The rotation rate of cylindrical objects during simple shear

The rotational behaviour of circular objects is modelled using a two-dimens ional finite element simulation. We present results for linear and non-line ar viscous theologies and explore parameter space for the theology contrast between object and matrix, eta, and for object size relative to the shear zone width, L. For high eta, our results confirm analytical descriptions fo r L --> 0 in that the rotation rate of the object is half of the bulk shear strain rate. However, we show that for 0.1 > L < 0.9, the rotation rate as a function of L can have a minimum, a maximum, or it can decrease or incre ase, depending on the stress exponent and <eta>. In fact, for some theologi es, the rotation rate may decrease to <0.3 of the shear strain rate at inte rmediate L <approximate to> 0.5 ( approximate to 20% area of porphyroblast in thin section). These counter intuitive results have important consequences for the interpr etation of the rotational behaviour of minerals in deforming rocks. For exa mple, in rocks where garnet constitutes a Volumetrically significant propor tion of the bulk rock, the effective shear zone boundaries may be given by neighbouring crystals corresponding to intermediate L as described here. Th us, the interpretation of the rotation rate of crystals in such rocks can o nly be performed if the distance to the effective system boundaries is know n. (C) 2001 Elsevier Science Ltd. All rights reserved.