Westward propagation of thrusts in the external Western Alps (France) reappraised from an updated chronostratigraphy of the Miocene Molasses

The fact that the western Alps Miocene foreland basin succession is poorly dated impacts directly our understanding of the deformation kinematics of that part of the external part of the Alpine belt (France). Here we propose a multidisciplinary approach aiming at building a robust tectono-stratigraphic framework of the Miocene deposits at the basin scale (northern subalpine massifs, southern Jura, Royans, Bas-Dauphiné and La Bresse basins). Sr isotopes stratigraphy combined with magnetostratigraphy and biostratigraphy enable sequence stratigraphy subdivisions S1 to S8 between the Upper Aquitanian (-21 Ma) and the Tortonian (-9 Ma) dated with a precision <0.5 Ma. These results highlight four different palaeogeographical domains during the Miocene: (i) the oriental domain with depositional sequences S1a to S3 (~21.3 to 15Ma), (ii) the median domain, in which sequences S2, S3, S4 and S5 occurred (~17.8 to 14Ma), (iii) the occidental domain with sequences S2 to S8 (~17.8 to ~9.5Ma); and (iv) the Bressan domain, in which sequences S6 to S8 are found (~ 11.5 to ~9.5Ma).

This revised chronostratigraphy was complemented with a structural and tectono-sedimentary study based on new fieldwork data and a reappraisal of regional seismic profiles, allowing to highlight five major faults zones (FZ). It appears that the oriental, median and occidental paleogeographical domains are delineated by FZ1, FZ2 and FZ3, therefore suggesting a strong interplay between tectonics and sedimentation. Evidences of syntectonic deposits and a westward migration of the depocenters impart the following deformation chronology : a Oligocene compressive phase (P1) corresponding to thrusting above FZ1 rooted east (above) Belledonne, which generated reliefs that limited the early Miocene transgression to the east; an Early- to Middle Miocene W-WNW/E-ESE-directed compressive phase (P2) involving the Belledonne massif basal thrust, which between 18.05 +/- 0.15 Ma and 12Ma successively activated the Salève thrust fault, and the FZ2 to FZ5 from east to west. P2 deeply impacted the Miocene palaeogeographical evolution by a rapid westward migration of depocenters in response to the exhumation of piggy-back basins above the growing fault zones; a last Tortonian phase (P3), less well constrained, apparently implied a significant uplift in the subalpine massifs, combined with the activation of the frontal Jura thrust.