The Formation of Similar Folds by Inhomogeneous Plastic Strain, with Reference to the Fourth Phase of Deformation Affecting the Dalradian Rocks in the Southwest Highlands of Scotland

Abstract

Recent work on the formation of folds is reviewed critically and found inadequate to account for the similar folding of inhomogeneous sequences. The geometry of the fourth-phase structures, typical of the later secondary phases of folding, is described from an area where the prior attitudes of the earlier primary structures can be inferred. Four stages in progressive secondary deformation can be recognized. The structures formed are, ideally, (1) conjugate sets of phyllitic (strainslip) cleavages, (2) conjugate sets of similar folds with axial planes parallel to one or other of the cleavage sets, (3) single sets of similar folds formed by the suppression of one or other of the conjugate sets of folds, and (4) single sets of composite folds. Flinn's (1962) methods indicate that the deformation was biaxial, at least in the less intense stages. A Compatibility Principle is outlined whereby the strains undergone by adjacent limbs of a similar fold can be related. It is applied to a simplified model of the c...