Duarte and his colleagues have spent eight years mapping geological activity off the Portuguese coast. "Slowly we started to realise that our data suggested a new subduction system is forming," he says.
It was already clear that the region is riddled with a series of thrust faults, small segments where rocks are forced beneath others. What Duarte's team has added is evidence that these thrusts are linked by "transform faults", where rocks grind past each other at the same level. Together, they create a large fault system hundreds of kilometres long – a subduction zone in the making, according to Duarte and his colleagues.
Most importantly, the new work reveals why this new subduction zone is forming. It lies only 400 kilometres west of the Gibraltar Arc, a pre-existing subduction zone in the western Mediterranean Sea – a former ocean now in its death throes as Africa collides with Eurasia. Duarte's team found transform faults linking the Gibraltar Arc with the new subduction zone. They say subduction appears to have spread from the dying Mediterranean into the relatively youthful Atlantic.
"We can say with some confidence that this is an example of subduction invasion," says Duarte. The Mediterranean in turn may have "caught" subduction from an even older ocean, and so on back through time. "Subduction can behave as an infectious disease," he says.