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-> Folding in geological structures
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Multilayer Structures and Chevron Folding
Multilayered structures have a tendency to form patterns with straight limbs
and sharp hinges, as shown in the picture on the right.
Since the rock is believed to behave elasticly during the deformation,
this is surprising. Think of a ruler, or an eraser - if you bend or buckle
one of these, it takes on a nicely smooth curved shape, not a sharp
hinge. This localization of the deformation into hinges is a consequence
of the multilayered structure.
The first step in understanding this phenomenon is by taking a stack of
layers, and buckling them without allowing any change in thickness:
Because of the geometry of a curved layer, the stack has to grow in thickness,
and voids are created between the layers. By changing the geometry we can change
the amount of void space that is created. Below we show three versions of the
stack, with different geometries. To save some space we only show a half-wave:
Note that it's the sharp hinge, straight limb geometry that has the least void
space.
In geological structures the overburden pressure (the pressure in the rock
that is due to the column of rock above) is large, preventing voids from
forming. The idea depicted in the figures above demonstrates how the deformation
should be localized in order to reduce the void space.
In summary, the overburden pressure limits the amount of voids that can open
during deformation. As a result, sharp hinged patterns are favoured above
rounded ones.
Last modified on October 22, 2004
by Mark
Peletier