Sinking ocean raised bridge
As the tectonic plate sank, it is thought to
have raised a chain of volcanic islands.
Image: iStockphoto

Monash geoscientist Mr Wouter Schellart and a team of international researchers have discovered an entire ocean that was destroyed 50 to 20 million years ago when its sea floor sank 1100 kilometres into the Earth between Australia and New Zealand.
Using new computer modelling programs, Mr Schellart was able to reconstruct the cataclysm that took place when the tectonic plate underlying the ocean basin sank (subducted) into the Earth’s interior and at the same time formed a long chain of volcanic islands at the surface. The researchers think that this chain of volcanic islands formed a geographical connection between New Caledonia and New Zealand.
Mr Schellart conducted the research, published in the world-leading journal Earth and Planetary Science Letters, in collaboration with Brian Kennett from ANU (Canberra) and Wim Spakman and Maisha Amaru from Utrecht University in the Netherlands.
“In our latest research we show for the first time evidence for such a connection by combining reconstructions of the tectonic plates that cover the Earth's surface with seismic tomography, a technique that allows one to look deep into the Earth's interior,” Mr Schellart said.
“This technique uses seismic waves that travel through the Earth's interior to map regions of high velocity versus regions with low velocity. Regions with a high velocity are thought to represent tectonic plates that have disappeared (subducted) into the Earth's interior,” Mr Schellart said.
“We are able to say the ocean basin was approximately 4 km deep and so contained 4 km of water, while the tectonic plate (which includes the ocean floor) that underlies the 4 km of water was on average 70 km thick. The ocean basin was originally some 2500 km by 700 km in lateral extent, so this means that the "plate" of 2500 km by 700 km by 70 km sank (subducted) into the Earth's interior.”
“Until now many geologists only looked at New Caledonia and New Zealand separately and didn't see a connection. In our new reconstruction, which looked at a much larger region (including eastern Australia, New Zealand, Fiji, Vanuatu, New Caledonia and New Guinea), we saw a large number of similarities between New Caledonia and northern New Zealand in terms of geology, structure, volcanism and timing of geological events,” Mr Schellart said. “We then searched deep within the Earth for evidence of such a connection using seismic tomography and found the evidence at 1100 km depth below the Tasman Sea in the form of a subducted tectonic plate.”

“The discovery means there was a geographical connection between New Caledonia and New Zealand between 50 and 20 million years ago by a long chain of volcanic islands. This could be important for the migration of certain plant and animal species at that time,” Mr Schellart said.
Mr Schellart said the new discovery diffuses the debate about whether the continents and micro-continents in the Southwest Pacific have been completely separated since 100 million years ago or not, and helps to explain some of the mysteries surrounding evolution.
“As geologists present more data, and computer modelling programs become more hi-tech, it is likely we will learn more about our Earth’s history and the processes of evolution.”

Editor's Note: Original news release not available online at time of writing.