Low-amplitude evolution of break-thrust folding

This paper proposes a mechanistic explanation for fault-related, flexural-slip folds that form predominantly by fixed-hinge folding (i.e. break-thrust folds). It also presents a stepwise evolutionary history for these folds which contrasts with the model suggested for self-similar, migrating-hinge, fault-propagation fold models. Both mechanical folding theory and rock-fabric evidence suggest that folding in advance of a growing thrust fault can evolve in a five-step process: (1) initial sinusoidal buckling instability ( < 2% permanent strain; < 10 ° limb dips), (2) hinge migration from the symmetric buckling instability to an asymmetric kink geometry at very low strains (2–6% strain; 12 °–20 ° forelimb dip) which is driven by interlayer shear stresses, (3) fold growth by limb rotation with fixed anticlinal and synclinal hinges, and propagation of a flat thrust beneath the fold (6–20% strain; 20 °–35 ° forelimb dip), (4) fold locking caused by space problems and the inability to overcome the increasing angular shear required to continue flexural slip folding (20–36% strain; 35 °–50 ° forelimb dip), and (5) partitioning of deformation accommodated by highly strained fold limbs, faulting on the flat basal décollement beneath the entire fold creating a detachment fold, or by continued faulting at the sub-fold ramp resulting in a fault breaking through the earlier fold.