FOLD SHAPE ANALYSIS: INSIGHTS FROM NUMERICAL MODELING OF FLEXURAL SLIP MULTILAYER BUCKLE FOLDS

Multilayer buckle folds form a variety of different shapes such as sinusoidal, parabolic, chevron, box and elliptical folds. These different fold shapes simultaneously occur in vertical succession in a multilayer, and their unique geometric features indicate specific deformation mechanisms during folding. As one of these mechanisms, flexural slip has been identified. Quantitative analyses of fold shape evolution during buckling use the relationship between aspect ratio, P, and dip angle at the inflection point to determine fold shape. In this study, 2D plane strain finite element analysis is used to provide a quantitative evaluation of the fold shapes based on visco-elastic buckling of effective single layers and true multilayers affected by flexural slip, with particular focus on chevron and box folds.

For effective single layer buckle folds, which amplify the dominant wavelength, slip initiates early, chevron folds do not develop, and a transition from sinusoidal or parabolic to box folds is common. For lower amounts of slip, rounded box folds develop; increasing amounts of slip result in flat-topped box folds including M-shaped subsidiary folds. For true multilayer setups, both box folds and chevron folds develop for a variety of model setups. Box folds gradually develop through a progression from sinusoidal to parabolic to box folds. Key control parameters for the development of box folds are the ratio of incompetent layer thickness to competent layer thickness, s/h, the viscosity contrast, the friction coefficient, and the number of layers.

Systematic chevron folds featuring hinge collapse develop regardless of the initial perturbation used. The ratio s/h is a crucial parameter: for s/h>1, flexural flow dominates and slip is not initiated; for s/h<1, flexural slip initiates, and the lower s/h becomes, the more slip surfaces are activated. The results also document that flexural slip initiates during the later stages of folding and that sinusoidal or parabolic folds instantaneously change shape to chevron folds during a short period of slip initiation and termination, thus confirming the phenomenon of limb lock up.

The fold shapes from the numerical models are in good agreement with observations from the field and laboratory experiments and thus enable a quantitative assessment of dynamic fold development.