Australasian Science: Australia's authority on science since 1938

The Big Twist

By Zheng-Xiang Li

Fossil magnetic needles in ancient Australian rocks have revealed that the continent underwent a 40° twist that split apart its most famous mineral provinces.

Everyone knows that the Australian continent is old and flat. Being such a vast and flat continent means there has been little tectonic activity within the continent for a very long time. In fact, the last major mountain-building event in Australia occurred more than 100 million years ago (Ma) along the continent’s younger east coast.

However, geological records reveal that this vast and peaceful continent, made of pits and pieces of continental blocks or fragments of various ages, had a rather violent past in deep geological times, and used to have gigantic mountain ranges, similar to those in central Asia today, running across it.

It has long been recognised that the western two-thirds of the Australian continent formed much earlier than the eastern one-third, which is younger than 540 Ma. This older part of the continent, commonly with basement rocks older than 1800 Ma, hosts the vast majority of Australia’s mineral wealth. It has been assumed to have been in its present-day shape since at least 1000 Ma, with relatively modest mountain ranges popping out here and there due to minor internal jiggling.

However, a recent reanalysis of geological and palaeomagnetic data from Australia has revealed a major mountain-building event within the older part of Australia during the period 650–550 Ma, leading to the formation of a major east–west divide cutting across the middle of the continent (Fig. 1). The event also laterally offset the northern and southern halves of the old continent by more than 500 km, dislocating major mineral belts like the Mt Isa and Broken Hills belts, causing major earthquakes along the mountain belt and forming new mineral deposits such as the Telfer gold deposit in Western Australia.

Palaeomagnetism uses tiny magnetic particles trapped in rocks to track down the movement of each continent on the Earth’s surface over geological timescales. Such magnetic records work like a magnetic barcode for each continent or even a part of a continent. When two parts of a continent travel together, their magnetic barcodes match exactly but if the they subsequently shift relative to one another their previous palaeomagnetic barcodes will split. This allows us to detect when continents joined together, when they broke apart, and how they moved relative to each other.

In a recent study with Prof David Evans at Yale University, we noticed a systematic offset in the palaeomagnetic record between the northern and southern halves of the ancient Australian continent. The two regions’ palaeo­magnetic barcodes share comparable shapes except for a systematic offset in their present configuration.

The two sets of fossil magnetic directions also show a systematic 40° difference. However, this offset disappears if we restore Australia to its likely “original” shape by rotating southern Australia about 40° relative to northern Australia (Fig. 2a).

This disjoint between the two halves of the ancient Australian continent co­incides with traces of a 650–550 Ma mountain belt that runs from the Paterson Ranges off the eastern boundary of the Pilbara to the Petermann and Musgrave ranges of Central Australia, and disappears into central Queensland (Fig. 1, 2b). This long-lost east–west great divide was likely a mighty mountain range comparable to the Tianshan ranges in central Asia.

Geologists have found rocks in Central Australia that were jacked up from 40 km deep to near the surface at that time. There were also webs of locally molten rocks (Fig. 1) caused by large earthquakes that occurred during this major upheaval.

Sediments shed off from the northern slope of this mighty mountain range formed the present-day cultural and landscape icons Uluru and Kata Tjuta (The Olgas).

It’s likely that Australia lost a bit of real estate during this event. Land that used to fill the narrow triangular gap in the western part of the continent was probably squeezed out to the west during this event (Fig. 2a).

However, Australia did gain in mineral endowment, such as the large Telfer gold deposit in Western Australia. There is potential for the discovery of more deposits along this long mountain belt.

This new interpretation of continental rotation within Australian also resolves one of the long-standing puzzles about where the southern extension of the Mt Isa mineral zone went. Geologists found many similarities between the Mt Isa and Broken Hill mineral zones, and some speculated that they might have once formed parts of a single mineral belt. However, most people thought that the southern extension of the Mt Isa mineral belt was located on another continent that was once joined with Australia: possibly South China or North America.

This new work proves that the Mt Isa and Broken Hill mineral zones were once part of a single belt that was cut into two halves and offset by more than 500 km during an intracontinental rotation some 650–550 million years ago. There is therefore scope to look for more Mt Isa or Broken Hill-type mineral deposits in regions between the two world-class mining centres.

On a global scale, the new geotectonic model resolves a longstanding controversy surrounding the configuration and breakup history of the supercontinent Rodinia (lifespan ca. 900–700 Ma), which consisted of almost all continents that existed at the time. It implies that Rodinia didn’t break up until much later than we thought, placing the breakup time much closer to the time of the hypothesised first “Snowball Earth” event, when ice covered most of the planet.

This makes it a more plausible hypothesis that the breakup of the supercontinent and accompanying geographical and chemical changes led to catastrophic global chilling events (opposite to global warming events) at 750–600 Ma. Both events likely played roles in the explosion of complex life on Earth during the so-called Cambrian explosion.

The formation and subsequent erosion of the mighty mountain range across Australia may also have contributed to the oxygenation of the atmosphere, thus making the Earth more habitable for complex life.

Zheng-Xiang Li is professor in geology and geophysics at the Institute for Geoscience Research, Curtin University.