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How Australia lost New Zealand: computer model explains abyss between the two countries



29 March 2010

There are thousands of jokes exploring the cultural differences between Australia and New Zealand - but what about their geological divide?

Associate Professor Patrice Rey and Professor Dietmar Müller, from the School of Geosciences, have created a mathematical computer model that calculates the immense forces of nature that tore New Zealand away from Australia 100 million years ago.
Associate Professor Patrice Rey and Professor Dietmar Müller, from the School of Geosciences, have created a mathematical computer model that calculates the immense forces of nature that tore New Zealand away from Australia 100 million years ago.

Geoscientists from the School of Geosciences at the University of Sydney have created a mathematical computer model that calculates the immense forces of nature that tore New Zealand away from Australia 100 million years ago and say a similar process may be underway in South America.

Australia was once part of a much larger land mass called Gondwana, which included the modern continents of Africa, South America, Antarctica and India. However, Gondwana was torn apart - with connections between Africa and South America broken first, followed by New Zealand's and New Caledonia's shift away from the eastern edge of the Australian plate.

In a paper in April's Nature Geoscience journal, Associate Professor Patrice Rey and Professor Dietmar Müller from the University's School of Geosciences explain for the first time the mechanism responsible for East Gondwana's disintegration into microcontinents.

Between 105 million and 90 million years ago, Australia and NZ were joined with Antarctica, but the Pacific tectonic plate dived under the supercontinent's east coast at the rate of 7cm to 8cm a year, about the same rate it now sinks beneath South America.

As the plates slowed down, the friction between the two plates (Pacific and Gondwana) got smaller and smaller and the mountain belt along East Gondwana began to collapse, spreading under its own weight.

At the same time, the mantle - the partly-molten layer of rock under the Earth's crust - became more buoyant, pushing on the crust above, causing it to stretch and fracture.

These cracks were the first wedges driving Australia and New Zealand apart and also tore off the continental fragments that eventually became Lord Howe Island and the Challenger Rise off Australia's east coast. The Tasman Sea formed as a consequence, filling the gap.

Professor Müller said, "We had long wondered why the Tasman Sea formed in the first place. It's quite different from so-called back-arc basins like the Philippine Sea, which form next to a volcanic arc and an active subduction zone."

If Australia had kept on heading east, instead of turning around on its current northward path, there would be no Tasman Sea and no difference between Australians and New Zealanders.

Professor Müller observed, "That would be a pity, as we'd be deprived not only of Kiwi jokes, but also of all the beachfront property that was created on both sides of the Tasman Sea after collapse of the East Gondwana mountain belt."

The new computer model illustrates how a mountain belt collapsing over partially molten mantle rocks underneath can result in continental fragmentation and sea floor spreading after a subduction zone has become extinct.

Associate Professor Rey said the model uses cutting edge numerical techniques linked to detailed knowledge on the formation and evolution of the ocean floor over the past 150 million years.

He said the model also applies in South America where the motion between the South American plate and the Pacific plate has been slowing down over the past 20 million years to 7-8cm per year.

"If the velocity of the Pacific plate and the South American plate continues to decrease, then the Andes will start to collapse and parts of South America will move away from the mainland," he said.

Associate Professor Rey said as the model sheds new lights on the formation and evolution of plate margins, it has significant consequences for the exploration and mining industry.

"How continental margins form is of significance to the oil and gas industry as most oil fields are found in continental margin basins. Mountain belts such as the South American Andes host huge reserves in mineral resources such as copper, zinc and nickel. Our model offers a new framework to understand the formation of economic hot spots," he said.


Contact: Katynna Gill

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