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Oliver Strimpel: This is Geology Bites with Oliver Strimpel. What events in the geological past were responsible for shaping Central Europe? In earlier episodes, I talked to Ulf Linnemann about the assembly of Central Europe during the Paleozoic, and with Rob Butler about the origin of the Alps in the much more recent past. Here we talk about two orogenies that also played a key role in the story: first, the Cadomian orogeny, which lasted from the late Neoproterozoic to the early Cambrian, i.e. from about 700 million years ago to about 525 million years ago; and second, the Variscan orogeny, which took place in the late Paleozoic, from about 380 to 280 million years ago. Jiří Žák has been studying the geology of Central Europe for over 25 years, using methods ranging from structural studies in the field to detrital zircon geochronology. He is a professor at the Institute of Geology and Palaeontology at Charles University in Prague. Jiří Žák, welcome to Geology Bites.

Jiří Žák: Thank you very much for inviting me. I'm glad to be here on this podcast.

Oliver Strimpel: Before we talk about individual orogenies, let's try to place this in the wider tectonic context of the assembly and breakup of supercontinents, the so-called Wilson cycle, in which a supercontinent breaks up and then reforms roughly every 500 million years or so. This period that we're talking about, how does that fit into the Wilson cycle?

Jiří Žák: This Wilson cycle would be positioned on a time axis between the supercontinent Rodinia and the supercontinent Pangaea. So in the Mesoproterozoic and Neoproterozoic, we have here the supercontinent Rodinia, which broke up, and one of the results of this breakup was Gondwana. And then, and we'll probably talk about it more, there was Pangaea, which formed during the late Paleozoic. So all the processes we will be talking about occurred in between. So in terms of the supercontinent cycle, we are in between Rodinia and Pangaea. In terms of the Wilson cycle, it's a little bit more complicated, because the Wilson cycle is only one of the forms of supercontinent breakup and assembly. It implies that you create an ocean and then you close the same ocean. What we have recorded here in Central Europe was much more complex. There were more oceanic basins and more terranes involved. So the history is more complex than the simple Wilson cycle. But in terms of the supercontinent cycle, we can track processes which happened from the breakup of Rodinia to the assembly of Pangaea.

Oliver Strimpel: So Europe today is part of Eurasia, which is itself a pretty massive continental block. So during the assembly of Europe, was this leading to greater and greater amalgamation of terranes, eventually including the Caledonian orogeny, and then, after the Variscan, basically leading to the formation of Pangaea? Is that really the very high-level picture?

Jiří Žák: On a larger scale, you are right, but on a smaller scale, we should not forget the Cadomian orogeny, which actually formed much of the crust which we see now in Western and Eastern Europe. And the younger Variscan orogeny simply recycled this material. So I would argue that the main crust-forming event, maybe even the most important crust-forming event, was the Cadomian orogeny, in late Proterozoic to early Paleozoic times. It really created new crust. The Variscan orogeny was collisional, and it largely recycled this Cadomian basement.

Oliver Strimpel: Okay, so let's talk about the Cadomian orogeny. Can you walk us through a little bit more about what happened?

Jiří Žák: So first of all, it should be mentioned that although the Cadomian orogeny was first defined in Europe, it was more likely a global-scale orogenic event, because it has been proposed that at the end of the Neoproterozoic and the beginning of the Paleozoic (we are talking about processes that happened about 750 to 500 million years ago), almost the entire Gondwana was surrounded by broadly coeval and linked orogenic belts. So what we now call the Avalonian–Cadomian belt was only one segment of a global orogenic system. And a key feature of this global orogen was subduction of large oceanic plates beneath the Gondwana mainland, forming a sort of subduction girdle. In effect, subduction controlled the formation of extensive tracts of accretionary wedges, volcanic arcs and back-arc basins, which were attached to the upper, overriding plate. So in terms of a recent analogy, this is pretty much similar to what we see now in the western Pacific.

Oliver Strimpel: In the western Pacific, then. Yeah, okay. You get subduction, and you get all the consequences, or corollaries, associated with subduction. So do we see those today? Can we find the remnants of those in Europe? I mean, can we see accretionary wedges, or where volcanic arcs used to be, or remnants of mountain chains?

Jiří Žák: Yes, we do. And actually, if you look at a geologic map of Europe, the fragments of the former Avalonian–Cadomian belt are now scattered all over Western and Central Europe, with the main exposures being the Iberian Massif in Spain and Portugal, the Armorican Massif in France, and of course the Bohemian Massif here in Central Europe. However, we also find them frequently within the Alpine orogenic system. So they were recycled during the Variscan orogeny and again recycled during the Alpine orogeny. And in this case, they extend farther east and south, into the Alps, the Carpathians, and even farther, into Bulgaria, Greece, Turkey and even Iran. So in all these places, we find the relics or fragments of the former accretionary orogenic belt, of the former Avalonian–Cadomian active margin. And I would argue that here in the Bohemian Massif, we have one of the best-preserved fragments of this margin, because we have here a relic of an ophiolite, we have a relic of an accretionary wedge, which is very well preserved, we have a relic of a volcanic island arc, and also probably the back-arc basin, all in their original relationships. So we can study the Cadomian processes very well here in the Bohemian Massif.

Oliver Strimpel: Okay, so there you have them still in their original relationships. I was going to ask you about that, because it sounds like there's been an awful lot of processing that's taken place afterwards, with fragments appearing from as far away as Spain, to France, and then to the central parts of Europe. So how do we reconstruct what happened during the orogeny? How do we manage to wind the clock backwards and see how all the parts fit together?

Jiří Žák: Yeah, there are several lines of evidence, and there are several principal things we can recognize in the rock record. First is what is called the Cadomian unconformity, which was discovered here in Central Europe. It was noticed in the second half of the 19th century and later formally defined in France, in 1921. It's the major evidence that the Cadomian orogeny took place before the Cambrian or Ordovician, because what we see is folded, deformed and metamorphosed basement complexes which are overlain by fossiliferous, undeformed Cambrian or even Ordovician strata. Nowadays we have a lot of data available, which consists of geochronologic data, usually, in sediments, detrital zircon ages, which allow us to define what's called a maximum depositional age. So we can put some time constraints on the age of deposition and deformation, even in rocks which do not contain any fossils. We have geochemistry, especially geochemistry of volcanic rocks, which allows us to interpret them as volcanic arcs built either on thin continental margin or on oceanic crust. We also have pieces, or fragments, of the subducting plate, which were incorporated into the accretionary wedge. Probably they formed seamounts on the seafloor, and of course, during subduction, it's very difficult to subduct these asperities, so they were accreted and mixed with the terrigenous sediments of the accretionary wedge. So there are enough pieces of evidence to argue that it was a Pacific-type margin with active, protracted subduction, where an unknown oceanic plate, which has almost completely gone into the mantle, was subducted beneath Gondwana, creating these accretionary wedges, island arcs, et cetera. So now, compared to 20 or 30 years ago, we have a lot of data. The geochronology also allows us to set a time frame for this process. If we focus on Central Europe here, the Cadomian orogeny and the subduction started probably around 630 or 620 million years ago. And what's interesting is that it continued into the Cambrian. The end of subduction was probably at around 525 million years ago. So at this stage of research, we have a lot of data.

Oliver Strimpel: What about paleomagnetism? Do we know roughly at what latitude all these events took place?

Jiří Žák: Paleomagnetism doesn't work well in these rocks, because they are quite old; they have been remagnetized several times. But if we want to estimate a paleolatitude, we can simply use the overlying strata, because we have Cambrian or Ordovician on top. So we can use paleontological data, biogeography. But also, in the Upper Ordovician, which is directly overlying the Cadomian basement, we have glaciomarine sediments, which argue for proximity to Gondwana, because there must have been a continental glacier available. So putting it all together, that's a good argument for southern latitudes, on the margin of Gondwana. And based mostly on detrital zircon geochronology, and comparison of the detrital zircon ages in the Cadomian basement with those in Africa and South America, so potential source areas, the most common view is that what constitutes Central Europe was somewhere close to West Africa, so sourced from the West African Craton or the Trans-Saharan belt, so northwest Africa. So we have pretty good constraints on where the terranes which are now in Central Europe were positioned at the end of the Neoproterozoic.

Oliver Strimpel: So let's run the geological clock forward now. After this period that you've been describing, at the end of the subduction, what happened next?

Jiří Žák: Yeah, that's actually a very difficult question, because how exactly the Cadomian orogeny ended has long been a matter of debate, and several contrasting models have been proposed, and I would say that it has not been resolved until now. For example, Damian Nance and Brendan Murphy proposed a model of an interaction of the Cadomian active margin with a migrating mid-ocean ridge, and a change to a transform regime, and later to a passive margin from the Ordovician onwards. And what also seems likely is that Avalonia rifted completely away from its original position next to South America, or Gondwana, opening the Rheic Ocean, and then collided with Laurentia and Baltica, in what we now call the Caledonian orogeny. However, great debate exists as to what happened to the other Cadomian terranes: whether they also were completely detached from Gondwana and drifted northwards as entirely separate islands within the Rheic Ocean, or remained attached to Gondwana at all times during the early Paleozoic. And this debate is actually rather important for the reconstruction of pre-Variscan paleogeography: how many oceans and how many continental blocks were involved in the subsequent Variscan orogeny. The model I prefer at the moment is the one with the migrating mid-ocean ridge, which intersected the Cadomian subduction zone, because it nicely explains the termination of arc activity, which was diachronous from west to east. But we cannot be sure.

Oliver Strimpel: So again, to step back a little bit: we had this series of events which, you say, we now have fairly good evidence to be able to constrain, what happened during the Cadomian orogeny, and we can roughly say how things looked at the end of that. And you say what's open to debate is what happened next. But at what point, running the clock still further forward, do we feel like we have enough evidence to say, well, at least now we know where all these terranes are positioned and what happened, and it's just that we have to somehow figure out what happened in between, starting with the end of the Cadomian orogeny?

Jiří Žák: If you look at the end of the Cadomian orogeny here in Central Europe, what we see is granitic plutonism, so large bodies of granitic magma intruding into the former accretionary wedge at about 525 to 500 million years ago. Which is sort of strange, because accretionary wedges are typically very cold regions of the Earth's crust, because they are underlain by cold subducting oceanic plates. So there is no heat source, and we need a heat source to explain the granitic plutonism. One of the models, the one I was talking about, is that a ridge intersected the subduction zone, which would explain the heat source. You can find more recent examples of mid-ocean ridges intersecting subduction zones and creating what's called a slab window. So the slab is sort of detached, and you can get hot asthenosphere into the base of the cold accretionary wedge, and that's what we think happened during the Cambrian. So we need to explain this heat source. And the next stage was the passive margin, which is evidenced by Ordovician, basal Ordovician, sediments, basal Ordovician beds, which overlie the Cadomian basement. So we have a very good record, actually, from the end of the Proterozoic to the beginning of the Ordovician. We see the end of the orogeny, some heat input, and granitic plutonism and volcanism, and then we see passive margin, clearly passive margin sediments of Early Ordovician age. But all these are indirect inferences.

Oliver Strimpel: So moving the clock forward a little bit, we then come to the Variscan orogeny, right?

Jiří Žák: So if we move onwards into the early Paleozoic, the key geodynamic process at the time was the continental drift of Gondwana to the north, together with the former Cadomian terranes at its margin. So Gondwana and anything which was north of it, so all these terranes formed previously during the Cadomian orogeny, moved north towards Laurentia, Baltica and Avalonia, which were assembled during the Caledonian orogeny to form Laurussia. So now we have two major converging megacontinents, Gondwana and Laurussia, moving towards each other and closing the Rheic Ocean in between, and other smaller oceanic basins. As we discussed before, it's not entirely clear how many continental fragments, or microplates, and how many oceanic basins were involved in the Variscan orogeny and in the Variscan collision. That's another matter of debate; it has been widely debated for decades. Nevertheless, I would emphasize that this process was significantly different from the Cadomian orogeny, because it was a truly collisional orogeny. So we had two major continents, on each side of an ocean, and after closure of the ocean, the continents collided, and this continental collision culminated during the Devonian to Carboniferous and resulted in the formation of the Variscan orogenic belt, and in turn in the formation of the supercontinent Pangaea. So what is now Central Europe was right in the heart of the Variscan belt and in the heart of Pangaea. And at the time, Central Europe was positioned somewhere near the equator, because we have Carboniferous coal basins which overlie the eroded Variscan basement.

Oliver Strimpel: So you said that the Variscan was really a kind of head-on collision of two continents, whereas the Cadomian, you say, was more accretionary. There wasn't actually the same kind of perpendicular motion, if you like, between the plates that created a Himalayan-scale mountain range, as the Variscan might have done at the time.

Jiří Žák: Yes, exactly. That's why we call it an accretionary orogeny, because there was probably a large oceanic plate subducting beneath Gondwana, but there was no major continent on the other side. So we have protracted oceanic subduction lasting for tens or maybe even more than a hundred million years. However, it should be noted that oceanic plates are never smooth. The oceanic plate which subducts carries, for example, seamounts; it can carry oceanic plateaus, it can carry intra-oceanic volcanic arcs or sedimentary basins, and all this material is very difficult to subduct. It makes the oceanic plate thicker, and it's buoyant, so it's difficult to subduct. So it rather accretes to the overriding continental margin, and this is exactly what we think might have happened during the Cadomian orogeny. In contrast, the Variscan orogeny was a classic collisional orogeny. So it has everything we know from other collisional orogenic belts, like the Alps and the Himalayas: closure of an ocean, first high-pressure metamorphism, creating high orogenic topography, maybe even an orogenic plateau, like present-day Tibet, up to final collapse, erosion, and exhumation of high-grade metamorphic complexes. All of this we have recorded here in Central Europe, and that's why the Variscan orogen is sort of a world-classic area for orogenic research.

Oliver Strimpel: Yeah. So the best evidence for the Variscan orogeny is in Central or Eastern Europe, is that what you're saying?

Jiří Žák: I wouldn't say the best, but we have all the orogenic zones very well preserved. So if you were to go across a collisional orogen, you would start in a foreland, and you would go to the central axial zone, which was uplifted, and then there is, of course, the deepest erosion. And we have this orogenic architecture very well preserved. So if you were to go from north to south across the Variscan orogenic belt, you would start in foreland basins, very low-grade, thick successions of siliciclastic sediments. Then you would enter low-grade metamorphic complexes, and then, if you went deeper, you could step into the orogenic interior, which is now exhumed: originally very deep rocks which formed the thickened orogenic root, with granulites and migmatites and other high-grade rocks. And all this is very well exposed and very well preserved here in the Bohemian Massif, and in Central Europe. So that's why it has been studied for decades.

Oliver Strimpel: So if you go from north to south, you say you go through all these regions. What kind of distance are we talking about? If you were going to do your transect, do a trek from top to bottom, where would you start and where would you finish?

Jiří Žák: You would start somewhere in the very south of Poland and then end up in Austria, northern Austria, and it's about, let's say, 300 kilometers. But it should be noted that it's not the end of the orogen. It's the end of the exposure of the orogen, because the Variscan orogen continues underneath the Molasse basin of the Alps, and goes farther south, and of course continues into the Alps. But there it was reworked during the Alpine orogeny, whereas to the north of the Alps, the Alpine overprint was only localized, and it was more reactivation; it was not affected by the Alpine orogeny.

Oliver Strimpel: But the Alpine orogeny was a bit further west. So you say if you start in Poland and you go down towards Austria... you said the Variscan orogeny extends to where the Alpine orogeny is today. So it actually extended further west, as well as north to south.

Jiří Žák: Yeah, you are right. You know, if you were to look at, let's say, a geologic map of the world in the Carboniferous, the Variscan orogen was rather extensive. The length could have been 8,000 to 10,000 kilometers, maybe even more. So it was a really extensive orogenic belt, and we find remnants or pieces almost everywhere. But I'm now talking about those parts or fragments which are exposed and which escaped the Alpine overprint. So you can see them emerging from below the younger Mesozoic and Tertiary sediments which largely cover much of Western Europe. So you have several places, we call them massifs, which project upward from the younger sediments, and that's where we can study the Variscan orogen at its best.

Oliver Strimpel: Is it the case, then, that even though we had a Himalayan-scale mountain range at the end of the Variscan, none of the topography that we see today, either in Central Europe going down from Poland to Austria, or indeed in the periphery of the Alps, is actually as old as the Variscan? That's all much more recently created. Is that right?

Jiří Žák: Yeah, that's right. Nothing is left from that, because it has been about 300 million years since the end of the Variscan orogeny, and even on a geological timescale, this is a very long time span, long enough for erosion to completely wipe out the original topography. So much of the present-day topography and much of the present-day relief which we see here in Central Europe is due to reactivation from the Alpine orogeny, because we are in the Alpine foreland, and this is still ongoing. So we see here several phases of compressional reactivation during the Cretaceous and Cenozoic. And we also had here an episode of Tertiary rifting, which led to the thinning of the lithosphere, some volcanism, and we have some rift-related coal basins. So that's the most recent relief. Nothing is left from the Variscan orogeny, of course; it's too long a time.

Oliver Strimpel: But the reactivation took place along faults that may have been active during the Variscan, because they were, I guess, zones of weakness.

Jiří Žák: Yes, exactly. Especially at the end of the Variscan orogeny, there was a major phase of strike-slip faulting, and some of these faults run for tens or even several hundreds of kilometers, and they penetrate through the whole crust. So these are large features which were there before the Alpine orogeny, and they are easy to reactivate. So compressional stress from the Alpine orogeny reactivated these faults quite significantly in some places. And we have a lot of evidence from fission track dating that there were also significant block uplifts during the Jurassic, Cretaceous and Tertiary. So there's plenty of evidence for movements in Central Europe. But all this was along these brittle faults. There was no metamorphism, no magmatism and no ductile, pervasive deformation related to the Alpine orogeny. So that's why we have the older orogenic cycle, the Variscan, very well preserved.

Oliver Strimpel: Oh, I see. So the Alpine orogeny, although it completely shifted things around and there's a lot of structural change, there wasn't so much metamorphic change.

Jiří Žák: Not at all, actually, because the metamorphism happened in the Alps, but we are too far from the orogen. But we still felt the compression. So there was some movement, but it was localized along, as you pointed out, zones of weakness, which were pre-existing faults.

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

Jiří Žák: With my colleagues, we are working on two main projects. One is focused on what we've been talking about a bit: it's reconstructing the provenance of terranes which formed at the northern margin of Gondwana, using detrital zircon geochronology. And we have a new project, which started this year, which is focused on the reconstruction of an Archean supercontinent. So the fieldwork will be in Canada, Wyoming and Finland.

Oliver Strimpel: So two very different time periods.

Jiří Žák: Very different time periods and very different field areas, but we enjoy both. Both are very interesting problems to solve.

Oliver Strimpel: Jiří Žák, thank you very much.

Jiří Žák: Thank you very much.

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