Discrimination of annealed and dynamic fabrics: Consequences for strain localization and deformation episodes of large-scale shear zones

Large-scale shear zones are often characterized by long lasting deformation with complex strain localization behavior. Based on a drill core across polymineralic carbonate mylonites of the Doldenhorn nappe (Helvetic Alps, Switzerland), the present study uses spatially resolved microstructural variations to discriminate unambiguously deformation microstructures from annealed ones, to infer the deformation mechanisms, and to reconstruct the strain localization history. These goals require quantitative microstructural and textural analyses of sample suites in sections across the shear zone. Particularly the occurrence of two distinct Zener trends, i.e., the relationship between grain size and volume fraction of matrix and second phase grains, allows for the identification of annealed from dynamic microstructures. The data presented here indicate that continued shearing after peak metamorphic conditions and during exhumation-induced cooling resulted in progressive strain localization at the base of the shear zone, while the inactive parts of the formerly high temperature shear zone were progressively overprinted by annealing.