ABSTRACT

Heterogeneous variation across flow lines in rate of flow within a deforming fluid body results in the development of flow folds, the axial surfaces of which parallel the flow lines. The direction of flow, and therefore the attitude of flow folds, is directly related to the change in shape, and varies from one part of the deforming body to another; the direction is related only indirectly to the attitude of the principal stress axes. Since the flow lines within a deforming fluid are almost invariably curved, deformation is commonly associated with the development of complex "refolded folds." Salt structures provide excellent examples of flow folds.

During orogenesis, especially during the early stages when pore pressures are high, metamorphic rocks commonly behave like viscous fluids. If orogenesis is accompanied by crustal shortening approximately normal to the trend of a linear orogenic belt, the resulting flow-fold pattern is characterized by much recumbent folding and the presence of root zones, and by a systematic variation in the attitude of the folds across the belt. This fold pattern, complicated considerably by subsequent deformation, may be seen in parts of the Caledonides, Appalachians, Alps and Himalayas, which suggests that orogenesis may well be associated with contraction.