The evolution of the Austroalpine domain

2.1.1 Variscan and post-Variscan evolution

In the Austroalpine realm, the Variscan orogenic evolution is mostly documented in the Ulten area, where crustal units reach peak metamorphic conditions at 350–325 Ma (U-Th-Pb on monazite; Langone et al., 2011). In the West, amphibolite-facies metamorphism is documented but remains yet undated (Bianchi Potenza et al., 1978a; Büchi, 1994; Halmes, 1991; Pace, 1966). In contrast, this western part and especially the Bernina unit develops a well-constrained Carboniferous plutonism of calc-alkaline affinity (Rageth, 1984; Spillmann & Büchi, 1993;

Von Quadt et al., 1994) with associated volcanics (Mercolli, 1989).

During the Permian, the continental crust was strongly affected by a magmatic and metamorphic episode that deeply modified its lithological composition and crustal architecture (Fig. V-1a).

In the upper crust, alkaline felsic magmas were emplaced and extruded between 295 and 288 Ma (Büchi, 1987; Spillmann & Büchi, 1993; Von Quadt et al., 1994). It is yet unclear in the study area if the volcanic rocks are filling basins.

In the same age range, intermediate levels (middle crust) were intruded by both mafic bodies (Tribuzio et al., 1999) and granitoïds (Bockemühl & Pfister, 1985; Guglielmin &

Notarpietro, 1997). Locally, the intrusions are rimed by a migmatitic contact aureole that may reach granulite-facies conditions in xenoliths (Braga et al., 2003; Braga et al., 2001).

152 Formation et exhumation des granulites permiennes

The lower crust was affected during the Permian by mafic underplating during emplacement of gabbros of tholeiitic affinity associated with felsic granulites (Hermann et al., 2001; Hermann et al., 1997; Müntener et al., 2000), indicating a emplacement and contact metamorphism at around 10 kbar. The emplacement of these gabbros occurred at 281-278 Ma (U-Pb on zircon; Hansmann et al., 2001).

2.1.2 Jurassic rifting history

This pre-rift lithospheric architecture was disrupted in Jurassic times by a rifting event that led to the opening of the Alpine Tethys and the formation of the Adriatic rifted margin.

Following Mohn et al. (2012), this process occurred in three stages.

(1) The stretching phase (220-185 Ma, Fig. V-1b) during the formation of basins bounded by high-angle normal faults over the complete rift domain (Eberli, 1988). The related normal faults are typically listric and rooted in mid-crustal levels (Bertotti, 1991).

During this stage, extension in the upper-crust is decoupled from deformation in the lower crust. The subcontinental mantle and the lower crust start to be exhumed and cooled at that time, as recorded by ⁴⁰Ar/³⁹Ar ages on amphibole (Müntener, 1997;

Villa et al., 2000).

(2) The thinning phase (185-175 Ma, Fig. V-1c) when extension localized within the future distal margin (Froitzheim & Eberli, 1990), and the Briançonnais domain starts to be uplifted (Decarlis et al., 2013; Lemoine et al., 1986). The basement starts to be exhumed at the surface by “thinning” faults and the crustal thickness of the distal domain is thinned below 10 km, by complete obliteration of the middle-crust. In the intermediate necking zone, mid-crustal levels are also expected to be exhumed along thinning faults.

(3) The exhumation phase initiates (175-165 Ma, Fig. V-1d) when the crust in the distal domain has been thinned below 10 km and is entirely brittle. Detachments faults cut into the lower crust and the mantle enabling its exhumation to the seafloor (e.g.

Mohn et al., 2012). The exhumation faults are cutting through a crustal section that has been thinned during the previous thinning phase.

Fig. V-1: Tectonic evolution model of the European and Adriatic rifted margins during the opening of the Alpine Tethys. (a) pre-rift crustal section mainly affected by the Permian lithospheric extension (290-240 Ma) affected by the three-stages rifting with (b) the stretching phase (220-190 Ma), (c) the thinning phase (180-175 Ma) and (d) the exhumation phase (175-161 Ma), lately reactivated during the Alpine compression (< 100 Ma). Modified after(Mohn et al., 2010) and Mohn et al. (2012), see text for details.

Note the relative position of the unit position before and after the rifting.

153

Chapitre V : Exhumation et refroidissement pendant le rifting

Eita shear zone Albula-Zebru movement zone

Campo Da2 movements & structures multiple movements & structures

(a) Permian lithospheric extension 290-240 Ma

(b) Stretching 220-190 Ma

(c) Thinning 180-175 Ma

(d) Exhumation 175-161 Ma

(e) Alpine compression <100 Ma

Diorite

154 Formation et exhumation des granulites permiennes

The final architecture of the hyper-extended margin is the result of the superposition of all extensional phases on the inherited pre-rift crust. As a consequence of thinning and exhumation, rock units that were prior to rifting on top of each other, were juxtaposed and occurred at the end of the rifting one next to each other. The evolution of the thinning and exhumation is documented by the P–T–t evolution of these rocks as well as by the syn-tectonic sediments associated with these events at the surface. In this work, we are mainly focusing on the evolution of the middle crust from the pre-rift Permian stage to the Jurassic rifting. These rocks were exhumed in between the distal and the proximal domains, in the so-called “necking”

zone recorded in the Grosina-Campo units.

10°20’E

Filladi di Bormio - phyllite

Grosina unit - orthogneiss & metapelite pre-rift upper crust

Campo unit - micaschist & paragneiss pre-rift middle crust

Movement zone Jurassic thinning structure

(inferred/observed) Location of samples used for Town, summit geochronology

Fig. V-2: Geological map of the study area compiled from maps of Del Moro & Notarpietro (1987), Del Moro et al. (1999), Mohn et al. (2011), the 1:10,000 and 1:25,000 geological maps of Italy and personal observations. Location of samples used for geochronology is indicated.

155

Chapitre V : Exhumation et refroidissement pendant le rifting

2.1.3 Alpine reactivation

From Late Cretaceous onward, the Jurassic margin architecture was reactivated and disrupted by Alpine compression (Fig. V-1e). The stacking of the nappes occurred from Late Cretaceous till Tertiary times, by multistage thrusting of proximal domains over more distal domains of the margin. The first major deformation phase, the Da1 phase (Trupchun phase of Froitzheim et al., 1994) consists of a W- to NW-directed thrusting occurring between 100 and 76 Ma (see Froitzheim et al., 1994; Handy et al., 1996), and related to the coeval closure of the Meliata-Vardar domain further west and the onset of the closure of the Alpine Tethys (Froitzheim et al., 1996; Mohn et al., 2011; Schmid et al., 2008). The second phase, the Da2 phase (Duncan-Ela phase of Froitzheim et al., 1994) is an extensional phase developing normal faults and associated recumbent folds (Froitzheim, 1992; Froitzheim et al., 1994; Handy et al., 1993) during Late Cretaceous times (80-67 Ma, Handy et al., 1996). The third phase, the Da3 phase (Blaisun phase of Froitzheim et al., 1994) is expressed by large-scale upright open-folds with sub-horizontal E-W trending axis in upper greenschist facies conditions (Hermann

& Müntener, 1992). Da3 has been estimated to be Eocene in age (Mohn et al., 2011) and is associated with the continental collision between Adria and Europe (Schmid et al., 1996).

The complete nappe stack is finally deformed by movement along the Engadine Line and the Insubric Line, both related to the Periadriatic fault system (Schmid et al., 1989; Schmid &

Froitzheim, 1993).