Alpine Fault's earliest activity revealed

Scientists have revealed the earliest-known stirrings of New Zealand's big-risk Alpine Fault, in a study that could hold implications for plate tectonics globally.

The new findings have shed new light on some of the first stages of the fault, at a time the Southern Alps hadn't yet risen from the Earth.

We know the Alpine Fault today as a major geological hazard and the on-land boundary between the constantly scrumming Pacific and Australian tectonic plates, spanning 600km between Milford Sound and Marlborough.

But in the prehistoric period to which scientists have been able to reach back to, between 20 and 25 million years ago, the fault ran through the great low-lying landmass that was Zealandia - and which New Zealand sits upon today.

Associate Professor Steven Kidder, a geologist at The City College of New York, described a landscape covered with forests of a completely different composition to those we know today.

It was only within the past seven to nine million years that the Southern Alps began rising – forced upwards by the same tectonic pressure that's unleashed massive quakes along the Alpine Fault over time.

While working at the University of Otago to study aspects of the fault, Kidder came upon a collection of ancient rocks, called mantle xenoliths, that were being studied by Otago scientist Associate Professor James Scott.

To geologists, these rocks are extremely valuable as they offer rare glimpses into the Earth's mantle, up to several thousand kilometres beneath our feet.

"The mantle is the layer of Earth beneath the crust and its physical properties are critical is supporting the topography of mountains and the longevity of continents," Kidder explained.

"So, geologists really need to know how the mantle behaves to comprehensively explain how continents and plate boundary faults evolve."

Only the size of fists, and coloured a striking green, mantle xenoliths only ever find their way to the surface by being plucked from the mantle and then carried up by volcanic magmas.

The scientists estimated that the xenoliths, discovered near Wanaka, were erupted out of the Earth about 23 million years ago – around the same time the Alpine Fault was in its infancy.

Kidder described them as providing a "fossilised" history of what was taking place at the time of their eruption.

"This is an insight into the deep Alpine Fault that cannot be gained in any other way," he said.

"Seismology, for example, can only determine what is present at great depth today – and as clever as seismologists are, they haven't yet figured out how to time travel."

Remarkably, one of the studied rocks happened to be 2.7 billion years old – making it the oldest rock ever recovered from Zealandia, and far more ancient than the oldest crustal rock found at the surface today, which dated only back as far as 500 million years.

But the rocks were all the more exciting as they bore signs of deformation, with the sizes, shapes and arrangements of minerals within revealing that the rock had once been faulted, deep beneath the crust along the plate boundary.

"Seismology tells us that today faulting can occur shallowly, but also that it can occur very deep Earth - in the mantle - at plate boundaries," Kidder said.

"Deformed mantle xenoliths like those we observed have only been described at seven other locations worldwide."

With the rocks, Kidder said he and his Otago colleagues had essentially stumbled upon the first physical evidence of deformation at extremely deep levels of the fault.

But that wasn't all: the rocks also helped them reconstruct what the fault environment would have looked like all those millions of years ago.

"We found that instead of there being a very wide zone of faulting as has been speculated, the deep Alpine fault was most likely made up of many narrow, probably linked, fault strands," Kidder said.

"This means that the Australia and Pacific plate boundary was not a single fault at depth, but rather that it was a series of linked fault zones formed as the two plates were faulted against one another."

But the scientists couldn't say whether that behaviour continued today, given the narrow zones were too small to currently resolve with seismic methods.

"This is also what makes looking at physical samples so important; we can actually touch and investigate the microscopic behaviour of old fault zones," Kidder said.

More generally, Kidder said the new insights meant that people must consider that major plate boundary faults, like the Alpine Fault, may evolve in ways that generally couldn't easily be studied with today's methods.

"This is important because deformation associated with narrow fault zones is very different from those associated with the wide zones," he said.

"The deep parts of faults have a big effect on earthquake type, for example."

Kidder pointed out that several seismological studies had predicted the existence today of a wide zone of mantle deformation beneath the modern Alpine Fault, spanning some 100km to 200km.

"Our discovery therefore raises questions such as how did the Alpine Fault actually evolve from narrow zones of deformation to a wide one. "

Beyond the fault, he added, the study had "major implications" for plate tectonics globally.

"Maybe other plate boundary faults could operate in this manner."

The study comes after new evidence gleaned from past earthquake behaviour led scientists to revise the chances of a major Alpine Fault rupture within the next 50 years from 30 per cent to 75 per cent.

They also calculated an 82 per cent chance the resulting quake would measure greater than 8.0 – big enough to cause widespread damage and disruption.