Abstract

The Tertiary rift basins of Thailand and adjacent countries show considerable variability in the timing of rift initiation, termination, the timing and magnitude of thermal subsidence, and the timing and intensity of inversion episodes. The rift basins developed on continental blocks that were extruded southeastwards. Hence their development must be tied into Himalayan extrusion tectonics. Current tectonic models propose that the Tertiary basins opened up as pull-apart basins associated with strike-slip faults. Published geochronology of strike-slip fault zone rocks, mapping of fault patterns in the Tertiary basins and mapped releasing-restraining bend geometries all indicate that in Thailand major sinistral strike-slip motion ceased at about 30 Ma, prior to the formation of most rift basins the Thailand. The effects of later dextral slip were minor and probably a result of reactivation during episodes of inversion during NW–SE to NE–SW (Himalayan) compression. Dextral slip was not responsible for opening most of the rift basins in Thailand. An alternative mechanism to open the rift basins is subduction rollback of the Indian plate to the west of Thailand. It is proposed that subuction rollback can help explain some of the characteristics of the rift basins such as non-uniform lithospheric extension, deep sag basins, and the diachronous onset and termination of rifting.

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