Experimental observations on the effect of interface slip on rotation and stabilisation of rigid particles in simple shear and a comparison with natural mylonites

For axial ratios R>∼3, porphyroclasts from three mylonites all show a very strong SPO with the long axis of the best fit ellipse at an antithetic angle of 5–10° to the shear direction. This is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear. The cause of this divergence is investigated in a series of high simple shear strain (γ>15) analogue experiments, performed in a ring-shear machine (couette flow) using a linear viscous matrix (PDMS). The rotational behaviour of elongate (R=∼5) rigid particles with elliptical and rhomboidal shapes, comparable with the natural examples, is modelled for both coherent and slipping particle–matrix interfaces. Interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction, but not stabilisation. Interface slip does result in stabilisation of the rhomboidal particle, with the long diagonal oriented at a small antithetic angle to the shear direction. For monoclinic particles, mirror image shapes (referred to here as Types 1 and 2) show different rotational behaviour. For the Type 1 particle (with a shape comparable to σ porphyroclast systems and mica fish), the long side rotates asymptotically into parallelism with the shear direction. Natural examples of Type 1 particles, such as hornblende and olivine porphyroclasts measured from the Finero mylonites (Southern Alps), show a very strong preferred orientation (for R>∼3), with the long side parallel or at a small (<5°) antithetic angle to the mylonitic foliation. For Type 2 particles, the short side stabilises close to the shear direction, or at a small synthetic angle, as also observed for sillimanite porphyroclasts from the Mont Mary mylonites (Western Alps). In this natural case, stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation. The analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles. In natural examples, decoupling from the matrix may be affected by extensional crenulation cleavage or C-planes in S–C fabrics.