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A Piece of North America Is Now in Queensland

Credit: Ruslan Olinchuk

Credit: Ruslan Olinchuk

By Adam Nordsvan, Zheng-Xiang Li & Bill Collins

Geologists have discovered rocks in northern Queensland that bear striking similarities to those found in North America, suggesting that part of northern Australia was actually part of North America 1.7 billion years ago.

Throughout the Earth’s four-billion-year history, as continents shifted around the globe they periodically massed together to form super­continents. Most recently this occurred about 300 million years ago when the southern continents (Africa, South America, Australia and Antarctica) and India joined Eurasia and North America to form the supercontinent Pangea.

Three supercontinents existed in the past two billion years: Pangea (320–170 Ma), Rodina (900–700 Ma) and Nuna (1600–1400 Ma). In Nuna, the least-known supercontinent of the three, it has been proposed that the east coast of Australia was adjacent to either North America (Laurentia) or Siberia. Researchers around the globe are working to advance this latest frontier in supercontinent research.

Reconstructing the ancient supercontinents is not easy. Each continent consists of discrete blocks, separated from one another by sutures: zones of deformation where the blocks collided. Geologists must characterise the blocks and match them with other blocks from different continents. They also need to determine the age of the sutures in order to work out when when the blocks came together. In this way, the continents are like pieces of a global jigsaw puzzle that have to be matched together at distinct periods of time. It’s a 3D problem, plus time!

What makes the problem worse is that many of these continental blocks are covered by younger sediments. For example, approximately 80% of the continental blocks in Australia are obscured. In Antarctica, almost 100% of the continent is covered by ice. However, geologists have tools to see through the cover, and the image of Australia’s jigsaw puzzle is truly amazing.

Scientists from the Earth Dynamics Research Group at Curtin University have focused on a large continental block in far north Queensland called the Georgetown Inlier. The block extends roughly from Charters Towers north towards Cape York and west to Mount Isa.

The oldest rocks of this ancient block are sedimentary and were deposited 1.7–1.6 billion years ago. We can observe and measure structures in the sedimentary rocks that tell us the type of environment in which the sediments were deposited and in which direction the currents were flowing when the sediments were deposited. We call these structures “palaeo­current indicators”.

We can also compare sedimentary rock layers to strata from elsewhere by extracting heavy minerals from the samples, especially zircon, and measure the age of tiny individual grains. By analysing hundreds of grains we can build up an “age spectra”, which is like a barcode that can be compared with any other sedimentary deposit on the planet. We can match the zircon age peaks with known ages of ancient mountain belts as a further means of linking sediments to continents.

Our new field data from the Georgetown Block, in conjunction with previous age data from these rocks, has revealed some surprising findings. Whereas the younger package of sedimentary rocks is comparable with rocks found further west in northern Australia, including the mineral-rich Mt Isa region, the older sedimentary package shows little similarities to known Australian rocks. Instead, they show a strong affinity with rocks in Canada.

Structures in the older sedimentary layers in Georgetown tell us that they were deposited on a shallow continental shelf. Importantly, the current ripple laminated structures tell us that the sediments were coming from a source east of the Georgetown Inlier, beyond the present-day coast instead of the ancient Australian continent to the west.

In addition, the age spectra of these rocks are almost identical to similar-aged rocks found in the Yukon Territory of north-west Canada. The combined palaeocurrent data and age spectra tell us that the oldest sediments deposited in the Georgetown Inlier must have been sourced from continental blocks in Laurentia – ancient North America.

However, the age spectra from the younger sedimentary package shows a much greater affinity to northern Australia, and the palaeocurrents switched from eastward to westward. Both lines of evidence are consistent with an Australian continental source at this later stage. We know the younger sediments were deposited approximately 1.5 billion years ago, which means the two continents were together by this stage as part of the supercontinent Nuna.

How did a piece of North America ended up becoming part of Australia, and what does this tell us about the formation and configuration of the supercontinent Nuna?

To understand how the Georgetown block became part of Australia, we need to look beyond the geological record from the sedimentary rocks and focus on other geological aspects. These include understanding how and when the sedimentary rocks evolved into the intervening mountain belt during the collision of the two continents.

Our new observations, plus evidence collected previously for both Australia and Laurentia, led us to propose that the Georgetown region was originally a piece of coastal Laurentia until 1.7 billion years ago. However, it then broke off Laurentia, migrated across a small ocean and then collided with Australia approximately 1.6 billion years ago. The migration of the Georgetown block across an ocean is similar to what we know about India crossing an ocean, when it broke off from Gondwana in the south and crashed into Asia in the north, forming the Himalayan mountain chain.

Interestingly, the mountain belt created when Georgetown crashed into Australia would not have been Himalayan in scale, but possibly much more subdued. To explain this “soft collision”, we evoke a double-sided subduction model, with oceanic crust subducting easterly below the Georgetown block but also westerly below the Mt Isa block. Alternatively, during this period the Earth’s mantle was hotter and may have been unable to support the immense mountain belts we see on modern Earth.

Nuna broke apart sometime after 1.4 billion years ago. Fortunately, the Georgetown region did not travel away with Laurentia. Instead, it has remained a permanent piece of Australian real estate ever since.

Adam Nordsvan is a PhD candidate in Curtin University’s Department of Applied Geology, where he is supervised by Zheng-Xiang Li and Bill Collins.