An imbrication model for the Rajang Accretionary Complex
in Sarawak, Borneo
E. Honza
a,*, J. John
b, R.M. Banda
caDepartment of Earth Sciences, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan
bBintulu Office, Minerals and Geosciences Department of Malaysia, Wisma Persekutuan, Jalan Bintulu-Miri, 97000 Bintulu, Sarawak, Malaysia cSarawak Office, Minerals and Geosciences Department of Malaysia, P.O. Box 560, 93658 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.q2000 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 Me´lange to the north of the Lupar Fault Zone consists of tectonic fragments and blocks of sedimen-tary, 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 me´lange, 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
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 accre-tionary complex in northern Sarawak (Fig. 1 inset) (Kirk, 1957; Wolfenden, 1960; Hutchison, 1996).
The Lupar Fault Zone and the me´lange belt can be traced westwards to Natuna Island and, together with the asso-ciated 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; Be´nard et al., 1990). The Lubok Antu Me´lange can be traced eastwards into Kalimantan as the Kapuas Me´lange. Here also there is no obvious fault zone between the me´lange and shallow marine to lagoonal, lacustrine and fluvial sediments to the south. The outcrop of the me´lange 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
Journal of Asian Earth Sciences 18 (2000) 751–759
1367-9120/00/$ - see front matterq2000 Elsevier Science Ltd. All rights reserved.
PII: S 1 3 6 7 - 9 1 2 0 ( 0 0 ) 0 0 0 4 4 - 4
www.elsevier.nl/locate/jseaes
* Corresponding author.
E. Honza et al. / Journal of Asian Earth Sciences 18 (2000) 751–759
Ketungau Basin in Kalimantan. The Ketangau Basin can be traced into the eastern part of the Mandai Basin to the south of the Kapuas Me´lange. A further me´lange belt, the Boyan Me´lange occurs further to the south along the southern margin of the Ketungau and Mandai basins. Williams et al. (1988) have suggested that the Boyan Me´lange was back-thrust from the northern me´lange zone.
The present authors have carried out a geological survey in southwestern Sarawak to investigate the strati-graphic 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 Silan-tek Formation is composed of three members: the Basal E. Honza et al. / Journal of Asian Earth Sciences 18 (2000) 751–759 753
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.
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 prob-ably 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 sand-stones interbedded with thin beds of siltstone and sandy to silty mudstone. The Upper Redbed Member consists of mudstone and shale, occasionally interbedded with fine-grained 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 over-turning of the folds near the fault.
The Lubok Antu Me´lange has a matrix of mudstone and shale interbedded with coarse grained sediments. The me´lange 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 me´lange. Native blocks include mudstone, shale and sandstone, while exotic blocks are serpentinite, gabbro and basalt, chert, and lime-stone, or their metamorphic equivalents.
Lower Eocene foraminifera and nannofossils, and reworked Upper Cretaceous coccoliths, occur within the me´lange 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 E. Honza et al. / Journal of Asian Earth Sciences 18 (2000) 751–759
754
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 me´lange (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 me´lange, originating as an olistostrome, or may
repre-sent 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 Me´lange.
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 E. Honza et al. / Journal of Asian Earth Sciences 18 (2000) 751–759 755
E. Honza et al. / Journal of Asian Earth Sciences 18 (2000) 751–759
756
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) distin-guished 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 palaeocur-rent 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).
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 inter-vals and duplicated by imbricate thrust faults (Fig. 5).
Bedding in the Lubok Antu Me´lange 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 Me´lange to the south with the Silantek Forma-tion and to the north with the Lupar FormaForma-tion 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 rela-tionships 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 deforma-tion, with much folding and faulting. In Profiles 3 and 4 the Layar Member is shown with steep to vertical dips, gener-ally towards the north, with frequent folds and faults as
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 10–15 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 land-ward-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 Me´lange 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).
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 consid-ered to have taken place between 32–16 Ma (late Oligo-cene–Early Miocene) (Briais et al., 1993). Chert in the Lubok Antu Me´lange 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.
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 908
since the latest Cretaceous, but has remained stable since the Early Miocene. On the other hand Indochina rotated 10–208 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 Indo-china and Borneo.
Compressional stress to form this arc may have been provided by the northward movement of the Indian–Austra-lian 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 criti-cal 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., 1989. The application of detailed reservoir geological studies in the D18 Field, Balingian Province, offshore Sarawak. Geological Society of Malaysia Bulletin 27, 137–159.
Banda, R.M., Honza, E., (in press). Miocene sediments and structure in the NW Borneo Basin. Geological Society of Malaysia Bulletin. Basir Jasin, 1996. Late Jurassic to Early Cretaceous radiolaria from chert
blocks in the Lubok Antu Me´lange, Sarawak, Malaysia. Journal of Southeast Asian Earth Sciences 13, 1–11.
Basir Jasin, Taj Madira Taj Ahmad, 1985. Some Palaeogene planktonic foraminifera from the Lubok Antu Complex, Sarawak. Warta Geologi 21, 147–151.
Be´nard, F., Muller, C., Letouzey, J., Rangin, C., Tahir, S., 1990. Evidence of multiphase deformation in the Rajang-Crocker Range (northern Borneo) from Landsat imagery interpretation: geodynamic implica-tions. Tectonophysics 183, 321–339.
Briais, A., Patriat, P., Tapponnier, P., 1993. 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, 6299–6328.
E. Honza et al. / Journal of Asian Earth Sciences 18 (2000) 751–759
758
Engebretson, D.C., Cox, A., Gordon, R.G., 1985. Relative motions between oceanic plates of the Pacific Basin. Geological Society of America Special Paper.
Fuller, M., Haston, R., Lin, J-L., Richter, B., Schmidtke, E., Almasco, J., 1991. Tertiary paleomagnetism of regions around the South China Sea. Journal of Southeast Asian Earth Sciences 6, 161–184.
Haile, N.S., 1957. The geology and mineral resources of the Lupar and Saribas Valleys, West Sarawak. British Borneo Geological Survey Memoir 7.
Haile, N.S., 1996. Note on the Enkilili Formation and the age of the Lubok Me´lange, West Sarawak, Malaysia. Warta Geologi 22, 67–70. Haile, N.S., Lam, S.K., Banda, R.M., 1994. Relationship of gabbro and
pillow lavas in the Lupar Formation, West Sarawak: implications for the interpretation of the Lubok Antu Me´lange and the Lupar Line. Geological Society of Malaysia Bulletin 36, 1–9.
Hamilton, W., 1979. Tectonics of the Indonesian Region. United States Geological Survey Professional Paper 1078.
Heryanto, R., Williams, P.R., Harahap, G.H., Pieters, P.E., 1993. Geology of the Sintang Sheet area Kalimantan (1:250,000). Geological Research and Development Centre, Bandung.
Ho, K.F., 1978. Stratigraphic framework for oil exploration in Sarawak. Geological Society of Malaysia Bulletin 10, 1–13.
Holloway, N.H., 1982. North Palawan Block, Philippines — its rela-tion to the Asian mainland and role in evolurela-tion of the South China Sea. American Association of Petroleum Geologists Bulletin 66, 1355–1383.
Honza, E., 1981. Subduction and Accretion in the Japan Trench (special volume). Oceanologica Acta, 251–258.
Honza, E., Murakami, F., 1986. Accretion in the Nankai Trough. In: Nasu, N., Kobayashi, K., Uyeda, S., Kushiro, I., Kagami, H. (Eds.). Formation of Active Ocean Margins. Terra Scientific Publishing Co, pp. 343–364. Hutchison, C.S., 1986. Formation of marginal seas in Southeast Asia by rifting of the Chinese and Australian continental margins and implica-tions for the Borneo region. Geological Society of Malaysia Bulletin 20, 201–220.
Hutchison, C.S., 1996. The ‘Rajang Accretionary prism’ and the ‘Lupar Line’ problem of Borneo. In: Hall, R., Blundell, D.J. (Eds.), Tectonic
Evolution of Southeast Asia. Geological Society of London, Special Publication, vol. 106, pp. 247–261.
Kagami, H., 1986. Internal structures of the accretionary wedge in the Nankai Tough off Shikoku, southwestern Japan. In: Nasu, N., Kobaya-shi, K., Uyeda, S., Kushiro, I., Kagami, H. (Eds.). Formation of Active Ocean Margins. Terra Scientific Publishing Co, pp. 193–219. Kirk, H.J.C., 1957. The geology and mineral resources of the Upper Rajang
and adjacent areas. British Borneo Geological Survey Memoir 8. Lee, T-Y., Lawver, L.A., 1995. Cenozoic plate reconstruction of Southeast
Asia. Tectonophysics 251, 85–138.
Liechti, P., Roe, F.W., Haile, N.S., 1960. The geology of Sarawak, Brunei and the western part of North Borneo. British Borneo Geological Survey Bulletin, 3.
Madon, M.B.H., 1994. The stratigraphy of northern Labuan, N.W. Sabah Basin, East Malaysia. Geological Society of Malaysia Bulletin 36, 19–30. Mutter, J.C., Hegarty, K.A., Steven, C.C., Weissel, J.K., 1985. Breakup between Australia and Antarctica: a brief review in the light of new data. Tectonophysics 114, 255–279.
Patriat, P., Achache, J., 1984. India–Eurasia collision chronology has implications for crustal shortening and driving mechanism of plates. Nature 311, 615–621.
Pieters, P.E., Abidin, H.Z., Sudana, D., 1993. Geology of the Long Pahan-gai Sheet area, Kalimantan (1:250,000). Geological Research and Development Centre, Bandung.
Tan, D.N.K., 1979. Lupar valley, West Sarawak (1:50,000) geological maps and explanations. Geological Survey of Malaysia Report 13. Tan, D.N.K., 1982. The Lubok Antu Me´lange, Lupar Valley, West
Sara-wak: a Lower Tertiary subduction me´lange. Geological Society of Malaysia Bulletin 15, 31–46.
von Huene, R., Klaeschen, D., Cropp, B., 1984. Tectonic structure across the accretionary and erosional parts of the Japan Trench margin. Journal of Geophysical Research 99, 22349–22361.
Williams, P.R., Johnston, C.R., Almond, R.A., Simamora, W.H., 1988. Late Cretaceous to Early Tertiary structural elements of West Kaliman-tan. Tectonophysics 148, 279–297.
Wolfenden, E.B., 1960. The geology and mineral resources of the Rajang Valley and adjacent areas. British Borneo Geological Survey Memoir 11.
