Examples from Namibia
1 – object-related flanking folds
The structures in Fig. 4 are developed in metaturbidites of Neoproterozoic age on the Lower Ugab Domain, Namibia, which were invaded by syenite plutons during ductile deformation at greenschist facies conditions in the Cambrian (Passchier et al. 2002, 2007). Close to the plutons, the metaturbidites were invaded by aplitic and pegmatite veins. Fig. 4 shows an outcrop where the tips of such veins fractured the layered metaturbidites and locally developed 1-2 cm wide zones of alteration in the wall rock (in blue in fig 4). The metaturbidite is locally enriched in albite. On the photograph it is clear that wherever alteration zones are present, hook-shaped flanking folds with neutral slip have formed such that the angle between CE and HE at the intersection points is nearly 90°. Where no alteration zones formed around the fractures, in the top of the outcrop, no flanking folds formed. We interpret this to indicate that the original angle between the fractures and HE was steep, possibly close to 90°. Ductile flow in the metaturbidites reduced this angle to ~30°, except where alteration zones were present; there, flow was deviant from the far field conditions, and preserved the original steep angle between CE and HE to some extent. We interpret the development of this structure to be similar to that shown in Movie 1.
2 – Andersonian flanking folds
We show two types of Andersonian flanking folds from Namibia, hook-shaped and shear band shaped, but both with reverse slip.
(a) The first example in Fig. 5 shows layering in turbiditic marbles from the lower Ugab domain, which was interrupted by minor faults, along which hook-shaped flanking folds developed with reverse slip. The white layer in the photograph was originally one continuous bed in the stratigraphy. The amount of slip along the faults decreases downward as can be seen in Fig. 5, and the faults terminate in the marble before reaching the lowest beds visible. No alteration zones are visible, and the faults do not show any deposition of vein material or alteration rims. We interpret this type of flanking fault to have formed by slip on the fault planes during ductile flow in the marble away from the faults; faults either formed by enhanced fluid pressure or relatively high strain rate. Since the marbles have statically recrystallised, it is impossible to estimate differential stresses during ductile flow. Faults may have generated as fractures nearly oblique to layering, after which the rocks underwent sinistral shear while the fractures remained open and acted as dextral faults. Sinistral shear sense can be determined independent of the flanking folds from the shape of quartz sigmoids (Passchier and Trouw 2005) in the same outcrop. The Anderson principle caused the marble close to the faults to flow by a different flow type than that in the far field, creating the flanking folds. We imagine that the evolution of this structure was very similar to that shown in movie 3. Hook-type flanking folds with reverse slip are amongst the most common types of flanking folds.
(b) Spectacular hook- and shear band type flanking folds develop along thin quartz veins in marbles near the "House of the German" (HOG) in the central Goantagab domain, Namibia (Passchier et al. 2002; Coelho et al. 2005). We refer to these features as HOG-structures after the location. Here, two sets of older quartz and carbonate veins have been boudinaged to asymmetric shear band boudins and sigmoids, indicating sinistral shear sense. This shear sense also fits shear sense on a regional scale determined during our mapping project (Passchier et al. 2002). Slip along the faults was dextral, opposite to the ductile bulk shear sense: This slip sense is indicated by layer offset along the faults, and the shape of quartz-filled jogs within the fault (Fig. 6). More than 60 faults, with lengths varying from 10-300cm were investigated, and all show the same geometry of flanking folds along the length of the faults. Figs. 6 and 7 show typical examples; in the centre of the faults, shear band geometry flanking folds with reverse slip dominate; these grade laterally into neutral and then hook-shaped flanking folds with normal slip. All flanking folds show dextral displacement of the layering with a gradation of tight folds to open folds as the amount of slip decreases from the tips of the faults into the wall rocks.
How can these enigmatic structures be explained? At first sight, the shear-band flanking folds suggest a two-phase evolution of sinistral slip and shear band formation, followed by dextral offset. However, the development of these structures can be explained by a single stage of tectonic evolution. The shear band shaped flanking folds with reverse slip can be explained if the angle between faults and bedding was originally small; a sinistral bulk shear sense in this case will steepen the faults with respect to the foliation, while inducing a dextral slip sense along them. Due to the Anderson principle, the layering close to the fault retains a relatively small angle to the fault and therefore develops a shear band shape, which is not due to shear band drag on the fault since the slip is reverse. At the tips of the fault, slip induces S-shaped folds that transform into hook-shaped flanking folds when the faults propagate outwards through them. (Fig. 7) This type of flanking folds seems to be rare in nature, but we compare its development with that shown in movie 4.
Figure 6. Shear band type folds
Shear band type folds along a fault with reverse slip formed by bulk sinistral shear sense. A quartz filled jog in the vein indicates slip sense is dextral on the fault. Location 14.4299E, -20. 6756S
Figure 7. Shear band type folds
Shear band type folds along a fault with reverse slip formed by bulk sinistral shear sense, overprinting isoclinal folds in marble. Location 14.4299E, -20. 6756S