Late Oligocene-Recent stress evolution in rift basins of northern and central Thailand: implications for escape tectonics

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 mix ed perspectives on how the basins evolved, some providing data on ages and structural geometries, others palaeostress data. In general, the region evo lved under approximately E-W extension, although the extension direction pr obably changed periodically within basins. Extension was episodically inter rupted by inversion events associated with sub-horizontal NW-SE to NE-SW si gma 1 direction. There is no evidence to suggest that the string of rift ba sins 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 differen t, although certain regional trends along the rift system are apparent as f ollows: (I) Oligocene-Lower Miocene extension is widespread in the region. (2) In central and northern Thailand Middle Miocene extension is also impor tant, and extension persisted into the Upper Miocene-Pliocene. (3) In the e xtreme south of the area (W. Natuna basin, Penyu basin, Malay basin) extens ion ceased in the earliest Miocene. (4) In the northern Gulf of Thailand ex tension ceased in the Middle Miocene. (5) Thermal subsidence is greatest (u p to 4 km) in the south (Malay, Pattani, W. Natuna and Penyu basins) and le ast 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 pat chy during the Miocene, but is strongest in the northwestern rift basins (p articularly the Li basin). The most widespread inversion event that affects the north occurred during the Plio-Pleistocene. It might be associated wit h 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 evolu tion is more complex than can be explained by simple escape tectonic models . (C) 2001 Elsevier Science B.V. All rights reserved.