3 Institute for Advanced Studies, University of Malaya, Kuala Lumpur, Malaysia. Abstract: Field ...... Geologists report of progress, Federated Malay States,.
Journal of the Geological Society, London, Vol. 147, 1990, pp . 11-26, 13 figs, 1 table. Printed in Northern Ireland
Structural evolution of Mesozoic Peninsular Malaysia 2
2 3 , I. METCALFE M. G. AUDLEY-CHARLES 2 & K. R. MOHAMED 1 Department of Geology, Birkbeck College London, Gresse Street, London WlP lPA, UK 2 Department of Geological Sciences, University College London, Gower Street, London WClE 6BT, UK 3 Institute for Advanced Studies, University of Malaya, Kuala Lumpur, Malaysia
N. A . HARBURY1,
M . E. JONES
,
Abstract: Field evidence from Upper Palaeozoic and Mesozoic strata exposed in Peninsular Malaysia demonstrates that the structural style, degree and orientation of folding, axial-plane cleavage , and faulting in the Triassic, Jurassic and Cretaceous rocks of the central basin are remarkably similar. In contrast, the Upper Palaeozoic strata, especially the Carboniferous rocks exposed in the eastern part of the peninsula, show multiple phase folding and regional metamorphism unlike anything seen in the Mesozoic rocks . These observations demonstrate that two important compressional events affected Peninsular Malaysia , one in Late Permian times and the other in the mid- to late Cretaceous. The Late Palaeozoic compressional event was a major orogenic mountain building phase with associated emplacement of major Permo- Triassic granite plutons that form the eastern and main ranges . No indications were found of the widely reported Triassic orogenic compression. The palaeotectonic and palaeogeographical implications of these discoveries are important for interpreting the evolution of eastern Gondwana and Tethys. The Raub-Bentong line was an important fault zone active during the Mesozo ic but doe s not appear to have been a major tectonic suture since the Late Palaeozoic . We postulate that the Permo-Triassic granites of the eastern belt and those presently exposed in the main range were originally about 30-50 km apart, and that Triassic and Jurassic crustal attenuation and subsidence led to the separation of these two granitic belts by more than 100 km .
The geology of Peninsular Malaysia is generally thought to reveal stratigraphical and structural evidence (Fig . 1) of a major Triassic compressional orogenic event (Burton 1973; Khoo & Tan 1983; Khoo 1983; Sengor 1986). Three separate lines of evidence support this interpretation: (i) the Triassic rocks of the peninsula have been observed over a wide area to be strongly folded and to display axial planar cleavage (Metcalfe 1988); (ii) the Mesozoic Tembeling Group and Gagau Group , the upper parts of which have been dated as Upper Jurassic to Lower Cretaceous, have been reported to be 'flat lying' , or 'only tilted or gently folded' and to rest unconformably on Triassic rocks (Rishworth 1974; Khoo 1983); (iii) some workers have reported a Triassic regional compressive event in adjacent areas including Thailand and Indochina (Sengor 1984; Sengor et al. 1988). Khoo (1983) suggested that the largely Triassic flysch sequences (e .g. Kaling , Lipis , Gunong Rabong , Telong , Semantan and Gemas Formations) are strongly folded, whereas what he called the Jurassic - Lower Cretaceous molasse deposits (mainly Tembeling Group and Gagau Group) are less strongly deformed . The structural history of Peninsular Malaysia during the Mesozoic is of regional importance because it is widely held that the peninsula , together with western Thailand and Burma (Fig. 2), collided with mainland Asia during the Late Triassic, after rifting from Australian Gondwanaland during the mid-Permian (Mitchell 1981; Sengor 1986; Metcalfe 1988). Metcalfe has also suggested an earlier suturing in the early Carboniferous (Metcalfe 1987). The Raub-Bentong line (Fig. 2) is claimed to represent the Triassic collision zone responsible for the orogenic compression which deformed the Triassic rocks but not the younger s\rata (Mitchell 1981; Sengor 1984). However \ Audley-Charles
(1983, 1984) came to a different interpretation of the tectonic history on the basis of regional stratigraphy, but without the benefit of field observations in Peninsular Malaysia. His view was that Peninsular Malaysia, western Thailand, Burma and South Tibet (Fig. 2) formed part of Australian Gondwana from where they were rifted during the Jurassic. He suggested that these continental blocks did not collide with mainland Asia until the Cretaceous . These contrasting views of the palaeogeographical and palaeotectonic evolution of Peninsular Malaysia have wide implications for the tectonic development of southeast Asia as well as for the evolution of eastern Gondwanaland and Tethys . The validity of these viewpoints was tested by examining the Mesozoic rocks of Peninsular Malaysia (Figs 1 & 2). Stauffer (1974) also recognized that the key to this problem lies in the Jurassic rocks. A joint University of London-Universiti Kebangsaan Malaysia expedition to the Tekai River, (Fig. 3) where Koopmans (1968) had described extensive exposures of the Tembeling Group of Jurassic to Lower Cretaceous age (Khoo 1983), was designed to investigate these critical questions.
Palaeomagnetic indications Metcalfe (1988) has reviewed the limited available palaeomagnetic data from Peninsular Malaysia, drawing heavily on the Haile & Briden (1982) review. Metcalfe pointed out that the palaeomagnetic data for the Palaeozoic rocks are extremely limited , the quality is highly suspect owing to poor age control, lack of fold tests and thermal and deformational effects. He noted that data from the Mesozoic rocks appear to be more reliable but still at a reconnaissance stage. 11
12
N . A . HARBURY
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Fig. 1. Interpretation of stratigraphic successions and main phases of deformation in Peninsular Malaysia.
Metcalfe (1988) cited the work of Bunopas et al. (1978), Barr et al. (1978) , Bunopas (1982) and Achache & Courtillot (1984) in support of his claim that the Sibumasu block (which includes most of Peninsular Malaysia) and the Indochina and South China blocks were united during the late Triassic and Jurassic . However , the palaeomagnetic data for Peninsular Malaysia on which that interpretation is based lacks fold tests , has inadequate sampling and includes some rocks of uncertain age. Palaeomagnetic data for rocks thought to be of Upper Jurassic and Lower Cretaceous age (Haile & Khoo 1980; Haile et al. 1983) indicate th at Peninsular Malaysia has rotated anticlockwise and moved south about 8° since the Cretaceous . Furthermore a major tectonic discontinuity separates Peninsular Malaysia from eastern Thailand and South China (Fig. 2) from where palaeomagnetic studies reveal that eastern Thailand and the Indo-China plate (Maranate & Vella 1986) have remained at nearly the same latitude , but rotated clockwise 37° ± 7° since late Triassic times .
Field expedition to Peninsular Malaysia
Fig. 2. The tectonostratigr aphic terranes which now constitute mainland southeast Asia (after Metcalfe 1988) .
In July 1986 the expedition navigated the Tekai river and its tributaries , the Termus , Kerum and Song river s. The results of these traverses of about 60 km total length (but about 35 km across strike) into the Tembeling Group and Kerum Formation are presented in the geological map and section (Figs 4 & 5) . The field mapping is supported by photogeological interpretation . We also sampled these strata for palaeomagnetic determinations by collecting 120 cores (including two fold tests). These samples have been measured by M . Fuller and E . Schmidke (University of
'
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Fig. 4. Geological Map of the Tekai River region of central Pahang , west Malaysia, based on field observations and aerial photographic interpretation.
MESOZOIC
STRUCTURES
: MALAYSIA
13
.A Gunung
Tahan 2187 m
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Fig. 3. Map showing the region surrounding the Tembeling and Tekai River systems, central Pahang and position of other localities mentioned in the text .
California at Santa Barbara) and will be the subject of a separate publication . Following the completion of the Tekai traverse we examined the stratigraphy , sedimentology and structure of a large number of Palaeozoic and Mesozoic sections throughout the Peninsular . Apart from information collected in the Tekai river, .most of the additional data presented in this paper were collected in south Pahang and northern Johore (Fig. 3). Rocks were sampled for palynological and petrographic studies. The Triassic rocks of the peninsula are the subject of a study by Kamal Rosian as part of a PhD project which has involved extensive sampling . The results of that continuing study will be reported later .
Stratigraphy The rocks studied in the area of Tembeling and Tekai Rivers of North Pahang were provisionally named the Tembeling Series (Scrivenor 1907), Tembeling Formation (Koopmans 1968) and the Tembeling Group (Khoo 1977, 1983). We agree with the upgrading of the succession to Group status but differ in our subdivisions (Fig. 1). On lithostratigraphic criteria we do not · include the Kerum Formation in the Tembeling Group . Our definition of the Kerum Formation as a separate lithostratigraphic element is in keeping with other parts of Peninsular Malaysia where Upper Triassic formations dominated by volcaniclastic material are separated from Jurassic-Cretaceous continental sediments (Khoo 1983). Koopmans (1968) recorded volcanic
rocks underlying the continental sediments and tentatively assigned them to the Triassic Lipis Group .
The Kerum Formation Although previous studies suggest a thickness of 2000 m for the Kerum Formation (Khoo 1983) we consider, on the basis of our mapping and structural interpretation of the region, that the formation does not exceed 1000 m in thickness, and that this formation may thin laterally within the central basin to a feather edge. For example, in the Jengka Pass region to the south of the Tekai River , the Kerum Formation or equivalent is entirely missing and the Tembeling Group lies with marked angular unconformity on steeply dipping Middle Permian limestones , very coarse sandstones, and shales (Koopmans 1968; Ichikawa et al. 1966; Ishii 1966). Field observations indicate that the Kerum Formation is composed of siltstones, sandstones, and conglomerates all dominated by volcaniclastic components. Locally red shales with steeply dipping cleavage predominate. The type locality for the formation is the river Kerum, a tributary of the Tekai river (Khoo 1983).
Age and correlation of the Kerum Formation. The absence of fauna in the Kerum Formation has prevented an accurate determination of its age. However, comparison with other Upper Triassic volcaniclastic rocks ( e .g. Kaling · and Semantan Formations) in Peninsular Malaysia (Ahmad 1976; Khoo 1983) suggests the Kerum Formation is likely to
LEGEND TERMU S SHALEFORMATION
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16
N . A . HARBURY
SECTION m
ET AL.
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Fig. 5. Cross-sections across the Tembeling Group and the Kerum Form ation in the Sungai Tek ai region. See Fig. 4 for location of cross-sections.
be Upper Triassic . This is supported by information on the age of the overlying Tembeling Formation presented below. The Kerum Formation may be a stratigraphi c equivalent of the volcanic rocks which lie directly on the pre-Gagau erosion surface and are overlain by the basal Gagau Group sediments (Rishworth 1974).
The Tembeling Group The Tembeling Group consists of sandstones , conglomerates and shales deposited in a predominantly continental environment. After Khoo (1983) we divide the Tembeling Group into three formations in younging order of succession : the Lanis Conglomerate, the Mangking Sandstone and the Termus Shale (Fig . 1). The detailed sedimentology of · these formations will be presented elsewhere . Cross bedding orientations were recorded in all formations along the length of the Tekai River traverse and give a very consistent palaeocurrent direction towards the SE . Previous workers have suggested thicknesses between 3000 m (Koopmans 1968) and 4800 m (Khoo 1983) for the rocks in the Tembeling Group . We conclude on structural evidence discussed below that these deposits are thinner than has been suggested , being between 1400-2000 m.
Lanis Conglomerate Formation . The Lanis Conglomerate is the oldest formation in the Tembeling Group as redefined
here (Fig . 1). The lower parts of this formation are poorly exposed but we suspect it lies conformably on the Kerum Formation, although an erosional disconformity may be present. The Gunung Lanis ridge in the eastern part of the mapped area is defined as the type locality for the formation (Khoo 1983). The thickness of the conglomerate is rather variable . In the region between the Kerum and Mangking Rivers the thickness of the formation is between 500 and 700 m; elsewhere it is slightly thinner. The formation consists predominantly of polymict conglomerate beds , while sandstones and siltstones are volumetrically less important. The conglomerates contain clasts of schist , arenite, red chert , vein quartz , volcaniclastics , granite , and basalt .
Mangking Sandstone Formation. The Mangking Sandstone Formation was named by Khoo (1983) who designated the Mangking river as the type locality . Cross-bedded sandstones form the most abundant facies within this formation. We estimate the thickness of this formation as 600-800 m. There is a wide range of grain sizes from pebbly sandstones down to fine sands. Termus Shale Formation . The Termus Shale Formation was first defined by Khoo (1983) who designated the Termus river as the type locality for the formation . We estimate that the formation has a thickness between 300 and 500 m and
MESOZOIC
STRUCTURES:
the palynological assemblage described below confirms a Lower Cretaceous age for the Termus Shale. The formation consists of red siltstones and shales with subordinate quartz arenites and poorly sorted breccias . The finer grained rocks frequently have a well developed axial planar cleavage which has destroyed primary sedimentary structures.
Age and correlation of the Tembeling Group. As noted by Koopmans (1968), the Tembeling Group is practically unfossiliferous and determination of its age is based upon recognition of a few poorly preserved plant fossils. Palynological analysis by Sarawak Shell Berhad, Miri, Sarawak , of samples we collected from the Tekai River was hampered by the high degree of organic metamorphism leading to disintegration of the palynomorphs , with only carbonized plant material remaining. However, one sample from the lower part of the Termus Shale Formation can be assigned to the Lower Cretaceous (Valanginian-Aptian), based on the rich assemblage of sporomorphs it contains which includes: Cyathidites australis, Schizaeopsis americana, Rousiporites gransopeciosus , Classopollis sp. , Schizosporis cf. parvus, Cingutriletes sp. Aequitradites sp., Cooksonites sp. and Biretisporites potoniaie. The age of this sample supports the Jurassic to Lower Cretaceous age previously assigned to the Tembeling Group (Koopmans 1968; Khoo 1977). The Mangking Sandstone yielded the plant fossil Gleichenoides Gaguensis (Konno) recorded by Khoo (1977) which indicates Upper Jurassic to Lower Cretaceous. In the same Geological Survey report Khoo records a lens of limestone containing Vivaparus sp . of Upper Jurassic to Holocene age. The age of the lower part of the Tembeling Group, i.e . the Lanis Conglomerate Formation and the lower part of the Mangking Sandstone may be considered to be within the range of Lower to Upper Jurassic. From all available description s of the Gagau Group (Rishworth 1974) we are led to suggest that the Upper Jurassic to Lower Cretaceous part of the Tembeling Group is stratigraphically equivalent to the Gagau Group in NE Pahang and S Terengganu. In the central part of the Peninsular the Raub Red Beds , previously considered to be Carboniferous (Haile et al. 1977), have recently been reinterpreted as stratigraphical equivalents of part of the Tembeling Group (Metcalfe 1988). Interpretation of the Tembeling Group. Koopmans (1968) described the Tembeling Formation as a fluviatile-deltaiclacustrine sequence . On the basis of the data presented here we suggest that deposition of the Tembeling Group was restricted to an alluvial system which changed its character with time . This is confirmed by an absence of marine fossils, and the presence of channels with unidirectional palaeocurrents in a coarse sand stone and conglomerate sequence . In addition, locally calcareous strata contain fresh -water gastropods (Khoo 1977). - Thick sequences - of · pebbly . alluvium such as the Tembeling Group are generated and preserved where there is topographic relief , which commonly implies tectonic activity during or immediately prior to deposition. Our observations in the Raub and Ben tong regions (discussed below under 'structure') suggest that during Triassic time s crustal extension was an important process in central Peninsular Malaysia . Dip-slip movements on the RaubBentong line created a basin east of Bentong into which
MALAYSIA
17
volcaniclastic sediments of the Kerum Formation were deposited. These were overlain by the continental Tembeling Group. We have not been able to examine the borders of the basin in which the Tembeling Group were deposited , although palaeocurrent data from the Mangking Sandstone Formation suggest that the source area was to the northwest. We have therefore not been able to determine whether the sedimentary succession was intimately associated with a fault line or was the fill of a basin flanking an uplifted area. The absence of outcrop between the type area of the Tembeling Group and the Raub-Bentong area means that lateral facies changes and thickness variations cannot be established. The Tembeling Group may have been restricted to the hanging wall block of a normal fault system to the east of the Raub-Bentong line , whilst other continental sediment s (e.g. Gaugau Group and Raub Red Beds) were deposited in adjacent hanging wall blocks.
Structure and metamorphism Evidence from this study, and previous work indicates that the Tembeling Group exposed in Peninsular Malaysia is of Jurassic-Cretaceous age. It is reported to be undeformed except for minor folding and tilting (Rishworth 1974; Mitchell 1981; Sengor 1984). These rocks have therefore been thought to post-date all major orogenic compressional events affecting the Malaysian Peninsula. Trias .sic rocks exposed in road sections through the central basin area (Fig. 3) are folded, faulted and cleaved and have been regarded as having been subjected to orogenic compression (Mitchell 1981; Sengor 1984). The deformed Triassic rocks themselves unconformably overlie far more intensely deformed Palaeozoic sedimentary rocks which crop out in the east of the peninsula and locally on structural highs in the central basin area. The deformation of these Palaeozoic rocks has been generally attributed to a major orogeny in Carboniferous times followed by intrusion of the Malay granites (Burton 1972). Recent work in western Thailand (Wolfart 1987) suggests that the orogenic event which deformed and metamorphosed the Carboniferous rocks was Late Permian. The accumulation of the Triassic deposits, which are dominantly volcaniclastic, seem likely to be broadly related to the magmatism associated with these granites . A second period of orogenesis has been postulated in Late Triassic or Early Jurassic times (Khoo & Tan 1983), prior to deposition of the Tembeling Group , in order to account for the reported strong deformation of the Triassic and undeformed state of the post-Triassic deposits. The development of a new palaeogeographic concept (AudleyCharles 1983, 1984) led to doubts about the reported structural history of Peninsular Malaysia and this field study was in part designed to test these conflicting views. Exposures of the Tembeling Group are not easily accessible ; the type section is located in the Tekai River and its tributary streams , and inspection of these Jurassic to Lower Cretaceous rocks therefore necessitates a field expedition into this uninhabited forested terr ain . Other outcrops of the Tembeling Group are located along the Tembeling River but are only marginally more accessible. Descriptions of the structural geology of the Tembeling rocks are therefore few (Koopmans 1968; Khoo 1983) compared with those of the well exposed and accessible Triassic sections (see Khoo 1983). A major aim of the
18
N. A. HARBURY
expedition was to re-examine the structural geology of the Tembeling Group (previously described as minor folding and tilting (Rishworth 1974; Khoo 1983)).
Deformation styles The structures (folds, faults, boudinage and cleavage) observed in the Permo-Carboniferous sections, in the Triassic volcaniclastic sediments, and in the various lithologies exhibited by the Tembeling Group , have been recorded . These are described in the following sub-sections , whilst a fourth section is devoted to observations of the texture and deformation observed in the Triassic rocks adjacent to the Raub-Bentong Line , which is widely
ET AL.
regarded as an important tectonic feature (Mitchell 1981; Sengor 1984). Deformation of the Permo-Carboniferous strata. Major outcrops of deformed pelitic rocks of possible Permian or Carboniferous age are found in the eastern part of Peninsular Malaysia , along road sections between Maran and Kuantan in the vicinity of Kampong Seri Jaya, on the coast at Mersing and Tanjung Kempit (Chakraborty & Metcalfe 1984), in less well preserved sections in the vicinity of Chukai and on the coast to the north of Chukai at various localities near Kerith . In all of these scattered localities the rocks exhibit intense deformation (most commonly represented by folding , faulting and foliation development)
Fig. 6. Examples of typical structural styles exhibited in Palaeozoic and Mesozoic rocks of Peninsular Malaysia. (A) Upright Fl folds refolded by F2 folds in Carboniferous psammites and pelites of the Kuantan Group between Chukai and Ketih on the east coast. (8) Asymmetrical close folds with fractured limbs in the Mangking Sandstone Formation, Tekai River region . (C) Pronounced cleavage in the Termus Shale Formation, Tekai River region . (D) Well developed cleavage in the Termus Shale Formation clearly showing the relationship between flat-lying beds and steeply dipping regional axial planar cleavage, Tekai River region .
MESOZOIC
STRUCTURES:
but the degree of recrystallisation and metamorphism is variable. Typically the rocks are pelites with at least one well-developed foliation , which is axial-planar to the dominant fold system in the outcrop . Where large exposures are available, however, (as at Tanjung Kempit and Mersing), it is clear that at least three phases of folding are represented (Chakraborty & Metcalfe 1984). The Fl folds are tight to isoclinal (Fig . 6A) with axial planes commonly dipping to the SW. The folds plunge moderately to the Nor NNE at Tanjung Kempit and gently towards the SE or NW at Mersing. The attitude of the Fl axial planes and associated pervasive axial planar foliation (Sl) vary due to deformation by F2 and F3 . The second phase folds (F2) predominate the Upper Palaeozoic rocks . Although the size and style of F2 folds are variable , they are mostly asymmetrical , (although overturned folds occur), and are usually tight and co-axially refold Fl folds . F2 axial planes have been deformed by D3 resulting in arcuate or sinuous outcrop patterns of many F2 folds. The Sl foliation is deformed to produce a pronounced crenulation cleavage (S2) which has a general N- S strike and steep dip easterly or westerly. The limbs of folds may be strongly boudinaged but it is not clear if the se boudin s were developed during Dl or D2. Where overprinting features can not be found there is some difficulty in distinguishing Fl and F2 folds . The third phase folds (F3), are common, though weakly expressed. They are asymmetrical to symmetrical open folds and have steeply dipping to subvertical axial planes which strike generally E-W . The orientation and plunge of the individual F3 folds vary according to their position on the limbs of the earlier folds. This intense deformation has been described by Chakraborty & Metcalfe (1984). Examples of the styles of deformation seen in these outcrops are shown in Fig. 7. New exposures of intensely deformed Palaeozoic met asediments (Singh 1985) have been created along the new east-west highway in the north of Peninsular Malaysia between Jeli and Grik (Fig . 3). These roadcuts reveal pelitic and psammitic schists , and marbles, and the structural styles shown by these rocks range from minor warping of bedding without any well developed folding, through to thick sequences where the succession is repeated by thrusts , and the beds are tightly folded and foliated. The degree to which the metamorphism was related to granite intrusion (which post-dates the deformation, the granites being undeformed) is unclear from field observation . All of the deformed rocks observed along the east-west highway in the north of the peninsula were deformed before the intrusion of the granite s and must therefore have been involved in the Permo-Carboniferous orogenic event. It is not possible to draw any overall picture of the Permo - Carboniferous orogeny from visits to the scattered outcrops along the east coast and in the north of Peninsular Malaysia , but it is clear that an orogenic event equivalent to a major mountain building phase must have occurred at this time with widespread deformation and metamorphism . Previous workers have suggested that this orogeny was a Carboniferous event (Burton 1970, 1972). However , the evidence available suggests that this major orogenic event is of late Permian age : because , firstly, there are no angular unconformities related to compressional deformation in the Carboniferous in Peninsular Malaysia (Metcalfe 1983, 1988); and , secondly, a post-Middle Permian orogenic event is suggested by the major unconformity at Jengka Pass in
MALAYSIA
19
®
............
F2
®
© lm
@ Fig. 7. Line drawings of the deformational styles observed in Carboniferous rocks recorded at Mersing and Tanjung Kempit, eastern Malaysia (A). Moderately plunging isoclinal folds at Tanjung Kempit. (B) Refolded steeply plunging isoclinal folds at Tanjung Kempit . (C) Upright isoclinal folds with boudinage in some layers near Mersing . (D) Same as C but refolded .
Pahang , where Jurassic continental strata lie on deformed Permian marbles and phyllites (Ichikawa et al. 1966; Kon'no & Asama 1970). Recent work in western Thailand (Wolfart 1987) records an orogeny in the late Permian. The granite plutons in Peninsular Malaysia range from Lower Permian (280 Ma) to Upper Triassic (200 Ma) age (Cobbing et al. 1986), which suggests these granites are related to a late Palaeozoic major orogenic event .
20
ET AL.
N. A. HARBURY
Deformation of the Triassic rocks. Major outcrops of deformed Triassic volcaniclastic sedimentary rocks occur along the road sections between Mantakab and Jengka Pass, and in the vicinty of Raub (Fig . 3). Between Mentakab and the Jenka Pass there are exposures of tuffaceous sedimentary rocks with a steeply dipping or vertical pronounced spaced cleavage striking between 160° and 170°. This cleavage is axial planar to a series of folds which are generally upright and gently plunging (up to 25° to the south) and are variously symmetrical and asymmetrical depending on their position within the major structure. Examples of the geometries of these folds are shown in Fig. 8. Thirty-eight kilometres east of Mentakab, on the road section known as the Jengka Pass, an angular unconformity occurs at the base of gently folded sandstones , shales , and conglomerates assigned to the Tembeling Group. These rocks rest on upright, tight to near-isoclinal folds (trend 155°) of limestones and shales of Mid Permian age (Ishii 1966; Fig. 9). The road cutting at Jengka Pass appears to be on the crest of a large (kilometre scale) anticline. Outcrops alongside the road just to the east of Jengka Pass expose folded Triassic volcaniclastic rocks (Singh 1985). Upright close folds predominate, though some open and tight folds may be found and axial planes and foliations are subvertical and strike NNW. Further east of Maran, Permo-Carboniferous carbonaceous pelitic rocks (Kuantan Group of Foo 1983) display three phases of folding . The deformation styles in these Permo-Carboniferous rocks are illustrated in Fig. 10, and consist of two phases of tight to isoclinal folds refolded into an open structure more typical of the Triassic strata. We consider these Permo-Carboniferous rocks have been deformed in both the late Palaeozoic and the post-Triassic orogenic events. Triassic volcaniclastic rocks are also exposed along road sections in the vicinity of Raub, notably between Raub and Kampong Lembah Bilut (Chakraborty & Metcalfe 1987) and also have a vertical cleavage with a strike of 010°. Further east these rocks are exposed in an asymmetrical synform with an axial planar cleavage . Plunge and axis orientation could not be determined accurately in this section but axes appeared to trend about 170° with a gentle plunge . The deformation style of the Triassic rocks differs from that of the underlying Upper Palaeozoic rocks in being dominated by a single phase of upright folds with an axial-planar spaced cleavage. Isoclinal folds, refolding, and boudinage structures, typical of the Carboniferous deformed rocks are not present in the Triassic rocks. The deformation of the Triassic rocks appears to have been restricted to strata which lay within the central basin. Granites, dated as 280-200 Ma, remain undeformed (Cobbing et al. 1986) suggesting that any post-granite deformation was not a major mountain building phase. The Permian deformation with metamorphism was far more severe than that which deformed the Triassic and Jurassic strata. Detailed examination of the Middle Permian strata at Jengka Pass indicates that the limestone units there may be blocks, as may be some of the incompetent strata (Fig. 9) . Although deformation of these Permian rocks has produced some tight folds and a cleavage in more argillaceous rocks, fusulinids, plants, and other fossils are well preserved. These Permian rocks beneath the unconformity may be
/ /
I
/ / I
I
///
I
/
/
I /
II II//
/
I/
//
A
1 E
0
N
j B
----- I
/'"-------,
---------------
c
0E
C!l~~;,= ;--/---~-
-
F/
~
__________
NJ
/
I
I
/
-20m
Fig. 8. Post-Triassic (Cretaceous) deformation fold styles exhibited by Triassic volcaniclastic sediments (Semantan Formation) of the Central basin. Where observed, cleavage is normally steeply dipping and has a strike of 160°. (A) Symmetrical tight cylindrical fold with a weak axial planar cleavage. (B) Tight asymmetric fold with well developed cleavage; (C) Monoclinal fold with a local fracture cleavage.
slump deposits (perhaps associated with basin extension). Local occurrences of Upper Permian strata not subjected to the main Palaeozoic orogenic event show the same styles of deformation as the Trias sic-Cretaceous strata . For example, the Gua Musang Formation in the Central Belt (Burton 1973; Foo 1983) exposed near the Kampong Awah and Kuala Lipis in N. Pahang and S. Kelantan, show only open folds without refolding. This implies these rocks were folded and cleaved at the same time as the Mesozoic rocks and were not affected by the major Permian orogeny.
MESOZOIC
STRUCTURES:
MALAYSIA
21
WEST
EAST
i
Q)
"'] E
If)
I
I Fault
D
Scree
~
Shale & siltstone
. [22] .
ffl
Tuffaceous sandstone, in claces conalomeritic
Black shale
Limestone
,-=----;,
~ Post-Permian
Fig. 9. Unconformity at Jengka Pass , central Pahang . Post-Permian Tembeling sandstones lie with marked unconformity on steeply dipping Permian marbles and shales (From Gobbet! & Hutchison 1973).
Metcalfe (1988) interpreted the Permian limestones of the west Malaysia block (his Sibumasu terrane) as conformable on the Carboniferous glacio-marine diamictites. Evidence from this region, and from the Jengka Pass exposures where there is a strong angular unconformity above the Middle Permian limestones, indicates that the late Palaeozoic orogenesis is no older than Middle Permian.
Deformation of the Jurassic-Upper Cretaceous Tembeling Group. The Tembeling Group was examined in the Tembeling, Tekai, Termus, Kerum and Song Rivers. The strata are commonly steeply dipping and folded on two scales (Figs 4 & 5): large scale folds, with a wavelength of about 7 km with upright axial planes which strike about 150-160°; superimposed smaller scale structures show a consistent trend of 160°, display a range of styles from monoclinal to symmetrical. Interlimb angles vary from gentle to isoclinal. The wavelength of these smaller folds ranges from 10 m to 2 km; they are best developed in the Mangking Sandstone Formation (Fig. 6B). A pronounced axial planar foliation occurs in the Tembeling Group. This is present in some of the sandstones as a spaced cleavage, but is best developed in the Termus Shale, where it variously forms a closely spaced cleavage, a pencil cleavage and a slaty cleavage (Fig. 6C & D). Its dip is normally steep and it strikes 160°. In strongly cleaved exposures of Termus Shale thin interbedded sandstones are boudinaged, whereas sandstone dominated outcrops show pinch and swell structures. The shortening of these sediments appears to have been in a SW-NE direction, and elongation NW-SE is indicated by the presence of a vertical mineralized joint set which trends approximately 045°. A comparative table of the deformation styles in the pre-Triassic rocks, the Triassic deposits and the Tembeling Group is given in Table 1.
Metamorphism Although published reports of the metamorphism of pre-Permo-Carboniferous rocks of Peninsular Malaysia are few, the data available (Hutchison 1973; Khoo & Tan 1983) suggest that the Upper Palaeozoic rocks were subjected to a . regional metamorphism producing schists and gneisses which commonly attained highest greenschist (North and East Pahang) and locally low amphibolite facies conditions (Raub Group). Khoo & Tan (1983) suggest that the metamorphism in the Western Belt probably ended in mid-Permian times whilst in the Central Belt, the Taku Schists in North Kelantan attained mid to . high amphibolite facies. In marked contrast, Jurassic mudstones sampled from the Tembeling Group yield white mica crystallinity values (Kubler Index) ranging from 0.45 - 0.28 ~ 2.These values span the anchizone and, on the basis of existing correlative studies (e.g. Roberts & Merriman 1985) strongly suggest that the Jurassic strata experienced only prehenitepumpellyite facies conditions . A metamorphic hiatus therefore probably exists between the pre-Upper Permian rocks and the Mesozoic successions. Supporting evidence is reported from the Sungai Siput region of Pahang, where Tian & Taylor (1974) record unmetamorphosed sandstones and conglomerates, which they suggest are lateral equivalents of the Tembeling and Gagau Groups, resting unconformably on metamorphosed sandstones and shales of unknown age. 0
The Raub-Bentong line The Raub-Bentong line is an important structural lineament which marks the western limit of the central basin. This feature is locally geologically complex, and generally exposes Triassic volcaniclastic rocks of the Semantan
N . A. HARBURY
22
Table 1. Comparison of deformation styles in the pre-Triassic , Triassic and post-Triassic rocks of Peninsula Malaysia
vertical plunge
Pre-Permian
A ___
---.J
-:-_ 1
F 3 monocline shows post-Triassic deformation
7
II
_____../ I
/ ___ ~tyle I
_______.,,
I
Ir
-----I
/
I
,,.. --------
j
I
I
I I
_______
I I I I
I
Ir----I
I
Folding Refolding Fold phases Axial planar Foliation Crenulation cleavage Isoclinal Folds Open folds Up right folds Fold plunge
/
I I II / ...---- . ---5 -_______ _,. I I ,,,.. B _____ /
ET AL.
(")
-
,---Foliation orientation s
Triassic
yes yes 3
yes no
yes
yes
yes no 1
yes
yes no yes no (Fl, F2) yes yes (F3) rare normal *55° to 010° 25° to 168° (Fl, F2) t 25° to 160° (Fl , F2) t90° ( axial plan e trend) 090° (F3) :j:55° to 098° (Fl) :j:90° ( axial plane trend) : 130° (F3) Various
Post-Triassic
90°/ 160°
no yes yes normal 20° to 168°
90/160 °
•, Tg. Kempit ; t, Mersing ; :j:, Kertih.
(1984) to suggest that the structure became an active norm al fault during the Triassic initiating and controlling the development of the Triassic sedimentary basin . We consider that the gravity data (Ryall 1982), the pattern of Mesozoic strata and the distribution of east and west granitic belts all support Tan 's (1984) interpretation of the central Mesozoic basin as a graben . This fault zone activity may have continued into the cretaceous , at which time sedimentation Fig. 10. Permo-Carboniferous carbonaceous pelitic rocks (Kuantan Group) east of Maran, Central Pahang exhibiting a pre-Triassic deformation style consisting of two phases of tight-isoclinal folds (A & C) which have subsequently been refolded into an open structure (8) typical of the post-Triassic deformation of the Triassic strata.
Formation on its eastern side (Ahmad 1976). The Lower Triassic deposits are notable for including polygenetic chaotic deposits which are very poorly sorted and of sedimentary origin (Chakraborty & Metcalfe 1987). Clasts include Permian and Triassic conodont-bearing limestones. These diamictites interfinger with finger grained rocks similar to those which are more typical of the Trias of the central basin which suggests fault-scarp influenced sedimentation during the early Triassic. The deposition of the chaotic and other elastic deposits may have been controlled by variations in scarp height . The elastic facies is interpreted as having been developed as fans against the scarp (Fig. 11). The Raub-Bentong line is widely interpreted as a major tectonic suture (Hutchison 1975; Mitchell 1981; Sengor 1984; Metcalfe 1988). Both Hutchison (1975) and Sengor (1984) have drawn attention to the presence of small ultramafic bodies associated with this fault system. The persistence of a fault scarp, and the position of this structure marking the limit of a large sedimentary basin, led Tan
w.
E. Raub- Bentong Line
Central
Basin
HANGING WALL BLOCK Tricssic debris flows
DEVONIAN/ ;;:;cARBONIFEROUS
i
TRIASSIC.
--
------.
Fig. 11. Schematic cross section across the Raub-Bentong fault in the Triassic. It is our view that this lineament represented a major scarp which strongly influenced sedimentation in the centra l basin in early Mesozoic times .
MESOZOIC
STRUCTURES:
in the basin ceased. Apparently limited strike-slip motion along this fault may indicate reactivation of fault activity in the late Mesozoic and Cenozoic. The Raub-Bentong line appears to be an important feature in controlling Mesozoic sedimentation in the central basin. The variation in geochemistry of the east and west granite belts (Cobbing et al. 1986), now separated by crustal extension beneath the graben , suggests that the Raub-Bentong line represents an older line of weakness in the crust which has been reactivated in various ways during the Phanerozoic .
Structural history The structural observations reported above clearly indicate that the central and east parts of Peninsular Malaysia have been affected by two orogenic episodes. The oldest affected rocks are of pre-Upper Permian , and are characterized by complex refolded structures, multiple cleavage and metamorphic assemblages which indicate that topmost greenschist-amphibolite facies conditions were attained . These events are closely associated with the early phase of Permo-Triassic granite intrusion, and represent the major orogeny that affected the Peninsula . Idris et al. (1987) suggest there were two deformation episodes in the Ulu Endau area in N . Mersing; namely between early and mid-Permian times, and a later Jurassic deformation . On the east coast, near Mersing and Tanjung Kempit , Chakraborty & Metcalfe (1984) record little-deformed acidic extrusives of probable Triassic age resting unconformably on multiply deformed Upper Palaeozoic metasediments described above . This late Palaeozoic deformation accompanied a major period of grnnite intrusion between 280 and 200 Ma ago (Cobbing et al. 1986). Subsequent crustal extension associated with formation of the central graben separated the granites into two main belts of intrusions in the east and west of the Peninsula . The extensional basin formation continued during the remainder of the Triassic period, and subsequently in the late Jurassic and Cretaceous. This interpretation implies that the central basin is a graben (Tan 1984) or half graben bounded by normal faults , although the possibility that these were oblique slip faults (in which case the basin is a partial pull apart) cannot be discounted. We estimate the thickness of the Mesozoic strata within the central basin to be about 2.4-3 .0 km, much less than recent estimates of 6 km (Khoo 1983). Our estimates of thickness imply that subsidence of the central basin during the Mesozoic was comparatively small. The relief of the granite bodies, which remained unroofed during the Triassic sedimentation, must have increased during the period of basin development for them to form now the elevated boundaries of the basin . This uplift could easily have been achieved on the faults bounding the basin, and have been a necessary isostatic response to the intrusion of the large volumes of low density rocks . The timing of the unroofing of the granites is an important question. In the southeast , apparently undeformed sandstones and clays of the Cretaceous Panti Formation (Burton 1973) lie directly on the granite bodies. Clearly the country rock into which these granites were intruded, and any overlying volcanic edifices, had been removed by erosion by that time. It is therefore probable that at least the eastern granites were structurally elevated, undergoing erosion during the deposition of the Tembeling
MALAYSIA
23
Group, and may have contributed detritus to that Group. Granite pebbles occur in the Lanis Conglomerate exposed in the Song River of the Tekai traverse. However , the Tembeling Group is highly deformed and the Panti Formation is hot. This may mean that the Panti Formation is younger and post-dates the second orogenic period which deformed the Tembeling Group in the central basin. Alternatively, the Panti Formation may be contemporaneous with the upper part of the Tembeling Group and may have been protected from the second orogenic event by its position on top of the east coast batholith which did not deform significantly (Cobbing et al. 1986). The later compressional deformation appears to have been restricted to the central basin, and to post-date the Tembeling Group , whose youngest part is Lower Cretaceous (Khoo 1983). This second compressional phase must therefore be mid-Cretaceous or younger in age . The effects of this late Mesozoic collision are also seen in the upper Palaeozoic rocks (where they are overprinted by a later deformation) as well as in the Triassic , Jurassic and Cretaceous rocks of the central basin, all of which must therefore have undergone a compressional or transpressional deformation during late Mesozoic or Cenozoic times. It seems unlikely that this deformation was of mountain building proportions because very little post-orogenic sediment is preserved. This second deformation could not have been responsible for the uplift of the Permo-Triassic granites because they had been unroofed before the Cretaceous deformation. Furthermore , the predominance of Jurassic and Cretaceous continental sediments associated with this granite belt in Sumatra, Peninsular Malaysia , Thailand , and Burma despite the 'world-wide' Cretaceous marine transgression, suggests that these regions were uplifted in the Jurassic. It seems most likely that the Cretaceous deformation was confined to the weakened, stretched and thinned continental crust between the Permo- Triassic granites, and may well be related to compression of the peninsula during the initial formation of the adjacent offshore basins. After this late Mesozoic compressional event only limited Tertiary sedimentation occurred within the peninsula. These sediments are faulted but do not show the more extreme deformation shown by the Tembeling Group. The deformation of the Tembeling Group is therefore pre-Tertiary , which corresponds to the mid- to late Cretaceous age of deformation deduced on the evidence of the Panti Formation. The late Cretaceous deformation of the Tembeling Group is also supported by evidence further south in Northern Johar, where folded Triassic volcaniclastic deposits (Semantan Formation) are overlain by sub-horizontal , largely undeformed Tertiary basalts (the Segamat Basalt of Bignell & Snelling 1977).
Palaeotectonics and palaeogeography One important palaeogeographical conclusion from this work is that we have found no indications of the Triassic or Jurassic orogenic compression in Peninsular Malaysia. There is therefore no evidence to support the view that the peninsula collided with the Asian mainland in the late Triassic or early Jurassic as Mitchell (1981) and Sengor (1984, 1986) have proposed. On the other hand , the indications of crustal extension in the central basin (Figs 12 & 13) associated with its subsidence during the Triassic and
---
-------
- ------
---
-
N. A. HARBURY
24
ET AL.
North MALAY AUSTRALIA KIMBERLEY
Timor-Tanimbar
BLOCK
km
Crotonic
0
basin
PENINSULA
RAUB-BENTONG FAULT SYSTEM Main Range
,{'fl
l ~I
East Coast Range Trench
100
Permo-Triassic
Time
0
Fig. U. Tectonic cartoon to illustrate interpretation of the crustal structure related to the Permo- Triassic volcanics and granites of Peninsular Malaysia assuming its palaeotectonic position on the northern edge of Australian Gondwana . Zircon ages (Liew & McCulloch 1985) in the granites reveal notably different suites in the two range which supports the view that the Raub-Bentong Line was an important Late Palaeozoic tectonic suture.
MALAYA
MALAYA MAIN RANGE •
-;
~~(13 Ill I (
EAST COAST RANGE MAIN RANGE R·B·F
,+h
-~~
+
+,
EAST COAST RANGE CENTRAL
BASIN +
~;S-:":}~~~~1 :::{~1~~ ~:- LITHOSPHERE
150km
/ 1}--!.)~:.,~-~1(1'!. \:~:-1_ 1:
150 km
0
50
lOOkm
IJurassic
Fig. 13. Schematic interpretation of Peninsular Malaysia illustrating Permo- Triassic magmatic activity linked to Late Permian and Triassic subduction of Tethys arid related crustal extension and thinning (Ryall 1982) producing the central basin (Tan 1984). RBF, Raub Bentong Fault.
j
MESOZOIC
STRUCTURES:
Jurassic could be related to arc and back-arc extensional processes associated with subduction at an active continental margin of eastern Gondwana (Audley-Charles 1983). In view of reports of Triassic-Jurassic major orogenic compression in central Thailand, NW Laos, west and southern Cambodia, NE Laos and North Central Vietnam (Workman 1975), the absence of these deformational events from Peninsular Malysia would seem to indicate that Peninsular Malayasia, western Thailand, and Burma were not attached to central and east Thailand and Indochina until after the Middle Jurassic. Some Permian and Lower Mesozoic invertebrate faunas of Peninsular Malaysia are said to have affinities with those of Indochina and N Tethyan provinces (Matsuda 1985; Metcalfe 1988). Furthermore Metcalfe (1988) pointed out that the Permian flora of his East Malaya block (Fig. 2) is Cathaysian in character. These fauna! and floral affinities would seem to indicate that the East Malaya and Sibumasu blocks had detached from Gondwana and were part of Asia by Middle Triassic times at the latest. However, Audley-Chades (1987) has suggested that Cathaysian flora may not have been geographically isolated from Gondwana, as is generally supposed. It may have been present in the warmer maritime parts of eastern Gondwana (AudleyCharles 1987) where warm Tethyan ocean currents could have influenced the climate (Robinson 1973). Furthermore, the mid-Permian brachiopod fauna being present on the Indochina block and Peninsular Malaysia offers an explanation for the mid~Permian reconstruction of eastern Tethys. Metcalfe (1988, fig. 10) shows Indochina located offshore Gondwana in eastern Tethys in the late Palaeozoic as has been also suggested by Audley-Charles (1983). Another explanation for the apparently conflicting fossil evidence is that the Malaya-Thailand Peninsula may be a composite of tectonic terranes which coalesced in midCretaceous times. Alternatively it may be that the Malay-Thailand Peninsula was rifted from Gondwana in the Late Palaeozoic, as Sengor (1986) and others have suggested, but this rifting may not have involved much new oceanic spreading until the Jurassic. Thus they may not have become distinct palaeogeographic provinces until the Late Jurassic. Palaeomagnetic data from the critical land areas will help resolve these problems. The palaeocurrent measurements in the Jurassic to Lower Cretaceous Tembeling Group indicate a major fluvial distributary system flowing parallel or sub-parallel to the central basin long axis which must have been roughly parallel to the proposed Jurassic rifting of the continental margin. Audley-Charles (1983, 1984, 1988) has argued that the present west coast region of Peninsular Malaysia was attached to northern Australia during the Triassic and early Jurassic, while a belt of calc-alkaline volcanoes and eastern Peninsular Malaysia occupied the adjacent South Tethyan margin. Vink et al. (1984) have argued that relatively long narrow continentalblocks, such as the Burma-W. Thailand and Peninsular Malaysia (or Sibumasu terrane) can be expected to result from lithospheric rifting. The mid-late Cretaceous compressional deformation event probably activated central basin inversion. It may have been related to transpression, associated with oblique convergence and collision between the northward moving Peninsular Malaysia (Sibumasu} continental block and the eastern Thailand- Indochina continental block of mainland Asia. This suggestion of oblique convergence between the
MALAYSIA
25
Malay (Sibumasu) block and the Asian mainland has been postulated by many palaeogeographical reconstructions, whatever date is suggested for the convergence, (e.g. Parker & Gealey 1985; Sengor 1984, 1986; Sengor et al. 1988; Audley-Charles 1988).
Conclusions (1) Triassic and Jurassic crustal extension led to the s·eparation of the Permo-Triassic granitic belts presently exposed in the main ranges. Subsidence and marine sedimentation in the central basin during the Triassic was followed by Jurassic and Cretaceous continental sedimentation associated with crustal attenuation. (2) The Raub-Bentong line was an important fault zone which controlled sedimentation during the Mesozoic. We suggest that this line has not acted as a major tectonic suture since Late Palaeozoic times. (3) There is no evidence for Triassic compressional deformation in the Peninsula. (4) The Mesozoic strata in the central basin of Peninsular Malaysia have a common structural style, degree and orientation of folding, and axial plane cleavage. Palaeozoic strata, especially the Carboniferous rocks have multiple phase folds and regional metamorphism attaining low amphibolite fades conditions, quite unlike anything seen in the Mesozoic rocks. These observations demonstrate that there were two important compressional events in Peninsular Malaysia: one major orogeny in the late Permian, and another much less severe deformation in the mid- to late Cretaceous. We are grateful to S. P. Sivam for making facilities at the Geology Department, University of Malaya available to us. H. Mohamad of Universiti Kebangssall Malyasia, encouraged the expedition and the participation of M. A. Abu Bakar who accompanied us into the field. A. R. Samsudin was responsible for some of the palaeomagnetic sampling . J. Cobbing helped with discussion of the Malaysian and related granites. We are grateful to B . Roberts, N. Haile and C .. Jones for constructive reviews of the manuscript. S. Hirons was responsible for the XRD work on the Tembeling Group rocks. Sarawak Shell Berhad and the University of London Central Research Fund gave us financial support for the fieldwork. Sarawak Shell Berhad also provided palynological identification.
References ACHACHE,J. & COURTILLOT,v . 1985. A preliminary Upper Triassic palaeomagnetic pole for the Khora! plateau (Thailand): consequences for the accretion of Indochina against Eurasia. Earth and Planetary Science Letters, 73, 147-57 . AHMAD, J. 1976. The geology and mineral resources of the Karak and Temerloh areas, Pahang. Geological Survey of Malaysia, Memoir 15. AUDLEY-CHARLES, M. G. 1983. Reconstruction of eastern Gondwanaland. Nature, 306, 48-50. 1984. Cold Gondwana, warm Tethys and the Tibetan Lhasa block. Nature, 310, 165-166. - ·- 1987. Dispersal of Gondwanaland: relevance to the evolution of the angiosperms. In: WHITMORE,T. C. (ed.) Biogeography of the Malay Archipelago. Oxford Monographs on Biogeography, 4, Clarendon Press, Oxford. 5-25. 1988. Evolution of the southern margin of Tethys (North Australian M. region) from early Permian to late Cretaceous. In: A UDLEY-CHARLES, G. & HALLAM,A. (eds), Gondwana and Tethys. Geological Society, Special Publication 37, 79-100. BARR, S. M., MACDONALD,A. S. & HAILE, N. S. 1978. Reconnasissance palaeomagnetic measurements on Triassic and Jurassic sedimentary rocks from Thailand . Bulletin of the Geological Society of Malaysia, 10, 53-62. BIGNALL , J. D. & SNELLING,N. J. 1977. K-Ar age on some basic igneous
26
N. A. HARBURY
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Received 17 August 1988: revised manuscript accepted 30 Jun e 1989
