Whether strain is physically meaningful surely depends on what it's used for. As a descriptor of change in shape it is very useful, and as Koenemann says, easy to measure and geometrically well-defined. It is not useful as a cause-effect relation, but that's not what it's used for.

This debate seems to have lost track of the basics of structural geology, in particular observations made of deformed rocks in the field. We already know (I think) that strain is a descriptive term, it does not describe the underlying deformation mechanisms involved in the change of shape it describes. Strain is also not a concept used to directly calculate stress or work done, this requires interpretations of rheology, as well as the deformation path which, as Koenemann rightly states, cannot be directly derived from a finite strain measurement.

I've found when teaching undergrad courses that strain is a very useful concept to describe changes in geometry, and it is also very useful to describe field observations of deformed rocks. I'm still to see any reference in Koenemann's discourses to observations relating to his theory. Unless the new theories can be used to improve our understanding and interpretations of field observations, I struggle to see how it can be more useful than strain, which is a very useful, and meaningful, quantity describing the observed geometry of deformed rocks.

----

Dr Ake Fagereng

Department of Geological Sciences

University of Cape Town

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>>> John Whalley <[log in to unmask]> 2011/04/07 12:57 PM >>>

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