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 860-8555, Japan
b
Bintulu Office, Minerals and Geosciences Department of Malaysia, Wisma Persekutuan, Jalan Bintulu-Miri, 97000 Bintulu, Sarawak, Malaysia
c
Sarawak 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. q 2000 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
1367-912000 - see front matter q 2000 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.nllocatejseaes
Corresponding author. E-mail address:
honzasci.kumamoto-u.ac.jp E. Honza.
E. Honza et al. Journal of Asian Earth Sciences 18 2000 751–759 752
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
