A newly-discovered subduction zone off the southwest coast of Portugal may one day close the Atlantic Ocean.
As the continents slowly drift over the eons, they can form areas where the oceanic crust is pushed under ground, to the mantle underlying the crust. The process by which these subduction zones form is still unknown. Although older crust is more rigid and has more strength than younger crust, it is also denser.
In order to learn how such zones form, geologists want to study a subduction zone in the process of forming. A pair of earthquakes in 1755 and 1969 off the coast of Portugal provided the clues Joao Duarte of Monash University in Melbourne needed to suspect the area might be forming a new subduction zone. He and his team created a 3D model of the ocean floor from sonar imaging and careful study of the seismic record.
"There should not be tectonic activity [in this area], but when we look at this area we see faults that are compressive. This is typical of a plate boundary that is converging," Duarte said.
Thrust faults - smaller areas where some rocks are driven beneath others - are common to the area, but Duarte and his team of researchers were able to show that these features are hitched together by places where rocks move past each other horizontally, called transform faults. They link together into a system hundreds of miles long. This long feature is also connected by transform faults to the Gibraltar Arc, a subduction zone in the western Mediterranean.
This connection between different faults and subduction zones has led some to characterize the features like an infection, spreading from ocean to ocean. The Mediterranean was once also an ocean, and it is slowly closing as Africa approaches Europe.
The Atlantic Ocean is young, showing few subduction zones. However, this newly-found zone, combined with two others in the South Atlantic and Carribean, could cause the crust under the Atlantic Ocean to shatter.
"These three subduction zones can be seen as defects. Fractures will propagate from these areas and ultimately cause the plate to break. We may well be at a turning point of the Atlantic's history," Duarte said.
The action of the new subduction zone is calculated to one day pull North America (on the North American Plate) toward Portugal, sitting on the Eurasian plate. Super-continents, the most famous of which is Pangaea, form every 400 million years or so before breaking apart again. According to Duarte, this new ridge will close the gap between North America and Portugal in 220 million years, ending the existence of the Atlantic Ocean.
