| Abstract: |
Thailand contains four north-south trending tectonic provinces. From west to east these are: 1. Downfaulted Palaeozoic-Jurassic unstable shelf (extending into Burma), 2. Upfaulted and arched Precambrian metasediments with Lower Palaeozoic stable shelf cover beds. This belt now forms most of the western mountain chain, 3. Palaeozoic-Triassic continental margin basin, thrust-faulted, overturned to east, now forming central part of Thailand, 4. Shelf basin of late Triassic to Cretaceous redbeds terminating with marine evaporites, gently folded except on the eastern margin, now forming the Khorat Plateau. Eastward from the Khorat Plateau, Laos and Vietnam contain Palaeozoic marine strata and Mesozoic redbeds that apparently overlie Precambrian basement. The three western provinces of Thailand are long narrow belts that extend north through the Shan States and south the the Malay Peninsula. The Precambrian represents a Shan-Thai Craton that during the Palaeozoic Era had a different drift history from the Indosinian Craton. Ultramafic rocks at Mae Nam Nan are regarded as a fragment of ophiolite indicating ocean floor crust underlying the enveloping Silurian-Devonian phyllite. The ultramafic body is thus considered to mark a suture (Nan Suture) between the Shan-Thai Craton and Indosinian Craton. Continental collision, resulting from sub-duction dipping westward beneath the Shan-Thai Craton probably commenced during the Permian and was completed by middle to late Triassic, and was accompanied by extrusive rocks and granite emplacement. Subsequently the united Shan-Thai and Indosinian continental block was sutured to the south side of the China Craton by closing of the south-west Yunnan-northern Vietnam Mesozoic Geosyncline where ophiolites were obducted along the line of the Red River.
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