Subalpine Chains Research Articles

Along-strike variations of structural style in the external Western Alps (France): Review, insights from analogue models and the role of salt

The European Alps result from the closure of a former Triassic to Cretaceous rifting system in which significative accumulation of Upper Triassic salt deposited. The thickness and spatial distribution of these salts had a major impact on the morphology and dynamic of the Alpine orogen all over the Western Alps. Following a bibliographical review, four regional cross-sections are revisited over the External Western Alps to emphasise along-strike variations of the orogenic wedge influenced by the presence or absence of Triassic salts. Each section is then compared with analogue models to examine the controlling factors explaining the structural styles variations. In the Northern Subalpine Chains, the absence of salt favoured a frictional decollement which promoted a narrow and thick orogenic wedge in the Bornes, Chartreuse, and Vercors massifs. Discrepancies in the spacing between the thrusts and the number of folds were however influenced by the thickness of incompetent layers in the sedimentary column. In the Jura and the SW Alps, the occurrence of Triassic salts promoted a viscous decollement and thus a wide and relatively thin orogenic wedge. However, in the SW Alps, thicker salt deposits allowed the development of pre-orogenic salt structures. These later constituted vertical heterogeneities having exerted a strong structural inheritance during the following compression, by accommodating a substantial part of the Alpine shortening, in squeezed diapirs especially. Triassic salt remobilization during the orogeny also locally promoted the development of syn-orogenic minibasins. This review highlights the significant role played by Triassic salt in shaping the Western Alps, and paves the way for possible new interpretations of the dynamics of the Alps regarding the exhumation of crystalline basement blocks, or new structural interpretations of Alpine domains, such as the Vocontian Domain.

The Early Cretaceous C-isotope record of shallow-marine carbonates: a transect across southern Alps, Apennines and Dinarides

Shallow-water carbonate platforms host a precious record of carbonate- associated proxies of palaeoceanographic conditions. The Apennine section of San Lorenzello, Valanginian–Barremian in age, exhibits interesting variations of geochemical, palaeontological and sedimentological proxies in the time interval after the Weissert and before the Selli OAEs. The C-isotope correlation, performed across a transect going from the Urgonian platform in the Subalpine Chains to the Apennine Carbonate Platform in the southern Apennines and to the Adriatic Platform in the Dinarides shows that the δ13C profile of the studied section potentially preserves the global marine C-isotope signature, but the above carbonate platforms show lateral thickness variations of time equivalent discrete stratal intervals and a not strictly synchronous distribution of some biomarkers. Biostratigraphy, carbon isotopes and sequence stratigraphy allow a chronostratigraphic calibration of the studied section and the interpretation of its depositional sequences (superbundles) and biotic events in the framework of global sequences and eustatic cycles. A recovery of the neritic biota is observed at the end of the Weissert OAE, coinciding with the Valanginian–Hauterivian sea- level rise and with a return to warmer temperatures after the late Valanginian cooling. The superbundles, considered also as expression of 400-kyr orbital cycles, can contribute to create a floating orbital time scale which quantifies the durations of the Hauterivian and Barremian stages.

Salt tectonics along a strike-slip fault system in the sub-Alpine chains of southeastern France, from the Triassic to the Oligocene

Salt tectonics along a strike-slip fault system in the sub-Alpine chains of southeastern France, from the Triassic to the Oligocene

Raman spectroscopy: an effective thermal marker in low temperature carbonaceous fold–thrust belts

Abstract Raman spectroscopy allows thermal maturation of carbonaceous sediments to be determined. The technique has been employed on metamorphic samples exceeding temperatures of 270°C, but recently has been shown to be effective at lower temperatures. Thermal maturation techniques commonly depend on sample size, have varying efficacies at different temperatures and in different conditions. The underlying processes are not well understood, thus data interpretation can be ambiguous. Here we show the efficacy of Raman as a low-temperature thermal marker in a thrust belt. The Bornes region, in a French Subalpine chain, provides an opportunity to test the technique against published vitrinite reflectance data and thermal modelling for the first time. In doing so we show that Raman is an effective thermal marker to temperatures as low as 75°C, has a small error and is consistent with previous work. The Raman data allow us to postulate the relative thickness of the sedimentary succession across the chain, the timing of thermal maturation and the timing and thickness variations of the over-thrust Prealpine nappe. The work establishes Raman as a low-temperature thermal marker for correlation with other techniques to ensure effective and robust interpretation, that can readily be applied to fold–thrust belts in hydrocarbon provinces.

Towards a Better Knowledge of Natural Methane Releases in the French Alps: A Field Approach

We report investigations performed at some hydrocarbon gas seeps located in the French Subalpine Chains in zones of outcropping Jurassic black shales, increasing the reported number of such occurrences in this part of the Alps. We present the characteristics of each of the seeps, based on soil flux measurements and soil gas measurements. Gases emitted are CH4-rich (87–94%) with the exception of one site (78.5% CH4 + 8.2% CO2) where an active landslide may induce dilution by atmospheric air. CO2 is generally measured at low levels (<1.6%). Concentrations in C2H6 are more variable, from less than 1% to more than 2.3%. Gas is emitted over areas of various sizes. The smallest gas emission area measures only 60×20 cm, characterized by a strong hydrocarbon flux (release of about 100 kg of CH4 per year). At a second site, hydrocarbon emissions are measured over a surface of 12 m2. For this site, methane emission is evaluated at 235 kg per year and CO2 emission is 600 kg per year, 210 kg being related to gas seepage. At the third site, hydrocarbons are released over a 60 m2 area but strong gas venting is restricted to localized seeps. Methane emission is evaluated at 5.1 tons per year and CO2 emission at 1.58 tons per year, out of which 0.53 tons are attributed to gas seepage. Several historical locations remain uninvestigated at present, and numerous others may still be unknown. We outline strategies to search for such unrecorded sites. Considering the topography of the potential alpine and perialpine emission areas, the possibilities to detect gas emissions appear of the size recorded so far seem to be restricted to ground-based methods or to methods offering the possibility to point orthogonally to the soil towards the seep maximum. If such sites are to be investigated in the future in the frame of Environmental Baseline Assessment (EBA), even establishing appropriate monitoring protocols will be challenging.

Fold–thrust structures – where have all the buckles gone?

Abstract The margins to evolving orogenic belts experience near layer-parallel contraction that can evolve into fold–thrust belts. Developing cross-section-scale understanding of these systems necessitates structural interpretation. However, over the past several decades a false distinction has arisen between some forms of so-called fault-related folding and buckle folding. We investigate the origins of this confusion and seek to develop unified approaches for interpreting fold–thrust belts that incorporate deformation arising both from the amplification of buckling instabilities and from localized shear failures (thrust faults). Discussions are illustrated using short case studies from the Bolivian Subandean chain (Incahuasi anticline), the Canadian Cordillera (Livingstone anticlinorium) and Subalpine chains of France and Switzerland. Only fault–bend folding is purely fault-related and other forms, such as fault-propagation and detachment folds, all involve components of buckling. Better integration of understanding of buckling processes, the geometries and structural evolutions that they generate may help to understand how deformation is distributed within fold–thrust belts. It may also reduce the current biases engendered by adopting a narrow range of idealized geometries when constructing cross-sections and evaluating structural evolution in these systems.

Thermal and exhumation histories of the northern subalpine chains (Bauges and Bornes—France): Evidence from forward thermal modeling coupling clay mineral diagenesis, organic maturity and carbonate clumped isotope (Δ47) data

AbstractAssessing the thermal evolution of sedimentary basins over time is a major aspect of modern integrated basin analysis. While the behavior of clay minerals and organic matter with increasing burial is well documented in different geological and thermal settings, these methods are often limited by the temperature ranges over which they can be precisely applied and by the available material. Here, we explore the emergent Δ47 clumped isotope geospeedometry (based on the diffusional redistribution of carbon and oxygen isotopes in the carbonate lattice at elevated temperatures) to refine time‐temperature paths of carbonate rocks during their burial evolution. This study provides a reconstruction of the thermal and exhumation history of the Upper Cretaceous thrust belt series in the western subalpine massifs (Bauges and Bornes, French Alps) by a new approach combining for the first time available data from three independent geothermometers. The investigated area presents two zones affected by contrasting thermal histories. The most external zone has undergone a relatively mild thermal history (T < 70°C) and does not record any significant clay mineral diagenetic transformation. By contrast, the internal zone has experienced tectonic burial (prealpine nappes) in response to thrusting, resulting in overheating (T > 160–180°C) that induced widespread clay mineral diagenetic transformations (progressive illitization from R0 to R1 and R3 illite‐smectite mixed‐layers), organic matter maturation (oil window) and Δ47 thermal resetting with apparent equilibrium temperatures above 160°C. The three employed geothermal indicators conjointly reveal that the investigated Upper Cretaceous rocks have suffered a wide range of burial temperatures since their deposition, with a thermal maximum locally up to 160–180°C. High temperatures are associated with the tectonic emplacement of up to 4 km of prealpine nappes in the northern part of the studied area. Finally, a forward thermal modeling using Δ47, vitrinite reflectance and clay mineral data, is attempted to precisely refine the burial and exhumation histories of this area.

Style of Alpine tectonic deformation in the Castellane fold-and-thrust belt (SW Alps, France): Insights from balanced cross-sections

This study proposes a reappraisal of the role of the basement tectonics in the structuration of the Alpine foreland, across the Castellane fold-and-thrust belt located in southwestern Alps. We construct three 30km length N–S balanced cross-sections across the entire fold-and-thrust belt, in order to quantify the amount of horizontal shortening due to the Pyrenean and Alpine deformations. We then assess the role of the basement inherited structures during the compressional phases which resulted in the exhumation of the Argentera–Mercantour External Crystalline Massif and the Barrot Dome. The construction of these balanced cross-sections suggests a dominant thick-skinned deformation style, which includes the reactivation of inherited Permian and Early Cretaceous basement normal faults. We reconstructed three restoration stages: the oldest one highlights the geometry of Permian and Early Cretaceous extensional structures; the intermediate one after the Late Cretaceous evidences a first compressional episode linked to the Pyrenean compression; and the latest one in the Oligocene shows the first Alpine folds resulting from the southward thrusting of the internal units over the external ones along of the Penninic Frontal Thrust. Balanced cross-sections suggest very moderate crystalline basement–sedimentary cover decoupling. On the western and central cross-sections, the estimated amount of shortening ranges from 9.5 to 10km (21%) whereas on the easternmost one shows ~5km of shortening (9%). These shortening values are consistent with previously published estimates in the surrounding foreland subalpine chains. They highlight a decreasing value of Pyrenean shortening toward the east, while the Alpine shortening dominates and amplifies this first phase in a similar direction. We interpret this dominantly thick-skinned structural style as a possible consequence of the Neogene thermal weakening in the European passive margin above the Ligurian slab rollback.

Carbon-isotope stratigraphy of Early Cretaceous (Urgonian) shoal-water deposits: Diachronous changes in carbonate-platform production in the north-western Tethys

Carbonate platforms are highly sensitive ecological systems that typically show rapid and characteristic response modes to environmental and climatic changes acting both on a regional scale and global scale. A widely accepted hypothesis proposes that the Late Barremian stepwise establishment of the Urgonian carbonate platform on the Northern Tethyan shelf was related to a gradual change from predominantly humid towards more arid greenhouse conditions. This climate change resulted in the reorganisation of the carbonate platform ecosystem from heterozoan towards photozoan-dominated assemblages and a decrease of organic-matter burial in Tethyan and Boreal basins. In order to decipher the palaeoenvironmental and climatic boundary conditions of these major changes in neritic and pelagic settings, a precise chronostratigraphy of Urgonian carbonate platform evolution is needed. Here, we provide an integrated stratigraphic framework of Lower Barremian to Lower Aptian Urgonian carbonate platform sections (Cluses, Forclaz) located at the northern rim of the Tethys (Subalpine Chains, ESE France), with special focus on sedimentological analyses and high-resolution carbon-isotope stratigraphy. A characteristic Barremian–Aptian carbon-isotope pattern permits precise platform-to-basin correlation with cyclostratigraphic and ammonite-dated pelagic and hemipelagic Tethyan sections in the Vocontian Trough (Angles/Combe-Lambert/Glaise) and Umbria Marche Basin (Gorgo a Cerbara). Similar to Helvetic shoal-water settings, the carbonate platform in the Subalpine Chains experienced a gradual transition from heterozoan- to photozoan-dominated ecosystems. This biogenic pattern points to a gradual change of the carbonate platform after the so-called “Early Barremian crisis” towards an oligotrophic rimmed platform system (Urgonian Limestone Formation). According to the chronostratigraphy of Urgonian carbonate platform evolution in the Subalpine Chains established here, the installation of a photozoan-dominated rudist-rich platform ecosystem, referred to as Urgonian limestones sensu stricto, occurred at the Early–Late Barremian transition. The onset of changes in the carbonate production mode (“heterozoan” versus “mixed heterozoan–photozoan” versus “photozoan”) clearly predates the onset of similar lithological changes in the Helvetic realm by about 1.0 to 1.2Myr. The established chronostratigraphic framework points to a link between the change towards photozoan-dominated limestones and fading palaeoenvironmental perturbations related to the mid-Barremian event, which is associated with major faunal turnovers and the onset of cyclic black-shale deposition in the central Tethyan realm. Judging from palaeoenvironmental proxies including kaolinite, phosphorus and black marl-limestone ratios, oscillating predominantly humid and relatively arid greenhouse conditions modulated the observed pattern in Urgonian carbonate platform production mode and the varying frequencies of black-shale deposits in the Tethyan and Boreal realms. Transient blooms of the orbitolinid foraminifera Palorbitolina lenticularis at the onset of a marked Upper Barremian positive carbon-isotope shift are interpreted to reflect increasing nutrient influx, most probably related to accelerated hydrological cycling and/or sea-level rise. These strata do not represent an over-regional correlatable lithostratigraphic unit, but likely an expression of progressive Northern Tethyan environmental and oceanographic change possibly related to Ontong Java large igneous province volcanism in the prelude of oceanic anoxic event 1a (OAE).

Allochthonous salt in the sub-Alpine fold–thrust belt of Haute Provence, France

Abstract In the sub-Alpine chains of Haut Provence, SE France, a very well-exposed Mesozoic sequence showing rapid thickness and facies changes associated with Jurassic and Cretaceous extension on the margin of the Ligurian Tethys has been deformed by ‘Alpine’ compression which occurred from the Late Cretaceous to the Pliocene. Although the geology has been very well known for decades, aspects of the structure remain enigmatic and cannot be explained by either Mesozoic extension or Alpine shortening alone. We infer that some deformation resulted from salt tectonics. A completely overturned, highly condensed Jurassic section near Barles village resembles the elevated roof of a Triassic salt body in a deep-marine basin. This carapace became overturned as a flap in the Middle Jurassic when salt broke out at the seafloor and overran the inverted flap as an allochthonous extrusion, comparable to those in the deepwater Gulf of Mexico or Angola. Later, Alpine compression exploited the weakness of the salt sheet as the Digne Thrust moved over the inverted flap. Although the flap is in the footwall of the thrust, evidence of soft-sediment deformation and other anomalous structures within the flap suggest that it did not originate as an overturned footwall syncline.

Geometry and sedimentary evolution of the transpresssive Roquebrune-Cap Martin basin: implications on the kinematics and timing of the Nice arc deformation during Miocene times, SW Alps

Abstract The Roquebrune-Cap Martin basin (RCMB), developed along the eastern rim of the Nice arc, represents an exclusive sedimentary marker constraining the timing of the deformation in the Nice arc (southern Subalpine chain) during Miocene times. Structural and sedimentological analyses as well as 3D geometrical modeling of the RCMB revealed morphological, structural and sedimentological features characterizing an active tectonic control of the sedimentary infills and the basin development. Structural and microstructural analyses along the eastern boundary of the Nice arc evidenced a N-S left-lateral strike-slip ‘en echelon’ faults system named Mont Gros-St Agnès Castillon relayed by the Biancon E-W thrusts and sheets. The formation of the RCMB appears to be genetically linked to these strike-slip ‘en échelon’ faults. Such characteristics include the presence of the Mont Gros strike-slip fault structural high relief bounding the RCMB to the West, the West-East asymmetry of the sedimentary infill with a laterally transition facies from breccias directly below the fault relief to conglomerates and sandstones in the central part of the basin and the presence of mass wasting in all structural levels of the basin. The onset and the evolution of the basin were driven by transpresssive tectonics, generating a deep and narrow tectonic depression, bounded by steep tectonically controlled slopes. The transpresssive character of the eastern Nice arc boundary where the syn-tectonic RCMB is hosted, accommodate a general southward translation of the Nice arc in response to a N-S shortening regime. The sedimentological and previous paleontological analyses suggest that the activity of the eastern Nice arc transpresssive boundary generating the RCMB and thus the southward motion of the Nice arc, started during the Early Miocene (Aquitanian), continuing through the Late Miocene (Tortonian). The style and the timing of the syn-sedimentary deformation of the Nice arc is coherent in space and time with the one affecting the Digne and Castellane arc.

Strontium and carbon-isotope chronostratigraphy of Barremian–Aptian shoal-water carbonates: Northern Tethyan platform drowning predates OAE 1a

A widely accepted hypothesis proposes that the Early Aptian demise of carbonate platforms in the northern Tethyan realm reflects the impact of environmental changes that eventually led to the deposition of organic-rich basinal sediments during oceanic anoxic event (OAE) 1a. In fact, the temporal correlation of oceanic anoxia and platform drowning has not been rigorously tested and evidence indicating causality is debated. The present paper provides a high-resolution carbon- and strontium-isotope chronostratigraphy combined with detailed sedimentological analysis applied to the Barremian–Aptian Urgonian carbonate platform development at the northern rim of the Tethys (Subalpine Chains, Haute-Savoie, ESE France). A characteristic Barremian–Aptian carbon- and strontium-isotope pattern below the onset of OAE 1a interval permits precise platform-to-basin correlation with the Barremian stratotype locality of Angles (SE France). This chemostratigraphic pattern equally allows for a correlation with well-studied shoal-water and pelagic records of the northern and central Tethys. Strontium-isotope stratigraphy provides a high-resolution numerical age-data set, indicating an earliest Aptian age (< 124.5 ± 0.4 Ma) of platform demise in the Subalpine Chains. The detailed platform-to-basin correlation obtained in France clearly illustrates that shoal-water carbonate production in the Urgonian platform ceased about 300 kyr before the most negative values of the characteristic negative carbon-isotope anomaly, which marks the beginning of OAE 1a black-shale deposition. The stratigraphic results confirm (i) the theory of an Early Aptian northern Tethyan platform drowning episode predating OAE 1a and (ii) document the temporal response of shoal-water carbonate platforms to the environmental perturbations prior to the OAE acme.

Messinian-Zanclean canyons in the Digne nappe (southwestern Alps): tectonic implications

AbstractMapping thick conglomerates in the subalpine chains around Digne allows reconstruction of a deeply incised Messinian-Pliocene paleo-drainage network. It shows that the main rivers (Bléone, Bès, Mardaric, Eaux-Chaudes) have existed since at least the Messinian. Pollen analysis reveals that the Pliocene-Early Quaternary continental infill of these canyons did not start at 5.3 Ma, like in the Zanclean rias, but in the Late Pliocene (between 3.4 and 2.6 Ma). This 2 m.y. delay results in part from the timing of the retrogradation of the alluvial wedge, and in part from the inland shift of the river mouths that accompanied the Zanclean flooding of Messinian canyons. This shift corresponds to a shortening of hundreds of kilometers of the river profiles that caused the river to gain more erosive energy. This strong Zanclean erosion is contemporaneous with the main infill of the rias downstream.We describe sections of paleo-canyons that were uplifted, tilted, overthrusted or truncated by strike-slip faults. The geometry of this dissected drainage network provides new constraints on the tectonic evolution of the southern subAlpine chains. An about 2.3 km strike-slip separation of the Bléone river paleo-canyon confirms that the front of the Digne nappe is an oblique ramp of the south-verging thrust system of the ‘Arc de Castellane’. This nappe moved south-southwestward along the N015 trending tear fault. Fault striation analysis in the Late Pliocene-Quaternary conglomerates shows that this displacement was characterized by ENE-WSW compression, along the Digne oblique ramp, consistent with its dextral slip. Most of the Digne nappe displacement (~20 km) occurred prior to the Late Pliocene as shown by the La Bonnette section of the Bléone paleo-canyon that incises a block of the overthrusted “residual basin”. It was followed by ~2.3 km southward displacement along out-of-sequence faults during the Late Pliocene and the Quaternary. This out-of-sequence reactivation of the Digne nappe is contemporaneous with the formation of the Barles half-window nappe anticline as shown by the 345 m uplift of the l’Escuichière paleo-canyon. The reconstructed paleo-drainage network attests for the tectonic activity of the Barles nappe anticline, the Bès tear fault, the Saint Benoît tear faults and the Digne nappe during the Pliocene and the Quaternary.

Cretaceous folding in the Dévoluy mountains (Subalpine Chains, France): gravity-driven detachment at the European paleomargin versus compressional event

AbstractLocated in the southern Subalpine Chains, the Dévoluy-Bochaine (D-B) area displays an “ante-Senonian” (Turonian) unconformity classically interpreted as the result of a compressional event. We argue that the D-B unconformity results from a major synsedimentary collapse on the slope of the European paleomargin. During the Tithonian-Late Cretaceous interval, the D-B area was located on a steep slope between the European platform (Vercors) and the Vocontian basin of SE France where mass-flows and resedimented deposits accumulated. Under the unconformity, the Dévoluy folding affects the Kimmeridgian-Cenomanian series detached on the Mid-Upper Jurassic marl-rich “Terres Noires”. Truncation of the top of the collapsed unit resulted from normal faulting and fragmentation of the poorly consolidated sediments and from their erosion by deep sea currents. Neither subaerial deposits nor shallow water conglomerates occur at the unconformity interface, where fault scarp breccias are locally exposed. The subsequent, mostly Coniacian-Maastrichtian onlapping formation (“Calcaires en Lauzes”) consists of channelized sandy calci-turbidites where slumping events occur again. To conclude, the Dévoluy-Bochaine massifs offer the first example exposed in the External Western Alps of a major synsedimentary collapse comparable to the Niger or Israel offshore study cases.

Toward a new < 250 °C pyrrhotite–magnetite geothermometer for claystones

We investigate the effects of burial and moderate experimental heating on claystones from three regions with different degrees of maturation: immature (burial temperature ∼ 40 °C) of Bure Callovo–Oxfordian claystones in the Basin of Paris (France); early mature (burial temperature ∼ 85 °C) of Opalinus Lower Dogger claystones from the Mont Terri anticline in front of the Jura fold belt (Switzerland); and mature to overmature (burial temperature < 170 °C) of Chartreuse Callovian–Oxfordian claystones from Chartreuse Sub-Alpine chains. To have information about the nature of the magnetic assemblage, we perform low-temperature (10 K–300 K) investigation of an isothermal remanent magnetization. In a first set of laboratory heating experiments, we aim to impart a chemical remanent magnetization (CRM) at 95 °C for several weeks in Bure and Opalinus claystones. Thermal demagnetization of the CRM reveals that magnetite is formed by heating the Opalinus claystones while an assemblage of magnetite and iron sulphide is formed in Bure claystones. Further, we document the appearance of a magnetic transition at ∼ 35 K in Bure claystones after heating. We name this transition the P-transition and we propose that it is related to the formation of fine-grained pyrrhotite (Fe 7S 8). The P-transition is also detected in early mature to mature Opalinus and Chartreuse claystones. We conduct additional experimental heating of natural Opalinus claystones. One set of experiments is referred to as short-term heating (1 h) from 100 °C to 200 °C. It is dedicated to an investigation of the effect of short-lived heating processes in geology. A second set of heating experiments is designed to approach burial conditions using a gold capsule. In burial-like experiments, we heated Opalinus claystones from 150 °C to 250 °C for several weeks under a pressure of 100 MPa. In both experiments, we observe a correlative diminution of the pyrrhotite signature at 35 K with increasing temperature. We interpret this trend as the appearance of magnetite. We derive a parameter PM from the warming curve of a saturated isothermal remanent magnetization acquired at 10 K (ZFC). We report on a consistent evolution of PM with temperature in the range of 40 °C to 250 °C, including natural samples, heated samples at 95 °C, and burial-like heated samples. PM first increases between ∼ 40 °C up to ∼ 85 °C, implying that pyrrhotite gradually dominates the magnetic assemblage at low temperature. For temperatures above 85 °C, PM decreases up to 250 °C, implying that the formation of magnetite gradually overshadows the magnetic input of pyrrhotite. PM values obtained from mature to overmature claystones from the Chartreuse are lower than the PM values obtained from the burial-like heated Opalinus claystones, suggesting that the formation of magnetite is driven by kinetics. The continuous trend of the PM parameter suggests that the magnetic properties of pyrrhotite–magnetite claystones can be used to infer paleo-temperatures and we propose to name this geothermometer MagEval.

Stratigraphic, sedimentological and palaeoenvironmental constraints on the rise of the Urgonian platform in the western Swiss Jura

Urgonian-type carbonates are a characteristic feature of many late Early Cretaceous shallow-marine, tropical and subtropical environments. The presence of typical photozoan carbonate-producing communities including corals and rudists indicates the prevalence of warm, transparent and presumably oligotrophic conditions in a period otherwise characterized by the high density of globally occurring anoxic episodes. Of particular interest, therefore, is the exploration of relationships between Urgonian platform growth and palaeoceanographic change. In the French and Swiss Jura Mountains, the onset and evolution of the Urgonian platform have been controversially dated, and a correlation with other, better dated, successions is correspondingly difficult. It is for this reason that the stratigraphy and sedimentology of a series of recently exposed sections (Eclepens, Vaumarcus and Neuchâtel) and, in addition, the section of the Gorges de l’Areuse were analysed. Calcareous nannofossil biostratigraphy, the evolution of phosphorus contents of bulk rock, a sequence-stratigraphic interpretation and a correlation of drowning unconformities with better dated sections in the Helvetic Alps were used to constrain the age of the Urgonian platform. The sum of the data and field observations suggests the following evolution: during the Hauterivian, important outward and upward growth of a bioclastic and oolitic carbonate platform is documented in two sequences, separated by a phase of platform drowning during the late Early Hauterivian. Following these two phases of platform growth, a second drowning phase occurred during the latest Hauterivian and Early Barremian, which was accompanied by significant platform erosion and sediment reworking. The Late Barremian witnessed the renewed installation of a carbonate platform, which initiated with a phase of oolite production, and which progressively evolved into a typical Urgonian carbonate platform colonized by corals and rudists. This phase terminated at the latest in the middle Early Aptian, due to a further drowning event. The evolution of this particular platform segment is compatible with that of more distal and well-dated segments of the same northern Tethyan platform preserved in the Helvetic zone of the Alps and in the northern subalpine chains (Chartreuse and Vercors).

Cretaceous folding in the Dévoluy mountains (Subalpine Chains, France): gravity-driven detachment at the European paleomargin versus compressional event

Cretaceous folding in the Dévoluy mountains (Subalpine Chains, France): gravity-driven detachment at the European paleomargin versus compressional event

The Global Boundary Stratotype Section and Point (GSSP) for base of the Bathonian Stage (Middle Jurassic), Ravin du Bès Section, SE France

The Bathonian GSSP is defined at the base of limestone bed RB071 in the Ravin du Bes Section, Bas-Auran area, southern Subalpine Chains (France). This section satisfies most of the erquirements recommended by the International Commission on Stratigraphy. The exposure extends over 13 m in thickness. No vertical biofacies, ichnofacies or taphofacies changes, stratigraphic gaps or hiatuses have been recorded at the Bajocian-Bathonian transition. Structural complexity, synsedimentary and tectonic disturbances, or important alterations by metamorphism are not relevant constraints. There is a well-preserved, abundant and diverse fossil record, with key markers (ammonites and nannofossils). The base of Bathonian Stage and Zigzag Zone corresponds to the first occurrence level of Gonolkites convergens Buckman that coincides with the first occurrence of Morphoceras parvum Wetzel. Calcareous nannofossils are present in all beds. The GSSP is below the Tethyan calcareous nannofossil NJT10b/11 zonal boundary (lowest Watznaueria barnesiae) and above the Bo real nannofossil NJ10/11 boundary (lowest Pseudoconus enigma). Regional analyses of sequence stratigraphy and manganese chemostratigraphy are available. Spectral gamma-ray data corroborate interpretation of an Early Bathonian deepening half-cycle of second order. The criteria of accessibility, conservation and protection are assured by the “Reserve Naturelle Geologique de Haute Provence”. The Cabo Mondego Section (Portugal) is the Bathonian auxiliary section and point (ASSP) .T he proposal was voted by the Bathonian Working Group in December, 2007, and by the International Subcommission on Jurassic Stratigraphy in March, 2008, approved by the ICS in June 2008, and ratified by the IUGS in Ju,ly 2008.

Kinematic models of deformation at the front of the Alps: new data from map-view restoration

Map-view restoration of the frontal part of the French Alps was done in order to test two hypotheses about the propagation of deformation. We discussed two models of the ‘rooting’ of the Jura-Molasse basal detachment either along basement thrusts beneath the External Basement Massifs (EBM-rooting model) or along basal thrusts of the Subalpine Chains above the External Basement Massifs (SAC-rooting model). The different sequences of deformation are described for the two models with their kinematics and geochronological constraints. The geochronological data available on the uplift timing of the External Basement Massif are then discussed. Due to their range of uncertainty, geochronological data on the exhumation of the External Basement Massifs are not decisive to discriminate between the two models. Using either the maximum or the minimum age in the range of their uncertainty allows fitting the timing constraints of both models. Interpretation of map-view restoration reveals a weakness of the EBM-rooting model linked to the fact that a sharp virgation is predicted between the two main cross sections. This does not discard this model as such virgation may be linked to paleostratigraphic or tectonic effects but this must be taken into account when using this model. Structural data are also discussed as field geology on the mechanism of uplifting of the external basement massif or geophysical data as deep seismic lines shot in the frame of the ECORS and NRP20 research programs. Such data favor one or the other model without discarding one of them. We conclude that, with the available data, both models seem possible and that the less popular one (SAC-rooting model) shows the more coherent restoration field, the reason why we favor it at the present time.

Charophytes from the Barremian-lower Aptian of the Northern Subalpine Chains and Jura Mountains, France: correlation with associated marine assemblages

Interdigitation of shallow marine limestone containing orbitolinids (benthic foraminifera), echinoids and dasycladales with lacustrine limestone and marl with charophytes in the Northern Subalpine Chains and Jura Mountains, France, enables us to improve the correlation between Early Cretaceous continental and marine biozonations. Early Barremian charophytes were found in marlstones intercalated in the Urgonian Limestone at Pas du Frou (Chartreuse Massif). This flora belongs to the Triquetra charophyte biozone (European Charophyte Biozonation) and is interlayered with rocks containing a characteristic assemblage of orbitolinids and dasycladales of the mid- to early Barremian (uppermost Hugii, Nicklesii and lowermost Caillaudianus ammonite biozones). Younger charophyte associations occur in the overlying Urgonian Limestone and Lower Orbitolina Beds above the major regional unconformity Δ 1 from the Chartreuse Massif, Vercors Massif and Southern Jura Mountains. They are formed by a charophyte association that belongs to the charophyte biozone Cruciata. The flora is interlayered with marine layers rich in orbitolinids and echinoids which enables us to redefine the Cruciata biozone as Cruciata-Paucibracteatus zone, ranging from the late Barremian (Vandenheckii ammonite biozone) to the early Aptian (ammonite biozone Weisii). An interval of uncertainty occurs in the beginning of the Cruciata biozone, which is equivalent to the upper part of the Caillaudianus ammonite biozone.