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

Abstract

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.

Probing the transition between seismically coupled and decoupled segments along an ancient subduction interface

“…Our field and petrological observations document the existence of multiple generations of brittle events with distinct microstructural expression. Semibrittle to brittle deformation responsible for fabric A formation took place first within a cool, prograde regime as attested by the coeval presence of Mg‐riebeckite, rutile and high‐silica phengite crystals (Figure c; T = 400–450°C, P = 1.2–1.4 GPa) [ Ayrton et al ., ; Angiboust et al ., ; Manzotti et al ., , and references therein; Strøm, unpublished MS thesis, 1990]. The mineral modal difference between fabrics A and B (in particular the absence of blue amphibole) does not necessarily indicate a pressure drop.…”

Permian high-temperature metamorphism in the Western Alps (NW Italy)

“…The Dent Blanche Tectonic System (Manzotti et al 2014b) is the most external Adria-derived continental unit in the Western Alps (Fig. 1), surfacing north of the Aosta Ranzola fault, resting on top of remnants of the Piemonte-Liguria Ocean.…”

“…1), surfacing north of the Aosta Ranzola fault, resting on top of remnants of the Piemonte-Liguria Ocean. It comprises the Mont Mary nappe and the Dent Blanche nappe, which are separated by a 25 km long Alpine high-strain zone (the Roisan Cignana Shear Zone, Manzotti et al 2014b). The Mont Mary nappe and the Dent Blanche nappe consist of slices of continental basement rocks, representing sections of late-Palaeozoic upper crust (the Arolla Series in the Dent Blanche nappe and the Lower Unit in the Mont Mary nappe) and lower crust (the Valpelline Series in the Dent Blanche nappe and the Upper Unit in the Mont Mary nappe; e.g., Manzotti et al 2014a).…”

Permian magmatism and metamorphism in the Dent Blanche nappe: constraints from field observations and geochronology

“…Fig. 1 Geological map of the Dent Blanche Tectonic System (DBTS), showing the main undeformed gabbroic and granitic intrusives (modified after Manzotti et al 2014b). The blue star indicates the location of Fig.…”

“…The DBTS is made of the Dent Blanche (DB) and the Mont Mary-Cervino (MM) nappes ( Fig. 1) that are separated by the Roisan-Cignana Shear Zone (RCSZ), an Alpine km-scale high strain zone (Manzotti 2011;Manzotti et al 2014b). The Dent Blanche nappe comprises the Valpelline Series and the Arolla Series (Diehl et al 1952), as well as remnants of a Mesozoic metasedimentary cover (i.e., the Mt.…”