Geodynamic of French bauxite through (U-Th)/He thermochronology on Fe-oxyhydroxides

Constraining quiescence intervals in tectonically active regions remains challenging, particularly in tectonic reconstructions, as these periods are often overprinted by extensional and/or compressional processes that remobilize geological materials, rendering access and dating of these intervals difficult.

Recent studies in tectonically stable regions on laterites and bauxites formed through weathering under tropical climates have demonstrated the efficacy of (U-Th)/He geochronology on Fe-oxyhydroxides (hematite and goethite) in constraining tectonic quiescence periods.

In the Mediterranean region, numerous bauxites have been preserved due to their remobilization into karst systems, allowing for their burial and protection during subsequent tectonic processes. This preservation offers a unique opportunity to better understand the geodynamics of the region. This study focuses on French bauxites from Bédarieux, Les Baux-de-Provence, and Brignoles, which constitute the Durancian Isthmus—a supposed Cretaceous paleosurface bordered by large inherited Variscan structures (Cévennes, Nîmes, and Durance faults)—whose geodynamic implications are still poorly understood.

The only available temporal constraints on the formation of this dismantled weathering profile rely on the sedimentary context of the karsts where they are trapped, with the most reliable timeframes established between the Hauterivian and Turonian. This transitional period is still poorly understood in the region, as various tectonic processes are at play, including Pyrenean rifting and its inversion, Alpine Tethys, and Massif Central exhumation.

We combined petrological investigations on nine different Fe-Al-bauxitic duricrust samples, allowing for the determination of different hematite and goethite generations, prior to conducting (U-Th)/He dating on the identified sub-generations to quantify bauxite formation and evolution. The ages obtained for hematite and goethite pisolites span from the Cretaceous to the Oligocene, encompassing all generations, enabling the placement of Cretaceous bauxites within their Mediterranean geodynamic context—from their initial formation via basement alteration, to their reworking within karsts, sedimentary burial, and subsequent exhumation.