This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: 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 Faccenna 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 Tre University. Claudio Faccenna, welcome to Geology Bites.

Claudio Faccenna: Thank you, Oliver. I'm very happy to be here.

Oliver Strimpel: We've talked about subduction zones in previous episodes of the podcast, especially the ones with Geoff Abers, David Bercovici, and Laurent Jolivet. But can you refresh us on what a subduction zone is and what the basic mechanism is that drives them?

Claudio Faccenna: Yeah, sure. At a large scale, we know mantle convection is a way to dissipate the heat inside the Earth. One of the consequences of that is the formation of a cold shell we call the lithosphere on the outer part of the Earth. Oceanic lithosphere is created at a ridge, at a mid-ocean ridge, where mantle material rises, spreads apart and cools down, forming new oceanic seafloor. Now, as the lithosphere ages, it becomes colder and eventually denser than the hotter, softer material that is beneath it. After 20 or 30 million years, approximately, it's going to be denser, and it gets dense enough to start sinking back into the mantle. Imagine a slab of metal or any dense material falling into honey. The slab is sinking because it's denser, but its motion is resisted by the viscosity of the mantle, almost like honey would resist a heavy object falling into it. So the speed at which subduction goes depends on two things: the density contrast, which scales with the age of the lithosphere, and the viscosity of the mantle. Now, the slab is still attached to the lithosphere, and so as it sinks, it has to bend over, and this requires energy. So the bending resistance is a big part of what controls the dynamics of subduction. Put simply, a subduction zone is where cold, dense lithosphere dives back into the mantle, driven by gravity and resisted by the viscosity of the mantle and by plate strength.

Oliver Strimpel: I've said that subduction zones can last a long time. How long are we talking?

Claudio Faccenna: Subduction is the longest-running tectonic process on Earth, so it can last millions or even hundreds of millions of years. For example, the Andean subduction zone, where the Nazca plate is now sinking below South America. We know that before it was the Farallon plate, and it's been subducting for almost a couple of hundred million years. And also where you have India colliding with Eurasia, along this big subduction zone, we know that subduction went on for more than a hundred million years. But here's the key: subduction is unsteady. It changes its speed, even the way it happens can shift dramatically, and sometimes it lasts for only a very few million years.

Oliver Strimpel: Can you categorize the various ways in which subduction zones evolve?

Claudio Faccenna: Sure. Subduction is a highly dynamic and highly unsteady process. It's a 3D system, so a slab is never fixed in one place. Slabs don't just sink straight down into the mantle by gravity; they also move sideways, or laterally. The sideways motion is driven by plate motion, mantle flow, and the forces within the slab itself. Now, there are several ways that the trench and the slab move. One, for example, is slab rollback: the slab moves backward into the mantle, and we call it slab rollback. And the trench, which is the surface expression of subduction, also moves backward; it retreats toward the subducting plate. The trench can also move forward, or it can even be stationary. 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 subduction in this case can shift to a new location, or flip direction. This is a process we call polarity reversal. There are two important takeaways here. First, the movement of a subduction zone is not steady but changes over time. And second, a subduction zone never acts alone, but interacts with neighboring slabs. So the behavior of one slab can be affected by another one close by. In many regions, there are several subduction zones next to each other, and they can be, for example, facing in opposite directions, outward or inward. So the interaction between the slabs in this case becomes really complicated.

Oliver Strimpel: 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?

Claudio Faccenna: This is a fascinating tectonic process that shapes some of the most active zones on Earth, so it's very important. So what is it? Slab rollback happens when a subducting plate sinks into the mantle and starts moving backward. At the surface, this causes the trench to retreat. You can think of it like pulling a carpet down and back: the edge moves 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 it down, the tendency will be for it to retreat. Now, all subduction zones in theory should have the tendency to migrate backward for this reason. One of the key features that we see when the slab rolls back is that you pull the trench away from the upper plate, and so you create extension and the formation of what we call back-arc basins.

Oliver Strimpel: When you say back-arc basin, that's happening in front of the subduction zone, and how does that work?

Claudio Faccenna: The overriding plate stretches and even splits apart, creating a new ocean. And there are several places where we can observe that. For example, along the Tonga trench, you have the Lau Basin, which is actually the fastest rollback area on the planet, or in the Aegean Sea, but also in the Sandwich, just south of South America. In the past, almost all of the western Pacific underwent extension. But there is something very interesting in the story. When we start looking at the ages of these basins, we realize that the stretching of the upper plate, which reflects the retreat of the trench, doesn't last forever; it usually lasts for 10 or 20 million years. That means that for some time the trench was retreating strongly, and then it stopped or slowed down almost completely. And after a pause of, say, a few million years, it started again. This stop-and-go behavior sometimes happens three times in some regions, like along the Izu–Bonin–Mariana system. This is, I think, very fascinating, because it gives us the opportunity to better understand the dynamics of the system. If you wonder where slab rollback is happening right now, then Tonga is probably the best place to look, as it is one of the fastest.

Oliver Strimpel: So you mentioned the Tonga Trench, and you also mentioned the Izu–Bonin Trench. Where is that?

Claudio Faccenna: The Tonga Trench is in the southwest Pacific, and the Izu–Bonin–Mariana is just south of Japan; it's a very long feature that runs almost north–south. This whole area of the western Pacific, bounded by the subduction zone of the Pacific plate, which is one of the fastest plates on Earth, underwent enormous back-arc extension between, say, 30 or 40 million years ago and now, and it is still active. That is one of the most spectacular pieces of evidence of slab rollback on Earth.

Oliver Strimpel: What happens when subduction zones do the opposite and they advance? Why does that occur, and how does that manifest on the surface?

Claudio Faccenna: Well, this is a little more complicated to understand. In some regions, again in the Izu–Bonin–Mariana, which was retreating for a long time, in the last 5 million years it started advancing. It's probably related to the fact that you have multiple subduction zones lined up side by side, and they are dipping in the same direction. So the motion of one of the slabs can help pull the other forward, like a little train. If the one in front moves and is pulling down the Philippine Sea plate, then it drags the other along. And so in the last 5 million years, the Izu–Bonin–Mariana, which for a long time was retreating, has changed its motion and started advancing.

Oliver Strimpel: Sorry, when you say dragging along, is it because it's next to it laterally, dragging it, or is it in front of it?

Claudio Faccenna: In front of it. And this was recognized a long time ago: the effect of the slab in front, which is the Ryukyu slab, which is retreating at the moment, is consuming the plate between the two and is then kind of sucking the other trench along, which is incredible. And so you have a complete reversal of motion. So those slabs are really moving laterally through the mantle. There are other extreme cases of trench advance, and those happen, for example, during continental collision. One of the best examples is not a subduction zone; it's a collisional zone. In this case, it's continental lithosphere that is advancing toward the upper plate: India is indenting into Asia, creating this enormous mountain belt. So you've got India crashing into Asia, but at the same time the Australian plate is subducting next to it, to the southeast, beneath Java and Sumatra. This sideways, or lateral, subduction, combined with the mantle flow, pushes India forward, causing the creation of the largest mountain belt in the world. So what does it look like at the surface? In cases of trench advance, you often have intense crustal deformation, uplift and compression. You can have closure of basins and powerful compressional forces, very different from the stretching during rollback. So, in short, trench advance is less common than rollback, but when it does happen, it usually means that the tectonic system is changing completely, moving from extension to compression. And it's also often caused by the influence of neighboring slabs and mantle dynamics.

Oliver Strimpel: 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?

Claudio Faccenna: Sure, I will try. Let's talk about what happens in the mantle as the slab sinks into it. As the slab dives into the mantle, the mantle itself flows from areas of high pressure toward areas of low pressure created by the slab itself. We usually break this mantle motion into two main components. The first one we call the poloidal component, and it's just the vertical flow, the classical up and down. But then in 3D, we have this other component, the toroidal component, which is the horizontal swirling flow that tends to show up along the edges of the slab. To picture this toroidal flow, if you move a spoon inside a pot full of honey, you will see little vortices around the edge of the spoon. And this is actually the kind of motion we are talking about and that we expect to have in the mantle. If there are slab windows, areas where the slab has broken off, or a gap in the plate, or near the slab edge, the mantle has space to flow inside and around the side, and this creates the toroidal flow. It's a 3D feature, and what is really fascinating is that this flow is not just deep in the mantle, but it has effects on the surface, and it leads, for example, to lateral deformation: for example, large strike-slip faults that produce large earthquakes, rocks shifting sideways, twisting or stretching in very unusual patterns. So it's sometimes very complicated to imagine, when slabs break into pieces or tear apart, that the slab is not a continuous feature. These effects are especially pronounced there. For example, as Laurent mentioned, in the Mediterranean the slabs are very small, and so you have edges where the mantle is flowing during trench rollback. You're basically pushing the material that is below the slab to the side, and this flows around the subducting slab, completely changing the composition of the mantle there too. Take the Tonga slab. As we mentioned before, this is an area where you really have a lot of toroidal flow; it's been recognized near Samoa. You also have it, for example, in the Antilles, or if you go to the edge of the subducting slab below the Andes in Patagonia, at the Drake Passage between Antarctica and South America, we have a slab break, and in this area we too have a big flow of Pacific mantle into the Atlantic. So even if those flows are really deep in the mantle, their fingerprints on the surface are quite impressive. And so understanding them helps us to really piece together the big picture of mantle circulation in the Earth.

Oliver Strimpel: Are there present-day examples of subduction zones that move laterally, sideways, or in a generally more complex fashion than either forward or backward?

Claudio Faccenna: Yes, lateral slab motion is happening everywhere, Oliver. Subduction zones rarely just sink straight down, and the trenches are never stationary. What really makes things tricky is how the different parts of a slab interact with each other and with nearby slabs, and reading the motion of the trench gives us the possibility to understand the slab dynamics. For example, one portion of the slab is advancing while another one is retreating, and this creates vortices and complex dynamics in the overriding plate. For example, if you go to New Zealand, and we will discuss that further on, near the Southern Alps we have strong collision. But when you move northward, the trench is transpressional and is advancing a little bit. And if you move further north, then you have a back-arc region. So you move from compression to extension in the upper plate, and the trench is advancing or moving laterally in the south and is retreating in the north. The same picture is observed in other areas, like in the Mediterranean. For example, if you go from the Hellenic trench in Greece, which is retreating, and move north along the Adriatic plate, going toward, say, Venice, below the Dinarides, then you have compression. So you're moving laterally to a compressional zone where the trench is stationary or advancing, and in the south, at the Hellenic trench, the trench is retreating very fast and producing back-arc extension. So lateral slab motion, combined with the interaction between adjacent slabs and the overriding plate, creates a patchwork of tectonic settings, from mountain building to rifting and extension, all connected in this 3D dynamic picture.

Oliver Strimpel: 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?

Claudio Faccenna: Well, this is quite a common mechanism as well. It happens when a buoyant feature arrives at a trench and doesn't want to subduct, and so subduction somehow stops. This can be continental crust, a volcanic arc, or a thick plateau that is more buoyant and so cannot descend into the mantle. Now, if subduction stops, then it is probably followed by break-off. The slab falls down and subduction ends. Now, if the compressional forces keep going, then subduction restarts in another place nearby. And one of the things that we observe happening is that you can have a reversal of subduction, now in the opposite direction. So subduction occurs on the other side and in the opposite direction. All this can occur over some millions of years; it's not very rapid, because you have to stop one subduction zone and initiate another one. One example is the Vanuatu trench in the South Pacific, which is north of Tonga. There was a plateau arriving at the trench, and it somehow stopped subduction. That subduction was of the Pacific plate going below the Australian plate. After the plateau arrived, we had a reversal of subduction. Now the subduction is of the Australian plate going below the Pacific, with the formation of the Vanuatu trench, of the New Hebrides. So this is a case where we now observe a lateral transition in polarity, from Tonga, which is plunging down toward the west, to the New Hebrides, which is plunging down to the northeast. And so it occurred because of polarity reversal. Another case that is pretty well known is Taiwan, where you had the arrival of a volcanic arc, which choked the subduction zone, and then the start of the new subduction zone of the South China Sea.

Oliver Strimpel: This reversal of polarity seems to have happened during the Caledonian orogeny. 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.

Claudio Faccenna: Well, it is very common, maybe even more common than we thought. Also in Kamchatka, for example, we had that. Also in the Caribbean, we had that: subduction below what is now Mexico, Costa Rica and Panama was interrupted by the arrival of a big plateau, the Caribbean plateau, with a reversal of subduction. Now, the only problem we have is that during subduction, sometimes the deformation of the upper plate is really intense, and some of the elements that testify to the older subduction polarity reversal can be lost in this process. And so for us geologists, it's sometimes a little difficult to unravel the history in a very complete fashion. The subsequent subduction episode may obscure the traces of the older one, and a process like that may also occur rapidly, in 10 or 20 million years on the geological timescale. Sometimes this is a rapid process.

Oliver Strimpel: 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.

Claudio Faccenna: It's actually the same thing. It's triggered by the collision of a buoyant feature, like an oceanic plateau or a volcanic arc. Sometimes it just reflects the paleogeographic distribution, the way the oceanic basins were distributed or arranged in the past, and this sets the stage for how and where subduction will start later. One example is the transition from the Alps to the Apennines in Europe, where we observe a change in subduction direction over a relatively short distance. In the Apennines, you have subduction of the Adria plate, which belongs to the African domain, below Eurasia, whereas when you go to the Alps, it's the other way around: it's Eurasia that is subducting below. Another great example, as I said, is Taiwan. In the south, the South China Sea plate is being subducted, but to the north, it's the Philippine Sea plate that's subducting under the Ryukyu trench. So that's another clear change in polarity along strike. And Vanuatu is the other example that I mentioned before. So it's the same process, along strike and in time, that drives this.

Oliver Strimpel: 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?

Claudio Faccenna: Oh yes, sure. This is one of my passions. The mantle is not uniform in terms of viscosity. The deep layer, especially the part below 700 kilometers, is more viscous than the layer above, perhaps 30 to 50 times more viscous. So when a slab reaches this depth, it can behave in two ways. Some slabs do not penetrate and flatten out; others may penetrate. And this also depends on the way the slab arrives at this transition zone, whether it's more vertical or not. If the slab enters the lower mantle, then it is more anchored; it cannot really move, because it's restricted by the more viscous layer. Now, there is also another effect. Once it penetrates into the lower mantle, it triggers a very large-scale convection cell, more vigorous than the one before in the upper mantle. In this case, the trench becomes stationary, and it could be stationary for some tens of millions of years, until something new happens, maybe the slab breaking off. This is intriguing, because we think a good example of this case, and of the impact of anchoring on the deformation of the upper plate, is the formation of the Andean Cordillera in South America, at least in the northern part, where the Andes go up, forming the Altiplano at 4,000 meters elevation. In this area, the trench of the Nazca plate remained relatively stationary, and we think this is because the slab penetrated and anchored deep into the mantle, and so it's resisting the motion of South America. That's the reason why you have a sort of collision where you create a mountain belt, even though you don't have a collision between two continental plates; it's just because the trench is stationary and anchored deep into the mantle.

Oliver Strimpel: 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 tens or even hundreds of millions of years. So it's still subducting downwards, driving continued volcanism in the Andes and releasing volatiles.

Claudio Faccenna: Exactly. And the velocity can increase even further when the slab penetrates into the lower mantle. We think that the slab penetrates after a period of stagnation, where you accumulate a lot of dense material, and then it may avalanche down into the lower mantle. In this case, you really can have an acceleration of plate motion too. So that is super interesting. This is ultimately, in my mind, the reason why you get everything under compression until you create a supercontinent. But that is another story.

Oliver Strimpel: What methods and observations do you use in your research to better understand the dynamics of subduction zones?

Claudio Faccenna: This is a great question. One of the major tools for imaging subduction zones at depth is obviously seismology. Seismology can give us the current structure of a subduction zone, not only from the distribution of earthquakes, which go down to 700 kilometers, but also using tomography. By looking at the distribution of cold material revealed by the paths of seismic waves, which go faster through it, you can unravel the whole story of subduction. And there is also active seismology, where we make shots and record them with seismometers, and this illuminates the upper part of the subduction zone, the first 10 or 15 kilometers, in more detail. You know, subduction zones release most of the seismic energy on Earth, and they cause big earthquakes. Only subduction zones can have them; none of the other faults will ever reach this kind of magnitude, just because the geometry of a subduction zone is so large that the rupture can propagate for hundreds of kilometers, with motions of tens of meters. We really spend a lot of energy and time to understand how they work, and so for that we're installing seismic stations and geodetic stations on the seafloor, trying to image the motion of the plate with great accuracy. Now, subduction zones also do something different. So it's not only seismology and geodesy; it's also about recycling material and moving rocks. A subduction zone is like a machine that takes sediment, fluids and rock down tens of kilometers, and when it does that, it releases fluids and volatiles that are recycled back into the volcanic arc. And so petrology is a very important tool as well, because with it you can understand much better what kind of material is subducting, and how metamorphism reflects the temperature of the subduction process at the interface between the two plates.

Oliver Strimpel: So is that because you're studying the detailed geochemistry of the material that comes out of the volcanoes in the volcanic arc?

Claudio Faccenna: This tells us a lot about how much sediment is subducting and how much fluid is subducting. And this story helps us to understand the cycle, the recycling of elements like carbon dioxide, metals and many others. So there's a whole budget of chemical elements that you can trace using the chemistry and petrology of the volcanic arc. But also, sometimes the subduction interface, down to 30 or 40 kilometers, is exhumed later. And so when we go into the field, we can look at that and reconstruct the pressure, the temperature and the budget of fluids during the motion of one plate below the other. And when you have all this information, then you can get into modeling. Now, modeling is a fundamental tool, both experimental and numerical modeling, to try to mimic and understand the dynamics. Once we have reconstructed the kinematics and the parameters controlling the process, we can go to our machines, to our laboratory, and understand what causes the motion of the slab in the mantle and what processes govern slab dynamics. So it's teamwork: you have to combine observation, experiment and theory to build a complete picture of a subduction zone. And that's also what makes subduction zones so exciting, I think.

Oliver Strimpel: 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, i.e. the different speeds of differently polarized waves, to reveal some of the mantle flows you've been telling us about around the subducting slab?

Claudio Faccenna: Oh, absolutely. Seismic anisotropy is a fundamental tool, actually, because seismic anisotropy gives us an integral of deformation. It's not only an instantaneous picture, like the one you can get from earthquakes; you can go back in time, because to have a strong signal, you have to deform the minerals in a way that polarizes the waves sufficiently. And so I think this is also a very useful tool, because it gives us a long-term signal. It's not an instantaneous one.

Oliver Strimpel: What are you working on at the moment?

Claudio Faccenna: So, Oliver, the driving stimulus of my research is understanding the coupling between deep mantle processes and what we see at the surface, the connection between surface processes and the deep. And I do that especially on subduction margins and active zones, because when you work in active areas, you can actually use information from geophysics to better constrain what is happening now, and with that, you can go back into the past. At the moment, I'm looking closely at deformation around slab edges, for example in Greece, where the dynamics are really fascinating, moving from compression to extension, but also in the Andes. Some years ago, we installed an observatory in northern Chile, in the Atacama Desert, where we have a big seismic gap, and we hope to be able to capture signals from the preparatory phase. But we're also working in the Panama–Colombia region, where the Caribbean meets the Nazca slab. So it's a highly three-dimensional problem. In this context, we are really trying to understand how subduction is also shaping the surface and the landscape, and how this is reflected in biodiversity, for example through the formation of the Isthmus of Panama, dividing the Pacific and the Atlantic. And we don't stop there, because we're also working in collisional areas. The region where we're working now is northern Pakistan. There, in the Karakoram, we had two subduction zones 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 traces of both subduction zones and try to understand what brought about the formation of this enormous mountain belt. I love mountains, so I'm moving to regions with high elevations. There you can see the surface expression of deep deformation, and try to understand the best expression of subduction and collision at the surface.

Oliver Strimpel: Claudio Faccenna, thank you very much.

Claudio Faccenna: Thanks a lot, Oliver.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.