Fluid-assisted spreading of thickened continental crust: preliminary data from the Variscan belt of South Brittany (France)

Abstract

During post-thickening extensional collapse, deformation can be localised within flat-lying decollement zones where fluids are present. General questions concern the nature and origin of fluids, the scale of their transfer, and the coupling between strain localisation and fluid availability. We present preliminary results from a major decollement zone from the Variscan belt of South Brittany. In this zone, we have studied a particular unit essentially made of acid metavolcanites deformed under low to moderate thermal conditions. The study combines structural analysis, petrography, geochemistry (major and trace elements, stable isotopes), and fluid inclusions analysis. At regional scale, the deformation resulted in very large pervasive strains accumulated by dissolution–crystallisation of the quartz and feldspar phases, combined with mechanisms of crack-seal type. Data suggest that both the amount of fluids and the scale of fluid transfers was limited. From this, we infer that the decollement zone may have acted as a trap for fluids rather than as a crustal-scale fluid channel.