Geological Context

Metamorphic evolution of the internal Alps

The Alpine belt (Figure 1) is the result of the convergence between the European and the African plates during the closure of the western Tethyan oceanic domain. The Maurienne valley cross cuts the main units of the European margin and of the oceanic units, and then records the complete alpine evolution from Cretaceous subduction to late basement domes formation. Subduction of the oceanic unit occurred between 60 and 55 Ma (Chopin and Maluski, 1980, Cliff, 1998, Agard et al., 2002) and was responsible for the burial of the oceanic crust down to the eclogite facies metamorphic conditions (Rolland et al., 2000). The European continental margin was involved in the subduction processes at circa 45 Ma (Chopin and Maluski, 1980, Ganne et al, 2005) and continental rocks from the Gran Paradiso massif were affected by eclogite metamorphism (Ballèvre, 1988, Borghi et al., 1996, Ganne et al., 2005). This metamorphic event is associated with a top to the north nappe stacking in the Briançonnais and Schistes Lustrés zones (Ganne et al., 2005). Part of these rocks was next exhumed in greenschist metamorphic facies conditions at circa 35 Ma (Agard et al., 2002, Reddy et al., 2003) along top to the east shear zones (Platt and Lister, 1985, Ganne et al., 2004). These shear zone are interpreted as tilted thrusts (Platt and Lister, 1985, Butler and Freeman, 1996) or as normal faults (Wheeler et al., 2001, Ganne et al., 2004).

Figure 1. Structural map of the Alps

Structural map of the Alps

Structural map of the Alps (after Chantraine et al., 1996 and Schmid et al., 2004). A. : Ambin, A.G. Aar and Gothard, A.R. Aiguilles Rouges, Arg. Argentera, Bel. Belledonne, D.M. : Dora Maira, D.B. Dent Blanche, G.P. Gran Paradiso, L.D. Lepontine Dome, M.B. Mont Blanc, M.R. : Monte Rosa, N.V. : Northern Vanoise, Pel. : Pelvoux, S.L. Sesio-Lanzo, S.V. : Southern


Basement dome formation.

The major structures that complicate the nappe stack of the internal Alps are basement domes that crop out through their sedimentary cover and the overlying nappe units (Figure 1). The basement domes of the internal Alps are the Dora Maira, Gran Paradiso and Monte Rosa Internal Crystalline massifs (ICM) associated with more external structures like the Ambin massif and the Southern and Northern Vanoise dome. The basement dome of the internal Alps are characterized by largely open folds of the metamorphic foliations and the occurrence of surrounding normal faults (Rolland et al., 2001, Ganne et al., 2004). The dome formation began under ductile conditions and ended under brittle conditions (Rolland et al., 2000, Ganne et al., 2004). Many models of alpine basement dome formation have been proposed. The first models gave a major role to the thrust tectonics: the basement culmination of the ICM is explained in this case by the formation at depth of an anticline (Malavielle et al., 1983, Bucher et al., 2004) which formed and was exhumed along a crustal scale ramp structure. In this hypothesis, the formation of both the Ambin and Vanoise basement domes would have been formed in the same shortening context but on two different thrust ramps. However, such early formation of the domes is in contradiction with the observation that the east verging folds affected by the doming, developed under greenschist facies conditions. In the next models, an important place is given to the extensional processes that occurred during exhumation. The culmination of the Monte Rosa massif could be related to an extensional tectonic event that occurred between 42 and 35 Ma (Reddy et al., 2003). Vertical pinching is also proposed in the case of the Gran Paradiso Massif (Rolland et al., 2000). In the Dora Maira massif, both early (Henry et al., 1993) and recent extensional processes are proposed (Tricart et al., 2004, Schwartz et al., 2005). Thus, the dome shape of the ICM has been related to exhumation processes: extension and vertical shortening is frequently proposed. However, the non cylindrical shape of these structures remains to be explained.

Post-metamorphic faults in the internal Alps.

Geological maps of the Alps (Chantraine et al., 1996, Schmid, 2004) do not show many post-metamorphic faults. Three mains structures (Figure 1) are to be highlighted: the Insubric line, the Simplon fault and the Aosta fault.

The Insubric Line is a 200 km long dextral fault along which a magmatic activity is recorded. This fault forms the limit between the South Alpine and the Austroalpine domains on its eastern part and the limit between the Austroalpine, Brianconnais and Piemontese domains on its western part. The strike slip motion of this fault occurred between 32 and 20 Ma (Stipp et al., 2004) and estimation of the lateral offset ranges from 30 to around 100 km (Lacassin 1989, Schmid and Kissling, 2000). The magmatic activity is related both to crustal anatexis produced by shear heating on the fault (Rosenberg, 2004) and to mantle-derived magmas.

The Simplon fault is a 40 km long normal fault. It forms the western boundary of the Lepontine domain. Ages and duration of activity on the Simplon fault are still debated: on te one hand, some authors propose that the normal motion of the fault is linked with the dextral motion of the Insubric line implying that the motion of the Simplon fault occurred between 32 and 20 Ma (Stipp et al., 2004). On the other hand, 5 ± 2 Ma K/Ar age on phengite fraction lower than 2µm sampled along the Simplon fault indicates that its motion occurred until at least 5Ma (Zwingmann and Mancktelow, 2004)

The Aosta fault is a 30 km long E-W fault with northward dip ranging from 50 to 70° (Bistacchi et al., 2001). It forms the southward limit of the Dent Blanche Austroalpine klippe (Figure 1). Estimates of the vertical displacement along this fault vary from 3000 to 400 m (Bistacchi et al., 2001 and reference therein). Age and duration of the Aosta fault motion are still debated It has been proposed that motion may started at the time of andesitic and lamprophyric dikes emplacement i.e. between 31 and 29 Ma (Dal Piaz et al., 1979) and gold bearing veins between 32 and 30 Ma (Diamond, 1990). Differences in age between apatite fission track analysis across the fault suggest a motion restricted between 28 and 12Ma (Hunziker et al., 1992).