This is Geology Bites with Oliver Strimpel.
A subduction zone can last for a very long time.
During that time, it can move forward, backward, or sideways
and get stuck in place. It can even change direction.
What governs how a subduction zone evolves, and how do the
various kinds of evolution manifest themselves on the
surface? Claudio Fachena has been
studying how convergent margins evolve for over 30 years,
concentrating particularly on the Mediterranean region.
He is head of the Lithospheric Dynamics section at the
Helmholtz Centre for Geosciences at GFZ in Potsdam in Germany,
and also a professor at the Department of Science at Roma
Trey University. Claudio Fachena, welcome to
Geology Bytes. Thank you, Oliver.
I'm very happy to be here. We've talked about subduction
zones in previous episodes of the podcast, especially the ones
with Jeff Abbas, David Bercavici, and Laurent Jolivet.
But can you refresh us on what a subduction zone is and what the
basic mechanism is that drives them?
Yeah, sure. At larger scale we know mantle
convection is a drive to dissipate the heat inside the
Earth. So one of the consequence of
that is the formation of a cold shell we call lithosphere on the
outer part of the earth. And so those plates we call it
oceanic Lithosphere is created a Ridge and then once it started a
mid oceanic Ridge where the moth material rise spread apart and
cooled down forming new oceanic sea floor.
Now the leaders fear age became colder and eventually denser
than the hotter softer material that is beneath it.
And so after 20 or 30 million years approximately, it's going
to be denser. And so it get dense enough to
start sinking back into the mantle cell.
Imagine to have a slab of metal or any dense material sinking
in, falling into honey. So the slab is sinking because
it's denser, but its motion is resisted by the viscosity of the
mantle, almost like honey could resist a heavy object falling
into that. So the speed at which the
subduction zone go depends on two things, the density
constraints that is scaling with the age of the literature and
the viscosity of the mantle. Now the slab is now attached to
the lidosphere and so for it sinks it has to bend over and
this require energy. So the bending resistance is a
big part of what control the dynamic of subduction.
So in simple way, a subduction zone is where a cold, cold and
dense lidosphere dive back into the mantle, driven by the
gravity and resisted by the viscosity and by plants trend.
I've said that subduction zones can last a long time.
How long are we? Talking, subduction is the
longest running tectonic process on Earth, so it can last
millions or even hundreds of millions of years.
For example, the unden subduction zone where the Nazca
play now is sinking down below South America.
We know before it was the Farayam play, it's been
subducting for almost a couple of hundreds of million year.
And also where you have India colliding with Eurasia along
this big subduction zone, we know that the subduction was
more than hundreds of million years.
But here's the key. Subduction is unsteady.
It change its speed, even the way it happened can shift
dramatically and sometimes it's last for very few millions of
years. Can you categorize the various
ways in which subduction zones evolve?
Sure, subduction zone, a highly dynamic process and highly
unsteady. It's a 3D system, so a slab is
never fixed in one place. So slab don't just sink straight
down into the mantle of gravity but they move also sideways or
laterally. The sideways motion is driven by
play motion, mantle flow and the force within the slab itself.
Now there is several way that the trench in the slab is
moving. For example, one is slab
rollback. So the slab move backward into
the mantle and we call it slab rollback.
And when the trench, which is the surface expression of
subduction, also move backward, it retreats toward the
subducting plate. The trench can also move forward
or can be even stationary. So in forward motion, the slab
is advancing toward the upper plate.
Occasionally we can also have the case where the slab might
break off, twist or stop and so the subduction in this case can
shift to a new location, flip direction.
And this is a process could we call polarity reversal.
There are two important takeaways here.
First, the movement of the subduction zone, it's not steady
but it change over time. And 2nd subduction zone and
never act alone, is never alone, but interact with the
neighbourhood's lab. So the behaviour of one slab can
be affected by another close by one.
It's many regions, there are several subduction zone next to
each other, and this can be for example in opposite direction,
outward or inward. So the interaction between the
slabs in this case became really complicated.
Let's talk about each of these in turn.
Then. What causes slab rollback or
retreat of the trench? And when that happens, how does
that manifest in the surface geology?
This is a fascinating tectonic process that shapes some of the
most acted zone on Earth, so it's very important.
So what is that? A slab rollback happen when a
subducting plate sink into the mountain and start moving
backward. So at the surface, this goes the
trench to retreat. So you should think like pulling
a carpet down and back and the hedge move away from you as it
rolls up. And this is simply because the
slab is inclined toward the upper plate.
So if you're pulling down even the tendency will be to retreat.
Now all subduction zone in theory should have the tendency
to Mcgrive backwards for this reason.
The problem is that one of the key feature that we see when the
slab rollback is that you push the trench away from the upper
plate and so you create extension of formation, what we
call Becker Basin. When you say Becker Basin,
that's happening in front of the subduction zone and how, how
does that work? So they're overriding plates,
stretches and even split apart creating a new ocean.
And there are several place where we can observe that.
For example, along the Tonga branch, you have the Low basin,
which is actually the fastest rollback area on the planet or
in the GNC, but also in the Sandwich just South of South
America. In the past, almost all the
Western Pacific underwent extension.
But there is something that is very interesting in the story.
When we start looking at the age of this basin, we realize that
this tension stretching on the upper plate, which are reflected
by the fact that you have the trench retreat, don't last
forever, but they're usually lasting for 10 or 20 million
year. So that means that the motion of
the trench for some time was highly retreating and then stop
or slow down almost completely. So and after a pose of say few
millions of year is start again. So this stop and go behaviour
sometimes happen three times in some region like along the
Ejuboni Mariana system. This is, I think, very
fascinating because it's giving us the opportunity to understand
better the dynamic of the system.
If you wonder where the slab Robeck is happening right now,
then you can look at probably the Tonga is the best place,
which is one of the fastest. So you mentioned the Tonga
Trench and you also mentioned the Izu Bonin Trench.
Where is that? The Tonga Trench is the
Southwest Pacific and the Izu Bonin Mariana.
It's just South of Japan and then it's a very long feature
that is standing almost north-south.
So all this area, the western Pacific is bounded by the
subduction zone of the Pacific, which is one of the fastest
plate on Earth, underwent an enormous black carcass tension
between say 30 million year, 40 million year and now is still
active. That is one of the most
spectacular evidence of lab rollback on Earth.
What happens when subduction zones do the opposite and they
advance? Why does that occur and how does
that manifest on the surface? Well, this is only more
complicated to understand in some region again in the Izuboni
Mariana, which is was retreating for a long time and in the last
5 million years start advancing. It's probably related to the
fact that you have a multiple subduction zone lining up side
by side and they are dipping on the same direction.
So the motion of one of the slab can help pull the other forward
like a little train. If the one in front move and
it's pulling down the flipping slates, then it drags the other
along. And so in the last five million
year, the Izuboni Mariana for a long time was retreating, now
changes motion and start advancing.
Sorry, when you say dragging along, is it because it's next
to it laterally dragging it or is it in front of it?
Front of it, in front of it, and this has been recognized long
time ago, but the effect of the front was lab, which is the
Lucas lab, which is retreating at the moment, is consuming the
plate between the two and then is kind of sucking the other
trench, which is incredible. And so you have a complete
reverse of moisture. So those labs are really
floating into the mount all laterally.
There are other extreme case of trench advance and those are,
for example, happening during continental collision.
One of the best example of it, it's not a subduction zone, it's
a collisional zone. In this case, it's a continental
lithosphere that is advancing toward the upper plate that
India is indenting into Asia, creating this enormous mountain
belt. So you got India crashing into
Asia, but at the same time the Australian plate is subducting
on the net from the southeast Java, Sumatra.
So this sideway or lateral subduction, combine it with the
mantle flow will push India frontward and crossing creation
of the largest mountain belt in the world.
So what does it look like at the surface?
In trench advanced cases you often have intense crustal
deformation, uplift, compression.
You can have closure of Bayesian that it's a power spool way to
having compression of force, very different from stretching
during rollback. So in short, trench advance is
less common than rollback, but when it does happen, it usually
means that the tonic system is changing completely and moving
from extension to compression. But it's also often caused by
the influence neighborhoods, lab and Montal dynamics.
I'm sure in the real world it's a bit of an oversimplification
to say that subduction zones are either retreating or advancing.
In his podcast episode, Laurent Jolivet describes some quite
complex motions, including toroidal ones in 3D.
Can you describe some of these more complicated motions?
Sure, I will try. Let's talk about what happened
in the mantle as his labs sink into it.
So as the slab dive into the mantle itself is flowing from
areas of high pressure toward areas of low pressure created by
the slab itself. We usually break this mantle
motion in two main component. The first one we call the
poloidal component and just the vertical flow, the classical up
and down. But then in 3D we have this
other component, that is the toroidal component, that is the
horizontal zwiggling flow, which tend to show up along the edge
of the slab. So when this toroidal flow
happened, if you're moving a spoon inside of a pot full of
honey, you can see around the edge of the spoon, you will see
the little vertex. And this is actually the kind of
motion we are talking about and we are expecting to have into
the mantle. If there are slab windows, so
area where the slab is broken off, also a gap in the plate or
near the slab edge, the mantle has space to flow inside and
around the side. And this is creating the
toroidal flow. It's a 3D feature and what is
really fascinating is that this flow is not just deep into the
mantle, but we have effect on the surface and it LED, for
example, lateral deformation. For example, a large trice leaf
folds on that produce a large earthquake rock shifting
sideways, twisting or stretching in a very unusual pattern.
So sometimes are very complicated to imagine when slab
break into pieces or tear apart that slab is not a continuous
feature. This effect are specially very
pronounced. And so for example, Lauren was
mentioned in the Mediterranean, this is a case where the slab
are very small and so you have edges where this mantle is
flowing during trench rollback. And so you're pushing basically
the material that is below the slab on the side.
And this is flowing over this abductance lab changing
completely also the composition of the mountain.
They had the Tongas lab. As we mentioned before, this is
an area where you really have a lot of toroidal fluid that's
been recognized nearby Samoa. Now, for example, you can have
it in the Antilles or or for example, if you go at the edge
of the abductance lab below the Andes in Patagonia on the Drake
Passage between Antarctica and the South America, we have a
slab break. And in this area we to have a
big flow of Pacific mantle into different.
So even if those flow are really deep into the mantle, their
fingerprint on the surface are quite impressive.
And so understanding them help us to really piece together the
big picture of the mantle circuit on the Earth.
All that present day examples of subduction zones that move
laterally sideways or in a generally more complex fashion
than either forward or backward. So yes, lateral motions lab is
happening everywhere. Oliver subduction zone rally
just sinks straight down and the trench are never stationary.
Really what makes things tricky is how the different part of the
slab interacting with each other with nearby slabs and reading
the motion of the trench gave us the possibility to understand
the slab dynamics. For example, one portion of the
slab is advancing while the other one is retreating, and
this created vortex and a complex dynamic in the
overriding plate. For example, example, if you go
to New Zealand, and we will discuss about that further on,
then you go near the Alps, we have a strong collision of the
plateau. But when you move northward, so
the trench here is transpressional and is advancing
a little bit. And if you move N, then you have
a Becker region. So you move from compression to
extension on the upper plate and the trench is more advancing or
moving laterally to the South and is retreating to the north.
The same picture is observed in other area like in the
Mediterranean for example, if you go in Greece from the Lanic
trench, which is retreating, you move north or along the other
the Attic plates say going to where Venice say, and below the
Dinarites, then you have compression.
So you're moving lateral from compression zone where the
trench is stationary is advancing toward the South.
In the Lanic the trench is retreating very fast and
producing backpack extension. So the lateral slab motion
combined with interaction between adjacent slab and
overriding play create a patchwork of tectonic setting
that from mountain building to rifting and extension and so all
connected on this 3D dynamic picture.
I think the most surprising kind of subduction zone evolution is
when the polarity of the subduction reverses.
That is, a down going plate subducting in One Direction
flips to becoming the overriding plate, with the formerly
overriding plate subducting under it in the opposite
direction. What makes that happen?
Well this is a quite common mechanism as well.
So it happen when you have a buoyant Fisher that is arriving
a trench and it doesn't want to subduct and so you stop
subduction somehow. In this case what you can have
this is because I'm in a continental crust or a volcanic
arc or a thick buoyant plateau is more buoyant and so it cannot
descend down into the mountain. And now if you stop subduction,
then you probably is followed by break off.
And so this lab is falling down and the subduction will end.
Now if compressional forces keep going in this case, then you
have the restart of subduction in another place nearby.
And one of the things that that we observe happening is that you
can have a reversal of subduction now on the opposite
direction. So subduction will occur on the
other side and an opposite direction.
So all this can occur in some millions of years, not very
rapid because you have to stop 1 subduction and initiate another
one. For example Banatu trench in the
South Pacific. So it's Nordo Tonga now.
There was a a plateau arriving a trench, it stopped subduction
somehow. The subduction was of the
Pacific that was going below the Australian plate.
Now after the subduction, the plateau arrived, we have a
reversal of subduction. Now the subduction is is of the
Australian plate that go below the Pacific somehow and
formation of the Vanatu trench of the New Hebrides.
So this is a case where we now observe A lateral transition of
the reversal polarity from the Tonga that is plunging down
toward the West to the New Hebrides that is plunging down
to the northeast. And so it occurred because of
polarity reversal. Now in other cases which is
pretty well known is Taiwan where you had the arrival of
volcanic arc which is the chalk, the subduction zone and then you
have the start of the new subduction zone of the South
China Sea now. This reversal of polarity seems
to have happened during the Caledonian originy.
Rob Strachan told us about that in his podcast episode.
So it sounds from what you're saying that polarity reversal
actually is fairly common. Well, it is very common, maybe
even more common than what we thought actually also in
Camchaca, for example, we had that also in the Caribbean, we
had that the subduction below what is now Mexico, Costa Rica,
Panama was interrupted for and then arrival of the big plateau,
which is the Caribbean plateau with the reversal of that.
Now the only problem we have is that during subduction,
sometimes the formation of the upper plate is really intense
and some of this element that tests for the holders, the
subduction polarity reversal can be lost into this mechanism.
And so for us geologists now becoming a little bit difficult
sometimes to unravel the history in a very complete fashion.
And so the subsequent subduction episode may obscure the phrases
of the oldest one or process like that that may occur also
rapidly like in 10, so 20 million year in geological time
scale. Sometimes this is a rapid
process. Another phenomenon that we see
quite often is the reversal of subduction polarity, not in
time, but in space. In other words, as you follow
the subduction zone along the surface, the subduction flips
direction at a certain point. And I think you already just
mentioned the example of this happening in New Zealand, where
to the north of New Zealand and in North Island, the Pacific
Plate is subducting westward under the Australian Plate.
And then as you go South and cross the Alpine Fault, the
direction reverses with the Australian Plate subducting
eastward under the Pacific Plate.
Do we understand how this happens?
It sounds from what you were suggesting that these two
subduction zones used to be going in the same direction, but
then one got blocked up and reversed.
It's actually the same things. So it's triggered by the
collision of a buoyant feature like oceanic plateau, a volcanic
arc. Sometimes it just reflect the
presence of a polygeographic distribution, so the way the
oceanic basin were distributed or ranged in the past and this
set the stage for how and where subduction will start down
later. One example is the transition
from the Alps to the Apennine in Europe where we observe a change
in subduction direction over a relatively short distance.
So in the Apennine you have a subduction of the Adria plate
which is belonging to Africa domain say below Eurasia.
Whereas when you go in the Alps is the other way around.
So the there is the Eurasia that is subducting below.
Another great example as I said is Taiwan.
So in this East China Sea plate is being subducted back to the
north is the Philippine plate that's subducting under the UQ
trench. So another clear change in
polarity along strike. And the Vanetto is the other
example that I was mentioned before.
So it's the same process along strike and on time that drive
that. When we spoke earlier, you
mentioned slab anchoring and how it plays a role in mountain
building and basically in freezing a subduction zone in
place. Can you explain that?
Oh yes, sure. This is one of my passion.
The health mantle is not uniform in terms of viscosity.
So that means that the deep layer, especially the one below
700 kilometers are more viscous than the other one, like 50 or
30 times more viscous. So when a slab reached this
depth, you can behave in two ways.
Some of them do not penetrate and flatten out, other may
penetrate. And this also depending on the
way the slab arrive to hit this transition zone, whether it's
more vertical or not. So if this lab I enter into the
lower mantle, then it is more anchor, it cannot really move
because it's more strict by the more viscous layer.
Now there is also another effect.
Once it penetrate into the lower mantle, then it trigger a very
large scale convection cell more vigorous than what the one
before in the upper mantle. So in this case the trench
became in stationary and could be stationary for some 10s of
millions of year until something new happened, maybe the slab
broken off. This is intriguing because a
good example of this case and the impact of the anchoring on
the deformation on the upper plate we think is the case of
the formation of the Undiscord era in South America.
So there at least in the northern part where the Andes go
up forming the Altiplano 4000 meters elevation.
So in this area, the trench of the Nazca plate remain
relatively stationary. And we think this is occurring
because the slab penetrated anchors deep into the mountain
and so you're resisting the motion of South America.
So that the reason why you have the sort of collision where you
create a mountain belt even if you don't have the collision
between 2 continental plate, it's just because the trench is
stationary and anchor deep into the mountain.
Wow that's interesting. So instead of just creating a
regular volcanic arc as most subduction zones do, we create a
giant mountain chain? Because the slab is stuck in
place continually generating volcanism in the same place over
10s or even hundreds of millions of years.
So it's still subducting downwards, driving continued
volcanism in the Andes and releasing volatiles.
Exactly if the motion of the velocity of that even further
increase when the slab penetrate into the lower mantle dioza, we
think that the slab penetrate after a period of stagnation
where you accumulate a lot of dense material and then they may
avalanche down into the lower mantle.
In this case, you really can have also an acceleration and
outplay motion. So that is super interesting.
This is ultimately in my mind the reason why you created
everything under compression until you create a
supercontinent. But this is another story.
What methods and observations do you use in your research to
better understand the dynamics of subduction zones?
This is a great question. One of the major tool used for
imagining subduction zone adapt obviously is a seismology.
So seismology can give us the current structure of subduction
zone, not only the distribution of earthquake that go down to
700 kilometre, but also using tomography.
And so looking at the distribution of the cold
material revealed by the path of seismic way they go faster then
you can unravel the whole phase of subduction story.
And there is also the active seismology where we're shooting
and then we can record by seismometer and this is
illuminating the upper part of the subduction zone.
So checking at the first tenths of 15 kilometers more in detail
what happened into the subduction zone, You know,
subduction zone release most of the seismic energy on Earth and
it's causing big earthquake. Only subduction zone can have
none of the other faults will never reach this kind of
magnitude. Just because the geometry of
subduction zone is so large that you can start propagating the
rupture for hundreds of kilometers with motion that are
10s of meters. We really spend a lot of energy
and time to understand how they work and so for that we're
installing seismic station on the sea, geodetic station on the
sea, and trying to images the mush on the plate with great
accuracy. Now seductions are also doing
something different. So not only seismology but
geodesy, but also it's about recycling material and moving
rocks. So like a machine that takes
sediment, fluids and rock down to 10s of kilometers, and when
you do that, they're releasing fluid and they're releasing
volatiles that are recycled backward into the volcanic arc.
And so petrology is a very important tool as well, because
with that you can understand much better what kind of
material is subducting and how metamorphism reflect the
temperature of the subduction process at the interface between
the two plates. So is that because you're
studying the detailed geochemistry of the material
that comes out of the volcanoes in the volcanic arc?
This will tell us a lot about the how much sediment are
subducting and then how much fluid are subducting.
And so this story will help us to understand the circuit that
you can have at the recycle of element like carbon dioxide,
metals and many others. So there's a whole budget of
chemical elements that you can trace it using chemistry on
petrology of volcanic arc. But also sometimes the
subduction interface down to 4 and 40 and 30 kilometer is
exhumed later. And so when we go in the field
that we can look at that and reconstruct the pressure, the
temperature and the budget of fluid that occur during the
motion on one plate below the other.
And when you have all this information, then you can get
into modeling. Now modeling is a fundamental
tool that both experimental and numerical model to try to mimic
and understanding the dynamic. Once we have reconstructed the
kinematic and reconstructing the parameters controlling the
process, we can go to our machine, to our laboratory,
reconstructing and understanding what caused the motion of this
lab into the mantle and what process are governing this lab
dynamic. So it's a teamwork you have to
combining observation, experiment and theory to build
that complete picture of a subduction zone.
And that's also what makes subduction zone so exciting, I
think. The seismic data we've been
talking about so far gives us information about seismic wave
speeds, which is usually interpreted to reflect the
temperature of the lithosphere and mantle traversed by the
seismic waves. But can we also use the
anisotropy of the seismic waves, IE the different speeds of
differently polarized waves, to reveal some of the mantle flows
you've been telling us about around the subducting slab?
No, absolutely. Seismic anisotropy is a
fundamental tool actually, because seismic anisotropy
actually gave us an incremental of deformation.
So it's not only an instantaneous picture like the
one you can have with earthquake, but you can go back
in time because to have a strong signal then you have to deform
our mineral in a way to polarize sufficiently the waves.
And so also for that, I think this is a very useful tool
because this gave us a long term signal.
It's not an Eastern tennis one. What are you working on at the
moment? So Oliver, my driving research
stimulus is understanding the coupling between deep mantle
process and what we see at the surface, so the connection
between surface process and the deep.
And I do that especially on subduction margin and active
zones because when you look at working in active area, then you
can actually use information from geophysics to better
constrain what is happening now. And with that, you can go back
in the past. So at the moment, looking close
at the formation around slab edges, for example, in Greece,
where the dynamics are very completely fascinating, moving
from compression to extension, but also the Andes.
We have installed some 10s of years ago an observatory along
the Northern Chile in the Atacama desert, where we have a
big seismic gap and we hope to be able to capture signal for
the preparatory phase. But we also working in Panama,
Colombia region, where the Caribbean meet the Nazca's labs.
So it's a highly 3 dimensional problem.
But in this context, we are really trying to understand how
subduction is modelling also on the surface and the landscape
and how this reflect on biodiversity for the crew and
and formation of the Panama strength dividing the Pacific
and the Atlantic. Also we don't stop here because
we're also working in collisional areas.
So the region where we're working now is Pakistan in the
northern Pakistan. There in the Karakoram, we have
two subduction zone in the past. Now they are pretty close to
each other in the region of Nanga Parbat.
And in this area we can find the trace of both subduction zone
and try to understand what bring this formation or this enormous
mountain belt. I love mountains, so I'm moving
region where there are high elevation.
There you can see the surface expression of deep deformation
and try understanding the best expression of subduction and
collision on the surface. Claudia Fachena, thank you very
much. Oliver, thanks a lot for tuning
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