Local deformation in compacting flows: development of pressure shadows

Small scale structures produced by deformation and compaction are common in igneous, metamorphic, and sedimentary rocks, and are often used to define foliation and lineation directions. Analytic solutions for models of flow round a rigid stationary sphere are obtained using the equations that describe viscous compaction. The finite deformation associated with these flows shows many of the features observed in deformed rocks, some of which resemble those generated when the ratio of the bulk viscosity ζ to the shear viscosity η is small. Under these conditions the particle paths of the matrix are almost unaffected by the presence of the sphere. It is encouraging that the continuum compaction equations can describe the observed behaviour, even though the length scale on which the deformation occurs is only mm to cm. It is unlikely that the observations can be used to distinguish between D’Arcy flow of the low viscosity phase and mass transport by diffusion.