How does the subduction process begin? That question has been open to debate for years: some scientists propose that eventually the cold ocean crust at the edges of the ocean becomes so dense that it sinks (or founders), initiating subduction. Not everyone has agreed, however – scientists have argued that such spontaneous action is not physically possible. And now Dr Duarte and his team believe they have identified an alternative mechanism in action.
The Atlantic: a Mature Ocean
A look at the tectonic setting of the Atlantic shows that it is dominated by the spreading ridge along its centre, creating new crust and pushing the old apart. Unlike the Pacific and the Indian Oceans, there is no sign of major active subduction apart from the two small arcs of the Antilles, in the Caribbean, and the Scotia Arc in the south Atlantic.
Both of these are relatively closely associated with existing major subduction zones in the eastern Pacific.
There, is, however, a third subduction zone in the Atlantic, though it is as yet poorly developed. Though described as ‘controversial’ by Yeats, the area off southwestern Iberian has been the focus of major earthquakes in the past (most notably the great Lisbon earthquake of 1755). The current nature of the tectonic faulting there is poorly understood and close study of the area by Duarte and his colleagues reveals structures typical of a subduction zone including faulting, deformation and mud volcanoes.
According to Dr Duarte, these findings are significant: “We may be witnessing the very beginning of a subduction in an Atlantic-type passive margin” he said.
Subduction of the Gibraltar Arc and its Implications
If subduction is indeed beginning at the Gibraltar Arc, the discovery has implications for plate tectonic theory. Dr Duarte believes that the findings support an alternative mechanism for the initiation of subduction, with the zone propagating outwards from areas of subduction within the remnant ocean of the Mediterranean – just as the Antilles and Scotia arcs may have propagated from the destructive margins of central and south America.
“There is simply no force available in a passive margin to cause its spontaneous collapse (unless you have something anomalous like a plume or a meteorite impact)” he explained to Decoded Science. “The only process that can produce forces with magnitudes high enough to break a passive margin is another subduction zone or a collision belt.”
Of course, the closure of the Atlantic is too long-term to affect current society. But subduction zones are associated with major earthquakes and the Atlantic sea floor off the Iberian peninsula has been the source of major tremors, among which the 1755 tremor, with a possible magnitude of 8.6-8.8, is the largest, has been the source of several.
Giant Earthquakes and Subduction
Although we can never rule out the possibility of a major earthquake at any time, the chances of a repeat of so large a tremor, at least in the near future, are small. “This is a process that began a few million years ago and may last for at least another 10 to 20 million years. Giant earthquakes should be expected and we need to be prepared,” notes Dr Duarte. “However, because the convergence rate is much slower than the Pacific the frequency [of tremors] is much lower. Quakes like that of 1755 should be expected every 2000 years or so.”
Duarte, J.C., Rosas, F.M., Terrinha, P., Schellart, W.P., Boutelier, D., Gutscher, M-A. and Ribeiro, A. Are subduction zones invading the Atlantic? Evidence from the southwest Iberia margin. (2013). Geology. Accessed 19 June 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press.