Caledonian orogeny
The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly 490–390 million years ago (Ma).
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.
[2] Current understanding has it that the Caledonian orogeny encompasses a number of tectonic phases that can laterally be diachronous, meaning that different parts of the mountain range formed at different times.
[8] 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."
[9][10] According to van Roermund and Brueckner (2004), there was a distinct orogenic event which was separate and slightly younger than that of the Finnmarkian one, which they dated at 455 Ma.
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.
It formed the Shelve Anticline and Rytton Castle Syncline and was the most important tectonic event in the area between the Cambrian and Devonian.
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).
[11] 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 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.
[15] The easternmost part of Eastern Avalonia amalgamated with Baltica through an oblique soft docking governed by dextral strike-slip convergence and shear, rather than through an orogen-causing hard continental collision.
It came to comprise Silesia in Poland, northern Germany, the Netherlands,Belgium and part of north-eastern France (the Ardennes Mountains).
This is because this Devonian event postdated the collision of Avalonia with Laurentia by 15–20 million years and was coeval with the early phase of the Variscan orogeny (Eo-Variscan or Ligerian) and because it was not related to the Iapetus Ocean.
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.
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.
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.
[26] In addition, the Southern Uplands accretionary wedge lacks evidence of the presence of a volcanic arc as usually found near subduction zones.
[26] Nelison et al. (2009) propose an Iapetus Ocean subducting slab breakoff model to account for the intrusive rocks in the Grampian terrane being emplaced post-subduction.
However, Miles at al. (2016) note that the intrusive rocks in the Trans-Suture Suite and in all the terranes in the region are similar in age and geochemistry.
Generally, Acadian deformation metamorphosed mudrocks throughout various geologic formations of the district into slates by creating slaty cleavages.
Folds are transected clockwise by their cleavage, major strike-parallel sinistral faults and ductile shear zones thought to be related to this transpression.
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 latter one implies that it is the toe end of the Southern Uplands turbidite accretionary wedge onlapping or thrust onto the Avalonia continental margin.
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.
[31] In Ireland the Acadian Orogeny affected the four main terranes of the island: Grampian, Midland Valley, Longford-Down and Leinster.
There are indications that the Bohemian Massif started moving northward from the Ordovician onward,[44] 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).
