Evolution of the Chumphon Basin, Gulf of Thailand

Anongporn Intawong, completed PhD project

Large scale strike-slip faults, such as the NW-SE trending Red River and Mae Ping Faults, and an apparently conjugate set of NE-SW trending faults, such as the Ranong and Khlong Marui Faults, form a prominent part of the geology of SE Asia. In the past it had been assumed that basin formation in the Gulf of Thailand is a result of dextral shear between parallel NW-SE trending strike-slip faults, although more recently extensional models have been proposed to explain the origin of these basins.

In the western Gulf of Thailand there is a close relationship between basins such as the Chumphon Basin and the NE-SW trending Khlong Marui Fault. The fault is commonly shown on maps as cutting the Thai-Malay Peninsula and is usually shown with a sinistral sense of movement. However, there are no documented field descriptions on which to base a deformational history of the Khlong Marui and Ranong Faults. The thesis seeks to address this question and is based on an integrated interpretation of digital elevation data, satellite imagery and observations of field outcrops along the Khlong Marui Fault zone. The relationship between the Khlong Marui Fault and the formation of sedimentary basins is addressed using 2D seismic data from the Krabi, Khien Sa and Kantang Basins that are located on the Thai Peninsula to the south of the Khlong Marui Fault, and from the offshore Chumphon Basin which is located at the NE end of the fault.

The southern Thailand basins revealed a regional mode of extension. An early period of rifting was initiated in the Late Eocene and became widespread by the Oligocene. A period of inversion took place in the eastern basins (Chumphon, Songkhla and possibly the Khien Sa Basins) at the start of Miocene, and maybe related to the formation of an unconformity at this time in the Krabi Basin. Later extension was renewed in the Early Miocene, and the Middle Miocene corresponds to a period of more general basin subsidence in the Chumphon, Songkhla, and Andaman Sea Basins. Subsequently, extension was renewed within the basin sag sequences in the Late Miocene, or slightly earlier in the Songkhla Basin, resulting in the development of complex conjugate fault splays with minor displacements. A further period of inversion during the Pliocene is recorded in the Chumphon Basin, possibly coinciding with a period of uplift on the Thai Peninsula and in the Andaman Sea.

The Khlong Marui Fault marks a significant change in geology, topography and heat flow and there is evidence that it has continued to be active until relatively recently. Within the Thai Peninsula, Tertiary basins are only found to the south of the fault, and there is a noticeable change in the dip polarity of basins across it.

Field evidence and thin section analysis show that the Khlong Marui Fault experienced an initial phase of sinistral ductile shear, contrary to the predictions of the extrusion tectonics model used to explain the origin of the more widely studied Red River and Mae Ping Faults. This early phase of deformation probably predates basin formation. Later brittle sinistral shear may be related to the period of initial basin extension when the Khlong Marui Fault acted as a major accommodation zone between the Chumphon and Khien Sa Basins. A later phase of brittle dextral shear is related to subsequent basin inversion in the latest Oligocene or earliest Early Miocene.