Washington  - Despite now being located on the opposite sides of the globe, southern Australia and the North West United States used to be neighbours.
That’s what a study of Tasmania’s oldest rocks has revealed. Minerals present in rocks in the island state’s north, dating from an ocean more than 1.3 billion years ago, contain traces of other minerals - monazite and zircon.
The minerals were extracted from sedimentary rocks from the Rocky Cape Group in North West Tasmania, where they were once deposited in an ancient ocean between 1.45 and 1.33 billion years ago – making them the oldest rocks in Tasmania. Researchers from the University of Tasmania and Mineral Resources Tasmania dated the minerals and found a pattern common to sedimentary rocks from the U.S. states of Montana and Idaho, as well as parts of Canadian province British Columbia.
According to the study, published in the journal Precambrian Research, the result was ‘a strong genetic fingerprint and evidence’ that Tasmania’s Rocky Cape was geographically close about 1.4 billion years ago to a basin that stretches through parts of Canada and the U.S.
Lead researcher Jacqueline Halpin explained: ‘At this time, both Tasmania and North America were part of a supercontinent called Nuna. ‘As plate tectonics and the supercontinent cycle started to rift Nuna apart, a large sedimentary basin formed that included the Rocky Cape Group and Belt-Purcell Supergroup rocks.’ The continued breakup of Nuna eventually dispersed parts of this ancient sedimentary basin to opposite sides of the Earth.
Dr Peter McGoldrick, also at the university, said that the new mineral dates also provide an age constraint for the Horodyskia fossils that were recently discovered in the Rocky Cape Group. These ‘string of beads’ fossils have also previously been found in the Belt-Purcell rocks. Fossils visible to the naked eye are exceedingly rare from rocks older than 635 million years. Horodyskia from the Rocky Cape Group and the Belt-Purcell Basin are nearly twice this age,’ he said. Researchers have revealed a fascinating glimpse of how the world could have looked - if a shift in the Earth’s tectonic plates had taken a slightly different turn.
Australian researchers said in March that the break-up of the supercontinent Gondwana about 130 Million years ago could have lead to a completely different shape of the African and South American continent. They explained that the move would have left a huge ocean south of today’s Sahara desert, creating a South Atlantic and a Saharan Atlantic Ocean. Geoscientists from the University of Sydney and the GFZ German Research Centre for Geosciences used sophisticated plate tectonic and three-dimensional numerical modelling to recreate the shift - but with a different twist.
The researchers said that the shift that left Africa it its present location was almost very different. ‘Extension along the South Atlantic and West African rift systems was about to split the African-South American part of Gondwana North-South into nearly equal halves, generating a South Atlantic and a Saharan Atlantic Ocean’, geoscientist Sascha Brune said.
‘In a dramatic plate tectonic twist, however, a competing rift along the present-day Equatorial Atlantic margins, won over the West African rift, causing it to become extinct, avoiding the break-up of the African continent and the formation of a Saharan Atlantic ocean.’ The team believe that their study highlights the importance of rift orientation relative to extension direction as key factor deciding whether an ocean basin opens or an aborted rift basin forms in the continental interior. For hundreds of millions of years, the southern continents of South America, Africa, Antarctica, Australia, and India were united in the supercontinent Gondwana. While the causes for Gondwana’s fragmentation are still debated, it is clear that the supercontinent first split along the East African coast in a western and eastern part before separation of South America from Africa took place.