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

Published online by Cambridge University Press:  01 May 2009

Wes Gibbons
Wes Gibbons
Affiliation:
Department of Geology, University of Wales Cardiff, P.O. Box 914, Cardiff CF1 3YE, U.K.
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Abstract

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.

Type
Articles
Information
Geological Magazine , Volume 126 , Issue 4 , July 1989 , pp. 363 - 372
Copyright
Copyright © Cambridge University Press 1989

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References

Bates, D. E. B. 1974. The structure of the Lower Palaeozoic rocks of Anglesey, with special reference to faulting. Geological Journal 9, 39–60.Google Scholar
Beckinsale, R. D., Evans, J. A., Thorpe, R. S., Gibbons, W. & Harmon, R. S. 1984. Rb–Sr whole-rock isochron ages, δ18O values and geochemical data for the Sarn Igneous Complex and the Parwyd gneisses of the Mona Complex of Llŷn, N. Wales. Journal of the Geological Society of London 141, 701–9.Google Scholar
Beckly, A. J. 1987. Basin development in North Wales during the Arenig. Geological Journal 22, 19–30.Google Scholar
Blake, J. F., 1888. On the Monian System of rocks. Quarterly Journal of the Geological Society of London 44, 463–546.CrossRefGoogle Scholar
Coward, M. P. & Siddans, A. W. B. 1979. The tectonic evolution of the Welsh Caledonides. In The Caledonides of the British Isles – Reviewed (ed. Harris, A. L., Holland, C. H. and Leake, R. E.), pp. 187–98. Geological Society of London Special Publication no. 8.Google Scholar
Crimes, T. P. 1970. A facies analysis of the Arenig of Western Lleyn, North Wales. Proceedings of the Geologists' Association 81, 221–39.CrossRefGoogle Scholar
Dallmeyer, R. D. & Gibbons, W. 1987. The age of blueschist metamorphism in Anglesey, North Wales; evidence from 40Ar/39Ar mineral dates of the Penmynydd Schists. Journal of the Geological Society of London 144, 843–50.CrossRefGoogle Scholar
Fortey, R. A. & Owens, R. M. 1987. The Arenig Series in South Wales. Bulletin of the British Museum (Natural History) 41, 69–303.Google Scholar
Gibbons, W. 1983. The Monian Penmynydd zone of metamorphism in Llŷn, North Wales. Geological Journal 18, 1–21.Google Scholar
Gibbons, W. 1987. The Menai Strait Fault System: an early Caledonian terrane boundary in North Wales. Geology 15, 744–7.Google Scholar
Gibbons, W. 1989. Transcurrent ductile shear zones and the dispersal of the Avalon superterrane. Tectonophysics (in press)Google Scholar
Greenly, E. 1919. The Geology of Anglesey. Memoir Geological Survey of the United Kingdom (2 vols), London.Google Scholar
Hawkins, T. R. W. 1983. Structure of the Ordovician succession around Aberdaron, south-west Llŷn, North Wales. Geological Journal 18, 169–81.Google Scholar
Hicks, H. 1879. On the pre-Cambrian (Dimetian, Arvonian and Pebidian) rocks in Caernarvonshire and Anglesey (with appendix by T. G. Bonney). Quarterly Journal of the Geological Society of London 35, 295ff.Google Scholar
Matley, C. A. 1928. The Pre-Cambrian Complex and associated rocks of S.W. Lleyn (Caernarvonshire). Quarterly Journal of the Geological Society of London 84, 440–504.CrossRefGoogle Scholar
Matley, C. A. 1932. The geology of the country around Mynydd Rhiw and Sarn, SW Llŷn, Caernarvonshire. Quarterly Journal of the Geological Society of London 88, 238–73.CrossRefGoogle Scholar
Matley, C. A., Nicholas, T. C. & Heard, A. 1939. Summer field meeting to the western part of the Lleyn Peninsula. Proceedings of the Geologists' Association 50, 83–100.CrossRefGoogle Scholar
Matley, C. A. & Smith, B. 1936. The age of the Sarn Granite. Quarterly Journal of the Geological Society of London 92, 188–200.CrossRefGoogle Scholar
Muir, M. D., Bliss, G. M., Grant, P. R. & Fischer, M. 1979. Palaeontological evidence for the age of some supposedly Precambrian rocks in Anglesey, North Wales. Journal of the Geological Society of London 136, 61–4.CrossRefGoogle Scholar
Peat, C. J. 1984. Comments on some of Britain's oldest microfossils. Journal of Micropalaeontology 3, 65–71.CrossRefGoogle Scholar
Shackleton, R. M. 1956. Notes on the structure and relations of the Pre-Cambrian and Ordovician rocks of south-western Lleyn (Caernarvonshire). Geological Journal 1, 400–9.CrossRefGoogle Scholar
Soper, N. J., Webb, B. C. & Woodcock, N. H. 1987. Late Caledonian (Acadian) transpression in NW England: timing, geometry and geotectonic significance. Proceedings of the Yorkshire Geological Society 46, 175–92.CrossRefGoogle Scholar
Wilkinson, I. & Smith, M. 1988. Basement fractures in North Wales: their recognition and control on Caledonian deformation. Geological Magazine 125, 301–6.CrossRefGoogle Scholar
Wood, M. & Nicholls, G. D. 1973. Pre-Cambrian stromatolitic limestones from Northern Anglesey. Nature, London 241, 65.Google Scholar