Engineering geology

Kinking and chevron folding

Kink bands are common in well-laminated and anisotropic rocks rich in phyllosilicate minerals, and some field occurrences are shown in Figure 1. Kink bandsare centimetre to decimeter wide zones or bands with sharp boundaries across which the foliation is abruptly rotated. Wider zones are sometimes referred to as kink folds. Kink bands and kink folds are characterized by their strong asymmetry and their Class 2 fold geometry.
They are closely related to chevron folds, which also are Class 2 folds, but differ in terms of symmetry.

Both are relatively low-temperature (low metamorphic grade) deformation structures where there is a significant mechanical anisotropy represented by lamination or repeated competent–incompetent layers and both imply
layer shortening.

a)
b)
chevron folding
c)
Figure 1. (a) Conjugate kink bands in mylonitized
anorthosite gabbro, Bergen, Norway. (b) Kink folds related to
Laramide thrusting in north Wyoming (Dead Indian Summit)
(c) Kink-like folds in oceanic sediments in Oman.

Chevron folds/kink bands form

Classic kink bands have very angular hinges and lack even the narrow hinge zone found in the outer arc of chevron folds. There is another important difference between the two. While chevron folds initiate with their axial surface perpendicular to the shortening direction, kink bands form oblique to this direction, typically in conjugate pairs.

When conjugate sets of low-strain kink bands are observed, such as the examples portrayed in Figure 1.a, b, s1 or ISA1 is commonly assumed to bisect the sets, as shown in Figure 2. As stated before, going from strain to stress is not straightforward, but the smaller the strain the better the correlation.

When a single set of kink bands occurs, we know that s1 is oblique to the band, but its precise orientation is unknown because kink bands may rotate during progressive deformation. In addition, we still do not understand kink band formation in detail, and there seem to be several mechanisms that apply.

Figure 2 (a) The orientation of s1 can be determined
from the orientation of conjugate sets of kink bands.
(b) Continued kink band growth can produce chevron folds.

Kink folds generated by bending do not directly reveal the orientation of stress. Such kink folds have orientations that are controlled by the local geometries of ramps or fault bends. Hence, in such cases the bisecting axis between two kink zones does not in general represent s1 or ISA1.

Experiments have shown that conjugate sets can nicely merge to form chevron folds if strain is high enough (around 50%) (Figure 2.). However, 50% shortening is not commonly achieved by kinking in naturally deformed rocks, so this way of forming chevron folds may not be the most common one after all.

Classic chevron folds with beds on the centimeter scale are more likely to form by flexural slip of multilayered rocks during layer-parallel shortening, as illustrated in Figure 3. The typical setting is where competent beds are separated by thin incompetent layers, for instance, quartzite or chert separated by shale or phyllite.

Flexural slip then occurs between the competent layers, which are strained only in the thin hinge zones. Just like buckle folds, the hinges have to stretch in the outer arc and shorten in their inner parts. Figure 4 shows an example of this, where extension veins have formed in the outer arc and (less obvious) contractional structures dominate the inner arc.

Figure 3 Chevron folds forming by the flexural slip
mechanism imply a space problem in the hinge zone that is
resolved by ductile flow of the incompetent (dark) layers or
collapse of the competent layers in the hinge zone. Strained
parts of competent layers are marked in red, showing that layer
thickness is maintained on the limbs.

Furthermore, geometric problems in the hinge zone require flow of the incompetent rock into the hinge, or alternatively inward hinge collapse of the competent bed as seen in Figure 3.
Hinge collapse is particularly common in relatively thick competent layers that occur between thinner ones.

Figure 4 Extension fractures (veins) in the outer,
stretched arc of folded competent layers. Chevron folds,
Varanger, northern Norway.

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