Rheological Bridge Zones: The Initialization of Strain Localization

Strain localization occurs across the crust in both brittle and viscous regimes, but the exact causes remain debated. Natural rock observations suggest that changes in phase properties (such as physical properties, phase distribution, and grain geometry) are more influential in weakening than variations in stress and temperature. Investigating the early stages of strain accumulation in various pressure-temperature conditions leads to a better understanding of these causes. Our study focuses on three weakly deformed rocks showing zones of localization on a millimeter or smaller scale, which we term “bridge zones”. These localized zones appear to mechanically connect weak domains and typically exhibit finer grain sizes within a narrow band. Importantly, these zones occur in less deformed rocks from the margins of shear zones. They result from both in-situ grain size reduction and chemical processes leading to phase mixing or element mobility on a limited spatial scale. Numerical modeling aligns high-stress areas with these zones, supporting their impact on reducing rock strength. We propose a conceptual model linking far-field loading to microscale changes in developing these zones. Characterization of bridge zones aids in elucidating the microstructural processes driving deformation localization, which is fundamental for plate tectonics, metamorphism, seismicity, and other lithospheric processes. This research reveals microscale mechanisms driving weak domain development, improving our knowledge of rheological changes and laying the groundwork for predictive models regarding strength evolution in the lithosphere.