11 Structural geology of Thailand during the Cenozoic CHRISTOPHER K. MORLEY, PUNYA CHARUSIRI & IAN M. WATKINSON This chapter begins with a brief review of the data available for understanding the Cenozoic deformation of Thailand and a description of the different Cenozoic structural provinces of the country. The two dominant Cenozoic structural styles present in Thailand are strike – slip faulting (found predominantly in the western half of onshore Thailand and the Andaman Sea) and exten- sion; folds, thrusts and inversion structures are also present in places (Fig. 11.1). The timing of deformation (Fig. 11.2) and typical struc- tural styles present in outcrop and the subsurface are described. Particular attention is paid to the inheritance of fabrics on later fault geometries, since these are very distinctive and well developed in many areas. The structural geology of Thailand is very important for under- standing Cenozoic orogenic processes marginal to the Himalayan orogen. For those less interested in regional tectonics, there are also individual examples of structures that are outstanding and could grace any structural geology textbook, including: (1) A beautifully documented opencast coal mine (Mae Moh, Fig. 11.1) 3  4 km in area, that reveals Miocene rift-related normal faults. (2) Extensively developed and large-scale strike – slip fault zones with multiple strike – slip duplex geometries. Cenozoic mid-crustal level mylonites and migmatites are exposed in places, with recently described outcrops along the Ranong Fault (Watkinson et al. 2008) that rival the Red River– Ailao Shan Fault Zone in scale. (3) The best examples seen anywhere on seismic reflection data of extensively developed, linked, high-length: displacement ratio normal faults (Gulf of Thailand). (4) Abnormally deep (6þ km), high-subsidence rate post-rift basins that challenge our understanding of simple rift to post-rift basin subsidence mechanisms (Morley & Westaway 2006). Modern tectonic setting of Thailand Detailed discussion of the regional tectonics is given in other chap- ters, in particular Barber et al. (2011) and Searle & Morley (2011). Here a very brief review of the tectonic setting is necessary to place Cenozoic structural development in a regional context (Figs 11.3 & 11.4). Thailand is located at the northern end of the continental crustal core of SE Asia called Sundaland (Fig. 11.3), which is composed of a series of terranes that were rifted off Australia– Antarctica and re-assembled on the Asian margin by collisional events during the late Palaeozoic – Triassic (e.g. Metcalfe 2002). Global Positioning Satellite (GPS) data have confirmed that present day Sundaland is moving approximately as a single block (Simons et al. 2007). There is a sharp transition in movement direction and velocity between Thailand/eastern Myanmar across the north – south trending Sagaing Fault (Fig. 11.5) and western Myanmar. The Sagaing Fault is one of the largest and most active strike–slip faults in the world, with dextral motion of the order c. 2.4 cm a 21 (Vigny et al. 2003; Curray 2005). The motion accom- modates about two-thirds of the northwards motion of India relative to Indochina. A less dramatic boundary, apparent from the GPS data, lies between northern Thailand and the Yunnan/Golden Triangle area to the north (Simons et al. 2007; Fig. 11.5). In Figure 11.5 the marked north–south change in velocity occurs across the Ailao–Shan–Red River Fault and a series of ENE– WSW trending sinistral strike – slip faults. Northern Thailand marks a region where the development of north–south to NE– SW trending basins commences and the prominent region of predo- minantly strike – slip seismicity diminishes. GPS data indicate that Thailand is moving as a single block today (Fig. 11.5). For much of the Cenozoic, Sundaland has been the overlying plate to a great belt of subduction zones stretching from the Andaman Sea through Indonesia to the Philippines. The northern end of Sundaland has also interacted with the Indian Plate as it moved north to collide with Eurasia. Consequently, the structural evolution of Thailand during the Cenozoic reflects both subduction- related processes to the west and SW and the effects of the Himalayan Orogeny. Today Myanmar effectively forms a diffuse north – south striking transform plate boundary defined by the dextral Sagaing Fault and the Andaman Trench (Fig. 11.4). This boundary is due to the coupling of western Myanmar with the Indian Plate which is consequently being dragged northwards rela- tive to Sundaland (see review by Curray 2005). Today, passing northwards from northern Thailand towards the southern margin of the East Himalayan syntaxis, there is increasing strike – slip fault- dominated deformation (Lacassin et al. 1998; Socquet & Pubellier 2005; Morley 2007) (Fig. 11.5). The modern tectonic setting provides a snapshot in time of a rapidly evolving region. To determine the evolution of Thailand during the Cenozoic, it is necessary to establish the timing and kin- ematics of deformation, metamorphic history, cooling history and sedimentary basin evolution. Our understanding of these aspects has significantly improved over the last 10 – 15 years due to increas- ing availability of coal and oil industry data, improved radiometric dating techniques and new outcrop studies from coal mines where the deformation of Cenozoic sediments can be seen in context.