Two mechanisms – one result
Flanking folds form in response to local inhomogeneous deformation in the rock. Two groups of mechanisms of formation have been proposed, which are of different nature, but which can produce very similar geometries:
1 – Object-related flanking folds
These are structures that form in a continuum where a planar object or CE of different rheology is present oblique to the HE. If a CE of lithology B is embedded in lithology A, flow in both units will be different because of different responses to the imposed external stress field. Refraction of foliation through layers, and the deflection of foliations around rigid porphyroclasts in metamorphic rocks are examples of this kind of behaviour. Flanking folds can form at the contact to a CE with a different rheology, even if the rheology is uniform in A. A CE with different lithology will produce "strain shadows" on both sides where the stress field will be affected by the presence of the CE; as a result, folds will develop there. This kind of flanking fold is least predictable in the types of geometries that will form; fold shape depends on the shape, rheology and orientation of the CE. In the case of dykes or fractures, the wall rock may have been altered adjacent to these, which effectively gives a CE composed of three parallel rheological zones; flanking folds commonly develop in such alteration zones.
2 – Andersonian flanking folds
As first recognised by Anderson (1905,1905a), principal axes of stress must be parallel and normal to "free surfaces", external surfaces of a solid body where these are in contact with a medium that cannot support a shear stress, such as air, water or magma. The Earth´s surface is such a free surface, and the orientation of brittle structures that interrupt it or form close to it is therefore affected as a boundary effect. This is visible in faults that have preferential orientations at the Earth´s surface, and in secondary mudcracks, which usually impinge on primary ones at right angles. What is not often explicitly said or realised, however, is that the "Anderson principle" also applies to free surfaces that occur deep within the Earth’s crust or upper mantle. Fluid-filled cavities or faults can form if fluid pressure equals lithostatic pressure, and they can form at any depth. If such a fluid-filled inclusion transects a marker such as a foliation or bedding, deformation close to the inclusion is governed by the Anderson principle and stress axes will rotate to be parallel and normal to the contact. As a result, fluid inclusions will strongly deflect the orientation of the stress field and the far field deformation will be different from the deformation close to the inclusion. However, in this case the Anderson principle governs the orientation of ductile deformation structures rather than brittle faults. In many cases, this leads to the development of flanking folds (Arslan et al. 2008). In fact, in the absence of a clear lithology contrast, the presence of flanking faults along a planar surface in the rock implies that it acted as an open and probably fluid-filled fracture during ductile deformation in the wall rock. An extreme example of such faults are foliation boudins, where flanking folds develop in opposite geometry on both sides of a lozenge-shaped cavity that can reach a fluid-filled volume of several cubic metres, even in the middle crust (Arslan et al. 2008).