Structure of Sumatra and its implications for the tectonic assembly of Southeast Asia and the destruction of Paleotethys - Barber 2009.pdf

Island Arc (2009) 18, 3–20Thematic Article Structure of Sumatra and its implications for the tectonic assembly of Southeast Asia and the destruction of Paleotethys ANTHONY J. BARBER1* AND MICHAEL J. CROW2 Southeast Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK (email: [email protected]), and 228A Lenton Road, The Park, Nottingham NG7 1DT, UK 1 Abstract It is now generally accepted that Southeast Asia is composed of continental blocks which separated from Gondwana with the formation of oceanic crust during the Paleozoic, and were accreted to Asia in the Late Paleozoic or Early Mesozoic, with the subduction of the intervening oceanic crust. From east to west the Malay peninsula and Sumatra are composed of three continental blocks: East Malaya with a Cathaysian Permian flora and fauna; Sibumasu, including the western part of the Malay peninsula and East Sumatra, with Late Carboniferous–Early Permian ‘pebbly mudstones’ interpreted as glaciogenic diamictites; and West Sumatra, again with Cathaysian fauna and flora. A further unit, the Woyla nappe, is interpreted as an intraoceanic arc thrust over the West Sumatra block in the mid Cretaceous. There are varied opinions concerning the age of collision of Sibumasu with East Malaya and the destruction of Paleotethys. In Thailand, radiolarites have been used as evidence that Paleotethys survived until after the Middle Triassic. In the Malay peninsula, structural evidence and the ages of granitic intrusions are used to support a Middle Permian to Early Triassic age for the destruction of Paleotethys. It is suggested that the West Sumatra block was derived from Cathaysia and emplaced against the western margin of Sibumasu by dextral transcurrent faulting along a zone of high deformation, the Medial Sumatra Tectonic Zone. These structural units can be traced northwards in Southeast Asia. The East Malaya block is considered to be part of the Indochina block, Sibumasu can be traced through Thailand into southern China, the Medial Sumatra Tectonic Zone is correlated with the Mogok Belt of Myanmar, the West Burma block is the extension of the West Sumatra block, from which it was separated by the formation of the Andaman Sea in the Miocene, and the Woyla nappe is correlated with the Mawgyi nappe of Myanmar. Key words: Malay peninsula, Myanmar, Paleotethys, Permo–Triassic, Sibumasu, West Sumatra block. INTRODUCTION Southeast Asia is considered to be made up of a number of continental blocks or terranes (Fig. 1), which separated from the northern margin of the Gondwana supercontinent in the Late Paleozoic, and accreted to the southeastern margin of the Eurasian continent later in the Late Paleozoic or in *Correspondence. Received 10 May 2007; accepted for publication 19 February 2008. © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd the Mesozoic. According to the synthesis proposed by Metcalfe (1996, 2005), the Indochina block, forming the core of Southeast Asia, separated from Gondwana in the Late Devonian, moved northwards driven by the expansion of Paleotethys, to collide and amalgamate with the South China block in the Early Carboniferous. East Malaya, the eastern part of the Malay peninsula, forms part of the Indochina block and, like the rest of the block, is characterized by a Cathaysian Permian flora and fauna. The Sibumasu terrane, doi:10.1111/j.1440-1738.2008.00631.x with the opening of Mesotethys in the Early Permian. it is worth re-iterating the evidence put forward originally by Metcalfe (2000) from the Malay peninsula. interpreted as glaciogenic diamictites or tillites (Stauffer & Lee 1986). 2005) recognition of microcontinental terranes among the Woyla terranes has been disputed (Wajzer et al. Barber & Crow 2003). 1). Sibumasu moved rapidly northwards to collide and amalgamate with the Indochina block in the Late Permian. J. Metcalfe’s (1996. a separate West Sumatra block with Cathaysian affinities has been recognized (Hutchison 1994.W. Barber & Crow 2003). Subsequently. 1995. separated from northern Gondwana. These issues are addressed in this paper. the timing of the separation from Gondwana of the West Burma block and its accretion to Asia needs to be determined. 2005) is disputed by Japanese radiolarian researchers from evidence of Middle Triassic Paleotethyan Oceanic sediments in the northwest Malay peninsula and Thailand (Sashida et al.b. Also. Kamata et al. is characterized by ‘pebbly mudstones’. with the addition of supporting evidence from the Langkawi Islands and Bangka Island. which was then in a glacial environment. BORNEO RA R INDIAN PLATE Te ct o re utu bS d ia at ra ST Me EA -R a u Bentong ANDAMAN SEA 0° SOUTH CHINA SEA CH ? 10° 120° SOUTH CHINA AO SIM lt k Be M og o A RM W E S T BU INDIA 100° which includes the western part of the Malay peninsula and eastern Sumatra. Barber and M. and were accreted to the southwest margin of Southeast Asia in the mid Cretaceous. Crow 100° 20° IN DO IN A SIBUMASU um LA OY ine YA LA W nic L MA lS TE W ES TS UM NE AT S RA S. There are several contentious issues in that synthesis: the timing of the collision of Sibumasu with Indochina with the destruction of Paleotethys as proposed by Metcalfe (1996. it is necessary to determine the origin and time of emplacement of this block. Since Metcalfe’s (1996) synthesis.4 A. © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd Fig. 1 Continental tectonic blocks in Southeast Asia (after Metcalfe 2005). TIMING OF COLLISION BETWEEN SIBUMASU AND EAST MALAYA Since there is discussion concerning the timing of the collision between the Sibumasu and East Malaya blocks and the destruction of Paleotethys. incorporated in the Woyla terranes and West Burma (Fig. 1991. separated from Gondwana in the Late Triassic to Early Jurassic. . Barber 2000. 2000a. Metcalfe (2005) suggested that the Sikuleh terranes. 2002). J. Carb. ages.Carb. U.Devonian L. 2 (a) Peninsular Malaysia and the Bentong–Raub Suture Zone with locations of dated bedded radiolarian chert. M.-U. L. A characteristic feature is the occurrence of bodies of mélange containing randomly arranged blocks of chert. 50 melange POS MERING granite/schist contact zone melange schist chert L. From that study Metcalfe (2000) concluded that Paleotethys opened in the Late Devonian and that its ocean-floor sediments were incorporated into an accretionary complex immediately prior to the collision of Sibumasu with the western margin of East Malaya in the Late Permian.Dev. 2).Perm. Metcalfe (2000) describes the Bentong–Raub Line as a 13 to 20-km wide zone of imbricated ribbon-bedded cherts and schists containing elongated blocks of serpentinized mafic and ultramafic rocks. but no radiolarian cherts of Triassic age were found. WEST U. Kuala Lumpur L.Triassic 5°N 5° Semanggol Section in B EAST MALAYA L/U.Carb. The bedded cherts are interpreted as deep-sea oceanic sediments deposited on the Paleotethys floor. Jengka BENTONG L. the (a) Middle Triassic 102°E INDOCHINA BLOCK Middle Permian Lower Permian Alor Star L. L. limestone. MALAYA RAUB L.Permian.Permian.Tectonic assembly of Southeast Asia Hutchison (1975) identified the north–south Bentong–Raub Line separating the eastern and western parts of the Malay peninsula as the suture zone marking the site of the collision and amalgamation of Sibumasu (Sinoburmalaya: Hutchison 1994) with the East Malaya block (Fig. The granite is a member of the Main Granite Suite giving Late Triassic to Early Jurassic. In places. Cobbing et al. Metcalfe et al. L.Devonian SIBUMASU Semanggol Formation Bentong-Raub Suture Zone 0 (b) 100 150km 102°E 101°35’ 4°45’ schist serpentinite thrust Triassic Granite roa d Singapore 101°45’ 101°40’ schist ) an se Vi t( er ch Fig. 230–270 Ma. and volcanic and volcaniclastic 5 rocks in a fine-grained mud and silt matrix. (1992).Carb. after Metcalfe (2000).Carb. (b) map along forest road section across the Bentong–Raub Suture Zone near Pos Mering. (1999) found that radiolaria from the bedded cherts ranged in age from Late Devonian to Late Permian (Fig. U. 2). Permian) mudstone 0 5km © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd .Carb. The evidence from the radiolarian cherts and the intrusive granites provides constraints for the age of the collision as Late Permian to Early Triassic. implying imbrication or isoclinal folding of the chert. and Middle Triassic ribbon-bedded cherts and lensoid conodontbearing siliceous limestones in the lower Chert Member of the Semanggol Formation near Alor Star in the northwest Malay peninsula (Ahmad et al. representing the unconformity. In the north they consist of slates and schists with isoclinal folding on northwest–southeast axes. Kimura and Jones (1967) showed Lower Paleozoic rocks thrust westwards over the Upper Paleozoic. According to Ko (1986) the oldest rocks on the island are of Permo–Carboniferous age. 1995. and limestones. corresponding to the Bentong– Raub suture of peninsular Malaysia. limestones. and broken by eastward-dipping thrusts. which includes Permian pebbly mudstones (Fig. (Van Overeem 1960). in Perlis. Triassic foraminiferal–algal limestones and possibly the associated radiolarian cherts were deposited in a shallower water environment. the Permo– Carboniferous rocks include ‘pebbly mudstones’. A similar constraint is provided by the relationships between the Permo–Carboniferous and Triassic rocks and intrusive granites on the island of Bangka to the southeast of Sumatra (Fig. In the Langkawi Islands the major thrust plane is cut by an intrusive Middle Triassic (242 ! 10 Ma) granite (Fig. In the adjacent island of Billiton a poorly preserved Cathaysian flora was found in shales and a limestone contained Schwagerina sp. In the western Malay peninsula. therefore. Late Permian. black pyritic shales. following the deposition of the complete sequence in the lower Chert Member of the Semanggol Formation. northwest Malay peninsula. J. While Permian cherts and limestones are interpreted as having been deposited in a pelagic environment. However. north of Alor Star. marking the transition between the shelf and slope sediments of the Sibumasu terrane and the deposits on the floor of the Paleotethys Ocean. Metcalfe (2000) therefore interpreted the Permian cherts and limestones as pelagic sediments deposited on the Paleotethys ocean floor and which were imbricated into an accretionary complex. . and units ranging in age from Lower Paleozoic to Upper Paleozoic have been identified. The Carboniferous and Permian rocks are interpreted as Paleotethyan ocean-floor sediments imbricated into an accretionary complex. 2). Barber and M. distal turbidites. Crow Bentong–Raub Zone is intruded by granites. andesites. One of the limestone blocks yielded Permian fusulinids (De Roever 1951). Confirmatory evidence for the timing of this collision event is provided by radiolarian studies of Middle Permian. On the other hand. providing a further constraint on the age of the collision of Sibumasu with East Malaya. Meor and Lee (2005) described a steeply eastward-dipping Silurian to Carboniferous sequence folded with western vergence. (1995) reported that the Permian cherts dip steeply and are tightly folded. The latest Permian and the Early Triassic are missing. 3). which have been dated as having been intruded in the Late Triassic to Early Jurassic (230–207 Ma). 1987. which must have been completed before the Middle Triassic. are deformed into an accretionary complex and a foreland fold-and-thrust belt. on the geological map stratigraphic units appear to be arranged © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd randomly. at Toboali in the far south of Bangka. Sashida et al. Sashida et al. that the collision between Sibumasu and East Malaya occurred after the Late Triassic. One problem in understanding the geology of the western Malay peninsula has been that although fossiliferous units are common. J. They concluded. 3). imbricated with basalts. In their account. the Triassic cherts and overlying turbidites are only gently folded. bedded cherts. Metcalfe 2000) (Fig.6 A. Sashida et al. These structures are characteristic of foreland fold-and-thrust belts. prior to the collision between Sibumasu and East Malaya. In the offshore Langkawi Islands. representatives of the Main Range granites of the western Malay peninsula. indicating affinities between these rocks and the East Malaya block. Paleozoic (Cambrian–Permian) sandstone–limestone–shale continental margin sediments of Sibumasu and the Paleotethyan ocean floor bathyal shales and cherts. indicating that the Permian and Triassic cherts are separated by an unconformity. so that it has neither proved possible to recognize continuous stratigraphic sequences. In part this is due to the discontinuity of exposures. Metcalfe (2000) pointed out that the ages of rocks in the sequence are repeated. followed by the deposition of Middle Triassic cherts. nor to reconstruct a coherent depositional or structural basin. (1995) supposed that the Permian and Triassic rocks formed part of a continuous sequence. extending all the way from the Bentong– Raub suture to the west coast of the Malay peninsula and into the offshore Langkawi Islands. However. and turbiditic sandstones in a successor basin. 4). Also © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd . (2005) recently published a revised synthesis for the nature and origin of the structural units that comprise Sumatra and peninsular Malaysia. In eastern Sumatra. and shales with interbedded ‘pebbly mudstone’ units. and Cretaceous ages. 1992) (Fig. 4). Both groups of rocks are cut and extensively hornfelsed by granites yielding ages ranging from 250 to 200 Ma (Cobbing et al. (b) map and cross-section of the Langkawi Islands after Kimura and Jones (1967). STRUCTURE OF SUMATRA Barber et al. 0 100m Silurian 6°23’ glaciogenic diamictites characteristic of Sibumasu (Ko 1986). Middle Triassic granite cuts thrust plane.Tectonic assembly of Southeast Asia 7 (b) (a) 100° 8° 101° 1 PERLIS THAILAND LANGKAWI ISLANDS Kimura & Jones (1967) 2 6° Langkawi Islands 4° th ru st Alor Star SIBUMASU granite KEDAH 2° Meor & Lee (2005) i Ab ai ng Su Locality 1 64° Devonian HILL C 40m 20m 60° Devonian-Carboniferous se cti on 0 5km NW SE granite 1km HILL B 50° 72° 0 100m Locality 2 60m 40m Middle Triassic Granite (242±10Ma) Permo-Carboniferous (with ‘pebbly mudstones’) Lower Palaeozoic 100°2’3” Devonian-Carboniferous 6°23’2”N Sibumasu continental margin sediments in a foreland fold-and-thrust belt 70° Fig. building on earlier syntheses by Hutchison (1994) and Metcalfe (1996). Permian. Late Carboniferous to Early Permian rocks of the Tapanuli and Tigapuluh groups are quartz-wackes. 3 (a) Northwest peninsular Malaysia with maps of localities 1 and 2 after Meor and Lee (2005). given the difference in their structure. showing that the collision between Sibumasu and East Malaya had already occurred before latest Permian times. Although a stratigraphic contact has not been described. Throughout the island highly deformed Permo–Carboniferous rocks are in fault contact with gently folded Middle to Upper Triassic sandstones. the relationships between the Permo– Carboniferous and Triassic rocks must be unconformable. Jurassic. Triassic. siltstones. interpreted as glaciogenic diamictites. The pre-Tertiary rocks of mainland Sumatra are of Carboniferous. phlogopite. quartz–feldspar augen gneiss. andesites. phyllite. and rhyolites of the Palepat Formation. of Cordaites. Early Carboniferous rocks of the Kluet and Kuantan formations are quartzwackes. Vachard 1989a. The Mengkarang Formation contains roots and large tree trunks up to 2 m in length. The southern segment of the zone is less well exposed. Asselian–Yahtashian (Artinskian) age. In western Sumatra. sandstones. 2005).b). older than early Middle Permian CarboniferousEarly Permian 0 KULUR PERMISAN 213Ma Tempilang Sandstone BEBULU BATHOLITH PADING Pemali Group 'Pebbly Mudstone' Tapanuli Group 50km 106°E TOBOALI 225Ma included in the Tapanuli Group are the Alas limestones (Cameron et al. that are considered to have formed in tropical intertidal and shallow marine environments (Fontaine & Gafoer 1989. At its northern end the zone includes limestones of the Alas Formation associated with lenses of massive marble. The tree trunks do not show tree rings. mylonite. The tropical Early Permian fauna and flora of West Sumatra contrast with the Early Carboniferous temperate fauna and Early Permian glaciogenic sediments of the East Sumatra (Sibumasu) block against which it is juxtaposed. Barber et al. Permo–Carboniferous rocks in the north of the island are imbricated Paleo-Tethyan ocean floor materials and limestones with fusulinids. traceable as a zone of highly deformed rocks including schists and gneisses. after Ko (1986). while Permo–Carboniferous rocks at Tobaoli in the south contain ‘pebbly mudstones’ and are related to Sibumasu. 2005) (Fig. The intercalated limestones contain oncolites and an abundant fusuline fauna of © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd 3° 107° Fig. also indicating a tropical environment (Ueno et al. Permian rocks include basalts. and a cyclic sequence of conglomerates. and conodonts identified as a temperate fauna of Viséan age (Metcalfe 1983. In other parts of the zone rock types include slate. and shales containing fossiliferous limestone lenses (Fig. MEDIAL SUMATRA TECTONIC ZONE The contact between the Sibumasu block and the West Sumatra block is marked by the Medial Sumatra Tectonic Zone (MSTZ). as much of its course is covered by Tertiary and Quaternary sediments. 5). Crow BELINYU 251Ma PENANGAS 252Ma 106°E BANGKA ISLAND KLABAT BATHOLITH KELAPA Very Late Permian to Early Jurassic granites 2° 2°S MENUBING 200Ma TEMPILAN MANGOL SUMATRA Faults Triassic Slates. 4 Geological map of Bangka Island. comprising the ‘Jambi flora’ (Jongmans & Gothan 1925. 1935). and southern China (VozeninSerra 1989). pillow lavas etc. 1982) containing corals. Indochina. 2006). J. together with the western Malay peninsula and the Langkawi Islands. cherts. . sandstones. 5). and pisolites.. extending the whole length of Sumatra from the Andaman Sea to Palembang. quartzite. related to the East Malaya block.8 A. sometimes in situ. are regarded as part of the Sibumasu block. a distance of 1760 km (Hutchison 1994. and the plant remains are characteristic of the Cathaysian flora found in East Malaya. and cataclasite. 1980). Barber and M. and Taeniopteris with abundant plant fragments and leaves in siltstones. East Sumatra. and chiastolite. all overlain unconformably by Middle Triassic sandstones and intruded by very Late Permian to Early Jurassic granites. and shales interbedded with limestones and thin coals of the Mengkarang Formation (van Waveren et al. brachiopods. Fontaine 1989). scapolite–calc-silicate schist and garnetiferous augen gneiss (Cameron et al. However. biotite–andalusite hornfels with cordierite. indicating that they grew in a tropical environment. biotite–garnet–sillimanite schist. intertidal algal mats. the latter with horizontal slickensides. Cathaysian–Tethyan affinity and Early Permian. and graphitic marble. migmatite. J. Calamites. oolites. The limestones include fossil corals of Viséan age. sometimes forming reef structures. see Figure 8 for location. 5 Distribution of the pre-Tertiary stratigraphic and tectonic units in Sumatra. a foliated syntectonic granitoid. Suwarna et al. and quartz schists. The Pawan Member is intruded by the Rokan granite. based on data from the 1:250 000 geological map sheets published by the Indonesian Geological Research and Development Centre. 2005). Batumilmil. 2005). 2005). Further to the southeast it can be traced beneath Tertiary sediments only through oil company boreholes (De Coster 1974. 1983). between Pekanbaru and Lubuksikaping it is represented by intensely folded muscovite. The MSTZ does not represent a suture zone formed from a subducted oceanic crust as it does not contain any significant ophiolitic components. The MSTZ is interpreted as a major transcurrent shear zone. As is the case with the present Sumatran Fault and the Quaternary volcanic arc. lighter tones indicate that the pre-Tertiary is overlain by Tertiary and Quaternary sedimentary and volcanic rocks. in © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd . Mengkarang 95°E 0° Tuhur Triassic 2° 1° Pawan Rokan Granite (189Ma) Palepat 2° Mengkarang Kluet. mapped as the Pawan and Tanjungpuah members of the Kuantan Formation (Clarke et al. Tigapuluh Group (pebbly mudstones) MEDAN Batumilmil 3° 200km EAST SUMATRA BLOCK (SIBUMASU) Kaloi k oro Kl h Bo Alas 4° 100 3° 2° SIBOLGA 1° WOYLA NAPPE JurassicWoyla Group Cretaceous Island arc and accretionary complex 0° LUBUKSIKAPING PEKANBARU Tanjungpuah Ku WEST SUMATRA BLOCK (CATHAYSIAN) 1°S PADANG an ta Carboniferous 96° Tigapuluh Hills n 1° Tuhur unconformity Permian Palepat. chlorite. 1991. This and other deformed granitoids of Triassic–Jurassic–Early Cretaceous age. are interpreted as syntectonic intrusions related to a magmatic arc formed during eastward subduction beneath the western margin of the West Sumatra block and emplaced in an active shear zone (Barber et al. The occurrence of syntectonic granitoids and of a distinct tin anomaly related to the zone. 1982. carbonate. Bandung (after Barber et al. Eubank & Makki 1981). Darker tones indicate areas of outcrop. Kualu Triassic unconformity Permo-Carboniferous Alas Limestone (Visean) (Tapanuli Group) Bohorok. However. 1991). Barber et al. dated by the K/Ar method at 189 ! 2 Ma (Early Jurassic) (Rock et al. tremolite.9 Tectonic assembly of Southeast Asia 95°E 96° BANDA ACEH 97° 98° 99° 104° 103° 102° 105° 106° PRE-TERTIARY TECTONIC UNITS IN SUMATRA Situtup 5°N 101° 100° 0 ue t Lake Toba Kualu MSTZ Medial Sumatra Tectonic Zone 2° 5° 4° Kaloi. analogous to the present Sumatran Fault zone. (Barber & Crow 2003. surrounded by metamorphic aureoles. 5). Barber et al. Further to the southeast. composed of intensely quartz-veined slates and phyllites (Simandjuntak et al. the intense deformation of the rocks in the zone indicates that it represents a major shear zone between the Sibumasu and West Sumatra crustal blocks. indicate that the MSTZ is a structure of crustal scale. Kuantan (with Visean limestone) 97° 98° 99° 100° 101° 102° 103° 104° PALEMBANG 106° Fig. 1982) (Fig. the zone is represented by the Gangsal Formation of the Tigapuluh Group on the southwestern side of the Tigapuluh Mountains. determined from stream sediment samples (Stephenson et al. 2005). in present day co-ordinates. Evidently in the Triassic the West Sumatra block formed the southwestern margin of Southeast Asia passing out westwards. 1983). identified by fossils from spot samples (Bennett et al. including cherts. There is a problem in accounting for present position of the Cathaysian West Sumatra block. Metamorphic rocks derived from different crustal depths are juxtaposed along the MSTZ. and Tuhur basins.g. overlain by rhythmites composed of finely-bedded alternating tur© 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd biditic sandstones and shales. In the western part of the Malay peninsula and in Sumatra shallowwater carbonates were deposited on horst blocks. Turner 1983. 5). and no unconformable contacts have been recognized. The deposits of the Tuhur Basin. Direct evidence of an unconformable relationship between the Permian and Triassic is provided by limestone clasts containing Middle Permian fusulinid fossils in Middle Triassic sediments (Tuhur and Limau Manis formations: Silitonga & Kastowo 1975. The MSTZ incorporates slices of the western margin of the Sibumasu block. Kualu and Tuhur formations). and Batumilmil formations) (Fig. (1991) as derived from the carbonate cap of a seamount in Mesotethys. They found that all Permian and Triassic stages are represented. It is inferred that by Middle Triassic times the Sibumasu and West Sumatra blocks were substantially in their present relationship. separated from the Cathaysian East Malaya block. J. sediment-starved graben far from a clastic sedimentary source (Barber et al. occurring in both the West Sumatra and Sibumasu terranes. carbonate deposits were formed on horst blocks formed by the extension of continental crust. Situtup and Batumilmil formations) or a chert–clastic facies (e. and have been re-metamorphosed in contact aureoles around the syntectonic granitoids. Triassic carbonates were sometimes deposited on top of Permian carbonates. 1981. A magmatic arc related to subduction beneath the western margin of the West Sumatra block was superimposed across the MSTZ in Late . The MSTZ is interpreted as a major transcurrent fault along which the West Sumatra block was translated from its original position as part of Cathaysia and emplaced against the western margin of the Sibumasu block (Hutchison 1994. finer at the base and coarsening upwards. Barber & Crow 2003). Barber & Crow 2003) (Figs 6. with cherts at the base. including possibly the basement as well as its sedimentary cover. Kualu. particularly along their margins. Barber and M.8). Cameron et al. J. Since Middle to Late Triassic sediments of the same facies rest unconformably on deformed Carboniferous and Permian rocks forming the Sibumasu and West Sumatra blocks on either side of the MSTZ. Barber et al. and lying outboard the Sibumasu block (Barber & Crow 2003. However. 2005). accumulated in the Semantan Basin. the MSTZ is considered to have been initiated in latest Permian or Early Triassic times. were deposited in the intervening subsided basins. Kaloi limestone. was interpreted by Wajzer et al. with granitoid magma chambers emplaced in an active shear zone. apart from the uppermost Permian and the lowermost Triassic. within the shear zone itself (Figs 5. a feature of major transcurrent fault zones. and also occur. In Europe as in Asia. The chert–clastic facies resembles the Semanggol Formation of West Malaya. while to the west the eroded sediments. only gently folded.10 A. but the relationships between the Permian and Triassic carbonates have not been described. 1982. were deposited on the southwestern margin of the West Sumatra block. The breakup of the European margin of Tethys during the Triassic–Jurassic (Bernouilli & Jenkyns 1974) provides an analogy for the tectonic environment on the southwestern margin of Southeast Asia during the Triassic.g. The distribution of these facies is plotted on Figure 8 and is interpreted as representing a submarine horst and graben structure which extended across the whole of Sumatra and the Malay peninsula. Sometimes Permian and Triassic limestones occur in the same group of outcrops (Situtup limestone. Crow the Jurassic–Cretaceous the MSTZ was also the site of an active magmatic arc. including radiolarites. 2005). which perhaps formed local elevations of the seafloor. PERMO–TRIASSIC UNCONFORMITY Fossiliferous Permian and Triassic rocks are widespread throughout Sumatra. The chert facies was deposited in the intervening Semanggol–Mutus. Middle to Upper Triassic sediments in Sumatra are either carbonates (e.7). A limestone block containing Triassic foraminifera in mélange in the accretionary complex which forms part of the Woyla Group emplaced on West Sumatra during the mid Cretaceous. In Malaya the easternmost horst appears to have been an area of uplift and erosion related to the intrusion of granites. while pelagic deposits. onto the Mesotethys Ocean floor. Fontaine and Gafoer (1989) made a systematic study of all the outcrops and their fossil content. WOYLA NAPPE The West Sumatra block is overlain to the west by the Woyla Group. Subsequently. (2005. composed of an east-facing accretionary complex of ophiolitic ocean floor material and pelagic and volcaniclastic sediments and mélanges. This interpretation has been contested. since the zone is displaced by 200 km along the Sumatran fault zone. Jurassic–Early Cretaceous times. and arguments have been put forward for regarding the Woyla Group as an intraoceanic island arc developed offshore West Sumatra on the Triassic– Cretaceous oceanic crust of Mesotethys (Wajzer et al. 6 Stratigraphic relationships between tectonic and stratigraphic units in Sumatra and East Malaya. the Woyla Group collided with. and was thrust over. by the subduction of the intervening part of the Mesotethys Ocean floor. providing con© 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd . and may have continued into the Cenozoic. the Natal and Bengkulu microcontinents were postulated among the ‘Woyla terranes’ further south (Metcalfe 1996). Barber & Crow 2003). West Malaya–East Sumatra (Sibumasu) blocks. the Medial Sumatra Tectonic Zone and the West Sumatra block and that all tectonic units are cut by Triassic and younger granites (after Barber et al. interpreted as a Late Jurassic to Early Cretaceous volcanic arc with carbonate fringing reefs. the West Sumatra block to form the Woyla nappe (for a full discussion of this controversy see Barber et al. and similar microcontinental fragments. 52–53). 2005). 1980). Jurassic–Cretaceous syntectonic granites in the MSTZ indicate that minor movements along the shear zone continued through the Mesozoic. and basaltic–andesitic volcanic rocks. Triassic rocks in similar facies extend across the East Malaya. pp. 1991. Because the volcanic arc is intruded by the Sikuleh granite it was interpreted initially as an Andean Arc. developed on a fragment of continental crust identified as the Sikuleh microcontinent (Cameron et al.Tectonic assembly of Southeast Asia 11 Fig. and the emplacement of the nappe resulted in the deformation and metamorphism up to amphibolite grade of rocks in West Sumatra. The overthrust assemblage includes carbonates of Albian–Aptian age. ranging in age from Carboniferous to mid Cretaceous. As has already been described. Crow Fig. Mitchell et al. including Late Carboniferous–Early Permian diamictites (Mergui Group) and so forms part of the Sibumasu (Shan–Thai) block (Fig. The marbles included in the Mogok Belt are regarded as the metamorphosed equivalents of the Cambrian to Devonian limestones of the plateau succession in the Shan Plateau to the east. can be traced from Bangka Island through eastern Sumatra. to Early Permian succession. 2005). and marbles with (?) Early Carboniferous fossils (Mitchell et al. As has already been mentioned. Curray 2005) (Fig. J. © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd but. the Sibumasu block. Barber and M. Note the apparent absence of fossils corresponding to the uppermost Permian and the lowermost Triassic stages. The Mogok Belt is intruded by . mylonites. Here. J. data taken from Fontaine and Gafoer (1989). which contains high to low grade metamorphic rocks. the continental crust has been extended and replaced by oceanic crust. and that clasts containing derived Middle Permian fusulinids are incorporated in Middle Triassic stratigraphic units (after Barber et al. the Shan Plateau is composed of a Proterozoic. peninsular Thailand and eastern Myanmar (Burma) to Yunnan in southwest China. RELATIONSHIP BETWEEN SUMATRA AND SOUTHEAST ASIA Crustal blocks identified in Sumatra and the Malay peninsula can be recognized in the mainland of Southeast Asia to the north. 9). 2007). the East Malaya block is considered to be part of the Indochina block. (2007) suggested that the Mogok Belt represents the deformed and uplifted margin of the Shan–Thai (Sibumasu) block. 2005). migmatites. 1979. Cambrian. the western part of the Malay peninsula. since the Miocene. separating western Myanmar from Sumatra by some 460 km (Curray et al. characterized by Permo–Carboniferous diamictites.12 A. to the west. the northward continuity of structural units is interrupted by the Andaman Sea. *. 7 Paleontology of Permo–Triassic stratigraphic units in Sumatra. fossils diagnostic of these stages occur in these areas. 2005). 1400 km long. 1). The western margin of the block is marked by the Mogok Metamorphic Belt. subduction polarity was reversed and these granites are related to eastward subduction of Mesotethys beneath West Sumatra and the accreted island arc (Barber 2000. Barber et al. Evidently following collision. straints on the age of collision (Barber et al. In eastern Myanmar. The Woyla nappe is intruded by Late Cretaceous and younger granites. where the carbonates were deposited on the horsts. which is part of the Sibumasu block. from which it became separated by the development of the Andaman Sea. northern Myanmar (Oo et al. The Karmine and West Sumatra faunas both include fusulinid genera which also occur in South China. 2007). 225) and formerly termed Mount Victoria Land (Mitchell 1989). and therefore. has no fusulinid genera in common with the fauna of the Karmine area. and the Murghabian fusilinid faunas from Situtup. based on the distribution of the carbonate and the chert–clastic facies. Myanmar (Garson et al. In Myanmar the West Burma block lies to the west of the Mogok Belt. belong to the Cathaysian faunal province. This correlation is supported paleontologically by a Middle Permian (Murghabian) fusulinid fauna described from limestones at Karmine. and the cherts and rhythmites were deposited in the intervening sediment-starved graben (after Barber et al. In Figure 10 the fusulinid fauna from Karmine is compared with the faunas of the Thitisipin limestone in the Shan Plateau. © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd . If the correlation of the Mogok Belt with the MSTZ is accepted. it raises the possibility that the West Burma block is the north- BINTAN KUNDUR PEKANBARU Horst Block (land area) 300km KUALA LUMPUR Sibaganding Fig. and Rugoschwagerina sp. 2005). In Sumatra the West Sumatra block lies to the west of the MSTZ. and Silungkang in West Sumatra (Fontaine & Gafoer 1989). 2002) (Fig. The Mogok Belt is the same age and has similar characteristics to the MSTZ (although the gemstones found in the northern part of the Mogok Belt have not been recorded in Sumatra). 1975. 200 13 LINGGA SINGKEP BANGKA Belinyo PALEMBANG BENGKULU BANDAR LAMPUNG ward extension of the West Sumatra block. The West Burma block. The Thitispin limestone Formation. while Parafusulina sp.Tectonic assembly of Southeast Asia Situtup TAKENGON KU U AL Kaloi LAKE TOBA MEDAN AS IN B Batumilmil N SI H BA HIG YA ALA T M ault N TA EAS ir F N A Leb n ta SEM Hamu H SI G HIGKer BA HI E ) ne ANG RA Zo MAIN R US T UR AT nic MU H M ecto OL SEMANGG TU SUtra T L ma IA l Su ED dia M (Me BANDA ACEH 100 0 LANGKAWI Kodiang MALACCA Cubadak N Fa ul t Sawahlunto PADANG JAMBI N SI BA Horst Block (shallow marine with carbonate banks) Sediment-starved graben (with cherts) syntectonic granitoids (augen gneiss) which have yielded Middle Jurassic isotopic ages and by undeformed granitoids of Late Cretaceous and younger ages (Mitchell et al. 2004. especially along their margins. The West Burma block is largely covered by Cenozoic sedimentary and volcanic rocks. as metamorphic rocks overlain by Triassic turbidites are exposed in Mount Victoria and a late Cenozoic high-K calc-alkaline volcanic arc was extruded through it. 8 Middle to late Triassic paleogeography in the Malay peninsula and Sumatra. but is underlain by continental crust. defined by Hutchison (1989. p. 9). occur both at Karmine and in the Situtup Formation and the Muarasipongi area in the West Sumatra block. Muarasipongi. 2002. Mitchell et al. 2007). is bounded by the Sagaing strike–slip fault in the east and the Indo–Burman Ranges to the west. Oo et al. A submarine horst and graben structure is postulated. Mica schists and gneisses in the eastern Indo–Burman Ranges. mudstones. Jade Belt . along the eastern margin of the Indo–Burman Ranges Mitchell (1993) described the Mawgyi andesites as a belt of basaltic andesites and basaltic pillow lavas. The Mawgyi andesites and associated rocks were emplaced on the western margin of Myanmar as the Mawgyi nappe in the mid Cretaceous. the Katha–Gangaw Range and in the Jade Mines area may represent the metamorphic footprint of the © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd Fig. Mawgyi nappe. as has been suggested for metamorphic rocks in a similar relationship to the Woyla nappe in West Sumatra (Barber et al. which is interpreted as a Jurassic–Cretaceous intraoceanic arc. which was emplaced on the western margin of the West Sumatra block in the mid Cretaceous (Cameron et al. Barber and M. Although no cherts are recorded amongst these turbidites they occupy a similar position along the western margin of the West Burma block to the Triassic Tuhur . The arc is duplicated in the Upper Irrawaddy Basin by dextral movements of about 300 km along the post-Miocene Sagaing Fault (Fig. which are deformed. this correlation is strengthened by the occurrence of Triassic turbidites. Medial Sumatra Tectonic Zone. 1980. The implication of this correlation is that the whole of western Myanmar. underlain by ophiolitic rocks. 2005. and mudstones. 28°N 24° Mogok West Burma Block = West Sumatra Block Mawgyi Nappe = Woyla Nappe Ophiolites Mandalay Mount Victoria metamorphics SHAN Mount Victoria PLATEAU 20° Fault Sagaing 20° YANGON THAILAND Cenozoic Accretionary Complexes Deformation Front Strike-Slip Fault Moulmein 16° 16° ANDAMAN SEA 0 200km 98° In western Myanmar. Wajzer et al. 1991). talc-schists. serpentinites. p. Mitchell (1993) correlated the Mawgyi andesites with the Woyla Group of western Sumatra. J. and the West Burma block of Hutchison (1989) and Metcalfe (1996). is the northward extension of the West Sumatra block. Unfortunately. However. MSTZ. Crow 94°E la Hima 96° 28° yas Karmine CHINA Ga nga w Magma tic Ar c NAPP E R. with its basement and associated accretionary complex. from the Mogok Belt to the Indo–Burman Ranges and the Mount Victoria Land of Mitchell (1989). 9). cherts. 249).14 A. J. phyllites. western Myanmar is covered largely by Tertiary and Quaternary sediments and volcanics. so that exposures of the underlying basement are rare. metamorphosed.BURMAN R INDO ANG ES MAWGYI Mogok Metamorphic Belt = MSTZ Kum INDIA 24° Shan Thai (Sibumasu) Block Late Cretaceous Granites an R. and limestones along the western margin of the West Burma block. 9 Tectonic units in Myanmar (after Mitchell 1993). and overthrust by ophiolites of the Mawgyi nappe. 2006) argued that the presence of Triassic cherts indicates that the collision of Sibumasu with the Indochina Block. Sashida and co-workers (Sashida et al. Formation on the western margin of the West Sumatra block. Ueno et al. and Tuhur basins Middle to Late Triassic cherts were deposited in extensional basins on a basement of deformed Carboniferous to Permian rocks. 10 Comparison of the Permian Fusilinacean faunas of Central Tethys (Paleotethys). with the destruction of Paleotethys. Barber 2000). Earle 1983. resting on crystalline basement and passing up into turbidite deposits in Southeast Asia. Kualu. In addition. the West Burma block and the Mawgyi nappe. (2006). in Sumatra cherts were deposited in intracontinental basins on continental crust of the Sibumasu and West Sumatra blocks. Data from Fontaine and Gafoer (1989). CONCLUSIONS AGE OF COLLISION OF SIBUMASU WITH INDOCHINA BLOCK In a series of papers on radiolarian-bearing bedded cherts and siliceous shales of Triassic age from the northern Malay peninsula. occurred after the Middle Triassic. As has been pointed out here and elsewhere. 1994). are intruded by Late Cretaceous granitoids. It is not the presence of oceanic or continental crust that controls the deposition of radiolarian © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd . have been described previously from the Cretaceous of Sulawesi and Timor (Haile et al.b. 2002. while Parafusulina sp. the West Sumatra block and South China. as has also been suggested for Sumatra (Wajzer et al. It can not therefore be taken for granted that radiolarian cherts were deposited on oceanic crust. Radiolarian cherts in a continental environment. Note that in Myanmar the fauna of the Thitispin limestone Formation has no fusulinid genera in common with the fauna in the Karmine area. In the Semanggol–Mutis. occur at Karmine and in the Situtup Formation and the Muarasipongi areas in the West Sumatra block. Myanmar. (2002). Mitchell (1993) interpreted these intrusions as due to the reversal of subduction polarity after the emplacement of the nappe. The assumption in this interpretation is that the cherts were deposited on Paleotethyan oceanic crust. 1979. 1995. Wakita et al. over 400 km to the west of the Sibumasu–Indochina collisional suture (Fig. 2000a. Ross (1995) and Ueno et al.Tectonic assembly of Southeast Asia 15 Fig. 8). Kamata et al. in the case of the Tuhur Basin. The western part of the Indo–Burman Ranges constitutes material accreted from the subduction of the Indian Ocean Plate which can be traced into the Andaman and Nicobar islands and southwards into the outer arc islands and the current subduction system offshore western Sumatra (Acharyya 2007). like the West Sumatra block and the Woyla nappe in Sumatra. southern and western Thailand. Oo et al. and Rugoschwagerina sp. 1991. In many of the occurrences of Triassic radiolarian chert studied by Sashida and co-workers in Thailand and Malaysia (e. after the Early Juras- . with the cherts being deposited on the continental slope leading down onto the deep ocean floor. but are rarely internally deformed. 1993). They proposed that these rocks form an accretionary complex within a previously unrecognized suture zone between the Shan–Thai and Indochina blocks. Helmcke and co-workers (e. but must have arrived later. (2000a. or where the supply of clastic sediment is restricted by the aridity or submergence of potential source areas. and black shales and green claystones. 1994) described deformed Carboniferous and Permian rocks overlain unconformably by undeformed Triassic rocks which were deposited in extensional half-graben (Drumm et al. In northern Thailand.16 A. J. 1995. and to changes in the level of the CCD in basins developed on continental crust. The relationship between the cherts and the other lithologies could equally well be interpreted as the result of fluctuations in the environment. interpreted as pelagic deposits. Sashida et al. fig. and sometimes closely interbedded. in areas remote from a terrigenous source. less deformed Triassic cherts and clastics may have been deposited on the top of the deformed rocks subsequent to the collision. In many occurrences of radiolarian Triassic chert studied by Sashida and colleagues (Sashida et al. together with evidence from the struc© 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd ture. They went onto to propose that the Middle Devonian cherts. The radiolarian fauna ranges across the Permo– Triassic boundary from the Dorashamian (Latest Permian) to the Middle Triassic. Kamata et al. older stratigraphic units of Carboniferous or Permian ages show evidence of internal deformation by intense shearing or cleavage. but the environmental conditions in a sedimentary basin. This locality is shown by Sashida et al. together with the Carboniferous. the increased supply of terrigenous sediment. and this view has recently been supported by Ueno et al. with terrigenous siltstones or fine sandstones. chocolate. Wonganan and Caridroit (2005) described radiolarian cherts of Middle and Late Devonian ages from the same area. But clearly the problems of the age of collision of the Sibumasu and Indochina blocks and the position of the major suture marking the destruction of Paleotethys have not yet been satisfactorily resolved and further study is required. These possibilities. 1995. should be taken into account in assessing the tectonic significance of occurrences of radiolarian chert. Barber and M. Helmcke 1984. and where the supply of carbonate material is restricted by the carbonate compensation depth (CCD) and its relationship to the depth of the basin floor. as in the major ocean basins. J. and Permian to Middle Triassic cherts in the same area (‘Fang Cherts’) represent a wide paleo-ocean which once separated the Shan–Thai (Sibumasu) and Indochina continental blocks.b). Ueno et al. It is the age of the internal deformation which indicates the age of the collision between Sibumasu and Indochina. where the cherts are interbedded with red. the cherts are associated. Crow cherts. (2006) had their conclusion concerning the age of collision of Sibumasu and Indochina based on the occurrence of Triassic cherts to argue that the West Sumatra block could not have arrived at its present position against Sibumasu in Triassic time. AGE OF EMPLACEMENT OF WEST SUMATRA BLOCK There is also a dispute concerning the timing of transcurrent movement along the Medial Sumatra Tectonic Zone and the time of emplacement of the West Sumatra block. 1) as lying within the continental Shan–Thai (Sibumasu) block or within the Inthanon Zone of Ueno (1999). It may be that the collision of the Sibumasu and Indochina blocks occurred later in northern Thailand than in peninsular Malaya and Sumatra. Sashida et al. 2000a. They interpret this association as indicating deposition on a continental margin. as it depends on the chemistry and the temperature of the seawater (cf. Heggemann et al. leading to increased erosion of the land areas. (2000a) also describe an occurrence of radiolarian cherts at Chang Dao in northern Thailand. with the destruction of Paleotethys occurred in the Late Permian or Early Triassic. 1985. Radiolaria are deposited on the seafloor where other sedimentary material is not available. and carbonates. while the Triassic rocks may be folded or faulted. Helmcke et al. by changes in sealevel or climate. (2006). and concluded that the collision of the Shan–Thai (Sibumasu) and Indochina blocks. The CCD can vary locally and over time. These deposits were clearly affected by a major tectonic event after the Middle Triassic.g. 2002). Hutchison (1994) suggested that the major phase of movement occurred in the Cenozoic. occurring as blocks in an olistostrome and with an ‘overall nappe-like structure’.g. but the sequence is inverted. Triassic–Jurassic European margins of Tethys: Bernouilli & Jenkyns 1974). 2005) Indochina separated from Gondwana in the Late Devonian and was accreted to South China in the Early Carboniferous. Warta Geologi 13. 1978. Hisada have stimulated a reassessment of some of our earlier conclusions and are much appreciated. & MILSOM J. 19th HKT Workshop. 229–42. J. Evaluation of plate tectonic models for the development of Sumatra. © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd . occur on the Sibumasu and West Sumatra blocks on either side of the shear zone. The MSTZ was dissected and displaced by the present Sumatran Fault system during the late Cenozoic. BARBER A. suggesting that all these blocks had reached something like their present relationship at that time. 6). The comments and suggestions of Guest Editor K. it will have shared the same tectonic history as Indochina and the other components of Cathaysia. with similar facies and similar sequences. 5). (2005). J. Collisional emplacement history of the Naga-Andaman ophiolites and the position of the eastern Indian suture. According to Metcalfe (1996. & CROW M. Journal of Asian Earth Sciences 29. and occasionally within it (Cameron et al. Sumatra: Geology. 2003. as has already been pointed out.. J. London.. the central segment displaced dextrally by 50 km along the Lokop–Kutacane Fault. Note on the occurrence of limestone in the Semanggol Formation. The Woyla Arc was certainly accreted to Southeast Asia as part of the Woyla nappe in the mid Cretaceous (Barber 2000. AHMAD J. IBRAHIM A. The origin of the Woyla Terranes in Sumatra and the Late Mesozoic evolution of the Sundaland margin. 1–28. At this stage both West Sumatra and West Burma would have formed part of the Indochina block. REFERENCES ACHARYYA S. K. AGES OF SEPARATION AND ACCRETION OF WOYLA MICROCONTINENTS AND WEST BURMA BLOCK In the formulation of a synthesis for the formation of Southeast Asia by the separation of continental blocks from Gondwana and their accretion to the southwest margin of Asia.. This conclusion requires further confirmation. but if the volcanic arc is identified correctly as an intraoceanic rather than a continental Andean arc. Wakita (Geological Survey of Japan) and M. Metcalfe (1996. 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