Geological setting

The southern part of the Piemont zone of the Western Alps (Figure 1) consists of three major tectono-metamorphic units (Figure 2) coming from the Piedmont-Liguria domain of the Western Tethys that was actively spreading during the Upper Jurassic period (e.g. Lombardo et al 2002 for review) and closed in the Late Cretaceous to Early Tertiary (e.g. Agard et al. 2002, Schettino and Scotese 2002). The uppermost unit (Chenaillet massif) consists of a weakly metamorphosed ophiolite obducted onto the European continental margin (Mével et al. 1978). The intermediate unit corresponds to the External Piedmont zone, namely the Schistes Lustrés units (Deville et al., 1992). It consists of metric to kilometric lenses of ophiolitic blocks embedded in dominant Jurassic to Lower Cretaceous clastic metasedimentary rocks (Lagabrielle et al. 1984, Lemoine et al. 1987). They were metamorphosed to blueschist facies conditions during the period from 75 to 40 Ma (Deville 1986, Schwartz 2001, Agard et al. 2002, Schettino and Scotese 2002) and are commonly considered an accretionary wedge that was later tilted during the Alpine collision (Tricart et al., 2004). The maximum thickness of the wedge during its deposition was calculated to be between 40 and 65 km, dependent on pressure estimates used for the blueschists (Schwartz et al. 2001, Agard et al. 2002). The maximum thickness of 65 km is probably overestimated as it is essentially based on the poorly constrained thermodynamic data of phengite (Vidal et al. 2001).

Figure 1. Map of the Western Alps

Map of the Western Alps

Map of the Western Alps showing the main tectonic units, modified after Polino et al. (1990). AM: Argentera-Mercantour, AR: Aiguilles Rouges, B: Belledonne, DB: Dent-Blanche, DM: Dora Maira, GP: Gran Paradiso, MB: Mont-Blanc, MR: Monte Rosa, P: Pelvoux, SA: South Alps, SE: Sezia, SL: Schistes Lustrés.

Figure 2. Detailed geological map

Detailed geological map

Detailed geological map of the studied area showing the relationships between the Monviso ophiolite and surrounding units (after Schwartz 2000).

The Monviso ophiolitic massif, 35 km NS and 35 km EW, forms the structural base of the unit (Figure 3) and is separated from the blueschist ‘Schistes Lustrés’ by a ductile normal fault within the western serpentinite unit (Traversetta unit) (Ballèvre et al. 1990) and to the east from the continental Sampeyre and Dronero units of the Dora Maira massif by a second normal ductile fault (Blake and Jayko 1990).

Figure 3. Vertical x-sections

Vertical x-sections

Schematic vertical sections across the studied area from the Briançonnais to the Dora Maira (after Schwartz 2000).

Geophysical data shows the root of this massif at a depth of about 10 km (Schwartz 2001, Paul et al. 2001, Fig. 4). The root of this unit in the eastern part is buried below the Tertiary sediments of Po Plain. However, northward along the Gran Paradiso or Monte Rosa transects, the ophiolitic unit has a vertical extension along the Insubric line (Escher et al., 1997). The evidence suggests that the Monviso unit likely extends vertically east of the Dora Maira massif (Fig. 4).

Figure 4. Monviso x-section

Monviso x-section

Cross section of the Monviso ophiolite showing the different units with eclogitic lenses embedded within the serpentinite. The tectonic contact between the different units correspond to normal shear zones under greenschist facies conditions. 1: Greenschist foliated metabasalts (prasinites); 2: eclogitic lenses composed of undifferentiated metagabbros, massive metabasalts and pillow lavas; 3 serpentinites. The shaded envelop corresponds to the geometry of the Monviso unit rooted below the Schistes Lustrés (Paul et al. 2001) and possibly rooted east of the Dora Maira massif, along the Insubric line.

In the central part of the massif, the Monviso massif is comprised of six distinct west-dipping tectono-metamorphic units of metabasalts or metagabbroic lenses, each of which is separated by west dipping normal shear zones in the serpentinite matrix (Fig. 4 and Fig. 5). Each unit records different eclogitic facies metamorphic conditions between 1.2 and 2.6 GPa and retrogression under similar blueschist facies conditions of about 0.7 GPa and 420°C (Lombardo et al. 1978, Philippot 1990, Blake et al. 1995, Schwartz et al. 2000, Lombardo et al. 2002) (Fig. 6 and Fig. 7).

Figure 5. Monviso map

Monviso map

Schematic geological map of the Monviso unit after Blake et al. (1995) and Schwartz et al. (2001) showing the relationships of main tectonic units. The boundaries are estimated by image analysis. Notice that the serpentines (SP) represent about 50% of the total surface area and caps (OR overlies) all the eclogitic units. CT: Costa Ticino, LS: Lago Superiere. PG: Passo Gallarino, VM: Viso Mozzo.

The age of the eclogitization varies. The Lago Superior unit yielded a Sm-Nd isochron age of 61 ± 10 Ma from garnet + omphacite (Cliff et al. 1998), a Lu-Hf isochron age of 49 ± 1 (Duchêne et al. 1997), and a U-Pb zircon age of 45 ± 1 Ma by Rubatto and Hermann (2003). These ages are comparable to an Ar-Ar age of 50 ± 1 Ma (Monié and Philippot 1989). The exhumation rate of the unit was estimated by Schwartz et al (2000) as fast at the beginning (1 cm.yr-1) and slower at the blueschist to greenschist-facies transition (1 mm.yr-1).

Figure 6. Monviso central map

Monviso central map

Geological map of the central part of the Monviso, after Schwartz et al. (2001). The P-T estimates on the eclogite facies conditions are from Blake et al. (1995), Messiga et al. (1999) and Schwartz et al. (2000).

Figure 7. PT paths

PT paths

P-T paths of eclogitic units in Monviso, showing that different units show similar retrogression paths under blueschist facies conditions. Note that the P-T paths yield geothermal gradients of 4 to 6°/km, typical of subduction zone conditions.

The basal serpentinite unit is 400 meters in thickness. The serpentinites have a lherzolitic protolith with only minor harzburgite and dunite, and are cut by sheared dykes of rodingitized gabbro and basalt. The serpentinite layer commonly contains metric to hectometric lenses of foliated eclogitic gabbro, ferrogabbro and metamorphosed plagiogranite. These lenses are tectonically incorporated as a tectonic melange and they show widely varied metamorphic conditions, from 1.3 GPa and 470 °C to 2.0 GPa and 575°C (Blake et al. 1995, Schwartz et al. 2000, Castelli et al. 2002). Considering its geometry, this basal serpentinite unit likely had an initial size of about 50 km x 10 km (Fig. 4).

The Lago Superiore unit is a discontinuous layer of intensely deformed and recrystallized metagabbros with local occurrences of chromian omphacite (smaragdite gabbro). The metagabbros host small bodies of ultramafic cumulates and hydrated mantle peridotites (Messiga et al. 1999). This unit recorded the highest pressure and temperature conditions of 2.4 GPa and 620 ± 50°C in Cr-rich magnesiochloritoid-bearing eclogites.

The Viso Mozzo unit consists of greenschists and banded glaucophane-epidote metabasalts with local preservation of pillow lava structures (Lombardo et al. 2002). Metric eclogitic lenses are embedded in the strongly foliated metabasalts, recording pressure–temperature conditions of 1.2 ± 0.2 GPa and 460 ± 30°C (Schwartz et al. 2000). In the upper part of the unit outcrops thin layers of carbonate micaschists (Schistes Lustrés) interbedded with the metabasites.

The Passo Gallarino unit is a 100 m thick layer of eclogite and omphacite-bearing metagabbro (1.3 ± 0.3 GPa, 450 ± 40°C) hosted in sheared serpentinites associated with quartzite and mica schist. This unit is characterized by intense syn-eclogitic shearing (Lardeaux et al. 1986) superimposed by flattening under blueschist facies conditions (Schwartz et al. 2000).

The Costa Ticino unit is the thickest unit (~1.2 km) and composed from the base to the top of basalt breccia, pillow lavas, metagabbro and slices of serpentinites that have been metamorphosed under blueschist facies conditions (~0.7 GPa, ~420°C) (Lombardo et al. 1978).

The Vallanta unit structurally overlies the Costa Ticino unit. It is also an eclogitic unit that consists of fine-grained metabasalts, carbonate-bearing mica schists and serpentinites. Metamorphic conditions are not available for this unit.