Tectonic evolution of eastern Indonesia and its bearing on the occurrence of hydrocarbons

Abstract

The evolution of the eastern Indonesian island arcs over the last 30 million years has been strongly affected by the northward drift of the Australian continent and the westward thrust of the Pacific Plate. As Australia drifted northward, New Guinea first collided with the Sepik island arc about 30 million years ago. About 20 million years ago, a major reorganization in the subduction pattern of the Indonesian region took place, resulting in the formation of an 8000 km long, east-west trending arc-trench system stretching from the western tip of Sumatra to Buru and even further to the Indonesian arc by way of Java, the Lesser Sunda Islands, Timor, Tanimbar, Kai and Seram. Prior to the arrival of the Australian continent at the Southeast Asian continental margin, a north-south oriented Sulawesi-Mindanao volcanic arc existed about 800 km east of Borneo. Further to the southeast, New Guinea and Sepik, now welded into a bigger microcontinent, collided with the Inner Melanesian island arc. This opened the Australian Plate to the influence of the WNW moving Pacific Plate. About 10 million years ago a south dipping subduction zone broke through north of Irian Jaya but no volcanism accompanied this process. The most dramatic event in the geologic history of Indonesia took place about 5 million years ago when the anticlockwise rotation of Irian Jaya and continuous northward movement of Australia trapped the Banda Sea and caused ‘tectonic shaving’ of the Birdhead of New Guinea along the Sorong transform fault system. Subsequently, Buton and Sula and other microcontinents collided with Sulawesi and Halmahera transforming the double island arcs into a K-shaped form. A small west-dipping subduction zone developed in northern Sulawesi accompanied by active volcanoes in Minahasa and the Sangihe island. Other small subduction zones with reverse polarities subsequently developed in northwest Sulawesi and Halmahera, which can be held responsible for the generation of, respectively, the active Una-Una volcano in the Gulf of Gorontalo, Central Sulawesi, and the volcanoes in the western Halmahera arc. Active collision accompanied by emplacement of ophiolites is presently taking place between the west-facing Halmahera arc and the east-facing Minahasa-Sangihe arc. The tectonic evolution of eastern Indonesia, which clearly envisaged a structural continuation of the Sunda-Banda arc and the existence of the north-south trending Sulawesi-Mindanao arc during the Tertiary period, could conveniently be used to explain the hydrocarbon occurrence in complex paleosubduction or collision zones of Timor, Seram and eastern Sulawesi. Whereas oil and gas accumulation in the pull-apart basins of Irian Jaya are found mostly in Tertiary deposits, the source rocks in the collision zones are likely to be of Mesozoic age. Main exploration target areas in eastern Indonesia are the intracratonic basins of the Arafura Shelf, the marginal (rift) basins skirting the southern and eastern section of the Banda arc, the collision zones of Timor, Seram, East Sulawesi and the thrustbelt of Irian Jaya.