The Unique Tectonic Setting of

Sulawesi, Indonesia

Origin

Many theories as to the origin of Sulawesi suggest the possibility of a double arc system. This is due to Sulawesi being composed primarily of two distinctly different igneous assemblages. The eastern arc has an abundance of mafic and ultramafic rocks, whereas the constituents of the western arc include granite and granodiorite associations, divided by the Median-zone, a north--south trending depression which is filled with Tertiary sediments. Sulawesi is thought to have emerged as a north--south striking east facing island arc 800 km east of Kalimantan (Borneo) during the Miocene. This arc originated from a spreading center which was located in the Pacific Ocean, which may have developed due to a change in the movement of the Pacific Plate during Eocene--Oligocene time.

A north-south trending subduction zone in the region 25 Ma collected some unusual trench deposits such as Alpine ultramafics, serpentine, tholeitic basalt and gabbro, Mesozoic carbonates, and radiolarian cherts, in addition to the normal pelagic sediments. The subduction zone was also the site of intensive volcanism and plutonism which produced much of Sulawesi's western arc.

As this episode of subduction came to a close towards the end of the Miocene, isostatic compensation caused the area to uplift which created a north--south trending double island arc. At this point this double island arc was located 600 km east of Sulawesi's present position. Then during Pliocene time the Australian continent, proceeding on its northern trend joined the counter clockwise rotation of New Guinea. This union, accompanied by a sharp westward thrust along the Sorong transform fault system transfigured Sulawesi into its unique K-shaped pattern, while also bending the Banda arc in a westerly direction and drove the islands of Sula Spur and Buton against the eastern and southeastern arms of Sulawesi.

There is another theory as to the origin of Sulawesi which is based on similarities in lithology and structures (see Collision with Kalimantan). The theory suggests that Sulawesi might have actually been a part of Kalimantan that rifted off at the end of the Pliocene.

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Simplified geologic map of Sulawesi representing the distinct lithologies of the two arcs (Katila 1978)

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