Particles in shear enclose important information about a rock's past and
can potentially be used to decipher the kinematic history and mechanical
behavior of a certain outcrop or region. Isolated rigid clasts in shear
zones often exhibit systematic inclinations with respect to the
shear-plane at small angles, tending towards the instantaneous
stretching direction of the shear zone. This shape preferred orientation
cannot be easily explained by any of the analytical theories used in
geology. It was recently recognized that a weak mantle surrounding the
clast or a slipping clast-matrix interface might be responsible for the
development of the observed inclinations. Physical considerations lead
us to conjecture that such mantled, rigid clasts can be effectively
treated as voids that are not allowed to change their shape. The
resulting equivalent void conjecture agrees well with numerical and
field data and has the following important geological implications. (i)
Clasts in shear zones can have stable positions in simple shear without
the requirement of an additional pure shear component. (ii) The stable
orientation can be approached either syn- or antithetically; hence, the
clast can rotate against the applied shear sense. (iii) The strain
needed to develop a strong shape preferred orientation is small (gamma =
1) and therefore evaluations based on other theories may overestimate
strain by orders of magnitude. (iv) The reconstruction of far-field
shear flow conditions and kinematic vorticity analysis must be modified
to incorporate these new findings. (C) 2004 Elsevier B.V. All rights
reserved.