WHAT MICROSTRUCTURAL PROCESSES CONTRIBUTED TO THE FORMATION OF RHEOLOGICAL BRIDGE ZONES?

Strain localization occurs throughout the crust, in both the brittle and viscous regimes. The causes of strain localization remain under discussion, with strain, shear heating, and stress being the most commonly suggested causes. Determining the cause(s) of localization at the macroscale requires investigation of the earliest stages of strain concentration. Our study focuses on mid-crustal rocks that exhibit localization on the millimeter or smaller scale. We combine optical and electron beam petrography with chemical mapping and electron backscatter diffraction to characterize these rheologically important domains. Morphologically, these localized zones appear to mechanically link rheologically weak phases or domains. These “bridge” zones typically comprise reduced grain sizes and an aggregation of relatively fine grains in a narrow band. Importantly, this phenomenon occurs not only in rocks inside the shear zones, but also in the less deformed margin outside the shear zones. Traditionally, the process of grain size reduction is thought to result from in-situ recrystallization or cataclasis. However, almost all the bridge zones, observed or in literature, contain more than one mineral phase and therefore require some process for phase mixing on a short spatial scale. Externally-constrained estimates of strain suggest that the mixtures are unlikely to form only by mechanical processes and that some reaction or chemical process, even in water-poor granulite facies rocks, operated. After characterizing these bridge zones in rocks with different mineralogy, P-T conditions of formation, and degrees of deformation, we calculate the rheological impact of these bridge zones on bulk rock properties.