DEFORMATION AROUND RIGID PARTICLES - THE INFLUENCE OF SLIP AT THE PARTICLE MATRIX INTERFACE

Theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent. In natural rocks, the boundary behaviour may cover the whole range from free-slip to complete coherence. Examples where slip may be promoted include crack-seal deformation during the development of pressure shadows, core-mantle structures, where the mantle is composed of much weaker, fine recrystallized material, and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase. Analogue scale-model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non-slipping particle/matrix interfaces. When slip does occur, particles rotate faster in pure shear and slower in simple shear than for non-slip, leading to the more rapid development of a shape preferred orientation. The preferred orientation tends to be bimodal in simple shear flow. The flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses, dependent on the original particle orientation. In multi-particle experiments, slip on the interfaces enhances strong strain localization in non-linear viscous material, to produce patterns very similar to natural S-C fabrics.