Structure and collision history of the Buton continental fragment, easternIndonesia

Although commonly reservoired within Tertiary rocks, hydrocarbons in easter n Indonesia have generally been sourced from Mesozoic sediments deposited o n the continental margin of northern Australasia. Fragments of this margin are now widely dispersed as allochthonous terranes throughout the area, one of the most far traveled examples being the island of Buton, southeast of Sulawesi. Asphalt reserves on Buton support a significant local industry an d exploration continues for oil and natural gas. In common with other prosp ective Australian-derived terranes around the margins of the Banda Sea, But on is now separated from Australia by the active plate boundary marked by t he Java Trench and the collision trace along the Timer, Tanimbar, and Seram troughs. Buton differs from these other terranes in its distance from that boundary and its consequent insulation from the effects of the Pliocene-Pl eistocene collision between Australia and the Banda arcs. Reconstruction of the geological history of the Buton terrane thus has an important role in guiding future exploration in the other Australasian fragments in eastern I ndonesia. Geophysical studies of the Buton region have used seismic reflection, gravi ty, and magnetic (including paleomagnetic) techniques. Seismic reflection i mages generally record extension rather than compression as dominating the recent history of the area. Gravity data define the present-day western lim its of the Buton terrane and suggest that in the east the terrane includes the almost entirely submerged Tukang Besi platform. The gravity surveys als o demonstrate that the ophiolitic rocks exposed on Buton are not attached t o deep roots, but are thin and isolated overthrust sheets. Therefore they d o not mark a terrane boundary and their presence has little bearing on the prospectivity of the area. Paleomagnetic results document the independent m ovements of thrust sheets on Buton during the Pliocene-Pleistscene. The combined data from Buton record its separation from Australia as part o f a microcontinental block in the Jurassic or Late Triassic, followed by co llision with the Eurasian margin in southeastern Sulawesi in the Oligocene or early Miocene. Collision was followed by extension (as in Sulawesi itsel f) producing minor separation of Tukang Besi from Buton and much greater di spersion of other fragments of the microcontinent, some of which have since been incorporated in the new collision zone in the Outer Bands are. The oi l seeps and asphalt deposits of Buton are proof that hydrocarbons in the Ba nda are fragments can be sourced from within these fragments and are not ne cessarily derived front the underthrusting Australian margin.