Effects of confinement on matrix flow around a rigid inclusion in viscous simple shear: insights from analogue and numerical modelling

We use analogue and numerical modelling to show that the flow of a Newtonian viscous fluid around a rigid body, in simple shear, depends strongly on the degree of confinement, i.e. the ratio between the shear zone width (H) and the rigid inclusion's least axis (e2) (S=H/e2). It also depends on how closely we look at the inclusion, which leads to the definition of an effective channel length and an effective flow pattern, compatible with micro-tectonics observations. If we consider a long channel, the flow pattern is bow tie-shaped, but tends to become eye-shaped as S approaches infinity. If we zoom in to an effective channel no longer than 10 inclusion diameters, the flow pattern is effectively bow tie-shaped for low to medium S values, but becomes effectively eye-shaped at medium to high S values. These changes may have great influence on the geometry of tails around a rigid inclusion. Therefore, special care must be taken when trying to infer rock rheology (e.g. viscous Newtonian or non-Newtonian) from geometrical patterns (e.g. geometry of a mantle and tails of recrystallized material around a rigid body), which are assumed to reflect the flow type.