Monday, July 1, 2013

Will New Tectonic Fault System Kill the Atlantic?

Well maybe. Otherwise I expect the Atlantic to grow a great deal more. I am not comfortable that we really have an understanding of all this. Recall that the Pacific occupies an entire half of the globe and I recognize no available mechanism able to send a continent onto that slab. It behaves as if we would have to observe massive outpourings that quickly built up a continent.

This extension is the natural extension of a long established East West subduction zone that swallowed the Tethys Sea as the Pacific expanded. What appears to be happening is that continents are now beginning to break up along North South Axis. The next up is plausibly the African rift.

None of this matters much as we are describing possibilities over a billion years. However it shows up the weakness in our own knowledge by identifying the Pacific as a question mark. Recall that our West coast has been assembled from a lot of Pacific Island arcs or so we think. Supposedly, the Americas will cruise across the Pacific to ultimately crash into the Asian shore to build up a monster mountain range there. This will produce an Atlantic and a Rift Ocean to compensate.

Will new tectonic fault system kill the Atlantic?

17 June 2013 by Colin Barras

The dying Mediterranean Sea may have contaminated the Atlantic with a subduction zone. One day, it could help destroy the vast ocean.

Oceans come and go over hundreds of millions of years. New ones are born when continents are ripped apart, allowing hot magma to bubble up and solidify into oceanic crust. They die when continents collide and force oceanic crust back down into the mantle.

An enduring geological mystery, though, is how the ocean-swallowing subduction zones form in the first place. Oceanic crust cools and becomes more dense as it ages, so older crust may spontaneously buckle, sink into the mantle and form a subduction zone. But older crust is also stronger and more rigid – features that should prevent it either buckling or subducting.

To get to the bottom of this puzzle we need to find a subduction zone that is still forming, says João Duarte at Monash University in Melbourne, Australia. Now he and his colleagues may have found this missing piece of plate tectonics evidence in the oceanic crust off south-west Portugal.

Telltale quakes

The Atlantic is a relatively young ocean and contains almost no subduction zones, and so it is geologically quiet. Yet a couple of huge earthquakes hit Portugal in 1755 and 1969, fuelling suspicion that something unusual is going on under the waves.

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.

Beginning of the end?

Jacques Déverchère at the University of Brest, France, says the "infection" theory could explain how new subduction zones form. But he thinks it is too early to say for sure that a new subduction zone is opening up in the region.

If Duarte and his colleagues are correct, though, the Atlantic could be about to turn from a young, growing ocean into an old, dying one. It is already being subducted in the Caribbean and the far south.

"These three subduction zones can be seen as defects," says Duarte. "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."

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