When do dragfolds not develop into sheath folds in shear zones?

Using a unified model for high-strain zones, rotation paths of material lines in different flow types are investigated and presented as stereographic projections. A material fabric element such as a mineral lineation or a fold axis can be plotted on such stereograms and the rotation path revealed. Using this method, we examine the rotation of folds and the development of sheath folds in general three-dimensional zonal deformation. We show that the evolution of dragfold geometry in a high-strain zone depends on the flow type and the initial perturbation of the fold axis. In simple shear zones and thickening zones (monoclinic or triclinic), dragfolds will evolve into sheath folds. In monoclinic thinning zones with biaxially stretching boundaries, if the shear direction is parallel to the maximum stretching direction of the boundary, sheath folds will develop. If the shear direction is parallel to the minimum stretching direction of the boundary, dragfolds become more cylindrical as deformation advances. In thinning zones with boundaries equally stretched in all directions, the likelihood of sheath fold development depends on the ratio of the shear strain rate to the thinning rate. In triclinic thinning zones, fold evolution depends on the details of the flow and the value of initial fold axis perturbation. (C) 1999 Elsevier Science Ltd. All rights reserved.