Australia–SE Asia collision: plate tectonics and crustal flow

The Sundaland core of SE Asia is a heterogeneous assemblage of Tethyan sutures and Gondwana fragments. Its complex basement structure was one major influence on Cenozoic tectonics; the rifting history of the north Australian margin was another. Fragments that rifted from Australia in the Jurassic collided with Sundaland in the Cretaceous and terminated subduction. From 90 to 45 Ma Sundaland was largely surrounded by inactive margins with localized strikeslip deformation, extension and subduction. At 45 Ma Australia began to move north, and subduction resumed beneath Sundaland. At 23 Ma the Sula Spur promontory collided with the Sundaland margin. From 15 Ma there was subduction hinge rollback into the Banda oceanic embayment, major extension, and later collision of the Banda volcanic arc with the southern margin of the embayment. However, this plate tectonic framework cannot be reduced to a microplate scale to explain Cenozoic deformation. Sundaland has a weak thin lithosphere, highly responsive to plate boundary forces and a hot weak deep crust has flowed in response to tectonic and topographic forces, and sedimentary loading. Gravity-driven movements of the upper crust, unusually rapid vertical motions, exceptionally high rates of erosion, and massive movements of sediment have characterized this region. Eastern Indonesia is at the centre of the convergent region between the Eurasian, Australian and Pacific plates (Fig. 1). It is the site of the gateway between the ancient deep Pacific and Indian Oceans which disappeared in the Early Miocene as Australia began to collide with the Sundaland margin of Eurasia. Today it is the passageway for water which continues to move from the Pacific to the Indian Ocean, by complex routes reflecting the evolution of the collision zone since the Early Miocene. This tectonically complex region is known to biologists as Wallacea, with a biota and diversity as complex as the geology. Wallace (1869) recognized in the 19th century that biogeographical patterns in some way reflected geology but we are still very far from understanding the links between geology, palaeogeography, ocean–atmosphere circulation and climate which may have influenced the evolution of life. Unravelling the geology is a first step, but remains a difficult one. Here I discuss this first step: the geological development of the Australia–Asia collision, particularly in eastern Indonesia. The Cenozoic, particularly Neogene, development was strongly influenced by what was present before collision, so this paper begins with an outline of the Mesozoic and Early Cenozoic history of SE Asia, the Jurassic breakup of the northern Australian part of Gondwana and the assembly of Gondwana fragments in SE Asia in the Cretaceous. Rifting of fragments, now in Indonesia, from Gondwana was the first control on the Australian margin and the character of Sundaland, affecting both the shape of the continental margins and the distribution of different types of crust within them. The nature of the Mesozoic Pacific margin is also touched upon, and the possible contribution of Cathaysian fragments to SE Asia. In contrast to most previous reconstructions of the region, the docking of different fragments is interpreted to have terminated subduction around SE Asia from the mid-Cretaceous until the Eocene, except for a short Paleocene episode of subduction beneath West Sulawesi. The effects of the assembly of different blocks, with their different internal structures, and separated by sutures, is then considered. The history of subduction resulted in an unusual lithosphere, and a high regional heatflow, and these features, combined with the heterogeneous nature of the basement were a major influence on Cenozoic deformation. It is argued that the Sundaland continent is not a craton or shield, but is a large region of generally weak lithosphere with weak and strong parts responding in a complex way to movement of the rigid plates that surround it. This determined the way in which the Australia–Asia collision proceeded and the deformational response of the upper crust to the movements of major plates, and the collision history is next reviewed, particularly the important subduction rollback into the Banda embayment of the Australian margin. Finally, I consider if plate tectonics can be reconciled with deformation of the crust and suggest that the region is not behaving as plates or microplates, as illustrated by different parts of eastern Sundaland From: Hall, R., Cottam, M. A. & Wilson, M. E. J. (eds) The SE Asian Gateway: History and Tectonics of the Australia–Asia Collision. Geological Society, London, Special Publications, 355, 75–109. DOI: 10.1144/SP355.5 0305-8719/11/$15.00 # The Geological Society of London 2011. and Wallacea. I outline an alternative model explaining why the surface topography and bathymetry, and palaeogeography, have changed very rapidly during the late Neogene with important consequences for ocean currents, local climate, and probably global climate.