Two mechanisms for forming flanking folds

Cees Passchier, Vincent Heesakkers, and Sara Coelho

Flanking folds are fold trains or shearband-like structures in layered rocks that are symmetrically arranged around a cross-cutting element (CE) such as a vein, alteration zone, or fault. The folds only occur close to these CE and seem to "flank" them, hence the name. Most flanking folds form by either of two very different mechanisms, although the structures produced can be very similar: (1) Disc-shaped planar high-viscosity CE such as veins, dykes or alteration zones around faults or thin veins can partly protect the adjacent part of the host rock from ductile deformation (object-related flanking folds). If the CE are originally oblique to a marker horizon such as layering, and if the entire assemblage is deformed in non-coaxial flow, flanking folds will develop in the marker horizon because of the difference in flow geometry in the far field and close to the CE. (2) If a brittle fault is active during ductile non-coaxial flow in the wall rock, principal stresses along the fault must be oriented parallel or normal to the fault surface because of the Anderson principle (which states that principle stress must be parallel or perpendicular to a "free" surface not to a fault surface). As a result, flanking folds develop in the marker horizon (Andersonian flanking folds). The presence of flanking folds along a fault plane without alteration zones are therefore an indication that a brittle fault was active during ductile deformation. Analogue experiments and field examples from Namibia are used to illustrate both types of flanking folds.