John Ramsay
Cratoule, Issirac,
F-30760 St. Julien de Peyrolas
France
Tel.: +33 4 66 82 32 28
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Dear John, Mark and Tim
I am very surprised that the controversy of whether geological processes
are of simple shear or of pure shear nature still continues to come up in
discussions. These two types of deformation are singular end members of a
great spectrum of possibilities for finite and progressive strain. Simple
shear, or something close to it, can occur in certain types of shear zones
but pure or irrotational strains are most unlikely to be important in
tectonic processes. Most finite strains arising from tectonic processes lie
between the two end members and almost all will have significant rotational
components
The reason for this conclusion is clear when one studies the types of
displacement equations arising in tectonic processes. If rocks involved in
deformation processes form a continuum (which from my field experience is
generally the case), if originally planar marker surfaces such as bedding
are curved or folded and if the finite strain states one observes are
heterogeneous then practically all strains must have rotational components.
This is not to say that these strains have the specific rotations arising in
simple shear and, in fact, generally have rotational components less than
those of simple shear. These conclusions arise directly from the nature of
the displacement gradient equations. Irrotational finite strains can only
arise when the displacement gradient matrix is symmetric, a situation that
is generally not attained with the geometric constraints of all practical
tectonic situations. Discussions of this irrefutable mathematical argument
can be found in Ramsay and Graham 1970, (pp. 792,794 and 795) and Ramsay and
Lisle 2000 (pp 917 and the following worked examples).
References:-
Ramsay, J.G. and Graham, R.H., 1970. Strain variation in shear belts.
Canad J; Earth Sci. 7, 786-813.
Ramsay, J.G. and Lisle, R.J., 2000. The Techniques of modern structural
geology Vol.3, Applications of continuum mechanics in structural geology.
John Ramsay
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