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 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 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
