A subduction zone forming off the coast of Portugal is the beginning of a cycle that will eventually see Europe move closer to America, geologists say.

Researchers at Australia's Monash University report they've detected the first evidence a passive margin in the Atlantic Ocean is becoming active as a subduction zone.

Subduction zones are areas where one of the tectonic plates that cover the Earth's surface dives beneath another plate into the planet's mantle.

The new activity could signal the start of a new phase of the so-called Wilson Cycle, where plate movements break up supercontinents and open oceans, stabilize and then form new subduction zones which close the oceans and bring the scattered continents back together, researchers said.

Monash researchers said mapping of the ocean floor showed the beginnings of fracturing, indicating tectonic activity around the formerly passive South West Iberia plate margin was starting.

"What we have detected is the very beginnings of an active margin -- it's like an embryonic subduction zone," geoscientist Joao Duarte said.

Break-ups and reformations of supercontinents has happened at least three times over more than 4 billion years on Earth, the researchers said, and the Iberian subduction will gradually pull Iberia towards the United States over about 220 million years.