  Miscible tracer dispersion measurements in transparent model fractures with
different types of wall roughness are reported. The nature (Fickian or not) of
dispersion is determined by studying variations of the mixing front as a
function of the traveled distance but also as a function of the lateral scale
over which the tracer concentration is averaged. The dominant convective
dispersion mechanisms (velocity profile in the gap, velocity variations in the
fracture plane) are established by comparing measurements using Newtonian and
shear thinning fluids. For small monodisperse rugosities, front spreading is
diffusive with a dominant geometrical dispersion (dispersion coefficient $D
\propto Pe$) at low P\'eclet numbers $Pe$; at higher $Pe$ values one has either
$D \propto Pe^2$ ({\it i.e.} Taylor dispersion) for obstacles of height smaller
than the gap or $D \propto Pe^{1.35}$ for obstacles bridging the gap. For a
self affine multiscale roughness like in actual rocks and a relative shear
displacement $\vec{\delta}$ of complementary walls, the aperture field is
channelized in the direction perpendicular to $\delta$. For a mean velocity
$\vec{U}$ parallel to the channels, the global front geometry reflects the
velocity contrast between them and is predicted from the aperture field. For
$\vec{U}$ perpendicular to the channels, global front spreading is much
reduced. Local spreading of the front thickness remains mostly controlled by
Taylor dispersion except in the case of a very strong channelization parallel
to $\vec U$.
