Evidence for middle Triassic to Miocene dual subduction zones beneath the Shan–Thai terrane, western Thailand from magnetotelluric data Songkhun Boonchaisuk a, b , Weerachai Siripunvaraporn a, b, ⁎, Yasuo Ogawa c a Department of Physics, Faculty of Science, Mahidol University, 272 Rama 6 Road, Rachatawee, Bangkok 10400, Thailand b ThEP Center, Commission on Higher Education, 328, Si Ayutthaya Road, Rachatawee, Bangkok 10400, Thailand c Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152–8551, Japan abstract article info Article history: Received 8 February 2012 Received in revised form 20 August 2012 Accepted 22 August 2012 Available online 31 August 2012 Handling Editor: A. Aitken Keywords: Magnetotelluric Shan–Thai Western Thailand Deep resistivity structure Kanchanaburi province, western Thailand, is in the middle of Shan–Thai terrane. Paleomagnetic and geolog- ical data support the hypothesis of dual subduction zones beneath the Shan–Thai terrane. During the late Triassic there was a westward subduction in the east where the Lampang-Chiang Rai block subducted beneath the Shan–Thai terrane. In addition, in the early Tertiary, the western Burma terrane subducted underneath the Shan–Thai in the west resulting in an eastward subduction. A pioneer survey of this region was therefore conducted using magnetotelluric (MT) techniques with the aim of finding deep structures associated with these ancient subduction zones. 39 MT stations were deployed to cover most of the province. Resistivity struc- tures are obtained from the 3-D inversion. The near surface resistivity structures correspond well with the sur- face geology. The mid and lower crusts are conductive and this is interpreted as the crusts being composed of mafic granulites of 3% porosity in which the remnant dehydrated fluid was accumulated during the subduc- tion. In addition, the westward and eastward subduction zones generated mafic/ultramafic intrusions which appear as two conductive zones on the east and west, respectively. These two conductors and the conductive lower crust support the hypotheses of the middle Triassic to Miocene dual subduction zones. © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 1. Introduction The tectonics of Thailand is complicated and very intriguing. Thailand is located on the southernmost of the Eurasian plate and is enclosed by the Indian plate on the west and the Philippine plate on the east (Fig. 1). It currently consists of two major terranes or micro-plates (Bunopas, 1981; Bunopas and Vella, 1983; Charusiri et al., 2002; Ferrari et al., 2008; Metcalfe, 2011). The east and northeast of Thailand is part of the Indochina terrane. On the west of Thailand and eastern Myanmar is the Shan–Thai terrane which is shown in Fig. 1. Note that the Shan–Thai is sometimes also called the Sinobuma terrane (e.g., Hisada et al., 2004; Hirsch et al., 2006; Ferrari et al., 2008, among many others). Others (e.g. Metcalfe, 2011) regard the Sinobuma terrane as a larger region which includes the Shan–Thai terrane. Both terranes originate from Gondwana. Many publications (e.g., Bunopas, 1981; Barr and Mac Donalds, 1987; Hada et al., 1991) have proposed that the Nan suture zone divides the two terranes (Fig. 1). However, Ueno and Hisada (2001) proposed instead that the Nan suture zone is the remnant of a closed back-arc basin. Metcalfe (2011) suggested that the smaller Sukhothai terrane and Chantaburi terrane divided these two main terranes. The tectonic evolution of Thailand is still not completely understood after the original scheme proposed by Bunopas (1981). Since then, many further studies resulting in other proposed schemes have been car- ried out (see e.g., Lee and Lawver, 1995; Charusiri et al., 2002; Metcalfe, 2002; Morley, 2002; Hirsch et al., 2006; Ferrari et al., 2008; Metcalfe, 2011; among many others). It is also difficult to judge which theory is the most suitable and beyond the scope of this paper. In this paper, we closely follow Thailand tectonic evolution scheme developed by Charusiri et al. (2002). They divided the tectonic evolution of Thailand into four main stages: (1) Precambrian to early Cambrian, (2) Cambrian to early Triassic, (3) middle Triassic to Miocene, and (4) early Tertiary to present. Selected schematic diagrams for each stage are re-plotted from Charusiri et al. (2002) in Fig. 2. From Precambrian to Cambrian (Fig. 2a), Shan–Thai terrane was in Gondwana and was close to what is now northwestern Australia. Indochina was part of the Pan-Cathaysia continent which is adjacent to what is now northern and northeastern Australia. Since then both Shan–Thai and Indochina had gone through many tectonic develop- ments (Bunopas, 1981; Charusiri et al., 2002). During the Cambrian to Permian, many events occurred in the Paleotethys between the Shan– Thai and Indochina terranes. These include the rifts that separated the Shan–Thai from Gondwana and the development of two small tectonic blocks: the subducted Nakorn Thai oceanic crust beneath the Gondwana Research 23 (2013) 1607–1616 ⁎ Corresponding author at: Department of Physics, Faculty of Science, Mahidol University, 272 Rama 6 Road, Rachatawee, Bangkok, Thailand. Tel.: +66 2 2015764; fax: +66 2 3547159. E-mail address: wsiripun@gmail.com (W. Siripunvaraporn). 1342-937X/$ – see front matter © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gr.2012.08.009 Contents lists available at SciVerse ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr