Structural analysis of the accretionary complex in Sirikit Dam area, Uttaradit, Northern Thailand

Journal of Southeast Asian Earth Sciences, Vol. 8, Nos I-4, pp. 233-245, 1993

0743-9547/93 $6.00+ 0.00 © 1993 Pergamon Press Ltd

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Structural analysis of the accretionary complex in Sirikit Dam area, Uttaradit, Northern Thailand SAMPAN SINGHARAJWARAPAN* a n d RONALD F. BERRY'~ *Department of Geological Sciences, Chiang Mai University, Chiang Mai 50002, Thailand; and tGeology Department, University of Tasmania, G. P. O. Box 252C, Hobart, Tasmania 7001, Australia Al~tract--The structure, strain history, and metamorphism of a multiple deformed sequence of metagreywacke and phyllite in the Sirikit Dam area have been investigated. Tectonically enclosed in this sequence is the complex zone of mafic and ultramafic rocks known as the "Nan River Ophiolite Belt" which has been interpreted as the suture between Shan-Thai and Indo-China terranes. The whole sequence is in thrust contact with less deformed massive volcanics in the eastern part of the study area. The dominant structure in the sequence consists of strongly developed foliation (Sd, slaty cleavage in phyllite and spaced cleavage in metagreywacke) which is axial planar to tight to isoclinal folds and dips shallowly towards the northwest. Transposition of the bedding is so strong that folds are rarely observed. The stretching lineation on the foliation surface (Sd) plunges down-dip towards the west-northwest. S-C fabrics, asymmetric porphyroclasts and asymmetric pressure shadows developed in narrow zones of higher strain consistently indicate reverse sense of shear. The slaty cleavage (Sd) is invariably crenulated and superimposed by a set of asymmetric kinks. The last ductile deformation style is a series of uptight open folds trending north-northeast. Brittle deformation occurred later and produced northeast striking normal faults which are better developed in massive volcanics and are locally reactivated as strike-slip faults. The lithology, style of deformation, and metamorphism of the Sirikit Dam metagreywackes bear strong similarity to those observed in ancient and modern accretionary complexes in many parts of the world. The tight-isoclinal folds, Sd, stretching lineation, and crenulations are interpreted as the direct result of non-coaxial progressive deformation due to east-directed aecretionary thrusting and kinks are probably related to late accretionary thrusting. We correlate the uptight open folds in the study area with the upright folds in the Triassic turbiditic greywacke sequence exposed along Lampang-Phrae and Phrae--Uttaradit highways. This late deformation is possibly the result of the collision between Shan-Thai and Indo-China terranes. Normal and strike-slip faults are due to Tertiary tectonic events related to the northward movement of the Indian plate.

INTRODUCTION THIS paper deals mainly with styles and development of structures in the multiple deformed sequence of subgreenschist to lower greenschist facies rocks in Sirikit Dam area which has been inferred to represent sediments of inner-trench slope (Bunopas, 1981). The sequence resembles several ancient accretionary prisms in many respects, e.g. Kodiak Complex in Alaska and Calaveras Complex in California (Paterson and Sample, 1988), Shimanto Complex in Japan (Needham and MacKenzie, 1988), Torlesse terrane in New Zealand (MacKinnon, 1983), New England fold belt, eastern Australia (Fergusson et al., 1990), Southern Uplands in Scotland (Knipe and Needham, 1986), and Dunnage zone in Newfoundland (Van de Pluijm, 1987). In comparison with these well studied terrains, we have found that the development of the structures in the Sirikit Dam area can be reasonably explained in the context of accretion tectonics and can be interpreted in terms of processes responsible for the evolution of accretionary prisms as proposed by Knipe and Needham (1986). In addition, metamorphic processes involved in the development of the complex have been considered, as this has been proved to be a very useful tool to complement the structural work and aid interpretation of the accretionary process (Kemp et al., 1985; Offler et al., 1987). Accretionary complexes are the product of transfer of material from the downgoing plate to the overriding

plate during subduction. They are characterized by imbricate fault systems containing either intensely disrupted strata or virtually coherent sedimentary sequences. In earlier models, it has been proposed that turbiditic trench-sediments are accreted to the toe of the accretionary prism by offscraping process (Seely et al., 1974; Karig and Sharman, 1975) causing the seaward growth of the prism. However, it has recently been realized that a significant amount of sediment is also underthrust to a substantial depth before being incorporated to the base of the overlying prism by underplating or subcretion process (Moore et al., 1982; Silver et al., 1985; Sample and Fisher, 1986). Duplexing has been proposed as a mechanism for underplating (Fisher, 1984; Silver et al., 1985). Deformation processes operating within the accretionary complexes can be extremely diverse and complex (Moore et al., 1985). However, Sample and Moore (1987) have noted that structures and fabrics commonly developed during offscraping are stratal disruption, cataclastic shear zones and scaly foliation whereas rocks deformed at greater depth are likely to possess well developed cleavage as a result of diffusional mass transfer or recrystallization of quartz or phyllosilicates and other phases. In ancient accretionary complexes, additional complication may be introduced by later deformation due to post-accretion collision and uplift (Knipe and Needham, 1986). Contemporaneous with deformation is the metamorphism of rocks that have been

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SAMPANSINGHARAJWARAPANand R. F. BERRY

carried downwards and subjected to higher temperature and pressure. Vrolijk (1987) and Paterson and Sample (1988) have shown that the geothermal gradient in some accretionary complexes is higher than that predicted by standard conductive heat flow models (e.g. Turcotte and Schubert, 1973). Several accretionary complexes have medium P-T metamorphism rather than the anormalously high P-T metamorphism of typical blueschist terranes.

GEOLOGICAL SETTING The Sirikit Dam area covers part of the inferred "Nan River suture zone" which consists of a belt of ophiolitic mafic-ultramafic rocks (Barr and Macdonald, 1987) flanked by metasedimentary rocks on both sides (Fig. 1). This belt extends northeastwards from Uttaradit along the Nan River to Luang Prabang and Dien Bien Phu in Laos (Hutchison, 1989) and has long been believed to represent the suture zone that mark the boundary be-

tween Shan-Thai terrane to the west and Indo-China terrane to the east. The timing of collision between the two terranes is still a subject of controversy due to the lack of reliable biostratigraphic and radiometric dating of the key lithological units. Some workers believe that the collision took place in Middle-Late Triassic (Hutchison, 1975, 1989; Bunopas and Vella, 1978, 1983; Panjasawatwong, 1991) or Early Triassic (Thanasuthipitak, 1978; Cooper et al., 1989) while the others proposed Middle Permian (Helmcke and Lindenberg, 1983; Helmcke, 1985, 1986; Barr and Macdonald, 1991). Recent geological mapping and petrological studies by Hada (1990) and Panjasawatwong (1991) have shown that the belt is in fact a serpentinite melange zone characterized by chaotic blocks of a great variety of rocks of diverse tectonic settings and ages enclosed in serpentinite matrix. The Nan River ophiolite belt is indeed composed of a mixed variety of rocks. The igneous rocks are composed of andesite, basalt, dolerite, microgabbro, gabbro, hornblendite, pyroxenites, and peridotites. Peridotites are

Tertiary-Qusternary Sediments

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Sirikit Dam area, Uttaradit, Northern Thailand almost completely altered to serpentinite. The metamorphic rocks are mainly amphibolite, epidote amphibolite, and garnet amphibolite (Macdonald and Barr, 1984; Barr and Macdonald, 1987; Panjasawatwong, 1991). Chert is a major sedimentary rock present in the melange and limestone is a minor component. The ages of these rocks vary markedly, e.g. Hada (1990) reported that most of the radiolaria in red chert blocks indicate Early to Late Permian while some are possibly Triassic. Panjasawatwong and Crawford (in preparation) using Ar-Ar dating found that basic lavas and associated plutonic rocks have the age range of 356-256 Myr (Early Carboniferous-Early Permian) with the exception that one rock sample yields the age of 97 _+ 7 Myr (Middle Cretaceous). Bordering the ophiolite suite on both sides are very low to low grade metamorphic rocks belonging to Pha Som Group of Bunopas (1981). The dominant rocks of the Pha Sore Group are banded quartzite, quartzitic phyllite, phyllite, muscovite~luartz schist, epidotequartz schist, actinolite~luartz schist, chlorite schist (Thanasuthipitak, 1978; Bunopas, 1981), and epidotecrossite schist (Barr and Macdonald, 1987). On the basis of regional lithological correlation, Bunopas (1981) proposed a Silurian-Devonian age for this rock unit whereas Hess and Koch (1975) assigned it a Carboniferous-Permian age. Barr and Macdonald (1987), however, suggested Early Permian (269 +_ 12 Myr) as a minimum metamorphic age according to K-Ar dating of associated actinolite~luartz schist in the Pha Som Group. The Nan River suture zone became active as a major dextral strike-slip fault during Tertiary time (Hutchison, 1989) and its extension to the southwest of the study area was displaced eastwards by northwest-trending sinistral Mae Ping fault. Polachan and Sattayarak (1989), in contrast, suggested that the movement of these strike-slip faults changed to the opposite sense in the Late Miocene leading to the formation of widespread northsouth trending pull-apart basins in northern Thailand.

LITHOLOGY AND STRATIGRAPHY The rocks in the study area can be divided into six lithological units, namely, mafic-ultramafic rocks, piemontite-bearing quartz schist, metagreywackes and phyllite, and greywackes and slate of the Carboniferous-Permian age, Permo-Triassic volcanic and volcaniclastic rocks and Early Triassic sandstone and shale which are in fault contact with one another as illustrated in Figs 2 and 3. Mafic and ultramafic rocks consist of pyroxenite, amphibolite, zoisite amphibolite and garnet amphibolite which show strong foliation. The amphiboles in garnet amphibolite are magnesio-hornblende to tschermakite whereas those in zoisite amphibolite are edenitic hornblende according to the amphibole classification of Leake (1978). Clinopyroxene in amphibolite is diopside. Garnets are almandine with high proportion of grossular (AI45Py~sGr40). Peridotites including dunite are found

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in some outcrops and are always serpentinized. Also included in this unit are chromitite and green chert. The age of this unit varies from Early Carboniferous to Late Permian (Panjasawatwong and Crawford, in preparation; Hada, 1990). The piemontite-bearing quartz schist is composed mainly of quartz with subordinate platy phengitic white mica and prismatic piemontite (Pm~sPs~sCz67) which commonly show parallel alignment that give rise to schistose texture of the rock. Colourless chlorite (clinochlore) is a minor constituent together with accessory hematite and ilmenite. The age of this rock unit is not known but it is likely to be pene-contemporaneous with mafic-ultramafic rocks. Metagreywackes and phyllite are the dominant rock types in the study area. They consist of banded quartzite, quartz semischist, quartzitic phyllite and phyllite. The metagreywackes are well-foliated and composed of quartz, white-mica, albite, chlorite, calcite, epidote, with accessory sphene, apatite, pyrite and other opaque phases. Phyllite consists of white-mica, chlorite, quartz, albite, calcite, and accessory pyrite and graphite. The compositions of white-micas vary from slightly phengitic to ideal muscovite. Chlorites are either brunsvigite or ripidolite according to the classification of Foster (1962). Following Barr and Macdonald (1987) the age of this unit may be assigned as Late Carboniferous Early Permian. Greywackes with associated slate are moderately foliated. They consist mainly of white-mica and quartz with a large amount of detrital albitized plagioclase, biotite (replaced by chlorite) and rock fragments. Sedimentary textures are largely preserved. The age of this rock unit is also not well constrained due to the unfossiliferous nature of the rocks but it is possibly Late Carboniferous-Early Permian as suggested by regional stratigraphy. Volcanic and volcaniclastic rocks are basically dacitic to rhyolitic in composition. The rocks are relatively altered. Large fragments of limestone, conglomerate, granite, and volcanics are abundant in volcaniclastic rocks. A Permo-Triassic age has been inferred for this unit (Bunopas, 1981). The sandstone and shale unit is relatively unmetamorphosed marine turbidite sequence. Fossil algae found in this unit indicates an Early Triassic age (Lumjuan and Sinpoolanant, 1987).

ACCRETION-RELATED STRUCTURE To simplify the deformation scenario and for the sake of clarity, an attempt has been made to group the structures into generations bearing in mind that the deformations in this area are probably progressive rather than episodic in nature. The generation of structures is based upon their styles and overprinting relationships recognizable on an outcrop or microscopic scale. The structural map and cross section of the study area are shown in Figs 2 and 3, respectively.

236

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D~ structures

Structures developed during this phase of deformation cannot easily be recognized in the field. This is because the effect of D 2 deformation especially the development

of very strong cleavages (Sd or $2) which transpose and largely obliterate the pre-existing structures. However, at one locality in the outcrop of piemontite-bearing quartz schist, Fj folds are still preserved. The style of this folding phase is characterized by an isoclinal fold with

237

Sirikit Dam area, Uttaradit, Northern Thailand x NW SIRIKIT RESERVOIR

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a sharp angular hinge (Fig. 4a) which is refolded by an F2 fold. In metagreywackes and phyllite, the existence of F~ folds can only be inferred from the relics of axial plane cleavage (S~) cut across by slaty cleavage ($2) as seen in thin section. Presumed original lamination (So) is generally parallel to discrete crenulation cleavage (S~). D2 structures

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Fig. 4. Profile sketches of folds: (a) an isoclinal F, fold in piemontitebearing quartz schist. Stippled layers are quartz-rich and unstippled layers are white-mica-rich; (b) a tight F z fold. Isogon patterns with 10° interval are convergent in quartzitic layer (stippled) and divergent in phyllosilicate-rich layer (unstippled). SEAES 8/1-4---M

D 2 deformation produced the dominant fabric elements in the Sirikit Dam area. It is characterized by a well developed slaty cleavage in phyllite and spaced cleavage/differentiated layering in metagreywackes (i.e. quartz semischist and banded quartzite). These two types of foliation are grouped together and designated as Sd or Sz. It can be seen from the stereographic projection (Fig. 5a) that $2 is remarkably uniform in orientation. In hand specimen the slaty cleavage is apparently continuous (using the terminology of Hobbs et al., 1976; Powell, 1979; Borradaile et al., 1982) but appears to be domainal in thin section. It transposed a discrete crenulation cleavage (S~). The slaty cleavage is defined by strong parallelism of white-micas in combination with the alignment of inequant grains of quartz and albite. Thin seams of opaque phases and graphite accentuate the cleavage and are axial planar to F 2 folds. Folding of thin dark seams which can be traced out into unfolded seams sometimes occurred probably due to perturbation in the flow regime. Diffusional mass transfer (i.e. pressure solution) played an important role as the dominant deformation mechanism in this event. This is clearly evident particularly in quartz semischist where large quartz grains were dissolved along the high-pressure sides and the dissolved materials precipitated/ recrystallized in the pressure shadows forming beards of fibrous quartz, calcite and minor chlorite (Fig. 6a). Differentiation between phyllosilicates and quartz-rich

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SAMPANSINGHARAJWARAPANand R. F. BERRY

a

b

POLES TO SPACED AND SLATY CLEAVAGES ($2)

c

STRETCHING LINEATIONS (I.2)

CRENULATION AXES (L3)

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f

KINK AXES (L4)

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POLES TO KINK BANDS ($4)

g

POLES TO BEDDING SURFACES IN GREYWACKE AND SLATE (SO)

h

POLES TO BEDDING SURFACES IN SANDSTONE AND SHALE (SO)

Fig. 5. Lower hemisphere equal-area projections of structural elements in the Sirikit Dam area.

domains are ubiquitous giving rise to the schistose appearance of the rocks. In some samples, especially banded quartzite, differentiated layering of calcite-rich and quartz-phyllosilicate-rich is present. Asymmetry of quartz porphyroclasts and pressure shadows in quartz

semischist signify the non-coaxial deformation path though they do not always give reliable kinematic information (Figs 6a,b). Quartz porphyroclasts are relatively flattened parallel to the cleavage plane. Some of them, especially in white-mica-chlorite phyllite, are highly

Sirikit Dam area, Uttaradit, Northern Thailand elongated and slightly rotated resulting in the formation of asymmetric boudins (Fig. 6c). Folding associated with D2 deformation folded original lamination or bedding (So) and cleavage/layering (S~) into tight to isoclinal folds. The limbs in some of these folds are strongly sheared and thinned while the hinge zones are thickened. Axial planar to these folds are $2 surfaces which generally transpose the layering. Based on dip-isogen pattern (Ramsay, 1967; Huddleston, 1973), the folds in quartzitic layers can be classified as class 1C (flattened parallel fold) and those in phyllitic layers belong to class 3 (divergent dip-isogen) as illustrated in Fig. 4b. These two classes of folds appear to approach class 2 (similar fold). Huddleston (1973) and Grey (1979) state that this style of folding develops while the layers are still mechanically active and hence buckling would be the principal mechanism rather than slip

239

folding (passive folding) as in case of an ideal similar fold. The asymmetric nature of the folds is reasonably clear so it can be used as a kinematic indicator where the sense of shear is deduced on the basis of fold vergence. All of these small-scale folds are east verging. This phase of folding probably affected the greywacke sequence as well because the bedding (So) dips uniformly towards the northwest (Fig. 5g) in the same sense as $2 orientation. Thrusts associated with F2 folding are rather shallow dipping. In an outcrop of metagreywackes near the Sirikit damsite, thrust surfaces apparently truncate layering ($2) and curve into parallelism with the layering at the upper part which clearly demonstrate the flat-ramp-flat geometry of thrusting that forms the outcrop-scale duplex structure. This suggests that duplexing is an important mechanism in the development of the accretionary wedge in this fold and thrust belt. Another prominent structural feature produced during D2 deformation is mineral stretching lineation (L S or L2) which is well developed in high strain zones. This feature has also been recognized in phyllitic rocks of Pha Som Group in the area to the west of Tha Pla District by Lumjuan and Sinpoolanant (1987) where they called it "chert augen". Stretching lineation (Ls) has been widely recognized on the cleavage surface ($2) especially in the phyllitic layer. Stereographic projection in Fig. 5b shows that L~ plunges shallowly towards the northwest. The direction of movement that produced this structure trends northwest-southeast. The vergence of small-scale folds, weakly developed S-C fabrics and shear bands, and the asymmetry of porphyroclasts and pressure shadows, suggest that the sense of movement is toward the southwest. D 3 structures

b

Fig. 6. Sketches from photomicrographs of metagreywackes: (a) asymmetric pressure shadow (stippled) around large quartz grain in quartz semischist. The materials in the pressure shadow are finegrained quartz, white-mica and chlorite. Note cleavage $2 anastomosed around quartz grains; (b) asymmetric pressure shadow around quartz porphyroclast in white-mica-chlorite phyllite. Note quartz fibres form pressure shadow around rhomb-shaped grains of ankerite. S2 in this specimen is defined by quartz-rich layer (stippled) alternate with white-mica-rich layer (unstippled); (c) boudinage of a stretched quartz grain with syntaxial growth of fibrous calcite in the space between boudins. S2 in this specimen is defined by quartz-rich layer (stippled) alternating with white-mica-rich layer (unstippled).

This deformation episode produced F 3 crenulations (microfolds) which crenulate $2 cleavage (Figs 7a-d) and is recognized almost everywhere on the $2 surface in the phyllitic layer. However, the associated crenulation cleavage ($3) is only incipiently to moderately developed and very finely spaced. This makes it difficult to be recognized in the outcrop and thus only the crenulation lineation has been measured in the field. Crenulation cleavages, as seen in thin section are either of discrete or zonal type according to Grey (1977). In most cases discrete crenulation cleavage can be traced into a zonal type and dies out over short distances as shown in Figs 7c and d, respectively. Folding is typically characterized by rounded-hinge microfolds in contrast to later angular folds (F4) shown in Fig. 8a. Crenulations developed during this deformation event are not strongly affected by later folding as seen in stereographic projection in Fig. 5c. The crenulation axes generally trend northeast and plunge towards the northeast or southwest. D 4 structures

Kinks (FD are the only recognized structural feature produced during the D4 deformation event. The shape of

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SAMPANSINGHARAJWARAPANand R. F. BERRY

most kinks is asymmetric open angular fold (Fig. 8b). Kink bands ($4) are rather narrow, ranging from a few centimetres to a few tens of centimetres. In the outcrop of metagreywackes, asymmetric close angular folds with amplitudes of 0.2-0.4 m and wavelengths of 0.2-0.3 m are associated with kinks (Fig. 8a). Though their styles are not exactly the same, the close spatial relation and similar orientation strongly suggest that they belong to the same generation. In general, kinks bands strike northwest and dip steeply towards the southwest or northeast (Fig. 5d) and kink axes (L4) plunge shallowly between north and west (Fig. 5e).

POST-ACCRETION STRUCTURE D5 s t r u c t u r e s

This deformation phase is characterized by low amplitude upright open F 5 folds in metagreywackes. Wavelengths of these folds range from a few centimetres to tens of centimetres. Fold axes plunge more or less to the north and axial planes dip steeply to sub-vertically. They overprint the pre-existing structures and thus postdate all ductile deformational features recognized in the Sirikit Dam area. This phase of folding probably belongs to the same event as the folding of Early Triassic sandstone and shale in the southeastern corner of the study area. In the latter, outcrop-scale folds cannot be recognized but it can be inferred from stereographic projection (Fig. 5h) that map-scale folds should be upright and trending northeasterly. Thrusting associated with Fs folding is clearly evident where Permo-Triassic volcaniclastics thrust over Early Triassic sequence with high angle reverse fault. This reverse fault differs from D 2 thrusts in that it has a steeper dip and has a brittle character. D 6 structures

This deformation phase is essentially of brittle style. High-angle faults were developed and can be recognized commonly in the volcanic and volcaniclastic rocks in the southeastern portion of the study area. Fault surfaces are steeply dipping or almost vertical and strike northeast. Fault striations defined by slickensides or fibrous minerals (e.g. quartz and calcite fibres) are numerous. In most cases, field observation of fault striations using the techniques of Petit (1987) indicates normal slips that are apparently overprinted by strike-slip movement. These overprinting features are also detected in the PermoTriassic volcanic and volcaniclastic rocks in Lampang and Phrae area suggesting that this brittle deformation phase is of regional significance.

METAMORPHISM

Greywackes composed of high variance assemblage quartz + white-mica + albite + chlorite + sphene invari-

ably contain relic detrital biotite (generally replaced by chlorite), phengite and albitized plagioclase. This mineralogically is indicative of sub-greenschist facies or anchizonal metamorphic grade. The metagreywackes are composed of quartz + whitemica + chlorite + albite + calcite + epidote _+ sphene whereas in more pelitic rocks (i.e. phyllite) the assemblage white mica + quartz + chlorite + albite _ calcite _+ankerite is the most common. These assemblages coupled with the absence of biotite, suggest that the rocks belong to chlorite zone of greenschist facies. Electron microprobe analyses of white-micas show that the variation in the composition of white-micas in crenulation cleavage (S~) and slaty cleavage ($2) is not very significant. This implies that both cleavages were developed at about the same P-T condition provided that there was no significant change in fluid composition. The plagioclase-muscovite thermometer of Green and Usdansky (1986) applied to phyllite (rock $2F591) indicates a metamorphic temperature of 320--350°C at pressure 3-4kbar. Using the computer program "THERMOCALC" (Holland and Powell, 1990), the geobarometer similar to that of Powell and Evans (1983) gives a pressure of around 3~4 kbar at 350°C. Piemontite-bearing quartz schist reflects slightly higher P-T condition that metagreywackes and phyllite. This rock is probably the product of metamorphism of hemipelagic sediments associated with trench sediments that was subducted deeper beneath the accretionary prism. Late thrust fault during D 5deformation may have transferred these metahemipelagites to the present shallow structural level. The presence of piemontite with accessory hematite in the rocks reflects high oxygen fugacity, probably higher than that of magnetitehematite buffer (Smith and Albee, 1967). This highly oxidized condition is believed to be inherited from the protolith. In similar piemontite-bearing schist of the Haast Schist terrane in western Otago, New Zealand, P-T conditions of metamorphism have been estimated to be about 350-400°C and 6-7 kbar (Yardley, 1982). Kawachi et al. (1983) noted that this relatively high P/T metamorphism has largely been overprinted by regionally developed greenschist facies metamorphism (with estimated pressure of 4-5 kbar) that may result from thermal relaxation and/or decompression due to rapid uplift or tectonic unroofing. Garnet amphibolite, zoisite amphibolite, and amphibolite with middle amphibolite facies assemblages are common within the serpentine melange. Temperature estimates using garnet-hornblende thermometry (Graham and Powell, 1984) and garnet-clinopyroxene thermometry (Ellis and Green, 1979; Pattison and Newton, 1989) for garnet amphibolite (rock $9F791) indicate that the rock equilibrated at the temperatures 690°C, 720°C, and 570°C, respectively, assuming a pressure of 6 kbar. We prefer the latter value because this thermometer has been calibrated to deal specifically with high grossular mole fraction in garnet as in our case (Xc, gr~= 0.40). It is also interesting to note that Tschermak's substitution between coexisting

Sirikit Dam area, Uttaradit, Northern Thailand

Fig. 7. Scanning electron micrographs of crenulations (F3) and crenulation cleavage ($3) in phyllite; (a) symmetrical crenulation; (b) detail of hinge zone of crenulation. Note inequant quartz grain (Q) aligns parallel to $2; (c) asymmetric crenulation. Note crenulation cleavage coincides with appressed limbs; and (d) detail of crenulation cleavage,

241

Sirikit Dam area, Uttaradit, Northern Thailand

243

stacked together and cause the uplift of the accretionary prism forming what is termed "trench-slope break" (Karig and Sharman, 1975) or "fore-arc ridge" (Hamilton, 1988). The diffusional mass transfer process is the $2 dominant deformation mechanism in this particular ~--C.d"/' ~",'~"~qp(! environment where the P-T condition is of sub-greenschist to lower greenschist facies. Temperature in the range of 300-400°C and pressure of 3-4 kbar obtained from the geothermobarometer in the previous section are compatible with the style and mechanism of deformation I, I recorded in the rocks. Crenulations and crenulation cleavages produced by b $4 D3 deformation are asymmetric features. These can be interpreted either as the extensional structures developed under rotational deformation regime (Platt and Vissers, 1980) or the result of layer shortening with non-parallel minimum principal strain axis. Inasmuch as the available evidence may indicate, the appropriate explanation would be the latter model because some crenulations are symmetrical in profile as shown in Fig. 7a and it can be postulated that crenulations and crenulation cleavage are the result of intra-wedge shortening. We proposed that the structural features described above are the result of duplex accretion and can be Fig. 8. Profilesketchesfromphotographs:(a) asymmetricangularfold (F4) with associatedkinks; and (b) kinks bands ($4). compared with the accretionary models put forward by Sample and Fisher (1986), Sample and Moore (1987), and Needham and MacKenzie (1988). clinopyroxene and hornblende may be used as a geoD4 kinks are interpreted as the result of late thrusting thermometer. The temperature of 600°C obtained when and uplift of the accretionary complex due to the ingress applied this thermometer to zoisite amphibolite (rock of the leading edge of Indo-China terrane at the subduc$4F791), is in good agreement with that of garnet tion site. amphibolite. A calculated minimum equilibration pressD 5 upright open folds and thrusts may be the result of ure of this rock is 5kbar based on the barometer the main collision as they are similar in style and involving assemblage hornblende + clinopyroxene + orientation to the open to close upright folds in marine zoisite + plagioclase. Serpentinization of the pyroxenite and peridotites may Triassic sedimentary rocks exposed along Lampangbe due to hydrothermal metamorphism at the ocean Phrae and Phrae-Uttaradit highways. The Triassic rocks floor and/or retrogressive metamorphism during thrust- are apparently subjected to only a single phase of folding ing where water was able to percolate through faults or with which the associated regional cleavage strikes shear zone as evidence of high strain is always present northeasterly. In general, fold axes plunge shallowly towards the northeast or southwest. In some localities in the serpentine. steeply plunging cleavage/bedding intersections are recognized. These features are thought to reflect local variation of the original bedding due either to softSTRUCTURAL AND TECTONIC SYNTHESES sediment deformation or small-scale faulting. The development of the first generation fold is atHigh angle faults with normal displacement that are tributed to layer-parallel shortening of subducting sedi- partly overprinted by strike-slip movements are interments that have suffered only partial stratal disruption. preted here as Tertiary structures postdating the colThe deformation mechanism prevailing over that period lision. An alternative interpretation is that the normal would have been diffusional mass transfer to accommo- faults were formed as the result of extension related to date large bulk strain associated with compression. relaxation after the main collision and were reactivated Nevertheless, this interpretation requires more detailed by later Tertiary strike-slip faults. investigation as D~ structures are almost completely obliterated by D2 deformational features. D2 structures discussed in the foregoing section are CONCLUSIONS interpreted as being the result of movement due to accretionary thrusting where thrusts, F2 folds, stretching The rocks in the Sirikit Dam area recorded a multiple lineation (Ls or L2) and cleavages (Sd or $2) developed. deformation history. Among the deformational features Under the process of duplexing that enable the accretion developed in this area, the most pronounced structures of the material from the lower plate to the overriding are strongly developed foliations (Sd or $2) which were plate, the thrust packets separated by thrust faults are produced during a D2 deformational event. These a

0.2~

244

SAMPAN SINGHARAJWARAPAN a n d R. F. BERRY

cleavages are axial p l a n a r to tight to isoclinal folds a n d dip shallowly towards the northwest. They invariably transpose pre-existing structures (i.e. the bedding (So) a n d cleavage/layering ($1)). The transposition is so strong that outcrop-scale F2 folds are rarely recognizable. The stretching lineation (L~ or L2) plunges d o w n - d i p towards the west-northwest. The available S~C fabrics, asymmetric porphyroclasts and asymmetric pressure shadows developed in n a r r o w zones of higher strain suggest that a reverse sense of shear is d o m i n a n t . The slaty cleavage ($2) is overprinted by crenulation cleavage ($3) a n d later by a set of kink b a n d s ($4). The last ductile d e f o r m a t i o n style is a series of upright open folds (Fs) trending n o r t h - n o r t h e a s t . Brittle d e f o r m a t i o n occurred later a n d produced northeast trending n o r m a l faults which are more c o m m o n in massive volcanic and volcaniclastic rocks in the eastern part of the study area. The n o r m a l - s l i p striations are locally overprinted by strike-slip striations. The lithology, style of deformation, a n d m e t a m o r phism of the Sirikit D a m metagreywackes can be compared with those of uplifted ancient and submerged m o d e r n accretionary complexes in m a n y parts of the world. The tight to isoclinical folds (Fz), slaty and spaced cleavage (S~ or $2 ), stretching lineation (L~ or L 2), a n d crenulations (F 3) are interpreted as the direct result of non-coaxial progressive d e f o r m a t i o n due to eastdirected accretionary thrusting. K i n k s (F4) are p r o b a b l y related to late thrusting or the b e g i n n i n g of collision. The upright open folds (Fs) in the study area could be correlated with upright folds in the Triassic turbiditic greywacke sequence exposed along L a m p a n g - P h r a e and P h r a e - U t t a r a d i t highways. This late d e f o r m a t i o n is possibly the result of the m a i n collision between S h a n Thai a n d I n d o - C h i n a terranes. N o r m a l and strike-slip faults were active d u r i n g the Tertiary tectonic events where widespread pull-apart basins occurred in n o r t h e r n Thailand. Acknowledgements--This research was supported by the Australian International Development Assistance Bureau (AIDAB) Postgraduate Training Award and partly by the University of Tasmania. We would like to thank the Electricity Generating Authority of Thailand for accommodation and for access to work in the Sirikit Dam area. Technical assistance on electron microprobe and the scanning electron microscope by Wieslaw Jablonski is highly appreciated. Thanks are also extended to Simon Stephens for the preparation of thin sections and to Keith Harris for an introductory sessionon electron microprobe to S. Singharajwarapan. Constructive comments from Clive Burrett and Pol Chaodumrong were particularly valuable.

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