We consider the problem of a turbulent mixing zone (TMZ), initially submitted to coupled effects of axisymmetric strain and stratification, then subsequently accelerated. The TMZ grows in the latter stage due to a rapid mixing induced by the Rayleigh-Taylor instability. It is shown that the short time dynamics is simply determined by only two parameters expressing the structure of the turbulent density field, one related to the mixing, the other to the dimensionality of the flow. These quantities are studied by rapid distortion theory and by several homogeneous direct numerical simulations performed in the moving frame of the mean flow. The implications for modeling are discussed, the influence of anisotropy is presented.