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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