Abstract

The Indonesian Archipelago is a collection of microcontinents, rifted and drifted continental blocks, island arcs and ophiolite fragments, partly enclosed by material accreted on active plate margins and overlain by younger sediments commonly called tectonic cover, all welded together into the current configuration. In the geological evolution of Southeast Asia active plate margins resulting in island arc development have played an important role since Late Triassic time. In contrast the northwestern margin of Australia has always been a passive margin. Based on paleontology, paleomagnetics and stratigraphy the Indonesian Archipelago is provisionally subdivided into 13 terranes. Material accreted along active margins commonly called accretionary terrane is excluded from the terrane classification. The terranes are Proto-Kalimantan, Sumatera Basement, southwest Sulawesi, Australia, northern Irian Jaya Arc, Sulawesi Ophiolite, Sumba, Seram, Timor, Buton, Banggai-Sula, Buru and Bacan. The continental fragments Sikuleh and Natal have recently been reported to occur in the southwestern coast of Sumatera. Proto-Kalimantan and Sumatera basement, both including island arcs, were amalgamated during Late Triassic time along the Bentong-Raub suture to form the Sunda Platform. This has continued to develop as an active margin up to the present day, producing the Banda Arc. During the Paleogene southwest Sulawesi rifted away from East Kalimantan to collide with oceanic crust to the east and in the Tertiary west Sulawesi magmatic arc came into existence.

Sulawesi Ophiolite originates from oceanic crust pushed westward by the migrating Banggai-Sula terrane and blocked by the Tertiary West Sulawesi magmatic arc. The terranes Sumba, Buton, Seram and Timor are a result of a rift-drift event affecting northwest Australia during Jurassic time. The terranes Banggai-Sula, Bacan and Buru have been formed by the Sorong Fault slicing off the northernmost portion of Irian Jaya and moving the slices westward. The northwestern Australian continent, including Irian Jaya, has acted continuously as a passive margin moving northward behind a front of oceanic crust. It collided with the northern Irian Jaya Island Arch during Oligocene time after the oceanic crust had been consumed by subduction. The polarity of the subduction changed after the collision, becoming southward.