Subduction zone initiation at passive margins plays a central role in plate tectonic theory; however the process by which a passive margin becomes active is poorly understood.
The Wilson Cycle, where plate movements break up supercontinents like Pangaea and open new oceans, describes the evolution of an Atlantic-type margin in three phases 1) Rifting and seafloor spreading; 2) spontaneous foundering of the passive margins and development of a new subduction zone; 3) consumption of ocean floor and eventual ocean closure. This break-up and reformation of supercontinents has happened at least three times, over more than four billion years. The absence of oceanic lithosphere older than 200Ma suggests that new subduction systems are likely to initiate close to passive margins; it is traditionally assumed that subduction occurs when the cold, gravitationally unstable, oceanic lithosphere collapses into the asthenosphere as a consequence of aging and thermal contraction. The geological record does not record explicit examples of an Atlantic-type margin evolving spontaneously into an Andean-type one.
The search for an incipient subduction zone at an Atlantic-type margin has long been a major challenge in plate tectonics- the Atlantic Ocean is almost entirely bounded by passive margins. A research group, lead by Monash University may have detected the first evidence of a passive margin becoming active. In several regions, including the Gibraltar Arc, the Atlantic oceanic lithosphere is being consumed by subduction. The Gibraltar Arc is a remnant of the subduction zone responsible for the closure of the western part of the Tethyan Ocean, and extends into the Azores- Gibraltar fracture zone. Mapping of the ocean floor has revealed fractures, indicating tectonic activity around the apparently passive South West Iberian Plate margin. Anomalous high magnitude seismicity in the region (M>8.5, Lisbon 1755; M 8 1969) indicates that the Iberian passive margin is not as passive as its name may suggest.
Currently there are two tectonic mechanisms in operation in the region: the westward migration of the Gibraltar Arc and the convergence of the African and Eurasian plates. The team, led by Dr Duarte, believe that a new subduction zone is forming as a result of the combination of stresses from these tectonic regimes. It is thought that the Gibraltar Arc will continue to migrate westwards along the Azores-Gibraltar fracture zone until it reaches the open Atlantic. Deformation has resulted in thrust fault systems forming away from the Gibraltar Arc and propagating along the passive margin for ~300km. These structures are thought to represent the onset of margin inversion and the nucleation of a new subduction zone. The lithosphere in the deformation zone largely consists of highly serpentinized mantle peridotites and gabbros providing an efficient weakening mechanism, facilitating lithospheric rupture.
The findings provide a unique opportunity to observe a passive margin becoming active- a process that will take around 20 million years. Eventually, Iberian subduction will gradually drag Iberia towards the United States- a slow process which will take around 200Ma.
The full paper may be found here.