Abstract

The Tertiary structural evolution of the North Sumatra basin is described as a consequence of its position along the trailing edge of the counter-clockwise rotating "Sunda Microplate".

Eocene to Lower Oligocene highly acute angle convergence between the Sunda Microplate and the Indian-Australian Plate generated northward propagating, dextral, overstepping wrench faults along the western edge of the continental microplate. The configuration of the ensuing North Sumatra basin was a consequence of the geometrical inter-relationship of these wrench fault systems.

Counter-clockwise rotation of the Sunda Microplate commenced during the Late Oligocene as a result of crustal attenuation and rifting in the Thai and Malay basins. Movement of the microplate was effected by resultant right lateral strike-slip movement along the Ranong and Khlong Marui Fracture Zones in the Malay Peninsula. The North Sumatra basin developed during the Late Oligocene through Early Miocene as tensional horst and graben structures formed between reactivated dextral wrench faults located along the western edge of the continental microplate.

A phase of Lower to Middle Miocene uplift, which heralded breakup in the Andaman Sea reactivated the earlier rifted structures of the North Sumatra basin and caused widespread erosion resulting in region-wide unconformities. The uplift was closely followed by rapid basinal subsidence which allowed invasion of the North Sumatra basin by marine waters for the first time in the Tertiary.

A second phase of counter-clockwise rotation of the Sunda Microplate, commencing in the late Middle Miocene and continuing to the present day, was caused by the emplacement of oceanic crust in the Andaman Sea.

Renewed inter-plate convergence since the late Middle Miocene between the Sunda Microplate and the Indian-Australian Plate, has taken place at a less acute angle, causing regional tectonic compression and the inception of: a subduction complex along the western edge of Sumatra, uplift of the Barisan Mountains, and a period of regressive sedimentation across the North Sumatra basin.

Late Middle Miocene to Recent opening of the Andaman Sea has greatly altered the tectonic complexion of the North Sumatra basin where oceanic transform faults have intersected the North Sumatran landmass as major wrench faults.

The consequences of the changing stress pattern, from a simple Paleogene tensile regime to a complex Neogene compressive regime, are of major significance to an understanding of the structure of North Sumatra. Consideration of the incremental development of any fault system must involve the counter-clockwise rotation, as well as the overall change from a tensile to a compressive regime.

The tectonic evolution of the Northern, Central and Southern Sumatran basinal areas is essentially identical, varying only as a function of geographical location within the deforming Sunda Microplate. Likewise, interpretation of the temporal development of any one area is a function of its spatial position within the deforming microplate.

This structural model explains the orientation of the North Sumatra sedimentary basins, their stratigraphic thicknesses and sedimentological characteristics. It also shows areas of increased heat flow, hydrocarbon migration, and trap configurations.