We investigate the behavior of the isolated mantled porphyroclast in a
shear zone. The method employed is a finite element model. Three
distinct phases, clast, mantle and matrix are present, the rheologies
are power-law with exponents ranging from 1 to 5 and the far-field
boundary condition is simple shear. The effective viscosity of the
mantle is assumed to be less than those of the clast and the matrix. We
show for which parameter sets mantled porphyroclasts reach
super-horizontal stabilization with respect to the shear plane and
sense. Clasts in natural mylonites frequently exhibit similar
orientations, which are interpreted as stable inclinations. The
systematic examination of the matrix-mantle-clast system allows for the
construction of attractor maps that can be directly used as gauges for
(i) the effective viscosity contrast between matrix and mantle, (ii) the
production rate of mantle material around the clast as a function of the
bulk shear strain, and (iii) for the total shear strain. The necessary
data required to use the attractor maps are simple geometrical
parameters that can be measured in the field, i.e. clast aspect ratio,
clast inclination versus the shear plane, mantle thickness, and mantle
and clast area. This new method successfully reproduces the
characteristics of natural porphyroclasts and is in good agreement with
data from natural shear zones. (c) 2005 Elsevier Ltd. All rights
reserved.