Superposed deformations and their hybrid effects: the Rhoscolyn Anticline unravelled

Treagus, Susan H.; Treagus, J. E.; Droop, G. T. R.

doi:10.1144/0016-764901-166

Cited by 9 publications

(21 citation statements)

References 25 publications

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“…1) where it resulted in metamorphism and fabric development in the Monian Supergroup (e.g. Treagus et al, 2003Treagus et al, , 2013. Unconformities occur during this interval in the remainder of the Welsh Basin.…”

Section: Introductionmentioning

confidence: 99%

Peri-Gondwanan terrane interactions recorded in the Cambrian–Ordovician detrital zircon geochronology of North Wales

et al. 2015

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Section: Introductionmentioning

confidence: 99%

Peri-Gondwanan terrane interactions recorded in the Cambrian–Ordovician detrital zircon geochronology of North Wales

et al. 2015

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“…Here a weakly deformed Old Red Sandstone succession, the Peel Sandstone Group, with Early Devonian palaeomagnetic declinations, microflora and trace fossils (Crowley et al 2009), is faulted against strongly deformed early Ordovician and mid-Silurian (Wenlock) rocks (Woodcock et al 1999). The inference is that these thrusts are Acadian, but that the D1 and D2 deformations in the Isle of Man are late Silurian (Ludlow or Pridoli) in age, possibly comparable with the D1 and D2 deformations on Holy Island, Anglesey (Treagus, Treagus & Droop, 2003). The inference is that these thrusts are Acadian, but that the D1 and D2 deformations in the Isle of Man are late Silurian (Ludlow or Pridoli) in age, possibly comparable with the D1 and D2 deformations on Holy Island, Anglesey (Treagus, Treagus & Droop, 2003).…”

Section: Discussion: Significance Of the Lligwy Structuresmentioning

confidence: 99%

Major folds affecting the Lower Old Red Sandstone Group at Lligwy, Anglesey, North Wales, and their regional significance

Treagus

Woodcock

2011

Geol. Mag.

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The Old Red Sandstone on Anglesey, North Wales, presumed Lower Devonian in age, is folded and locally cleaved, but the intensity of this deformation has previously been understated. We describe two S-verging anticline-syncline pairs, one with a strongly overturned middle limb, their associated minor folds and an axial-planar cleavage. The intensity of the deformation calls into question a proposed link to Variscan fault displacements, and the angular unconformity below the Old Red Sandstone precludes the deformation being part of a continuous 'late Caledonian' phase. We consider this deformation of the Old Red Sandstone to be mid-Devonian, correlating with the Acadian phase in mainland Wales. It is predated by a Silurian shortening deformation on Anglesey that is possibly related to the closure of the Iapetus Ocean, absent in mainland Wales and the Lake District, but perhaps preserved also on the Isle of Man.

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“…In detail, these consist from oldest to youngest of (e.g. Treagus et al 2003 and2013): South Stack Formation (alternating centimetre-metre scale pelites, semi-pelites and psammites), Holyhead Quartzite Formation (typically poorly bedded ortho-quartzites) and Rhoscolyn Formation (alternating centimetre-metre scale pelites, semi-pelites and psammites) of the Holy Island Group and the New Harbour Group (mainly finely laminated green semi-pelites). A (deep-water) turbidite interpretation has been suggested for the original depositional environment of the Rhoscolyn Formation and New Harbour Group, although sedimentary structures indicative of shallower marine environments are present in the South Stack Formation (e.g.…”

Section: Geological Settingmentioning

confidence: 99%

Syntectonic quartz vein evolution during progressive deformation

Lloyd

2018

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Two models to explain the progressive deformation of syntectonic quartz veins are derived from conventional theories for simple and pure shears. The simple shear model is based on reorientation and changes in length of linear vein elements and predicts initial orientations of veins for imposed shear strains, elongations and strain ratios. The pure shear model considers changes in length of lines variably oriented relative to the maximum compression direction and yields estimates of elongation strains and strain ratios. Expectations of both models are different, as illustrated by analysis of quartz veins from the Rhoscolyn Anticline, Anglesey, NW Wales. The simple shear model recognises three distinct initial orientations, which predict different strains across the fold; the pure shear model suggests veins were initially subparallel to the principal compression direction and predicts effectively constant strains across the fold. In addition, both models predict different patterns of fold vergence: for simple shear, vergence depends on magnitude and direction of shearing and may exhibit complex patterns; for pure shear, vergence patterns are predicted to be essentially constant. In general, the predictions of either model are critically dependent on the origin of the veins, particularly relative to the formation of the Rhoscolyn Anticline. The publication of 'Folding and Fracturing of Rocks' (Ramsay 1967) represented a step change in Structural Geology. It cemented the combination of rigorous field investigations augmented by mechanical and numerical analyses in the structural geological psyche and prepared the ground for ever more sophisticated investigations that persist to this day. However, it must be recognised that historically, the book represented a continuous progression of ideas that had developed albeit slowly over more than a century. Although folds and folding are central themes of Ramsay's book (Chapters 7-10), fracturing is actually of relatively minor consequence (i.e. there are no chapters dedicated to it specifically); in essence, the presentation is predicated on the significance of stress and particularly strain during geological deformation (Chapters 1-6). This contribution follows the philosophy inherent in 'Folding and Fracturing of Rocks' to interpret the evolution of syntectonic quartz veins during progressive deformation. The veins occur within (semi-)pelitic units folded by the Rhoscolyn Anticline, Anglesey, NW Wales (Fig. 1), a well-known location for both structural geology research and teaching. It begins with a brief description of the general geology of Rhoscolyn, including the recognition of various models to explain the evolution of the kilometre scale Rhoscolyn Anticline (e.g.

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Superposed deformations and their hybrid effects: the Rhoscolyn Anticline unravelled

Cited by 9 publications

References 25 publications

Peri-Gondwanan terrane interactions recorded in the Cambrian–Ordovician detrital zircon geochronology of North Wales

Peri-Gondwanan terrane interactions recorded in the Cambrian–Ordovician detrital zircon geochronology of North Wales

Major folds affecting the Lower Old Red Sandstone Group at Lligwy, Anglesey, North Wales, and their regional significance

Syntectonic quartz vein evolution during progressive deformation

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