This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Over the course of Earth history, collisions of continental terranes have led to many mountain-building events that created mountains on the scale of today's Himalaya. But several episodes of mountain building stand out because of their spatial extent and duration. One of these is the Caledonian orogeny, which left its imprint on parts of North America, the northern British Isles, Greenland and Norway, and took place in various phases over about 100 million years, from the Ordovician to the Early Devonian. How did this orogeny unfold, and what can we learn about such mountain-building events from studying the formerly buried lower reaches of this orogen?

Rob Strachan has studied the rocks of the Caledonian orogen for over 40 years, focusing on unraveling the history of the orogen in what is Scotland today. He is Emeritus Professor of Geology at the University of Portsmouth. Rob Strachan, welcome to Geology Bites.

Rob Strachan: Thank you very much, Oliver. It's great to be talking to you today.

Oliver Strimpel: Even though the Caledonian orogeny had many phases spanning a period of about 100 million years, can we place it into an overarching tectonic picture of what was happening at the time?

Rob Strachan: Yes. It's best to think about the orogeny as occurring in between two major supercontinents: Rodinia, which formed at about 1,000 million years, and Pangaea, which formed by about 300 million years or so. If we go back to 1,000 million years or so, most of the Earth's continents were assembled in this big supercontinent called Rodinia, and that supercontinent remained fairly stable for a couple of hundred million years. But then, as most supercontinents do, it became unstable and started to fragment. And part of that supercontinent involved large continental masses such as Laurentia, Amazonia and Baltica. And those three in particular rifted apart from each other at around 600 million years ago.

That rifting gave rise to what we call the Iapetus Ocean, which gradually widened to become several thousand kilometers across. And about 500 million years ago or so, subduction zones developed within the Iapetus Ocean, and it then started to close, which meant that the surrounding continents then started to converge with each other. And around about 430 to 400 million years or so ago, those three continents actually collided back together again to form the Caledonian mountain belt. The three continents in question are what we call Laurentia, which basically means North America, Greenland and Scotland; Baltica, which corresponds to present-day Scandinavia; and Avalonia, which corresponds to southern Britain and parts of mainland northern Europe.

And that collision, resulting in the Caledonian orogeny, was fairly quickly followed by the closure of another ocean to the south that we call the Rheic Ocean. And that closure led to the Variscan orogeny, which is best developed in mainland Europe, and that culminated in the formation of the next supercontinent, Pangaea. So the Caledonian orogeny represents an early stage in the assembly of Pangaea.

Oliver Strimpel: So for the Caledonian orogeny, then, the closure of a major ocean, the Iapetus, preceded the collision of the continental blocks you mentioned. Do we see geological evidence in the field for the subduction zone or zones that swallowed up the Iapetus?

Rob Strachan: Yes, geologists can do some detective work to look for certain rock associations that might provide direct evidence for subduction. In particular, we might look for rock associations known as ophiolites, which are regarded as remnants of the ocean that was destroyed by continental collision. If we go to present-day ocean ridges, a piece of oceanic lithosphere might typically include ultrabasic mantle rocks at the base, overlain by lower crustal rocks such as gabbros, then basaltic dikes, pillow lavas, and deep-sea sedimentary rocks. And if we're lucky, in between, where continents have collided together, in zones that we call suture zones, we might have bits and pieces of oceanic lithosphere squeezed up along those suture zones. And these are what we call ophiolites.

There are various places in Norway, Scotland and Ireland where we can see these remnants of oceanic lithosphere. It's rare for any of these ophiolites to contain a complete sequence through oceanic lithosphere from mantle to oceanic sediments, because quite often bits and pieces of oceanic lithosphere get faulted out during the actual collision process. If we go up to Shetland, for example, the ophiolite only comprises mantle and lower crustal rocks, so we don't see the pillow lavas and the deep-sea sediments.

That's one rock association. The second important rock association that is associated with the convergence and ultimate closure of oceans is the sorts of igneous rocks that are formed by melting of mantle rocks above a subduction zone. So, for example, in the Lake District in northwest England, we see thick successions of volcanics that were erupted above a subduction zone, which is thought to have dipped southwards underneath Avalonia during Ordovician times, at about 500 to 450 million years ago. There are also many granitic plutons in Scotland that are thought to have been intruded into Laurentian crust on the other side of the closing ocean, above a subduction zone that dipped northwards underneath the margin of Laurentia. So we have plenty of geological evidence for the different subduction zones that, as you say, swallowed up the Iapetus Ocean.

Oliver Strimpel: We said that the Caledonian lasted 100 million years, which is an awfully long time. Do the various phases of the Caledonian orogeny correspond to collisions of different blocks of crust?

Rob Strachan: Yes, absolutely. The Caledonian orogeny is not just one collision, but several different collisions, and each collision may not itself have occurred at the same time along the length of the mountain belt. If we go back and look at the history of research over the last 30 or 40 years or so, traditionally we can divide the Caledonian orogeny into two main phases in our neck of the woods in Scotland and Ireland.

The first of these occurred at about 490 to 470 million years ago. Now, the context of that is that the early subduction zone that developed within the Iapetus Ocean dipped away from Laurentia, and as a result of that subduction, a juvenile volcanic arc developed on Iapetan oceanic crust. It is thought that the remnants of that arc lie underneath what we call the present-day Midland Valley of Scotland. Now, if we imagine a subduction zone dipping out into the Iapetus Ocean and the volcanic arc forming above that, eventually the edge of Laurentia is going to arrive at that subduction zone. That's exactly what happened. The edge of Laurentia arrived at the subduction zone and collided with the volcanic arc. That resulted in the thrusting of intervening bits of oceanic crust up onto the edge of Laurentia, and that resulted in widespread deformation and metamorphism of rocks in Scotland and Ireland north of the Highland Boundary Fault.

That early phase of the Caledonian orogeny is known as the Grampian orogenic event, mainly because its effects are particularly well developed in the Grampian Highlands, where we see the Dalradian rocks extensively deformed and metamorphosed during that period of the orogeny. And as I mentioned a moment ago, the Highland Boundary Fault is particularly important, because it is thought to correspond to the suture between the edge of Laurentia to the north and the Midland Valley to the south, beneath which we think we have remnants of that volcanic arc.

Now, once the edge of Laurentia had collided with that volcanic arc, the effect was to jam that subduction zone up. Continental crust is relatively buoyant, so it can only be subducted a small amount, and therefore that subduction zone became jammed up. What we think happened next is that a new subduction zone developed on the south side of the Midland Valley, but dipping in the opposite direction, this time northwestwards underneath Laurentia. And this ushers in the second major phase of the Caledonian orogeny, because it is thought that between about 460 and 420 million years ago, there was more or less continuous subduction of the Iapetus Ocean northwards, which brought Avalonia northwards to eventually collide with Baltica and Laurentia. That final collision is often referred to as the Scandian orogenic event. The suture between Avalonia and Laurentia is located along the Solway Firth, but it is completely covered by younger rocks, so unfortunately we're not actually able to access it at the surface.

Oliver Strimpel: Apart from the ophiolites and the structural elements you mentioned and the metamorphism, what evidence actually enables us to reconstruct this history and the movements of all these blocks as you've just outlined?

Rob Strachan: Well, a lot of the early clues came from paleontology. Very early in the 20th century, a well-known paleontologist, Charles Walcott, worked on fossil trilobites in Scotland, Newfoundland, Scandinavia and southern Britain. He identified that the fossil trilobites of Scotland and western Newfoundland in rocks dated at around 520 million years ago, that's Cambrian in age, were very different in their main morphological characteristics from those in Scandinavia, southern Britain and eastern Newfoundland. At that time, the prevailing wisdom was that an elongate trough that geologists called a geosyncline separated the various continental masses, which had their different trilobite faunas. Now, at that point those continental masses were thought to be fairly static, because this was before seafloor spreading and plate tectonic theory. However, by the time we get into the 1950s, the emerging science of paleomagnetism provided really good evidence that the continents had been moving around the Earth's surface. And this morphed into plate tectonic theory, which enabled scientists to understand the Caledonian orogeny as resulting from the closure of the Iapetus Ocean by the process of subduction, leading to continental collision. So those are some of the very early lines of evidence that led to the current theories that we talk about today.

In terms of the age of the orogeny, we can constrain it between the time of deposition of the youngest rocks to be affected by the orogeny, which are about 470 million years old, and the oldest rocks to sit unconformably on the eroded remnants of the mountain belt, which are about 400 million years old. So those very broad constraints tell us that the Caledonian orogeny occurred between about 470 and 400 million years ago. But with the advent of geochronology, we can now precisely date individual mountain-building events. We can date quite precisely the metamorphic minerals and igneous rocks formed at different stages of an orogeny. For example, we can use uranium–lead isotopes to date zircon, which is a common accessory mineral in many igneous rock types, to a precision of less than a million years. And we can use slightly different isotopic techniques to date very common metamorphic minerals such as garnet to a similar sort of precision. So we can date igneous rocks and metamorphic rocks, and we can date ophiolites and volcanic arc rocks, all to a great deal of precision, and therefore build up a history of magmatism, deformation and metamorphism through these different collisions that ultimately make up the whole orogenic story.

Oliver Strimpel: OK, well, let's talk about the various present-day locations that bear evidence of the Caledonian. This episode's web page at geologybites.com has geological maps of the regions we'll be discussing that might be helpful for what follows. The most extensive record is on the east coast of Greenland and the west coast of Norway. Let's start there.

Rob Strachan: Yes, you're absolutely right, Oliver. The most spectacular manifestations of the Caledonian orogeny are indeed in East Greenland, where I worked for a few years, and western Norway. Throughout much of the time period between 500 and 400 million years ago, those two pieces of continental crust lay on either side of the northern part of the Iapetus Ocean, and the northern part of the Iapetus Ocean was being consumed by subduction which was dipping westwards underneath East Greenland, which was part of Laurentia. So Baltica was approaching East Greenland and was forming the downgoing plate, and eventually, from about 430 million years onwards, those two pieces of continental crust collided and formed a mountain belt that would have been on the same scale as the Himalaya.

If we look at cross sections through East Greenland and Scandinavia at the present day, we see that both parts of the orogenic belt are made up of major thrust sheets. Those in East Greenland were emplaced to the west, and those in Scandinavia were emplaced to the east, so overall there is a divergent symmetry. Now, I said that Baltica was forming the downgoing plate as it approached that subduction zone, and as it entered that subduction zone a number of important things happened. Firstly, the leading edge of Laurentia was thrust up on top of Baltica, and that leading edge of Laurentia included not only continental crust, but volcanic arcs and bits and pieces of ocean crust. These were all thrust up onto Baltica, and we can see those bits of Laurentian crust in present-day western Norway. So if we go and look at the geology of western Norway, we can see Baltica crust overlain by thrust sheets and thrust nappes of material that was derived from East Greenland.

A second major thing that happened as Baltica crust entered the subduction zone is that the continental crust was subducted to depths of at least 100 kilometers and was subjected to ultrahigh-pressure metamorphism. We know that to be the case because there are pieces of Baltica continental crust that contain microdiamonds and a high-pressure form of quartz called coesite. These ultrahigh-pressure rocks are exposed in what is known as the Western Gneiss Region of Norway, which is north of Bergen along the coast there. And that's a really classic area for the study of ultrahigh-pressure metamorphism, as well as the thrusting of Laurentian crust onto Baltica and the subduction to great depths of Baltican crust down the subduction zone.

The basement rocks in East Greenland also underwent high-pressure metamorphism, and across a great deal of eastern Greenland we see high-pressure rocks known as eclogites, which formed at a depth of at least 60 kilometers. So we've got evidence for high-pressure metamorphism on both sides of the mountain belt there, but particularly on the Baltican side. Now, ultimately that mountain belt became unstable, and it started to spread sideways and become tectonically thinned, which then resulted in the exhumation of the central, deeply buried parts of the mountain belt, which were more or less back up towards the surface by about 380 to 350 million years ago.

Oliver Strimpel: What manifestation do we see of the Caledonian in North America?

Rob Strachan: Well, things are slightly different in North America. We do see evidence for the early Caledonian arc–continent collision on the edge of Laurentia, although there, in places like Newfoundland, for example, it is referred to as the Taconic orogeny, and it occurs slightly later, at 450 million years. And after that there are a number of collisions of smaller continental blocks with the edge of Laurentia, resulting in repeated deformation and metamorphism. The net effect is that the edge of Laurentia then starts to grow outwards by a process of accretion. So in Newfoundland, for example, what is known as the Gander microcontinent collided first with the edge of Laurentia at about 430 million years, and then Avalonia at about 400 million years.

Oliver Strimpel: You've worked extensively on the Caledonian rocks in Scotland. Can you describe what we see there?

Rob Strachan: OK. Well, the best way of approaching this is to think that we're going to traverse from the northwestern seaboard of Scotland down towards the Iapetus Suture along the Solway Firth. If we go up to the northwest seaboard, some of the low-lying rocks along the coast west of Assynt, for example, are part of what we call the foreland to the Caledonian orogeny. We use the term foreland to refer to the continental crust on either side of a mountain belt that has not been affected by the mountain building. So the area in the northwesternmost part of Scotland is what we call the Caledonian foreland. There we see ancient Archean gneisses known as the Lewisian gneisses, and these are overlain by several sedimentary successions, some of which are as old as 1,000 million years, others as young as 500 million years. But the important thing is that those rock sequences on the foreland are not actually affected by the Caledonian orogeny.

If we go east, we then encounter a major structure called the Moine Thrust, and this is the western margin of the Caledonian mountain belt. Bits and pieces of the foreland rocks are stacked up within that thrust zone, a fairly narrow zone of thrusting which we call the Moine Thrust Zone. The Moine Thrust Zone dips eastwards underneath the Northern Highlands, and at a late stage in the Caledonian orogeny, all the rocks to the east were pushed many tens of kilometers towards the northwest over the foreland.

Now, if we were to traverse from the Moine Thrust across the Northern Highlands, across the Great Glen Fault and down towards the Highland Boundary Fault, we would see that a great deal of the bedrock is made up of metamorphosed sedimentary rocks. These are the Moine and Dalradian successions, and the Great Glen Fault chops this extensive metamorphic terrain in half. These sedimentary successions are fairly old. They were laid down on the edge of the Rodinia supercontinent, and if we look at the metamorphic minerals within them, we can see that they were quite highly metamorphosed during the Caledonian orogeny. In places, temperatures were as high as 700 to 800 degrees, and the rocks actually started to melt. In other places, typically east of the Great Glen, the metamorphic temperatures were perhaps as low as 400°C, so rather lower. And towards the end of the Caledonian orogeny, these metamorphosed sedimentary rocks were intruded by huge granite plutons, and these probably formed as a result of the melting of mantle and crustal material above the subduction zone that was dipping underneath Laurentia from about 460 million years onwards.

If we then pop across the Highland Boundary Fault, what we see at the surface within the Midland Valley are mainly Devonian and Carboniferous sedimentary rocks. These were deposited after the Caledonian orogeny. However, we do get some insights into the rocks that they overlie at depth. There are various bits and pieces of older rocks brought up within younger volcanic vents, for example, and we've got evidence that at depth there exists the volcanic arc which I talked about earlier as having collided with the edge of Laurentia during the Grampian phase of the orogeny.

If we go to the south edge of the Midland Valley, we cross the Southern Upland Fault, and between the Southern Upland Fault and the Iapetus Suture there is a huge wedge, many, many kilometers thick, of folded and thrusted deep-sea sedimentary rocks, and together these are thought to form what we call an accretionary prism. At the present day, accretionary prisms develop above a subduction zone as sediments are scraped off the downgoing ocean plate rather than being taken down into the Earth's mantle. You'll recall I said earlier that the collision of the Midland Valley arc with Laurentia was followed by the development of a new subduction zone on the southeast side of the Midland Valley. Well, the sedimentary rocks of the Southern Uplands date from about 460 to 430 million years, and they record that period of subduction that was directed towards the northwest and was ultimately terminated by the collision of Avalonia south of the Solway Firth. That collision was probably fairly soft, as the rocks in the Southern Uplands are not particularly metamorphosed, so they were never deeply buried or strongly deformed during that final collision. So I think that provides a basic traverse from the northwest foreland down to the Iapetus Suture.

Oliver Strimpel: So it seems that this part of the Caledonian orogen has been quite intensively scrutinized by you and others in the field, and I'm wondering if this enables us to construct a more detailed picture of the sequence of events and what actually happened in that part of the orogeny.

Rob Strachan: We still haven't really arrived at a finally agreed picture, which is what makes research so exciting. We know that the Moine and Dalradian rocks clearly underwent several periods of Alpine-scale folding, shearing and medium- to high-grade metamorphism. So one thing we can do is try to date the various structures and metamorphic minerals which formed during specific events, so that we can distinguish those that formed in the earlier arc–continent collision from those which formed during the later continental collision. As I said earlier, we can try, for example, to date metamorphic minerals such as garnet, and we can identify some phases of garnet growth at around 470 million years, for example, but we can also identify younger phases of garnet growth at 450 million years or so. Then we can try to tie those periods of garnet growth to different sets of geological structures, such as folds and shear zones, and try to erect some sort of chronology of the order in which all these different minerals and structures formed, and ultimately tie them in to the bigger picture.

Earlier on, I suggested that traditionally there was a picture of a two-phase development of the orogeny: an early arc–continent collision followed by a later continent–continent collision. Is that absolutely accepted by all workers? Not necessarily. Another significant problem concerns the role and importance of some of the major faults, such as the Great Glen Fault, and the amount of lateral, or sideways, movement along these structures at a late stage in the Caledonian orogeny. There's quite a lot of structural evidence that the final collision between the continents wasn't head-on, but slightly oblique. And as a result of that slight obliquity in the way the continents came together, there may have been many tens of kilometers of movement along some of the big faults, such as the Great Glen, Highland Boundary and Southern Upland faults.

Indeed, one model has suggested that there might have been as much as 500 kilometers of movement along the Great Glen Fault, such that northern Scotland was once way to the north, and that northern Scotland north of the Great Glen actually collided with Baltica in the final continental collision stage of the orogeny. It was then moved southwestwards by displacement along the Great Glen Fault at around 400 million years or so ago, to bring it into its present position relative to the Grampian Highlands east of the Great Glen Fault. In another interpretation, rather than the process I've just described, the orogeny might be viewed as a continuous process related to long-lived, southeast-directed subduction and collision of Laurentia and the Midland Valley arc, with very little significant movement along the Great Glen Fault. So there are still a number of different controversies and some big uncertainties which we need to look at.

Oliver Strimpel: Despite these being some of the most closely studied rocks in the world, then, isn't it a bit surprising that we still have no consensus on some fundamental aspects of their history, relatively recent in geological terms at least?

Rob Strachan: Well, there are a number of reasons why this is the case. One reason is that the Caledonian orogen as a whole has been split up and dispersed over a wide area as a result of the breakup of Pangaea and the formation of the much younger Atlantic Ocean. In many areas, such as East Greenland, we only see a very deeply eroded cut through the orogenic belt, so we're missing quite a lot of the geological record. If we've got a slice through the orogenic belt which is very much dominated by high-grade metamorphic rocks, it's probably quite easy to conceal important geological structures and sutures between different colliding blocks. So a late fault might apparently be separating two quite innocuous groups of basement gneisses, but it could in fact be a major suture. These sutures can be very difficult to detect in high-grade basement rocks.

East Greenland in particular is very remote, and it's really only been mapped at a reconnaissance level. In contrast, Scotland has been mapped in great detail, but exposure is often rather incomplete, and the orogenic belt in Scotland is divided up by these big late faults, which often complicates interpretation. Paleomagnetism isn't actually a great deal of help in resolving questions such as the amount of movement on these late faults, perhaps because the magnetic records of the different fault blocks have been overprinted later on in their geological history. It might be that we could go to areas in the Himalaya, for example, and map those in much, much more detail, and we would still find that we wouldn't be able to understand those areas either. So it's often the case that the more we find out, the more problems we find.

Oliver Strimpel: As we've been discussing, during the Caledonian orogeny, Avalonia and Baltica joined up with Laurentia in the early stages of the formation of the Pangaea supercontinent. Pangaea subsequently rifted apart, and the rocks of the Caledonian orogen were transported to their present-day far-flung locations on opposite shores of a newly formed ocean, the Atlantic. What factors controlled the locations of the lines along which this part of Pangaea eventually split up?

Rob Strachan: Well, this is a good example of what we call structural inheritance. Orogenic belts generally form as a result of the collision of old cratons, and very often these old cratons are composed of rocks as old as Archean, over 2.5 billion years or so. These are often very thick and strong, so orogenic belts, once they're formed, appear to represent major zones of weakness, even after many hundreds of millions of years. And these zones of weakness can be the preferred sites for later rifting when supercontinents break up. In the North Atlantic area, when Pangaea did split up, it didn't do so exactly along the lines of the Caledonian orogen, but it did more or less, and that separated North America and Greenland, which were all part of Laurentia, from Norway and southern Britain, which remained tied together. There was an abortive attempt at rifting in the North Sea, but that never actually led to very much.

A much better example of structural inheritance, perhaps, is evident in Africa. If we look at the East African Rift zone, for example, its location is very much controlled by the structural grain of mountain belts which formed around 700 million years ago. And the East African Rift nicely skirts around in between the much older and stronger cratons in that part of Africa, all of which formed more than two and a half billion years ago.

Oliver Strimpel: In the regions that manifest the remnants of the Caledonian orogeny, do we still see topography that was generated during the Caledonian?

Rob Strachan: No, almost certainly not, because in many parts of the Caledonides, in Scotland and East Greenland for example, we see sedimentary rocks deposited not far off sea level by about 350 million years or so ago. So a great deal of that mountain topography had been reduced within 50 million years or so after the actual mountain-building event. It's a common misconception that the so-called Caledonian mountains in Scotland are inherited from the actual Caledonian orogeny, and in fact that's not the case. A great deal of the topography around the present-day North Atlantic region was produced as a result of underplating of igneous rocks during Paleogene times, as the northern part of Pangaea split up to form the North Atlantic Ocean.

Oliver Strimpel: The Caledonian orogeny is one of the youngest extinct major orogenies. Of course, we want to reconstruct the movement and the sequence of docking or collisions of the pieces in the tectonic plate jigsaw puzzle. But more generally, what can we learn from the Caledonian rocks exposed today about how mountain-building events occur?

Rob Strachan: Well, the study of ancient mountain belts does allow us to better understand the way in which the lower to middle parts of the continental crust deform and recrystallize during orogeny. The present-day exhumed outcrops of ancient mountain belts, such as in East Greenland, for example, are almost exclusively made up of high-grade metamorphic rocks that we know were recrystallized and deformed in the lower to middle crust. So these are potentially very good analogues for the way in which, for example, the deep crust is deforming at the present day as India collides with Asia. We don't see the deep crust there at the present day, because it's all deeply buried under the core of the mountain belt. But GPS measurements tell us that large parts of Asia are being squeezed sideways and eastwards away from the Himalayan collision.

Now, at the surface, there are a number of brittle faults that may be assisting that eastward squeezing. But equally, it seems very likely that some sort of plastic or ductile flow in the lower to middle crust may also be accommodating the eastward movement of these large blocks of crust. If we look at East Greenland, for example, there are very large areas in the northern sector of the East Greenland Caledonides that show evidence for ductile flow towards the north, parallel to the length of the orogenic belt. And maybe that is also telling us that there was an oblique collision, and parts of the northern sector of the East Greenland Caledonides were being squeezed sideways out of the way of that collision zone as Baltica and Laurentia collided together again. So ultimately, we can study ancient mountain belts to try to understand what might be happening at depth in orogenic belts that are currently developing.

Oliver Strimpel: Rob Strachan, thank you very much.

Rob Strachan: Well, thanks, Oliver. It's been great talking to you.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find the 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.