RHEOLOGICAL BRIDGE ZONE, STRESS AMPLIFICATION, AND INITIALIZATION OF STRAIN LOCALIZATION

The phenomenon of strain localization occurs throughout the crust, in both brittle and viscous regimes. However, the precise underlying causes remain a subject of ongoing debate. Observations in natural rocks suggest that alterations in phase properties, encompassing physical attributes, phase distribution, and grain geometry, wield a substantial influence on weakening processes. We evaluate that claim through a comprehensive exploration of the initial stages of strain accumulation at various pressure-temperature conditions. Our study focuses on three instances of weakly deformed rocks that exhibit localized zones, which we term "bridge zones," on a millimeter or smaller scale. These localized zones function as mechanical connectors between weak domains and consistently feature relatively small grain sizes within a narrow band. Rather than having a dominantly mechanical origin, we find that these zones appear to have developed largely from chemical processes, inducing phase mixing and reactions, with mechanical processes emerging as a secondary deformation mechanism. The dominant influence of chemical processes is substantiated by core-rim cathodoluminescence (CL) variations observed in fine-grained plagioclase and the occurrence of new phases (e.g., K-feldspar, quartz, titanite, etc.) at the triple-junctions of fine-grained feldspar. These findings suggest that reactions, dissolution-precipitation, and neocrystallization are pivotal in the development of fine grains within the bridge zones. Crucially, these zones form both within shear zones and in adjacent less-deformed rocks. Numerical modeling illustrates a spatial coincidence of high-stress areas with bridge zones, emphasizing their impact on reducing rock strength. These observations lead to a conceptual model in which far-field loading and microscale alterations catalyze the development of these bridge zones. Bridge zone characteristics likely scale up and relate to macroscale deformation localization, a fundamental component of plate tectonics, metamorphism, seismic activity, and other first order lithospheric processes.