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

Manzotti, Paola; Zucali, Michele; Ballèvre, Michel; Robyr, Martin; Engi, Martin (2014). Geometry and kinematics of the Roisan-Cignana Shear Zone, and the orogenic evolution of the Dent Blanche Tectonic System (Western Alps). Swiss journal of geosciences, 107(1), pp. 23-48. Birkhäuser 10.1007/s00015-014-0157-9

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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.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences

UniBE Contributor:

Manzotti, Paola; Robyr, Martin and Engi, Martin

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

1661-8726

Publisher:

Birkhäuser

Funders:

[4] Swiss National Science Foundation

Language:

English

Submitter:

Martin Engi

Date Deposited:

10 Aug 2015 09:57

Last Modified:

12 Aug 2015 21:39

Publisher DOI:

10.1007/s00015-014-0157-9

Uncontrolled Keywords:

Austroalpine nappes, Early Alpine tectono-metamorphic development, Basement-cover relations, Italy, Switzerland

BORIS DOI:

10.7892/boris.70829

URI:

https://boris.unibe.ch/id/eprint/70829

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