Chronology of thrust propagation from an updated tectono-sedimentary framework of the Miocene molasse (western Alps)
Authors: Kalifi Amir , Leloup Philippe Hervé , Sorrel Philippe , Galy Albert , Demory François , Spina Vincenzo , Huet Bastien , Quillévéré Frédéric , Ricciardi Frédéric , Michoux Daniel , Lecacheur Kilian , Grime Romain , Pittet Bernard , Rubino Jean-Loup 
Published: December 2021

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