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Grain Shape and Preferred Orientation Changes

The pure shear deformation seen in the movies produces a sub-horizontal foliation defined by an alignment of grain shapes.

Many of the original grains, both in the movies and in the FLAC model below that displayed higher order birefringence colours have disappeared. This reflects the development of a more permissive preferred-orientation within the sample and the evolution of a new fabric
 
Figure 1. 4. 1
Figure 1. 4. 1. Polycrystalline aggregate showing: Undeformed shape of grains and slip-plane traces and initial random two-dimensional orientation of slip-plane normal (c-axes) with respect to the specimen orientation. Deformed by 29% shortening, the grains in the aggregate show elongation parallel to the extension direction and the c-axes are concentrated in a bi-symmetrical pattern about the shortening axis. This 2D FLAC model is taken from Wilson & Zhang (1996).
 

A two-dimensional view of Undeformed grains in a polycrystalline aggregate and slip-plane traces and initial random orientation of slip-plane normal (c-axes).

Deformed aggregate shows grain-shape alignment parallel to the extension direction and the c-axes are concentrated in a bi-symmetrical pattern about the shortening axis.

 
Figure 1. 4. 2
Figure 1.4.2. Stereographic projections that contrast the random c-axis preferred-orientation in undeformed verses strong fabric in deformed sample. The broken line represents the grain-shape orientation and hence foliation in the sample.

Three-dimensional change from randomly oriented c-axes in undeformed aggregate (see fig. 1.4.2) to a distribution containing two concentrations of c-axes symmetrically distributed about the foliation plane.

These concentrations are symmetrically related to the shortening axis and the plane of flattening defined by the grain-shape distribution.

In this pure-shear environment we have the situation where the principal axes of the strain ellipsoid do not rotate.

The plane of flattening in the corresponding strain ellipsoid will coincide with the alignment of grain-shapes.

The c-axis fabric development is symmetrically related to the shortening direction.

Figure 1. 4. 3
Figure 1.4.3. The c-axis distributions in pure shear versus a simple shear regime.
 
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Created: August 23, 1999
Last modified: March 15, 2004
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