A simple parameterization of strain localization in the ductile regime dueto grain size reduction: A case study for olivine

We propose a simple parameterization of the transition between dislocation creep and grain-size-sensitive creep under conditions characteristic of the lithospheric mantle and derived from the results of laboratory experiments on olivine-rich rocks. Through numerical modeling and linear stability ana lysis, we determine the conditions under which this transition takes place and potentially leads to strain localization. We pay particular attention t o the effect of cooling rate and strain rate which are likely to be dominan t parameters in actively deforming tectonic areas. We conclude that at cons tant temperature, strain localization can only take place if the rheology o f the material is nonlinearly related to grain size; that strain localizati on is facilitated by syndeformation cooling; that there is only a narrow re gion in the strain rate versus cooling rate parameter space where localizat ion is likely to take place; and that grain growth inhibits strain localiza tion at fast cooling rates but may lead to "grain growth localization" at l ow cooling rates. We draw attention to the potential. consequences of our a nalysis of strain localization for the style of plate motions at the Earth' s surface.