Role of Microscale Stress in the Viscous Regime

In what is considered the lithosphere's viscous regime, strain localization occurs across scales, resulting in plate boundaries, shear zones, shear bands, and similar structures. Whether entirely brittle, entirely viscous, or involving a combination of the two, localization involves crossing some type of threshold, resulting in a change from a stable, non-localized state. In the upper crust, we typically consider the threshold to be stress, as in Mohr-Coulomb behavior. Deeper, the most commonly proposed parameters are strain and energy. However, geological observations, such as the relationship between fractures and localization, suggest that despite their theoretical validity, strain- and thermal-dependence alone do not appear to explain the geological observations leading to viscous localization. We present microstructural observations, including microfracturing and stress-controlled changes in deformation mechanisms, that support the argument that stress is a dominant factor driving weakening in the viscous regime. Numerical modeling augments the microstructural observations by demonstrating that due to geometrical and crystallographic heterogeneity, microscale stress magnitudes can greatly exceed the stress values at the bulk, homogenized scale. These microscale stress values are more than sufficient to cause the observed damage.