We are pleased to see that our recent publication (Ha'nmer and Passchier 1991) has come to the attention of Derek Flinn . However, we are surprised by his negative attitude, as well as by the form taken by his contribution. As clearly stated in our title Shear-sense indicators: a review, we reviewed a large body of work by countless researchers in the field of vorticity analysis. Yet Derek Flinn appears to take issue with the concepts, and thereby their originators, rather than our presentation of them. Indeed, much of his contribution expresses his disagreement with researchers whose work is not even the subject of our review. Furthermore, he contends that we reinforce the 'widespread view that monoclinic structures are due to simple shear and that all deformation [sic] is simple shear'. This is surprising, given our insistence on general non-coaxial flow, as opposed to idealized flow types such as simple shear, and our treatment of the kinematic vorticity number of flow ( W,; ), a useful quantitative expression of deviation from both simple and pure shear flows. Far from ignoring the possibility that deformation structures may result from progressive pure shear, we were explicitly concerned with structures which form in more complex, general non-coaxial flows. Clearly the subject requires further elaboration.
Take the case of sheath folds (Cobbold and Quinquis 1980), just one of the examples mentioned by Derek Flinn. It was not our intention to examine the myriad of possible strain paths leading to their formation, but to caution against their injudicious use as shear-sense indicators even where they had formed in non-coaxial flow. Flinn states that we consider that 'shear zones are zones of simple shear, possibly accompanied by a minor component of two-dimensional pure shear . . .'. Again, this is surprising as a kinematic vorticity number ( Wk) approaching zero (our Figure ) is equivalent to pure shear with a minor component of simple shear. We present many natural examples of winged inclusions whose long axes have come to rest at an orientation which is not a stable position of rest in non-coaxial flow unless the kinematic vorticity number of the flow ( Wk) approaches zero (see our Figure , and the gist of the entire first chapter of our paper). One of those examples was even used to illustrate the cover of the paper. We also discuss, at some length (pp. 44-52), the now common observation that some structures may appear to rotate backwards, whereas others really do rotate backwards with respect to the kinematic axes and the bulk vorticity of the flow. With the aid of 14 illustrations, we carefully explain that the former case is well known in pure shear flow (e.g. Ramsay 1967), whereas the latter case requires a sign$hznt deviation from simple shear.
Derek Flinn concludes that we 'divorce [our] determinations of shear sense from any interpretation of the rock mass . . .'. However, five pages of the first chapter are dedicated to examining the influence of CCC 0072-1 050/95/020 197-02