Silurian Closure Of The Iapetus Ocean: Geological Effects On Britain

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Introduction

The Silurian period was undoubtedly a time of significance for Britain as the tectonic activity that occurred during this time triggered the two halves of Britain to converge and became a fundamental factor in the geology of the Isles. The continents Baltica and Avalonia, throughout the Ordovician time, both had comparable drift history as they moved northwards together. Becoming more significant during the Silurian period, the continents collided to the supercontinent Laurentia and eventually, the Iapetus ocean, which separated the two halves of Britain, had closed (Aldridge et al, 2000). Therefore, the Silurian time became an essential factor in why Britain was formed. Furthermore, the Caledonian Orogeny, a notable consequence, caused the period to become vital for future fieldwork – providing valuable understanding and explanations for modern-day Britain. It formed key geological portions of Scotland and other parts of Isles. The Silurian time was the leading and pivotal reason in the creation of Britain as well as the essential geology we study in the present time. This makes it one of the most vital geological time periods in relation to its significance for Britain.

However, there is still some discussion amongst scientists on the geological timing of events. Pickering (1988) suggested the late Ordovician was the final closure of the Iapetus ocean while Soper & Woodcock (1990) and McKerrow (1988) proposed Silurian and early Devonian closure, respectively. With the support of palaeomagnetic data, drift history and sediment dispersal patterns, this has established a critical debate within geological investigations. Furthermore, subsidence analysis has been used to further verify the timing of the closure (King, 1994).

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Silurian closure of the Iapetus Ocean – the formation of Isles

There is some complexity when discussing the closing of the Iapetus Ocean – it had an undeniable impact as it formed Isles. Initially, there was a collision between the microcontinent Eastern Avalonia, comprising of southern Britain and Laurentia including both northern Britain and north American and Baltica consisting of Scandinavia (King, 1994). This is evident in useful maps; figure 1a illustrates the beginning of the Ordovician-Silurian boundary where Eastern Avalonia has moved northwards already beginning to close the Iapetus Ocean (Aldridge et al, 2000).

At around 443 Ma, the Ordovician-Silurian boundary, Avalonia combined with Baltica, with a sinistral displacement, as they both moved in the same direction. These continents collided diachronous during the Silurian to form the supercontinent Laurussia (Cocks & Torsvik, 2002). This can be seen in Figure 1b where the Iapetus Ocean has relatively closed, and the line of the eventual Iapetus Suture is established. Furthermore, oblique plate convergence had occurred, accompanied by anticlockwise rotation, allowing Eastern Avalonia and Baltica to dock sinisterly against Laurentia which eventually joined the 2 halves of Isles (Dewey & Strachan, 2003). This can be defined as closing in a ‘scissor like’ method where closure advanced to the northeast as it started in the southwest. Evidence that supports the gradual underthrusting of Avalonia underneath Laurentia is seen in the dispersal of the Wenlock and Ludlow foreland basin deposits in Northern England (King, 1994). This can be reinforced by figure 1c which shows the Iapetus ocean completely disappearing and the merging of Laurentia and Eastern Avalonia to complete Britain.

These maps are key evidence that the Silurian period was extremely significant in reference to the formation of Isles. It shows a stage by stage procedure of tectonic activity and visually proves that without the collision, the 2 halves of Britain would not have merged. Therefore, this emphasises that the silurian period contributed greatly to the formation and geology of Isles. Moreover, figure 4 shows a broader perspective regarding the tectonic activity that took place in the late Ordovician and Silurian – underlining the merging of the two halves of Britain. Cherns (2006) had stressed that Avalonia united with Baltica during the Ordovician-Silurian boundary which is evidently clear in figure 2a as Avalonia is moving towards Baltica and in figure 2b Eastern Avalonia colliding with Laurentia during the Llandovery or Wenlock time period.

The East Mendips inlier has been used to help with the positioning of continents as they have rocks that help produce primary palaeomagnetic directions. These rocks have suggested a palaeolatitude of 13 ± 5ºS for the occupation of Eastern Avalonia (Trench & Torsvik 1992). Furthermore, figure 3 shows continental reconstruction for around 425 Ma is positioned using the mean mid-silurian pole from Baltica given the probability of local rotation affecting the East Mendips inlier – the poles for reconstruction pole for Baltica are 20ºS, 348ºE and for Laurentia 20ºS 344ºE (Torsvik & Smethurst 1992). Therefore, altogether these results prove that by Wenlock times Baltica and Eastern Avalonia had similar latitudinal positions and must have combined during Early Silurian before the collision to Laurentia. Furthermore, it strengthens the idea that the Iapetus Ocean had relatively closed as well as the formation of Britain taking place during the Silurian period

Verification of tectonic activity can also be seen with the analysis of subsidence of Eastern Avalonia sequences to suggest for the Iapetus closure – seen in in Figure 4 Northern England has a convex-upward profile as its subsidence curve displays accelerating subsidence rates. (King, 1994) – around 430 Ma, subsidence had begun to increase rapidly and towards the end of the Silurian period, subsidence (km) was more than 6.0. These results show that tectonic activity was highly concentrated and prevalent during the Silurian time. This further increases the validity of maps proposing that the Silurian period included the closing of the Iapetus Ocean due to tectonic activity and therefore the formation of Britain.

Silurian geological effects on Britain

The geological effects of tectonic activity during the Silurian period were long-lasting and significant. The collision of Baltica and Laurentia generated the Scandian event in the North while the collision between Avalonia and Laurentia generated the Southern Uplands in the South (Mendum, 2012). Furthermore, the collision between Eastern Avalonia and the Laurentian margin led to the Moine Thrust and other key ductile thrusts. This occurred because the collision caused the deformed and metamorphosed Neoproterozoic rocks of the Moine Nappe to overthrust north-westerly for 50-100 km over the older foreland Lewisian gneisses complex and their Torridonian + Cambrian sedimentary rocks instigating the formation of the Moine Thrust belt (Mendum, 2012). These Lewisian gneisses and Torridonian + Cambrian rocks were heavily thrust-faulted and folded during this event and they overlaid unconformably over the Lewisian Gneiss Complex (McClay & Coward, 1981; Chew & Strachan 2013). This further developed the geology of Scotland and verifies that the Silurian time had great significance. The Moine thrust belt ranges from Loch Eriboll south-westerly to the Sleat Peninsula of Skye (Mendum, 2012) which is approximately 200km. This displays how large the belt is which therefore supports how important it is to the overall geology of the Scottish Uplands. Mylonitic rocks. with brittle deformation features, are greatly present in the Moine belt (Mendum, 2012) which evidently happened due to the tectonic activity during the Silurian time. This is further seen as the consequence of tectonic activity such as thrusts. were transporting various parts of the Foreland succession in the north-westerly direction (Mendum, 2012). Consequently, displaying the substantial and complex events taking place as a result of the tectonic activity. Furthermore, it can be evidenced that deformation in relation to the Moine Thrust can be dated to the Silurian period as verified by Goodenough, et al (2006). Radiometric dating has been used by several scientists to prove this. For instance, Freeman et al (1998) used the Rb-Sr method to obtain the ages of 437-425 Ma by using micas from mylonites above the Moine thrust in southern Assynt (Goodenough, 2006). Moreover, they used the same method in a different location, Dundonnell, and attained 429-408 Ma ages. Therefore, this shows that main ductile deformation on the Moine Thrust was around 430 Ma (Goodenough, 2006). This can be seen again with Goodenough again (2011) as new figures suggest that movement within the Moine Thrust Belt yielded a date of around 430.7 ± 0.5 Ma and deformation within the Moine Thrust Belt was completed by approximately 429.2 ± 0.5 Ma.

Powel & Phillips (1985) proposed that ductile thrusting of the Moine rocks and other rocks were kinematically separate and occurred during the Grampian Orogeny, which was throughout the Ordovician time, with the evidence of weak isotopic figures. However, by using U-Pb Zircons it has been concluded that a sequence of nonkinematic granites, that were located during displacement near the Naver Thrust in Sutherland, generated emplacement ages of 435-425 Ma. Therefore, discrediting the Grampian Orogeny involvement and meaning that deformation of these rocks had to have taken place when the Scandian Orogeny was occurring as the ages indicate that these events were unquestionable during the Silurian time (Dewey & Strachan 2003). Consequently, strengthening the Silurian periods significant involvement in the geology of Britain.

Moreover, as the Scandian event occurred, upright folding in the central part of the Moine outcrop caused the formation of the Northern Highland Steep Belt (Roberts & Harris 1983; Chew & Strachan, 2013). These upright folds are predated due to U-Pb TIMS zircons attained from the Glen Scaddle Metagabbro showing age of 426+3. This suggests that the deformation of rocks took place during the final stages of the Scandian event (Strachan & Evans, 2008; Chew & Strachan, 2013). Therefore, further evidencing that major folding and other key geological consequences occurred in Scotland during the Silurian period.

Another outcome of the collision of Avalonia and Laurentia were sedimentary basins along the Eastern Avalonian margin. These basins prove there was accelerating subsidence throughout the Silurian period as the basin, named the Windermere Group, shows sediment accumulation rates (projected using graptolite time-scale) exceeding 1mm per year around 427-423 Ma (Kneller, 1991). Therefore, this means there was accelerating subsidence hence it was caused by the collision between Eastern Avalonia and Laurentia. Again, proves this major tectonic event did in fact occur throughout the Silurian period and had a permanent geological effect. There was also a reactivation of pre-existing fault systems which triggered compression, uplift and renewal of sedimentary sources in specific areas as well as extension, subsidence and deposition in further areas in Scotland. (Open University, 2018). The NW-direction ductile thrusting that settled in the expansion of the Moine Thrust Zone had gave rise to widespread folding and fabric development under amphibolite to greenschist facies conditions (Krabbendam et al 2011, Chew & Strachan 2013).The Southern uplands accretionary complex was formed after segments of the oceanic sequence and sedimentary cover were taken from the subducting Avalonian plate beneath the Laurentian margin, instigating the accretionary complex. The accretionary complex ranges and is maintained across southern Scotland to a southwestward direction towards around Northern Ireland (Stone, 2012). This displays its long-lasting and effective impact on the Geology of Britain. Lastly, magmatism had occurred in the Southern Uplands due to the melting of the subduction of Iapetus. This has can be seen in the alkali-basalts found in the Southern Uplands, inferred to be fragments of ocean islands (McMurtry, 1980).

The effects of tectonic activity have become significant in the essential and critical geology of Scotland and other parts of Britain. This makes it a fundamental factor in the present-day geology of Britain. Without the tectonic collisions between the three plates, the major Moine Thrust wouldn’t have formed along with other key parts of the Southern Uplands.

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