Tectono-metamorphic evolution of the Briançonnais zone (Modane-Aussois and Southern Vanoise units, Lyon Turin transect, Western Alps)

https://doi.org/10.1016/j.jog.2011.11.010Get rights and content

Abstract

In the central Western Alps, a combined structural, petrological and 40Ar–39Ar geochronological study of the Modane-Aussois and Southern Vanoise units yields important constraints on the timing of deformation and exhumation of the Briançonnais zone. These data help to decipher the respective roles of oceanic subduction, continental subduction and collision in the burial and exhumation of the main units through time. In the Modane-Aussois unit top to the NW thrusting (D1) was followed by top to the east shearing (D2) interpreted by some as normal faulting and by others as backthrusting. Pseudosection calculations imply that D1 deformation occurred at 1.0 ± 0.1 GPa and 350 ± 30 °C. Analysis of chlorite–phengite pairs yield P–T estimates between 0.15 and 0.65 GPa and between 220 and 350 °C for the D2 event. Phengites along the D1 schistosity (sample M80) yields an 40Ar–39Ar age of 37.12 ± 0.39 Ma, while D2 phengites yield ages of 35.42 ± 0.38 (sample M173) and 31.60 ± 0.33 Ma (sample M196). It was not possible to test whether these ages are altered by excess argon or not. Our interpretation is that the D1/D2 transition occurred at ∼37 Ma at the beginning of decompression, and that D2 lasted until at least ∼32 Ma. Pseudosection calculation suggests that the Southern Vanoise unit was buried at 1.6 ± 0.2 GPa and 500–540 °C. D1 deformation occurred during exhumation until 0.7–10.5 GPa and 370 ± 30 °C. Published ages suggest that D1 deformation possibly started at ∼50 Ma and lasted until ∼37 Ma. D2 deformations started at P–T conditions close to that recorded in Modane-Aussois unit and lasted until 0.2 ± 0.1 GPa and 280 ± 30 °C at ∼28 Ma. The gap of 0.6 ± 0.3 GPa and 150 ± 130 °C between peak metamorphic conditions in the two units was concealed by thrusting of the South Vanoise unit on top of the Modane-Aussois unit during D1 Deformation. Top to the east deformation (D2) affects both units and is interpreted as backthrusting.

Based on these data, we propose a geodynamic reconstruction where the oceanic subduction of the Piedmont unit until ∼50 Ma, is followed by its exhumation at the time of continental subduction of the continental Southern Vanoise unit until ∼45 Ma. The Southern Vanoise is in turn underthrusted by the Modane-Aussois unit until ∼37 Ma (D1). Between 37 and 31 Ma the Modane-Aussois and Southern Vanoise units exhume together during backthrusting to the east (D2). This corresponds to the collision stage and to the activation of the Penninic Thrust. In the ∼50 Ma to ∼31 Ma time period the main thrusts propagated westward as the tectonic context switched from oceanic to continental subduction and finally to collision. During each stage, external units are buried while internal ones are exhumed.

Introduction

Although the formation of high pressure (HP) and ultrahigh pressure (UHP) rocks is an integral process occurring in oceanic or continental subduction (Ernst, 2001), their exhumation is a transient processes occurring during oceanic subduction or during continental collision (Ernst, 2001, Agard et al., 2008). The transition from oceanic subduction to continental collision is marked by the subduction of the continental margin, still attached to the downgoing oceanic slab, when HP To UHP rocks of continental origin are produced (Chopin, 1987) and exhumed (Guillot et al., 2009). Moreover, this period is crucial in the evolution of mountain belt as it records a decrease of the plate convergent rate, the progressive transition from marine to continental sedimentation due to continental uplift of the lower plate and the transition from low temperature to middle temperature geothermal gradient (Guillot et al., 2003). Understanding the exhumation of high and ultra-high pressure (HP to UHP) rocks is a major challenge in our knowledge of plate convergence and mountain building processes. Exhumation of HP to UHP rocks results from the interaction of boundary forces, buoyancy, rheology, geometry of the subduction channel and surface processes (Jolivet et al., 2003, de Sigoyer et al., 2004, Agard et al., 2008, Guillot et al., 2009). The timing of exhumation with respect to the onset of continental subduction has important bearings on the exhumation processes (Brun and Facenna, 2008, Guillot et al., 2009). Models proposed for the exhumation depend upon the orogenic context i.e. subduction or collision. Pro- and back-thrustings coupled with strong erosion and the formation of foreland basins take place during collision. A wide variety of exhumation model have been proposed during the subduction stage: channel flow (Cloos, 1982), corner flow (Platt, 1986), extensional collapse (Dewey et al., 1993), thrusting towards the foreland (Steck et al., 1998), buoyancy assisted by erosion and tectonics (Chemenda et al., 1995), compression of a soft zone between two rigid blocks (Thompson et al., 1997), serpentinite channel (Guillot et al., 2001), and coaxial extension associated with a decoupling fault (Jolivet et al., 2003).

The Western Alps are a good example for studying the exhumation processes of HP to UHP metamorphic rocks as early HP-LT metamorphic relics have been widely preserved. It is a curved orogenic belt consisting of a nappe stack of continental terranes, that are from the top to the bottom Austroalpine, Internal Crystalline Massifs, Briançonnais zone and External Alps (Fig. 1). Two oceanic domains separate these continental domains (Fig. 1): the Piedmont zone between the Austroalpine and the Internal Crystalline Massifs and the Valais oceanic unit squeezed between the Briançonnais zone and the external Alps along the Penninic Thrust (e.g. Schmid and Kissling, 2000, Rosenbaum et al., 2005).

In the internal part of the belt, HP to UHP metamorphic rocks formed and exhumed during distinct periods: 65 Ma for the Austroalpine massif (Duchene et al., 1997), between 65 and 45 Ma for the Piedmont zone (Agard et al., 2002, Lapen et al., 2003), between 45 Ma and 35 Ma for the Internal Crystalline Massifs (Duchene et al., 1997, Meffan-Main et al., 2004) and the Briançonnais zone (Markley et al., 1998, Freeman et al., 1997) and at 35 Ma for the Valais unit (Bousquet et al., 2002). The variation in metamorphic ages and a geothermal gradient lower than 10 °C km−1 in these rocks suggest that such nappes formed in a subduction wedge from 65 to 35 Ma (Rosenbaum et al., 2005, Ford et al., 2006, Lardeaux et al., 2006, Gabalda et al., 2008). The transition from subduction to collision is dated at ca. 35 Ma and is associated with the activation of the Pennine thrust (Schmid and Kissling, 2000, Pfiffner et al., 2002, Leloup et al., 2005, Rosenbaum et al., 2005, Beltrando et al., 2010, Dumont et al., 2011). Recently this age has been confirmed on the basis of P–T–t estimates of alpine metamorphism in the External zone (Rolland et al., 2008, Simon-Labric et al., 2009). Such event is associated with the formation of backthrusts from the internal part of the belt (Tricart, 1984, Platt et al., 1989, Schmid and Kissling, 2000, Tricart and Sue, 2006) to the boundary between the Pô plain and the Alpine belt (Carrapa and Garcia-Castellanos, 2005, Escher and Beaumont, 1997, Roure et al., 1990).

In the internal part of the Western Alps, tectonics associated with exhumation is polyphased (e.g., Lanari et al., 2012). Early, top to N or NW direction of nappe emplacement and shearing accommodated the earliest and rapid exhumation of the HP and UHP continental units. This tectonic phase (D1) is observed and interpreted everywhere as a thrusting phase (Agard et al., 2002, Markley et al., 1998, Bousquet et al., 2002, Reddy et al., 2003, Bucher et al., 2003, Ganne et al., 2007, Wheeler et al., 2001, Le Bayon and Ballèvre, 2006).

The D1 nappe stack is often affected by top to the east or SE shearings (D2). In the Piedmont zone, these D2 structures accommodate a significant part of the exhumation in a context of extension (Agard et al., 2002, Reddy et al., 1999, Rolland et al., 2000, Ganne et al., 2006, Ganne et al., 2007). A late Eocene age (>35 Ma) is proposed for these structures (Agard et al., 2002, Reddy et al., 1999).

Others top to the east or southeast structures occurred after the major exhumation phase. Some of these structures are responsible for the fan shape of the Western Alps and are interpreted as back-thrusts (Tricart, 1984, Platt et al., 1989, Escher and Beaumont, 1997, Le Bayon and Ballèvre, 2006, Tricart and Sue, 2006). An Oligocene Age (∼33–25 Ma) is attributed to these structures by analogy with other ones observed further SE at the rear of the Pô plain (Carrapa and Garcia-Castellanos, 2005, Roure et al., 1990) and that are coeval with the formation of foreland basins (Schmid and Kissling, 2000, Pfiffner et al., 2002, Ford et al., 2006). Backfoldings related to backthrusting or to normal faulting are also described in the Briançonnais units (Bucher et al., 2003, Tricart and Sue, 2006, Ganne et al., 2006). Following the successive phases of ductile deformation, two phases of brittle deformation took place, producing orogen parallel extension followed by orogen perpendicular extension (Strzerzynski et al., 2004, Malusa et al., 2005, Champagnac et al., 2006, Sue et al., 2007).

In the present study, we focus on the intermediate zone of the continental orogenic system between the internal zone and the external zone. In this area both subduction and collision related structures are found (Tricart, 1984, Tricart and Sue, 2006, Gabalda et al., 2008, Ganne et al., 2007), giving the opportunity to decipher their respective role in the exhumation of HP units. We conducted a combined structural, petrological and geochronological study in order to relate the deformation phases with the P–T–t evolution and to discuss how and when the continental crust is exhumed in the Western Alps. We review the stratigraphy, structure and metamorphic evolution of the area, and present new P–T estimates and 40Ar–39Ar ages. We finally propose a tectonics and metamorphic evolution of the internal Western Alps between 45 and 30 Ma.

Section snippets

Location of the studied area

The Studied area encompasses Modane and Aussois cities in the Maurienne Valley (Fig. 1). It consists of Briançonnais basement and cover over-thrusted to the south and the east by the Piedmont (schistes lustrés) and Gypse nappes (Fig. 2). The Piedmont nappe emplacement took place during the early top to the NW tectonic event (Ganne et al., 2007). To the West, a tectonic contact separates the Briançonnais and the Houiller zones (Fig. 1). This contact is interpreted either as a major detachment

Methods

We conducted a structural analysis based on a micro-, meso-structure analysis, geological mapping, and a metamorphic study associated with 40Ar/39Ar dating. Samples were taken from the basement and the metasedimentary cover both at the surface and from drill holes performed in the frame of the Lyon-Turin tunnel project (Fig. 5). Mineral compositions (Table 2) were determined using the CAMECA SX100 microbeam of the Brest University (15 kV – 20 nA). Standards were albite (Na), orthoclase (K),

D1: nappe stacking and duplex formation

In the Modane-Aussois unit, the expression of the D1 deformation slightly differs from the Clarea and Ambin Groups to the cover. Clarea and Ambin Groups present relics of D1 folds at various scales in the field and along borehole (Fig. 4, Fig. 7). The original large-scale geometry of the D1 folds is difficult to access because of later deformation phases. However, correlation between boreholes on the eastern part of the section C–D (Fig. 6B), suggests that at least three recumbent and isoclinal

Micro structures and mineral chemistry

D1 and D2 deformation phases are associated with different mineral assemblages. Rocks from the Clarea group show glaucophane and white mica crystallizing along the D1 foliation both in the Southern Vanoise and the Modane-Aussois units (Fig. 9, Fig. 10). Garnet is only present in the Southern Vanoise unit (Fig. 9, Fig. 10). Within the Ambin group, the D1 foliation is underlined by chlorite and white mica in the Modane-Aussois and the Southern Vanoise units (Fig. 9D). In the Etache and the white

Pressure and temperature conditions of the deformation phases

P–T estimates were performed on samples from the Modane-Aussois unit: glaucophane bearing micaschist (M266 sample) and chlorite bearing micaschists (F21-5, M290, M259 samples), and from the Southern Vanoise unit (M278 sample).

Sample M266 is located on the sole of a D1 thrust within the Briançonnais cover (Fig. 5). It is a dark micaschist that has been interpreted as belonging to a slice of the Clarea group pinched on the sole of a D1 trust duplicating the white quartzite layer (Fig. 5). Two

Geochronological constraints on the Modane-Aussois unit

Three samples have been selected for dating: M80, M173 and M196 (Fig. 5). It has been demonstrated that there is a relationship between the paragonite component of phengite and the 39Ar excess (Gerber, 2008). To avoid 39Ar excess problem, we only selected phengites from samples of the Brianconnais cover that are characterized by the absence of paragonite component (Fig. 12). Classical step heating was performed and plateau ages were calculated. Very little 36Ar has been extracted, precluding

Discussion

The P–T conditions for D1 correspond to geothermal gradients between 13 and 18 °C km−1 in the Modane-Aussois unit and between 9 and 14 °C km−1 in the Southern Vanoise unit (Fig. 13). Such values are in good agreement with those obtained in the internal Briançonnais zone (Bucher et al., 2003). This suggests that the Modane-Aussois unit and the Southern Vanoise units were buried in a similar context as the rest of the Briançonnais zone.

In the following discussion, we build a P–T–t–d path for the

Conclusion

By combining structural analyses, metamorphic P–T estimates, 40Ar/39Ar dating of micas, we propose a P–T–t–d path for the Alpine evolution of the Modane-Aussois and the Southern Vanoise units. The Alpine tectonics are polyphased and occurred in a context of exhumation of HP rocks. For each unit, a cold geothermal gradient is obtained for the pressure peak suggesting that continental subduction is responsible of the burial of these units.

The southern Vanoise unit is buried before and at greater

Acknowledgements

This study was supported by the CNRS, the EMERGENCE program of the Rhône-Alpes region and the ANR-08-BLAN-0303-01 ERD-Alps: Erosion and Relief Development in the western Alps. We thank Mary Ford, Keiko Hattori, Patrice Rey, Benoit Saumur, Pierre Tricart, Marcel Bohn, Yann Rolland, Stephanie Duchene and one anonymous reviewer for fruitful suggestions.

References (85)

  • B. Le Bayon et al.

    Deformation history of a subducted continental crust (Gran Paradiso, Western Alps): continuing crustal shortening during exhumation

    Journal of Structural Geology

    (2006)
  • S.M. Reddy et al.

    Kinematic reworking and exhumation within the convergent Alpine Orogen

    Tectonophysics

    (2003)
  • Y. Rolland et al.

    Extension syn-convergence, poinçonnement vertical et unités métamorphiques contrastées en bordure Ouest du Grand Paradis (Alpes Franco-Italiennes)

    Geodinamica Acta

    (2000)
  • M.D. Schmitz et al.

    U–Pb zircon and titanite systematics of the Fish Canyon Tuff: an assessment of high-precision U–Pb geochronology and its application to young volcanic rocks

    Geochimica and Cosmochimica Acta

    (2001)
  • M.D. Schmitz et al.

    Comment on “Precise K–Ar, 40Ar–39Ar, Rb–Sr and U–Pb mineral ages from the 27.5 Ma Fish Canyon Tuff reference standard” by M.A. Lanphere and H. Baadsgaard

    Chemical Geology

    (2003)
  • P. Agard et al.

    Exhumation of the Schistes Lustres complex; in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps

    Journal of Metamorphic Geology

    (2002)
  • P. Agard et al.

    Discontinuous exhumation of oceanic crust: insights from blueschists and eclogites into the subduction channel

    Earth-Science Reviews

    (2008)
  • Aillères, L., 1996. Structure et cinématique de la zone Houillère Briançonnaise entre Arc et Isère (Alpes Française):...
  • N. Arnaud et al.

    Evidence for Mesozoic shear along the western Kunlun and Altyn-Tagh fault, northern Tibet (China)

    Journal of Geophysical Research – Solid Earth

    (2003)
  • M. Beltrando et al.

    (Ultra-) high-pressure metamorphism and orogenesis: an Alpine perspective

    Gondwana Research

    (2010)
  • R.G. Berman

    Thermobarometry using multi-equilibrium calculations: a new technique, with petrological applications

    Canadian Mineralogist

    (1991)
  • J.-M. Bertrand et al.

    Granitoides d’age Paleozoique inferieur dans le socle de Vanoise meridionale: geochronologie U–Pb du metagranite de l’Arpont (Alpes de Savoie France)

    Comptes Rendus de l’Académie des Sciences – Series IIA – Earth and Planetary Science

    (1997)
  • J. Bertrand et al.

    SHRIMP and IDTIMS U–Pb zircon ages of the pre-Alpine basement in the Internal Western Alps (Savoy and Piedmont)

    Schweizerische Mineralogische Und Petrographische Mitteilungen

    (2000)
  • J. Bocquet et al.

    Dating of blue amphibole, micas and associated minerals from the western Alps

    Contributions to Mineralogy and Petrology

    (1974)
  • A. Borghi et al.

    Composite P–T paths in the internal Penninic massifs of the Western Alps; petrological constraints to their thermo-mechanical evolution

    Eclogae Geologicae Helvetiae

    (1999)
  • R. Bousquet et al.

    The tectono-metamorphic history of the Valaisan Domain from the Western to the Central Alps; new constraints on the evolution of the Alps

    Geological Society of America Bulletin

    (2002)
  • J.P. Brun et al.

    Exhumation of high-pressure rocks driven by slab rollback

    Earth and Planetary Science Letters

    (2008)
  • S. Bucher et al.

    Late-stage deformation in a collisional orogen (Western Alps); nappe refolding, back-thrusting or normal faulting?

    Terra Nova

    (2003)
  • R. Caby

    Low-angle extrusion of high-pressure rocks and the balance between outward and inward displacement of middle Penninic units in the Western Alps

    Eclogae Geologicae Helvetiae

    (1996)
  • J.D. Champagnac et al.

    Regional Brittle extension in Quaternary sediments of Lanslebourg (Haute-Maurienne valley, western Alps)

    Bulletin de la Société Géologique de France

    (2006)
  • C. Chopin

    Very high pressure metamorphism in the western Alps: implications for subduction of continental crust

    Philosophical Transactions of the Royal Society of London

    (1987)
  • M. Cloos

    Flow melanges: numerical modelling and geological constraints on their origin in the Franciscan subduction complex

    Geological Society of America Bulletin

    (1982)
  • J.A.D. Connoly

    Multivariable phase diagrams: an algorithm based on generalized thermodynamics

    American Journal of Science

    (1990)
  • J. de Sigoyer et al.

    Exhumation processes of the high-pressure low-temperature Tso Morari dome in a convergent context (eastern-Ladakh, NW-Himalaya)

    Tectonics

    (2004)
  • Debelmas, J., Desmons, J., Ellenberger, F., Goffé, B., Fabre, J., 1989. Carte géologique de la France, feuille de...
  • J.F. Dewey et al.

    Orogenic uplift and collapse, crustal thickness, fabrics and metamorphic phases changes: the role of eclogites

  • S. Duchene et al.

    The Lu–Hf dating of garnets and the ages of the Alpine high-pressure metamorphism

    Nature

    (1997)
  • T. Dumont et al.

    Lateral termination of the north-directed Alpine orogeny and onset of westward escape in the Western Alpine arc: structural and sedimentary evidence from the external zone

    Tectonics

    (2011)
  • F. Ellenberger

    Etude géologique du pays de Vanoise (Savoie)

    (1958)
  • M. Ford et al.

    Two-phase orogenic convergence in the external and internal SW Alps

    Journal of the Geological Society, London

    (2006)
  • S. Freeman et al.

    Dating deformation using Rb–Sr in white mica: greenschist facies deformation ages from the Entrelor shear zone Italian Alps

    Tectonics

    (1997)
  • S. Gabalda et al.

    Thermal structure of a fossil subduction wedge in the Western Alps

    Terra Nova

    (2008)
  • Cited by (24)

    • Tectono-metamorphic evolution of an evaporitic décollement as recorded by mineral and fluid geochemistry: The “Nappe des Gypses” (Western Alps) case study

      2020, Lithos
      Citation Excerpt :

      This confirms our structural observations indicating that the whole nappe stack is folded during D2 event. Such an event has been dated between 43 Ma and 35 Ma (Gerber, 2008; Strzerzynski et al., 2012). In the surrounding units, D3 phase is characterized by brittle structure (Ganne et al., 2007; Gerber, 2008; Lanari et al., 2014).

    View all citing articles on Scopus
    View full text