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94-818: The orogeny is an early phase of the Caledonian orogeny and overlapped in time with the orogenies that formed the Appalachian Mountains . It was the only orogeny in Laurentia at that time which resulted in deformation, folding and metamorphism. The Fleur de Lys rocks in Newfoundland may have been affected by the Grampian orogeny as well. In the Cambrian and early Ordovician, shallow water carbonates and deep water turbidite basins, which formed

188-426: A magnetometer towed on the sea surface or from an aircraft. The stripes on one side of the mid-ocean ridge were the mirror image of those on the other side. By identifying a reversal with a known age and measuring the distance of that reversal from the spreading center, the spreading half-rate could be computed. In some locations spreading rates have been found to be asymmetric; the half rates differ on each side of

282-642: A rift in a continental land mass , similar to the Red Sea - East Africa Rift System today. The process starts by heating at the base of the continental crust which causes it to become more plastic and less dense. Because less dense objects rise in relation to denser objects, the area being heated becomes a broad dome (see isostasy ). As the crust bows upward, fractures occur that gradually grow into rifts. The typical rift system consists of three rift arms at approximately 120-degree angles. These areas are named triple junctions and can be found in several places across

376-588: 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

470-405: A base or reference level h b {\displaystyle h_{b}} , the depth of the ocean d ( t ) {\displaystyle d(t)} is of interest. Because d ( t ) + h ( t ) = h b {\displaystyle d(t)+h(t)=h_{b}} (with h b {\displaystyle h_{b}} measured from

564-768: 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

658-586: A constant temperature T 1 . Due to its continuous creation, the lithosphere at x > 0 is moving away from the ridge at a constant velocity v , which is assumed large compared to other typical scales in the problem. The temperature at the upper boundary of the lithosphere ( z = 0) is a constant T 0 = 0. Thus at x = 0 the temperature is the Heaviside step function T 1 ⋅ Θ ( − z ) {\displaystyle T_{1}\cdot \Theta (-z)} . The system

752-591: A failed arm that was opening more slowly than the other two arms, but in 2005 the Ethiopian Afar Geophysical Lithospheric Experiment reported that in the Afar region , September 2005, a 60 km fissure opened as wide as eight meters. During this period of initial flooding the new sea is sensitive to changes in climate and eustasy . As a result, the new sea will evaporate (partially or completely) several times before

846-406: A few kilometers to tens of kilometers wide, a crustal accretion zone within the boundary zone where the ocean crust is youngest, and an instantaneous plate boundary – a line within the crustal accretion zone demarcating the two separating plates. Within the crustal accretion zone is a 1–2 km-wide neovolcanic zone where active volcanism occurs. In the general case, seafloor spreading starts as

940-482: A hypothesis of continental drift in 1912, he suggested that continents plowed through the ocean crust. This was impossible: oceanic crust is both more dense and more rigid than continental crust. Accordingly, Wegener's theory wasn't taken very seriously, especially in the United States. At first the driving force for spreading was argued to be convection currents in the mantle. Since then, it has been shown that

1034-527: A late Caledonian phase and as having been driven by the closure of the Iapetus Ocean . However, there is also an argument that it would more appropriate to regard it as a proto- Variscan orogeny. 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

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1128-468: 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

1222-538: A rate less than 40 mm/year. The highest known rate was over 200 mm/yr during the Miocene on the East Pacific Rise . In the 1960s, the past record of geomagnetic reversals of Earth's magnetic field was noticed by observing magnetic stripe "anomalies" on the ocean floor. This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred from data gathered with

1316-420: 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. In addition, the Southern Uplands accretionary wedge lacks evidence of the presence of a volcanic arc as usually found near subduction zones. This has led to

1410-752: A series of tectonically related events. In the Neoproterozoic most of the Earth's landmasses were united in the Rodinia supercontinent . The majority of its bulk consisted of the landmass of Gondwana . Near the end of the Neoproterozoic, during the breakup of this supercontinent, Laurentia and Baltica rifted from the western ( Amazonian craton ) and northern (African) margins of Gondwana respectively. Laurentia first drifted westward away from Gondwana and then migrated northward. This led to

1504-585: A stretching lineation perpendicular to the fold hinges. The Southern Belt and the rest of the Central belt underwent sinistral transpression . This reflects a Late Ordovician – Silurian change from an orthogonal to an oblique tectonic plate collision. In the Central Belt the cleavage transects folds in a clockwise sense and is accompanied by a sub-horizontal stretching lineation. In the Southern belt

1598-535: A transition from orthogonal compression to transpression during the later stages of Acadian deformation. This makes the island more similar to the Southern Uplands terrane of Scotland than the Lake District inlier in this respect. In Ireland the Acadian Orogeny affected the four main terranes of the island: Grampian, Midland Valley, Longford-Down and Leinster. Tectonic deformation was mild as

1692-641: 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

1786-598: Is a large basement massif. It is part of a magmatic belt which, starting from the Lake District , to the north of this massif, bears record of the subduction of part of the Tornquist Sea beneath Avalonia and its closure. The closure of the Rheic Ocean , which took place soon after, occurred through subduction along the southern margin of this massif. The Trans-European Suture Zone or Tornquist Zone

1880-401: Is added to each tectonic plate on either side of a mid-ocean ridge is the spreading half-rate and is equal to half of the spreading rate). Spreading rates determine if the ridge is fast, intermediate, or slow. As a general rule, fast ridges have spreading (opening) rates of more than 90 mm/year. Intermediate ridges have a spreading rate of 40–90 mm/year while slow spreading ridges have

1974-436: 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

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2068-549: Is assumed to be at a quasi- steady state , so that the temperature distribution is constant in time, i.e. T = T ( x , z ) . {\displaystyle T=T(x,z).} By calculating in the frame of reference of the moving lithosphere (velocity v ), which has spatial coordinate x ′ = x − v t , {\displaystyle x'=x-vt,} T = T ( x ′ , z , t ) . {\displaystyle T=T(x',z,t).} and

2162-799: Is buried under younger sediments in Scotland's Midland Valley. During the collision at the edge of the Laurentian continent, an ophiolite nappe was overthrusted, possibly preserved in Unst , in the Shetland Islands . Other ophiolite zones at the Highland Border and Ballantrae may be from the same event. The Grampian orogeny took place at close to the same time as the Taconic orogeny . In both cases, more extensive ophiolite nappes may have eroded away. The Grampian orogeny deformed

2256-589: Is composed mainly of the Manx Group and the Dalby Group which were deformed in a sinistral transpression zone during the sinistral, oblique closure of the Iapetus Ocean. Folds are transected clockwise by their cleavage , major strike-parallel sinistral faults and ductile shear zones thought to be related to this transpression. All primary folds have the same style and are associated with

2350-641: Is forced under either continental crust or oceanic crust. Today, the Atlantic basin is actively spreading at the Mid-Atlantic Ridge . Only a small portion of the oceanic crust produced in the Atlantic is subducted. However, the plates making up the Pacific Ocean are experiencing subduction along many of their boundaries which causes the volcanic activity in what has been termed the Ring of Fire of

2444-477: Is found within the crustal accretion zone. The differences in spreading rates affect not only the geometries of the ridges but also the geochemistry of the basalts that are produced. Since the new oceanic basins are shallower than the old oceanic basins, the total capacity of the world's ocean basins decreases during times of active sea floor spreading. During the opening of the Atlantic Ocean , sea level

2538-976: 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

2632-510: Is small compared to L 2 / A {\displaystyle L^{2}/A} , where L is the ocean width (from mid-ocean ridges to continental shelf ) and A is the age of the ocean basin. The effective thermal expansion coefficient α e f f {\displaystyle \alpha _{\mathrm {eff} }} is different from the usual thermal expansion coefficient α {\displaystyle \alpha } due to isostasic effect of

2726-597: 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

2820-480: Is the density of water. By substituting the parameters by their rough estimates: gives: where the height is in meters and time is in millions of years. To get the dependence on x , one must substitute t = x / v ~ Ax / L , where L is the distance between the ridge to the continental shelf (roughly half the ocean width), and A is the ocean basin age. Rather than height of the ocean floor h ( t ) {\displaystyle h(t)} above

2914-793: 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

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3008-676: 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,

3102-634: 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 , meaning that different parts of the mountain range formed at different times. The name "Caledonian" can therefore not be used for an absolute period of geological time, it applies only to

3196-881: 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. As 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

3290-675: 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 . Late Caledonian orogeny is another term used in reference to this phase. This phase involved a soft docking or soft collision rather an orogen -causing hard continental collision like the Eastern Avalonia docking with Baltica. This orogenic event has been interpreted as

3384-603: 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

3478-651: 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. According to van Roermund and Brueckner (2004), there

3572-718: 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

3666-424: The heat equation is: where κ {\displaystyle \kappa } is the thermal diffusivity of the mantle lithosphere. Since T depends on x' and t only through the combination x = x ′ + v t , {\displaystyle x=x'+vt,} : Thus: It is assumed that v {\displaystyle v} is large compared to other scales in

3760-520: The opening of the Iapetus Ocean between Laurentia, Baltica and Gondwana. Its initial opening phase was between the adjacent Laurentia and Baltica (to the West and East respectively) and caused the two to breakup c. 615 Ma or 590 Ma. Then the part between Laurentia and Gondwana (to the east), opened c. 550 Ma. Further spreading of the Iapetus Ocean also caused Laurentia and Baltica to move away from each other. Baltica drifted northward, too. This involved

3854-482: 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

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3948-512: 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. There are several ductile shear zones which run subparallel to the Manx Group northeast-oriented boundary faults which indicate predominantly sinistral shear and possibly

4042-500: The Hercynian orogeny. Seafloor spreading Seafloor spreading , or seafloor spread , is a process that occurs at mid-ocean ridges , where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Earlier theories by Alfred Wegener and Alexander du Toit of continental drift postulated that continents in motion "plowed" through the fixed and immovable seafloor. The idea that

4136-476: 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

4230-490: 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

4324-429: The Lakesman terrane and north Wales . Transpression resulted in regionally clockwise transecting sinistral transpressive cleavages which were superimposed on pre-existing structures. Folding northwest of the Iapetus Suture is weak and this northward weakening of deformation may indicate that it is linked with Rheic Ocean subduction rather than Iapetus Ocean closure. The Lake District in north-western England

4418-455: 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

4512-420: The North Pacific): Assuming isostatic equilibrium everywhere beneath the cooling plate yields a revised age depth relationship for older sea floor that is approximately correct for ages as young as 20 million years: Thus older seafloor deepens more slowly than younger and in fact can be assumed almost constant at ~6400 m depth. Parsons and Sclater concluded that some style of mantle convection must apply heat to

4606-413: The Ordovician; these continents were by then 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

4700-428: The Pacific Ocean. The Pacific is also home to one of the world's most active spreading centers (the East Pacific Rise) with spreading rates of up to 145 ± 4 mm/yr between the Pacific and Nazca plates . The Mid-Atlantic Ridge is a slow-spreading center, while the East Pacific Rise is an example of fast spreading. Spreading centers at slow and intermediate rates exhibit a rift valley while at fast rates an axial high

4794-425: 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)

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4888-460: 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

4982-400: The Southern Highland Group, dominated the section of the Laurentian coast that would be later separate to form Scotland. The Grampian orogeny stopped sedimentation. The discovery of volcanic arc rocks in western Ireland indicated a possible island arc collision with a subduction zone during the closure of the Iapetus Ocean to form Pangea . Some proposals in 1983 and 1984 suggested that the arc

5076-412: 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

5170-452: The Tinure Fault is the surface expression of the Iapetus Suture zone. 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

5264-463: 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

5358-533: The change in water column height above the lithosphere as it expands or retracts. Both coefficients are related by: where ρ ∼ 3.3   g ⋅ c m − 3 {\displaystyle \rho \sim 3.3\ \mathrm {g} \cdot \mathrm {cm} ^{-3}} is the rock density and ρ 0 = 1   g ⋅ c m − 3 {\displaystyle \rho _{0}=1\ \mathrm {g} \cdot \mathrm {cm} ^{-3}}

5452-466: The collision was strongly oblique with sinistral transpression and without substantial crustal thickening . Devonian to Carboniferous rocks rest unconformably on Cambrian to Silurian folded and cleaved rocks. There were igneous intrusions with plutons and batholiths . The terrane has three relief belts. The northern belt and the northernmost part of the Central Belt underwent pure shear deformation with an axial planar cleavage and

5546-419: 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

5640-413: The continent is completely severed, then a new ocean basin is created. The Red Sea has not yet completely split Arabia from Africa, but a similar feature can be found on the other side of Africa that has broken completely free. South America once fit into the area of the Niger Delta . The Niger River has formed in the failed rift arm of the triple junction . As new seafloor forms and spreads apart from

5734-399: The continental crust is attenuated as far as it will stretch. At this point basaltic oceanic crust and upper mantle lithosphere begins to form between the separating continental fragments. When one of the rifts opens into the existing ocean, the rift system is flooded with seawater and becomes a new sea. The Red Sea is an example of a new arm of the sea. The East African rift was thought to be

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5828-417: 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

5922-565: 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

6016-488: The elevated mid-ocean ridges a process called ridge push . At a spreading center, basaltic magma rises up the fractures and cools on the ocean floor to form new seabed . Hydrothermal vents are common at spreading centers. Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone. Spreading rate is the rate at which an ocean basin widens due to seafloor spreading. (The rate at which new oceanic lithosphere

6110-399: The elevation of the rift valley has been lowered to the point that the sea becomes stable. During this period of evaporation large evaporite deposits will be made in the rift valley. Later these deposits have the potential to become hydrocarbon seals and are of particular interest to petroleum geologists . Seafloor spreading can stop during the process, but if it continues to the point that

6204-442: 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. 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

6298-461: The intrusive rocks in the Trans-Suture Suite and in all the terranes in the region are similar in age and geochemistry. Thus, they argue that the common mechanism for the whole region involved an Iapetus Ocean slab which did not just break off. It also peeled back below the Iapetus Suture for c. 100 km to the SE below Avalonia. Thus they invoke a model of slab drop-off caused by lithospheric mantle delamination . The Lakesman terrane covers

6392-414: The mantle half-space model, the seabed height is determined by the oceanic lithosphere and mantle temperature, due to thermal expansion. The simple result is that the ridge height or ocean depth is proportional to the square root of its age. Oceanic lithosphere is continuously formed at a constant rate at the mid-ocean ridges . The source of the lithosphere has a half-plane shape ( x = 0, z < 0) and

6486-406: The mid-ocean ridge it slowly cools over time. Older seafloor is, therefore, colder than new seafloor, and older oceanic basins deeper than new oceanic basins due to isostasy. If the diameter of the earth remains relatively constant despite the production of new crust, a mechanism must exist by which crust is also destroyed. The destruction of oceanic crust occurs at subduction zones where oceanic crust

6580-405: The motion of the continents is linked to seafloor spreading by the theory of plate tectonics, which is driven by convection that includes the crust itself as well. The driver for seafloor spreading in plates with active margins is the weight of the cool, dense, subducting slabs that pull them along, or slab pull. The magmatism at the ridge is considered to be passive upwelling, which is caused by

6674-409: The north of England down to the Wensleydale in North Yorkshire and crosses the Irish Sea passing by the Island of Anglesey off Wales . Its continuation in eastern Ireland is the Leinster terrane. The combined terrane is termed Leinster-Lakesman terrane. It lies on the southern margin of the Iapetus Suture . It includes the Lake District and the Isle of Man . The Acadian Orogeny affected

6768-448: The northern margin of Gondwana ( Amazonia and northwest Africa) close to the original position of Baltica which had been to its north. Its rifting involved the opening and spreading of the Rheic Ocean to its south, which separated it from Gondwana. This rifting and opening were coeval with and may be related to subduction onset in the Iapetus Ocean. The drift of Avalonia was towards the positions where Baltica and Laurentia had been in

6862-454: The ocean surface): The depth predicted by the square root of seafloor age derived above is too deep for seafloor older than 80 million years. Depth is better explained by a cooling lithosphere plate model rather than the cooling mantle half-space. The plate has a constant temperature at its base and spreading edge. Analysis of depth versus age and depth versus square root of age data allowed Parsons and Sclater to estimate model parameters (for

6956-641: The opening of the Tornquist Ocean which separated it from the northern margin of Gondwana to the south. The onset of Baltica rifting and the Tornquist Ocean opening are difficult to date due to insufficient palaeomagnetic data but must have occurred in similar times as those of Laurentia and the Iapetus Ocean. Either in the Late Precambrian or Early Ordovician , the Avalonia microcontinent started to drift northwestward from

7050-740: 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

7144-418: The plates are sliding apart over the mantle upwelling in the process of ridge push. The depth of the seafloor (or the height of a location on a mid-ocean ridge above a base-level) is closely correlated with its age (age of the lithosphere where depth is measured). The age-depth relation can be modeled by the cooling of a lithosphere plate or mantle half-space in areas without significant subduction . In

7238-558: The plates being pulled apart under the weight of their own slabs. This can be thought of as analogous to a rug on a table with little friction: when part of the rug is off of the table, its weight pulls the rest of the rug down with it. However, the Mid-Atlantic ridge itself is not bordered by plates that are being pulled into subduction zones, except the minor subduction in the Lesser Antilles and Scotia Arc . In this case

7332-452: The problem; therefore the last term in the equation is neglected, giving a 1-dimensional diffusion equation: with the initial conditions The solution for z ≤ 0 {\displaystyle z\leq 0} is given by the error function : Due to the large velocity, the temperature dependence on the horizontal direction is negligible, and the height at time t (i.e. of sea floor of age t ) can be calculated by integrating

7426-565: 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

7520-414: The ridge crest by about five percent. This is thought due to temperature gradients in the asthenosphere from mantle plumes near the spreading center. Seafloor spreading occurs at spreading centers, distributed along the crests of mid-ocean ridges. Spreading centers end in transform faults or in overlapping spreading center offsets. A spreading center includes a seismically active plate boundary zone

7614-607: The rocks of the Grampian Terrane and the Dalradian Supergroup . Fold traces extend for hundreds of kilometers, with a complex formation of nappes and fold stacks. Caledonian orogeny 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

7708-528: 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

7802-554: The seafloor itself moves and also carries the continents with it as it spreads from a central rift axis was proposed by Harold Hammond Hess from Princeton University and Robert Dietz of the U.S. Naval Electronics Laboratory in San Diego in the 1960s. The phenomenon is known today as plate tectonics . In locations where two plates move apart, at mid-ocean ridges, new seafloor is continually formed during seafloor spreading. Seafloor spreading helps explain continental drift in

7896-409: 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

7990-402: The theory of plate tectonics . When oceanic plates diverge , tensional stress causes fractures to occur in the lithosphere . The motivating force for seafloor spreading ridges is tectonic plate slab pull at subduction zones , rather than magma pressure, although there is typically significant magma activity at spreading ridges. Plates that are not subducting are driven by gravity sliding off

8084-436: The thermal expansion over z : where α e f f {\displaystyle \alpha _{\mathrm {eff} }} is the effective volumetric thermal expansion coefficient, and h 0 is the mid-ocean ridge height (compared to some reference). The assumption that v is relatively large is equivalent to the assumption that the thermal diffusivity κ {\displaystyle \kappa }

8178-753: 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 than through an orogen-causing hard continental collision . This

8272-435: The world today. The separated margins of the continents evolve to form passive margins . Hess' theory was that new seafloor is formed when magma is forced upward toward the surface at a mid-ocean ridge. If spreading continues past the incipient stage described above, two of the rift arms will open while the third arm stops opening and becomes a 'failed rift' or aulacogen . As the two active rifts continue to open, eventually

8366-713: Was 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

8460-627: 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

8554-707: Was at the north-western margin of the English part of Eastern Avalonia which converged and collided with Scotland and was thus involved in the Acadian phase. Generally, Acadian deformation metamorphosed mudrocks throughout various geologic formations of the district into slates by creating slaty cleavages . The Early Palaeozoic rocks in the Isle of Man in the Irish Sea crop out close to or probably on Iapetus suture . The island lies immediately to its SE. The island

8648-459: Was not related to the Iapetus Ocean. It also has been argued that, although the Acadian orogeny in the British Isles involved the Iapetus Ocean closure, its driving force was actually a push from the south caused by the northward subduction of the Rheic Ocean which lied to the south of Avalonia and separated it from Gondwana . The closure of this ocean involved the (early) Eo-Variscan collision of Gondwana-related terranes in which Eastern Avalonia

8742-465: Was peripherally involved. 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

8836-593: Was so high that a Western Interior Seaway formed across North America from the Gulf of Mexico to the Arctic Ocean . At the Mid-Atlantic Ridge (and in other mid-ocean ridges), material from the upper mantle rises through the faults between oceanic plates to form new crust as the plates move away from each other, a phenomenon first observed as continental drift. When Alfred Wegener first presented

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