The role of flexural slip during the development of multilayer chevron folds

Abstract

Flexural slip has long been recognized as a crucial mechanism during the development of chevron folds and a conceptual understanding of its role has been presented by analogue and kinematic models and field studies. This study uses 2D finite element analysis to provide a thorough quantitative evaluation of flexural slip based on buckling of effective single layers1 and true multilayers1. The analysis of the results describes the spatial and temporal evolution of flexural slip and documents the resulting fold shapes. The results show that effective single layer buckle folds amplifying the dominant wavelength, λd, do not result in chevron folds in any of the situations considered - varying initial perturbation, viscosity contrast and friction coefficient. Slip initiates early, and a transition from sinusoidal or parabolic to box folds is common. Reducing the amplified wavelength (~10% of λd), results in systematic chevron folds. For true multilayer buckle folds, systematic chevron folds featuring hinge collapse develop regardless of the initial perturbation used. The ratio of incompetent to competent layer thickness, s/h, is a crucial parameter: for s/h > 1, 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 occurrence of slip is localized near the hinge zone, which reflects the mechanical decoupling of the competent and incompetent beds. The results also document that flexural slip initiates during the later stages of folding (for limb dip angles >53°) and that sinusoidal or parabolic folds nearly “instantaneously” change shape to chevron folds during a short period of slip initiation and termination, thus confirming the phenomenon of limb lock up. For models incorporating permeability contrasts, the development of deformation induced overpressure leads to an earlier initiation of flexural slip.