Rob Butler on the Origin of the Alps
Transcript
Note, transcripts are not fully edited for grammar or spelling.
Oliver Strimpel
This is Geology Bites with Oliver Strimpel. With the help of plate tectonics, we have a relatively straightforward picture of how major mountain chains formed. For example, at a very high level, the Himalaya resulted from the collision of two continental plates, India and Eurasia. And the Andes are a volcanic chain that formed where the oceanic Nazca Plate subducts beneath the South American plate. But what about the Alps? They are the most intensively studied of all mountain chains, being readily accessed from the geological research centers of Europe. But, despite this, there remains considerable uncertainty as to how the Alps were formed. One of the major challenges in Alpine research is to understand how rocks deform on various scales, from individual mineral grains, up to the scale of mountains. But the picture’s complicated by the presence of several sedimentary basins and tectonic fragments along the southern margin of Europe in the Eocene epoch, about 40 million years ago, when the Alps started to form. Rob Butler has devoted much of his research career to the Alps, and particularly to discovering how deformation within Alpine rocks has been concentrated over the past 40 million years. It turns out that while recent defamation is clearly visible as folds and faults in the rocks today, it was the structure of the southern margin of Europe before the Alps started to form that played a critical role in the early evolution of the Alps.
Rob Butler is Professor of Tectonics at the University of Aberdeen. Rob Butler, welcome to Geology Bites.
Rob Butler
Thank you. It's great to be here.
Oliver Strimpel
Since I just mentioned the importance of the situation on the southern margin of Europe before the Alps started to form, let's start with that. What did the geography of the region look like in the Eocene about 40 million years ago?
Rob Butler
Yeah, it was a great starting point. I mean, first of all, we have to be quite clear what we mean by Europe at that stage and, the Italians won't like this, but we're not gonna include them in Europe at this stage. They're gonna be part of a small continental block that geologists generally refer to as Adria, or Apulia. And, that was a distinct fragment of similar continental blocks that decorated or got caught up between the two major continental masses of Africa and Europe. So, when we're talking about Europe at that stage, about 40 million years ago, in terms of land masses, the bits I'm interested in, in Western Europe, so that's very much southeast France, if you like, running round into Western Switzerland, or at least the northern part of western Switzerland. So, we have that being an area of low-lying ground, some of it above, some of it below sea level, some very simple bits of proto-mountain range beginning to develop in there, nothing very substantial. Either side of that, you've got mountain ranges beginning to develop in the sense of topography and ranges. So, in the eastern Alps, running into Austria and so forth, modern day Austria. And beyond, there would be more conventional types of mountain range, perhaps like the modern day Caucasus or somewhere like that. Not a big wide belt, but narrow strips. Similarly, a sort of proto-Pyrenees between Spain and Liberia and southern France. So, we have a complicated surroundings of Proto mountain ranges. But in the Western Alps, we've got essentially very low-lying area, most of it under water.
Oliver Strimpel
So, what events set the mountain building process in motion?
Rob Butler
So, probably the preceding 50+ million years, there's been convergence between Africa and Europe, and the bits of these small continental blocks are getting shuffled and pushed together. So, there's an area of general convergence and reshaping. The geological Europe, not the land, that we would call it. So, if you were pottering around in the Eocene, you'd see this seaway, but actually, the geology of Europe continued and begun to subduct or get carried beneath this northern margin of Apulia or Adria. So that's the preceding part. As with any mountain range, pretty much there's a period of seduction of the ocean areas and thin bits of continent being taken down into the upper mantle. So, that's the preceding game, and you just let the tape run so that the convergence continues and eventually you collide mountains. And, that's the Himalayan situation. That's the same in any collision mountain belt. But in the Eocene, we're not quite at that stage yet in the Alps. So, 40 million years ago, it's all set to go.
Oliver Strimpel
So, wait a minute. So, you have this shallow sea that you talked about this low-lying area, part above sea level, part below it, and that is subducting in which direction?
Rob Butler
Broadly southwards or south and east, underneath this continental fragment that you can think of as Italy, modern-day Italy.
Oliver Strimpel
Or Adria, as you call it.
Rob Butler
Quite so.
Oliver Strimpel
OK, so that then swallowed up that low-lying area. So, during this period, there was no actual topography created and then it was only when that area got fully consumed that you then started a more conventional mountain building process?
Rob Butler
Yeah. So, we need to think a little bit about what this area that's already going down the plug hole, if you like, what that actually consists of. And that's thin crust. The old continental margin, the people think, rather like the modern-day Atlantic margin of Iberia or of Western France. So, you have an area of thin tapering wedge of continental sedimentary basins that are being gradually taken down into the system. So, you've got thin crust and a big root of mantle underneath it, and that's the driver that allows the continental crust, the thin crust, to be taken down and subducted. In contrast, as you go further out into what you might think is stable Europe, the normal Europe, which, if you like, would be the sort of Massif Central of France, in terms of what you see in the outcrop today. That's normal thickness continental crust, and the change will happen when that enters the subduction zone and chokes it. So, that thicker crust is too buoyant, too thick, to be carried down by its mantle root into the mantle, and it's then that you'll start thickening that up and making mountain ranges.
Oliver Strimpel
OK, so now we've wound the clock forward to about 25 million years ago. Would that be in the early Miocene, roughly?
Rob Butler
Yes, end Oligocene, early Miocene, is when there's evidence in the geological record for a change in the surrounding basin areas that are going to collect the detritus that erodes off the proto-Alpine chain. So, as you begin to make mountains, they poke up in the air, they erode, and the signal of that is detritus being washed into the surrounding areas. And that really gets going into the early Miocene; virtually nothing before that.
Oliver Strimpel
Is the initiation of the topography in this region still driven by what's happening with the subduction zone and how it gets clogged up with this thick crust, or are we talking about the more regional convergence of Africa up north towards Europe?
Rob Butler
Well, it's both really because that convergence happens because you're able to have the subduction going on, and, the likelihood is that the motor for the convergence as it continues is the pull of that subducted slab of mantle lithosphere that lay under the rifted margin and beyond, so that that's an extra pull that pulls the material in together. So, that's so-called slab pull process, which is one of the main drivers of plate tectonics. Anyway, it's much easier to think of processes in terms of slab pull, rather than, if you like, a car crash of continents.
Oliver Strimpel
OK, So, you have this slab pull operating which consumes these low-lying areas and then, you say, you get choked-up by the Massif Central-type thick crust. So, what happens next?
Rob Butler
That thick crust is not completely thick. It actually consists of slightly weaker rifted basins, perhaps like the margins of intercontinental rifts like the North Sea, for example. So, it's got some scope to deform and squash and to thicken up, a process we call inversion tectonics, because what was basins have now become mountains. And, in bygone days, people really didn’t recognize the presence of these old basin sections in the Alps. And the reason we now know this is because there a lot of detailed work done by Alpine stratigraphists and paleontologists actually (not by structural geologists) who recognized earlier rift basins: the sort of things we now know exist because of seismic and so forth; places like the North Sea. And we recognize those in the stratigraphy, now squashed-up, in the car crash of the Alps. So, you've thickened the crust by squashing together those old basins.
Oliver Strimpel
OK, I'm still a little confused about the mechanism by which the basins get squashed together and form the topography, if the subduction process has essentially ground to a halt.
Rob Butler
The subduction is continuing in terms of what's happening for the upper mantle, rather like pulling a rug. And the material on the rug, which is the crust, gets rammed together. So, you could imagine having a rug on the floor and you're pulling. Let's imagine you pull a rug under a bed. The furniture that's on that rug gets bashed into the bed frame. That's the concept of crust being bashed together. But the pull, the rug, the slab pull, continues until of course the amount of furniture has rammed-up against the bed, and you can't pull the rug anymore. And that's essentially what's happening in a collision process that the crust that's coming in clogs the system up.
Oliver Strimpel
But the process continues to the extent that it then creates topography and the squashing, as you refer to it.
Rob Butler
Yeah, because the crust is thick, it doesn't take much extra squash to generate mountain ranges. The interesting feature of the thin crust that preceded it in the system is that it looks like that can be just carried down into the mantle and doesn't really do much topographically. It doesn't thicken much, it just gets entrained-in, and we see that recorded by high pressure metamorphism in it: the record mineralogically of the material being taken down to great depths and changing character in mineralogy. It then returns in that subduction zone by its residual buoyancy, and it sort of squirts back up rather like toothpaste, if you like, being squirted out of the toothpaste tube. But, that's a subduction process and isn't doing very much to generate topography. It's certainly making some neat structures in the rocks, but isn't really manifest in landscape processes.
Oliver Strimpel
So, let's wind the clock forward a bit.
Rob Butler
Well, that process we can date it radiometrically from cooler gauges and crystallization of minerals. So that return flow, most of it seems to have happened by about 30-ish million years. So, towards the end of the Oligocene, and that represents the change when that thicker content or crust is arriving on the scene. So, you get this return flow, you pack stuff up in the upper crust as it crushes-in and at the same time the thick continental crust arrives and begins to thicken up as well. And that process is clogging the whole system-up. Continental crust is having a hard time getting down the subduction zone now. You've got too much of it. It's too buoyant. And so that thickens. And now you start creating a range - topographic mountain range. So, you elevate the rocks that have come up this channel, you start seeing metamorphic rocks at the surface. They generate detritus that appears in the surrounding sedimentary records. And so you can chart not only that, but also the bits of detritus coming off the thickening continental crust as well, and you see them all coming out together into the surrounding areas.
Oliver Strimpel
And where do we see that detritus today?
Rob Butler
One of the really great records of it, actually, is in a package of rocks called the Marnosa Arenacea, which is a Miocene succession in northern Italy, as you could probably guess from the name, notwithstanding my bad pronunciation. And it's caught up in what is now the Northern Apennines. And, that package is a sequence of so-called turbidites, which are relatively deep-water rocks (that area was low-lying) and is essentially submarine fans building out into what is to become northern Italy. That's the smoking gun for these processes.
Oliver Strimpel
Is that part of the Po Basin today?
Rob Butler
It went underneath the Po Basin. The Po Basin is yet younger still to detritus that've come off the Alps and northern Apennines and is still accumulating today.
Oliver Strimpel
I said earlier that the pre-existing structure along the southern margin of Europe played a critical role in the early evolution. Are you referring there to the transition from the thin crust to the thick crust, or is there more fine-grained detail that we see reflected in the deformation of the Alps today that is a testament to that situation 40 million years ago?
Rob Butler
You're right, first time. The general distribution of crustal thickness is fundamentally related to a rifting process in the Mesozoic which was forming Tethys, the precursor ocean to the Alps and other chains. But, in detail, you can see that quite a lot of the contractual structures, the folds and so forth you see certainly in parts of the Alps like the Écrins National Park area, to the east of the city of Grenoble. The folds there, lots of the folds there you can relate to rocks being squashed up against fault blocks; pre-Jurassic age fault blocks. So, the structures you see when you wander around in the Alps, many of them have their spatial distribution controlled by these pre-existing basin structures. And the areas that didn't have very many pre-existing normal faults have a much simpler stratigraphy, much more like a continuous layer cake. And those areas permitted thrust slip to go out rather like having a tablecloth moving out across a table top. If that table is broken up and has lots of different steps on it, that's much harder, of course. So, the areas that have those big so-called decollement or unsticking tectonics are places where there were very weak developments of these pre-existing rift basins.
Oliver Strimpel
And what regions would those correspond to today?
Rob Butler
The classic example at the Jura Hills in northern Switzerland, running round to that corner of France, that's the continuation. So, outboard from Geneva, if you like, going right round towards outboard of Zurich. So, that's the classic example. But, you can see hints of it elsewhere in the geology of the Alps where strata that deposited at a large scale across the top of blocks, have been peeled off and carried out as thrust sheets.
Oliver Strimpel
The term that people use all the time to describe these sheets are nappes. That's what you're referring to, is it?
Rob Butler
Yeah, nappe sheet. So, that's basically a tablecloth slipping around the place. Now, of course, if you squash any rocks enough, they can sort of squirt out. So, that's not the exclusive reason why you get thrust sheets. Some of the sedimentary basins, when they're squashed a lot, they essentially you're squirting the contents like a jam sandwich, you sort of squirt the contents out and those can become far-traveled thrust sheets as well, a slightly inelegant sort.
Oliver Strimpel
Let's talk a bit about the different deformation styles, then. So, you already mentioned how the Jura are this tablecloth, or, I don't know if there's a more technical term for it – tectonics. Could you just describe the different parts of the Alps a little bit in terms of how they've deformed and how that manifests in what we see there today.
Rob Butler
Yes, decollement tectonics would be the fanciest way you'd say for tablecloth tectonics - ungluing detachment tectonics. And, that means, just like a tablecloth, if you make that slip surface slightly sticky, it can rock up and make folds, and the folds have a very simple type of form: kilometer-wide structures. They make the hills of quite a lot of the Jura. The landscape actually mimics the bedrock geology. As you go deeper into the Alps, those rocks have had material taken off the top. They were hotter therefore when they deformed. They were once deeper underground. And in those situations, you get different types of folds, shorter wavelengths, much shorter wavelength folds. I've already touched on the idea that if they're pre-existing normal faults, that the sedimentary strata in the little basins gets squashed-up against the normal faults and that will generate much shorter wavelengths: upright folds; much more intense folding than you see in the decollement systems of the Jura. So, there's a fold-scale and fold-shape variation that relates to these different settings in part related to where they lay on the old lifted continental margin, whether they were normal faults or not, and, in part where they lay in the crust or in the roots of the mountain range, what level they lay, which controls some of the ductility of the rocks when they were deforming. So, there's dual controls. It is quite difficult to tease these out, actually. What are the relative importance of each. Part of the issue is that structural geologists traditionally have only been really that bothered about the structure. They haven't thought very much about the basins - the pre-existing basins’ stratigraphy. These days, of course they do. But you go back 50 years when a lot of the ideas of folds relating to ductility were developed, the idea there were rift basins in the Alps was barely thought of.
Oliver Strimpel
Is that partly because the structural features are so much easier to see? I mean, even the untrained eye, like mine, you can see the glorious thrust, for example. It's just a great big line across the mountain. Is that because there's a sort of selection effect based upon what It's easy to see when you go hiking in the mountains?
Rob Butler
I'm sure that's true, and there's a tendency also for people to work on things that they can recognize. Things you can't necessarily make your mind up about straight away, I think this tends for all of us to put to one side. And, often the stratographic relationships make for slightly more complicated structures because the layering wasn't nice and continuous to start with. So, people have tended to overlook them. Nowadays, people are going after them all over the place because those are the key. And it's an interesting challenge in an investigation of any region. Something we have in the. sciences, of course, is that the world is our oyster, but it's a big world and a lot of oysters. And so, the question is, which bits do you concentrate on? Which do you think are important? For me, I think, the important stuff is always the stuff you can't understand. It's the stuff that doesn't leap out at you, because that's almost certainly where you're gonna find the solution. The things that are most obvious are usually not the most important.
Oliver Strimpel
And in this case, the less obvious stuff is the pre-existing stratigraphy.
Rob Butler
It's the areas where there's pre-existing stratographic variation. Those in rift systems. Those will happen most dramatically from one side of a normal fault to another, from a fault block to a basin. So, you wanna find those… It's a holistic approach. You want to try and put that strategically together at the same time as you are trying to unravel the structure.
Oliver Strimpel
One of the ways in which people try to disentangle the various contributions to this accommodation if you like, of one continent moving next to another one, is really the overall amount of thrust or shortening that's taken place, and at least in the case of Himalayas, there's often a debate about to what extent that's accommodated by huge boundary faults, or by integral shortening through folds and maybe smaller faults on many different scales. Is that a similar debate in the Alps, with respect to the fact that these early major faults were detected first and now we're finding out much more about the finer scale structure?
Rob Butler
Understanding distributed deformation. An individual hand block won't have that much convergence between Italy and France recorded in it, but you put a lot of blocks together. That's a lot of convergence. And, the catch is to capture that as an insight. It's quite difficult. You can't take a photograph of it. You can take a photograph of a little bit of it, but you can't capture the essence in the same way as you can by taking a photograph of the Glarus Thrust. So, I think in structural geology, I think this goes for many other scale dependent parts of the science, there's a tendency to deal with the things that you can capture in a photograph or capture with the view, not necessarily capture the distributed nature of… deformation. That's point 1. However, the question really is whether you can evaluate the amount of shortening in mountain belts, to actually get the convergence. It's something I tried a long time ago. In fact, my first postdoc was to try and do exactly that in the Alps. And, I came up with a number of hundreds of kilometers shortening, `cause if you unravel all these structures, that's sort of number you get. People have had similar numbers for centuries. But, how much of that is because of the return flow in our subduction channel. So, you essentially are double accounting. You've got the material that's gone down our subduction zone, so that's recording true plate conversions. But if you also add in the return flow coming back up, you're counting it twice because it's come back up. So, I think there's been an awful lot of double accounting going on, and the challenge is to recognize it, because there's obviously real convergence as well. So, separating those two components… Well, I don't think you can and maybe you try and find other explanations or other tools to establish the far-field tectonic convergence in the system.
Oliver Strimpel
What kind of tools might those be?
Rob Butler
For example, the Himalayan collision with Tibet and the rest of Asia, we understand how that works, because we can reconstruct the plate motion on a global scale and reconstruct India's relative motion through the opening of the Indian Ocean Basin, for example, and tied it into a whole earth plate tectonic model. And we can do a similar thing between the whole of Africa and if you like stable Europe. But what you can't do, is then define the movements of the individual little blocks caught up in the car crash between, which is the Alps, the other Mediterranean chains. You need to sort all those out, and that's tough – anymore than it would be to you know, if you have a car crash, and to work out where a particular piece has come out of the car and where it's gone to. You couldn't forecast where that wing mirror went when you had a car crash. Not easily. So, that's the challenge we have, is having a marker we can work at, at a scale that's appropriate to the scale of Alpine system.
Oliver Strimpel
So, then, what you're suggesting is that there were many more fragments that came into play in determining what happened in this crash, than there were in, for example, the Himalayan case.
Rob Butler
Let's take that in two parts, certainly in an Alpine-Mediterranean context, there is great debate about what the distribution of the various pieces of older content of fragments (by old I mean pre-Tethys, so that's Triassic and older fragments) - what their distribution was. You can identify where they. Are there's a huge controversy about where, for example, the Calabria, the foot of Italy, exactly sat - nowhere near where it is now. Similarly, the other fragments that make up the Italian peninsula, where do those sit? Let alone the southern margin of the Alps going into northern Italy - where precisely was that? Precisely in this case is ±100 kilometers. Well, 100 kilometers is quite a lot of shortening and Alpine contact, so that uncertainty of where you're gonna put it feeds into the far-field determinations of strain in the mountain belt. And the prize is in that value. It's all about the 10s of kilometers, not the hundreds of kilometers when it comes to understanding the structure at the scale of those mountain ranges. Now the question is: “How much of that is also true of the Himalayas?” and we just don’t know it yet.
Oliver Strimpel
People talk about the presence of volcanic arcs that were between India and Eurasia, and whether those collided with Eurasia first or collided with India first. But, we're not really talking about volcanic arcs hear, we're just talking about where all these little terrains came to be after the previous phase of rifting.
Rob Butler
That's right. But the question is then, what do you mean by terrain? So, you could think of an individual fault block which has had a bit of stretch away from another bit of fault block on a rifted margin as being a micro-continental block. It was all joined together by bits of continent. But understanding where all those fragments exactly sit, is the prize, and so you need to be able to unravel that. So, OK, in a Himalayan case, what did that rifted margin look like, that's been caught-up? The amount of geological investigation of that problem in the Himalayas is miniscule compared to the understanding of what's been built in the Alps, where there's been two centuries of stratographic studies rather than a handful of expeditions into rather remote parts of the northern Himalayas.
Oliver Strimpel
Coming back to the Alps then, what are the most pressing research themes that people are engaged in today?
Rob Butler
There's a lot of effort trying to add geochronological precision to the timing of things. Trying to get better estimates of the conditions in the subduction zone, partly because subduction zones are continuing to be a major topic in our science research. Understanding how continental fragments get entrained into subduction zones is a global issue and the Alps is a place that you can try and look at that. Part of the challenge, though, is that many of these works rely on quite historical reconstructions of the large scale structure. So, the question is, it's all very well having a very precise number on the deformation, but if you're imprecise about where it is in space, it doesn't necessarily help you… that there remains this problem of how you deal with the diversity of data you need to solve these sorts of problems, and you've alluded to the idea that one of the Alps’ great assets is the sheer amount of data. How do you handle it? Most of it's not real data. Most of it's first order interpretation derived from observations and caught up by all those issues we've been touching on, such as selection bias. And so, how do you see through all that? It's a challenge. I mean it's challenge I have in trying to teach Alpine geology. How do you really get into what is knowledge? What is inference. What is known? What is unknown? What do we think we know, but actually we just assume. And I think there's a real challenge with knowing too much or not being able to filter.
Oliver Strimpel
Are the Alps still growing today?
Rob Butler
Going up? Yes, they are. They're also wearing away and sending detritus down the river Po whenever it rains, which this year it did a lot. So, there's still Africa convergence going on that is, that is squeezing the crust a little bit, although hardly at all. And most of those earthquakes are not in the Alps, they're in places like Sicily and elsewhere in the Mediterranean area. But there's still a weak convergence going on. There's still earthquakes because of rebound. Rebounding crust is coming up and that's not a steady process. It's creaky, so, you get earthquakes, you get mountains that are growing. We know that not only because of geological research, it's also had a loss of topographic surveys for centuries, but also over the last 30 years from GPS records. But you can also see the uplift in some of the long railway tunnels in Switzerland, which started off horizontally and now bowed, so that there's relatively short kilometers wavelength differential uplift happening across the Alps as well.
Oliver Strimpel
And that uplift has a tectonic origin.
Rob Butler
Principal driver is actually erosion, so that as you erode with material and take it away from the Alps, you're taking a load off the Alps, which means the bits that haven't eroded, which are the mountain peaks rather than the mountain valleys rise. So, it's an isostatic response to erosion.
Oliver Strimpel
That's fascinating. So, actually by wearing down the mountains, we're just really creating more relief. We're not actually wearing down the peaks.
Rob Butler
Yeah, we're not going down the mountains. We're eroding the valleys, the valley sides, particularly in a glaciated time. Then you're wearing out to the bottoms and the sides of the valleys, and the spiky bits go higher. Obviously, they fall down catastrophically every now and then, but actually overall you generate, exactly, more relief.
Oliver Strimpel
But that's really because, overall, the Alps are still fairly young. I mean eventually they'll be flattened like the Urals or something, won’t they?
Rob Butler
Presumably, once you've taken all the load off your ship. It just goes back to where it was. So. yeah.
Oliver Strimpel
Are you currently engaged yourself in any Alpine research?
Rob Butler
One of the things I've been playing with is trying to use this sedimentary rocks and the strata that deposited around the time, particularly of the Eocene-Oligocene to reconstruct relief. But it's submarine relief, because these were submarine deposits: the turbidites. And they were fed into kilometer-deep seaways that surrounded the Alps (the things I've mentioned before). The earliest ones weren't derived from the Alpine chain, `cause the Alps weren’t generating topography. They were derived from other places and came into this seaway, and so if you can reuse this record, you could potentially get a very detailed and quite subtle story of how the symmetry, submarine topography, varied around the edge of the Alps. I think it will provide information about the slab processes; the subduction zone processes that were just coming to an end at this sort of time. And it will tell us potentially how those slabs coupled with the adjacent, not so rifted, margin of Europe. I'm a believer in serendipity and the importance of doing curiosity research. I don't know if it will have any tectonic value at all, but if you don't do it, you'll never find out.
Oliver Strimpel
Whereabouts are you actually looking at these sedimentary structures?
Rob Butler
I'm looking at it in a system that's generally known as the Grès d’Annot, which is quite a famous set of turbidites in southeast France, and you can find its continuation around that rim through the Dauphine and maybe even up to that corner of western Switzerland, right around the western Alpine Arc. It's a nice place to go as well, and it's always good to have a hobby that provides good wine.
Oliver Strimpel
Rob Butler, thank you very much.
Rob Butler
Thank you. It's been fun.
Oliver Strimpel
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