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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. [ABSTRACT FROM AUTHOR]

The Hauterivian–Barremian series of the Jura Mountains were measured over more than 60 sections along a 200 km long transect between Aix-les-Bains (Savoie Department, France) and Bienne (Bern Canton, Switzerland), which prompted the need for a revision and improvement of the current lithostratigraphic scheme for this stratigraphic interval. A new formation, the Rocher des Hirondelles Formation, is proposed in replacement of the unsuitable Vallorbe Formation, while the Gorges de l'Orbe Formation is formally described. The Gorges de l'Orbe Formation, equivalent to the well-known "Urgonien jaune" facies, consists of two members, namely Montcherand Member and Bôle Member. The Rocher des Hirondelles Formation, equivalent to the "Urgonien blanc" facies, consists of three members, i.e. Fort de l'Écluse Member, Rivière Member and Vallorbe Member. The marly Rivière and Bôle members appear to present time-equivalent lithostratigraphic units, recording a major sedimentological event affecting contemporarily both formations. This study proposes a new sedimentary model opening a new point of view on the long-living controversies about the age of the Urgonian series from the Jura Mountains. The data point to strong diachronic ages of lithostratigraphic units with a late Hauterivian to early Barremian occurrence of the "Urgonian blanc" facies in the Meridional Jura area versus a latest Barremian age in the Central Jura area, reflecting a general progradation of the Urgonian shallow-water carbonate platform from the present-day Meridional Jura area toward external deeper-water shelf environments of the present-day Central Jura area and Molasse basin. [ABSTRACT FROM AUTHOR]

In this paper, we present new exhumation ages for the imbricated proximal molasse, i.e. Subalpine Molasse, of the northern Central Alps. Based on apatite (U-Th-Sm)/He thermochronometry, we constrain thrust-driven exhumation in the Subalpine Molasse between 12 and 4 Ma. This occurs synchronously to the main deformation in the adjacent Jura fold-and-thrust belt farther north and to the late stage of thrust-related exhumation of the basement massifs (i.e. external crystalline massifs) in the hinterland. Our results agree with other findings along the north Alpine foreland. While site-specific variations in the mechanical stratigraphy of the molasse deposits influence the pattern of thrusting at the local scale, we observe that late-Miocene thrusting is a long-wavelength feature occurring along the north Alpine foreland roughly between Lake Geneva and Salzburg. The extent of this thrusting signal as well as the timing suggests that late-Miocene thrusting in the north Alpine foreland coincides with the geometries and dynamics of the attached Central Alpine slab at depth. Interestingly, this implies that the slab geometry at depth does not coincide with the boundary between the Eastern and Central Alps as observed in the surface geology. Using this observation, we propose that thrusting in the Subalpine Molasse and consequently also the late stage of thrust-related exhumation of the external crystalline massifs, as well as the main deformation in the Jura fold-and-thrust belt are at least partly linked to changes in slab dynamics. [ABSTRACT FROM AUTHOR]

A comprehensive sedimentological study was undertaken in the Miocene of the subalpine massifs and southern Jura (France) with the aim to constrain the evolution of process changes in third‐order sequences of peripheral foreland basins during the overfilled phase (i.e. sediment supply higher than accommodation space). Fieldwork analyses based on 35 sedimentological sections allowed the identification of four depositional models: wave dominated, mixed wave‐tide, river to tide and river dominated. The sections were dated using chemostratigraphy (i.e. marine strontium isotopic ratios), revealing three‐third‐order sequences between the Upper Aquitanian and the Langhian. Chronostratigraphical and sedimentological results document prominent and recurrent changes in depositional models along third‐order sequences: (i) in the earliest stage of the transgression, mixed‐energy coastal environments influenced by the local coastal morphology prevailed (in palaeo‐highs or incised valleys); (ii) during the course of the transgression, Gilbert delta deposits suggest a prominent steepening linked to a tectonic uplift in the proximal depozone (between the tectonically active frontal part of the orogenic wedge and the proximal foredeep). Instead, in the distal depozone (between the proximal foredeep and the proximal border of the flexural uplifted forebulge), deposits were characterized either by wave‐dominated or mixed wave‐tide environments and are likely eustatically‐driven; (iii) during the maximum flooding stage, water depth remained shallow below the storm‐weather wave base; and (iv) during the regression, the proximal depozone is characterized by the progradation of gravel‐rich fan deltas. In the distal depozone, mixed wave‐tide systems preceded the development of river to tidal depositional environments. These results were integrated and compared with facies models from other basin analogues worldwide. A model tackling the evolution of process changes within third‐order sequences (of the overfilled phase) of foreland basins is proposed, thereby improving sequence stratigraphic predictions in foreland basins. [ABSTRACT FROM AUTHOR]

Additional to classical nappe tectonics, the Oligocene to mid-Miocene post-collisional evolution of the Central European Alps was characterized by vertically directed tectonics, with backthrusting along the Insubric Line and the subsequent uplift of the External Crystalline Massifs (ECMs). Thereafter, the orogen experienced axis-perpendicular growth when deformation propagated into its external parts. For the North Alpine foreland between Lake Geneva and Lake Constance, in the past, this has been kinematically and spatially linked to the uplift and exhumation of the ECMs. Based on apatite (U-Th-Sm)/He thermochronometry, we constrain thrusting in the Subalpine Molasse between 12-4 Ma, thus occurring coeval to main deformation in the Jura fold-and-thrust belt (FTB) and late stage exhumation of the ECMs. However, this pattern of tectonic activity is not restricted to areas which are bordered by ECMs, but is consistent along the northern front of the Alps between Geneva and Salzburg. Therefore, late Miocene foreland deformation is not necessarily a consequence of uplift and exhumation of the ECMs. While the local geometry of the Subalpine Molasse results from lateral variations of the mechanical stratigraphy of the foreland basin sediments, we suggest that the large-wavelength tectonic signal is the response to a shift in tectonic forces possibly caused by deep-seated geodynamic processes. This resulted in a change from dominantly vertical to horizontal tectonics and orogen-perpendicular growth of crustal thickening. We constrain the onset of this major tectonic change to ca. 12 Ma in the North Alpine foreland, resulting in thrusting and folding in the Subalpine Molasse west of Salzburg and in the Jura FTB until at least 4 Ma. [ABSTRACT FROM AUTHOR]

Shales contain magnetic minerals generally at very low concentrations. In the early stages of diagenesis, the inherited magnetic minerals are altered, while magnetic nanominerals are formed. In this study, we proposed a study of shales over a stratigraphic thickness of 1.3 km from a borehole in the Paris basin (Borehole EST 433, France), and shales from the same formation (Opalinus Clay) collected in seven boreholes in the Jura molasse basin (Swiss). Magnetic measurements at experimental temperatures <30 K allowed the formation of a proxy of magnetite nanograins named PM. We showed that some of these nanograins formed around the pyrite grains, probably under the action of temperature and organic matter. PM was then compared to the maturity values of the organic matter. We found a correlation between PM and the percentage of reflectance of vitrinite. The shales from both Paris and molassic Swiss basins showed very comparable magnetic characteristics for a given maturity level. The magnetic study therefore provided constraints on the maturity level of the shales in the oil window area. Our study showed that PM can be used as a geothermometer in shales in which CaCO3 is lower than 60%. [ABSTRACT FROM AUTHOR]

This work presents new apatite fission track LA–ICP–MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) data from Mid–Late Paleozoic rocks, which form the substratum of the Swiss Jura mountains (the Tabular Jura and the Jura fold-and-thrust belt) and the northern margin of the Swiss Molasse Basin. Samples were collected from cores of deep boreholes drilled in North Switzerland in the 1980s, which reached the crystalline basement. Our thermochronological data show that the region experienced a multi-cycle history of heating and cooling that we ascribe to burial and exhumation, respectively. Sedimentation in the Swiss Jura Mountains occurred continuously from Early Triassic to Early Cretaceous, leading to the deposition of maximum 2 km of sediments. Subsequently, less than 1 km of Lower Cretaceous and Upper Jurassic sediments were slowly eroded during the Late Cretaceous, plausibly as a consequence of the northward migration of the forebulge of the neo-forming North Alpine Foreland Basin. Following this event, the whole region remained relatively stable throughout the Paleogene. Our data show that the Tabular Jura region resumed exhumation at low rates in early–middle Miocene times (≈20–15 Ma), whereas exhumation in the Jura fold-and-thrust belt probably re-started later, in the late Miocene (≈10–5 Ma). Erosional exhumation likely continues to the present day. Despite sampling limitations, our thermochronological data record discrete periods of slow cooling (rates of about 1°C/My), which might preclude models of elevated cooling (due to intense erosion) in the Jura Mountains during the Miocene. The denudation (≈1 km) of the Tabular Jura region and the Jura fold-and-thrust belt (≈500 m) has provided sediments to the Swiss Molasse Basin since at least 20 Ma. The southward migration of deformation in the Jura mountains suggests that the molasse basin started to uplift and exhume only after 5 Ma, as suggested also by previous authors. The data presented here show that the deformation of the whole region is occurring in an out-of-sequence trend, which is more likely associated with the reactivation of thrust faults beneath the foreland basin. This deformation trend suggests that tectonics is the most determinant factor controlling denudation and exhumation of the region, whereas the recently proposed "climate-induced exhumation" mechanism might play a secondary role. [ABSTRACT FROM AUTHOR]

The Western Alpine foreland is composed of two structurally different regions: to the S, the Western Alpine Molasse Basin (WAMB) located NW of the Penninic frontal thrust and to the N, the Jura foreland Fold-and-Thrust Belt (FTB) forming the external outlier of the Alpine orogeny. The WAMB stretches from Lake Constance in the E to the city of Chambéry (France) to the SW. It initiated as flexural foreland basin, and subsequently evolved to become a wedge-top basin and a part of the detached Alpine foreland. The development of the Jura FTB is associated to the formation of a main décollement level over the mechanically more rigid basement s.l., and can be linked to a "distant push" from the Alps (Fernschub hypothesis) associated with the exhumation of the External Crystalline Massifs in the Alps.The WAMB s.str. contains the Cenozoic erosional remnants of the Alpine orogenic wedge, resting on Mesozoic epicontinental platform carbonate series overlying a Paleozoic basement. The Jura FTB mainly develops in these Mesozoic series. The arc-shaped geometry of the Jura FTB and the trailing WAMB have been related to the lateral distribution and thickness changes of the Triassic evaporite layers. The Muschelkalk layers act as main décollement zone in the eastern area, and the Keuper layers in the western area. Strain is partitioned between several décollement levels and the brittle-ductile deformation is mainly concentrated in salt and evaporite layers of both ages. The structural style ranges from fault-and ramp-related folds, mostly in the Jura FTB, to evaporite-cored folds in the WAMB. In addition, secondary décollement levels have been proposed in the Liassic shale series. Strike-slip faults oriented N-S (sinistral) and NW-SE (dextral) cut from the Jura FTB into the WAMB, and are restricted to the detached sedimentary cover, as documented by seismic activity. Inherited basement structures main function as trigger for structures in the detached cover.The tectonic development of the foreland is generally considered to occur between 15 and 4 My, though recent work suggests that it may have started earlier, and is still actively deforming, as documented by ongoing seismicity. The thrust propagation in the orogenic wedge is dependent on the basal friction and the evolution of the topography and we observe a forward-backward oscillating sequence of thrusts and thrust-related folds in which the Jura FTB and the WAMB are linked. The entire foreland evolves as a tapered wedge. The very low taper and the weak décollement allow for a broad wedge geometry and the formation of numerous backthrusts.Paleostress analysis and recent earthquake data suggest a regionally uniform NW-SE oriented regional stress field that rotates with the arc of the Jura to be more W-E oriented. However, examinations on the major tear fault systems point to local deviations of the regional stress orientation. The broad strike-slip fault system are composed of 3 orders of self-similar imbricated Riedel shear faults that locally yield reorientations of the stress field, suggesting very strong strain partitioning. Ongoing distributed seismicity suggests that the foreland is possibly critically stressed. [ABSTRACT FROM AUTHOR]