Strontium (Sr) signal in the Mesozoic Southeastern French Basin (Alps) and its relation with pre-compression tectono-halokinetic activity

For decades, Mesozoic tectono-halokinetic structures have been increasingly recognized in the peripheral French alpine basins. However, reconstructing the full history and mechanisms of halokinesis during the Mesozoic in these regions remains challenging due to the overprinting effects of Cenozoic compressive tectonics, which have erased much of the evidences of earlier deformations.This severely limits our understanding of the interplay between diapirism, tectonics, and sedimentary processes in sedimentary basins, and the role of pre-compressional inheritances in shaping the internal deformation of orogenic wedges. In the Baronnies (southern subalpine Alps), several Triasic diapirs that were reactivated during Cenozoic compression are exposed, raising questions about the earlier halokinetic activity in the area.

In orogenic domains, salt-tectonics is generally inferred from geometric evidences, which are not always well preserved. To address this limit, we developed a geochemical and regional approach, applied to the Mesozoic deposits in the Baronnies. Specifically, we used the strontium (Sr) content of pelagic carbonates deposited in the Vocontian basin (today incorporated in the Alpine prism) as a tracer of potential salinity anomalies associated to submarine diapirism. Rocks samples were collected from Oxfordian to Turonian sedimentary sequences in the deep environments of the Vocontian basin and along its northern (Vercors and Chartreuse), southern (Ventoux) and western (Ardèche-Languedoc) shallow margins. Sr content was measured using X-ray fluorescence (XRF) in the field and then lab-based XRF on both bulk samples and their carbonate fractions. Selected samples were also measured using ICP-OES spectrometry. Sr values were compared to the mean contemporaneous oceanic values of the reference curve established by Renard (1975) to identify possible anomalies.

The Sr content exhibits spatial and temporal variability, with both normal and abnormal values relative to the reference curve. Normal values characterise the Late Jurassic and basal Cretaceous periods. In contrast, Valanginian to Aptian values are significantly higher than the reference curve. The largest anomalies are observed in the deep Vocontian basin and suggest local contamination of the sediments by saline material flows. Comparison of the geochemical signal, sedimentary remobilization events (slumps, calciturbidites,…) and the structural and paleo-stresses frameworks point to a renewed halokinesis activity after the Liasic rifting, with diapirs piercing or not the seafloor. In the Baronnies, this Mesozoic activity has significantly deformed the contemporaneous sedimentary sequences, with local overtuned dips and megaflap-type geometries associated with angular unconformities and pitching of the sequences close to the paleodiapir bodies. At the basin scale, wide synclines were flanked by EW submarine ridges which, together with the inherited NE-SW faults divided the Vocontian basin. We corelate these structures with the Early Cretaceous tectono-halokinesis activity in the South East French Basin, with the Vocontian rift forming a major structure between the Valaisan Ocean and the Altlantic rift.  This study supports the existence of sutured diapirs in the meridional subalpine Alps with an enhanced tectono-halokinetic activity during Early Cretaceous. The last one created regional weak salt inheritances in the pre-compression Mesozoic sedimentary pile, preconditioning it for deformation during the later compressional phases.