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Journal of Asian Earth Sciences 76 (2013) 137–151

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Journal of Asian Earth Sciences

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Discussion of tectonic models for Cenozoic strike-slip fault-affectedcontinental margins of mainland SE Asia

1367-9120/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jseaes.2012.10.019

E-mail address: [email protected]

C.K. MorleyPTTEP, Enco, Soi 11, Vibhavadi-Rangsit Road, Chatuchak, Bangkok 10900, Thailand

a r t i c l e i n f o a b s t r a c t

Article history:Available online 30 October 2012

Keywords:Strike-slipSplaySouth China SeaAndaman SeaPull-apart

Understanding the roles of Cenozoic strike-slip faults in SE Asia observed in outcrop onshore, with theiroffshore continuation has produced a variety of structural models (particularly pull-apart vs. obliqueextension, escape tectonics vs. slab-pull-driven extension) to explain their relationships to sedimentarybasins. Key problems with interpreting the offshore significance of major strike-slip faults are: (1) recon-ciling conflicting palaeomagnetic data, (2) discriminating extensional, and oblique-extensional faultgeometries from strike-slip geometries on 2D seismic reflection data, and (3) estimating strike-slip dis-placements from seismic reflection data.

Focus on basic strike-slip fault geometries such as restraining vs. releasing bends, and strongly splayinggeometries approach the gulfs of Thailand and Tonkin, suggest major strike-slip faults probably do notextend far offshore Splays covering areas 10,000’s km2 in extent are characteristic of the southern por-tions of the Sagaing, Mae Ping, Three Pagodas and Ailao Shan-Red River faults, and are indicative of majorfaults dying out. The areas of the fault tips associated with faults of potentially 100 km+ displacement,scale appropriately with global examples of strike-slip faults on log–log displacement vs. tip area plots.The fault geometries in the Song Hong-Yinggehai Basin are inappropriate for a sinistral pull-apart geom-etry, and instead the southern fault strands of the Ailao Shan-Red River fault are interpreted to die outwithin the NW part of the Song Hong-Yinggehai Basin. Hence the fault zone does not transfer displace-ment onto the South China Seas spreading centre. The strike-slip faults are replaced by more extensional,oblique-extensional fault systems offshore to the south. The Sagaing Fault is also superimposed on anolder Paleogene–Early Miocene oblique-extensional rift system. The Sagaing Fault geometry is complex,and one branch of the offshore fault zone transfers displacement onto the Pliocene-Recent Andamanspreading centre, and links with the West Andaman and related faults to form a very large pull-apartbasin.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Mainland SE Asia is a region of considerable Cenozoic tectonicactivity, which includes development of numerous strike-slipfaults (Fig. 1). The cause, displacement amount, and structural sig-nificance of these faults remains highly contentious, particularlyfor the Ailao Shan-Red River Fault zone and its relationship toextension in the South China Sea (see review in Searle and Morley(2011)). Two main tectonic scenarios have been proposed: one dri-ven by slab pull of the Proto-South China Sea (e.g. Taylor andHayes, 1980, 1983; Holloway, 1982; Hall, 1996, 2002; Hall et al.,2008) and the other driven by escape tectonics (e.g. Tapponnieret al., 1986; Leloup et al., 2001; Replumaz and Tapponnier,2003), see Cullen (2010) for a review.

The slab pull model involves subduction of the Proto-SouthChina Sea under Borneo (Hall et al., 2008). Extension in the SouthChina Seas area rifted the Dangerous Grounds region from Asia,creating sea floor spreading in the South China Sea during theOligocene–Lower Miocene (Briais et al., 1993; Barckhausen andRoeser, 2004). Spreading ceased when subduction of the Proto-South China Seas crust was terminated by collision of the Danger-ous Grounds block with NW Borneo. During extension right lateralshear occurred along the southern half of the NW–SE trendingVietnam margin (e.g. Hayes, 1985; Roques et al., 1997; Huchonet al., 1998; Morley, 2002; Clift et al., 2008).

The escape tectonics model proposes that narrow zones of high-displacement strike-slip faults bounded rigid (i.e. cold) crustalblocks. The major strike-slip faults (e.g. Ailao Shan-Red River(ASRR), Mae Ping and Three Pagodas Faults) are consideredresponsible for formation of basins on the margin of Indochina(Tapponnier et al., 1986; Fig. 1), including the Pattani, Malay,and Song Hong-Yinggehai Basins. The extreme thicknesses

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Thrust BeltHimalayan

Shillong Plateau

NW Borneo Tr

ough

Ailao Shan-Red River Fault zone

Khorat Plateau

Mae PingFault Zone

ChainatDuplex

Sagaing Fault

Three PagodasFault Zone

Khlong MaruiFault Zone

Ranong FaultZone

Gulf ofThailand

N. SumatraFault

B

A

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West AndamanFault

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So Hong-Yinggehai Basin

South China Seas Oceanic Crust

Cuu Long Basin

Nam Con Son Basin

Indo-BurmaRanges

Begu Yoma

AndamanSpreadingCenre

Predominantly Cenozoicstrike-slip faults

Cenozoic folds in Myanmar

Shelf edge

Oceanic Crust

Invisible Ridge (in western Andaman Sea)

Andaman-SumatraTrench

Fig. 2

Fig. 4

Fig. 7

10°

20°

°011°100

Gulf of Martaban

A

B

Sedimentary basin

i

i

i

Fig. 1. Regional map of Cenozoic strike slip faults in SE Asia, the map shows all fault (extensional and strike-slip) offshore Vietnam from Fyhn et al. (2009). Faults onshoreinterpreted from satellite images and geological maps. Areas of folding and thrusting are indicated for the Begu Yoma and Indo-Burma Ranges in Myanmar. Modified fromMorley et al. (2011). Inset maps show the two principal models for the Vietnam margin: (A) Escape tectonics (sinistral margin) , e.g. Leloup et al. (2001), Replumaz andTapponnier (2003); (B) northern sinistral , southern dextral margin (e.g. Roques et al., 1997; Morley, 2002), i = areas of apparent restraining geometries in the escape tectonicsmodel that are incompatible with sinistral strike-slip.

138 C.K. Morley / Journal of Asian Earth Sciences 76 (2013) 137–151

(�8–18 km) of these Cenozoic basins are a significant factor forinterpreting them as pull-aparts (e.g. Tapponnier et al., 1986; Pola-chan et al., 1991; Fyhn et al., 2009). The Mae Ping Fault is inter-preted to terminate in the fold and thrust belt offshore NWBorneo (Leloup et al., 2001; Replumaz and Tapponnier, 2003).While the ASRR Fault Zone has been linked with the developmentof the South China Sea spreading centre, which requires a sinistralsense of motion to open the Song Hong-Yinggehai Basin on theNW–SE Vietnam margin (Leloup et al., 2001).

Subduction of the Proto-South China Sea oceanic crust beneathNW Borneo has been invoked to explain Palaeogene–Early Miocenedeformation along the NW Borneo margin (e.g. Holloway, 1982;Sandal, 1996; Morley et al., 1998; Morley, 2002; Hall, 1996,2002). However, the Proto-South China Sea crust does not existin the escape tectonics model, and displacement caused by the500+ km extrusion of a large continental block is accommodatedby opening of the South China Sea (Leloup et al., 2001; Replumazand Tapponnier, 2003).

Recent arguments for an offshore extension of strike-slip faultsalong the NW–SE trending Vietnam coastal margin have led tohybridized versions of the escape tectonic model (Fyhn et al.,2009; Cullen, 2010). For example both authors do not extend the

Mae Ping Fault to the NW Borneo margin, and accept the presenceof a Proto South China Seas oceanic crust. In the Cullen (2010)model subduction ended during the Oligocene, not the early Mio-cene. Fyhn et al. (2009) note that their timing of inferred majorsinistral motion (predominantly between 40 and 30 Ma) alongthe entire NW–SE trending Vietnam margin is too early for extru-sion to transfer displacement into sea floor spreading in the SouthChina Sea. Despite numerous studies on the offshore data the long-standing absence of conclusive evidence from seismic reflectiondata for either displacement amount or sense of motion alongthe putative offshore sinistral extension of the ASRR Fault Zone re-mains a problem for resolving the appropriate tectonic model forthe region.

This paper discusses recent improvements in our understandingof strike-slip fault evolution and geometries in the Andaman Sea,Thailand and Cambodia and how these observations are relevantfor the ASRR fault zone and the South China Sea. The discussionthen focuses on the Song Hong-Yinggehai Basin and its relationshipto the ASRR fault zone as it passes offshore, whether its geometryfits with a typical pull-apart setting or not, and the implications ofthe basin geometry for the link between the ASRR fault zone andthe South China Sea spreading centre.

C.K. Morley / Journal of Asian Earth Sciences 76 (2013) 137–151 139

2. Regional fault patterns

The region considered in this paper is bound by the Ailao Shan-Red River Fault to the east, and the Sagaing Fault to the west(Fig. 1). The dextral Sagaing Fault passes into the Pliocene-RecentAndaman Spreading centre, which is bounded on its western mar-gin by the West Andaman Fault zone (WAF). The motion of westernMyanmar, north of the spreading centre is related to coupling ofthis continental block with the Indian continent as it moved north(e.g. Curray, 2005). Present day motion along the Sagaing Fault, theWAF and other faults is transferred southwards into the SumatraFault zone (Fig. 1), whose motion is linked to strain partitioningof the deformation caused by oblique subduction along theSumatra–Andaman subduction zone (Malod and Kemal, 1996;McCaffrey et al., 2000).

One of the key features of escape tectonics is that major strike-slip fault zones bound zones of rigid extruded blocks. However,with improved satellite imaging, gravity and magnetic data, andfrom fieldwork, the regions affected by strike-slip faults are veryextensive, with networks of strike-slip faults forming broad zones,duplex geometries and splays that cover a large part of SE Asia(Fig. 1; Morley, 2002, 2004, 2007; Morley et al., 2007, 2011;Watkinson et al., 2008; Ridd and Morley, 2011).

3. Andaman Sea

The Andaman Sea formed as a complex pull-apart basin duringthe Cenozoic, which led to the creation of a spreading centre dur-ing the early Pliocene (e.g. Curray et al., 1979; Curray, 2005;Kamesh Raju et al., 2004). The Pliocene structural developmentof the Andaman Sea links the Sagaing Fault links with theAndaman Spreading Centre on its eastern side, and transfersstrike-slip displacement onto a series of strike-slip faults on thewestern side (including the West Andaman Fault; Kamesh Rajuet al., 2004), and ultimately connects through to the SumatraFault in the south (Curray, 2005). Instantaneous motions fromGPS data along the Sagaing (18 mm/yr and Sumatra fault zones(23 mm/yr; Maurin et al., 2010; Genrich et al., 2000) are suffi-ciently similar to support the regional linkage of the fault sys-tems. Although the plate tectonic setting of the Andaman Sea isdifferent from the South China Sea, it offers the best analoguefor the ASRR fault zone if it was kinematically linked to the SouthChina Sea spreading centre. While strike-slip fault geometriesalong the margin of the eastern Andaman Sea are well docu-mented on 2D seismic reflection data and more limited in area3D oil industry seismic data, these data remain confidential.Consequently the comments made in this section are based onpublically available information that the author knows is wellsupported by a large volume of unpublished industry data. Somegeneral comments about the geometry of the Sagaing Fault arealso made based on the industry data.

3.1. Sagaing Fault zone

Onshore the Sagaing Fault is a strikingly linear feature (Fig. 1),clearly visible on satellite images. Locally there are splays, smallbends, and small jumps in the fault trace, but these are compara-tively minor features. There are no large-scale bends that give riseto clear pull-apart or releasing bend geometries, probably due tosmoothing of the fault trace due to large displacement, as demon-strated by analogue modeling of stepping geometries in strike-slipfaults (e.g. Dooley and McClay, 1997).

The western side of the Sagaing Fault underwent transpres-sional deformation during the Pliocene, which created a seriesof en echelon folds, and some thrusts, which form the hilly Begu

Yoma region (e.g. Pivnik et al., 1998; Bertrand and Rangin, 2003;Fig. 1). East of the Sagaing Fault is a low-lying basinal areabounded to the east by the transpressional Shan Scarp (Bertrandand Rangin, 2003). In southern onshore Myanmar, and offshore inthe Gulf of Martaban this basinal area becomes much wider anddeeper (Fig. 2). Offshore the Sagaing Fault comprises three, andfurther south, two N–S trending main fault traces or principal dis-placement zones that lie at the deepest part of a synformal depo-centre (Fig. 2). This predominantly offshore located strike-slipbasin is remarkable in that its axis lies parallel to the SagaingFault zone, and there is no obvious releasing bend geometry toexplain the location of the basin. The basin depocentre is not con-fined to one side of any of the principal displacement zones, butoverall the basin is thickest west of the strike-slip faults (Figs. 2and 3).

There are several factors contributing to the high subsidenceand location of the basin. First the N–S trending PeninsularThailand margin underwent oblique extension/transtension fromthe Late Eocene–Early (or in places Middle) Miocene (Morley andRacey, 2011; Morley et al., 2011). This extension evolved into athermally subsiding N–S trending passive margin segment. Secondis the later E–W trending Miocene rifting, which in the Plioceneevolved into the Andaman Spreading Centre (Fig. 2). The E–W seg-ment is also now in the early stages of post-rift subsidence, andintersects the older N–S rift trend in the vicinity of the SagaingFault zone. Third, the Sagaing Fault zone is a profound fault zonethat accommodates over half of the northwards motion of Indiarelative to Indochina (Vigny et al., 2003). Since it is effectively aplate boundary the fault zone is likely to be of lithospheric extent(e.g. Searle and Morley, 2011). Such a fault zone in the context ofisostatic modeling would have greatly weakened the crust andconsiderably reduced the effective elastic thickness of the crustin comparison with adjacent areas. Fourth is the very high sedi-ment supply focused along the synformal trough by the Sittoungand Salween rivers. A rough estimate of the modern sediment sup-ply to the eastern Gulf of Martaban by these rivers is around240 million tons/year (Robinson et al., 2007). This combination ofdeepwater passive margin segments, high sediment supply, andlow effective elastic thickness, N–S trending strike-slip fault zoneserved to focus sediment along a N–S trending synformal basin,where offshore the Pliocene-Recent section can exceed the six sec-onds two way travel time on the 2D seismic record (i.e. thicknessesexceeding �8 km).

Offshore the Miocene-Recent fault pattern in the Gulf ofMartaban can be related to a combination of three factors: crustalextension, delta-type gravity tectonics, and localized strike-slipfaulting (particularly the Sagaing Fault; Fig. 3). The two main prin-cipal displacement zones that comprise the Sagaing Fault zone off-shore are narrow, vertical features that pass up to the sea floor.Close to the principal displacement zones (i.e. about 1–2 km eitherside) there is intense deformation, uplift of beds and formation ofpressure ridges (which are well exposed in places onshore). Locallyaccompanying the fault zone there are overpressured fluids (on-shore these are manifest as sand and shale injection features).More than a few kilometres away from the principal displacementzones there is a general absence of any faults that could be de-scribed as R (Riedel) or P shear trends (i.e. faults oriented at anacute angle of about 16�± to the master strike-slip fault; Fig. 2 in-set). Instead the offshore fault pattern is dominated by ENE–WSWtrending extensional faults, and N–S trending strike-slip faults(Figs. 2 and 3). This pattern is broken passing towards the tips ofthe strike-slip fault trends where a fanning, horse tail pattern offaults is developed. The western principal displacement zone diesout north of the eastern principal displacement zone, and it isthe eastern one that transfers its displacement onto the AndamanSpreading Centre (Fig. 2).

Fig. 2. Structural features of the northern Andaman Sea, in part based on Curray (2005) and Searle and Morley (2011). See Fig. 1 for location.


?

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Mesozoic-Palaoegeneaccretionary prism complex

50 km 2 km

Peninsular MyanmarWest SagaingFault

East SagaingFault

Yadana ArchCoco Islands

Top of subducted Indian Oceanic crust (largely inactive subduction zone)

W E

Sea floor

Palaeogene-Early Miocene sedimentary section

Middle Miocene-Recentsedimentary section

Fig. 3. Cross-section across the offshore Gulf of Martaban, showing the main features of the Cenozoic sedimentary basins and tectonic elements, based on 2D seismicreflection data. See Fig. 2 for location.


3.2. Evolution of the Andaman Sea

The present day spreading centre of the Andaman Sea has sep-arated a region of deepwater, rugged topography, which forms theSewell and Alcock Rises (Figs. 1 and 2). The crust underlying theserises is of uncertain crustal type and age (Curray, 2005). Curray(2005) envisioned the rises as oceanic crust formed during Neo-gene extension. However, this age for oceanic crust formation isprobably incorrect. East of the rises a thick N–S sedimentary troughdeveloped, it is undated by wells, but based on regional 2D seismiccorrelations is probably of Oligocene–Lower Miocene age. The ba-sin is characterized by ENE–WSW trending extensional faults andN–S trending strike-slip faults. A Middle Miocene unconformityseals these faults. This marks the time of tectonic reorganizationwhen extensional activity shifted northwards, and crustal exten-sion led to development of the Pliocene-Recent Andaman Spread-ing Centre (Curray, 2005; Morley et al., 2011).

The ocean-continent boundary in the eastern Andaman Seadetermined from seismic refraction data by Curray (2005) followsthe southern projection of the Sagaing Fault. In offshore Thailandthis boundary is exactly followed by a large NNE–SSW trendingstrike-slip fault that ceased activity in the Middle Miocene. Henceit is suggested in Fig. 2 (dashed line labeled A) that an older ‘SouthSagaing’ Fault segment defines what Curray (2005) interpreted asthe ocean-continent boundary. The consequences of the sedimen-tary basin interpretation are two fold. First it is most likely that ifoceanic crust exists it is older than the Neogene. One possibility isthat it is of Late Cretaceous age, similar to the Andaman Islandsophiolite (Pedersen et al., 2010). Subsequently the crust has beenthickened by plateau basalt igneous activity during the Miocene-Quaternary (Curray, 2005; Iyer et al., 2012). Secondly since the‘oceanic-continental crust boundary’ actually coincides with a ma-jor strike-slip fault, there is the possibility that the strike-slip faultdefines thinner crust on the western side than the eastern side,without causing an abrupt change in crustal type (i.e. the westernside of the fault is composed of more highly attenuated continentalcrust than the eastern side. An unpublished gravity inversion mod-eling study of SE Asia crust using the technique described byChappell and Kusznir (2008), suggests the Alcock and Sewell Risesare formed from extended continental crust (Kusznir, N., pers.comm., 2012).

One unusual feature of the entire strike slip system from theEast Andaman Basin in the south to the Begu Yoma in the northis that the major strike-slip faults are accompanied by very numer-ous ENE–WSW trending extensional faults, not the more typical Rand P shears (these fault systems are shown schematically in Fig. 2,and their precise extent is not well defined). If a conventional pri-mary strike-slip fault feature label were applied, these faults wouldbe called R0 shears.

In the East Andaman Basin area a Late Oligocene–Early Miocenestrike-slip basin, very similar in location, and structural style ispresent. The large strike-slip fault (here referred to as the South

Sagaing Fault) bounding this basin appears to have become inac-tive during the Middle Miocene. Probably the offshore deformationlinked with transtensional deformation along the Shan Scarp areadescribed by Bertrand and Rangin (2003). In the Alcock and Sewellrises superimposed on this older system is the Pliocene-Recentpull-apart system of the Sagaing–West Andaman–Sumatra faultzones.

The Late Eocene(?)–early Middle Miocene rifting of the MerguiBasin in the western Andaman Sea is associated with early rifting(until the Late Oligocene) along N–S to NNE–SSW trending faults(e.g. Morley et al., 2011). Which suggests that extension was moreE–W to WNW–ESE early on in the rift development then theextension direction later rotated to a more NW–SE to NNW–SSEdirection in the Miocene, which promoted the development ofstrike-slip faults (such as the South Sagaing Fault).

4. Strike-slip faults of eastern Myanmar, Thailand andCambodia

A number of N–S, NW–SE and NE–SW trending strike-slip faultstrands are present in eastern Myanmar and Thailand (Fig. 1).These faults have been attributed to Oligocene–Neogene escapetectonics (e.g. Tapponnier et al., 1986; Polachan et al., 1991;Huchon et al., 1994; Lacassin et al., 1997; Leloup et al., 2001;Replumaz and Tapponnier, 2003). The largest of these fault zonesare the Mae Ping, Three Pagodas, Ranong and Khlong Marui. Thesefault zones have a later history that can be attributed to escape tec-tonics, and also an older history of deformation during the Eocene,and episodically as far back as the Late Cretaceous (Morley, 2004;Morley et al., 2007, 2011). Watkinson et al. (2008, 2011) studiedthe Khlong Marui and Ranong faults, and on the basis of cross-cutting relationships between Late Cretaceous–Palaeogene gran-ites and ductile fault strands, 40Ar/39Ar and U–Pb SHRIMP ages con-cluded that multiple stages of ductile, dextral strike-slipdeformation could be identified as follows: D1, low grade ductiledextral strike-slip prior to 81 Ma, and between 59 and 49 Ma(D2). A major period of medium to high-grade ductile strike-slipdeformation at temperatures between 300 and 500 �C occurredduring the Middle Eocene (48–40 Ma, D3; Watkinson et al.,2011). Brittle sinistral and sinistral-reverse oblique strike-slip oc-curred after 37 Ma (D4), and brittle dextral strike-slip occurredaround 23 Ma (D5). The D2 and D3 events fit with the regionalPalaeogene transpressional deformation seen elsewhere inThailand. Between the Ranong and Khlong Marui Faults, and westof the Khlong Marui Fault folded Mesozoic rocks are presentpredominantly in synclinal areas. The deformation in some areasproduced broad, open folds, while elsewhere it is more intensewith rotation of beds to near vertical.

The regional pattern of strike-slip faults in eastern Myanmarand Thailand is one of broad fault zones composed of numerousfault strands that branch, splay, and form duplexes (Figs. 1 and4). In eastern Thailand the fault patterns for branches of the Three

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Pagodas and the Mae Ping fault zones are strongly splaying (Fig. 4;Morley et al., 2007; Ridd and Morley, 2011). In the escape tectonicsmodel the Mae Ping Fault zone is inferred to extend beyond thesplays of eastern Thailand to the NW margin of Borneo (Leloupet al., 2001; Replumaz and Tapponnier), while Fyhn et al. (2010a)proposed that a high displacement (100+ km) branch of the ThreePagodas Fault zone extends into the eastern Gulf of Thailand. Con-versely the splaying geometries suggest the faults are dying outand not continuing as high-displacement faults to the south andsoutheast.

Along the same fault system mapped by Fyhn et al. (2010a), andto the north of their study area an unpublished 3D seismic survey(mapped by the author) shows that the faults bound a half grabendepocentre, and switch polarity along the structure. The faults tendto dip between 25� and 60�. The large right-stepping bend (Fig. 5,

Fig. 4. Regional map of the Gulf of Thailand and surrounding areas. Basins where MesoBasin, W = Western Basin, NK = Nakhon Basin, CPB = Central Pattani Basin, NMB = North

location A) in the main west-dipping bounding fault would be arestraining bend feature under sinistral displacement that shouldhave produced considerable uplift, if strike-slip displacement wassignificant, but no large inversion is present (until the Early Mio-cene), instead a deep (6+ km) basin is present north of the bend.The fault dips, gaps in the fault traces, and switching dip directionsare entirely consistent with extensional basins and inconsistentwith a strike-slip fault zone with 10’s km of horizontal displace-ment. The main problem with Fyhn et al’s (2010a) interpretationis the insufficient spacing and resolution of the 2D seismic datato properly resolve fault geometries. The unusually narrow and lin-ear nature of the (extensional) fault zone suggests strike-slip faultcharacteristics, but they are insufficient discriminators by them-selves. Lake Rukwa in East Africa is a rift basin bound by a linearfault trace suggestive of strike-slip deformation, but seismic

zoic sediments underlie the Cenozoic rift section, CHP = Chumphon Basin, K = KraMalay Basin. Modified from Morley (2012). See Fig. 1 for location.

Fig. 5. Fault map for the eastern Gulf of Thailand, based in part on Fyhn et al.(2010a) (southern part), and in part from mapping by the author (northern part)from 2D and 3D seismic data. See Fig. 4 for location.


activity and fault kinematic data demonstrate almost pure exten-sion, and in profile the fault has a listric geometry (see reviewsin Morley (2012) and Delvaux et al. (2012)). Similarly the linearfault traces in the eastern Gulf of Thailand could be inherited frompre-rift fabrics (structural trends associated with the mainIndosinian terranes tend to be N–S to NNW–SSE in this area).Consequently in eastern Thailand there is no strong evidence forstrike-slip faults passing far offshore into the Gulf of Thailand assuggested by Fyhn et al. (2010a) and the splaying geometry ofthe Three Pagodas Fault in Eastern Thailand is probably indicativeof the fault dying out in the northern-most part of the Gulf.

The Mae Ping Fault exhibits a broad splaying zone in easternThailand (Fig. 4; Morley et al., 2011; Ridd and Morley, 2011). How-ever, little was known about the fault zone geometry into the TonLe Sap Lake area of Cambodia. Although a branch of the Mae PingFault zone has been inferred to run through the Lake to pass off-shore in the vicinity of the Mekong Delta, and Cuu Long Basin(see review in Morley (2002)). Recently a gravity survey and areconnaissance accelerated weight drop seismic reflection survey(Hendrick et al., 2009) have provided some general informationabout the basin configuration, which can be viewed at theCambodian National Petroleum Authority website (http://cnpa-cambodia.com/pages/petroleum-exploration/seismic-data.php).Some 2D seismic lines are presented on the website, but no exactlocations are given. In the absence of well data, and specific seismicline locations only some general comments can be made. Never-theless the data represents an important glimpse into the struc-tural development of the area. A number of basins, expressed asnegative gravity anomalies are present, and suggest that the faultpattern is a continuation of the strongly splaying geometry seenin the Mae Ping Fault zone passing towards Ton Le Sap (Fig. 4).

The age of the basins remains uncertain. Seismic data shows anumber of angular unconformities, and from regional consider-ations these unconformities may correspond with the top of Meso-zoic, Eocene–Oligocene, and Neogene sequences (Fig. 6). But theseages are, at best, educated guesses at this time, and possibly theentire basinal sequence is of Cenozoic age. Alternatively CHplusResources interpret the section as predominantly of Mesozoic age(http://www.slideshare.net/chris_newport/block-xi-information-memorandum-march-2010-c). Given that the Mesozoic sequenceis well imaged on offshore seismic reflection data (e.g. Fyhnet al., 2010b) it seems reasonable to expect that the Mesozoic se-quence is also imaged onshore, particularly since it forms hillsimmediately adjacent to the basins. The seismic data at least pointto a prolonged, episodic development of basins, with phases ofextension/transtension and inversion/transpression. If Mesozoicrocks are imaged on the seismic data, as seems likely, then theTon Le Sap area differs from the Khorat Plateau, in that the Meso-zoic basins are fault controlled.

The Ton Le Sap area displays a complex fault pattern not a sin-gle, simple fault strand of the Mae Ping Fault zone. If a Cretaceoushistory of basin development is present in the area, then either: (a)the faulting is unrelated to the Mae Ping Fault, and it just becamelinked with the fault during the Cenozoic, or (b) it points to a veryearly stage of the fault development followed by later reactivation.A multi-stage history (b, above) is supported by the evolution ofthe Ranong Fault zone described by Watkinson et al. (2008,2011). In both cases the splaying geometry of the Mae Ping Faultzone near the border with Thailand suggests the Cenozoic MaePing Fault zone is losing a considerable amount of displacementin this region.

4.1. Summary

The Mae Ping and Three Pagodas fault zones splay and die out inthe Gulf of Thailand and Ton Le Sap area and are replaced by exten-sional basins. Possibly a strand of the Mae Ping Fault zone reachesthe Cuu Long Basin on the Vietnam South China Seas margin(Fig. 4), but given the amount of splaying to the NW, this is likelyto be a very low displacement fault strand. The strike-slip faultzones may have reactivated under escape tectonics, but they alsodisplay earlier Cretaceous and Palaeogene histories unrelated toescape tectonics. The Mae Ping Fault splays into basins in theTon Le Sap area that may have both a Mesozoic and Cenozoic his-tory of development. The very complex fault networks, splays andnested duplex structures associated with the fault zones suggestthe faults are losing displacement to the south and east.

5. The ASRR in the Gulf of Tonkin

In many parts of SE Asia the occurrence of basins at releasingbends in strike-slip is well documented, for example at numerousNE–SW trending strike-slip faults in Yunnan, northern Myanmarand northern Laos (Morley, 2007), and along the Three PagodasFault zone in western Thailand (Morley, 2002; Morley and Racey,2011). Uplift associated with the restraining bend Chainat Duplexalong the Mae Ping Fault zone (Fig. 1) has also been described(Morley et al., 2007). Splaying at the terminations of strike-slipfaults is also widespread as has been discussed above. Hence itseems reasonable to apply basic our fundamental understandingof strike-slip geometries to the Ailao Shan-Red River Fault zoneas it passes into the Gulf of Tonkin. However, in doing so the inter-pretation of the Song Hong-Yinggehai Basin as forming in a pull-apart basin setting (e.g. Clift and Sun, 2006; Zhu et al., 2009) arechallenged.

http://www.slideshare.net/chris_newport/block-xi-information-memorandum-march-2010-c

http://www.slideshare.net/chris_newport/block-xi-information-memorandum-march-2010-c

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5.1. Song Hong-Yinggehai Basin characteristics

The Song Hong-Yinggehai Basin (Fig. 1) is one of the deepest ba-sins in SE Asia, attaining a maximum thickness in the order of17 km. According to Clift and Sun (2006) the basin initially openedaround 44 Ma, with dominantly transtensional deformation begin-ning after 34 Ma. Extension along the margin ended around 21–30 Ma. The basin was diachonously inverted between 21 and14 Ma. The late Miocene was a period of reduced subsidence inthe basin. After 5.5 Ma the subsidence rate increased again in thebasin, particularly during the late Pliocene (Clift and Sun, 2006).This late increase in sediment supply reflects fast erosion causedby the glacial–interglacial climate, and erosion of Hainan as itwas uplifted in response to magmatism (Tu et al., 1991; Floweret al., 1998). In sections along the basin axis (NW–SE) and acrossthe basin (NE–SW) the basin is dominated by a synformal geome-try (Clift and Sun, 2006). Fault control of the Miocene-Recent basingeometry is not particularly obvious in any published cross-sectionor seismic section. However, some authors infer a period of dextralmovement after 5 Ma (e.g. Sun et al., 2003).

The general geometry of the basin comprises NW–SE trendingfault strands, and important N–S trending fault segments at theSE and NW margins of the basin (Fig. 7). The depth to the Moho be-neath the Song Hong-Yinggehai Basin may be as shallow as 20 km,which given the great thickness of sediment present in the basin(�17 km) implies a very high degree of crustal attenuation (Xieet al., 2006; Zhang et al., 2008). However, the area of greatest thin-ning does not coincide with the greatest sediment thickness, hencethe minimum thickness of crystalline crust is about 6 km (Fig. 7;Zhang et al., 2008) and lies just south of the area of maximum crus-tal thickness. Possibly the region of hyper-extension is partly re-

lated to the propagating tip of the older, northern spreadingcentre of the South China Sea.

5.2. Strike-slip timing and displacement

The escape tectonic model requires that hundreds of kilometresof sinistral strike-slip along the Ailao Shan-Red River Fault zone iskinematically linked with the South China Sea spreading centre(e.g. Tapponnier et al., 1986; Leloup et al., 2001; Replumaz andTapponnier, 2003). The timing of sinistral activity of the ASRRshear zone (e.g. Leloup et al., 2001) approximately matches thetiming of opening of the South China Sea (Briais et al., 1993), andranges between 34 and 17 Ma. The timing of both the ASRR shearzone and South China Sea spreading may vary somewhat from 34to 17 Ma., Searle (2006) proposed sinistral motion along the ASRRshear zone could be limited to 32–22 Ma, or possibly even a shorterperiod (also see discussion in Leloup et al., 2007). While Barckhau-sen and Roeser (2004) limit the timing of seafloor spreading in theSouth China Sea to 31–20.5 Ma. Initially spreading occurred on thesmaller, northern spreading centre (31–25 Ma), and was followedby a jump to the the southern spreading centre (25–20.5 Ma)(using the timing of Barckhausen and Roeser, 2004).

An early (Middle Eocene?–Lower Oligocene) phase of widelydistributed transtensional depocentres is present in the SongHong-Yinggehai Basin, which are sealed by an unconformity datedat 30 Ma (Rangin et al., 1995a,b). This deformation occurred largelyprior to onshore activity of the ASRR shear zone (Leloup et al.,2007), and suggests initial deformation was independent of thisshear zone. Between 30 Ma and 15 Ma deformation became morefocused in the central part of the basin, related to transtensionaldeformation associated with the ASRR shear zone, and was fol-

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Fig. 7. Structural map showing how the Red River Fault zone splays and interacts with the offshore basins in the Gulf of Tonkin. Compiled from Morley (2007), Rangin et al.(1995a) and Zhu et al. (2009). Arrows associated with faults in the Song Hong-Yinggehai Basin show possible oblique opening following the style of strike-slip fault-rift basintransition shown in Fig. 8. See Fig. 1 for location. Grey contour lines, with labels 6 km, 8 km, 10 km and 14 km are thickness of crystalline crust estimated from depth to Mohoand sediment thickness data in Zhu et al. (2009). Palaeomagnetic study localities from Liu and Morinaga (1999), X = Hainan , Y = Xinlong, Guangxi.


lowed by transpressional deformation between 15.5 and 5.5 Ma(Rangin et al., 1995a,b). These authors also noted there was no evi-dence for Plio-Pleistocene dextral strike-slip motion seen onshore,although minor faulting is present in the basin (Clift and Sun,2006). According to Rangin et al. (1995a,b) in the SongHong-Yinggehai Basin there is no evidence for large sinistraldisplacements in the order of 500 km, and between 30 and5.5 Ma strike-slip motions did not exceed a few tens of kilometres.

5.3. Palaeomagnetic data

Palaeomagnetic data has produced some mixed conclusionsabout extrusion in SE Asia. A number of studies identified south-erly, post Late Cretaceous displacement of Indochina betweenabout 500 and 900 km, which supports the extrusion model(Charusiri et al., 2006; Otofuji et al., 2012; Kondo et al., 2012).Other studies suggest that southern extrusion is harder to define,for example Cung and Dorobek (2004) concluded that the southerndisplacement is about 6.5 ± 5.1�, i.e. the error bar is very large, andthat complex internal deformation characterizes the region.

The Da Lat area of Southern Vietnam shows anomalous lowrotations of the area �2.7 ± 4.2� with respect to Eurasia, whereaselsewhere in Indochina clockwise rotations in the order of 10–24� are interpreted (Otofuji et al., 2012). Assuming that the DaLat area also underwent such rotations, Otofuji et al. (2012) sug-

gest that locally 27 ± 9.8� counter clockwise rotation was later im-posed on the region. To achieve this they propose Indochinaunderwent clockwise rotation prior to 32 Ma, and that during theescape tectonic phase southeastern Vietnam underwent counterclockwise rotation. The Aliao Shan-Red River Fault trends intothe N–S striking East Vietnam Boundary Fault (the N–S restrainingbend B, Fig. 1) in the south. For sinistral displacement this restrain-ing bend should enhance clockwise rotation (e.g. Replumaz andTapponnier, 2003), not promote anticlockwise rotation. Possiblydistributed deformation at the southern tip of the fault might helpto explain the anomalous rotation. However, it is difficult to recon-cile the expected rotations from escape tectonics with the modelproposed by Otofuji et al. (2012). It is also possible that SE Vietnamsimply did not undergo clockwise rotation, and hence there is noneed for later large counter clockwise rotation.

Although the palaeomagnetic data generally supports southernextrusion, it does not support that motion occurring along thesouthern part of the Red River Fault zone. Studies by Takemotoet al. (2005), Geissman et al. (2008) and Cung and Geissman (inpress) indicate that there was little or no latitudinal translationof the Son Da terrane with respect to the South China Block acrossthe Red River Fault Zone. Takemoto et al. (2005) concluded that adifferent fault zone or zones to the west of the Red River Fault zonemust have accommodated the southerly displacement ofIndochina.

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Fig. 8. Plot of strike-slip fault displacement vs. area of strike-slip fault (splaying)termination zone. 1 = Glacier Lakes Fault, Kirkpatrick et al. (2008); 2 = Bolfin Faultzone (Jensen et al., 2011), note fault displacement estimated from length-displacement relationships only; 3 = unnamed fault zone, Central Iran (Morleyet al., 2009); 4 = North Mergi Fault, Mergui Basin, Andaman Sea, (based on mappingof seismic reflection data by the author); 5 = Hosgri Fault, California, (Sorlien et al.,1999); 6 = North Island Fault System (Mouslopoulou et al., 2008); 7 = Har Us NuurFault, Jargalant Nauruu restraining termination, Central Asia, (Cunningham, 2007;Nissen et al., 2009); 8 = Ubiernera Fault, Pyrenees, Spain, (Tavani et al., 2011);9 = Priestely Fault, northern Victoria Land, Antarctica (Storti et al., 2001); 10 = KarlikTagh, Central Asia (Cunningham, 2007); 11 = North Anatolian Fault zone-AegeanSea (Umhoefer et al., 2007); 12 = Mae Ping Fault Zone (this paper), 13 = Ailao Shan-Red River Fault zones discussed in this paper, with large range in potential sinistraldisplacement amount .


The island of Hainan, which lies on the eastern side of the RedRiver Fault Zone is not expected to show evidence for significantsoutherly displacement in the extrusion model. Yet according toLiu and Morinaga (1999) Hainan has been translated approxi-mately 7� north of the present latitude, while onshore at Xinlong,Guangxi about 200 km north of Hainan, was translated 7.8 ± 6.9�south (see Fig. 7 for locations). In a more recent study about 6� lat-itude southerly displacement was calculated for Hainan (Zang andTan, 2011). Liu and Morinaga (1999) explained the data in terms ofhighly distributed strike-slip deformation related to escape tecton-ics (which tends to support the arguments for splaying fault tips inthis paper). Alternatively Zang and Tan (2011) interpret the data asresulting from strain-induced particle rearrangement within hae-matite, which is the dominant magnetic carrier. They concludedthat the data is not an accurate record of Cretaceous paleolatitude,since no large faults have been identified which could have trans-lated Hainan to the south.

Haematite is the dominant magnetic carrier in the IndochinaBlock as well (e.g. Otofuji et al., 2012), which begs the questionis it correct to accept the palaeomagnetic data on the IndochinaBlock because it supports a model, but reject the data from Hainanbecause it does not? Or does the Hainan and Xinlong data indicatethe magnetic data is regionally unreliable as a paleolatitude indica-tor? For example Cung and Geissman (in press) note how thechoice of reference palaeopole can significantly reduce the magni-tude of Khorat Plateau southward displacement. Due to the ambi-guity of the palaeomagnetic results it is argued here that these datado not preclude, and some cases supports, the interpretation thatthe Ailao Shan-Red River Fault is losing displacement into the Gulfof Tonkin.

5.4. Song Hong-Yinggehai Basin fault geometries

There are two areas along the Vietnam margin where NNW–SSEto N–S trending fault segments exist along the main (inferred)strike-slip fault segment (Figs. 1, locations A and B, and 7, locationA). These areas would be restraining bends under sinistral dis-placement, and under 500+ km displacement they should be re-gions of considerable uplift, and basin compression andinversion. Yet there are no such features on the published seismicdata. It is very difficult to explain the Song Hong-Yinggehai Basinas a pull-apart when the main bend in the fault trace is appropriatefor a restraining not a releasing bend under sinistral motion. Thisproblem is seen in papers that interpret the basin as opening upas a dextral pull apart (correct sense of motion for the restrainingbend, but wrong sense of motion for the ASRR Fault zone, e.g.Harder et al., 1992; Zhang and Hao, 1997; Li et al., 1998).

If a pull-apart geometry could be created, 500 km displacementwould inevitably lead to widespread creation of oceanic crustalong the NW–SE trending margin and cause intense igneous activ-ity. The crust is thin, perhaps as little as 6 km of crystalline crust insome areas of the basin (Fig. 7). The area of thinning is localized tothe tips of the N–S trending faults and elongate parallel to the mar-gin. It is certainly debatable whether the crust is highly attenuatedcontinental crust or oceanic crust. However, the location andgeometry of the thinned crust are not consistent with a sinistralpull-apart setting, nor is the zone of thinning extensive enoughto conclusively be associated with 500 km of sinistraldisplacement.

The observed depocentre migration during the syn-rift stage(i.e. 44–21 Ma), and time of sea floor spreading in the South ChinaSeas, only shows an eastwards younging shift of 50 km, while thepost-rift migration of depocentres occurs in a more southeasterlydirection and results in a shift of about 200 km (Zhu et al., 2009).Consequently the timing and amount of depocentre migration isincompatible with hundreds of kilometres of sinistral displace-

ment offshore. The shift in depocentre location is not clearly re-lated to structure, and instead may reflect shifting sedimentationpatterns.

There remains virtually no hard evidence from published seis-mic data in the form of long, linear faults, correct releasing bendgeometries, or relationships of depocentres to faults to supportthe large-displacement strike-slip model. In Fyhn et al. (2009)the evidence for flower structure geometries on seismic data wasover emphasized by displaying lines that were vertically exagger-ated by about 4:1 to 5:1.

5.5. Comparison of Mae Ping and Ailao Shan-Red River fault zonesplays with other examples

The loss of displacement for major strike-slip faults occurs overa substantial portion of the fault (�10–20% of the length). Forexample measurements of recent activity along the Kunlun Fault,in east Central Tibet has revealed a central region about 800 kmlong of fairly constant slip between 10 and 12 mm/yr, which de-creases to near zero over the eastern 200 km of the fault (Kirbyet al., 2007). How the strain decrease is accommodated in the adja-cent area remains uncertain (Kirby et al., 2007). It is remarkablydifficult to find large displacement (i.e. 100’s m to kms) examplesof strike-slip fault tips whose area of deformation approachingthe fault termination, and maximum displacement can be reason-ably estimated. Nevertheless a few examples are given in Fig. 8 totest whether the suggested areas of splays for the fault zones dis-cussed in this paper are sufficient to have accommodated sinistraldisplacements in the order of 150 km (Mae Ping Fault zone) to500 km (Ailao Shan-Red River Fault zone). The relatively low dis-placement:splaying area ratio examples in Fig. 8 are related tothe active fault zones, which suggests that the area of the faulttip splays is likely to remain fairly constant for some time butthe fault displacements will increase as the faults develop. An addi-tional factor for the Anatolian Fault zone is that its western tip ter-minates in the Aegean Sea. Although Umhoefer et al. (2007) show


the Aegean as representing the tip deformation of the AnatolianFault, strike-slip is not the only cause of extension since subduc-tion rollback is also a significant component of the deformationin that region (e.g. Jolivet et al., 2008).

Large strike-slip faults display considerable complexity relatedto pre-existing fabrics, variable rock mechanical properties andinteraction with different tectonic or structural provinces. Forexample the Priestly Fault, Antarctica, splays approaching thecoast, and in part loses slip by reactivation of an older rift system(Storti et al., 2001). The reactivation of pre-existing trends (e.g.Priestley Fault), and mixing of extensional systems of different ori-gins (e.g. Aegean Sea) appear to be features of the Mae Ping, ThreePagodas and Ailao Shan-Red River Fault zones. The splaying areasof the Mae Ping and Ailao Shan-Red River Fault zones are in linewith their estimated displacement amounts and the trends fromother strike-slip fault zones (Fig. 8). Hence it does not appear tobe necessary to infer that these fault zones can only lose displace-

Fig. 9. Regional fault map showing how the dextral strike-slip North Island Fault SystemNew Zealand basin, redrawn from Mouslopoulou et al. (2008).

ment by transfer into a plate boundary (such as a trench or spread-ing centre).

5.6. Preferred structural model for the Song Hong-Yinggehai Basin

From the published literature the Song Hong-Yinggehai Basinlacks the characteristics of a pull-apart basin. There is no clearlymapped large-scale releasing bend or left stepping fault geometryto cause a pull-apart basin under sinistral motion. The largestfault-bends are actually restraining bends under sinistral motion.Also much of the large subsidence along the fault occurred duringthe post-rift stage, not during fault motion. Sedimentation has al-ways been in shallow water. In this regard the Song Hong-YinggehaiBasin is reminiscent of extensional and post-rift basins in the Gulf ofThailand (see review in Morley and Westaway (2006)).

Morley (2002, 2004) stressed that the Three Pagodas and MaePing fault zones in Thailand and Myanmar have anastomosing

divides into N–S oriented splays, that pass northwards into the Taupo Rift system,

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geometries in many places, and that the Three Pagodas Faultstrongly splays passing into the Gulf of Thailand. More recentinterpretations of the Mae Ping fault geometry from satelliteimages, fieldwork and gravity data show a strongly splaying geom-etry in eastern Thailand and Cambodia (Fig. 4; Ridd and Morley,2011). These splays are inferred to occur at the tips the major faultsand occupy very large regions about 25,000–60,000 km2, whichseem to be of sufficient size to absorb strike-slip fault displace-ments along the major faults strands of tens to 100+ km (see Sec-tion 5.5). Approaching the Gulf of Tonkin, the ASRR fault zone alsoshows a strongly splaying geometry (Fig. 7; Rangin et al., 1995a;Morley, 2007), which covers an area of about 100,000 km2. Conse-quently, given the absence of evidence for large displacementsbased on seismic reflection data and palaeomagnetic data dis-cussed above, and the splaying fault geometries it seems reason-able to conclude that the Song Hong-Yinggehai basin did notform as a classic pull-apart basin, but instead formed at thesouthern splaying tip of the ASRR fault zone (Fig. 7).

The splaying tip geometry permits stress rotation at fault tips,which can allow transtensional depocentres to form on the easternsides of the splaying N–S trending faults (Fig. 7; Chinnery, 1966;Kim and Sanderson, 2006). Conversely if the N–S trending faultsare part of a through-going connected fault system then the N–Strends would form restraining bends, prohibiting Oligocene–EarlyMiocene transtensional basins development (Rangin et al., 1995a).

A possible analogue for the Song Hong-Yinggehai Basin comesfrom North Island Fault System of New Zealand. Mouslopoulouet al. (2008) describe how the North Island Fault System is aNNE–SSW trending dextral fault that terminates by dividing intoN–S oriented splays that pass northwards into the Taupo Rift sys-tem (Fig. 9). Instead of acting as a restraining bend, the N–S trendsdisplay increasingly oblique (extensional) displacement passingnorthwards (Fig. 9). A similar scenario is invoked here for the ASRRas it passes southwards within the Song Hong-Yinggehai Basin(Fig. 7). The model is not too dissimilar from the one proposedby Sun et al. (2003) who viewed the Song Hong-Yinggehai Basinas terminating in a zone where the Indochina Block underwentclockwise rotation with respect to the South China Block. However,whether wholesale regional block rotation (Sun et al., 2003), or justmore localized rotations near the fault tips (Fig. 7) is required isuncertain.

Roques et al. (1997), Morley (2002) and Clift et al. (2008) arguedthat fault geometries further south along the margin are better ex-plained by transtensional dextral motion than sinistral motion. Inthe absence of large displacements on the ASRR fault zone, thereis no compelling reason from seismically mapped fault geometriesto call for sinistral motion along the margin. Indeed the geometryof rifting is much more compatible with a dextral component ofmotion, than a sinistral component (Fig. 1). This change from sinis-tral to dextral deformation along the NNW–SSE trending margin issupported by data to the west, onshore, where Rangin et al.(1995b) identified a sinistral strike-slip regime, particularly inthe northern part of the area, while right-lateral deformation be-came dominant further south (and also in places is later than thesinistral deformation). Rather than being a conventional pull-apartbasin the Song Hong-Yinggehai Basin appears to have developed atthe southern tip of the ASRR fault zone to the north (related to es-cape tectonics), and the dextral strike-slip system to the south, re-lated to oblique extension of the margin.

The Indosinian suture zones in Thailand are zones of weaknessthat are exploited by later structures, particularly rifts (e.g. Morley,2009). The NNW–SSE trending Song Ma suture in Vietnam is sug-gested to have also acted as a zone of weakness that became a fo-cus for highly oblique rifting. This rifting acted as the westernboundary to the more regional ENE–WSW to E–W fault trends.

Given the ambiguity of interpreting seismic reflection deep in thebasin, and correlating faults on 2D seismic the highly oblique zoneof transtension gave the impression that the ASRR shear zone couldbe extended along the margin, which, it is argued here, is the incor-rect interpretation.

6. Conclusions

Seismic reflection data has been used to argue both for andagainst the presence of major strike-slip faults in the Gulf ofThailand and Gulf of Tonkin. This paper infers that the map viewgeometries of major fault zones using the presence of splays, andbends in strike-slip faults requires the major faults seen on landto not extend far into the offshore areas. The linkage of large-strike-slip fault displacement with sea floor spreading applies tothe Andaman Sea (Sagaing–West Andaman faults), where the ma-jor faults form relatively simple, very linear traces and set up alarge pull-apart basin. Such clarity of linkage, and linearity of majorfaults is not observed along the proposed offshore extension of theASRR Fault Zone. If ultimately the ambiguity in the palaeomagneticdata is resolved in favour of very large displacements along the off-shore Aliao Shan-Red River Fault zone then explaining how thefault zone geometries, which are counter-intuitive with basicstructural models for strike-slip faults, can be reconciled needs tobe addressed.

The Late Cretaceous–Palaeogene story of the South China Seas-Borneo-Gulf of Thailand area is important for understanding theonset of rifting in the South China Sea, crustal conditions, andpre-existing structural fabrics. This story is not particularly wellunderstood at present. For example, did some of the rifting in theregion originate as a result of extensional collapse of crust thick-ened during transpressional deformation that initially establisheda number of the strike-slip fault in Thailand and eastern Myanmar(Morley, 2004, 2012)? In SE Asia major strike-slip faults exhibitsome narrow zones but also commonly flare out into much broaderzones of anastomosing fault strands. The ‘rigid blocks’ between thestrike-slip faults actually appear to be significantly deformed inmany places by very numerous extensional, oblique-slip andstrike-slip faults. An exception is the Sagaing–West Andaman Faultsystem, where there is clearly slip of a large, discrete continentalblocks past each other. Elsewhere in Indochina deformation ap-pears to be much more of a continuum, with extensive regionaldeforming by slip on numerous fault strands than by slip along dis-crete blocks.

The South China Sea margin rifting has an older history ofextension (Late Cretaceous–Palaeogene) than the Oligocene–LowerMiocene age for sinistral motion on the ASRR. Consequently itseems unnecessary to invoke a phase of strike-slip faulting to ex-plain part of the extension, when at other times extension mustbe explained by different mechanisms. This study is not arguingagainst the effects or possible magnitudes of displacement of majorfaults involved in escape tectonics onshore. However, the faultgeometries are inconsistent with pull-apart explanations for off-shore basin geometries (except in the Andaman Sea). In the gulfsof Thailand and Tonkin either the displacement on large strike-slipfaults is not large enough to necessitate connection of the faults toa plate boundary in order to terminate the faults, or the splayinggeometries occupy such a large area (10,000’s km2), that hundredsof kilometres of displacement can be lost in a large rock volume,without involving a plate boundary. The area of displacement lossseems appropriately large compared with global examples ofstrike-slip displacement-tip splay area characteristics (Fig. 8). Thesinistral splaying tip geometries passing into the Song Hong-Yinggehai Basin fit with previous models for the offshore ASRR

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Fault zone that suggest it passes SE into oblique dextral extension,or transtensional motion (Taylor and Hayes, 1980, 1983; Roqueset al., 1997; Huchon et al., 1998; Morley, 2002; Sun et al., 2003;Clift et al., 2008). Given that the history of extension along the Gulfof Tonkin margin (45 Ma+) is older that the age of initiation ofsinistral slip onshore along the ASRR Fault Zone (�32–30 Ma),the ASRR Fault Zone is inferred here to have propagated into analready existing rift system, which developed independently ofescape tectonics, and produced a hybrid basin (Song Hong-Yinggehai Basin) at its splaying fault tips.

In order to develop general models for how hydrocarbons aredistributed in different basin types, it is necessary to correctly clas-sify the basin type. In the most comprehensive and recent reviewof such settings, the Pattani, Malay and Song Hong-Yinggehai Ba-sins have been classified as pull-apart basins (Mann et al., 2003).The conclusions of this paper indicate the basins are not pull-aparts. It is suggested here that the Mae Ping, Three Pagodas faultzones for part of their history were involved with escape tectonic,and died out into an extensional province (including the Pattaniand Malay basins) on their southern (splaying) terminations. How-ever the extensional basin probably owe their origins more to re-gional stresses associated with subduction margins, and localgravity collapse of thickened crust, rather than to pull-apart geom-etries (Morley et al., 2011).

Acknowledgements

I would like to thank the three reviewers who had very differentperspectives on this paper. They all three made valuable commentsthat helped improve paper in different ways. PTTEP and the Petro-leum Geoscience program at Chulalongkorn University are thankedfor the opportunity to study the regions discussed in this paper. JoeCurray and Claude Rangin provided very helpful discussions of theAndaman Sea geology.

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