ROTATION BY PURE AND SIMPLE SHEAR

Pure shear and simple shear - irrotational and rotational strain paths. Mixtures of these two strain histories exist - these are two endmembers. Start out with pure shear since it is simplest: :

1 principal strain ellipses axes have same orientation throughout deformation and remain the same material lines: 2 direct correlation between stress and finite strain axes

Occurs during:

1 compaction.perhaps during forceful intrusion.perhaps during some types of cleavage formation.

2 Simple shear deformation path best modeled by incremental slip of a stack of cards. Note that

a principal strain axes rotate position with time, a basic vorticity

b throughout history different material lines rotate in and out of principal axis position.

c long axes approaches shear plane throughout history

d a line can start out in shortening field and then rotate into the extension field

It occurs during:

1 ductile shear zone development.

2 certain types of folding.

3 basically anywhere have rock competency contrasts localizing shear at boundaries.