Although fundamental to the understanding of crustal dynamics in extensional setting, the relationships between the emplacement of granitic intrusions and activity of detachments still remain very elusive. Through a multiscale approach, we here document a continuous deformation history between the monzogranitic intrusion of Naxos and the Naxos‐Paros Detachment System (Cyclades, Greece). Field observations first show an early magmatic deformation followed by solid‐state, ductile, and then brittle deformation when approaching the detachment zone, as evidenced by the overprinting of mylonites by cataclastes and pseudotachylites. From these observations, we define six strain facies that characterize a positive strain gradient from core to rim of the Naxos monzogranite. Based on field pictures, X‐ray tomography, and electron backscatter diffraction (EBSD) analyses along the strain gradient, we then quantify the intensity of mineralogical fabrics in 2‐D and 3‐D and better characterize the deformation mechanisms. Our measured shape variations of the strain ellipsoid corroborate the large‐scale strain gradient, showing a good correlation between qualitative and quantitative studies. In addition, EBSD data indicate that dislocation creep was predominant during cooling from more than 500°C to temperature conditions of the ductile‐to‐brittle transition. However, (1) a weakening of quartz lattice preferred orientation with increasing strain and (2) evidence of numerous four‐grain junctions in high‐strain shear bands also indicate that grain boundary sliding significantly contributed to the deformation. Although the source of grain boundary sliding remains to be constrained, it provides a consistent approach to account for strain localization in Naxos.

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The reduction of grain size is believed to play a critical role in strain localization to form shear zones. Although many mechanisms have been proposed, the source of grain size reduction remains debated. The South Armorican Shear Zone (SASZ) is a crustal-scale strike-slip fault that deforms granitoids at mid-crustal conditions. The SASZ records the transition from protolith to ultramylonite, representative of increasing ductile shear strain. To investigate the evolution of strain localization, the different states of deformation were studied using a combination of detailed microstructural, chemical and electron backscatter diffraction analyzes. Increasing strain from protolith to ultramylonite resulted in (1) grain size reduction, (2) the development of interconnected monophase layers of mica and incipient mixed-phase zones composed of phengite-quartz ± K-feldspar in protomylonite and lowstrain mylonite, and (3) the formation of fine-grained mixed-phase zones composed of K-feldspar