Hostname: page-component-77c89778f8-5wvtr Total loading time: 0 Render date: 2024-07-18T14:30:48.825Z Has data issue: false hasContentIssue false
  • English
  • Français

Palaeomagnetism of Late Ordovician igneous intrusions from the northern Welsh Borderlands: implications to motion of Eastern Avalonia and regional rotations

Published online by Cambridge University Press:  01 May 2009

J. D. A. Piper
J. D. A. Piper
Affiliation:
Geomagnetism Laboratory, Department of Earth Sciences, University of Liverpool, Liverpool L69 3BX, UK
Get access Rights & Permissions [Opens in a new window]

Abstract

The palaeomagnetism of a Late Ordovician dolerite suite widely distributed in the northern Welsh Borderlands has been investigated to evaluate (i) regional rotations associated with later deformations and (ii) Lower Palaeozoic latitudes of Eastern Avalonia. Only local effects of Acadian and Variscan overprinting are observed in this region and most bodies appear to preserve a primary cooling-related magnetization. Magnetic properties in the Breidden Dolerite are related to primary igneous differentiation and a dual polarity characteristic remanence (D/I = 333.1/−71.6°, 6 sites, α95 = 10.4°) predates Late Ordovician folding. Dolerites elsewhere in the Breidden Hills Inlier record the same reversal of remanence and the combined mean (D/I = 314.3/−72.8°) is rotated clockwise from the mean remanence (D/I = 291.5/−59.5°, 21 sites, α95 = 6.0°) in a similar dolerite suite within the adjoining Shelve Inlier which is also of dual polarity and predates Late Ordovician folding. The difference in magnetic declinations is similar to the rotation of Late Ordovician fold axes and fracture systems between the two inliers and is probably the resultant effect of local rotation, possibly about a constraining bend in the Severn Valley Fault System. The Moel-y-Golfa Andesite of Caradocian (Soudleyan) age has a remanence (D/I = 294.3/−50.4°, 13 samples, α95 = 2.8°) which study of an adjoining volcanic conglomerate shows to be of primary cooling-related origin. The equivalent palaeolatitude is ˜ 31° S in early Caradocian times. A similar estimate is derived from a probable diagenetic overprint in the contemporaneous Castle Hill Conglomerate at Montgomery. The Mynydd-Bryn Andesite in the Berwyn Inlier, of probable similar origin and age to Moel-y-Golfa, has a magnetization of opposite polarity but similar inclination (D/I − 239.1/47.4°, 14 samples, α86 = 5.5°). Declination differences between these volcanic outcrops are a possible signature of later block rotations between the Lower Palaeozoic inliers and identify the importance of terrane definition in analysis of Lower Palaeozoic palaeomagnetic results from the Caledonides. A dolerite sheet complex at Hendre in the Berwyn Hills has a younger remanence (D/I − 182.3/ − 3.0°, 20 samples, α95 = 7.8°), probably of mid-Carboniferous age.

The collective Upper Ordovician results define a rapid movement of the Avalonian Plate, probably spanning no more than 10 Ma, from ˜ 30° S in early Caradocian times to ˜ 57° S prior to folding of the dolerites in late Caradocian to early Ashgill times. This motion into higher southerly palaeolatitudes marks a reversal of the motion of Avalonia into more shallow southerly palaeolatitudes between Arenig and Llanvirn times and shows that Ordovician closure of the Iapetus Ocean was more complex than recognized hitherto.

Type
Articles
Information
Geological Magazine , Volume 132 , Issue 1 , January 1995 , pp. 65 - 80
Copyright
Copyright © Cambridge University Press 1995

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Brenchley, P. J., 1978. The Caradoc rocks of the north and west Berwyn Hills, North Wales. Geological Joumal 13, 137–64.CrossRefGoogle Scholar
Brenchley, P. J., & Pickerill, R. K., 1980. Shallow subtidal sediments of Soudleyan age in the Berwyn Hills, North Wales and their palaeogeographic context. Proceedings of the Geologists Association 91, 177–94.CrossRefGoogle Scholar
Cave, R., Dean, W. T., & Hains, B. A., 1988. The age of the andesite conglomerate at Castle Hill, Montgomery, Eastern Wales. Geological Journal 23, 205–10.CrossRefGoogle Scholar
Cave, R., & Dixon, R. J., 1993. The Ordovician and Silurian of the Welshpool Area. In Geological Excursions in Powys, Central Wales (eds Woodcock, N. H. & Bassett, M. G.), pp. 51–84. Cardiff: University of Wales Press.Google Scholar
Channell, J. E. T., & McCabe, C., 1992. Palaeomagnetic data from the Borrowdale Volcanic Group: volcanotectonics and Late Ordovician palaeolatitudes. Journal of the Geological Society of London 149, 881–8.CrossRefGoogle Scholar
Channell, J. E. T., McCabe, C., Torsvik, T. H., Trench, A., & Woodcock, N. H., 1992. Palaeozoic palaeomagnetic studies in the Welsh Basin — recent advances. Geological Magazine 129, 533–42.CrossRefGoogle Scholar
Channell, J. E. T., McCabe, C., & Woodcock, N. H., 1992. Early Devonian (pre-Acadian) magnetisation directions in Lower Old Red Sandstone of South Wales (UK). Geophysical Journal International 108, 883–94.CrossRefGoogle Scholar
Dixon, R. J., 1988. The Ordovician (Caradoc) volcanic rocks of Montgomery, Powys, N. Wales. Geological Journal 23, 149–55.CrossRefGoogle Scholar
Dixon, R. J., 1990. The Moel-y-Golfa andesite: an Ordovician (Caradoc) intrusion into unconsolidated conglomeratic sediments, Breidden Hills inlier, Welsh Borderlands. Geological Journal 25, 35–46.CrossRefGoogle Scholar
Harland, W. B., Cox, A. V., Llewllyn, P. G., Pickton, C. A. G., Smith, A. G., & Walters, R., 1990. A Geologic Time Scale, 1989. Cambridge University Press, 279 pp.Google Scholar
Lomax, K., & Briden, J. C., 1977. Palaeomagnetic studies of the Longmyndian and other British late Precambrian/early Palaeozoic rocks, and their regional tectonic implications. Journal of the Geological Society of London 133, 5–21.CrossRefGoogle Scholar
Lynas, B. D. T., 1983. Two new Ordovician volcanic centres in the Shelve inlier, Powys, Wales. Geological Magazine 120, 535–42.CrossRefGoogle Scholar
Lynas, B. D. T., 1988. Evidence for dextral oblique-slip faulting in the Shelve Ordovician inlier, Welsh Borderland: implications for the south British Caledonides. Geological Journal 23, 39–57.CrossRefGoogle Scholar
Lynas, B. D. T., Rundle, C. C., & Sanderson, R. W., 1985. A note on the age and pyroxene chemistry of the igneous rocks of the Shelve Inlier, Welsh Borderland. Geological Magazine 122, 641–7.CrossRefGoogle Scholar
Mankinen, E. A., Prevot, M., & Grommé, C. S., 1985. The Steens’s Mountain (Oregon) geomagnetic polarity transition 1: directional history, duration of episodes and rock magnetism. Journal of Geophysical Research 90, 10393–416.CrossRefGoogle Scholar
McCabe, C., & Channell, J. E. T., 1990. Palaeomagnetic results from volcanic rocks of the Shelve Inlier, Wales: evidence for a wide Late Ordovician Iapetus Ocean in Britain. Earth and Planetary Science Letters 96, 58–68.CrossRefGoogle Scholar
McCabe, C., & Channell, J. E. T., 1991. Reply to comment of A. Trench and T. H. Torsvik on ‘Palaeomagnetic results from volcanic rocks of the Shelve Inlier, Wales: evidence for a wide Late Ordovician Iapetus Ocean in Britain’. Earth and Planetary Science Letters 104, 540–4.CrossRefGoogle Scholar
McCabe, C., Channell, J. E. T., & Woodcock, N. H., 1992. Further palaeomagnetic results from the Builth Wells Ordovician Inlier, Wales. Journal of Geophysical Research 97, 9357–70.CrossRefGoogle Scholar
McElhinny, M. W., 1964. Statistical significance of the fold test in palaeomagnetism. Geophysical Journal of the Royal Astronomical Society 8, 161–7.Google Scholar
Piper, J. D. A., 1978. Palaeomagnetic survey of the (Palaeozoic) Shelve inlier and Berwyn Hills, Welsh Borderlands. Geophysical Journal of the Royal Astronomical Society 53, 355–71.CrossRefGoogle Scholar
Piper, J. D. A., Atkinson, D., Norris, S., & Thomas, S., 1992. Palaeomagnetic study of the Derbyshire lavas and intrusions, central England: definition of Carboniferous apparent polar wander. Physics of the Earth and Planetary Interiors 69, 37–55.CrossRefGoogle Scholar
Piper, J. D. A., & Stearn, J. E. F., 1975. Palaeomagnetism of the Breidden Hill (Palaeozoic) Inlier, Welsh Borderlands. Geophysical Journal of the Royal Astronomical Society 43, 1013–16.CrossRefGoogle Scholar
Setiabudidaya, D., Piper, J. D. A., & Shaw, J., 1994. Palaeomagnetism of the (Early Devonian) Lower Old Red Sandstones of South Wales: implications to Variscan overprinting and differential regional rotations. Tectonophysics 231, 257–80.CrossRefGoogle Scholar
Smith, R. D. A., 1987. Structure and deformation history of the Central Wales synclinorium, northeast Dyfed: evidence for a long-lived basement structure. Geological Journal 22, 183–98.CrossRefGoogle Scholar
Smith, R. L., & Piper, J. D. A., 1984. Palaeomagnetic study of the (Lower Cambrian) Longmyndian sediments and tuffs, Welsh Borderlands. Geophysical Journal of the Royal Astronomical Society 79, 875–92.CrossRefGoogle Scholar
Trench, A., & Torsvik, T. H., 1991. Comment on ‘Palaeomagnetic results from volcanic rocks of the Shelve Inlier, Wales: evidence for a wide Late Ordovician Iapetus Ocean in Britain’ by C. McCabe and J. E. T. Channell. Earth and Planetary Science Letters 104, 535–9.CrossRefGoogle Scholar
Trench, A., Torsvik, T. H., Smethust, M. A., Woodcock, N. H., & Metcalfe, R., 1991. A palaeomagnetic study of the Builth Wells — Llandrindod Wells Inlier, Wales: palaeogeographic and structural implications. Geophysical Journal of the Royal Astronomical Society 105, 477–89.CrossRefGoogle Scholar
Trench, A., Torsvik, T. H., Dentith, M. C., Walderhaug, H., & Traynor, T. J., 1992. A high southerly palaeolatitude for Southern Britain in Early Ordovician times: palaeomagnetic data from the Treffgarne Volcanic Formation, SW Wales. Geophysical Journal International 108, 89–100.CrossRefGoogle Scholar
Watts, W. W., 1885. On the igneous and associated rocks of the Breidden Hills. Quarterly Journal of the Geological Society of London 41, 532–46.CrossRefGoogle Scholar
Watts, W. W., 1925. The geology of South Shropshire. Proceedings of the Geologists Association 36, 321–63.CrossRefGoogle Scholar
Wedd, C. B., 1932. Notes on the order of rocks at Bausley, Montgomery. Summary of Progress of the Geological Survey for 1931 pt. II, 49–55.Google Scholar
Wedd, C. B., Smith, B., King, W. B. R., & Wray, D. A., 1929. The Country around Oswestry. Explanation of Sheet 137. Memoirs of the Geological Survey of England and Wales. London, H.M. Stationary Office, 234 pp.Google Scholar
Woodcock, N. H., 1984. The Pontesford Lineament, Welsh Borderland. Journal of the Geological Society of London 141, 1001–14.CrossRefGoogle Scholar
Woodcock, N. H., Awan, M. A., Johnson, T. E., Mackie, A. H., & Smith, R. D. A., 1988. Acadian tectonics of Wales during Avalonia/Laurentia convergence. Tectonics 7, 483–95.CrossRefGoogle Scholar