A QUANTITATIVE STUDY OF HINGE THICKNESS OF NATURAL FOLDS - SOME IMPLICATIONS FOR FOLD DEVELOPMENT

A quantitative analysis of some natural folds showing thickened hinges-designated here as "T-folds"-has been carried out. Two parameters of a fold have been recognized here: (a) the axial angle (alpha) is the angle for a given trace or arc as subtended at its apex (hinge); and (b) the thickness ratio (T) is the ratio between the hinge (T(h)) and limb (T1) thicknesses. The T-alpha-plots for a variety of natural folds yield a very typical curve that not only reflects interdependence of hinge thickening and axial angle but also bears significant implications for fold development. Assuming the initiation of folding due to buckling, it appears that, under a system of compressive forces, the naturally deformed buckled folds may retain their parallel fold geometry down to the axial angle (alpha) of about 130-degrees. Thereafter, i.e. alpha between 130-degrees and 70-degrees, the hinge thickening appears to be a rather slow process. With more enhanced deformation, i.e. alpha between 70-degrees and 30-degrees (when elevated temperatures also appear to play their part), the folds develop great mechanical instability and undergo marked flowage and mass transfer towards the hinge zone ("flattening"). At alpha < 30-degrees or so, the graph shows an asymptotic behaviour, possibly implying that with much enhanced deformation, the follow-up processes occur in a highly ductile environment, leading to gradual coalescence of the two limbs.