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Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Several previous episodes of Geology Bites, such as the ones with Damian Nance and David Evans, have discussed the formation and breakup of supercontinents. But how does something so apparently strong and permanent as a continental block actually break up? Folarin Kolawole studies the early stages of continental rifting by using a combination of field observation, geophysical imaging and mechanical study of rocks. His research has focused on rifting in East Africa, southeastern Brazil and Death Valley in the US. He is assistant professor of earth and environmental sciences at Columbia University, and also works in the Seismology, Geology and Tectonophysics division of the Lamont–Doherty Earth Observatory of Columbia University. Folarin Kolawole, welcome to Geology Bites.

Folarin Kolawole: Thank you for having me.

Oliver Strimpel: Can you give us an overview of where around the world we see ongoing rifting happening today?

Folarin Kolawole: We have quite a few of them. The largest and the most enigmatic one is the East African Rift System, which extends from the northeastern corner of East Africa all the way south into the northern part of Botswana in southern Africa. We also have active rifting in Greece, in the Gulf of Corinth. We have the Shanxi Rift in China; we have the Lake Baikal Rift in Siberia, Russia. We have the Rhine Graben system in Germany; we have the Rio Grande Rift here in the United States. And there are many other parts of the world where we have rifting going on.

Oliver Strimpel: Well, there are really quite a lot of them. And how do we know that rifting is happening when there is as yet no actual rift, I mean, no actual new oceanic crust being formed between separating continents?

Folarin Kolawole: Right. I would mention a few key observations. One is surface observations. We have faults breaking the surface of the Earth, and as the faults slip, they create deep valleys, and as they evolve even further, the flanks of the rift are uplifted and form big mountains on the landscape. So that is one important observation. You could have lakes forming within these depressions, and these lakes could be as huge as Lake Tanganyika, which is one of the largest continental lakes on Earth today, or Lake Malawi. Common observations are also volcanoes, because the stretching of continents allows for melting in the mantle that can inject huge volumes of magma straight through the lithosphere to the surface. So you see these big active volcanoes lining up along the axis or on the margins of the rift valleys. These are classic indicators of melt and thermal fluxes into the lithosphere. And in places in the rift valley where you don't yet have volcanoes, you commonly see hot springs along the traces of the big faults, and geysers in some places, which tell us that there are huge thermal fluxes into the lithosphere from the mantle.

And when we look deeper into the subsurface, using geophysical imaging techniques like seismology, gravity and magnetics, one common observation we make is that the base of the lithosphere has been rising up at the axis, or near the axis, of these rift valleys. We're able to delineate the lithosphere–asthenosphere boundary, which is the boundary between the thick plate itself and the more mobile mantle material beneath it, and we see that that boundary shallows near the axis of the rift compared with areas farther away from the rift. So we see that the lithosphere has been thinning along the axis of the rift valley, and oftentimes we see geophysical signatures of hot material right at the zone of shallowing of the lithosphere–asthenosphere boundary. There are many places where we also see features that look like magma within the lithosphere itself, or deep down in the Earth's crust. And one other important feature that we interpret as indicating active stretching and rifting is earthquakes. Active rifts are regions of intense seismic activity. So these are common observations that we make in these regions, which tell us that the lithosphere is thinning, not just by the creation of a depression at the surface through the fault activity there, but also by thinning from below, the shallowing of the lithosphere–asthenosphere boundary.

Oliver Strimpel: Well, that's actually quite a lot of telltale characteristics to look for, both on the surface and below. I'm wondering if all these things happen at the same time, or if they represent different phases. So perhaps you could run through the various phases of the continental rifting process.

Folarin Kolawole: The first one is that as the lithosphere starts to stretch, the entire landscape and shallow crust is dominated by small normal faults. They are just all over the place, and they're slipping. They grow laterally to connect to one another and also penetrate deeper into the crust. We think that these faults nucleate somewhere in the middle crust, the boundary between the brittle, shallower crust and the more ductile lower crust, because that is the strongest part of the crust. When they are in the vicinity of one another, the faults start to interact, because their strain fields begin to overlap, and they can connect. They can merge and become longer, over thousands of years to potentially millions of years. One of these faults cuts through the crust, penetrates into the mantle and becomes the principal plate boundary fault.

At that point, the rift transitions to the next phase, which is what we call the stretching phase. The stretching phase is typically prolonged, because this is really when the lithosphere starts to accommodate a lot of the extensional strain. It just keeps stretching. For the majority of this phase, the border fault takes up most of the extension. We can also have a case where we have two border faults that are inclined towards each other and penetrate down into the mantle, and in that case, those two faults will dominate the strain field, with continuous stretching over millions of years. As the lithosphere thins along the axis of the rift, it's going to get to a point where it has lost most of its material, and that means it has lost most of its strength. As it gets to that point, the strain on the border faults begins to migrate into the axis of the rift, because that is where most of the thinning is happening, and that is where most of the mechanical strength loss is focused. At that point, the border faults become abandoned, and the intra-rift faults now become the dominant faults. The rift is now in the necking phase, and that point is very critical, because it marks the point at which catastrophic events start to happen along the rift axis, which could potentially cascade down to continental breakup.

So past the necking phase, we think that a rift could go to continental breakup right away, or transition into another phase of extension that we call the hyperextension, or mantle exhumation, phase before it eventually gets to continental breakup. And we think what determines whether the hyperextension or mantle exhumation phase comes into the picture is the magnitude of melt. If you have a lot of magma shooting through the lithosphere after the necking phase, it could weaken the lithosphere enough and push it faster to continental rupture and breakup. But if you don't have a lot of melt in the system, it is possible that you stretch out the entire crust first, and then you start to pull out the mantle part of the lithosphere before you eventually get to continental breakup.

Oliver Strimpel: OK, so first you have lots of small normal faults, which grow and start interacting and merging with each other. Then a principal plate boundary fault forms, and we have the stretching phase, where the border fault takes up a lot of the extensional strain and the lithosphere thins. Then the strain migrates back to the rift axis, which is thinnest and therefore weakest, and the necking phase begins. And this can lead directly to continental breakup, especially if a lot of melt is present. Otherwise, there can be a further hyperextension, or mantle exhumation, phase before breakup.

Folarin Kolawole: Exactly.

Oliver Strimpel: So all those examples of rifts that you gave us around the world, those are all in an earlier phase, prior to the necking. Is that right?

Folarin Kolawole: That is correct. Most active rifts on Earth today are in the stretching phase. We also have some rifts that are undergoing the transition to breakup, or undergoing incipient seafloor spreading. The Red Sea, for example, is undergoing incipient seafloor spreading in the south. The Afar segment of the East African Rift System, which is in the northernmost tip of the East African Rift System, is undergoing oceanization. We know it has not yet broken up, but it is getting close, and it's definitely past the necking phase and definitely in the final phases of extension. The northern part of the Gulf of California is also undergoing the transition to breakup. And when we talk about rifts that are in the earliest phase of rifting, like the rift nucleation phase, or that have just started the stretching phase, we commonly talk about the Okavango Rift Basin in the northern part of Botswana. It's our poster child for incipient rifting.

Oliver Strimpel: What determines where and when a continent starts to rift?

Folarin Kolawole: The consensus in the scientific community is that rifts nucleate where we have ancient zones of weakness in the continental lithosphere, and a very good example of these are suture zones within ancient orogenic belts. At the largest scale, we observe that continental rifts mostly develop within orogenic belts, which are ancient zones of deformation that developed due to the collision of cratonic blocks or island arcs. So you have all this welding of different continental and oceanic masses with one another, and the boundaries between them become crumpled up into folds, thrusts and mountain belts. Within these belts, we typically have the principal zones of suturing, of merging, which we call suture zones in continents. Structural geologists identify ancient suture zones in areas where we see ophiolites. Where we see ancient oceanic crust on land, it means there must have been an oceanic basin here in the past that got subducted. That's one important indicator of the presence of a suture within a continental region. Another one would be eclogites, which are metamorphic rock facies that form along the subduction interface. When we see rocks that have mineral assemblages that speak to that temperature and pressure condition, we call them eclogites, and if you see them anywhere in a mountain range, an ancient orogenic belt, we say there has to be a suture somewhere here.

In my own studies of active rifts, like in East Africa, I see a common association of rifts with suture zones: the rifts are in the vicinity of suture zones, or centered on them. And in areas where we've not even identified a suture, you see the rifts and the rift faults following shear zones, which are boundaries between different terranes within the ancient orogenic belt. The consensus is that rifts like to form in ancient orogenic belts and like to avoid cratons, and within the ancient orogenic belts themselves, the rift faults like to form along pre-existing shear zones and suture zones.

Oliver Strimpel: So you mentioned avoiding cratons. Is that because they're super hard and would not rift easily?

Folarin Kolawole: Yes. Cratons are continental blocks that are older than 2 billion years, and due to their longevity, these continental blocks are colder than their surrounding edges. The edges are dominated by mobile belts that formed when those cratons collided with other cratons, but the interiors of the cratons themselves are colder. The cratons are also thicker than the neighboring mobile belts, so this makes them stronger mechanically. So it makes sense for rifts to avoid continental blocks like that.

Oliver Strimpel: One theory put forward by Damian Nance in his episode was that continents form a kind of thermal blanket that causes the mantle below to heat up, which in turn rises and weakens the lithosphere, and that initiates thinning and rifting as you described. Do you have a view as to whether this process occurs?

Folarin Kolawole: The question for me is the source of instability. The idea that continents act as thermal blankets over the mantle is indicative of steady-state behavior over long timescales, over millions of years. We have radiogenic minerals in the mantle that produce heat, and this heat gets trapped there, but that is steady state. To get rift nucleation or initiation in the lithosphere above it, we need instability in that lithosphere. And if the mantle everywhere is generating heat, we would have rifting everywhere in the continents; rifts would be everywhere, or they'd be in all orogenic belts. OK, but we don't have rifting in all orogenic belts. And there are metacratonized cratons: metacratons are cratons that are not intact cratons anymore. They don't have mobile belts cutting through them, but they've been deformed by multiple cycles of collision on their margins, so they have all kinds of shear zones breaking through them. So for me, I would imagine that those metacratonic settings are areas where such a thermal buildup in the mantle would break through, but we do not see that happening. So for me, the question will be, OK, what is the source of instability within a certain location that nucleates a rift there?

Oliver Strimpel: And as you said, you put it down to the relative strengths of the different parts of the continental lithosphere, with the previous orogenic belts and suture zones being zones of weakness that then get exploited by the mantle to initiate the rift.

Folarin Kolawole: Right. I would also say that melting is a very important mechanism for rift initiation. Far-field stretching, or subduction zones pulling plates down, can create some tensile forces within the down-going slab. But those far-field stresses, and even processes that could drive buoyancy of the lithosphere and make it rise over long wavelengths, which is what such a thermal buildup in the mantle would do, creating long-wavelength uplift, do not provide enough force to nucleate a rift. In my own studies, I'm beginning to see that even in magma-poor rifts, there appear to be some amounts of melt deep down in the lower crust and upper mantle of these settings.

Oliver Strimpel: So you mentioned force balance and far-field stresses. Can you explain a little bit what forces and what force balance you mean? Is that to do with the idea that a very thick continent would tend to smear itself out and collapse? And what far-field stresses are we talking about here?

Folarin Kolawole: The idea of force balance just relates to resisting forces versus imposed forces that are trying to initiate deformation. The resisting forces would be the inherent strength of the lithosphere itself, and the imposed forces would be the external forces that are trying to break it. There are two common sources of external forces in the context of rift initiation. One of them is the far-field stress component. If you have a plate that is being pulled in one or two directions, say you have a subduction zone somewhere, that process of pulling a plate down into the mantle could induce tension at some distance into that down-going plate.

Oliver Strimpel: Like pulling the continent behind it.

Folarin Kolawole: Exactly. The continent behind it experiences tension. That is what we call far-field stresses, and we know that they can even induce reactivation of ancient faults. So that is one mechanism that has been proposed for a long time that could potentially explain the initiation of many rifts on Earth, like the Red Sea, for example. The Red Sea likely initiated due to far-field stresses from the Zagros fold belt. The other mechanism that we think could initiate a rift is the one provided by plumes. Hot plumes rising up from the core–mantle boundary towards the base of the lithosphere exert a vertical traction that pushes the lithosphere up. Also, just by being a thermal source, a plume can make the lithosphere thermally rise above it. By flexing the lithosphere, the outer part starts to stretch, and as you heat it from below, you can easily nucleate a rift right there.

Well, I should mention the third potential source of forcing, which is orogenic collapse. It's been observed that there are many places on Earth, particularly in fossil rifts, where orogenesis was directly followed by rift initiation in the same region. It makes sense to imagine that the development of the orogenic belt had something to do with the nucleation of rifts at that location, because there's no evidence that the rift at that location was initiated by a plume or by some far-field stresses. A classic example of that would be the Basin and Range in the western US. There is no evidence that says the Basin and Range initiated due to a plume, but the extension appears to have initiated immediately after the Sevier orogeny.

Oliver Strimpel: Which orogeny?

Folarin Kolawole: The Sevier orogeny, S-E-V-I-E-R. So it totally makes sense that the high elevations that were attained due to collision, folding and compression allowed the continent to attain an elevation that is not sustainable because of gravity. And that change in force balance, due to the gravitational pull of a high orogenic belt, could initiate rifting due to gravitational spreading of the mountain belt. The best examples of this are areas where we see rifting immediately following orogenesis, within less than 10 million years or less than 20 million years, where the lithosphere has not thermally equilibrated from the orogenic event before rifting initiated. And if we cannot explain the rifting by the initiation of a subduction zone somewhere that is pulling the North American plate underneath it, or by a plume coming up to the base of the lithosphere, because we don't see any evidence of a plume, or the earliest volcanics in such a rift do not record plume geochemistry, then we cannot infer that the rift was initiated via a plume or far-field stresses. The most reasonable candidate would be orogenic collapse. And there are scientists who are showing the presence of active rifts initiating along the axis of the Himalaya in the present day. The faults are breaking the surface, and we have earthquakes there that record that style of motion, normal-fault-style motion, within a compressive mobile belt, the Himalaya. Of course, these are not as huge as the thrust faults that are driving the mountain building, but they do define a distinct system of deformation. And the potential explanation for that would be gravitational instability due to the elevation of the orogenic belt.

Oliver Strimpel: We talk about failed rifts, where we see the early stages of rifting, but then the rifting doesn't proceed. Can you give some examples of failed rifts? And do we know what determines whether rifting succeeds or fails in a particular location?

Folarin Kolawole: There are quite a few failed rifts. The biggest one is the Midcontinent Rift, which extends from southern Canada all the way south to northern Oklahoma. So that's one. We also have the Benue Trough in West Africa. We have the Reelfoot Rift in Tennessee in the US, which hosts the New Madrid Seismic Zone, the most active seismic zone in the central and eastern US today, and the earthquakes are reactivating the ancient faults of the failed rift. What seems to be very common in terms of observations of these failed rifts is that the structures show compression after rifting. It's almost as if you develop normal faults, you develop these rift valleys, fill them with sediments and lavas, all kinds of stuff, the typical features that we see in active rifts today, and then at some point you compress the lithosphere, compress it back, such that the faults that were acting as normal faults back then now look like thrust faults today. But the rock types and the geometry of the basins all indicate a confined valley that was bounded by normal faults and got compressed and inverted.

That observation leads us to interpret that perhaps changes in the far-field force balance could make a stretching continental lithosphere start to contract. If there was a subduction zone somewhere away from an active rift, and there were changes in the activity, or the style of movement, along that subduction zone, that could induce compression in the plate that has been pulled down along the subduction zone. In the same way that the continent being pulled behind the subducting plate undergoes tension, if there is slab rollback, for example, which we know often happens along subduction zones, or anything that changes the style of motion along that subduction zone, it could impose compressional stresses within the continent that was being pulled behind it. And we think that a change in far-field external forces could shut off the extension and induce compression, which leads to closing, or failure, of the rift. The other possible idea would be that the thermal state of the lithosphere changes. Let's say it was a rift that had a lot of magma coming out, and it's still in its early stages of development, and then the source of melt gets shut off. The lithosphere can start to cool, and the cooling induces strengthening in the lithosphere, makes it stronger, and that could make the rifting fail. These are hypotheses, potentially supported by many geodynamic models that we have today, but I think the far-field stresses are the easiest way to shut off a rift.

Oliver Strimpel: We have the Red Sea, which is a successful rift, if you like, and not too far away to the south, you have all the rifting that's going on in East Africa, and, as you say, all the way down as far as Botswana. Do we think that the rifting in Africa is going to succeed, or is it going to fail?

Folarin Kolawole: I think the rift in East Africa is going to succeed, and I'm not saying this because I'm biased, because I study there. The reason why I think so is this. East Africa is not driven by far-field stresses, because to the west of the African continent we have the Mid-Atlantic Ridge, which is a spreading center pushing Africa towards the east, and on the eastern flank of the African continent we have the Mid-Indian Ridge, which is also a spreading center, pushing East Africa towards the west. So Africa should be in a state of compression, but it's rifting, and that speaks to the magnitude of influence that the African superplume has. The rifting in East Africa is driven by plume activity. Even though we have magma-rich branches and magma-poor branches, the system is driven by the plume pushing up, and I think that single source of force is going to drive the system to success. We see evidence, even in the magma-poor parts of the system, that things are cascading. We're seeing gradual focusing of deformation into the rift axis. We see earthquakes in the lower crust and upper mantle that are migrating upward. Whenever the earthquakes initiate, they go from the upper mantle straight up into the lower crust, and the rate of migration of these events follows the diffusion rate of fluids, given the low permeability of the lower crust. The best explanation for that is that we have melts or volatiles migrating up from the lithospheric mantle into the lower crust along the axis of these rifts. These are rifts that do not have volcanoes at the surface yet, but we see that the faults are big; we have big earthquakes there. We have deep lakes there, rift lakes, confined by these big faults. So we see all this activity along the rift axis, which says that at some point this rift is going to break. For me, the persistent supply of fluids from the mantle into the crust beneath this rift, and the obvious deformation along the axis of the rift itself, which we see in our geophysical images of the upper crust, tell me that there is a high chance that this rift will make it to success.

Oliver Strimpel: Folarin Kolawole, thank you very much.

Folarin Kolawole: It's a pleasure. Thank you for having me.

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.