Tectono-metamorphic map of the Mont Morion Permian metaintrusives (Mont Morion—Mont Collon—Matterhorn Complex, Dent Blanche Unit), Valpelline—Western Italian Alps
Abstract:The presented map displays the structural and metamorphic evolution of lithotypes from pre-Permian to present. We distinguish pre-Permian rocks (e.g., amphibolite, biotite-bearing gneiss and acid granulite) preserved as roof pendants (i.e., xenoliths) within Permian intrusives. Permian intrusives and hosted xenoliths are then re-equilibrated during Alpine evolution, producing coronitic to mylonitic metaintrusives, due to meter to kilometer-scale fabric gradients, and associated white mica-, glaucophane-bearing… Show more
“…In the II DK large rock volumes (1 km 3 ) still preserve pre-Alpine structures Ridley 1989;Vuichard 1989), and the same is true for the Valpelline Series (10 9 5 9 2 km) (Diehl et al 1952;Nicot 1977;Gardien et al 1994;Manzotti and Zucali 2013). In the Arolla Series and in the Lower Unit of the Mont Mary, mapping has revealed a few undeformed granitoids and gabbros as well (Diehl et al 1952;Elter 1960;Roda and Zucali 2008;Manzotti 2011;Roda and Zucali 2011). By contrast, the GMC shows pervasive Alpine deformation, undeformed volumes being conspicuously absent.…”
International audienceThis study reviews and synthesizes the presentknowledge on the Sesia–Dent Blanche nappes, the highesttectonic elements in the Western Alps (Switzerland andItaly), which comprise pieces of pre-Alpine basement andMesozoic cover. All of the available data are integrated in acrustal-scale kinematic model with the aim to reconstructthe Alpine tectono-metamorphic evolution of the Sesia–Dent Blanche nappes. Although major uncertainties remainin the pre-Alpine geometry, the basement and coversequences of the Sesia–Dent Blanche nappes are seen aspart of a thinned continental crust derived from the Adriaticmargin. The earliest stages of the Alpine evolution areinterpreted as recording late Cretaceous subduction of theAdria-derived Sesia–Dent Blanche nappes below theSouth-Alpine domain. During this subduction, severalsheets of crustal material were stacked and separated byshear zones that rework remnants of their Mesozoic cover.The recently described Roisan-Cignana Shear Zone of theDent Blanche Tectonic System represents such a shearzone, indicating that the Sesia–Dent Blanche nappes representa stack of several individual nappes. During thesubsequent subduction of the Piemonte–Liguria Oceanlarge-scale folding of the nappe stack (including the Roisan-Cignana Shear Zone) took place under greenschistfacies conditions, which indicates partial exhumation of theDent Blanche Tectonic System. The entrance of the Brianc¸onnais micro-continent within the subduction zone ledto a drastic change in the deformation pattern of the Alpinebelt, with rapid exhumation of the eclogite-facies ophiolitebearingunits and thrust propagation towards the foreland.Slab breakoff probably was responsible for allowing partialmelting in the mantle and Oligocene intrusions into themost internal parts of the Sesia–Dent Blanche nappes.Finally, indentation of the Adriatic plate into the orogenicwedge resulted in the formation of the Vanzone back-fold,which marks the end of the pervasive ductile deformationwithin the Sesia–Dent Blanche nappes during the earliestMiocene
“…In the II DK large rock volumes (1 km 3 ) still preserve pre-Alpine structures Ridley 1989;Vuichard 1989), and the same is true for the Valpelline Series (10 9 5 9 2 km) (Diehl et al 1952;Nicot 1977;Gardien et al 1994;Manzotti and Zucali 2013). In the Arolla Series and in the Lower Unit of the Mont Mary, mapping has revealed a few undeformed granitoids and gabbros as well (Diehl et al 1952;Elter 1960;Roda and Zucali 2008;Manzotti 2011;Roda and Zucali 2011). By contrast, the GMC shows pervasive Alpine deformation, undeformed volumes being conspicuously absent.…”
International audienceThis study reviews and synthesizes the presentknowledge on the Sesia–Dent Blanche nappes, the highesttectonic elements in the Western Alps (Switzerland andItaly), which comprise pieces of pre-Alpine basement andMesozoic cover. All of the available data are integrated in acrustal-scale kinematic model with the aim to reconstructthe Alpine tectono-metamorphic evolution of the Sesia–Dent Blanche nappes. Although major uncertainties remainin the pre-Alpine geometry, the basement and coversequences of the Sesia–Dent Blanche nappes are seen aspart of a thinned continental crust derived from the Adriaticmargin. The earliest stages of the Alpine evolution areinterpreted as recording late Cretaceous subduction of theAdria-derived Sesia–Dent Blanche nappes below theSouth-Alpine domain. During this subduction, severalsheets of crustal material were stacked and separated byshear zones that rework remnants of their Mesozoic cover.The recently described Roisan-Cignana Shear Zone of theDent Blanche Tectonic System represents such a shearzone, indicating that the Sesia–Dent Blanche nappes representa stack of several individual nappes. During thesubsequent subduction of the Piemonte–Liguria Oceanlarge-scale folding of the nappe stack (including the Roisan-Cignana Shear Zone) took place under greenschistfacies conditions, which indicates partial exhumation of theDent Blanche Tectonic System. The entrance of the Brianc¸onnais micro-continent within the subduction zone ledto a drastic change in the deformation pattern of the Alpinebelt, with rapid exhumation of the eclogite-facies ophiolitebearingunits and thrust propagation towards the foreland.Slab breakoff probably was responsible for allowing partialmelting in the mantle and Oligocene intrusions into themost internal parts of the Sesia–Dent Blanche nappes.Finally, indentation of the Adriatic plate into the orogenicwedge resulted in the formation of the Vanzone back-fold,which marks the end of the pervasive ductile deformationwithin the Sesia–Dent Blanche nappes during the earliestMiocene
“…11). Regional-scale Alpine ductile shear zones, comparable with the RCSZ, have been recognized along the Arolla-Valpelline boundaries in the Dent Blanche s.s. nappe (Diehl et al 1952;Pennacchioni and Guermani 1993), as well as within the Arolla and Valpelline Series (Roda and Zucali 2011). These shear zones represent the oldest Alpine structures recognized in the DBTS; they were folded by subsequent deformational phases.…”
Section: Early Alpine (Ductile) Evolutionmentioning
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 postnappe 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 precollisional rifting at the Adriatic continental margin preconditioned the efficiency of convergent processes, i.e. accretion, subduction, and orogenic exhumation.
“…Figure 1.(a) Tectonic map of the Alps (modified after Marotta & Spalla, 2007). (b) Simplified geological map of the Dent Blanche tectonic system (Roda & Zucali, 2011). (c) NW–SE geological cross-section along the Western Alps: SL – Sesia-Lanzo Zone; MR – Monte Rosa; DB – Dent Blanche; PI – Penninic; SB – Grand Saint Bernard; MB – Mont Blanc; LPN – Lower Penninic; H – Helvetic; MO – Molasse (Zucali, 2011).…”
-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.
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