Structural Evolution of the Phrae Basin, Northern Thailand

Phumee Srisuwon, completed PhD project

The Phrae Basin is a Tertiary fault-bend basin covering approximately 1,100 square kilometres that formed during the Late Oligocene-Early Miocene. Sinistral strike-slip movement of the Phrae-Thoen Fault was a major control on the creation of accommodation space in the Phrae Basin. Three sub-basins have been revealed from 2D seismic and Landsat data. The Phrae and Rong Kwang sub-basins cover most of the Phrae Basin. They are separated by a basement high which may be part of a transfer zone or may be created by right overstep of the controlling left lateral Phrae-Thoen Fault. The Song sub-basin is separated from the Rong Kwang sub-basin by the Long Fault.
2D seismic data shows that the Phrae Basin contains three main genetic sequences. There are rift fill, rift fill associated with inversion and post-rift sequences. The rift fill sequence can be separated into early rift fill which occurs locally in the central area and the main rift fill that was deposited throughout the basin. The rift fill associated with inversion is seen into two stages. The first stage is represented by folding in the Rong Kwang sub-basin and in the Phrae sub-basin is expressed by folds and reversal of the eastern fault margin. The second stage is represented by a broad anticline in the rift fill sequence, continued reversal of the eastern border fault and reverse movement on the fault in the centre of the basin. The post-rift sequence covers almost all the basin, except in the central and eastern parts where this unit was uplifted.
Borehole data shows that alluvial fans and braided channels are dominant during early stage of deposition of the Phrae Basin Formation, while in the middle units lacustrine, peat swamp and poorly drained floodplain deposits alternate with braided channels and alluvial plains. Coal facies are commonly associated with lake margin and peat swamp environments in the middle units, although the western area contains thicker lacustrine and peat swamp sequences while alluvial plains and braided channels are dominant in the eastern part of the basin. In the Rong Kwang sub-basin no core is available but 2D seismic characteristics show that this part of basin was probably dominated by high energy fluvial deposits.
The regional controls on the evolution of the Phrae Basin are extension associated with subduction of the Indian Oceanic Plate beneath the Sunda Shelf and strike-slip displacement on major regional strike-slip faults. Inversion is associated with the interaction of the Philippine and Australian Plates.