FLEXURAL FLOW FOLDING REVISITED

We revisit the topic of an earlier paper on flexural flow folding, and whether it is an important mechanism for parallel folds in nature. This is one of two mechanisms/models for internal deformation associated with parallel folding, flexural flow (FF) and tangential longitudinal strain (TLS), that have become classics in structural geology, used widely in text books and teaching. We concluded from finite element modelling (FEM) of single-layer folding, (i) that FF would only be approximated if the layer was highly anisotropic (with normal viscosity/shear viscosity > 50), which is unlikely for most competent rocks folding as single layers, and (ii) that the predominant mechanism in buckle folding of isolated competent layers is TLS. However, a subsequent study of folded carbonate layers, and new methods of modelling of folds lead us to re-examine these two conclusions.

We suggest three ways in which deformation in an isotropic competent layer might develop a pattern of deformed markers suggestive of flexural flow. 1) As the fold grows to large amplitudes, layer-parallel shear in the limbs increases, causing deflection of originally orthogonal markers. This tendency is enhanced when the wavelength/thickness ratio is small. 2) Late-stage ‘flattening’ deformation that changes a parallel fold (1B) to 1C geometry will produce layer-parallel shear in limbs (unless isoclinal). 3) Deflection of a competent layer obliquely intersected by a shear zone of comparable width to the layer may induce localized strains similar to those predicted for flexural flow. All these processes are enhanced if there is any intrinsic anisotropy in the competent layer.

In analyses of real folds, the approach becomes one of comparing finite strain markers such as cleavage fabrics, and/or incremental strain or stress markers such as fractures and veins, with geometric and kinematic models such as TLS, FF, or in various combinations with additional volume changes and/or ‘flattening’ strain^. The results of such analyses provide useful information on how certain natural folds could have developed, but likely will not provide a full or accurate picture of the entire deformation history of the fold, nor of its mechanics.