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Australia's north slammed into its middle

Finke River

Long before Gondwana existed it would have been possible to surf in Central Australia, in areas that now look like this (Image: Kate Selway)

Northern Australia was once a separate land mass to the rest of the continent and it crashed into central Australia, wedging underneath it, researchers say.

Kate Selway, a geophysicist and PhD student at the University of Adelaide, and colleagues, have been studying the collision that is estimated to have happened 1.64 billion years ago.

"If you looked south from Alice Springs before that time, you would have seen an ocean," Selway says.

Most scientists have long suspected that the Australian continent we know today is formed from pieces of land that were previously attached to other continents.

For example, comparison of different rock types, ages and chemistry suggest that, long before eastern Australia existed, northern, western and central Australia were all discrete landmasses, says Selway.

When they collided, some time less than two billion years ago, mountains were formed, which have long since eroded away, she says.

But Selway says no one has really known where exactly the boundaries of these separate plates were and how exactly they collided.

"People had theories that these boundaries are there but they've been hard to see because they're largely covered by dirt and sediment," she says.

Which edge under which?

When continents collide, the edge of one can wedge under the other in a process called subduction.

The prevailing theory has been that central Australia subducted under northern Australia, says Selway.

But she says there was never much data to back this up.

She and colleagues used a technique called magnetotellurics to peer hundreds of kilometres beneath the Earth's surface in the vicinity of the presumed boundary between northern and central Australia, just south of Alice Springs.

The technique relies on the fact that the Earth's magnetic field causes current to flow through rocks.

It measures the strength of the current as it changes with the electrical conductivity of different rock types.

Selway and colleagues plotted the changing profile of electrical conductivity along a 380 kilometre track north of the Northern Territory-South Australian border, down to a depth of 200 kilometres.

The profile shows that the northern plate was more conductive than the central one. And it also shows that the northern plate is wedged under the central one, and not the other way around.

Because boundaries between plates can be places where minerals concentrate, this kind of "big picture geology" helps the narrow down places to explore for minerals, says Selway.

Some of the research, which was funded by the Northern Territory Geological Survey, was reported in Geophysical Research Letters earlier this year. The rest has been submitted to the journal Geology.

Selway is one of 16 young scientists presenting their research to the public for the first time under the Fresh Science program.

Tags: environment, geology, palaeontology