Effects of Compression on Concentric Folds and Mechanism of Similar Folding

The effect of compressive strain on the thicknesses of concentrically folded beds is discussed. Two sets of graphs have been given illustrating the thickness variation due to compression. If the fold axis is parallel to one of the principal axes of strain the two dimensional strain on the profile plane can be directly determined from these graphs. When this is not so the numerical value obtained from the graphs will only give a qualitative idea of the two dimensional strain. If the fold axis has got a uniform orientation in space it will not be possible to determine the three dimensional strain from thickness measurements. Equations have been given by which the three dimensional strain can be computed if the fold axis has a variable orientation.

It has been shown that in compressed concentric folds there is a critical value of θ (θ being the angle between the bedding plane and the normal to the axial plane), below which the fold will show a nearly perfect similar geometry. If in natural compressed concentric folds the maximum value of θ observed falls below this critical value the fold will show a similar geometry. Thus, it is possible to produce similar fold by compressing an initially concentric one.