Late Oligocene±Recent stress evolution in rift basins of northern and central Thailand: implications for escape tectonics C.K. Morley a,1, * , N. Woganan b,1 , N. Sankumarn c,1 , T.B. Hoon d,1 , A. Alief e,1 , M. Simmons a a Department of Geology and Petroleum Geology, King's College, Meston Building, Aberdeen University, Aberdeen AB24 3UE, UK b Department of Geological Sciences, Chiang Mai University, Chiang Mai 50200, Thailand c Department of Geotechnology, Faculty of Technology, Khon Kaen University, 40002, Thailand d Amerada Hess, Suite 9.02, Level 9 Menara Tan and Tan, 207 Jalan Tun Razak 50400, Kuala Lumpur, Malaysia e Unocal Balikpapan, P.O. Box 276, Pasir Ridge, Balikpapan 76102, East Kalimantan, Indonesia Received 23 May 2000; accepted 18 December 2000 Abstract The Tertiary rift basins of Thailand are known through sub-surface seismic and borehole data collected for hydrocarbon and coal exploration, outcrops in open-cast coal mines, and a few natural outcrops. These data provide mixed perspectives on how the basins evolved, some providing data on ages and structural geometries, others palaeostress data. In general, the region evolved under approximately E±W extension, although the extension direction probably changed periodically within basins. Extension was episodically interrupted by inversion events associated with sub-horizontal NW±SE to NE±SW s 1 direction. There is no evidence to suggest that the string of rift basins running from N. Thailand and Laos into the Gulf of Thailand evolved in a similar way. On the contrary, the evolution of each basin seems different, although certain regional trends along the rift system are apparent as follows: 1) Oligocene±Lower Miocene extension is widespread in the region. 2) In central and northern Thailand Middle Miocene extension is also important, and extension persisted into the Upper Miocene±Pliocene. 3) In the extreme south of the area W. Natuna basin, Penyu basin, Malay basin) extension ceased in the earliest Miocene. 4) In the northern Gulf of Thailand extension ceased in the Middle Miocene. 5) Thermal subsidence is greatest up to 4 km) in the south Malay, Pattani, W. Natuna and Penyu basins) and least in northern Thailand a few tens to hundreds of metres). 6) Inversion in the southern Gulf of Thailand was intense and occurred during the Lower and Middle Miocene. In the northern Gulf of Thailand inversion is very mild and occurred during the Lower and Middle Miocene. Onshore inversion is patchy during the Miocene, but is strongest in the northwestern rift basins particularly the Li basin). The most widespread inversion event that affects the north occurred during the Plio-Pleistocene. It might be associated with the change from left to right lateral motion on the Red River Fault. The relationship between strike-slip faults and rift basins in terms of timing of extensional and inversion events, and palaeostress orientation and evolution is more complex than can be explained by simple escape tectonic models. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Thailand; escape tectonics; extension; inversion; strike-slip 1. Introduction An extensive Tertiary rift system runs through Thailand and adjacent areas of Laos and Myanmar, Tectonophysics 334 2001) 115±150 0040-1951/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0040-195100)00300-0 www.elsevier.com/locate/tecto * Corresponding author. E-mail addresses: chrismmorley@hotmail.com C.K. Morley), c.morley@abdn.ac.uk C.K. Morley). 1 Formerly at Department of Petroleum Geoscience, University of Brunei Darussalam, Bandar Seri Begawan 2028, Negara Brunei Darussalam.