Researchers shed new light on the pulse of plate tectonics

This NW Argentian outcrop

Monash researchers have shown for the first time that the melting of rocks within the Earth’s continental crust occurs in ‘pulses’.

This NW Argentinian outcrop shows several generations of vein formation, indicating multiple melting events that occurred spaced in time by 10 to 15 million years.

The discovery, led by researchers from the School of Earth, Atmosphere and Environment, and the School of Mathematics, is outlined in an international study published in Nature Geoscience.

Convergent plate margins - the area where two major plates that form the outer shell of the planet collide - are sites of major magmatic activity; a good example is the Pacific Ring of Fire.

Often this occurs on the edges of large continental masses (Australia does not have one, but to the North, Indonesia does). South America has had one of these margins for the last 500 million years or so.

The research team analysed tiny zircon crystals of approximately 100 micrometers separated from rocks out of Argentina. They dated the zircon crystals with a laser attached to a mass spectrometer and the U-Pb radioactive decay method.

“We found that a single hand-specimen of a rock from NW Argentina had zircons recording five distinct melting events in the continental crust, one every 10-15 million years, lasting 60 million years,” said lead author Professor Roberto Weinberg, from the Monash School of Earth, Atmosphere and Science.

“The research is significant because for the first time we have been able to show how the heat input from the mantle into the crust in active continental margins pulsates,” he said.

“Heat pulses control what happens to the crustal rocks and therefore control the nature of the continents, including when and how metal deposits such as copper, tin and silver form in these tectonically active margins.”

According to Professor Weinberg, periods of intensive crustal heating, occurring every 10 or 15 million years at the active continental margin, have led not only to crustal melting but also to pulses of mountain growth.

“This is because hot rocks are weaker and deform into mountains at a faster rate,” he said.

Co-author Associate Professor Oliver Nebel said the study was significant because for the first time scientists were able to show that melting of solid rock deep within the Earth’s continental crust occurred in ‘pulses’.

“What this means is that our outer crustal shell matures in a more complex way than previously thought and that the delving of oceanic plates into the mantle can affect continent evolution more severely,” Associate Professor Nebel said.

“This process is unique to Earth and one more piece of the puzzle that explains why our planet is so different in appearance to others out there,” he said.

A Monash mathematician was also involved in the study by contributing statistical analysis to the research.

"The use of statistics in this study provides a solid basis to distinguish the different groups of zircon ages and shows that they are unlikely to be a sampling artefact,” said study co-author Professor Kais Hamza from the Monash School of Mathematics.

For further information or to arrange an interview with Professor Weinberg or Professor Nebel, contact:
Silvia Dropulich
Marketing, Media & Communications Manager, Monash Science
T: +61 3 9902 4513 M: +61 (0) 0435138743