Recent tectonics around the island of Timor, eastern Indonesia

Abstract

Eastern Indonesia is the site of active collision between the Banda Arc and the Australian continent. The zone of collision is a poorly understood area and, in particular, controversy exists regarding the sites and styles of Holocene deformation in the vicinity of the island of Timor. This paper describes evidence for late Holocene to historically active sea-floor and shallow subsurface tectonic activity in the area, mapped using the GLORIA sidescan sonar system and single-channel seismic reflection data. Our main conclusion is that the zone of plate contact and major compressional deformation, which lies along the Java Trench west of 119° 45′E, continues directly eastward into the Timor Trough. There is little evidence for recent upper plate shortening north of this major plate boundary in the vicinity of the islands of Savu and Timor. Changes in deformational style, from small-scale uniform deformation at the Java Trench to mud diapirism and broad thrust-bounded folds further east, partly disguise the continuity of the deformation zone. Lateral changes in the style of deformation appear to be controlled only by the thickness and facies of the subducting sedimentary section. Where thin, such as at the Java Trench, uniform small-scale folding results. Further east, in the Timor Trough, a thicker sedimentary sequence deforms into buckled thrust sheets. The location of a large mud diapir field in the Timor Trough south of Savu appears to correlate with a thick sequence of Early Cretaceous marine shale that underlies the Australian shelf. The same shale may be the source of mud diapirs on Timor, demonstrating that, within the convergence zone, sediment facies rather than precise tectonic setting is the principal control on the distribution of the diapirs. A zone of strike-slip, thrust, and possibly normal faults observed to the north of Timor, between the islands of Alor and Wetar, appears to have a complex origin. Some evidence suggests that it is part of a left-lateral offset which cuts across the entire arc, through Timor. However, the apparent south-easterly thrusting of the Banda Sea beneath Wetar, and the deep pull-apart basin developed south-east of Alor, seems also to indicate eastward translation of Wetar relative to the arc further west. This may be driven by deformation of the Banda Sea under north-south compression. We see no evidence for major thrusting north of Wetar, indicating that, at this longitude, little cross-arc compression is accommodated by shortening on its northern side.