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Orthogonal thickness

This is designated by t' and it is the minimum (or perpendicular) distance between two tangents of the same limb dip value on two surfaces. Since orthogonal thickness varies in naturally developed folds from the hinge to the inflexion point, the thickness for each a value is different. Therefore this is more properly designated as t'. The orthogonal thickness in the hinge region is measured perpendicular to the two tangents of a at the hinge point where limb dip is zero. This is known as hinge thickness and is denoted to. The ratio talpha /to for various values of limb dip is a useful parameter to describe the thickness changes in the folded surface relative to that in the hinge region. The values of t' may be plotted against a and curves drawn to determine the geometry of a fold. In subclass 1A folds t' exceeds unity, in subclass 1B folds t' has a value of 1 throughout, this being on ideal parallel fold; in subclass 1C folds the value of t' is less than 1 but greater than cos a . In class 2 folds the value of t' equals cos a while in class 3 folds the value of t' is greater than zero but less than cos a . It is not necessary that the limb of a fold analysed in this manner will result in a curve that would wholly trace its path through only one of the classes or subclasses. Since the mechanism of buckling in complex, the curve of t'/a may traverse almost through all the fields. For example the geometry of a fold in the hinge zone may vary between subclass 1A and 1B while in the limb areas it may be close to 1C. This is the most commonly encountered feature in slightly flattened concentric folds. The various mechanisms operating during the evolution of natural folds are responsible for such complex geometric forms of most of the fold structures.

t'>1 subclass 1A folds

t'=1 subclass 1B folds

cos alpha<t'<1 1C folds

t'=cos alpha Class 2 folds

t'<cos alpha Class 3 folds