Relationships between distributed and localized shear in the tectonic evolution of a Caledonian fold and thrust zone, northwest Ireland

Broad zones of distributed shear operating through mid-crustal regions of orogenic belts may incorporate narrow horizons of intense localized deformation culminating in discrete, large magnitude displacements. The relative importance and relationship between distributed and localized shear are influenced by a variety of factors including lithological variation, pre-existing structural anisotropy, strain rate and migration of fluids. Rigorous structural analysis of lower amphibolite facies Dalradian metasediments in northwestern Ireland demonstrates that an early (D1) discrete ductile detachment was subsequently reactivated during distributed non-coaxial D2 deformation operating in a broad zone through the structural pile. Regional shear was directed towards the southeast and resulted in the generation and translation of kilometre-scale, isoclinal, recumbent sheath folds which close and face towards the transport direction. The D1 detachment is clearly folded around the hinges of these major folds, whilst on fold limbs it was reactivated and acted as a local decollement within the zone of distributed shear. Shear criteria along the detachment indicate a southeast-directed translation of the major folds, in sympathy with regional shear. A broad zone of D3 translation operating through the nappe pile resulted in coaxial refolding of large scale F2 folds by the D3 Ballybofey Nappe producing a complex fold interference pattern. Non-coaxial D3 deformation resulted in continued reactivation of local decollements, together with the initiation of east-southeast directed oblique thrusts and partial dismemberment of D2 folds. Detailed structural investigation allows concepts of distributed and localized shear to be evaluated and models of crustal deformation to be assessed.