Central Indonesia consists of an assortment of micro continents amidst micro- oceanic plates. The map in Fig. 2.0-1 is based mainly on interpretation of various authors, checked for accuracy by bathymetric, Bouger gravity, and surface geology maps as well .. Deep sea basins are generally underlain by an oceanic crust. This is supported by Bouger gravity values of more than 140 mgal, Values of less than 140 mgal are subject to interpretation of geologic data, such as continental crust, an extended crust, or accreted crust. Extended crust is related to passive continental margin or rifted continental crust (precursor to spreading centres). Accreted crust generally involved a thickening of the crust. There are two types of accreted crust, one related to continuous subduction /volcanism and one related to collision. Volcanic island arcs involved a melange of oceanic and volcanic materials, while collision belts involved metamorphic and broad ophiolite zones. Typically collision related accreted crust has a negative Bouger gravity values. Negative gravity values are also by sedimentary basins with thick sedimentary fill or in subduction zones.

Oceanic Platelets: Oceanic crustal plates are definitely shown by the Sulawesi Sea, North Banda Sea, Banda Sea, Flores Sea which are oceanic basins and the South Lombok Basin, a fore-arc basin, and of course the Indian Ocean in the south. The Sulawesi Sea and Banda Sea basins are interpreted to be spreading oceanic spreading centres of less than 5 Ma old. The North Moluccas Sea between the North Sulawesi-. Sangihe island arc and the Ternate island arc systems is interpreted as underlain by an oceanic crust with accreted material on top giving a Bouger gravity values of between 60 and 90 mgal, and even negative values The crustal composition below the Gulf of Tomini (Gorontalo) and Gulf Bone is probably a transition from a continental crust to an accreted crust, and will be discussed later. Negative values indicate mass deficiency, and the crust is interpreted not to be in isostatic equilibrium, such as being pressed down such as in subduction zones and collision zones.

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