Abstract
Ramsay’s classification of folds based on the variation of layer thickness as a function of layer attitude is extended. The different patterns of layer thickness variation which characterize Ramsay’s classes are brought out by plotting a polar graph of inverse thickness against layer orientation. On this graph, each fold type produces a diagnostic curve or pachymetric indicatrix.
It is shown that a wide range of class 1 fold geometries can potentially be produced by the homogeneous flattening modification of a common fold style, the parallel fold. The subdivision of class 1 folds can therefore be implemented by referring to a single parameter, the flattening index F, which describes the amount and general orientation of the flattening. This index describes the axial ratio of the elliptical pachymetric indicatrix on the inverse thickness polar diagram. Class 1C folds have F values greater than unity. Class 1A folds have F values greater than 0 but less than 1, this denoting the fact that the flattening required is oriented normal to the axial trace.
Class 3 folds have morphologies which are incompatible with an evolution involving homogeneous strain modification of parallel folds. Instead the basic form of a class 3 fold is defined as one which yields an equilateral hyperbola (hyperbola with unit axial ratio) on the inverse thickness polar graph. This basic morphology is designated a class 3B fold and has an F value of −1. In analogous fashion to the case of class 1 folds, the other subclasses of class 3 are defined as the products of the flattening modification of 3B folds. The axial ratio of the hyperbolas, i.e. the flattening index, for class 3A folds is less than −1, while the F values for class 3C folds fall in the range 0 and —1. Class 3A forms occur naturally with much greater frequency than 3C folds.
Graphs and a computer program are presented to allow the practical implementation of the new classification.
Acknowledgements: I thank Deepak Srivastava (Roorkee) and Cardiff students Michelle Dance and Tony Morton for discussion.
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Lisle, R.J. (1997). A fold classification scheme based on a polar plot of inverse layer thickness. In: Sengupta, S. (eds) Evolution of Geological Structures in Micro- to Macro-scales. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-5870-1_19
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DOI: https://doi.org/10.1007/978-94-011-5870-1_19
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