The use of a combined structural, stable isotope and fluid inclusion studyto constrain the kinematic history at the northern Variscan front zone (Bettrechies, northern France)

A detailed structural, stable isotope and fluid inclusion study of distinct vein generations has been performed in a kilometre-scale syncline to corre late the temperature-pressure conditions of Vein formation with specific de formation episodes. The structural relationship of the Veins with cleavage, folds, faults and with one another, allows the identification of pre-, syn - and post-Variscan carbonate vein generations. The stable isotope composit ion of the vein calcites and dolomites is very similar to that of the surro unding limestones and dolostones, respectively, This indicates that the cem ents were precipitated from fluids buffered by the host-rock and implies th at the temperature-pressure characteristics of the fluids reflect the therm al history of the deformation history within the syncline. Microthermometri c data of primary fluid inclusions were used to estimate the temperature-pr essure conditions at precipitation time, and thus of the deformation. Press ure-corrected trapping temperatures for the veins show a temperature evolut ion from pre-Variscan (less than or equal to 310 degrees C) to Variscan (26 0-200 degrees C) and post-Variscan (75 degrees C). This evolution indicates that the syncline developed not at a specific depth, but rather as an acti ve, progressive deformation process during transport along the northern Var iscan thrust front. (C) 2000 Elsevier Science Ltd. All rights reserved.