Geographical repartition, ages and duration of the brittle tectonic phases

Since 10 years, many studies on the brittle deformation of the alpine belt have been performed. North to the Maurienne valley, two successive stress-states are described (Grosjean et al., 2004, Champagnac et al., 2004): the oldest one has a 3 axis parallel to the strike of the belt (Fig 8) and a latest one with a 3 axis perpendicular to the strike of the belt (Figure 7a,b). Immediately to the south of the Modane-Aussois area, two deformation phases are also described (Malusa 2004). The first one is defined by 3 axis oriented parallel to the strike of the belt, followed by a multidirectional extension phase (Figure 8). South of the city of Briançon, a multidirectional extension has been inferred from brittle micro tectonics (Sue and Tricart, 2003). However, on a recent synthesis on the Alpine belt, Champagnac (2004) proposed that the brittle tectonics of this part of the Alps is also characterized by polyphased tectonics where an extension direction parallel to the strike of the belt occurred before an extension direction perpendicular to the strike of the belt (Figure 8). It thus appears as proposed by Champagnac (2004) that polyphase brittle tectonic evolution is not limited to the Modane-Aussois-Lanslebourg area but is widespread in the whole western internal Alps. It is first characterized by a 3 axis parallel to the strike of the belt (F1) and the second one perpendicular to it (F2).

Figure 7. Detail view of a fault system located in the Contamines quarry

Detail view of a fault system located in the Contamines quarry

Detail view of a fault system located in the Contamines quarry. Both apparent normal and reverse faults coexist.

Figure 8. Age and spatial evolution

Age and spatial evolution

Age and spatial evolution of the 3 axes for F1 and F2 brittle event at the western Alps scale. (1) Grosjean et al., 2004, (2) Bistacchi et al., 2000, (3) Champagnac et al., 2004, (4) Malusa 2004, (5) Sue and Tricard, 2002, (6) Tricart et al., 2004, (7) This study.

Age and duration of the brittle tectonic phase is bounded by the youngest age obtained on ductile deformation and the present day stress field. Classically, an age of 35 Ma is given for the latest ductile deformation in the internal part of the Alps (Hunziker 1992 and references therein). This deformation is related to the top to the east shearing of the whole nappe stack. However youngest ages are also proposed for late ductile-brittle structures like extensional crenulation cleavage and gouge formation or reactived foliation (Bistacchi and Massironi, 2000). The same kind of structures that postdate the top to the east shearing are also described in the Ambin (Ganne, 2004) and in the Gran Paradiso massifs (Rolland et al., 2000). Near the Aosta fault, Gold bearing veins and calc-alkaline dikes associated with these structures give ages between 32 and 29 Ma (U/Pb zircon). Another age of 31.6 ± 0.33 Ma (Ar/Ar on separated phengite) has been obtained on the latest ductile structures of the Modane Aussois area (Strzerzynski et al, in prep).

In order to estimate the age of the end of the F1 event and the beginning of the F2 event, two different geochrological methods can be used. The first one consists of dating minerals that crystallize along faults and the second one consists of dating the latest stage of cooling of the rocks by apatite fission track measurement. Further north to the Maurienne valley, the motion of the Insubric fault (Figure 1) is estimated by dating the emplacement of syn-tectonic granite. The dextral movement of this fault occurred between 32 and 20 Ma (Stipp et al., 2004). Some authors (Lacassin, 1989, Hubbard and Mancktelow, 1992, Schmid and Kissling, 2000) proposed that the normal motion of the Simplon fault is related to the lateral motion of the Insubric line. In this hypothesis, the extension parallel to the chain axis, represented by the Simplon fault motion, occurred between 32 and 20 Ma. If the motion of the Insubric and the Simplon faults are not linked, Ar/Ar on phengite (Markley et al., 1998) and K/Ar age on lower than 2µm fraction of phengite (Zwingmann and Mancktelow, 2004) suggest that the normal motion of the Simplon faults occurs between 22 Ma and 5 ± 2 Ma. In this case, the F1 tectonic event spans from 22 to 5 Ma. Thermal modelling using apatite fission track length patterns to estimate the temperature-time travel of rocks for the last 150°C of the exhumation. Results on the internal zone of the Alpine belt cover a period since around 30 Ma (Hunziker et al., 1992). Results of Malusa (2004) close to the studied area suggest that the late cooling of rocks occurred in three stages: the first stage is related to rapid cooling from 150 to 100°C ended at 22 Ma. The second stage consists of a period of thermal stability until 5 Ma. The third stage is characterized by a rapid cooling of rocks since 5Ma from ~ 100°C to surface temperature. We propose that this thermal-time travel coincides with the whole brittle evolution i.e. that the F1 tectonic event ended at around 22 Ma and the F2 tectonic event started at 5 Ma.

The two independant methods of dating suggest that the F2 event start at 5Ma from the Simplon fault zone to the Gran Paradiso area (i.e. the central part of the Western Alpine belt). This age is also supported by fission track ages (Figure 8) for the denudation of the External Crystalline massifs (Mt Blanc-Belledonne-Pelvoux) that occurred at around 6 Ma (Seward and Mancktelow, 1994, Seward et al., 1999, Tricart et al., 2001, and Fugenschuh and Schmid, 2003, Leloup et al., 2005) and that may reflect the tectonic inversion of the Penninic Front as a normal fault (Tricart et al., 2001). Duration of the first brittle tectonic phase remains unclear: according to thermal modelling of apatite fission tracks (Malusa 2004) and the link between Insubric and Simplon faults, the first brittle tectonic phase ended at around 22 Ma. Taking into account the 5 ± 2 Ma K/Ar age on a fraction of phengite lower than 2µm (Zwingmann and Mancktelow, 2004), the first brittle tectonic phase ended at around 5Ma. This difference in age may result from the migration of the deformation from the south-western to the north-eastern part of the belt. This hypothesis is also supported by the repartition of the apatite fission tracks in the Houillère zone (Figure 2) as noticed by Fugenschuh and Schmid (2003) where ages increase from north-east to south-west. Then the first brittle tectonic phase started between 32-28 Ma and ended at ~ 22 Ma, south of the Simplon fault and started at 32 Ma and ended at ~ 5 Ma further north, along the Simplon fault zone. Wherever, the end of the F1 tectonic event occured, the second event F2 starts everywhere at ~ 6-5 Ma and is still active.

Tilting, Fault pattern and basement dome formation.

At the scale of the Modane Aussois area, the formation of a dome is related to the local southward tilting of the Briançonnais and Piemontese domain. For both tectonic events F1 and F2, the 3 axes cluster around a common orientation, while the orientations of the 1 and 2 stress axes vary from one station to another (Figure 4). This variation amounts ~60° for F1 and ~ 90° for F2. Different processes such as stress axis permutation, tilting related to folding of faultings, and a inhomogeneous stress field at the Modane Aussois scale can be proposed to explain the variation of axes orientations. The values of  significantly lower than 1 (Table 1), indicate triaxal (oblate) stress ellipsoids with 1 significantly different from 2. Permutation of the 1 and 2 axes because 1 ~ 2, does thus not appears a satisfactory explanation for 1 and 2 variations in direction. These variations cannot be related to folding because at the outcrop and at the map scale all the brittle structures cut the folds (Figure 3). Tilting of the bedrock related to faulting is necessarily limited to few tens of degrees an canot explain tilt angles reaching 90°. An inhomogeneous stress field at the Modane Aussois scale thus appears to be the main explanation for 1 and 2 stress axes variation of orientation during F1 and F2.

3D modelling (Strzerzynski et al., 2005) and structural analyses suggest that the whole studied zone, including the main fold axis, has been tilted by ~20° towards the south. One may wonder if this tilting was antecedent or posterior to F1 and F2. The F1 3 axes trend ~N-S and are thus optimally oriented to record any southward tilting around an ~E-W axis. If one assumes that during F1 3 axes were horizontal, it follows that tilting after F1 was maximum in station 10 (~25°) and negligible in stations 2, 3 and 6 for an average value of ~10°. The E-W directions of the F2 3 axes, and the great dispersion of 1 and 2 axes, render any discussion on the post F2 southward tilting based on the paleostress orientations, difficult. However, the pattern of 3 axis suggests that an episode of local E-W tilting may have occurred during or after F2. We thus suggest that tilting towards the south of the southern flank of the South Vanoise dome occurred after the last folding phase and during the first brittle tectonic event F1. Later some local tilting on ~N-S axes may have occurred during F2.

At the Modane Aussois scale, faults are related both to F1 and F2 tectonic events (Figure 3). Faults related to the F2 event are mainly oriented N-S and dip to the east or the west. As the orientation of the F2 faults planes is perpendicular to the direction of the tilting axis, the southward tilting cannot have been only accommodated by motion along these faults. The orientation and the kinematics observed along the F1 faults are mostly compatible with this southward tilting: most of the F1 faults have orientation around an E-W direction and dip to the south or the north. Then a part of the tilting of the whole Modane Aussois area is related to an early normal motion of ENE-WSW and ESE-WNW faults. Then the tilting of the Modane Aussois is probably related to the normal motion of the F1 main faults. However, as the tilting probably affects both the F1 and the F2 event, a part of the tilting may be more recent.

At the Vanoise domain scale, most of the boundaries of the basement domes are underlined by post metamorphic faults (Figure 2). These are mainly NE-SW faults (i, j, k, l on figure 2) and the N-S fault (m on figure 2) that occurred at the ductile-brittle transition. Late N-S normal fault (i, b on figure 2) locally formed the eastward and westward boundaries of the basement domes. As NE-SW normal faults can be related to the F1 tectonic event, it appears that the formation of most of the southward limit of the basement dome is related to this tectonic event. The formation of the eastward and westward limits of the basement dome appears to be polyphased: an early step occurs before the F2 event and is well recorded at the western boundary of the Gran Paradiso and Dora Maira massifs. A second step occurs after the D1 event and is possibly related to the F2 tectonic event. This last event is well recorded on the most external part of the Vanoise domain i.e. in the Briançonnais units.