Sense and non-sense of shear in flanking structures with layer-parallel shortening: implications for fault-related folds

We investigated the effects of fault-related folding mechanisms along a single fault that is pinned on both ends, upsection and downsection. Using a numerical model we produced flanking structures under plane strain transtension for the whole range between simple shear, general shear and pure shear, with layer-parallel shortening parallel and shear zone widening normal to the shear zone boundaries. Under these boundary conditions, contractional flanking folds with thrusting kinematics are the structures that are most likely to form and s-type flanking folds develop stable orientations. Comparison with natural examples reveals that contractional flanking structures occur from the outcrop scale within ductile shear zones, where they can be used as kinematic indicators in special cases, up to the mesoscopic scale within fold and thrust belts. The fundamental differences of our model to existing fault-related fold models like fault-propagation folds, fault-bend folds or break-thrust folds are: (1) the fault does not necessarily maintain a stable orientation but may rotate during progressive development; (2) the drag can change from reverse to normal along the fault; (3) the displacement along the fault has its maximum in the centre of the fault and decreases in both directions, downsection and upsection towards fixed fault tips.