Since the time of Bruno Sander (Sander 1948). most structural textbooks have commenced with a section on deformation explained in terms of pure shear and simple shear. With time, the pure shear has been treated increasingly cursorily, and reduced to a two-dimensional phenomen hardly mentioned in the rest of the book. The position has now been reached where an author can state. even in the abstract to a paper, that pure shear is 'uncommon' in deformed rocks . On the other hand. simple shear has become increasingly popular as a model for rock deformation, possibly because of the ease with which a pack of cards can be deformed by the imagination.
The book by is a survey of much of structural geology, which nowhere envisages the possibility of any structures being the result of pure shear. In particular, the authors adopt the position that all shear zones are zones of simple shear, possibly accompanied by a minor component of two-dimensional pure shear so oriented that its axis of shortening is normal to the shear plane of the simple shear. This very special-case of deformation they name 'general noncoaxial flow', although they do refer to it as 'ZD-general noncoaxial flow' three times in the book (but not where they define the term).
This may seem to most readers a reasonable terminology as 'shear' and 'shear zone' are in such wide general use as synonyms for simple shear. However, their use of the term 'shear zone' in this book is not defensible on these terms, because in the second sentence in the book the authors define the term 'shear zone' to include 'contact zones between tectonic plates' and 'boundaries between allochthonous masses occurring at all scales within orogenic belts', thus extending shear zones to cover crustal-scale rock deformation including environments where pure shear is dominant. Yet they present simple shear as the invariable or, at least, dominant rock-deforming process. They accept as of universal applicability the results of theoretical studies of simple shear deformation, such as the work of Simpson and de Paor (1993) and many .earlier studies. They apply them (kinematic analysis) to natural rocks with no attempt to determine the true nature of the deformation (deformation analysis).
The deformation expected to occur on such a large scale as that of 'contact zones between tectonic plates', and indeed in many similar cases on a much smaller scale, is simple shear and pure shear combined in any (including zero) proportions. Furthermore, simple shear and pure shear are not equal and opposite types of deformation as presented by the authors and most structural textbooks since Sander (1948). Simple shear is two dimensional ( k = 1) and pure shear is three dimensional ( O < k < a ) so that the 'general' field between these extremes is infinitely larger than the '2-D general field' adopted by the authors. The authors devote one-third of the book (24 pages) to expounding their (two-dimensional) general non-coaxial deformation, but nowhere offer a justification for restricting the study of deformation in our three-dimensional geological world to two dimensions, nor indeed for presenting simple shear as the dominant rock deformation. Furthermore, although two-dimensional rock deformation, where it does take place, occurs in three-dimensional rocks, they only consider two-dimensional deformation within the two-dimensional deformation plane. Out of 8 1 figures only six show three dimensions, and all of these are of folds and none of fabrics.
On page 24, at the end of the theoretical section, they supply instructions enabling the reader to apply their methods in the field. The reader is told to examine the deformation plane (normal to the foliation and parallel to the extension lineation) for shear-sense criteria 'in a shear zone corresponding to either progressive simple shear, or 2-D general noncoaxial flows of the type discussed in the text'. That is all! No hints are given on how to recognize a simple shear zone and in none of the examples in the rest of the book is such a deformation analysis carried out. The reader is not reminded that eight pages back he or she has been told to ensure that the lineation is 'truly an extension lineation and . . . subparallel to the direction of transport', although, according to them a long list of quoted authors have questioned this relationship between the shear direction and the extension lineation. Nor is the reader told