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The Antrim–Galway Line: a resolution of the Highland Border Fault enigma of the Caledonides of Britain and Ireland

Published online by Cambridge University Press:  01 May 2009

P. D. Ryan ,
N. J. Soper ,
D. B. Snyder ,
R. W. England  and
D. H. W. Hutton
P. D. Ryan
Affiliation:
Geology Department, University College, Galway, Ireland
N. J. Soper
Affiliation:
Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
D. B. Snyder
Affiliation:
B.I.R.P.S., Bullard Laboratories, Madingley Rise, Madingley Road, sCambridge CB3 0EZ, UK
R. W. England
Affiliation:
B.I.R.P.S., Bullard Laboratories, Madingley Rise, Madingley Road, sCambridge CB3 0EZ, UK
D. H. W. Hutton
Affiliation:
Department of Geological Sciences, Science Laboratories, South Road, Durham DH1 3LE, UK
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Abstract

The westward continuation of the Highland Border fault of Scotland (HBFZ) into Ireland is problematic. It is widely thought to follow a pronounced magnetic and gravity lineament, the Fair Head-Clew Bay Line (FCL). The advantage of this interpretation is that it places all the Ordovician ophiolitic complexes and associated sedimentary basins to the south of the FCL, which would represent the contact between Laurentia and the outboard terranes. Its main shortcomings are that both the deep structure and timing of strike-slip are different on the HBFZ and FCL. In Ireland the FCL is a north-dipping feature that can be traced to the Moho on BIRPS profiles, while the HBFZ has no such signature. Terrane amalgamation in western Ireland was completed by the late Ordovician, while in Scotland the Midland Valley terrane did not finally dock until the early Devonian. These considerations suggest that in western Ireland a branch of the HBFZ exists, which was active in post-Ordovician time and must lie south of Connemara. An examination of Irish geological, geophysical and image-processed magnetic data shows that a profound lineament can be traced from Antrim to Galway Bay (the Antrim–Galway Line). Stitching plutons date movement on it as pre-405 Ma. We propose that the Antrim–Galway Line represents the continuation of the Scottish HBFZ, while the FCL is a preserved Ordovician splay of the HBFZ system whose northdipping geometry is a product of Ordovician collapse of the orogen in western Ireland.

Type
Articles
Information
Geological Magazine , Volume 132 , Issue 2 , March 1995 , pp. 171 - 184
Copyright
Copyright © Cambridge University Press 1995

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