Subalpine Chains Research Articles

Contrasting thermal evolution of thrust systems: An analytical and modeling approach in the front of the western Alps

In foreland fold and thrust belts, the study of the maturity of the organic matter and kinematic and thermal modeling techniques is useful in unraveling the thermal history and maturation of source rocks. The case study presented here is the classical example, the western Alps. This work evidences that contrasting thermal evolutions can develop in relatively closely adjacent areas of the same thrust belt. Analytical results show that the Mesozoic rocks have undergone a wide range of temperatures up to 250C locally. The higher temperature values are associated with the tectonic emplacement of early, widespread allochthonous nappes that were emplaced obliquely in the northern part of the area, prior to the thrust tectonics of the Subalpine chains. Folding and offsets of the isomaturity surfaces are observed, which implies that in some parts of the thrust belt, the thermal peak was reached before fold and thrust deformation. Erosion rates vary greatly along the thrust front. Extensive erosion (probably more than 4 km [2.5 mi] of sediments from the Tertiary and the Prealpine nappes) occurred in the north of the area studied along the Bornes transect. Away from the area of influence of the Prealpine nappes, numerical modeling shows differences in the timing of maturation of the source rocks. According to the area considered, maturation was attained either early, before thrust tectonics (most of the Vercors area), or during the deposition of the syntectonic flexural series preceding thrust tectonics (Molasse Basin), or else, during thrust tectonics (most of the Chartreuse area).

Geometry and petrophysical parameters of a calcarenitic turbidite lobe (Barremian-Aptian, Pas-de-la-Cluse, France)

Several bioclastic resedimentation systems, late Barremian to early Aptian in age, outcrop in southeastern France. The upslope portion of a calcarenitic turbidite lobe outcrops as a cliff, 2.3 km wide and up to 45 m high, oriented roughly perpendicular to the direction of palaeoflow. It has been studied through 31 sections. The studied body is point-sourced and corresponds to a channel/lobe transition located at the toe-of-slope of a Urgonian carbonate platform on the western edge of the Vocontian Trough (French Subalpine Chains). The internal geometry of the lobe is the result of repeated erosion and filling of channels, creating well-defined stacked units. Filling phases result in lobe-shaped morphology. Units evolve from base to top in an ordered way. First, the sediments exported from the platform and deposited in the basin are represented by decimetre-thick, sheet-like and fine-grained Bouma a-e and ab-e sequences. In this first unit, erosion is limited and represented by shallow and symmetrical channels. This initial phase of deposition corresponds to the formation of a lobe with a positive morphology. The next phase of system development is more erosional and this is dominated by migrating, sinuous channels several metres deep filled with coarse-grained, poorly graded turbidites. These channels are first filled through lateral accretion and then sediment spills over to create a sub-lobe. The mixture of channel and lobe features could be considered as a carbonate equivalent of the channel–levee system of siliciclastic deposits. Finally, the system ends with the deposition of a flat-based unit. This unit extends for more than 2 km and is composed of the southwards lateral accretion of hundreds of metres wide sequences each composed of a debris-flow deposit capped by a coarse-grained calcarenite bed. These sigmoidal bodies could be the ultimate forms of a spreading out phenomenon of the lobe. Overall, the Pas-de-la-Cluse system is a coarsening-upward body. As the grain-size is related to the nature of the material, this coarsening-upward implies either an evolution in the nature of the carbonate material produced on the platform (from grainy to bioconstructed), a displacement of the source sediment (from inner platform to margin) or a displacement of the channel head through time (from distal to proximal). The upward evolution from classic turbidites to debris-flow deposits can be explained either by the way the platform facies evolved with time in a depositional sequence or through retrogressive erosion by successive slope failures that cut deeper and deeper into the platform slope. Although its present-day porosity is low, the size of the system and the fact that it is encased in marls make it an interesting outcrop analogue for hydrocarbon exploration.

La migration de la plate-forme urgonienne entre le Jura plissé et les Chaînes subalpines septentrionales (France, Suisse)

The stratigraphic study of the Urgonian carbonate platform presented here was carried out on a directed transversal NW-SE and passing by Geneva. This transversal extends on 100 km length; it lies between the face of the “folded Jura” (Champagnole, the Jura) and the septentrional subalpine chains (Aravis, Haute-Savoie). Essentially based on the biostratigraphy (ammonites, orbitolinids, echinids) and on sequence stratigraphy, this study illustrates the progradation of the Urgonian platform from the “folded Jura” (top of the Lower Hauterivian) to the Aravais chain (lowermost Barremian).

Geometry, sedimentation and diagenesis of Barremian-Aptian lowstand prograding wedges and basin floor fans of southern Vercors (Subalpine chains, SE France)

Geometry, sedimentation and diagenesis of Barremian-Aptian lowstand prograding wedges and basin floor fans of southern Vercors (Subalpine chains, SE France)

Tectonic rotation and diverse remanence carriers revealed by the paleomagnetism of Eo-Oligocene turbidites from two Ultrahelvetic units (Western Alps, Haute-Savoie, France)

Our paleomagnetic study of two Ultrahelvetic units from the Western Alps shows that: (1) one unit has experienced an important vertical-axis tectonic rotation, and (2) in turbidite deposits, the nature of the remanence carrier can differ between coarse-grained and fine-grained intervals. Samples were collected at close intervals from weakly deformed turbidites in two different areas (Dérochoir and Marais). Characteristic remanent magnetization (ChRM) was measured with alternating field and thermal demagnetization, whereas clues for identifying the main remanence carrier were provided by coercivity spectra analysis. In the Dérochoir area, the bulk of the section consists of fine-grained rocks. Magnetic intensity values are small, directions are scattered and no polarity reversals were detected. Remanence was probably acquired during late diagenesis or regional tectonic deformation. In coarser lithologies, depositional ChRM has probably been preserved by an early phase of cementation. In the Marais turbidites, remanent magnetization is mainly carried by magnetite. Although some samples show clustered directions, several polarity inversions have been observed, suggesting that remanence was acquired soon after sediment deposition. Magnetic directions values indicate a counterclockwise rotation of about 110° of the Marais sediments with respect to the stable European craton. This movement, which partly affects the underlying Subalpine chains, is probably related to sinistral strike–slip motion along a major regional accident during the Late Oligocene or Miocene.

Tertiary foreland sedimentation in the Southern Subalpine Chains, SE France: a geodynamic appraisal

Tertiary foreland sedimentation in SE France occurred along the western sidewall of the Alpine orogen during collision of the Apulian indentor with the European passive margin. A detailed reappraisal of the stratigraphy and structure of the Southern Subalpine Chains (SSC) in SE France shows that Tertiary depocentres of differing character developed progressively toward the foreland during ongoing SW‐directed shortening. The geodynamic controls on each of four stages of basin development are evaluated using a flexural isostatic modelling package of thrust sheet emplacement and foreland basin formation. (1) The initial stage (mid to late Eocene) can be explained as a flexural basin that migrated toward the NW, closing off to the SW against the uplifting Maures–Esterel block. This broad, shallow basin can be reproduced in forward modelling by loading a lower lithospheric plate with an effective elastic thickness of 20 km. (2) The end of detectable flexural subsidence in the early Oligocene coincides with the emplacement of the internally derived Embrunais–Ubaye (E‐U) nappes, which caused 11 km of SW‐directed shortening in the underlying SSC. The lack of Oligocene flexural subsidence dictates that the E‐U units were emplaced as gravitational nappes. Within the SSC, Oligocene sedimentation was restricted to small thrust‐sheet‐top basins recording mainly continental conditions and ongoing folding. Further west, Oligocene to Aquitanian NNW–SSE extension generated the Manosque half‐graben as part of the European graben system that affected an area from the Gulf of Lion to the Rhine graben. (3) Following the Burdigalian breakup of the Gulf of Lion rift, a marine transgression migrated northward along the European graben system. Subsequent thermal subsidence allowed 1 km of marine sediments to be deposited across the Valensole and Manosque blocks, west of the active SSC thrust belt. (4) Mio‐Pliocene conglomeratic deposits (2 km thick) were trapped within the Valensole basin by the uplifting Vaucluse block to the west and the advancing Alpine thrust sheets to the east. Late Pliocene thrusting of the SSC across the Valensole basin (approx. 10.5 km) can be linked along a Triassic detachment to the hinterland uplift of the Argentera basement massif.

Neogene remagnetization of normal polarity in the Late Jurassic black shales from the southern Subalpine Chains (French Alps). Evidence for late anticlockwise rotations

This study presents paleomagnetic investigations of 23 sites from the Late Jurassic black shales, the so-called Terres Noires, that outcrop in the southern Subalpine Chains (SSC). Six sites are located in the Diois, six sites in the Trièves and Dévoluy, and eleven sites in the Digne nappe. The magnetizations are weak and are carried by titanomagnetites. We found generally one component per site within the medium range of temperature (200° to 500°C). All magnetizations are characterized by the lack of reverse polarity. The directions of magnetization are generally better grouped in geographic coordinates in the Trièves, the Dévoluy and the Digne nappe. Fold testing at one site from the Trièves demonstrated a late synfolding acquisition of magnetization. In contrast, pre-tilting magnetizations are suggested in the Diois and locally, in the Digne nappe. Given an average inclination close to late Tertiary expected inclinations in geographic coordinates, we propose that these magnetizations are Neogene overprints. Our results point to two major results. (1) The lack of reverse polarity which is puzzling during the Neogene where 50% of the magnetic field is reverse. (2) The discovery of quite systematic anticlockwise rotations. These anticlockwise rotations do not occur in the Diois and are larger in the southern part of the Digne nappe. We presume that these rotations were active during the late stage of Alpine folds emplacement and are still active today.

The magnetic fabric of weakly deformed Late Jurassic shales from the southern subalpines chains (French Alps): evidence for SW-directed tectonic transport direction

Extensive magnetic fabric investigations have been conducted on the Late Jurassic sedimentary black shales (37 sites, 362 samples), the so-called Terres Noires Formation, from the Southern Subalpine Chains (western Alps). This study completes a previous study done in the Northern Subalpine Chains which demonstrated that the Terres Noires Formation preserves a subtle magnetic lineation related to tectonic transport direction. In the Southern Subalpine Chains (Diois, Dévoluy, Embrunais, Digne nappe), similar magnetic fabrics have been found. The shape of the magnetic fabric is oblate and locally mirrors the gradient of deformation in the Embrunais footwall. The magnetic foliation is parallel to the bedding or to the slaty cleavage. About 50% of the magnetic lineations have down-dip magnetic directions while the other magnetic lineations are sub-horizontal or parallel to the intersection bedding/cleavage. The tectonic magnetic fabrics recorded in the Embrunais confirm the ability of the Terres Noires to record tectonic transport direction. The magnetic fabric geometry suggests SW-directed tectonic transport in the Southern Subalpine Chains. The Diois appears as a tip zone where transport directions are abruptly clockwise rotated from NW-directed in the Northern Subsalpine Chains to SW-directed. The magnetic fabric from the Digne nappe exhibits a fan pattern of transport directions in close agreement with the arcuate shape of the thrust.

TheML5.3 Épagny (French Alps) earthquake of 1996 July 15: a long-awaited event on the Vuache Fault

The ML 5.3 Épagny earthquake that occurred on 1996 July 15 in the vicinity of Annecy (French Alps) was the strongest event to shake southeastern France in the last 34 years. Moderate to serious damage in the Annecy area is consistent with MSK intensities of VII–VIII. This earthquake occurred on the Vuache Fault, a geologically well-known, morphologically clear, NW-SE-trending strike-slip fault that links the southern Jura Mountains with the northern Subalpine chains. The hypocentre was located in Mesozoic limestones at shallow depths (1–3 km). The focal mechanism indicates left-lateral strike-slip motion on a N136°E-striking plane dipping 70° to the NE. Abundant field evidence was gathered in the days following the main shock. Several hundred aftershocks were recorded thanks to the rapid installation of a 16-station seismic network. All aftershocks occurred along the southernmost segment of the Vuache Fault, defining a 5-km-long, 3.5-km-deep, N130°E-striking rupture zone dipping 73° to the NE. The fault plane solutions of 60 aftershocks were found to be consistent with left-lateral slip on NW-SE-striking planes. At the SE tip of the aftershock zone we found ground cracks parallel to the fault close to the Annecy-Meythet airport runway; at the NW tip, near Bromines, we observed left-lateral displacement of concrete walls in a building. We also noticed flow changes in two springs close to that locality. Geodetic levelling across the fault revealed about 1 cm of uplift for the region north of the fault. The recording of aftershocks with a six-station accelerometric network showed that lacustrine deposits locally amplified the ground motion up to eight times, which explains how this moderate-magnitude shock could cause such heavy damage. Historical records draw attention to the central segment of the Vuache Fault, which has been locked for at least 200 years. Situated NW of the 1996 aftershock zone, between the Mandallaz and Vuache mountains, this segment forms a 12-km-long potential seismic gap where other M5 events or one single M6 event might occur.

Horizontal shortening control of Middle Miocene marine siliciclastic accumulation (Upper Marine Molasse) in the southern termination of the Savoy Molasse Basin (northwestern Alps/southern Jura)

Abstract Between the southern Jura and the Subalpine Chains (Bornes, Bauges, and Chartreuse Massifs), the Burdigalian-Langhian-Serravalian tidal siliciclastic ‘Upper Marine Molasse’ (UMM) outcrops in N-S synclines between N-S-trending ramp anticlines separating southward-narrowing synclines. A surface survey and an interpretation of transverse seismic-reflection profiles across the latter confirm that they represent progressively individualized and separated sedimentation areas. Combined seismic configuration and surface geometry show westward migration of depo-axes, and coeval progressive steepening close to each ramp, upon blind thrusts. Several thick earthquake-disturbed layers offer further evidence of a syntectonic sedimentation. A regime of flexure-subsidence prevailing during the Chattian-Aquitanian (the so-called Lower Freshwater Molasse, LFM) was overprinted by thin-skin shortening. The latter, combined with Burdigalian-Langhian higher eustatic sea-level, favoured the accumulation and the preservation of abundant tidal bedforms and sequences, which are generally poorly preserved in stable settings.

Sequential restoration of the external Alpine Digne thrust system, SE France, constrained by kinematic data and synorogenic sediments

Abstract The Tertiary foreland basin of the southern Subalpine chains preserves a stratigraphic record of late Alpine deformation, both ahead of the thrust front in the Valensole basin, and in a series of thrust-sheet-top basin remnants. Stratigraphy and growth structures in these basin remnants have been used to identify the location and timing of deformation and hence to constrain the sequential restoration of a cross section through the region. The thrust belt developed as a single large thrust sheet riding on a weak Triassic evaporite layer. Minor breaching thrusts occur, in particular at Mesozoic normal faults. Kinematic studies of the Digne thrust sheet show a dominantly SW direction of tectonic transport. Where reactivated structures lie at a slightly oblique angle to the transport direction there has been a partitioning of deformation into SW-directed thrusting and a component of dextral strike-slip taken up on minor faults. The total shortening across the fold and thrust belt is 21.5 km, a much lower value than previously estimated but more in line with regional tectonics. The foreland basin stratigraphy demonstrates that deformation within the Digne sheet occurred in three stages. Firstly, during the late Eocene, Alpine collision in the hinterland caused flexure of the foreland plate to generate a simple broad marine foredeep. Slight detachment above the Triassic evaporites allowed gentle buckling of the floor of the foredeep. Secondly, in the Early to Mid-Oligocene overthrust shear associated with the SW emplacement of the internally derived Embrunais-Ubaye nappes into the foredeep caused substantial deformation in the underlying Digne sheet. A component of flattening strain was associated with overthrust shear beneath the nappes, but ahead of the nappe front gentle, upright folding and thrusting continued. Sedimentation stepped westward, became dominated by continental facies and confined to small thrust sheet top basins (e.g. Barrême). The Mio-Pliocene stage of deformation involved a change from thin-skinned to a thick-skinned out-of-sequence deformation when basement blocks were uplifted on deepseated structures across the region (e.g. dôme de Chateauredon). Shortening associated with the uplift of the main Argentera basement massif at the internal boundary of the external fold and thrust belt was transferred onto the Triassic detachment from below, and transferred forward to reactivate the frontal edge of the Digne sheet, which was thrust out over the Valensole basin.

Porosity Associated with Sequences Boundaries in the Lower Cretaceous Carbonate Platforms of Subalpine Chains (Alps, France) : ABSTRACTS

Porosity Associated with Sequences Boundaries in the Lower Cretaceous Carbonate Platforms of Subalpine Chains (Alps, France) : ABSTRACTS

Present-day deformation of the Dauphine Alpine and Subalpine massifs (SE France)

The positions of 20 geodetic points surveyed in 1946–1952 by the Institut Géographique National in the Grenoble area (western Alps) were remeasured in 1993–1994 using GPS. We evaluate the displacements of the common points of the networks between the two surveys, and calculate the strain-rate tensors for different sets of three adjacent points. The significant N70° shortening direction observed at the boundary of the Belledonne and Taillefer massifs suggests that the dextral strike-slip motion between the External Crystalline Massifs (ECM) and the Subalpine domain is still active. Geological evidence suggests that this deformation, which is compatible with the few focal mechanisms available for the area, occurred throughout the late Neogene. Our data also suggest a roughly 3–5 mm yr−1 active shortening concentrated at the external front of the Subalpine chains. This thrust motion results in a principal axis of compression orientated N130°, oblique to the direction of shortening observed in the ECM. This change in direction is also seen in in situ stress measurements. This motion may mark the southward continuation of the active fault observed in the Jura mountains by Jouanne, Ménard & Darmendrail (1995).

Displacement localization and palaeo-seismicity of the Rencurel Thrust Zone, French Sub-Alpine Chains

The palaco-seismicity and history of strain accumulation within the Rencurel Thrust Zone, French Sub-Alpine Chains, has been investigated by examining fracture-filling cements. Two generations of fracture-filling cement with distinct petrographic characteristics, cation geochemistries and 13C and 18O stable isotopic compositions have been distinguished within the Rencurel Thrust Zone. The hanging-wall rocks contain fracture-filling calcite cements, whereas ferroan calcite fills fractures within the Central Gouge Zone. Calcitecemented faults include frontal, oblique and lateral ramps, as well as faults dipping in the movement direction, whereas, ferroan calcite-cemented faults include only frontal ramps and faults dipping in the movement direction. Fragments of calcite cement occur as clasts within the Central Gouge Zone, indicating that the calcite cements formed prior to precipitation of ferroan calcite in the Central Gouge Zone. Lineation data indicate that precipitation of both generations of cement occurred during a single phase of thrusting. The lack of ferroan calcite cements in the hanging-wall rocks suggests that the hanging-wall of the Rencurel Thrust was not fractured during displacements within the Central Gouge Zone. This contrasts with seismogenic faults where rocks surrounding major faults are fractured during fault slip, and it is inferred that the exposed portion of the Rencurel Thrust may have experienced aseismic fault displacements. The aseismic fault slip may have been the consequence of the shallow burial depths (<3 km) experienced by the exposed portion of the Rencurel Thrust during faulting.

Deformation conditions in the northern subalpine chain, France, estimated from deformation modes in coarse-grained limestone : Ferrill, D A; Groshong, R H J Struct GeolV15, N8, Aug 1993, P995–1006

Deformation conditions in the northern subalpine chain, France, estimated from deformation modes in coarse-grained limestone : Ferrill, D A; Groshong, R H J Struct GeolV15, N8, Aug 1993, P995–1006

Biostratigraphy and Evolution of the Urgonian Platform from the Jura to the Northern Subalpine Chains (SE France)

Biostratigraphy and Evolution of the Urgonian Platform from the Jura to the Northern Subalpine Chains (SE France)

Deformation conditions in the northern Subalpine Chain, France, estimated from deformation modes in coarse-grained limestone

In low-temperature experiments, the deformation mode and ductility in a single rock type are most directly controlled by the confining pressure and strain magnitude. A graphical technique is developed from published experimental microstructural data that allows for the determination of deformation mode for coarsegrained limestone on the basis of microstructural point-count data. The deformation mode and strain are used to constrain the confining pressure during thin-section scale deformation by comparison with experimental deformation results. The technique is applied to coarse-grained samples of Cretaceous and Tertiary limestone from the northern Subalpine Chain, France, and reveals that the majority of samples deformed mostly by twinning in the uniform flow mode at minimum depths, estimated from deformation mode and strain, of between 0 and 6.0 km and maximum depths, interpreted from peak metamorphic temperatures and geothermal gradient, of about 2.7–9.7 km. In the remaining samples, brittle failure probably occurred at relatively shallow depths (< 2.5 km) or in response to lowered effective confining pressure due to elevated fluid pressures.

Kinematic model for the curvature of the northern Subalpine Chain, France

Analyses of cross-sectional shortening, mapped faults and calcite twin strain are used in conjunction with geometric-kinematic modeling to interpret the mechanism of curvature formation in the northern Subalpine Chain of France. There is a 45° change in strike around the Chain that is convex towards the foreland (northwest). Five models for fold-thrust belt curvature are applied to the northern Subalpine Chain. They can reproduce the geometry and the kinematic history of a curved fold-thrust belt and can be used to predict the magnitude and orientations of strains and the three-dimensional geometry of the curvature. The models are: pure bending; radial thrusting; curve-parallel simple shear; uniform displacement-uniform shortening; and transport-parallel simple shear. Two principal criteria for distinguishing among the models geometrically are the horizontal tangential strain (extension or contraction parallel to strike) and cross-sectional shortening. The transport-parallel simple shear model predicts no tangential extension in the central domain and greater radial shortening and significant tangential extension in the northeastern and southern domains. This is the pattern observed in the northern Subalpine Chain. Therefore, the curvature of the northern Subalpine Chain is attributed to a transport-parallel simple shear mechanism.

Organic maturation in the French Subalpine Chains: regional differences in burial history and the size of tectonic loads

The French Subalpine Chains of southeastern France are the foreland fold/thrust belt of the Western Alps, formed in the mid- to late Tertiary as a result of the continental collision between the European and Italian-North African plates. With the change from passive margin to destructive margin, certain areas of the Subalpine Chains experienced enhanced temperatures through being buried to greater depths. Burial of these areas has been investigated using vitrinite reflectance (R o ). Levels of organic maturity of Mesozoic and Tertiary horizons have been established using vitrinite reflectance (R o ) and spore fluorescence. In the southwestern part of the chain, values of R o for Mesozoic units from the Vercors and Chartreuse massifs are generally low, with all Cretaceous units having values &lt;0.5% R o . Older Jurassic horizons have higher values, up to 1.25% for the mid-Jurassic (Oxfordian). When compared to values from the same horizons, and from younger Tertiary units (published data) from the northeastern part of the chain, it is clear that this part of the belt has encountered higher temperatures, produced by greater depth of burial. In the Haut-Giffre and Aravis massifs Mesozoic units have R o values as high as 2.7% and, in terms of hydrocarbon generation, can be considered mature or overmature. Two causes of greater depth of burial of the northeastern part of the belt are indicated by field studies and by burial history modelling of specific localities of the Subalpine Chains: first, the earlier initiation of the Tertiary foredeep in the northeast compared to the southwest; and second, differences in the size of tectonic loads emplaced onto the Subalpine Chains during the final stages of Alpine compression, with the greater structural deformation of the Subalpine Chains occurring in the northeast.

Hydrocarbon maturation, migration and tectonic loading in the Western Alpine foreland thrust belt

Abstract The Western Alpine foreland fold and thrust belt developed in late Miocene times through the Neogene foredeep basin, superimposed on an early Mesozoic and local Palaeogene shelf-basin system. The relative timing between subsidence, tectonic burial and thermal maturation is examined along a section line through the Annecy district of the French Alps. Available stratigraphic data are used to define burial-time paths, using suitable decompaction parameters. The tectonic burial history is determined from regional structural sections. These burial histories are used with a simple thermal model (geothermal gradients: 28°C km −1 before thrusting, 30°C km −1 for the post-orogenic period) to establish hypothetical time-temperature histories. Both the Lopatin method and peak temperature model of source rock maturation suggest that the Jura anticlines are undermature while the Subalpine chains are overmature. However, Subalpine maturation occurred during foredeep subsidence with overmaturation during thrust sheet loading. Large-scale lateral migration of hydrocarbons is required to explain the distribution of bitumen accumulations in the foreland basin offering the possibility of a series of structural and stratigraphic traps in the palaeo-foreland which are not associated with the thrust belt. A complex secondary and remigration history would be needed if liquid hydrocarbons are to have accumulated in traps associated with compressional structures in this part of the thrust belt.