The Permo-carboniferous Facies Development in Thailand: a Plate-tectonic Discussion

T h e Permo-Carboniferous sedimentary facies development in Thai land reflects the late V a r i s c a n orogeny in S E A s i a . During the Carboniferous and Permian, a N S trending trough separated the " S h a n T h a i C r a t o n " and the " Indosinia C r a t o n " . In this pelagic basin, deposition of ribbon cherts continued from pre-Asselian to Kubergandian. I n L o w e r Permian, carbonatic and tuffitic turbidites were transported from the neighbouring platforms into the basin. T h e Middle Permian flysch sedimentation resulted from a strong orogenic activity. T h e basin was E-vergent, isoclinally folded and pverthrusted. Parts of the basin are metamorphosed into the greenschist facies. I n the eastern marginal parts Kubergandina to M i d i a n ; molasse was deposited from the new rising fold belt. T h e intensity of folding of the molasse decreases towards the east or the younger strata. T h e total width of the basin was probably not greater than 200 km. T h e pelagic sediments, f lysch and molasse represent a thick pile of a coarsening-upwards sequence, typical of subduction related sutures. Folding affected a marginal marine basin and w a s caused by a westward directed subduction (A-subduction) under the volcanic arc. West of the arc, pebbly mudstones were laid down on the trench slope or the continental margin of Paleoeurasia. T h e deposition of these mixtites continued through the Carboniferous to L o w e r Permian and came to an end contemporary with the relative uplift of the " S h a n T h a i C r a t o n " and the onset of the A-subduction under the Petchabun marginal basin. T h e Benioff-subduction must have been located west of the depositional area of the pebbly mudstones and directed towards the east.