  We present a simple and efficient implementation of a viscous creep rheology
based on diffusion creep, dislocation creep and the Peierls mechanism in
conjunction with an elasto-plastic rheology model into a shock-physics code,
the iSALE open-source impact code. Our approach is based on the calculation of
an effective viscosity which is then used as a reference viscosity for any
underlying viscoelastic (or even visco-elasto-plastic) model. Here we use a
Maxwell-model which best describes stress relaxation and is therefore likely
most important for the formation of large meteorite impact basins. While common
viscoelastic behavior during mantle convection or other slow geodynamic or
geological processes is mostly controlled by diffusion and dislocation creep,
we showed that the Peierls mechanism dominates at the large strain rates that
typically occur during meteorite impacts. Thus, the resulting
visco-elasto-plastic rheology allows implementation of a more realistic mantle
behavior in computer simulations, especially for those dealing with large
meteorite impacts. The approach shown here opens the way to more faithful
simulations of large impact basin formation, especially in elucidating the
physics behind the formation of the external fault rings characteristic of
large lunar basins.
