Double-edge fault-propagation folding: geometry and kinematics

Fault-propagation folding is a common folding mechanism in thrust-and-fold belts and accretionary prisms. Several geometrical models relating the fold shape to the ramp shape have been proposed. In all these models, ramps always emanate from a basal fault and propagate upwards. We have developed a new kinematic and geometric model of fault-propagation folding, named double-edge fault-propagation folding. The model simulates folding at thrust ramps as a function of their nucleation site and propagation history within the folded multilayer. The fold shape depends on the initial length and location of the ramp, its dip, and the S/P ratio (i.e. incremental ramp slip versus propagation) of both the upper and lower ramp tips. This solution increases the geometrical flexibility of fault-propagation folding reducing, for example, the direct dependence between the backlimb dip and the ramp dip, as double-edge fault-propagation folding is characterised by a backlimb panel not necessary parallel to the ramp. Non-parallelism between the ramp and the backlimb is commonly observed in thrust-related anticlines, within fold-and-thrust belts and accretionary prisms. The excess layer-parallel shear imposed by the development of double-edge fault-propagation folding can be easily accommodated by discrete faulting and/or penetrative deformation. The dependence of the fold shape on the fault behaviour provides a tool for including the role of mechanical stratigraphy and environmental conditions of deformation into kinematic models. Natural examples of anticlines that could be modelled by double-edge fault-propagation are presented.