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| Titel |
Localised Folding and Axial Plane Structures |
| VerfasserIn |
Alison Ord, Bruce Hobbs |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250047606
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| Zusammenfassung |
| The development of folds in layered rocks is commonly analysed using Biot’s theory of
folding. This theory expresses the deflection, w, of a single layer embedded in a weaker
medium in terms of the equation -4w4
-x + P-2w2
-x + F(w,x) = 0 Â where x is the distance
measured along the layer, P is a function of the mechanical properties of the layer and F(x)
is a function that represents the reaction force exerted by the embedding medium on the layer
arising from the deflection of the layer. In Biot’s theory F(w, x) is a linear function
of w and is independent of the wavelength of the deflection. The result is strictly
sinusoidal folding even at high strains with, most importantly, no localised deformation
in the embedding medium and hence no development of axial plane structures.Â
However, if the embedding medium weakens as the layer deflects or shows other
more complicated deformation behaviour then F is no longer linear and can depend
on the wavelength of the deflection. We explore the resulting behaviour in this
paper.
For a simple weakening response of the embedding layer to deflection, the initiation of
folds follows Biot’s theory and the initial folding response is sinusoidal. However, as the
folds grow and weakening develops in the embedding material the fold profile ceases to be
sinusoidal and the folds localise to form packets of folding along the layer. This behaviour is
reflected in the embedding medium as a series of localised deformation zones parallel to
the deflection direction, wsghat is, parallel to the axial plane of the folds. These
zones constitute micro-lithons, or in an initially finely layered material, crenulation
cleavages.
We also explore the situation where a layering arising from metamorphic differentiation
forms oblique to folding multi-layers early in the folding history. Now the function F also
depends on x and a variety of localised folds with associated axial plane structures develops.
An important observation is that shear displacements form parallel to the axial plane
structures during folding.
Our conclusion is that the response of the embedding medium to deflection of the layer(s)
to be folded is fundamental in controlling the style of folding and the types of axial plane
structures that form and future research should concentrate on the forms of these responses.
Most importantly, these axial plane structures form parallel to the deflection vector, w, and
hence are controlled by the kinematics of the deformation and are not parallel to a principal
plane of strain. |
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