On tangential longitudinal strain folding

Intrinsic two-dimensional analysis of the geometry of fold profiles origina ted by tangential longitudinal strain has allowed us to examine several fea tures of this folding mechanism. The strain distribution in, the folded lay ers indicates that the direction of the strain ellipse axes has a deviation with respect to the directions normal and tangential to the boundaries of the folded layer, which mainly depends on the curvature changes. The geomet ry of the folded layers involves layer thickening in the hinge zone (single hinge folds of class 1C, and double hinge folds composed of classes 1A and 1C); when migration of the neutral line is allowed, layer thinning occurs in the hinge zone (single hinge folds of class 1A, and double hinge folds c omposed of classes 1C and 1A). Nevertheless, in most cases, the folds are c lose to parallel folds. The bulk shortening due to tangential longitudinal strain mainly depends on the amplitude of the folds and scarcely depends on the shape of the folded surfaces. When the original thickness of a layer i s large in relation to the radius of curvature, two gentle minor antiforms separated by a. synform appear in the inner are close to the hinge zone of the major antiform. This minor folding involves local thickening in the hin ge zone, which is observed in some natural folds as a protuberance in the i nner are of the hinge zone. (C) 2000 Elsevier Science B.V. All rights reser ved.