The use of a combined structural, stable isotope and fluid inclusion study to 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 correlate the temperature–pressure conditions of vein formation with specific deformation 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 composition of the vein calcites and dolomites is very similar to that of the surrounding limestones and dolostones, respectively. This indicates that the cements were precipitated from fluids buffered by the host-rock and implies that the temperature–pressure characteristics of the fluids reflect the thermal history of the deformation history within the syncline. Microthermometric data of primary fluid inclusions were used to estimate the temperature–pressure conditions at precipitation time, and thus of the deformation. Pressure-corrected trapping temperatures for the veins show a temperature evolution from pre-Variscan (≤310°C) to Variscan (260–200°C) and post-Variscan (75°C). This evolution indicates that the syncline developed not at a specific depth, but rather as an active, progressive deformation process during transport along the northern Variscan thrust front.