CONICAL FOLDS: A POINTLESS MISREPRESENTATION OF PERICLINES

Reliance on π-diagrams limits the geometrical description of fold surfaces to simple geometric forms, a cylinder for "cylindrical folds” represented by a great circle and cones for “non-cylindrical” or conical folds represented by a small circle, and this has led to misclassification of periclines (doubly plunging domes/basins) and overrepresentation of conical folds in the literature. Analysis of virtual dynamically modeled periclines using synthetic stereograms, tangent-plots, SCAT, and Geologic Curvature Analysis, demonstrates small portions of a pericline are cylindrical whereas a significant portion are non-cylindrical. We show that while cones are a mathematically permissible solution to the small circle on a π-diagram they are an extremely poor geometric representation of the actual shape of real folds (e.g., periclines) especially fold terminuses. “Conical folds” terminate at a point. This requires the plunge and the ratio of the amplitude (A) to the width (W) of the fold to remain constant (A/W =½) along the crestal line to the terminus of the fold. Periclines terminate by a gradual dissipation of the fold and A/W decreases and the plunge varies along the crestal line of the fold. These differences have important implications for the rheological modelling of folds, and while realistic dynamic models for periclines exist, models for conical fold formation remain conceptual. Utilizing patterns of synthetic stereograms, it appears over reliance on interpretation of small circles in π-diagrams has led to overrepresentation and an elevated status for conical folds at the expense of periclines. We suggest that in order to advance our understanding of how folds form it is pointless to continue the misconception of conical folds as an accurate geometric representation for how folds end.