THE IMPACT OF STRESS THRESHOLDS ON LOCALIZATION IN THE VISCOUS REGIME

Strain localization is one of the most ubiquitous processes in the solid Earth, occurring across scales and in nearly every geologic setting. The community has considered several different drivers for weakening leading to localization in the viscous regime, with the most common ones being strain and energy (via shear heating). However, a realistic look indicates that strain and energy are not as likely as stress to be the dominant factor for weakening. Stress variations within a rock result from a combination of elastic, brittle, and viscous deformation, and are controlled by the phases present and their physical properties, phase distribution, grain shapes and geometry, crystal orientations, and pressure-temperature-loading conditions, among other factors. Stress perturbations can cause weakening in at least two ways: by inducing either (1) fracturing that promotes fluid mobility and drives reactions or hydrolytic weakening, 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 can make the following observations: (1) all shear zones involve a change in bulk constitutive law parameters across the strain gradient; (2) most or all weakening mechanisms involve a stress-dependent process; (3) many weakening mechanisms involve a strain-dependent process; (4) most shear zones form at rheological boundaries; (5) shear zones are mechanically heterogeneous at all scales. 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.