Australian crust in Indonesia

Abstract

It is now generally accepted that the core of Southeast (SE) Asia was assembled from continental blocks that separated from Gondwana in the Paleozoic and amalgamated with Asian blocks in the Triassic. Fragments of these Gondwana/Cathaysia blocks rifted and separated from Asia and later re-amalgamated with the SE Asian continental core. Mesozoic rifting of fragments from the Australian margins followed by Cretaceous collisions, and Cenozoic collision of Australia with the SE Asian margin added more continental crust. There can be no doubt that there is Australian crust in SE Asia, including Indonesia, but where this crust came from, and when it arrived, continues to promote discussion. Argoland has been variously identified in Tibet, West Burma, and Borneo. Other fragments supposed to have rifted from Australia are claimed to be in Sumatra, Java, Sulawesi and Sumba. The Banda region is the site of even more controversy because pieces of Australian crust are found in Sulawesi, the North Moluccas, the Banda Sea, and in numerous islands forming the Inner Banda Arc on both sides of the supposed subduction boundary separating Indonesia from Australia. There is increasing evidence that fragments of Cathaysian/Asian continental crust form parts of northwest Borneo and the offshore shelf to the north of Sarawak and east of Vietnam, and that Australian blocks underlie much of Borneo, West Sulawesi and Java. These fragments rifted from Australia in the Jurassic and arrived in their present positions during the Cretaceous. The rifting led to formation of a continental promontory, the Sula Spur, that extended west from New Guinea on the north side of the Banda embayment. This collided with the SE Asian margin in the Early Miocene and has subsequently been fragmented by subduction-driven extension. The Sula Spur and its fragmentation, and the history of subduction of the Banda embayment, are the causes of many of the controversies about collision ages, and account for the unusual distribution of continental crust in the Banda Arc.

KEY WORDS:

• Indonesia
• continental crust
• extension
• zircon
• Sundaland
• Gondwana

Acknowledgements

We thank the consortium of oil companies that supports the SE Asia Research Group, and many colleagues for advice and discussion, in particular Mike Cottam, Ian Watkinson and Simon Suggate. We are grateful to Mike Audley-Charles and Tony Barber for discussion and Marcelle Boudagher-Fadel and Brian Rosen for advice. We also thank our collaborators in SE Asia especially Ben Sapiie and colleagues at ITB, Bandung. Interpretations presented here were significantly supported by zircon U–Pb age data collected by Ben Clements, Lorin Davies, David Evans, Indra Gunawan, Helen Smyth, Marco van Hattum, Theo van Leeuwen, Duncan Witts, and other SEARG students and colleagues to whom we are grateful. It would have not been possible to acquire these data without the help of colleagues at University College London, Australian National University and the GEMOC ARC Key Centre.