Abstract

It is suggested that the multiple orogenic phases identified in the Cainozoic of the islands of the Outer Banda Arc resulted from a succession of collisions between northward drifting Australian continent and a series of northward dipping subduction trenches. The evidence for these multiple orogenic phases is discussed and some refinements to their age suggested in the light of recent studies.

Another tectonic model proposed that the Outer Banda Arc is a tectonic wedge of melange composed of imbricated and chaotically mixed elements separated from the Australian shelf by a northward dipping Benioff zone. It has been argued that the recognition of multiple orogenic phases in the Outer Banda Arc islands can have no stratigraphical validity because it is not possible to resolve the original relationships in such a chaotic mixing as a tectonic melange.

This paper recognises that the effects of imbrication and compressional folding have been profound in the islands of the Outer Banda Arc, but finds the concept of the tectonic melange unacceptable here, in the light of unambiguous field geology, which exposes the chaotic element as a superficial olistostrome. The tectonic interpretation put forward here regards these islands as the imbricated northern margin of the Australian shelf and slope on which has been superimposed various overthrust Asian elements and a major olistostrome, all emplaced from the north.

Any tectonic model of the Banda Arcs has to account for the anomalous strike of Sumba, the relationship of New Guinea, the sinuosity of the Arcs and the occurrence of Permian and Mesozoic Asian elements superimposed on Australian facies. While these problems can be accommodated by the proposed model the almost total absence of any available information about the structure and age of the Banda Sea inhibits confidence in any tectonic interpretation of this region.

This model does not exclude the presence of petroleum prospects in the Mesozoic and Permian sections of the Australian shelf and slope facies that occur below the thrust sheets. Similar tectonic situations in the foothills of fold mountains have been productive. The concept of the Outer Banda Arc as a tectonic melange excludes the possibility of petroleum prospects from all but the post-melange Plio-Pleistocene molasse deposits.

The gradual eastward diminution of the Inner (volcanic) Banda Arc is considered here as the result of the convergent plate boundary between Asia and Australia becoming a transcurrent boundary in the Banda Sea region as the arcs acquired their eastern sinuosity during the Plio-Pleistocene.