Subduction-zone birth may be happening off Portuguese coast

New Scientist : Subduction zones occur where one tectonic plate is forced under its neighbor until it melts into the mantle. If an ocean lies between the plates, subduction can cause it to be squeezed out of existence within a gigayear. How the zones form is uncertain because the rock involved appears to be too strong to break or be subducted. Now João Duarte at Monash University in Melbourne, Australia, and his colleagues believe they have found an area off the southwestern coast of Portugal that is in the process of becoming a subduction zone. Portugal experienced major quakes in 1755 and 1969, despite the lack of major fault lines in the area. Duarte’s team mapped geological activity off the coast for eight years. They found that the known thrust faults were linked by previously unknown transform faults, resulting in a system of faults hundreds of kilometers long. The reason they think that this is a protosubduction zone is that it is just 400 km away from the Gibraltar Arc, a subduction zone in the western Mediterranean responsible for the shrinking of that sea as Europe and Africa collide. They believe that as the two continental masses approach each other, the subduction zone spreads into nearby areas of tectonic weakness. If they are correct, it could be that the Atlantic Ocean, which is a relatively young ocean, could begin shrinking in the geologically near future.