  Intermittent motion, called stick--slip, is a friction instability that
commonly occurs during relative sliding of two elastic solids. In adhesive
polymer contacts, where elasticity and interface adhesion are strongly coupled,
stick--slip results from the propagation of slow detachment waves at the
interface. Using \emph{in situ} imaging experiments at an adhesive contact, we
show the occurrence of two distinct detachment waves moving parallel
(Schallamach wave) and anti-parallel (separation wave) to the applied remote
sliding. Both waves cause slip in the same direction and travel at speeds much
lesser than any elastic wave speed. We use an elastodynamic framework to
describe the propagation of these slow detachment waves at an elastic-rigid
interface and obtain governing integral equations in the low wave speed limit.
These integral equations are solved in closed form when the elastic solid is
incompressible. Two solution branches emerge, corresponding to opposite moving
detachment waves, just as seen in the experiments. A numerical scheme is used
to obtain interface stresses and velocities for the incompressible case for
arbitrary Poisson ratio. Based on these results, we explicitly demonstrate a
correspondence between propagating slow detachment waves and a static
bi-material interface crack. Based on this, and coupled with a recently
proposed fracture analogy for dynamic friction, we develop a phase diagram
showing domains of possible occurrence of stick--slip via detachment waves
vis-\'a-vis steady interface sliding.
