Upper Triassic sandy horizons in the Paris Basin were sampled at depths ranging from outcrop in the northeast to 2,700 m in the centre of the basin. The smallest clay sub-fractions (<0.2 μm) from the deepest central samples consist mainly of illite and chlorite whose K-Ar age is ∼ 190 Ma. These minerals formed at a relatively high temperature of 220-250°C, as determined from the oxygen isotope fractionation between authigenic illite and associated quartz overgrowths, but at a burial depth of only 500 m. In a nearby drillhole that crosscuts a fault zone reactivated during the 190 Ma event, illite is less well-crystallized and has a higher δ18O value suggesting different physical and chemical conditions of formation during the same hydrothermal episode. Two other generations of illite-smectite mixed-layer clays formed in the same Triassic horizon: one at ∼150 Ma and the other at ∼80 Ma. These younger clays have higher δ18O values and thus may have formed at somewhat lower temperatures. The δ18O values of the fluids from which the different illite-smectite mixed-layer minerals form range from +9 to +13.5 per mil (SMOW).
An Ar diffusion code was used to estimate, on the basis of the Ar loss of the clay-type material, the duration of these events. The results suggest that the duration of the hydrothermal events at 190 Ma and 150 Ma were rather short, <1 Ma, whereas the youngest event was protracted over a much longer period of ∼37 Ma. Comparison between K-Ar ages of the different mixed-layer minerals and sedimentation rates of the sediments since the Palaeozoic shows significant accelerations of the rates at ∼ 190-200 Ma and 150 Ma, and a less important one at ∼80 Ma. These observations provide additional evidence that the first two events were promoted by basement-related tectono-thermal activities. The third event is considered to be of diagenetic origin.