NONCYLINDRICAL FOLDS

The best way to study the geometry of folds is in their profiles as we have seen in the discussion on fold geometry. But in non-cylindrical folds the geometry would vary in each of the multitude of profiles all along the fold hinge. We know that the non-cylindrical folds can be classified into (i) plane non-cylindrical folds in which the non-cylindricity is restricted within a perfectly planar axial plane and the only variable is the up and down curving fold hinge; and (ii) non-plane non-cylindrical folds in which the axial plane is curviplanar and the hinge too is curvilinear. Geologists did not attempt geometrical classification of non-cylindrical folds for a long time until Williams and Chapman made a break-through in 1979. A single non-cylindrical fold may be represented on a triangular diagram by a single point. Measurements required are b, or hinge angle which is the angle made by the culmination of hinge line between two relatively straight segments of the hinge measured within the axial plane; the interlimb angle a measured at the point of highest culmination of the hinge (representative of the highest degree of non-cylindricity).

In case of non-plane non-cylindrical folds, a third angle g is measured in the fold profile between two relatively straight segments of the axial surface. The data is plotted on a triangular diagram in terms of P, Q and R where P= a/180 and varies between 0 and 1 and represents the degree of planarity of fold (or reverse of interlimb angle), R= (180 -b)/180 where R is the degree of non-cylindricity varying between 0 for cylindrical folds and 1 for totally non-cylindrical folds. Q equals 180 - [a+ (180 - b) /180 where Q varies between 1 for cylindrical isoclines and 0 for domical isoclines These relationships are clear from figure given. For a non-plane non-cylindrical fold, the same diagram may be used except that against the point plotted, the value of g may be simply written, or the plotted point circled, the size of circle directly related to the amount of the g angle.

To be consistent with the classification the degree of non-plane non-cylindricity should be given by a parameter S which would equal 180 -g /180. S may be shown on the PQR diagram as a circle whose area is proportional to the size of S. One of the shortcomings of this classification is that it is shorn of the orientational characters of fold elements. On the whole the diagram is useful since a large-scale cylindrical fold may contain a large number of congruous non-cylindrical folds, which should be carefully analyzed to understand the fold evolution and strain variability on smaller domains. Fig. given herein shows plots of some hypothetical non-cylindrical folds (a to d) on the basis of measurements of two or three angles.