Alpine rift: Scientists reveal earliest known activity of a major earthquake maker

The Alpine Fault is a geological fault that runs roughly the length of the South Island of New Zealand. YouTube / GNS Science

Scientists have revealed the oldest known movements of a dangerous Alpine fault in New Zealand, in a study that could have implications for the movement of tectonic plates globally.

The new findings shed light on some of the early stages of the fault, at a time when the Southern Alps had not yet risen from the ground.

Today we know that the Alpine Fault is a major geological hazard and the land boundary between the Australian and the ever-quiet tectonic plates, which stretches for 600 km between Milford Sound and Marlborough.

But in the prehistoric period that scientists have access to, between 20 and 25 million years ago, the fault ran through the large lowland massif that was Zealandia – on which New Zealand sits today.

Associate Professor Stephen Kidder, a geologist at the City College of New York, described a landscape covered in forests with a composition very different from that we know today.

It was only in the past seven to nine million years that the Southern Alps began to rise—and were forced upward with the same tectonic pressure that unleashed massive earthquakes along the Alpine Fault over time.

While working at the University of Otago studying rift aspects, Kidder stumbled upon a group of ancient rocks, called mantle xenoliths, that were being studied by Otago scientist Assistant Professor James Scott.

For geologists, these rocks are of great value because they offer rare glimpses of the Earth’s mantle, which reaches several thousand kilometers beneath our feet.

Alpine crack today. Source / GNS Science

Kidder explained that “the mantle is a layer of the Earth under the crust and its physical properties are very important, and it supports the topography of mountains and the longevity of the continents.”

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

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Just the size of a fist, and strikingly green, mantle xenoliths find their way to the surface only by being yanked from the mantle and then carried by volcanic magma.

Scientists have estimated that xenolithic inclusions discovered near Wanaka erupted from the Earth about 23 million years ago – around the same time the Alpine rift was in its infancy.

New Zealand is located on the ancient and only recently recognized continent of Zealandia. Image / GNS Science

Kidder described them as providing a “petrified” history of what was happening at the time of their eruption.

“This is an insight into the deep collapse of the Alps that cannot be gained any other way,” he said.

“Seismology, for example, can only determine what is at great depth today — and as smart as seismologists are, they haven’t yet figured out how to travel through time.”

Remarkably, one of the rocks studied is 2.7 billion years old – making it the oldest rock ever discovered from Zealandia, and much older than the oldest crustal rocks found at the surface today, dating back only 500 million years. .

But the rocks were even more exciting because they bore signs of deformation, as the sizes, shapes, and arrangements of the minerals within revealed that the rocks had once fractured, deep under the crust along the plate boundaries.

University of Otago students Marshall Palmer and Felix Schmidt watch a rock discovered near Wanaka that contains xenoliths from within the Earth’s mantle. The attached photo

“Seismology tells us that today the fault can occur superficially, but it can also occur deep in the Earth — in the mantle — at plate boundaries,” Kidder said.

“A deformed mantle xenoliths like the one we observed have only been described at seven other sites around the world.”

With the rocks, Kidder said he and colleagues in Otago have essentially found the first physical evidence of deformation at very deep levels of error.

Xenoliths erupted from the Earth about 23 million years ago – around the same time the Alpine Rift was in its infancy. The attached photo

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 rather than having a very wide fault zone as predicted, the deep alpine fault was most likely composed of many narrow, interconnected fault threads likely,” Kidder said.

“This means that the boundary of the Australia-Pacific plate was not a single fault in depth, but rather a series of related fault zones that formed when the two plates collide with each other.”

But scientists were unable to determine if this behavior persists today, since the narrow areas were too small to currently be resolved by seismic methods.

The finger points to the point in the rock in the middle of the Alpine rift on the South Island where two tectonic plates split. Photos / Supplied, Alpine Faults Tours

“This is also what makes examining physical samples so important; we can actually touch and investigate the microscopic behavior of ancient fault zones,” Kidder said.

Overall, Kidder said, the new insights mean that people should take into account that major plate boundary faults, such as the Alpine fault, may evolve in ways that are not easily studied using today’s methods.

“This is important because the deformation associated with narrow fault zones is very different from that associated with broad areas,” he said.

“Deep parts of the faults have a big influence on the type of earthquake, for example.”

Kidder noted that several seismic studies have predicted a wide region of mantle deformation under the modern Alpine fault, extending from 100 kilometers to 200 kilometers.

Franz Josef, located on the west coast of the South Island, is especially threatened by The Big Mistake. The attached photo

“Our discovery thus raises questions such as how the Alpine fault actually evolved from narrow regions of deformation to a large region.”

Other than the error, he added, the study had “significant effects” on the movement of tectonic plates globally.

“Perhaps other defects at the plate boundaries could act in this way.”

The study comes after new evidence from previous earthquake behavior led scientists to revise the chances of a major rupture in the Alpine Fault over the next 50 years from 30 percent to 75 percent.

They also calculated an 82 percent chance that the resulting earthquake would be greater than 8.0 — large enough to cause widespread damage and disruption.

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