(e-mail: [email protected])2Department of Petroleum Geoscience, Universiti Brunei Darussalam,
Bandar Seri Begawan, Brunei Darussalam3School of Earth Sciences, Birkbeck College, University of London, UK
4Department of Geology, Chulalongkorn University, 10330, Bangkok, Thailand5Department of Geological Sciences, Chiang Mai University, Thailand
Abstract: The c. 500-km-long Mae Ping fault zone trends NW–SE across Thailand into easternMyanmar and has probably undergone in excess of 150 km sinistral motion during the Cenozoic.A large, c. 150-km-long, restraining bend in this fault zone lies on the western margin of theChainat duplex. The duplex is a low-lying region dominated by north–south-trending ridges ofMesozoic and Palaeozoic sedimentary, metamorphic and igneous rocks, flanked by flat, post-rift basins of Pliocene–Recent age to the north and south. A review of published cooling-agedata, plus new apatite and zircon fission-track results indicates that significant changes in patternsof exhumation occurred along the fault zone with time. Oldest uplift and erosion (Eocene)occurred in the Umphang Gneiss region, west of an inferred thrust-dominated restraining-bendsetting. From 36 Ma to 30 Ma, exhumation was strongest north of the duplex, along the NW–SE-trending segment of the fault zone at the (northern) exiting bend of the Chainat duplex.This region of the fault zone is characterized by a mid-crustal level shear zone 5–6 km wide(Lan Sang Gneisses), that passes to the NW into an apparent strike-slip duplex geometry. Thedeformation is interpreted to have occurred during passage around the northern restrainingbend, which resulted in vertical thickening, uplift, erosion and extensional collapse of the northernside of the shear zone. This concentration of deformation at the bends at the ends of the restrainingbend is thought to be a characteristic of strike-slip-dominated restraining bends. Following LateOligocene–Early Miocene extension, there is apatite fission-track evidence for 22–18 Ma exhu-mation in the Chainat duplex, that coincides with a phase of inversion in the Phitsanulok Basin tothe north. The Miocene–Recent history of the Chainat duplex is one of minor sinistral and dextraldisplacements, related to a rapidly evolving stress field, influenced by the numerous tectonicreorganizations that affected SE Asia during that time.
Restraining bends (cf. Crowell 1974) in strike-slipzones have been identified in many parts of theworld (e.g. Anderson 1990; Corsini et al. 1996;Laney & Gates 1996; Curtis 1998), but in particularthose in California, China and Mongolia have beenthe subject of numerous and diverse investigations(e.g. reviews in Cunningham et al. 1996, 2003;Cowgill et al. 2004;Wakabayashi et al. 2004). Typi-cally, in these areas, major restraining bends formregions tens of kilometres up to several 100 kmlong. Models for restraining-bend behaviour havehighlighted two end members: thrust dominated,and strike-slip dominated (e.g. Hauksson & Jones1988; Cowgill et al. 2004). Restraining-bend struc-tural evolution displays a range of trends, includingsynchronous movement on faults within a strike-slip
duplex (McClay & Bonora 2001); progressiveoutward propagation of the active fault systemwithin a duplex away from the original restrainingbend (Wakabayashi et al. 2004), and complexrotations of faults during simple shear whichcauses changes in their sense of motion, anddegree of activation (Cunningham et al. 2003).However, the number of well-documented examplesof major restraining bends for developing andtesting such models remains low.
Eastern Myanmar and western Thailand displayan extensive network of Cenozoic strike-slip faults(e.g. Le Dain et al. 1984; Lacassin et al. 1997;Morley 2004; Fig. 1). One of the major faultswithin this system is the Mae Ping fault zone(also known as the Wang Chao fault zone). The
From: CUNNINGHAM, W. D. & MANN, P. (eds) Tectonics of Strike-Slip Restraining and Releasing Bends.Geological Society, London, Special Publications, 290, 325–349.DOI: 10.1144/SP290.12 0305-8719/07/$15.00 # The Geological Society of London 2007.
Mae Ping fault zone trends predominantly NW–SE,but displays important north–south-trendingsegments (Morley 2004; Figs 1, 2 & 3). FromMyanmar to central Thailand, the Mae Ping faultzone is 500 km long; its continuation to the SE isuncertain. Some interpretations extend the MaePing fault zone over 1000 km further to the SE, toreach the Mekong Delta of southern Vietnam(Lacassin et al. 1997; Leloup et al. 2001). In
regional restorations of SE Asian rigid-blockmotions during the Cenozoic, Replumaz &Tapponier (2003), building of a model publishedin Leloup et al. (2001) required the Mae Pingfault zone to extend to the NW Borneo margin.However, Morley (2002) viewed such an extensiveMae Ping fault zone as unsupported by availabledata, and contradictory to the known geologicalhistory of NW Borneo. There is actually no hard
Fig. 1. Regional location map, modified from Morley (2004). Terrane boundaries are from Barr & Macdonald (1991).
C. K. MORLEY ET AL.326
evidence for even extending the Mae Ping faultzone through Cambodia and Vietnam to theMekong Delta, except for the presence of con-venient NW–SE-oriented linear features such asthe lake, Ton Le Sap. It is even possible that thefault zone splays and ends in eastern Thailand/western Cambodia (Fig. 1).
The Mae Ping fault zone has undergone predomi-nantly sinistral strike-slip motion (Lacassin et al.1993, 1997) where the north–south segmentswould have acted as restraining bends within theoverall NW–SE trend. Horizontal sinistral displace-ment is estimated at about 150 km (Lacassin et al.1997). Hence, the restraining bends should haveexperienced considerable strain, uplift and erosion.Strongly entrenched in the literature is the idea of alater, simple reversal to dextral strike-slip motionalong the major NW–SE-trending strike-slip faults
of SE Asia during the Miocene or Pliocene (e.g.Huchon 1994; Leloup et al. 1995; Lacassin et al.1997, 1998). Specifically for the Mae Ping faultzone, Lacassin et al. (1997) suggested that theswitch occurred post-23 Ma. Smith et al. (2007)discuss the evidence for this dextral motion withinthe Chainat duplex area of the Mae Ping faultzone, and so the subject is not addressed in detailin this volume. However, the conclusions are thatevidence for dextral motion can be found, but displa-cement is minor (a few kilometres of displacement).Dextral displacement increases in magnitude andimportance, passing westward into Myanmar. Inthe Chainat duplex area, dextral motion has alter-nated with long periods of quiescence, andoccasional left-lateral motion during the Miocene,and does not have the history of timing, displacementmagnitude or correct detailed structural geometries
Phitsanulok Basin
Phetchabune Basin
SuphanBuriBasin
Ayutthaya Basin
Mae Sot Basin
Sing Buri Basin
Granite-Gneissregion outside ofmain strike-slip zone
Sheared granite-gneissregion inside mainstrike-slip zone
Cenozoic basin
Cenozoic basin depocentre
90°00' 100°00' 101°00'
0 100 km
N
UmphangGneiss
ChainatDuplex
Lan Sang Park
Khlong LhanGneiss
Khlong LhanRestrainingbend
Fig. 3
Normal faults inPhitsanulok basin
Strike-slip faults within Mae Ping Fault zone
17°00'
16°00'
15°00'
Fig. 2. Regional map of the Mae Ping fault zone (modified from Smith et al. 2007, this volume).
MAE PING FAULT ZONE, WESTERN THAILAND 327
to be the cause of rift basin development as pull-apartbasins (Morley 2002; Smith et al. 2007, paper 11,this volume). The absence of large basins withinthe Chainat duplex area indicates that dextralmotion reactivation of the area as a releasing bendmust have been minor, and deep basins adjacent tothe duplex were formed by extension, not by strike-slip (Morley 2002).
In this paper we show that the 33–30 Ma exhu-mation documented by Lacassin et al. (1997) is onlyseen locally in the Lan Sang area, and probably rep-resents the results of transpression at the exitingbend of the Khlong Lhan restraining bend, withevolution from a thrust-dominated to strike-slipdominated type restraining bend. The structuralevolution of the fault zone during the Cenozoic isdefined by summarizing the existing thermo-chronology data from the region around the MaePing fault zone in western and central Thailand,and from new apatite (AFT) and zircon (ZFT)fisson-track ages. This model is compared withexisting models for restraining bend evolution.
Regional geological setting
The Mae Ping fault zone in western Thailand isclearly seen on satellite images to form a
pronounced NW–SE-trending feature (Figs 1 &2), with north–south-trending splays branchingfrom it. The NW–SE trend of the fault zone slicesthrough older Palaeozoic–Early Mesozoic terraneboundaries (as defined by Barr & Macdonald1991), that trend predominantly north–south(Fig. 1). However, several of the splays appear tocoincide with the terrane boundaries, hence theMae Sariang splay (Fig. 1) coincides with theboundary between the Western Zone and the Intha-non Zone, whilst the major bend in the fault zonethat is the focus of this study (here termed theKhlong Lhan bend, Fig. 1), lies between theSukhothai Zone and the Inthanon Zone. Hence,the influence of major crustal pre-existing fabricsdescribed for examples of large releasing orrestraining bends elsewhere in the world (e.g.Corsini et al. 1996; Tommasi & Vauchez 1997;Curtis 1998), also appears to have influenced theirlocation along the Mae Ping fault zone.
Recently, Morley (2004) has suggested that theMae Ping fault zone first developed during a LateCretaceous–Early Cenozoic transpressional eventrelated to collision of the Burma Block with thewestern margin of Sundaland. This early transpres-sion was a precursor to the main Indian–Eurasiancollision when the fault zone underwent further(probably the greatest) sinistral motion during the
4
3
4
1
2
3
2
3
1
2
3
4
Small extensionalbasins
5 km
N
Mae Ping Fault zone
B 33.1±0.4B33.0±0.2B31.3±0.7
B29.0±1 MaB28±3 Ma
B23.2±1
B25.4±0.8
A19±1
A 24±2Z28Ma
A 22±1
A24±1
A 18.7±1.2A 24.5±2.1Z 35.9±1.3
A 21.2
Bhumibol dam
B47.5 Ma
MS = Mae Sot BasinML = Mae Lamao BasinMT = Mae Tuen coal mineLSNP = Lan Sang National Park
MTMS ML
Lan Sang Gneiss(mylonitic shear zone)
Cenozoic rift or pull-apart basin
Cenozoic basin depocentre definedfrom gravity data
Cenozoic basin relatedto movement on horse?
Strike-slip'antiformal' stackduplex?
LSNP
Normal fault thataided unroofing ofLan Sang Gneiss
B = Biotite 39Ar/40Ar ageA = Apatite fission track central ageZ = Zircon fission track central age
Upton (1999)
This study
Lacassin et al. (1997)
Charusiri (1988)
B 30.6±0.3
Mae Ramat-Banli road
Fig. 3. Map of the Lan Sang to Mae Sot area, showing the antiformal geometry of the Lan Sang Gneisses, basedon satellite image interpretation and maps in Lacassin et al. (1997).
C. K. MORLEY ET AL.328
Oligocene (Lacassin et al. 1997). The best exposureof the mid-crustal levels of deformation associatedwith the Mae Ping fault zone is the 5–6-km-widemylonitic to ultramylonitic shear zone in the LanSang national park (Lacassin et al. 1993, 1997;Fig. 2). North of this area is a north–south-trendingregion of gneisses and granites which form thehighest ranges of hills in Thailand (Fig. 2). Theseranges include the hills called Doi Inthanon andDoi Suthep, which have been interpreted as meta-morphic core complexes exposed by top-to-the-eastshear on low-angle east-dipping detachments (e.g.MacDonald et al. 1993; Rhodes et al. 1997).However, differences in ages between the datingof the detachment (Eocene) and the timing of exhu-mation (Early–Middle Miocene) mean that thehistory of metamorphic core complex is in doubt(e.g. Rhodes 2002).
The Lan Sang Gneisses within the Mae Pingfault zone NW of Lan Sang national park have a dis-tinctive pattern to the trend of their foliation(Fig. 3). Although the main shear zone trendsNW–SE, the foliations are curved and lie within aconvex stretch of the NE northern boundary (Lacas-sin et al. 1997; Fig. 3). The pattern of foliations andshear zones suggests the strike-slip equivalent of anantiformal duplex geometry (e.g. Woodcock &Fischer 1986). The small Cenozoic sedimentarybasin that opened up on the northernmost segmentof the duplex suggests that one horse block movedindependently from those to the south (Fig. 3).The minor road from Mae Ramat to Banli, whichcuts the northern part of the duplex, reveals smalloutcrops of gneiss and augen gneiss that do nothave an imposed sinistral mylonitic fabric, and afew strongly weathered outcrops with a subverticalfoliation – a pattern consistent with horses within aduplex. However, the duplex (and the road) lies inremote, jungle-covered, hilly country, and detailedresolution of the structural geometry from outcropsis unlikely to be possible. If the foliation pattern onsatellite images does represent an antiformalduplex, then the area would have evolved in away similar to that illustrated in stages 1–4 ofFigure 3. The first horse to move block (1) thenceased motion and became overridden by succes-sive horses that each were transported further tothe NW than previous ones. What was possiblythe final motion on horse 4 set up a small releasing-bend geometry, resulting in the creation of a minorCenozoic basin.
Southeast of Lan Sang national park and thewestern highlands are the broad Central Plains(Fig. 1). This region is a flat-lying area whichrepresents a post-rift basin overlying several LateOligocene–Miocene rift basins (e.g. Morley et al.2001). The Mae Ping fault zone east of Lan Sangnational park broadens and splays into the Central
Plains area. In one large region of the CentralPlains, some 200 km north–south and 100 kmwide, Palaeozoic and Mesozoic sedimentary,metasedimentary and igneous rocks are exposedas isolated hills. These hills tend to trend eithernorth–south or NW–SE. This area between theCenozoic rift basins was called the Chainat Ridgein O’Leary & Hill (1989). Morley (2002, 2004)interpreted the region as a strike-slip duplex andrenamed it the Chainat duplex. A detailed discus-sion of the evidence for strike-slip deformation inthe Chainat Ridge area is provided in a companionpaper to this one (Smith et al. 2007). Adjacent to theChainat duplex are the Phitsanulok, Ayutthaya andSuphan Buri rift basins of Late Oligocene–Mioceneage (O’Leary & Hill 1989). The timing and struc-tural history of the basins are constrained by welland seismic reflection data gathered for hydro-carbon exploration (e.g. Flint et al. 1988; O’Leary& Hill 1989; Wongpornchai 1997; Ronge &Surarat 2002). The history of these basins helps tofurther constrain the activity of the Mae Ping faultzone. This paper focuses on the exhumationhistory of the region around the poorly exposedantiformal duplex in the NW illustrated inFigure 3, and the much better-constrained Chainatduplex to the SE (Fig. 1).
Methods and results
This study is based on collating available publishedand unpublished cooling age data for NW Thailand,and providing additional ZFT and AFT data whichinfill key areas where there was little publishedinformation. The aim of the work is to determinewhether the patterns of uplift are consistent withone or more mechanisms of uplift, and specificallyto determine patterns of uplift that might be associ-ated with motion along the Mae Ping fault zone.
A number of radiometric dating studies havebeen conducted in western Thailand, with a rangeof aims. The locations and cooling ages determinedfrom these studies are shown in Figure 4. Severalstudies have focused on the uplift and erosion ofgneisses in the Doi Suthep and Doi Inthanon areas,with regard to documenting the denudation historyof putative metamorphic complexes associatedwith low-angle extensional detachments (Dunninget al. 1995; Rhodes 2002). Ahrendt et al. (1993,1997) have regionally dated granites and gneissesin Thailand, and have related the ages to orogenicevents. Charusiri (1989) obtained 40Ar/39Ar radio-metric age dates from micas and feldspars fromparts of the Three Pagodas fault zone and the MaePing fault zone, in order to understand the timingand genesis of ore deposits (Fig. 4). Upton et al.(1997) and Upton (1999) collected samples for
MAE PING FAULT ZONE, WESTERN THAILAND 329
B30.6
B 33.1B33.0B31.3
(B 33.4±0.4)
B29.0 MaB28 Ma
B23.2
A20±1
A29±1
A19±1
A 24±2Z28±1Ma
A 21±1
A24±1
A 37±2
A 80±6
Tak
Chiang Mai
Lacassin et al. (1997)Biotite ages, Ar/Ar
Charusiri (1989)Biotite ages, Ar/Ar
Upton (1999)A = apatiteZ = zircon
This study (numbers denote sample number in Table 1)A = apatiteZ = zircon
A 18.7A 24.5Z 35.9±1.3
A 21.2
A 19.3 ± 2.2
A 19.8 ±1.5
A 22±1
A 19.7
A 18.2±4
13
9
8
32
5 14
Z29.7±1.4
Bhumibol dam47.5 Ma
70.9 Ma69.5 Ma72.0 Ma
65.7 Ma69.5 Ma66.3 Ma
M 84±2M 72±1
B 16±0.2
B 26±1-21±1 Ma
A 22±1
46
UmphangGneiss
KhlongLhan Gneiss
Dunning et al. (1995)M = Monazite age
Barr et al. (2002)Biotite ages, Ar/Ar
Chainat Duplex
Mae SariangFault strand
d
c
b
d'
c'
b'
a'
a
Mae PingFault zone
A20±1Z40±1
= location numbers used in Figure 8
4
A 40±2Z 47±3
A18 ±1Z 19±1
Z 52±4A 22±1
A14 ±1
A43 ±3
A13 ±1
A64 ±4
A60 ±3A58 ±2
A34 ±2
A14 ±1
A23±2
A23 ±1
A51 ±3
A19 ±2
A 23 ±2
A73 ±7
A39 ±2
A15 ±1
0 250 km
5
Myanmar
Thailand
98 100o o
19
18
17
o
o
o
B24.4
A 22.4±2.8
A 18.2±1.6
9
1011
87
6
5
6
4
1 2
3
Fig. 4. Regional map of NW Thailand, showing the location of cooling-age data used in this study.
C. K. MORLEY ET AL.330
apatite and zircon fission-track analysis in order tobuild a regional denudation history for Thailand,as well as focusing on more detailed local tectonicand exhumation problems in some areas (such asmore concentrated sampling in the regions of theproposed metamorphic core complexes in thewestern highlands, and around the Mae Ping faultzone). Lacassin et al. (1997) specifically sampledthe Mae Ping and Three Pagodas fault zones todetermine the timing of strike-slip deformation;their results are discussed separately below.
For this study, samples were taken for apatiteand zircon fission-track dating from the Lan Sangarea into the Chainat Ridge area (Fig. 4) to deter-mine whether any systematic change in agesoccurred along the strike, and perpendicular to theMae Ping fault zone passing away from the LanSang area (Table 1). The samples were analysedin the laboratories at the University College ofLondon. The results of this work were partially suc-cessful; however, a systematic spread of data couldnot be obtained, due to unsuitable outcrop litholo-gies and insufficient apatite or zircon in somesamples (UBDA-7, 8, 10, 11 and 12). New dateswere obtained for two localities within theChainat duplex (samples UBDA-9 and UBDA-13,Table 1, Fig. 4). Samples (UBDA-4 , 5, and 6,Table 1, Fig. 4) within the Lan Sang area validatedprevious results and established similarity betweenbiotite 40Ar/39Ar and ZFT cooling ages (Table 2).However, east and NE of the Mae Ping fault zone,around Tak, a cluster of cooling ages (samplesUBDA-1, 2 and 3, Table 1, Fig. 4) showed AFTcentral ages around 19–20 Ma.
Cooling-age studies in western Thailand
The cooling ages available from the studies men-tioned above are mostly from 40Ar/39Ar biotiteages, zircon and apatite fission-tracks. Compli-cations arising from mineral structure, grain sizeand previous cooling rates mean that the conceptof ‘closure temperatures’ (Dodson 1979) for manymineral/isotopic systems (e.g. 40Ar/39Ar) is anoversimplification. For example, chemical compo-sition and the presence of large quantities of fluidinclusions can cause significant changes to standardclosure temperatures form micas (e.g. McDougall& Harrison 1999; Dunlap 2003). However, thetemperature range below which many of thesesystems effectively become stable can yield quali-tative estimates of cooling rates experienced by asample. In this context, stability means retention,within the crystal system, of some measurableproduct of various radioactive decay reactions. Asan approximate guide, the temperature range belowwhich the system is effectively stable is as follows
(Carter 1999; McDougall & Harrison 1999;Dunlap 2003): 40Ar/39Ar for muscovite ! 400–250 8C; 40Ar/39Ar for biotite 300+ 50 8C, zirconfission-track 320–2008C, and apatite fission-track110–60 8C. Consequently, for the high-temperaturecooling age map (Fig. 5), dates for biotite 40Ar/39Arand zircon fission-track were combined. Whilst thisis clearly a great approximation, where zirconfission-track and biotite 40Ar/39Ar ages have beenobtained from the same or nearby localities (e.g.Lan Sang, Fig. 4), the resulting cooling ages arevery similar (Fig. 4; Tables 1 & 2). The low-temperature cooling-age map (Fig. 6) is entirelybased on apatite fission-track ages from Upton(1999) and this study (Table 1).
Determination of the cooling history along
the Three Pagodas and Mae Ping fault
zones, by Lacassin et al. (1997)
Evidence for dating motion on the Mae Ping (WangChao) and Three Pagodas fault zones relies con-siderably upon the work by Lacassin et al. (1997)who specifically dated synkinematic micas andfeldspars from metasediments and orthogneisseswithin mylonitic shear zones, using the 40Ar/39Artechnique. Biotite cooling ages for the ThreePagodas fault zone suggested that the dates of theonset and end of sinistral motion were !36 Ma to33 Ma, and for the Mae Ping fault zone !33 Mato 30 Ma (Figs 4 & 7). Lacassin et al. (1997) mod-elled the cooling histories of the Lan Sang samples,calibrated by 40Ar/39Ar step-heating of a K-feldspar. The results indicate that cooling from400 8C to 185 8C was rapid between 32.5 Ma and31 Ma, in order to fit the last 16% of argonrelease. These authors also identified a secondcooling step at about 23.5 Ma, before a final isother-mal step (about 758C). Lacassin et al. also stressedthat the 33 Ma to 30 Ma dates probably documentedthe last increments of ductile sinistral deformation.The authors also suggest that late-stage exhumationof the Lan Sang Gneisses might be explained bynormal faulting within a transtensional setting.The onset of dextral strike-slip motion was placedat about 23 Ma, but was not constrained by anyradiometric dating.
The work by Lacassin et al. (1997) also obtainedbiotite cooling ages between 29 Ma and 23 Ma insome gneisses away from the strike-slip faultzones, including the Bhumipol Dam to the north(Fig. 4). The gneisses at Bhumipol Dam show noevidence for Cenozoic shear, and hence are inferredto represent denudation between 29 Ma and 23 Ma,possibly related to a Cenozoic basin-boundingnormal fault (Sam Ngao Fault) to the east (Lacassinet al. 1997).
MAE PING FAULT ZONE, WESTERN THAILAND 331
Table 1. Results of apatite and fission-track dating conducted for this study
1Track densities are ("106 tr cm2) numbers of tracks counted (N ) shown in brackets.2Analyses by external detector method using 0.5 for the 4p/2p geometry correction factor.3Ages calculated using dosimeter glass CN-5; (apatite) zCN5 ! 338+ 4; CN-2; (zircon) zCN2 ! 127+ 5, calibrated by multiple analyses of IUGS apatite and zircon age standards (see Hurford 1990).4Px2 is the probability of obtaining the x2 value for v degrees of freedom, where v ! number of crystals – 1.5Central age is a modal age, weighted for different precisions of individual crystals.
C.K.MORLEY
ETAL.
332
Patterns of cooling ages in western
Thailand
Introduction
The highest density of cooling-age data clustersaround the Mae Ping fault zone and the area tothe north. The area south of the Mae Ping faultzone, to the Three Pagodas fault zone, is muchmore sparsely sampled (Fig. 4). Therefore muchof this discussion will focus on the northern halfof western Thailand. To understand the uplift anderosion history of the Mae Ping fault zone, it isnecessary to investigate not only the timing of exhu-mation around the fault zone, but also theregional pattern.
Upton (1999) sampled outcrops in westernThailand extensively for the purposes of apatitefission-track analysis, and produced compositecooling paths for a subset of those samples, usingzircon fission-track and published K–Ar dates.For the apatite fission-track data, Upton et al.(1997) and Upton (1999) identified two samplesuites that differ in both age and cooling history.The largest subset displayed results that mostlyranged between 24 Ma and 13 Ma. However, threesamples with central ages of 40, 34 and 29 Mawere also included. These samples in the first setare characterized by narrow s.d. 1–1.5 mm, unimo-dal and long (.14 mm) mean track-length distri-butions, consistent with rapid cooling through thepartial annealing zone. Upton (1999) estimated theaverage cooling rates as between 8.5 8C Ma and25 8C Ma. The second subset ranged between80 Ma and 37 Ma, and shows broad track-lengthdistributions, with standard deviations between2.03–2.49 mm. Mean track-length distributionsare relatively short (12–13 mm), typical ofsamples exposed to short-lived, high temperatures,or prolonged residence in the partial annealingzone to reduce track length. The second suite of
samples exhibited much slower average coolingrates, of about 1.85+ 0.55 8C Ma. The youngAFT ages of the first subset form an extensivenorth–south-trending region along the western high-lands of Thailand (Fig. 6). On either side of thisnorth–south trend, in central northern Thailand andalong the Thailand–Myanmar border region, olderages of the second subset are found (Figs. 6 & 7).
The variations in cooling-age history in theregion are discussed in the context of three differentprovinces: the Chainat duplex; the Mae Ping faultzone; and the putative metamorphic core complexarea between the Mae Sariang–Hot Highway andChiang Mai. These provinces are exemplified infour cooling-age traverses (Fig. 7), and arediscussed below.
Chainat duplex area (Fig. 7 a–a0)
The southern traverse (Figs 4 & 7 a–a0) crosses theKhlong Lhan restraining bend and passes into theChainat duplex. Location 1 is from the UmphangGneiss area (Upton 1999; Fig. 4), The deepestcrustal rocks exposed in the duplex, called theUmphang Gneiss, lie on the western side ofthe restraining bend. The U–Pb analysis of theUmphang Gneiss, indicates that the paragneissunderwent high-grade metamorphism during theLate Triassic (Mickein 1997). A ZFT central ageof 47+3 was obtained for the Umphang Gneissby Upton (1999; Fig. 4). An apatite central age of40+ 2 Ma for the Umphang Gneiss (Upton 1999)suggests rapid exhumation on the western marginof the Chainat Ridge during the Eocene (Fig. 7 a–a0). At the NW corner of the duplex, where theMae Ping fault zone splays to the SE, are theKhlong Lhan Gneisses. The U–Pb dating ofzircon indicates a slightly younger age for high-grade metamorphism in the Khlong LhanGneiss compared with the Umphang Gneiss, of174+ 5–6 Ma, whilst a monazite age of117+ 3 Ma (Mickein 1997) indicates a subsequenthigh-temperature metamorphic event during theCretaceous. ZFT and K–Ar analyses by Upton(1999) from both gneisses show overlap at the2s-error level, thus indicating that they hadcooled below the 350–260 8C isotherm (Fig. 7)by the end of the Eocene (c. 40–43 Ma).However, the Khlong Lhan Gneiss has a40+ 1 Ma ZFT central age, and a 20+ 1 AFTage, indicating either slower exhumation between40 Ma and 20 Ma or a younger exhumation eventimposed on the older Eocene one (Fig. 7 a–a0).Whatever the precise scenario, the cooling historyis unlike the Umphang Gneiss, which just showsrapid cooling during the Eocene (Fig. 7).
Within the main part of the Chainat duplex thereare only a few Triassic granitic outcrops. Five
Table 2. Comparison of high-temperaturecooling ages determined for the Lan SangGneisses using zircon fission-track and40Ar–39Ar of biotite from threeseparate studies
Fig. 5. Map of high-temperature cooling ages for NW Thailand, mostly from zircon fission-track and 40Ar/39Ar biotitecooling ages. Some of the older apatite fission-track dates are included because they provide minimum ages forhigh-temperature cooling.
C. K. MORLEY ET AL.334
Rifts with Late Oligocene-EarlyMiocene syn-rift section
Post -EarlyMiocene upliftof Mae LamaoBasin (palynologyand Ro=0.45%)
Na HongLate Oligocene-Early Miocenecoal with Ro = 0.67.Early Miocene oryounger uplift of2-3 km
Mae LaiLate Oligocene-Early Miocenelignite. EarlyMiocene oryounger uplift of 1-2 km?
ChiangMai
(B 33.1) (B33.0) (B31.3)
(B 33.4±0.4)
B29.0 MaB28 Ma B23.2
A20±1
A29±1
A19±1
A, 21±1
A 22±1
A24±1
A 37±2
A 80±6
Tak
Chainatduplex
Lacassin et al. (1997)Biotite ages, Ar/Ar
Upton (1999)A = apatiteZ = zirconThis study
A 18.7A 24.5
A 21.2
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Mae Sot Basin
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Thoen Basin
Li Basin
Chiang Mai Basin
Mae SariangSplay
UmphangGneiss
Lan Sang National Park
KhlongLhanGneiss
A20±1Z40±1
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A15 ±140 Ma
0 250 km98° 100°
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Fig. 6. Map of low-temperature cooling ages for NW Thailand, from apatite fission-track data.
MAE PING FAULT ZONE, WESTERN THAILAND 335
localities were sampled, and two yielded usableAFT data (Table 1). AFT central ages of22.4+ 2.8 and 18.2+ 1.6 were obtained (Fig. 4).It is uncertain from these results alone whether the
Early Miocene cooling ages represent regionaluplift and erosion, or a specific structural eventrelated to strike-slip deformation within theduplex. Seismic data from the Lahan graben of
Fig. 7. Exhumation history for four transects through NW Thailand. See Figure 4 for locations of the transects andsources of the data. a–a0 southernmost transect through the Umphang Gneiss area (1), Khlong Lhan Gneiss (2), andChainat duplex area (3). Location 1 shows the rapid 50–40 Ma exhumation. A transect along the Mae Sot–Tak road isshown in b–b0. Along the western part of the road, cooling began early (locations 4 and 5) in the Late Cretaceous–EarlyCenozoic and the Lan Sang Gneiss displays one phase of exhumation between about 35 Ma and 30 Ma (6) and a secondphase around 25–19 Ma. Transect c–c0 runs from a large north–south-trending splay of the Mae Ping fault zone in thewest, to a putative metamorphic core complex in the east. In the west, exhumation is early (Late Cretaceous–EarlyCenozoic) and relatively slow. There is no indication of rapid, strike-slip-related uplift. In the east (location 8)exhumation was extremely rapid at around 20–18 Ma. The northernmost transect (d–d0) shows a similar pattern toc–c0, except that the rapid exhumation in the east is younger, from about 16–13 Ma.
C. K. MORLEY ET AL.336
the Phitsanulok Basin show that the basin ceased tobe active in the latest Early Miocene, coincidentwith the AFT ages (Smith et al. 2007, paper 11,this volume). This uplift is not seen in the sedimen-tary section of the Ayutthaya or Suphan Buri basinson the southern margin of the duplex. Hence, thepresent available data suggest that uplift occurredin a NW–SE-trending belt, in the northern part ofthe duplex (Fig. 6).
Mae Ping fault zone (Fig. 7 b–b0)
Passing westward into Myanmar along the MaePing fault zone, the muscovite 40Ar/39Ar coolingages increase – a similar trend to that establishedby the AFT ages further north (Figs 6 & 7 c–c0
and d–d0). Charusiri (1989) obtained 40Ar–39Arcooling ages from micas in granites adjacent tothe Mae Ping fault zone in westernmost Thailand(Fig. 4). The oldest ages obtained lie furthest tothe west (69.5–72 Ma), and young eastward(69.5–65.7 Ma and 47.5 Ma). A sample of hydro-thermal muscovite collected from a wolframite-bearing quartz vein (collected underground) yielded40Ar–39Ar spectra with well-defined plateau ages ofc. 69.5+ 0.68 Ma and 70.6 Ma. Muscovite fromyounger, cross-cutting scheelite–fluorite–calcite–quartz and sphalerite–muscovite–quartz veinsyielded fusion dates of c. 69.2 Ma and 71.9 Ma. Thehydrothermal muscovite probably crystallized attemperatures between 300 and 425 8C, undermaximum confining pressures of about 170 to200 MPa (i.e. depths of 6–7 km).
Further ESE along the trend of the Mae Pingfault zone, Charusiri dated samples from the MaeSuri Mine. Hydrothermal muscovite from atungsten-rich quartz vein was dated using totalfusion and step-heating methods. The total fusionage is c. 45.2 Ma, and the integrated agec. 47.5+ 0.51 Ma. The 40Ar–39Ar age spectrumdisplays a well-defined plateau of 46 Ma. Theminimum at the first step (21.5 Ma) may be aresult of thermal resetting. There was evidence ofshearing within the mine that suggested emplace-ment of the veins during sinistral displacement ofthe Mae Ping fault zone.
The cooling ages obtained by Charusiri (1989)are not as directly linked to the Mae Ping faultzone as the Lan Sang ages (Lacassin et al. 1997).But they are close to the fault zone, and thusshow that passing west, close to the fault zone,there is no evidence for large-scale Oligoceneregional exhumation related to strike-slip faultingthat would have obliterated the older ages. Hence,the exhumation of mid-crustal rocks at Lan Sangbetween 33 and 30 Ma (Fig. 7 b–b0 location 6) isan atypical and localized feature of the fault zonethat requires specific explanation.
The pattern associated with high-temperaturecooling (Fig. 6) is very consistent with the AFTresults of Upton (1999). The contour patterns(Figs 5 & 6) suggest that, for much of the lengthof the Mae Ping fault zone, strike-slip motiondoes not equate with significant Oligocene upliftand erosion, and that if the exhumation is associatedwith the Mae Ping fault zone, then it is of Late Cre-taceous–Palaeogene age (Fig. 7 b–b0). The overallcooling pattern indicates that the strike-slip defor-mation has not dominated the cooling history. Thelow-temperature cooling pattern shown inFigure 6 reveals a predominantly north–south-trending exhumation pattern. This pattern may rep-resent both more local tectonic effects such asextensional or inversion related uplift and erosion,and more regional uplift and erosion at least par-tially related to climate change (Morley & West-away 2006). For example, the syn-rift basins ofnorthern Thailand show a switch from palyno-morphs associated with a temperate climate to tro-pical forms in the Early Miocene (Songtham2000; Ratanasthien 2002); this change is also seenin peninsular Malaysia (Morley 1998).
Exhumation of the Lan Sang area can either beinterpreted as part of a regional Late Oligocene–Early Miocene north–south-trending event, or asa composite of strike-slip-related deformationsuperimposed on a north–south striking regionaltrend. We feel that there is sufficient evidence aspresented by Lacassin et al. (1997) to justifymuch of the exhumation at Lan Sang as beingrelated to strike-slip deformation.
Hot–Mae Sariang highway-region west
of Chiang Mai (Fig. 7 c–c0 & d–d0)
The two northernmost traverses (Fig. 7 c–c0, d–d0)are through the putative metamorphic core complexarea (MacDonald et al. 1993; Rhodes et al. 1997,2002). Traverse c–c0 is along the Hot–MaeSariang highway (Figs 6 & 7). The pattern ofcooling was defined by Upton (1999) using AFT,ZFT and K–Ar biotite cooling ages. Passing west-ward, four AFT ages progressively become older,ranging from 19+ 2 Ma and 18+ 1 Ma, through22+ 1 Ma in the east to 37+ 2 Ma in the west(Fig. 7). For the 18+ 1 Ma sample, Upton (1999)also obtained a 19+ 1 ZFT age, and nearby a con-cordant K–Ar biotite cooling age of 20+ 1 Ma,was reported. For the 22+ 1 Ma sample, Upton(1999) obtained a ZFT age of 52+ 4 Ma, and aK–Ar biotite cooling age of 67+ 2 Ma, indicatinga much slower and more prolonged cooling historytoward the west. Using isotopic dating, Mickein(1997) also identified a younging-to-the-eastpattern along the Hot to Mae Sariang highway.
MAE PING FAULT ZONE, WESTERN THAILAND 337
On the northernmost line (Fig. 7 d–d0) a similarpattern of cooling ages is seen, with slow, pro-longed exhumation in the west (AFT central ageof 80+ 6 Ma) and rapid exhumation in the east.The timing of the eastern rapid exhumationbecomes younger passing north, along line c–c0
(Fig. 7) with the AFT central ages being EarlyMiocene. Along line d–d0, the AFT central agesare Middle Miocene, with the nearest biotite40Ar/39Ar age being 16+ 0.2 Ma. The rapidEarly and Middle Miocene cooling occurs in theregion identified as the metamorphic-core-complexarea. However, one problem with a simple core-complex story is that the rapid exhumation isyoung, compared with the age of the shearingdefined by dating of biotite within the detachementzone, which is of Eocene age (Rhodes 2002).
One possible explanation of the cooling-agepattern lies in the model for basin subsidence inresponse to sediment loading, a model proposedby Morley & Westaway (2006). There, erosion ofthe sediment source area and deposition in the sedi-mentary basin triggers a return flow in the lowercrust, from beneath the basin toward the sedimentsource area. Applied to Thailand, the model pre-dicts lower-crustal flow from beneath the basinsof the Gulf of Thailand toward the sedimentsource areas of the western highlands (Morley &Westaway 2006). If the pattern of erosion andlower-crustal flow shifted northward with timethen this may explain the pattern of young, rapidcooling on the eastern side of the highlands.
Before discussing a model for the structuralhistory of the Mae Ping fault zone, other constraintson timing of deformation and exhumation associ-ated with the Mae Ping fault zone from adjacentsedimentary basins and the Chainat duplex areaare reviewed.
Significance of Cenozoic basins
for exhumation history
Sedimentary basins that were sites of subsidencesynchronous with the areas of exhumation provideimportant constraints for the location and origin ofexhumation. Here the basins within the westernhighlands are discussed.
Mae Lamao
The main exposures of the Mae Lamao basin arecoarse conglomerates and sandstones along themain Tak–Mae Sot road, and deeper levels ofthe basin exposed in a very small coal mine thatlies north of the main Tak–Mae Sot road (Fig. 3).The main coal seam is mined from the footwall ofa normal fault. This fault strikes 3258 and dips
608. It displays pure dip-slip striations that plunge608 2388SW. Bedding dips range between3068208SW and 3308248WSW. The mine liesvery close to the Mae Ping fault zone, just akilometre or two south of one of the main faultstrands (Fig. 3). There is little evidence in theoutcrop for strike-slip deformation. Instead, themain normal fault and two secondary faults showalmost pure dip-slip motions.
In the Mae Lamao Basin, deposits over 500 mthick comprise conglomeratic claystone andsandstone at the base, overlain by shales, oilshales, coal and sandstone. The palynology indi-cates a Late Oligocene–Early Miocene age (Rata-nasthien 1989), which is presumably also the ageof the normal faulting. The coal seams pass abruptlylaterally into thick conglomeratic sequences, indi-cating that the sediment from an adjacent upliftedarea was dumped into the basin. There is little post-Miocene deposition except for fluvial deposits, indi-cating Early Miocene or later uplift and erosion.Vitrinite reflectance values from the coals average0.45 (Ratanasthien 1989), i.e. the rocks experiencedmaximum temperatures of about 120 8C. If a 30 8Csurface temperature is assumed, then, for ageothermal gradient of 3 8C/100 m, burial to3 km is indicated. If a much higher rift-typegeothermal gradient of 6 8C/100 m is assumed,then burial to 1.5 km is indicated. These numberssuggest that a considerably thicker, more extensivebasin existed in the past and was removed by EarlyMiocene or later uplift and erosion. The basingeometry appears to be that of a simple upliftedand eroded rift. Hence, it is uncertain whetherstrike-slip motion was responsible for basin uplift,or whether it was just part of the more regionaluplift and erosion event.
Mae Sot Basin
The Mae Sot Basin is one of the larger rift basins innorthern Thailand, and lies just south of the MaePing Fault (Figs 2 & 3); hence, its evolution is ofgreat interest for understanding the activity of theMae Ping Fault. Unfortunately, there is little infor-mation in the public domain about the basin.Gibling et al. (1985) show a Bouguer gravity mapfor the Mae Sot Basin which comprises two enechelon NNW–SSE-trending gravity lows, whichare 10–20 milligals less in magnitude than theareas of outcropping pre-Cenozoic basement. Thelargest anomaly indicates a Cenozoic basincentred around Mae Sot about 20 km long and10 km wide. They also report on a drill-hole(DDH 3-5) made by the Department of MineralResources (DMR). The drill-hole penetrated833 m of Cenozoic strata, dominated by carbonate
C. K. MORLEY ET AL.338
mudstones and oil shales, without reaching base-ment. The well was drilled near an outcrop of oilshales reported in Gibling et al. (1985).Huminite-reflectance values for the section rangebetween 0.25 and 0.34%, equal to soft brown coalrank (Gibling et al. 1985), and vitrinite reflectancevalues for the outcrops range between 0.25 and0.4%, suggesting a sedimentary cover 800–1100 mthick that has subsequently been uplifted anderoded. Watanasak (1989) sampled the DMR IMS1borehole in the Mae Sot Basin from 866–454 mand, on the basis of palynology, determined anearly Middle to late Early Miocene age for thesection (i.e. probably in the age range of 18–13 Ma). The outcrop described by Gibling et al.1985) is folded into a syncline. This outcrop indi-cates that compression/transpression affected thebasin sometime after the Early Miocene.
A 1997 vintage seismic line across the Mae SotBasin is presented on website http://www.ccop.or.th/epf/thailand/thailand_petroleum.html. Figure 8is a line drawing of the seismic line, showing thatthe basin has a half-graben geometry, expandingto the west. The seismic line also shows foldingassociated with basin inversion, and confirms theoutcrop observations made around Mae Sot aswell as the general basin geometry as determinedby gravity data. Assuming an interval velocity of3000 m sec–1, then the maximum depth of thebasin on the seismic line is about 2600 m. Hence,it is very likely that a Late Oligocene–EarlyMiocene section is present in the basin, below thesection penetrated by the IMS-1 well.
Mae Tuen coalfield
The Mae Tuen coalfield is located in a small basinthat lies along the northern trend of the Mae Pingfault zone, north of Lan Sang national park(Fig. 3). Ratanasthien (1990) describes the MaeTuen coalfield as having early coals (LateEocene–Early Oligocene, i.e. probably in therange of 36–32 Ma) unconformably overlain byLate Oligocene–Early Miocene strata. Vitrinitereflectance values from the coals are high (about0.66% Ro, Ratanasthein, pers. comm., 2005).These values suggest uplift in the order of 2 km ifa high (6 8C/100 m) geothermal gradient isassumed. Modern geothermal gradients associatedwith Thailand rift basins range between about3 8C and 7 8C (see Morley et al. 2001 for areview). The modern values occur at a time whenrifting has largely ceased or is very minor, yetthey are high, and would seem to indicate a rangeof gradients appropriate for syn-rift times as well.
The relatively old age of the Mae Tuen Basin isunusual considering that other coal mines in north-ern Thailand exploit the reserves in basins of LateOligocene–Miocene age (e.g. Ratanasthien 2002).The Eocene–Early Oligocene age is concomitantwith biotite 40Ar/39Ar and zircon fission-trackcooling ages in the Lan Sang area (Fig. 5). Hence,there is a strong indication that extensional collapseand basin formation occurred on the northern sideof the Lan Sang Gneiss region during strike-slipdeformation. Widening of the Tak–Mae Sot roadjust south of the Lan Sang national park has cut
Pre-CenozoicBasement
Early Middle Miocene-late Early Miocene (probably about 18Ma to 13 Ma)W E
About 2.5 kmdepth
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Fig. 8. Line drawing of a seismic line through the Mae Sot Basin, on website http://www.ccop.or.th/epf/thailand/thailand_petroleum.html
MAE PING FAULT ZONE, WESTERN THAILAND 339
into sediments of the Mae Tuen Basin. The road cutting has revealed a poorly sorted conglomerate,including boulder-sized clasts composed of meta-morphic rocks typical of the Lan Sang area, cutby minor normal faults. There are no shalespresent that could be used for dating. However,the coarse, immature deposits of metamorphicrock clasts are consistent with deposition adjacentto a rapidly uplifted and eroded region.
Structural evolution of the Mae Ping
fault zone
The still limited, but more regional, data reviewundertaken in this study shows that the region ofthe Lan Sang Gneisses where the Oligocene micacooling ages have been obtained (Lacassin et al.1997) is very limited geographically (Fig. 5). TheOligocene ages from Lan Sang are bracketed tothe NW and SE by Eocene–Late Cretaceousbiotite, ZFT and AFT cooling ages (Figs 4, 5 &7). Hence, the area with the highest number ofcooling ages from the fault zone is not really repre-sentative of the history of exhumation along theentire fault zone. This is a demonstrably largefault zone in outcrop and on satellite images, azone which extends hundreds of kilometres intoMyanmar (Lacassin et al. 1997; Morley 2004).However, accurate quantification of the displace-ment has not yet been achieved: Lacassin et al.(1993) estimated a minimum 40 km of sinistral dis-placement based on shear-zone geometries. Usingthe offset of the regional geological markers,Lacassin et al. (1997) estimate about 150 km sinis-tral displacement, whilst the regional rigid-platereconstructions of Replumaz & Tapponnier (2003)require up to 240 km of 40–30 Ma sinistralmotion. The Replumaz & Tapponnier (2003) esti-mate is model-driven and is not constrained by geo-logical markers, whereas the 150 km estimate isbased on a generalized, but reasonable, offset ofgranitic outcrops, and is the preferred estimatehere. However, detailed geochemical typing ofoffset granites is really necessary to demonstrateoffset of the same granite body and to obtain areasonably constrained offset estimate. Despite theprobable large displacements, the exhumationhistory along the fault zone is highly variable andcertainly not consistently in the range of the 33–30 Ma ages determined by Lacassin et al. (1997)(Fig. 5). This section discusses how the availablecooling ages can be used to explain the structuralevolution of the Mae Ping fault zone.
The early history of a Mae Ping fault zone aspart of a transpressional orogen spanning the LateCretaceous–Palaeogene, related to collision of theBurma Block with the Shan Thai Block, has been
discussed by Morley (2004), and is not discussedin detail here. The oldest cooling ages in western-most Thailand (Figs 4 & 5) are part of the evidencefor that orogenic event. The starting point for thisdiscussion is the Eocene–Oligocene history of thefault zone. Within the Chainat Ridge area theUmphang Gneiss to the west shows rapid exhuma-tion within the time span of 50 Ma to 40 Ma(Figs 7 & 9). Cooling-age data south of theUmphang Gneisses are sparse (Fig. 4), but region-ally appear to fit a north–south trend of Late Oligo-cene–Early Miocene AFT ages that extend frompeninsular Thailand up to northern Thailand(Morley 2004). Hence, the Umphang Gneissesappear to be a patch of locally older exhumation,on the western margin of the Mae Ping fault zone,consistent with exhumation at the restraining bendof a sinistral strike-slip fault system (Fig. 9).However, the subsequent history of the Mae Pingfault zone in the area does not follow such asimple interpretation.
The outcropping geology in the Lan Sang areashows that the deepest crustal levels exposedalong the Mae Ping fault zone are not in therestraining-bend area to the SE, but along a NW–SE segment of the fault. This uplift occurred from36 Ma to 30 Ma (Lacassin et al. 1997; Upton1999; Figs 4, 5, 9 & Table 1), with the ages young-ing from the NW to the SE. The most intenselydeformed part of the fault zone is a belt of gneissesand mylonitic metasediments about 6 km wide,within which are more highly deformed zones ofultramylonite (particularly calc-silicates andmarbles) typically about 1 km wide (Lacassinet al. 1993). Assuming simple shear, Lacassinet al. (1993) estimated lower bounds of 7 to 9+ 3for the shear strain (g)% within the mylonitezones. The strain estimate implies a minimumof 35–45 km sinistral displacement within ac. 5-km-wide shear zone (Lacassin et al. 1993).The latest sinistral shear occurred along a retro-grade P/T path, and progressed from ductile defor-mation to below the brittle–ductile transition(Lacassin et al. 1993; 1997). Commonly, small-scale conjugate brittle faults cross-cut the ductileshear zone fabrics. They tend to strike east–west(sinistral shear sense) and NNE–SSW (dextralshear sense). Displacements are typically in theorder of centimetres to metres, although a fewmay display tens of metres of displacement. Theconjugate brittle faults indicate that the horizontalprincipal stress was (at least locally) approximatelyperpendicular to the strike of the gneissic foliationduring their formation. In Figure 10b, the shearedmid-crustal rocks seen in Lan Sang are restored toa position east of the Umphang Gneiss. In this pos-ition they would have occupied the first Chainatduplex area. During progressive simple shear, the
C. K. MORLEY ET AL.340
Fig. 9 Proposed evolution of the Mae Ping fault zone to fit the cooling ages and outcrop patterns discussed in this study. Note: motions during the later stagesof deformation (30 Ma–present) were probably small (a few kilometres at most), and hence appear insignificant on the maps.
MAEPIN
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Umphang Gneiss region First Chainat duplex
(a)
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. X
Present
~34-32 Ma
~40 Ma
Fig. 10. Schematic cross-section illustrating the structural evolution of the Mae Ping fault zone. The eastern half(ii) of the cross-section is kept in a constant location, equivalent to the location of the Lan Sang Gneisses today,whereas the western half (i) of the cross-section changes with time as section (ii) moves north to NW duringsinistral displacement. Hence, for section (c), (i) is through the Umphang Gneiss area, whilst, for section (a),(i) is through the Mae Sot and Mae Lamao basins. Section a shows the present-day configuration; however, most ofthe uplift of the Lan Sang Gneiss was completed by the Late Oligocene, and apart from some erosion and minorstrike-slip motion, and development of the Mae Sot and Mae Lamao basins, the geometry of the strike-slip faultzone is likely to have been similar from the Late Oligocene onward. The near-surface Lan Sang Gneissgeometry is based on the cross-section in Lacassin et al. (1997). Section (b) represents c. 34 Ma to 32 Ma ago. Sincethe Lan Sang Gneiss had to undergo vertical thickening and uplift to be exposed today, restoration of the gneissesthrough the bend requires that the region of strike-slip deformation becomes broader, with the amalgamated shear zones
C. K. MORLEY ET AL.342
duplex was translated, became subject to horizontalsimple shear, and became narrower. Verticalthickening is required to produce exhumation ofthe Palaeozoic–Mesozoic cover and retrograde P/T conditions within the Lan Sang Gneisses.
Whilst some erosion of the Chainat duplex hasoccurred, it is noticeable that the area of theduplex is dominated by Palaeozoic–Mesozoic sedi-mentary, metasedimentary and igneous rocks.Deeper crustal levels are exposed only where theKhlong Lhan Gneisses crop out in the NWcorner of the duplex. The Khlong Lhan Gneissesshow a cooling history (ZFT ! 40+ 1 Ma,AFT ! 20+ 1 Ma) different from the adjacentUmphang Gneiss (ZFT ! 47+ 3 Ma, AFT!40+ 2 Ma) (Fig. 7). In Figure 9, the history ofthe Khlong Lhan Gneiss is explained as early exhu-mation occurring during entry into the restraining-bend area in the south of the duplex, and later exhu-mation where the gneisses entered the northernbend of the duplex. This interpretation of thehistory of the Khlong Lhan Gneiss implies thatthe main exhumation of the Lan Sang Gneissesdid not occur at the obvious restraining-bend geo-metry, but as the rocks entered and turned thecorner of the bend, passing from a north–south toNNW–SSE-striking fault segment to the NE–SW-striking segment. The cooling-age data are con-sistent with this interpretation, the oldest (36–33 Ma) ZFT and biotite cooling ages in the LanSang Gneisses come from the NW area, whilst theyoungest (30 Ma) come from the SE. Whilst thedata-set is not sufficient to be definitive, thesedata fit with rocks entering the bend and thenbeing uplifted and eroded. Probably the exhumed,cooled, and thus relatively strong, region of theUmphang Gneiss acted as a hard anvil or buttressat the bend in the Mae Ping fault trace, and servedto focus stresses, as rocks to the NE were translated,uplifted and flattened when passing throughthe bend.
One of the key questions arising from the modelfor exhumation of the Lan Sang Gneiss is why wasthe Umphang Gneiss area exhumed first, then failedto continue reactivating, but instead acted as thehard, resistant buttress against which the Lan SangGneisses were flattened and sheared? The answer
may lie in the granite intrusions prevalent inwestern Thailand. Malaysia, Myanmar andThailand are famous for their extensive suites ofgranitic rocks, in particular those formed duringTriassic and Cretaceous orogenic events (e.g. Beck-insale et al. 1979; Charusiri et al. 1993). TheUmphang Gneiss region is a mixture of para- andortho-gneisses intruded by granites. Formation ofgranite melts depletes the lower crust of radiogenicmaterials and concentrates them in the granites (forexample, see the discussion by Sandiford &McLaren 2002). Granite intrusion then transfersthose radiogenic materials to higher levels of thecrust. Uplift and erosion such as that seen in theUmphang Gneiss region would then remove muchof the radiogenic granite to sedimentary basins,and elevate the remaining granite to very highlevels in the crust. Consequently, the underlyingarea of gneiss is likely to be depleted in radiogenicmaterial; have lower geothermal gradients than thesurrounding regions; and thus be relatively cold andstrong. Hence, the exhumation to the highest levelsof the crust of the Umphang Gneiss would havebrought the stronger granitic rocks west of theKhlong Lhan restraining bend into contact withweaker sedimentary and metasedimentary rockseast of the Khlong Lhan restraining bend. Deeperin the upper and middle crust, once the depleted,less-radioactive crust cooled, the Umphang Gneissarea would have been colder than the adjacentradioactive granitic Khlong Lhan gneiss regioneast of the Khlong Lhan restraining bend. Thus,the buttressing effect of the Umphang Gneisswould have developed, due to both mechanicaland thermal variations in the upper crust, west andeast of the Khlong Lhan restraining bend.
One feature of the northern boundary of the LanSang Gneisses is a sharp contact with an adjacentCenozoic basin, mapped as a northward dippingnormal fault by Lacassin et al. (1997) (Fig. 3).This Cenozoic basin contains the Mae Tuen coal-field with its Late Eocene–Early Oligocene coalsunconformably overlain by a Oligocene–EarlyMiocene section (Ratanasthien 1990). Lacassinet al. (1997) interpreted the normal fault as a LateOligocene feature. However, the coalfield data indi-cate that the normal fault probably operated during
Fig. 10. (Continued) seen today at Lan Sang becoming more widely separated as they enter the northern bend ofthe Khlong Lhan restraining bend. Section (c) represents c. 40 Ma ago, prior to translation of the Lan SangGneisses around the northern bend. The Umphang Gneiss area was uplifted, cooled and became inactive. Therestraining bend had begun to develop a number of strike-slip fault strands east of the Umphang Gneiss, whichwould develop into the Lan Sang Gneiss region. The principal strike-slip fault zones are shown as brittle faults in theupper crust passing into narrow, but broader shear zones in the brittle–ductile transition and the upper part ofthe lower crust. It is uncertain whether the fault zone should be drawn as a narrow, discrete zone all the way throughthe crust to the Moho (e.g. Leloup et al. 2001), or whether the fault zone passes into typical lower-crustal flat-lyingshear zones as strike-slip motion becomes accommodated by lower-crustal flow. Since the crust is likely to be hot inthis area (Hall & Morley 2004), the lower crust is depicted as deforming along low-angle shear zones.
MAE PING FAULT ZONE, WESTERN THAILAND 343
sinistral displacement as well. Hence, the inferenceis made here that whilst passing through the bendthe sheared and uplifted, hot and thickened area ofLan Sang Gneisses underwent extensional collapseon the northern side, whilst being overthrust to theSW on the southern side of the shear zone(Fig. 10). The evidence from the modelled coolinghistories using K-feldspar (Lacassin et al. 1997)indicates rapid cooling from 400 to 185 8Cbetween 32.5 Ma and 31 Ma, which is consistentwith exhumation occurring in a short burst, andnot progressively throughout the strike-slip historyof the fault zone. Movement through the bend,uplift, and concomitant extensional unroofing areinterpreted here to be the reason for the narrowrange of cooling ages.
During the Late Oligocene–Early Miocene (i.e.c. 28–22 Ma) there was a period of extensive rift-basin formation, from the Gulf of Thailand, all theway up to northern Thailand (as reviewed byMorley et al. 2001). Adjacent to the Mae Pingfault zone, several rift basins developed (the MaeSot, Mae Lamao, Phitsanulok, Suphan Buri andAyutthaya basins). The regional extent of thesebasins suggests a major change in regional stress,probably from an approximately east–west SHmax
direction favourable for sinistral strike-slip defor-mation, to a north–south SHmax direction appropri-ate for east–west extension (e.g. Huchon et al.1994; Morley 2002).
The Chainat duplex area is a region of uplift,but, east of the Umphang Gneiss, deep levels ofthe crust are not exposed, despite having arestraining-bend geometry under sinistral motion.Relatively young uplift is supported by the 22–18 Ma range of three AFT central ages from theduplex area. As discussed in Smith et al. (2007)uplift within the duplex approximately coincideswith the cessation of extension in the Lahangraben immediately north of the duplex, and aphase of inversion within the southern PhitsanulokBasin (Bal et al. 1992). The interpretation thereforeimplies a short-lived phase of minor (in the order ofkilometres of horizontal displacement) sinistralmotion occurred along the Mae Ping fault zone inthe Early Miocene and contributed to the presentduplex geometry.
The Mae Lamao and Mae Sot basins may haveopened under dextral motion on the Mae Pingfault zone, but an oblique extensional origin isalso possible. Satellite images show fault strandsbranching off the Mae Ping fault zone and linkingwith basin-bounding faults. However, whether thebasins are just reactivating older strike-slip trendsor are kinematically linked remains uncertain. Inthe basins the youngest rift fill is of EarlyMiocene to early Middle Miocene age; there areinversion structures in the Mae Sot Basin; and
coal maturity points to removal of somewherebetween 1.5 and 3 km of section from the MaeLamao Basin. These data point to an uplift eventof Middle Miocene or younger age. A known LateMiocene inversion event associated with sinistraldeformation on NW–SE-trending faults affectsthe Phitsanulok and Ayutthaya basins (Bal et al.1992; Smith et al. 2007, this volume), and hencemay also fit with the Mae Sot and Mae Lamaouplift history.
Vertical extent of strike-slip shear zones
There are two main models for the way that thelarge escape tectonics related shear zones mightbe behaving in SE Asia. In one model the shearzones penetrate the entire crust and upper mantle,and a broadening – but comparatively narrow anddiscrete – zone of simple shear (e.g. the RedRiver fault zone model of Leloup et al. 1995).The alternative model considers the fault zones tobe essentially upper-crustal features that die outinto broadly distributed shear within the middle orlower crust (e.g. England & Houseman 1989). Themodel in Figure 10 shows the Mae Ping fault zoneas dying out within the lower crust. This is notbecause there is definitive evidence for eithermodel, but because on balance it is the modelmost favoured by the data at present. First, thereare no melts along the Mae Ping fault zone that indi-cate that magma of mantle origin was being tapped,unlike the model for the Red River fault zone(Leloup et al. 1995). Second, in Yunnan, wherethere are numerous important strike-slip zones,there does appear to be evidence for strike-slipfaults dying out in the middle to lower crust –both from magneto-telluric data which indicatethe presence of a middle-crustal detachment layer(e.g. Bai & Meju 2003) and from seismic tomogra-phy which shows no evidence for any deep pertur-bation of layers vertically beneath the majorstrike-slip fault zones (Liu et al. 2000). In thecase of the Red River fault zone, it may follow amajor suture zone at the surface, but tomographyindicates that a relict Tethyan subduction zone atlower-crustal and mantle levels lies fifty or morekilometres west of the Red River fault zone (Liuet al. 2000). Hence, the upper-crustal zone of weak-ness does not appear to extend downward verticallythroughout the crust to favour the development of adeep-penetrating strike-slip fault zone.
Comparison with other models
of restraining-bend development
Cowgill et al. (2004) describe large restrainingbends in terms of thrust- and strike-slip-dominated
C. K. MORLEY ET AL.344
types. Thrust-dominated restraining bends displaymaximum uplift along the main length of therestraining bend, producing restraining-bend‘pop-ups’ (e.g. Wakabayashi et al. 2004). Thrust-dominated earthquake focal mechanisms fromrestraining bends in California, such as the SantaCruz bend, indicate that the vertical principalstress is the minimum principal stress (Hauksson& Jones 1988; Cowgill et al. 2004). Instrike-slip-dominated restraining bends, the verticalprincipal stress axis is the intermediate principalstress. Strain and uplift are focused on the areas ofchanging fault orientation entering and leaving therestraining bend (Cowgill et al. 2004). There isalso a tendency for the strike-slip fault to undergovertical-axis rotation to reduce the bend angle(Cowgill et al. 2004). The Akato Tagh bend alongthe Altyn Tagh Fault in China, is cited by Cowgillet al. (2004) as such an example.
The Mae Ping fault zone does not appear toshow a simple or constant pattern of deformationassociated with the restraining-bend geometry ofthe Chainat duplex area. The oldest documenteduplift began in the Umphang Gneiss region on thewestern margin of the duplex (Fig. 2), and mayhave spanned the time from about 50 Ma to40 Ma (Fig. 9). This uplift suggests a thrust-dominated Santa Cruz-type restraining-bendsetting (e.g. Hauksson & Jones 1988; Cowgillet al. 2004), where uplift of the gneisses occurredalong the restraining bend in the hanging wall of asteeply inclined, west-dipping transpressionalfault zone.
The next phase of deformation, during the Oli-gocene, appears to be very different in character,and involved extensive shearing and translationof the Lan Sang Gneiss around the northern bendin the fault zone just west of Tak (Fig. 9). TheKhlong Lhan Gneiss underwent uplift movinginto the restraining bend at about 40 Ma,and then appears to have been translated withonly moderate cooling until a second uplift eventoccurred at 20 Ma at the exiting bend of theChainat duplex. Conversely, the Lan SangGneisses moving around the exiting bend displayrapid Late Eocene–Early Oligocene cooling ages(Figs 5 & 7). This concentration of uplift at theentering and exiting bends is consistent with thestrike-slip-dominated restraining-bend model(Cowgill et al. 2004), with transpressional defor-mation just being locally concentrated at theexiting bend. The two styles are also consistentwith the regional tectonics, where early faultdevelopment occurred within a Late Cretaceous–Palaeogene transpressional orogen (Morley2004), whilst Oligocene reactivation occurredduring Himalayan escape tectonics (Lacassinet al. 1997).
McClay & Bonora (2001) presented analoguemodels for restraining-bend duplex geometries,and they thus generated a range of deformationstyles that changed according to: the amount of dis-placement; the angle between the restraining bendand the main strike-slip trend; and the width ofthe restraining bend. The last major stage of theChainat restraining-bend development is the for-mation of the present-day Chainat duplex, and itsgeometry appears to be quite appropriate for com-parison with the McClay & Bonora (2001) analoguemodels. In the Chainat duplex, the angle made bythe restraining bend with respect to the main faulttrend is about 358, hence the 308 stepover modelshown in McClay and Bonora (their fig. 3) is themost appropriate. In this model, the duplex is domi-nated by internal faults striking subparallel to therestraining bend, unlike higher stepover angles,where a wider range of fault angles is developed.The model pattern is reminiscent of the dominantNNW–SSE to north–south strike of ridges withinthe Chainat duplex, bounded by NW–SE-strikingfaults to the north and south (Fig. 2). It is quiteapparent from analogue models and descriptionsof natural examples of strike-slip duplexes (e.g.Laney & Gates 1996; McClay & Bonora 2001;Cunningham et al. 2003) that the relativelysimple, classic strike-slip duplex geometrybecomes complicated by a wide range of faulttrends, rotation of faults, and variable fault kin-ematics once large displacements becomeimposed. Whilst the comparative simplicity of theChainat duplex geometry might be misleading(and a function of exposure), the lack of stronguplift (and exposure of higher metamorphic-graderocks) within the duplex; the long, linear, uninter-rupted trend of the Jurassic ridge on the west sideof the duplex (Smith et al. 2007, paper 11, thisvolume); and the 22 Ma to 18 Ma AFT ages, allindicate that it is a comparatively young featurethat developed late in the history of the fault zone.It appears to represent the third incarnation ofuplift at the restraining bend.
During the Late Oligocene to Pliocene, the riftbasins of central and northern Thailand documenta series of extensional phases punctuated byperiods of inversion (e.g. Morley et al. 2000,2001), and testify to a rapidly evolving stressregime. Two episodes of inversion during theEarly Miocene and the latest Miocene to Early Plio-cene appear to be quite widespread (Morley et al.2000; 2001), but at least four episodes of inversionhave been recorded in some basins (Bal et al. 1992;Morley et al. 2000). Probably the dominant stressregime was extensional, with SHmax orientedapproximately north–south, as it is today (Bottet al. 1997). The orientations of inversion-relatedfolds, inverted normal faults, and episodically
MAE PING FAULT ZONE, WESTERN THAILAND 345
active strike-slip faults indicate that during episodesof inversion the stress regime may have rangedfrom strike-slip to compression, and the SHmax
direction ranged between north–south and east–west (Morley et al. 2000, 2001). This briefsummary of regional data indicates that the latesthistory of the Chainat duplex was characterizedby short episodes of activity during phases ofbasin inversion, and there is clear structural evi-dence for sinistral motion within the duplex, fromfolded Mesozoic rocks and fault kinematic data(Smith et al. 2007).
The NW–SE trending Three Pagodas Fault tothe south has Late Cenozoic basins developed atnorth–south releasing-bend geometries (Morley2002). The low-level earthquake activity thataffects northern and western Thailand today isdominated by dextral strike-slip fault-plane mech-anisms on NW–SE-striking faults and sinistralfocal mechanisms for NE–SW-striking faults; theSHmax direction is approximately north–south(e.g. Bott et al. 1997; Morley 2004). From thesetwo lines of evidence and the observed dextralslickensides within the duplex, it is concludedthat the Chainat duplex was also reactivatedepisodically under minor dextral motion.
Conclusions
The study by Lacassin et al. (1997) remains vitallyimportant to our understanding of the Mae Pingfault zone, but highlights the problem of drawingconclusions from a geographically limited area ofthe fault zone. Other parts of the fault do notshow the same cooling-age histories. Both to theSE (Umphang and Khlong Lhan Gneisses, Fig. 7)and the NW (Fig. 5) of the Lan Sang Gneissescooling ages become older. The rapid coolingages of the Lan Sang area do not appear to be repre-sentative of the entire fault zone, or even a longsegment of it, but instead record an unusual exhu-mation event, interpreted here to be a passagearound the exiting restraining bend. In addition,the regional north–south trend of cooling-age pat-terns seen for biotite, ZFT and AFT data (Figs 5& 6) indicates that, at least in part, exposure ofthe Lan Sang Gneisses is related to more regionalexhumation patterns than strike-slip specific upliftand erosion.
Given the available range of major structures inthe area (large rift basins, pull-apart basins, low-angle extensional detachments, major strike-slipfaults, strike-slip duplexes, the ‘extensional col-lapse’ normal fault north of Lan Sang) and theavailable range of cooling ages (and associateddata such as sedimentary-basin history), ourability to construct the structural model remains
limited, and numerous questions remain outstand-ing. Considerably more supplementary data isrequired to test the models presented in this paperand to develop a good understanding of the relation-ships between different structural styles. Forexample, the way that the region of ‘metamorphiccore complexes’ west of Chiang Mai, down to theMae Sariang–Hot highway (between arrows c–c0,Fig. 4) connects with the Mae Ping fault zone isuncertain. The Umphang Gneiss appears to be anisland of Eocene exhumation in the westernranges, surrounded by Oligocene–Miocenecooling ages, but again data south and west of thegneisses are very sparse and additional informationis required to fill in the gaps in our knowledge.
Despite the caveats associated with the interpret-ation of the data and its limitations, a fairly detailedmodel for the evolution of the fault zone has beenproposed in this paper, and can be tested in futurestudies. The Cenozoic history of the predominantlysinistral Mae Ping strike-slip fault zone shows con-siderable strain in the vicinity of the Khlong Lhanrestraining bend. This deformation can be under-stood in terms of models proposed for otherrestraining beds (strike-slip v. thrust-dominatedrestraining bends Cowgill et al. 2004) and analoguemodes of early restraining-bend deformation(McClay & Bonora 2001). Initial uplift anderosion on the western side of the restraining bendunroofed the Umphang Gneisses during theEocene, probably in a thrust-dominated restraining-bend context. Later, as regional deformationevolved from a transpressional orogen related toterrane collision, to escape tectonics associatedwith the main India–Eurasia collision (Morley2004) the restraining bend shows strike-slip-dominated characteristics (Cowgill et al. 2004).Passing through the northern (exiting) bend in therestraining bend, the northern side of the faultzone was subject to extensive simple shear and ver-tical thickening, resulting in uplift, erosion andextensional unroofing during passage through thebend. The resulting 5–6-km-wide mid-crustalshear zone exposed at Lan Sang records coolingages consistent with this passage through thebend. Possibly prior to flattening and simple shearpassing through the bend, this zone was originallysome 40–50 km wide. The final phase ofrestraining-bend deformation (Late Oligocene–Recent) occurred under a complexly evolvingstress field when episodically relatively small dis-placements (probably totalling a few kilometres ofmotion) with both sinistral and dextral sense ofmotion affected the Chainat duplex area.
M. Smith would like to acknowledge the Universiti ofBrunei Darussalam and the AAPG Foundation Grants inAid (2003) scheme for providing the funding for fieldwork
C. K. MORLEY ET AL.346
and associated analytical costs. C. Morley thanks the Uni-versiti of Brunei Darussalam for funding for fieldwork andsample analysis. Sarawute Chantraprasert was funded bythe Faculty of Science, Chiang Mai University. Residualmagnetic-anomaly data used in fault interpretation wereprovided by the Department of Mineral Resources, Minis-try of Natural Resources and Environment, Thailand.
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