A cavitation-seal mechanism for ultramylonite formation in quartzofeldspathic rocks within the semi-brittle field (Vivero fault, NW Spain)

摘要: Abstract Bands of multi-phase mixtures with an extreme grain size reduction called ultramylonites are a common feature in shear zones from the middle crust to upper mantle conditions. Indeed, they one most typical manifestations strain localization high-strained polymineralic rocks lithosphere, especially semi-brittle field. Yet many questions remain on their origin, and existing models either do not explain all observations or require introduction physical processes that difficult prove, particularly for inter-grain mixing phases. Here we present case study ultramylonite bands developed coarse-grained granite deformed at conditions near base seismogenic zone, i.e. showing coseismic deformation microstructures overprinted by thermally activated ones. We found mylonite transition, two mechanisms coexist promoting phase through Kfs precipitation: (1) dislocation-accommodated boundary sliding quartz (2) diffusion-accommodated feldspar. The model proposed is based cavitation-seal mechanism, which strongly dependent opening transient micro-cavities diffusivity potassium feldspar progressive sealing.