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:

principal strain ellipses axes have same orientation throughout deformation and remain the same material lines

direct correlation between stress and finite strain axes

Occurs during:

compaction.

perhaps during forceful intrusion.

perhaps during some types of cleavage formation.

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

principal strain axes rotate position with time, a basic vorticity

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

long axes approaches shear plane throughout history

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

It occurs during:

ductile shear zone development.

certain types of folding.

basically anywhere have rock competency contrasts localizing shear at boundaries.

We have computer program that model these different behaviors, separately and in combo. You should play with them.