Abstract K/Ar ages and oxygen isotope compositions were determined on illitic clay fractions of Purbeckian marls from the Swiss and French Jura Mountains in order to examine their syn-depositional origin which had previously been suggested based on sedimentological and mineralogical data. Measured ages range between 122.3+ or -2.3 Ma and 136.5+ or -2.5 Ma, thus being either coeval or slightly younger than the accepted age of sedimentation (Jurassic/Cretaceous boundary). Because total burial depth was less than 1000 m, delta 18 O values around 20 per mil are compatible with a formation of illite by replacement of smectite in an evaporated sea-water. Therefore, in addition to the two common types of illite resulting either from the fragmentation of pre-existing micas or diagenetic formation in deeply-buried sediments, illitic clay minerals formed at low temperature do exist and may be useful for age determination.
Une sequence de produits, superposant coulees pyroclastiques, coulees de lave et projections soudees (dalle soudee), affleure sur les versants nord, ouest et sud du Piton des Neiges, ou leur volume est estime a 20km3. De nature mugearitique, ils ont ete emis lors d'une phase eruptive majeure, terminee par la formation de la caldeira II. Dates de 223000 ans (K/Ar) et grâce a leurs relations chronostratigraphiques, ils permettent de preciser le schema d'evolution morphostructurale du massif lors de sa periode a laves differenciees alcalines
We investigate the relationship between rift propagation and volcanism in the Afar Depression in the last 4 Myr. Potassium‐argon and thermoluminescence dating allow detailed reconstruction of the temporal evolution of volcanism. Volcanic activity is almost continuous since 3.5 Ma, with intervals characterized by more intense activity, especially around 2 Ma. Spatial distribution of ages reveals that Stratoid Series volcanism migrated northward along a 200‐km trend between 3 and 1 Ma, at about 10 cm/yr, linked to northward propagation of the Gulf of Aden Ridge, after it had cut across the Danakil horst at 4 Ma. Our work underlines the role of rhyolitic volcanism in initiation of rifting. Acid volcanoes, initially formed near the axes of extensional zones, have been subsequently dissected and are presently located on both sides of active rift segments. These lavas were the first to be erupted in areas of low extensional strain and were followed by basaltic lavas as extension increased. Differentiated volcanoes acted as zones of local weakness and guided localization of fractures, then leading to fissural magmatism. This regional‐scale, composite style of rifting, including volcanic and tectonic components, can be compared to the large‐scale continental breakup process itself. Deformation occurs through propagation of faults and fissures under a regional stress field. These become localized because of weakening of the crust (or lithosphere) due to emplacement of magmas, under the influence of a plume in the large‐scale case, or of silicic centers linked to magma chambers in the regional‐scale case.
