HINGE FRACTION PARAMETER

The figure shows Bayly's hinge fraction parameter, which is the ratio of the hinge fraction, or portion that can be said to belong to the hinge since the limb dip is gradually increasing, to the limb fraction, or the portion up to the inflexion point that can be said to belong to the limb since the change of limb dip is very rapid. Generally, the arc length from the hinge point to inflexion point is set to unity. For example in cuspate folds, the hinge fraction is very large and the parameter may have a value as large as 0.9 or more. In contrast, in angular hinged or chevron folds, the hinge fraction is very small and limb fraction too large and the parameter may have a value as small as 0.05 or even less. This classification was proposed by Bayly to overcome the flaws in Fleuty's classification that even though the interlimb angle is the same, the hinge forms can be different and therefore the interlimb angle is only a measure of the flattening in folds in which extreme shortening has occurred. But tightness alone cannot define the fold shape properly. For example a very broad hinge zone fold with parabolic shape can also be very tight. Bayly (1974) devised a parameter called hinge fraction parameter. If the total arc length between hinge point and inflexion point are equally divided, hinge fraction is then that fraction of total length which contributes to the hinge area, i.e. the limb dip in this zone varies extremely gradually at infinitesimally small intervals, the rest being the limb fraction i.e. limb dip up to the inflexion point remains fairly constant or increases at a very rapid rate. Hinge fraction is maximum for outer arcs of cuspate folds while it is minimum in chevron folds.