Paola Manzotti

The tectonometamorphic evolution of the Sesia–Dent Blanche nappes (internal Western Alps): review and synthesis

AbstractThis study reviews and synthesizes the present knowledge on the Sesia–Dent Blanche nappes, the highest tectonic elements in the Western Alps (Switzerland and Italy), which comprise pieces of pre-Alpine basement and Mesozoic cover. All of the available data are integrated in a crustal-scale kinematic model with the aim to reconstruct the Alpine tectono-metamorphic evolution of the Sesia–Dent Blanche nappes. Although major uncertainties remain in the pre-Alpine geometry, the basement and cover sequences of the Sesia–Dent Blanche nappes are seen as part of a thinned continental crust derived from the Adriatic margin. The earliest stages of the Alpine evolution are interpreted as recording late Cretaceous subduction of the Adria-derived Sesia–Dent Blanche nappes below the South-Alpine domain. During this subduction, several sheets of crustal material were stacked and separated by shear zones that rework remnants of their Mesozoic cover. The recently described Roisan-Cignana Shear Zone of the Dent Blanche Tectonic System represents such a shear zone, indicating that the Sesia–Dent Blanche nappes represent a stack of several individual nappes. During the subsequent subduction of the Piemonte–Liguria Ocean large-scale folding of the nappe stack (including the Roisan-Cignana Shear Zone) took place under greenschist facies conditions, which indicates partial exhumation of the Dent Blanche Tectonic System. The entrance of the Briançonnais micro-continent within the subduction zone led to a drastic change in the deformation pattern of the Alpine belt, with rapid exhumation of the eclogite-facies ophiolite-bearing units and thrust propagation towards the foreland. Slab breakoff probably was responsible for allowing partial melting in the mantle and Oligocene intrusions into the most internal parts of the Sesia–Dent Blanche nappes. Finally, indentation of the Adriatic plate into the orogenic wedge resulted in the formation of the Vanzone back-fold, which marks the end of the pervasive ductile deformation within the Sesia–Dent Blanche nappes during the earliest Miocene.

Petro-structural map of the Dent Blanche tectonic system between Valpelline and Valtournenche valleys, Western Italian Alps

Abstract Please click here to download the map associated with this article. The mountain ridge between Valpelline and Valtournenche valleys (Western Italian Alps) consists of a high-strain zone, several kilometres wide, in which basement and cover of the Dent Blanche and Mont Mary tectonic units (nappes of the Austroalpine domain) show complex interrelations. New detailed petro-structural maps across the ridge help to update the structural and metamorphic evolution of these Austroalpine units. The Alpine imprint, under blueschist facies onditions, is evident only locally, notably in shear zones. Such high-strain zones developed in the basement, and in a post-Permian cover (Roisan metasedimentary Zone) where the polyphase Alpine overprint is strong. The pre-Alpine structural and metamorphic evolution is commonly well-preserved in many domains with little Alpine strain. The presented maps from 1:2,500 to 1:20,000 scales, and cross sections, are the primitive data source of this work. They synthesize the geological and structural setting of this area and improve knowledge of the Western Alpine evolution by attributing tectonic structures to the evolving Alpine metamorphic stages.

The pre-Alpine tectonic history of the Austroalpine continental basement in the Valpelline unit (Western Italian Alps)

The Valpelline unit is a large slice of continental crust constituting the Austroalpine Dent Blanche nappe (NW Italy). The pre-Alpine evolution of this unit holds important clues about the Palaeozoic crustal structure at the northern margin of the Adria continent, about the history of rifting in the Alpine region, and thus about the thermomechanical conditions that preceded the Alpine convergent evolution. Several stages of the deformation history and of partial re-equilibration were identified, combining meso- and micro-structural analyses with thermobarometry. Reconstructed pre-Alpine P–T–t –d paths demonstrate that the Valpelline unit experienced an early stage at pressures between 4.5 and 6.5 kbar followed by migmatite formation. A subsequent stage reached amphibolite to granulite facies conditions. This stage was associated with the development of the most penetrative fabrics affecting all of the Valpelline lithotypes. The pre-Alpine evolution ended with a weak deformation associated with a local mineral-chemical re-equilibration under greenschist facies conditions at ≈ 4 kbar and T < 450°C. A Permo-Mesozoic lithospheric extension is thought to be responsible for asthenosphere upwelling, thereby causing high temperature metamorphism at medium pressure and widespread partial melting, which led to upper crustal magmatic activity.

Geometry and kinematics of the Roisan-Cignana Shear Zone, and the orogenic evolution of the Dent Blanche Tectonic System (Western Alps)

The Dent Blanche Tectonic System (DBTS) is a composite thrust sheet derived from the previously thinned passive Adriatic continental margin. A kilometric high-strain zone, the Roisan-Cignana Shear Zone (RCSZ) defines the major tectonic boundary within the DBTS and separates it into two subunits, the Dent Blanche s.s. nappe to the northwest and the Mont Mary nappe to the southeast. Within this shear zone, tectonic slices of Mesozoic and pre-Alpine meta-sediments became amalgamated with continental basement rocks of the Adriatic margin. The occurrence of high pressure assemblages along the contact between these tectonic slices indicates that the amalgamation occurred prior to or during the subduction process, at an early stage of the Alpine orogenic cycle. Detailed mapping, petrographic and structural analysis show that the Roisan-Cignana Shear Zone results from several superimposed Alpine structural and metamorphic stages. Subduction of the continental fragments is recorded by blueschist-facies deformation, whereas the Alpine collision is reflected by a greenschist facies overprint associated with the development of large-scale open folds. The post-nappe evolution comprises the development of low-angle brittle faults, followed by large-scale folding (Vanzone phase) and finally brittle extensional faults. The RCSZ shows that fragments of continental crust had been torn off the passive continental margin prior to continental collision, thus recording the entire history of the orogenic cycle. The role of preceding Permo-Triassic lithospheric thinning, Jurassic rifting, and ablative subduction processes in controlling the removal of crustal fragments from the reactivated passive continental margin is discussed. Results of this study constrain the temporal sequence of the tectono-metamorphic processes involved in the assembly of the DBTS, but they also show limits on the interpretation. In particular it remains difficult to judge to what extent pre-collisional rifting at the Adriatic continental margin preconditioned the efficiency of convergent processes, i.e. accretion, subduction, and orogenic exhumation.

Continental gabbros in the Dent Blanche Tectonic System (Western Alps): from the pre-Alpine crustal structure of the Adriatic palaeo-margin to the geometry of an alleged subduction interface

Several Permian gabbro bodies are known in the continental-derived Dent Blanche Tectonic System (Western Alps). A new gabbroic complex, the Berrio Gabbro, is described in this paper, and compared with the other gabbro bodies of the Adriatic palaeomargin, as well as of the Tsaté Unit, a remnant of the Piemonte–Liguria ocean, immediately underlying the Dent Blanche Tectonic System. The Berrio Gabbro comprises ultramafic cumulates (clinopyroxenite, wehrlite and hornblendite) and mafic rocks (leucocratic and mesocratic gabbros). Ultramafic (serpentinite) and mafic rocks (gabbro) are also present in the oceanic Tsaté Unit of the Combin Zone. The ultramafic–mafic rocks of the Dent Blanche and the Tsaté are clearly distinguished by their mineralogy and their geochemistry. In addition, their behaviour during the Alpine deformation is distinct. Detailed mapping associated with geochemical characterization of the protoliths allows us to infer the structure of the Adriatic crust during the Permian extension and refute a model of mixing between oceanic and continental material along the hypothesized subduction interface. Supplementary material: Additional supporting information (deformation–mineral growth relationships and Alpine mineral analyses of the Berrio Gabbro basic and ultrabasic rocks) is available at https://doi.org/10.6084/m9.figshare.c.3673039.

Pre-Alpine (Variscan) inheritance: a key for the location of the future Valaisan Basin (Western Alps).

The boundary between the Helvetic and the Penninic (= Brianconnais) Zones has long been recognized as a major fault (“Penninic Front”) in the Western Alps. A narrow oceanic domain has been postulated at least along part of this boundary (the Valaisan ocean). However, the information provided by the pre-Triassic basement has not been fully exploited, and will be discussed here in detail. The igneous and metamorphic history of the pre-Triassic basement shows significant differences between the External Massifs from the Helvetic Zone, with abundant late Carboniferous granites, and the basement of the Brianconnais Zone, including the Internal Massifs (Dora-Maira, Gran Paradiso, Monte Rosa), devoid of Carboniferous granites. A major coal-bearing basin, the “Zone Houillere”, opened along this boundary. This limnic intramontane basin has never been properly investigated. The Zone Houillere is not comparable with the external, paralic, flexural, basins on both sides of the Variscan belt, but shows similarities with the Saar-Saale basin. Like the latter, we interpret the Zone Houillere as a transtensional basin opened along a major, crustal-scale, fault zone, namely the East Variscan Shear Zone. The Permian magmatism and sedimentation displays contrasting distributions, being absent or very localized in the Helvetic Zone, and widespread in the Penninic Zone. The above data indicate that the structural inheritance from the Variscan belt plays a major role in defining the future location of the Valaisan basin, i.e. the boundary between the European palaeomargin and the Brianconnais microcontinent.

From maps and cross-sections to kinematic models: the impact of metamorphic petrology

In the Western Alps, more than a century of geological mapping has provided an unrivalled basis for geometrical and kinematical analysis of a collision belt. Metamorphic petrology has complemented geological mapping by providing key information on (i) the structure and history of the pre-Alpine crust, (ii) the amount of subduction recorded by mineral assemblages, and (iii) the relative timing and amount of exhumation of the different slices involved in the nappe stack. Integration of these data with isotopic ages allows a discussion of kinematic models, and some inferences on the dynamics of the mountain belt.