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_Journal of Structural Geology 32 (2010) 235–248_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg _Present-day stress orientation in Thailand’s basins_ _Mark R.P. Tingay a,*, Chris K. Morley b, Richard R. Hillis c, Jeremy Meyerd a Department of Applied Geology, Curtin University of Technology, WA, Australia b PTT Exploration and Production, Bangkok, Thailand c Australian School of Petroleum, University of Adelaide, Adelaide, Australia d JRS Petroleum Research, Adelaide, Australia_ _article info_ _Article history: Received 10 December 2008 Received in revised form 17 November 2009 Accepted 30 November 2009 Available online 7 December 2009_ _Keywords: Present-day stress Thailand Borehole breakout Neotectonics_ _abstract_ _The Cenozoic tectonic evolution of Thailand is widely considered to have been primarily controlled by forces generated at the eastern Himalayan syntaxis. This hypothesis is supported by earthquakes in northern Indochina and southern China, which reveal a fan-shaped present-day maximum horizontal stress (SHmax) pattern centered on the eastern Himalayan syntaxis. However, the distance to which forces generated by the Himalayan syntaxis influence the stress pattern in Indochina is not known. We analyzed caliper and image logs from 106 petroleum wells for borehole breakouts and drilling-induced fractures. A total of 558 breakouts and 45 drilling-induced fractures were interpreted in six basins, indicating that a north–south regional present-day SHmax exists in central and southern Thailand and the Gulf of Thailand. The N–S SHmax orientation suggests that forces generated at the Himalayan syntaxis are a major control on the stress pattern throughout Thailand, extending approximately 1000 km beyond the outer limit of syntaxis-associated seismicity. Despite the influence of the Himalayan syntaxis on the present-day stress field, the sedimentary basins of central, southern and offshore Thailand are characterized by structural styles that are somewhat inconsistent with those predicted to result from India–Eurasia collision. Furthermore, localized variations in SHmax orientation, and the predominance of structures associated with purely extensional rifting, indicate that other processes also influence the stress field in Thailand. We suggest that stresses generated by the Sumatran–Andaman subduction zone may also have resulted in significant deformation in offshore Thailand and that the stress pattern may also be perturbed at very local (several km) scales by mechanically weak faults._ _? 2009 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Thailand lies in the heart of one of the most tectonically active regions on Earth and displays an extensive history of Cenozoic deformation (Morley, 2002; Hall and Morley, 2004). Consequently the present-day stress field provides insight into a region of continental crust that is actively deforming (Morley, 2001; Tingay et al., in press). Understanding present-day stress orientations is important for several reasons including: testing tectonic and fault evolution models for the region, hazard prediction associated with fault reactivation, and for the petroleum industry with regard to borehole stability and predicting the orientation of open fracture systems (Hall and Morley, 2004; Morley et al., 2004; Vigny et al., 2005; Tingay et al., 2009)._ _The Cenozoic tectonic evolution of Indochina is often considered to be controlled by stresses and strains arising from the ongoing collision of India with Eurasia (Molnar and Tapponnier, 1975; Morley, 2002; England and Molnar, 2005). The nature of Himalayan extrusion into SE Asia remains a topic of debate, with some authors proposing rigid block escape tectonics (Molnar and Tapponnier, 1975; Tapponnier et al., 1982; Leloup et al., 2001; Replumaz and Tapponnier, 2003) whereas other authors suggest that deformations can be better matched by viscous or visco–elastic flow (England and Molnar, 2005; Shen et al., 2005). Regardless of the nature of Himalayan extrusion, all models predict a fan-like present-day maximum horizontal stress (SHmax) pattern in Indochina centered on the eastern Himalayan syntaxis, with present-day SHmax oriented NNW–SSE to NNE–SSW throughout much of Indochina (Huchon et al., 1994; Kong and Bird, 1997; Morley, 2007). The modeled NNW–SSE to NNE–SSW SHmax orientation in Indochina is supported by stress orientations estimated from earthquake focal mechanisms solutions in northern Thailand and the Yunnan Region of China (Fig. 1; Holt et al., 1991; Huchon et al., 1994; Morley et al., 2001; Morley, 2007). However, there is a relative absence of seismicity south of approximately 17 x14N latitude in Indochina and thus it is not known whether the fan-shaped stress_ _236_ _M.R.P. Tingay et al. Journal of Structural Geology 32 (2010) 235–248_ _Fig. 1. The main Cenozoic–Recent tectonic and structural features of the Myanmar–Western Thailand region of the back-arc mobile belt. Complied from Pivnik et al. (1998), Morley (2004) and Curray (2005)._ _M.R.P. Tingay et al. Journal of Structural Geology 32 (2010) 235–248_ _237_ _pattern observed in regions adjacent to the Himalayan syntaxis extends into southern Thailand and offshore Indochina._ _Models of extrusion tectonics predict different distances at which forces generated at the Himalayan syntaxis should influence the stress field in SE Asia (Molnar and Tapponnier, 1975; Morley, 2002; Hall and Morley, 2004). For example, the rigid block escape tectonics model predicts that the stress pattern, and associated deformations, throughout much of the Sunda plate would be controlled by Himalayan extrusion (Molnar and Tapponnier, 1975; Huchon et al., 1994). However, the Cenozoic tectonic evolution of Indochina has also been strongly influenced by processes other than Himalayan extrusion, most notably stresses arising from the Java–Sumatra–Andaman subduction zone to the south and west of Thailand, gravitational collapse of thickened continental crust in Indochina and the coupling between the Burma block in Myanmar with India (Morley, 2001; Hall and Morley, 2004; Morley et al., in press; Searle and Morley, in press). Hence, the primary aim of this study is to determine the present-day SHmax orientation in central and southern Thailand and the Gulf of Thailand (south of 17 x14N latitude) in order to better establish the forces controlling the present-day stress field in Indochina and examine the distance at which forces generated by the Himalayan syntaxis influence the stress pattern in SE Asia._ _Thailand displays some exceptional examples of extensional fault geometries that can be seen from satellite images, open cast coal mines and from 2D and 3D reflection seismic data (e.g. Rigo De Rhigi et al., 2002; Uttamo et al., 2003; Morley et al., 2004). These fault patterns commonly display multiple orientations, and complex fault propagation and linkage patterns that indicate inheritance of older fabrics, and complex evolution of the stress field with time (e.g. Morley and Wonganan, 2000; Kornsawan and Morley, 2002; Morley et al., 2004, 2007). Investigation of the modern stress field can help determine whether fault orientations oblique to the main rift trend can be explained by simple reactivation of deeper structures, or whether other factors, such as localized stress rotations need to be considered. Hence, a secondary aim of this study is to examine the stress field at small-scales within sedimentary basins in order to investigate whether complex fault patterns in Thailand may be the result of local stress perturbations or the reactivation of deeper structures._ _2. Geological summary_ _Central and southern Thailand and the Gulf of Thailand are tectonically significant regions of Southeast Asia because they lie along the north–south transition from the orogenic region of the Himalayan syntaxis to the subduction-dominated Java–Sumatra margin further south. The tectonic development of Indochina is often considered to be dominated by widespread, large-scale strike-slip faulting associated with Himalayan extrusion tectonics (Molnar and Tapponnier, 1975; Tapponnier et al., 1986). More recent work has, however, established that the Cenozoic tectonic evolution of Thailand is considerably more complex (Hall and Morley, 2004; Morley et al., in press; Searle and Morley, in press). The geography of Thailand very strongly expresses Cenozoic deformation that can be divided into seven main provinces, three in the south and four in the north (Morley et al., in press). The three southern provinces comprise Peninsular Thailand, the Gulf of Thailand and the Andaman Sea, whilst the northern provinces consist of the Western Highlands, the central region, northern central region and the Khorat Plateau (Fig. 1). The provinces are described below from south to north._ _The Gulf of Thailand is dominated by Cenozoic rift systems, and has been an area of subsidence, and extensive sedimentation since the Eocene (e.g. Lockhart et al., 1997; Jardine, 1997; Morley and Westaway, 2006). Some basins are large, extremely deep and subsided rapidly, such as the super-deep Pattani and Malay Basins, which in places contain over 7 km of Neogene section (Morley and Westaway, 2006). The present-day shape of the gulf is due to a sea-level highstand covering an extensive, broadly subsiding intracontinental post-rift basin (maximum water depth of 80 m), which extends onshore as the Central Basin._ Ключевые слова: north malay, stress pattern, peninsular thailand, molnar, hillis, evolution, malay, simons, himalayan extrusion, tingay, geophysical, central southern, standard deviation, gulf, log, eastern himalayan, press, indochina, press searle, petroleum exploration, northern, post, image, khorat group, extensional basin, northern thailand, gulf thailand, morley, stress eld, image log, tapponnier tapponnier, stress generated, borehole wall, journal structural, hall, stress orientation, earth, eastern, ?eld, stress ?eld, hickman, ha, structure, sirikit field, northern indochina, binh, southern, time-structure map, elsevier, pattani basin, bell, locally deected, fault, breakout observed, difs, n–s, condence level, post-rift fault, elongation, tensile failure, day, super-deep pattani, sinistral motion, borehole breakout, outer limit, pattani basins, khorat plateau, schlumberger, syntaxis, science, syn-rift, gps analysis, harrison, pattern, earthquake, shmax orientation, sd, broadly consistent, force generated, shmax, thailand journal, rift, gps data, horizontal, breakout difs, deformation, hinthong, rotation, thailand basin, resistivity, andaman sea, tectonophysics, drilling-induced fracture, syn-rift fault, fracture, offshore thailand, maximum, southeast asia, himalayan, displacement, ptt exploration, international conference, geological society, london memoir, kirsch, four-arm caliper, rift basin, arm, difs observed, observed, journal structural geology, slip, journal, structural style, combined, north, onshore thailand, region, platongpladang trend, generated, aadn?y, central, tectonic evolution, western thailand, data, present-day stress, caliper, strike-slip, formation, dain, tool, himalayan syntaxis, nnw sse, zone, structural geology, situ stress, breakout, basins, result, coblentz, stress, null hypothesis, deformation style, phitsanulok pattani, late, tectonic, major control, journal geophysical, surrounding rock, phitsanulok basin, borehole, tectonics, zoback, upton, structural, geological, faulting, force, ller, eds, burma block, length, wellbore wall, regional, continental crust, shmax direction, asia, curray, rock, magnitude, localized rotation, horizontal stress, strike, relative absence, direction, basin, morley morley, yunnan region, geology, phitsanulok basins, western, oriented, tingay journal, indiaeurasia collision, thailand, cenozoic, vigny, meyer, trend, dextral motion, post rift, southern thailand, ranong fault, elongation zone, doi, bird, phitsanulok, tectonic implication, average, motion, malay basin, sedimentary basin, strike slip, collision, maximum horizontal, cross-sectional shape, tapponnier, sagaing fault, bulletin, indian plate, south, petroleum, open fracture, combined length, post-rift, pattani, map, analysis, orientation, andaman, present-day