the lower part of the formation is Late Eocene (Haile, 1957). The upper part of the formation probably extends into the Oligocene (Tan, 1979). The maximum thickness of the Silantek and Plateau formations, as calculated from Profile 1, is about 5000 m in the Batu Lintang area and approximately 4000 m in the Sri Aman area (Fig. 2). The Basal Sandstone Member, the lowermost unit of the Silantek Formation, outcropping along the Lupar Zone, consists of sandstone and alternations of sandstone and shale. The sandstones are turbidites, with some of the beds exhibiting grading and sole marks (Fig. 3). The Temodok Member consists of coarse to fine grained sandstones interbedded with thin beds of siltstone and sandy to silty mudstone. The Upper Redbed Member consists of mudstone and shale, occasionally interbedded with finegrained sandstone. This member is overlain conformably by the Plateau Sandstone Formation. The Silantek and Plateau Formations were deposited in shallow marine to near-shore environments (Haile, 1957; Tan, 1979). Coal beds in the upper part of the Silantek Formation are exposed repeatedly along the main road to Sri Aman as the result of folding. In 1995 these coal seams were being mined. The beds are gently folded, the folding becoming steeper towards the Lupar Fault Zone, with overturning of the folds near the fault. The Lubok Antu Mélange has a matrix of mudstone and shale interbedded with coarse grained sediments. The mélange matrix is highly deformed and sheared, faulted and folded. Some beds are overturned, occurring as faulted blocks, a few to several tens of metres in size. Both native and exotic rock types occur as blocks in the mélange. Native blocks include mudstone, shale and sandstone, while exotic blocks are serpentinite, gabbro and basalt, chert, and limestone, or their metamorphic equivalents. Lower Eocene foraminifera and nannofossils, and reworked Upper Cretaceous coccoliths, occur within the mélange matrix (Tan, 1979) (Fig. 4). Radiolaria obtained from the chert blocks can be grouped into three assemblages: of upper Tithonian (Upper Jurassic); middle Valanginian to Barremian and upper Albian to

5 E. Honza et al. / Journal of Asian Earth Sciences 18 (2000) Fig. 4. Stratigraphy in the area surveyed in western Sarawak. Age data from Haile (1957), Liechti et al. (1960), Tan (1979), Basir Jasin (1996) and Hutchison (1996). Data for the younger formations are from Kirk (1957), Wolfenden (1960), Ho (1978), Almond et al. (1989), Madon (1994) and Banda and Honza (2000). Cenomanian (Lower Cretaceous) ages (Tan, 1979; Basir Jasin, 1996). Blocks of unaltered and unsheared calcareous shale and limestone of Early Eocene age occur in a belt along the southern margin of the mélange (Tan, 1979; Basir Jasin and Taj Madira Taj Ahmad, 1985). The origin of these blocks has not yet been determined. They may be part of the mélange, originating as an olistostrome, or may represent a stratigraphic unit in situ. Haile (1996) designated this zone the Enkilili Formation, and suggested that the Lupar fault lay to the north of this unit, between the Enkilili Formation and the Lubok Antu Mélange. The Lupar Formation consists of turbidites and igneous rocks. The turbidites show rhythmic alternations of fine-grained, graded sandstones and shale beds. Either sandstone or shale may be dominant in particular

6 756 E. Honza et al. / Journal of Asian Earth Sciences 18 (2000) Fig. 5. Geological profiles in the surveyed area along the route shown in Fig. 1. The thin vertical bar crossing the profiles marks the boundary of each profile.

7 E. Honza et al. / Journal of Asian Earth Sciences 18 (2000) sections. Palaeocurrent indicators in the sandstones show that the sediments came from the SW (Tan, 1979). Igneous rocks within the Lupar Formation form the Pakong Mafic Complex, composed of gabbro, basalt, porphyritic basalt and andesite. Tan (1979) distinguished two zones within the complex: mostly layered and granular plutonic rocks of the gabbro zone; and fine-grained, massive or pillowed, spilitised volcanic rocks of the basalt zone (Haile et al., 1994). Layering in the gabbros dips steeply towards the north. The Layar and Kapit members of the Belaga Formation consist of slate and phyllite, interbedded with turbiditic sandstones and siltstones. Our observations from palaeocurrent indicators in the Layar Member in the Bukit Tiban area of central Sarawak show that the sediments were derived from the SW. This direction, as well as the palaeocurrent directions in the Lupar Member, are oblique to the trend of the outcrops of these stratigraphic units. This relationship suggests that sediments were supplied from a direction slightly oblique, rather than directly perpendicular to the subduction trench. There is very little direct indication of the age of the Belaga Formation. Several benthonic foraminifera from the Layar Member suggest an Upper Cretaceous age, while the Kapit Member probably extends from the Upper Palaeocene to the lowermost Eocene (Tan, 1979). illustrated in the profiles (Fig. 5a). The dip is mainly vertical in Profile 5 but towards the south in Profiles 6 to 8 (Fig. 5b). The rocks dip uniformly within blocks, which are separated at intervals by major faults. In the southern part of the outcrop of the Layar Member the dip is steeply to the north, in the middle part steeply to the south, and in the northern part relatively gently to the south. The same pattern is observed in the eastern outcrop of the Layar Member in the northern extension of Profile 4. In these profiles major thrusts are found to separate blocks at intervals of km with each block becoming younger towards the north. These blocks resemble the imbricate thrust blocks imaged on seismic reflection profiles in modern offshore accretionary complexes, such as the Japan Trench (Honza, 1981; von Huene et al., 1984) or the Nankai Trough (Honza and Murakami, 1986; Kagami, 1986), where blocks of similar size are separated by landward-dipping reflectors, representing bedding or thrust surfaces. By comparison with the imbricate structures seen in modern accretionary prisms it is possible to construct a schematic profile across the Rajang Accretionary Complex. Back-thrusting, affecting the Lubok Antu Mélange and the Lupar Formation, as observed in the field and shown on Profile 2, is also indicated in the southern part of this profile (Fig. 6). 3. Imbricated thrusting of the Rajang Group The Rajang Group is highly deformed, with high angles of dip and much folding and faulting. The general trend of the strike of the beds is from WNW ESE, parallel to the Lupar Fault, with dips generally to the south throughout the area surveyed (Fig. 5). Although the regional dip is towards the south, the formations and members of the Rajang Group become younger towards the north. The explanation for this relationship is that the rocks are broken into blocks at intervals and duplicated by imbricate thrust faults (Fig. 5). Bedding in the Lubok Antu Mélange dips to the north in Profile 2, except immediately adjacent to the Lupar Fault Zone, where the beds are sheared and strongly deformed, becoming vertical or even overturned. The contacts of the Lubok Antu Mélange to the south with the Silantek Formation and to the north with the Lupar Formation are both probably faults or shear zones. The Lupar Formation is also strongly deformed and sheared, with much folding and faulting. minor faults and igneous intrusions within the sediments are offset by faults which dip to the north, rather than to the south. These relationships show that the Lupar Formation has been affected by back-thrusting, as well as by imbricate thrusting. The Layar and Kapit Members also show strong deformation, with much folding and faulting. In Profiles 3 and 4 the Layar Member is shown with steep to vertical dips, generally towards the north, with frequent folds and faults as 4. The evolution of the Rajang Accretionary Complex The formation of the Rajang Accretionary Complex was not contemporaneous with the opening of the South China Sea. The opening of the South China Sea Basin is considered to have taken place between Ma (late Oligocene Early Miocene) (Briais et al., 1993). Chert in the Lubok Antu Mélange ranges in age from uppermost Jurassic to Lower Cretaceous, while the younger members of the Rajang Group have ages from Early Cretaceous to Eocene. The Rajang Group is overlain unconformably by the Late Eocene Tatau Formation. Therefore all the formations in the Rajang Group were deposited and imbricated into the Rajang accretionary prism before the initiation of spreading in the South China Sea. Fragments of ocean floor incorporated in the Lupar Zone are believed to have been accreted by the subduction of an ocean which lay on the Pacific side of the island of Borneo (Williams et al., 1988). In the Late Jurassic this ocean floor was part of the Izanagi Plate. During the Cretaceous this plate was being subducted along the eastern margin of Asia to form a magmatic arc, indicated by the occurrence of Cretaceous intrusive and extrusive rocks all along the margin, from Japan in the north, through South China and South Vietnam, to Kalimantan in the south. In the latest Cretaceous the Izanagi Plate separated into the Kula and Pacific Plates (Engebretson et al., 1985). During this plate reorganisation some Jurassic and Cretaceous oceanic crust

8 758 E. Honza et al. / Journal of Asian Earth Sciences 18 (2000) Fig. 6. Interpretation of the Rajang Accretionary prism in western Sarawak based on the structural data shown in Profiles 1 8 in Fig. 5. may have survived, either as part of the Pacific Plate, or as a separate Danau Sea, to the north of the Rajang Group (Hamilton, 1979; Holloway, 1982; Hutchison, 1986; Haile, in Hutchison, 1996). Palaeomagnetic studies have shown that Borneo has rotated counterclockwise through approximately 90 since the latest Cretaceous, but has remained stable since the Early Miocene. On the other hand Indochina rotated clockwise between the Mesozoic and the Early Tertiary (Fuller et al., 1991). However, there is no obvious plate boundary between Indochina and Borneo (Hamilton, 1979; Hutchison, 1996). If Indochina and Borneo shared the same depositional and tectonic history in the Palaeozoic and Mesozoic, subsequently Borneo must have been bent into an arc, convex towards the east, in the same way as in the eastern Banda arc at the present time. The development of such an arc would explain the conflicting palaeomagnetic data from Indochina and Borneo. Compressional stress to form this arc may have been provided by the northward movement of the Indian Australian Plate, with the hard collision of India with the southern margin of Asia since 43 Ma (Patriat and Achache, 1984; Mutter et al., 1985; Lee and Lawver, 1995). The bending of the arc may have accelerated the development of the Rajang accretionary prism in northern Borneo. Subduction ceased by the time that bending of the arc was completed in the Late Eocene. Hutchison (1996) has suggested that the cessation of subduction was due to the collision of the Balingian Luconia Continent with the Rajang Accretionary Complex, initiating the Sarawak Orogeny, with the peripheral uplift of the Rajang Group and the Silantek Formation. By the Late Eocene, in northern Sarawak, shallow water deposits of the Tatau Formation were being laid down unconformably on the eroded surface of the Rajang Group (Ho, 1978; Almond et al., 1989; Madon, 1994). Acknowledgements We should like to thank the staff and former staff of the Geological Survey of Malaysia, Mr Fateh Chand, Mr Chen Shick Pei, Mr Victor Hon and Mr Alexander Unya anak Ambun for their assistance during the field survey. We should also like to thank Prof. N.S. Haile, Prof. C.S. Hutchison and Mr Loganathan for their critical reading and useful comments on the manuscript. Dr A.J. Barber suggested improvements in presentation and the use of English. References Almond, J., Vincent, P., Williams, L.R., The application of detailed reservoir geological studies in the D18 Field, Balingian Province, offshore Sarawak. Geological Society of Malaysia Bulletin 27, Banda, R.M., Honza, E., (in press). Miocene sediments and structure in the NW Borneo Basin. Geological Society of Malaysia Bulletin. Basir Jasin, Late Jurassic to Early Cretaceous radiolaria from chert blocks in the Lubok Antu Mélange, Sarawak, Malaysia. Journal of Southeast Asian Earth Sciences 13, Basir Jasin, Taj Madira Taj Ahmad, Some Palaeogene planktonic foraminifera from the Lubok Antu Complex, Sarawak. Warta Geologi 21, Bénard, F., Muller, C., Letouzey, J., Rangin, C., Tahir, S., Evidence of multiphase deformation in the Rajang-Crocker Range (northern Borneo) from Landsat imagery interpretation: geodynamic implications. Tectonophysics 183, Briais, A., Patriat, P., Tapponnier, P., Updated interpretation of magnetic anomalies and sea-floor spreading stages in the South China Sea: implications for the Tertiary tectonics of Southeast Asia. Journal of Geophysical Research 98,

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