Basement fractures in North Wales: their recognition and control on Caledonian deformation

Wilkinson, Ian P.; Smith, Martin J.

doi:10.1017/s0016756800010244

Cited by 14 publications

(8 citation statements)

References 27 publications

(22 reference statements)

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“…The coincidence of the magnetic Euler anomaly with the surface disturbance, however, implies that more than one structure is involved. The continuity and narrowness of the strong gravity and aeromagnetic Euler anomalies, and their distribution across Wales, the Borderlands and off-shore, strongly suggest that they define a network of deep faults supporting the views of other researchers (for example : Jones, 1938;Shackleton, 1954;James & James, 1969;Woodcock, 1984;Kokelaar et al 1984;Smith, 1987;Fitches & Campbell, 1987;Wilkinson & Smith, 1988;Kokelaar, 1988;Woodcock & Gibbons, 1988;Howells, Reedman & Campbell, 1991). The majority of the Euler solution depths are less than 4 km which indicates that the anomalies generated in the cover sequences are overprinting any deeper solutions from the basement.…”

Section: Discussionsupporting

confidence: 72%

“…It has been suggested for many years that deep structures in the Precambrian basement have strongly influenced the framework, sedimentation, volcanism and tectonics of the Lower Palaeozoic Basin of Wales (for example ;Jones, 1938;Shackleton, 1954;James & James, 1969;Woodcock, 1984;Kokelaar et al 1984;Smith, 1987;Fitches & Campbell, 1987;Wilkinson & Smith, 1988;Kokelaar, 1988;Woodcock & Gibbons, 1988;Howells, Reedman & Campbell, 1991). The existence of these structures has been inferred mainly from the geological modelling of the exposed rocks except in the few cases where seismic data or major changes in gravity and magnetic parameters have allowed more positive identification.…”

Section: Introductionmentioning

confidence: 99%

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Interpretation of the regional gravity and magnetic surveys of Wales, using shaded relief and Euler deconvolution techniques

et al. 1992

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Regional gravity and aeromagnetic data of Wales have been processed using a variety of techniques. Image processing has greatly assisted qualitative interpretation, whilst automated procedures have provided additional quantitative information. The shaded relief images emphasize gradients in the potential fields, and are useful for displaying strong linear features. The Euler deconvolution method produces plotted solution maps, which define the position of the source of the gravity and magnetic anomalies. Euler solution maps of Wales and the adjacent continental shelf are presented for the first time. These maps are interpreted in relation to the known geology, with special emphasis on the Lower Palaeozoic Welsh Basin. It is proposed that the Euler solutions define a network of fault-bounded blocks within the Precambrian basement.

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Section: Discussionsupporting

confidence: 72%

Section: Introductionmentioning

confidence: 99%

Interpretation of the regional gravity and magnetic surveys of Wales, using shaded relief and Euler deconvolution techniques

et al. 1992

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“…However, Jackson & Mulholland (1993) and Needham & Morgan (1997) argue that the N-S fault pattern is mismatched to primary Permo-Triassic extension directions, and is instead inherited from the pre-Carboniferous subcrop. Faults with this N-S strike certainly cut Lower Palaeozoic rocks in North Wales, where Wilkinson & Smith (1988) argue their pre-Acadian ancestry. South of the Bala Fault, in Mid-Wales, NNE-striking normal faults had been operating as turbidite-basin bounding faults in sinistral transtension for much of Silurian time (e.g.…”

Section: D Early Devonian Extensional Faults?mentioning

confidence: 95%

The lost Lower Old Red Sandstone of England and Wales: a record of post-Iapetan flexure or Early Devonian transtension?

Soper

Woodcock

2003

Geol. Mag.

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Illite crystallinity data from the Silurian slate belts of England and Wales indicate anchizone to low epizone metamorphism during the Acadian deformation in late Early Devonian time. This metamorphic grade implies a substantial overburden, now eroded, of Lower Devonian non-marine sediments of the Old Red Sandstone (ORS) magnafacies. A minimum 3.5 km pre-tectonic thickness of ‘lost’ ORS is estimated in the southern Lake District and comparable thicknesses in North Wales and East Anglia. Tectonically driven subsidence of the underlying Avalonian crust is required to accommodate such thicknesses of non-marine sediment. One proposed mechanism is flexure of the Avalonian footwall during convergence that continued from Iapetus closure in the Silurian until Acadian cleavage formation in the Emsian. The evidence for this model in the critical area of northwest England is reviewed and found to be unconvincing. An alternative model is developed following a recent suggestion that the Early Devonian was a period not of continued convergence but of orogen-wide sinistral transtension. Transtensional accommodation of the lost ORS is evidenced by Early Devonian extensional faults, by synchronous lamprophyric magmatism, and by compatibility with previously diagnosed sediment provenance patterns. A summary of Siluro-Devonian tectonostratigraphy for Britain south of the Highland Border emphasizes that, unlike the Scottish Highlands, this area was not affected by the Scandian Orogeny, but was by the Acadian. An important period of sinistral transtension in the Early Devonian (c.420–400 Ma) was common to both regions. This was a time of high heat flow, lamprophyric and more evolved magmatism, and major southward sediment transport, involving mainly recycled metamorphic detritus from the Highlands and from contemporaneous volcanicity. Old Red Sandstone, deposited in coalescing transtensional basins over much of Britain from the Midland Valley to the Welsh Borders, was largely removed and recycled southward during Acadian inversion.

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“…Monian basement fault control over the development of narrow, NE-SW oriented basins is a characteristic of the Ordovician sequences in Anglesey (Bates, 1974), and in this respect the southwest Llyn Arenig topography appears to have been very similar to the west of the Daron Fault. Although sedimentation within the main area of the Welsh Basin to the southeast of Llyn was less closely confined within such narrow basins, the reactivation of important basement structures does appear to have influenced Acadian deformation patterns (Wilkinson & Smith, 1988).…”

Section: Basement Control On Cover Deformationmentioning

confidence: 96%

Basement–cover relationships around Aberdaron, Wales, U.K.: the fault-reactivated northwestern margin of the Welsh Basin

Gibbons

1989

Geol. Mag.

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On mainland North Wales basement rocks emerge from beneath a Lower Ordovician cover along the west side of the Llyn peninsula. The basement contains steeply dipping mylonites (Llŷn shear zone) that separate plutonic and gneissic rocks (Sarn Complex) from a melange (Gwna mélange). The western edge of the Ordovician outcrop follows the basement shear zone, and new trenching data confirm that only a faulted relationship exists between cover and basement along this northwestern extremity of the Welsh Basin. Deformation along this margin has propagated into the Arenig cover to produce southeasterly verging thrusts, asymmetric folds and northwesterly dipping cleavage. A prominent steep fault (Daron Fault) cutting the Ordovician succession follows an eastern splay of the Llŷn shear zone and again therefore records brittle reactivation of an underlying mylonite belt. The likelihood of syn-Arenig fault movements is provided by the presence of a prominent late Arenig coarse clastic unit, containing boulders of the basement mylonites, that is found only to the west of the Daron Fault. Steep basement structures such as the Llŷn shear zone, initially generated as major transcurrent faults, are interpreted as having exerted a strong control over the deposition and subsequent deformation of the Ordovician cover sequence.

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Basement fractures in North Wales: their recognition and control on Caledonian deformation

Cited by 14 publications

References 27 publications

Interpretation of the regional gravity and magnetic surveys of Wales, using shaded relief and Euler deconvolution techniques

Interpretation of the regional gravity and magnetic surveys of Wales, using shaded relief and Euler deconvolution techniques

The lost Lower Old Red Sandstone of England and Wales: a record of post-Iapetan flexure or Early Devonian transtension?

Basement–cover relationships around Aberdaron, Wales, U.K.: the fault-reactivated northwestern margin of the Welsh Basin

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