Tectonic analysis of progressive secondary deformation in the hinge of a major Caledonian fold nappe, north-western Ireland

Abstract

Polyphase deformation chronologies established within the mid-crustal portions of orogenic belts have classically been attributed to regional-scale ‘events’ which generate distinct structural sequences that can be directly correlated across large tracts of the orogenic belt. However, concepts of progressive deformation in which minor structures may be continually generated, amplified and redeformed within a unifying kinematic framework suggest that regional correlation of minor structures is both misguided and misleading. Detailed structural analysis of lower amphibolite facies Dalradian metasediments in north-west Ireland does, however, demonstrate that a coherent and meaningful deformation chronology can be established within the framework of individual fold nappes. Protracted deformation has resulted in the generation of a series of overprinting, secondary structures (D4–D9), which are kinematically linked to the continued structural evolution and south-east directed translation of the crustal-scale (D3) Ballybofey (fold) Nappe. Secondary (D4) crenulation axes initiated at an oblique angle to the direction of nappe transport both rotate and amplify into larger scale folds, which are subparallel to transport and demonstrate successive stages of diachronous folding. Continued nappe-related deformation induces southwards verging contractional (D5) folds, which are particularly well developed and focused into reactivated ductile (D3) thrust zones generated during the initial stages of nappe translation. Subsequent to thickening-induced ductile extension and collapse of the nappe, a return to contractional tectonics is marked by major episodes of broad, open buckle folding developed orthogonal to both the overturned limb (D7) and upper limb (D8) of the nappe. Detailed structural analysis and investigation of secondary folds and overprinting fabrics provides a valuable insight into the protracted kinematic evolution of major fold nappes.