We consider in this paper an isothermal model of nonlinear elasticity. This
model is described by two conservation laws that define a problem of mixed
type, both elliptic and hyperbolic. We restrict ourselves to the linearly
degenerate case, and consider Riemann data that lies in the hyperbolic regi
ons. The lack of uniqueness of the Riemann problem is solved by the introdu
ction of a so-called kinetic relation, used to narrow the set of admissible
subsonic phase transitions. In this situation, we consider the Riemann pro
blem for any data lying in the hyperbolic region, using either explicit com
putations or geometric arguments. This construction allows us to give suffi
cient conditions on the kinetic relation in order that the generated Rieman
n solver possesses properties of uniqueness, globality, and continuous depe
ndence on the initial data in the L-1 distance.