A CLASSIFICATION OF FOLDS : ROLE OF AXIAL ANGLE AND THICKNESS RATIO

Abstract

Geometric parameters/features of folds play a major role in classifying the folds in several ways as well as in making any systematic and comparative study of the various fold types in a structurally deformed terrain. The following parameters of fold geometry enable a simple classification of single-layer, symmetric, natural folds : thickness ratio (R), i.e. the ratio existing between the hinge thickness and the limb thickness ; axial angle (α) which is the angle for a given (outer or inner) trace or arc of a fold as subtended at the apex (hinge), and outer axial angle (αo) and inner axial angle (αi) which are the acute angles subtended by the outer and inner arcs respectively at the apex (hinge). Folds which are assumed to have initiated by buckling and have undergone thickening at the hinges are thus considered here as T (thickened) folds. Folds showing uniform layer thickness and with both the outer (αo ) and inner (αi ) axial angles of equal value constitute P (parallel) folds, while those in which the hinge thickness is less than the limb thickness - and therefore the inner axial angle ((αi) is smaller than the outer one (αo) - constitute S (supratenuous) folds. Folds which practically do not show any geometric relation between the above parameters constitute F (flow) folds that may form from any of the above (T, P or S) fold types at later/advanced stages of deformation. Each of these four types of folds thus show some definite values/conditions of the above-mentioned geometric parameters : for P-folds, T = 1, αo = αi ; for T-folds, T > 1, αo < αi ; for S-folds, T < 1, αo > αi , while for F-folds, no definite geometric relations amongst the above parameters exist.