Abstract On the 50th anniversary of the publication of the book Folding and Fracturing of Rocks by John G. Ramsay we use an analysis of citations to assess the book's impact on research in the field of structural geology. Five topics dealt with in the book have had special success in terms of the number of citations: (1) superimposed folding; (2) fold classification based on layer thickness variation; (3) simple shear deformation; (4) the R f / φ method; and (5) the flexural-slip mechanism of folding. The great impact of this book can be seen in several examples where the book has inspired new lines of research, such as: progressive deformation, strain analysis, shear zones and folding analysis. Ramsay's great merit in writing his book was to have wisely known to apply the quantitative methodology of continuum mechanics to the analysis of the geological structures. We conclude that Folding and Fracturing of Rocks is the most influential textbook in the field of structural geology research.

Abstract New and existing methods for the analysis of finite strains in shear zones involving volume changes are reviewed. By assuming that the wall rocks are undeformed, the position gradients tensor can be determined from data derived from deformed passive markers with or without knowledge of the orientation of cleavage within the shear zone. The new methods are both algebraic and graphical and include those based on off-axis Mohr circles. Application of these methods to deformed sandstones at Marloes Sands reveals important volume changes that would invalidate an approach that assumes simple shear.

Abstract The geometries and densities of fractures associated with fold structures can be predicted by assuming that the strains accommodated by fractures mimic the bulk strains induced in the strata during folding. This paper examines, from a theoretical standpoint, the distributions of bedding-plane strains expected in folds formed by various folding mechanisms. The relationship between the state of bedding-plane strain and fold-surface geometry is found to vary according to different fold types, distinguished on the basis of their curvature properties. The first type are developable fold surfaces, which have Gaussian curvature equal to zero. Folding mechanisms which are dominated by the mechanical strength of the layering, such as buckling, produce surfaces of this type. Folds of this type allow the possibility of estimating the bedding-plane strains from the geometrical features of the folded layer. Neutral surface folds and flexural-slip folds are discussed as examples. The other main class of folds have non-developable surfaces, which have non-zero Gaussian curvature. Folded surfaces with this form arise predominantly from mechanisms that involve the passive deflection of the layering in response to displacement gradients originating outside of the layer, e.g. drape folding. Although the geometry of these surfaces implies the presence of bedding plane strains, the quantification of these strains cannot be made from the fold geometry but requires additional information on these displacement patterns.