Abstract

The pre-Tertiary framework of Sumatra consists of a mosaic of continental and oceanic microplates accreted in the late Triassic when the Mergui, Malacca and East Malaya Microplates were joined together to form Sundaland. Further accretion involving the west coast Woyla Terrains followed in the late Mesozoic. Concurrent magmatism and faulting affected much of Sundaland.

The continental Mergui Microplate occupies the central core of Sumatra from Aceh to southern Jambi and is characterised by a complex history which included older and younger Palaeozoic granite plutonism, late Permian arc volcanism and the widespread deposition of Permo-Carboniferous "peobly mudstones". A NW–SE to N–S trending Triassic suture complex, traceable at subcrop from Riau to the Palembang district and named the Mutus Assemblage, separates the continental Malacca Microplate to the east. This microplate is poorly known, but appears to be dominated by low grade metasediments cut in the east by granites that represent the continuation of the Triassic Main Range Granites of the Malay Paninsula. The East Malaya Microplate is characterized by Permo-Triassic arc magmatism and lies east of a line joining Kundur and NE Bangka. The boundary is a complex one associated with mafic and ultramafic rocks related to the Raub-Bentong Line of peninsula Malaya. The Woyla Terrains consist of tectonised, Jurassic and Cretaceous arc volcanics and ophiolites.

The major zone of basement weakness during the development of the back-arc Central and South Sumatra Basins was the Mutus Assemblage. This region experienced Miocene alkaline magmatism and, due to a combination of high heat flow and the early growth of structures, is the site of ± 95% of the two basin's oil production. Young Tertiary structures in this zone are related to wrenching in the north; and in the south, to compressional reactivation of cross cutting WNW–ESE faults formed during the accretion of the adjacent Woyla Terrains.