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PGEOLA-291; No. of Pages 17

Proceedings of the Geologists’ Association xxx (2013) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Proceedings of the Geologists’ Association

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Review paper

The Phuket-Slate Belt terrane: tectonic evolution and strike-slip emplacement of a

major terrane on the Sundaland margin of Thailand and Myanmar

a, b

Michael F. Ridd *, Ian Watkinson

a

43 Prospect Quay, Point Pleasant, London SW18 1PR, UK

b

Southeast Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK

A R T I C L E I N F O A B S T R A C T

Article history: The Phuket-Slate Belt terrane can be traced for 1700 km from Phuket to Mandalay, and has a distinct

Received 26 November 2012

stratigraphy and tectonic history. It is characterized by a very thick Carboniferous-Lower Permian

Received in revised form 30 January 2013

succession which includes diamictites interpreted as glacio-marine rift-infill deposited when the

Accepted 31 January 2013

Sibumasu block separated from Gondwana. It was emplaced in the Late Cretaceous-Palaeogene by

Available online xxx

dextral strike-slip movement on a fault system which includes the Khlong Marui and Panlaung Faults.

Southwards the Khlong Marui bounding-fault and its close associate, the Ranong Fault, are postulated to

Keywords:

extend to Sumatra where they align with the restored proto-Indian Ocean location of the India–Australia

Phuket Terrane

transform at the time that both were undergoing dextral displacement and Greater India was moving

Slate Belt

toward its collision with Eurasia. It is suggested that emplacement of the Phuket-Slate Belt terrane was

Phuket-Slate Belt terrane

Western Granite Province the result of its coupling with the north-going India plate, resulting in up to about 450 km of dextral shift

Mogok on the terrane’s bounding fault system. Post-emplacement sinistral movement on the cross-cutting Mae

Strike-slip faults Ping and Three Pagodas Faults offset the terrane boundary resulting in its present outline.

Khlong Marui Fault ß 2013 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.

Three Pagodas Fault

India–Australia transform

Contents

1. Introduction ...... 000

2. Thailand segment of the Phuket-Slate Belt terrane ...... 000

3. Myanmar segment of the Phuket-Slate Belt terrane ...... 000

3.1. Paunglaung Mawchi Zone ...... 000

3.2. Relations of the Slate Belt with the Mogok Metamorphic Belt...... 000

4. Western Granite Province ...... 000

5. History of the Phuket-Slate Belt terrane...... 000

5.1. Emplacement of the Phuket-Slate Belt terrane ...... 000

5.2. Postulated cross-cutting displacement of the Phuket-Slate Belt terrane ...... 000

6. Speculations on the southern extension of the Phuket-Slate Belt terrane ...... 000

7. Conclusions ...... 000

Acknowledgements ...... 000

References ...... 000

1. Introduction understanding their role in the tectonic history of mainland SE

Asia. Garson and Mitchell (1970) described the Khlong Marui Fault

Since the discovery of major strike-slip faults in Thailand over belt which coincides with the prominent bend of the Thai

four decades ago, important advances have been made in Peninsula and separates the upper- from the lower Peninsula,

while Ridd (1971a) identified the NW–SE-trending Mae Ping and

Three Pagodas Faults which extend from western Thailand into

Myanmar. Since then the broader network of faults into which they

* Corresponding author. Tel.: +44 020 8877 9717.

E-mail address: [email protected] (M.F. Ridd). fit has emerged (Fig. 1), and studies of the mineralogy and internal

0016-7878/$ – see front matter ß 2013 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.pgeola.2013.01.007

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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2 M.F. Ridd, I. Watkinson / Proceedings of the Geologists’ Association xxx (2013) xxx–xxx

they were single strands.) He showed that whereas the upper

Carboniferous-Lower Permian succession west of the Khlong

Marui Fault (i.e. in the upper Peninsula) is characterized by very

thick intervals of diamictite (named by Mitchell et al., 1970 the

Phuket Group), such intervals are thin or absent east of the fault, in

the lower Peninsula. And moreover, whereas east of the fault the

Carboniferous-Lower Permian succession is underlain by Devonian

and Lower Palaeozoic rocks, pre-Carboniferous rocks have not

been seen anywhere on the west side of the fault as the base of the

Phuket Group does not reach the ground surface (Fig. 2).

Northwards and westwards the Phuket Terrane passes into

Myanmar where its approximate continuation was called the

Karen–Tenasserim Unit by Bender (1983), although the Slate Belt

(Mitchell et al., 2002, 2004, 2007) is more clearly the continuation

of the Phuket Terrane and is the name adopted here. The name

Phuket-Slate Belt terrane is used here for the combined entity,

while also retaining the component names as appropriate.

A feature of this terrane is that it hosts the Western Granite

Province of SE Asia (Mitchell, 1977; Cobbing et al., 1992; Charusiri

et al., 1993; Putthapiban, 2002). Insofar as those granite (sensu lato)

intrusions are largely confined to the Phuket-Slate Belt terrane it

raises the question: is that because the terrane in some way

favoured the intrusion of those plutons, or were they intruded over

a wider area and then isolated in the terrane by strike-slip faulting

which displaced both terrane and intrusions?

The aim of this paper is to demonstrate the existence of the

Phuket-Slate Belt terrane northward from Thailand into Myanmar,

to comment on its relations with the Western Granite Province of

SE Asia, and to suggest how the terrane was emplaced in its present

position. It is tentatively suggested that dextral strike-slip

movement on its eastern boundary-fault occurred in the Late

Cretaceous–Palaeogene when the terrane became coupled with

the north-going India plate, ceasing only after India itself had

collided with Eurasia. It is suggested, furthermore, that a later

phase of cross-cutting sinistral strike-slip faulting resulted in the

outline of the terrane we see today.

2. Thailand segment of the Phuket-Slate Belt terrane

That the Phuket Terrane is a terrane in the full sense of that term

was argued by Ridd (2009): (i) it has a different and distinctive

stratigraphy, geological and magmatic history from that of

adjacent blocks (Figs. 2 and 3), (ii) it is linear and extensive

(800 km in Thailand alone), and (iii) it is bounded by one of the

major fault systems in Thailand, the Khlong Marui and Three

Pagodas Faults.

As shown in Fig. 3 the Kaeng Krachan Group is the stratigraphic

Fig. 1. Simplified map showing the principal strike-slip faults identified in Thailand

unit which shows the pronounced difference between the Phuket

and part of Eastern Myanmar. Also shown is the eastern limit of Sibumasu, the

Terrane and the adjacent block. In the Phuket Terrane several

Gondwana-derived composite block which includes the Phuket Terrane and Slate

attempts have been made to subdivide it into component

Belt (combined under the name Phuket-Slate Belt terrane). Sundaland was created

by the Triassic collision of Sibumasu with the combined Sukhothai zone and formations (Piyasin, 1975; Raksaskulwong and Wongwanich,

Indochina block. 1993; Chaodumrong et al., 2007). But beneath the top few

hundred metres the succession lacks fossils and marker beds are

not persistent, and therefore stratigraphic units cannot be

fabric of the main fault zones, together with radiometric dating, correlated over any distance and so are unmappable. Mitchell

have thrown light on the timing and the sense of movement on et al. (1970) and Garson et al. (1975), who adopted the name

them (e.g. Lacassin et al., 1993, 1997; Watkinson et al., 2008, 2011; Phuket Group for the entire succession beneath the Ratburi

Watkinson, 2012; Nantasin et al., 2012; Kanjanapayont et al., Limestone in upper Peninsular Thailand, found that they could

2012). divide the succession into only two mappable units which they

Few of those earlier studies considered the fault-bounded named informally the ‘Upper Formation’ up to 200 m thick

blocks themselves, and differences between adjacent blocks which comprising mudstones, thick-bedded sandstones and shales with

might exist. The present study concentrates on just one: the block a bryozoa-rich shelly fauna in its lower part, and the ‘Lower

bounded on its east by the Khlong Marui Fault in Thailand, for Formation’ of unknown thickness which is essentially barren of

which Ridd (2009) adopted the name Phuket Terrane. (This and fossils. (The name Phuket Group has been dropped among

many of the other named faults in Southeast Asia are fault belts geologists in Thailand, but is retained here as it highlights the

with multiple strands, but for convenience are described here as if different successions in the upper and lower Peninsula). The

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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Fig. 2. Simplified geological map of part of Peninsular Thailand showing the contrasting stratigraphy west and east of the Khlong Marui Fault belt. RF is Ranong Fault, and KMF

is Khlong Marui Fault belt.

characteristic lithology of the ‘Lower Formation’ is what Mitchell Island and they concluded that the rocks are glacio-marine as well

et al. (1970) and Garson et al. (1975) called ‘pebbly mudstone’ as slump deposits. Structural complexity and the lack of marker

which generally lacks bedding and occurs as intervals up to scores beds or marker horizons have made it impossible to measure the

of metres thick. A valuable sedimentological study of the ‘Lower thickness of the Phuket Group, and the estimate of >3000 m by

Formation’ was carried out by Ampaiwan et al. (2009) on Phuket Mitchell et al. (1970) and Garson et al. (1975) is probably

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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called the Mergui Series, since named the Mergui Group. Bender

(1983) reviewed the lithological descriptions of earlier workers

and described the Mergui Group as comprising (a) blue-grey to

black splintery argillite with sandstone units tens of metres thick;

(b) ‘dark grey greywacke and/or fine-grained agglomerates with

angular fragments of quartz, slate, quartzite and feldspar in an

argillitic matrix’; (c) conglomerate intercalations in the greywacke

and agglomerate with clasts of granite, quartz and quartzite up to

25 cm diameter; and (d) thin, impure limestone intercalations,

mostly in the argillites. Bender (1983) does not mention pebbly

mudstone nor diamictite but there is little doubt that his

‘greywacke and/or fine-grained agglomerates. . . in an argillitic

matrix’ refers to this rock type. This region is therefore clearly part

of the Slate Belt.

If the Three Pagodas Fault is the NE boundary of the Slate Belt

(as it is the boundary of the Phuket Terrane in Thailand), then it can

be traced nearly as far north as Latitude 168N (Fig. 4). Here the fault

runs close to the sea and the topography is flat-lying over wide

areas, but the strike of the rocks forming the ridges north and south

of Moulmein suggests that the Three Pagodas Fault continues NW

Fig. 3. Simplified stratigraphy of Peninsular Thailand, showing contrasting

near the coast and then connects to the N–S-trending Bilin Fault.

successions west of the Khlong Marui Fault (the Phuket Terrane) and east of the

Insofar as the term ‘Slate Belt’ is used here as a terrane, it is

fault. Grey tone is mudstone and sandstone; brick pattern is limestone or dolomite;

pebble symbol is diamictite. important to consider also that part of the succession which

overlies the Mergui Group. It is clear from Rau (1930) that a

limestone unit overlies the diamictite-bearing Mergui Group and

that it is the same unit as the Ratburi Limestone of upper

conservative. For a comprehensive review of the Phuket Group, as Peninsular Thailand. Rau (1930) gave it the name ‘Moulmein

well as the Kaeng Krachan Group of the wider area, the reader is limestones’, but without proposing a type-section. Brunnschweiler

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referred to Ridd (2009). (1970) studied the Pa-an district (approx 16854 N, 97838 E, which

All of the upper Peninsula north and west of the Khlong Marui is therefore outside the Slate Belt boundary) and adopted the name

Fault is occupied by the Phuket Terrane. Further north the strike of Moulmein Limestone for the carbonate succession there.

the terrane’s rocks swings from SSW–NNE to SE–NW, as does the The geological map of Myanmar (Earth Sciences Research

trend of the long linear faults within the terrane which are visible Division, 1977) groups the ‘Mergui Series’ with the ‘Mawchi Series’,

on satellite images. The bounding fault of this segment of the and shows this combined unit continuing northward bounded on

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terrane is the NW–SE trending Three Pagodas Fault. Again, the the east by the Bilin Fault as far as Papun (18803 N, 97826 E). In the

stratigraphic relations on opposite sides of the fault are similar to Papun area (Fig. 4) a number of faults, including the Mae Ping Fault

those on either side of the Khlong Marui Fault described above. SW belt, converge to form a plexus. The diamond-shaped fault block

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of the Three Pagodas Fault the succession comprises unmeasured (between c. 18825 and 19835 N) which includes the town of

but probably very thick Phuket Group with prominent diamictite Mawchi is important, since it falls either within or else

intervals, overlain by Middle-Late Permian Ratburi Limestone; NE immediately east of the Slate Belt. This area was described by

of the fault there is a more complete succession with Lower Hobson (1941), and its succession of varied terrigenous clastic

Palaeozoic, Devonian and Carboniferous-Permian rocks devoid of rocks with frequent limestone units was given the name Mawchi

any diamictite intervals, overlain by the Middle–Upper Permian Series and appears not to include any units which might be

Doi Phawar Formation limestone (Ridd, 2011; Hagen and Kemper, diamictite. It is interpreted to lie east of the Slate Belt for reasons

1976; Vimuktanandana et al., 2008; Ueno and Charoentitirat, discussed below, meaning that the Mawchi Series of Hobson

2011). (1941) correlates with Permian and younger ‘Plateau sequence’

rocks and not with the Mergui Group. The Slate Belt’s eastern

3. Myanmar segment of the Phuket-Slate Belt terrane boundary in that case is the Taungoo Fault, which passes north to

join the southern part of the Panlaung Fault. A further point of

Tracing the Phuket Terrane of Thailand into Myanmar is interest is the predominantly shale unit mapped by Hobson (1941)

hampered by a scarcity of modern geological data over wide areas, as the Yinyaw Beds (Fig. 5). It contains a shelly fauna including

and by some confusion arising in part from a retention of productid brachiopods of Permian age and apparently underlies

traditional but outmoded terminology. Thus some workers have the Plateau Limestone, seeming therefore to correlate with the

used the term ‘Slate belt’ as synonymous with Mergui Group (see Pharaka Formation of western Thailand, and so strengthening the

below), ignoring the fact that in some areas the Mergui Group is view that this area mapped by Hobson (1941) lies outside the Slate

overlain by a succession including Permian limestone and younger Belt.

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beds which must therefore also be part of the Slate Belt if ‘belt’ is to Between about latitude 18845 N and 21845 N the Panlaung

1

have an areal significance. In this account the term Slate Belt is Fault is very prominent on air photographs and satellite images as

used to mean the Myanmar segment of the Phuket-Slate Belt a deep, rectilinear, series of valleys trending NNW–SSE. From about

terrane. As with its continuation in Thailand, the Phuket Terrane, it

1

is similarly fault-bounded and has a stratigraphy which distin- Confusingly, three similar names but with different geological meanings have

guishes it from adjacent blocks. appeared in the Myanmar literature. In this paper they have the following

meanings. The Paunglaung Mawchi Zone is a graben with a distinctive stratigraphy

Over much of peninsular Myanmar, including the islands of the

which bounds the west side of the Shan Plateau. The Pan Laung Formation is a

Mergui archipelago, a similar succession is present to that across

lithostratigraphic unit which occurs in and adjacent to the Paunglaung Mawchi

the border in Thailand. Rau (1930) described the suite of rocks

Zone. The Panlaung Fault is the prominent western boundary fault of the

making up this region, identifying the oldest rocks as what he Paunglaung Mawchi Zone.

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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Fig. 4. Part of Eastern Myanmar, showing the most prominent faults identified as linear topographic depressions from Google Earth terrain maps and satellite imagery.

Stippled area is the Slate Belt. The inset map shows additionally the strike of bedding, interpreted from linear topographic ridges; the swing of these lineations east of the Bilin

Fault suggests dextral strike-slip movement.

0

latitude 19830 N northwards the Panlaung Fault marks the western of mostly uplifted rhomboidal slivers between the Panlaung Fault

boundary of the Shan Plateau (Garson et al., 1976; Mitchell et al., and the Kyauk Kyan Fault further east. The geometry of many of

2004, 2007, 2012). A number of approximately NW–SE-trending these splays and displacement of rock fabrics suggests that they

faults also splay on to the adjacent blocks (Fig. 5), forming a region may have originated as synthetic strands of a sinistral system that

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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Fig. 5. Compilation map (based on sources shown on the inset map) of part of the western margin of the Shan Plateau and adjacent segment of the Slate Belt. The faults are

interpreted from satellite imagery and Google terrain maps. The map shows the northern part of the Paunglaung Mawchi Zone which forms the western boundary of the Shan

Plateau, separating it from the Slate Belt in which the Mergui Group, Mogok metamorphic complex and Western Province granite intrusions predominate. Overlap of

Hobson’s (1941) map with that of Lain (1973) allows much (if not all) of the Mawchi Series to be correlated with Plateau-sequence rocks together with the overlying

Shweminbon Fm. and younger Mesozoic rocks. PF is Panlaung Fault.

linked to the Mae Ping Fault (Morley, 2004). Present-day fault systems are deep-seated lines of weakness that have

geomorphology, including uplift at restraining double bends and experienced a prolonged period of strike-slip deformation, which

subsidence at releasing double bends, is consistent with a likely began before the Cenozoic.

subsequent period of dextral slip, perhaps related to Neogene Along much of the western margin of the Shan Plateau north

0

dextral tectonics associated with India–Sundaland motion (e.g. of latitude 19830 N the Panlaung Fault marks the western side of a

Vigny et al., 2003; Morley, 2004; Curray, 2005). It is clear that these 2–5 km wide linear belt which Mitchell et al. (2002, 2004) called

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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Group is at the base, in possible fault contact with an ENE-SSW

trending Mong Long Schist Group belt, which in turn passes north

into rocks of the Mogok metamorphic complex, i.e. the so-called

Mogok Metamorphic Belt discussed below. Further NE, in Yunnan,

diamictite re-occurs in the Lower Permian Dingjiazhai Formation

of the Baoshan block but, unlike the Phuket Terrane and Slate Belt,

older Palaeozoic rocks underlie it (Cai and Li, 2001; Jin, 2002). The

Baoshan block lies north of the Keiji-Nangdinghe Fault in Yunnan,

but there is no evidence that this fault is a significant terrane

boundary where it continues into Myanmar as the Kunlong Fault

(Fig. 4).

3.1. Paunglaung Mawchi Zone

One of the earliest studies of the Shan Scarps region, by Garson

et al. (1976), noted a major zone of crushed rocks, the Nwalabo

Fault Complex, over 2 km wide, running nearly N–S and separating

the Shan Plateau with its distinctive stratigraphy, from granite-

intruded Cretaceous and older rocks in the west, the subsequently

Fig. 6. Simplified stratigraphy west and east of the Paunglaung Mawchi Zone in the

named Slate Belt. They noted that the western bounding fault of

Shan Scarps region of Myanmar. The stratigraphy of the intervening Paunglaung

that complex was a high-angle structure which they called the

Mawchi Zone is discussed in the text. On the left-hand side of the figure the Slate

Belt is assumed here to include the Mogok metamorphic complex, and it is Panlaung Fault. Later workers on the Shan Scarps region inferred a

tentatively suggested that the protoliths of these schists, gneisses and marbles are

number of thrusts, particularly eastward-directed ones, separating

the Mergui Group and other sedimentary rocks of the Slate Belt. Dark grey tone is

the Slate Belt from the Shan Plateau, and noted that the Nwalabo

mudstone, siltstone and sandstone; brick pattern is limestone or dolomite; and

Fault Complex was present within this plexus of thrusts (United

pebble symbol is diamictite.

Nations, 1978). Mitchell et al. (2002, 2004) later reinterpreted the

surface geology, describing a narrow, linear, belt of Mesozoic rocks

between the Slate Belt and the Shan Plateau as the Paunglaung

the Paunglaung Mawchi Zone (Fig. 5). The rocks in this zone are Mawchi Zone (Figs. 5 and 6). It included the Nwalabo Fault

structurally and stratigraphically complex and are discussed Complex as well as a number of stratigraphic units which they

0

below; in places units mapped as being part of the Paunglaung described extending south from about latitude 21815 to beyond

Mawchi Zone extend a few kilometres beyond the zone as mapped, Mawchi (Fig. 4).

onto the adjacent Slate Belt and the Shan Plateau. For example, Structural complexity, discontinuous outcrops and a general

Garson et al. (1976) mapped the Jurassic–Cretaceous Pan Laung lack of fossils or marker beds have hampered a full understanding

Formation cropping out several kilometres west of the Panlaung of the Paunglaung Mawchi Zone. It is a succession of shales, cross-

Fault. bedded sandstones, conglomerates, occasional limestones, with

Diamictite is a distinctive lithology of the Mergui Group in this red-beds and coal seams indicating parts of the sequence are non-

segment of the Slate Belt, as it is in southernmost Myanmar. But marine. What is thought to be the lowest unit, the Kyaukhsu Taung

the width of the Mergui Group outcrop is very much narrower than Formation, has yielded marine bivalves, foraminifera and algae

further south, continuing to narrow northwards from about 15 km from several localities, indicating an age within the range mid-

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at latitude 20830 N to zero at latitude 21815 N. The western Jurassic to Aptian (Garson et al., 1976; Mitchell et al., 2012).

marginal area of the Shan Plateau is widely referred to as the Shan Turbidite intervals are also present, as are debris-flow conglom-

Scarps, and the stratigraphy of this region, west and east of the erates. An interval of volcanigenic conglomerates associated with

0

Paunglaung Mawchi Zone (Fig. 6), is based on Thein (1973), Garson diorite and rhyolite occurs in one area (at around latitude 21800 ).

et al. (1976), United Nations (1978), Wolfart et al. (1984), Oo et al. The thickness of the entire succession making up the Paunglaung

(2001), Mitchell et al. (2002, 2007, 2012), and Mitchell (pers. Mawchi Zone is not clear, but the Kyaukhsu Taung Formation alone

comm., 2012). is about 1000 m thick (United Nations, 1978). The fossiliferous

The contrasting stratigraphy is very apparent, as noted by Jurassic–Cretaceous Pan Laung Formation is described in some

Mitchell et al. (2012, fig. 4). The Slate Belt stratigraphy is similar to detail by Thein (2004) and Saing (2004) as a >1500 m-thick unit

that of southern Myanmar and upper Peninsular Thailand. deposited in a basin whose eastern boundary was on the western

However, a short distance to the east, across the Paunglaung marginal zone of the Shan Plateau.

Mawchi Zone, the succession in this part of the Shan Plateau is On the western margin of the Shan Plateau the Late Triassic-

broadly similar to lower Peninsular Thailand (south and east of the Early Jurassic Shweminbon Formation is considered by Mitchell

Khlong Marui Fault) and the part of Thailand north of the Three et al. (2002, 2004, 2012) to be distinct from the Paunglaung

Pagodas Fault. Notable differences from the latter are (a) the Mawchi Zone. It is a turbidite succession containing the bivalves

presence of a thick succession of undated but assumed Upper Halobia sp. and Astarte sp. (Oo, pers. comm., 2012) which overlies

Proterozoic rocks including turbidites beneath a Cambrian and is intimately deformed with the Plateau Limestone (Fig. 5),

unconformity (Garson et al., 1976), neither of which crop out both showing east-vergent folds. Hobson (1941) mapped large

anywhere in Thailand and peninsular Malaysia, and (b) a major areas of the Mawchi region (Fig. 5) as the Mawchi Series but failed

hiatus at the base of the Middle–Upper Permian Plateau Limestone to determine its age or even whether it underlies or overlies the

(Garson et al., 1976; Wolfart et al., 1984; Oo et al., 2001) and, less Permian-Triassic Plateau Limestone. Later mapping by Lain (1973)

certainly, beneath the Permian Yinyaw Beds of Hobson (1941). overlaps Hobson’s (1941) map, and shows the Shweminbon

The Mergui Group is absent from the northern margin of the Formation and other Mesozoic formations extending from the

Shan Plateau (La Touche, 1913; Mitchell et al., 1977), where a Shan Plateau into the Mawchi region and apparently confirming

‘Plateau type’ succession broadly similar to the succession in the that the Mawchi Series correlates with the Permian and younger

Shan Scarps (Fig. 6) is present. The Proterozoic Chaung Magyi succession. The limestone units within the Mawchi Series mapped

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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by Hobson (1941) in that case are probably tightly folded and by Searle et al. (2007) who carried out U–Pb (isotope-dilution-

faulted outcrop slivers of Plateau Limestone. thermal-ionization-mass-spectrometry) and U–Th–Pb (laser-abla-

The rocks of the Paunglaung Mawchi Zone are interpreted here tion-multicollector-inductively-coupled-plasma-mass-spectrom-

to have been deposited in a linear basin or basins which developed etry) analyses on the Mogok metamorphic complex cropping out

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along a mobile belt. Initially this belt may have been a zone of between 21830 N and 22810 N North. They interpreted their

sagging as a back-arc basin developed, but with time a line of results as indicating two Tertiary metamorphic events, each

rupturing developed with pull-apart basins in which non-marine culminating in granite intrusions, the first in the Palaeocene and

deposition was periodically interrupted by marine incursions. the second spanning the Late Eocene to Oligocene boundary. More

Seeking an analogue for the Paunglaung Mawchi Zone, the recently Mitchell et al. (2012) described U–Pb zircon analyses on

0

1000 km-long Median Tectonic Line of Japan has obvious 18 intrusive and metamorphic rock samples between 22821 N and

0

similarities (Taira et al., 1989; Taira, 2001; Tanimoto, 2005; 16837 N, interpreting the results as showing protolith ages up to

Isozaki et al., 2010). The marine and non-marine Upper Cretaceous 491 Ma and an earliest Cretaceous minimum age for the regional

sediments of the latter were deposited while sinistral strike-slip metamorphism. The implications of the different authors’ findings

displacement on the Median Tectonic Line was active along the are discussed below.

northwestward-subducting eastern margin of the Eurasia plate. At Relict fossils in the Mogok metamorphic complex have been

least 1500 km of sinistral shift is interpreted to have taken place. mentioned by several writers but details of them and their precise

The analogy cannot be taken too far, however, as the Median locations are uncertain. According to Mitchell (pers. comm., 2012)

Tectonic Line has a forearc-basin setting whereas the Phuket-Slate at least some of them were found in the metamorphosed

Belt terrane bounding-fault is suggested to have developed on the Palaeozoic rocks of the western fringe of the Shan Plateau in the

site of a back-arc basin. What it does highlight is the important role Mandalay area mentioned above, and so throw no light on the

of shifting ribbon-terranes along the margin of Eurasia in a Mogok metamorphic complex itself. At several localities in the

transpressional setting. Slate Belt the Mergui Group has acquired a schistosity (Mitchell

et al., 2007; Mitchell, 2012) and it is considered here that the

3.2. Relations of the Slate Belt with the Mogok Metamorphic Belt Mergui Group diamictite and slates, as well as such older

Palaeozoic rocks as may be concealed beneath the Mergui Group

Gneisses, schists and marbles of upper amphibolite and locally including the Lower Palaeozoic, are the protoliths of the Mogok

granulite facies extend in an irregular outcrop belt from about metamorphic complex. An orthogneiss from Sedawgyi, north of

0

latitude 19815 N (Hobson, 1941) northwards through the Manda- Mandalay, yielded a protolith U–Pb age of 491 4 Ma (Cambrian)

lay area before swinging to a NE trend towards Yunnan, and then which lends weight to this interpretation (Mitchell et al., 2012).

continuing northwards to the eastern Himalayan syntaxis (Mitch- The western boundary of the Slate Belt including the Mogok

ell, 1993; Mitchell et al., 2007). (For present purposes the rocks metamorphic complex, is concealed beneath an alluvial and locally

themselves are referred to here informally as the Mogok a Neogene cover, but most writers on the subject (e.g. Barley et al.,

metamorphic complex, since the term ‘belt’ can be confused with 2003; Bertrand and Rangin, 2003; Searle et al., 2007; Mitchell et al.,

the Slate Belt, of which the rocks of the Mogok metamorphic 2004, 2007, 2012) assume that it extends west to the still-active

complex are here considered to be a part.) Recent work on this Sagaing Fault (Fig. 1). That same fault has in the past been called

group, including radiometric dating, has concentrated on the the Shan Boundary Fault but insofar as it bounds the Slate Belt and

southern part, i.e. south of the latitude of the gemstone-mining not the Shan Plateau, that name is considered inappropriate.

town of Mogok (Fig. 4). South of Mandalay the rocks of the Mogok

metamorphic complex have a close spatial relationship with the 4. Western Granite Province

Mergui Group (and possibly also with the ‘Moulmein Limestone’

overlying the Mergui Group). Their contact is mostly obscured by At the beginning of this paper it was stated that a feature of the

alluvium but in places a fault has been inferred (e.g. the Sakhanya Phuket-Slate Belt terrane is that it hosts the Western Granite

Taung Fault; Mitchell et al., 2007, fig. 8) and elsewhere granite Province of Southeast Asia (Cobbing et al., 1992; Charusiri et al.,

intrusions intervene (Fig. 5). 1993; Putthapiban, 2002). The distribution of the major plutons of

In the Mandalay area the eastern boundary of the Mogok that province are shown in Fig. 7.

metamorphic complex is unclear. Here the Mergui Group rocks To place the Western Province in context, the adjacent province

including its characteristic diamictite intervals are not present, (the Central or Main Range Province) extends from easternmost

having wedged out further south, and the rocks of the westernmost Myanmar and northern Thailand to the southern part of peninsular

part of the Shan Plateau are themselves partly metamorphosed to Malaysia. The plutons in the northern Thailand part of this

phyllite, quartzite and marble (Mitchell et al., 2007, fig. 4). The province are petrologically variable and Cobbing et al. (1992)

picture is complicated further by uncertainty over whether any divided them into a central belt of deformed and migmatitic

fossiliferous Permian limestone observed in the field is the Plateau granites, a western marginal belt, and an eastern marginal belt.

Limestone of the Shan Plateau province or the Moulmein With rare exceptions they are S-type granites predominantly of

Limestone of the Slate Belt. Triassic age which were intruded as so-called stitching plutons at

The age of metamorphism of the Mogok metamorphic complex the time of the collision of Sibumasu with the combined terrane of

is controversial and the history of earlier studies has been outlined the Sukhothai block and Indochina block (Cobbing et al., 1992;

by Mitchell et al. (2012). Mitchell et al. (2007) reported on the Charusiri et al., 1993; Putthapiban, 2002; Metcalfe, 1999; Sone and

relationships of the metamorphic rocks and associated intrusive Metcalfe, 2008). A number of plutons in the northern Thailand part

rocks, concluding that there were at least two metamorphic events, of the Central or Main Range Province have yielded Cretaceous ages

one before and one after the intrusion of late Jurassic to early and on the face of it would seem to be outliers of the Western

Cretaceous calc-alkaline igneous rocks. The first sensitive-high- Province. However, they were examined by Cobbing et al. (1992)

resolution-ion-microprobe (SHRIMP) U–Pb studies on the Mogok who state ‘Apart from their age, none of these granites are in any

metamorphic complex were reported by Barley et al. (2003) who way similar to those of the Western Province.’ The criteria for their

determined zircon ages of Jurassic, mid-Cretaceous and Eocene, judgement are not known. Palin et al. (2013) describe evidence

and concluded that an Andean-type southern margin of Eurasia that the magmatic protolith of the Lansang Gneiss close to the Mae

was present throughout that period. Those findings were followed Ping Fault was emplaced between c. 123 and 114 Ma (Early

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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Cretaceous), although whether that would also be considered by

Cobbing et al. to be different from the granites of the Western

Province is not known. Another apparent anomaly is the Khao

Phanom Bencha pluton east of the Khlong Marui Fault and north of

0 0

Krabi (8815 N, 98856 E) (Fig. 2). This is a hornblende adamellite

which gave a K–Ar age of 55 Ma, although Garson et al. (1975) who

collected the sample concluded that this age indicates a heating

event after the rock was intruded; Cobbing (2011) subsequently

referred to it as an I-type granite.

The granites which make up the Western Province are of S- and

I-type, and although they were thought all to be of Cretaceous age

that age bracket has now been expanded, in part because many

previously determined ages (e.g. Charusiri et al., 1993) were based

on mica Ar–Ar cooling spectra. U–Pb zircon ages of 212 2 Ma and

214 2 Ma (both Late Triassic) have been obtained from an I-type

granite from Phuket in Thailand, albeit with zircon rims showing a

Late Cretaceous thermal overprint of 81.2 1.2 and 85–75 Ma (Searle

et al., 2012). And associated with the Mogok metamorphic complex in

Myanmar there are numerous examples of granite-emplacement ages

extending up to the Eocene (Searle and Morley, 2011; Mitchell et al.,

2012).

Whereas the granite plutons of the Central Province are spread

over a broad region straddling the suture at the eastern boundary

of Sibumasu (Fig. 1), those of the Western Province are confined to

the Phuket-Slate Belt terrane, and are bounded on the east by the

faults described above, pace Cobbing (2011) who speculates that

the Khao Phanom Bencha pluton may be an outlier of the Western

Province (Fig. 7). The northwards extension of the Western

Province through northern Myanmar is uncertain, although

Cobbing et al. (1992) conjectured that it continues northeastward

beyond Mogok. Southwards, beyond Phuket, any extension must

lie beneath the waters of the Malacca Strait, although one small

0

pluton in Sumatra, the Hatapang Granite SE of Lake Toba (c. 2815 N,

0

99830 E) is considered by Cobbing (2005, p. 61) to be possibly the

sole representative of the Western Province in Sumatra.

The question arises: is confinement of the Western Granite

Province to this long fault-bounded terrane because in some way it

favoured intrusion of the granites, or were they emplaced (perhaps

in a broader belt than we now see) and then displaced from their

original location by strike-slip shift on the bounding fault system?

For the latter explanation to be plausible a strike-slip displacement

of many hundreds of kilometres would be required, and this paper

is unable to demonstrate displacement over such a distance. The

answer to the question therefore remains unresolved for the time

being.

5. History of the Phuket-Slate Belt terrane

There is a wealth of stratigraphic and fossil evidence that the

Sibumasu plate (including the diamictite-bearing terrane which is

the subject of this paper) was formerly part of Gondwana from

which it rifted in the Late Carboniferous-Early Permian and began

to drift northwards (see reviews by Metcalfe, 1999, 2011; Ueno

and Charoentitirat, 2011; Barber et al., 2011). The evidence for that

Fig. 7. The Western Granite Province of Thailand and Myanmar (plutons shown in

rifting in Thailand was examined by Ridd (2009) who concluded

colour), and the Central Granite Province (plutons shown black). Granite plutons in

Myanmar are from Earth Sciences Research Division (1977), and those in Thailand that the diamictites were the product of Gondwana glaciers

are based on Department of Mineral Resources (1999) and Cobbing et al. (1992). The dropping their load at the western margin (in present-day

assignment of plutons to one or other province is based on Cobbing et al. (1992),

orientation) of the Phuket Terrane from where it was re-deposited

although note that they studied many fewer plutons than are shown on this map,

by mass-flow in the rapidly subsiding marine rift basin. While he

and the boundary faults were not recognized by them at that time; the faulted

boundary shown here between the two provinces is nevertheless consistent with recognized that the bounding fault system of the Phuket Terrane

the boundary of Cobbing et al. (1992). (For interpretation of the references to colour (the Khlong Marui and Three Pagodas Faults) probably followed

in this figure legend, the reader is referred to the web version of the article.)

reactivated ancient rift-boundary faults, he did not consider the

possibility that there was later, major, strike-slip displacement

along those faults. That possibility is discussed here, and Fig. 8 is a

cartoon illustrating the inferred history of the Phuket-Slate Belt

terrane and the Western Granite Province of which that terrane is

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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sub-tropical as its drift continued (reviewed by Ueno and

Charoentitirat, 2011). Through that period the trailing edge of

Sibumasu was a passive margin, but at the end of the Middle

Triassic the leading edge of Sibumasu collided with the Indochina

block (including in Thailand the Sukhothai magmatic arc)

resulting in the SE Asia continental block we know as Sundaland.

Ridd (2012) recently documented evidence for the Middle-Late

Triassic timing of this collision in the Thailand-Malaysia border

area, although this timing remains contentious, some arguing for

an end-Permian to Early Triassic collision (Sevastjanova et al.,

2011; Barber and Crow, 2005). Sibumasu’s northward drift was

halted, but sea-floor spreading would have continued behind it, in

Mesotethys, and the consequence was that subduction would

have switched from the now-closed Palaeotethys margin to

Mesotethys. An Andean magmatic arc formed along the southern

margin of Eurasia, of which Sibumasu was now a part. Magmatic

ages of 212 2 and 214 2 Ma (Late Triassic) from I-type granites

at Phuket, reported by Searle et al. (2012), can thus be explained

although it is noteworthy that no other direct evidence of a

magmatic arc older than Jurassic has yet come to light. If confirmed,

it represents the birth of the Western Granite Province. The near

absence of coeval volcanic rocks over large parts of the Western

Granite Province might be explained by deep erosion having

removed them, although rare occurrences of quartz porphyry were

observed by Rau (1930) associated with granite in peninsular

Myanmar, and Mitchell et al. (2002) describe dacites, rhyodacites,

silicic porphyries, basalts and andesites associated with granites in

0

the Yebokson area (c. 20845 N) which they cite as evidence of a late

Jurassic-early Cretaceous magmatic arc west of the Shan Plateau.

Cropping out in a belt up to about 30 km wide along the western

margin of the Shan Plateau in Myanmar, a late Triassic-early

Jurassic succession of tightly folded beds including turbidites has

been mapped by Lain (1973) and Mitchell et al. (2002, 2004, 2007,

2012) as the Shweminbon Formation, overlying the Permo-Triassic

Plateau Limestone (Fig. 5). Lain (1973) shows the Shweminbon

Formation continuing south to become part of what Hobson (1941)

called the Mawchi Series. Mitchell et al. (2004, 2007, 2012)

interpret these rocks as having been deposited in an ocean which

Fig. 8. Cartoons (at different scales) to illustrate the history of the Phuket-Slate Belt

they name neo-Tethys II and which they propose existed between

terrane. (a) In the Early-Middle Permian, Sibumasu having rifted from Gondwana

the Shan Plateau and the Slate Belt at that time. They invoke

preserved deep, rift-bounded basins of glacio-marine sediments. (b) An Andean-

westward subduction of the neo-Tethys II oceanic crust beneath

type margin developed with intrusion of I-type granites, possibly as early as the Late

Triassic, which persisted at least to the Cretaceous and probably into the the Slate Belt in the Jurassic, and final closure of the ocean by the

Palaeogene. The back-arc basin is conjectural, but might account for the deep- mid-Cretaceous. It is an ingenious model but a simpler explanation

marine Shweminbon Formation in Myanmar. (c) Dextral strike-slip faulting was

of the Shweminbon Formation turbidites is that they were

probably most active in the Late Cretaceous–Palaeogene; by the end-Palaeogene

deposited on the western margin of the Sundaland block (if there

the Phuket-Slate Belt terrane had been dislocated from its original location, and

was no magmatic arc west of the Shan Plateau at that time - see

together with its granites, i.e. the Western Granite Province, was translated along

the margin of Sundaland. PMZ is the Paunglaung Mawchi Zone. caveat, above), or if separated from the Mesotethys by a magmatic

arc they were possibly deposited in a back-arc basin on the site of

what would later become the strike-slip fault boundary of the Slate

the host. One caveat should be borne in mind when attempting to Belt, as shown on Fig. 8. Hyndman et al. (2005) have pointed out

reconstruct this history: the terrane was translated an unknown that back-arc basins, because of their thinned, hot and weakened

distance to its present position and has not always been there. lithosphere, tend to become long-lived mobile belts. An absence of

The quantity of terrigenous material now represented by the ophiolite in the Paunglaung Mawchi Zone suggests that in that

Upper Carboniferous to Lower Permian Phuket Group and Mergui region, if the magmatic arc and back-arc basin existed at that time

Group is immense, and it has usually been interpreted to have been the latter is likely to have been incipient, although between the

sourced in the west (then the south) from the Gondwana craton Tengchong and Baoshan blocks in Yunnan the Nujiang-Luxi

(Ridd, 1971b, 2009). The presence of offshore detrital diamonds ophiolite (Cai and Li, 2001) is evidence of a more fully-developed

thought to have eroded out of the Phuket Group (Garson et al., basin there, floored by oceanic crust.

1975) can best be explained as having a source in the kimberlite Highly deformed rocks in the Mogok metamorphic complex,

intrusions of Australia, but more work is needed to seek matches including amphibolites and migmatites, are sealed by an

between the diamictite clasts and possible source areas. unfoliated biotite granite dyke emplaced at 59.5 0.9 Ma (Searle

By Middle Permian times the rifting was complete and the et al., 2007). Metamorphism may have occurred during the Late

Sibumasu block began its drift northwards across Palaeotethys Cretaceous and Palaeocene up to about the time of dyke emplace-

while Mesotethys widened behind it. A predominantly carbonate ment, as suggested by a U–Th–Pb monazite age of 58 1 Ma from an

succession was deposited across Sibumasu, the marine faunas augen gneiss of the Mogok metamorphic complex (Searle et al., 2007).

reflecting the progressive changing environment from cool to Metamorphism and I-type granitoid magmatism in the Mogok

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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metamorphic complex indicate that crustal thickening along the Ar–Ar plateau age of host mylonitic migmatites (Thanetian-

Andean margin of Sibumasu persisted into the Palaeogene (Barley Ypresian). Both periods pre-date the India–Eurasia collision, but

et al., 2003; Searle et al., 2007; Searle and Morley, 2011). The presence can be related to rapid, highly oblique, subduction of the

also of S-type granites in the Phuket Terrane-Slate Belt was explained Mesotethys ocean plate ahead of India under northern Sundaland.

by Morley (2004) as the result of the Late Cretaceous–Palaeogene Both periods of dextral movement pre-date the 33–30 Ma mica

collision of the West Burma block (i.e. west of the Sagaing Fault) with Ar–Ar plateaux from the Three Pagodas and Mae Ping Faults,

Sibumasu, an interpretation he later withdrew (Morley, pers. comm., interpreted to represent the timing of sinistral shear (Lacassin

2012). et al., 1993, 1997) or regional cooling prior to sinistral shear

(Morley, 2004; Searle and Morley, 2011).

5.1. Emplacement of the Phuket-Slate Belt terrane (e) Mylonitic granitoids within the Khlong Marui Fault yield Late

Triassic to Late Cretaceous zircon U–Pb outer core ages,

Evidence which points towards fault emplacement of the surrounded by 55 3 Ma to 45.6 0.7 Ma (Ypresian–Lutetian)

Phuket-Slate Belt terrane includes: rims (Kanjanapayont et al., 2012). Those authors interpret the

rims as metamorphic overgrowths formed during dextral strike-

1. Its linearity and essential stratigraphic uniformity over a slip shearing under amphibolite facies conditions (Kanjanapayont

distance of at least 1700 km, from Phuket to Mandalay. et al., 2012). Alternatively, they may be magmatic rims related to

2. The presence of great thicknesses of diamictite only in this Ypresian acid magmatism along the peninsula (Watkinson et al.,

terrane, distinguishing it from adjacent terrane(s) to the east 2011) prior to further dextral shear.

where diamictite is absent, or if present is thin (c.f. Singa (f) Lutetian to Rupelian dextral shear along the Ranong and

Formation at Langkawi, NW Malaysia, Stauffer and Lee, 1986). Khlong Marui Fault zones is recorded by Ar–Ar ages from syn-

3. The apparent coincidence of this terrane with the Western tectonic (e.g. inter-boudin) mica grains, solid-state mylonitisa-

Granite Province. tion of a granitoid with a zircon U–Pb emplacement age of

4. Presence of a major, deep-seated, bounding fault or fault system 47.4 0.5 Ma (Watkinson et al., 2011), and mica whole-rock Rb–

along its eastern side. Sr isochrons from mylonitic rocks (Kanjanapayont et al., 2012).

5. Presence in Myanmar of the Paunglaung Mawchi Zone, a long, (g) A study of ductile boudinage in the Ranong and Khlong Marui

narrow, structurally complex basin preserving a mixed marine Fault zones of Peninsular Thailand led Watkinson (2012) to

and non-marine late Mesozoic succession interpreted to overlie calculate that >220 km of dextral shift occurred on this pair of

a mobile belt which was to become the locus of strike-slip faults in the period 47.6 0.8 Ma to 40.85 0.26 Ma (Middle faulting. Eocene).

0 0

6. Long, steeply dipping, wide and complex fault systems (h) The Thabsila metamorphic complex (approx. 14815 N, 99810

bounding uplifted and subsided rhomboidal slivers, which are E) is a high-grade metamorphic enclave in the low-grade Three

strong indicators that the terrane-bounding faults are strike- Pagodas Fault shear zone. It was studied by Nantasin et al.

slip. Close to the intersection between the Mae Ping and Three (2012) who describe U–Pb zircon-rim ages of 51–57 Ma (Late

Pagodas Faults at Papun, the rotated strike of bedding (Fig. 4, Palaeocene–Early Eocene) from high-grade Thabsila gneiss

inset map) is a strong indicator of dextral strike-slip movement. which they interpreted as the age of peak metamorphism,

7. Further south, detailed structural and geochronological studies while Rb–Sr biotite cooling ages of 32–36 Ma (Late Eocene–

on the Khlong Marui and Ranong Faults which show a prolonged Early Oligocene) they relate to exhumation due to sinistral

history of dextral strike-slip movement, much of which pre- strike-slip movement on the Three Pagodas Fault. Although

dates the India–Asia collision (Watkinson et al., 2008, 2011; Nantasin et al. (2012) see both these events in the context of

Watkinson, 2012; Kanjanapayont et al., 2012). the India–Eurasia collision, the peak metamorphic event can

also be explained by Late Palaeocene–Early Eocene dextral shift

As regards the timing, there are several lines of evidence to along the Phuket-Slate Belt terrane bounding fault (Khlong

suggest that dextral movement on the Phuket-Slate Belt terrane- Marui Fault), its associate, the Ranong Fault, and partly along a

bounding fault system occurred continuously or intermittently dextral pre-cursor to the sinistral Three Pagodas Fault.

over an extended period (Fig. 9): Subsequent Late Eocene–Early Oligocene sinistral slip along

the Three Pagodas Fault displaced the Khlong Marui and

(a) Granitic orthogneisses in the Mogok metamorphic complex Ranong faults and the Phuket-Slate Belt terrane, as discussed

yielded a U–Pb zircon age of 170 Ma (Middle Jurassic), below.

leading Barley et al. (2003) to conclude that an Andean-type (i) Morley et al. (2011) concluded that dextral strike-slip

margin was present along the Eurasia margin from that time. deformation along the Ranong and Khlong Marui Fault zones

(b) Zircon and apatite fission-track studies in Thailand led Morley began some time during Late Cretaceous–Palaeogene granite

(2004) to postulate a broad regional uplift from about 80 Ma to emplacement, a time when subduction-related processes

40 Ma, which, in combination with a wide, braided and dominated the western margin of Sundaland, and thermally

splaying fault pattern, he interpreted as evidence of a Late weakened the continental crust.

Cretaceous–Palaeogene transpressional period at the Sunda-

land margin. Although a detailed chronology does not emerge from the

(c) Bertrand and Rangin (2003) state that along the Panlaung Fault above lines of evidence, it is hard to escape the conclusion that the

‘microtectonic analyses. . . provide strong evidence for a dextral bounding fault system of the Phuket-Slate Belt terrane underwent

strike-slip component of motion.’ dextral strike-slip movement in the Cretaceous and continued to

(d) A detailed study of the Ranong and Khlong Marui Faults in the Eocene.

Peninsular Thailand (Watkinson et al., 2008, 2011) showed that

the dominant Lutetian mylonitic fabric was preceded by two 5.2. Postulated cross-cutting displacement of the Phuket-Slate Belt

periods of dextral strike-slip shear: (1) before the 79.9 0.7 Ma terrane

(Coniacian) zircon U–Pb emplacement age of an interkinematic

granite; and (2) after the 58.7 0.6 Ma biotite Ar–Ar cooling age In view of the angular relationship of the Khlong Marui and

of an interkinematic granite, but before the 51.2 0.7 Ma biotite Ranong Faults with the Three Pagodas and Mae Ping Faults it was

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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Fig. 9. Summary of radiometric age determinations on the Khlong Marui and Ranong Fault belts compared with other relevant age determinations in the region. Numbers in

brackets are references: 1, Watkinson, 2012; 2, Palin et al., 2013; 3, Dunning et al., 1995; 4, Barley et al., 2003; 5, Searle et al., 2012; 6, Morley, 2004; 7, Searle et al., 2007; 8,

Mitchell et al., 2012; 9, Searle and Morley, 2011; 10, Bertrand and Rangin, 2003; 11, Nantasin et al., 2012; 12, Kanjanapayont et al., 2012; 13, Watkinson et al., 2011.

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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here to examine its likely continuation north into China although it

is known that Late Carboniferous-Early Permian diamictite

intervals crop out there. However, in order to stimulate discussion

and encourage further work, the following speculations are offered

on the terrane’s possible southern extension.

Whether or not the thesis outlined in this paper has any

validity, structures of the magnitude of the Khlong Marui Fault and

its associate, the Ranong Fault, can be expected to extend

southwards beneath the Strait of Malacca. Most authors who

have contemplated this matter, including Garson and Mitchell

(1970) who first described it, have extrapolated the fault SSW to

follow the 200 m bathymetric contour which marks the edge of the

Malacca Strait continental shelf (e.g. Wood, 1985; Hutchison, 1989,

1994; Hall and Morley, 2004). However, offshore petroleum

exploration in the Thai sector of the Andaman Sea has revealed that

horst and graben structures with a predominantly NNE-SSW trend

are also present to at least 200 km west of the continental shelf

edge (Morley and Racey, 2011). One of the most prominent is the

eastward-downthrowing offshore continuation of the Ranong

Fault (Fig. 11). A direct link with the mapped faults onshore

Sumatra is not possible although N–S trending horsts and grabens

characterize this part of the northern Sumatra offshore (Liew,

1994; Clure, 2005). Liew (1994) and Barber and Crow (2005)

reproduce petroleum industry mapping which shows an arcuate,

westward-downthrowing Cenozoic hinge line, the Rayeu Hinge,

Fig. 10. (a) Present outline of the Sibumasu block and the Phuket-Slate Belt terrane,

0

from Fig. 1. (b) Proposed disposition of the terrane after an unknown amount of intersecting the coast at about 97830 E. It is not possible to state

dextral movement on the Khlong Marui and Panlaung Faults had emplaced them, with any confidence whether this hinge line is the re-activated

and before Eocene–Oligocene sinistral movement on the Mae Ping and Three

Ranong or Khlong Marui Fault, but the surface expression of the

Pagodas Faults dislocated both the boundary of the Phuket-Slate Belt terrane and

hinge line is probably the Lokok Kutacane Fault of Stephenson et al.

the eastern boundary of Sibumasu.

(1982), Liew (1994) and Barber et al. (2005) (Fig. 11).

Hutchison (1994) interpreted the Lokok Kutacane Fault as a

previously assumed that they were parts of a conjugate fault terrane boundary, seeming to terminate the chain of diamictite-

system (e.g. Polachan and Sattayarak, 1989; Lacassin et al., 1997; bearing Bohorok and Mentulu Formation outcrops which run the

Tapponnier et al., 1986; Nantasin et al., 2012). But this study has length of the island (Fig. 11). But he considered this fault then to

concluded that the two sets of faults have different origins and extend northwards offshore to bound the diamictite-bearing

were active at different times, and so they cannot be regarded as a Phuket-Slate Belt terrane on its west, not east. That way he was

conjugate system. The evidence cited above supports the able to contain all the diamictite occurrences of Southeast Asia,

interpretation that the sinistral Three Pagodas and Mae Ping including the outlying Singa Formation at Langkawi, in a single

Faults post-dated the strike-slip emplacement of the Phuket-Slate broad terrane. But Hutchison’s (1994) thesis raises difficulties: (a)

Belt terrane. Fig. 10 shows the terrane forming the western part of he projects the Lokok Kutacane Fault northwards offshore and has

Sibumasu, and it shows the prominent displacement of its outline to assume its continuation west of the Phuket-Slate Belt terrane;

by the Mae Ping and Three Pagodas Faults amounting to a total of and (b) it fails to acknowledge the presence of the major fault

350 km. The Mae Ping Fault also cuts the eastern boundary of the system which, from Phuket to Mandalay, bounds the distinctive

Sibumasu block, but insofar as this boundary runs along a segment stratigraphy of the Phuket-Slate Belt terrane on its east.

of the fault itself the amount of displacement cannot be quantified. A case can be made for linking either the Khlong Marui Fault or

Individually, the Mae Ping Fault’s sinistral dislocation of the the Ranong Fault with the Lokok Kutacane Fault; the Ranong Fault

0

Phuket-Slate Belt terrane boundary (x to x in Fig. 10) is about is the connection favoured by Stephenson et al. (1982). Of the two

80 km. Sinistral displacement of the boundary by the Three faults in Peninsular Thailand the greater displacement was on the

0

Pagodas Fault (y to y ) is 270 km. These figures compare with a Ranong Fault (Watkinson et al., 2008, 2011) but it is the Khlong

total of 300 km of Eocene–Oligocene movement on both faults Marui Fault which is the terrane boundary. The region of

estimated by Lacassin et al. (1997). Chumphon in upper Peninsular Thailand (Fig. 2) may represent

The radiometric studies on the Thabsila metamorphic complex a relay zone, north of which the Ranong Fault becomes the

in the Three Pagodas shear zone by Nantasin et al. (2012) cited bounding structure.

above, lend support to the two-stage model described here: first It is suggested that both faults are related to a discontinuity in

the emplacement of the Phuket-Slate Belt terrane by dextral shift the deeply buried slab which subducted beneath Sumatra in the

on its bounding faults in the Late Palaeocene–Early Eocene (51– Late Cretaceous to Palaeogene. Hall (2012), in his reconstruction of

57 Ma), followed by dislocation of the terrane boundary and the history of the Indian Ocean, postulated that the Khlong Marui

exhumation of the older metamorphic rocks by sinistral shift on Fault and the Ranong Fault formed from 90 to 45 Ma above the

the major NW–SE faults in the Late Eocene–Early Oligocene (32– subducted transform where there was a change from the

36 Ma). northward subduction of India on the west side, to a cessation

of the northward subduction of Australia on the east side. Our

6. Speculations on the southern extension of the Phuket-Slate preferred interpretation goes one step further than Hall (2012) in

Belt terrane suggesting that the entire Phuket-Slate Belt bounding-fault system

as far north as Mandalay is the landward extension of that dextral

This paper has argued for the existence of a distinct fault- India–Australia transform (Fig. 12). Such an extension would

bounded terrane in Thailand and Myanmar. No attempt is made explain why there is no apparent decrease in dextral shear-zone

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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14 M.F. Ridd, I. Watkinson / Proceedings of the Geologists’ Association xxx (2013) xxx–xxx

Fig. 12. Relations of the Phuket-Slate Belt terrane with the reconstruction by Hall

(2012) of the Indian Ocean at 60 My. The alignment of the India–Australia (I–A)

transform with the Phuket-Slate Belt terrane bounding fault system (shown red) is

Fig. 11. Postulated southward extension of the Khlong Marui and Ranong Faults to

clear. S is Sumatra, M is Malaysia, T is Thailand. Timing of emplacement of the West

Sumatra. The grey lines in the Thai sector of the Andaman Sea are mapped faults

Burma block (WB) is controversial. The grey zone north of Greater India 2 is the

from Morley and Racey (2011) of which the easternmost is the offshore

Incertus Arc (Hall, 2012), and the narrow strip of ocean south of the arc is the

continuation of the Ranong Fault. The Ranong Fault is shown here to extend on

remnant of Cenotethys. (For interpretation of the references to colour in this figure

to Sumatra as the Lokok Kutacane Fault (Stephenson et al., 1982).

legend, the reader is referred to the web version of the article.)

width or shear-strain magnitude at the northern end of the Ranong close to its margin. The same interpretation could apply to the

fault – it does not naturally terminate in the north but has simply outlying Singa Formation diamictite intervals on the Langkawi

been truncated mid-length by the Three Pagodas Fault. In other Islands (Fig. 11).

words, the Phuket-Slate Belt terrane was translated northwards

because it was coupled with the north-going India plate, and it 7. Conclusions

ceased only with the onset of orogenesis ahead of continental

India, or when dextral shift on the India–Australia transform The principal findings of this study are:

ceased as subduction of the Australia plate resumed under Sumatra

in the Eocene (Hall, 2012). That continental northward continua- 1. The Phuket Terrane can be traced north into Myanmar as the

tion of the oceanic transform would have been facilitated by the Slate Belt. Throughout its length it has a distinctive stratigra-

presence of a belt of rift-boundary faults dating from the Early phy characterized by a very thick Upper Carboniferous-Lower

Permian separation of Sibumasu from Gondwana. Segmentation of Permian sequence including diamictite-bearing units (the

the strike-slip faults may reflect inherited segmentation of the rift- Phuket Group in Thailand and the Mergui Group in Myanmar);

boundary fault systems. The West Burma block lies west of the these rocks are thought to have been deposited in a glacio-

Sagaing Fault and the timing of its emplacement is controversial marine rift setting as the Sibumasu block parted from

(e.g. Hall, 2012). If, as argued by Mitchell (1992), Hutchison (1989) Gondwana.

and Barber and Crow (2005) it became part of Sundaland in the 2. Bounding this combined Phuket-Slate Belt terrane on its east, a

Triassic it would have been present outboard of (i.e. west of) the dextral fault system can be traced from the Khlong Marui Fault

Phuket-Slate Belt terrane during the latter block’s Cretaceous- in Peninsular Thailand to the Panlaung Fault in Eastern

Palaeogene emplacement. But the model presented here throws no Myanmar, a distance of 1700 km (exceeding the length of

light on how much movement, if any, took place between the West the better-known San Andreas Fault). It was along this fault

Burma block and the Phuket-Slate Belt terrane as the latter was belt that the terrane was emplaced. The faults are thought

emplaced. broadly to follow the trend of the Late Permian rifting, but

If this model is correct, it will be seen from Figs. 11 and 12 that possibly also follow the trend of a line of weakness, probably an

the dextral displacement on the terrane-bounding fault system incipient back-arc basin postulated to have been present along

could not have exceeded about 450 km, i.e. the distance from the the Andean-type margin of Sundaland.

SW margin of Sumatra to a point somewhere south of the present 3. It is suggested that the Mergui Group and such underlying

southern tip of the onshore Phuket Terrane. On the Indian Ocean rocks as may have been present (since the base of the Mergui

side of Sumatra itself, dextral displacement along the Sumatra Group and Phuket Group has nowhere been seen) were the

Fault since its commencement in the Late Oligocene (Barber et al., protoliths of the metamorphic rocks of the Mogok metamor-

2005) has removed any trace of the Phuket-Slate Belt terrane phic complex.

which may have existed there. An implication of this hypothesis is 4. On regional grounds Late Cretaceous–Palaeogene timing of the

that the diamictite occurrences in the Bohorok and Mentulu boundary-fault movement best fits the field and radiometric-

formations on Sumatra fall outside the terrane and can be viewed dating evidence. But within that range, it is suggested the age

as having been deposited on the hinterland of Sundaland, albeit of metamorphism of the Mogok metamorphic complex

Please cite this article in press as: Ridd, M.F., Watkinson, I., The Phuket-Slate Belt terrane: tectonic evolution and strike-slip

emplacement of a major terrane on the Sundaland margin of Thailand and Myanmar. Proc. Geol. Assoc. (2013), http://dx.doi.org/ 10.1016/j.pgeola.2013.01.007

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M.F. Ridd, I. Watkinson / Proceedings of the Geologists’ Association xxx (2013) xxx–xxx 15

provides a more specific timing. Although Mitchell et al. (2012) particularly the Kaeng Krachan Group of Thailand’s lower

controversially propose a minimum age of metamorphism of Peninsula, the better to understand the palaeogeography.

128 Ma, Searle et al. (2007) proposed two Tertiary events ii. Insofar as the Middle–Upper Permian carbonate sequence of

(59 Ma, and 47–29 Ma) The older of the ages of Searle et al. the Phuket-Slate Belt terrane is interpreted as having been

(2007), i.e. 59 Ma, is close to the 51–57 Ma age of deposited perhaps as much as several hundred kilometres

metamorphism described by Nantasin et al. (2012) from the distant from the equivalent limestone east of the terrane

Thabsila schist and gneiss enclave in the Three Pagodas Fault bounding fault, a search may reveal stratigraphic differences

belt, and it is suggested here that this metamorphic event was between them.

caused by dextral transpression and associated crustal iii. An integrated study of the Late Triassic-Early Jurassic Shwe-

thickening as the Phuket-Slate Belt terrane was emplaced. minbon Formation in Myanmar could achieve a better

The 55 Ma re-heating event of the Khao Phanom Bencha understanding of the palaeogeography at that time.

granite (Fig. 7) reported by Garson et al. (1975) might also be iv. Rigorous testing of the postulation that the terrane bounding

explained by this event. fault system is related to the India–Australia transform is

5. The Shweminbon Formation turbidites of possible Late recommended.

Triassic-Early Jurassic age cropping out along the west flank v. A search for worldwide analogues of this 1700 km-long terrane.

of the Shan Plateau had been thought to indicate the presence

at that time of a closing ocean between the Shan Plateau and

Slate Belt (Mitchell et al., 2007, 2012). That is now thought Acknowledgements

unlikely, although the palaeogeography of the western Shan

Plateau at the time the Shweminbon Formation was deposited

We are most grateful to Dr. Andrew Mitchell for the information

is a conundrum.

on Myanmar he has freely provided. He along with Dr. Tony Barber

6. New evidence has emerged from Phuket that some of the

and Dr. Mike Crow gave helpful comments on an early draft. The

granites of the Western Granite Province of Southeast Asia

paper was reviewed by Dr. Chris Morley and Professor Robert Hall

were intruded as early as the Late Triassic (Searle et al., 2012);

whose comments have led to further improvements.

it had been known for some time that most of the granites of

that province were intruded in the Cretaceous and Palaeogene. References

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