Mesozoic and Cenozoic tectono-halokinetic evolution in the Baronnies Provençales (Alps, France)

Evaporite distribution and mobility is a key parameter in the structuration of salt bearing sedimentary basins where these layers can generate halokinetic deformations. Recent revisions of compressive basin models, including those in the external Alps, highlight the significant role of salt tectonics. However, identifying pre-compression halokinetic deformations is often challenging due to erosion or misinterpretation. The “Baronnies provençales”, located in the Vocontian basin in the external western Alps (France), display a unique structure characterized by large E-W oriented synclines oblique to the NNW-SSE trend of the subalpine Alps, bordered by very tight E-W oriented anticlines, whether faulted or not, the origin of which remains debated. Several outcropping diapirs involving Triassic evaporites have been identified in this area, indicating halokinetic activity, either recent or ancient. This study aims to characterize the structural style of the region and the Mesozoic halokinetic structures in order to assess their impact on subsequent deformations.

Field observations, bedding measurements, and cross-sections illustrate that the sedimentary series in the synclines became abruptly steeper near the anticline axes, often adopting overturned dips, sometimes forming megaflap-type geometries. Several angular unconformities have been identified within the Early Cretaceous sequence, notably between the Barremian-Aptian and the Hauterivian, as well as between the Coniacian and the Turonian along strike of the present day anticline axes. N-S oriented slumps within the cretaceous succession highlight the presence of paleo-morphologies parallel to the modern anticline structures that were thus developing during the Early Cretaceous. E-W sediment gravity-flow (e.g. marly calcareous slumps, sandy- and calci- turbidites and debris flow) following submarine palaeocanyons more numerous than the N-S ones and located at the core of the synclines, reflect the regional paleoslope of the region. The structural analysis of the region shows that major thrust faults are located along and with the same orientation as the E-W oriented tight anticlines, thus positioned between the multi-kilometer wide and flat synclines. These thrust faults exhibit lateral variations in their vergence. Microtectonic analysis indicate normal faulting predating the formation of E-W folds, associated with a WNW-ESE extension likely linked to the reactivation of the major inherited NE-SW faults within the South French basin. A ~N-S compression, probably corresponding to the Pyrenean-Provençal phase, as well as a ~NE-SW to ENE-WSW compression, likely associated with the Alpine phase, have also been identified. These results highlight that the ‘Baronnies provençales’ area has thus recorded the main regional deformation phases of the Meso-Cenozoic, but has also been affected by renewed halokinetic activity during the Early Cretaceous, which was responsible for layer tilting forming the ‘megaflap’ – type structure and the formation of topographic anticline-like ridges that induced episodes of lateral sedimentary reworking. The Cretaceous salt-tectonics thus played a key role in shaping the structural style of the region and also probably in the thrust vergence. The paleo-diapirs were sutured while accommodating the shortening of later compressive phases.

Key words: Halokinesis, Structural inheritance, Tectonics, Vocontian basin.