EXPERIMENTAL MODELING OF MANTLED PORPHYROCLASTS

Mantled porphyroclasts are commonly used as sense of shear indicators in mylonites. Analogue experiments have been carried out to model the development of such objects. Rigid spheres mantled by a viscous Plasticene-putty mixture with power-law behaviour were suspended in an optically transparent Newtonian polymer matrix. The samples were deformed between two cylinders, one placed within the other, and with coinciding axes. This induces a non-coaxial flow between the cylinders and allows the achievement of high finite strain. The viscous mantle around the rigid sphere remained undeformed, deformed into an ellipsoid, or developed wings. The final geometry depends mainly on the induced viscosity contrast between the mantle and the matrix, which is a function of the flow-induced stress on the surface of the mantle. Winged mantles only develop if the induced viscosity contrast between the mantle and the matrix polymer is sufficiently small. If this 'critical' induced viscosity contrast is exceeded, the mantled sphere remains undeformed or obtains a stable ellipsoidal shape, even at high finite strain. 6-shaped winged mantles only develop in simple shear flow if the mantle around a rigid sphere is thin.