# The Geology of Indonesia/Regional Tectonic Framework

Regional Tectonic Framework The Indonesian archipelago is situated on the southeastern extension of the Eurasian Plate. It is bounded to the south and west by the Indon-Australian (Indian Ocean) Plate, and to the east by the Philippine Sea and Pacific Plates. The margin of the plates are in collision, resulting in the consumption of plates along subduction zones, the creation of volcanic arcs, and the formation of compressional and oblique slip structures. It is generenally acknowledged that the physiographic setting of the Inndonesian archipelago is dominated by two so called continental shelfal regions. The Sunda shelf area (or Sundaland to erverl authors) lies to the west, and the Sahuls shelf area to the east, separated by a geologically complex region of deep sea basins and island arcs.
Both shelfal areas provide some semblance of stable continental cores to the eastern and western halves of the archipelago. The Sahul shelf area, part of Indin Ocean-Australian continental plate, extends through most of Irian Jaya, the Arafura Sea and the soutern part of the Timor Sea and southward on towards the present Australian landmass. Th Sunda shelf area represents the submerged souteastern outgrowth of the Eurasian contienantal plate and comprises peninsular Malaysia, most of Sumatra, Java and Borneo, mosr of the Java Sea and the southern part of the South China Sea.
The shelfal area, consisting of intensely deformed pre-Tertiary sedimentary and crystaline igneous and metamorphic rocks, has been tectonically stable since Teritary time. The more unstable marginal parts has been subjected to Tertiary mountain building and accompanying subsidence movements and presently expresses itself as a volcanic inner arc of islands (trench-slope break). The volcanic arc comprises Sumatra, Java and extends furhter estward into the Lesser Sunda Islands of Bali, Lombok, Sumbawa, Flores and the smaller islands rimming the Banda Sea to the east and northeast. The non-volcanic outer arc comprises the islands owest of Sumatra and a submarine ridge south of Java, with the non-volcanic islands of Timor, Tanimbar, Kai and Seram believed to bits eastern continuation.
Western Indonesia is chiefly an area of Tertiary sedimentary deposition, whereast eastern Indonesia was a major depocentre throughout late Paleozoic and Mesozoic time. Belief in the structural continuity of the Sumatra, Java, Banda Sea arcs (also known as Sunda arc) with the two continental domains of the Sunda Shelf and the Sahul Shelf has been widely accepted in the past but it now appears that these arcs are the resulting features of long term plate convergence. Actually, it was not until the late 1960's that the new concepts were introduced to describe the geologic evolution of the Indonsian archipelago. these ideas negated the older tectonic models developed in the 1930's and the 1940's which followed the concept of several orogenic belts forming an arcuate (or concentric) pattern around the core of Sundaland in ever widening areas towards the Indian Ocean.
When the new concept of global plate tectonics were introduced in 1967, western Indonesia (i.e. Sumatra and its surrounding regions) became a major focus of attention for further investigation. This region, with its deep sea trenches, volcanic chains, sedimentary basins and cratonic continental areas, is situated at the convergence of a northward moving Indian Ocean plate subduction beneath the Eurasian continental plate.
Studies were made by Hatherton and Dickinson (1969), Fitch (1970), Hamilton (1970, 1979), and Katili (1971) but it was not until 1973 that the first plate tectonic model of western Indonesia was published by Katili. In this model, the following structural zones are listed along transverse sections of Sumatra & Java:
* The active subduction zone
* The magmatic of volcanic arcs
* The foreland (back-arc) basins.
The subduction zones have systematically moved farther away from the continent towards the Indian Ocean. The magmatic zones also show a zonal arrangement but the ages of the volcanics and granitic zones do not necessarily become younger towards the ocean. This posed problems to earlier investigators postulating the theory of a concentric arrangement of orogens but is explained by the supposition that the dips of the Benioff zones varied with time (Katili, 1980), Lithospheric descent in western Indonesia (i.e. Sumatra) also appears to have occurred during Permian, Jurassic, Cretaceous, Miocene and Pliocene times and continuous today. Broadly speaking, the back-arc (foreland) basins occurring within this plate tectonic setting determine the position of the oil bearing Tertiary basins. In a more detailed subdivision, the following structural elements may be noted:
* the trench
* the non-volcanic outer island arc
* The fore-rac basinsn (on the arc-trench gap)
* the volcanic / magmatic rec
* the back arc basins.
* The Sunda continental craton