Scientists may have discovered the world's oldest arc-slicing fault in Northwestern Australia's remote deserts. The finding demonstrates that plate tectonic processes were operational at least 3 billion years ago, fueling the ongoing scientific debate.
"This study clearly demonstrates horizontal plate movements before 3 billion years ago," study co-author Timothy Kusky, director of the Center for Global Tectonics at the China University of Geosciences, told Live Science.
In the new study, published July 15 in the journal Geology, researchers revealed that around 3 billion years ago, large, city-size rock blocks moved horizontally past each other by at least 19 miles (30 kilometers). The patterns resemble what geologists call arc-slicing transform faults, seen in active volcanic arcs like the Andes and Sumatra. If the findings are correct, these battered rocks might be the earliest evidence of horizontal plate movements, the researchers said, although not all experts are convinced.
Plate tectonics, the theory that underpins Earth's geological activity, shapes our planet with mountains, shifting continents, and seismic upheavals. Yet pinpointing the origins of this fundamental process remains a contentious debate.
Models indicate that early Earth had less-developed convection currents necessary to drive plate tectonics, suggesting that a thick and rigid outer crust formed a "stagnant lid," limiting dynamic horizontal movements. While magma bodies may have risen and solidified, rigid plates could not collide or subduct to form the volcanic mountain chains observed today. The debate centers around when convection currents developed, allowing Earth's "stagnant lid" to break into individual tectonic plates.
Some scientists argue plate tectonics started in the Hadean, over 4 billion years ago. Others believe the primitive "single lid" or "stagnant lid" dominated early Earth until about 1 billion years ago.
Recent AI modeling suggests tectonic activity may date back to the Hadean eon, over 4 billion years ago. However, validating models with direct clues from Earth's oldest and rarely preserved rocks is a monumental challenge.
Studying these early processes is difficult due to the scarcity of ancient rocks. But Australia's Pilbara Craton, with its 3.59 billion-year-old rocks, is a vital region for understanding the origins of plate tectonics. "The Pilbara Craton is where geologists first defined the 'stagnant lid' hypothesis," Kusky said. The Mulgandinnah shear zone — a broad region of intense deformation, including horizontal faulting, within the Pilbara Craton — could offer crucial insights into this debate.
The researchers used classic field observations and high-resolution magnetic data to connect buried features with surface geology. They built on previous studies that dated the movement to around 3 billion years ago, employing structural geology techniques to reconstruct the displacement of large, once-connected rock bodies by at least 19 miles (30 kilometers).
When plates collide at odd angles in today's volcanic arcs, arc-slicing transform faults develop, enabling horizontal and vertical movement. Because the Mulgandinnah shear zone's rock types and destruction patterns are so similar to modern volcanic arcs, Kusky explained that only deep subduction, where one plate slides beneath another, could account for these observations. Consequently, these findings validate recent AI models suggesting that plate tectonics were active at least 3 billion years ago, and possibly over 4 billion years ago.
These studies "represent the last nails in the myth that a stagnant lid dominated early earth," Kusky said.
Not everyone agrees that this new study settles the debate. Taras Gerya, a professor of Earth sciences at the Swiss Federal Institute of Technology Zurich who was not involved in the study, remains cautious. "There is no consensus about subduction evidence in Pilbara," he told Live Science. He suggested that other processes could produce similar observations. "This fault pattern could also develop in a so-called squishy-lid regime," he added, noting an intermediate condition where Earth's lithosphere behaves like a "squishy" or semi-rigid layer rather than a fully rigid plate.
However, Simon Lamb, an associate professor of geology at Victoria University of Wellington Te Herenga Waka in New Zealand who reviewed the study, finds the evidence persuasive. "It is hard to envisage how such large displacements could have occurred without subduction. Thus, I see this as convincing evidence for plate tectonics," Lamb told Live Science.
Kusky sums it up: "If it looks like it, smells like it, it probably is it."
