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

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 conjugate folds, and the states of infinitesimal and finite strains are determined quantitatively. Simple shear parallel to the axial planes, and not "flexure gliding" parallel to the layers, is the mechanism of folding. By relating the stress field directly to the state of infinitesimal strain, it is confirmed that the axial planes of conjugate folds lie at approximately 45° to the maximum principal stress. Various modifications of this model account for the actual geometry of conjugate folds. The present hypothesis is discussed in relation to the suppression of one of the conjugate fold sets in the third stage, and the conclusions reached are shown to be able to account satisfactorily for the development of single sets of folds and their axial planar cleavages from the start of deformation. The kinematic and dynamic interpretation of such folds is discussed. The development of phyllitic cleavages is discussed in relation to the folding. Such cleavages cannot be interpreted as "shear fractures" but are shown to form by the microfolding of the primary mica fabric accompanied by the pressure solution of quartz against mica. The apparently anomalous displacements on such cleavages are compatible with the strain undergone during the folding. It is suggested that the behavior of rock masses undergoing deformation is plastic in nature, rather than viscous. By analogy with the plastically deformed layers within metals, which form by simple shear parallel to the theoretical slip planes of plastic strain, it is concluded that secondary folds are initiated by local stress concentrations exceeding the yield stress of the rocks involved. The general implications of this hypothesis are considered.