combined escape tectonics and subduction rollback–back arc...

Journal of the Geological Society, London, Vol. 158, 2001, pp. 461–474. Printed in Great Britain.

Combined escape tectonics and subduction rollback–back arc extension: a model forthe evolution of Tertiary rift basins in Thailand, Malaysia and Laos

C. K. MORLEY1

1Department of Geology and Petroleum Geology, Meston Building, Kings College, University of Aberdeen, AB24 3UE,UK (e-mail: [email protected])

Abstract: The Tertiary rift basins of Thailand and adjacent countries show considerable variability in thetiming of rift initiation, termination, the timing and magnitude of thermal subsidence, and the timing andintensity of inversion episodes. The rift basins developed on continental blocks that were extrudedsoutheastwards. Hence their development must be tied into Himalayan extrusion tectonics. Currenttectonic models propose that the Tertiary basins opened up as pull-apart basins associated with strike-slipfaults. Published geochronology of strike-slip fault zone rocks, mapping of fault patterns in the Tertiarybasins and mapped releasing-restraining bend geometries all indicate that in Thailand major sinistralstrike-slip motion ceased at about 30 Ma, prior to the formation of most rift basins the Thailand. Theeffects of later dextral slip were minor and probably a result of reactivation during episodes of inversionduring NW–SE to NE–SW (Himalayan) compression. Dextral slip was not responsible for opening mostof the rift basins in Thailand. An alternative mechanism to open the rift basins is subduction rollback ofthe Indian plate to the west of Thailand. It is proposed that subuction rollback can help explain some ofthe characteristics of the rift basins such as non-uniform lithospheric extension, deep sag basins, and thediachronous onset and termination of rifting.

Keywords: Thailand, Himalayan Orogeny, rift zones, strike-slip faults, subduction.

Tapponnier et al. (1982) and Molnar & Tapponnier (1975)greatly influenced geological thought about the evolution ofChina and SE Asia in their classic work on escape tectonicsas a result of the India–Eurasia Himalayan collision. Theyconstructed analogue models, the first of which intruded awooden block into a cake of plasticine with one constrainedmargin (to the west) and an unconstrained margin (to the east)(Tapponnier et al. 1982). Later models with sand, silicone andsyrup employed more realistically scaled materials (e.g. Jolivetet al. 1990). The resulting patterns of deformation predictedextrusion of continental blocks east and southeast of theHimalayas (Indochina) towards the unconstrained marginresulting in hundreds of kilometres of strike-slip motion onseveral major shear zones (Fig. 1). The two most importantfaults bounding the crustal blocks discussed in this paper arethe Sagaing fault (c. 450 km dextral displacement, Mitchell1981; Torres et al. 1997) and the Red River fault (c. 500 kmsinistral motion, followed by dextral slip in the order of tens ofkilometres, Leloup et al. 1995).

The Tertiary rift basins of Thailand and adjacent countries(Malaysia, Myanmar and Laos) evolved on continental blocksinvolved in escape tectonics (Fig. 1). Some prominent NW–SE(Mae Ping, Three Pagodas) and NE–SW (Klong Mariu,Ranong, Uttaradit) trending strike-slip faults mapped in pre-Tertiary rocks strike into areas covered by Tertiary rift basins.Tectonic models currently applied to the rift basins linkregional escape tectonics, with locally prominent strike-slipfaults to explain the formation of the rift basins (Polachan &Sattyarak 1989; Packham 1993; Hall 1996; Madon 1997a, b).Using offsets of Triassic granite belts in western Thailand,Tapponnier et al. (1982) inferred several hundred kilometres ofsinistral displacement on the Three Pagodas and Mae Pingfault zones associated with escape tectonics (i.e. Oligocene–Miocene). The timing of sinistral displacement has beendisputed. Collisions of continental blocks during the

Permo-Triassic and the late Cretaceous–Early Tertiary col-lision of western Burma with the Shan Thai block may alsohave caused left-lateral slip (e.g. Bunopas 1981; Cooper et al.1989; Polachan & Sattayarak 1989). However more recentstudies indicate sinistral motion on the Mae Ping fault zoneended at about 30 Ma (Ahrendt et al. 1993; Lacassin et al.1997). Polachan et al. (1991) proposed that Oligocene–Recentstrike-slip movements in Thailand were dominated by right-lateral slip on NW–SE-striking faults and left-lateral slip onNE–SW–striking faults, with transtensional dextral shearbeing dominant. Maximum horizontal displacements wereestimated in the order of tens of kilometres. Polachan et al.(1991) proposed that the Tertiary basins formed at releasingbends within a virtually pervasive system of conjugate strikeslip faults. This remains the model most widely applied to thebasins of Thailand today.

This paper discusses a new tectonic model for the regionbased on evaluation of a considerable amount of oil and coalcompany subsurface data from the Tertiary rift basins and onoutcrop work. Information about the subsurface basins inCentral Thailand and the Gulf of Thailand has come fromMSc research projects conducted at the University of BruneiDarussalam by employees of PTTEP and Unocal using 2Dand 3D seismic reflection data sets (e.g. Morley et al. 2000;Watcharanantakul & Morley in press; Morley & Wognanan inpress). A comprehensive review of the structural evolution ofthe Tertiary rift basins based on the new data mentioned aboveand published data is presented in Morley et al. (in press).

Evolution of Tertiary rift basinsThis section provides a summary of the main characteristics ofTertiary rift basin evolution that must be incorporated intoany regional tectonic model.

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Timing of rift activityPatchy Eocene–Oligocene rifting is known from onshoreThailand (e.g. Krabi basin) and the northern Gulf of Thailand(e.g. Chaodumrong et al. 1983; Ratanasthien 1990; Ducroqet al. 1991; Jardine 1997; Vilaihongs & Areesiri 1997).The extent and geometry of these basins is poorly defined:Lockhart et al. (1997) states that the oldest well penetrations todate in the Gulf of Thailand are possibly Late Oligocene andthe lowest syn-rift units are poorly imaged on seismic data.Eocene–Late Oligocene extension is best documented in theWest Natuna basin (Ginger et al. 1993; Phillips et al. 1997).The onset of widespread extensional basins appears to youngnorthwestwards from the West Natuna basin.

Rifting onshore and in the northern Gulf of Thailandbecame widespread in the Late Oligocene–Early Miocene (e.g.Watanasak 1990; Jardine 1997; Lockhart et al. 1997; Morleyet al. in press) (Figs 2 and 3). Basins in the northern Gulf ofThailand show greatest extension-related deposition during theOligocene–Early Miocene. Depocentre locations changed inthe Mid-Miocene, and faults tend to be less active (e.g.Lockhart et al. 1997), and the transition to thermal subsidencebegan (e.g. Jardine 1997).

In the Central Plains area (Bangkok and to the north)extension lasted from the Late Oligocene to the Pliocene, butthe timing of maximum subsidence varies considerably frombasin to basin (Morley et al. in press). For example thePhitsanulok basin underwent greatest subsidence and highest

subsidence rates in the Mid-Miocene (Knox and Wakefield1983; Flint et al. 1988); while the Phetchabun basin underwentmost of its subsidence in the Oligocene–Early Miocene (Remuset al. 1993). Oligocene–Early Miocene subsidence rates forthe Phetchabun and Phitsanulok basins were similar, andconsiderably slower for the Suphan Buri basin.

In northern Thailand the geometry and ages of the riftbasins are less well known than to the south. Howeverpalynology and vertebrate fossils have yielded reliable ages fora few basins (for a review see Morley et al. in press). Apreliminary study which provides additional supporting datafor palynological age dating of some key mines in northernThailand has been deposited with the Society Library andthe British Library Document Supply Centre, Boston Spa,Wetherby, N. Yorks LS23 7BQ, UK as Supplementary Publi-cation No. SUP 18157 (9 pp.). In particular a Late Oligoceneage for the oldest exposed section in the Li basin is confirmed(Watanasak 1990; Ratanasthien 1992). Some basins are domi-nated by Late Oligocene–Early Miocene coal deposition (MaeLamao, Mae Lai, Mae Chaem, Li basin), indicative of rela-tively early rifting (Watanasak 1990; Ratanasthien 1990, 1992;Figs 2 and 3). Other basins have late Early Miocene predomi-nantly fluvial deposition and Middle Miocene coal develop-ment, or just a Middle Miocene history (Mae Moh basin,Lampang basin, Phrae basin, Mae Tha, Chiang Muan basin;Ginsburg et al. 1988; Watanasak 1990; Ratanasthien 1992;Ratanasthien et al. 1993). Some basins span both periods ofdevelopment and tend to show several angular unconformites(Fang, Mae Teep, probably the Mae Sot basin and possibly theChiang Mai basin; Pradidtan 1989; Ratanasthien 1990). Astructural reorganization during rifting in the late EarlyMiocene–early Middle Miocene resulted in the creation ofnew, younger basins east of the largest existing rift basins (Figs3 and 5).

Onset of thermal subsidenceThe termination of rifting clearly youngs northwards from theearliest Miocene in the south (West Natuna basin, Malaybasin), to the Late Miocene and Pliocene in the Central Plains(Fig. 2). Northern Thailand underwent little or no thermalsubsidence due to late cessation of rifting and persistent uplift(Fenton et al. 1997; Upton et al. 1997). Uplift mechanismsinclude: magmatic underplating (Late Cretaceous–Tertiaryigneous activity), crustal shortening (compression and strike-slip events), isostatic uplift due to normal faulting associatedwith metamorphic core complexes (Palaeogene?) and rifting.Uplift was both relatively slow, long-term uplift from the LateCretaceous onwards and more local short-term rapid upliftover a few million years (Upton et al. 1997). Central Thailandis a low relief area of relatively low crustal temperatures (3–4�C100 m). Thermal subsidence began during the Pliocene (Flintet al. 1988). The Gulf of Thailand began thermal subsidenceduring the Mid-Miocene (Bustin & Chonchawalit 1995;Jardine 1997; Fig. 2). The western half of the Gulf of Thailandis relatively cool (heat flow up to 70 mW m2; geothermalgradients between 4–5�C 100 m; thermal sag basin thickness upto 2 km), while to the east the Pattani and Malay basins haveanomalously high geothermal gradients for thermal sag basins(heat flow 78 mW m2 to 101 mW m2; geothermal gradient6–7�C 100 m; thermal sag basins up to 4 km thick, Bustin& Chonchawalit 1995; Madon 1997b; Watcharanantakul &Morley 2000).

Fig. 1. Regional tectonic map of western Southeast Asia, modifiedfrom Packham (1996) and Leloup et al. (1995). The rift basins ofThailand discussed in this paper are in dark grey.

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Synchronous onset of the three main rift stages (rift initi-ation, rift climax, transition to thermal subsidence) acrossThailand has been cited as support for regional tectonic events(e.g. Polachan & Sattayarak 1989; Chinbunchorn et al. 1989).However, as reviewed above increasing documentation of theTertiary basin evolution shows that the notion of synchronousrift evolution is far too simplistic.

Inversion

Basins affected by inversion can be divided into three provinceswith gradational borders based on the intensity and timing ofinversion. They comprise (1) Northern Thailand, (2) CentralPlains and the northern Gulf of Thailand and (3) the south-eastern province comprising the Malay, Penyu and West

Fig. 2. North–south to NW–SE variations in gross rift structure along the strike of the rift system passing from the West Nantuna basin in thesouth to the Fang basin in the north. Note there is considerable variation in the onset of thermal subsidence (Early Miocene in the south, latePliocene–Pleistocene in the north if it occurs at all), and the duration of extension. Correlation of structural events passing from north to southin the region. Some palaeostress orientations are shown for the north; such detail cannot be established for Central Thailand and the Gulf ofThailand where the rifts are only known in the subsurface. Compiled from Knox & Wakefield (1983); Flint et al. (1988); O’Leary & Hill (1989);Pradidtan (1989); Chinbunchorn et al. (1989); Ratanasthien (1990, 1992); Watanasak (1990); Remus et al. (1993); Bustin & Chonchawalit(1995); Phillips et al. (1997); Madon (1997a); Jardine (1997); Wongpornchai (1997); Morley et al. (2000).

RIFT-BASIN TECTONICS, THAILAND–MALAYSIA 463

Natuna basins (Morley et al. in press). Relatively stronginversion tectonics characterize the northern Thailand andsoutheastern provinces, while the Central Plains and northernGulf of Thailand experienced relatively mild inversion (Fig. 2).

Northern Thailand experienced patchy, locally intenseinversion at various stages from the Late Oligocene–LateMiocene (Morley et al. 2000, 2001). The timing and frequencyof inversion episodes within and between basins varies con-siderably. Some coal mines reveal four or five episodes ofinversion which span the Late Oligocene–Early Miocene to thePlio-Pleistocene (e.g. Li basin, Morley et al., 2000; Figs 2 and3). While other mines reveal only one episode of inversion(e.g. Mae Moh, Morley et al. 2001). The most widespreadinversion event in northern Thailand occurred during thePlio-Pleistocene and is associated with a NW–SE to NE–SWoriented maximum horizontal compression direction (Morleyet al. in press). Modern stresses determined from earthquakes(Bott et al. 1997) indicates a similar stress system operates

today, as predicted by finite element models of the Himalayancollision (Huchon et al. 1994).

Central Thailand and the northern Gulf of Thailand under-went only minor, local inversion during the Early, Mid- andLate Miocene (Figs 2 and 3). For example some localizedminor inversion events occurred around the Oligocene–Miocene boundary (e.g. Jardine 1997). In the Kra basinseismic data shows evidence for very local, mild inversion inthe Middle Miocene (Areeanchuleekorn 1998). Minor inver-sion affected the Phitsanulok basin in the Late Miocene (Knox& Wakefield 1983) while inversion is strongest during thePliocene in the Phetchabun basin (Remus et al. 1993).

In the Malay, Penyu and West Natuna basins, considerableinversion occurred during a number of episodes in the Earlyand Mid-Miocene while the Late Miocene and Pliocene was aperiod of tectonic quiescence, with no inversion after about10 Ma (Ginger et al. 1993; Phillips et al. 1997; Madon 1997a;Fig. 2). Estimates of (NNW–SSE) shortening associated withthe inversion are: 10 km for the southern Malay basin (fromMadon 1997b, Fig. 6) and 12–30 km for the West Natunabasin (from balanced cross-sections in Ginger et al. 1993;Fig. 5).

The central region is relatively free from more intenseinversion events to the north and southeast, possibly becausestresses driving the inversion events were either generated tothe SE (e.g. the rotation of Borneo affecting the southernbasins as reviewed by Hall 1996) or to the NW by theHimalayan orogeny.

Fig. 3. Location map of Tertiary rift basins of northern and centralThailand. Basin data complied from: Knox & Wakefield (1983);Gibling et al. (1985); Chinbunchorn et al. (1989); O’Leary & Hill(1989); Pradidtan (1989); Remus et al. (1993); Muraoka et al.(1997); Vilaihongs & Areesiri (1997); Wongpornchai (1997);earthquake data from Bott et al. (1997); faults from Dunning et al.(1995) and Fenton et al. (1997). Palaeostress summary from thispaper and Morley et al. (2000). L.O.-E.M, Late Oligocene–EarlyMiocene; M.M., Mid-Miocene; L. M., Late Miocene; P.P.,Plio-Pleistocene.

Fig. 4. Geological map of the main Oligocene–Miocene rift basins inthe Gulf of Thailand. Basins and faults compiled from Madon(1997a), Madon et al. (1997), Brown et al. (1981); Lockhart et al.(1997), Ginger et al. (1993); Phillips et al. (1997). Estimates ofshortening associated with the inversion for the southern Malaybasin from Madon (1997a, fig. 6) and for the West Natuna basinfrom balanced cross-sections in Ginger et al. (1993). Extension inthe northern Gulf of Thailand from extrapolation of �=1.3 acrossthe gulf (Watcharanantakul & Morley in press).

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Differences in the timing of extension, and basin orientationbetween the Thailand rifts and the Penyu and West Natunabasins seem to reflect different origins for the extension. ThePenyu and West Natuna basins are probably related to riftingassociated with the South China seas spreading centre, whilethe Thailand rifts are related in some way to the Himalayanorogeny as discussed in this paper. Quite how these differentrift systems blend together via the Malay basin is unclear.

Metamorphic core complexesMuch recent work on the Tertiary tectonic evolution ofnorthern Thailand has focused on uplifted pre-Tertiary ‘base-ment’ rocks (Cretaceous and Triassic granites, Palaeozoicsedimentary and metasedimentary rocks and gneisses). Untilrecently mylonitic amphibolite-grade ortho- and paragneissesand granitic rocks of the region were thought to be Precam-brian (e.g. Suensilpong et al. 1982). Now U–Pb geochronologyindicates a late Cretaceous and early Miocene age (Ahrendt etal. 1993; Dunning et al. 1995). Dating of uplift using thepassage of micas through the 300�C isotherm (Ahrendt et al.1993, 1997; Lacassin et al. 1997) and apatite-fission trackanalysis (Upton et al. 1997) indicates the gneisses west of theChiang Mai-Tak line underwent about 10 km uplift duringthe late Oligocene–Early Miocene, if an average geothermalgradient is assumed. Macdonald et al. (1993); Dunning et al.,(1995) and Rhodes et al. (1997) attribute the uplift, at least inthe Doi Suthep and Doi Inthanon areas west and southwest ofChiang Mai to isostatic footwall uplift associated with thedevelopment of metamorphic core complexes bounded bylow-angle normal faults (Fig. 5).

Driving mechanismsAs previously discussed, many workers attribute the formationof Tertiary rift to pull-apart structures associated with strike-slip faults (O’Leary & Hill 1989; Polachan & Sattayarak 1989;Polachan et al. 1991; Fig. 6). The model was proposed whenlittle detailed data were available about the exact relationshipsbetween strike-slip faults and extensional faults. In this section

the evidence for strike-slip faults is re-examined and then analternative driving mechanism (subduction rollback) discussed.

Testing the strike-slip model(1) Timing of strike-slip fault activity. Understanding the roleplayed by NW–SE- and NE–SW-trending faults in Thailandrequires establishing whether the strike-slip faults playedan active or passive role in rift structure (Morley 1999;Watcharanantakul & Morley 2000). In the active mode theyare the dominant fabric and create a through-going strike- oroblique- slip fault zone. The entire length of the fault zoneshould be active, have a consistent sense of displacement andcross-cut any other trend. A passive oblique fabric bears norelation to the regional extension direction, it is a pre-existingfabric that is partially reactivated by younger faults (Morley1999). Segments of the oblique fabric form the preferredorientation for an unlinked or partially linked collection offaults. Watcharanantakul & Morley (2000) discuss evidence inthe Pattani basin for active and passive strike-slip fabrics andconclude that syn-rift normal fault geometries, largely mappedfrom 3D seismic reflection data (e.g. Lockhart et al. 1997;Watcharanantakul & Morley 2000, fig. 4) support passivereactivation by late Oligocene–early Miocene normal faults.There is no evidence for pronounced strike-slip fault activity inthe northern Gulf of Thailand after about 30 Ma.

(2) To open up the Pattani basin as a pull-apart in theOligocene requires dextral motion on the Three Pagodas faultand displacement transfer across right-stepping faults bound-ing the Malay basin (Polachan & Sattarayat 1989; Fig. 1). Thismodel is incompatible with the broad NW–SE-trending sinis-tral shear proposed to drive Oligocene extension in the Malaybasin (Madon 1997a, b). Later in the Early Miocene extensioncontinued in the Pattani basin, but ceased in the Malay, W.Natuna and Penyu basins to the SE (Madon 1997a; Gingeret al. 1993; Phillips et al. 1997; Fig. 2). Hence there is aproblem transferring dextral strike-slip motion on faults open-ing up the Pattani basin into the (thermally subsiding) Malaybasin.

Fig. 5. Geological cross-section across the northern Thailand rift basins illustrating the following characteristics: (1) mixed origins for thebasins—possible strike-slip development of Mae Sariang Basin, metamorphic core-complex upper plate (Na Hong?), later extensional riftdevelopment associated with high-angle normal faults, in Chiang Mai, Lampang and Phrae basins, (2) changing age of basins, in generalyounging to the east, (3) young apatite fission-track (AFT) ages (Early Miocene) only associated with the metamorphic core complex (ages fromUpton et al. 1997), (4) mica cooling ages through 300�C geotherm at 30 Ma (Ahrendt et al. 1993) requires 5–10 km of subsequent uplift.However, Na Hong basin of similar age (Late Oligocene) has high grade coal and requires burial of about 2 km followed by uplift. (See Fig. 4for location, but note the section extends about 40 km west from the western edge of Fig. 3.) The cross-section around the metamorphic corecomplex is based on Dunning et al. (1995).


(3) If the Tertiary basins of northern Thailand formed aspull-aparts then offsets of pre-Tertiary lithology markers andadjacent Tertiary basins should be consistent with a strike-slipsystem of faults as sketched by Polachan & Sattayarak (1989)and Polachan et al. (1991) (Fig. 6). However, rarely is itpossible to construct pull apart geometries with associatedstrike-slip faults that show a consistent sense of offset ofpre-Tertiary marker units or adjacent Tertiary rift basins andthe correct releasing bend or stepping geometry. In fact moststrike-slip fault trends do not offset the mapped boundaries ofpre-Tertiary units (Fig. 6c).

(4) Palaeostress studies of the basins in northern Thailandshow predominantly east–west extension (Morley et al. 2000,2001; Fig. 3). Some faults have undergone extension followedby inversion indicating a change in stress conditions with timerather than a simple strike-slip story where structures ofdifferent orientations can synchronously display differentsenses of motion. The model of Polachan et al. (1991) predictsthat NNE–SSW- to north–south-striking faults should displaya dextral sense of motion (Fig. 6b). Yet at the Mae Lai mine,which lies along one of these trends, palaeostress data indicatesinistral strike-slip motion during inversion of normal faults(Morley et al. 2001). Consequently the structural evolution ofsome basins shows greater complexity or different kinematicsto those predicted by the Polachan et al. (1991) model. Somestrike-slip fault activity has occurred during the late Oligocene-Miocene activity of rift basins in northern Thailand, but not inthe way predicted by Polachan et al. (1991). Strike-slip faultingmay have more than one origin (Morley et al. 2001) including:(i) reactivation of older strike-slip trends during phases ofinversion, (ii) strike-slip transfer faults between large exten-sional faults; (iii) oblique-slip transtensional faults, whereextensional faults followed pre-existing fabrics oblique to theregional extension direction.

(5) The rift trend onshore and offshore in Thailand ispredominantly north–south and linear, hence the overall

appearance suggests an extensional rift, it would require apeculiar set of circumstances to localize strike-slip basins toa linear rift system.

(6) Palaeomagnetic data show late Neogene differentialrotation between western and central Thailand (24.4��7.7�clockwise rotation), and eastern Thailand and VietnamThailand (13.5��5.8� clockwise rotation; McCabe et al. 1993).However within the central and eastern block McCabe et al.(1993) found no evidence for Neogene differential blockrotation associated with the major strike-slip faults (the MaePing and Three Pagodas faults), as would be expected forimportant strike-slip zones. However more detailed palaeo-magnetic studies would be beneficial to conclusively definewhether Tertiary block rotations can be detected.

(7) The post-rift thermal subsidence story does not comfort-ably fit with a strike-slip setting. No strike-slip province hasbeen documented as being associated with thermal subsidencebasins, yet a widespread thermal subsidence basin post-15 Macovers the whole of the northern Gulf of Thailand and is up to4 km thick (e.g. Bustin & Chonchawalit 1995; Madon 1997b;Watcharanantakul & Morley 2000). Thermal subsidencebasins are characteristic of extensional tectonics (e.g. Kuszniret al. 1995).

Strike-slip faults and rift basins have interacted, but thestrike-slip faults do not appear to be the cause of rift basinformation in most cases. Possible alternative driving mech-anisms are limited. Extension of areas with enhanced gravi-tational potential due to crustal shortening and thickening, oruplift by a mantle plume (e.g. England & Houseman 1989;Buck 1991; Bott 1992) is one possibility. The passage of theIndian continent past the Burma block resulted in compres-sional events as early as the Eocene, (e.g. Mitchell 1993), henceenhanced gravitational potential due to crustal thickening mayhave played a role in metamorphic core complex developmentin northern Thailand and Burma (e.g. Dunning et al. 1995;Bertrand et al. 1999). However it seems a poor explanation for

Fig. 6. Strike-slip structural model fornorthern Thailand. The model byPolachan et al. (1991) shows thescattered Tertiary rift basins of northernThailand as opening a pull-apart basinsunder transtensional dextral shear (a).However palaeostress analysis from thecoal mines does not support this model(b). For example the Mae Lai coal mineis predicted to show evidence for dextralslip in the model, but the invertednormal faults were affected by left-lateralstrike-slip (Morley et al. 2001). Theswarms of strike-slip faults predicted tocause opening of the basins cannot bereconciled with regional geological mapswhere the faults fail to cause offset ofpre-Tertiary geological boundaries (c).

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Fig. 7. Geological evolution of SE Asia from the Oligocene to Present based on the map in Figure 1, modified with the plate reconstructions ofLee & Lawver (1995) and illustrating details of the evolution of the Tertiary rift basins discussed in this paper. Block rates of slip from Avouac& Tapponnier (1993); Andaman Sea convergence v. transform motion from Moore et al. (1980).


extension in the topographically low Gulf of Thailand andCentral Plains area. There is no documentation of a majorEarly Tertiary regional thickening event in southern Thailandand Malaysia that could have later led to gravitational col-lapse. Nor is there the widespread magmatism which thermallyweakens the crust and leads to gravitational collapse. Litho-spheric stresses that can generate extension include slab pull,and slab rollback (e.g. Bott 1992, 1993). There is no sub-ducting slab attached to the Shan Thai or Indochina blocks,hence no slab pull mechanism is possible. That leaves the onlyremaining mechanism as subduction rollback associated withsubduction of Indian Plate oceanic crust eastwards under theWest Burma block (e.g. Bunopas 1981).

Subduction rollbackThe record of subduction of the Indian oceanic crust beneathBurma is somewhat uncertain. Mitchell (1993) reviewed theevolution of the area as follows: a Cretaceous magmatic arc isfollowed by a gap in volcanic activity between 60 and 70 Ma.In the Mid-Eocene the Manipur Ophiolite was obducted ontothe western margin of Burma (arc reversal). East-dippingsubduction then became re-established. South of BanmaukEarly Oligocene subduction is indicated by a granodioriteintrusion. The Guwa Chaung granite yielded Late Oligocene-Early Miocene K–Ar age dates. Late Neogene–Quaternaryvolcanism best documents east-dipping subduction. Satyabala(1998) noted that the paucity of late Oligocene–early Miocenearc volcanics could indicate a variety of causes including nosubduction, slow subduction and obduction.

In this section reasons for accepting a subduction rollbackmodel are discussed. However the setting is far from an ideal,simple back arc model; there are two major deviations. First

the north-northeasterly movement of the Indian Plate meansthat subduction is highly oblique along the West Burmasegment of the trench (Moore et al. 1980; Lee & Lawver 1995),and so the importance of subduction rollback can be ques-tioned. Secondly the belt of extension lies some 700–1000 kmaway from the trench, this is rather a long separation. It is not,however, completely unreasonable. For example in Sumatrathe distance between the trench and region of back-arcextension is 300–800 km. Despite these reservations when asubduction rollback model is applied, it seems to providea solution to many of the detailed aspects of rift and thermalsag basin evolution as discussed below.

The modern subduction zone geometry is less steeply in-clined under Sumatra (about 30�) and steepens up passingnorthwards, where under Myanmar it dips steeply (about 50�)and is apparently inactive (Guzman-Speziale & Ni 1996),although the active-inactive nature of the zone is disputed(Satyabala 1998; Guzman-Speziale & Ni 2000). Thus themodern slab geometry is at least compatible with a subductionrollback explanation. A subducting slab model can also ex-plain some of the major features of the Gulf of Thailand andonshore extensional system. If subduction rollback began atthe southern end of the system (Fig. 7) then extension in theGulf of Thailand could precede extension further north. Assubduction rollback moved north in the Late Oligocene soextension propagated north. One pronounced coincidence isthat the cessation of extension in the Gulf of Thailand in theMiddle Miocene is marked by the onset of extension in theAndaman Sea (Curray et al. 1982), while extension in northernThailand continued during the Mid-Miocene. Hence if thedriving mechanism for extension lies to the west of theAndaman Sea (i.e. subduction rollback) then the debut ofextension in the Andaman Sea explains the onset of thermalsubsidence in the Gulf of Thailand, whilst extension continuedto the north. Finally in the latest Miocene–Pliocene thesubduction zone became completely inactive and extensionterminated in northern and central Thailand. This coincideswith the final docking and collision of the West Burma blockand the northeastern Indian continent.

Subduction rollback and geothermal gradientsThree striking features of the Tertiary rift basins are: (1) theirvariable geothermal gradients, which range from average (3�C/100 m) to very high (7–9�/100 m, Watcharanantakul & Morley2000); (2) high amounts of thermal subsidence in the Gulf ofThailand (Bustin & Chonchawalit 1995; Madon 1997b); (3)low amounts of syn-rift crustal extension based on fault heaves(�=1.2–1.3, Watcharanantakul & Morley 2000). Estimates oflithospheric extension from backstripping require thinning ofthe mantle lithosphere in the order of �=2–3 (Bustin &Chonchawalit 1995; Madon 1997b; Watcharanantakul &Morley 2000). These data strongly suggest non-uniform litho-spheric thinning. One mechanism to accomplish this isan asthenospheric mantle thermal anomaly such as mantleplume (Watcharanantakul & Morley 2000) or propagatingspreading centre.

An alternative to an active mantle thermal anomaly isgreater stretching of the mantle lithosphere than the crust. Thisdifferential or non-uniform stretching could be accomplishedby subduction rollback. If the slab steepens as it rolls backthen the deeper the slab is from the (surface) hinge line, thegreater the horizontal component of extension (Fig. 8). For

Fig. 8. Schematic illustration of how subduction roll back and slabsteepening could lead to non-uniform extension of the lithosphere.To cause non-uniform shortening under the rift the upper plate mustbe coupled with the slab as it rolls back, which requires flow of thelower lithosphere towards the slab. If the upper plate and slab arenot coupled then asthenosphere can flow into the space createdabove the slab (e.g. Doglioni 1992).

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example steepening of a slab from 15� to 17� results in 19 kmextension at 40 km depth, and 55 km extension at the base ofthe lithosphere (120 km).

The zone of extension in the Gulf of Thailand is up to about300 km wide, if crustal extension is assumed to be a uniform�=1.3 this equates to about 90 km upper crustal extension.However the thermal subsidence basin requires a �=2+, i.e.300 km extension of the mantle lithosphere. 90 km can berelated to passive extension, the remaining 210 km of extensioncould be explained by slab steepening. For example an increasein slab dip from 17� to 35� would generate c. 220 km of lowerlithosphere extension. The effect is greatest for gently inclinedslabs and decreases for higher dips. Lower-lithosphere exten-sion at least partially independent of surface extension can helpexplain why high geothermal gradients are associated with theMalay, Pattani, Chiang Mai and Fang basins and not others.The modest amount of upper crustal extension only requiresup to 100 km westerly rollback of the trench, which is a veryminor distance for the scale of structures involved (Fig. 7).

Eocene-Recent tectonic evolution of SE AsiaThis section uses the aspects of basin evolution discussedabove and fits them into a regional tectonic model. The main

structural elements considered are illustrated in Figure 1,which has been modified in Figure 7 to include the platereconstructions of Lee & Lawver (1995). The evolution isdiscussed for four time periods (Eocene–Late Oligocene, LateOligocene–Early Miocene, Mid–Late Miocene and Pliocene–Recent). The passage of the Indian continent past the Burmablock resulted in compressional events as early as the Eocene,probably as a result of obduction of oceanic crust (e.g.Mitchell 1993). If the Indian continent and the Burma blockbecame coupled in Eocene-Oligocene times the suture wouldlie north of the Thai rift basins. Subduction of oceanic crustbetween the southern part of the Burma block and the IndianPlate could have continued (Mitchell 1993; Satyabala 1998)and possibly driven extension in Thailand and Myanmar, butdiminished towards the north.

Eocene–Early OligoceneBrias et al. (1993) link motion along the Red River faultwith extension in the South China Sea, which requires earlyOligocene activity on the Red River Fault. However otherworkers argue against a kinematic link (e.g. Rangin et al.1995b; Wang & Burchfiel 1997) The latter view is followed hereso the Eocene to early Oligocene is characterized by little

Fig. 9. Schematic illustration of the mainevolution of blocks and structuralprovinces during late Tertiary escapetectonics in SE Asia.


activity on the Red River shear zone and greatest activity(sinistral) on the Mae Ping fault zone (Leloup et al. 1995;Lacassin et al. 1997) (Fig. 7).

As reviewed in the timing of rift activity section there isscattered evidence for the initial creation of some rift basins inThailand during the Eocene–Late Oligocene. Eocene–LateOligocene extension is best documented in the West Natunabasin (Ginger et al. 1993; Phillips et al. 1997). Samples ofultramylonites at Lan Sang national park indicate that theductile, sinistral deformation along the Mae Ping fault zoneended around 30 Ma (Lacassin et al. 1997).

The apparent patchy development of Eocene–Late Oli-gocene rift basins in Thailand contemporaneous with sinistralstrike-slip fault activity could mean they have a strike-sliporigin (unlike most later basins). Unfortunately there is nowell-documented basin geometry from seismic or outcrop datato help determine the origins of the early basins. An approxi-mately east–west oriented maximum horizontal principal stressdirection is required to cause sinistral motion on the NW–SE-trending strike-slip faults and dextral motion on the NE–SWfaults. Similar observations have been made for the Red Rivershear zone (Rangin et al. 1995a). The origin of the east–westcompression probably lies with the motion of the NE corner ofthe Indian continental crust past the Burma block (Fig. 7;Huchon et al. 1994). While there was some northerly motion ofBurma relative to the Shan Thai block in the Oligocene, it wasrelatively small compared with the Miocene-Pliocene motion(Lee & Lawver 1995). Strain partitioning commonly occursassociated with major strike-slip faults in the upper plates ofoblique convergent subduction zones (e.g. McCaffry 1992;Maclod & Kemal 1996), and the Oligocene history of theSagaing fault appears to represent such an occurrence. Obliqueconvergent motion was partitioned into strike-slip motion onthe Sagaing fault, while compression/transpressional stressesaffected the Shan Thai and Indochina blocks. In the northernGulf of Thailand motion on the Three Pagodas fault zone, andcounter clockwise rotation of Peninsula Malaysia along NE–SW-striking faults created a complex network of strike-slipfault trends.

Late Oligocene–Early MioceneThe Late Oligocene–Early Miocene represents a markedchange in structural activity in the Shan Thai and Indochinablocks. Strike-slip deformation was largely abandoned andextensive north–south-trending extensional basins developedrunning from the Gulf of Thailand up into Northern Thailand,Myanmar and Laos (Fig. 7).

According to Macdonald et al. (1993), Dunning et al. (1995)and Rhodes et al. (1997) metamorphic core complexes devel-oped during the late Oligocene–Early Miocene in the DoiSuthep and Doi Inthanon areas, of northern Thailand (Fig. 6).The timing is similar to that proposed for the Mogok meta-morphic belt which lies adjacent to the Sagaing fault inMyanmar (Bertrand et al. 1999). Data is still very patchy inthis region, but the suspicion must at least be entertained thatregionally a number of metamorphic core complexes weredeveloped in the Late Oligocene–Early Miocene betweennorthern Thailand and the Sagaing Fault. One explanationcould be that crustal thickening, uplift and igneous intrusionsassociated with the Eocene–Early Oligocene transpressioncreated sufficient gravitational potential and lithosphericinstability to generate metamorphic core complexes. This

instability may have been promoted by a decrease in east–westcompressive stress as India moved further north. To the southand east of the core complexes rift basins associated withrelatively high-angle normal faults and low amounts of exten-sion were developed. Episodically these extensional basinswere subject to varying degrees of inversion (Morley et al.2000, in press).

Another possible explanation of the metamorphic corecomplexes is that non-uniform lithospheric stretching relatedto subduction rollback resulted in anomalous lower litho-spheric thinning under the Doi Suthep and Doi Inthanonareas. Consequently the crust was heated, isostatically up-lifted and partially melted (resulting in the emplacementof Oligocene granites) this situation triggered low-angledetachment faulting.

Extensional stresses may not have been continually imposedon the Thai rift basins. The ease of motion along strike-slipfaults of adjacent blocks (South China and West Burmablocks) would presumably have an effect on how stresses weredistributed in the continental crust. Episodic inversion affectedthe region in the Late Oligocene–Early Miocene (e.g. Li Basin,Morley et al. 2000). Both east–west-directed compression andNW–SE to north–south compressional events are recorded,which may reflect transpressive events from the West Burmablock (east–west), and from the South China block (NW–SEto north–south) during periods of less active displacement onthe Sagaing and Red River strike-slip fault zones. When thesefaults locked compressional stresses may have built up in thevicinity of the Thai rift basins and led to inversion.

Evidence for brittle, right-lateral slip on the Mae Ping faultzone can be seen in well developed stepped striations on theapproach road to the Lan Sang national park in NW Thailand.Their age is after or at the later stages of uplift that followedsinistral motion on the Mae Ping fault zone which terminatedaround 30 Ma (Lacassin et al. 1997). The magnitude of dextraldisplacement is not known, nor is it known whether the faultwas active during the extensional or inversion stages of riftbasin development. However following the arguments given inthe driving forces section for the Three Pagodas fault, it issuggested that the Mae Ping fault played no significant activerole in the development of most of the large Late Oligocene–Miocene rift basins with the possible exception of the MaeSariang basin (Fig. 6). Certain small basins found along thetraces of the strike-slip faults are also probably of strike-sliporigin. Episodes of inversion where NW–SE to north–southcompressive stresses affected the region are the favoured causeof right-lateral motion on the Mae Ping fault.

Regionally during this time the Red River shear zoneexperienced its greatest sinistral displacement (in the order of500+ km, Leloup et al. 1995). In terms of relative motionthe Shan Thai-Indochina block was expelled southeastwardsrelative to the South China block.

Mid-Miocene–Late MioceneThe Mid-Miocene marks another important reorganization ofextension. At the beginning of the Mid-Miocene extensionceased in the northern Gulf of Thailand and was replaced bythermal subsidence. Continental extension in the AndamanSea area followed by sea floor spreading in the Late Miocenecoincides with this change, and could reflect the transfer ofsubduction rollback-related extension from the Gulf ofThailand to the Andaman Sea. Onshore in Thailand extension

470 C. K. MORLEY

continued, and in northern Thailand new Late Miocene–Mid-Miocene extensional basins (e.g. Lampang, Phrae, Mae Mohbasins) tended to open eastwards of existing Late Oligocenerift basins (e.g. Chiang Mai, Li, Fang basins). Episodic inver-sion affected some basins (e.g. Fang and Mae Lai, Morleyet al. in press) but appears to be much less significant than thepreceding and following inversion events.

Regionally during the Mid–Late Miocene the Red Rivershear zone experienced little uplift and erosion after 15 Ma(Leloup et al. 1995). However left-lateral deformation isrecorded on the Red River fault zone offshore up to 5 Ma(Rangin et al. 1995b). In terms of relative motion theShan Thai–Indochina blocks and South China block weremore or less coupled and expelled to the SW at similar rates.There was a little differential motion (probably kilometresto 10+ km) between the Peninsula block and the Shan Thaiblock as recorded by the dextral motion on the Mae Pingfault, which reflects episodic differential motion between theblocks.

The Pattani basin is pervaded by strands of minor conjugateextensional faults which affect the post-rift section and exhibit�<1.1. They are a very distinctive and unusual structural style.The faults develop semi-independently of the syn-rift faults,but nevertheless are closely related to them in terms oflocation. They probably represent a short-lived oblique exten-sion event. Further to the southeast inversion in the Malay andWest Natuna basins is thought to be due to episodic broaddextral shear which trended NW-SE (Ginger et al. 1993;Madon 1997b; Phillips et al. 1997). The origins of this shearare uncertain. Shortening is estimated to be about 20–40 kmfor the Early–Mid-Miocene inversion (Ginger et al. 1993;Madon 1997b; Fig. 5).

Pliocene–RecentExtension ceased in most of the onshore Thai rift basins by theLate Miocene–Early Pliocene (Fig. 2). Thermal subsidencebegan to affect the onshore Central Plains area of Thailand. Innorthern Thailand Plio-Pleistocene inversion is widespread inmany rift basins including the Li, Lampang, Mae Moh andFang basins (Morley et al. 2001). The inversion events oc-curred during renewed (right-lateral) activity on the Red Rivershear zone, where the South China block was expelled south-eastwards faster relative to the Shai–Thai and Indochinablocks (Allen et al. 1984; Leloup et al. 1995; Rangin et al.1995a). Inversion features and modern earthquakes both indi-cate NW-SE to NE-SW horizontal maximum principal com-pressive stress directions (Figs 1 and 3). These orientations fitwith the predicted stress orientations from finite elementmodels of the collision of the indenting Indian Plate with theEurasian Plate (e.g. Huchon et al. 1994, and Kong & Bird1996).

Discussion and conclusionsThe results of palaeomagnetic studies indicate that during theLate Tertiary Borneo rotated counter clockwise by about 45�and peninsula Malaysia rotated 15� counter clockwise (Fulleret al. 1991, 1999; Hall 1996). It has been convenient to use thestrike-slip model for Thailand (e.g. Polachan et al. 1991) tohelp explain how both regions have rotated. Hall (1996) showsabout 200 km dextral strike-slip displacement through the

GOT during the Miocene from 20 to 10 Ma. This amount ofdisplacement is based upon accommodating the counter-clockwise rotation of Borneo not on the geological require-ments in Thailand. Active strike-slip deformation wasimportant prior to 30 Ma. From 30 Ma to the Recent largestrike-slip displacements cannot exist because as discussedin the driving forces section. (1) It is impossible to drive athrough-going strike-slip fault zone through the predomi-nantly north–south-trending overlapping extensional fault setsfound in the Gulf of Thailand. Segments of extensional faultsmay well follow pre-existing NW–SE and NE–SW pre-existinginactive strike-slip fabrics in pre-rift ‘basement’. (2) After15 Ma in the northern Gulf of Thailand, and 25 Ma in theMalay and West Natuna basins thermal subsidence dominatedthe region. Thermal subsidence is a characteristic of exten-sional, not strike-slip deformation and post-rift basin wouldseal any pull-part activity if it had occurred previously. Hencethe proposed strike-slip deformation and associated exten-sional faulting is completely mis-timed. (3) Some broadregional right-lateral shear or NW–SE compression did affectthe Malay and West Natuna basins in the Early and Mid-Miocene in order to generate predominantly east–west- toNE–SW-trending inversion anticlines and faults (Ginger et al.1993; Madon 1997b; Phillips et al. 1997). However shorteningis in the order of only 40 km on overlapping, predominantlysoft-linked fault systems, (Fig. 4). Consequently for thePeninsular Malaysia–Thailand region the plate reconstructionsby Lee & Lawver (1995) are favoured because they minimizestrike-slip motion on the Mae Ping and Three Pagodas faultsduring the Miocene. However it leaves a question mark overthe palaeomagnetic data and how the rotations of PeninsulaMalaysia and Borneo can be accommodated.

The initial wooden block and plasticine model ofTapponnier et al. (1982) showed extrusion of blocks withconstant senses of slip on the major strike-slip faults and anortherly younging in the onset of block activity as thecollision zone developed. The escape tectonics of SE Asia aspresently understood shows the following characteristics (Figs7 and 9).

(1) A progressive northerly younging of initiation ofTertiary strike-slip faulting. This is best characterized by thecessation of sinistral slip on the Mae Ping fault zone at 30 Ma(Lacassin et al. 1997), with similar timing on the Klong Mariuand Three Pagodas faults; while the main period of sinistralactivity on the Red River fault zone was from 27 to 17 Ma(Leloup et al. 1995; Harrison et al. 1996; Lacassin et al. 1997).This progression coincides with the northerly motion of India(Lacassin et al. 1997).

(2) Progressive extrusion of more northerly crustal blockswith time. The more southerly blocks were squeezed out faster,relative to their more northerly neighbours. This gave rise tosinistral strike slip. The blocks left behind as India movedfurther northwards became less active or inactive and devel-oped extensional basins over portions of the older strike-slipfaults (particularly the Shan–Thai, Indochina and Peninsulablocks). As the northerly propagation of block extrusioncontinued faults that were initially sinistral (when the southerlyblock was extruded) switched to dextral (as predicted byHuchon et al. 1994). The switch resulted from the morenortherly block taking over extrusion from the block to thesouth, or extruding at a relatively faster rate. After reducedactivity during the Late Miocene (activity is recorded inoffshore sedimentary basins, Rangin et al. 1995b), Pliocenedextral motion on the Red River fault zone (Allen et al. 1984;


Leloup et al. 1995) reflects the continued progressive north-wards activation of crustal blocks.

(3) A modification to the simple extrusion model is thepercentage of India–Asia convergence accommodated byextrusion v. crustal thickening in Southern Tibet and theHimalayas. Harrison et al. (1996) suggested that the slowing ofRed River fault activity in the Early Miocene reflects aprogressive increase in the amount of convergence accom-modated by crustal thickening. The evidence for inversion ofrift basins in northern Thailand also indicates that a shorterperiod tectonic cyclicity existed during the Miocene wheneast–west extension was episodically interrupted by inversion.These minor crustal-thickening epidodes, presumably occurredduring periods when strike-slip faults to the north becamelocked.

(4) In addition to extrusion tectonics, regional east–westextension has been superimposed on the extruding blocks. Thelithosphere has a complex thermal structure and thermalhistory as indicated by the diachronous onset of thermalsubsidence, marked variations in heat flow and geothermalgradient between rift basins, the very deep thermal sag basinsin the Gulf of Thailand, and rapid uplift associated withmetamorphic core complexes west of Chiang Mai. The originof the east–west extension is not completely clear, but astrike-slip pull-apart mechanism is completely inadequate.Subduction roll-back of the Indian Plate seems to offer at leastsome solutions to the timing and origin of the basins. Finiteelement models of the India–Asia collision have only modelledthe effect of the Indian plate indentor on Asia (e.g. Huchon etal. 1994; Kong & Bird 1996). Consequently the modelledpredictions about stress distribution do not entirely explain thecomplex rift and inversion history described in this paper. Themodels do, however, demonstrate that episodes of inversionare generally compatible with stresses originating from theHimalayan collision zone. Huchon et al. (1994) also showedthat early in the collision history (Eocene–Oligocene) Thailandwas likely to have been associated with east–west-trendingmaximum horizontal principal stresses, which fits well withearly sinistral strike-slip on NW–SE-trending faults. Subse-quently the maximum horizontal principal stresses in Thailandtrended NW–SE to north–south, which fits well with laterinversion structures.

(5) The assumption of rigid crustal blocks with strainconcentrated on major strike-slip bounding faults (e.g.Tapponnier et al. 1982) simplifies plate reconstructions (e.g.Lee and Lawver 1995; Hall 1996). However using rigid modelscreates problems with the plate restorations and as Hall (1996)noted, results in overestimation of displacements in the Gulf ofThailand. Further evidence of strain internal to the blocks isthe inability to take large displacements onland on the RedRiver fault zone into the offshore area (Rangin et al. 1995b;Wang & Burchfiel 1997). Inversion and extension in Thailandis another example of non-rigid behaviour.

This regional review has been based on data from many sources, inparticular the theses of MSc students at the University of BruneiDarussalam, including those by: N. Woganan, N. Sankumarn, T. B.Hoon, A. Alief, A. Areeanchuleekorn, C. Haranya, W. Phoosongsee,Y. Gornecome, R. Watcharanantakul and S. Buayai. The ElectricityGenerating Authority of Thailand, BANPU P. C. and Lanna Lignitemining companies are thanked for providing data and logistical help.B. Ratanasthien and S. Singharajwarapan (Chiang Mai University) arethanked for very helpful discussions of the regional geology. PTTEPand Unocal provided data on the onshore and offshore basins of

Thailand for the MSc Projects, in particular S. Praditan, C. Tongtaow,K. Doyle and M. Turk are thanked for their help. P. Brenac (BrenacStratigraphy International) is thanked for the palynological analyses.The University of Brunei Darussalam is gratefully acknowledged forproviding funds for scholarships and research. Finally, the manuscriptwas considerably helped and improved by extensive, detailed andconstructive reviews by R. Hall and A. Collins. R. Hall drew attentionto the non-rigid nature of block behaviour.

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Received 26 June 2000; revised typescript accepted 7 November 2000.Scientific editing by Alex Maltman.

474 C. K. MORLEY

combined escape tectonics and subduction rollback–back arc...

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