Kinematic analysis of symmetrical natural folds developed in competent layers

Abstract

Analysis of symmetrical folds developed in competent layers yields a kinematic folding model based on the study of the geometric characteristics of the folded layer, finite strain measurements and c-axis preferred orientation of detrital quartz. The meter-scale folds that we studied were formed during the first phase of Variscan deformation in the northwestern Iberian Massif. The strain sequence of the proposed model begins with initial layer shortening (ILSH) followed by tangential longitudinal strain (TLS), flexural flow (FF) and, finally, flattening (FL). In each fold, the intensity of each strain pattern has a small variation. FF is much less important than TLS and usually occurs after the latter due to the geometric incompatibilities developed during TLS. Neutral surface migration cannot completely explain the misfit between the strain measurements of the natural folds and the strain values predicted by the combination of the different strain patterns. This problem may be solved by considering a heterogeneous distribution of ILSH and FF, together with a modified tangential longitudinal strain. Heterogeneous area change across the fold profile could have also had an influence.