The South Mayo Trough: tiny grains record huge events

Posted on 4 August, 2013 by Metageologist

Sedimentary basins have been described as ‘tape recorders’1 that preserve evidence of past events. Some sedimentary basins contain ‘recordings’ of grand tectonic events – plate collisions and mountain building. The information is stored as subtle but compelling patterns in the type of sand grains. Combined with studies of linked metamorphic and igneous rocks, they allow us to form a very rich understanding of past – to ‘listen in’ to dramatic stories from earth history.

Sandstones are made of sand, that has become stuck together to make rock2. These sand grains come from the breakdown of rocks that no longer exist. Whole mountain ranges are brought low by slow everyday processes. The mountains may be gone, but the stuff they were made of remains, as humble sand.

Most sand grains are overwhelmingly made up of quartz, followed by feldspar and fragments of rock. Other types of grain are there, but they make up a tiny proportion of the rock. To study these less common grains, geologists bash the sandstone back into sand and pour that into a heavy liquid. The common grains float to the top and many of the rarer ones sink. Geologists can then separate out the ‘heavy minerals’ and identify them under a microscope.

A dramatic example of the usefulness of heavy minerals comes from Ireland’s South Mayo trough. This area of Ordovician volcanic and sedimentary rocks sits within a complex collage of rocks shuffled around by the Caledonian orogeny. It is broadly a syncline in shape, with two sets of outcrops – a northern and a southern limb.

Location of South Mayo Trough. Figure 1 from Mange et al. (2010)

Today, the South Mayo Trough sits within the Eurasian plate, but it was born in the middle of an ocean that no longer exists.

All at sea

The oldest sediments are found in the Letterbrock formation3. The sediment matches what you would expect from erosion of the rocks found immediately north of the basin, the Killadangan formation which has been interpreted as an accretionary prism.

In the south, the sediments correlate with the Lough Nafooey group of volcanic rocks. These formed as part of an oceanic island arc within the Iapetus ocean. Together, this evidence suggests the South Mayo Trough formed as a forearc basin.

South Mayo Trough forming within the Iapetus ocean

The oldest sediments in the north also includes ‘ophiolite detritus’ – grains such as epidote and chromite that are typical of the erosion of oceanic crust. In the overlying Derrymore and Sheefry the ophiolitic debris becomes dominant – serpentine and chromite are so abundant that some beds are unusually heavy and have a ‘soapy’ feel. The volume of chromite increases up through the Sheefry and its chemistry becomes richer in Cr/Ni/Mg.

Grains of mica and zircon in these rocks have been dated. They show a variety of ages, all Precambrian, consistent with being derived from the old rocks of the Laurentian margin.

During this time, the volcanic rocks of the south show a change in composition. The earliest rocks are basaltic. Patterns of rare earth elements and other geochemical signatures are consistent with an oceanic island-arc origin (only oceanic crust involved). Over time the rocks become progressively more acidic, moving into andesitic and ultimately rhyolitic compositions. Rare earth elements show that the tectonic environment changes. Initially the oceanic island arc magmas were formed from melting of oceanic crust only. Progressively, more and more melt is derived from melting of Laurentian continental crust.

Collision but no mountains

The volcanic rocks therefore record that subduction zone has run out of oceanic crust – the island arc has collided into the continent – the leading edge of which was subducted and melted to feed the volcanic arc.

At the same time as these events recorded in the South Mayo trough, sediments formed on the edge of the Laurentian continent the Dalradian Supergroup) were being buried, heated and deformed in Grampian/Taconic orogeny. The sediments were buried underneath the oceanic crust (ophiolite) and oceanic island arc as they collided with the continent.

South Mayo trough as part of arc-continent collision

This implies the South Mayo trough itself was part of the upper plate, thrust onto the continent. The work orogeny is synonymous with mountain building, but here we have a sedimentary basin sitting on top of an orogeny, not only being preserved, but continuing to fill up with sediment. Various explanations have been given: the subducting slab and the ophiolitic upper nappe may have been unusually dense. The sedimentary basin, packed with serpentine and chromite certainly was. The sea-level at this time (mid-Ordovician) was unusually high, between 250-500m higher than at present. It’s possible the South Mayo trough was only plastered onto the side of the orogen, not thrusted completely over the top.

Whatever the reasons we should certainly be grateful that the sedimentary tape recorder was preserved. It was still rolling and about to record some more remarkable events.

A change of direction

Plate tectonics is a global phenomena. The closure of the subduction zone and the arc collision did not stop the overall convergence between the Laurentian continent and the Iapetan oceanic crust. In time another subduction zone formed, this time putting oceanic crust underneath the continent – a change of direction.

While this flip of subduction was taking place, conditions in the South Mayo trough at first didn’t change. The Lower Derrylea formation contains ophiolite debris from north (chrome spinel and purple zircons) and arc debris (clear zircons) from the south.

Diagram showing links events in the South Mayo Trough and other areas. Supporting Appendix. Key to columns: A, Western Newfoundland Ordovician Shelf; B, Notre Dame Bay arc stratigraphy; C, West Newfoundland ophiolites; D, Notre Dame arc ages; E, Quebec.New England; F, Scottish Highlands; G, Achill; H, Connemara; I, Clew Bay Complex; J, Scottish ophiolites; K, north limb of SMT; L, south limb of SMT (thicknesses in K and L in meters), detrital mica ages [71] in SMT; M, Derryveeny; N, Mweelrea; O, Derrylea; P, Rosroe; Q, Maumtrasna; R, Sheefry; S, Southern Uplands accretionary prism; T, detrital mica ages [70] in Southern Uplands accretionary prism.

Diagram showing links events in the South Mayo Trough and other areas. See Dewey (2005) for detailed explanation

Suddenly, around 466 million years ago, while the upper Derrylea formation was being deposited, a massive change occurs. Starting with a single massive thick turbidite bed there is an influx of different heavy minerals. In comes staurolite, almandine and chloritoid, along with floods of muscovite. These are metamorphic minerals and they show Ordovician ages – they come from the metamorphic rocks of the Dalradian.

Dating of these minerals shows that they were hot only 5-10 million years before they ended up as sand grains. Such rapid unroofing of metamorphic rocks suggests something more potent that erosion is at work. The Dalradian rocks in this area show rapid cooling at this time also suggesting something was bringing them rapidly towards the surface. What tectonic mechanisms could explain this?

With the creation of a new subduction zone to the south, the force of the converging plates was no longer supporting the thickened rocks of Taconic/Grampian orogeny. Now in a back-arc position, they extended rapidly. Major faults rapidly brought deep rocks to the surface sending metamorphic minerals cascading into the South Mayo Trough.

Once the Dalradian debris starts flowing, there are no more dramatic changes in the recording. It ends fairly soon after – the whole area is covered by unconformable Silurian sediments – but there is one more thing.

By 464Ma, the whole area is now in an ‘Andean’ type of tectonic environment, with intermediate vulcanism associated with the new subduction zone. This is recorded as ignimbrite layers in the South Mayo Trough, but also as granite intrusions within the nearby Connemara terrane. Once more we are able to make links between the surface and deep processes.

This is what makes these techniques and these rocks, so special. Linking surface to deep processes, resolving timescales to within a million years – these are very powerful ways of understanding how the earth really works.

References

Dewey J.F. (2005). Inaugural Article: Orogeny can be very short, Proceedings of the National Academy of Sciences, 102 (43) 15286-15293. DOI: 10.1073/pnas.0505516102
Mange M., Idleman B., Yin Q.Z., Hidaka H. & Dewey J. (2010). Detrital heavy minerals, white mica and zircon geochronology in the Ordovician South Mayo Trough, western Ireland: signatures of the Laurentian basement and the Grampian orogeny, Journal of the Geological Society, 167 (6) 1147-1160. DOI: 10.1144/0016-76492009-091
Brown D., Ryan P.D., Ryan P.D. & Dewey J.F. (2011). Arc-continent collision in the Ordovician of western Ireland: stratigraphic, structural, and metamorphic evolution, Arc-Continent Collision, 373-401. DOI: 10.1007/978-3-540-88558-0_13

Telling stories about Irish Geology

Posted on 5 June, 2013 by Metageologist

I clearly remember the most important moment of my geological career. I was resting my back on a glacially-polished wall of gabbro, my feet in an Irish bog, talking to myself in the sunshine. As a young man with bushy hair and beard, tattered field gear, wellington boots and a battered rucksack held together by darning and staples, I was recognisably a geologist, a ‘hammer-man’ in local parlance.

I was looking around as I talked to myself. This is a natural enough thing to do in beautiful Connemara. I’ve spent a lot of time admiring the interplay of sun and cloud and rain and glacial peaks there. I can especially recommend gazing out to sea, enjoying the sense of being at the end of the world, with the whole Atlantic before you. On a practical note, the west is where the showers come from; a glance that way gives you your own personal very-short-range weather forecast.

This time I was looking right at the low bobbles of the Dawros peninsula and left to the rounded hump of Currywongaun and Doughruagh’s majestic black stack. My gaze wasn’t aesthetic but geological, conceptual. In my mind these were no longer hills, but bodies of molten rock, injected into the beating heart of a mountain range millions of years ago.

I was talking because I was telling myself a story. A scientific story, where every detail is backed with evidence and armoured against the necessary pedantry of the scientific process. My story was a good one – good enough to turn into a scientific paper – I had not wasted the last 2 years of my life on pointless data collection after all. Best of all, my story felt like The Truth. I could dare to let myself believe that I understood events that happened miles underground millions of years ago. It was a good feeling.

The Dalradian in 1995

The modern consensus about the rocks of Connemara is that they are Dalradian sediments, deformed and metamorphosed over a short period of time by the Acadian/Grampian orogeny. Back in 1995 when I was a scruffy PhD student1 this was far from being a consensus. Some scholars (including, not incidentally, one of my supervisors John Dewey) were promoting the short orogeny model, but others disagreed. They had evidence on their side, too.

Understanding the timing events in deformed metamorphic rocks is helped by correlating phases of deformation and metamorphism. For Dalradian rocks there was a consensus that the ‘D2’ phase involved burial, heating and intense folding. D2 is associated with the original Barrovian style of metamorphism. Later, D3 involves less intense folding and in some areas Buchan style metamorphism linked to heating following the intrusion of gabbro intrusions into already hot rocks.

In the 80s and 90s, radiometric dating began to be applied to these problems. Direct dating of metamorphism was not then possible and the most reliable source of ages were zircons crystallised in igneous intrusions. Intrusions like the Ben Vuirich granite in Scotland – in 1989, Rogers et al. dated this as 590 million years old. Previous studies had interpreted the granite as being older than D2, but younger than D3. Scottish gabbros linked to D3 had been dated to around 490 million years, meaning that D2 and D3, far from being part of a single quick orogeny must represent, not a single quick orogeny, but entirely different mountain building events separated by 100 million years. In 1994, remapping of the Ben Vuirich granite showed that it was in fact pre-D2 (Tanner and Leslie, 1994), allowing advocates of the quick-Taconic model to argue it was pre-orogenic and therefore not relevant to these debates.

This was going on slightly before and during my PhD research. The challenge of linking igneous intrusions to deformation sequences was core to my work.

D3 folds of a D2 fabric. Sillimanite blebs aligned axial to the D3 folds. Connemara. Wellington boots for scale courtesy of the Geological Survey of Ireland

The Geology of Connemara in 1995

The rocks of Connemara correlate with Scottish events. There are extensive gabbro and calc-alkaline intrusions associated with an intense D3 phase of metamorphism. Most of these intrusions are in the south and the pattern of metamorphism reflects this, with sillimanite grade metamorphism close to them and lower temperatures further north.

These gabbros were originally correlated with D2 deformation, but in 1990 Geoff Tanner, partly in response to the Ben Vuirich date, argued that they were post-D2 and pre-D3.

For my PhD, I focused on a set of gabbro intrusions in the north of Connemara known as the Dawros-Currywonguan-Doughruagh2-Complex (DCDC). I produced detailed maps of the structures in these deformed rocks, focussing particularly on fabrics and shear-sense. I also did a lot of metamorphic petrology, describing the large metamorphic aureole around the intrusions. After a few years of this, I had enough data to start putting my story together.

The Currywongaun contains xenoliths of partially melted granulite facies sedimentary rock. Within these xenoliths are fragments of folded rocks, suggesting that had been deformed before the gabbro was intruded. The gabbro was intruded into rocks that were already at amphibolite grade (550 °C). There is abundant evidence that the magma was affected by deformation during its intrusion – it is syntectonic. Small intrusions within the DCDC are deformed by a fabric that is the same as the D2 fabric in the sedimentary rocks surrounding it. Overall, this evidence points to a syn-D2 page for the intrusion of the gabbros.

Folded fabric in a block of quartzite within a gabbro intrusion. The whole area is a xenolith, fractured by partial melting

There’s more. The larger gabbro bodies are extensively hydrated. Gabbro intrusions start off relatively dry, but if they are intruded into already hot metamorphic rocks they drive even more metamorphic reactions that produce water. As the gabbro cools a little (down from an initial 1200 °C) the water surrounding it makes its way into the intrusion. This water metamorphoses the gabbro into wet amphibolite, which is weaker than the gabbro and so is preferentially deformed. The west edge of Currywonguan is the site of a fantastic shear-zone that was active at high temperatures. In the country rocks, these fabrics cross-cut granulite facies D2 fabrics and can be correlated with D3 folding.

So the gabbros were intruded during D2, but affected by D3 folding while still hotter than the surrounding rocks. A spot of primitive (but effective) thermal modelling allowed me to show that the pulse of heat associated with these relatively small intrusions would have vanished within half a million years. So D2 and D3 were close in time – consistent with the ‘quick-Taconic’ model.

A sketch map of the Dawros-Currywongaun-Doughruagh intrusion from my paper.

Academic hurly-burly

Every good story needs a bit of conflict. For my story, it came from the radiometric dates. A 1988 paper dated the Connemara gabbros at 490Ma. Whereas 1993 and 1996 papers gave dates of around 470Ma for the D3 metamorphism. This gap of 20 million years doesn’t fit my story.

At this point, you may well be thinking that I must be wrong. Hard science, numerical analysis of isotopes must surely trump my hand-wavy field-based studies? No. Remember the paper was from 1988. Back then people still thought digital watches were a pretty neat idea. They dated zircons, not with high-precision laser beams but by chucking a bunch of them into acid. They didn’t even abrade them first. The date was based on the assumption that there was no inherited lead in them. I had no doubt that the date was wrong and I said so in public, at conferences and eventually in an academic paper.

Not everyone agreed. At a conference, after a talk I was told by a angry geochronologist that I couldn’t expect to be taken seriously if I went around saying radiometric dates were wrong. He hadn’t produced the date I disagreed with, but he clearly took it personally. This incident, the venom with which he disagreed with me, marked the start of the end of my geological career.

Closure

Some scientific disagreements remain unresolved for years. Not this one.

Anke Friedrich, a German PhD student at MIT started working on Connemara a few years after me. Soon after I’d been told off for doubting a published radiometric age, she proved it was wrong by redating the same rocks. The whole suite of dates she produced was powerful evidence for the ‘quick-Taconic’ model. All of the magmatism in Connemara and therefore the associated metamorphism and deformation lasted only 12 million years.

I was pleased to be proved right, of course, but I don’t actually remember when I first heard about it. I was so certain that I had to be correct that my reaction would have been quite mild. In this, I wasn’t unusual – I am far from the only scientist to have been hugely certain that they are correct. What strikes me now is how far away this is from the way science is ‘supposed’ to work. How can this be?

Leaning against my syn-tectonic gabbro, feeling I knew The Truth felt great. It helped motivate me: I’m only human. When I was ‘doing science’, writing papers, this feeling was irrelevant to the process of presenting evidence and suggesting hypotheses. Further, Anke Friedrich’s paper is rightly much more highly cited than mine. Her range of radiometric dates is the best scientific evidence for the ‘quick-Taconic’ model within Ireland. ‘Science’ is bigger than what goes on in scientists’ heads – its a process, not just a bunch of people’s opinions.

References

ROGERS, G., DEMPSTER, T., BLUCK, B., & TANNER, P. (1989). A high precision U-Pb age for the Ben Vuirich granite: implications for the evolution of the Scottish Dalradian Supergroup Journal of the Geological Society, 146 (5), 789-798 DOI: 10.1144/gsjgs.146.5.0789
TANNER, P., & LESLIE, A. (1994). A pre-D2 age for the 590 Ma Ben Vuirich Granite in the Dalradian of Scotland Journal of the Geological Society, 151 (2), 209-212 DOI: 10.1144/gsjgs.151.2.0209
WELLINGS, S. (1998). Timing of deformation associated with the syn-tectonic Dawros Currywongaun Doughruagh Complex, NW Connemara, western Ireland Journal of the Geological Society, 155 (1), 25-37 DOI: 10.1144/gsjgs.155.1.0025
Friedrich, A., Bowring, S., Martin, M., & Hodges, K. (1999). Short-lived continental magmatic arc at Connemara, western Irish Caledonides: Implications for the age of the Grampian orogeny Geology, 27 (1) DOI: 10.1130/0091-7613(1999)0272.3.CO;2

Structural Geology by the Deformation numbers

Posted on 1 April, 2013 by Metageologist

Structural geologists seek to understand how rocks have changed shape, in order to better understand wider processes such as how mountains are formed. Sometimes they use a terminology called ‘Deformation-numbers’ which I will now explain via a series of pretty pictures.

Structural geologists spend their day measuring the orientations of things. These can be planar things, like sedimentary bedding, fault planes, cleavage planes and other metamorphic fabrics; or linear things like fold axes or mineral stretching lineations. All these things interact in various ways, but given time and a compass-clinometer a good geologist will work it all out.

The trouble is that rocks are complicated. Take these gorgeous pics from northern Norway, kindly provided by Stephen Daly.
It’s clear that there is tight folding in metamorphosed sediments. There are things to measure the orientation of, such as hinge line (where the fold is tightest) and the axial plane, (the surface joining the hinge lines, here flat lying).

But, as so often in field geology, a closer look from the same area reveals a more complicated picture. What can you see here?

There is a clear set of folding – the obvious wavy pattern of the dark and light layers. Try tracing the individual layers – they are not even. In fact there are two sets of folding visible in these rocks. Let’s trace it out.

The orange lines are the axial traces of the obvious set of folding. The straight blue lines are axial traces of another set of folding. The curved blue lines are the faint traces of folded sedimentary layering.

This rock has enjoyed two phases of folding – therefore any description of the deformation in such complicated rocks has to introduce the concept of a sequence of events. Geologists love this sort of thing. I remember a seminar in the 1990s about the first detailed images of topography from Venus, showing linear structures. The lecturer said that as a geophysicist his first reaction was to perform a mathematical analysis of their spacing, but that a geologist’s first reaction was to look for cross-cutting relationships. This fundamental geological instinct applies to folds as well. One of the ‘blue’ folds is clearly folded by an ‘orange’ fold, meaning that ‘blue’ is older than ‘orange’.

A simple way of expressing this is to label the folds F1 and F2. The smaller the number, the older the structure. The same applies to other types of structures – a planar structure is know as S, starting with bedding which is know as S0. In our example S0 is folded by F1 and F2. A metamorphic fabric formed at the same time as F1, but folded by F2 would be S1. Most likely F1 and S1 were formed by the same deformation event, which we would call D1.

An Irish example

Another thread through much of geology is scale. Let’s move away from the outcrop scale to one of kilometres. Here is a classic cross-section through the Connemara, in Western Ireland.

If you give brilliant structural geologists enough time and enough Guinness, this is what you get. This is from the classic 1979 paper on Connemara by Tanner and Shackleton (1979). The different shades of grey each present a different group within the Dalradian Supergroup, each of which contain distinctive layers of sediment. This varied package of sediments allows the complex folding to be worked out.

In blue I’ve highlighted the trace of the Derryclare anticline, a tight structure that folds the sediments associated with a phase of deformation know as D2. In orange I’ve highlighted a few later D3 folds that contort the Derryclare anticline. Note that these are themselves bent over by a major D4 structure that covers the whole of Connemara.

On an outcrop scale in Connemara, most outcrops show clear D3 folding as in this rather splendid marble outcrop.

In Connemara, a view on the kilometre scale is the best way to see D2 and D4 structures, but sometimes a closer look is best. If you glue a slice of rock to a piece of glass, grind it down to a very thin slice and shine light through it then you can look at it under the microscope. This is another way of finding structures, more often fabrics than folds.

Photomicrograph of garnet-mica schist. Image courtesy of British Geological Survey Geoscenic archive

The image above is of a deformed schist. The large grey lump is a garnet crystal, a porphyroblast that grew during metamorphism. Look at it like a structural geologist – what do you see? I see this:

The orange lines show a fabric within the main body of the rock, visible in aligned quartz and mica. This is likely to correspond to the fabric visible in a hand specimen of the rock. In blue I’ve sketched a fabric visible only in the garnet. As the garnet grew it swallowed up fragments of quartz and other minerals that were themselves flattened into a fabric. This older fabric is now preserved only in the rigid garnet. Outside the older blue fabric has been destroyed, partly by metamorphic recrystallisation, partly by a later deformation phase that squashed the minerals to form the orange fabric

Wider implications

Looking at thin sections allows us to find fine structures that may no longer be visible in an outcrop or a hand specimen. It also let’s us link metamorphic and structural histories together. In our thin-section example the garnet (or at least its core) was growing while the older structure still existed, that is before the later orange deformation episode. If we found a mineral that grew across the orange fabric, it is probably younger than it.

I hope you can see that this allows us to build up two mutually-supporting sequences of events, the structural and the metamorphic. Metamorphic events are sometimes referred to as M1 and M2 and correlated with deformation events. Once you’ve done this, its possible to start linking D and M numbers to tectonic events: an arc colliding with a continent, for example.

Careful now!

Discovering one fold twisting another is an observation. Talking about D1 and D2 and then correlating that with rocks kilometres away is an interpretation. Geologists are excellent at carefully turning multiple observations into rigorous interpretations, but there are various reasons to treat them with care.

The first one is that modern models of mountain building processes (where many deformed rocks form) emphasise gradual processes. The continuous readjustment of an orogenic wedge, maybe switching into channel flow and out again, all this predicts a bewilderingly complex sequence of events for the whole orogen. The second related issue is that a single outcrop may not preserve the entire structural history. Structural and metamorphic processes will not affect an entire mountain belt at once – deformation and metamorphism may be focused into particular areas (perhaps rich in heat and water) and leave the surrounding rocks untouched. Maybe correlations of deformation episodes over wide areas are simply wrong. Maybe ‘D2’ is consistently a strong deformation followed by ‘D3’ folding, but these events happened at different times in different places?

There’s an analogy here with the study of separate sedimentary basins. In the absence of dateable fossils, the age of a sedimentary basin may be poorly known. Even so, geologists will attempt to correlate separate basins based on events preserved in the rocks – we’ve got to do something, even if we know the correlations may be incorrect. If a dateable fossil is found, it may show us we’ve made a mistake, but more likely the sedimentary history and correlations will make the fossil more useful. It doesn’t just tell us the age of a particular layer, but by inference it can illuminate the history of a much wider suite of rocks.

Within deformed metamorphic rocks, we can’t use fossils, but we can use the isotopes within minerals to tell us the age of events. For a while we’ve been able to do this accurately for zircons, but recently we can also directly date metamorphic minerals. Sometimes, for example in Connemara, the metamorphic dates are consistent with a single sequence of structural and metamorphic events that can be linked to arc-continent collision. However increasingly detailed studies of many areas are finding that apparently similar fabrics were formed in different mountain-building episodes, millions of years apart. Single grains of garnet have been found that contain a core that grew hundreds of millions of years earlier than the rim1

The concept of deformation sequences, as a set of observations, is invaluable for linking a particular isotopic age to a wider tectonic context. To say that a particular mineral grain grew at a particular time is not in itself very interesting. But it seems that interpreted correlations of D numbers without isotopic dating should be treated with care.

I’ll be illustrating these concepts with a specific example in my next post on the great Dalradian D2-D3 controversy and my part in it.

References

Argles, T., Prince, C., Foster, G., & Vance, D. (1999). New garnets for old? Cautionary tales from young mountain belts Earth and Planetary Science Letters, 172 (3-4), 301-309 DOI: 10.1016/S0012-821X(99)00209-5
Tanner, P., & Shackleton, R. (1979). Structure and stratigraphy of the Dalradian rocks of the Bennabeola area, Connemara, Eire Geological Society, London, Special Publications, 8 (1), 243-256 DOI: 10.1144/GSL.SP.1979.008.01.25

The Grampian / Taconic orogeny in Ireland – when arcs attack

Posted on 20 March, 2013 by Metageologist

Ever since the plate tectonic paradigm-shift of the 1960s, geologists have strived to understand ancient rocks in terms of the movements of plates. The geology of north-western Ireland can be explained by what happened when a subduction zone ran out of oceanic crust back in the Ordovician. Let me take you back to before that happened.

Imagine you are floating in the sea. In 480 million years time the crust below will be in Ireland. The sea is warm – CO₂ levels are high and you are fewer than 30° south of the equator. Apart from cheeky trilobites nibbling your toes, it is an idyllic place to be. You are near a large land-mass. It’s barren looking – plants are just about to learn how to survive outside the sea 1 so there is little to soften the landscape. The tumbler of gin and tonic2 you clumsily dropped has sunk into sediment that will one day be part of the Dalradian Supergroup.

Your mood has soured. Not only has your drink gone, but you’re getting creeped out by the cloud on the horizon out to sea. It’s not a fluffy, friendly one but a dark spreading plume of volcanic ash. Far out to sea, there is a line of volcanoes and they spell D-O-O-M for the peaceful spot you’ve found. Every year the volcanic arc gets a little bit nearer. Eventually it will smash into the continent behind you, grinding over the top. The peaceful sand and mud washing around below your feet will be sediment no more. The collision between the volcanic arc and the continent will transform the Dalradian sediments into the contorted metamorphic rocks that today make up much of NW Ireland.

An accident waiting to happen

Here’s a diagram showing a section through the land below. Black is oceanic crust, yellow stippled is sediment3.

Let’s start from the left. Notice that the edge of the continent is thin and tapering – it was stretched out when the ocean basin formed. The oceanic crust attached to the continent is being stuffed down back into the deep earth. As it sinks, it is squashed and heated, starting a complex process that ends with molten rock reaching the surface. Over time this magma has formed a volcanic arc, a small piece of thickened crust.

This situation can’t last for ever. Once all of the oceanic crust of the lower plate has run out, normal subduction comes to an end. The upper plate slides over the thinned continental margin and ends up lying on top of the continent – a process called obduction. The island arc and forearc basin are squashed against the continent. Like a bulldozer hitting a pile of sand, the arc collision compresses and thickens the crust. The Dalradian sediments become thoroughly deformed and heated and are now a wildly complex set of schists and marbles. This process creates a mountain belt, an event known as the Grampian orogeny.

As shown in the diagram below 4, the orogeny does not stop plate convergence. The old subduction zone has been destroyed, but another one is created in the opposite direction. This process, called ‘subduction flip’ changed the tectonic stress regime; it’s believed to have led to a process of ‘orogenic collapse’ whereby the thickened crust extends and thins, bringing metamorphism to an end. The overall tectonic event was remarkably quick, around 15 million years.

Where are they now?

This type of orogeny can be recognised as it leaves distinctive rock types behind. Ophiolites, pieces of oceanic crust within continents are found in several places in Ireland; the largest example is the Tyrone ophiolite, but traces of oceanic crust can also be found in Mayo, near Westport, in the Clew Bay and Deerpark Complexes. The story goes that there are no snakes in Ireland because Saint Patrick, while fasting on top of Croagh Patrick, threw them down into Clew Bay. That the sea-facing side of the mountain contains a lot of green serpentinite suggests the origins of the story have a geological angle5.

Map of NW Ireland. Red – continental basement, Yellow-Dalradian, Blue – lower-mid Ordovician volcanics, Brown – Ordovician sediments, purple serpentinite & melange. Data from Geological Survey of Ireland, displayed using Google Earth

Parts of the volcanic arc can now be found around Lough Nafooey, just to the south of the South Mayo trough, which corresponds to the fore-arc basin. The Dalradian sediments and underlying older crust make up most of the land to the north west of these rocks.
Let’s revisit the cross-sections, labelling the modern day equivalents. Before the collision:

and immediately afterwards:

A wider context

Ireland was only a small section of a continental margin that stretched from rocks now in Greenland down into the eastern US and Canada. The Grampian orogeny also affected Scotland and the same arc-collision event is found in eastern Northern America where it is known as the Taconic orogeny.

Figure 1 from Hollis et al. 2012

Recognising the connection across the Atlantic helped geologists understand the causes of these complicated patterns in the rocks – different sections show different parts of the orogeny and synthesising research across a wider area leads to a richer understanding. The patterns are complex – more so than I’ve shown above. There were probably multiple ‘accretion events’ where different arcs collided at different times. The pattern of plates in modern day SE Asia is seen as an analogy here – they may have been multiple subduction zones and arcs within the wider ocean.

Geological is 4-dimensional, and the Grampian/Taconic orogeny reminds us of this. The timing of the various accretion events varies because the edge of the continent was not straight. Promontories (sticky-out bits) were hit by the arc sooner than parts where there was more oceanic crust to be consumed. Another complication is that continents don’t break cleanly: fragments of continental crust can end up far from the main continent (for example Rockall in the Atlantic). The arc whose collision caused the Grampian orogeny in Scotland (now found buried in the Midland Valley) is thought to sit on continental crust.

These 15 million years were a very important time for the crust of the northern half of Ireland, but are only a small part of the wider geological history of Ireland. I’ll leave the story unfinished here and tell you what happened when the new subduction zone ran out of oceanic crust in future posts.

References

RYAN, P., & DEWEY, J. (1991). A geological and tectonic cross-section of the Caledonides of western Ireland Journal of the Geological Society, 148 (1), 173-180 DOI: 10.1144/gsjgs.148.1.0173
Hollis, S., Roberts, S., Cooper, M., Earls, G., Herrington, R., Condon, D., Cooper, M., Archibald, S., & Piercey, S. (2012). Episodic arc-ophiolite emplacement and the growth of continental margins: Late accretion in the Northern Irish sector of the Grampian-Taconic orogeny Geological Society of America Bulletin, 124 (11-12), 1702-1723 DOI: 10.1130/B30619.1
Bird, A., Thirlwall, M., Strachan, R., & Manning, C. (2013). Lu-Hf and Sm-Nd dating of metamorphic garnet: evidence for multiple accretion events during the Caledonian orogeny in Scotland Journal of the Geological Society, 170 (2), 301-317 DOI: 10.1144/jgs2012-083