Chronology of thrust propagation from an updated tectono-sedimentary framework of the Miocene molasse (western Alps)

Abstract. After more than a century of research, the chronology deformation external part western Alpine belt (France) is still controversial for Miocene epoch. In particular, poor dating foreland basin sedimentary succession hampers comprehensive understanding kinematics. Here we focus on molasse deposits northern subalpine massifs, southern Jura, Royans, Bas-Dauphiné, Crest, and La Bresse basins through multidisciplinary approach to build basin-wide tectono-stratigraphic framework. Based sequence stratigraphy constrained by biostratigraphical, chemostratigraphical (Sr isotopes), magnetostratigraphical data between late Aquitanian (∼ 21 Ma) Tortonian 8.2 Ma), chronostratigraphy revised with precision ∼ 0.5 Ma. The sediments encompass four different paleogeographical domains: (i) oriental domain outlined depositional sequences S1a S3 15 (ii) median characterized S2 S5 17.8 12 (iii) occidental in which S2a S8 were deposited, (iv) Bressan where sedimentation restricted S6 Ma). A structural tectono-sedimentary study conducted based new field observations reappraisal regional seismic profiles, thereby allowing identification five major faults zones (FZs). oriental, median, domains are clearly separated FZ1, FZ2, FZ3, suggesting strong interactions tectonics during Miocene. evolution time space within well-constrained framework reveals syntectonic westward migration depocenters, proposing three phases at front. first compressive phase (P1) corresponding thrusting above Chartreuse thrust (FZ1), was likely initiated Oligocene rooted east Belledonne. This tectonic generated reliefs that limited transgression east. second WNW–ESE-directed (P2) lasting 18.05 ± 0.25 Ma Ma, thrusts Belledonne basal thrust. Thrusts activated from west: Salève (SAL) Gros Foug (GF) then successively FZ4, FZ5. Along two WNW–ESE balanced cross sections amount horizontal shortening 6.3 6.7 km, average rates 1.2 km Myr−1 toward west rate 2.9 Myr−1. During 6 Myr, Sea forced regress rapidly westwards response active exhumation piggyback atop fault zones. Phase P2 thus deeply shaped area appears as prominent scale Alps Swiss Rhodano–Provencal one. third 300 m uplift Bas-Dauphiné (P3) probable age 10 would have induced southward sea retreat been coeval folding Jura north possibly back-thrusting massif.