As the floor of the Southern Ocean between Antarctica and Australia grows by sea-floor spreading, the Australian continent is being carried away from Antarctica on its own plate. This process began about 60 million years ago. Over the next few tens of millions of years, Australia will continue to drive northwards and will eventually collide with the Asian mainland. In the process, the various volcanic island chains and continent fragments that make up most of modern SE Asia will be caught up, and crushed between the two continental blocks. Such was the fate of arcs and continental fragments set within the Iapetus Ocean, which closed during the Caledonian orogeny some 500-400 million years ago. This bulldozing of volcanic arcs has begun for Australia.

	Topography of Papua New Guinea. The Finisterre range can be seen in the very north-central region of the country with the Markham river valley lying to the south between the Finishterre Range and the Hiighlands in the centre: © Dr Brains

 

In the late Miocene (about 8 million years ago) the continent collided with the Finisterre island arc. The effect at depth was for subduction to flip, so that now the sea floor to the north of the island of New Guinea (part of the Caroline Sea plate) is plunging southwards under the northern edge of the Australian continent. The southern part of New Guinea is the northern-outcropping portion of the Australian continent, uplifted from below the shallow Arafura Sea as the collision has progressed. The suture – or tectonic join – between the Finisterre arc and the Australian continent lies along the Ramu-Markham Fault, which runs up the Markham river valley in eastern Papua New Guinea. Just as with the Iapetus suture in western Ireland, it forms a prominent landscape feature – visible from space.

Further reading:

Baldwin, S.L., Fitzgerald, P.G., and Webb, L.E. 2012. Tectonics of the New Guinea Region. Ann. Rev. Earth Planet. Sci.. 40:495–520