Caledonian orogeny

Mountain building event caused by the collision of Laurentia, Baltica and Avalonia

The Caledonian orogeny was a mountain building cycle recorded in the northern parts of the British Isles, the Scandinavian Caledonides, Svalbard, eastern Greenland and parts of north central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly 490–390 million years ago (Ma). It was caused by the closure of the Iapetus Ocean when the Laurentia and Baltica continents and the Avalonia microcontinent collided. The orogeny is named for Caledonia, the Latin name for Scotland. The term was first used in 1885 by Austrian geologist Eduard Suess for an episode of mountain building in northern Europe that predated the Devonian period. Geologists like Émile Haug and Hans Stille saw the Caledonian event as one of several episodic phases of mountain building that had occurred during Earth's history. Current understanding has it that the Caledonian orogeny encompasses a number of tectonic phases that can laterally be diachronous. The name "Caledonian" can therefore not be used for an absolute period of geological time, it applies only to a series of tectonically related events.

In the Neoproterozoic most of the Earth's landmasses were united in the Rodinia supercontinent. Gondwana formed its bulk. or 590 Ma. Then the part between Laurentia and Gondwana (to the east), opened c. 550 Ma. Its drift was towards the Baltica and Laurentia Ordovician positions which by then were further north. It also involved the consumption of both the Iapetus Ocean and the Tornquist Ocean along its northern margin. Avalonia's motion was related to slab pull created by the subduction of the Iapetus Ocean beneath the margin of Laurentia to its northwest and possibly also by ridge push created by the spreading of the Rheic Ocean. It migrated across the Iapetus Ocean orthogonally (at a right angle). Its drift included an up to 55° counterclockwise rotation with respect to the subduction zone to its north, mainly in the 470–450 Ma timeframe. It moved significantly faster than Baltica but slowed down to a rate comparable to that of the latter in the Late Ordovician when it got close to it. The main phases of the Caledonian orogeny resulted from the convergence of Baltica, Laurentia and Avalonia which led to the closure of the Iapetus Ocean.

McKerrow et al. (2000) give a definition of the Caledonian orogeny which includes "all the Cambrian, Ordovician, Silurian and Devonian tectonic events associated with the development and closure of those parts of the Iapetus Ocean which were situated between Laurentia (to the NW) and Baltica and Avalonia (to the SE and east) and each tectonic event throughout this 200 million years can be considered as an orogenic phase." This includes tectonic events which were smaller, localised and predated the more well known main phases of this orogeny. In this definition, the Taconic and Acadian orogenies in what today is North America are included in the phases of the Caledonian orogeny. Some early phases of deformation and metamorphism are recognised in the Scandinavian Caledonides. The first phase that is often included in the Caledonian orogeny is the Finnmarkian Orogeny, which was an early deformation event in Arctic (northern) Norway which preceded the Scandian phase (see below) in this area. Its onset has been dated at c. 500 Ma (Late Cambrian). It continued to c. 460 Ma and was reactivated in the Scandian phase at ~425–415 Ma. van Roermund and Brueckner (2004) proposed a distinct orogenic event which was separate and slightly younger than that of the Finnmarkian one, which they dated at 455 Ma. They named it the Jämtlandian Orogeny. It involved the Seve Nappe Complex of the Swedish Caledonides in central Sweden which is interpreted as the stretched outermost edge of Baltica. Contrary to the previous opinion that it had been subducted beneath an oceanic island arc, they propose that it involved a collision with a continental fragment. The Shelveian Orogeny occurred particularly in the Shelve area in Shropshire, in eastern Wales and in the English Midlands in the Late Ordovician and was related to the Taconic orogeny. It formed the Shelve Anticline and Rytton Castle Syncline and was the most important tectonic event in the area between the Cambrian and Devonian. Folding was accompanied by late stage igneous intrusions. The event caused a major unconformity in Shropshire with considerable erosion before the deposition of sediments in the Llandovery Epoch of the Silurian (444–443 Ma). There was no break in sediments in the area until the end of the Early Devonian, which was caused by the Acadian Orogeny in the British Isles. It was associated with dextral (right lateral) strike slip movement in the Pontesford Linley fault system and folding in pre Ashgill strata, uplift of the adjacent Towi Anticline and igneous activity.

The main orogenic events or phases of the Caledonian orogenic cycle were related to the final closure of the Iapetus Ocean. They were, in sequential order, the Grampian phase, the docking of Eastern Avalonia with Baltica, the Scandian phase and the Acadian phase. The latter involved: A) the docking of England and Wales (which were part of eastern Avalonia) with eastern and southern Ireland with Scotland and the rest of Ireland (which were part of Laurentia). B) the amalgamation of terranes of Western Avalonia with the eastern margin of the main landmass of Laurentia (see Acadian orogeny article for this orogeny). During the final part of its northwestward migration, Avalonia converged with Baltica and Laurentia to its northeast and northwest respectively. After its amalgamation with Eastern Avalonia, Baltica converged with Laurentia in a westward direction. The combined convergence of this microcontinent and the two continents created continental collisions between them, the mentioned orogenic events and the closure of the Iapetus and Tornquist oceans. Continental collisions started in the Mid Silurian and mountain building and ended in the Early Devonian (420–405 Ma).

The Grampian orogeny involved collisions between two landmasses of Laurentia and an oceanic island arc in the Iapetus Ocean outboard the main margin of the Laurentia tectonic plate (the future North America). There two Laurentian landmasses were Scotland
and northern and western Ireland. The other parts of the British Isles (England and Wales and the rest of Ireland) were part of the Avalonia microcontinent.

Two parts of Avalonia have been distinguished, a western and an eastern one. The term Western Avalonia refers to the westernmost part of the microcontinent which amalgamated the east coast of the main part of the Laurentia tectonic plate (what is now North America) to the west in the area of the northern Appalachians and the Maritimes. Eastern Avalonia refers to a) the part which amalgamated with Baltica, b) England a Wales and eastern and south eastern Ireland which amalgamated with Scotland and the north and west of Ireland (which were part of Laurentia).

The easternmost part of Eastern Avalonia amalgamated with Baltica through an oblique soft docking governed by dextral strike slip convergence and shear, rather an orogen causing hard continental collision. This is indicated by the absence of orogenic structures or high pressure metamorphic rocks, which are either not present or buried. This event occurred close to the end of the Ordovician, 440 Ma. It docked with the Baltica margins in southern Denmark, the south western corner of the Baltic Sea and Poland. It came to comprise Silesia in Poland, northern Germany, the Netherlands,Belgium and part of north eastern France (the Ardennes Mountains). The Anglo Brabant massif or London Brabant Massif in central and southern England and in Belgium is a large basement massif.

The Trans European Suture Zone or Tornquist Zone is the area of the suture of Baltica and Eastern Avalonia. It runs from a portion of the North Sea close to Denmark, through southern Denmark, a portion of the Baltic Sea between Denmark and Poland (by Germany's Rügen Island), and through Poland. It then follows the eastern margin of the Eastern Carpathian Mountains in western Ukraine. Finally, it runs to the Black Sea. However, in the Sudetes Mountains and the Eastern Carpathians, it evolved through the Variscan and the Alpine orogenies, rather than the Caledonian one.

The Scandian phase involved a collision between eastern Greenland on the eastern margin of Laurentia and the margin of the Baltoscandian platform of the Fennoscandian peninsula of Baltica. It involved the Scandinavian Caledonides in what is now Norway and the Swedish areas by its border. It occurred from the Wenlock Epoch of the Silurian to the Mid Devonian (430–380 Ma). Gee et al. (2013) and Ladenberger et al. (2012) propose a revised onset dating set at 440 Ma, however, there is no consensus about this. The Scandian orogenic event also led to the formation of mountains of Queen Louise Land (or Dronning Louise Land) in north eastern Greenland. It is an exposed N–S trending thrust zone which marks the western limit of intense Caledonian deformation. The dominant structures are interpreted as having resulted from sinistral transpression, which involved strain partitioning of regional deformation between sinistral strike slip movements in the east and NW directed oblique thrusting and folding further to the west. This orogenic event also affected Scotland and the outer Hebrides, causing thrusting in the Northern Highlands which culminated in the development of the Moine Thrust Belt, Ben Hope Thrust and Naver Sgurr Beag Thrust (435–420 Ma) and led to igneous intrusion in Galloway and the Southern Uplands (c. 400 Ma) in Scotland and the enlargement of the Lake District batholith in northern England. All this spanned the Iapetus Suture zone (see below). It also caused northeast trending strike slip faults, such as the Great Glen Fault which affected the Moine Supergroup and the Dalradian rocks in Scotland and the Shetland Islands through the Walls Boundary Fault, which is the northeast ward extension of the Great Glen Fault.

A mentioned above, the British Isles were separated and belonged to two different tectonic plates: Laurentia (Scotland and northern and western Ireland) and Avalonia (England and Wales and the rest of Ireland). The Early Devonian Acadian event in this area saw the amalgamation of these landmasses to form the British Isles as they are now. This occurred through NW dipping subduction of Avalonian oceanic crust beneath the southern margins of the Laurentian landmasses. Since the 1980s the term Acadian, which referred to the Late Silurian to Early Devonian orogeny in the Northern Appalachians, and the Maritime Provinces of Canada has been applied to the early Devonian deformation phase in the British Caledonides by analogy with the one that occurred in what is now North America. Subduction of the Iapetus Ocean occurred beneath the Midland Valley terrane of Scotland. There is a Trans Suture Suite of intrusive plutons which straddle both sides of the trace of the Iapetus Suture in the Southern Uplands terrane of Scotland (to the north of the suture) and the Lakesman Leinster terrane of northern England and eastern Ireland (to the south of the suture) which were at the Laurentia and Avalonia margins respectively. The emplacement of the plutons occurred after the subduction of the Iapetus Ocean ended. The Southern Uplands terrane is thought to be an accretionary wedge. Deep marine sedimentation here in response to subduction begun 455 Ma and marked the switch from an initial SE dipping Iapetus subduction under Avalonia to a NW dipping one beneath Laurentia. About 430 Ma accretion in the Southern Uplands and Ireland switched from being orthogonal (at a right angle) to a sinistrally (left lateral) transpressive one as indicated by cleavage transecting folds counterclockwise. Turbidite deposition in the oceanic trench overlapped onto the Lakesman Leinster terrane. Laurentia Avalonia convergence and Iapetus Ocean subduction ceased by C. 420 Ma as indicated by a mid Silurian weakening of deformation in the accretionary wedge. Magma production should be larger in convergent tectonic regimes during subduction and markedly reduced with the change to post subduction collisional regimes. However, during Iapetus subduction (455–425 Ma) this was low and intrusive rocks were largely absent across all terranes in the concerned area in this period. Most Acadian magmatism occurred post subduction (425 390 Ma) in a regional tectonic setting with alternating transpression and transtension phases. High rates of magma generation coincided with a c. 418–404 Ma Early Devonian sinistral transtension phase. This decreased during the 404–394 Ma Acadian transpression. This has led to the hypotheses that arc rocks were eroded and thus have not been preserved, that the arc was displaced by lateral movement along strike slip faults or that this is due to flat–slab subduction, which reduces magmatism rates. In the Early to Middle Ordovician the Skiddaw Group was a deep submarine sedimentary basin whose sediments were largely derived from an earlier continental margin volcanic arc. It underwent subduction related tectonic uplift which brought it above water. Convergence eventually caused the ranges of the Southern Uplands of Scotland to extend across the sutured Iapetus Ocean. This caused thrust imbrication of the group and a penetrative slaty cleavage with a broadly Caledonian trend superimposed on the earlier fabric of the group. The final stage of compression may have also involved the reactivation of thrust faults. In the Middle to Late Ordovician the Borrowdale Volcanic Group (BVG) and Eycott Volcanic Group (EVG) underwent caldera collapse as indicated by block faulted tracts. The superimposed Acadian deformations are regional monoclines north of the EVG and south of the BVG, which are part the Lake District anticline whose core is the Skiddaw Group (which is overlain by the EVG). The BVD is dominated by synclines and anticlines are generally absent. This suggests that the Acadian deformation tightened the main basins of the BGV and folded its strata and that the major synclines were reactivated and tightened volcanic basins. A cleavage with fabrics varying from spaced to slaty and penetrative was superimposed locally. The Windermere Supergroup was a foreland basin on the Avalonia margin which was created by the subduction of Avalonia continental crust beneath Laurentia. It underwent accelerating subsidence through the Middle to Late Silurian. It was related to indentation of a rigid basement block that was driven northward during Acadian continental collision creating transpressive strain. The Acadian orogeny created a single penetrative cleavage throughout the Supergroup. It underwent a small clockwise transection of the folds in the west of the outcrop and small anticlockwise one in the east which is part of a wider transecting cleavage pattern across the Avalonian rocks of Britain and Ireland. The Westmorland Monocline, which lies above the southern margin of the Lake District batholith (which underlies most of the central part of the district) is a more extensive Acadian deformation which affects the BVG outcrop southern margin overlying Windermere Supergroup strata. Cleavage is strongly developed in the volcanic rocks close to its hinge zone. The Honister slate belt in the north of this area is another enhanced cleavage zone. Deformation is more marked at the edges of the batholith as the granite it is made of is resistant to tectonic stress. All primary folds have the same style and are associated with the same regional cleavage suggesting that they are roughly coeval. There is ductile deformation in some localities and a broad shear zone in the Langness Peninsula which deform the primary cleavage and are thought to have formed during or soon after the main deformation phase. The Dalby Group was overthrust onto the Manx Group, probably in the early Devonian. During the final stage of the Iapetus Ocean closure its turbidites were deposited from the NE into a marine basin which bridged the Avalonia and Laurentia margins. The tectonic contact between the two groups has been correlated either with the Windermere Supergroup (Lake District) turbidites or the Riccarton Group, (Southern Uplands terrane). The former hypothesis implies that the Dalby Group was originally deposited on the Manx Group and was subsequently faulted into its present day relationship. The latter one implies that it is the toe end of the Southern Uplands turbidite accretionary wedge onlapping or thrust onto the Avalonia continental margin. The broad deformation style and age of the Manx Group are very similar to the equivalent features of the Skiddaw Group in the Lake District and the Ribband Group in SE Ireland. This group is thought to be their regional equivalent. It underwent two main deformation phases which also affected the Dalby Group: a) a pervasive slaty cleavage associated with gently to moderately plunging folds which also affected many of the minor igneous intrusions, b) a gently dipping crenulation cleavage associated with small folds verging towards the bedding dip direction. The Midland Valley terrane is continuation of the terrane of the same name in Scotland. It underwent more ductile regional strain than the Grampian terrane, with upright folds, cleavage and polyphase deformation. Acadian deformation resulted in steeply dipping to vertical Silurian rocks and a NE trending syncline in the Curlew Mountains inlier, folds and a SE dipping thrust fault in the Pomeroy inlier (County Tyrone) and NE trending open buckle folds in the Early Old Red Sandstone rocks by Cushendall, in County Antrim. The Longford Down terrane is the SE continuation of the Southern Uplands terrane in Scotland and the Southern Uplands Fault all the way to Galway Bay in County Galway, western Ireland. It lies to the SE of the Grampian and Midland Valley terranes. In underwent most of its deformation during the subduction and closure of the Iapetus Ocean prior to the Acadian orogeny. It has Early Palaeozoic deep marine rocks deposited on the NW margin of the Iapetus Ocean. The Southern Upland Longford Down terrane has been interpreted as having being either an accretionary wedge or a back arc basin. In the Dingle Peninsula (County Kerry) in south western Ireland, the Dingle Group underwent Acadian deformation with a high strain penetrative cleavage prior to the deposition of the Upper Devonian Slieve Mish Group in the same peninsula. The Early Devonian Dingle Basin was the source of detritus deposition in the Late Devonian Munster Basin to the south (which covered the Kerry and Cork counties). The Dingle basin underwent sediment recycling into the Munster Basin following Mid Devonian Acadian basin inversion.

The Iapetus Suture is the lineament where the Caledonian collision closed the Iapetus Ocean. In Ireland it runs from the estuary of the River Shannon on the Atlantic coast to Clogherhead on the Irish Sea. It crosses this sea and is exposed in the Niarbyl Fault in the southern part of the northern coast of the Isle of Man. In Britain it runs roughly parallel to the Anglo Scottish border. It consists of a series of faults with no traces of subduction, such as ophiolite remnants or oceanic trench derived rocks. The Iapetus Suture also extends along the margin of the Baltoscandian platform of the Fennoscandian Peninsula which collided with the eastern margin of Greenland along the eastern margin of Laurentia in the Scandian orogeny.

According to some authors, the Caledonian continental collisions involved another microcontinent, Armorica (southern Portugal, most of the north of France and parts of southern Germany and the Czech Republic), even smaller than Avalonia. This microcontinent probably did not form one consistent unit, but was instead a series of fragments, of which the current Armorican and Bohemian Massifs are the most important. The ocean between the combined continental mass of Laurentia, Baltica and Avalonia (called Euramerica, Laurussia or Old Red Continent) and Armorica is called the Rheic Ocean. The paleogeographic position of the Armorica crustal fragments between the Ordovician and Carboniferous is highly disputed though. There are indications that the Bohemian Massif started moving northward from the Ordovician onward, but many authors place the accretion of the Armorican terranes with the southern margin of Laurussia in the Carboniferous Variscan orogeny (about 340 million years ago). The Rhenohercynian basin, a back arc basin, formed at the southern margin of Euramerica just after the Caledonian orogeny. According to these authors, a small rim from Euramerica rifted off when this basin formed. The basin closed when these Caledonian deformed terranes were accreted again to Laurussia during the Hercynian orogeny.