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Tectonic and kinematic evolution within mid-crustal orogenic root zones: a case study from the Caledonides of northwestern Ireland

G. I. ALSOP^*,*, R. BRYSON and D. H. W. HUTTON

^* Crustal Geodynamics Group, School of Geography & Geosciences, University of St Andrews, Fife, Scotland KY16 9AL, UK
Formerly: Dept. of Geosciences, Queen’s University, Belfast BT7 1NN, UK
School of Earth Sciences, University of Birmingham, Birmingham B15 2TT, UK

^* Author for correspondence: gia{at}st-andrews.ac.uk

The Slieve League Peninsula of northwest Ireland lies on the western limb of a major orogenic strike-swing in which regional foliation trends have deviated from the northeast–southwest trends typical of much of Scotland, to west–east orientations. Across-strike coastal exposures on the western tip of the peninsula through Neoproterozoic Dalradian metasediments enable a detailed examination and analysis of the structural evolution of a Caledonian orogenic root zone which has been previously correlated with the Loch Awe Syncline of southwest Scotland. Minor structural development may be evaluated in terms of regional strain profiles and overprinting relationships. Over much of the area, a composite, steep northeast–southwest-trending S2–S3 foliation containing a gently southwest-plunging quartz mineral elongation lineation is the dominant fabric at outcrop, and is associated with MP2 almandine–amphibolite facies metamorphism. F2 folds are isoclinal with curvilinear hinges and similar geometry. They typically plunge steeply towards the southwest and display variable (dextral) or north-directed vergence, whilst minor F3 fold hinges plunge moderately towards the southwest and typically verge (sinistrally) towards the south. Major, composite D1–D3 tectonic slides are developed in the Argyll Group. Structural and stratigraphic relationships indicate that D1 induced large-scale reversals in younging across tectonic slides, resulting in reversals in subsequent F2 and F3 facing patterns. Tectonic sliding is associated with an intensification of strain demonstrated by increasingly intrafolial and curvilinear folding, together with extensional crenulations, sheared quartz pods and metre-scale asymmetric boudinage of metadolerites, all of which indicate dextral (D2) and sinistral (D3) shear. After unfolding subsequent folds (F4), this corresponds to top-to-the-north (D2) and top-to-the-south (D3) translations. D4 results in regionally northwest-verging structures, with minor crenulations and the S4 cleavage transecting fold hinges in an anticlockwise sense, suggesting a dextral component of deformation. The detailed kinematic data indicate that the overall geometry of this western, deep-level arm of the root zone is not a product of the classic mushrooming fountain of nappes model, but rather major interference between consistent northerly directed D2 thrusting and a later phase of southeast-directed (D3) retrocharriage (‘back-folding’) which intensifies towards the south.

This article has been cited by other articles:

				C.L. Kirkland, G.I. Alsop, and A.R. Prave The brittle evolution of a major strike-slip fault associated with granite emplacement: a case study of the Leannan Fault, NW Ireland Journal of the Geological Society, January 1, 2008; 165(1): 341 - 352. [Abstract] [Full Text] [PDF]

				V. CHATZARAS, P. XYPOLIAS, and T. DOUTSOS Exhumation of high-pressure rocks under continuous compression: a working hypothesis for the southern Hellenides (central Crete, Greece) Geological Magazine, November 1, 2006; 143(6): 859 - 876. [Abstract] [Full Text] [PDF]

				D.J. Condon, K.V. Hodges, G.I. Alsop, and A. White Laser ablation 40Ar/39Ar dating of metamorphic fabrics in the Caledonides of north Ireland Journal of the Geological Society, March 1, 2006; 163(2): 337 - 345. [Abstract] [Full Text] [PDF]

				P.W.G. Tanner, G.I. Alsop, and D.H.W. Hutton Discussion on evidence for a major Neoproterozoic orogenic unconformity within the Dalradian Supergroup of NW Ireland Journal of the Geological Society, January 1, 2005; 162(1): 221 - 224. [Full Text] [PDF]

				D. H. W. Hutton, D.H.W. Hutton, and G.I. Alsop Evidence for a major Neoproterozoic orogenic unconformity within the Dalradian Supergroup of NW Ireland Journal of the Geological Society, July 1, 2004; 161(4): 629 - 640. [Abstract] [Full Text] [PDF]