2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/170543Dispersion experiments are compared for two transparent model fractures with identical complementary rough walls but with a relative shear displacement $\vecδ$ parallel ($\vecδ\parallel \vec{U}$) or perpendicular ($\vecδ \perp \vec{U}$) to the flow velocity $\vec{U}$. The structure of the mixing front is characterized by mapping the local normalized local transit time $\bar t(x,y)$ and dispersivity $α(x,y)$. For $\vecδ \perp \vec{U}$, displacement fronts display large fingers: their geometry and the distribution of $\bar t(x,y)U/x$ are well reproduced by assuming parallel channels of hydraulic conductance deduced from the aperture field. For $\vecδ \parallel \vec{U}$, the front is flatter and $α(x,y)$ displays a narrow distribution and a Taylor-like variation with $Pe$.GeophysicsFluid Dynamicstext