Recipes for Rheologic Change in the Viscous Regime

The development of many of fundamental geologic structures in the viscous regime, such as shear zones and even plate boundaries, require rheologic change. In principle, that change can arise from reversible variation in environmental variables, such as stress and temperature. In practice, we find that material properties are the dominant cause for irreversible, spatially variable strength changes in the continental crust. Within that broad category, phase changes appear to be the leading cause, more important than mechanical grain size reduction or alterations in phase arrangement. This spatially variable metamorphism may arise from microscale stress heterogeneity. We calculate that typical continental rocks amplify the bulk load stress by a factor of ~1.5, but specific phase arrangements and phase combinations can produce close to an order of magnitude stress variation over small scales. These variations can drive neocrystallization and other forms of metamorphism, which in turn focus deformation and correlative processes. Most geodynamic models do not incorporate stress amplification and this form of rheologic change, hindering their ability to reproduce localization-related phenomena.

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