The Jura mountains are a classic example of a fold-and-thrust belt formed via thin-skinned deformation during the Alpine convergence from ca. 15 Ma to 4 Ma. The Mesozoic sedimentary cover is detached from the basement along Triassic evaporites. However, recent U-Pb dating of syn-tectonic calcite mineralization (44-48 Ma) and 3D seismic data suggest a more complex polyphased evolution. Reactivation of basement-rooted faults and their propagation into the cover challenges the traditional model of thrusts solely rooted at the base of the sedimentary pile.

The « Avant-Monts » zone, at the front of the Jura near Besançon, is key to understanding the influence of inherited structures. It contains hidden Permian grabens and major crustal faults oriented ~N070, as well as ~N010 to N030 faults. This area also coincides with the Rhine-Bresse left-lateral transtensive transfer zone formed during the Oligocene rifting. All these structures may have experienced inversion during later Jura thrusting.

We divided the area in three structural domains : (1) the “La Serre zone” characterized by N070 basement faults and horst-graben geometry; (2) The Corcondray–Miserey fault system, marked by dense N020-030 faults exposing Triasic formations; and (3) The “Chailluz domain”, with a more classic fold-and-thrust geometry. All are bounded to the north by the Ognon normal fault.

In-situ LA-ICP-MS U-Pb dating of calcite veins from the N020-030 faults yields consistent late Eocene - early Oligocene ages (32- 38 Ma). Samples near the Ognon fault show two age groups: btained : Middle Eocene (39–47 Ma) and Early Oligocene (32–35 Ma), matching those from the N020–030 faults. No Miocene-Pliocene were detected. Clumped isotope thermometry indicates formation temperatures of 40–70°C, implying deep-seated, basement-rooted faulting.

We propose that the Avant-Mont finite geometry is largely predetermined by successive Middle Eocene (ca. 40-45 Ma) and Early Oligocene (ca. 32-34 Ma) tectonic events, linked to the far field tectonic effect of Alpine orogeny and the onset of the European Cenozoic Rift System with the development of the Rhine-Bresse transfer zone respectively.