Tectonic development of East Indonesia

Abstract

The outer non-volcanic arc in eastern indonesia appears to have been formed as a marginal part of the Australian continent in the southern hemisphere before Upper Jurassic. Timor and Sumba did not reach their present positions until Mid Miocene or later. The major tectonic event in Timor and Seram occured during Mid Pliocene and has been correlated with the collision between the margin of the Australian continent and the subduction zone of the volcanic arc. Data from trace elements, particularly Sr and Nd isotopes, suggest that the continental crust was subducted beneath the island arc. At the end of the Pliocene, the subduction zone shifted from the northern part of Timor to the Timor trough. A paper in 1975 proposed the Sumba fracture within the Indonesian Island Arc and a further paper in 1980 modified this proposal. These interpretations of eastern Indonesia have claimed that a continuous arc system extends from east of the Sumba fracture through Tanimbar and then swings north. The present report, however, describes a tectonic fracture at Pantar Strait. The evidence for this Pantar Strait fracture is discussed and it is interpreted as being initially a wrench fault at about 3 Ma. Paleomagnetic data indicate that northern arm of Sulawesi has rotated clockwise more than 90°. In the Miocene, Seram and the northern arm of Sulawesi collided with the western edge of the present Sula island, which was then just beginning to move westward along left-lateral transform faults after the Lower Miocene continent-arc collision in New Guinea. As a result of this later collision, the northern arm of Sulawesi and Seram became detached from the trench-transform junction and moved continuously in northwest and southwest direction along inferred transform faults. The volcanic arcs along both sides of the Molucca Sea increase in K from south to north without corresponding changes in Sr-isotope composition. These changes are interpreted as resulting from decreasing degress of partial melting northward because of arc-arc collisions. These tectonic movement are also related to the back-arc spreading of the Banda Sea and the Celebes Sea.