Subduction zones, where one tectonic plate dives underneath another, drive the world’s most devastating earthquakes and tsunamis. How do these danger zones come to be? A study in Geology presents evidence that subduction can spread like a contagion, jumping from one oceanic plate to another—a hypothesis previously difficult to prove.
This result “is not just speculation,” says University of Lisbon geologist João Duarte, who was not involved in the research. “This study builds an argument based on the geological record.”
Because subduction drags crust deep into the earth, its beginnings are hard to examine. The new study provides a rare ancient example of potential subduction “infection.” Its authors say they’ve discovered evidence that neighboring collisions triggered East Asia’s “Ring of Fire,” a colossal subduction system currently fueling earthquakes and volcanoes from Alaska to the southern Indian Ocean.
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Nearly 300 million years ago China was a scattering of islands separated by the ancient Tethys and Asian oceans. Established subduction zones consumed these oceans, welding the landmasses into a new continent and raising mountains from Turkey to China. By 260 million years ago this subduction seems to have spread and begun pulling down the neighboring Pacific plate.
Ripley Cleghorn; Source: USGS Earthquake Catalog (data)
“The dying act of those closing oceans may have been to infect the Pacific plate and start it subducting westward under the Asian continent,” says study lead author Mark Allen, a geologist at Durham University in England. “In one form or another, it’s been diving down ever since.”
The smoking gun in this case is the “Dupal anomaly,” identified by a geochemical fingerprint from the ancient Tethys Ocean and what is now the Indian Ocean. When the study authors unexpectedly found this signature in volcanic rocks from the western Pacific, they surmised that material from the Tethys had spread eastward across a plate boundary from one subduction zone to another—triggering the neighboring plate’s descent. “It’s like seeing someone’s fingerprint at a crime scene,” Allen says.
But the mechanism of spread remains mysterious. The researchers suspect that transform faults—boundaries where plates slide past one another, like the San Andreas Fault—may act as weak spots where slight changes in collision angle or speed can destabilize dense oceanic crust, causing it to sink. Duarte compares the scenario to aluminum foil in water. “The foil floats,” he says, “but the slightest tap will cause it to sink.”
If subduction spreads this way, could the Atlantic Ocean’s relatively quiet plate margins be next? The massive 1755 Lisbon earthquake hints at early subduction invasion there. Duarte suggests parts of Iberia and the Caribbean are undergoing this process’s initial stages: “In another 100 million years a new Atlantic ‘Ring of Fire’ may form—just as it once did in the Pacific.”