Brittle tectonics and fluids overpressure during the early stage of the Bay of Biscay opening in the Jard-sur-Mer area, (northern Aquitaine Basin, France)

Abstract : Ba, F, Pb, Ag, Zn mineral deposits are widespread at the northern and eastern boundaries of the Aquitaine Basin. In most cases, they are hosted within high permeability carbonates that rest over the Hercynian basement and below an impermeable layer. Such a position suggests a Mississippi Valley Type (MVT) model for the formation of these deposits. This model is characterized by the lateral flow of sedimentary fluids expelled from the deeper part of the basin and mixed with other sources of water as they reach the basin boundaries. In the Jard-sur-Mer area, which sits in the north of the Basin, these deposits are also found higher in the sedimentary series suggesting that fluids have flown through the impermeable layer. Our field observations demonstrate that a brittle deformation episode, compatible with an upper-Jurassic N-S direction of extension, occurred as the mineralizing fluids were over pressured. The overpressure was the result of a large input of hydrothermal water ascending along inherited faults affecting the Hercynian basement and released at the onset of the tectonics event. When compared with the rest of the basin, these new results at the northern boundary suggests that the Aquitaine Basin recorded several stages of fluid overpressure both at the onset and during the opening of the Bay of Biscay.