Some considerations on the kinematics of chevron folds

Geometrical modelling and field analysis of chevron folds suggest that these structures are a result of the combination of several kinematical mechanisms, whose magnitude and order of application vary within certain limits. A possible mechanism operating early in the fold growth is homogeneous layer shortening, whose contribution is restricted by the high competence contrast that the multilayers developing chevron folds usually exhibit. In the early stages of folding, when the curvature is small, equiareal tangential longitudinal strain (ETLS) is an essential mechanism, since the operation of parallel tangential longitudinal strain (PTLS) or flexural flow (FF) would give rise respectively to area changes or strain in the limbs of the final fold that are too high to be geologically realistic. After folding by ETLS, probable mechanisms are PTLS and FF, which can operate in this order or simultaneously. In general, FF is necessary at the last stage of buckling, although the increment of folding due to this mechanism can be very small. High values of slip between layers and area change produced in the later stages of chevron folding can bring an end to buckling, probably at an interlimb angle value of 60–70°, and induce the onset of homogeneous strain (HS). This strain is not coaxial in many cases, with simple shear playing an important role, and gives rise to asymmetrical folds.