structure of sumatra and its implications for the tectonic assembly of southeast asia and the...

Thematic ArticleStructure of Sumatra and its implications for the tectonic assembly of

Southeast Asia and the destruction of Paleotethys

ANTHONY J. BARBER1* AND MICHAEL J. CROW21Southeast 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

Abstract It is now generally accepted that Southeast Asia is composed of continentalblocks which separated from Gondwana with the formation of oceanic crust during thePaleozoic, and were accreted to Asia in the Late Paleozoic or Early Mesozoic, with thesubduction of the intervening oceanic crust. From east to west the Malay peninsula andSumatra are composed of three continental blocks: East Malaya with a CathaysianPermian flora and fauna; Sibumasu, including the western part of the Malay peninsula andEast Sumatra, with Late CarboniferousEarly Permian pebbly mudstones interpreted asglaciogenic diamictites; and West Sumatra, again with Cathaysian fauna and flora. Afurther unit, the Woyla nappe, is interpreted as an intraoceanic arc thrust over the WestSumatra block in the mid Cretaceous. There are varied opinions concerning the age ofcollision of Sibumasu with East Malaya and the destruction of Paleotethys. In Thailand,radiolarites have been used as evidence that Paleotethys survived until after the MiddleTriassic. In the Malay peninsula, structural evidence and the ages of granitic intrusions areused 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 emplacedagainst the western margin of Sibumasu by dextral transcurrent faulting along a zone ofhigh deformation, the Medial Sumatra Tectonic Zone. These structural units can be tracednorthwards in Southeast Asia. The East Malaya block is considered to be part of theIndochina block, Sibumasu can be traced through Thailand into southern China, theMedialSumatra Tectonic Zone is correlated with the Mogok Belt of Myanmar, the West Burmablock is the extension of the West Sumatra block, from which it was separated by theformation of the Andaman Sea in the Miocene, and the Woyla nappe is correlated withthe Mawgyi nappe of Myanmar.

Key words: Malay peninsula, Myanmar, Paleotethys, PermoTriassic, Sibumasu, WestSumatra block.

INTRODUCTION

Southeast Asia is considered to be made up of anumber of continental blocks or terranes (Fig. 1),which separated from the northern margin of theGondwana supercontinent in the Late Paleozoic,and accreted to the southeastern margin of theEurasian continent later in the Late Paleozoic or in

the Mesozoic. According to the synthesis proposedby Metcalfe (1996, 2005), the Indochina block,forming the core of Southeast Asia, separatedfrom Gondwana in the Late Devonian, movednorthwards driven by the expansion of Paleo-tethys, to collide and amalgamate with the SouthChina block in the Early Carboniferous. EastMalaya, the eastern part of the Malay peninsula,forms part of the Indochina block and, like the restof the block, is characterized by a CathaysianPermian flora and fauna. The Sibumasu terrane,

*Correspondence.

Received 10 May 2007; accepted for publication 19 February 2008.

Island Arc (2009) 18, 320

2008 The AuthorsJournal compilation 2008 Blackwell Publishing Asia Pty Ltd

doi:10.1111/j.1440-1738.2008.00631.x

which includes the western part of the Malaypeninsula and eastern Sumatra, is characterizedby pebbly mudstones, interpreted as glaciogenicdiamictites or tillites (Stauffer & Lee 1986), sepa-rated from northern Gondwana, which was then ina glacial environment, with the opening of Meso-tethys in the Early Permian. Subsequently, Sibu-masu moved rapidly northwards to collide andamalgamate with the Indochina block in the LatePermian. Metcalfe (2005) suggested that theSikuleh terranes, incorporated in the Woyla ter-ranes and West Burma (Fig. 1), separated fromGondwana in the Late Triassic to Early Jurassic,and were accreted to the southwest margin ofSoutheast Asia in the mid Cretaceous.There are several contentious issues in that syn-

thesis: the timing of the collision of Sibumasu withIndochina with the destruction of Paleotethys asproposed by Metcalfe (1996, 2005) is disputed byJapanese radiolarian researchers from evidence ofMiddle Triassic Paleotethyan Oceanic sedimentsin the northwest Malay peninsula and Thailand(Sashida et al. 1995, 2000a,b; Kamata et al. 2002).

Since Metcalfes (1996) synthesis, a separate WestSumatra block with Cathaysian affinities has beenrecognized (Hutchison 1994; Barber &Crow 2003);it is necessary to determine the origin and time ofemplacement of this block. Metcalfes (1996, 2005)recognition of microcontinental terranes amongthe Woyla terranes has been disputed (Wajzeret al. 1991; Barber 2000; Barber & Crow 2003).Also, the timing of the separation from Gondwanaof the West Burma block and its accretion toAsia needs to be determined. These issues areaddressed in this paper.

TIMING OF COLLISION BETWEEN SIBUMASU ANDEAST MALAYA

Since there is discussion concerning the timingof the collision between the Sibumasu and EastMalaya blocks and the destruction of Paleotethys,it is worth re-iterating the evidence put forwardoriginally by Metcalfe (2000) from the Malay pen-insula, with the addition of supporting evidencefrom the Langkawi Islands and Bangka Island.

Fig. 1 Continental tectonic blocks inSoutheast Asia (after Metcalfe 2005).

INDIA SOUTH CHINA

INDOCHINA

SIB

UM

AS

U

EASTM

ALAYA S.W.BORNEOINDIAN

PLATEWEST SUMATRA

ANDAMAN SEA

WE

ST

BU

RM

A

SIMAO

20

120

10

0

100

SOUTH CHINA SEA

?

Bentong-R

aubSuture

Medial Sumatra Tectonic Line

100

WO

YLATERRANES

Mogok

Belt

4 A. J. Barber and M. J. Crow


Hutchison (1975) identified the northsouthBentongRaub Line separating the eastern andwestern parts of the Malay peninsula as the suturezone marking the site of the collision and amal-gamation of Sibumasu (Sinoburmalaya: Hutchison1994) with the EastMalaya block (Fig. 2). Metcalfe(2000) describes the BentongRaub Line as a 13to 20-km wide zone of imbricated ribbon-beddedcherts and schists containing elongated blocks ofserpentinized mafic and ultramafic rocks. A char-acteristic feature is the occurrence of bodies ofmlange containing randomly arranged blocks ofchert, limestone, and volcanic and volcaniclastic

rocks in a fine-grained mud and silt matrix.Metcalfe et al. (1999) found that radiolaria fromthe bedded cherts ranged in age from Late Devo-nian to Late Permian (Fig. 2), but no radiolariancherts of Triassic age were found. The beddedcherts are interpreted as deep-sea oceanic sedi-ments deposited on the Paleotethys floor. Fromthat study Metcalfe (2000) concluded that Paleo-tethys opened in the Late Devonian and that itsocean-floor sediments were incorporated into anaccretionary complex immediately prior to thecollision of Sibumasu with the western margin ofEast Malaya in the Late Permian. In places, the

Fig. 2 (a) Peninsular Malaysia and theBentongRaub Suture Zone with locations ofdated bedded radiolarian chert, (b) map alongforest road section across the BentongRaubSuture Zone near Pos Mering. The granite is amember of the Main Granite Suite giving LateTriassic to Early Jurassic, 230270 Ma, ages,Cobbing et al. (1992), after Metcalfe (2000).

INDOCHINA BLOCK

melange

thrust melange

102EMiddle Triassic

Middle PermianLower Permian

AlorStar

Semanggol

RAUB

BENTONGJengka

KualaLumpur

Singapore0 50 100 150km

102E

5N

Section in B

L.Carb.

M.Triassic

L.Carb.

U.Devonian L.Carb.

L.Carb. L/U.Permian.

U.Dev.

U.DevonianL.Carb.

L.Carb.

L.Perm.

L.Permian.

EASTMALAYA

WESTMALAYA

SIBUMASU

10135 10140 10145

445

Granite

schist

serpentinite

chert (Visean)

0 5km

mudstone

schist schist

road

Triassic

granite/schistcontact zone

5

POS MERING

chertL.-U. Permian)

(a)

(b)

Semanggol Formation

Bentong-Raub Suture Zone

Tectonic assembly of Southeast Asia 5


BentongRaub Zone is intruded by granites, rep-resentatives of the Main Range granites of thewestern Malay peninsula, which have been datedas having been intruded in the Late Triassic toEarly Jurassic (230207 Ma). The evidence fromthe radiolarian cherts and the intrusive granitesprovides constraints for the age of the collision asLate Permian to Early Triassic.Confirmatory evidence for the timing of this

collision event is provided by radiolarian studiesof Middle Permian, Late Permian, and Middle Tri-assic ribbon-bedded cherts and lensoid conodont-bearing siliceous limestones in the lower ChertMember of the Semanggol Formation near AlorStar in the northwest Malay peninsula (Ahmadet al. 1987; Sashida et al. 1995; Metcalfe 2000)(Fig. 2). While Permian cherts and limestones areinterpreted as having been deposited in a pelagicenvironment, Triassic foraminiferalalgal lime-stones and possibly the associated radiolariancherts were deposited in a shallower water envi-ronment. Sashida et al. (1995) supposed that thePermian and Triassic rocks formed part of a con-tinuous sequence, marking the transition betweenthe shelf and slope sediments of the Sibumasuterrane and the deposits on the floor of the Paleo-tethys Ocean. They concluded, therefore, thatthe collision between Sibumasu and East Malayaoccurred after the Late Triassic, following thedeposition of the complete sequence in the lowerChert Member of the Semanggol Formation.In their account, Sashida et al. (1995) reported

that the Permian cherts dip steeply and are tightlyfolded. Metcalfe (2000) pointed out that the ages ofrocks in the sequence are repeated, implyingimbrication or isoclinal folding of the chert. On theother hand, the Triassic cherts and overlying tur-bidites are only gently folded, indicating that thePermian and Triassic cherts are separated by anunconformity. Metcalfe (2000) therefore inter-preted the Permian cherts and limestones aspelagic sediments deposited on the Paleotethysocean floor and which were imbricated into anaccretionary complex. The latest Permian and theEarly Triassic are missing, representing theunconformity, followed by the deposition of MiddleTriassic cherts, limestones, and turbiditic sand-stones in a successor basin.One problem in understanding the geology of

the western Malay peninsula has been thatalthough fossiliferous units are common, and unitsranging in age from Lower Paleozoic to UpperPaleozoic have been identified, on the geologicalmap stratigraphic units appear to be arranged

randomly, so that it has neither proved possible torecognize continuous stratigraphic sequences, norto reconstruct a coherent depositional or struc-tural basin. In part this is due to the discontinuityof exposures. However, in Perlis, northwest Malaypeninsula, north of Alor Star, Meor and Lee (2005)described a steeply eastward-dipping Silurian toCarboniferous sequence folded with western ver-gence, and broken by eastward-dipping thrusts. Inthe offshore Langkawi Islands, Kimura and Jones(1967) showed Lower Paleozoic rocks thrust west-wards over the Upper Paleozoic, which includesPermian pebbly mudstones (Fig. 3). These struc-tures are characteristic of foreland fold-and-thrustbelts.In the western Malay peninsula, Paleozoic

(CambrianPermian) sandstonelimestoneshalecontinental margin sediments of Sibumasu andthe Paleotethyan ocean floor bathyal shales andcherts, extending all the way from the BentongRaub suture to the west coast of the Malay penin-sula and into the offshore Langkawi Islands, aredeformed into an accretionary complex and a fore-land fold-and-thrust belt. In the Langkawi Islandsthe major thrust plane is cut by an intrusiveMiddle Triassic (242! 10 Ma) granite (Fig. 3),providing a further constraint on the age of thecollision of Sibumasu with East Malaya, whichmust have been completed before the MiddleTriassic.A similar constraint is provided by the relation-

ships between the PermoCarboniferous andTriassic rocks and intrusive granites on the islandof Bangka to the southeast of Sumatra (Fig. 4).According to Ko (1986) the oldest rocks on theisland are of PermoCarboniferous age. In thenorth they consist of slates and schists with isocli-nal folding on northwestsoutheast axes, imbri-cated with basalts, andesites, bedded cherts, distalturbidites, black pyritic shales, and limestones.One of the limestone blocks yielded Permianfusulinids (De Roever 1951). In the adjacent islandof Billiton a poorly preserved Cathaysian florawas found in shales and a limestone containedSchwagerina sp. (Van Overeem 1960), indicatingaffinities between these rocks and the EastMalaya block. The Carboniferous and Permianrocks are interpreted as Paleotethyan ocean-floorsediments imbricated into an accretionarycomplex, prior to the collision between Sibumasuand East Malaya, corresponding to the BentongRaub suture of peninsular Malaysia. However, atToboali in the far south of Bangka, the PermoCarboniferous rocks include pebbly mudstones,



glaciogenic diamictites characteristic of Sibumasu(Ko 1986). Throughout the island highly deformedPermoCarboniferous rocks are in fault contactwith gently folded Middle to Upper Triassic sand-stones. Although a stratigraphic contact hasnot been described, given the difference in theirstructure, the relationships between the PermoCarboniferous and Triassic rocks must be uncon-formable. Both groups of rocks are cut andextensively hornfelsed by granites yielding agesranging from 250 to 200 Ma (Cobbing et al. 1992)(Fig. 4), showing that the collision between Sibu-masu and East Malaya had already occurredbefore latest Permian times.

STRUCTURE OF SUMATRA

Barber et al. (2005) recently published a revisedsynthesis for the nature and origin of the struc-tural units that comprise Sumatra and peninsularMalaysia, building on earlier syntheses by Hutchi-son (1994) and Metcalfe (1996). The pre-Tertiaryrocks of mainland Sumatra are of Carboniferous,Permian, Triassic, Jurassic, and Cretaceous ages.In eastern Sumatra, Late Carboniferous to

Early Permian rocks of the Tapanuli and Tigapu-luh groups are quartz-wackes, siltstones, andshales with interbedded pebbly mudstone units,interpreted as glaciogenic diamictites. Also

Fig. 3 (a) Northwest peninsular Malaysiawith maps of localities 1 and 2 after Meor andLee (2005), (b) map and cross-section of theLangkawi Islands after Kimura and Jones(1967). Middle Triassic granite cuts thrustplane.

section

Meor & Lee (2005)

PERLIS THAILAND

KEDAH

Alor Star

1

2

Langkawi Islands

LANGKAWI ISLANDSKimura & Jones (1967)

8

6

4

2

100 101

SIBUMASU

0 5km

NW SE

1km

granite

granite

Permo-Carboniferous

Lower Palaeozoic

64

60

Locality 1

Locality 2

HILL B

Sibumasu continental margin sediments in a foreland fold-and-thrust belt

Middle Triassic Granite (24210Ma)

(with pebbly mudstones)

Sungai Abi

20m

40m

5072

40m0 100m

10023

6232N

623

0 100m

70

Devonian

Devonian-Carboniferous

Silurian

Devonian-Carboniferous

60m

HILL C

thrust

(a) (b)



included in the Tapanuli Group are the Alas lime-stones (Cameron et al. 1982) containing corals,brachiopods, and conodonts identified as a temper-ate fauna of Visan age (Metcalfe 1983; Fontaine1989). East Sumatra, together with the westernMalay peninsula and the Langkawi Islands, areregarded as part of the Sibumasu block.In western Sumatra, Early Carboniferous rocks

of the Kluet and Kuantan formations are quartz-wackes, sandstones, and shales containing fossilif-erous limestone lenses (Fig. 5). The limestonesinclude fossil corals of Visan age, sometimesforming reef structures, intertidal algal mats,oolites, and pisolites, that are considered to haveformed in tropical intertidal and shallow marineenvironments (Fontaine & Gafoer 1989; Vachard1989a,b). Permian rocks include basalts, andesites,and rhyolites of the Palepat Formation, and acyclic sequence of conglomerates, sandstones, andshales interbedded with limestones and thin coalsof the Mengkarang Formation (van Waveren et al.2005). The Mengkarang Formation contains rootsand large tree trunks up to 2 m in length, some-times in situ, of Cordaites, Calamites, andTaeniopteris with abundant plant fragments andleaves in siltstones, comprising the Jambi flora(Jongmans & Gothan 1925, 1935). The tree trunksdo not show tree rings, indicating that they grew ina tropical environment, and the plant remains arecharacteristic of the Cathaysian flora found in EastMalaya, Indochina, and southern China (Vozenin-Serra 1989). The intercalated limestones containoncolites and an abundant fusuline fauna of

CathaysianTethyan affinity and Early Permian,AsselianYahtashian (Artinskian) age, also indicat-ing a tropical environment (Ueno et al. 2006). Thetropical Early Permian fauna and flora of WestSumatra contrast with the Early Carboniferoustemperate fauna and Early Permian glaciogenicsediments of the East Sumatra (Sibumasu) blockagainst which it is juxtaposed.

MEDIAL SUMATRA TECTONIC ZONE

The contact between the Sibumasu block and theWest Sumatra block is marked by the MedialSumatra Tectonic Zone (MSTZ), traceable as azone of highly deformed rocks including schistsand gneisses, extending the whole length ofSumatra from the Andaman Sea to Palembang, adistance of 1760 km (Hutchison 1994; Barber et al.,2005) (Fig. 5). At its northern end the zoneincludes limestones of the Alas Formation associ-ated with lenses of massive marble, phlogopite,and graphitic marble, scapolitecalc-silicate schistand garnetiferous augen gneiss (Cameron et al.1980). In other parts of the zone rock types includeslate, phyllite, biotitegarnetsillimanite schist,biotiteandalusite hornfels with cordierite, andchiastolite, quartzite, quartzfeldspar augengneiss, migmatite, mylonite, and cataclasite, thelatter with horizontal slickensides.The southern segment of the zone is less well

exposed, as much of its course is covered byTertiary and Quaternary sediments. However,

Fig. 4 Geological map of Bangka Island,see Figure 8 for location, after Ko (1986).PermoCarboniferous rocks in the north ofthe island are imbricated Paleo-Tethyan oceanfloor materials and limestones with fusulin-ids, related to the East Malaya block, whilePermoCarboniferous rocks at Tobaoli in thesouth contain pebbly mudstones and arerelated to Sibumasu, all overlain unconform-ably by Middle Triassic sandstones andintruded by very Late Permian to EarlyJurassic granites.

Triassic TempilangSandstone

Pemali Group

106E 107

2S 2

3

106E

KLABATBATHOLITH

BELINYU

BEBULUBATHOLITH

PENANGAS KELAPA

MENUBINGTEMPILAN MANGOL

PERMISAN

TOBOALI

PADING

'Pebbly Mudstone' Tapanuli Group

Carboniferous-Early Permian

0 50km

Faults

BANGKA ISLAND

Slates, cherts,pillow lavas etc.older than earlyMiddle Permian

200Ma

252Ma

251Ma

213Ma

225Ma

KULUR

Very Late Permian toEarly Jurassic granites

SUMATRA



between Pekanbaru and Lubuksikaping it is rep-resented by intensely folded muscovite, tremolite,chlorite, carbonate, and quartz schists, mapped asthe Pawan and Tanjungpuah members of theKuantan Formation (Clarke et al. 1982) (Fig. 5).The Pawan Member is intruded by the Rokangranite, a foliated syntectonic granitoid, dated bythe K/Ar method at 189! 2 Ma (Early Jurassic)(Rock et al. 1983). This and other deformed grani-toids of TriassicJurassicEarly Cretaceous age,surrounded by metamorphic aureoles, are inter-preted as syntectonic intrusions related to a mag-matic arc formed during eastward subductionbeneath the western margin of the West Sumatrablock and emplaced in an active shear zone(Barber et al. 2005). Further to the southeast, thezone is represented by the Gangsal Formation ofthe Tigapuluh Group on the southwestern side ofthe Tigapuluh Mountains, composed of intenselyquartz-veined slates and phyllites (Simandjuntak

et al. 1991; Suwarna et al. 1991). Further to thesoutheast it can be traced beneath Tertiary sedi-ments only through oil company boreholes (DeCoster 1974; Eubank & Makki 1981).The MSTZ does not represent a suture zone

formed from a subducted oceanic crust as it doesnot contain any significant ophiolitic components.However, the intense deformation of the rocks inthe zone indicates that it represents a major shearzone between the Sibumasu and West Sumatracrustal blocks. The occurrence of syntectonicgranitoids and of a distinct tin anomaly relatedto the zone, determined from stream sedimentsamples (Stephenson et al. 1982; Barber et al.2005), indicate that the MSTZ is a structure ofcrustal scale. The MSTZ is interpreted as a majortranscurrent shear zone, analogous to the presentSumatran Fault zone, (Barber & Crow 2003;Barber et al. 2005). As is the case with the presentSumatranFault and theQuaternary volcanic arc, in

5N

95E

4

3

2

1

0

1S

2

95E

96 97 98 99BANDA ACEH

Situtup

Kaloi

MEDANBatumilmil

Kualu

Pawan

Tanjungpuah

PADANG

PEKANBARU

Palepat

Mengkarang

PALEMBANG

Tuhur

Rokan Granite (189Ma)

Bohorok

KluetAlas

PRE-TERTIARY TECTONIC UNITS IN SUMATRA

EAST SUMATRA BLOCK (SIBUMASU)

Kuantan

Triassic Kaloi, Batumilmil, Kualu

Permo-Carboniferous (Tapanuli Group)

unconformity

Bohorok, Tigapuluh Group(pebbly mudstones)

Alas Limestone (Visean)

WEST SUMATRA BLOCK(CATHAYSIAN)

Triassic

Permianunconformity

Carboniferous

Tuhur

Palepat, Mengkarang

Kluet, Kuantan (with Visean limestone)

LakeToba

MSTZ Medial SumatraTectonic Zone

96 97 98 99 100 101 102 103 104 106

5

4

3

2

1

0

1

2

100 101 102 103 104 105 106

WOYLA NAPPE

Woyla Group Jurassic-Cretaceous

Island arc and accretionary complex

Tigapuluh Hills

SIBOLGA

0 100 200km

LUBUKSIKAPING

Fig. 5 Distribution of the pre-Tertiary stratigraphic and tectonic units in Sumatra, based on data from the 1:250 000 geological map sheets publishedby the Indonesian Geological Research and Development Centre, Bandung (after Barber et al. 2005). Darker tones indicate areas of outcrop, lighter tonesindicate that the pre-Tertiary is overlain by Tertiary and Quaternary sedimentary and volcanic rocks.



the JurassicCretaceous the MSTZ was also thesite of an active magmatic arc, with granitoidmagma chambers emplaced in an active shear zone.There is a problem in accounting for present

position of the Cathaysian West Sumatra block,separated from the Cathaysian East Malaya block,and lying outboard the Sibumasu block (Barber &Crow 2003; Barber et al. 2005). TheMSTZ is inter-preted as a major transcurrent fault along whichthe West Sumatra block was translated fromits original position as part of Cathaysia andemplaced against the western margin of theSibumasu block (Hutchison 1994; Barber & Crow2003). The MSTZ incorporates slices of thewestern margin of the Sibumasu block, includingpossibly the basement as well as its sedimentarycover. Metamorphic rocks derived from differentcrustal depths are juxtaposed along the MSTZ,a feature of major transcurrent fault zones, andhave been re-metamorphosed in contact aureolesaround the syntectonic granitoids.

PERMOTRIASSIC UNCONFORMITY

Fossiliferous Permian and Triassic rocks are wide-spread throughout Sumatra, occurring in both theWest Sumatra and Sibumasu terranes. SometimesPermian and Triassic limestones occur in the samegroup of outcrops (Situtup limestone, Kaloi lime-stone, and Batumilmil formations) (Fig. 5), identi-fied by fossils from spot samples (Bennett et al.1981; Cameron et al. 1982, 1983), but the rela-tionships between the Permian and Triassiccarbonates have not been described, and no uncon-formable contacts have been recognized. However,Fontaine and Gafoer (1989) made a systematicstudy of all the outcrops and their fossil content.They found that all Permian and Triassic stagesare represented, apart from the uppermostPermian and the lowermost Triassic. Direct evi-dence of an unconformable relationship betweenthe Permian and Triassic is provided by limestoneclasts containing Middle Permian fusulinid fossilsin Middle Triassic sediments (Tuhur and LimauManis formations: Silitonga & Kastowo 1975;Turner 1983; Barber & Crow 2003) (Figs 6,7).Middle to Upper Triassic sediments in Sumatra

are either carbonates (e.g. Situtup and Batumilmilformations) or a chertclastic facies (e.g. Kualuand Tuhur formations). The chertclastic faciesresembles the Semanggol Formation of WestMalaya, with cherts at the base, overlain by rhyth-mites composed of finely-bedded alternating tur-

biditic sandstones and shales. The distribution ofthese facies is plotted on Figure 8 and is inter-preted as representing a submarine horst andgraben structure which extended across the wholeof Sumatra and the Malay peninsula. In Malayathe easternmost horst appears to have been anarea of uplift and erosion related to the intrusion ofgranites, while to the west the eroded sediments,finer at the base and coarsening upwards, accumu-lated in the Semantan Basin. In the western partof the Malay peninsula and in Sumatra shallow-water carbonates were deposited on horst blocks,particularly along their margins. Triassic carbon-ates were sometimes deposited on top of Permiancarbonates, which perhaps formed local elevationsof the seafloor. The chert facies was deposited inthe intervening SemanggolMutus, Kualu, andTuhur basins, sediment-starved graben far froma clastic sedimentary source (Barber et al. 2005).The deposits of the Tuhur Basin, including cherts,were deposited on the southwestern margin of theWest Sumatra block. A limestone block containingTriassic foraminifera in mlange in the accretion-ary complex which forms part of the WoylaGroup emplaced on West Sumatra during the midCretaceous, was interpreted by Wajzer et al.(1991) as derived from the carbonate cap of a sea-mount in Mesotethys. Evidently in the Triassic theWest Sumatra block formed the southwesternmargin of Southeast Asia passing out westwards,in present day co-ordinates, onto the MesotethysOcean floor.The breakup of the European margin of Tethys

during the TriassicJurassic (Bernouilli & Jenkyns1974) provides an analogy for the tectonic environ-ment on the southwestern margin of SoutheastAsia during the Triassic. In Europe as in Asia,carbonate deposits were formed on horst blocksformed by the extension of continental crust, whilepelagic deposits, including radiolarites, weredeposited in the intervening subsided basins.Since Middle to Late Triassic sediments of

the same facies rest unconformably on deformedCarboniferous and Permian rocks forming theSibumasu and West Sumatra blocks on either sideof the MSTZ, and also occur, only gently folded,within the shear zone itself (Figs 5,8), the MSTZ isconsidered to have been initiated in latest Permianor Early Triassic times. It is inferred that byMiddle Triassic times the Sibumasu and WestSumatra blocks were substantially in their presentrelationship. A magmatic arc related to subductionbeneath the western margin of the West Sumatrablock was superimposed across the MSTZ in Late



JurassicEarly Cretaceous times. As has alreadybeen described, JurassicCretaceous syntectonicgranites in the MSTZ indicate that minor move-ments along the shear zone continued throughthe Mesozoic, and may have continued into theCenozoic, since the zone is displaced by 200 kmalong the Sumatran fault zone.

WOYLA NAPPE

The West Sumatra block is overlain to the west bytheWoylaGroup, composed of an east-facing accre-tionary complex of ophiolitic ocean floor materialand pelagic and volcaniclastic sediments andmlanges, and basalticandesitic volcanic rocks,interpreted as a Late Jurassic to Early Cretaceousvolcanic arc with carbonate fringing reefs. Becausethe volcanic arc is intruded by the Sikuleh granite itwas interpreted initially as an Andean Arc, devel-oped ona fragment of continental crust identified as

the Sikuleh microcontinent (Cameron et al. 1980),and similar microcontinental fragments, the Nataland Bengkulu microcontinents were postulatedamong the Woyla terranes further south (Metcalfe1996). This interpretation has been contested, andargumentshavebeenput forward for regarding theWoyla Group as an intraoceanic island arc devel-oped offshore West Sumatra on the TriassicCretaceous oceanic crust of Mesotethys (Wajzeret al. 1991; Barber & Crow 2003).Subsequently, by the subduction of the interven-

ing part of the Mesotethys Ocean floor, the WoylaGroup collided with, and was thrust over, the WestSumatra block to form the Woyla nappe (for a fulldiscussion of this controversy see Barber et al.(2005, pp. 5253). The overthrust assemblageincludes carbonates of AlbianAptian age, and theemplacement of the nappe resulted in the deforma-tion and metamorphism up to amphibolite gradeof rocks in West Sumatra, ranging in age fromCarboniferous to mid Cretaceous, providing con-

Fig. 6 Stratigraphic relationships between tectonic and stratigraphic units in Sumatra and East Malaya. Triassic rocks in similar facies extend across theEast Malaya, West MalayaEast Sumatra (Sibumasu) blocks, the Medial Sumatra Tectonic Zone and the West Sumatra block and that all tectonic units arecut by Triassic and younger granites (after Barber et al. 2005).



straints on the age of collision (Barber et al. 2005).The Woyla nappe is intruded by Late Cretaceousandyounger granites.Evidently following collision,subduction polarity was reversed and thesegranites are related to eastward subduction ofMesotethys beneath West Sumatra and theaccreted island arc (Barber 2000; Barber et al.2005).

RELATIONSHIP BETWEEN SUMATRA ANDSOUTHEAST ASIA

Crustal blocks identified in Sumatra and theMalaypeninsula can be recognized in the mainland ofSoutheast Asia to the north. As has already beenmentioned, the East Malaya block is considered tobe part of the Indochina block; the Sibumasu block,characterized by PermoCarboniferous diamic-tites, can be traced from Bangka Island througheastern Sumatra, the western part of the Malaypeninsula, peninsular Thailand and easternMyanmar (Burma) to Yunnan in southwest China;

but, to the west, the northward continuity of struc-tural units is interrupted by the Andaman Sea.Here, since the Miocene, the continental crust hasbeen extended and replaced by oceanic crust, sepa-rating western Myanmar from Sumatra by some460 km (Curray et al. 1979; Curray 2005) (Fig. 1).In eastern Myanmar, the Shan Plateau is com-

posed of aProterozoic,Cambrian, toEarlyPermiansuccession, including Late CarboniferousEarlyPermian diamictites (Mergui Group) and so formspart of the Sibumasu (ShanThai) block (Fig. 9).The western margin of the block is marked by theMogok Metamorphic Belt, 1400 km long, whichcontains high to low grade metamorphic rocks,migmatites, mylonites, and marbles with (?) EarlyCarboniferous fossils (Mitchell et al. 2007). Themarbles included in theMogokBelt are regardedasthemetamorphosed equivalents of theCambrian toDevonian limestones of the plateau succession inthe Shan Plateau to the east. Mitchell et al. (2007)suggested that the Mogok Belt represents thedeformed and uplifted margin of the ShanThai(Sibumasu) block. The Mogok Belt is intruded by

Fig. 7 Paleontology of PermoTriassic stratigraphic units in Sumatra, data taken from Fontaine and Gafoer (1989). *, fossils diagnostic of these stagesoccur in these areas. Note the apparent absence of fossils corresponding to the uppermost Permian and the lowermost Triassic stages, and that clastscontaining derived Middle Permian fusulinids are incorporated in Middle Triassic stratigraphic units (after Barber et al. 2005).



syntectonic granitoids (augen gneiss) which haveyielded Middle Jurassic isotopic ages and by unde-formed granitoids of Late Cretaceous and youngerages (Mitchell et al. 2007).The Mogok Belt is the same age and has similar

characteristics to the MSTZ (although the gem-stones found in the northern part of theMogokBelthave not been recorded in Sumatra). In MyanmartheWest Burma block lies to the west of theMogokBelt. TheWest Burma block, defined by Hutchison(1989, p. 225) and formerly termed Mount VictoriaLand (Mitchell 1989), is bounded by the Sagaingstrikeslip fault in the east and the IndoBurmanRanges to the west. The West Burma block islargely covered by Cenozoic sedimentary and vol-canic rocks, but is underlain by continental crust, asmetamorphic rocks overlain by Triassic turbiditesare exposed in Mount Victoria and a late Cenozoichigh-K calc-alkaline volcanic arc was extrudedthrough it. In Sumatra theWest Sumatra block liesto the west of the MSTZ. If the correlation of theMogokBeltwith theMSTZ is accepted, it raises thepossibility that the West Burma block is the north-

ward extension of the West Sumatra block, fromwhich it became separated by the development ofthe Andaman Sea.This correlation is supported paleontologically

by a Middle Permian (Murghabian) fusulinid faunadescribed from limestones at Karmine, northernMyanmar (Oo et al. 2002) (Fig. 9). In Figure 10the fusulinid fauna from Karmine is comparedwith the faunas of the Thitisipin limestone in theShan Plateau, Myanmar (Garson et al. 1975; Ooet al. 2002; Mitchell et al. 2004, 2007), and theMurghabian fusilinid faunas from Situtup,Muarasipongi, and Silungkang in West Sumatra(Fontaine & Gafoer 1989). The Thitispin limestoneFormation, which is part of the Sibumasu block,has no fusulinid genera in common with the faunaof the Karmine area, while Parafusulina sp. andRugoschwagerina sp. occur both at Karmine andin the Situtup Formation and the Muarasipongiarea in the West Sumatra block. The Karmine andWest Sumatra faunas both include fusulinidgenera which also occur in South China, and there-fore, belong to the Cathaysian faunal province.

Fig. 8 Middle to late Triassic paleogeogra-phy in the Malay peninsula and Sumatra. Asubmarine horst and graben structure is pos-tulated, based on the distribution of thecarbonate and the chertclastic facies, wherethe carbonates were deposited on the horsts,especially along their margins, and the chertsand rhythmites were deposited in the inter-vening sediment-starved graben (after Barberet al. 2005).

Horst Block (land area)

Horst Block (shallow marinewith carbonate banks)

Sediment-starved graben (with cherts)

0 100 200 300km

BANDA ACEH

TAKENGON

LAKETOBA

MEDAN

PADANG

BENGKULU

PALEMBANG

BANDARLAMPUNG

JAMBI

PEKANBARU

MALACCA

LANGKAWI

BINTAN

KUNDUR

LINGGA

SINGKEP

BANGKA

EA

ST

MA

LAYAH

IGH

SE

MA

NTAN

BASIN

MA

INR

AN

GE

HIG

H

SE

MA

NG

GO

L- M

UTU

SBASIN

MEDIAL

SUMATRA

HIGHTUHURBASIN

Lebir Fault

Keramutan

Fault

(Medial Sum

atraTectonic Zone)

KU

ALU

BASIN

Situtup

Kaloi

Batumilmil

Sibaganding

Cubadak

Sawahlunto

Belinyo

Kodiang

KUALALUMPUR



In western Myanmar, along the eastern marginof the IndoBurman Ranges Mitchell (1993)described the Mawgyi andesites as a belt of basal-tic andesites and basaltic pillow lavas, underlain byophiolitic rocks, serpentinites, talc-schists, cherts,phyllites, and mudstones, which is interpreted asa JurassicCretaceous intraoceanic arc, with itsbasement and associated accretionary complex.The arc is duplicated in the Upper IrrawaddyBasin by dextral movements of about 300 kmalong the post-Miocene Sagaing Fault (Fig. 9). TheMawgyi andesites and associated rocks wereemplaced on the western margin of Myanmar asthe Mawgyi nappe in the mid Cretaceous. Mitchell(1993) correlated the Mawgyi andesites with theWoyla Group of western Sumatra, which wasemplaced on the western margin of the WestSumatra block in the mid Cretaceous (Cameronet al. 1980; Wajzer et al. 1991). Mica schists andgneisses in the eastern IndoBurman Ranges, theKathaGangaw Range and in the Jade Mines areamay represent the metamorphic footprint of the

Mawgyi nappe, as has been suggested for meta-morphic rocks in a similar relationship to theWoyla nappe in West Sumatra (Barber et al. 2005,p. 249). The implication of this correlation is thatthe whole of western Myanmar, from the MogokBelt to the IndoBurman Ranges and the MountVictoria Land of Mitchell (1989), and the WestBurma block of Hutchison (1989) and Metcalfe(1996), is the northward extension of the WestSumatra block.Unfortunately, western Myanmar is covered

largely by Tertiary and Quaternary sediments andvolcanics, so that exposures of the underlyingbasement are rare. However, this correlation isstrengthened by the occurrence of Triassic turbid-ites, mudstones, and limestones along the westernmargin of the West Burma block, which aredeformed, metamorphosed, and overthrust byophiolites of the Mawgyi nappe. Although nocherts are recorded amongst these turbidites theyoccupy a similar position along the western marginof the West Burma block to the Triassic Tuhur

Fig. 9 Tectonic units in Myanmar (afterMitchell 1993). MSTZ, Medial SumatraTectonic Zone.

INDIA

Himalayas

Cenozoic AccretionaryComplexes

94E 96

28N

Mogok

SHAN

PLATEAU

24

INDO

-BU

RMAN

RANG

ES

Moulmein

20

16

0 200km98

CHINA

THAILAND

28

24

20

16

ANDAMAN SEA

Shan Thai(Sibumasu) Block

Mogok Meta-morphic Belt = MSTZ

Late CretaceousGranites

West Burma Block= West Sumatra Block

Ophiolites

Mawgyi Nappe= Woyla Nappe

Deformation Front

Strike-Slip Fault

MAW

GYI N

APPE

Mount Victoriametamorphics

Mag

mat

icAr

c

Gang

awR.

Kum

an

R.

Jade Belt

Karmine

MountVictoria

Saga

ing

Fault

YANGON

Mandalay



Formation on the western margin of the WestSumatra block. In addition, the West Burma blockand the Mawgyi nappe, like the West Sumatrablock and the Woyla nappe in Sumatra, areintruded by Late Cretaceous granitoids. Mitchell(1993) interpreted these intrusions as due to thereversal of subduction polarity after the emplace-ment of the nappe, as has also been suggested forSumatra (Wajzer et al. 1991; Barber 2000).The western part of the IndoBurman Ranges

constitutes material accreted from the subductionof the Indian Ocean Plate which can be traced intothe Andaman and Nicobar islands and southwardsinto the outer arc islands and the current subduc-tion system offshore western Sumatra (Acharyya2007).

CONCLUSIONS

AGE OF COLLISION OF SIBUMASU WITHINDOCHINA BLOCK

In a series of papers on radiolarian-bearing beddedcherts and siliceous shales of Triassic age from thenorthern Malay peninsula, southern and westernThailand, Sashida and co-workers (Sashida et al.

1995, 2000a,b; Kamata et al. 2002; Ueno et al. 2006)argued that the presence of Triassic cherts indi-cates that the collision of Sibumasu with theIndochina Block, with the destruction of Paleo-tethys, occurred after the Middle Triassic. Theassumption in this interpretation is that the chertswere deposited on Paleotethyan oceanic crust. Ashas been pointed out here and elsewhere, inSumatra cherts were deposited in intracontinentalbasins on continental crust of the Sibumasu andWest Sumatra blocks. In the SemanggolMutis,Kualu, and Tuhur basins Middle to Late Triassiccherts were deposited in extensional basins on abasement of deformed Carboniferous to Permianrocks, in the case of the Tuhur Basin, over 400 kmto the west of the SibumasuIndochina collisionalsuture (Fig. 8). Radiolarian cherts in a continentalenvironment, resting on crystalline basement andpassing up into turbidite deposits in SoutheastAsia, have been described previously from theCretaceous of Sulawesi and Timor (Haile et al.1979; Earle 1983; Wakita et al. 1994).It can not therefore be taken for granted that

radiolarian cherts were deposited on oceanic crust.It is not the presence of oceanic or continentalcrust that controls the deposition of radiolarian

Fig. 10 Comparison of the Permian Fusilinacean faunas of Central Tethys (Paleotethys), Myanmar, the West Sumatra block and South China. Data fromFontaine and Gafoer (1989), Oo et al. (2002), Ross (1995) and Ueno et al. (2006). Note that in Myanmar the fauna of the Thitispin limestone Formation hasno fusulinid genera in common with the fauna in the Karmine area, while Parafusulina sp. and Rugoschwagerina sp. occur at Karmine and in the SitutupFormation and the Muarasipongi areas in the West Sumatra block.



cherts, but the environmental conditions in a sedi-mentary basin. Radiolaria are deposited on theseafloor where other sedimentary material is notavailable, in areas remote from a terrigenoussource, as in the major ocean basins, or where thesupply of clastic sediment is restricted by thearidity or submergence of potential source areas,and where the supply of carbonate material isrestricted by the carbonate compensation depth(CCD) and its relationship to the depth of the basinfloor. The CCD can vary locally and over time, as itdepends on the chemistry and the temperatureof the seawater (cf. TriassicJurassic Europeanmargins of Tethys: Bernouilli & Jenkyns 1974).In many of the occurrences of Triassic radiolar-

ian chert studied by Sashida and co-workers inThailand and Malaysia (e.g. Sashida et al. 1995,2000a,b), older stratigraphic units of Carbonifer-ous or Permian ages show evidence of internaldeformation by intense shearing or cleavage, whilethe Triassic rocks may be folded or faulted, but arerarely internally deformed. It is the age of theinternal deformation which indicates the age ofthe collision between Sibumasu and Indochina;less deformed Triassic cherts and clastics mayhave been deposited on the top of the deformedrocks subsequent to the collision. In northernThailand, Helmcke and co-workers (e.g. Helmcke1984; Helmcke et al. 1985; Heggemann et al. 1994)described deformed Carboniferous and Permianrocks overlain unconformably by undeformed Tri-assic rocks which were deposited in extensionalhalf-graben (Drumm et al. 1993), and concludedthat the collision of the ShanThai (Sibumasu)and Indochina blocks, with the destruction ofPaleotethys occurred in the Late Permian orEarly Triassic.In many occurrences of radiolarian Triassic

chert studied by Sashida and colleagues (Sashidaet al. 1995; Kamata et al. 2002), the cherts areassociated, and sometimes closely interbedded,with terrigenous siltstones or fine sandstones, andcarbonates. They interpret this association as indi-cating deposition on a continental margin, with thecherts being deposited on the continental slopeleading down onto the deep ocean floor. The rela-tionship between the cherts and the other litholo-gies could equally well be interpreted as the resultof fluctuations in the environment, by changes insealevel or climate, leading to increased erosion ofthe land areas, the increased supply of terrigenoussediment, and to changes in the level of the CCD inbasins developed on continental crust. These pos-sibilities, together with evidence from the struc-

ture, should be taken into account in assessing thetectonic significance of occurrences of radiolarianchert.Sashida et al. (2000a) also describe an occur-

rence of radiolarian cherts at Chang Dao in north-ern Thailand, where the cherts are interbeddedwith red, chocolate, and black shales and greenclaystones, interpreted as pelagic deposits. Theradiolarian fauna ranges across the PermoTriassic boundary from the Dorashamian (LatestPermian) to the Middle Triassic. This locality isshown by Sashida et al. (2000a, fig. 1) as lyingwithin the continental ShanThai (Sibumasu)block or within the Inthanon Zone of Ueno (1999),but the sequence is inverted. These deposits wereclearly affected by a major tectonic event after theMiddle Triassic. Wonganan and Caridroit (2005)described radiolarian cherts of Middle and LateDevonian ages from the same area, occurring asblocks in an olistostrome and with an overallnappe-like structure. They proposed that theserocks form an accretionary complex within a pre-viously unrecognized suture zone between theShanThai and Indochina blocks. They went ontoto propose that the Middle Devonian cherts,together with the Carboniferous, and Permian toMiddle Triassic cherts in the same area (FangCherts) represent a wide paleo-ocean whichonce separated the ShanThai (Sibumasu) andIndochina continental blocks.It may be that the collision of the Sibumasu and

Indochina blocks occurred later in northernThailand than in peninsular Malaya and Sumatra.But clearly the problems of the age of collision ofthe Sibumasu and Indochina blocks and the posi-tion of the major suture marking the destructionof Paleotethys have not yet been satisfactorilyresolved and further study is required.

AGE OF EMPLACEMENT OF WEST SUMATRA BLOCK

There is also a dispute concerning the timing oftranscurrent movement along the Medial SumatraTectonic Zone and the time of emplacement of theWest Sumatra block. Hutchison (1994) suggestedthat the major phase of movement occurred in theCenozoic, and this view has recently been sup-ported by Ueno et al. (2006). Ueno et al. (2006) hadtheir conclusion concerning the age of collisionof Sibumasu and Indochina based on the occur-rence of Triassic cherts to argue that the WestSumatra block could not have arrived at itspresent position against Sibumasu in Triassic time,but must have arrived later, after the Early Juras-



sic. However, as has already been pointed out, inthe Triassic the tectonic and depositional environ-ment of horsts and graben extended from EastMalaya, across the western Malay peninsula andeast Sumatra onto the West Sumatra block, sug-gesting that all these blocks had reached some-thing like their present relationship at that time.The continuity of the MiddleUpper Triassic

Kualu and Tuhur formations composed of unde-formed Triassic sediments, with similar facies andsimilar sequences, occur on the Sibumasu andWest Sumatra blocks on either side of the shearzone, and occasionally within it (Cameron et al.1978, appendix), indicating that the major move-ment along the MSTZ occurred before the MiddleTriassic (Fig. 6). But the occurrence of syntectonicgranitoids of Triassic to Jurassic age along theMSTZ suggests that movement to some extentcontinued throughout these periods. The MSTZwas dissected and displaced by the present Sumat-ran Fault system during the late Cenozoic; thecentral segment displaced dextrally by 50 kmalong the LokopKutacane Fault, and the southernsegment displaced by 150 km along the mainstrand of the Sumatran Fault near Sibolga (Fig. 5).At the present day on digital elevation modelimages the MSTZ forms a topographic lineament,so it is probable that the shear zone has beenre-activated as a strikeslip fault during thesemovements and may still be active.

AGES OF SEPARATION AND ACCRETION OF WOYLAMICROCONTINENTS AND WEST BURMA BLOCK

In the formulation of a synthesis for the formationof Southeast Asia by the separation of continentalblocks from Gondwana and their accretion to thesouthwest margin of Asia, Metcalfe (1996, 2005)suggested that the Sikuleh microcontinental ter-ranes and the West Burma block separated in theLate Jurassic to Early Cretaceous and wereaccreted in the mid Cretaceous. The Woyla Arcwas certainly accreted to Southeast Asia as part ofthe Woyla nappe in the mid Cretaceous (Barber2000; Barber & Crow 2003), but if the volcanic arcis identified correctly as an intraoceanic ratherthan a continental Andean arc, it is not necessaryto look for an age of separation of microcontinentsfrom Gondwana. This conclusion requires furtherconfirmation.Arguments have been put forward in this

account for identifying the West Burma block asthe northward extension of the Cathaysian WestSumatra block, now separated by the development

of the oceanic crust of the Andaman Sea. Theimplication is that it is unnecessary to consider theseparation and accretion of the West Burma blockas an independent continental block; it will haveshared the same tectonic history as Indochina andthe other components of Cathaysia. According toMetcalfe (1996, 2005) Indochina separated fromGondwana in the Late Devonian and was accretedto South China in the Early Carboniferous. At thisstage both West Sumatra and West Burma wouldhave formed part of the Indochina block. Accord-ing to the hypothesis proposed by Barber et al.(2005), during the Triassic an elongated slice,including West Sumatra and West Burma, becamedetached from Cathaysia (Indochina) along amajor transcurrent fault and was translated alongthe western margin of Southeast Asia to itspresent position outboard of the Sibumasuterrane. Further studies are required to confirmor refute this hypothesis.

ACKNOWLEDGEMENTS

The authors have benefited from discussions withAndrew Mitchell (Ivanhoe Myanmar HoldingsLtd), K. Ueno (University of Fukuoka), K. Wakita(Geological Survey of Japan) and M. Ridd duringthe preparation of this paper. The comments andsuggestions of Guest Editor K. Hisada have stimu-lated a reassessment of some of our earlier conclu-sions and are much appreciated.

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