Geologists reunite prehistoric siblings

New Zealand and New Caledonia were once connected through a long chain of volcanic islands, a team of international researchers has found.

The team discovered the existence of an ocean floor that was destroyed 50 to 20 million years ago, proving that New Caledonia and New Zealand are geographically connected.

One of the researchers, Wouter Schellart, said: "Until now, many geologists have only looked at New Caledonia and New Zealand separately and didn't see a connection."

Using new computer modelling programs, Dr Schellart and the team reconstructed the prehistoric cataclysm that took place when a tectonic plate between Australia and New Zealand was forced 1100km into the Earth's interior, which formed a long chain of volcanic islands at the surface.

Dr Schellart conducted the research, published in the 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 reconstruction, which looked at a much larger region including eastern Australia, New Zealand, Fiji, Vanuatu, New Caledonia and New Guinea, we saw a number of similarities between New Caledonia and northern New Zealand in terms of geology, structure, volcanism and timing of geological events," said Dr Schellart.

"We then searched deep within the Earth for proof of a connection and found the evidence 1100km below the Tasman Sea in the form of a subducted tectonic plate.

"We combined 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 using seismic waves that travel through the Earth's interior to map different regions. We are now able to say a tectonic plate about 70km thick, some 2500km long and 700km wide, was subducted into the Earth's interior.

"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," Dr Schellart said.

He said the new discovery defused the debate about whether the continents and micro-continents in the southwest Pacific had been completely separated since 100 million years ago and helped to explain some of the mysteries surrounding evolution in the region. "As geologists present more data, and computer modelling programs become more high-tech, it is likely we will learn more about our Earth's history and the processes of evolution."