  The dynamical mechanism at the origin of the non-local rheology of dense
granular flows is investigated trough discrete element simulations. We show
that the influence of a shear band on the mechanical behavior of a distant zone
is contained in the spatial variations observed in the network of granular
contacts. Using a micro-rheology technique, we establish that the exponential
responses hence obtained, do not proof the validity of a mechanical activation
process as previously suggested, but stem from the spatial relaxation of the
shear rate as a direct consequence of a macroscopic non-local constitutive
relation. Finally, by direct visualization of the local relaxation processes,
we dismiss the kinetic elasto-plastic picture, where a flow is conceived as a
quasi-static sequence of localized plastic events interacting through the
stress field. We therefore conclude in favor of the jamming scenario, where
geometrical constrains lead to coherent non-affine displacements along floppy
modes, inherently non-local.
