MIXING IN FLOW PERTURBATIONS - A MODEL FOR DEVELOPMENT OF MANTLED PORPHYROCLASTS IN MYLONITES

The presence of a rigid object such as a porphyroclast in simple shear flow induces a flow perturbation with a relatively simple geometry. Domains of closed and open flow lines are bounded by a 'separatrix' that surrounds the porphyroclast. The concept of a separatrix to describe heterogeneous flow gives rise to some new insights in the development of porphyroclasts with a mantle of recrystallized material in mylonites. The final shape of the recrystallized mantle depends on separatrix shape, but also on the degree of intersection of the mantle and the separatrix, and therefore on relative mantle width. This model can explain the coexistence of several types of mantled porphyroclast in a single mylonite sample, the occurrence of porphyroclasts that lack wings at high strain values, and the presence of symmetric winged mantled porphyroclasts in ductile shear zones.