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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