The Unique Tectonic Settingof Sulawesi, IndonesiaIntroductionSulawesi is located in Indonesia, east of Kalimantan (Borneo). In this region a junction exists between the Philippine and Australian Sea Plates, as well as a number of smaller plates which together form the Southeast Asian and Eurasian margins. The predominate theory as to the origin of Sulawesi is that it developed as a double island arc. The main contention for this thought is that the two halves of Sulawesi, the western arc and the eastern arc have very different rock types. The arc is thought to have developed in the Pacific Ocean during Eocene--Oligocene time. A subduction zone in that region some 25 Ma collected some very unusual types of rocks including ultramafics, various sediemtary rocks and pelagic sediments. This subduction zone also developed intense volcanism which created Sulawesi's western arc. Following the end of that period of subduction, tectonics involving a northward moving Australian continent inverted Sulawesi into its unique K shape. The western arc of the island, which includes the southern and northern arms, as well as the western portion of central Sulawesi, is composed of Cretaceous aged subduction complexes which have been overlain by sediments that may be from an outer arc basin. The arc basin sediments are overlain as well by Upper Paleogene continental shelf deposits. The ages of these clastics range from Lower Paleogene to Upper Eocene. These basin and shelf deposits have been overlain by volcanic and sedimentary rocks of Neogene age, and intruded by those same Neogene volcanics. The eastern arc, which is composed of eastern central Sulawesi and the East and Southeast Arms has a much different assemblage of rocks. This part of Sulawesi is made up of subduction complexes and ophiolite fragments. On the western portion of central Sulawesi is a belt of high pressure, high temperature blueschist-facies rocks which are juxtaposed upon both unmetamorposed rocks and metamorphic rocks of completely different temperature/pressure gradients. There is an eastern belt of sedimentary and low grade metamorphic rocks that contains both pelagic and terrigenous clastic rocks. The clastics include limestone, shale, red and gray chert, and sandstone. Throughout its history Sulawesi has gone through many tectonically induced alterations. At one point it was driven into the eastern coast of Kalimantan, and then it was pushed away as the ocean opened again. During this period of extension Sulawesi was twisted and altered some more. This is a very unique tectonic setting, but, also an extremely compllecated one.
Map of Indonesia (http://www.govacation-indonesia.com/01engl/gv_indo/02indomp/indompsm.html)
Australian and Euarasian Sea Plates in Eastern Indonesia (Hall et al. 1995) |
