Editing Acadian Orogeny, Devonian, Northern England

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[[File:P916065.jpg|thumbnail|Pattern of faults cutting the Windermere Supergroup in the southern Lake District (after Soper and Woodcock, 2003). P916065.]]
 
[[File:P916065.jpg|thumbnail|Pattern of faults cutting the Windermere Supergroup in the southern Lake District (after Soper and Woodcock, 2003). P916065.]]
  
Stratigraphically, the Acadian Orogeny is constrained as post-Pridoli, pre-Late Devonian. Cleavage development overlapped the generation of metamorphic aureoles around the Devonian granites at Skiddaw and Shap, and these have been dated radiometrically at about 399 and 404<br>Ma respectively. Further south, in the Ribblesdale (Craven) inlier, metamorphic white mica developed along the cleavage in a Ludlow bentonite has given radiometric ages in the 397–418<br>Ma range. All the evidence points to the Acadian Orogeny being an Early to Mid Devonian event, most probably of Emsian to Eifelian age.
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Stratigraphically, the Acadian Orogeny is constrained as post-Pridoli, pre-Late Devonian. Cleavage development overlapped the generation of metamorphic aureoles around the Devonian granites at Skiddaw and Shap, and these have been dated radiometrically at about 399 and 404 Ma respectively. Further south, in the Ribblesdale (Craven) inlier, metamorphic white mica developed along the cleavage in a Ludlow bentonite has given radiometric ages in the 397–418 Ma range. All the evidence points to the Acadian Orogeny being an Early to Mid Devonian event, most probably of Emsian to Eifelian age.
  
 
The Acadian tectonic deformation was superimposed on pre-existing, Late Ordovician and Silurian disruption of the strata in both the Skiddaw and Manx groups. Not surprisingly, the structures of both groups are highly complex, and their apparently polyphase character led initially to proposals for a protracted, polyphase deformation history. The recognition of the earliest deformations as soft-sediment, slump-related phenomena, and definitive confirmation that the first, regional tectonic cleavage was indeed Acadian, have largely resolved the regional interpretational controversies. The possibility of a diachronous cleavage being developed in a southward prograding thrust belt, in continuity with the Southern Uplands accretionary complex and linked to the Windermere Supergroup foreland basin, cannot be sustained. However, at a more local scale, the detailed structures of the Manx and Skiddaw groups still remain open to different interpretations.
 
The Acadian tectonic deformation was superimposed on pre-existing, Late Ordovician and Silurian disruption of the strata in both the Skiddaw and Manx groups. Not surprisingly, the structures of both groups are highly complex, and their apparently polyphase character led initially to proposals for a protracted, polyphase deformation history. The recognition of the earliest deformations as soft-sediment, slump-related phenomena, and definitive confirmation that the first, regional tectonic cleavage was indeed Acadian, have largely resolved the regional interpretational controversies. The possibility of a diachronous cleavage being developed in a southward prograding thrust belt, in continuity with the Southern Uplands accretionary complex and linked to the Windermere Supergroup foreland basin, cannot be sustained. However, at a more local scale, the detailed structures of the Manx and Skiddaw groups still remain open to different interpretations.
  
The Borrowdale and Eycott Volcanic groups were affected by synvolcanic faulting associated with piecemeal caldera collapse. The most important Acadian structures superimposed on the previously block-faulted volcanic tracts are regional monoclines facing north in the Eycott Volcanic Group and south in the Borrowdale Volcanic Group. These structures define the regional Lake District ‘anticline’ of which the Skiddaw Group forms the core, and are asymmetrically disposed above the margins of the subvolcanic batholith [[Media:P916060.jpg|(P916060)]]. The northern ‘monocline’, affecting the Eycott Volcanic Group may well have been initiated much earlier, with geophysical evidence and age relationships suggesting that at least some of the volcanic rocks were rotated to a near-vertical attitude soon after their eruption. In this case, Acadian effects may simply have been to modify the existing structure. A better-defined and more extensive Acadian structure is the southern, Westmorland Monocline; it affects the southern margin of the Borrowdale Volcanic Group outcrop and the overlying Windermere Supergroup strata, and lies above the southern margin of the batholith. Cleavage is strongly developed in the volcanic rocks close to the monocline hinge zone, forming the Tilberthwaite slate belt in which the ‘green slate’ industry is concentrated. Another zone of enhanced cleavage, the Honister slate belt, lies further north and may coincide with a step, downwards to the north, in the top surface of the batholith. Overall, the style of deformation is strongly influenced by the disposition of the more rigid rocks in the volcanic succession and by the protective effects of the underpinning batholith.
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The Borrowdale and Eycott Volcanic groups were affected by synvolcanic faulting associated with piecemeal caldera collapse. The most important Acadian structures superimposed on the previously block-faulted volcanic tracts are regional monoclines facing north in the Eycott Volcanic Group and south in the Borrowdale Volcanic Group. These structures define the regional Lake District ‘anticline’ of which the Skiddaw Group forms the core, and are asymmetrically disposed above the margins of the subvolcanic batholith [[Media:P916060.jpg|(P916060)]]. The northern ‘monocline’, affecting the Eycott Volcanic Group may well have been initiated much earlier, with geophysical evidence and age relationships suggesting that at least some of the volcanic rocks were rotated to a near-vertical attitude soon after their eruption. In this case, Acadian effects may simply have been to modify the existing structure. A better-defined and more extensive Acadian structure is the southern, Westmorland Monocline; it affects the southern margin of the Borrowdale Volcanic Group outcrop and the overlying Windermere Supergroup strata, and lies above the southern margin of the batholith. Cleavage is strongly developed in the volcanic rocks close to the monocline hinge zone, forming the Tilberthwaite slate belt in which the ‘green slate’ industry is concentrated (see Chapter 12). Another zone of enhanced cleavage, the Honister slate belt, lies further north and may coincide with a step, downwards to the north, in the top surface of the batholith. Overall, the style of deformation is strongly influenced by the disposition of the more rigid rocks in the volcanic succession and by the protective effects of the underpinning batholith.
  
 
Across the southern Lake District the steep limb of the Westmorland Monocline forms the northern limb of the Bannisdale Syncline, the large-scale, regional structure that is the primary influence on disposition of Windermere Supergroup strata [[Media:P916060.jpg|(P916060)]]. A single slaty cleavage is ubiquitous and congruous to the folding, though the relationship is commonly transecting (albeit by only a small amount) rather than axial planar.
 
Across the southern Lake District the steep limb of the Westmorland Monocline forms the northern limb of the Bannisdale Syncline, the large-scale, regional structure that is the primary influence on disposition of Windermere Supergroup strata [[Media:P916060.jpg|(P916060)]]. A single slaty cleavage is ubiquitous and congruous to the folding, though the relationship is commonly transecting (albeit by only a small amount) rather than axial planar.
  
 
== Skiddaw Group structure ==
 
== Skiddaw Group structure ==
The dominant structural feature is a series of polyphase fault-and-thrust zones that trend approximately east-north-east across the main Skiddaw Group outcrop, roughly parallel to the regional strike of bedding and cleavage. Of these, the Causey Pike Fault has the longest demonstrable history and a geographical extent that spans the main Lake District inlier and extends as far as the Cross Fell inlier 30&nbsp;km to the east. It has a profound stratigraphical effect (equivalent to at least 2&nbsp;km vertical downthrow to the south), separating discrete parts of the Skiddaw Group succession with marked differences in sedimentary provenance and slump-fold orientation; hence it may well have had a synsedimentary role, partitioning the original depositional basin. Some sinistral strike-slip movement seems likely during the Early Devonian transpressive phase, whilst south-directed thrust movement cutting across the approximately 400&nbsp;Ma Crummock Water thermal aureole is likely to be an Acadian (sensu stricto) effect.
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The dominant structural feature is a series of polyphase fault-and-thrust zones that trend approximately east-north-east across the main Skiddaw Group outcrop, roughly parallel to the regional strike of bedding and cleavage. Of these, the Causey Pike Fault has the longest demonstrable history and a geographical extent that spans the main Lake District inlier and extends as far as the Cross Fell inlier 30 km to the east. It has a profound stratigraphical effect (equivalent to at least 2 km vertical downthrow to the south), separating discrete parts of the Skiddaw Group succession with marked differences in sedimentary provenance and slump-fold orientation; hence it may well have had a synsedimentary role, partitioning the original depositional basin. Some sinistral strike-slip movement seems likely during the Early Devonian transpressive phase, whilst south-directed thrust movement cutting across the approximately 400 Ma Crummock Water thermal aureole is likely to be an Acadian (sensu stricto) effect.
  
 
Farther north, the Gasgale, Loweswater and Watch Hill thrust faults were all most probably initiated as the slide planes of large-scale, synsedimentary slumps. Thereafter, subsequent reactivation during Acadian deformation has created a situation such that, in the hanging walls, minor, upright folds have an axial-plane cleavage that progressively swings into an alignment parallel with the thrust plane, but is then crenulated by a uniformly shallow-dipping cleavage. The Watch Hill thrust is a particularly complex structure and is most probably a compound plexus of faults having different attitudes, movement senses, and ages.
 
Farther north, the Gasgale, Loweswater and Watch Hill thrust faults were all most probably initiated as the slide planes of large-scale, synsedimentary slumps. Thereafter, subsequent reactivation during Acadian deformation has created a situation such that, in the hanging walls, minor, upright folds have an axial-plane cleavage that progressively swings into an alignment parallel with the thrust plane, but is then crenulated by a uniformly shallow-dipping cleavage. The Watch Hill thrust is a particularly complex structure and is most probably a compound plexus of faults having different attitudes, movement senses, and ages.
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The interpretation of thrust geometry in the Skiddaw Group is important in establishing the overall convergence regime. The sequence of thrusts seen at outcrop was previously modelled as an imbricate network extending southwards from the leading edge of the Southern Uplands thrust belt [[Media:P916061.jpg|(P916061)]] but, in order to accommodate the apparently steep dips that some of these structures show at outcrop, subsequent reorientation was required. It could only be achieved by compression buttressed against the subvolcanic Lake District Batholith during the propagation of a deep thrust dislocation at the base of the batholith. This in turn was thought to have driven the Westmorland Monocline into mountain front proportions, with commensurate acceleration of subsidence in the Windermere Supergroup foreland basin phase. Such reorientation would have been essentially pre-Acadian and is effectively ruled out by the relationship of the major fault structures to the Acadian cleavage.
 
The interpretation of thrust geometry in the Skiddaw Group is important in establishing the overall convergence regime. The sequence of thrusts seen at outcrop was previously modelled as an imbricate network extending southwards from the leading edge of the Southern Uplands thrust belt [[Media:P916061.jpg|(P916061)]] but, in order to accommodate the apparently steep dips that some of these structures show at outcrop, subsequent reorientation was required. It could only be achieved by compression buttressed against the subvolcanic Lake District Batholith during the propagation of a deep thrust dislocation at the base of the batholith. This in turn was thought to have driven the Westmorland Monocline into mountain front proportions, with commensurate acceleration of subsidence in the Windermere Supergroup foreland basin phase. Such reorientation would have been essentially pre-Acadian and is effectively ruled out by the relationship of the major fault structures to the Acadian cleavage.
  
A more likely, alternative view of the Skiddaw Group thrusts sees them as early, synsedimentary slump planes caught up in wholly Acadian reactivation entirely unrelated to the Southern Uplands thrust belt. In this interpretation the dominant Acadian components are part of a compressional flower structure generated along the northern margin of the batholith [[Media:P916062.jpg|(P916062)]]. If this view is accepted, it follows that a considerable thickness of Skiddaw Group strata probably exists below the oldest exposed level; the tentative recognition of a single Cambrian-aged locality may be supportive. It also follows that the southward propagation of thrusts at the leading edge of the Southern Uplands thrust belt must have largely taken place at a structural level higher than that currently exposed.
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A more likely, alternative view of the Skiddaw Group thrusts sees them as early, synsedimentary slump planes caught up in wholly Acadian reactivation entirely unrelated to the Southern Uplands thrust belt. In this interpretation the dominant Acadian components are part of a compressional flower structure generated along the northern margin of the batholith [[Media:P916062.jpg|(P916062)]]. If this view is accepted, it follows that a considerable thickness of Skiddaw Group strata probably exists below the oldest exposed level; the tentative recognition of a single Cambrian-aged locality (see discussion in Chapter 2) may be supportive. It also follows that the southward propagation of thrusts at the leading edge of the Southern Uplands thrust belt must have largely taken place at a structural level higher than that currently exposed.
  
 
== Manx Group structure ==
 
== Manx Group structure ==
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Several ductile shear zones run subparallel to the north-east-orientated tract boundary faults within the Manx Group and are generally marked by narrow zones of intense fabric development with indications of predominantly sininstral shear. They would seem to indicate a transition from orthogonal compression to transpression during the later stages of Acadian deformation. In this respect the Isle of Man has more in common with the Scottish Southern Uplands terrane than with the Lake District inlier.
 
Several ductile shear zones run subparallel to the north-east-orientated tract boundary faults within the Manx Group and are generally marked by narrow zones of intense fabric development with indications of predominantly sininstral shear. They would seem to indicate a transition from orthogonal compression to transpression during the later stages of Acadian deformation. In this respect the Isle of Man has more in common with the Scottish Southern Uplands terrane than with the Lake District inlier.
  
Geophysical data suggest a zone of complex structure in the mid crust, underlying and trending parallel to the zone of faulting in the north-western part of the Isle of Man. This, together with the apparent north-westward increase in the structural complexity of the Manx Group, may be related to the proximity of the Isle of Man to the Iapetus Suture. Deep seismic profiles across the suture show a number of north-dipping reflectors in its footwall and these might link with the tract boundary faults in the Isle of Man — and also, by inference, with the major fault structures in the Skiddaw Group.
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Geophysical data suggest a zone of complex structure in the mid crust, underlying and trending parallel to the zone of faulting in the north-western part of the Isle of Man (see Chapter 2). This, together with the apparent north-westward increase in the structural complexity of the Manx Group, may be related to the proximity of the Isle of Man to the Iapetus Suture. Deep seismic profiles across the suture show a number of north-dipping reflectors in its footwall and these might link with the tract boundary faults in the Isle of Man — and also, by inference, with the major fault structures in the Skiddaw Group.
  
 
== Borrowdale Volcanic Group structure ==
 
== Borrowdale Volcanic Group structure ==

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