Mimicking Alpine thrusts by passive deformation of synsedimentary normal faults: a record of the Jurassic extension of t
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Swiss Journal of Geosciences
Open Access
ORIGINAL PAPER
Mimicking Alpine thrusts by passive deformation of synsedimentary normal faults: a record of the Jurassic extension of the European margin (Mont Fort nappe, Pennine Alps) Adrien Pantet* , Jean‑Luc Epard and Henri Masson
Abstract The Mont Fort nappe, former uppermost subunit of the Grand St-Bernard nappe system, is an independent tectonic unit with specific structural and stratigraphic characteristics (Middle Penninic, NW Italy and SW Switzerland). It consists in a Paleozoic basement, overlain by a thin, discontinuous cover of Triassic-Jurassic metasediments, mainly breccias, called the Evolène Series. The contact of this Series over the Mont Fort basement is debated: stratigraphic or tectonic? We present new observations that support the stratigraphic interpretation and consequently imply that the Evolène Series belongs to the Mont Fort nappe. We moreover show that the Mont Fort nappe was strongly affected by normal faulting during Jurassic. These faults went long unnoticed because Alpine orogenic deformation blurred the record. Alpine strain erased their original obliquity, causing confusion with an Alpine low-angle thrust. These Jurassic faults have been passively deformed during Alpine tectonics, without inversion or any other kind of reactivation. They behaved like passive markers of the Alpine strain. Detailed field observations reveal the link between observed faults and specific breccia accumulations. Areas where the Evolène Series is missing correspond to sectors where the fault scarps were exposed on the bottom of the sea but were too steep to keep the syn- to post-faulting sediments. The Mont Fort nappe thus represents an example of a distal rifted margin. The succession of synsedimentary extensional movements followed by orogenic shortening generated a situation where passively deformed normal faults mimic an orogenic thrust. Keywords: Western Alps, Paleofaults, Orogenic deformation, Tethyan distal margins, Hyper-extended continental margin, Penninic, Briançonnais, Prepiedmont 1 Introduction The Pennine Alps (SW Switzerland and NW Italy) show a complex tectonic architecture resulting from the Cenozoic collision of the European and Adriatic plates
Editorial handling: Stefan Schmid *Correspondence: [email protected] Institute of Earth Sciences, University of Lausanne, Géopolis building, CH‑1015 Lausanne, Switzerland
(e.g. Argand 1911, 1924, 1934; Escher et al. 1988, 1993, 1997; Escher and Beaumont 1997; Dal Piaz 1999; Lemoine et al. 2000; Steck et al. 2001, 2015; Schmid et al. 2004, 2017; Beltrando et al. 2010, 2014; Mohn et al. 2014; McCarthy et al. 2018, 2020). The orogen-parallel axial plunge of the large structures, bringing all the main tectonic and paleogeographic domains of the Alps to the surface, provides one of the most complete sections through the Alpine nappe stack (Figs. 1 and 2) which consists from bottom to top of:
© The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International L
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