New Zealand’s Alpine Fault biggest mover in the world

A group of researchers from Victoria University of Wellington and GNS Science has shown that New Zealand’s Alpine Fault has moved more in the last 25 million years than any other known onland fault on Earth.

The findings reveal that over this time the two sides of the South Island have shifted relative to each other by more than 700 kilometres along the Alpine Fault, dramatically changing the understanding of New Zealand’s tectonic movements.

The next largest known fault displacement on land is on the Altyn Tagh Fault in Tibet, with a total movement of about 475 kilometres.

“It was widely accepted that the Alpine Fault had shifted 450 kilometres over time, based on the offset of distinctive rocks either side of the fault. But according to evidence found in the nature of the ocean floor, we knew the shift should be much larger,” says Associate Professor Simon Lamb from Victoria’s School of Geography, Environment and Earth Sciences.

“It had been thought that the answer might lie in the shape of the belts of rock that run across New Zealand, starting south of Dunedin and curving up to the Alpine Fault, twisting and stretching over time. But nobody had been able to prove it.

“By simply looking at it another way—at geological maps of New Zealand together with studies of the direction of magnetisation in the rocks—we discovered that the missing displacement was actually on the Alpine Fault itself.”

Instead of the 450 kilometre shift, the new findings show that the movement on the Alpine Fault is 700 kilometres over the last 25 million years.

“The key to this new understanding is the realisation that the rocks had actually shifted about 250 kilometres in the opposite direction prior to this, and so to make up for this, the more recent motion is much bigger than we had thought. At this time New Zealand was part of Antarctica and Gondwana,” says Dr Lamb.

“I don’t think anybody in their wildest dreams would have thought that displacements on the fault could be so large, and also change direction so dramatically through time.”

The findings, recently published in the American Geophysical Union journal G³, bring new light to our understanding of plate movements, says Dr Lamb.

“It’s a great demonstration of the power of plate tectonics as it offers answers to a bigger puzzle about what happened in Antarctica over 65 million years ago, on the other side of an ocean from New Zealand and when dinosaurs roamed.

“It may also have implications for why the Alpine Fault is the main hazard for earthquakes in the South IsIand.”

Dr Lamb worked in collaboration with Victoria’s Emeritus Professor Euan Smith and Associate Professor Gillian Turner, and Dr Nick Mortimer from GNS Science.