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Progressive deformation and the rotation of contemporary fold axes in the Ballybofey Nappe, north‐west Ireland
Author(s) -
Alsop G. I.
Publication year - 1992
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3350270305
Subject(s) - lineation , geology , fold (higher order function) , nappe , crenulation , shear (geology) , dalradian , sinistral and dextral , clockwise , shear zone , geometry , seismology , pure shear , oblique case , simple shear , petrology , metamorphic rock , tectonics , mechanical engineering , linguistics , philosophy , mathematics , engineering
Abstract Internal regions of orogenic belts may be characterized by an alignment of fold axes with mineral elongation lineations. This relationship is commonly interpreted as representing progressive tightening and rotation towards the shear direction of early buckle folds, the hinges of which were initiated orthogonal to this direction. Detailed structural analysis of lower amphibolite facies Dalradian metasediments of the Ballybofey (fold) Nappe, north‐west Ireland, shows that an intense S 3 schistosity is developed axial planar to mesoscopic and minor F 3 folds. In areas of low D 3 strain, F 3 fold axes plunge gently towards the north‐east, whereas in regions of greater strain plunges are towards the south‐east subparallel to the constant mineral lineation. Minor folds which initiated at angles of 70–80° from the mineral lineation subsequently rotated towards the shear direction in a consistent clockwise sense. Progressive and variable non‐coaxial deformation oblique to the original mean F 3 orientation has resulted in a unimodal distribution pattern of fold axes. Analysis of the angular rotation of fold axes enables estimates of the bulk shear strain to be evaluated and models of progressive deformation to be assessed.