  Mixing fronts, where fluids of different chemical compositions mix with each
other, are typically subjected to velocity gradients, ranging from the pore
scale to the catchment scale due to permeability variations and flow line
geometries. A common trait of these processes is that the mixing interface is
strained by shear. Depending on the P\'eclet number $Pe$, which represents the
ratio of the characteristic diffusion time to the characteristic advection
time, and the Damk\"ohler number $Da$, which represents the ratio of the
characteristic diffusion time to the characteristic reaction time, the local
reaction rates can be strongly impacted by the dynamics of the mixing
interface. This impact has been characterized mostly either in kinetics-limited
or in mixing-limited conditions, that is, for either very low or very high
$Da$. Here the coupling of shear flow and chemical reactivity is investigated
for arbitrary Damk\"ohler numbers, for a bimolecular reaction and an initial
interface with separated reactants. Approximate analytical expressions for the
global production rate and reactive mixing scale are derived based on a
reactive lamella approach that allows for a general coupling between stretching
enhanced mixing and chemical reactions. While for $Pe<Da$, reaction kinetics
and stretching effects are decoupled, a scenario which we name "weak
stretching", for $Pe>Da$, we uncover a "strong stretching" scenario where new
scaling laws emerge from the interplay between reaction kinetics, diffusion,
and stretching. The analytical results are validated against numerical
simulations. These findings shed light on the effect of flow heterogeneity on
the enhancement of chemical reaction and the creation of spatially localized
hotspots of reactivity for a broad range of systems ranging from kinetic
limited to mixing limited situations.
