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)

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.

Protracted Subduction of the European Hyperextended Margin Revealed by Rutile U‐Pb Geochronology Across the Dora‐Maira Massif (Western Alps)

“…West of the Schistes Lustrés and in the Acceglio half-window at the same latitude, the "proximal" Briançonnais crops out along a restored section of ∼80 km (e.g., Ballèvre et al, 2020;Michard et al, 2004;Michard & Henry, 1988). Conservative estimates of the width of the Briançonnais s.l., including its distal margin (i.e., Dora-Maira) sum up to 210 ± 60 km, comparable to the ∼200 and ∼240 km restored sections proposed by Ballèvre et al (2020) and Pantet et al (2020), respectively. This structure is mainly inherited from the Early Jurassic rifting, which has impacts far into the Dauphinois domain (e.g., in Oisans, Bellahsen et al, 2012).…”

“…(2020) and Pantet et al. (2020), respectively. This structure is mainly inherited from the Early Jurassic rifting, which has impacts far into the Dauphinois domain (e.g., in Oisans, Bellahsen et al., 2012).…”

The Maira-Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolution of the Briançonnais distal margin

“…Units derived from the Briançonnais s.l. domain form the classical Briançonnais nappes of the Briançon area (Briançonnais sensu stricto) and its northern equivalents, The Maira-Grana Allochthons the Siviez-Mischabel Nappe (Genier et al, 2008;Pantet et al, 2020;Scheiber et al, 2013). The Mesozoic-Eocene series of the Briançonnais s.str.…”

“…The combination of published and new structural and petrological-thermometric data presented below allow us to discuss the relationships of the Maira and Grana allochthonous units (MGA) with other similar, so-called "Prépiémontais" (Prepiedmont, Prepiemonte), Triassic-Liassic units (Deville, 1986;Ellenberger, 1958;Lemoine, 1967;Pantet et al, 2020) as well as with the Schistes lustrés and various units derived from the Briançonnais sensu lato domain (Acceglio s.l. and Dora-Maira units).…”

Missing lawsonite and aragonite found: P–T and fluid composition in meta-marls from the Combin Zone (Western Alps)

“…Further work is required to integrate in this scheme the "Série Rousse", which is considered in our scheme (Fig. 7) as part of the detached cover from the Middle Penninic Units (see also Pantet et al 2020).…”