Rob Strachan on the Caledonian Orogeny
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The Caledonian orogeny took place during a multiphase collision of crustal blocks in the early stages of the assembly of Pangea between about 490 million years ago and 390 million years ago. In the process, Himalayan-scale mountains were formed. While these mountains have been worn down today, we still see plenty of evidence for their existence in locations straddling the Atlantic and the Norwegian Sea. In the podcast, Rob Strachan describes the tectonic movements that led to the orogen and explains how we can reconstruct the sequence of events that occurred and what we can learn about today’s mountain-forming processes by studying the exhumed rocks of ancient orogens.
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.
Podcast Illustrations
Courtesy of Rob Strachan unless otherwise indicated.
Supercontinent Context of the Caledonian Orogeny
Supercontinent timeline. The Caledonian orogeny took place after the breakup of Rodinia at about 750 million years ago (Ma) when three of the continental blocks that would ultimately form part of Pangea assembled. These blocks were (i) Laurentia, which corresponds to present-day North America, Greenland, and Scotland; (ii) Baltica, corresponding to present day-Scandinavia; and (iii) Avalonia, corresponding to southern Britain and parts of mainland northern Europe.
Evans, D. A. D., et al. (2016), Geol. Soc. London, Special Pub. 424, 1
Video showing the reconstructed motions of tectonic plates over a billion years of Earth history. At about 700 Ma, Rodinia breaks up, and the Iapetus Ocean formed between the dispersing continental blocks. The Caledonian orogeny starts during the closure of the Iapetus and accompanies the assembly of Laurentia, Baltica, and Avalonia from about 490 Ma to 390 Ma.
EarthByte, University of Sydney
Tectonic Plate Movements Leading Up to the Caledonian Orogeny
Reconstruction of Rodinia about 1,000 million years ago before it broke up and oceans, such as the Iapetus formed between the dispersing continental blocks. Au: Australia; Am: Amazonia; Ba: Baltica; La: Laurentia; RP: Rio de la Plata; Si: Siberia; Wa: West Africa. S: Scotland; EG: East Greenland; Sv: Svalbard; P Pearya; red lines: areas affected by a prior orogeny (Grenville-Sveconorwegian) about one billion years ago.
After Cawood, P.A, et al. (2016), Earth and Planetary Science Letters 449, 118
Reconstruction of continental positions from the Ediacaran to the Devonian. (a) Laurentia drifts away from Gondwana (part of the supercontinent Rodinia) as the Iapetus Ocean grows from a mid-ocean spreading ridge. (b) A south-dipping subduction zone develops within the Iapetus Ocean and starts to consume the Iapetus oceanic crust. Avalonia rifts away from Gondwana. (c) The Iapetus is almost entirely swallowed up by subduction zones, and Avalonia collides with Baltica and then with Laurentia. (d) Baltica and Avalonia have joined Laurentia to form Larussia, and the Caledonian orogeny comes to an end.
Woodcock, N.H, et al. (2012) Geological History of Britain and Ireland (second edition), Wiley-Blackwell
Rocks Associated With the Subduction of the Iapetus Ocean
In the podcast, Strachan explains that some oceanic crust is thrust onto the margin of Laurentia during the early (Grampian) phase of the closure of the Iapetus Ocean to form ophiolites. The remains of the Caledonian ophiolites are often missing some of the full sequence of oceanic lithosphere as represented in the cross-section shown here. In the Shetlands, for example, only the mantle and lower crustal rocks are present, and the pillow lavas and deep-sea sediments are absent.
Dilek, Y., et al. (2014), Elements 10(2), 93
Layered ultramafic rocks in the Unst ophiolite of Shetland. These correspond to the light-blue layer just below the Moho shown in the cross-section above.
Lake District Ordovician volcanics and interbedded volcaniclastic rocks. In the podcast, Strachan describes the igneous rocks that are associated with the convergence and ultimate closure of oceans. These are formed by melting of mantle rocks above a subduction zone. The volcanic rocks seen in the Lake District in Northwest England comprise thick successions of volcanic rocks that were erupted above a subduction zone, which is thought to have dipped southwards underneath Avalonia during the closure of the Iapetus Ocean.
Toward the end of the Caledonian orogeny, the metamorphosed rocks adjacent to the suture zone between Laurentia and Avalonia were intruded by huge granite plutons, which were probably formed as a result of the melting of mantle and crustal material above the subduction zone that was dipping underneath Laurentia. Top and bottom right: granite in the Cairngorm mountains of Scotland; bottom left: Shap granite, Lake District, England.
Laurentian Margin in Britain and Eastern Greenland
Caledonian tectonic model. (a) Rifted margin of Laurentia approaching an intra-oceanic subduction zone above which has developed the juvenile Midland Valley arc (volcano at right). (b) Large-scale folding of Dalradian and Moine strata on Laurentia following its collision with the Midland Valley arc, and the switch in the direction of subduction zone dip so that oceanic lithosphere is now being subducted under Laurentia (note the developing accretionary prism that is now exposed in the Southern Uplands) (c) Final collision of Avalonia and Baltica with different sectors of the Laurentian margin during the final closure of Iapetus (note also the development of the main strike-slip/lateral faults). (GGF = Great Glen Fault; HBF = Highland Boundary Fault; SUF = Southern Uplands Fault).
Searle, M. P. (2021), Geological Magazine https://doi.org/10.1017/S0016756821000947
Siccar Point, one of the most famous of the sites where Hutton and Lyell first understood the magnitude of geological time, is an unconformity between Silurian greywackes underneath and Upper Devonian sandstone above. The Silurian greywackes are part of the accretionary prism that was uplifted and then eroded during the final closure of Iapetus and the collision of Avalonia with Laurentia. A long period of non-deposition then followed before the Upper Devonian sandstones were laid down unconformably on the older strata.
Neoproterozoic to Cambrian sedimentary rocks of the Eleonore Bay Supergroup that were laid down on the edge of Laurentia as the Iapetus Ocean widened. This succession is broadly time-equivalent to the Dalradian Supergroup in Scotland; they were thrust many 10s of km westwards during the Caledonian orogeny.
The folded rocks here are Paleoproterozoic basement gneisses in East Greenland, and the huge fold that can be seen probably formed during the Caledonian orogeny. The image is viewed to the north, so the sense of overturning of the fold is to the left, consistent with thrusting and overfolding toward the foreland in the west.
Baltican Margin and the Western Gneiss Region of Norway
This hypothetical cross-section restores the sedimentary strata that are now stacked up in Norway as a series of thrust nappes into what is thought to have been their original location on the Baltica passive margin during the Silurian just as it is about to enter the Iapetus subduction zone that was dipping west under Laurentia (East Greenland).
Jamtveit, B., et al. (2018), Nature Scientific Reports, 8, 17011.
During the collision of Laurentia and Baltica, continental rocks of the Baltican margin were subducted to depths of about 100 km where they underwent high-pressure metamorphism. They then returned to the surface where we see them today as the ultra-high pressure (UHP) rocks in the Western Gneiss Region of Norway.
Eclogite in Western Gneiss Region. Eclogite is a rock formed at high temperatures and pressures by metamorphosing a rock of basaltic composition Minerals: garnet (red); omphacite, a sodium-rich pyroxene (geen); quartz (colorless, glassy). The garnets are about 10 mm across.
Courtesy of Haaken Fossen
Scandian collision and east-directed nappe stacking (left) followed by slab detachment, uplift, extension, and exhumation of Western Gneiss Region (right)
Brueckner, H.K. et al. (2013), Lithosphere, doi: 10.1130/L256.1
The Caledonian History of Scotland
The main terranes in the British Isles and the names of the main faults. The terranes north of the Highland Boundary Fault (HBF) are thought to have always been part of Laurentia; those south of the Iapetus Suture are thought to have always been part of Gondwana (Avalonia). Between the Iapetus Suture and the Highland Boundary Fault are two terranes (the ‘intermediate terranes’) that are thought to have evolved within the Iapetus Ocean. OIT: Outer Isles Thrust; MTZ: Moine Thrust Zone; GGF: Great Glen Fault; SUF: Southern Uplands Fault; MSF: Menai Straits Fault; WBF: Welsh Borderlands Fault.
Courtesy of the Open University
Moine Thrust at Knockan Crag, a Geopark in Scotland where the Moine Thrust forms a sharp boundary. Mylonitic Moine rocks lie above buff-colored Cambro-Ordovician carbonates, viewed east.
Map showing location of granites intruded in the late stages of the Caledonian orogeny. As discussed in the podcast, these rocks were formed from the melting of mantle and crustal material above the subduction zones that consumed the Iapetus.
Miles, A.J. et al. (2016), Gondwana Research, 39, 250
This shows the tectonic framework on the northwest margin of the Iapetus Ocean during the Silurian. Laurentian crust north of the Highland Boundary Fault is out of view to the left (northwest). The top diagram shows Iapetan oceanic lithosphere being subducted northwestwards, resulting in a) construction of the Southern Uplands accretionary prism made up of oceanic sediments that were progressively scraped off the subducting oceanic plate, and b) magmatism in the Midland Valley due to melting of the mantle wedge above the subduction zone. Sedimentary rocks also accumulated in the Midland Valley between the main plutonic/volcanic centers. The bottom diagram shows the complexity of the Southern Uplands accretionary prism, which is typical of modern examples. Thrust faults (in red) separate different packages of oceanic sediment that have been progressively scraped off the downgoing plate; hence these packages get younger from 1 to 6. Within each package, the order of the strata is still more or less right-way-up as shown by the inverted Y sign.
Courtesy of the Open University
Reconstruction of the end of the Caledonian following ~700-500 km of movement along the Great Glen Fault. In the podcast, Strachan discusses a model in which the Northern Scottish Highlands were further north during Caledonian times, collided with Baltica, and then were displaced to the south by ~700-500 km along the Great Glen Fault. (Dewey & Strachan 2003).
Open University 2003
Rocks Associated With the Caledonian Orogen Before and After the Opening of the North Atlantic
This very simplified reconstruction represents the point at which Pangaea had assembled by approximately 330 million years ago (Ma). In Scotland, the light blue represents the deformed margin of Laurentia (the Northern Highland and Grampian terranes), and the dark green the terranes that developed just outboard of Laurentia within the Iapetus Ocean (the Midland Valley and Southern Uplands terranes). The corresponding units are shown to the west in Maritime Canada and the Appalachians, to the north in East Greenland, and on the Baltica side, an area of green represents Laurentian arcs that were thrust onto Baltica crust during the final continental collision. On the far left, a key includes five different units, one of which is Avalonia. This reflects the very complex geology of the various pieces of crust that collided with Laurentia. The simplified view adopted in the podcast is that all the crust south of the Iapetus Suture in Britain and Ireland can be referred to loosely as ‘Avalonia’ with the proviso that it may in fact be rather more complex.
Waldron, J. W. F. et al. (2022), Earth Science Reviews, 233, 104163
Reconstruction of the continental blocks before the opening of the northern Atlantic about 54 million years ago. The green shading shows the areas that were caught up in the Caledonian orogeny, which at the time of the orogeny formed the margins of Laurentia, Baltica, and Avalonia. Today, these areas run from the Appalachian mountains and Newfoundland in North America through the northern British Isles to eastern Greenland and western Norway.
Dispersal of different parts of the Caledonides during the opening of the North Atlantic Ocean, which did not start until 54 Ma, about 350 million years after the end of the Caledonian orogeny.
Gasser, D. 2014, Geological Society of London Special Publication 390, 93
General Reading
Higgins, A. K. et al. (2008), The Greenland Caledonides, Geological Society of America, Memoir 202
Ramberg, I. B. et al. (2008), The Making of a Land: Geology of Norway
Smith, M. & Strachan, R.A. (2024), The Geology of Scotland, Geological Society, London
Stephens, M.B. & Weihed, J.B. (2020), Sweden: Lithotectonic Framework, tectonic evolution and mineral resources, Geological Society, London, Memoir 50