Ilse Kenis

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)

Abstract 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.

The "boudinage" question in the High-Ardenne Slate Belt (Belgium): a combined structural and fluid-inclusion approach

Abstract The terms “boudin” and “boudinage” 1 were first introduced, as descriptive terms, for particular geometric features associated with quartz veins in competent metaquartzitic layers in the High-Ardenne Slate Belt (Belgium). The occurrence of the “boudins” has subsequently been used to draw kinematic conclusions on a pre-Variscan extensional event in the Rhenohercynian belt. In this paper, a combined structural and fluid-inclusion approach allows proposing a new and more representative kinematic model for the development of the veins and associated structures. Moreover, the kinematic significance of “interboudin” veins is emphasized. In this regard, microthermometry of primary CO 2 –N 2 fluid inclusions in the quartz of the “interboudin” veins suggests that the origin of vein development is hydraulic fracturing in an already compressional setting. This suggestion implies the presence of fluid and near-lithostatic fluid pressures to permit fracturing at great depths where the rocks otherwise would be either stable or in the realm of ductile behaviour. The vein filling fluid had a local origin and was most likely related to the formation of bedding-parallel dissolution seams within the host rock. Veining is followed by layer-parallel shortening, resulting in the development of “double-sided” mullions at both interfaces of metaquartzitic layers, pinned by pre-existing veins. Subsequently, these polyphase structures behave passively in the Variscan deformation.