Guntoro, Agus; (1995) Tectonic evolution and crustal structure of the central Indonesian region from geology, gravity and other geophysical data. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Geographically, the Indonesian archipelago is often divided into eastern and western parts, the boundary between them being placed at the 200 m bathymetric contours passing through the Makassar Strait in the north to Lombok Strait in the south. In this study, a new subdivision is proposed, introducing a Central Indonesian Region (CIR) which represents a transition between the largely Eurasian elements of Western Indonesia and the Pacific and Australian related elements of Eastern Indonesia. The CIR is bounded by two major subduction zones; in the west by pre-Tertiary subduction zone at the southeastern margin of the Sundaland, and to the east by the Early Tertiary subduction zone. The latter is marked by the Selayar-Bonerate ridge. One of the most interesting features of the CIR is the existence of outcrops of deformed pre-Tertiary basement complexes in the West and Central Java, SE Kalimantan and SW Sulawesi, which are similar in age, lithology and structure (Katili 1978; Hamilton 1979; Parkinson 1991). They suggest that these terranes are fragments of a microcontinent, which accreted eastwards and was dismembered in the Late Cretaceous. The eastward migration of a subduction system during the Late Cretaceous and Early Tertiary is suggested by the eastward growth of melange terranes, by the position of the Neogene magmatic arc to the east of the Cretaceous one and by the separation of the Western Arc of Sulawesi from Kalimantan. These events are thought to have been responsible for the formation of the basins in the CIR. As part of this study, a geological and gravity survey has been carried out on the Flores Sea Islands. The results of this survey were integrated with the published geological and geophysical data and with commercial seismic sections to allow examination of the crustal structure and tectonic development in the CIR. On the basis of gravity, magnetic and structural maps the CIR and vicinity can be divided into five major provinces, these being the Bone Bay, the Makassar Strait, the Central Java Sea, the East Java Sea and the Flores Sea provinces. Each province is examined and their stratigraphy, structural and tectonic styles correlated in order to have a complete understanding of their evolution. Variations in gravity values and models demonstrate that the continental crust in the CIR was attenuated by subduction roll-back and then subjected to rifting by extensional forces. The extension in the Makassar Strait, Central Java Sea and East Java Sea took place in the Eocene, forming marginal basins. Bone Bay opened due to collision between the Banggai- Sula microcontinent and Sulawesi in the Middle Miocene and was followed by clockwise rotation of Java, Sumbawa and Flores which cause the opening of the Flores Sea. The present configuration of the CIR is influenced by the collision between the Indo-Australian Plate and the Banda-Sunda arcs.

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