Deformation style of the Mesozoic sedimentary rocks in southern Thailand

• The Mesozoic rocks have been folded to form two huge synclinoria.
• The folds have steeply dipping eastward limbs and gently dipping westward limbs.
• These folds were transected by brittle fractures in four major directions.
• They indicate WNW–ESE to E–W contraction with top-to-the-east simple shear.

Mesozoic sedimentary rocks in southern Thailand are widespread from NNE–SSW and N–S in Chumphon and Trang provinces. The Mesozoic stratigraphic units are the marine Triassic Sai Bon Formation and the non-marine Jurassic–Cretaceous Thung Yai Group, the latter subdivided into Khlong Min, Lam Thap, Sam Chom, and Phun Phin Formations. These units overlie Permian carbonate rocks with an angular unconformity, and are overlain unconformably by Cenozoic units and the Quaternary sediments. The Mesozoic rocks have been folded to form two huge first-ordered syncline or synclinoria, the Chumphon and Surat Thani–Krabi–Trang synclinoria. These synclinoria are elongated in NNE–SSW to N–S direction, and incorporate asymmetric lower-order parasitic folds. The folds have moderately to steeply dipping eastward limbs and more gently dipping westward limbs. These folds were transected by brittle fractures in four major directions. These geologic structures indicate WNW–ESE to E–W contraction with top-to-the-east simple shear at some time before the deposition of the Cenozoic sedimentary units. No major deformation has affected the rocks subsequently, apart from the formation of the fault-controlled Cenozoic basins.

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