Mantled porphyroclast gauges

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.