Abstract
Abstract The Tertiary basins of the northern Sunda Shelf are underlain by normal and attenuated continental crust that is characterized by moderate to high average geothermal gradients in excess of 5°C/100 m. In the Malay basin, upper Oligocene and younger sediments are more than 12 km thick; in the other basins, such sediments are between 4 and 8 km thick. The Malay, Penyu and West Natuna basins are aulacogens meeting at a triple junction that marks a Late Cretaceous hot spot in the centre of the Malay Dome. Sub-basins (commonly half-graben) developed as pull-apart basins within regional, north to northwest-striking, wrench fault zones. The NW-striking Three Pagodas fault most probably extends farther southeast as the Axial Malay fault in the basement along the length of the Malay basin. Pre-late Oligocene sinistral slip along this fault developed east-west half-graben that fixed the position and orientation of inverted anticlines that later developed in the basin-filling sediments through strike-slip reversal along the fault. Initial basin subsidence took place during Eocene-Oligocene time. Regional tensional conditions prevailed until the early Miocene. During the middle to late Miocene, regional compressional stresses caused reversals of the sense of motion on the major wrench faults, and structural inversion of the basin-filling sediments. On some of the north-striking wrench faults there are indications of up to 45 km of right-lateral displacement which is possibly post-Miocene. The regional wrench faults have acted as domain boundaries with each tectonic domain characterized by different stress fields. The stress systems evolving during the Cenozoic are attributed to varying degrees of interference of plates coupled with changes in convergent directions and/or rates of motion of the Pacific plate, the Indian Ocean-Australian plate, and continued, differential extrusion of SE Asian crust following the collision of the Indian plate with the Eurasian plate.
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