Fluid-assisted large strains in a crustal-scale d&ollement

Abstract: Crustal deformations often occur along gently-dipping shear zones, where the scale of fluid transfers and the coupling between deformations and fluid availability are important questions. We present a structural nd geochemical study of felsic rocks deformed at about 350-400°C within a crustal-scale d&ollement zone from the Hercynian Belt of South Brittany. At regional scale, deformations resulted in large distributed strains accumulated by dissolution-crystallization of quartz and feldspars. Mass transfers produced veins grow-ing parallel to the foliation. Veins were major precipitation zones, while adjacent rocks were the main zones of dissolution. Deformation patterns, vein mineralogy, and oxygen isotope data show that both the amount of fluids and the scale of fluid flow were limited, and that the dominant transfer mechanism was diffusion. Results suggest tha he deformation zone has acted as a trap for early fluids rather than as a syn-kinematic fluid channel. The existence of close relationships between fluid movements and crustal deformation is wel