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FLEXURAL SLIP Folds may form by a mechanism called flexural slip or simply by sliding of successive layers, one above the other. The individual layers themselves are not particularly deformed i.e. they do not undergo significant internal deformation and their mineral and chemical composition at the end of buckling process may not be much different from that of the original. These folds are particularly common in rocks of alternate competent and incompetent layers with the viscosities of the competent layers remaining more or less uniform across the complex. These folds are true parallel folds and they have a subclass 1B geometry, their orthogonal thickness remaining more or less constant across the multilayered complex. In a flexural slip fold, the boundaries of less competent layers act as boundaries of shear zones. In other words, each less competent layer can be considered to be a minor shear zone. Most of the internal deformation in flexural slip folds occurs within the less competent layers by partial recrystallization and development of cleavage. Most of the competent layers undergo only mechanism such as pressure solution and development of sigmoidal extension fissures. Thus in flexural slip folds the overall strain is heterogeneous. While cleavage developed in less competent layers is usually parallel to the principal plane of the finite or incremental strain ellipsoid and may have sigmoidal trajectories in fold profile, the spaced fractures developed in more competent units may be parallel to the planes of no finite longitudinal strain of the finite strain ellipsoid, as shear displacements are sometimes observed along these planes. If the less competent layers are of infinitesimally small thicknesses,
The competent layers simply slide over one another and when this happens, the gaps between them act as shear veins and slickensided quartz fibres are produced together with extension veins(see fig in chevron folds page). If the less competent layers are thick enough, the extension veins developing across the layer boundaries are drawn along the cleavage planes in less competent layers giving them a pseudo-sigmoidal form and an anomalous curvature, which is not compatible with the typical sigmoidal tension gash veins and are opposed to the shear sense. At advanced stages of folding by flexural slip, the outer arcs show extension fissures and inner arcs cleavage development but under special circumstances, the inner arcs of competent layers may show extension fissures and the outer arcs fissures parallel to folded layering. In flexural slip folds the overlying bed moves towards the antiformal hinge
or away from the synformal hinge. If the simple shear is non-uniformly
distributed throughout the buckled layers, the mechanism is called flexural
flow. Thus the shear is inhomogeneous in flexural flow folds but more or less
homogeneous in flexural slip folds. The minor structures associated with
flexural slip folds are extension veins, sometimes extension faults, slickensided quartz, chlorite or calcite fibres
in fine acicular form on bedding planes, sigmoidal cleavage in less competent
layers and they can be associated with extension of fold hinges during late
stages of fold development. The slickensided fibres may lie perpendicular to the fold hinge if the fold
elements are symmetrically related to the principal axes of strain, otherwise
folds develop by oblique flexural slip, that is, the fibre growth direction on
the limbs is not perpendicular to the hinge line but at some angle that
departs from 90. Dubey has shown that initially as the fold develops at a
point of perturbation in a layer, the fibre growth is in various directions
and at various angles to the culmination point which develops over the initial
perturbation but with progressive development of folds both in the direction
of amplification, and along the hinge, the fibre growth becomes eventually
perpendicular to the fold hinge. As the amount of shear in the hinge area
decreases to zero, no slickensided fibre growth can be noticed on the hinges
of flexural slip folds. In flexural slip folds, the amount of slip changes during folding producing
curved slickensided fibres on layer surfaces and as the folds develop, the
shear strain or angular shear strain increases with increase in the limb dip.
The amount of slip is also directly proportional to the thickness of the
layer. Flexural slip folds are important as their mechanism enables
mathematical formulae regarding the amount of slip to be derived. The amount
of slip has no relation to the degree of curvature of the hinge area. Flexural
slip folds can also be produced in material which is not perfectly layered as
experimentally shown by model analogue experiments by Kuenen(1938) and de
Sitter (1958). Most of the chevron folds and conjugate kink-bands are examples
of flexural slip folds, in anisotropic material and since they have ideal
shapes, they have been (Ramsay 1974; Lloyd and Whalley, 1986, Sanderson,1974;
Dubey 1982, Behzadi & Dubey, 1981; Ghosh, 1968)studied by rigorous
experimental and theoretical modeling.
FLEXURAL SLIP, OBLIQUE The slickensided fibres may lie perpendicular to the fold hinge if the
fold elements are symmetrically related to the principal axes of strain,
otherwise folds develop by oblique flexural slip, that is, the fibre
growth direction on the limbs is not perpendicular to the hinge line but
at some angle that departs from 90. Dubey has shown that initially as the
fold develops at a point of perturbation in a layer, the fibre growth is
in various directions and at various angles to the culmination point which
develops over the initial perturbation but with progressive development of
folds both in the direction of amplification, and along the hinge, the
fibre growth becomes eventually perpendicular to the fold hinge. As the
amount of shear in the hinge area decreases to zero, no slickensided fibre
growth can be noticed on the hinges of flexural slip folds. In flexural slip folds, the amount of slip changes during folding
producing curved slickensided fibres on layer surfaces and as the folds
develop, the shear strain or angular shear strain increases with increase
in the limb dip. The amount of slip is also directly proportional to the
thickness of the layer. Flexural slip folds are important as their
mechanism enables mathematical formulae regarding the amount of slip to be
derived. The amount of slip has no relation to the degree of curvature of
the hinge area. Flexural slip folds can also be produced in material which
is not perfectly layered as experimentally shown by model analogue
experiments by Kuenen(1938) and de Sitter (1958). Most of the chevron
folds and conjugate kink-bands are examples of flexural slip folds, in
anisotropic material and since they have ideal shapes, they have been
(Ramsay 1974; Lloyd and Whalley, 1986, Sanderson,1974; Dubey 1982, Behzadi
& Dubey, 1981; Ghosh, 1968)studied by rigorous experimental and
theoretical
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