PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Sixth AMWI Convention, May 1977

317

PAST AND PRESENT GEOTECTONIC POSITION OF SULAWESI, INDONESIA.

JOHN A. KATILI *I

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 a x , trapping of old oceanic crust caused by the bending of an island arc, formation of a marglnal 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 north- ward 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 beaioff zones dip under the northern Sulawesi and Halma- hera 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 Mio- cene 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 Aus- tralian-New Guinea plate which occurred in early Pliocene time severely transformed Sulawesi into an island with its convex side turned towards the coritiaent, at the same time causing ibduction of opfriolite in the eastern arc of this island.

The movement of the Pacific piate 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 tne western arc of Sulawesi and eastern Kalimantan causing obduction of ophiolites in the Meratus Mountains and slight deformation of the sediments in the eastern Kalimansan 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 sourhern part of the west arc of Sulawesi. Two small spreading centers then developed, one each on the northern and southern part of the Mabassar Strait, connected by the Passernoster fault

*) **)

Director General of Minerals, Ministry of Mines, Jakarta, undoIresia) Abstract only is presented with illustrations. The full text will be published in Tecronophysks.

© IPA, 2006 - 6th Annual Convention Proceedings, 1977

, 318

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 east- dipping small subduction zone which could be held responsible for the formation of the late Quaternary Lompobatamg 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-south- easterly 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.

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