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

Abstract

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.

Laser ablation 40 Ar/ 39 Ar dating of metamorphic fabrics in the Caledonides of north Ireland

“…2a). A regionally extensive phase of late biotite porphyroblasts has been documented throughout central and north Donegal (MP 4 of Pitcher & Berger 1972;Alsop et al 2001) and these are probably correlative with the post-S L porphyroblasts in our sample from the Inishowen Peninsula.…”

Temporal and spatial changes of the submarine Cretaceous paleoslope in Northern Tunisia, inferred from slump folds analysis

“…Recently, other studies have focused on the study of slump folds and have described other methods estimating the paleoslope direction from these structures (e.g. Hansen, 1965Hansen, , 1967Woodcock, 1979;Bradley and Hanson, 1998;Alsop et al, 2001;Strachan and Alsop, 2006;Strachan, 2008;Debacker and Meester, 2009;Alsop and Marco, 2011, 2012a, b, 2013, 2014Debacker, 2012;Yong et al, 2013;Sharman, 2014).…”

Late‐stage deformation in a collisional orogen (Western Alps): nappe refolding, back‐thrusting or normal faulting?

“…Retro‐deformation of the latest stages of deformation is a prerequisite for unraveling the main stages of the tectonic and metamorphic evolution of an orogen (Dewey, 1988; Platt et al ., 1989; Escher et al ., 1993; Alsop et al ., 2001). Such late‐stage deformation may severely modify pre‐existing large‐scale structural and metamorphic patterns, preferentially within the more internal and metamorphic parts of the pre‐existing nappe stack (Müller, 1983).…”