The relationships between fluid circulation and deformation are one of the issues of the sedimentary basin study. In the Cretaceous flysch of the Saint-Jean-de-Luz basin, the evolution of folds geometry and the increasing volume of calcite-filled fractures and veins evidence a northward deformational gradient along the French Basque coast. A combined approach is proposed to assess the corresponding physical and chemical conditions: the microthermometric study of fluid inclusions in calcite sampled in different generations of veins and fractures and the X-ray diffraction analysis of clay minerals from adjacent marl layers. Salinity of the trapped H2O-CaCl2-NaCl fluids increases with depth in the series, in good agreement with salinity gradients reported in sedimentary basins. Dispersion of the data also increases with depth from 0.3 to 1.3wt.% NaCl eq. in the shallowest formation (Haizabia flysch) to 9.1 to 23.0wt.% NaCl eq. in the deepest formation (Guéthary flysch). Minimal trapping temperatures of the fluids in the Haizabia and Socoa flyschs (79 and 102°C, respectively) are consistent with the temperatures estimated from the depth of burial, which did not exceed 5km, in good agreement with the stability of the smectite-illite-kaolinite assemblage found in marls. In addition, the kaolinite proportion significantly decreases with depth in the series, as a potential consequence of climate changes and diagenetic transformations, whereas the increasing dispersion of illite crystallinity data might indicate fluid-mineral interactions. We propose a synthetic model of fluid circulation in the folded series that involves the mixing of mainly-horizontal fluid circulation (potentially meteoric) with an upward flow of high-salinity fluid throughout the deepest formations (potentially related to underlying evaporite-rich layers). The northward deformational gradient, as exposed along the French Basque coast, is likely to be responsible for such a vertical circulation, by increasing the volume of fracture (particularly cross-cutting fractures) in the deeply buried formations.