Refolding by flexural flow

Abstract

The existing method of derivation of the magnitude of simple shear (γ) in flexural slip and flexural flow folds has been extended to the general case of folds in which the radius of curvature varies continuously over a fold arc of any shape. In agreement with earlier conclusions, the present analysis shows that γ is equal to the dip angle (θ) measured in radians. This result is applied to the situation in which an early fold (F₁) on a passively behaving layer is refolded by flexural flow on the axial planar cleavage of F₁. Both the theory and the folding experiments with paper stacks show that the F₂ folds on the passively behaving layers show an unusual pattern of thickness variation, with the orthogonal thickness continuously increasing or continuously decreasing from one limb to the other without a maximum or a minimum at the hinge. Flexural slip or flexural flow folds may be identified from this characteristic pattern of thickness variation. The asymmetry of the smaller F₁ folds on the layering is strongly modified by later folding on the axial planar cleavage.