Editing Geological framework of the North-west Highlands - structural units - the Moine Supergroup

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In east Sutherland there is evidence for metamorphism at ''c''.470 Ma (Kinny ''et al''., 1999), but Caledonian regional metamorphism in the Morar Group in the area of this guide has been shown to have occurred at 435– 420 Ma (Kinny ''et al''., 2003) and to be approximately coeval with movement on the Moine Thrust.
 
In east Sutherland there is evidence for metamorphism at ''c''.470 Ma (Kinny ''et al''., 1999), but Caledonian regional metamorphism in the Morar Group in the area of this guide has been shown to have occurred at 435– 420 Ma (Kinny ''et al''., 2003) and to be approximately coeval with movement on the Moine Thrust.
  
== The Moine Thrust Zone  ==
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== (3) The Moine Thrust Zone  ==
 
The Moine Thrust Zone is a structurally complicated belt that stretches from Loch Eriboll in the north to the Isle of Skye in the south (Excursions 5 to 10 and 14 to 15). It is defined as the zone lying below the Moine Thrust (which carries the rocks of the Moine Supergroup, with local basement inliers), but above the Sole Thrust which separates the thrust zone from the undisturbed foreland. The Moine Thrust is everywhere a distinctive structure, but the Sole Thrust is rather variable, and locally includes structures with very little displacement. The rocks within the thrust zone are derived from the foreland, but show varying states of defor-mation. In some cases, the strain and accompanying recrystallization has been so intense as to make direct correlation with specific foreland units difficult.
 
The Moine Thrust Zone is a structurally complicated belt that stretches from Loch Eriboll in the north to the Isle of Skye in the south (Excursions 5 to 10 and 14 to 15). It is defined as the zone lying below the Moine Thrust (which carries the rocks of the Moine Supergroup, with local basement inliers), but above the Sole Thrust which separates the thrust zone from the undisturbed foreland. The Moine Thrust is everywhere a distinctive structure, but the Sole Thrust is rather variable, and locally includes structures with very little displacement. The rocks within the thrust zone are derived from the foreland, but show varying states of defor-mation. In some cases, the strain and accompanying recrystallization has been so intense as to make direct correlation with specific foreland units difficult.
  
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Elliott and Johnson (1980) presented a ‘piggy-back’, foreland-propa-gating model for the Moine Thrust Zone; that is, the upper thrusts moved first, and these thrust sheets were carried further by subsequent movement along lower thrusts. In this model, the earliest movement in the Moine Thrust Zone was along the Moine Thrust itself – although it should be noted that a number of important earlier thrusts (including the Naver and Sgurr Beag thrusts) occur within the rocks of the Moine Supergroup further to the east. Displacement on the thrusts was broadly towards the WNW. The thrust sheet carried by the Moine Thrust was large; on the basis of current exposure on the mainland, it was over ''c''.200km in strike length and ''c''.10–20km in thickness. Furthermore, the presence of a klippe (outlier) of Moine rocks at Faraid Head (Excursion 14) shows that the Moine sheet extended westwards over the foreland for a distance of at least 10km beyond its present outcrop.
 
Elliott and Johnson (1980) presented a ‘piggy-back’, foreland-propa-gating model for the Moine Thrust Zone; that is, the upper thrusts moved first, and these thrust sheets were carried further by subsequent movement along lower thrusts. In this model, the earliest movement in the Moine Thrust Zone was along the Moine Thrust itself – although it should be noted that a number of important earlier thrusts (including the Naver and Sgurr Beag thrusts) occur within the rocks of the Moine Supergroup further to the east. Displacement on the thrusts was broadly towards the WNW. The thrust sheet carried by the Moine Thrust was large; on the basis of current exposure on the mainland, it was over ''c''.200km in strike length and ''c''.10–20km in thickness. Furthermore, the presence of a klippe (outlier) of Moine rocks at Faraid Head (Excursion 14) shows that the Moine sheet extended westwards over the foreland for a distance of at least 10km beyond its present outcrop.
  
Although elegant, the simple ‘piggy-back’ model does not account for (a) the dual nature of the Moine Thrust, which is an early ductile shear zone in some places and a late brittle fault in others; and (b) the apparent truncation of lower faults by higher ones at some localities (e.g. in the klippen to the east of Knockan, south Assynt, Excursion 6). It is clear that the Moine Thrust Zone represents a rather more complex system. A variety of models have been proposed to explain some of these features, including late-stage extensional faulting (particularly in southern Assynt; Coward, 1982, 1983); synchronous movement along imbricate thrusts and roof thrusts (Butler, 2004); and extensional collapse episodes during the largely compressional evolution of the thrust wedge (Holdsworth ''et al''., 2006). Recent work has shown that detailed mapping of specific localities is essen-tial to understand the different processes that have operated in the Moine Thrust Zone (Butler, 2004; Krabbendam and Leslie, 2004; Holdsworth ''et al''., 2006).
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Although elegant, the simple ‘piggy-back’ model does not account for
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(a) the dual nature of the Moine Thrust, which is an early ductile shear zone in some places and a late brittle fault in others; and (b) the apparent truncation of lower faults by higher ones at some localities (e.g. in the klippen to the east of Knockan, south Assynt, Excursion 6). It is clear that the Moine Thrust Zone represents a rather more complex system. A variety of models have been proposed to explain some of these features, including late-stage extensional faulting (particularly in southern Assynt; Coward, 1982, 1983); synchronous movement along imbricate thrusts and roof thrusts (Butler, 2004); and extensional collapse episodes during the largely compressional evolution of the thrust wedge (Holdsworth ''et al''., 2006). Recent work has shown that detailed mapping of specific localities is essen-tial to understand the different processes that have operated in the Moine Thrust Zone (Butler, 2004; Krabbendam and Leslie, 2004; Holdsworth ''et al''., 2006).
  
 
The Moine Thrust has traditionally been defined as the thrust that forms the base of the Moine Supergroup (and its Lewisianoid basement, where exposed), but this structure varies in character along its length. In places it is a ductile shear zone, represented by a thick pile of mylonites, as seen at the Stack of Glencoul and at Loch Eriboll (Excursions 11 and 15); elsewhere (e.g. at Knockan Crag, Excursion 6), it is a polyphase brittle-ductile structure, the mylonites being brecciated by late, lower-tempera-ture deformation (Coward, 1983). Mylonites are fine-grained, strongly layered rocks, formed by dynamic recrystallisation during ductile defor-mation (e.g. White, 1980), and they were first defined on the basis of examples from the Moine Thrust Zone (Lapworth, 1885).
 
The Moine Thrust has traditionally been defined as the thrust that forms the base of the Moine Supergroup (and its Lewisianoid basement, where exposed), but this structure varies in character along its length. In places it is a ductile shear zone, represented by a thick pile of mylonites, as seen at the Stack of Glencoul and at Loch Eriboll (Excursions 11 and 15); elsewhere (e.g. at Knockan Crag, Excursion 6), it is a polyphase brittle-ductile structure, the mylonites being brecciated by late, lower-tempera-ture deformation (Coward, 1983). Mylonites are fine-grained, strongly layered rocks, formed by dynamic recrystallisation during ductile defor-mation (e.g. White, 1980), and they were first defined on the basis of examples from the Moine Thrust Zone (Lapworth, 1885).
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Direct dating of micas from mylonites along the Moine Thrust, using Rb-Sr, K-Ar, and Ar-Ar techniques, has obtained a broader spread of results. Mylonitisation of the Moine rocks was accompanied by green-schist-facies metamorphism, at a temperature of approximately 400˚C (Freeman ''et al''., 1998). This ductile deformation continued until at least 430 Ma, but locally appears to extend until 408 Ma (Kelley, 1988; Freeman ''et al''., 1998; Friend ''et al''., 2000; Dallmeyer ''et al''., 2001).
 
Direct dating of micas from mylonites along the Moine Thrust, using Rb-Sr, K-Ar, and Ar-Ar techniques, has obtained a broader spread of results. Mylonitisation of the Moine rocks was accompanied by green-schist-facies metamorphism, at a temperature of approximately 400˚C (Freeman ''et al''., 1998). This ductile deformation continued until at least 430 Ma, but locally appears to extend until 408 Ma (Kelley, 1988; Freeman ''et al''., 1998; Friend ''et al''., 2000; Dallmeyer ''et al''., 2001).
  
In general terms, the majority of displacement within the Moine Thrust System appears to have been confined to the interval from the middle to late Llandovery (''c''.435–428 Ma), with some displacement persisting into the early Devonian, and this timing is remarkably synchronous from Scot-land to eastern North Greenland. Although the general pattern is clear, there continues to be considerable discussion about detailed relationships within this well-preserved ancient mountain belt.
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In general terms, the majority of displacement within the Moine Thrust System appears to have been confined to the interval from the middle to late Llandovery (''c''.435–428 Ma), with some displacement persisting into the early Devonian, and this timing is remarkably synchronous from Scot-land to eastern North Greenland. Although the general pattern is clear, there continues to be considerable discussion about detailed relationships within this well-preserved ancient mountain belt.  
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==[[References - Geological excursion guide to the North-west Highlands of Scotland|References]] ==
 
==[[References - Geological excursion guide to the North-west Highlands of Scotland|References]] ==
 
{{EGwalks}}
 
{{EGwalks}}
 
[[Category:2. Northern Highlands]]
 
[[Category:2. Northern Highlands]]

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