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Journal of Asian Earth Sciences xxx (2013) xxx–xxx

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Journal of Asian Earth Sciences

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Timing of metamorphism of the Lansang gneiss and implications forleft-lateral motion along the Mae Ping (Wang Chao) strike-slip fault, Thailand

R.M. Palin a,⇑, M.P. Searle a, C.K. Morley b, P. Charusiri c, M.S.A. Horstwood d, N.M.W. Roberts d

a Department of Earth Sciences, Oxford University, South Parks Road, Oxford OX1 3AN, UKb PTT Exploration and Production, 555 Vibhavadi Rangsit Road, Chatuchak, Bangkok 10900, Thailandc Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailandd NERC Isotope Geoscience Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK

a r t i c l e i n f o

Article history:Available online xxxx

Keywords:ThailandLansang gneissMae Ping faultMonazite geochronology

1367-9120/$ - see front matter � 2013 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.jseaes.2013.01.021

⇑ Corresponding author. Tel.: +44 1865 272000; faxE-mail addresses: [email protected]

[email protected] (M.P. Searle), [email protected] (P. Charusiri), [email protected] (M.S.A. H(N.M.W. Roberts).

Please cite this article in press as: Palin, R.M., etPing (Wang Chao) strike-slip fault, Thailand. Jou

a b s t r a c t

The Mae Ping fault (MPF), western Thailand, exhibits dominantly left-lateral strike-slip motion andstretches for >600 km, reportedly branching off the right-lateral Sagaing fault in Myanmar and extendingsoutheast towards Cambodia. Previous studies have suggested that the fault assisted the large-scaleextrusion of Sundaland that occurred during the Late Eocene–Early Oligocene, with a geological offsetof �120–150 km estimated from displaced high-grade gneisses and granites of the Chiang Mai–Lincangbelt. Exposures of high-grade orthogneiss in the Lansang National Park, part of this belt, locally containstrong mylonitic textures and are bounded by strike-slip ductile shear zones and brittle faults. Geochro-nological analysis of monazite from a sample of sheared biotite-K-feldspar orthogneiss suggests two epi-sodes of crystallization, with core regions documenting Th–Pb ages between c. 123 and c. 114 Ma and rimregions documenting a significantly younger age range between c. 45–37 Ma. These data are interpretedto represent possible magmatic protolith emplacement for the Lansang orthogneiss during the Early Cre-taceous, with a later episode of metamorphism occurring during the Eocene. Textural relationships pro-vided by in situ analysis suggest that ductile shearing along the MPF occurred during the latter stages of,or after, this metamorphic event. In addition, monazite analyzed from an undeformed garnet-two-micagranite dyke intruding metamorphic units at Bhumipol Lake outside of the Mae Ping shear zone produceda Th–Pb age of 66.2 ± 1.6 Ma. This age is interpreted to date the timing of dyke emplacement, implyingthat the MPF cuts through earlier formed magmatic and high-grade metamorphic rocks. These new data,when combined with regional mapping and earlier geochronological work, show that neither metamor-phism, nor regional cooling, was directly related to strike-slip motion.

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1. Introduction

A prominent tectonic model proposed to explain the structuralevolution of Tibet and SE Asia following the India–Asia collision isthat of ‘continental extrusion’, which hypothesizes that large tractsof thickened continental crust have been extruded to the east andsoutheast away from the northward-moving Indian indenter viasystems of lithospheric-scale strike-slip faults (Tapponnier et al.,1986; Armijo et al., 1989; Leloup et al., 1995). In this model, theeastward extrusion of Tibet has purportedly occurred along theright-lateral Karakoram and Jiale faults, in Pakistan and southernChina respectively, and by left-lateral motion along the Altyn Tagh

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al. Timing of metamorphism ornal of Asian Earth Sciences (2

fault in northern China (e.g. Tapponnier et al., 1986). By contrast,the extrusion of Sundaland to the southeast is reported to have lar-gely occurred as a result of right-lateral motion along the Sagaingfault in Myanmar and the left-lateral Ailao Shan–Red River (AS–RR)shear zone passing through Yunnan and Vietnam (Leloup et al.,1995 and others). By contrast, some recent studies of majorstrike-slip faults in Tibet and along the AS–RR shear zone refutethe large-scale extrusion hypothesis (e.g. Searle, 2006; Yeh et al.,2008; Searle et al., 2010, 2011). These studies have shown thatthe total geological offsets are generally much less than previouslythought and that strike-slip motion was largely not coincidentwith the initial India-Asia collision (c. 50 Ma; Green et al., 2008).

The extent of Quaternary and recent (active) motion along afault system is readily determined by interpreting geomorpholog-ical offsets, GPS motion and interferometric synthetic aperture ra-dar (InSAR). However, elucidation of the pre-Quaternary history isoften more problematic and requires accurate determination ofgeological offsets and obtaining ages on the formation of such

f the Lansang gneiss and implications for left-lateral motion along the Mae013), http://dx.doi.org/10.1016/j.jseaes.2013.01.021

http://dx.doi.org/10.1016/j.jseaes.2013.01.021

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2 R.M. Palin et al. / Journal of Asian Earth Sciences xxx (2013) xxx–xxx

features. As such, metamorphic and magmatic rocks exposed with-in ductile shear zones involved in the continental extrusion modelhave been targeted in the past for detailed geochronological anal-yses, as these lithologies provide the best opportunity to constrainthe timing of pre-Quaternary fault motion. In and around Tibetthese include the Karakoram fault (e.g. Searle et al., 1998; Phillipsand Searle, 2007; Robinson, 2009), the Jiale fault (e.g. Lee et al.,2003) and the Red River fault (e.g. Leloup et al., 1995; Searle,2006; Searle et al., 2010). In SE Asia high-grade metamorphic rocksare also exposed along the Sagaing fault in Myanmar (e.g. Barleyet al., 2003; Searle et al., 2007) the Mae Ping fault (Lacassinet al., 1997; Morley et al., 2007) and the Khlong Marui and Ranongfaults (Watkinson et al., 2008; Morley et al., 2011). The occurrenceof high-grade metamorphic and magmatic rocks in these litho-spheric-scale strike-slip shear zones is under debate. Some studiessuggest that metamorphism is directly related to strike-slip shearheating (cf. the Red River fault in Leloup and Kienast (1993) andLeloup et al. (1995)), whereas others suggest that transpressionaluplift merely exhumes older (and therefore) unrelated metamor-phic rocks (cf. the Karakoram and Red River fault in Searle et al.(1998, 2010), Searle and Philips (2007) and Streule et al. (2009)).

In this paper we address this issue in consideration of the prom-inent Mae Ping fault (MPF) in western Thailand. Geochronological

Fig. 1. Simplified tectonic map of SE Asia showing regional fault patterns (modified from(1982): ITSZ, Indus–Tsangpo suture zone (approximate position marked by bold dashedRed River fault zone; MPFZ, Mae Ping fault zone. Dashed box in inset shows approxima

Please cite this article in press as: Palin, R.M., et al. Timing of metamorphism oPing (Wang Chao) strike-slip fault, Thailand. Journal of Asian Earth Sciences (2

analysis of monazite within a sample of mylonotized orthogneissprovides constraints upon the timing of peak metamorphism andductile shearing within Sundaland in relation to the India–Asia col-lision. Our new U–Th–Pb monazite age data are combined withpublished structural and geochronological results along other ma-jor strike-slip faults in Thailand to show that the latest metamor-phic episode in the mid-Eocene pre-dated left-lateral motionalong the MPF. Furthermore, we suggest that widespread Late Cre-taceous leucogranite dykes exposed away from the fault zone re-cord a regional anatectic event prior to the collision of India withthe Myanmar microplate, and the subsequent collision of boththese welded terranes with Asia.

2. Geological background

Strike-slip fault complexes in SE Asia show orientations that in-fer an intimate relation to the northward-progression of the Indianplate and clockwise rotation of Asian plate blocks around the East-ern Himalayan Syntaxis (Huchon et al., 1994). Two prominent andwell documented examples outside of Thailand include the N–Strending right-lateral Sagaing fault in Myanmar and the NW–SEtrending left-lateral AS–RR fault zone in Yunnan–Vietnam(Fig. 1). Although comparable large-scale fault zones in Thailand

Watkinson et al. (2011) and Morley (2012)). Inset figure is after Tapponnier et al.line); ATF, Altyn Tagh fault; KFJZ, Karakoram–Jiale fault zone; ASRRFZ, Ailao Shan–te location of main figure.



R.M. Palin et al. / Journal of Asian Earth Sciences xxx (2013) xxx–xxx 3

may also play an important role in the tectonic evolution of SE Asia,they are currently poorly documented. These structures includethe NW–SE aligned left-lateral MPF, also called the Wang Chaofault, and associated splays, together constituting the Mae Pingfault zone (MPFZ), the NW–SE aligned left-lateral Three Pagodasfault zone and, to a lesser extent, the NNE-aligned right-lateral Ra-nong and Khlong Marui fault zones (Fig. 1). These latter two faultzones in southern Thailand extend from the continental marginand Mergui basin in the west to the Gulf of Thailand in the east un-til their traces are lost offshore. The MPF and Three Pagodas faultlikely both splay off the Sagaing fault, although the region in whichthey should merge contains very poor exposure, hence their inter-relationship remains uncertain. All four Thai faults have apparentlyshown episodic reversals in shear sense during the Oligocene–Qua-ternary such that the MPF and Three Pagodas faults are now right-

Fig. 2. Tectonic–structural map of Thailand showing major geologic fe


lateral and the Ranong and Khlong Marui faults are now left-lateral(Polachan et al., 1991; Lacassin et al., 1997; Morley, 2002; Watkin-son et al., 2008). The geological significance of Thailand’s majorfault zones and major metamorphic and magmatic belts withinthe framework of the tectonic evolution of SE Asia is given below.

2.1. Chiang Mai–Lincang belt

The Chiang Mai–Lincang belt (CM–LB) is a N–S aligned range ofhigh-grade metamorphic rocks and granites, locally up to 70 kmwide, prominently exposed in NW Thailand (Fig. 2) and Yunnan.Some metamorphic and magmatic units in this belt are highlysheared and occur within the MPFZ (e.g. the Lansang gneiss, westof Tak), whereas others exist as relatively undeformed massifsand core complexes in the surrounding regions (Fig. 2). Examples

atures (modified from Morley et al. (2011) and Cobbing (2011)).




of the latter include the Doi Inthanon and Doi Suthep massifs, westof Chiang Mai, and the Khlong Lhan and Umphang gneisses furthersouth in the CM–LB (Fig. 2). Although smaller exposures of de-formed granite and gneiss occur south of Bangkok and along Pen-insular Thailand, these units are generally not considered to be partof the CM–LB.

Numerous studies have described the structural evolution ofthese CM–LB core complexes, whereby top-to-the-east movementon bounding low angle detachments appears to have commencedin the Late Eocene and uplift and exhumation occurred duringthe Early Miocene (Rhodes et al., 1997, 2000; Morley, 2009; Mac-donald et al., 2010). Despite this, it is uncertain whether the Eocenedeformation represents a period of thrusting or the start of exten-sional detachment, or whether the period of extensional detach-ment only began during the Miocene (Morley, 2009). By contrast,the metamorphic and magmatic evolution of CM–LB units (andconstituent Lansang gneiss) is relatively poorly understood, withsome studies recording episodes related to the Indosinian orogeny(between �250 Ma and �190 Ma), whereas other studies docu-ment events of Cretaceous-Palaeogene age (see review by Macdon-ald et al. (2010)).

U–Pb zircon geochronology performed by Dunning et al. (1995)determined that the Doi Inthanon massif contains orthogneisseswith protolith I-type granite emplacement ages of c. 211–203 Ma, which had undergone subsequent metamorphism andpartial melting at c. 84–72 Ma. A younger age of 26.8 ± 0.5 Ma(U–Pb zircon and monazite) obtained from a sample of cross-cut-ting leucogranite on the eastern margin of the massif (the MaeKlang granite) was also reported (Dunning et al., 1995). Unpub-lished ages for the Umphang gneiss (Mickein, 1997, quoted in Mor-ley et al., 2007) also indicate a Late Triassic high-grademetamorphic event; with the Khlong Lhan gneiss (Fig. 2) havingslightly younger ages of 174 ± 6 Ma (U–Pb zircon) and 117 ± 3 Ma(U–Pb monazite). Fission track ages of 47 ± 3 Ma (zircon) and40 ± 2 Ma (apatite) from the Umphang gneiss suggest that thesedeep-crustal metamorphic rocks were exhumed to within 2–3 km of the surface by the Early Eocene (Upton, 1999; Morleyet al., 2011). These younger ages overlap with similar data fromthe Mogok metamorphic belt in Myanmar that were interpretedto represent evidence for high temperature metamorphism occur-ring between c. 47–29 Ma (U–Pb zircon and monazite; Barley et al.,2003; Searle et al., 2007). Furthermore, undeformed granites cross-cutting various components of the Mogok belt have been dated atc. 72, 48 and 44 Ma (U–Pb zircon, Mitchell et al., 2012), suggestingthat large-scale crustal melting, metamorphism and deformationcould have been occurring on a regional scale at these times. How-ever, the majority of the central and north Thailand sedimentarybasins and core complexes in the CM–LB developed during the LateOligocene–Miocene (Morley, 2002, 2009; Macdonald et al., 2010;Morley and Racey, 2011) and thus provide a limit on the cessationof metamorphism in the Thailand region. Work on currentlyundocumented high-grade metamorphic core complexes of theCM–LB is required in order to provide additional constraints uponthe tectonometamorphic history of the SE Asia basement.

2.2. Mae Ping fault zone

The NW–SE orientated MPFZ stretches for >600 km across Thai-land (Fig. 2) and contains many outcrops where mylonitic fabricshave been superimposed onto gneissic CM–LB components in theductile shear zone with left-lateral strike-slip kinematic indicators(Lacassin et al., 1997; Morley et al., 2011; Searle et al., 2011). Northof Tak and the Lansang region of Thailand the MPFZ enters a highlyfaulted area where it forms a number of strike-slip duplex patternsand splays (Morley, 2004) including the N–S trending Mae Yuamfault zone that lies to the west of the Doi Inthanon and Doi Suthep


massifs (Fig. 2). Apparent displacement along the MPFZ in this re-gion has been estimated from pinning points along the westernmargin of exposed granite-gneiss complexes, producing a geologi-cal offset of �120–150 km (Lacassin et al., 1997; Fig. 2). Immedi-ately south of Tak, the MPFZ splays into the strike-slip Chainatduplex. This duplex is a major feature along the fault zone, about100 km in length, which contains numerous imbricate strands con-necting the main fault as it trends southwards towards Bangkok(Morley, 2002; Smith et al., 2007). However, the continuation ofthe MPF cannot be traced accurately further east beyond the ChaoPhraya basin, north of Bangkok, due to poor exposure.

Some studies have suggested that the MPFZ stretches >1200 kmin length, continuing eastward from Bangkok across southern Cam-bodia and Vietnam to reach the Mekong delta (Tapponnier et al.,1986; Lacassin et al., 1997), although recent geological and seismicevidence from eastern Thailand cast doubt on this proposition(Morley et al., 2007, 2011). In particular Ridd and Morley (2011)describe evidence for the MPFZ continuing into eastern Thailandwhere the fault zone displays a strongly splaying geometry, a fea-ture typically associated with fault terminations. This geometrycontinues into Cambodia, where gravity and seismic reflection dataindicates the presence of numerous basins of unknown age in theTonle Sap area where the MPFZ is presumed to finally dissipate(Fig. 2; Morley, 2012).

2.3. Three Pagodas fault zone

Akin to the MPFZ, the Three Pagodas fault zone is a NW–SEtrending left-lateral strike-slip fault complex that is manifestedas a series of sub-parallel splays, located south of the Westernhighlands from the Three Pagodas Pass on the Thai–Myanmar bor-der towards Bangkok, where any trace of the fault is buried be-neath young sediments of the Chao Phraya Basin (Fig. 2). Basedupon magnetic anomaly profiles Morley (2002) proposed that theThree Pagodas fault zone splays into an E–W trending fault zoneas it trends SE towards the Gulf of Thailand, running eastwardsthrough Bangkok and passing into the Klaeng fault zone. Othersplays have been shown to continue to run NW–SE, and N–S, thelatter set joining with the northern extension of the Ranong faultzone (Ridd, 2012).

A zone of exhumed CM–LB high-grade metamorphic rocks oc-curs along the Three Pagodas fault in western Thailand (the Thabs-ila metamorphic complex). U–Pb dating of upper amphibolites-facies metamorphic units produced zircon rim ages between 51and 57 Ma, whereas biotite Rb–Sr isochron and 40Ar–39Ar coolingages show that strike-slip-related exhumation (through tempera-tures of 350–300 �C) occurred during 32–36 Ma (Lacassin et al.,1997; Nantasin et al., 2012). High-grade gneisses also occur ineastern Thailand on the Klaeng fault segment of the Three Pagodasfault at Nong Yai (e.g. Ridd, 2012). These units, known as the NongYai Gneiss or Khao Chao Gneiss (Areesiri, 1983), do not containlarge-scale strike-slip-related mylonite fabrics such as those seenat Lansang.

The combination of Landsat and SPOT image interpretation withstructural and 40Ar–39Ar geochronology indicates that �300 km ofleft-lateral strike-slip motion took place along the Mae Ping andThree Pagodas faults between approximately 40–30 Ma (Lacassinet al., 1997). These authors also suggested that a reversal toright-lateral slip motion along these fault zones initiated at c.23.5 Ma, and that this inversion may have been coeval with aprominent regional scale E-W extension across Thailand.

2.4. Ranong and Khlong Marui faults

Both the Ranong and Khlong Marui strike-slip faults trend NNE–SSW and are exposed along Peninsular Thailand (Figs. 1 and 2).




Field investigation by Garson and Mitchell (1970) identified a left-lateral phase of strike-slip motion along the Ranong fault that post-dates the emplacement of c. 111–113 Ma adamellites, these intru-sions having been dated via the whole rock Rb–Sr method by Bur-ton and Bignell (1969). Watkinson et al. (2008) suggested thathigh-grade gneisses and mylonites exposed along the Ranong faultshowed two phases of ductile right-lateral shearing, separated bythe intrusion of granitic pegmatite dykes at 71.7 ± 0.6 Ma (Ar–Armuscovite; Charusiri, 1989). Ductile shear must have ended beforethe emplacement of cross-cutting Palaeocene–Eocene post-kine-matic granites at c. 56–52 Ma (Ar–Ar muscovite, Charusiri, 1989;K–Ar biotite, Bignell, 1972) leading Watkinson et al. (2008) to con-clude that the faults were mostly active during the Late Cretaceous,prior to India-Asia collision. A subsequent study of deformed gran-ite plutons by Watkinson et al. (2011) suggested that later motionalong these fault systems has also occurred between c. 48–40 Ma(U–Pb zircon). This more recent Eocene–Oligocene reactivation ofthe faults during regional extension was broadly contemporaneouswith basin development offshore (Hall and Morley, 2004; Morleyand Westaway, 2006).

3. Petrography and mineral chemistry

Samples of sheared Lansang orthogneiss and undeformed cross-cutting granite dyke were collected from two separate outcrops inorder to provide constraints upon the magmatic, metamorphic anddeformational history of western Thailand and the MPFZ. Multiplethin sections of each sample were examined with a polarizingmicroscope in order to determine microstructural relationshipsand characteristic mineral assemblages. Mineral compositionaldata were obtained from representative thin sections on a JEOLJSM-840A scanning electron microscope (SEM) equipped with anOxford Instruments Isis 300 energy-dispersive analytical system,situated in the Department of Earth Sciences, Oxford University.Operating conditions included an acceleration voltage of 20.0 kV,beam current of 6.00 nA, a working distance of 15 mm and a livecounting time of 100 s. Calibrations were performed every120 min with a range of natural and synthetic standards and aZAF correction procedure (provided by the JEOL software) was ap-plied. A more detailed account of analytical procedures for compo-sitional analysis is given in Palin et al. (2012). Anhydrous mineralcompositions were calculated to the standard numbers of oxygensper formula unit (Deer et al., 1992), whereas mica compositionswere recalculated to a total of 11 oxygens and chlorite to a totalof 14 oxygens with H2O assumed to be present in stoichiometricamounts. The proportion of Fe3+/Fetotal, where relevant, was calcu-lated using the software AX (Holland, 2009). Mineral abbreviationson Fig. 3 are after Whitney and Evans (2010).

3.1. TH04 – granite dyke

Sample TH04 is an undeformed garnet-two-mica granite dykethat cuts across older leucocratic dykes and foliated Triassic gran-ites from the Bhumipol dam lakeside region, northeast of the MPFat GPS co-ordinates 17�14053.000N and 98�57024.700E (Figs. 2 and 3a).Similar garnet-two-mica granite dykes located away from the damsite intrude lower Palaeozoic marbles. Sample TH04 is largely iso-tropic, containing approximately equal proportions of quartz, K-feldspar and plagioclase with occasional garnet (Fig. 3b). Rare bio-tite and muscovite flakes also occur (Fig. 3c) alongside accessoryzircon, apatite and monazite. Phenocrysts of plagioclase exhibit acompositional range Ca/(K + Na + Ca) = XCa = 0.16–0.22 and K-feld-spar contains K/(K + Na + Ca) = XK = 0.92–0.93 (Table 1). Garnet oc-curs as subhedral phenocrysts up to 3 mm in diameter that exhibitinclusion-rich core regions with skeletal growth around coarse


polygonal quartz (Fig. 3b). Such grains lack significant composi-tional zoning from core to rim, with an approximately constantcomposition of 66–67% almandine, 24–25% spessartine, 5–7% py-rope and 2–3% grossular (Table 1). Biotite in the groundmass hasMg/(Mg + Fe) = XMg = 0.31–0.32 and muscovite has Si = 3.08–3.12pfu (Table 1).

3.2. TH01 – orthogneiss

Sample TH01 is a sheared orthogneiss collected from beside thePa Pueng waterfall in the Lansang National Park, west of Tak, atGPS co-ordinates 16�46047.300N and 99�00050.700E (Figs. 2 and 3d).Further upstream of the sample locality, similar orthogneiss andinterlayered calc-silicates exhibit excellent outcrop-scale strike-slip kinematic indicators. A macroscopic example is shown inFig. 3e with competent quartz-feldspar pods forming sinistral roll-ing structures in a highly sheared ductile marble matrix. At themicroscopic scale in orthogneisses this mylonitic microstructureis defined by elongate ductiley deformed quartz ribbons and thinbiotite folia that wrap abundant clasts of plagioclase and K-feld-spar (Fig. 3f). Quartz in these ribbons exhibits a medium-strongcrystallographic preferred orientation (Fig. 3g). Accessory mineralsinclude rutile, zircon, apatite and monazite. Biotite located in themylonitic foliation has XMg = 0.37–0.39 and is occasionally par-tially chloritised. Chlorite also occurs as individual clumps in thematrix and has XMg = 0.37–0.38 (Table 1). Clasts of K-feldsparare occasionally perthitic with albite stringlets (XCa = 0.01) thathave exsolved into a host crystal with XK = 0.94–0.97. Clasts of pla-gioclase have XCa = 0.23–0.34 (Table 1).

4. U–Th–Pb monazite geochronology

Monazite occurs as an accessory mineral in both samples TH01and TH04 and was chosen for geochronological analysis due to theincorporation of high concentrations of Th and U alongside low ini-tial levels of common lead (Parrish, 1990; Kohn and Malloy, 2004).Additionally, monazite exhibits a strong resistance to internal U–Th–Pb diffusion at temperatures below �900 �C (Cherniak et al.,2004) and various grains may preserve evidence of multiple meta-morphic or magmatic events in a single sample (e.g. Kohn et al.,2005). All grains were analyzed in situ from thin section using laserablation inductively-coupled plasma mass spectrometry in orderto provide petrographic context to the ages obtained; an advantagethat the analysis of separated grains cannot always provide. Thistechnique allows tight constraints to be placed upon the tectonic,magmatic and metamorphic evolution of this region of westernThailand.

4.1. Analytical techniques

Compositional and textural investigation of monazite was per-formed at the Department of Earth Sciences, Oxford Universityon a JEOL JSM-840A SEM with working conditions and analyticaltechniques as described previously for major mineral analysis.Multiple point analyses were obtained throughout each grain andwere used to identify compositional zonation or distinct internaldomains, where present. Compositional data were used in conjunc-tion with enhanced-contrast back scattered electron (BSE) imagesof each grain to visually identify potential spot ablation localitiesfor geochronological work. Such domains were often small enoughto allow ablation of separate domains and therefore minimize po-tential compositional or age mixing; however, subsequent analysisof ablated spot locations led to some geochronological data beingeliminated (see later).



Fig. 3. Outcrop localities and representative photomicrographs of both samples utilized in this study. (a) Field photograph of multiple generations of leucogranite dykes inthe Bhumipol dam lakeside region. Sample TH04 was collected from the youngest generation and cuts across earlier fabrics. Field of view 3 m. (b) Photomicrograph of sampleTH04 showing phenocrysts of garnet with skeletal core regions that include quartz (plane polars). (c) Photomicrograph of sample TH04 showing the undeformedmicrostructure comprised of quartz, K-feldspar and plagioclase with minor biotite (crossed polars). (d) Field photograph of sheared orthogneiss exposures in the LansangNational Park. Occasional post-kinematic leucogranite dykes cross-cut the mylonitic foliation. Field of view 5 m. (e) Sheared calc-silicates with quartzofeldspathic pods androlling structures, Lansang National Park. Field of view 6 m. (f) Photomicrograph of sample TH01 showing clasts of feldspar, ribbons of quartz and aligned folia of biotite(plane polars) (g). Photomicrograph of sample TH01 showing swaths of mutual extinction in quartz ribbons (crossed polars).


All geochronological work was performed at the Natural Envi-ronment Research Council Isotope Geosciences Laboratory, UK,using a Nu Atom single collector inductively coupled plasma massspectrometer (SC-ICP-MS) coupled to a New Wave ResearchUP193FX laser ablation system. Subsequent reference validationages for monazite standard materials are reported without correc-tion for common lead. Monazite was analyzed with an ablationspot size of 20 lm diameter and U–Th–Pb data were normalizedto the primary monazite reference material ‘Stern’(512.1 ± 1.9 Ma (2 S.D.), ID-TIMS 238U–206Pb average age; in-housedata, Palin et al., in press). Uncertainty propagation followed the


method of Horstwood (2008), which involves a contribution fromthe external reproducibility of reference materials for the ratios206Pb/238U and 208Pb/232Th, which is �3% (2r) and �4% (2r)respectively. 204Pb intensity was below the detection limit in allanalyses and the accuracy of 238U–206Pb ages are within 3% accord-ing to measurement of secondary reference materials; 238U–206Pbages were calculated to be 524 ± 15 Ma for ‘Moacyr’(515.6 ± 1.4 Ma (2 S.D.), ID-TIMS 238U–206Pb average age; in-housedata, Palin et al., in press) and 566 ± 16 for ‘Manangotry’(559 ± 1 Ma (2 S.D.), ID-TIMS 238U–206Pb age based on two concor-dant points; in-house data, Palin et al., in press).



Table 1Representative mineral compositions for samples TH01 and TH04.

Sample TH04 TH01

Mineral Grt Kfs Bt Pl Ms Bt Kfs Ab Pl ChlLocation Rim Phenocryst Groundmass Phenocryst Groundmass Foliation plane Clast Perthite Clast Matrix

SiO2 36.07 64.21 35.16 63.78 45.68 34.52 64.20 68.86 61.39 25.47TiO2 0.01 0.01 1.01 0.00 0.12 3.87 0.01 0.04 0.03 0.11Al2O3 20.49 18.56 19.97 23.44 33.53 16.55 18.58 19.80 24.66 20.64Fe2O3 1.89 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00FeO 28.86 0.00 23.72 0.00 2.22 22.76 0.00 0.00 0.00 30.88MnO 10.86 0.00 0.52 0.00 0.02 0.18 0.00 0.00 0.04 0.20MgO 1.16 0.00 6.01 0.00 0.76 7.86 0.04 0.00 0.00 10.51CaO 0.89 0.00 0.09 4.38 0.02 0.03 0.05 0.17 6.05 0.07Na2O 0.00 0.92 0.07 8.24 0.11 0.00 0.64 10.77 7.82 0.01K2O 0.00 15.92 10.16 0.24 11.14 10.21 16.43 0.65 0.14 0.09

Total 100.23 99.61 96.71 100.08 93.60 95.98 99.95 100.29 100.15 87.98Si 2.95 2.98 2.70 2.81 3.12 2.68 2.98 3.00 2.72 2.76Ti 0.00 0.00 0.06 0.00 0.01 0.23 0.00 0.00 0.00 0.01Al 1.98 1.02 1.81 1.22 2.70 1.52 1.02 1.02 1.30 2.63Fe3+ 0.12 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Fe2+ 1.98 0.00 1.53 0.00 0.13 1.48 0.00 0.00 0.00 2.79Mn 0.75 0.00 0.03 0.00 0.00 0.01 0.00 0.00 0.00 0.02Mg 0.14 0.00 0.69 0.00 0.08 0.91 0.01 0.00 0.00 1.69Ca 0.08 0.00 0.01 0.21 0.00 0.00 0.00 0.01 0.29 0.01Na 0.00 0.07 0.01 0.74 0.02 0.00 0.05 0.93 0.67 0.00K 0.00 0.94 1.00 0.01 0.97 1.01 0.96 0.04 0.01 0.01

Sum 8.00 5.01 7.84 4.99 7.02 7.84 5.01 4.99 4.99 9.93Oxygen 12 8 11 8 11 11 8 8 8 14XMg 0.07 – 0.31 – 0.38 0.38 – – – 0.38XCa – – – 0.22 – – – 0.01 0.30 –XK – 0.93 – – – – 0.95 – – –Al(iv) – – 1.30 – 0.88 1.32 – – – –Al(vi) – – 0.52 – 1.82 0.20 – – – –Alm 0.67 – – – – – – – – –Prp 0.05 – – – – – – – – –Grs 0.03 – – – – – – – – –Sps 0.25 – – – – – – – – –


Data processing for all analyses used the time-resolved functionon the Nu Instruments’ software, an in-house MS Excel spread-sheet for data reduction and error propagation and the MS Exceladd-in Isoplot (Ludwig, 2003) for data presentation. The interpre-tation of U–Pb sample data was made using Tera–Wasserburg plots(Fig. 4) to allow for the presence of variable amounts of commonlead and regressions assumed a 207Pb/206Pb ratio of 0.83 ± 0.02, acomposition representing that acquired during mineral growthover the past �220 Ma (Stacey and Kramers, 1975). All quoteduncertainties and ellipses on Tera–Wasserburg plots are at the2r confidence interval.

4.2. Results

Geochronological investigation in the U–Th–Pb isotope systemallows two age determinations to be obtained for each analysisthrough the independent U–Pb and Th–Pb decay schemes, provid-ing a check upon the reliability of each result. However, in themajority of monazite analyses from both samples (below) commonlead corrected 238U–206Pb ages are invariably older than commonlead corrected 232Th–208Pb ages (Fig. 4a). This relationship (‘reversediscordance’) is indicative of disequilibrium partitioning of 230Th,an intermediate nuclide in the 238U–206Pb decay series, during ini-tial crystallization (Schärer, 1984; Parrish, 1990). Thus, the produc-tion of excess of 206Pb causes calculated 238U–206Pb ages to beincorrectly older than that of actual crystallization. Although232Th–208Pb ages are unaffected by excess 206Pb, so should providea minimum estimate for ages of crystallization, they carry signifi-cantly larger associated uncertainties than U–Pb ages. Hence inthis study we use both the U–Pb and Th–Pb isotope systems to pro-


vide geochronological constraints on the evolution of bothsamples.

4.2.1. TH04 – granite dykeMonazite in sample TH04 is typically small (up to 100 lm in

length), displays distinct euhedral and prismatic outlines and con-tains concentric compositional zoning (Fig. 5a). These features areindicative of a magmatic origin (Ayers et al., 1999). Core regionsare rich in Th (11.92–12.83 wt.%) and U (2.05–2.38 wt.%), with con-centrations systematically decreasing towards relatively Th-poor(9.75–11.88 wt.%) and U-poor (0.89–1.47 wt.%) outer rim regions.Such a relationship is consistent with Rayleigh fractionation duringa single episode of monazite growth (Kohn and Malloy, 2004). A to-tal of 15 spot analyses were obtained from 6 grains, which exhibit aco-linear relationship with some scatter on a Tera–Wasserburgplot (Fig. 4b). These data are interpreted using a single regressionresulting in a U–Pb lower intercept age of 69.8 ± 1.2 Ma(MSWD = 4.9; Fig. 4b). As a result of the reverse discordance be-tween analyses (Fig. 4a) and large calculated MSWD for this data-set, Th–Pb ages were also investigated. Common lead correctedTh–Pb ages produced a weighted average of 66.2 ± 1.6 Ma withan MSWD = 1.4 (Fig. 4c and Table 2).

4.2.2. TH01 – orthogneissMonazite morphology in sample TH01 ranges from sub-

rounded to sub-idioblastic and grains occasionally contain patchyinternal zoning patterns with swirling bright and dark domains(Fig. 5b), these features being typical of a metamorphic origin(Ayers et al., 1999). Grains are typically large (�50–250 lm) andmostly allowed individual compositional domains to be analyzed;however, data for 12 ablation points were subsequently discarded



Fig. 4. U–Th–Pb geochronological plots for all samples. (a) Correlation between common lead corrected U–Pb and Th–Pb ages for both samples. Reverse discordance isindicated by results lying above the 1:1 line. (b) Tera–Wasserburg plot for sample TH04. (c) Weighted average Th–Pb age box plot for sample TH04. (d) Tera–Wasserburg plotfor sample TH01 showing all analyses. (e) Tera–Wasserburg plot for sample TH04 showing distribution of young U–Pb ages along concordia. (f) Weighted average Th–Pb agebox plot for sample TH01.


upon re-examination of spot localities under the SEM as a result ofsignificant overlap between adjacent domains. Spot analyses fromthe cores of two different grains produced U–Pb lower interceptages of 135.2 ± 3.4 Ma and 134.1 ± 5.1 Ma (grains 2 and 12; Table 2)although equivalent Th–Pb analyses for these spots producedyounger ages of 113.5 ± 9.0 Ma and 123.1 ± 12.6 Ma (Table 2). Spotanalyses on the remainder of grains produced significantly youngerU–Pb ages ranging between c. 53 Ma and c. 38 Ma (Fig. 4d and e),


which spread along concordia with no clear populations. Further-more, spot analyses on individual grains situated within the matrixshow ranges in common lead corrected U–Pb ages of up to �5 Myr(e.g. grain 11; Fig. 5c). In order to resolve these data, weightedaverages of common lead corrected Th–Pb ages were determinedfrom multiple spot analyses on individual monazite grains. Resultsusing this method produced ages of 41.1 ± 8.8 Ma (grain 4),37.1 ± 1.5 Ma (grain 5), 41.3 ± 1.0 Ma (grain 6), 44.5 ± 6.1 Ma (grain



(a) (b)

(c) (d)

Fig. 5. Representative SEM BSE contrast images of monazite and petrographic positions pre- and post-ablation. Dashed circles indicate the position of individual spot analysesand U–Pb ages given are those after correction for common lead. Scale for grain images is given by spot sizes (20 lm diameter). (a) TH04 monazite 4. (b) TH01 monazite 12.(c) TH01 monazite 11. (d) Monazite grain 11 in sample TH01 aligned with the mylonitic foliation. Field of view �1 mm.


9), 41.2 ± 1.4 Ma (grain 11) and 41.0 ± 1.9 Ma (grain 13), whereas asingle analysis on grain 7 produced a Th–Pb age of 43.5 ± 3.6 Ma(Fig. 4f). Additionally, one single rim analysis of grain 12 produceda Th–Pb age of 39.5 ± 3.7 Ma (Table 2), distinctly younger than thec. 114 Ma Th–Pb core age.

4.3. Geological interpretation

Monazite from granite dyke sample TH04 exhibits composi-tional and textural characteristics suggesting that growth occurredduring one single crystallization event. This event has been datedat 69.8 ± 1.2 Ma in the U–Pb isotope system and 66.2 ± 1.6 Ma inthe Th–Pb system; however, as a result of ubiquitous reverse dis-cordance between analyses (Fig. 4a), the latter Th–Pb age(66.2 ± 1.6 Ma) is preferred and taken to reliably represent the tim-ing of dyke intrusion. The slightly older U–Pb age is thus inter-preted to be caused by a proportion of excess 206Pb.

The interpretation of geochronological data from shearedorthogneiss sample TH01 is more complex. Examination of twoseparate monazite core regions reveals the existence of old U–Pblower intercept ages (c. 134 and c. 135 Ma); however, commonlead corrected Th–Pb data suggests younger ages of c. 114 Maand 123 Ma (Table 2). This discrepancy in common lead correctedages in both isotope systems is most likely a result of excess 206Pbin the monazite structure, therefore Th–Pb ages are again pre-ferred. It is uncertain as to why additional old ages were not foundthroughout other grains in TH01 and it is duly noted that the inter-pretation of two individual points in a study must be treated withcaution, as opposed to the larger population of younger ages ob-tained that clearly represents a significant monazite-formingevent. The observation that these old ages exist as core regions sur-rounded by younger rims (cf. Fig. 5b) suggests that this later epi-sode of young monazite growth could have resulted from partialrecrystallization of older grains, with very few relics being pre-


served. These points notwithstanding, we prefer not to proposeor affix definite tectonic significance to these data, but simply re-port them with a possible interpretation (see discussion) withthe intention of initiating further investigation to corroborate theresults.

Geochronological analysis of younger monazite within sampleTH01 produced common lead corrected U–Pb ages that exhibitedranges up to �5 Myr within individual grains. This feature is againtaken to represent variable levels of excess 206Pb distributed non-uniformly throughout each grain, a conclusion supported by thereverse discordance exhibited by almost all individual data pointsin this study (Fig. 4a). Consequently Th–Pb age ranges are preferredand, due to the metamorphic textural and zoning characteristicsexhibited by all grains, we interpret this total range of ages be-tween c. 45 and c. 37 Ma to represent an extended period of pro-grade metamorphic heating and monazite growth. As a result ofthe textural relationship of young grains parallel to (or in) themylonitic foliation (Fig. 5d), we suggest that the mylonitic fabricwas superimposed on the orthogneiss after metamorphism butwhilst the rock was still at high temperature (see below). Thesedata therefore provide a maximum age for strike-slip motion alongthe MPF.

5. Conditions of mylonitization

Textural analysis of microfabrics in sheared orthogneiss sampleTH01 allows constraints to be placed upon the thermal conditionsof strike-slip deformation. Plagioclase feldspar clasts exhibit re-peated twins on the albite and pericline laws, which are regularlybent (Fig. 6a), whereas K-feldspar lacks twinning but commonlyexhibits undulose extinction (Fig. 6b). Both features are character-istic of the occurrence of crystal-plastic deformation processes(dislocation creep) as opposed to deformation by brittle fracture(Simpson, 1985). This deformation mechanism requires tempera-



Table 2U–Th–Pb geochronological data for samples TH01 and TH04.

Analysis Position 206Pb(mV)

Pba

(ppm)Tha

(ppm)Ua

(ppm)Th/Uratio

Calculated ageb (Ma) Corrected agec (Ma)

238U/206Pb

1r(%)

207Pb/206Pb

1r(%)

208Pb/206Pb

1r(%)

207Pb/235U

1r(%)

206Pb/238U

1r(%)

Rho 208Pb/232Th

1r(%)

207Pb/206Pb

2r(abs)

206Pb/238U

2r(abs)

207Pb/235U

2r(abs)

208Pb/232Th

2r(abs)

208Pb/232Th

2r(abs)

206Pb/238U

2r(abs)

TH04 mnz4_1

Groundmass 5.8 39 7372 566 13 84.9 1.2 0.12385 1.9 3.57 1.34 0.2011 2.3 0.01178 1.2 0.53 0.0031 1.6 2012 34 75.5 1.8 186.1 7.7 63.0 2.1 59.4 4.1 68.0 1.7

TH04 mnz4_2

Groundmass 12.1 80 8690 1247 7 90.7 1.1 0.05363 0.8 2.14 1.36 0.0815 1.4 0.01102 1.1 0.83 0.0033 1.6 356 17 70.7 1.6 79.6 2.1 65.8 2.1 65.3 4.4 70.0 1.6

TH04 mnz4_3

Groundmass 8.7 57 8657 880 10 89.9 1.0 0.04779 0.5 2.99 1.13 0.0733 1.2 0.01112 1.0 0.89 0.0033 1.6 89 13 71.3 1.5 71.8 1.6 66.1 2.1 66.0 4.2 71.1 1.5

TH04 mnz4_4

Groundmass 5.2 35 8000 551 15 92.3 1.1 0.04908 0.7 4.57 1.53 0.0734 1.3 0.01084 1.1 0.85 0.0033 1.4 152 16 69.5 1.5 71.9 1.8 66.2 1.9 66.2 4.4 69.1 1.5

TH04 mnz5_1

Groundmass 10.8 81 6330 1305 5 91.1 1.2 0.05161 0.8 1.44 1.29 0.0781 1.4 0.01098 1.2 0.81 0.0032 2.0 268 19 70.4 1.6 76.4 2.1 65.0 2.6 64.6 5.0 70.0 1.7

TH04 mnz5_2

Groundmass 12.1 91 7634 1089 7 68.4 1.2 0.20518 0.7 2.04 1.35 0.4138 1.4 0.01463 1.2 0.86 0.0043 1.9 2868 11 93.6 2.2 351.6 8.0 85.8 3.3 68.5 5.3 74.9 1.8

TH04 mnz5_3

Groundmass 7.5 57 8719 891 10 91.5 1.2 0.04869 0.7 2.93 1.37 0.0733 1.3 0.01092 1.2 0.87 0.0033 1.9 133 16 70.0 1.6 71.9 1.9 66.2 2.6 66.1 5.2 69.8 1.6

TH04 mnz5_4

Groundmass 6.7 51 8585 711 12 80.4 1.2 0.12387 0.7 3.37 1.39 0.2125 1.4 0.01244 1.2 0.85 0.0035 1.9 2013 13 79.7 1.9 195.7 4.9 70.4 2.7 66.3 5.1 71.8 1.7

TH04 mnz1_1

Groundmass 9.2 69 8957 1136 8 93.9 1.2 0.04855 0.7 2.44 1.49 0.0713 1.4 0.01064 1.2 0.87 0.0033 1.9 126 16 68.3 1.6 69.9 1.8 66.0 2.5 66.0 5.2 68.1 1.6

TH04 mnz2_1

Groundmass 6.6 50 7944 715 11 81.6 1.2 0.08153 0.7 3.17 1.27 0.1378 1.4 0.01226 1.2 0.87 0.0034 2.0 1234 14 78.5 1.9 131.1 3.4 69.3 2.8 67.3 5.3 75.0 1.8

TH04 mnz2_2

Groundmass 5.3 40 9212 637 14 90.6 1.2 0.05240 0.7 4.42 1.47 0.0797 1.4 0.01103 1.2 0.85 0.0033 1.9 303 17 70.7 1.6 77.9 2.1 67.0 2.6 66.8 5.3 70.0 1.7

TH04 mnz3_1

Groundmass 6.0 45 8370 681 12 87.2 1.2 0.07995 1.3 3.79 1.43 0.1264 1.8 0.01147 1.2 0.67 0.0035 1.9 1196 26 73.5 1.7 120.8 4.0 70.8 2.7 69.2 5.3 70.3 1.7

TH04 mnz3_2

Groundmass 5.4 41 9221 636 15 88.8 1.2 0.05123 0.7 4.60 1.38 0.0795 1.4 0.01126 1.2 0.87 0.0036 2.0 251 16 72.2 1.7 77.7 2.0 72.6 2.8 72.4 5.7 71.5 1.7

TH04 mnz3_3

Groundmass 4.8 36 8765 573 15 91.1 1.2 0.04989 0.7 4.75 1.46 0.0755 1.4 0.01098 1.2 0.85 0.0034 1.9 190 17 70.4 1.7 73.9 2.0 69.0 2.7 68.9 5.5 69.9 1.7

TH04 mnz4b_1

Groundmass 6.7 51 5430 824 7 93.3 1.2 0.05698 0.8 2.06 1.23 0.0842 1.5 0.01072 1.2 0.85 0.0033 2.2 491 17 68.7 1.7 82.1 2.3 67.4 2.9 66.6 5.5 67.8 1.7

TH01 mnz2_2

Enclosed withinquartz matrix

11.7 88 6042 713 8 47.4 1.9 0.05067 0.7 2.44 1.42 0.1475 2.0 0.02112 1.9 0.94 0.0061 2.7 226 16 134.7 5.0 139.7 5.2 123.5 6.7 123.1 12.6 134.1 5.1

TH01 mnz4_1


0.6 4 6107 85 72 113.1 1.3 0.14873 1.8 17.48 1.17 0.1813 2.3 0.00884 1.3 0.59 0.0022 2.3 2331 32 56.7 1.5 169.2 7.1 44.4 2.1 43.7 3.8 47.6 1.3

TH01 mnz4_2


0.4 3 4904 45 109 84.6 1.9 0.30065 2.3 17.95 1.12 0.4898 3.0 0.01182 1.9 0.64 0.0019 2.2 3474 35 75.7 2.9 404.8 19.8 39.2 1.7 37.8 3.1 48.6 2.0

TH01 mnz4_3


0.7 5 5787 110 53 118.5 1.3 0.06072 1.3 13.39 1.16 0.0707 1.8 0.00844 1.3 0.69 0.0022 2.3 629 29 54.2 1.4 69.3 2.5 43.6 2.0 43.5 3.7 51.9 1.4

TH01 mnz5_1


1.9 14 7770 400 19 162.5 1.2 0.05164 1.1 5.86 1.26 0.0438 1.6 0.00615 1.2 0.75 0.0019 2.2 269 24 39.6 1.0 43.5 1.4 38.1 1.7 38.0 3.2 38.9 1.0

TH01 mnz5_2


2.0 15 7109 444 16 167.5 1.2 0.05160 0.8 5.05 1.28 0.0425 1.4 0.00597 1.2 0.83 0.0019 2.1 268 18 38.4 0.9 42.2 1.2 37.4 1.6 37.3 3.0 37.8 0.9

TH01 mnz5_3


1.7 13 7001 374 19 169.1 1.3 0.05315 0.9 5.76 1.15 0.0433 1.6 0.00591 1.3 0.81 0.0018 2.1 335 21 38.0 1.0 43.1 1.3 36.7 1.6 36.6 3.0 37.3 1.0

TH01 mnz5_5


2.3 18 7151 520 14 170.0 1.2 0.05329 1.2 4.25 1.26 0.0432 1.7 0.00588 1.2 0.69 0.0018 2.2 341 28 37.8 0.9 43.0 1.4 36.8 1.6 36.7 3.1 37.3 0.9

TH01 mnz6_1


2.0 15 7216 426 17 161.0 1.2 0.05262 0.8 5.31 1.22 0.0451 1.4 0.00621 1.2 0.84 0.0019 2.2 312 18 39.9 1.0 44.8 1.3 38.9 1.7 38.8 3.2 39.3 1.0

TH01 mnz6_2


2.6 20 7282 526 14 154.7 1.2 0.06017 1.2 4.18 1.16 0.0536 1.7 0.00647 1.2 0.69 0.0020 2.2 610 26 41.5 1.0 53.1 1.7 39.8 1.7 39.4 3.3 40.6 0.9

TH01 mnz6_3


1.3 9 7684 237 32 140.8 1.1 0.05121 0.9 9.62 1.31 0.0501 1.5 0.00710 1.1 0.78 0.0021 2.0 250 21 45.6 1.0 49.7 1.4 42.5 1.7 42.5 3.3 44.6 1.0

TH01 mnz6_4


1.2 9 7689 212 36 136.6 1.2 0.05971 1.0 10.10 1.20 0.0603 1.6 0.00732 1.2 0.79 0.0020 2.0 593 21 47.0 1.2 59.4 1.8 40.1 1.6 39.9 3.1 45.4 1.2

TH01 mnz6_5


1.1 8 7658 198 39 141.5 1.2 0.05383 1.0 11.42 1.28 0.0525 1.6 0.00707 1.2 0.78 0.0021 2.0 364 22 45.4 1.1 51.9 1.6 42.2 1.7 42.1 3.3 44.1 1.1

TH01 mnz Enclosed within 1.0 7 7473 174 43 139.0 1.2 0.05385 1.4 12.65 1.23 0.0534 1.8 0.00719 1.2 0.65 0.0021 2.1 365 31 46.2 1.1 52.8 1.8 43.1 1.8 43.0 3.5 44.8 1.1

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Table 2 (continued)

Analysis Position 206Pb(mV)

Pba

(ppm)Tha

(ppm)Ua

(ppm)Th/Uratio

Calculated ageb (Ma) Corrected agec (Ma)

238U/206Pb

1r(%)

207Pb/206Pb

1r(%)

208Pb/206Pb

1r(%)

207Pb/235U

1r(%)

206Pb/238U

1r(%)

Rho 208Pb/232Th

1r(%)

207Pb/206Pb

2r(abs)

206Pb/238U

2r(abs)

207Pb/235U

2r(abs)

208Pb/232Th

2r(abs)

208Pb/232Th

2r(abs)

206Pb/238U

2r(abs)

6_6 quartz matrixTH01 mnz

6_7Enclosed withinquartz matrix

1.8 14 7187 370 19 152.1 1.2 0.05155 0.9 6.22 1.21 0.0467 1.5 0.00657 1.2 0.80 0.0021 2.2 265 20 42.2 1.0 46.4 1.4 42.2 1.8 42.2 3.5 41.6 1.0

TH01 mnz6_8


2.1 16 7291 434 17 155.6 1.2 0.05524 0.9 5.17 1.26 0.0490 1.5 0.00643 1.2 0.81 0.0020 2.1 422 19 41.3 1.0 48.5 1.4 40.4 1.7 40.2 3.2 40.5 1.0

TH01 mnz6_9


1.2 9 7975 225 35 139.3 1.2 0.05203 1.1 11.12 1.29 0.0515 1.6 0.00718 1.2 0.76 0.0022 2.1 287 24 46.1 1.1 51.0 1.6 45.2 1.9 45.2 3.7 45.0 1.1

TH01 mnz6_10


1.4 10 7670 261 29 144.5 1.2 0.05004 1.0 8.76 1.23 0.0478 1.6 0.00692 1.2 0.76 0.0021 2.0 197 24 44.5 1.1 47.4 1.5 41.5 1.6 41.5 3.2 43.6 1.1

TH01 mnz6_11


1.5 11 7386 284 26 144.0 1.2 0.05310 1.2 7.57 1.22 0.0509 1.7 0.00695 1.2 0.68 0.0020 2.2 333 28 44.6 1.0 50.4 1.7 40.6 1.8 40.6 3.4 43.7 1.0

TH01 mnz7_1

Inclusion in feldsparporphyroclast

1.2 9 6194 187 33 125.2 1.6 0.05434 1.1 8.93 1.20 0.0598 2.0 0.00798 1.6 0.84 0.0022 2.2 385 24 51.3 1.7 59.0 2.2 43.6 1.9 43.5 3.6 50.0 1.7

TH01 mnz9_1


0.7 5 4555 123 37 141.4 1.3 0.05223 1.3 10.41 1.16 0.0509 1.8 0.00707 1.3 0.70 0.0020 2.4 295 30 45.4 1.1 50.4 1.8 40.1 1.9 40.1 3.5 44.2 1.1

TH01 mnz9_2


0.7 5 4812 115 42 132.7 1.3 0.05538 1.1 12.08 1.14 0.0575 1.8 0.00754 1.3 0.75 0.0022 2.3 427 26 48.4 1.3 56.8 1.9 43.8 2.0 43.7 3.7 46.9 1.3

TH01 mnz11_1

Aligned withinbiotite shear band

0.9 7 4952 179 28 155.0 1.3 0.05318 1.1 8.60 1.14 0.0473 1.7 0.00645 1.3 0.75 0.0020 2.3 336 25 41.5 1.0 46.9 1.5 40.8 1.9 40.7 3.5 40.5 1.0

TH01 mnz11_2


0.8 6 5013 154 33 139.1 1.3 0.05155 1.1 9.50 1.28 0.0511 1.7 0.00719 1.3 0.76 0.0021 2.1 265 25 46.2 1.2 50.6 1.7 41.8 1.7 41.7 3.4 45.1 1.2

TH01 mnz11_3


0.6 4 5251 101 52 136.7 1.3 0.05638 1.6 15.46 1.21 0.0569 2.1 0.00732 1.3 0.61 0.0022 2.2 467 36 47.0 1.2 56.2 2.3 44.1 1.9 44.0 3.7 45.1 1.2

TH01 mnz11_4


1.0 7 5373 186 29 144.2 1.2 0.05240 1.1 8.36 1.16 0.0501 1.7 0.00693 1.2 0.74 0.0020 2.2 303 26 44.5 1.1 49.6 1.6 40.3 1.8 40.3 3.4 43.6 1.1

TH01 mnz11_5


1.3 10 5461 250 22 141.9 1.2 0.04920 1.0 6.06 1.18 0.0478 1.6 0.00705 1.2 0.77 0.0019 2.2 158 24 45.3 1.1 47.4 1.5 38.9 1.7 38.9 3.3 44.7 1.1

TH01 mnz11_6


0.8 6 5613 131 43 125.5 1.4 0.16913 1.8 11.82 1.14 0.1859 2.3 0.00797 1.4 0.62 0.0021 2.3 2549 30 51.2 1.4 173.1 7.2 43.4 2.0 42.2 3.6 42.2 1.2

TH01 mnz12_1


1.9 14 4980 379 13 151.3 1.2 0.05581 1.2 3.86 1.12 0.0509 1.7 0.00661 1.2 0.73 0.0020 2.5 445 26 42.5 1.1 50.4 1.7 39.8 2.0 39.5 3.7 41.8 1.1

TH01 mnz12_3


4.4 33 5093 271 19 47.0 1.3 0.05074 0.7 4.98 1.36 0.1488 1.5 0.02127 1.3 0.87 0.0056 2.0 229 17 135.7 3.4 140.9 3.8 113.7 4.5 113.5 9.0 134.8 3.4

TH01 mnz13_1


1.3 10 5472 260 21 147.9 1.2 0.06166 1.1 6.51 1.20 0.0575 1.6 0.00676 1.2 0.75 0.0021 2.2 662 23 43.4 1.1 56.8 1.8 42.1 1.9 41.9 3.5 42.2 1.0

TH01 mnz13_2


1.5 11 5729 281 20 146.6 1.2 0.06441 1.1 6.33 1.17 0.0606 1.6 0.00682 1.2 0.75 0.0021 2.2 755 23 43.8 1.1 59.7 1.9 42.4 1.9 42.1 3.5 42.4 1.1

TH01 mnz13_3


1.6 12 5926 322 18 157.4 1.3 0.05592 0.9 5.74 1.33 0.0490 1.6 0.00635 1.3 0.82 0.0019 2.0 449 20 40.8 1.0 48.6 1.5 39.4 1.6 39.2 3.2 40.0 1.0

a Concentration uncertainty c. 20%.b Data not corrected for common-Pb.c Data corrected for common-Pb using Stacey and Kramers (1975) 208Pb/206Pb or 207Pb/206Pb composition at the intial measured 206Pb/238U age; U–Pb data also corrected for Th-disequilibrium assuming Th/U whole rock = 3.

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Fig. 6. Representative photomicrographs of sample TH01 illustrating mineralogicaldeformation mechanisms, all viewed under crossed polars. (a) Plagioclase clastsexhibiting bent twin lamellae. (b) K-feldspar clasts exhibiting undulose extinction.(c) Quartz ribbons exhibiting internal grain boundary migration.


tures above the feldspar brittle-ductile transition, which is esti-mated to lie between 450 and 500 �C (Hanmer, 1982; Simpson,1985). This interpretation is supported by the occurrence of grainboundary migration within quartz (Fig. 6c), which indicates tem-peratures above �500 �C (Stipp et al., 2002). Due to the absenceof pressure sensitive minerals or deformation textures, the crustaldepth at which shearing took place remains unknown. We inter-pret that mylonitization along the MPF occurred after peak meta-morphism, here dated between c. 45 and 37 Ma. Althoughmetamorphism clearly extends outside of the shear zone, and is re-gional in extent, the effects of mylonitization are restricted to theshear zone and the exhumed rocks within it.


6. Discussion

6.1. Regional tectonothermal events in Thailand

The main tectonic events previously identified in Thailand arethose associated with the Indosinian orogeny (between �250 Maand �190 Ma), the onset of escape tectonics at �36 Ma (Lacassinet al., 1997; Morley, 2004), and the development of rift basins dur-ing the Late Eocene–Miocene (Morley, 2002; Morley and Racey,2011). In this region of Thailand, the ‘Indosinian orogeny’ is associ-ated with the collision of the Sibumasu block with the Sukhothaiblock (Barber et al., 2011), whilst Cenozoic deformation is mostlyassociated with India-Asia collision (Hutchison, 1996, 2007). In be-tween these two events, the western margin of Sundaland fromMyanmar to Sumatra experienced Jurassic–Early Cretaceous sub-duction (see Hutchison, 1973, 1983, 2007; Hall et al., 2009; Hall,2012; Morley, 2012, for reviews). During the Mid Cretaceous, is-land arc collision occurred in Sumatra forming the Woyla Nappeand possibly also in Myanmar forming the Mawgyi Nappe (Barberand Crow, 2009). Following this island arc collision with Sunda-land, subduction of oceanic crust beneath Myanmar, Thailandand Malaysia resumed in the Late Cretaceous, although subductiondoes not appear to have operated beneath Sumatra (Hall et al.,2009). This suggests that transform faults in the Indian Plate serveto separate a fast, northwards-moving segment of the Indian plateto the west, from an essentially stationary segment to the east(Hall et al., 2009; Hall, 2012). Our age data supports the existenceof additional regional-scale tectonothermal events in the tectonicevolution of Thailand, as hinted at by previous studies, the implica-tions of which are discussed below.

6.1.1. Cretaceous magmatismGeochemical and geochronological studies of granitoid plutons

along the Mogok Belt and the Slate Belt, eastern Myanmar (location6 on Fig. 7), were conducted by Barley et al. (2003) and Searle et al.(2007). Results indicated the presence of a Late Jurassic–Early Cre-taceous (170–120 Ma; U–Pb zircon) magmatic arc active along thesouthern margin of Asia prior to the India-Asia collision. This arc,up to 200 km wide, was interpreted to be part of a much larger An-dean-type margin that extended along the Eurasian margin fromPakistan across southern Tibet to Myanmar–Thailand and possiblyfurther south to Sumatra (see Hall et al., 2009). By contrast, Wat-kinson et al. (2011) presented similar U–Pb zircon data from fourgranites associated with the Ranong fault zone in Peninsular Thai-land, indicating that major pluton emplacement occurred in theLate Cretaceous between c. 80 and 71 Ma, with additional minorgranite intrusions dated at c. 48–37 Ma. Those authors interpretedthat some granites were emplaced during right-lateral shear alongthe Khlong Marui and Ranong fault zones between c. 48 and 40 Ma.A study of mylonitized granites from within the Khlong Marui faultzone by Kanjanapayont et al. (2012) documented Late Triassic(200 ± 10 Ma) and Late Cretaceous (c. 70–62 Ma) U–Pb ages fromzircon middle and outer core regions respectively, interpreted asrepresenting episodes of granite intrusion. In addition, Early Eo-cene (c. 55–50 Ma) ages from rim regions alongside Late Eocene(36 ± 0.4 Ma) Rb–Sr biotite cooling ages were interpreted as repre-senting an amphibolite-facies metamorphic overprint followed byexhumation to low-medium greenschist-facies as a result of dex-tral transpression along the Khlong Marui fault (Kanjanapayontet al., 2012). Other 40Ar–39Ar mica ages of 88–50 Ma for granitesin western Thailand (including those around Phuket) have been re-ported by Charusiri et al. (1993), although the significance of thesedata is uncertain. Such ages may represent the timing of coolingthrough the respective closure temperature shortly after plutonemplacement; the timing of exhumation through the respective



Fig. 7. Regional map of SE Asia showing interpreted Cretaceous magmatic arc trends. The western volcanic arc is related to subduction of the Neo-Tethys and was activeduring the Late Cretaceous–Palaeogene, while the eastern volcanic arc is related to subduction of the palaeo-Pacific ocean and was active until the Late Cretaceous (see Hallet al., 2009; Morley, 2012, for reviews). The area of probable Late Cretaceous–Palaeogene crustal thickening (indicated by monazite U–Pb or Th–Pb ages) is inferred based onthe localities indicated: (1) Stong migmatite complex (Umor et al., 2010; Hutchison and Tan, 2009; Ghani, 2009); (2) Khao Taphao, Triassic granite (Geard, 2008); (3) Lansang(this study); (4) Bhumipol dam (this study); (5) Doi Inthanon (Macdonald et al., 2010); (6) Mogok metamorphic belt (Searle et al., 2007); (7) Gaoligong–Tengliang–Yingjianarea, western Yunnan (Xu et al., 2012). (See above-mentioned references for further information.)


closure temperature associated with Eocene deformation, or alter-natively they may document episodes of recrystallization due tohydrothermal activity (Watkinson et al., 2011). Xu et al. (2012)document evidence for additional magmatism further north withU–Pb zircon data describing Early Cretaceous (126–121 Ma) andEarly Cenozoic (66–52 Ma) plutonism in the Gaoligong–Tengli-ang–Yingjian area in western Yunnan (location 7 on Fig. 7). Thesedata were interpreted via a comparison to a northern American


Cordilleran tectonic setting, with magmatism in two belts (a wes-tern I-type dominated belt and an eastern S-type dominated belt)related to eastwards subduction of the Neo-Tethys and further eastto crustal thickening in a region that had been weakened and dri-ven by subduction processes.

It is clear from these data that regional-scale magmatism wasoccurring during the Cretaceous in the region of eastern Myanmar,southwest Yunnan and western Thailand. It is therefore unsurpris-




ing that orthogneiss sample TH01, representative of the Lansanggneiss of northwest Thailand, produced Early Cretaceous (c. 123–114 Ma) Th–Pb geochronological evidence for monazite growth,interpreted here to represent initial magmatic protolith emplace-ment. This sample provides additional constraint upon the hypoth-esized position of the western Cretaceous N–S trending volcanicarc, extending this region down the Myanmar–Thailand border(location 3 on Fig. 7). This new age range approximately coincideswith the Early Cretaceous phase of subduction outlined above andthe putative collision of the Mawgyi nappe with western Sunda-land in Myanmar (Mitchell, 1993).

6.1.2. Late Cretaceous–Palaeogene migmatizationFollowing the intrusion of Cretaceous granites, a thermal over-

printing event has also been identified in exhumed granite–migmatite–gneiss complexes in Thailand. In some radiometric dat-ing campaigns this event (placed between 80 and 60 Ma) was ob-served and interpreted as a result of partial lead-loss from zircon,leading to a subsequent lowering of apparent ages (Darbyshire,1988; Charusiri et al., 1992; Ahrendt et al., 1997; Hansen andWemmer, 2011). More recently these observations have been cor-roborated by U–Pb dating of monazite, which indicates upperamphibolite-facies metamorphism and partial melting during theLate Cretaceous at c. 84 Ma and c. 72 Ma at Doi Inthanon (Macdon-ald et al., 2010; location 5 on Fig. 7), and 79.1 ± 2.2 Ma and76.8 ± 2.6 Ma in the Chonburi–Rayong area (Geard, 2008; location2 on Fig. 7). Additionally, Searle et al. (2007) report evidence formigmatization, ductile deformation and biotite granite sill injec-tion associated with Early Palaeogene regional metamorphism inthe Mogok metamorphic belt (location 6 on Fig. 7).

In this study, a sample of garnet-two-mica granite dyke (TH04)from the Bhumipol Lake (location 4 on Fig. 7) that cross-cuts Trias-sic granite yielded a Th–Pb monazite age of 66.2 ± 1.6 Ma, with ourfield observations showing that similar dykes in the vicinity alsocross-cut lower Palaeozoic marble. Biotite 40Ar–39Ar cooling agesfrom granites and hydrothermal quartz veins near to the Myanmarborder, just north of the MPF, also indicate igneous activity be-tween c. 72 Ma and c. 65 Ma (unpublished ages from Charusiri,1989, reported in Morley et al., 2007). Sample TH04 from this studyestablishes evidence for similar activity further to the east in thisbelt, with this age of magmatism filling in an apparent gap be-tween similar age plutonic activities noted further north in Yunnanand to the south in Peninsular Thailand (Fig. 7). Although this agegap may alternatively be an artefact of sample collection or anexample of subduction without magmatism, it is plausible thatLate Cretaceous–Early Palaeogene activity can be correlated alongthe region highlighted in Fig. 7 running from north Malaysia toYunnan. Further research into suitable samples has the potentialto corroborate or dismiss this hypothesis.

6.1.3. Eocene metamorphismTh–Pb monazite ages from the Lansang gneiss (sample TH01)

record an Eocene metamorphic event during the period c. 45–37 Ma, the lower limit of which provides a maximum age of forma-tion of left-lateral shear fabrics and initiation of the MPF. This agerange is similar to the youngest sillimanite-grade metamorphicevent recorded in the Mogok belt in Myanmar (c. 47–29 Ma; Searleet al., 2007). It is possible that both metamorphic belts are con-nected at depth and parts have been exhumed by transpressionalong the Sagaing fault (Mogok belt) and the MPF (with additionalcore complex structures) in northwest Thailand.

Morley (2004, 2012) and Morley et al. (2011) discuss the possi-bility that widespread compressional (or transpressional) deforma-tion was occurring in this region during the Early Cenozoic and,although the actual timing is poorly constrained, deformation musthave occurred after Mid-Cretaceous deposition and before Oligo-


cene rifting. The best evidence for the timing of this event has pre-viously come from eastern Thailand where uplift, erosion andfolding of the Khorat Group has been constrained between 65 Maand 35 Ma (apatite fission track; Racey et al., 1997; Upton, 1999).The c. 45–37 Ma metamorphic event identified in this study fur-ther confirms interpretations presented by Charusiri et al. (1993),Dill et al. (2008), Searle and Morley (2011), Crow and Zaw (2011)and Nantasin et al. (2012) for associated deep crustal metamor-phism and mineralization in Thailand.

6.1.4. Timing of shear along the MPF and basin developmentDeep-crustal regional metamorphic rocks are known to have

been transpressionally uplifted along the MPF (the Lansang gneiss),but many exposures also occur outside of the shear zone, includingthe Umphang and Khlong Lhan gneisses (Fig. 2). Zircon fission trackages of 47 ± 3 Ma and 40 ± 1 Ma, and apatite fission track ages of40 ± 2 Ma and 20 ± 1 Ma have been reported for these massifs (Up-ton, 1999), indicating that localized uplift was diachronous inlocalities only 50 km apart. In general, apatite fission track geo-chronology along the western ranges of Thailand shows a regionalLate Oligocene–Early Miocene N–S trending cooling pattern alongthe eastern margin of the ranges that is unrelated to shear alongthe MPF (Upton, 1999; Morley et al., 2007). Instead, this exhuma-tion pattern is likely related to Late Oligocene–Neogene extensionand associated high rates of erosion. Lacassin et al. (1997) docu-ment a c. 33–30 Ma episode of cooling from 40Ar–39Ar ages of bio-tite in the Lansang region that contrast with younger biotitecooling ages from the Bhumipol dam area (c. 29–23 Ma; Morleyet al., 2007), which lies north of the shear zone. Lacassin et al.(1997) inferred that the c. 33–30 Ma cooling ages were related tothe final stage of left-lateral strike-slip deformation during whichmylonites that were forming at mid-crustal levels were being ex-humed. The youngest Th–Pb monazite age obtained from orthog-neiss sample TH01 (37 Ma) gives a maximum age constraint ondeep crustal exhumation and the c. 33–30 Ma 40Ar–39Ar biotitecooling ages reported by Lacassin et al. (1997) may constrain thetiming of transpressional uplift of the mylonite zone.

Geochronological data presented in this study therefore supportthe hypothesis that Early Cenozoic deformation, unrelated to es-cape tectonics, affected Thailand (e.g. Morley, 2004; Watkinsonet al., 2008; Searle and Morley, 2011). It can be seen that theMPF, and perhaps other major strike-slip faults in Thailand, cutacross pre-existing metamorphic terranes and gneiss domes thatpost-date the Triassic Indosinian orogeny. The suggested 120–150 km of left-lateral offset along the MPF (Lacassin et al., 1997)may record some extrusion-related tectonics, although the amountof extrusion is relatively minor. The Chainat strike-slip duplexformed under left-lateral transpression, but was later episodicallyreactivated under right-lateral transpression from the Late Oligo-cene onwards (Morley et al., 2007; Smith et al., 2007).

From the Late Oligocene to present, the stress regime in CentralThailand has varied considerably resulting in prolonged phases ofextension in rift basins (e.g. Phitsanulok, Ayutthaya, Suphan BuriBasins) punctuated by episodes of basin inversion, particularlyduring the Early-Middle Miocene and around the Miocene-Plio-cene boundary (Morley and Racey, 2011; Morley et al., 2011). Pass-ing northwards, the Neogene extensional deformation transitionsinto one dominated by strike-slip (Morley, 2007). The start ofextension in the Gulf of Thailand and possibly in the AndamanSea (Morley and Racey, 2011) is contemporaneous with the endingof the Eocene-Early Oligocene phase of metamorphism (c. 37 Ma;sample TH01), 40Ar–39Ar biotite cooling ages of c. 33–30 Ma fromthe Lansang gneiss (Lacassin et al., 1997), and by U–Th–Pb agesfrom the Mogok metamorphic belt in Myanmar (Barley et al.,2003; Searle et al., 2007). Extension in the Gulf of Thailand hasbeen related to pull-apart geometries along major strike-slip fault




trends (e.g. Tapponnier et al., 1986; Polachan et al., 1991). How-ever, the link between extension and strike-slip deformation isproblematic given: (1) the strongly splaying strike-slip fault geom-etries passing towards the gulf that suggest termination of strike-slip activity passing into the gulf, (2) the contradiction inherent insustaining basin extension in a pull apart setting over the time per-iod when strike-slip motion switched from sinistral to dextral, and(3) the lack of evidence for strike-slip faults in the gulf from seis-mic reflection data (Morley, 2002, 2004). The spatial and temporaltransitions from extension to strike-slip deformation in Thailandare more complex than the escape tectonics model alone can ex-plain, although it is part of the history. Extensional collapse of crustthickened during the Palaeogene in response to changing plateboundary forces (perhaps including subduction rollback) is analternative scenario for explaining rift basin development (Morley,2004; Searle and Morley, 2011). The data presented in this papersupport Late Cretaceous-Palaeogene crustal thickening as animportant precursor event to rift basin development.

7. Conclusions

The lack of recent U–Th–Pb age dating of high-grade crystallinerocks in Thailand has hindered the interpretation of its tectonicevolution; however, this reconnaissance study adds informationto the growing body of regional data that indicates important sub-duction-related tectonic activity affected Thailand and Myanmarduring the Cretaceous and Early Palaeogene. Th–Pb geochronologyof a sample of sheared Lansang gneiss (TH01) suggests the possibil-ity of magmatic protolith emplacement between c. 123 and114 Ma. Although further investigation is required to verify thesedata, these ages correlate with similar lithologies in Yunnan andMyanmar that report widespread magmatism occurring at thistime.

A garnet-two-mica granite dyke from the Bhumipol damyielded a weighted average Th–Pb monazite age of 66.2 ± 1.6 Ma,correlating with regional trends that suggest an Andean-type crus-tal thickening tectonic regime affected western Yunnan, easternMyanmar and western Thailand during the Late Cretaceous–EarlyPalaeogene (Fig. 7). This pre-collisional Andean-type margin wassubsequently affected by an episode of Eocene–Oligocene crustalthickening and amphibolite-facies metamorphism associated withthe India–Asia collision documented between c. 45–37 Ma in theLansang region (Th–Pb monazite, this study), c. 45–29 Ma in theMogok metamorphic belt (U–Pb monazite, Searle et al., 2007), c.55–45 Ma along the Khlong Marui fault (U–Pb zircon, Kanjanapa-yont et al., 2012) and 51–57 Ma for the Thabsila metamorphiccomplex in the Three Pagodas fault zone (Nantasin et al., 2012).Similarly, Geard (2008) obtained a 42.54 ± 0.88 Ma monazite agefor the Nong Yai Gneiss, east of Bangkok, as well as two U–Pb titan-ite ages of 35.5 ± 3.1 and 37.8 ± 4.8 Ma. Together, these data sug-gest that a large region of Thailand appears to have undergonethrusting, folding and basin inversion after deposition of the Khor-at Group continental sediments (mid to Late Cretaceous), proposedto be related to regional Palaeogene transpressional deformationprior to strike-slip deformation associated with escape tectonics(Morley, 2004).

Acknowledgements

The authors acknowledge a Natural Environmental ResearchCouncil postgraduate grant to R.M. Palin for research funding, ref-erence number NE/H524781/1. Work performed at NIGL wasfunded by grant number IP/1263/1111. Constructive and helpfulreviews by M. Ridd and M. Crow are gratefully acknowledged


and I. Metcalfe is thanked for editorial handling. J. Hyde is thankedfor thin section preparation in Oxford.

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