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Quantitative Finite Strain Analysis of the Quartz Mylonites Within the Three

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Quantitative Finite Strain Analysis of the Quartz Mylonites Within the Three ZOBODAT - www.zobodat.at Zoologisch-Botanische Datenbank/Zoological-Botanical Database Digitale Literatur/Digital Literature Zeitschrift/Journal: Austrian Journal of Earth Sciences Jahr/Year: 2018 Band/Volume: 111 Autor(en)/Author(s): Kanjanapayont Pitsanupong, Ponmanee Peekamon, Grasemann Bernhard, Klötzli Urs S., Nantasin Prayath Artikel/Article: Quantitative finite strain analysis of the quartz mylonites within the Three Pagodas shear zone, western Thailand 171-179 download https://content.sciendo.com/view/journals/ajes/ajes-overview.xml Austrian Journal of Earth Sciences Vienna 2018 Volume 111/2 171 - 179 DOI: 10.17738/ajes.2018.0011 Quantitative finite strain analysis of the quartz mylonites within the Three Pagodas shear zone, western Thailand Pitsanupong KANJANAPAYONT1)*), Peekamon PONMANEE1), Bernhard GRASEMANN2), Urs KLÖTZLI3) & Prayath NANTASIN4) 1) Basin Analysis and Structural Evolution Special Task Force for Activating Research (BASE STAR), Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand; 2) Department of Geodynamics and Sedimentology, University of Vienna, Althanstrasse 14, Vienna 1090, Austria; 3) Department of Lithospheric Research, University of Vienna, Althanstrasse 14, Vienna 1090, Austria; 4) Department of Earth Sciences, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand; *) Corresponding author: [email protected] KEYWORDS Finite strain; Kinematic vorticity; Sinistral; Three Pagodas shear zone; Thailand Abstract The NW–trending Three Pagodas shear zone exposes a high–grade metamorphic complex named Thabsila gneiss in the Kanchanaburi region, western Thailand. The quartz mylonites within this strike–slip zone were selected for strain analysis. 2–dimensional strain analysis indicates that the averaged strain ratio (Rs) for the lower greenschist facies increment of XZ– plane is Rs = 1.60–1.97 by using the Fry’s method. Kinematic vorticity analysis of the quartz mylonites in the shear zone showed that the mean kinematic vorticity number of this increment is Wk = 0.75–0.99 with an average at 0.90 ±0.07. The results implied that the quartz mylonites within the Three Pagodas shear zone have a dominant simple shear component of about 72% with a small pure shear component. A sinistral shear sense is indicated by kinematic indicators from macr o– to micro–scale. We conclude that the Three Pagodas shear zone deformed in the process of sinstral shear–dominated transpression, which is similar to the Mae Ping shear zone in the north. 1. Introduction Cenozoic strike–slip systems have played an important in the west, and the NE–SW dextral Ranong and Khlong role in the tectonic evolution of SE Asia (e.g. Tapponnier Marui shear zones in the south, respectively (Kanjanapa- et al., 1982, 1986). Their kinematics fit to the continental yont et al., 2012a; Kanjanapayont, 2015). The Mae Ping extrusion model that explains the structural evolution of and Three Pagodas shear zone with the generally sinis- Tibet and Southeast Asia (Tapponnier et al., 1982, 1986). tral strike–slip kinematics possible splays out from the This continental extrusion model is based on northward Sagaing fault in Myanmar. However, the area where these drift of the Indian plate and clockwise rotation of the strike–slip systems mechanically interact is still poorly Asian plate around the Eastern Himalayan Syntaxis (e.g. documented. Many authors reported that the Mae Ping Tapponnier et al., 1982; 1986; Leloup et al., 1995). It pre- strike–slip zone is splayed to the Klaeng fault zone in sumes that the large thickened continental crusts of eastern Thailand, and the Three Pagodas strike–slip zone Sundaland, Tibet, and South China have been extruded by is extended to the Ranong shear zone in the southern the Indian subduction through the motion of strike–slip Thailand (Lacassin et al., 1993, 1997; Leloup et al., 1995; systems to the E and SE direction (Tapponnier et al., 1986). Gilley et al., 2003; Geard, 2008; Nantasin et al., 2012; Ridd, The N–S striking dextral Sagaing fault in Myanmar and 2012; Ponmanee et al., 2016). In other geological inter- the NW–SE striking sinistral Ailao Shan–Red River shear pretations (e.g. Morley, 2002; Morley et al., 2007; Smith et zone in China are the most prominent structures of these al., 2007), the Three Pagodas strike–slip zone is possibly strike–slip systems (e.g. Tapponnier et al., 1990; Lacassin connected to the Klaeng fault zone, which documents et al., 1997; Morley et al., 2007). High–grade metamor- ductile sinistral shearing in the Eocene (Kanjanapayont et phic and plutonic rocks have been exhumed representing al., 2013). In southern Thailand, the Ranong and Khlong the mid–crustal deformation history during continental Marui shear zones extend from the continental margin extrusion within these crustal scale strike–slip shear and Mergui basin to the Gulf of Thailand (Tapponnier et zones (Lacassin et al., 1997). However, some studies have al., 1986; Lacassin et al., 1997; Charusiri et al., 2002). All reported that the ductile strike–slip shearing did not co- of the major strike–slip shear zones in Thailand possi- incide with the India–Asia collision (e.g. Palin et al., 2013), bly were active during the Eocene (Lacassin et al., 1997; whereas some studies in Tibet and the Ailao Shan–Red Watkinson et al., 2011; Kanjanapayont, 2015; Kanjanapa- River shear zone disagree with the continental extrusion yont et al., 2012b; Nantasin et al., 2012; Palin et al., 2013). model (e.g. Searle, 2006; Searle et al., 2010, 2011). Moreover, the high–grade metamorphic and plutonic ex- In Thailand, the main strike–slip structures are the posures in such crustal scale strike–slip zones represent an NW–SE sinistral Mae Ping and Three Pagodas shear zones important source for investigating the kinematics and the Open Access. © 2018 Pitsanupong KANJANAPAYONT et al., published by Sciendo. This work is licensed under the Creative 171 Commons Attribution NonCommercial-NoDerivatives 4.0 License. download https://content.sciendo.com/view/journals/ajes/ajes-overview.xml Quantitative finite strain analysis of the quartz mylonites within the Three Pagodas shear zone, western Thailand timing of the tectonic evolution of SE Asia (e.g. Lacassin Resources, 1982). The Three Pagodas shear zone is char- et al., 1997; Palin et al., 2013). Especially in Thailand, acterized by a lenticular slice of the high–grade met- syndeformational metamorphic rocks are exposed amorphic rocks named “Thabsila gneiss” or “Thabsila along the main ductile strike–slip shear zones (Lacassin metamorphic complex” in the Kanchanaburi region in et al., 1997; Kanjanapayont, 2015; Kanjanapayont et al., western Thailand (Nantasin et al., 2012). It has been in- 2012a; Nantasin et al., 2012; Palin et al., 2013; Ponmanee ferred as Precambrian basement rock (Bunopas, 1981; et al., 2016) (Fig. 1). In this study, we report quantitative Department of Mineral Resources ,1982). The Thabsila structural investigations of relatively homogeneously gneiss, which shown as mylonites in Fig. 2, is made up deformed rocks from the Three Pagodas shear zone. of four units based on lithology and structural features: (1) Unit A: marble, mica schist, fine-grained biotite gneiss, 2. Geological setting and quartzite (Fig. 3), (2) Unit B: mylonitic gneiss and The mylonites of the NW–SE striking Three Pagodas mylonite, (3) Unit C: calc–silicate, and (4) Unit D: biotite shear zone is 250 km long and 25 km wide and extends gneiss, orthogneiss, and sillimanite gneiss (Nantasin et from the Three Pagodas Pass on Thai–Myanmar bor- al., 2012). Augen structures within a very strongly devel- der towards central Thailand (Kanjanapayont, 2015). oped foliation can be observed in the mylonitic gneisses This ductile strike–slip shear zone cuts the sedimentary of Unit B. Calc–silicates are typically intercalated with the units of the Palaeozoic (Silurian–Permian) and Mesozoic gneisses of Unit B and C as boudins or strongly folded slic- sedimentary units, and subordinate low–metamorphic es, and contain diopside–rich layers alternating with mar- units of Cambrian–Ordovician (Department of Mineral ble layers. The transitional zone of Unit C toward Unit D is characterized by a narrow band of garnet–free amphibo- lite, which gradually changed into biotite gneiss. Based on petrology and mineral chemis- try (Nantasin et al., 2012), the Thabsila gneiss was formed during single metamorphic event at upper amphibolite to lower granulite facies condi- tions. The rocks experienced constrictional strain and sinis- tral shearing during exhuma- tion (Nantasin et al., 2012). In the past, the sharp contact between less metamorphic Paleozoic rocks and the high– grade metamorphic rocks led to the interpretation that these high–grade and partly ana- tectic metamorphic rocks are Precambrian in age (Depart- ment of Mineral Resources, 1982). These high–grade met- amorphic rocks were typically intruded by Permo–Triassic and Cretaceous–Tertiary granitic plutons (Lacassin et al., 1997). Previous studies of 40Ar/ 39Ar dating, however, reveal that the deformation of the Mae Ping and the Three Pagodas shear zones terminated around 30.5 Ma (Oligocene), and the lower Mesozoic metamorphic and magmatic belt of north- Figure 1: Simplified regional tectonic map of Thailand and adjacent areas showing the major shear ern Thailand experienced rapid zones and related structures (modified after Macdonald et al., 2010; Mitchell et al., 2012; Morley, 2002; Polachan and Sattayarak, 1989). Black, metamorphic
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