Folds and Folding*

Abstract

The geometry and the internal features of folds reflect the mechanisms that produce folds. The close relation between mechanism and resultant structure permits fold characteristics to be used in determining the mechanisms; in turn, the mechanisms can be used to infer general environmental conditions under which deformation has occurred. The relative ductilities and the nature of inherent anisotropy (primarily layering and inter-layer cohesion) in the involved rocks determine which mechanism of folding will operate. Flexural mechanisms are dependent on the presence of mechanical anisotropy; passive mechanisms operate when anisotropy is absent or ineffective. As the ductilities of the involved rocks increase, the effect of layering in the folding process decreases. Flexural folding represents a true bending of layers and can be produced by slip between layers, flexural slip, or by flow within layers, flexural flow. Passive folding reflects relative displacements of layer boundaries by flow or slip across the boundaries; the layering does not control the configuration of the deformed mass—it merely reflects the deformation. Passive mechanisms are designated passive flow and passive slip. Where irregular flow-occurs within and across layers, certain layers are flexed in response to passive behavior in the associated rocks. This gradational mechanism is called quasi-flexural folding; the geometry and more obvious features of the fold are flexural in general aspect, but the overall behavior is predominantly passive. Individual fold mechanisms are not mutually exclusive but may operate singly or in combination to produce folds.