  We propose an elasto-plastic inspired friction model which incorporates
interfacial stiffness. Steady state sliding friction is characterized by a
generic nonmonotonic behavior, including both velocity weakening and
strengthening branches. In 1D and upon the application of sideway loading, we
demonstrate the existence of transient cracklike fronts whose velocity is
independent of sound speed, which we propose to be analogous to the recently
discovered slow interfacial rupture fronts. Most importantly, the properties of
these transient inhomogeneously loaded fronts are determined by steady state
front solutions at the {\em minimum} of the sliding friction law, implying the
existence of a new velocity scale and a "forbidden gap" of rupture velocities.
We highlight the role played by interfacial stiffness and supplement our
analysis with 2D scaling arguments.

Materials Science;Soft Condensed Matter