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AAPG Bulletin

Abstract


Volume: 64 (1980)

Issue: 6. (June)

First Page: 868

Last Page: 915

Title: Arc-Continent Collision in Banda Sea Region

Author(s): Carl Bowin (2), G. M. Purdy (2), Chris Johnston (3), George Shor (4), Lawrence Lawver (5), H. M. S. Hartono (6), Peter Jezek (7)

Abstract:

A 2-month marine geophysical study of the Banda arc region was conducted in late 1976 using the R/V Atlantis II of WHOI and the R/V Thomas Washington of SIO; 19 seismic refraction lines were successfully completed. Oceanic crust underlies the Banda Sea and Weber Deep. Continental crust 35 to 40 km thick underlies the Australian Shelf. Thick continental crust is also present beneath the Timor and Aru Troughs. Low-amplitude magnetic anomalies are present over the Australian Shelf and extend to near the western edge of the Banda outer arc and, together with the presence of metamorphic rocks, suggest that continental crust may extend to the eastern lip of the Weber Deep. Continuous seismic reflection profiling shows the Australian Shelf sedimentary sequence dipping beneath th accretionary prisms of the outer Banda arc at the Timor and Seram Troughs: the tectonic front of the subduction zone lies along the axis of these troughs. The bathymetric profile on the outer flank of the Timor and Seram Troughs is unusual in that the profile asymptotically approaches a shallow depth near sea level and no outer rise is present as at oceanic trenches. An elastic-flexure analysis of this topographic profile indicates that an elastic plate is an incorrect model for the lithosphere at this plate-convergence zone. The Aru Trough, although structurally on trend with the Timor Trough, is not presently a site of subduction and compression tectonics. Instead, it is now a place of crustal extension, and is an example of graben formation that is separating a block of Australian co tinental crust (beneath the Kai Islands) from the Australian platform. The present lack of structural continuity between the Seram Trough and the Aru and Timor Troughs is best demonstrated by the pattern of gravity anomalies. The discontinuity between the Seram and Aru Troughs supports the seismic evidence that the Seram subduction zone is separate from the Southern Banda subduction zone that is continuous with that beneath Java on the west. The Bouguer gravity anomaly pattern indicates a division of the Banda arc into four segments: a southern segment, Timor to Babar, with relative plate convergence possibly trending about N20°W between the Banda and Australian plates; a central segment from Tanimbar to about Kasiui beneath which the Java to Timor Benioff zone is bent to a northwar trend; a Seram segment that is converging with the Australian platform along approximately a S70°W trend; and a Buru segment that has rifted away from the Sulu Islands. The southern segment has been undergoing compressional deformation distributed across the width of the arc for the last 3 m.y. with attendant slowing of the differential-slip rate between the leading edge of the accretionary wedge and the underthrust Australian margin crust. Oblique convergence and the bend of the Benioff zone produce in the Tanimbar to Kasiui segment a slower convergence rate normal to the arc at the southern end which diminishes northward and changes to extension normal to the arc at the Aru Trough. Thrust focal mechanisms indicate that subduction is presently active at the Seram Trough. Seafloor- preading magnetic anomalies appear to have been found in the south Banda basin. The trend approximately N60 to 70°E, similar to Cretaceous anomalies in the eastern Wharton basin, suggests the possibility that the Banda Sea may be trapped oceanic crust. Water depths of 5 km and low heat flow (generally 1.5 HFU or less) are compatible with an old age (greater than 25 to 60 m.y.) for the Banda Sea crust.

We conclude that the Outer Banda arc from Buru around to Timor, and possibly to Sumba, contained Australian continental crustal blocks and fragments prior to its collision with the Australian margin in the last 3 to 6 m.y. Continuous convergence following the addition of a thick Australian margin sedimentary sequence to the south Banda subduction zone has led to deformation being distributed over the width of the arc and not simply being taken up on a single thrust surface. This scenario helps reconcile the geologic relations on Timor, Seram, and Buru with the structural continuity of the Timor Trough with the Java Trench.

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