Stress thresholds and strain localization in the viscous regime

Christopher Gerbi, University of Maine
Scott Johnson, University of Maine
He Feng, University of Maine

Abstract

Strain localization in the viscous regime occurs across scales and tectonic regimes, driving numerous small- and large-scale geologic processes. "Shear heating" and "strain weakening" are two processes commonly invoked to initiate localization. However, at a micromechanical level, we find that stress - and specifically a stress threshold - is a more likely candidate. Stress heterogeneity within a rock results from a combination of elastic, brittle, and viscous deformation, with the magnitude and pattern controlled primarily by the phases present and their spatial distribution. Stress perturbations can cause weakening in at least two ways: by inducing either (1) fracturing that promotes fluid mobility and drives reactions, or (2) microstructural damage and a change in the operative deformation mechanisms and local constitutive law. From a compilation of published studies and our own work, we suggest that most or all weakening mechanisms in shear zones involve a stress-dependent process, though many also involve a strain-dependent process. The formation of most shear zones at rheological boundaries is a result of stress amplification in these areas. Although thermal anomalies can potentially exploit stress–strain-rate heterogeneity, the widespread presence of microstructures consistent with a constitutive law variation across strain gradients suggests that energy balance cannot fully explain the observations. Strain alone has a theoretically high potential to weaken a rock, but natural systems do not appear likely to exploit that potential. Thus, we consider stress thresholds to be the primary factor driving localization in the viscous regime. As such, we suggest that shear zone formation has a mathematical similarity to fault initiation.

Session

Session 2: Rheology of the Lithosphere