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 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.
Highlights
In axial parts of subduction-collision orogens, fragments of continental crust are commonly found intermingled with meta-sediments and meta-ophiolites, both oceanic and continental units displaying evidence of high-pressure metamorphism
The Roisan-Cignana Shear Zone (RCSZ) formed during several Alpine deformation phases and is characterized by tectonites and mylonites derived from protoliths of the Dent Blanche s.s. nappe (Arolla Series) and of the Mont Mary nappe (Lower and Upper Units) basements, from pre-Alpine meta-sediments and from a Mesozoic cover of unknown but presumably Austroalpine origin (Roisan Zone)
The Mesozoic Roisan Zone occurs only within the RCSZ and its thickness ranges from 30–50 m to 400–500 m. It is dismembered into bands and pods from metres up to 100 m in lateral extent, embedded within mylonites derived from the Arolla Series (Dent Blanche s.s. nappe) and from the Lower Unit (Mont Mary nappe)
Summary
In axial parts of subduction-collision orogens, fragments of continental crust are commonly found intermingled with meta-sediments and meta-ophiolites, both oceanic and continental units displaying evidence of high-pressure metamorphism. Further mapping has shown that the Mesozoic sediments are imbricated and folded together with intensely strained pre-Alpine orthogneisses, with the whole complex defining the RoisanCignana Shear Zone (Elter 1960; Canepa et al 1990; De Giusti et al 2003; Dal Piaz et al 2010; Manzotti 2011). This kilometric high-strain zone divides the DBTS into two nappes. We critically assess to what extent pre-collisional rifting (Mohn et al 2011) and post-collisional tectonic unroofing may account for the present position of the DBTS in the Western Alps, scrutinizing the evidence for the relevant processes, discerning their relative timing in each part of the evolution, and showing the present limits of understanding
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