ORIGIN OF THE SUMBA FORE-ARC BASEMENT

An interpretation of acoustic basement structures in the eastern Sunda/western Banda forearc region suggests structural continuity between Sumba and Northern Timor. The area between Sumba and Timor is delineated as a zone of continuous structures, without major strike-slip offsets. These structures trend east-west in the west but gradually change towards the east into NE-SW trending structures. A major NE-SW trending discontinuity west of Sumba between 117-degrees-30' and 118-degrees-30'E marks the transition between an intraoceanic volcanic arc system in the west and a volcanic arc-continent collision zone in the east. The extent of the Sumba basement suggests either a common (Late Jurassic) rift/drift history for both Sumba and Northern Timor or an (Early Miocene) magmatic welding of two continental fragments of different origin, resulting in structural continuity between two microplates. The first explanation implies that Sumba originated from the north-west Australian continental margin south of Tethys, from which it drifted away in the Late Jurassic. The second hypothesis, which is favoured, proposes a northern provenance of Sumba. The island would then originate from the south-east Asian continent, north of Tethys, from which it was detached in the Eocene.