Abstract

Sulawesi with its peculair K-shaped pattern is situated in an area where the Eur-Asian, Indian-Australian and Pacific plates interact and collide.

Complex geological processes in this area resulted in the transformation of a normal island arc structure into an inverted one. Deformation of an already tectonized belt, sweeping of fragments against unrelated terrain, thrusting of oceanic and mantle material over the island arc, closing of deepsea basins behind the arc, trapping of old oceanic crust caused by the bending of an island arc, formation of a marginal basin by the spreading of the seafloor behind the arc, development of small subduction zones with reverse polarities etc. illustrate this phenomenon.

Small deepsea basins surrounding Sulawesi such as the Gulf of Bone and the Gulf of Gorontalo originally formed the arc-trench gap of the Sulawesi island arc.

The Banda Sea is considered as an oceanic crust trapped by the rolling up of the east west trending Banda arc due to the northward drift of Australia combined with the westward movement of the Pacific Plate. Similarly the Sulawesi Sea consists of an old Pacific crust trapped by the westward bending of the Sulawesi island arc caused by the spearheading westward thrust along the Sorong transform fault system. Later a minor spreading center became active in the central part of the Sorong fault zone. The Molucca Sea comprises a tectonic melange situated between the two colliding arcs of northern Sulawesi and western Halmahera. While the benioff zones dip under the northern Sulawesi and Halmahera arcs in normal fashion, the melange thrusts over them. The Strait of Makassar is a marginal basin which was brought into existence by the spreading of the seafloor between Kalimantan and Sulawesi.

The evolution of Sulawesi started in Miocene time or even earlier when 800 km east of Kalimantan a northsouth trending east facing island arc came into existence originating from a spreading center located in the Pacific Ocean.

Collision between Sulawesi and the Australian-New Guinea plate which occurred in early Pliocene time severely transformed Sulawesi into an island with its convex side turned towards the continent, at the same time causing ibduction of ophiolite in the eastern arc of this island.

The movement of the Pacific plate continued and gradually pushed Sulawesi towards the Asian continent resulting in the closing of the sea between Kalimantan and Sulawesi at the end of Pliocene. Collision took place, this time between the western arc of Sulawesi and eastern Kalimantan causing obduction of ophiolites in the Meratus Mountains and slight deformation of the sediments in the eastern Kalimantan oil basins.

Opening of Makassar Strait took place in Quaternary time along the Pasternoster and Palu-Koro transform faults.

The first movement leading towards the opening of the strait began with a sinistral transcurrent shift along the northwest-southeast trending Pasternoster fault, which affected both Kalimantan and the southern part of the west arc of Sulawesi. Two small spreading centers then developed, one each on the northern and southern part of the Makassar Strait, connected by the Pasternoster fault which now acted as a dextral transform fault and moved the northern part of the western Sulawesi arc towards the east.

Spreading in the southern part of the Makassar Strait was accomodated by an eastdipping small subduction zone which could be held responsible for the formation of the late Quaternary Lompobatang and Barupu volcanoes in southern Sulawesi.

In the northern part of the Makassar Strait the eastward movement of Sulawesi was checked by the newly formed Palu-Koro transform fault which moved the part of Sulawesi west of the fault in a south-southeasterly direction until the island gained its present position. This movement presumably caused destruction of the small spreading centers in the Makassar Strait and consequently terminated the activities of the late Quaternary volcanoes in southern Sulawesi.

Some unsolved problems in relation to the geology of Sulawesi and the surrounding areas are also discussed.