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Swiss Journal of Geosciences

, Volume 110, Issue 2, pp 503–521 | Cite as

Polyphase greenschist-facies reactivation of the Dent Blanche Basal Thrust (Western Alps) during progressive Alpine orogeny

  • Frederik Kirst
Article

Abstract

This study assesses the significance, geometry, and kinematics of greenschist-facies deformation along the Dent Blanche Basal Thrust (DBBT), a major tectonic contact in the Internal Western Alps of Switzerland and Italy. The DBBT separates continental units of the Dent Blanche nappe, the structurally highest unit in the Western Alps, from underlying Piemont-Ligurian ophiolites. Mylonites and deformation structures along the contact provide a record of its retrograde greenschist-facies evolution after earlier high-pressure metamorphism. A first phase of foreland-directed, reverse-sense, top-(N)W shearing (D1) occurred between ca. 43 and 39 Ma, related to exhumation of the Dent Blanche nappe from high-pressure conditions. It led to the formation of mylonitic fabrics under high- to medium-grade greenschist-facies conditions along the entire DBBT. A phase of ductile normal-sense top-SE shearing (D2) at ca. 38–37 Ma was mainly localized within underlying ophiolitic units and only partly affected the DBBT. Another phase of ductile deformation (D3) under medium- to low-grade greenschist-facies conditions at ca. 36–35 Ma occurred in response to underthrusting of European continental margin units and resulted in the updoming of the nappe stack. Especially the southeastern DBBT was characterized by bulk top-NW shearing, partly conjugate top-NW/top-SE shearing, and resulting orogen-perpendicular crustal extension. Subsequently, the DBBT was affected by a phase of orogen-perpendicular shortening (D4) and formation of folds and crenulations at ca. 34–33 Ma due to increasing compressional tectonics. Finally, a phase of semi-ductile to brittle normal-sense top-NW and conjugate shearing (D5) from ca. 32 Ma onwards particularly affected the southeastern segment and indicates exhumation of the DBBT through the ductile–brittle transition. This was followed by brittle NW–SE extensional deformation. This study suggests that the DBBT experienced a polyphase deformation and reactivation history under decreasing greenschist-facies metamorphic conditions during which different segments of this major shear zone were variably affected.

Keywords

Internal western Alps Dent Blanche Basal Thrust Shear zone reactivation Greenschist-facies deformation Retrograde shearing Continental collision 

Notes

Acknowledgements

This study was funded and supported by scholarships from the University of Bonn and the DAAD (Deutscher Akademischer Austauschdienst) as well as DFG (Deutsche Forschungsgemeinschaft) grant FR700/15-1. I thank Marco Herwegh and two anonymous reviewers whose constructive reviews helped to significantly improve the manuscript. Roland Oberhänsli and Uwe Ring are thanked for very helpful comments on an earlier version of this manuscript. I am indepted to Niko Froitzheim for help and support during my Ph.D studies as well as for help in the field and stimulating discussions. I thank Thorsten Nagel for help and support as well as for managing electron microprobe measurements. I am indebted to Jan Pleuger for constant support, fruitful discussions, and for getting me started on literature and maps at the beginning of my Ph.D studies. Comments and advice by Derya Gürer are greatly acknowledged. I thank guest editor Christian Sue and editor Stefan Schmid for very kind editorial handling.

Supplementary material

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Supplementary material 3 (XLSX 14 kb)

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© Swiss Geological Society 2017

Authors and Affiliations

  1. 1.Steinmann-InstitutUniversity of BonnBonnGermany

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