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

The Slieve League Peninsula of northwest Ireland lies on the western limb o f a major orogenic strike-swing in which regional foliation trends have dev iated from the northeast-southwest trends typical of much of Scotland, to w est-east orientations. Across-strike coastal exposures on the western tip o f the peninsula through Neoproterozoic Dalradian metasediments enable a det ailed examination and analysis of the structural evolution of a Caledonian orogenic root zone which has been previously correlated with the Loch Awe S yncline of southwest Scotland. Minor structural development may be evaluate d in terms of regional strain profiles and overprinting relationships. Over much of the area, a composite, steep northeast-southwest-trending S2-S3 fo liation containing a gently southwest-plunging quartz mineral elongation li neation is the dominant fabric at outcrop, and is associated with MP2 alman dine-amphibolite facies metamorphism. F2 folds are isoclinal with curviline ar hinges and similar geometry They typically plunge steeply towards the so uthwest and display variable (dextral) or north-directed vergence, whilst m inor F3 fold hinges plunge moderately towards the southwest and typically v erge (sinistrally) towards the south. Major, composite D1-D3 tectonic slide s are developed in the Argyll Group. Structural and stratigraphic relations hips indicate that D1 induced large-scale reversals in younging across tect onic slides, resulting in reversals in subsequent F2 and F3 facing patterns . Tectonic sliding is associated with an intensification of strain demonstr ated by increasingly intrafolial and curvilinear folding, together with ext ensional crenulations, sheared quartz pods and metre-scale asymmetric boudi nage 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. D3 results in region ally northwest-verging structures, with minor crenulations and the S4 cleav age transecting fold hinges in an anticlockwise sense, suggesting a dextral component of deformation. The detailed kinematic data indicate that the ov erall geometry of this western? deep-level arm of the root zone is not a pr oduct of the classic mushrooming fountain of nappes model, but rather major interference between consistent northerly directed D2 thrusting and a late r phase of southeast-directed (D3) retrocharriage ('back-folding') which in tensifies towards the south.