Superposed folding in the Silurian Rocks of Co. Mayo, Eire

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Abstract
Silurian strata of Upper Llandoverian and Wenlockian age, lying in a complex synclinorium south of the Highland Boundary Fault Zone in Co. Mayo, are affected by two main structural episodes. A tight, curving, E–W synclinorium, verging to the south, was developed during the first episode of deformation (F1). Large‐scale cross‐buckling, followed by variable flattening in the axial‐planar schistosity, was responsible for a considerable plunge variation of F1 folds. A mechanism is described whereby primary downward‐facing minor folds may be produced in an upward‐facing major structure. Three slides, of considerable magnitude, are co‐planar with the schistosity. During the second episode of deformation (F2) a slip‐cleavage, dipping steeply to the NNE, was superimposed upon the F1 synclinorium. It is deduced, partly from an analysis of the geometry of F2 minor structures, that the arcuate curvature of the F1 synclinorium is pre‐F2 and was developed during the F1 deformation as a response to the presence of an early or pre‐F1 granite pluton. F2 folds show a great variability in plunge and facing direction, depending upon their position in the F1 major structure.

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