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