Drag-folds and parasitic folds

Abstract

Drag-folds, literally plis d’entraînement, but termed by Moret (1947) plis de frottement (friction folds), are asymmetrical folds produced in the weaker beds by differential movements of the more resistant rocks above and below them. They are disharmonie folds, usually confined to definite zones or groups of rocks. The weaker strata, being unable to resist the frictional stress of effect induced by the movements of stronger beds above and below them are dragged into these overturned or asymmetrical minor folds, the axes of which lie at right angles to the direction of movement. The sense of the movement is indicated by the inclination of the axial planes or the regard of the folds: they are typical monoclinic structures (Fig. 3.1c).