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

During post-thickening extensional collapse, deformation can be localised w ithin flat-lying decollement zones where fluids are present. General questi ons concern the nature and origin of fluids, the scale of their transfer, a nd the coupling between strain localisation and fluid availability. We pres ent preliminary results from a major decollement zone from the Variscan bel t of South Brittany. In this zone, we have studied a particular unit essent ially made of acid metavolcanites deformed under low to moderate thermal co nditions. 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 strai ns accumulated by dissolution-crystallisation of the quartz and feldspar ph ases, combined with mechanisms of crack-seal type. Data suggest that both t he 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 ra ther than as a crustal-scale fluid channel. (C) 2000 Elsevier Science B.V. All rights reserved.