  Dispersion experiments are compared for two transparent model fractures with
identical complementary rough walls but with a relative shear displacement
$\vec{\delta}$ parallel ($\vec{\delta}\parallel \vec{U}$) or perpendicular
($\vec{\delta} \perp \vec{U}$) 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 $\alpha(x,y)$. For $\vec{\delta} \perp
\vec{U}$, 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
$\vec{\delta} \parallel \vec{U}$, the front is flatter and $\alpha(x,y)$
displays a narrow distribution and a Taylor-like variation with $Pe$.
