2-D shape preferred orientations of rigid particles in transtensional viscous flow

Ghosh and Ramberg [Tectonophysics 34 (1976) 1–70] studied the two-dimensional (2-D) rotational behaviour of individual rigid particles embedded in a viscous medium subject to simple shear and transpression (Sr>0), at low/medium shear strains. We now extend this theoretical study to transtension (Sr<0) by deriving a new analytical solution, to non-interacting populations of rigid particles and to high shear strains. We used different initial orientations of particles in the same graph to simulate populations of particles in an originally isotropic rock. Our results show that: (1) shape preferred orientations (SPOs) developed in viscous simple shear flow are transient and cyclical, except for particles with aspect ratio=∞ (or material lines), which tend to the shear direction; (2) stable SPOs can develop in transpression; they dip towards the shear sense but, when defining an S/C structure with the shear foliation, this composite fabric is always antithetic and, thus, indicates the wrong sense of shear; (3) stable SPOs can also develop in transtension; they dip opposite to the shear sense but, when defining an S/C structure with the shear foliation, this composite fabric is always synthetic and, thus, indicates the correct sense of shear; (4) SPOs produced in transpression or transtension can be used as vorticity gauges only when they represent stable orientations, which must be demonstrated a priori.