<p>The creep response of composite salt rock depends on mineral composition, however, quantitative constitutive models that account for composition effects remain limited. In this paper, composition-controlled synthetic salt rock cores were prepared by high-temperature axial compaction using halite as the matrix and anhydrite or sylvite as accessory minerals. Anhydrite-halite specimens contained 0-80 wt.% anhydrite, while sylvite-halite specimens contained 10-100 wt.% sylvite. Triaxial creep experiments were conducted for 20 hours under a confining pressure of 15&#xa0;MPa at different temperatures and deviatoric stresses. The results indicate that increasing anhydrite content reduced accumulated creep strain, lowered the steady-state creep rate, weakened stress and temperature sensitivity, and shortened the transition from primary to steady-state creep. In contrast, increasing sylvite content increased creep deformation and produced a more gradual transition, indicating a stronger viscosity-dominated response. To describe these composition-dependent trends, a modified nonlinear Burgers model was formulated by replacing the Maxwell dashpot with a Heard unit, allowing nonlinear stress- and temperature-dependent steady-state creep to be represented. A staged parameter-identification procedure was used to separate the primary and steady-state creep contributions, and the model parameters were correlated with halite, anhydrite, and sylvite mass fractions. Validation using natural Lower Fars Formation salt cores from the Missan Oilfield showed that the predicted parameters reproduced the measured creep curves with relative strain errors within 20%. The framework provides a practical basis for modeling composition-dependent creep of composite salt rock.</p>