Proceedings of the International Symposia on Geoscience Resources and Environments of Asian Terranes (GREAT 2008), 4th IGCP 516,and 5th APSEG; November 24-26, 2008, Bangkok, Thailand

Parallel Tethyan Sutures and the Sukhothai Island-arc System in Thailand and Beyond Masatoshi Sone1* and Ian Metcalfe2 1. School of Engineering and Science, Curtin University of Technology, Sarawak Campus, 98009 Miri, Malaysia 2. School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia * Corresponding author e-mail: [email protected]

Abstract Sub-parallel tectonic sutures of Permian-Triassic ages, representing the Palaeo-Tethys Suture Zone and the former back-arc basin, have been recognised in NW Yunnan, Thailand and Cambodia of mainland SE Asia. The Sukhothai Island-arc System is then delineated between the two sutures. This longitudinal system is now extended to the ThaiMalay Peninsula in the south beyond the Gulf of Thailand. The Bentong–Raub Suture Zone and the East Malaya Terrane in the peninsula are defined as the southern extensions of the Palaeo-Tethys Suture Zone and the Sukhothai Island-arc System, respectively.

Keywords: island-arc, back-arc basin, Tethys, Thailand, Malaysia. 1. Introduction In mainland SE Asia, parallel tectonic sutures of contrasting Tethyan histories are recognised between two major continental blocks, the Indochina Terrane and the Sibumasu Terrane (Fig. 1). These sutures represent the main Palaeo-Tethys Ocean and a short-lived backarc basin, respectively; they resulted from prolonged subduction of the Palaeo-Tethys beneath Indochina during the Late Carboniferous–Triassic and subsequent Sibumasu collision (Sone & Metcalfe, 2008). The location of the Palaeo-Tethys Suture recognised here primarily corresponds to the collisional boundary delineated by Mitchell (1977), that is, the major suture in Thailand is located west of the Nan Suture. Contrasting chronostratigraphic ranges of pelagic/deep-sea sediments in the parallel sutures clearly distinguish the two closed marine basins. That is, the Palaeo-Tethys Suture Zone with Devonian–Triassic oceanic cherts and the back-arc suture with restricted Permian deep-sea records. The emplacement ages of pre-Cretaceous S- and I-type granitoid belts also support this interpretation. The Sukhothai Island-arc System, characterised by its I-type granitoid belt, is defined between the sub-parallel sutures.

132

2. Thailand The Nan–Uttaradit Suture and/or the Sra Kaeo Suture have traditionally been regarded as representing the Palaeo-Tethys Suture (e.g. Bunopas, 1982; Hada et al., 1999). However, some authors (Ricou, 1994; Ueno & Hisada, 1999; Ueno & Hisada, 2001) argued that they only represent a closed back-arc basin with the true Palaeo-Tethys subduction zone (suture) located to the west. Sone and Metcalfe (2008) favoured this view. The Sukhothai Terrane of Thailand is no longer treated as part of the Sibumasu (Shan-Thai) Terrane, but is defined as a Permo-Triassic magmatic arc (Sukhothai Arc of Sone & Metcalfe, 2008). It emerged as an islandarc (the Sukhothai Island-arc System) developed off the western margin of the Indochina Terrane as early as the Early Permian. The southern extension of the Sukhothai Islandarc System is controversial. Sone and Metcalfe (2008) proposed the Chanthaburi Terrane to be the southern extension of the Sukhothai Terrane and the PalaeoTethyan Inthanon Suture Zone extending southwards, bounded by the Klaeng Fault in East Thailand.

Proceedings of the International Symposia on Geoscience Resources and Environments of Asian Terranes (GREAT 2008), 4th IGCP 516,and 5th APSEG; November 24-26, 2008, Bangkok, Thailand

3. Yunnan The Sukhothai Arc and parallel Tethyan sutures are extended to western Yunnan in the north. The PalaeoTethys Suture Zone is located to the west, represented by the Changning–Menglian and Inthanon sutures (e.g. Sone & Metcalfe, 2008, Ueno, 1999). Ueno (1999) correlated the southern Lancangjiang Suture in western Yunnan with the Nan Suture as a back-arc suture. Sone and Metcalfe (2008) supported this view and then renamed the former Yunnan suture to be “the Jinghong Suture” to avoid confusion with the Lancangjiang Suture of the main Palaeo-Tethys lineage in Tibet. The Jinghong Suture so far yields only Permian deep-water records, suggesting its alliance to the Nan to Sra Kaeo back-arc basin, not the Palaeo-Tethys Ocean. 4. Cambodia The extention of the new Chanthaburi Terrane and the bounding Sra Kaeo Suture to western Cambodia is still controversial, due to a shortage of key information from Cambodia. Nevertheless, a probable extension of the Sra Kaeo Suture is traceable to the Pailin ophiolite just beyond the border, then to the Kampot ophiolite in southern Cambodia (see Bach & Thang, 1995).

Fig. 1.

New tectonic subdivision of SE Asia (modified after Sone & Metcalfe, 2008). C-M S.Z. = Changning–Menglian Suture Zone, Lincang T. = Lincang Terrane.

5. Peninsula Malaysia The Palaeo-Tethys Inthanon Suture Zone extends southwards over the Gulf of Thailand to the greater Bentong–Raub Suture Zone which we define in the Thai-Malay Peninsula. The southern equivalence of the Sukhothai Island-arc System in Peninsular Malaysia is considered as being represented by the East Malaya Terrane, which also consists of Permo-Triassic I-type granitoids (see Cobbing et al., 1986). The Cenozoic opening of the Thai Gulf to the Malay Basin, following the pre-existing Palaeozoic and Mesozoic fabrics (Morley et al., 2004), probably placed East Malaya apart from mainland SE Asia. Palaeomagnetic results also suggest such dislocation (see Richter et al., 1999).

133

Proceedings of the International Symposia on Geoscience Resources and Environments of Asian Terranes (GREAT 2008), 4th IGCP 516,and 5th APSEG; November 24-26, 2008, Bangkok, Thailand

References Bach, L.D. Thang, N.G. 1995. Phanerozoic ophiolites in Indochina. Journal of Geology Series B, 5-6, 212–221. Bunopas, S. 1982. Paleogeographic History of Western Thailand and adjacent parts of Southeast Asia: A Plate Tectonics Interpretation. Department of Mineral Resources Thailand, Bangkok, 810 p. Cobbing, E.J., Mallick, D.I.J., Pitfield, E.J. Teoh, L.H. 1986. The granites of the Southeast Asian tin belt. Journal of the Geological Society, London, 143, 537-550. Hada, S., Bunopas, S., Ishii, K. Yoshikura, S. 1999. Rift-drift history and the amalgamation of Shan-Thai and Indochina/East Malaya blocks. In: Metcalfe, I., Ren, J., Charvet, J. Hada, S. (Eds.), Gondwana Dispersion and Asian Accretion. A.A. Balkema, Rotterdam, pp. 67-87. Mitchell, A.H.G. 1977. Tectonic setting for emplacement of Southeast Asian tin granites. Bulletin of the Geological Society of Malaysia, 9, 123-140. Morley, C.K., Haranya, C., Phoosongsee, W., Pongwapee, S., Kornsawan, A. Wonganan, N. 2004. Activation of rift oblique and rift parallel pre-existing fabrics during extension and their effect on deformation style: examples from the rifts of Thailand. Journal of Structural Geology, 26, 1803–1829. Richter, B., Schmidtke, E., Fuller, M., Harbury, N. Samsudin, A.R. 1999. Paleomagnetism of Peninsular Malaysia. Journal of Asian Earth Sciences, 17, 477-519. Ricou, L.E. 1994. Tethys reconstructed: plates, continental fragments and their boundaries since 260 Ma from Central America to South-eastern Asia. Geodinamica Acta, 7, 169-218. Sone, M. Metcalfe, I. 2008. Parallel Tethyan sutures in mainland Southeast Asia: New insights for PalaeoTethys closure and implications for the Indosinian orogeny. Comptes Rendus Geosciences, 340, 166–179. Ueno, K. 1999. Gondwana/Tethys divide in East Asia: solution from Late Paleozoic foraminiferal paleobiogeography. In: Ratanasthien, B. Rieb, S.L. (Eds.), Proceedings of the International Symposium on Shallow Tethys 5, Chiang Mai University, pp. 45-54. Ueno, K. Hisada, K. 1999. Closure of the Paleo-Tethys caused by the collision of Indochina and Sibumasu. Chikyu Monthly, 21, 832-839 (in Japanese). Ueno, K. Hisada, K. 2001. The Nan-Uttaradit-Sa Kaeo Suture as a main Paleo-Tethyan suture in Thailand: Is it real? Gondwana Research, 4, 804-805. 134